Master Sciences Technologies Santé mention Sciences de la Terre spécialité Paléontologie Sédimentologie Paléoenvironnements

Stage de recherche M2 The coralline alga Leptophytum fœcundum (Kjellman): a new paleoenvironmental proxy for the Arctic Ocean? par

BOUGEOIS Laurie

Sous la direction de : Jochen Halfar, Assistant Professor of Geology

Department of Chemical & Physical Sciences, University of Toronto at Mississauga

Informations destinées à la Bibliothèque des Sciences de la Terre Date de présentation orale : 10 Juin 2011 Mots-clés : Océan Arctique, géochimie, sclérochronologie, algues corallines encroûtantes, paléoenvironnements.

Université Claude Bernard Lyon 1

École Normale Supérieure de Lyon -2011-

Laurie Bougeois

L’algue coralline Leptophytum fœcundum (Kjellman) : un nouveau proxy paléoenvironnemental pour l’océan Arctique ?

Résumé Connaître le climat passé à des échelles annuelles et décennales est essentiel à la compréhension des changements climatiques observés aujourd’hui. Du fait de leur localisation polaire, les régions arctiques sont déterminantes pour le climat global sur Terre. Dans le cadre de cette étude, je me suis intéressée à la mer de Beaufort, secteur de l’Océan Arctique qui joue un rôle essentiel dans le climat de la région. J’ai utilisé l’algue coralline encroûtante Leptophytum fœcundum pour reconstruire l’évolution de la composition isotopique de la mer de Beaufort de 1991 à 2006. Le but de cette étude est de déterminer si cette espèce peut être définie comme un nouveau proxy paléoenvironnemental pour l’Océan Arctique. L’enregistrement des isotopes de l’oxygène et du carbone dans le squelette calcaire pourrait nous permettre de connaître la conséquence du dégel du pergélisol sous-marin de la mer de Beaufort sur un relargage potentiel de méthane provenant du réservoir d’hydrates de gaz. Cela nous permettrait également d’estimer l’importance de l’absorption de CO2 de l’atmosphère vers l’océan Arctique, l’augmentation de la productivité primaire et l’influence des eaux de ruissellement des rivières arctiques dans l’océan. Tous ces paramètres jouent un rôle essentiel dans le climat arctique et donc dans le climat global. La proximité du site avec le delta d’une rivière a malheureusement perturbé l’enregistrement isotopique de cette étude et l’influence océanique sur la géochimie de l’algue n’a pas pu être distinguée de l’influence de la rivière. Par ailleurs, à cause de sa vitesse de croissance très faible, Leptophytum fœcundum ne semble pas être un proxy fiable pour reconstruire les paléoclimats et les paléoenvironnements océaniques arctiques du dernier siècle. Mots-clefs : Océan Arctique, géochimie, sclérochronologie, algues corallines encroûtantes, paléoenvironnements.

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Master Thesis

The coralline alga Leptophytum fœcundum (Kjellman): a new paleoenvironmental proxy for the Arctic Ocean?

Abstract Knowing the past climate on annual to decadal scales is essential to understand the climate change observed today. Because of their pole location, Arctic regions are determinant for the global climate in Earth. Here I am interested in the Beaufort Sea in the Arctic Ocean which plays a main role in the Arctic climate. I use the crustose coralline Leptophytum fœcundum to reconstruct the evolution of the isotopic composition of the Beaufort Sea from 1991 to 2006. The aim of this study is to determine if this species can be defined as a new paleoenvironmental proxy for the Arctic Ocean. The record in calcitic skeleton of carbon and oxygen isotopic compositions may allow us to know if the subsea permafrost thawing in Beaufort Sea yields a methane release from gas hydrate reservoir. But it also allows us to estimate the importance of the CO2 uptake from the atmosphere into the ocean, the increase of primary production, and the influence of runoff from Arctic rivers in the ocean. All of these parameters play a role in the Arctic climate, then in global climate. Because of a proximity of river drainage in the site area, the isotopic record is too much disturb and the oceanic influence has not been distinguish from the river influence. Furthermore, because of their very low growth rate, Leptophytum fœcundum appears to not be a reliable proxy to reconstruct the paleoclimate and the Arctic oceanic paleoenvironments for the last century. Keywords: Arctic Ocean, geochemistry, sclerochronology, crustose coralline algae, paleoenvironments.

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Contents Introduction 1

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Geological Setting 1.1 Arctic environments and study area . . . . . . . . . . . . . . . . . . . . 1.1.1 Location of the study area . . . . . . . . . . . . . . . . . . . . . . 1.1.2 The Arctic climate and its oceanic environments . . . . . . . . . 1.2 The oceanic Suess effect in Arctic Ocean . . . . . . . . . . . . . . . . . . 1.3 Impact of shrinking Arctic ice cover on the oceanic primary production 1.4 The Beaufort Sea submarine permafrost as a gas hydrate reservoir . . . 1.4.1 The permafrost environments . . . . . . . . . . . . . . . . . . . . 1.4.2 Permafrost and gas hydrates: role in the methane cycle and climate change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Methane release on Arctic shelf and climatic consequences . . . Material and Methods 2.1 Leptophytum fœcundum (Kjellman) . . . . . . . . . . . . . . . . . . . . . . 2.1.1 General aspects of crustose coralline algae as paleoenvironmental archives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Coralline collection and taxonomy . . . . . . . . . . . . . . . . . 2.2 Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Geo.TS Microscope Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Electron Microprobe Analysis . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Development of chronology . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Isotope Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.1 Micromill Drilling . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6.2 Mass Spectrometer and Isotope Analysis . . . . . . . . . . . . .

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9 9 9 10 11 12 13 13

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17 17 19 19 19 23 23 23 24

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Results 25 3.1 Chronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.2 Microprobe results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.3 Isotope results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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Discussion 4.1 Development of chronology . . . . . . . . . . . . . . 4.2 Sea surface temperature deduced from Mg/Ca ratio 4.3 Record of surface water oxygen composition . . . . 4.4 Record of surface water carbon composition . . . . . 4

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Master Thesis 4.4.1 4.4.2 4.4.3

Contents Gas hydrate dissociation and methane release . . . . . . . . . . . 32 Influence of local and regional oceanic effects . . . . . . . . . . . . 33 Growth rate influence on the carbon isotopic record . . . . . . . . 36

Conclusion and Prospects

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Acknowledgements

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Tables of values

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Bibliography

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Changes in climate that have already taken place are manifested in the decrease in extent and thickness of Arctic sea ice, permafrost thawing, coastal erosion, changes in ice sheets and ice shelves, and altered distribution and abundance of species in polar regions (...). Climate change in polar regions is expected to be among the largest and most rapid of any region on the Earth, and will cause major physical, ecological, sociological, and economic impacts, especially in the Arctic, Antarctic Peninsula, and Southern Ocean (...). Polar regions contain important drivers of climate change. Once triggered, they may continue for centuries, long after greenhouse gas concentrations are stabilized, and cause irreversible impacts on ice sheets, global ocean circulation, and sea-level rise (...). IPCC (2001)

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Introduction Present and future climate change and the human impact on these are a main issue for scientists and politicians. To understand actual changes and model future changes, climate variations during the last centuries need to be understood. Because of its importance in modulating climate during the Quaternary (Ruddiman (2002); Kennett (2003)), Arctic paleoenvironments are in the center of interest of climatologists (Kvenvolden et al. (1993); IPCC (2001); A.C.I.A. (2005); Shakhova and Semiletov (2007); Schuur et al. (2008); Shakhova et al. (2010); Kerr (2010); Heimann (2010); Grosse et al. (2011)). In these regions, the impact of global warming on sea-ice melting and thus on the Arctic Ocean salinity, plays a primary role in future climate change. Because of its main role in the sinking of deep waters in the northern North Atlantic Ocean, change in salinity in the Arctic Ocean might have serious consequences on the thermohaline circulation. Or, this circulation determines the heat flux across latitude and is an essential factor in the Earth climate (Ruddiman (2002)). Furthermore, the positive or negative feedback through albedo or carbon sink change linked to sea-ice melting could determine the climate evolution for the next decades, or the next centuries (IPCC (2001); A.C.I.A. (2005); Perovich et al. (2007)). Natural proxy archives that give indirect information on past climatic conditions can help scientists to extend climate data in space and time. Reconstructions of past climates and environments can extend the sparse record of instrumental measurements, and thus provide us with a deeper understanding of past global changes (Ruddiman (2002)). In high latitudes, most annual and decadal climate reconstructions are based on terrestrial proxies such as ice cores from glaciers, tree rings, or varved lake sediments (Overpeck et al. (1997); Steffensen et al. (2008); Kaufman et al. (2009)). These data do not provide any marine environmental conditions and little is known about the evolution of Arctic Ocean environments for last centuries. The majority of annual to decadal resolution surface ocean climate reconstructions are almost exclusively based on corals (Nozaki et al. (1978); Swart et al. (2010)), sclerosponges (Swart et al. (1998)), and bivalves (Butler et al. (2010); Schöne et al. (2011)). However, hermatypic coral and sclerosponges are limited to tropical and subtropical seas, excluding the possibility of high latitude climate reconstructions. Although bivalves provide the majority of extratropical near surface marine climate data (Wanamaker Jr et al. (2011)), they experience an ontogenetic decline in shell growth as the organism ages, decreasing the resolution than can be measured with time (Butler et al. (2010)). Thus, due to the biological constrains of bivalves and the limited biogeographical range of coral species, these biological archives are unsuitable for reconstructing long-term environmental trends in arctic regions on multi-decadal timescales. Previ7

Introduction

Laurie Bougeois

ous studies have shown that coralline algae are an ideal biogenic marine climate archive because of their common occurrence in mid to high-latitude oceans, their longevity, and their incremental growth pattern (Halfar et al. (2007, 2008, 2010); Kamenos et al. (2008, 2009); Hetzinger et al. (2009, 2011); Gamboa et al. (2010); Williams et al. (2010); Chan et al. (2011)). These sub-arctic studies focus on the genus Clathromorphum sp. or Lithothamnion glaciale and demonstrate the use of this new proxy for climatic, and ocean current reconstructions. Recently, the evidence of the effects of the East Siberian subsea permafrost thawing and methane release from gas hydrate buried under the permafrost (Shakhova and Semiletov (2007); Shakhova et al. (2010)) alerted scientists due to potential consequences on the carbon cycle in Arctic regions and therefore global climate. Hence, as part of this study I tested if this methane release can also be detected in other parts of the Arctic ocean, such as the Beaufort Sea. In order to achieve this I have tested the suitability of using the crustose coralline alga Leptophytum fœcundum (Kjellman) as a paleoenvironmental proxy for detecting an isotopic signal into the Beaufort Sea. However this species has not been studied previously as an paleoenvironmental proxy. Hence, the aim of this study was to determine if the coralline algae L. fœcundum can be used as a paleoenvironmental archive for the Arctic Ocean. This will be achieved by comparing the geochemical record in the calcitic skeleton with the recent measure in-situ or by satellite of Arctic Ocean properties. In this way, I studied growth rates and growth increment formation of L. fœcundum to verify if a chronology can be built for the past decades. I have also investigated the carbon and oxygen isotopic composition (δ 18 O and δ 13 C 1 ) and trace element composition of the calcitic skeleton. If L. fœcundum proofs to yield a reliable record of Arctic Ocean parameters, futures studies may use older specimens in order to know the evolution of Arctic Ocean during the past centuries during which we have not direct data. To this date it remains unresolved if the environmental change in Arctic Ocean is part of natural multidecadal climate variability or an unique feature of anthropogenically induced warming. This way we may understand the recent evolution of the Arctic Ocean concerning the salinity and the vertical stratification, or the surface and deep currents (Aagaard and Carmack (1989); Macdonald et al. (2002); Woodgate and Aagaard (2005)). After a presentation of the Arctic environments and the geological setting of this study, I will present the crustose coralline L. fœcundum and the methods used. The development of a chronology across the coralline skeleton allows me to establish the isotopic and trace element variations recorded through time. Finally, I will discuss Arctic Ocean properties which can influence the composition of the algal skeleton.

1

Notation: δ(%) = (Rsample /Rstd − 1) × 1000, with R = isotopic ratio 18 O/16 O or 13 C/12 C and std = international standard, here Peedee Belemnite Standard (PDB)

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Chapter 1 Geological Setting 1.1

Arctic environments and study area

1.1.1

Location of the study area

The Arctic Ocean is located around the North pole. With two main basins of more than 4000 m depth: the Eurasian and Canada basin, the Arctic Ocean is almost completely landlocked (Figure 1.1a). The total ocean area is 11.5 millions km2 , of which 60% is continental shelf (A.C.I.A. (2005)). The Arctic ocean is composed of eight geographic sectors defined according to their longitude: Chukchi (180 − 160◦ W), Beaufort (160 − 100◦ W), Baffin (100 − 45◦ W), Greenland (45◦ W − 15◦ E), Barents (15 − 55◦ E), Kara (55 − 105◦ E), Laptev (105 − 150◦ E) and Siberian (150 − 180◦ E) (Pabi et al. (2008)). This study focuses on the Arctic inner shelf in the Beaufort Sea (147◦ 40′ W, 70◦ 20′ N), Alaska, US (Figure 1.1a). The Beaufort Sea shelf has a relatively narrow (∼ 50 km) continental shelf in contrast to the adjacent Chukchi Sea shelf (Bates and Mathis (2009)). The area of the Beaufort Sea shelf is 178, 000 km2 , with a mean depth of 124 m, and the residence time of the waters is 0.5 to 1.0 year (Macdonald et al. (2009)). The Beaufort Sea is characterized by very low annual salinity between 28 to 30 and annual temperature between −1.5 to −0.5◦ C (Pabi et al. (2008); Arrigo et al. (2010)). The study area is close to Prudhoe Bay, off the Sagavanirktok River (often called Sag River) delta (Figure 1.1b). This river is 289 km long and originates on the north slope of the Brooks Range (between the Endicott and the Philip Smith mountains), in northern Alaska. The river flows across the Arctic Slope province before entering the Beaufort Sea of the Arctic Ocean. The Sag River, principally fed by snowmelt with some groundwater and glacial influence (Benke and Cushing (2005)), is frozen between September and May and thaws in late May to June. Paradoxically, even if the Sag River is the most easily accessible river on the Alaskan North Slope (because of the Trans-Alaskan Pipeline construction and development), few studies dealing with isotopic or trace-elements composition are available. Actually, the deltaic nature of the river makes total measurements difficult (Benke and Cushing (2005)). Most of the readily accessible hydrologic information for the Sag River comes from US Geological Survey (USGS) gaging stations. 9

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Beaufort Sea

147°40'W 70°20'N

Prudhoe Bay

Kavik River Colville River Sagavanirktok River

Alaska (US) N W

20 km

(a) Map of Arctic Ocean showing bathymetry, Beaufort Sea and study area (red circle). Adapted from Pabi et al. (2008)

E S

(b) Detail of Prudhoe Bay and Sagavanirktok river showing the proximity of the study area to the Sag River delta. Adapted from http://www.maps.google.com

Figure 1.1: The study area

1.1.2

The Arctic climate and its oceanic environments

Because of a permanent anticyclone above the ocean, Arctic climate is characterized by extremely low annual precipitation (around < 15 cm/yr, similar to deserts), which falls mostly as snow (Wagner (2008)). Mean monthly temperatures in the Alaska arctic range from about −40 to 0◦ C in January and from −10 to 10◦ C in July (Benke and Cushing (2005)). Temperatures vary according to latitude and longitude, and have increased by +0.09◦ C/decade from 1900 to 2003 and +0.40◦ C/decade from 1966 to 2003 (A.C.I.A. (2005)). About 70% of the Arctic Ocean is ice-covered throughout the year (Bauch et al. (2000); A.C.I.A. (2005)), but recent global warming since pre-industrial age has diminished the sea ice extend. The increased freshwater flux from both glacial melt and the melting ice, which has increased in the Arctic from 8000 km3 in 1980 to 17000 km3 in 1997, reduces surface salinity and increases the solubility of CO2 (Arrigo et al. (2010) and references therein).

Numerous properties of the Arctic Ocean changed or may change since the preindustrial age. This study is mainly concerned with changes in the carbon isotopic composition of the Arctic Ocean. Hence, my thesis will focus on the Suess effect, the impact of warming on the primary production and the consequences of the subsea permafrost thawing. 10

Master Thesis

1.2

1. Geological Setting

The oceanic Suess effect in Arctic Ocean

During photosynthesis, autotrophic organisms incorporate into their living tissues preferentially 12 C rather than 13 C (Ruddiman (2002)). Thus organic matter (OM) has a negative isotopic signature for δ 13 COM relative to atmospheric CO2 (Craig (1953)). Since the industrial age, humans burn isotopically light fossil fuels (such as oil or coal), which decreases the value of carbon reservoirs of the atmosphere (Nozaki et al. (1978); Williams et al. (2010)). This trend is called δ 13 C-Suess effect (DIC = Dissolved Inorganic 2− Carbon = CO2 +H2 CO3 +HCO− 3 +CO3 ), named after the Austrian chemist Hans Suess, who noted the influence of fossil fuel burning on radiocarbon dating (Suess (1953)). During the last 200 years, the δ 13 Catm (stable carbon isotope ratio of atmospheric CO2 ) has decreased from ca −6.3% to −8.1% (Schöne et al. (2011)). Approximately 118 ± 19PgC (1PgC = 1015 gC = 1Gt carbon), i.e. ca. 30% of the total carbon dioxide emitted to the atmosphere by human activity between 1800 and 1994 has been absorbed by the oceans in the form of DIC (Sabine et al. (2004)). Because of this equilibrium between atmospheric and hydrospheric reservoirs, δ 13 CDIC values record the δ 13 CDIC -Suess effect also called oceanic Suess effect. For the past years the oceanic Suess effect has been of great interest to scientists because the rate of change of the δ 13 CDIC signature can be used to estimate the oceanic uptake rate of atmospheric CO2 (Gruber et al. (2002)). The fluctuations of this uptake are important for climate modelling, as changes in the strength of the oceanic carbon sink are inversely correlated to the CO2 concentration in the atmosphere (Schöne et al. (2011)). Furthermore, the oceanic absorption of carbon lowers sea water pH. Such an acidification of the ocean could have an important impact on marine biosphere, as it triggers a shoaling of the calcite compensation depth (CCD) (Zachos et al. (2005)). Nozaki et al. (1978) were the first to note the oceanic-Suess effect in calcareous coral skeletons in low latitude. Recent studies on coral skeletons (Swart et al. (2010)) confirm that the δ 13 C of the atmosphere controls ambient δ 13 CDIC of the dissolved inorganic carbon which in turn is reflected in the calcareous skeletons. Williams et al. (2010) showed that the coralline algae Clathromorphum nereostratum records a δ 13 CDIC -Suess effect in Northern North Pacific Ocean an Bering Sea (Figure 1.2a). A δ 13 CDIC decrease of −0.9% from 1850 to 1987 is also recorded in the Arctic Ocean in the polar planktic foraminifer Neogloboquadrina pachyderma (sinistral) (Bauch et al. (2000), Figure 1.2b). Such an anthropogenic decrease in δ 13 CDIC for the Arctic Ocean halocline waters is relatively high compared to values measured in other areas of the world oceans in low latitudes (Bauch et al. (2000)) such as −0.6% in West Africa from planktic foraminifera (Beveridge and Shackleton (1994)), or −0.7% Coral sea from demosponges (Bohm et al. (1996)). The amount of the surface ocean 13 C-Suess effect represents the degree of equilibration with the atmosphere and is thus a measure of the ventilation of the surface water with respect to CO2 (Bauch et al. (2000)). It appears that the Arctic Ocean, with only 2% of the global ocean area, is responsible for about 4 to 6 % of the global ocean’s CO2 uptake (Bauch et al. (2000)). It can be concluded that the Arctic Ocean halocline waters are well equilibrated with the atmosphere. This seems surprising because the Arctic Ocean is covered with sea-ice and shelf regions are free of sea-ice during the summer season only. But the spread of the halocline waters in a relatively thin layer of around 11

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(a)

(b)

Figure 1.2: Oceanic Suess effect recorded in paleoenvironmental archives (a.) Oceanic Suess effect for three Clathromorphum nereostratum specimens collected offshore of the Aleutian Islands with 3-year running mean in bold. The δ 13 C values are relatively constant from the 1940s to the 1960s then the decadal rates of decrease are 0.3% for the 1960s, 0.5% for the 1970s, 0.7% for the 1980s and 0.95% for the 1990s. From Williams et al. (2010). (b.) δ 13 C values of N. pachyderma (sin) from the water column and from core top sediments allow to determined the surface ocean Suess effect which is estimated to be about −0.9% (±0.2%) within the Arctic Ocean halocline waters according to Bauch et al. (2000). 50 m (Kandel and Pinot (2008)) and the large input of rivers water allows for a better isotopic exchange with the atmosphere (Bauch et al. (2000)).

1.3

Impact of shrinking Arctic ice cover on the oceanic primary production

Changes in climate during the last decades are manifested in the decrease in extent and thickness of Arctic sea ice (IPCC (2001)). The loss of sea ice increases in the last years, culminating in a 2007 summer minimum ice extent that was 23% below the previous low (Arrigo et al. (2008)). This long term decline in sea ice is attributed to: increased advection of warm water into the Arctic Ocean (Steele and Boyd (1998); Maslowski et al. (2001); Shimada et al. (2006)), atmospheric circulation favoring ice advection out of the Arctic (Rigor and Wallace (2004); Maslanik et al. (2007)), increased Arctic temperatures due to greenhouse warming (Rothrock and Zhang (2005); Lindsay and Zhang (2005)). Furthermore, the above processes result in a strong ice albedo feedback (Perovich et al. (2007)). Parallel to this decrease in ice extent, annual primary production in the Arctic has increased by ∼ 30% from 356 to 459 Tg.C.yr−1 between 1998 and 2006 (Pabi et al. (2008)). 12

Master Thesis

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It appears that 30% of this rise was due to increased open water habitat, and 70% to a longer phytoplankton growing season (Arrigo et al. (2008), Figure 1.3).

Figure 1.3: Open water area in the Arctic Ocean between 2003 and 2007 averaged over the entire year,

as well as corresponding increases in annual primary production in the Arctic. The solid horizontal line represents the mean annual primary production in the Arctic for the years 1998-2002. Adapted from Arrigo et al. (2008)

During the assimilation of carbon by ocean phytoplankton the net fractionation is near −22% for the δ 13 COM relative to the Peedee Belemnite (PDB) standard (Ruddiman (2002)). By this mechanism, primary production has an impact on the isotopic composition of dissolved inorganic carbon (DIC). When the primary production increases, the δ 13 CDIC increases at the same time.

1.4

The Beaufort Sea submarine permafrost as a gas hydrate reservoir

1.4.1

The permafrost environments

Permafrost is commonly defined on thermal and temporal criteria as ground that remains below 0◦ C for at least two consecutive years. It may comprise bedrock, sediment, soils or organic materials (Grosse et al. (2011)). Models and geophysical data allow to locate continental and sub-marine permafrost distribution in the Arctic shelf (Figure 1.4a, Wagner (2008)). The Arctic permafrost comes from iced ground formed during the last glaciation. The submarine (or subsea) permafrost consists of relict continental permafrost when sea levels were lower than today (Wagner (2008)). During the Holocene, sea level rise flooded the formerly terrestrial permafrost (Romanovskii et al. (2005)) which became a submarine permafrost. 13

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(a) Terrestrial and submarine permafrost distribution in the Northern Hemisphere. International Permafrost Association Standing Committee on Data Information and Communication 2003. Adapted from Wagner (2008).

(b) Idealized cross section through northern permafrost regions showing the major known and assumed carbon pools related to permafrost and their total carbon stores, in petagrams as known to date. Adapted from Grosse et al. (2011).

Figure 1.4: Location and cross section of Arctic permafrost .

1.4.2

Permafrost and gas hydrates: role in the methane cycle and climate change

The permafrost environments host diverse and abundant microbial communities, which have to overcome the combined action of extremely cold temperatures, freezethaw cycles, desiccation and starvation (Wagner (2008)). Methanogenic Archaea and methane-oxidizing bacteria are the object of particular attention because of their role in the Arctic methane (CH4 ) cycle and of their significance for the global methane budget. Actually, biogenic methane is produced almost exclusively by the methanogens (CH4 -producing Archaea) at the end of their metabolism to produce energy in anaerobic environment. Methane plays an important role in modulating climate since it is the third most abundant greenhouse gas after water vapor (H2 O) and carbon dioxide (CO2 ) (Heimann (2010)) and it is a powerful greenhouse gas 25 times more potent than carbon dioxide (Kerr (2010)). Mainly methane (99.5% after Weaver and Stewart (1981)), but also carbon dioxide and hydrogen sulfide produced by microbiota in permafrost environments are stocked in gas hydrate in contact with water. The ice formed by water creates a cage in which gas molecules are enclosed, hence the name of clathrate, from Latin clatratus for cage (Figure 1.5a, Maslin et al. (2010)). The metabolism of Archaea incorporate preferentially light carbon in organic matter (OM) leading to an isotopic signature of methanederived carbon of δ 13 COM between ∼ −60 % (Kvenvolden (1993); Dickens et al. (1995)) and ∼ −65 % (Kennett et al. (2000)) relative to the Peedee Belemnite (PDB) standard. Hence the isotopic composition of methane contained in gas hydrate is clearly distinct from the isotopic composition of other oceanic and atmospheric carbon reservoirs with 14

Master Thesis

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a δ 13 C = 0 ± 7% (Paull et al. (2003)).

(a)

(b)

Figure 1.5: Gas hydrate structure and phase diagram. (a) Gas hydrate structure: water molecules are linked together to form a cage trapping a gas molecule. From Maslin et al. (2010). (b) Methane hydrate phase diagram: free gas methane (white) and methane hydrate (grey) for a pure water and pure methane system. From Kvenvolden (1993) Methane hydrate exists in metastable equilibrium, thus small changes in pressure and temperature can cause it to dissociate (Figure 1.5b) and release gas into ocean and atmosphere (Kvenvolden (1993, 1999, 2000)). The stability of gas hydrates in polar regions is sustained by the existence of permafrost, especially in subsea permafrost (Rachold et al. (2007)), where surface temperatures are < 0◦ C (Kvenvolden (1993, 1999)). The Beaufort Sea shelf is particularly favourable for gas hydrate development since high concentration of methane exist beneath the submarine permafrost in the young sediments which are under anomalous pressure (Weaver and Stewart (1981)).

1.4.3

Methane release on Arctic shelf and climatic consequences

Dissociation of gas hydrate has likely operated in the past with devastating effects on global ecosystems. During the hyperthermal at the end of Paleocene (Paleocene-Eocene Thermal Maximum, PETM) 55.5 Ma ago, Antarctic sea surface temperature increased by 5◦ C, in less than 10000 years (Kennett and Stott (1991); Zachos et al. (2005, 2008)). This lead to the most important benthic foraminifera extinction of the last 90 million years (Thomas (2007)) and lots of change in biosphere, as the apparition of modern mammal (Gingerich (2006)). Because of the important negative carbon isotopic excursion (CIE) found in carbonate, Dickens et al. (1995) hypothesize that methane release from deep ocean gas hydrate would be responsible of abrupt temperature increase. The "Clathrate Gun Hypothesis" formulated by Kennett (2003) suggests also that the dissociation of gas hydrate was responsible for the abrupt temperature variations during late Quaternary climate change. During the Holocene, subsea permafrost has experienced a drastic change in this thermal regime because of seawater inundation (Shakhova et al. (2010)). Today, the annual average temperature of Arctic shelf bottom seawater is 12◦ to 17◦ C warmer than 15

1. Geological Setting

Laurie Bougeois

the annual average surface temperature over on-land permafrost. (Romanovskii et al. (2005)). Hence, submarine permafrost is more vulnerable to thawing than terrestrial permafrost and thus more permeable for gases (Shakhova et al. (2010)). Given the past climatic events and the role which gas hydrate dissociation can play, particular attention has to be paid to thawing of subsea permafrost and its consequences on clathrate stability. The large amount of ancient organic matter contained in permafrost might be involved in current biogeochemical cycling due to thawing of the upper permafrost and restoration of the activity of viable methanogens preserved inside the permafrost (see Shakhova and Semiletov (2007) and reference therein). Previous studies have shown a recent increase in methane being released by the subsea permafrost on the East Siberian Arctic Shelf (Shakhova and Semiletov (2007); Shakhova et al. (2010)). According to a study discussing field campaign between 2003 and 2008, Shakhova et al. (2010) suggest the annual out-gassing from this region at present amounts to about ∼ 8 × 1012 gram of carbon (0, 008 GtC). Therefore, the subsea permafrost is extremely vulnerable to ongoing regional warming which may be the cause of the methane hydrate dissociation. Considering the global emissions of ∼ 440 TgC of methane per year (Denman et al. (2007)), the Siberian Arctic Ocean emissions are negligible. Hence, the current climate change does not affect the natural methane cycle in a globally important way (Heimann (2010)). But we do not know if this will persist into the future under a sustained warming trend. Based on seismic reflection studies, Kayen and Lee (1991) established a map of the Beaufort Sea continental shelf showing the distribution of gas hydrates (Figure 1.6).

Figure 1.6: Map of the continental margin of the Beaufort Sea offshore Alsaka showing gas hydrate deposits. Adapted from Kayen and Lee (1991) The recent observations on the continental shelf of the Siberian Arctic Ocean (Shakhova et al. (2010)) show that gas hydrate dissociation due to the thawing of the sub-marine permafrost is possible. No study to date has dealt the Beaufort subsea permafrost. This explains my motivation to establish a carbon isotopic record in Arctic coralline algae, in order to evaluate the possibility of an ongoing methane release from the Beaufort subsea permafrost.

16

Chapter 2 Material and Methods 2.1

Leptophytum fœcundum (Kjellman)

2.1.1

General aspects of crustose coralline algae as paleoenvironmental archives

Corallines are among the most diverse and abundant organisms occupying intertidal and shallow subtidal zones worldwide (Steneck (1986)). They are shallow marine calcareous plants that commonly exist as nodular free-living rhodoliths or encrusting algae, both of which have been used for paleoclimate analysis (Halfar et al. (2000, 2007, 2008); Gamboa et al. (2010)). In this study, I utilize encrusting (nongeniculate) corallines, also known as “crustose corallines” which commonly grow on hard substratum(Steneck (1986)). Photosynthetic crustose coralline algae secrete a high-magnesium calcite skeleton. Seasonal decreases in temperature and light during winter periods reduce the calcification rate of the algae resulting in growth increment lines in the skeleton (Adey and McKibbin (1970); Halfar et al. (2007)). The skeletal morphology changes from small cells with dense walls to larger cells with thinner walls resulting in a growth line in late spring. Furthermore, sporangial (i.e. spore-producing) conceptacles (cavities in which the reproductive structures develop) form in fall and early winter (Adey (1966)). Parts of the conceptacles intrude downward by decalcification, so that the greatest width of the conceptacle cavity may be in an algal skeleton that has precipitated in the summer. In cases of minimal yearly growth, conceptacle bases can intrude into the previous year’s growth (Moberly Jr (1968)). Based on recent SEM (Scanning Electron Microscopy) studies of coralline algal cellular structure, it has been hypothesized that the formation of annual growth bands also occurs in the fall and early winter, coinciding with the formation of conceptacles (Adey, 2011 personal communication).

2.1.2

Coralline collection and taxonomy

Coralline algae used in this study were collected alive from 5 m depth during a research cruise in July 2007 by Brenda Konar (University of Alaska, Fairbanks) using SCUBA. According to Adey (1970b); Adey and Macintyre (1973); Athanasiadis and Adey (2006) and Walter Adey (personal communication), specimens collected in the Beaufort 17

2. Material and Methods

Laurie Bougeois

high magnesium calcite skeleton

pebble

conceptacle

1 cm

(a) L. fœcundum in life position growing on pebble. Young

conceptacle growth line

Growth direction

Old

Pebble = Substratum

5 mm

(b) Cross section of L. fœcundum (BSA2) perpendicular to growth direction showing growth direction, growth lines and conceptacles.

Figure 2.1: Anatomical characteristics of L. fœcundum calcareous skeleton

Sea are identified as Leptophytum fœcundum (Kjellman) which is an arctic species that extends into the subarctic deep water (Adey (1970b)) and grows on pebbles and stones (Athanasiadis and Adey (2006)) (Figure 2.1). L. fœcundum is a thin crustose species showing no major surface irregularity, except those created by overgrowing of an irregular substrate (Athanasiadis and Adey (2006)). Overgrowth of one plant by another seems to be quite rare in this species, and continued development of the perithallium is restricted (Adey et al. (2001)). Buried multiporate asexual conceptacles are very frequent and their chambers are 220-500 µm in diameter and 140-250 µm high (L. fœcundum var. sandrae) (Athanasiadis and Adey (2006)). Unfortunately no information about the timing of sexual conceptacle formation is available. Indeed, L. fœcundum was essentially studied in the Gulf of Maine by Walter Adey and collaborators (Adey (1966, 1970a,b); Adey and Macintyre (1973); Athanasiadis and Adey (2006)) where it is a relatively rare plant. One important characteristic about Beaufort Sea Leptophytum fœcundum is its very low growth. Because of the temperature and the pole’s proximity (involving winter darkness) the growth conditions are extreme. According to laboratory experiments (Adey (1970b)), growth rates of L. fœcundum range from 1 to 9 µm/day, depending on ambient temperature and light (Figure 2.2). 18

Master Thesis

2. Material and Methods

35L/8

50L/14

20L/8

Figure 2.2: Leptophytum fœcundum marginal growth rate as a function of temperature and light.

The three curves depend of the light intensity (lux) and the light cycle (hours of illumination per day). Red=50lux/14, Blue=35lux/8 and Green=20lux/8. Adapted from Adey (1970b)

2.2

Sample Preparation

After being separated from the pebbles, L. fœcundum specimens were prepared for geochemical analysis by cutting 1 cm thick slices perpendicular to the direction of growth using a circular diamond precision saw (Buehler IsoMet 1000). Samples were subsequently epoxyed onto glass slides and machine polished to 1 µm with a Logitech CL50 using diamond-polishing suspensions with grit sizes of 9, 3, and 1 µm on a Buehler polishing disk. Polished sections were cleaned in an ultrasonic bath for ten minutes with deionized water and dried overnight in between polishing steps. Sectioned and polished (to 1 µm) alga is shown in Figure 2.1.

2.3

Geo.TS Microscope Analysis

Digital images of the polished surface were produced using an Olympus reflected light microscope (BX51) attached to an automated sampling stage-imaging system equipped with geo.TS software (Olympus Soft Imaging Systems). This setup allows a 2-dimensional mapping of the surfaces of the polished specimens at various magnifications (Gamboa et al. (2010)). The resulting high resolution photomosaics enabled the identification of growth patterns over the entire sample and the subsequent selection of sampling locations (Figure 2.3). Using geo.TS images locations of conceptacles, annual growth increments and growth lines were identified.

2.4

Electron Microprobe Analysis

Although magnesium (Mg) ions are more than five times as abundant as calcium (Ca) in sea water (Moberly Jr (1968)), Ca is incorporated faster in a calcite skeleton than Mg. The substitution of Mg in calcite is an endothermic reaction, thus it is favoured by higher temperatures, providing the use of Mg/Ca ratio as a paleothermometer in foraminifera (Lea (2003)). 19

2. Material and Methods

Laurie Bougeois

5 mm

Growth dir ection

conceptacle high magnesium calcite skeleton

lu xy g epo

e

2 mm

Figure 2.3: Geo.TS images of BSA2 cross-sections. See detail of BSA2 showing growth direction, conceptacles and growth lines. For the Geo.TS images of the other specimens analyzed in this study, please refer to the Figure 2.4 Moberly Jr (1968) hypothesized that Mg content is mainly related to algal growth rates, which in turn are a function of water temperature and light (Adey (1970b)). Studies have concluded that seasonally changing Mg content within annually banded corallines positively correlates to the sea surface temperatures (SST) of the water in which they formed (Chave (1954); Chave and Wheeler Jr (1965); Kamenos et al. (2008, 2009); Hetzinger et al. (2009, 2011); Halfar et al. (2010)). These results support the use of Mg/Ca ratio in corallines skeleton as a valuable paleothermometer (Halfar et al. (2000); Kamenos et al. (2009)). Previous studies (Halfar et al. (2000); Kamenos et al. (2008); Hetzinger et al. (2009, 2011)) established equations displaying the relationship between water temperature and magnesium for coralline algae at subtropical and cold-water environments. I will use the equation established by Halfar et al. (2000), who observed a near perfect covariation of Mg and temperature in the cold-water coralline alga Lithothamnium glaciale: T(◦ C) = 0.9752 MgCO3 (Mol%) - 7.9051 (R2 =0.9907) (Mol% = M(MgCO3 )/M(MgO), with M = molar mass, g.mol−1 )

Moreover, the identification of the annual Mg cycles was used to confirm the annual character of banding observed in geo.TS images (cf section 2.5). 20

Master Thesis

2. Material and Methods

To analyze trace-element composition of the samples, individual growth increments were subsampled using a JEOL JXA 8900 RL electron microprobe at the University of Göttingen, Germany. The electron microprobe analyser is an instrument for the X-ray spectrochemical analysis of small areas on the surface of a solid specimen (Hetzinger et al. (2009)). For quantitative wavelength dispersive measurements, an acceleration voltage of 10 kV, a spot diameter of 1 to 2 µm, and a beam current of 12 nA were used. The electron beam was manually positioned on calcite within an individual algal cell. Sampling transects were selected on geo.TS images avoiding conceptacles cavities and recalcified portions of the skeleton (Figure 2.4), then images and algal specimens BSA3 and BSA7 were send to Andreas Kronz (University of Göttingen). Spot analyses were spaced at 10 µm along a transect parallel to the growth ranging in length from 862 µm (BSA3) to 1374 µm (BSA7). Transect passes were manually positioned in order to avoid conceptacles (both empty or secondarily crystallized).

a.

Old

epoxy glue

b.

200 µm

Young

5 mm

c.

d.

Old ep ox yg l ue

200 µm

Young 5 mm

Figure 2.4: Geo.TS overviews and detail scans showing transects for microprobe analysis. Overview (a.) and detail (b.) of BSA3 polished to 1 µm. Overview (c.) and detail (d.) of BSA7 polished to 1 µm. Note that transects avoid conceptacle cavities and recalcified portions of the skeleton.

Backscattered electron images obtained after the microprobe analysis allowed to follow exactly the single point transects analyzed (Figure 2.5). The mean standard deviations of multiple standard measurements were found to be no larger than 0.2 mass % for MgO and 0.4 mass % for CaO. Counting statistics errors vary between 1.8 relative % and 2.2 relative % for MgO and between 0.76 relative % and 0.84 relative % for CaO. The detection limit of Mg calculated from the background noise was found to be 0.02% (same counting statistics errors as in Hetzinger et al. (2011)). 21

2. Material and Methods

Laurie Bougeois

a

b

c

d

e

f

g

h

Figure 2.5: Backscattered electron images (BSE) of the sample surface. Small spots indicate the single point microprobe transect analysis. Figures a, b, c : BSA3 ; Figures d, e, f, g, h : BSA7. Red arrows show measurements spots. 22

Master Thesis

2.5

2. Material and Methods

Development of chronology

In order to study isotopic composition during time, a chronology across the sections studied was established. Chronologies were generated by two independent methods. First by counting annual growth lines on the mapped and digitized image of the specimens. All samples were live collected, hence the top layer was assigned the year of collection (2007). Thus, calendar years were assigned to annual growth increments starting from 2007 and extending back in time. Unfortunately, for L. fœcundum, yearly growth banding is not well-defined and the yearly growth layers are not strongly delineated as with other species such as Clathromorphum sp. (Hetzinger et al. (2011)). Thus, for the chronology, we assume that one series of almost parallel conceptacles is formed per year (Adey (1966)). In addition, yearly growth-increment widths were calculated from annual Mg/Caelement cycle widths that were obtained by electron microprobe for two specimens (BSA3 and BSA7). Age models were established on the basis of the pronounced seasonal cycle in algal Mg/Ca, with high Mg values within the skeleton interpreted to correspond to summer periods/growth. Maximum (minimum) Mg/Ca values were tied to July (January), which is on average the warmest (coolest) month at the study site (Benke and Cushing (2005); A.C.I.A. (2005)). The algal Mg/Ca time series were linearly interpolated between these anchor points using the AnalySeries software (Paillard et al. (1996)) to obtain an equidistant proxy time series with a resolution of 12 samples/year. The developed chronologies were refined and cross checked for possible errors in the age model by comparing annual extreme values in the Mg/Ca ratio time series to mapped growth increment patterns for each individual year of algal growth.

2.6

Isotope Analysis

2.6.1

Micromill Drilling

Skeletal material for stable isotopes analyses was removed by a high-precision, computer driven Micromill (New Wave Research) attached to an x, y and z stage using digitized milling path positions. Sampling paths were defined using geo.TS software, transferred to the Micromill, then interpolated to have a regular spacing of 70 µm between each scan. Based on a digital volume estimation, 2 to 3 milling paths were combined to collect between 100 and 200 µg of calcite powder per sample. Hence, the final sample spacing achieved was on the order of 150 − 200 µm, while sampling depth was 150 µm and sample path lengths averaged 1.30 cm for BSA2 and 1.15 cm for BSA7. Material was removed from the outside edge of the specimen which represented the most recent growth (year 2007) toward the oldest part of the sample (Figure 2.6). Using a razor blade, the milled powder was transferred into a glass vial used for the mass spectrometer. In order to avoid contamination compressed air was used to clean the crustose algal skeleton, the drillbit and the razor blade after each sample. 98 samples were milled from two specimens of L. fœcundum (56 from BSA2 and 42 from BSA7). 23

2. Material and Methods

Laurie Bougeois

Figure 2.6: BSA7 during (above) and after (below) the micromilling. The milling follows precisely the lines drawn with the Micromill software.

2.6.2

Mass Spectrometer and Isotope Analysis

Removed material was analyzed for δ 13 C (= % deviation of the ratio of stable carbon isotopes 13 C:12 C relative to Vienna Peedee Belemnite Limestone Standard (VPDB)) and δ 18 O (= % deviation of the ratio of stable carbon isotopes 18 O:16 O relative to Vienna Peedee Belemnite Limestone Standard (VPDB)) using a Thermo-Finningan MAT253 mass spectrometer connected to a gasbench and autosampler at the Geobiology Stable Isotope Laboratory, University of Toronto. One standard deviation of the mean of duplicate measurements was ±0.16% for δ 13 C and ±0.08% for δ 18 O. Using the established chronology (cf 3.1), a stable isotope time series was compiled for BSA2 and BSA7 (Figure 3.4). Measuring each growth line with geo.TS allows to determine annual growth rates. In order to correlate the chronology with the samples analyzed by mass spectrometry, I assume that within an individual year the growth rate is constant. Because of small sample amounts, samples between 1986 and 1994 for BSA2 did not yield reliable mass spectrometer results. Hence, only the isotope data from 1994 to 2006 were analyzed for this specimen.

24

Chapter 3 Results 3.1

Chronology

Using geo.TS images, we noted an important number of large conceptacles and a very low growth rate. This results in poorly delimited annual growth increments. Despite poorly defined growth increments, chronologies were established from 1991 to 2007 for BSA7 and from 1987 to 2007 for BSA2. With this method, average annual growth rates are 180 ± 26 µm (max=213 µm, min=117 µm) for BSA2 and 243 ± 49 µm (max=330 µm, min=160 µm) for BSA7 (Figure 4.3 and Appendix A.).

19 86 19 87 19 88 19 19 89 9 19 0 91 19 92 19 9 19 3 94 19 9 19 5 96 19 9 19 7 19 98 99 20 00 20 20 01 0 20 2 03 20 04 20 05 20 06

19 91 19 92 19 93 19 94

19 95

19 96 19 97 19 98

19 99 20 0 20 0 01 20 02 20 03 20 04 20 0 20 5 06

Figure 3.1: Chronologies established for BSA2 (a.) and BSA7 (b.). Blue lines delimit annual growth

increments. Calendar years were assigned to annual growth increments starting from 2007 (year of collection) and extending back in time.

The Mg/Ca cycles obtained by microprobe analysis were graphically superimposed on microprobe backscatter electron images (Figure 2.5) of the sample surface. Maxima in Mg/Ca were aligned with the center of each growth increment, and minima with growth increment lines (Figures 3.2a and 3.2b). On average ∼ 15 samples/year were obtained for BSA3 and ∼ 21 samples/year for BSA7. Based on the microprobe data annual growth rates averaged 215 ± 50 µm from 2001 to 2006 for BSA7 (Figure 3.2a) and 170 ± 47 µm from 2003 to 2006 for BSA3 (Figure 3.2b). In comparison the average growth rate from 2001 to 2006 according the image-based annual growth measurements is 210 ± 35 µm for BSA7 and 178 ± 26 µm for BSA3. These 25

3. Results

Laurie Bougeois

data therefore validate the chronology established from geo.TS images.

100MgO/CaO 1

2

3

4

5

6

7

8

9 10 11

2000

2001

100MgO/CaO 1 2 3 4 5 6 7 8 9 10 1112

2002

2002

2003

2003

2004

2004

2005

2005

2006 2006

100 µm

50 µm

(a) Microprobe analysis and algal chronology for (b) Microprobe analysis and algal chronology for BSA7. BSA3.

Figure 3.2: Microprobe analysis and algal chronology. Minimum and maximum peaks of the Mg/Ca ratio were tuned to annual bands observable in backscattered electron images (BSE) of the sample surface. Red and blue lines indicate position of measurement transects (see Figure 2.5 for detail)

3.2

Microprobe results

Using the equation established by Halfar et al. (2000) (cf section 2.4), algal Mg/Ca ratio were converted to temperature during the period of growth of BSA3 and BSA7 (Figure 3.3). Results indicate temperatures ranging from −5.6◦ C to 2.0◦ C (mean −1.3 ± 1.6◦ C) for BSA3 (period: 2001-2007) and from −6.3◦ C to 3.0◦ C (mean −1.7 ± 2.0◦ C) for BSA7 (period: 2000-2007). 26

Master Thesis

3. Results

E

EF

E

ABC ABC

DEF

Figure 3.3: Calculated sea surface temperature variations deduced from Mg/Ca data using the Halfar et al. (2000) equation.

3.3

Isotope results

The isotope data show annual variations since 1991. The δ 18 Oalga values range from −1.74 to −0.38% (BSA2) and from −1.71 to −0.32% (BSA7). The δ 13 Calga values range from −2.36 to −1.38% (BSA2) and from −2.47 to −0.83% (BSA7). Due to the very low growth rate of L. fœcundum, the best resolution achieved was 2 to 4 samples per year. This is not sufficient to determine the exact timing of the subannual variations. From 1994 to 1998 we note δ 18 Oalga values of BSA2 (average: −1.3%) were lower than values of BSA7 (average: −0.8%). Between 1998 and 2007, variations of δ 18 Oalga of BSA2 and BSA7 are significantly positively correlated (n = 19, r = 0.54, p = 0.008). (Figure 3.5 a.). Hence, δ 18 Oalga of both specimens record a common signal after 1998. Instead, from 1994 variations of δ 13 Calga of BSA2 and BSA7 are negatively correlated (n = 29, r = 0.44, p = 0.008) (Figure 3.5 b.).

27

3. Results

Laurie Bougeois

B CD EF CD F CD EF CD F

B

A A

AB

B

CF

F

B AB

B B AB

Figure 3.4: Variations of δ 18 Oalga and δ 13 Calga for BSA2 and BSA7. The isotope values appear to

be cyclic but the resolution of 2 to 4 samples per year is not sufficient to determine the exact timing of the subannual variations. The δ 18 Oalga values range from −1.74 to −0.38% (BSA2) and from −1.71 to −0.32% (BSA7). The δ 13 Calga values range from −2.36 to −1.38% (BSA2) and from −2.47 to −0.83% (BSA7).

F

AB

F

BB

!

B

CD

CD EF

F

F

F

F

F

B

B

B

A CD EF

F

B

B

A CD

F

F

F

F

Figure 3.5: Correlation between isotope values of BSA2 and BSA7. a. δ 18 Oalga of BSA2 and BSA7 are positively correlated (n = 19, r = 0.54, p = 0.008) b. δ 13 Calga of BSA2 and BSA7 are negatively correlated (n = 29, r = 0.44, p = 0.008).

28

Chapter 4 Discussion 4.1

Development of chronology

Because of the very low growth rates, annual growth lines are not necessarily distinctive. Furthermore, some years are not continuous laterally and growth interruptions distort the chronology because their length is unknown (Figure 4.1). Cross dating of both algae using the same method as for tree rings (i.e. to correlate years with the same thickness of growth line) is not possible, since a variety of biotic factors can be responsible for differential growth and growth interruptions. For example, growth can be affected by the shading of a specimen by another organism for an extended period, local differences in the nutrient concentration in the water (Swart et al. (2010)), differential grazing or some combination of all of these or other unknown factors. The annual chronology deduced from Mg/Ca ratios was established with difficulty because of noise in the trace-element signal. Variations between colder (January) and warmer (July) months are not always clear in the time series and in some cases the cold and warm months had to be chosen arbitrarily in the time series. The likely reason for the poor quality of the annual Mg/Ca signal is the small scale variability in annual growth banding.

growth interruption

growth interruption

discontinuity 1 mm

1 mm

Figure 4.1: Growth interruptions and discontinuities in the calcitic skeletons of BSA2 (a.) and BSA7

(b.).

29

4. Discussion

4.2

Laurie Bougeois

Sea surface temperature deduced from Mg/Ca ratio

Temperature of −5 or −6◦ C calculated above from the Mg/Ca ratios are not achievable in a marine setting at low salinities and differ significantly from satellite-derived temperatures for the Beaufort Sea (Figure 4.4). The equation used here was established by Halfar et al. (2000) for L. glaciale and might therefore not be applicable to L. fœcundum. Alternatively, salinity has previously been shown to have an effect on the incorporation of Mg into the algal skeleton (Chan et al. (2011)). Recent studies of high-Mg calcite in echinoderm skeletons and foraminiferal tests field-collected or cultured in different salinities have demonstrated a clear negative effect of salinity on Mg/Ca ratios (Ferguson et al. (2008); Kisakurek et al. (2008)). This effect might explain the cold temperatures calculated in this study as the salinity in Beaufort Sea is very low (between 28 and 30, cf Section 1.1.1). In addition, the collection site is located close to the mouth of Sagavanirktok River which can locally contribute to an additional lowering of salinity. More experiments of the species L. fœcundum are therefore necessary to establish a new calibration linking MgCaO (Mol%) and temperature in waters with low salinity.

4.3

Record of surface water oxygen composition

δ 18 Oalga values of BSA2 are lower than values of BSA7 from 1994 to 1998 (cf Section 3.3). This difference in values may be due to poorly containable local effects such as fresh water input. Given that annual variations in the oxygen isotopic record are not observed in the data, it can be assumed that the annual sea-ice melting does not influence the oxygen composition of the algal skeleton. Indeed, even if seawater becomes enriched in 18 O as it freezes, the isotopic fractionation observed in sea ice is typically small (Cooper et al. (2005)). The proximity of the Sagavanirktok River likely influences the oxygen isotopic composition and other water properties in the study area (such as temperature and salinity) and these localized river variations are recorded in the algal skeleton. The US Geological Survey (USGS) records different parameters of the Sagavanirktok River since 1982, at a station near Sagwon (http://nwis.waterdata.usgs.gov/ ). Data are available on gage height, stream flow and discharge of the river. Annual river data from 1987 to 2007 are shown in Figure 4.2. δ 18 Oalga variations of both samples are positively correlated with Sag river streamflow (BSA2: n = 13, r = 0.33, p = 0.1; BSA7: n = 16, r = 0.52, p = 0.02) and gage height variations (BSA2: n = 13, r = 0.44, p = 0.07; BSA7: n = 16, r = 0.58, p = 0.01) (Figure 4.3). Furthermore, δ 18 Oalga variations of BSA7 are positively correlated with Sag river discharge (n = 16, r = 0.50, p = 0.02). Hence, it can be assumed that the runoff from Sag River influences the isotopic composition off the delta. As the source of Sag river is located in high altitudes of the Brooks Range in Alaska with continental climate, the runoff is strongly depleted in 18 O (Cooper et al. (2005)). But with a δ 18 Oriver more negative than the δ 18 Osea waters (because of the salinity of Beaufort Sea and high altitude origin of Sag River runoffs), an anti-correlation between 30

Master Thesis

4. Discussion

CD

AEF

A

B

CD

ED

AB

D

A

B

FD

B B

A

A

AA

AA

AA

AA

AA

Figure 4.2: Sagavanirktok River parameters from 1987 to 2007. a. Streamflow, b. Gage height, c. Discharge. Data recorded near Sagwon by the US Geological Survey. !C"#C$C 'C($ &!C %!C"#C$C %'C($&& !C

$ &&!C)$ $ * !C)$

$ !C)$ $ * !C)$

%

%

/

A

!C"#C$C !C"#C$C 'C($ &!C %!C"#C$C %!C"#C$C %'C($&&% !C

A BCDECF

A

$ * !C)$

2

*

. 2- , CD

CD

+ , C- .,- CD

%

%C($ !C $ * !C !C %'C($ )$ "0, . (C .(0 . C ) 1C %C($ !C $ * !C)$

*

A BCDECF

&

A BCDECF

Figure 4.3: Correlation between Sag River properties and oxygen isotopes of BSA2 and BSA7. Both

samples are positively correlated with the Sag river (a) streamflow (BSA2: n = 13, r = 0.33, p = 0.1; BSA7: n = 16, r = 0.52, p = 0.02), (b) gage height variations (BSA2: n = 13, r = 0.44, p = 0.07; BSA7: n = 16, r = 0.58, p = 0.01) and (c) discharge (BSA7: n = 16, r = 0.50, p = 0.02).

variations of gage height, stream flow and discharge of the Sag River and the δ 18 Oalga would be expected. No continuous measurement of δ 18 Oriver since 1991 was found in literature, thus the influence of Sag River on δ 18 Oalga cannot be estimated. However, sea surface temperature also influences the δ 18 Oalga recorded in algal skeleton (see equation established with Clathromorphum compactum by Halfar et al. (2008)). The surface temperatures of the Sag River and the Beaufort Sea since 1991 have been partially recorded (Figure 4.4). Gaps in the temperature record are due to site access difficulties during the winter months. It can therefore be assumed that the annual temperature average (Beaufort Sea and Sag River) would be colder. Due to this 31

4. Discussion

Laurie Bougeois CDEF E

D

D

D EFD

!

A

A B

" #E E

B AAA

AA

AA

AA

AA

AA

AA

AA

DE

Figure 4.4: Beaufort Sea and Sag River water temperature from 1991 to 2007. Values from http://reason.gsfc.nasa.gov/OPS/Giovanni/ocean.aqua.2.shtml for the Beaufort Sea and from Mathew Schellekens (USGS) for the Sag River. These data show a large difference of temperature between the waters of the Sag River and that of the Beaufort Sea. . sparsity in data, no further correlation attempts are made. A large temperature difference exists between the Sag River waters (average for 1991-2007: 4.9 ± 3.8◦ C) and the Beaufort Sea waters (average for 2002-2007: 0.7 ± 1.4◦ C). It is possible that Sag River temperatures have a large influence on the δ 18 Oalga . This could explain the positive correlation between the gage height, the streamflow and the discharge of the Sag River and the δ 18 Oalga .

4.4

Record of surface water carbon composition

4.4.1

Gas hydrate dissociation and methane release

No significant negative carbon isotopic excursion (δ 13 Calga decrease) over time is found in the algal record (Figure 3.4). Thus no isotopic signature of a potential methane release from gas hydrates is recorded. Thereby, even if gas hydrate dissociation is observed on the East Siberian Arctic shelf (Shakhova and Semiletov (2007); Shakhova et al. (2010); Heimann (2010); Kerr (2010)), no isotopic trace of such a phenomenon is recorded in the corallines of the Beaufort Sea Arctic shelf. We assume that the thawing of subsea permafrost is slower off Alaska because of local climate influences. Pabi et al. (2008) showed that since 1998, open water area increases in all Arctic sectors except for the Beaufort sector. This ice cover appears to protect the subsea permafrost from the thawing. The sea-ice budget in the Arctic Ocean is linked to currents from the North Atlantic and North Pacific oceans and the salt transport by inflowing water masses (A.C.I.A. (2005)). Most Beaufort Sea surface waters come from Bering Strait throughflow, which accounts for approximately one third of the Arctic Ocean freshwater budget (Woodgate and Aagaard (2005); Chan et al. (2011)). This current dominates the upper ocean of the western Arctic, including the Beaufort Sea (A.C.I.A. (2005), cf Figure 4.5). One quar32

Master Thesis

4. Discussion

ter of Bering strait throughflow is determined by freshwater influx from the Alaskan Coastal Current (ACC) (Woodgate and Aagaard (2005)). Because of an excess of precipitation over evaporation in the North Pacific Ocean relative to the North Atlantic, salinity of the ACC is lower than water masses from North Atlantic ocean (Woodgate and Aagaard (2005); A.C.I.A. (2005); Chan et al. (2011)). This very cold and fresh current protects the sea-ice from melting in the Beaufort Sea while the currents from the North Atlantic Ocean, warmer and saltier (Figure 4.6), influence the open water area increase in other sectors.

Figure 4.5: Surface currents in the Arctic Ocean. Oceanic circulation is influenced by both Atlantic and Pacific waters. Atlantic currents (red): warm, salty, low nutrient, deep. Pacific (blue): cold, fresh, high, nutrient, shallow. From A.C.I.A. (2005) and personal communication (Kevin Arrigo).

In summary, the subsea permafrost is more protected in the Beaufort sector and the algal δ 13 C compositions give no indication of gas hydrate dissociation. A recent and unprecedented freshening trend of the ACC (Chan et al. (2011)) delivers large amounts of relatively low salinity water to the Beaufort Sea enhancing ice build up. If this trend continues, one can expect continued stability of Beaufort sea-ice and subsea permafrost on the Beaufort Sea shelf. Hence, in contrast to the Siberian Arctic, the Beaufort Sea oceanography might cause a prolonged stability of gas hydrate in this sector of the Arctic Ocean.

4.4.2

Influence of local and regional oceanic effects

As there is no decreasing trend for δ 13 Calga in BSA2 and BSA7, the oceanic Suess effect is not recorded in the algal skeleton. Several hypothesis are possible to explain the absence of a Suess effect signal. First, L. fœcundum does not record the variations of δ 13 CDIC because of lack of equilibrium between the DIC and the carbonates. However, Williams et al. (2010) showed that corallines Clathromorphum nereostratum record the Suess effect, and there is not much reason to believe that one genus would record it, whereas the other one not. 33

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Laurie Bougeois

Figure 4.6: Map of Arctic Ocean with mean values for (a) sea surface temperature and (b) sea surface

salinity (SSS). The large-scale spatial distribution of SSS in the Arctic Ocean is controlled by the balance between freshwater influx from river discharge and sea ice melt and the inflow of higher-salinity waters from the Atlantic Ocean (∼ 34.8) through the Fram and Nares straits as well as by Pacific waters (∼ 32.5) entering through the Bering Strait. Adapted from Arrigo et al. (2010)

Second, the oceanic Suess effect does not appear in the Beaufort Sea. According to Bauch et al. (2000), the δ 13 CDIC -Suess effect intensity depends of the ventilation and the CO2 exchange between atmosphere and ocean. Swart et al. (2010) showed a difference in the magnitude of the slope of the δ 13 CDIC between the Atlantic and Pacific which reflects that CO2 is being recharged into the oceans in the Atlantic, while in the Pacific Ocean, more deep water is being returned to the surface. Bauch et al. (2000) showed that the Arctic Ocean is well ventilated and a Suess effect is recorded in Barents sector of Arctic Ocean (Figure 1.1a) which is influenced by currents from the North Atlantic Ocean (Figure 4.5). In contrast, the the Beaufort Sea is influenced by the Beaufort Gyre and currents from the North Pacific (Figure 4.5), and it is partly sea-ice covered all year round (Pabi et al. (2008)). These parameters could decrease the exchange between atmosphere and ocean and can explain the absence of an oceanic Suess effect in the Beaufort Sea. An absence of a Suess effect in the Beaufort Sea can also be influenced by a variety of local and regional factors. For example an increase in marine primary production can cancel the oceanic Suess effect in the Arctic Ocean. Actually because of the increase of primary production in the Arctic ocean during the last years (Pabi et al. (2008); Arrigo et al. (2008, 2010); Mundy et al. (2009)), the isotopic composition of dissolved inorganic carbon (DIC) has to increase. While δ 13 Calga do not show an increase over time, it is possible that the primary production and the Suess effect cancel each other out. However, primary production has been shown to increase significantly everywhere in the Arctic Ocean except in the Beaufort sector (Pabi et al. (2008)), possibly due to the relatively low sea-ice shrinking during summer in this sector (Figure 4.7). The latter 34

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4. Discussion

can be explained by the freshening of the ACC observed in Chan et al. (2011).

Figure 4.7: Annual mean open water area and annual primary production in the Arctic Ocean for

each ecological province and geographical sector during 1998-2006. Note that open water area increases for all the sectors except in Greenland sector where the open water area stagnates and in Beaufort sector where it decrease significantly. In parallel, the primary production in Beaufort Sea decreases from 1998 to 2006 at a rate of 1.5 TgC.a−1 . DMIZ = Deep water Marginal Ice Zone, SMIZ = continental Shelf Marginal Ice Zone. From Pabi et al. (2008).

Moreover, local effects, such as coastal currents and riverine influx, can have an influence on the surface water δ 13 CDIC and might be recorded by the δ 13 Calga . In fact, the proximity of the Sagavanirktok River delta can influence both the oceanic Suess effect and primary production signals. As river waters tend to have a more negative carbon isotope signature than the ocean (Mook and Tan (1991); Schöne et al. (2011)), we expect an anti-correlation with gage height, streamflow or discharge in the δ 13 Calga recorded. However no correlation between the variations of the river’s regime and δ 13 Calga was identified. Finally, a combination of two or more of the above assumptions could lead to the observed absence of a declining δ 13 C trend. The most likely explanation is that the proximity of the Sagavanirktok River delta and sediment input (B. Konar, personal communication) in the study area disturb the record of the Beaufort Sea isotopic composition. In conclusion, no clear external cause for the observed carbon isotopic record can be identified here. 35

4. Discussion

4.4.3

Laurie Bougeois

Growth rate influence on the carbon isotopic record

It is possible that parameters intrinsic to each specimen control the variations of δ 13 Calga . Generally, more negative δ 13 Calga values are associated with faster rate of skeleton formation (Swart et al. (2010); Butler et al. (2010)). In fact, the δ 13 Calga may indicate kinetic isotopic disequilibrium at high growth rates. During the years of relatively low growth rates, the 13 C depletion may result from a relative increase in the proportion of metabolic carbon available for skeleton construction (Lorrain et al. (2004); Butler et al. (2010)). Thus an increase in extension rate is coupled with a decrease in δ 13 Calga and vice-versa. Over time the BSA7 growth rate decreases and relates to an increase in δ 13 Calga . Instead, the BSA2 growth rate increases slightly and relates to a small decrease in δ 13 Calga (Figure 4.8). A B

+,$#%& +,-

B

+,$#%& +,-

A B A B

A B

A

E

A B A B A B

CDEF CDEF

A

$#%& D' ()*+F

A B

CDEF

D !B" #F

EA

$#%& D' ()*+F

D !B" #F

CDEF

EA A B

E A B

A B

Figure 4.8: Evolution of growth rate and δ 13 Calga of BSA2 and BSA7. The trends of the growth rates and the δ 13 Calga are inverse.

However, no clear relationship emerges between growth rates and δ 13 Calga .

36

Conclusion and Prospects As no carbon isotopic decrease across time is observed in the algal skeleton, the Beaufort subsea permafrost thawing appears to be less important than in the East Siberian shelf (Shakhova and Semiletov (2007); Shakhova et al. (2010)). At same time, no oceanic Suess effect is recorded in the Beaufort Sea, even though other studies report Suess effect in the arctic Barents Sea (Bauch et al. (2000)). This absence of δ 13 C decrease can be explained by a recent increase of primary production in Arctic Ocean due to acceleration of sea-ice melting (Pabi et al. (2008); Arrigo et al. (2008, 2010); Mundy et al. (2009)). However, as the record of oxygen isotope appears to be strongly influenced by the Sagavanirktok River, the carbon isotopic composition of the algae studied may also be controlled by the river drainage. Due to the low number of specimens available for geochemical study, and the fact that the two specimens analyzed showed dissimilar results, no robust conclusions can be drawn at this point. The poor knowledge of the isotopic composition of the Beaufort Sea and the Sag River make a comparison with the isotopic and element data derived from the corallines difficult. In addition, the difficult access to the Arctic Ocean environment during the winter leads to a sparse documentation of the different parameters studied here. The disturbance of the isotopic signal by the nearby Sagavanirktok River cannot easily be distinguished from oceanic influences. Hence, future study sites need to be located far from riverine input, if a pure oceanic signal is desired. Furthermore, the short lifespan of L. fœcundum crusts (no more than two decades) does not allow to detect multidecadal signals. Moreover, the very low growth rate of this species and the high number of large conceptacle cavities provide additional problems for establishing precise chronologies. Hence, even if the use of crustose coralline as paleoenvironmental archives is promising, the species L. fœcundum does not appear to provide a reliable new paleoenvironmental proxy for the Arctic Ocean. In addition to L. fœcundum, the genus Clathromorphum, which has been shown to function as an accurate proxy in the North Atlantic and Pacific oceans (Halfar et al. (2007, 2008, 2010); Hetzinger et al. (2009, 2011); Gamboa et al. (2010); Williams et al. (2010); Chan et al. (2011)) also occurs in the Arctic Ocean (W. Adey, personal communication and http://www.algaebase.org/search/species/ ). Further studies using Clathromorphum which is better known and already well studied in sub-arctic environments may allow a record of sea surface conditions in the Arctic Ocean.

37

Acknowledgements First, I want to thank Jochen Halfar who supervised me during this whole internship. Thanks to for his trust to give me this difficult but very interesting topic. I appreciate his advice and the helpful discussions. I am grateful to Phoebe Chan and Branwen Williams for their advice during the laboratory work and data interpretation. I would like to thank the Geobiology and Paleoclimate Group of University of Toronto at Mississauga for integrating me into the lab: Jochen, Branwen, Phoebe, Benjamin, Emily, Milian. Thanks to Walter Adey, for assisting with specimen identification and to Brenda Konar who sampled the specimens of L. fœcundum during the summer 2007 and provided collection site information and images. Andreas Kronz conducted at the microprobe analyses at the University of Göttingen, Germany. Uli Wortman and Hong Li performed isotope analyses at the Geology Department at University of Toronto. Mathew Schellekens provided data from Sag River. Thanks to Sophie for her help and advice during my statistical analysis. Finally, thanks to Bruno who supported and sustained me every day.

38

Tables of values Tables of values are in the electronic file attached. 1. Isotope results of BSA2 and BSA7 with their chronology 2. Sagavanirktok River discharge, streamflow and gage height data from 1983 to 2010 from the US Geological Survey. 3. Electron microprobe results of BSA3 and BSA7 with their chronology

39

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Master Thesis

BIBLIOGRAPHY

Swart, P., Moore, M., Charles, C., and BOLUN, F.: Sclerosponges may hold new keys to marine paleoclimate, Eos, 79, 1998. Swart, P., Greer, L., Rosenheim, B., Moses, C., Waite, A., Winter, A., Dodge, R., and Helmle, K.: The 13C Suess effect in scleractinian corals mirror changes in the anthropogenic CO2 inventory of the surface oceans, Geophysical Research Letters, 37, L05 604, 2010. Thomas, E.: Cenozoic mass extinctions in the deep sea: What perturbs the largest habitat on Earth?, Large Ecosystem Perturbations: Causes and Consequences, pp. 1–24, 2007. Wagner, D.: Microbial communities and processes in Arctic permafrost environments, Microbiology of Extreme Soils, pp. 133–154, 2008. Wanamaker Jr, A., Hetzinger, S., and Halfar, J.: Reconstructing mid-to high-latitude marine climate and ocean variability using bivalves, coralline algae, and marine sediment cores from the northern hemisphere, Palaeogeography, Palaeoclimatology, Palaeoecology, 2011. Weaver, J. and Stewart, J.: In-situ hydrates under the Beaufort Sea Shelf, in: Proceedings of the 4th Canadian Permafrost Conference, pp. 312–19, 1981. Williams, B., Halfar, J., Steneck, R., Wortmann, U., Hetzinger, S., Adey, W., Lebednik, P., and Joachimski, M.: Twentieth century δ 13 C variability in surface water dissolved inorganic carbon recorded by coralline algae in the northern North Pacific Ocean and the Bering Sea, Biogeosciences Discussions, 7, 5801–5828, 2010. Woodgate, R. and Aagaard, K.: Revising the Bering Strait freshwater flux into the Arctic Ocean, Geophys. Res. Lett, 32, 2005. Zachos, J., Röhl, U., Schellenberg, S., Sluijs, A., Hodell, D., Kelly, D., Thomas, E., Nicolo, M., Raffi, I., Lourens, L., et al.: Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum, Science, 308, 1611, 2005. Zachos, J., Dickens, G., and Zeebe, R.: An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics, Nature, 451, 279–283, 2008.

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