C

Fig. 7. Total production of phytoplankton, 103 t C per day in the Greenland (a), Norwegian (b), Barents (c), Kara (d), Laptev (e), East Siberian (f), and Chukchi (g) seas.

Table 3. Trends of primary production and flux of organic carbon to seafloor in the Arctic Seas (% year-1 from mean value for period of observation).

Sea Number of observationsAverage primary Summary PP for Flux to seafloor

Table 3. Trends of primary production and flux of organic carbon to seafloor in the Arctic Seas (% year-1 from mean value for period of observation).

Sea Number of observationsAverage primary Summary PP for Flux to seafloor

production (PP)

year

Greenland

11.5

-2.3

7.8

5.8

Norwegian

7.6

-2.8

3.7

1.7

Barents

8.5

0.7

8.1

6.1

Kara

14.6

0.4

10.7

4.8

Laptev

10.7

0.6

10.7

9.2

East Siberian

19.4

-0.7

18.6

18

Chukchi

12.4

-3.7

9.6

5.5

1999 2000 2001 2002 2003 2004 2005 2006 2007 1"8 1"9 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

y = 0.18x - 0.035 r

^^^ n

i

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

25 20 15

c 10

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

25 20 15

c 10

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

y = 0.11x + 1.7

il n n

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Fig. 8. Flux of organic carbon to seafloor, 103 t C year 1, in the Greenland (a), Norwegian (b), Barents (c), Kara (d), Laptev (e), East Siberian (f), and Chukchi (g) seas.

Growth of primary production leads to increase of organic carbon flux to seafloor (Fig. 8). In some cases, trend of organic carbon flux to seafloor does not conform to that of the trends of the annual primary production. This is related to the annual variations of the primary production distribution over the test areas, and, if the increase in the annual primary production is accompanied by a displacement of the maximum photosynthesis activity to the seaward part of the test area, a lesser part reaches the bottom because of the great depths in these regions.

Analysis of trends of primary production in condition of open water with lest influence of ice conditions at polygons in the Barents, Kara and Chukchi seas (Vetrov and Romankevich 2008) also shows in some case negative trend of average primary production and positive trend of total primary production and organic carbon fluxes to seafloor for all polygons.

Conclusions

To summarize, one may note that the primary production of phytoplankton in the Arctic seas, sensitively responds not only to well-pronounced seasonal variations in the environment but also to the interannual changes. One may note the general trend of the primary production increase in the test areas in 1998-2007. The higher values of the total production trends and the greater number of observations, probably, are mainly caused by the gradual increase of the open water period. Because the phytoplankton primary production is the principal source of the autochthonous OM, the flux of organic carbon to seafloor and its burial in the bottom sediments varies with the changes in the organic matter produced by photosynthesis. Respectively, greater or smaller amounts of carbon dioxide are removed from the atmosphere, which is of considerable importance for the processes of climate formation. The process of the carbon burial in the bottom sediments acts as a negative feedback in the processes of climate change. However in concern to global climate processes burial of organic carbon in bottom sediments in the Arctic Seas (107 t C year-1; Vetrov and Romankevich 2004) is three order of values as low as technogenic emission of CO2 to atmosphere and 400 times as less as raise of CO2 in atmosphere.

Acknowledgments

The authors are grateful to the GSFS DAAC and the NASA GES DISK DAAC for the SeaWiFS and MODIS data they provided. This study was supported by the Russian Foundation for Basic Research (project nos. 06-05-6405 and 06-05-08039-OFI); Presidium Russian Academy of Sciences (Program "Fundamentals of oceanic processes in sub polar regions. VI-5. Polar branch of carbon cycle.), and by the Program of the President of the Russian Federation for the Support of Leading Scientific Schools (grant nos. NSh-1902.2003.5, NSh-5329.2006.5 and NSh-4687.2008.5).

References

Artem'ev VA, Burenkov VI, Grigor'ev AV, et al. (2003) Optics. In: Romankevich EA (ed) The

Pechora Sea, system studies. MORE, Moscow, 117-140 (in Russian) Bidigare RR, Ondrusek ME, Brooks JM (1992) Phytoplankton pigment distributions in surface waters. In: Izrael YA, Tsyban' AV Ecosystem study of the Bering and Chukchi Seas, 3rd edn. Gidrometeoizdat, St Petersburg, 250-269 (in Russian) Burenkov VI, Vedernikov VI, Ershova SV, et al. (2001) Application of the ocean color data gathered by the SeaWiFS satellite scanner for estimating the bio-optical characteristics of waters in the Barents Sea. Oceanology, 41(4): 461-468 (Okeanologiya 41(4): 485-492) Gleitz M, Grossmann S (1997) Phytoplankton primary production and bacterial production. Ber

Polarforschung, 226: 92-94 Hameedi MJ (1978) Aspects of water column primary productivity in the Chukchi Sea during summer. Mar Biol, 48(1): 37-46 Heiskanen A-S, Keck A (1996) Distribution and sinking rates of phytoplankton, deitritus, and particulate biogenic silica in the Laptev Sea and Lena River (Arctic Siberia). Mar Chem, 53: 229-245

Juterzenka KV, Knickmeier K (1999) Chlorophyll: a distribution in water column and sea ice duting the Laptev Sea freeze-up study in Autumn 1995. In: Kassens H et al. (eds) Land-ocean systems in the Siberian Arctic: dynamics and history. Springer, Berlin, 153-160 Kopelevich OV, Burenkov VI, Vazyulya SV, et al. (2003) An Assessment of the photosynthetically active radiation balance in the Barents Sea from the data of the SeaWiFS satellite color scanner. Oceanology, 43(6): 786-796 (Okeanologiya, 43(6): 834-845) Tseitlin VB (1993) Correlation between the primary production and the vertical organic matter flux in mesopelagic zones of the ocean. Oceanology, 33(2): 189-192 (Okeanologiya 33(2): 224-228)

Tuschling K (2000) Phytoplankton ecology in the arctic Laptev Sea - a comparison of three seasons. Ber Polarforschung, 347: 1-144 Vedernikov VI, Sukhomlin AV, Shaposhnikova MG (1990) Primary production and chlorophyll in the central regions of the Pacific Ocean in January-April 1987. In: Ecosystems of the eastern boundary currents and central regions of the Pacific Ocean. Nauka, Moscow, 80-99 (in Russian)

Vetrov AA (2008) Chlorophyll, primary production, and organic carbon fluxes in the Kara Sea.

Oceanology, 48(1): 33-42 (Okeanologiya 48(1): 38-47) Vetrov AA, Romankevich EA (2004) Carbon cycle in the Russian Arctic Seas. Springer, Berlin. Vetrov AA, Romankevich EA (2008) Interannual variability of the primary production and organic carbon fluxes in the Arctic Seas of Russia. Oceanology, 48(3): 362-370 (Okeanologiya 48(3): 371-380)

Vetrov AA, Romankevich EA, Belyaev NA (2008) Chlorophyll, primary production, fluxes, and balance oforganic carbon in the Laptev Sea. Geochemistry Int, 46(10): 1055-1063 (Geokhimiya 46(10): 1122-1130)

Vinogradov ME, Vedernikov VI, Romankevich EA, Vetrov AA (2000) Components of the carbon cycle in the Russian Arctic Seas: primary production and flux of Corg from the photic layer. Oceanology, 40(2): 204-215 (Okeanologiya 40(2): 221-233)

Reconstruction of oceanic circulation using mineralogical and isotopical (Nd/Pb) signatures of deep sea sediments: the case study of the northern North Atlantic and some perspectives for the Arctic

Nathalie Fagel

UR AGEs Argiles, Géochimie et Environnements sédimentaires, Department of Geology, University of Liege, B-4000 Liege, Belgium, [email protected]

Abstract

This work aims to reconstruct the evolution of deep ocean circulation patterns in the North Atlantic during the last 10-100 kyr using abiotic proxies. Nd and Pb isotopic ratios have been measured on the clay-sized fraction of two sediment cores drilled in the Labrador Sea off southern Greenland (MD99-2227, ODP646). At present this site is under the influence of the Western Boundary Undercurrent that drives the water masses involved in the formation of the North Atlantic Deep Water. Based on an identification of regional sources areas sedimentary isotopic signatures allow to determine the origin of the particles driven by the North Atlantic deep currents: any change in the sediment supplies reflects a relative change in the contribution of the deep water masses. Our isotopic dataset emphasizes several main changes in the relative contribution of the two major components of North Atlantic Deep Water, i.e. the North East Atlantic Deep water (NEADW) and the Denmark Strait Overflow Water (DSOW) throughout the last 12 kyr, and especially during the Late Holocene. The inception of the modern deep circulation seems to be quite recent, occurring during the last 3 kyr. Over glacial/interglacial time-scale deep current variability is less pronounced and/or partly masked by variable proximal supplies. Labrador Sea results emphasize that the application of mineralogical and isotopical tools on sediments allow to monitor variability in sedimentary supplies driven by deep currents. This indirect approach is further promising to identify deep currents pathways and reconstruct past circulation.

Introduction

The deep water formation is a critical component of the climate system (Fig. 1). In particular in the northern North Atlantic the production rate of North Atlantic Deep Water (NADW) is one of the puzzle in understanding the oceanic influence in climate changes (Broecker and Denton 1989). Variations in atmospheric CO2 concentrations have been related to Pleistocene glacial to interglacial changes in the exportation of NADW to the Southern Ocean (e.g., Boyle and Keigwin 1987).

Arctic basins

Labrador Sea

Labrador Sea y Surface circulation

y Surface circulation

Fig. 1. The global ocean circulation pattern. (Lower panel modified from Broecker and Denton 1989.) The Labrador Sea is one of the areas of formation of North Atlantic Deep Water (NADW). NADW takes part of the thermohaline circulation and therefore it is a key factor for the regulation in the Earth's climate. The upper panel emphasizes the role of the arctic basins in the global thermohaline circulation. Major features of North Atlantic and Arctic Ocean circulation: red arrows: warm water from lower latitudes entering the arctic; blue arrows: export of colder water from the arctic; shaded white: average area covered by sea ice. (Figure courtesy of G. Holloway, Institute of Ocean Sciences, Sidney, British Columbia. For more information see http://www-sci.pac.dfo-mpo.gc.ca/osap/.) http://nsidc.org/arcticmet/factors/land_sea_distribution.html. Accessed 23 June 2007.

Most paleoceanographic reconstructions derived from biogenic proxies (benthic fauna species distribution, elemental chemical ratios, carbon or oxygen isotopic composition of benthic assemblages) that are sensitive to ocean ventilation and water mixings (for a synthesis see e.g., Boyle 1995; Hillaire-Marcel and de Vernal 2007). Sedimentary abiotic components like magnetic properties, clay mineral assemblages or long period isotopic composition of clays were less investigated, even they bring indirect information on past circulation tracing the origin of the particles driven by the water masses (Fagel and Hillaire-Marcel 2006).

In this study, mineralogy and isotopic composition (Nd, Pb) of Labrador Sea deep sea sediments are coupled to reconstruct deep circulation patterns in northern North Atlantic. Since the 1960s clay mineralogy has been used as a tracer of provenance and transport mechanisms in studies of the world oceans in which the mineralogy of the fine detrital fraction generally reflects the intensity of continental weathering in the source areas (e.g., Biscaye 1965; Chamley 1989). Radiogenic isotopes of the detrital sedimentary fraction are also used to trace sediment provenance (e.g., Goldstein and O'Nions 1981). Radiogenic signature can therefore be used as indirect paleoceanographic tracers (Fagel et al. 2004, Paleoceanography and references therein; Fagel 2007). In this approach both clay mineralogy and isotope signatures constitute fingerprints of regional continental source-areas: they are proxies for deep current supplies. Changes in their relative contribution through time bring further information on the deep circulation pathways: they constitute indirect proxies for oceanic circulation.

Such approach has been applied on two cores from the deep Labrador Sea basin. Cores MD99-2227 and ODP646 were retrieved off Southern Greenland, at 3,460 m, at the inlet of deep circulation gyre into Labrador Sea (Figs. 1, 2). In both cores we focus on the clay-size fraction of the sediments: due to its cohesive behaviour, this fraction is less sensitive to winnowing after deposition (McCave et al. 1995). Clay minerals have been identified by X-ray diffraction analysis (Moore and Reynolds 1989); Nd isotopes have been measured by TIMS (Geotop, Montreal, Canada) and Pb isotopes have been measured by TIMS and/or MC-ICP-MS (DSTE, Brussels, Belgium). For the full analytical procedures, see Fagel et al. (1997), Innocent et al. (1997), Fagel et al. (2004), respectively.

In term of time window North Atlantic paleoceanography has been mainly investigated over the Last Glacial. Here different time-scales have been investigated: samples from core MD99-2227 allow to investigate Holocene variability (OIS 1-2) whereas glacial/interglacial cycles were investigated with ODP material. Age model are derived from oxygen isotope and paleomagnetic stratigraphies, constrained by 14C AMS dating (details in Fagel et al. 2004; Fagel and Hillaire-Marcel 2006). By comparing the Last Glacial with previous glacial/interglacial cycle we would like to document short term and long term variability in order to test the still debate representativity of last climate cycle for the whole Pleistocene (e.g., Raymo et al. 2004).

Present distribution of deep water masses in northern North Atlantic

In the modern North Atlantic, the deep circulation is driven by a contour-flowing bottom current, the Western Boundary Undercurrent (WBUC), which carries the main water masses involved in the formation of the North Atlantic Deep Water (NADW) along a counterclockwise gyre through the marginal North Atlantic basins (Fig. 2). These water masses include: (a) the Northeast Atlantic Deep Water 1 (NEADW1) flowing from East to West and bathing the shallower parts of Reykjanes Ridge; (b) the deeper Northeast Atlantic Deep Water 2 (NEADW2) whose circulation is constrained by the morphology of Reykjanes Ridge, and flowing from the Northeast Atlantic through the Charlie-Gibbs Fracture Zone into the Irminger Basin where it forms a counterclockwise gyre along the ridge and the

Greenland Margin; (c) the Denmark Strait Overflow Water (DSOW) that consists of cold surface waters from the Greenland and Norwegian seas that sink below 2,700 m along the Eastern Greenland Margin to fill most of the deepest parts of the Irminger and Labrador basins; (d) the Labrador Sea Water (LSW) characterized by cold water flowing from Davis Strait (Davis Strait Overflow, DSO) southwards along the Eastern Labrador Margin.

Fig. 2. Studied area: Core location, ocean circulation and geology. Location of cores MD99-2227 and ODP646 in the Labrador Sea, northern North Atlantic. The plain arrows indicate the pathways of deep or intermediate currents (modified from McCartney 1992; Schmitz and McCartney 1993; Dickinson and Brown 1994; Lucotte and Hillaire-Marcel 1994 and from Hansen and Osterhus 2000). The broken arrows indicate surface circulation adapted from Hansen and Osterhus (2000). The structural terranes of the continental crusts adjacent to the northern North Atlantic are indicated by different colors (North American and Greenland Shield, Panafrican and Variscan crusts) and by dots for mantle-derived material (Iceland volcanism).

Fig. 2. Studied area: Core location, ocean circulation and geology. Location of cores MD99-2227 and ODP646 in the Labrador Sea, northern North Atlantic. The plain arrows indicate the pathways of deep or intermediate currents (modified from McCartney 1992; Schmitz and McCartney 1993; Dickinson and Brown 1994; Lucotte and Hillaire-Marcel 1994 and from Hansen and Osterhus 2000). The broken arrows indicate surface circulation adapted from Hansen and Osterhus (2000). The structural terranes of the continental crusts adjacent to the northern North Atlantic are indicated by different colors (North American and Greenland Shield, Panafrican and Variscan crusts) and by dots for mantle-derived material (Iceland volcanism).

Calibration of proxies on surface sediments from northern North Atlantic

The clay mineralogical tool

In marine sediments detrital clays reflect the weathering conditions within the watershed (e.g., Chamley 1989). In surface and late Quaternary sediments the composition of clay assemblages depicts a peculiar spatial distribution between the Eastern and the Western North Atlantic basins (Fig. 3). In Eastern Irminger and Iceland sediments more than 50% of the clay fraction consists of smectites (Fagel et al. 1996 and references therein). The relative abundance of this clay species decreases in Western Labrador Sea basin, mainly replaced by illite and

Relative clay mineral abundance in Eastern basins

Relative clay mineral abundance in Labrador Sea

Fig. 3. Spatial and depth distributions of clay minerals in surface sediments from northern North Atlantic. Mean histogram for the western Labrador Sea and for the eastern Irminger and Iceland basins (data from Fagel et al. 1996). On average smectite is the dominant clay mineral in surface sediments from Eastern basins, it is usually replaced by illite and chlorite in western basin. As an exception, note the smectite enrichment in the clay-size fraction of WBUC-influenced Labrador Sea sediments.

Fig. 3. Spatial and depth distributions of clay minerals in surface sediments from northern North Atlantic. Mean histogram for the western Labrador Sea and for the eastern Irminger and Iceland basins (data from Fagel et al. 1996). On average smectite is the dominant clay mineral in surface sediments from Eastern basins, it is usually replaced by illite and chlorite in western basin. As an exception, note the smectite enrichment in the clay-size fraction of WBUC-influenced Labrador Sea sediments.

chlorite. In Labrador Sea the smectite abundance varies according to sediment depth, supporting for a link between clay assemblages and deep water masses stratification (Fagel et al. 1996). Along both Labrador Sea margins an enrichment in smectites (up to 50% of the clay fraction) is observed in sediments along the high velocity axis of the WBUC, i.e. at water depths of 2,800-3,400 m. In contrast, the clay assemblages of the upper slope and shelf off Greenland and Canada contain primarily illite and chlorite with no or only low amounts of smectites from adjacent continental areas. As smectites dominate the clay assemblages in the Eastern Irminger and Iceland basins, their occurrence in the deep Labrador Sea sediments has been related to distal supplies by deep currents through the WBUC. Based on this specific spatial and vertical mineralogical distribution, the smectite-enrichment will be used to track for WBUC-influenced supplies.

The Nd and Pb isotopical tool

Nd isotopic signature in sediments records the age and the composition of the rock material occurring in the source-area, with no significant change during erosion, transport and deposition (Goldstein and O'Nions 1981). Investigation of Nd isotopes in the clay-size fraction of surface and late Quaternary sediments of the northern North Atlantic allows to identify the main sources (Innocent et al. 1997). Based on characterization of potential geographical sources of particles, three main sources were identified (Fig. 2): an old Precambrian crustal material from Canada, Greenland and/or Scandinavia (North American Shield, NAS), a Paleozoic or younger crustal material from East Greenland, NW Europa, and/or West Scandinavia (Young crust, YC) and, a volcanic source from the mid-Atlantic oceanic volcanism grouping the Iceland, the Faeroe Islands and/or the Reykjanes Ridge (MAR). Eastern basins are characterized by higher Nd isotopic ratios than Western Labrador Sea, reflecting the relative contribution of the local isotopically-different sources (Fig. 4). Beside this spatial distribution the Nd composition of the clay fraction also changes through sediment depth. Like for clay mineralogy a significant shift in the Nd isotope signature is evidenced in sediments that are under the axis of the WBUC velocity core (Innocent et al. 1997) (Fig. 4). The WBUC is thought to be responsible for erosion and transport of clay particles from the western North Atlantic Iceland and Irminger basins, followed by redeposition in the Labrador Sea. Therefore Nd constitutes a suitable tracer of the origin of deep-sea sediments of the North Atlantic. Adding Pb isotopic measurements allows for a better discrimination between the different source-areas of

"young" crustal material (see Fagel et al. 2004). Changes in the Nd and Pb signatures of clay-size fraction of Late Glacial and Holocene sediments provide constraints on the different sources areas that supplied the fine clayey particles into the Labrador Sea.

143Nd/144Nd

0,511000 0,511200 0,511400 0,511600 0,511800 0,512000 0,512200 0,512400 0,512600 0,512800 0

1000

2500

3000

3500 4000

Fig. 4. Spatial and depth distributions of Nd isotope signatures in surface sediments from northern North Atlantic: Labrador Sea margins, circle; Greenland margin, open circle; Canadian margin, closed circle; eastern Iceland and Irminger basins, square (data from Innocent et al. 1997). Along both margins of the Labrador Sea note the significant shift in the Nd isotope signature in the WBUC-influenced sediments.

Results and discussion

Holocene variability of deep current supplies

Nd-Sm concentration and isotopic ratios (Fig. 5) as well as Pb isotopes (Fig. 6) have been measured on the fine fraction of Holocene and deglacial sediments from MD99-2227 core drilled in the Labrador Sea off southern Greenland (Fagel et al. 2004). The clay-size fraction significantly increases through the Holocene, representing up to 70% of the carbonate-free sediment. The smectite/illite ratio increases by a factor of 4 throughout the last 12 kyr. Such mineralogical trend is in agreement with higher smectite-rich Eastern supplies.

143Nd/144Nd

0,511000 0,511200 0,511400 0,511600 0,511800 0,512000 0,512200 0,512400 0,512600 0,512800 0

1000

2500

3000

Fig. 4. Spatial and depth distributions of Nd isotope signatures in surface sediments from northern North Atlantic: Labrador Sea margins, circle; Greenland margin, open circle; Canadian margin, closed circle; eastern Iceland and Irminger basins, square (data from Innocent et al. 1997). Along both margins of the Labrador Sea note the significant shift in the Nd isotope signature in the WBUC-influenced sediments.

Fig. 5. Nd mixing diagram. Nd isotopic signatures (143Nd/144Nd vs. 147Sm/144Nd) of the clay -size fraction of Holocene and deglacial sediments from core MD99-2227. Data are plotted with regard to the surface sediment sample (filled circle). The linear regression trends (dashed lines) are calculated for two groups of samples, i.e., older than 6.5 kyr (open squares) and younger than 6.5 kyr (plain squares). For the youngest samples, the grey squares characterize the samples between 6.5 and 3.3 kyr and, the black squares the samples <3.3 kyr. The regional continental end-members are defined in Fagel et al. (1999). (Data from Fagel et al. 2004.)

Fig. 5. Nd mixing diagram. Nd isotopic signatures (143Nd/144Nd vs. 147Sm/144Nd) of the clay -size fraction of Holocene and deglacial sediments from core MD99-2227. Data are plotted with regard to the surface sediment sample (filled circle). The linear regression trends (dashed lines) are calculated for two groups of samples, i.e., older than 6.5 kyr (open squares) and younger than 6.5 kyr (plain squares). For the youngest samples, the grey squares characterize the samples between 6.5 and 3.3 kyr and, the black squares the samples <3.3 kyr. The regional continental end-members are defined in Fagel et al. (1999). (Data from Fagel et al. 2004.)

147Sm/144Nd

Fig. 6. Pb mixing diagram. Pb isotopic signatures (207Pb/206Pb vs. 206Pb/204Pb) of the clay-size fraction of Holocene and deglacial sediments from core MD99-2227 (modified from Fagel et al. 2004). Data subdivided into the three same groups as for Fig. 5. The arrow indicates the regression trend defined by the samples older than 6.5 kyr. For the samples younger than 6.5 kyr, a calculated mixing-line between two types of young crusts, i.e. the Greenland and European Panafrican crusts, is used. For mixing, we take into account the median value for Greenland Panafrican Crust (GPC) and the mean value for the European Panafrican Crust (EPC). (See Fagel et al. 2004 for more explanation.)

147Sm/144Nd

Fig. 6. Pb mixing diagram. Pb isotopic signatures (207Pb/206Pb vs. 206Pb/204Pb) of the clay-size fraction of Holocene and deglacial sediments from core MD99-2227 (modified from Fagel et al. 2004). Data subdivided into the three same groups as for Fig. 5. The arrow indicates the regression trend defined by the samples older than 6.5 kyr. For the samples younger than 6.5 kyr, a calculated mixing-line between two types of young crusts, i.e. the Greenland and European Panafrican crusts, is used. For mixing, we take into account the median value for Greenland Panafrican Crust (GPC) and the mean value for the European Panafrican Crust (EPC). (See Fagel et al. 2004 for more explanation.)

The distribution of the sediment Nd and Pb signatures could be explained by a mixture between three sources (Fig. 2): the North American Shield, the Panafrican and Variscan crusts ("young" crustal source of Europe and Eastern Greenland), and the Mid-Atlantic Ridge (Iceland and Faeroe Islands). Basically we estimate for each sample the relative contribution of fine particle supplies carried by the North Atlantic deep components into the Labrador Sea.

The evolution of the relative contributions of sediment sources suggests major changes in relative contributions of the deep water masses carried by the WBUC over the past 12 kyr (Fig. 7). Clay isotopic signatures indicate two different mixtures of sediment sources succeeding in time. The first mixture is composed of proximal material from Labrador Sea margins and distal deep current-driven crustal source. The mixture is made by material eroded from the North American Shield and from a "young" crustal source. The Nd signatures shift according to the age of the samples (Fig. 5). Such trend reflects a progressive decrease of the proximal supplies during deglaciation. Changes in clay fluxes (Fagel et al. 1997) evidence an increase of distal supplies linked with a progressive intensification of the WBUC.

From 6 kyr onward, a "young" crustal component is still involved but mixed with a mantellic component characteristic of the Mid-Atlantic Ridge. In this interval Pb isotopes allow for a better discrimination between the different source areas of "young" crustal material (i.e., Panafrican crust of Europe and Greenland, Variscan crust of Europe). Sediment Pb isotopic data are plotted along a two end-member mixing, i.e. between the points representative of the Panafrican crust of Europe and of Greenland (Fig. 6). According to the geographical distribution of the identified source areas (reported on Fig. 2) we assume DSOW-like water mass supply the young crustal material from Greenland whereas NEADW brings rather European-like material. From 6 kyr onward the increase of Greenland Panafrican Crust contribution is probably related to the inception of the DSOW.

The establishment of the modern circulation is estimated at ~3 kyr. This last interval is characterized by a decrease of the crustal material. Note the reduced influence of the Denmark Strait Overflow Water is synchronous with the full appearance of the Labrador Sea Water mass (Solignac et al. 2004). We suggest the relative increase in contributions from the Mid-Atlantic Ridge follows the inception of the Iceland-Scotland Ridge Overflow Water mass (ISOW).

0 0

Post a comment