CARBON PRODUCTIVITY AND CARBON FLUX IN TH E LABRADOR SEA DURING THE LAST 40,000 YEARS

Geochemical and micropaleontological analyses of Labrador Sea sediment s allowed us to intercalibrate indicators of paleoproductivity and pal eofluxes of carbon and to reconstitute biogeochemical fluxes, notably in relation to deep sea circulation changes (Western Boundary Undercur rent and North Atlantic Deep Water). Linear or logarithmic relations a re observed between the subrecent fluxes of paleoproductivity indicato rs (coccoliths, diatoms, dinocysts, Th-230), the C-13 content of left- coiling or sinistral Neogloboquadrina pachyderma, the modern surface w ater carbon production, and the rates of organic and inorganic carbon (C(org), C(inorg)) burial. All indicators increase by a factor of 10(2 ) to 10(3) in response to the evolution of paleosea-surface conditions since the last glacial maximum. Correlations are observed between aut higenic C(inorg), Th-230, coccoliths, and C-13 content (left-coiling o r sinistral Neogloboquadrina pachyderma), and also between authigenic C(org), dinocysts, uranium, and sulfur. Relative C(inorg)/C(org) buria l rates in the deep sediments ( > 3000 m) allow us to calculate a net CO2 production in the basin. The net CO2 flux was slightly negative be tween approximately 20 and approximately 15 ka BP. It reached a first maximum at about 9 ka BP (approximately 30 mumol . cm-2 . a-1 CO2) fol lowed by a decrease at approximately 8 ka BP (approximately 20 mumol . cm-2 . a-1). Since about 7 ka BP, it stabilized at - 35 mumol . cm-2 . a-1. The minimum observed at about 8 ka BP corresponds to a signific ant rate of C(org) burial accompanied with a maximum flux of redox-sen sitive indicators (uranium, sulfur). This event does not correspond to any change in sea-surface temperature or salinity, nor in deep circul ation, but may be related to the reorganization of the atmospheric cir culation during a major breakup stage of the Laurentide ice sheet. The close correlation between the change in CO2 production in the Labrado r Sea with variations of atmospheric CO2 concentrations during the las t 20 ka BP suggests that the production of biogenic carbonates in subp olar basins plays a role in the global carbon cycle on the scale of gl acial-interglacial climate changes.