Productivité et flux de carbone dans la mer du Labrador au cours des derniers 40 000 ans

Abstract

Geochemical and micropaleontological analyses of Labrador Sea sediments allowed us to intercalibrate indicators of paleoproductivity and paleofluxes of carbon and to reconstitute biogeochemical fluxes, notably in relation to deep sea circulation changes (Western Boundary Undercurrent and North Atlantic Deep Water). Linear or logarithmic relations are observed between the subrecent fluxes of paleoproductivity indicators (coccoliths, diatoms, dinocysts, 230Th), the 13C content of left-coiling or sinistral Neogloboquadrina pachyderma, the modern surface water carbon production, and the rates of organic and inorganic carbon (Corg, Cinorg) burial. All indicators increase by a factor of 102 to 103 in response to the evolution of paleosea-surface conditions since the last glacial maximum. Correlations are observed between authigenic Cinorg, 230Th, coccoliths, and 13C content (left-coiling or sinistral Neogloboquadrina pachyderma), and also between authigenic Corg, dinocysts, uranium, and sulfur. Relative Cinorg/Corg burial 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 between ~ 20 and ~ 15 ka BP. It reached a first maximum at about 9 ka BP (~ 30 μmol∙cm−2∙a−1 CO2) followed by a decrease at ~ 8 ka BP (~ 20 μmol∙cm−2∙a−1). Since about 7 ka BP, it stabilized at ~ 35 μmol∙cm−2∙a−1. The minimum observed at about 8 ka BP corresponds to a significant rate of Corg burial accompanied with a maximum flux of redox-sensitive indicators (uranium, sulfur). This event does not correspond to any change in sea-surface temperature or salinity, nor in deep circulation, but may be related to the reorganization of the atmospheric circulation during a major breakup stage of the Laurentide ice sheet. The close correlation between the change in CO2 production in the Labrador Sea with variations of atmospheric CO2 concentrations during the last 20 ka BP suggests that the production of biogenic carbonates in subpolar basins plays a role in the global carbon cycle on the scale of glacial–interglacial climate changes.