Upper ocean mixing controls the seasonality of planktonic foraminifer fluxes and associated strength of the carbonate pump in the oligotrophic North Atlantic

[摘要] Oligotrophic regions represent up to 75% of Earth's open-oceanenvironments. They are thus areas of major importance in understanding theplankton community dynamics and biogeochemical fluxes. Here we presentfluxes of total planktonic foraminifera and 11 planktonic foraminiferspecies measured at the Oceanic Flux Program (OFP) time series site in theoligotrophic Sargasso Sea, subtropical western North Atlantic Ocean.Foraminifera flux was measured at 1500 m water depth, over two~ 2.5-year intervals: 1998–2000 and 2007–2010. We find thatforaminifera flux was closely correlated with total mass flux, carbonate andorganic carbon fluxes. We show that the planktonic foraminifera fluxincreases approximately 5-fold during the winter–spring, contributing upto ~ 40% of the total carbonate flux. This was primarilydriven by increased fluxes of deeper-dwelling globorotaliid species, whichcontributed up to 90% of the foraminiferal-derived carbonate during latewinter–early spring. Interannual variability in total foraminifera flux, andin particular fluxes of the deep-dwelling species (Globorotalia truncatulinoides, Globorotalia hirsuta and Globorotalia inflata), was related todifferences in seasonal mixed layer dynamics affecting the strength of thespring phytoplankton bloom and export flux, and by the passage of mesoscaleeddies. As these heavily calcified, dense carbonate tests of deeper-dwellingspecies (3 times denser than surface dwellers) have greater sinking rates,this implies a high seasonality of the biological carbonate pump inoligotrophic oceanic regions. Our data suggest that climate cycles, such asthe North Atlantic Oscillation, which modulates nutrient supply into theeuphotic zone and the strength of the spring bloom, may also in turnmodulate the production and flux of these heavily calcified deep-dwellingforaminifera by increasing their food supply, thereby intensifying thebiological carbonate pump.

[学科分类] 地球化学与岩石