Seasonal stratification, shell flux, and oxygen isotope dynamics of left-coilingN. pachydermaandT. quinquelobain the western subpolar North Atlantic

Abstract

[1] We present an almost 3 year long time series of shell fluxes and oxygen isotopes of left-coiling Neogloboquadrina pachyderma and Turborotalita quinqueloba from sediment traps moored in the deep central Irminger Sea. We determined their response to the seasonal change from a deeply mixed water column with occasional deep convection in winter to a thermally stratified water column with a surface mixed layer (SML) of around 50 m in summer. Both species display very low fluxes during winter with a remnant summer population holding out until replaced by a vital population that seeds the subsequent blooms. This annual population overturning is marked by a 0.7‰ increase in δ18O in both species. The shell flux of N. pachyderma peaks during the spring bloom and in late summer, when stratification is close to its minimum and maximum, respectively. Both export periods contribute about equally and account for >95% of the total annual flux. Shell fluxes of T. quinqueloba show only a single broad pulse in summer, thus following the seasonal stratification cycle. The δ18O of N. pachyderma reflects temperatures just below the base of the seasonal SML without offset from isotopic equilibrium. The δ18O pattern of T. quinqueloba shows a nearly identical amplitude and correlates highly with the δ18O of N. pachyderma. Therefore T. quinqueloba also reflects temperature near the base of the SML but with a positive offset from isotopic equilibrium. These offsets contrast with observations elsewhere and suggest a variable offset from equilibrium calcification for both species. In the Irminger Sea the species consistently show a contrast in their flux timings. Their flux-weighted Δδ18O will thus dominantly be determined by seasonal temperature differences at the base of the SML rather than by differences in their depth habitat. Consequently, their sedimentary Δδ18O may be used to infer the seasonal contrast in temperature at the base of the SML.