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 d 18 O 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 d 18 O of N. pachyderma reflects temperatures just below the base of the seasonal SML without offset from isotopic equilibrium. The d 18 O pattern of T. quinqueloba shows a nearly identical amplitude and correlates highly with the d 18 O 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 Dd 18 O 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 Dd 18 O may be used to infer the seasonal contrast in temperature at the base of the SML.

Chamber formation leads to Mg/Ca banding in the planktonic foraminifer Neogloboquadrina pachyderma

“…Foraminifera living during the winter period have thus not experienced any significant temperature, salinity or other environmental variability, making this period/site a natural laboratory to study biological effects on test geochemistry. Kohfeld et al, 1996;Stangeew, 2001) and most calcification is likely to take place in this zone (Jonkers et al, 2010a). Specimens from the warm season samples could thus have experienced up to ~5 °C temperature variability, whereas those from spring were formed in very stable conditions with changes in temperature ≤0.5 °C (Fig.…”

“…For an extended description of the sediment trap mooring and sample treatment see Jonkers et al (2010a;2013a;2013b). All tests were cleaned using repeated brief ultrasonication in deionized water and methanol prior to embedding in Araldite 20/20 resin.…”

Sediment flux and recent paleoclimate in JordanBasin, Gulf of Maine

“…Knowing this is the first step in determining where in the water column this species lives. The process we follow is similar to that described by Jonkers et al (2010) for a sediment trapping location in the Irminger Sea.…”

“…A simple regression analysis of the data shows that δ 18 O calcite is a significant linear function of SST (r 2 =0.95) but not of SSS (r 2 =0.00). At first this seems surprising, but applying the simple rule of thumb that a 1 ‰ change in δ 18 O corresponds to a ~4°C change in temperature (Figure 7) accounts for this very strong relationship (also see discussion of Jonkers et al 2010).…”

“…This contrasts with results from other North Atlantic studies. For example, in the Irminger Sea, maximum foraminiferal flux occurs during summer stratification (Jonkers et al, 2010), whereas in the Gulf of Maine the maximum occurs when stratification breaks down. In nearby waters it was reported that N. incompta grew close to the sea surface south of Cape Cod during the winter (Keigwin et al 2005), and studies off Ireland show it dominates the fauna in spring and summer (Chapman, 2010;Harbers et al 2010).…”