Stable carbon and oxygen isotope record of Neogloboquadrina pachyderma and Cibicides lobatulus from the southwestern Greenland Sea

A core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic delta13C excursions between 40 and 87 ka. The most pronounced event, event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom-water warming of up to 8 °C. The amplitude, timing, and geographic pattern of the delta13C events suggest that this bottom-water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of delta13CDIC (DIC- dissolved inorganic carbon). Since delta13C event 1 matches a major peak in atmospheric CH4 concentration, this clathrate destabilization may have contributed to the rise in atmospheric CH4 and thus to climate warming over marine isotope stage 5.1.

Multicentennial resolution proxy records from splicings of MD96-2080 and MD02-2594, Agulhas Bank Splice (ABS)

[1] Planktonic d18O and Mg/Ca-derived sea surface temperature (SST) records from the Agulhas Corridor off South Africa display a progressive increase of SST during glacial periods of the last three climatic cycles. The SST increases of up to 4°C coincide with increased abundance of subtropical planktonic foraminiferal marker species which indicates a progressive warming due to an increased influence of subtropical waters at the core sites. Mg/Ca-derived SST maximizes during glacial maxima and glacial Terminations to values about 2.5°C above full-interglacial SST. The paired planktonic d18O and Mg/Ca-derived SST records yield glacial seawater d18O anomalies of up to 0.8 per mill, indicating measurably higher surface salinities during these periods. The SST pattern along our record is markedly different from a UK'37-derived SST record at a nearby core location in the Agulhas Corridor that displays SST maxima only during glacial Terminations. Possible explanations are lateral alkenone advection by the vigorous regional ocean currents or the development of SST contrasts during glacials in association with seasonal changes of Agulhas water transports and lateral shifts of the Agulhas retroflection. The different SST reconstructions derived from UK'37 and Mg/Ca pose a significant challenge to the interpretation of the proxy records and demonstrate that the reconstruction of the Agulhas Current and interocean salt leakage is not as straightforward as previously suggested.

Shell size variation of Neogloboquadrina pachyderma sin. in the Norwegian-Greenland Sea during the last 1.3 Myr

We present measurements of the maximum diameter of the planktonic foraminifer Neogloboquadrina pachyderma sin. from six sediment cores (Ocean Drilling Program sites 643, 644, 907, 909, 985 and 987) from the Norwegian-Greenland Sea. Our data show a distinct net increase in mean shell size of N. pachyderma sin. at all sites during the last 1.3 Ma, with largest shell sizes reached after 0.4 Ma. External factors such as glacial-interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of N. pachyderma sin. We consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of N. pachyderma sin. to the cold water environment after 1.1-1.0 Ma. Probably, the Mid Pleistocene climate shift and the associated change of amplitude and frequency of glacial-interglacial fluctuations have triggered the evolution of this planktonic foraminifer. Oxygen and carbon stable isotope analyses of different shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis. For paleoceanographic reconstructions, the evolutionary adaptation of Neogloboquadrina pachyderma sin. to the cold water habitat has significant implications. Carbonate sedimentation in highest latitudes is highly dependent on the presence of this species. In the Norwegian-Greenland Sea, carbonate-poor intervals before 1.1 Ma are, therefore, not necessarily related to severe glacial conditions. They are probably attributed to the absence of this not yet polar-adapted species. Further, transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could, therefore, contain a large range of errors if they were applied to samples older than 1.1-1.0 Myrs.