Calcareous nannofloral response to Termination II at 45°N, 25°W (northeast Atlantic)

Abstract

Termination II (glacial to interglacial transition from oxygen isotope stage 6 to stage 5) is considered to be one of the most rapid and abrupt climatic changes during the late Quaternary. Our high resolution geochemical and nannofloral study of the interval from 140 to 100 ka in the northeast Atlantic core T90-9P (45°17.5′N, 25°41.3′W) shows that Termination II (135–125 ka) occurred in two steps of drastic changes, interrupted by a short period of stability, and followed by at least 25 ka of relatively stable, interglacial conditions, traditionally referred to as the Eemian age (substage 5.5). A primary productivity proxy based on coccolith accumulation rate (number cm −2 ka −1) shows higher values during the Eemian than during glacial stage 6 and interglacial substage 5.4, with the maximum values (70×10 9 coccoliths cm −2 ka −1) at 123–122 ka. The coccolith assemblage was dominated by two morphotypes of Gephyrocapsa alternating in dominance under different climatic conditions: Gephyrocapsa muellerae was more abundant in colder periods, whereas `small' Gephyrocapsa dominated in the Eemian. Emiliania huxleyi, Syracosphaera pulchra and Florisphaera profunda increased in the Eemian whereas Coccolithus pelagicus and Helicosphaera carteri decreased. `Small' Gephyrocapsa together with Emiliania huxleyi are associated with periods of higher primary productivity. The pelagic sedimentary record of Termination II in core T90-9P includes a 5 cm thick layer enriched in ice-rafted material, a Heinrich layer. In contrast to Termination I, where such a Heinrich layer (H1) is associated with the first meltwater pulse of the deglaciation, the record of core T90-9P suggests that the Heinrich event of Termination II (H11) occurred during the second stage of deglaciation (129.4–127.1 ka). It is characterised by accumulation of ice-rafted debris and low coccolithophore diversity. The coincident apparent drop in coccolith accumulation rate could be explained either by changes in primary productivity due to reduction in salinity and light, or by dilution by ice-rafted material. Five models are applied for the deposition of the Heinrich layer with changing thickness (3 cm) and duration (1000, 500 and 250 yr). These scenarios result in a lowered, constant or even slightly increased primary productivity during this event.