Changes in calcareous nannofossil assemblages during the Mid-Pleistocene Revolution.

The distribution of calcareous nannofossils are quantitatively analysed in high-resolution deepsea core samples from North Atlantic DSDP Site 607 and Eastern Mediterranean ODP Site 967, between marine oxygen isotope stage (MIS) 35 and 15, in order to observe the response of calcareous nannofossils to the Mid-Pleistocene Revolution (MPR). Nannofossil abundance patterns show major variations through MIS 25-22 and distinct higher amplitude changes from MIS 21 upward, sometime clearly correlatable to glacial-interglacial cycles. High amplitude, short-term, fluctuations of small placoliths and F. profunda characterize the interval through MIS 24 to 22/21, suggesting strong instability in nutricline dynamics and surface water productivity. Principal Component Analysis and Shannon-Weaver indices (diversity and dominance) highlight significant shifts, mainly related to variations in the characteristics of surface waters. In both sites a minimum in diversity is recorded at MIS 22/21, corresponding to a maximum in the dominance of Reticulofenestra spp. at Site 607 and of Pseudoemiliania lacunosa at Site 967. Above MIS 21 the increase in abundance of various taxa (C. leptoporus, Umbilicosphaera spp., Syracosphaera spp., Rhabdosphaera spp., Oolithotus spp.) and a general increase in diversity are related to a distinct trend toward more stable and oligotrophic surface waters in contrast to the higher productivity and less stratified surface waters of the Early Pleistocene. In addition, power spectral analysis was performed on calcareous nannofossil abundances, Shannon Index diversity values and Factor 1 of Principal Component Analysis. These show significant peaks in the obliquity (41-kyr) and eccentricity (100-kyr) frequency bands, indicating that the quasi-periodic oscillations in the climate proxy data are controlled by these orbital parameters. A relationship between changes in abundance of calcareous nannofossils and production of glacial North Atlantic Deep Water is inferred.