Sea surface dynamics and coccolithophore behaviour during sapropel deposition of Marine Isotope Stages 7,6 and 5 in Western Adriatic sea

Resum

A detailed calcareous nannofossil analysis was performed in the core PRAD1-2, recovered in the Mid-Adriatic Deep during the PROMESS 1 Cruise, in order to show fluctuations of several species during the Marine Isotope Stages 7, 6 and 5, crossing the S8eq.�S3eq. sapropel-equivalent layers. This study reports a decrease in the coccolithophore productivity, given by Gephyrocapsa spp. abundance decreases, during the deposition of MIS 7 and MIS 6 sapropel-equivalent layers and also during S5eq. Florisphaera profunda, on the contrary, shows abundance peaks within these intervals, reflecting a deeper position of the nutricline. Within the two last sapropel-equivalent layers of MIS 5 (S4eq. and S3eq.) the opposite can be observed. The deposition of these atypical sapropels may have been concomitant with some Adriatic Deep Water production and subsequent some water column mixing, in result of the activity of the north-easterly Bora wind, favouring the development of the upper photic zone species in relation to the lower photic zone inhabitant F. profunda. Similar general abundance patterns are evidenced by reworked specimens (Cretaceous-Tertiary), Coccolithus pelagicus and Helicosphaera carteri, with important increases during the sapropel-equivalent layers of MIS 7 and MIS 6 and during the latest part of the glacial stage 6, the coldest and the driest one. Due to the extended fossil record of C. pelagicus (Early Paleocene- Recent) and H. carteri (Miocene-Recent), reworked placoliths and helicoliths from a continental source, by precipitation and runoff into the Adriatic basin, or from redistribution by bottom currents, are of particular importance at certain moments (210-200, 145, 135, 110 and 85 kyr). Syracosphaera spp., Rhabdosphaera clavigera and Calciosolenia spp., usually related to warm and oligotrophic water masses, do not show a clear correlation with the isotope record. However, their important increases at the top or above the termination of some sapropel-equivalent layers is consistent with the end of dryer conditions that feature the sapropel deposition, and with the subsequent reestablishment of the general oligotrophic state of the surface water with a nutrient redistribution. At the same time a salinity reduction should occur due to the simultaneous presence of Braarudosphaera bigelowii. During S8eq. and S6eq. the presence of B. bigelowii coupled with its stratigraphic range (Cretaceous-Recent), may also indicate a continental source for some pentaliths.