Response of macrobenthic and foraminifer communities to changes in deep-sea environmental conditions from Marine Isotope Stage (MIS) 12 to 11 at the “Shackleton Site”

Abstract

Integrative research including facies characterization, ichnological composition and foraminifer analysis has been conducted on cores from Site U1385 of the IODP Expedition 339 to evaluate the incidence of Marine Isotope Stage (MIS) 12 and MIS 11 on deep-sea environmental changes. Four color facies groups have been differentiated, showing variable transitions between them (bioturbated, gradual and sharp contacts). Trace fossil assemblage, assigned to the Zoophycos ichnofacies, consists of light and dark filled structures, with Alcyonidiopsis, Chondrites, Nereites, Planolites, Spirophyton, Thalassinoides, Thalassinoides-like structures, and Zoophycos. A deep-sea multi-tiered trace fossil community is interpreted, revealing predominance of well-oxygenated bottom and pore-waters, as well as abundance of food in the sediment for macrobenthic tracemaker community. Changes in environmental parameters are interpreted to be associated with significant variations in trace fossil distribution according to the differentiated intervals (A to M). Benthic foraminifer concentration in the sediments and variations of the planktonic foraminifer assemblages suggest significant changes in surface productivity and food supply to the sea floor since the ending of MIS 13 to the end of MIS 11 that could be correlated with the registered changes in facies and trace fossil assemblages. At the end of MIS 13 values of annual export productivity were very low, that together with the presence of light-color sediments and the continuous presence of light Planolites and Thalassinoides, reveal lower organic carbon flux to the bottom and high oxygen conditions (interval A). Afterwards the organic matter supply increased rapidly and remained very high until Termination V, determining an eutrophic environment, expressed by high benthic foraminifer accumulation rates, and reduced availability of oxygen, that correlate with the record of Spirophyton and Zoophycos, and the presence of Chondrites, observed in intervals B and D. Lower benthic foraminifer accumulation rates during MIS 11 suggest an oligotrophic environment at the bottom consistent with lower inputs of organic carbon, associated with high oxygen content of bottom waters that agrees with the lighter color of the sediments as well as by the continuous presence of light Planolites and Thalassinoides in the differentiated interval M. The evolution of the macrobenthic tracemaker community during MIS 12 and MIS 11 responds to major changes in bottom water ventilation probably linked to variations in deep water (North Atlantic) thermohaline circulation, determining variations in oxygen and food availability.