Impact of productivity events on the benthic foraminiferal fauna in the Arabian Sea over the last 150,000 years

Abstract

We have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest Arabian Sea (covering the last 150,000 years), and compared the results with the stable isotopic record from both benthic (Uvigerina peregrina) and planktonic (Globigerinoides sacculifer) foraminifera. The δ18O record of the planktonic foraminiferal species shows excursions towards heavier δ18O values when compared to the benthic record and the SPECMAP stack, particularly in stage 3, between approximately 30 and 45–50 kyr. We call this the “stage 3 event.” Species diversity and relative abundance data on benthic foraminifera reveal that some species, e.g. Cibicidoides wuellerstorfi, Oridorsalis umbonatus, Pullenia bulloides, Pyrgo spp., and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event, whereas other (e.g., Astrononion novozealandicum, Cibicidoides bradyi, Eggerella bradyi, Gyroidina altiformis, Hoeglundina elegans, Pullenia subcarinata, and Sigmoilina edwardsii) decreased. Using correspondence analysis we determined that approximately 50% of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor. The relative abundance of some species appears to vary on a precessional cycle, perhaps responding to monsoon‐driven upwelling and associated increased food supply, whilst the abundances of other species are related to the 100 kyr ice volume cycle, and may be responding to changes in intermediate and deep water chemistry. Geochemical paleoproductivity tracers, such as the Ba content of the sediment, have a very similar distribution to specific foraminiferal factors in both time and frequency domains. Certain planktonic foraminifera, (e.g., Globigerina bulloides), also display obvious maxima in the stage 3 event. The stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index. We conclude, that there was a period 15 to 20 kyr duration during stage 3 with cooler‐than‐normal surface waters, enhanced productivity and rapid settling of biogenic matter.