New constraints on early Tertiary palaeoproductivity from carbon isotopes in foraminifera

Abstract

ONE of the most pronounced features of marine sections at the Cretaceous/Tertiary (K/T) boundary is the negative excursion in biogenic carbonate δ¹³C in the early Tertiary carbon isotope record^1–9. The coincidence of this excursion with the elimination of many marine plankton species supports predictions that large-scale reduction of primary productivity in the oceans would disrupt the biological carbon pump, resulting in a diminished surf ace-to-deep δ¹³C gradient¹⁰ (Δδ¹³C). Carbonate accumulation rates and other geochemical indices support the idea that the oceans were less productive in the early Palaeocene⁹. Here we present carbon isotope analyses of individual species of benthic and planktonic foraminifera spanning the K/T boundary in OOP Hole 690C (Weddell Sea, Antarctica) which demonstrate that primary productivity following the K/T extinctions may not have diminished to the extent or for the duration indicated by previous isotope studies. The magnitude of the δ¹³C gradient change is dependent on the species analysed, reflecting differing depths of calcification, seasonal contrasts and/or disequilibrium effects. The lack of any change in benthic δ¹³C at the K/T boundary indicates that the Antarctic may not have been a significant source of deep water during the early Palaeocene, contrary to previous suggestion^11–13.