Palaeoecology of the biserial planktonic foraminifer Heterohelix moremani (Cushman) in the late Albian to middle Turonian Circum North Atlantic

The Cretaceous Heterohelix moremani (Cushman) was the only biserial plankto nic foraminiferal species from its first appearance in the late Albian up t o the Cenomanian/Turonian boundary. Within that time, it increased graduall y in abundance relative to other planktonic foraminifera in five Circum-Nor th Atlantic sections. It is generally rare in upper Albian sediments, commo n in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overa ll abundance trend correlate with positive excursions in the bulk carbonate delta(13)C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but i ts overall distribution indicates that it is not necessarily a marker for v ery high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surfa ce water dweller at least in part of its geographic range, but incorporated C-13 out of equilibrium with ambient seawater. It is depleted in delta(13) C relative to other planktonic foraminifera, which is attributed to vital e ffects related to its opportunistic character. (C) 1998 Elsevier Science B. V. All rights reserved.