Neogene Multispecies Planktonic Foraminifer Stable Isotope Record, Site 871, Limalok Guyot

Abstract

Carbon (δ 13 C) and oxygen (δ l8 0) stable isotope measurements were made on over 30 species of planktonic foraminifers from the lower Miocene to Pleistocene interval of the pelagic cap on Limalok Guyot (ODP Site 871), a low-latitude site in the western Pacific. Two reference taxa, Globigerinoides ruber and "Globoquadrina" venezuelana, show a consistent relative offset in δ' 3 C and δ 18 θ indicative of shallow-and deep-water habitats, respectively. Globigerinoides ruber exhibits an apparent carbon isotope vital effect of-0.5‰ throughout the Neogene record. By comparing them to G. ruber and "G." venezuelana, other taxa are also interpreted in terms of their preferred classification depth, broadly confirming previously reported depth rankings. Many species show possible carbon isotope vital effects, including Globigerinella praesiphonifera, Fohsella spp., and Paragloborotalia mayeri. The isotopic record of "Dentglobigerina" altispira, which has usually been interpreted as a shallow-water dweller, bears little similarity to that of G. ruber, and it is speculated that it may be a relatively deep-dwelling species with a strong vital effect in both δ' 8 O and δ 13 C. Contrary to recent studies, no evidence for a change in depth habitat in the evolution of Fohsella was found. Similarly, the Globigerinoides bisphericus-Praeorbulina-Orbulina lineage apparently evolved without a change in depth habitat. Anomalously high paleotemperatures for an extended period in the early to middle Miocene are interpreted as recording the existence of a locally restricted warm current over the site. Oxygen isotope data for this period are the most negative so far reported. Long-term trends in the carbon isotope record probably reflect global trends in organic carbon burial. For example, the mid-Miocene carbon isotope "Monterey" excursion is well reflected in the data.