Carbon Isotopes in Deep-sea Benthic Foraminifera: Precession and Changes in Low-latitude Biomass

Detailed carbon isotope records have water enriched in i•C into the deep North Atlantic been obtained from benthic foraminifera in the during glacial periods alters the carbon isotopic North Atlantic (CHN 82 Sta 24 Core 4PC, 42øN, 32øW, signal of Atlantic cores relative to the global 3427 m, 244 kyr) and central equatorial Pacific mean [Streeter and Shackleton, 1979; Duplessy et (KNR 73 Sta 4 Core 3PC, OøS, 106øW, 3606 m, 410 al., 1975; Boyle and Keigwin, 1982; Curry and kyr). These data demonstrate that the North Lohmann, 1983; Shackleton et al., 1983; Berger and Atlantic site has always been nutrient depleted Keir, 1984]. The change in the oceanic average relative to the Pacific site. Although down-core carbon isotopic composition between the present and carbon isotope data in the Atlantic and Pacific the 18-kyr (1 kyr=103 years) glacial maximum is now reflect changes in dee?_-ocean circulation patterns, thought to be about 0.4•/o• [Shackleton et al., a larger fraction of •-SC variability in both 1983; E.A. Boyle and L.D. Keigwin, unpublished oceans is due to changes in the inventory of manuscr$•t, 1984], corresponding to a mass of about continental reduced carbon. The magnitude of this 50 x 10 -moles of reduced carbon, about carbon isotope biomass signal is nearly twice as one-quarter of the present-day mass of carbon large during oxygen isotope stages 4 and 6 as it is stored in plants and soils. during stage 2. Carbon isotope variability in both The carbon isotopic records of Shackleton [1977] oceans contains significant power at the 23-kyr and Shackleton et al., [1983 I only covered the last frequency band which is coherent with insolation 140,000 years, so it was not possible from these changes caused by precession of the earth's data to determine if this same pattern of change rotational axis. We propose that this correlation has occurred over several glacial/interglacial is due to orbitally driven variations in transitions. In addition, the North Atlantic low-latitude biomass, and that the lower amplitude record is based on analysis of three species and of the carbon isotope signal during the last 60 kyr core M12392 is located in the eastern basin of the is due to the reduction in precession parameter amplitude during this period.