Cibicides

Abstract: Ocean circulation and climate are strongly interconnected. Under climatic conditions very different from those of today, deep and intermediate water circulation was subject to drastic changes1–7. For instance, during the last glacial maximum (LGM), deep ocean water was cooler than now by several degrees8. These temperature changes in the deep ticean were associated with striking variations in chemical characteristics of the intermediate and deep water masses of the Atlantic and Pacific Oceans3,8–13. Here we reconstruct the hydrological structure of the deep and intermediate water of the Indian Ocean during LGM (∼18,000yr BP). The carbon and oxygen isotope analyses of the benthic foramiriifira genus Cibicides show that the water-column structure of the Indian Ocean during; LGM was marked by the presence of a deep front separating intermediate and deep-water masses with very different characteristics: Intermediate-water mass temperatures and §13C were similar to those of today. By contrast, the Jeep water was cooler than iiow by at least 1.5 °C, more depleted in 13C and poorly oxygenated.

Abstract: Oxygen and carbon isotope records measured in epibenthic foraminifers ( Cibicides wuellerstorÞ, Cibicides spp., and Nuttalides umbonifera ) are presented for Sites 925 (3041 m water depth), 927 (3315 m), and 929 (4356 m), recovered during Ocean Drilling Program (ODP) Leg 154 at the northeast slope of the Ceara Rise, western equatorial Atlantic. These records comprise the time interval from 2.6 to 0 Ma. Dating is based on tuning variations of the magnetic susceptibility records to orbital parameters using the 1,1 solution of Laskar. The isotope records of all sites are exceptionally coherent and show a cyclicity dominated by a 41-k.y. cycle during the late Pliocene and the early Pleistocene and a 100-k.y. cyclicity during the late Pleistocene. Carbon isotope records show a varying depth gradient during the studied period. For interglacials, a difference of 0.3a resects the modern pattern of admixing of Lower Circumpolar Deep Water into the overlying North Atlantic Deep Water (NADW). During glacials, carbon isotope values of shallow and deep sites differ by up to 1.0a, but converge since 1.6 Ma, and especially since 1.0 Ma. The evolution of Atlantic to PaciÞc carbon isotope gradients is examined using a record of the western equatorial PaciÞc ODP Site 806B (2520 m). From 2.6 to 1.6 Ma glacial reductions in NADW are less than those observed between 1.6 and 0 Ma. During the mid-Pleistocene (0.7Ð0.3 Ma), glacial carbon isotope values of even the shallower Atlantic cores are indistinguishable from PaciÞc values from approximately the same depth.