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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2008.08.001 Byline: Iris Wilke (a), Helge Meggers (b), Torsten Bickert (c) Keywords: Planktic foraminifers; Seasonality; Depth habitat; Stable oxygen isotopes; Disequilibrium effects; Ecology Abstract: Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29[degrees]N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios ([delta].sup.18O.sub.shell) of five planktic foraminiferal species. Both the [delta].sup.18O.sub.shell and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average [delta].sup.18O.sub.shell of G. ruber (w) from surface sediments is similar to the [delta].sup.18O.sub.shell values measured from the sediment-trap samples during winter. However, the [delta].sup.18O.sub.shell of G. ruber (w) underestimates sea surface temperature (SST) by 2[degrees]C in winter and by 4[degrees]C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in [delta].sup.18O.sub.shell values that underestimate SST by 2[degrees]C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline ([approximately equal to]180m) and deep surface water ([approximately equal to]350m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their [delta].sup.18O.sub.shell. Author Affiliation: (a) Ocean Sciences Department, University of California, 1156 High Street, Santa Cruz, CA 95064, USA (b) Fachbereich Geowissenschaften, Universitat Bremen, Klagenfurter StraAe, 28359 Bremen, Germany (c) Marum - Zentrum fur Marine Umweltwissenschaften, Universitat Bremen Leobener StraAe, 28359 Bremen, Germany Article History: Received 1 February 2008; Revised 29 July 2008; Accepted 11 August 2008

Abstract: The middle Miocene climate approximately 14 Ma ago was characterized by the glaciation of Antarctica, deep-ocean cooling and variations in the global carbon cycle. Although the Southern Ocean underwent significant oceanographic changes, there is limited information on their spatial extent and timing. However, such knowledge is crucial for understanding the role of the Southern Ocean and the Antarctic Circumpolar Current (ACC) for Antarctic glaciation and the coupling between the ocean and continental climate. We have reconstructed surface temperatures and seawater oxygen isotopes at Ocean Drilling Program (ODP) Site 1092 in the Polar Frontal Zone of the Atlantic sector of the Southern Ocean from foraminiferal oxygen isotopes (δ 18O) and magnesium to calcium ratios (Mg/Ca). Sea surface cooling by ~4 °C and freshening indicated by the ~1‰ reduction of seawater δ 18O (δ 18Osw) at 14.2 Ma precede the major step in Antarctic ice sheet growth at 13.8–13.9 Ma. This pattern qualitatively mirrors previous findings from the Pacific sector, and we interpret the surface hydrographic changes to reflect the circum-Antarctic northward shift of the Southern Ocean fronts and specifically at Site 1092 the passage of the Subantarctic Front. The magnitude of change in reconstructed δ 18Osw requires a δ 18Osw: salinity gradient significantly higher than the modern value (~0.52‰) and it possibly exceeded 1.1‰. This implies the Polar Frontal Zone was influenced by freshwater derived from Antarctica, which in turn confirms higher than modern continental precipitation. The latter has previously been suggested to have contributed to Antarctic glaciation. [Copyright &y& Elsevier]

Abstract: In many palaeoceanographic studies low carbonate contents in Neogene deep-sea sediments are interpreted as dissolution events related to prominent shifts in intermediate and deep-water circulation. In the middle to late Miocene such carbonate crash events were a widespread phenomenon throughout the world's ocean basins. However, low carbonate contents may also derive from terrigenous dilution and carbonate production changes in surface waters as shown for late Miocene records of the South Atlantic (Diester-Haass et al., 2004; Westerhold et al., 2005). In order to contribute to solve this question we investigated the youngest of the Miocene crash events (10.5Ma to 9.0Ma) at a prominent key location of global deep water circulation, i.e. the Equatorial Atlantic Ceará Rise depth transect (ODP Sites 926, 927 and 928). By analysing different carbonate preservation proxies like carbonate content, silt grain size parameters, sand content, fragmentation index and mass accumulation rates we were able to quantify dissolution, dilution and production, and to assess previous conflicting interpretations. Critical evaluation of the different dissolution proxies and comparison to former studies revealed that silt preservation proxies do not entirely reproduce the depth dependant dissolution in the Miocene sediments, as indicated by foraminifer fragmentation. In the investigated late Miocene time slice three minima of carbonate accumulation were detected. The predominant control on these events changed from dissolution of carbonate to lowered production. The last event at 9.6Ma is accompanied by an increased preservation but reduced carbonate accumulation. During this time the first prominent pulses of Northern Component Water (NCW) occurred, sourced most probably from the Labrador Sea. This may be understood as a first short term precursor of long time persistent and much stronger NCW production that is characteristic for younger periods and marks the turnover to the modern type of basin-to-basin fractionation between the Atlantic and Pacific. Furthermore, preservation trends of Caribbean and Ceará Rise records were in phase in contrast to former hypotheses that suggested antithetical preservation during carbonate crash events. The similarity of the preservation records in the studied ocean basins in conjunction with the inconsistency in timing of the CC-events and proposed NCW estimates indicates that changes in the carbon cycle other than dissolution controlled the CC events prior to ~9.9Ma. [Copyright &y& Elsevier]