ALBERT

Description: Automatic weather station data from a buoy deployed on sea ice station PS81/515-1 from Research Vessel Polarstern in the Antarctic Weddell Sea in austral winter 2013 (cruise leg ANT-XXIX/6, AWECS campaign). The automatic weather station provides data between 2013-07-26T19:00 and 2013-10-03T19:00. Time is in UTC.

Description: Stable isotopic analyses (18O/16O, 13C/12C) of both benthic and planktonic foraminifers from DSDP Holes 516A and 517 appear to reflect changing paleoclimatic and paleoceanographic conditions in the South Atlantic Ocean during the Pliocene. The d18O records of Cibicides wuellerstorfi and Globigerinoides sacculifer do not vary in phase with one another, indicating that the d18O of C. wuellerstorfi is reflecting changing glacial ice volumes, but the d18O of G. sacculifer is being influenced more by other factors, such as variable sea-surface temperatures and salinities. A net depletion in 18O and a series of very low-amplitude fluctuations characterize the late Pliocene planktonic d18O record. The stability of this section may be related to increasing surface-water temperatures in middle southern latitudes during northern hemisphere glacial ice buildup caused by differences in the phasing of the orbital precessional cycle between the northern and southern hemispheres. Hole 516A is presently located in an area of strong d13C gradients in the water column, and a similar situation probably prevailed during the Pliocene. A record of the difference between the d13C of C. wuellerstorfi and G. sacculifer (d13C) reflects times of increased or decreased similarity in d13C between Circumpolar Water and subtropical surface waters throughout the Pliocene. Periods of increased or decreased d13C may reflect: 1) changes in water mass boundaries because of expansion or contraction of a particular water mass, or 2) changes in the partitioning of 13C within the various components of the global carbon reservoir. These changes may be associated with glacial/interglacial climatic changes, such as the initiation of northern hemisphere glaciation.

Description: Pliocene changes in the vertical water mass structure of the western South Atlantic are inferred from changes in benthic foraminiferal assemblages and stable isotopes from DSDP Holes 516A, 517, and 518. Factor analysis of 34 samples from Site 518 reveals three distinct benthic foraminiferal assemblages that have been associated with specific subsurface water masses in the modern ocean. These include a Nuttalides umbonifera assemblage (Factor 1) associated with Antarctic Bottom Water (AABW), a Globocassidulina subglobosa-Uvigerina peregrina assemblage (Factor 2) associated with Circumpolar Deep Water (CPDW), and an Oridorsalis umbonatus-Epistominella exigua assemblage associated with North Atlantic Deep Water (NADW). Bathymetric gradients in d13C between Holes 516A (1313 m), 517 (2963 m), and 518 (3944 m) are calculated whenever possible to monitor the degree of similarity and/or difference in the apparent oxygen utilization (AOU) of water masses located at these depths during the Pliocene. Changes in bathymetric d13C gradients coupled with benthic foraminiferal assemblages record fundamental changes in the vertical water mass structure of the Vema Channel during the Pliocene from 4.1 to 2.7 Ma. At Site 518, the interval from 4.1 to 3.6 Ma is dominated by the N. umbonifera (Factor 1) and O. umbonatus-E. exigua (Factor 3) assemblages. The d13C gradient between Holes 518 (3944 m) and 516A (1313 m) undergoes rapid oscillations during this interval though no permanent increase in the gradient is observed. However, d13C values at Site 518 are clearly lighter during this interval. These conditions may be related to increased bottom water activity associated with the re-establishment of the West Antarctic Ice Sheet in the late Gilbert Chron (-4.2 to 3.6 Ma) (Osborn et al., 1982). The interval from 3.6 to 3.2 Ma is marked by a dominance of the G. subglobosa-U. peregrina (Factor 2) assemblage and lack of a strong d13C gradient between Holes 518 (3944 m) and 516A (1313 m). We suggest that shallow circumpolar waters expanded to depths of a least 3944 m (Site 518) during this time. The most profound faunal and isotopic change occurs at 3.2 Ma, and is marked by dominance of the N. umbonifera (Factor 1) and O. umbonatus-E. exigua (Factor 3) assemblages, a 1.1 per mil enrichment in d18O, and a large negative increase in the d13C gradient between Holes 518 and 516A. These changes at Site 518 record the vertical displacement of circumpolar waters by AABW and NADW. This change in vertical water mass structure at 3.2 Ma was probably related to a global cooling event and/or final closure of the Central American seaway. A comparison of the present-day d13C structure of the Vema Channel with a reconstruction between 3.2 and 2.7 Ma indicates that circulation patterns during this late Pliocene interval were similar to those of the modern western South Atlantic.