ALBERT

Description: This database contains the data resulting from a study of Antarctic Ice Sheet history, over the last ~500 thousand years. Iceberg-rafted debris (IRD), considered as an indicator of the presence of a continental ice sheet in the source area, is counted and analyzed in relation to the other components of the studied size fraction. The study materials are derived from two cores raised from a deep-water marine site in the southeastern Atlantic Ocean: the piston core TN057-6-PC4 and the associated drill core from the Ocean Drilling Program (ODP) Leg 177 Site 1090 (177-1090). Because the top portion of the 177-1090 drill core was apparently disturbed by the drilling, the piston core TN057-6-PC4 has been used to represent the most recent part of the record. The site is located at ~43 deg S, 9 deg E, about 2/3 of the way from the Antarctic coast to South Africa. The location from which both cores were raised is on the southern slope of the Agulhas Ridge, on the northern edge of the Agulhas Basin, in the southeastern Atlantic Ocean. The initial cleaning and preparation of samples from these cores was carried out by Kathryn Venz and David Hodell (UF). The methods used here to examine the IRD are those described by Allen and Warnke (1991, doi:10.2973/odp.proc.sr.114.118.1991), Stanton (1997), and Murphy et al. (2002, doi:10.1016/S0031-0182(01)00495-3). The samples were first weighed, then separated by sieving into four size fractions: 〈150 microns, 150-250 microns, 250 microns to 2 mm, and 〉2 mm. The medium to coarse sand size fraction (250 microns to 2 mm) was utilized for our study. Within this, a split of about 1000 grains was obtained; the grains in this split were counted. Grains that were counted within categories that included: quartz, feldspar, mafic minerals, lithic fragments, volcanic lithics, volcanic ash, diatoms, planktic forams, benthic forams, radiolarians, and 'other'. The counts of quartz, feldspar, mafic minerals and lithic non-volcanic fragments were then summed as IRD. Following the suggestion of Murphy et al. (2002), the final step of this stage was a brief overview of the non-counted portion of the 250 micron to 2 mm size fraction, primarily to look for 'background rafting'. Background rafting refers to the presence of IRD in the non-counted portion (i.e., outside of the counted split) of the size fraction. For this study, a note was made of the presence or absence only of any IRD clasts in the non-counted portion.

Description: Constraining the nature of Antarctic Ice Sheet (AIS) response to major past climate changes may provide a window onto future ice response and rates of sea level rise. One approach to tracking AIS dynamics, and differentiating whole system versus potentially heterogeneous ice sheet sector changes, is to integrate multiple climate proxies for a specific time slice across widely distributed locations. This study presents new iceberg-rafted debris (IRD) data across the interval that includes Marine Isotope Stage 31 (MIS 31: 1.081-1.062 Ma, a span of ~19 kyr; Lisiecki and Raymo, 2005), which lies on the cusp of the mid-Brunhes climate transition (as glacial cycles shifted from ~41,000 yr to ~100,000 yr duration). Two sites are studied - distal Ocean Drilling Program (ODP) Leg 177 Site 1090 (Site 1090) in the eastern subantarctic sector of the South Atlantic Ocean, and proximal ODP Leg 188 Site 1165 (Site 1165), near Prydz Bay, in the Indian Ocean sector of the Antarctic margin. At each of these sites, MIS 31 is marked by the presence of the Jaramillo Subchron (0.988-1.072 Ma; Lourens et al., 2004) which provides a time-marker to correlate these two sites with relative precision. At both sites, records of multiple climate proxies are available to aid in interpretation. The presence of IRD in sediments from our study areas, which include garnets indicating a likely East Antarctic Ice Sheet (EAIS) origin, supports the conclusion that although the EAIS apparently withdrew significantly over MIS 31 in the Prydz Bay region and other sectors, some sectors of the EAIS must still have maintained marine margins capable of launching icebergs even through the warmest intervals. Thus, the EAIS did not respond in complete synchrony even to major climate changes such as MIS 31. Further, the record at Site 1090 (supported by records from other subantarctic locations) indicates that the glacial MIS 32 should be reduced to no more than a stadial, and the warm interval of Antarctic ice retreat that includes MIS 31 should be expanded to MIS 33-31. This revised warm interval lasted about 52 kyr, in line with several other interglacials in the benthic d18O records stack of Lisiecki and Raymo (2005), including the super-interglacials MIS 11 (duration of 50 kyr) and MIS 5 (duration of 59 kyr). The record from Antarctica-proximal Site 1165, when interpreted in accord with the record from ANDRILL-1B, indicates that in these southern high latitude sectors, ice sheet retreat and the effects of warming lasted longer than at Site 1090, perhaps until MIS 27. In the current interpretations of the age models of the proximal sites, ice sheet retreat began relatively slowly, and was not really evident until the start of MIS 31. In another somewhat more speculative interpretation, ice sheet retreat began noticeably with MIS 33, and accelerated during MIS 31. Ice sheet inertia (the lag-times in the large-scale responses of major ice sheets to a forcing) likely plays an important part in the timing and scale of these events in vulnerable sectors of the AIS.