Palaeogeography, Palaeoclimatology, Palaeoecology
History of ice rafting at South Atlantic ODP Site 177-1092 during the Gauss and Late Gilbert Chrons
Introduction
The modern cryosphere developed between 3 and 2 million years ago, when large ice masses developed in the northern hemisphere. Prior to this time only Antarctica was covered by an ice sheet (Hodell and Warnke, 1991, Raymo et al., 1992). The time period studied in this paper centers on the Gauss Normal Chron (2.58–3.58 Ma) and part of the Gilbert Reversed Chron (3.58–5.89 Ma). Study samples are from Ocean Drilling Program (ODP) Site 177-1092, on the north-central Meteor Rise at 46°24.708′S, 7°4.792′E (Fig. 1, Fig. 2) in the South Atlantic. Site 177-1092 provides a record of pelagic deposits spanning the Miocene to Pleistocene (Shipboard Scientific Party, 1999).
Paleoclimatic data from this time are considered to be appropriate for use in general circulation models for outlining possible climatic events associated with future climate warming (Heusser and Morley, 1996). The Antarctic cryosphere is the largest ice accumulation on Earth. Its melting would raise sea level by about 70 m (Kennett and Hodell, 1993). How this cryosphere evolved and its stability during the Pliocene warm period are of more than academic interest because of consequences to coastal human communities should future global warming lead to a climate scenario similar to that of the Pliocene.
Prior drilling in the Atlantic Sector of the Southern Ocean (Legs 113 and 114) and in the Indian Sector of the Southern Ocean (Legs 119 and 120) provided a basic understanding of the paleoclimatic evolution of southern high latitudes. However, these drilling legs produced sections that were in many cases incomplete or disturbed by drilling. Leg 177 recovered complete sections that were used to construct a continuous, composite sedimentary record (Shipboard Scientific Party, 1999).
Section snippets
Previous paleoclimate studies
Most workers believe that large-scale glaciations of Antarctica extend back to approximately 36–40 Ma, when the continent became thermally isolated by the Circum-Antarctic Current (Hambrey and Barrett, 1993). Ice-rafted detritus (IRD) collected at ODP Sites 699 and 701 in the Atlantic sector of the Southern Ocean indicates the presence of Antarctic continental glaciers with marine termini beginning in early Miocene time (Warnke and Allen, 1991, Kennett and Hodell, 1993, Warnke et al., 1996).
Purpose of investigation
Previous research has not entirely resolved the issues of the stability and extent of the Antarctic Ice Sheets during the Pliocene and the history of sea-level changes. The principal objectives of this research were to:
• describe fluctuations in the delivery of IRD from ∼3.7 Ma to 2.6(?) Ma in South Atlantic Site 177-1092. This time interval is equivalent to the time interval covered by an earlier IRD study at nearby Site 114-704, and was intended to verify, modify, or reject the conclusions
General procedures
At each coring site, cores from the several holes are aligned using multisensor track and color reflectance data to create a composite section that avoids disturbances or distortions present in the individual cores (Shipboard Scientific Party, 1999). Samples from the individual cores that comprise the spliced section are assigned a meters composite depth (mcd) below the sea floor. Four holes were drilled at Site 177-1092.
Personnel at the Geology Department of the University of Bergen prepared
Stable isotopes
Stable isotopic measurements were made on the benthic foraminifer Cibicidoides wuellerstorfi (Andersson et al., 2002) or on Globocassidulina when Cibicidoides was not available. Stable isotopic values have been adjusted to equilibrium. The Holocene value (adjusted) is 3.64‰ (Andersson et al., 2002). Stage assignments are based on the magnetostratigraphy (see below). The lack of a strong transition to significantly heavier values at marine isotope stage (MIS) MG2, as at other sites, is puzzling (
Timing of IRD peaks compared with the δ18O record
Fig. 3 shows the relationship between IRD events and the oxygen isotopic record. IRD peaks below 51.3 mcd tend to coincide with positive peaks in the oxygen isotopic curve. This pattern is different from that seen in the North Atlantic. There, Stanton (1997) and Venz et al. (1999) determined that Terminal Ice-Rafting Events occurred just after large positive excursions in the benthic isotopic record (i.e. during the transition to a negative excursion), reflecting the quick disintegration of
Conclusions
IRD arrived frequently during the Early and early Late Pliocene at Site 177-1092. Only one interval during C2An3n and several brief intervals during the Gilbert Reversed Chron were free of IRD. IRD included quartz, feldspars, mafic minerals and lithic fragments. A comparison of the IRD peaks with benthic oxygen isotopes reveals that IRD peaks tend to coincide with positive excursions on the oxygen isotopic curve below 51.3 mcd. Up section, the reverse is true. The IRD peak near the top of the
Acknowledgements
We thank the Shipboard Scientific Party of ODP Leg 177 and the ODP curator staff at the Bremen Core Repository for their efforts in obtaining Leg 177 sediments. L.M., D.A.W., J.C., and J.S. thank ODP-USSAC for post-cruise support. For this research we used samples provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of the Joint Oceanographic Institutions. We gratefully acknowledge reviews by
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