History of ice rafting at South Atlantic ODP Site 177-1092 during the Gauss and Late Gilbert Chrons

https://doi.org/10.1016/S0031-0182(01)00495-3Get rights and content

Abstract

We have carried out a multiphase analysis of samples from ODP Site 177-1092, Meteor Rise, subantarctic South Atlantic. Samples were analyzed for ice-rafted debris (IRD) and stable isotopes from benthic foraminifera. Both analyses were performed on the same samples. Additional work was performed to identify the paleomagnetic stratigraphy. The analyzed samples range in age from about 2.6(?) Ma to 4.6 Ma, a time span that saw considerable global warmth, but witnessed overall global refrigeration and the transition to truly bipolar glaciations. A tentative oxygen isotopic stratigraphy was established by comparison with Shackleton et al. [R. Soc. Edinburgh Trans. Earth Sci. 8 (1990) 251–261] and shackleton et al. [Proc. ODP Sci. Results 138 (1995) 337–355]. Paleomagnetic results show that the Gauss Normal Chron, including subchrons, is identified, although uncertainties plague the exact definitions of the reversals. The subchrons of the Gilbert Reversed Chron, unfortunately, could not be identified. IRD arrived frequently during the Early and early Late Pliocene, but only as ‘background rafting’ (occasional grains per sample). The first identifiable IRD above background rafting is associated with marine isotope stage (MIS) KM4 (∼3.18 Ma). Successive IRD peaks become larger, the same pattern as noted at nearby Site 114-704. A very large peak near the top of the record, approximately 2.8 Ma, is considered to represent a hiatus. Peaks below 51.3 meters composite depth (mcd) coincide with positive excursions of the oxygen isotopic record, and with negative excursions of the carbon isotopic curve, a pattern also noted at Site 114-704. However, the reasonably large IRD peak at 51 mcd (tentatively identified with MIS G11) coincides with a positive excursion on the carbon isotopic curve and negative excursion on the oxygen isotopic curve. This relationship suggests a northern hemisphere interglacial, rising sea level, destabilization of the Antarctic margin, and delivery of Antarctic icebergs to the Southern Ocean. Such a mechanism has recently been suggested by Kanfoush et al. [Science 288 (2000) 1815–1818] for latest Pleistocene stadial/interstadial oscillations. Here we suggest that such a mechanism may have been in place on glacial/interglacial time scales as early as the Late Pliocene. One interval in the lowermost Gauss Normal Chron and several short intervals in the upper Gilbert Reversed Chron have no IRD. However, oxygen isotopic values of benthic foraminifera are only about 0.62‰ lighter than modern, and must be ascribed to temperature effects in the area of water-mass formation [Hodell and Warnke, Quat. Sci. Rev. 10 (1991) 205–214; Hodell and Venz, Antarctic Research Series 56 (1992) 265–310; Warnke et al., Mar. Micropaleontol. 27 (1996) 237–251]. The East Antarctic Ice Sheet was therefore stable – but the stability of the West Antarctic Ice Sheet may have been compromised.

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

References (44)

  • Barrett, P.J., 1991. Antarctica and global climatic change: a geological perspective. In: Harris, C., Stonehouse, B....
  • Berggren, W.A., Kent, D.V., Flynn, J.J., Van Convering, J.A., 1985. Cenezoic geochronology. GSA Bull. 96,...
  • F.C. Berggren et al.

    A revised Cenozoic geochronology and chronostratigraphy. Geochronology time scales and global stratigraphic correlation

    SEPM Spec. Publ.

    (1995)
  • Billups, K., 2001. Enhanced thermohaline circulation during the latest Miocene through Early Pliocene: Early response...
  • L.H. Burckle et al.

    Pliocene-Pleistocene diatoms in Paleozoic and Mesozoic sedimentary and igneous rocks from Antarctica: A Sirius problem solved

    Geology

    (1996)
  • P.F. Ciesielski et al.

    Early Pliocene temperature changes in the Antarctic seas

    Geology

    (1974)
  • Clark, D.L., 1990. Arctic Ocean ice cover: Geologic history and climatic significance. In: Grantz, A., Johnson, L.,...
  • Dowsett, H.J., 1994. Mid-Pliocene planktonic foraminiferal assemblages from ODP Site 704: Palaeoceanographical...
  • H.J. Dowsett et al.

    Micropaleontological evidence for increased meridional heat transport in the North Atlantic Ocean during the Pliocene

    Science

    (1992)
  • Dowsett, H.J., Barron, J.A., Poore, R.Z., Thompson, R.S., Cronin, T.M., Ishman, S.E., Willard, D., 1999. Middle...
  • B.L. Hall et al.

    Pliocene paleoenvironment and Antarctic ice sheet behavior: Evidence from Wright Valley

    J. Geol.

    (1997)
  • Hambrey, M.J., Barrett, P.J., 1993. Cenozoic sedimentary and climatic record, Ross Sea Region, Antarctica. In: Kennett,...
  • Cited by (23)

    • Antarctic Ice Sheet response to a long warm interval across Marine Isotope Stage 31: A cross-latitudinal study of iceberg-rafted debris

      2015, Earth and Planetary Science Letters
      Citation Excerpt :

      The assumptions and implications of this technique of normalization were discussed in Teitler et al. (2010); we use the same average dry bulk density (DBD) of 0.8 g/cm3 as before at this site. The remaining portion of the examined size fraction that was not part of the counted split was surveyed, and a note was made of the presence there of IRD; these grains are considered as ‘background rafting’ (Murphy et al., 2002). We tallied the absolute count of garnets found in either the counted split, or as background rafting.

    View all citing articles on Scopus
    View full text