Palaeogeography, Palaeoclimatology, Palaeoecology
Clay minerals in Cenozoic sediments off Cape Roberts (McMurdo Sound, Antarctica) reveal palaeoclimatic history
Introduction
Variations in the extent and volume of the Antarctic ice sheet influence the level of the world oceans, the global oceanographic circulation pattern, and the production of cold and dense bottom waters that fill the deep oceans and penetrate far into the northern hemisphere. This makes the Antarctic ice sheet to one of the most important features controlling the climate of our planet (e.g., Hambrey and Barrett, 1993, Bartek et al., 1996, Abreu and Anderson, 1998, Barker et al., 1998, Barker et al., 1999, Barrett, 1999, Lear et al., 2000, Zachos et al., 2001, Billups and Schrag, 2003, Mackensen, 2004). Therefore, the reconstruction of the long-term Cenozoic climatic and glacial history of Antarctica is of fundamental importance, and the reconstruction of the past conditions may be a key for assessing future developments.
Substantial progress in deciphering the behaviour of the Antarctic ice sheet has been made in recent years. Important contributions came from the Ocean Drilling Program (Legs 113, 114, 119, 177, 178, 188, 189; Barker et al., 1990, Barron et al., 1991, Ciesielski et al., 1991, Exon et al., 2001, O´Brien et al., 2001, Barker and Camerlenghi, 2002, Gersonde and Hodell, 2002) and other Antarctic drilling programmes (McGinnis, 1981, Barrett and Scientific Staff, 1985, Barrett, 1986, Barrett, 1989). These projects showed that the East Antarctic ice sheet abruptly developed at the Eocene/Oligocene boundary. The size and volume of the Cenozoic ice sheet, however, experienced large variations. A further cooling and development of Antarctic ice sheets occurred after the Mid-Miocene climatic optimum.
In this paper, we concentrate on sediments recovered by the Cape Roberts Project (Hambrey et al., 1998, Barrett et al., 2000a, Barrett et al., 2000b, Barrett et al., 2001a, Barrett et al., 2001b). Three cores (CRP-1, CRP-2/2A and CRP-3) were drilled off Cape Roberts in McMurdo Sound of the Ross Sea, Pacific sector of the Southern Ocean (Fig. 1, Table 1). The combined sedimentary sequence comprises some 1600 m and reaches back in time at least to the earliest Oligocene. Large parts of the present-day Antarctic ice sheets discharge into the Ross Sea, both through glaciers breaching the Transantarctic Mountains and through the Ross Ice Shelf. Thus, the sediment cores of the Cape Roberts Project are expected to document the influence of the Antarctic ice masses through time.
We present results of detailed clay mineral analyses on sediments of the cores CRP-1, CRP-2/2A and CRP-3. The method is based on the observation that the clay mineral assemblages of sediments around Antarctica generally consist of illite, chlorite, smectite and minor kaolinite. These minerals are mostly of detrital origin. Their distribution is mainly controlled by the composition of the source rocks and by the type of weathering. Illite and chlorite result from physical weathering under a cool and dry climate. In contrast, kaolinite and smectite are normally derived from chemical weathering and require humid conditions and warm temperatures for their formation. The climate therefore exerts major control on the clay mineral composition of the sediments. The weathering conditions furthermore influence the chemical character of the clay minerals.
Thus, several studies successfully used clay minerals in Cenozoic sediments off Antarctica for reconstructing source areas, weathering conditions and the glacial and palaeoclimatic history of Antarctica. The detrital clay minerals have proved to be an important proxy that can compensate for the lack of information normally gained by other proxies, such as stable isotope records or faunal assemblages (e.g., Claridge and Campbell, 1989, Robert and Maillot, 1990, Ehrmann et al., 1992a, Ehrmann et al., 2003, Ehrmann and Mackensen, 1992, Ehrmann, 1998a, López-Galindo et al., 1998, Hillenbrand and Ehrmann, 2003, Hillenbrand and Ehrmann, 2005). The occurrence of authigenic clay minerals, however, may lead to incorrect palaeoclimatic interpretations. Therefore it is essential to differentiate between detrital and authigenic phases.
In this paper we combine detailed XRD (X-Ray Diffraction) studies with FESEM (Field Emission Scanning Electron Microscope), TEM (Transmission Electron Microscope) and geochemical investigations in order to identify authigenic clay minerals. We present for the first time a complete and consistent set of clay mineral data for the combined sedimentary sequence of cores CRP-1, CRP-2/2A and CRP-3. Previous publications had only preliminary character and could not present the whole range of data. Most studies only investigated the smectite occurrence and the composition of the smectites. They concentrated on single cores and were based on much less samples. They could draw an only very rough picture on the clay mineral distribution and its implication. However, the studies could show that there is a major change from smectite-dominated assemblages in the Early Oligocene to illite-dominated assemblages in the younger sediments (Ehrmann, 1998b, Ehrmann, 2000, Ehrmann, 2001, Setti et al., 1998, Setti et al., 2000, Setti et al., 2001).
The main objectives of our present study are to use the percentage distribution and the composition of the individual clay minerals for a detailed reconstruction of the palaeoclimatic and glacial conditions on the Antarctic continent during deposition of the investigated sediments.
Section snippets
Methodology
Bulk sediment samples were crushed, then oxidized and disaggregated by means of a 5% H2O2 solution. The clay fraction (< 2 μm) was separated in settling tubes. The clay was dried and weighted. Clay percentages were calculated. Forty milligram of clay were dispersed and mixed with an internal standard consisting of a 0.4% MoS2 suspension. The samples were mounted as texturally oriented aggregates by rapidly filtering the suspensions through membrane filters of 0.15 μm pore width. The filter cakes
Geological setting
Neogene alkali volcanic complexes characterise the geology of the southern part of McMurdo Sound (Fig. 1). In contrast, the Transantarctic Mountains west of McMurdo Sound consist of a crystalline basement of Late Precambrian to Early Palaeozoic granites and mainly amphibolite-grade metamorphic rocks that crop out over wide areas (Haskell et al., 1965, Warren, 1969, Laird and Bradshaw, 1982, Tingey, 1991a). In the western part of the Transantarctic Mountains, sedimentary rocks, mainly non-marine
Stratigraphy
The cores CRP-1, CRP-2/2A and CRP-3 of the Cape Roberts Project were drilled from the sea ice, some 12 to 16 km off Cape Roberts on the western shelf of McMurdo Sound in Ross Sea (Fig. 1, Table 1).
CRP-1 penetrated to 148 mbsf (metres below sea floor) and recovered Quaternary and Early Miocene (17–22.5 Ma) sediments (Fig. 3). A major hiatus at 43 mbsf separates the two stratigraphical intervals (Cape Roberts Science Team, 1998).
CRP-2/2A (Fig. 4) drilled to a depth of 624 mbsf. It overlaps with
Lithology
The main part of the Cenozoic sedimentary sequence of cores CRP-1, CRP-2/2A and CRP-3 consists of an alternation of diamictites, gravelly sandstones, sandstones, and mudstones (Fig. 3, Fig. 4, Fig. 5). Massive diamictites are more common than stratified diamictites. Both types vary between clast-poor and clast-rich. Clasts are angular to rounded. The matrix may be muddy or sandy. Depending on the amount of matrix and clasts, the diamictites grade into sandstones or conglomerates. The sandstones
Results
The results are shown in Fig. 3, Fig. 4, Fig. 5, Fig. 6 and in Table 2. The description given in this chapter therefore concentrates on the main features only. We refrained from graphically presenting kaolinite percentages, because kaolinite occurs in minor amounts of 0–6% and does not show any significant downcore variations (Table 2).
The clay mineral assemblages allow a subdivision of the sedimentary sequence off Cape Roberts into seven units.
Unit I comprises the lower part of the Cenozoic
Origin of the clay mineral assemblages
The Cenozoic sediments recovered with cores CRP-1, CRP-2/2A and CRP-3 exhibit a large number of unconformity-bound fining-upwards cycles, which were attributed to sea level changes or to changes in the proximity of the ice masses (e.g., Naish et al., 2001, Hambrey et al., 2002). The clay mineral data do not mirror these cycles. Furthermore, no correlation exists between the sedimentary facies and the percentage distribution of individual clay minerals, their crystallinities and chemical
Summary and conclusions
The Cenozoic sedimentary sequence off Cape Roberts can be subdivided into seven clay mineral units. Although the stratigraphical information of the sediment cores is poor, the clay minerals allow reconstructing a stepwise change from humid climatic conditions in earliest Oligocene time to subpolar and polar conditions in younger times.
Unit I (ca. 35–33.6 Ma) is characterised by an almost monomineralic assemblage consisting of authigenic, very well to well crystalline smectites. They have an
Acknowledgements
The laboratory work for the XRD analyses was carried out at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany. Helga Rhodes is acknowledged for technical assistance. The Deutsche Forschungsgemeinschaft provided financial support to W.E. TEM and FESEM analysis were performed at the “Centro de Instrumentación Científica” of the University of Granada (Spain) with financial support of the Italian “Programma Nazionale di Ricerche in Antartide” (PNRA). We thank B.
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