Cyclic Sedimentation at Sites 745 and 746, Leg 119

Abstract

The upper Miocene to Pleistocene sediments recovered at ODP Sites 745 and 746 in the Australian-Antarctic Basin are characterized by cyclic facies changes. Sedimentological investigations of a detailed Quaternary section reveal that facies A is dominated by a high content of siliceous microfossils, a relatively low terrigenous sediment content, an icerafted component, low concentrations of fine sediment particles, and a relatively high smectite content. This facies corresponds to interglacial sedimentary conditions. Facies B, in contrast, is characteristic of glacial conditions and is dominated by a large amount of terrigenous material and a smaller opaline component. There is also a prominent ice-rafted component. The microfossils commonly are reworked and broken. The clay mineral assemblages show higher proportions of glacially derived illite and chlorite.A combination of four different processes, attributed to glacial-interglacial cycles, was responsible for the cyclic facies changes during Quaternary time: transport by gravity, ice, and current and changes in primary productivity. Of great importance was the movement of the grounding line of the ice shelves, which directly influenced the intensity of ice rafting and of gravitational sediment transport to the deep sea. The extension of the ice shelves was also responsible for the generation of cold and erosive Antarctic Bottom Water, which controlled the grain-size distribution, particularly of the fine fraction, in the investigated area.

A high‐resolution record of early Miocene Antarctic glacial history from ODP Site 1165, Prydz Bay

“…Facies 1 is a dark grey claystone with silt laminae and rare to absent IRD (Figure 3a); Facies 2 is a greenish‐gray bioturbated structureless claystone that contains IRD (Figure 3b). Similar facies alternations are described for late Miocene sediments at this site [ Grützner et al , 2003], and for Plio‐Pleistocene sediments at other locations around Antarctica [ Cowan , 2001; Grobe and Mackenson , 1992; Ehrmann and Grobe , 1991]. The similarity between the facies at Site 1165 and the analogous younger facies indicates active glaciation during the early Miocene, even though the overall volume of the ice sheet was smaller than during the late Pleistocene.…”

“…A third factor is the latitude at which the icebergs melt and hence where the IRD is deposited. It is unlikely that the icebergs melted further north than Site 1165, because early Miocene IRD is found only in trace amounts at Site 744 (61.6°S) and not at all at Site 751 (57.7°S) on the Kerguelen Plateau to the northeast of Site 1165 [ Ehrmann and Grobe , 1991; Breza , 1992]. In the present day, icebergs generally travel westward parallel to the coast just to the south of Site 1165 under the influence of the Polar Current and the prevailing winds; it is possible that a weaker current during Facies 2 deposition allowed more icebergs to drift northward over the site than during Facies 1 deposition.…”

“…The relative abundance of IRD offshore of the Antarctic Peninsula late Pleistocene interglacials was found by O'Cofaigh et al. [2001] to be caused by lowered input or winnowing of the other components of the sediment, rather than increased IRD supply . Ehrmann and Grobe [1991] found cyclic facies changes in the late Miocene to Pliocene of Sites 745 and 746 on the east side of the Kerguelen Plateau.…”

A 0.55‐Ma paleotemperature record from the Subantarctic zone: Implications for Antarctic Circumpolar Current development

“…This offset is linked to the 3.3–6.5 kyr lead of Southern Ocean SSST versus the Northern Hemisphere ice volume change and sea level. In more southerly locations with lower SSST the maximum IRD input occurs at the terminations in relation with increasing sea level and massive discharge of Antarctic ice into the Southern Ocean (e.g., cores PS2319‐1, PS2515‐3: Diekmann et al [2000]; Ocean Drilling program (ODP) Sites 745, 746: Ehrmann and Grobe [1991]). This results in a diachronous termination of IRD deposition in the Southern Ocean, depending on the latitude and surface water temperature regime, a model that was already proposed by Keany et al [1976].…”

Eucampia Index as an indicator of the Late Pleistocene oscillations of the winter sea-ice extent at the ODP Leg 119 Site 745B at the Kerguelen Plateau

“…abundance are well documented (e .g. Schrader 1976 ;Trinchitella & Dinkelman, 1980 ;Graves, 1982 ;Burckle & Cooke, 1983 ;Smith et al, 1983 ;Burckle & Burak, 1988 ;Ehrmann & Grobe, 1991 ;Jordan & Pudsey, 1992) . The "waxing and waning" of E .…”