Middle Miocene Cooling

Marine deep-sea oxygen isotope records indicate that a period of ice-sheet instability during the Early and Middle Miocene is superimposed on a long-term warming trend that culminated in the mid-Miocene climatic optimum from ~ 17 to 15 Ma (Miller et al., 1987; Zachos et al., 2001). This phase was followed by a gradual cooling with a major cooling event occurring at the Middle-Late Miocene boundary ~14Ma that continued for ~1-2m.y. (MMCT; Flower and Kennett, 1995).

The Prydz Bay area was visited during the Leg 119 (Barron et al., 1989, 1991) and Leg 188 (O'Brien et al., 2001; Cooper et al., 2004) drilling campaigns of the ODP. Middle Miocene-Pliocene sediments were drilled at Sites 739, 742, 1165 and 1166. An interesting stratigraphic record across this transition was recovered during ODP Leg 188 when three sites were drilled proximal to the EAIS across the Prydz Bay continental shelf (Site 1166), slope (Site 1167) and rise (Site 1165). Of these, Site 1165 (64°22'S; 67°13'E) records a history of sedimentation on the continental rise extending back to earliest Miocene times (about 22 Ma). Several long-term changes characterize this record, including an overall trend of decreasing sedimentation rates from the bottom to the top of the hole. There is a progressive decrease in the sedimentation rate above about 308 m b.s.f. (metres below sea floor), which is marked by a transition from predominantly dark-grey fissile claystones with abundant silt laminae to grey diatom-bearing clays with a higher abundance of IRD (Fig. 10.6). At this transition, the total clay content also increases. The chronology of this sequence indicates a Middle Miocene age (ca. 14.3 Ma) for the lithological transition. Correlation to ODP Hole 747A on the Kerguelen Plateau shows that this transition coincides with the base of the Mi-3/3a 818O event (Florindo et al., 2003). Middle Miocene

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LithologyAge

ODP Hole 1165B

Continental Rise - 64°S

ODP Hole 747A Kerguelen Plateau - 54'S

ODP Hole 1165B

Continental Rise - 64°S

ODP Hole 747A Kerguelen Plateau - 54'S

Figure 10.6: Lithostratigrapliic column of Site 1165 and correlation between Middle Miocene intervals to ODP Hole 747A from the Kerguelen Plateau (ODP Leg 120). The lithological transition at about 308 m b.s.f., which is characterized by an increase in ice-rafted debris, sand grains and total clay content, coincides with the base of Mid-Miocene glacial event (Mi-3/3a) (modified from Florindo et al., 2003).

changes in sedimentation at Site 1165 may have been caused by the development of a cooler ice sheet characterized by decreased rates of glacial erosion and decreased production of melt water. In such an event, less suspended sediment is delivered to the continental rise, but ice at sea level produces more icebergs and, hence, more ice rafting of debris.

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