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USGS Open-File Report 94-588

A direct, high-resolution, Southern Hemisphere record of late Neogene glacioeustasy, important constraints, and a possible stratotype for ice volume, sea-level and climate change

Gary S. Wilson
Byrd Polar Research Center, The Ohio State University
Two independent records of Late Neogene (2.0 - 6.0 Ma.) glacioeustasy are presented: one of Antarctic ice volume from East Antarctica and the other of eustatic sea level from the South Wanganui Basin, New Zealand. The sea level record from New Zealand is unequalled in its resolution and provides important constraints for other studies and models of individual events and extents of global climate change. Such a record has the potential to be a Late Neogene glacioeustatic and climate change stratotype.

Glacial deposits in the Transantarctic Mountains (Sirius Group; McKelvey et al., 1991) and sediments at the Antarctic continental margin provide direct evidence of Antarctic ice sheet fluctuation. Evidence for deglaciation includes the occurrence of Pliocene marine diatoms in Sirius Group deposits (Harwood 1986), which are sourced from the East Antarctic Interior (Webb et al., 1984, Webb and Harwood, 1991). K/Ar and 39Ar/40Ar dating of a tuff in the CIROS-2 drill core confirms their Pliocene age at high latitudes (78S) in Antarctica (Barrett et al., 1992). Further evidence for Antarctic ice volume fluctuation is recorded by glaciomarine strata from the Ross Sea sector cored by the CIROS-2 and DVDP-11 drill- holes (Ishman and Rieck, 1992, Barrett and Hambrey, 1992, Wilson, 1993). Magnetostratigraphy integrated with Beryllium-10, K/Ar and 39Ar/40Ar dating provides a high resolution (+ 50 k.y.) chronology of events in these strata.

In the Wanganui Basin, New Zealand, a 5 km thick succession of continental shelf sediments, now uplifted, records Late Neogene eustatic sea level fluctuation. In the Late Neogene, basin subsidence equalled sediment input allowing eustatic sea level fluctuation to produce a dynamic alternation of high-stand, transgressive, and low-stand sediment wedges. This record of Late Neogene sea level variation is unequalled in its resolution and detail. Magnetostratigraphy provides a high resolution chronology for these sedimentary cycles, as well as magnetic tie lines with the Antarctic margin record in McMurdo Sound.

These two independent records of Late Neogene glacioeustasy are in good agreement and record the following history: The Late Miocene and Late Pliocene are times of low 'base level' glacioeustasy (here termed glacialism, rather than glacial), with growth of continental-scale ice sheets on the Antarctic continent causing a lowering of global sea level. The Early Pliocene was a time of high 'base level' glacioeustasy (here termed interglacialism, rather than interglacial), driven by collapsing of continental-scale ice sheets to local and subcontinental ice caps. The middle Pliocene is marked by a move into glacialism with an increasing "base level" of glacioeustatic fluctuation. Higher-order glacial advances and associated eustatic sea-level lowering occurred at approximately 3.5 and 4.3 Ma, separating the Early Pliocene into 3 sea-level stages. Still higher-order glacioeustatic fluctuations are recognised in this study, with durations of 50 Ka and 100 - 300 Ka. The 100 - 300 Ka duration cycles are prominent during interglacialisms, and the 50 Ka duration cycles are prominent during glacialisms. These shorter duration fluctuations in glacioeustasy have already been recognised as glacial/ deglacial cycles from detailed studies of the Quaternary.

Four orders of sea-level fluctuation are recognized within the Late Neogene, these are of approximately 0.05 Ma, 0.1-0.3 Ma, 2 Ma, and 4 Ma in duration. The 2 Ma and 4 Ma duration cycles are subdivisions of the third order cyclicity recognized by Vail et al. (1991) (referred to here as cyclicity orders 3a and 3b). The 0.1-0.3 Ma duration cycles are a subset of the fourth order cyclicity recognized Vail et al. (1991), and the 0.05 Ma duration cycles are a subset of the 5th order cyclicity recognised by Vail et al. (1991). 3a, 3b and 4th order sea level fluctuations are driven by fluctuations in the volume of the Antarctic Ice Sheet. Fifth order sea level fluctuations are also suggested to be at least partially driven by fluctuations in the volume of the Antarctic Ice Sheet. Milankovitch cyclicities in glacioeustasy (<100 Ka., fifth order cyclicity) are prominent in the geologic record at times when there is large scale glaciation (glacialism) of the Antarctic continent (e.g., for the Pleistocene). Conversely, at times when the Antarctic continent is in a deglaciated state (deglacialism), fourth order cyclicity is more prominent, with Milankovitch cyclicities present at a parasequence level.

This new glacioeustatic composite record differs markedly from the lower resolution Haq Sea Level curve (Haq et al., 1987, 1988) for the same time interval, this is due to the increased resolution, more accurate dating methods and a new understanding of the sedimentary sequences recording sea level changes. At known time intervals the new record is in direct agreement with other records of sea level events, such as those from the U.S. Middle Atlantic Coastal Plain (Krantz 1991), Enewetak Atoll (Wardlaw and Quinn, 1991), the Sea of Japan (Cronin et al., in press), and from Eastern Kamtchatka, Russia (Gladenkov et al., 1991). In detail and extent the glacioeustatic composite record mimics the oxygen isotope proxy record of the same time interval (Shackleton et al., 1993), suggesting that the isotope signal is more closely driven by ice volume variation. However, changes in the late Pliocene sea surface temperature record (Dowsett et al., in press) also appear to be synchronous with the Wanganui Basin sea level record.

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