Palaeo Sea Levels

Sequence and seismic stratigraphy has provided a means of relating the geologic record of continental margins to global sea-level changes that are often related to ice-volume changes at high latitudes (Vail et al., 1977; Haq et al., 1987). Sea-level history is deduced by the recognition of unconformity-bounded units (i.e. depositional sequences) deposited in response to a cycle of falling and rising sea level. However, determining the relative role of tectonic subsidence and uplift versus rising and falling global sea-level and hence ice-volume fluctuations (glacioeustasy) is still unresolved, particularly in pre-Pleistocene records (e.g. Macdonald, 1991). Vail et al. (1977) and Haq et al. (1987) attempted to extract the glacioeustatic signal from the comparison of records from several continental margins (Fig. 9.4). However, the large amplitudes of sea level/ice volume, the limited resolution of the resulting record and the use of proprietary data to create the sea-level records, has spurred the scientific community to collect independent data to test the records of Vail et al. (1977) and Haq et al. (1987). The Oligocene-Miocene interval of these records has sparked particular interest. Both Vail et al. (1977) and Haq et al. (1987) predicted sea-level rises and falls of between ~50 and 100 m in the late Oligocene. Vail et al. (1977), however,

Figure 9.4: Eustatic sea-level curves derived from coastal onlap patterns for the Oligocene-Miocene boundary interval from Haq et al. (1987) and Miller et al. (2005). Calibration of Miller et al. (2005) curve is considered more realistic. Curves are adjusted to astronomical time scale of Billups et al.

Figure 9.4: Eustatic sea-level curves derived from coastal onlap patterns for the Oligocene-Miocene boundary interval from Haq et al. (1987) and Miller et al. (2005). Calibration of Miller et al. (2005) curve is considered more realistic. Curves are adjusted to astronomical time scale of Billups et al.

predicted a fall of some 60 m across the Oligocene-Miocene boundary, whereas Haq et al. (1987) predicted a rise of ~100m across the Oligocene-Miocene boundary. The proprietary nature of much of the data has precluded resolving this conundrum from the same data set.

An alternative passive margin stratigraphic data set is available from the New Jersey/New York Bight region of North America. Sea-level changes predicted from sequence stratigraphic analysis have recently been calibrated from coring as part of the Ocean Drilling Programme Legs 150X and 174AX (Miller and Mountain, 1996; Miller et al., 1997a, b). The glacioeustatic contribution to sea-level changes in the Oligocene and earliest Miocene was estimated by combining two-dimensional palaeoslope modelling of the foraminiferal biofacies and lithofacies with two-dimensional flexural back-stripping of the margin (Kominz and Pekar, 2001; Pekar and Kominz, 2001). The depth ranges of foraminiferal biofacies were determined from a combination of standard factor analysis techniques and the backstripped geometries. The geometry of the margin through time was determined using two-dimensional flexural backstripping. Foraminiferal biofacies and lithofacies were then used to constrain the depths of the Oligocene margin profiles obtained from backstripping. A eustatic fall of ~40 + 15 or 56m apparent sea level (apparent sea level is eustasy plus the effects of water loading on the margin, Pekar et al., 2002) was estimated across the Oligocene-Miocene boundary (Fig. 9.4), which is similar to the glacioeustasy predictions from oxygen isotopic and trace metal geochemical data (Paul et al., 2000; Lear et al., 2004).

Pekar et al. (2006) provided estimates of Antarctic ice volume and the resulting changes in global sea-level for the late Oligocene by applying 81SO-to-sea-level calibrations to deep-sea 81SO records from a number of ODP Sites. Their results indicate that the size of the Antarctic Ice Sheet increased from approximately 50% of the present-day EAIS during the latest Oligocene to as much as 25% larger than the present-day EAIS at the Oligocene-Miocene boundary. Ice volume returned to near late Oligocene size in the early Miocene (Pekar and DeConto, 2006).

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