Milankovitch cyclicity and sea-level change in the Late Eocene-Early Oligocene interval: evidence for rapid and extensive Antarctic glaciation at 33.5 Ma? (abstract of paper presented at EUG XI, Strasbourg, France, 8-12 April 2001)

Abstract

We have studied an expanded succession of coastal marine, estuarine and lacustrine sediments of Late Eocene-Early Oligocene age in the Isle of Wight southern England. In this succession, a strong Milankovitch signal (406, 100, 40 and weaker 20Ka) is recorded from the relative abundance of neoformed illite and illite-smectite, which formed in soils by seasonal wetting and drying. The orbital timescale is calibrated using magnetostratigraphic, and to a lesser extent, biostratigraphic data. Combined orbital calibration and sequence stratigraphic analysis allows us to identify the major control on sea-level as the 406Ka long eccentricity cycle, which caused sea-level to fluctuate by 10-15 m. These values have been determined from the amount of incision at observed at sequence boundaries on a regional scale. Minor sea-level changes of 1-3 m were controlled by obliquity. The position of the Early Oligocene heavy δ¹⁸O event can be inferred in the Isle of Wight from its magnetostratigraphic proxy (base of chron 13n). We have determined the sea-level fall at this level to be approximately  12 m, close in magnitude to drops associated with the preceding 3 Late Eocene 406 Ka sequences. This evidence does not support recent estimates of a 50-90 m sea-level fall within the Early Oligocene based on the calculation that a significant part of the oxygen isotope event was caused by rapid Antarctic ice buildup. Rather, orbitally driven sealevel changes throughout the Late Eocene-Early Oligocene, although probably glacio-eustatic in origin, remained of similar magnitude.

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