Abstract:
High-resolution stable isotope (4-10 k.y. resolution) and moderately low-resolution Mg/Ca ratio records were constructed for the late early Miocene (19-16 Ma) from ODP Leg 189 Site 1168, to evaluate deep-sea water-mass changes in the Tasman Seaway and ice-volume changes in Antarctica. Ice-volume changes of up to 80% of present-day EAIS occurred between 19 and 16 Ma based on paired Mg/Ca ratio and oxygen isotope records, indicating that a dynamic ice sheet existed during this time. Ice volume reached near present-day levels during the two isotopic events (Mi1b [17.9 Ma] and Mi2 [16.2 Ma]), while becoming far more reduced in size during the first climatic optimum (17.6-16.4 Ma). These ice-volume estimates are consistent with previous estimates from isotopic records calibrated to eustatic estimates obtained from backstripped stratigraphy. Isotope and Mg/Ca ratio records from Site 1168 indicate temperature fluctuations from 4 degrees to 10 degrees C occurred on the Tasmanian slope during the late early Miocene. Low temperatures (4 degrees -6 degrees C) are typically associated with high carbon isotopic values (>1.40/00) and are interpreted to represent Southern Component Waters originating from high southern latitudes. High bottom water temperatures (7 degrees -10 degrees C) indicate a warmer-water mass, which we speculate to have originated from Indian Ocean and may be analogous to warm saline deep waters that have been postulated to have originated from the eastern Tethys Sea during the early Miocene. Additionally oxygen isotopic values decreased at Site 1168 during the first climatic optimum, resulting in an isotopic gradient with isotope records from other southern ocean sites, suggesting that a warmer water mass bathed the Tasman Slope during this time. Large high-frequency isotopic excursions (low oxygen and carbon isotope values) occurred between 18.7 and 18.4 Ma and were originally thought to be due to either localized effects (e.g., disassociation of hydrates) or possible diagenesis. However, a high-resolution isotopic record from the Southern Ocean (Site 1090) also contains large isotopic excursions (e.g., >10/00 decrease in oxygen isotope values) at this time, suggesting that these events may not be due to diagenesis but may be transient global events.