Environmental and Climatological Evolution of the early Paleogene Southern Ocean

Abstract

Through application of palynological and organic geochemical tools, this thesis provides a detailed image of the early Paleogene greenhouse-icehouse transition of the Southern Ocean. The organic-walled dinoflagellate cyst (dinocyst) zonation for a large part of the early Paleogene may serve as guide for future biostratigraphy and age-assessment, which is crucial because few biostratigraphic tools are available in Eocene Southern high-latitude sediments. The sea surface temperature (SST) evolution of the South Pacific Ocean mimics that of benthic foraminiferal oxygen isotopes, suggesting that the South Pacific Ocean was a region of deep-water formation. However, absolute temperatures seem much higher than those inferred from the benthic foraminifera, which may be due to the effect of seasonal biases. The biotic response to the SST evolution in the Southern Ocean is controversial: dominance of endemic dinocyst assemblages seems unrelated to SST. The working hypothesis intuitively was that the proliferation of an endemic phytoplankton community at high latitudes was the result of outcompeting warm-water taxa when temperatures cool. Because SST seems unrelated to the onset of proliferation of endemic dinocysts, an alternative suggestion is proposed relating the shift to an increase of surface-water nutrient levels in the middle Eocene. Indeed, several independent other proxies conspire to suggest a switch in nutrient input from the late early Eocene (~50 Ma) onwards. The PETM interval in the South Pacific Ocean also stands out with rapid sea surface temperature increase: 5-8°C. Also an acme of the tropical dinocyst genus Apectodinium is observed. As for some other PETM sections, the PETM record from the South Pacific Ocean reveals major environmental change preceding the negative carbon isotope excursion (CIE). This is in concert with suggestions that initial climate changes preceded the release of carbon in the exogenic carbon pool that resulted in the CIE. In turn, this could suggest that the carbon input in the exogenic carbon pool is a response rather than a forcing factor. Another ongoing debate that was touched upon in this thesis is the role of oceanic gateway openings and its role in climate change. Newly drilled (2010) sediment archives from the Wilkes Land Margin, Antarctica, showed that through flow of the Antarctic Counter Current initiated around the early-middle Eocene boundary (~52-50 Ma), concomitant to amplified cooling of Antarctic continental and coastal temperatures. The opening of Tasmanian Gateway occurred at the time when the Earth started cooling gradually towards the icehouse state of the past 35 million years. Hence, Tasmanian Gateway opening may have set the stage for climate deterioration although atmospheric CO2 was likely still the major forcing factor of early Paleogene climate evolution. Indeed, we still see that transient climate warming at the MECO coincides with increased concentrations of atmospheric CO2, suggesting an important role for atmospheric CO2 as driver of climate.