Investigating stratigraphic evidence for Antarctic glaciation in the greenhouse world of the Paleocene, eastern North Island, New Zealand

Early Paleogene (Paleocene-Eocene) climate was significantly warmer than today, with the poles generally considered to have been free of ice. These greenhouse conditions ended in the earliest Oligocene when the formation of the circum-Antarctic seaway between Antarctica and Australia had widened sufficiently to allow thermal isolation of the Antarctic continent and expansion of ice sheets to sea level. The associated invigorated Southern Ocean circulation patterns have been deemed responsible for formation of the Marshall Unconformity (32-29 Ma), a prominent hiatus in many Early Oligocene sedimentary sections in the wider New Zealand region. However, some recent studies have suggested there may have been short periods of significant Antarctic glaciation prior to opening of the circum-Antarctic seaway, including in the Late Paleocene at c. 58-57 Ma, referred to in this study as the “Late Paleocene event” (LPE). This study investigates the origin(s) of sedimentary sequences from four eastern North Island early Paleogene sections with the aim of determining whether any links can be made between the nature of the sediment facies and their bounding contacts with ice sheet growth on Antarctica. A late Paleogene section that records a possible occurrence of the Early Oligocene Marshall Unconformity has also been studied with the aim of comparing lithologic features from the site with those in the early Paleogene study sections. Methods used include detailed logging and description of sedimentary strata, and petrographic, mineralogical, grain size, elemental, stable carbon and oxygen isotope, organic carbon content, micropaleontologic, and palynofacies analyses. In the study sections the LPE is represented by dark grey, brown-grey, or black, occasionally glauconitic, non-calcareous mudstone, often with relatively high organic matter contents. Geochemical proxies indicate that during deposition of these lithologies siliceous microbiota were the dominant primary producers in the overlying surface ocean waters, suggesting a period of cooling and enhanced upwelling occurred at the time. Additionally, relative sea-level curves formulated for the studied sections through the Paleocene show three falls in sea level, the last of which coincides with the LPE. The relative sea-level curves are considered to represent eustatic changes in sea level and thus the fall that coincides with the LPE may record development of continental ice on Antarctica. The two interpreted falls in sea level prior to the LPE are also commonly associated with dominantly siliceous productivity and may record precursor cooling events. The occurrence of possible “dropstones” in Early to Late Paleocene strata at one section may also record fluctuations between glacial and interglacial conditions prior to, and possibly within, the LPE. There is presently inconclusive evidence for the Akitio River section recording the Early Oligocene Marshall Unconformity, but the glaucony bearing unit that has been suggested to mark the event is similar to some LPE lithofacies and is associated with an episode of increased biosiliceous productivity, supportive of climate cooling. Collectively these features suggest that the LPE may be associated with marked cooling and continental ice development on Antarctica, well before the circum-Antarctic seaway had widened sufficiently to allow thermal isolation of the Antarctic continent. This suggests that Antarctic ice sheet growth is less reliant on ocean heat transport than currently believed, and other mechanisms, such as draw down of atmospheric CO2, may also be involved. Other results from this study of less relevance to early Paleogene marine climate but of interest to general eastern North Island geology include the possible identification of Awhea Formation and Waipawa Formation at the Pahaoa coastal section in Wairarapa; interpretations of the depositional environments for the lithofacies in the study sections, involving dominantly hemipelagic upper to lower slope mudstone with occasional mass emplaced sandstone and pelagic limestone deposits; a suggested mechanism that led to organic matter enrichment in the Waipawa Formation; a possible explanation for the lateral transition of thick deposits of organic rich mudstone to relatively thin deposits of greensand and glauconitic mudstone; possible explanations for lateral variations in redox conditions; the possible correlation of an Early Eocene olistrostromic unit at Pahaoa with an olistrostomic unit farther south, suggestive of a major tectonic episode; and confirmation of an anomalous trend, noted by previous studies on several Paleocene-Eocene sections, within the Wanstead Formation of increasing terrigenous sediment input during a transgressive period