Antarctic paleoenvironment and vegetation reconstruction during the early and middle Miocene using biomarkers from Ross Sea sediment drill cores

Abstract

<p><b>The Ross Sea during the Early to Middle Miocene was influenced by Antarctic Ice Sheets that were highly variable and potentially contributed to 60 m of sea level variations. This variability is proposed to be regulated by shifts in orbital configurations that influenced atmospheric, oceanic temperatures and circulation patterns, that in turn regulated shifts in atmospheric CO2 levels and ocean heat delivery to the Antarctic region, causing ice sheet loss or advance. The vegetation response to these Miocene changes is also proposed to highly variable, with advancing ice sheets restricting the real extent and elevation of land where higher order plants can grow. This thesis reinvestigates two Deep Sea Drilling Project cores from DSDP sites 272 and 273, which are located in the middle to outer continental shelf of the central Ross Sea, Antarctica, and capture a record of organic remains from inland vegetation that has been transported offshore by glaciers or meltwater outwash. Recent records from International Ocean Discovery Programme Site (IODP) U1521 and ANDRILL site AND-2A has provided records of Early Miocene to Middle Miocene. New age models from the DSDP Site 272 and 273 cores suggest they provide records that compliment these newer cores and fill in key time gaps for the Miocene time period, in particular leading into, and during the Miocene Climate Optimum, a period of notable global warmth between 17 and 14.6 Ma.</b></p> <p>This thesis presents biomarker and XRF datasets from DSDP Site 272 and 273 in the Ross Sea region, as well as a redescription of the lithological descriptions and to align with more recent lithological frameworks used for IODP site U1521. To construct this new description, XRF data and grain size data was used to define 4 main units in both cores. Only short “snapshot” intervals of the MCO were recorded in DSDP Site 272, between ~15.8 and 15.3 Ma, but contains an offshore hemipelagic signal thorough this time periods, as well as prior to the MCO (between 17-18 Ma). A longer record of the MCO is present in DSDP Site 273, but this core also records the transition into the MCO, while DSDP Site 273 captures a records of the Mid-Miocene climate transition (MMCT). </p> <p>Biomarker data showed an unexpected resulted, whereby vegetation diversity, or average chain length (ACL) does not increase, despite a warming signal in the carbon preference index (CPI) and other proxies during the mid-Miocene climate optimum compared to earlier times. Two hypotheses are proposed for this: 1) there was an extinction of higher order plants during a previously document glacial advance of West and East Antarctic Ice sheets between 17.8-17.4 Ma; or 2) this same advance is also known to have resulted in major erosion of low-lying land in West Antarctica to below sea level, leaving larger mountain ranges to dominate the terrestrial setting in Antarctica after that time. This resulted in reduced habitat available for higher order plants at low-elevation environments of the Ross Sea during the MCO, and thus higher elevation, shrub and tundra dominated the vegetation during the MCO, despite the warm climate state.</p>