Marine geochemical and micropaleontologic constraints on cenozoic antarctic cryosphere evolution [electronic resource]
Keywords: Geological and Environmental Sciences

The Antarctic continent plays a fundamental role in the modulation of global and regional circulation, climate, and sea level. In its present glaciated state, Antarctica increases the Southern Hemisphere temperature gradient, enhancing atmospheric circulation and generating strong, cold winds that contribute to the formation of Antarctic bottom water by cooling surface waters and sustaining sea ice production. Wind intensification may also contribute to an increase in Southern Ocean fertility, both by increasing surface circulation and upwelling at frontal zones, and by transporting iron-rich dust from the continent. On geological timescales, polar ice sheets are a major factor controlling regional and global sea level, planetary albedo, continental weathering rates, and ocean and atmospheric chemistry; each of these factors has the potential to trigger large-scale geochemical or physical feedbacks. Because the Antarctic system exerts such a dramatic influence on the planet as a whole, constraining Antarctic sensitivity to past intervals of rapid global change will directly inform our understanding of the potential for future climate aberrations. Existing paleoclimate reconstructions identify the late Pliocene, ~3 million years ago (Ma), as a promising analog in the recent geologic past for our anticipated planetary future. When forecasting global temperatures over the coming century, even the most conservative climate models predict a range of warming that was last encountered during the mid-Pliocene. The AND-1B marine sediment core, collected by the Antarctic Geological Drilling (ANDRILL) Program from beneath the McMurdo Ice Shelf, represents the most complete record to date of Neogene climate evolution proximal to the Antarctic continent. Diatom-rich lithologic units alternate with glacial sediments throughout the late Pliocene in AND-1B. Each diatom-rich unit within this oscillating record has distinctive geochemical and diatom assemblage characteristics and most are interpreted to preserve the interglacial interval of a 40-thousand-year glacial/interglacial cycle. Chaetoceros resting spore relative abundances, the radiation of a new Fragilariopsis flora, and environmental associations of extant diatoms in assemblages from ~3.2 and 3.0 Ma indicate that late Pliocene cooling initiated in the Ross Sea around 3.2 Ma. Additional environmental constraints are provided by the first-ever Pliocene diatom-bound nitrogen isotope ([lowercase Delta]15N) record of phytoplankton community dynamics and seasonal nutrient availability across the late Pliocene climate transition. Diatom-bound [lowercase Delta]15N in the AND-1B core diverges from bulk sedimentary [lowercase Delta]15N at 2.9 Ma. This divergence is accompanied by changes in the diatom assemblage and increased cyclical variations in bulk carbon isotopes ([lowercase Delta]13C) and sedimentary biogenic silica concentration, implying a shift from an interglacial surface ocean that is stratified by sea ice melt and dominated by diatom productivity prior to 2.9 Ma, to a deeply-mixed modern polynya-style interglacial system, dominated by the prymnesiophyte alga Phaeocystis antarctica, after 2.9 Ma. A broader context for Antarctic glaciation is also presented, using benthic foraminiferal [lowercase Delta]18O and [lowercase Delta]13C and bulk %CaCO3 from ODP Site 1263 to develop the highest-resolution record of the early Oligocene climate transition from the Atlantic Ocean to date. The two-step [lowercase Delta]18O transition, a prominent feature at Pacific Site 1218, is more subtle at Site 1263, suggesting a variable influence of deep water-mass circulation patterns in the two ocean basins across the transition. The pattern of ice accumulation inferred by the gradual-to-rapid transition into Oi-1 supports the influence of hysteresis in the pacing of Oligocene ice growth.