EARLY PLIOCENE ISOTOPIC MELTWATER RECORDS FROM THE SOUTHWESTERN ROSS SEA, ANTARCTICA

We examined the sedimentology, foraminiferal fauna, and foramininiferal geochemistry of late Neogene glaciomarine sediment cored at the mouth of Taylor Valley, Antarctica, adjacent to McMurdo Sound. Sediments within Taylor Valley deposited by Taylor Glacier and alpine glaciers on the flanks of Taylor Valley during the late Neogene indicate expansions of both glacier systems that were significantly larger than any of the late Pleistocene. The 30 m of late Neogene glaciomarine sediment and fauna studied, from Dry Valley Drilling Project cores 10 and 11, record mixing between large meltwater pulses expected during glacier retreat and Ross Sea water.

In DVDP-10 and -11, average benthic foraminiferal d18O decreases from 2‰ in the latest Miocene to -8‰ in the earliest Pliocene. Multiple taxa in both cores record the negative shift which is dated by a variety of techniques. Even after adjustment for some diagenesis, the DVDP d18O are significantly more negative than contemporaneous deep-sea benthic d18O because of the addition of isotopically light meltwater. DVDP benthic d18O increase to -5‰ in the mid-Pliocene. Average d18O of the same taxa in McMurdo Sound today is 4.5‰.

A key problem is determining how much warmer-than-present the Antarctic climate got in the early Pliocene to cause the melting. Additionally, how did the meltwater lower fjord bottom water d18O the measured amount without the salinity becoming prohibitively low for the benthic fauna?

We used simple models to seek convergence between fjord temperature and salinity, Taylor Glacier melting, Ross Sea water temperature and salinity, and regional climate. Here, we present box model results for scenarios that balance Ross Sea water inflow, Taylor Glacier meltwater production, and fjord circulation and outflow. Constraints include: water d18O, minimum salinity from benthic fauna, fjord dimensions, sea-level, and climate inferred from alpine glaciers. The scenarios draw on modern analogues such as Pine Island Glacier, Larsen Ice Shelf disintegration, George VI Ice Shelf, and present-day McMurdo Sound. We examine the strengths and weaknesses of these scenarios.