Graduation Year

2020

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

Amelia E. Shevenell, Ph.D.

Committee Member

Tim M. Conway, Ph.D.

Committee Member

Robert M. McKay, Ph.D.

Keywords

Foraminifers, Geochemistry, Mg/Ca, Paleoclimate, Stable isotopes

Abstract

Warm, intermediate-depth Southern Ocean waters are implicated in recent Antarctic ice mass loss. Direct observations of Antarctic Ice Sheet (AIS) retreat are temporally limited, necessitating paleoceanographic records of ocean-ice interactions during past warm climate intervals. Deepsea and ice-proximal sediments record orbitally-paced glacial-interglacial fluctuations in AIS volume during the Plio-Pleistocene (last 5 million years; Ma), but the total contribution of the AIS and the role of ocean heat in these fluctuations remain unresolved. To address the response of Antarctica’s ice sheets to changing ocean temperatures during the Plio-Pleistocene, International Ocean Discovery Program (IODP) Expedition 374 recovered sediments from the Ross Sea outer-shelf at Site U1523. Site U1523 is close to the shelf break and sensitive to incursions of warm intermediate-depth Southern Ocean waters. Site U1523 sediments include foraminifer-bearing/rich sands and muds, which enable development of benthic (Trifarina sp.) and planktic (N. pachyderma sinistral) foraminifer stable oxygen (δ18O) and carbon (δ13C) isotope records. Here we present foraminifer δ18O, δ13C, and Mg/Ca records from the upper 90 m of U1523, which spans the late Pliocene to Holocene (last 3.1 Ma). We provide a new chronology for the U1523 Pleistocene sedimentary sequence by correlating diatom-bearing muds with low magnetic susceptibility (MS) to interglacial periods of the last 0.65 Ma. Benthic and planktic foraminifer δ18O values increase by ~1.5‰ up-section, which reflects long-term global cooling and ice growth, with orbital-scale ice volume and temperature variability superimposed. Our isotope records include an abrupt shift at 16 m CCSF, which we interpret as a hiatus due to an increase in ice-volume and bottom water production during the mid-Pleistocene transition. To separate the ice volume and temperature signals contained in the δ18O signal, we analyzed Mg/Ca in benthic and planktic foraminifers. Our Mg/Ca records reveal a 4.4°C and 4.8°C cooling in deep and surface waters, respectively, over the last 3.1 Ma, consistent with Plio-Pleistocene cooling estimated from deep sea Mg/Ca records. This is the first application of Mg/Ca paleothermometry in ice-proximal sediment for the Plio-Pleistocene, and although temperature reconstructions are high, Mg/Ca fluctuations parallel changes in sedimentology, δ18O, and δ13C, suggesting retainment of a primary environmental signal. Pleistocene surface water estimates are up to 11°C high, and exhibit a distinct decrease at ~0.40 Ma to values similar to modern. Warm perturbations in bottom water temperatures of 4-5°C are synchronous with lithologic transitions at depths that may correlate to MIS 31, 13 and 10 through 5. These warm spikes coincide with low δ13C and planktic δ18O, and shifts in MS, suggesting a mechanistic link between warm ocean temperatures, meltwater input, and ocean circulation. These records provide the first iceproximal evidence of paired ocean temperature and ice volume fluctuations during the mid-late Pleistocene. We interpret the relationship between low δ13C and high Mg/Ca to reflect intervals of stronger modified Circumpolar Deep-Water presence on the Ross Shelf, that may be responsible for ice loss suggested by low δ18O. Despite assumptions regarding ocean temperatures and the δ18O of glacial ice, our δ 18Osw records demonstrate ~100 Kyr cyclicity in ice sheet fluctuations following the mid Pleistocene Transition, consistent with deep-sea records of eccentricity-paced climate oscillations. We conclude that Ross Sea circulation and temperature changes coincide with ice volume fluctuations during the mid-late Pleistocene, though improvements in the U1523 chronology are necessary to confirm the exact timing of iceocean interactions.

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