Dronning Maud Land ice sheet activity during the early stages of the Eocene Oligocene Transition

The benthic foraminiferal oxygen isotope record of the Eocene Oligocene Transition (EOT) outlines two abrupt increases in δ¹⁸O values—a smaller first step followed by a larger second step—separated by a terrace interval of intermediate δ¹⁸O values. The second step is generally attributed to formation of a large ice sheet as it is contemporaneous with eustatic sea level fall and extensive ice rafted debris (IRD) in Prydz Bay and elsewhere around Antarctica. Deep-sea radiogenic isotope records of sedimentary provenance and weathering flux changes revealed ice sheet activity in the Lambert Graben/Prydz Bay catchment during the first δ¹⁸O step, however the extent of Antarctic ice sheets during the first step and terrace interval is as yet unknown.

Hafnium-neodymium (Hf-Nd) isotope results of the lithogenic sediment and Nd isotopes from fossil fish teeth in ODP Site 689 (Maud Rise) were analyzed to assess changes in weathering on Dronning Maud Land (DML). Across the first δ¹⁸O step lithogenic ɛ_Nd values change by 1-2 units while ɛ_Hf values change by 7 units. These changes produce a near vertical array on the Hf-Nd diagram, which is consistent with either 1) an increase in the intensity of weathering on DML or 2) stronger bottom current transport of sediment grains to the study site. The absence of a fundamental change in fossil fish tooth ɛ_Nd values across the first step argues against a change in bottom current transport. During the terrace interval lithogenic Hf-Nd isotope data show no major changes while fossil fish tooth (i.e., bottom water) eNd values from typical Southern Ocean Deep Water (SODW) values of -8 to the lithogenic endmember value on DML of -10. These results strongly indicate that there were major pulses of weathering during the terrace interval on DML, suggesting dynamic ice sheets. Across the second δ¹⁸O step lithogenic ɛ_Nd values change by 6 units with no corresponding change in ɛ_Hf values. The changes across the second step produce a horizontal array on the Hf-Nd diagram indicating a shift in sedimentary provenance, probably to more distal sediment sources from Prydz Bay. Stronger sediment sorting along a longer flow path reduces the sensitivity of Hf isotopes at this location during the second step.