Bulk Sediment Hf-Nd Isotopic Composition Across the EOT, Northern Hemisphere Glaciation?

Abstract:

In recent decades, near and far field proxies of continental ice production indicate the presence of continental ice on Antarctica. Short Antarctic glaciations blinked in and out of existence throughout the middle and late Eocene, culminating in the formation of a continental ice sheet during the Eocene Oligocene Transition (EOT; ~34 Ma). Moreover, the onset of the Antarctic glaciation coincides with pCO₂ declining below a critical threshold for the accumulation of a continental ice sheet. New evidence suggesting bipolar glaciation (that is, northern and southern hemisphere) occurred through this period with ice sheets on Greenland and Antarctica. However, the pCO₂ threshold for the accumulation of ice on Greenland is not reached until the late Oligocene.

Preliminary hafnium-neodymium (Hf-Nd) isotope results of oxyhydroxide leachates from IODP Site U1411 on the Newfoundland Ridge points to increased weathering intensity coinciding with the EOT, marked by less radiogenic Hf isotope compositions. One interpretation of this data is that glaciation of the northern hemisphere (e.g. Greenland) coincides with that of Antarctica during the EOT. Hf-Nd isotopic composition of sediment on the Newfoundland ridge indicates a shift from incongruous chemical weathering to a more congruous mechanical weathering regime (i.e. glaciers). However, it could be suggested that the observed congruous Hf-Nd isotopic signal originates in the southern ocean and has been propagated north from the Antarctic.

We are using sediment core from the equatorial Pacific to determine if a signal of glacial weathering could be transmitted though deep waters from Antarctica. The core, IODP Site 1333, is in the equatorial Pacific positioned far from either pole thus, a shift towards a less radiogenic Hf isotopic compositions is not to be expected. The absence of a shift in Hf isotopes in the oxydroxide leachates, or a shift of lesser magnitude, will strengthen the possibility of northern hemisphere glaciation.