SGER: Development of the Paired Authigenic Neodymium-Hafnium Isotope Weathering Tracer From Marine Sediments in the Circum Antarctic Realm

Combined Nd and Hf isotope analyses have become widely used in studies of solid Earth geochemistry, but their application to the low-temperature environment is still limited (for recent reviews see van de Flierdt et al. (2004, 2007)). Both elements show variable compositions in seawater and their residence times in the ocean are thought to be similar (500 – 2000 years; David et al., 2001; Godfrey et al., 1996, 1997; Jeandel, 1993; Jeandel et al., 1995; Lee et al., 1999; McKelvey, 1994; McKelvey and Orians, 1998; Tachikawa et al., 1999; Tachikawa et al., 2003) making them potential tracers of ocean circulation. For Nd isotopes this potential has been taken advantage of in many studies addressing past circulation patterns in the ocean on million year to millennial time scales (e.g., Frank, 2002; Goldstein and Hemming, 2003; Piotrowski et al., 2004, 2005; Martin and Scher, 2004; Vance et al., 2004). Due to the very low concentrations of Hf in seawater (Godfrey et al., 1996; McKelvey et al., 1998) and associated analytical difficulties in measuring Hf isotope compositions, our overall understanding about fractionation processes and internal cycling of Hf in the ocean is not as well developed as it is for Nd. Based on ferromanganese crust studies, it has been suggested that the Hf isotopic composition of seawater can serve as a tracer for enhanced physical weathering resulting from glaciation because of the significant fractionation of Lu and Hf within the sedimentary system (Patchett et al., 1984; Piotrowski et al., 2000; van de Flierdt et al., 2002, 2004, in press). This fractionation is caused by the retention of Hf in zircons (low Lu/Hf ratio), which are very resistant against weathering, and lead to a “radiogenic” Hf signal being eroded from the continents to the ocean.

The circum-Antarctic realm offers a perfect setting to test the idea of incongruent weathering on the continents affecting the seawater Hf isotopic composition. The Southern Ocean plays a crucial role in the present-day global ocean current system, as the Antarctic Circumpolar Current (ACC) provides the connections among all three major ocean basins, permitting efficient global water-mass exchange. Secondly, the Southern Ocean offers the possibility to study input and transfer pathways of dissolved and particulate tracers from the Antarctic continent. Thirdly, Antarctica is an ice-covered continent and therefore the circum-Antarctic realm may have recorded a fundamental change in the run-off signature from Antarctica over the transition from the greenhouse world in the early parts of the Cenozoic to the icehouse world we are currently living in.

This project marks a first attempt to extract an authigenic (seawater-derived) Hf isotope signal from two sites in the Southern Ocean (ODP sites 689 and 738/744) in order to monitor the Hf isotopic composition of the Southern Ocean at the onset of Antarctic glaciation around the Eocene/Oligocene boundary. In preparation for the project we developed a sequential leaching method, which enabled us to extract seawater Hf from bulk sediments. The method is based on the sequential leaching procedure developed at L-DEO for Nd isotopes (Rutberg et al., 2000) and was modified in the way that we added sodium-ethylenediaminetetraacetic acid (Na2EDTA) to a mixture of hydroxylamine hydrochloride and acetic acid to extract authigenic trace metals from the carbonate-free sediment matrix. This leads to a complexation of Hf, preventing it from re-adsorption onto particles. We applied this method to nine samples across the Eocene/Oligocene boundary from Sites 744 and 689 on the Kerguelen Plateau and the Maud Rise.

In order to characterize provenance of sediments in the Southern Ocean and to establish the link between Antarctic geology and proximal sediment composition, we produced new Nd isotope data on 24 circum-Antarctic core top sediments to complement other bulk sediment and mineralogical analyses. We produced Hf isotope analyses on the same Circum-Antarctic sediments in order to compare them with the global array and with circum-Antarctic ferromanganese nodules. We produced new Hf isotope data for the Southern Ocean on a set of ferromanganese nodules, leading to the discovery of an isotopically distinct local bottom water source produced in the Indian Ocean sector of the Southern Ocean off Adélie Coast, which we identified as Wilkes Land - Adélie Coast bottom water (van de Flierdt et al., 2006). Within the context of these new data, we took the opportunity to re-evaluate global Nd-Hf isotope systematics, specifically to evaluate the alternative hypotheses that the “seawater array” of Nd-Hf isotopes is a product of hydrothermal sources rather than incongruent weathering from the continents. We used Lu-Hf and Sm-Nd evolution and mixing calculations to demonstrate that the seawater array can be explained by preferential weathering of “non-zircon portions” of the upper continental crust, implying retention of zircons in the solid residue of weathering. On the other hand, a predominantly hydrothermal origin of Hf in the ocean is not possible because the seawater Hf isotopic composition has significantly lower values than hydrothermal sources, and the composition requires a minimum of 50% continental Hf. While hydrothermal sources may contribute some Hf to seawater, continental contributions are required to balance the budget (van de Flierdt et al., 2007).