1.6.1 Previous work
The hydrography of the Southern Ocean is relatively well known. In terms of marine Hf and Nd isotope distributions in the Southern Ocean, most information has been derived from analyses of ferromanganese (FeMn) crusts and nodules (Albarède and Goldstein, 1992; Albarède et al., 1997; Albarède et al., 1998; Piotrowski et al.,
profiles for Nd isotopes (Piepgras and Wasserburg, 1982; Jeandel, 1993), whereas for Hf only two data points exist (Rickli et al., 2009). The poor data-coverage of modern Hf and Nd is one the prime motivations for this study. Additionally, the Southern Ocean is a key area of the global thermohaline circulation and the knowledge of the present day isotopic distributions of deep water formation sites are essential for understanding the input parameters as well as for a reliable application of radiogenic Nd and Hf isotopes as proxies for past ocean circulation.
The first comprehensive study on the Hf isotope composition of seawater based on FeMn deposits was carried out by White et al. (1986). They suggested that the Hf budget in the ocean is governed by a relative dominance of mantle derived radiogenic Hf by reduced fluxes from land through the retention of unradiogenic Hf in weathering resistant zircons. As described in section 1.2, zircons have relatively high Hf concentrations and can thus be responsible for a low Hf flux from land because of their indestructibility. The retention of unradiogenic Hf in resistant minerals being one of the reasons for the observed Hf isotope composition in FeMn-deposits, the idea arose that Hf isotopes, in combination with Nd isotopes, can be used as a potential weathering tracer (Piotrowski et al., 2000; van de Flierdt et al., 2002). These authors observed changes toward less radiogenic Hf isotope compositions recorded by FeMn deposits, which coincided with the onset of major glaciations. They concluded that enhanced physical weathering by glacial grinding has the potential to release unradiogenic Hf from resistant minerals. However, a reason for the observed general offset in the isotope composition obtained from available seawater data form FeMn deposits towards more radiogenic Hf values compared to the terrestrial array (see section 1.4, Fig. 1.3) has, so far, only been hypothesized. Bayon et al. (2006) showed for the first time in river water that preferential release of radiogenic Hf from readily weathered mineral phases, such as sphene or apatite, is a possible explanation for the Hf isotope composition of Atlantic seawater derived from FeMn crusts. This was largely confirmed by mass balance calculations, which revealed that if 65 % to 70 % of all Hf is sequestered in zircons, a weathering input of a “zircon-free” crust can fully explain the isotopic offset of the combined Hf-Nd isotope composition of seawater from terrestrial rocks (van de Flierdt et al., 2007). An obvious explanation for the offset in seawater Hf is also delivered by a potential longer oceanic residence time compared to Nd (White et al., 1986). Particulate scavenging of Hf in estuaries has been suggested to be the major reason of the observed
low concentration of Hf in seawater (Godfrey et al., 2008). According to these authors, if rivers were considered as the main contributor of Hf into the ocean, paradoxically, these sources would at the same time represent a major Hf sink and thus result in an estimated oceanic residence time of more than 7500 years.
The first real seawater Hf isotope data have only recently been published (Rickli et al., 2009; Zimmermann et al., 2009a;b; Rickli et al., 2010). In these studies it has been observed that the Hf concentration does not increase with the age of a water mass.
In contrast to previous estimates, they concluded that the oceanic residence time for Hf is only in the order of a few hundred years and that Hf isotopes are only a useful tracer on a basin-wide scale. Furthermore, Rickli et al. (2009) suggested that the Southern Ocean is a potential Hf sink in that biogenic opal acts as an effective scavenger. Due to the very low number of seawater Hf concentrations and isotope analyses currently available, there are still a lot of open questions on the factors controlling the behavior and distribution of Hf in seawater.
The first investigations on the Nd isotope composition of seawater have been carried out in the early 1980s by Piepgras and Wasserburg (1980; 1982). From comparison of the Nd isotope composition of the Atlantic and the Pacific, they concluded that Nd is a valuable tracer for ocean circulation patterns. These authors also calculated the rate of exchange between the Pacific and the Atlantic from Nd isotope compositions and concentrations derived from one full depth profile in the Drake Passage. They suggested that the Nd budget of the Southern Ocean is to about 70 % governed by Atlantic contributions. This estimate, however, contrasted with the findings of Jeandel (1993), who proposed a higher Pacific contribution based on a more radiogenic (i.e., Pacific sourced) Nd isotope composition of AAIW in the Argentine Basin. The relative importance of the contribution of the respective ocean basins and therefore, of the cold and warm route in the ocean conveyor belt explained above is crucial to investigate past and present ocean circulation. In addition, potential contributions from the Antarctic continent to the Nd isotope composition of the Southern Ocean have until now not been considered.
1.6.2 Motivation and research questions
The subject of this study is a detailed investigation of the distribution of dissolved
Nd and Hf isotopes has been successfully applied for the characterization of continental weathering regimes, i.e., it has been suggested that coupled Nd-Hf isotope analyses of Hf and Nd allow to distinguish weathering regimes dominated by chemical weathering from those dominated by physical weathering (e.g. van de Flierdt et al., 2002). So far, no data from the modern Southern Ocean are available to confirm this hypothesis. The Antarctic continent is positioned in latitudes where chemical weathering is subordinate to physical weathering (e.g. Ehrmann et al., 1992), therefore the Hf-Nd isotope compositions of Southern Ocean seawater are expected to be affected by this weathering regime.
To better understand the processes governing the Hf and Nd budget in this key area of ocean circulation, a more detailed study is needed to allow a more reliable interpretation of paleo-oceanographic data. Given the numerous uncertainties, which still exist in Hf and Nd isotope geochemistry in the world ocean, this study is aimed at answering the following crucial research questions: (Chapter 1) gives a general overview of the global thermohaline circulation with focus on the Southern Ocean and Hf and Nd isotope systematics. Furthermore, the basic ideas of using these isotopes as (paleo)oceanographic tracers are introduced and the motivation of this study is presented.
The second chapter presents the applied methodology. It explains the chemical procedures and the analysis of the two isotope systems. This chapter is essentially a
“cookbook” for the reliable extraction and measurement of Hf and Nd concentrations and isotope compositions from seawater samples. It also includes suggestions for