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43383-G2
Towards an Improved Paleoflux Proxy for Ocean Margins

Zanna Chase, Oregon State University

            ²³⁰Thorium in marine sediments has been used as a "constant flux proxy". As such, it is a very important tool in paleoceanographic studies that aim to reconstruct the flux of material to the seafloor. Such flux reconstructions are crucial for understanding the history of marine productivity and of dust inputs, among others.            Total ²³⁰Th in sediment consists of ²³⁰Th scavenged from seawater, ²³⁰Th present as detrital minerals, and, in some cases, ²³⁰Th that has been produced by the decay of authigenic U in sediments. As only the scavenged ²³⁰Th is useful as a constant flux proxy, it is necessary to correct for contributions from the other sources of ²³⁰Th. This can introduce errors and uncertainties where these corrections are large, such as in ocean margin sediments. The goal of this project is to come up with a way of chemically separating the scavenged ²³⁰Th from the detrital and "authigenic" ²³⁰Th. Our approach was to use a flow through leaching procedure, in contrast to batch methods that have been tried in the past. With a flow-through technique, the possibility of readsorption of leached Th is reduced, since leachate is constantly flowing over the sediment.            During the past year the PI and graduate student Jesse Muratli have developed the capability to accurately and precisely measure Th and U isotopes in marine sediments by isotope-dilution, ICP-MS. Work has focused on a set of sediment samples from the Chilean margin. The sediments were recovered during ODP Leg 202, and come from three sites: 1233, 1234, and 1235. We are interested in reconstructing biogenic particle flux at the deeper of these three sites (1233 and 1234) in order to determine whether changes in intermediate water oxygenation were driven primarily by changes in ocean circulation or changes in local productivity. Analysis of these sediments is challenging because ²³⁰Th concentrations are low and ²³²Th concentrations are very high, resulting in significant 'tailing' of ²³²Th counts into adjacent mass ranges. We have optimized the ICP-MS to deal with these challenges.

           Results from Holocene sediments at site 1234 illustrate the need for a procedure to separate scavenged from detrital ²³⁰Th. At this site, because particle flux is great (which dilutes ²³⁰Th), the water depth is shallow, and lithogenic fluxes are high, over half of the ²³⁰Th is of detrital origin (Figure 1). The amount of "excess ²³⁰Th", used to determine particle flux, is thus very sensitive to the correction for detrital ²³⁰Th. Uncertainty in the detrital correction accounts for 50-75% of the uncertainty in reconstructed particle flux at this site. In this example detrital ²³⁰Th (derived from decay of detrital ²³⁸U) was estimated from ²³²Th concentration assuming a U/Th ratio in lithogenic material of 0.7 +/- 0.1. A procedure to experimentally separate scavenged from detrital ²³⁰Th will help determine whether or not the detrital U/Th ratio was constant through time, or changed, perhaps in response to changing river drainage patterns.            This year we re-designed our flow-through leaching apparatus, because we found the previous design did not work well with all sediment types, and did not allow sufficient contact between sediment and the leaching solution. The new set-up (Figure 2) is very inexpensive, simple, and effective. It does not leak, there are no back-pressure issues, and we have verified that sediment mass is conserved. We have run a handful of leaching experiments with the system. We also ran a handful of experiments using batch leaching. The samples are dried and waiting to be analyzed.

           Note that PI Chase took maternity leave in 2007, which necessarily slowed progress on this project. A no-cost extension has been granted to this award.      

Figure 1: Concentration of ²³⁰Th in Holocene sediments at site 1234 (1015m) on the Chilean margin. This site represents an extreme case of applying the ²³⁰Th flux-normalization approach to reconstruct particle flux. Total ²³⁰Th concentrations are low, and the contribution of detrital ²³⁰Th is high. The excess ²³⁰Th is thus small and sensitive to the correction for detrital ²³⁰Th.

Figure 2: Apparatus for flow-through leaching of marine sediment. A 25mm polystyrene filter holder is placed inverted over a magnetic stirrer. Leaching solution is pumped through using a peristaltic pump at 1-3 mL/min. Sediment remains suspended through constant stirring, and the filter at the top prevents loss of sediment from the system.

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