Assessment of Sequential Extractions: Implications for Isotopic Composition of Authigenic Oxides and Detrital Residues as Paleoceanographic Proxies

Abstract

Constraints on characteristics of past oceans-- ocean circulation, sedimentation processes, and sources of dissolved trace metals-- are vital because they contribute to our understanding of heat transport, atmospheric conditions, and productivity. Proxies that record information about these processes in marine sediment are key to understanding the ocean’s role in past climate. This dissertation looks in depth at how the Neodymium (Nd) and Lead (Pb) isotopic composition (IC) of fish debris, sequentially extracted oxide coatings, and “detrital” residues are used as proxies for ocean circulation and sediment composition. We first present a reconstruction of ocean circulation in the Cenozoic Pacific Ocean employing Neodymium IC of fossil fish Debris and oxide coatings based on Deep Sea Drilling Program (DSDP), Ocean Drilling Program (ODP), and Integrated Ocean Drilling Program (IODP) sites spanning the North and South Pacific. The South Pacific becomes slowly less radiogenic, after 35Ma as the gradient between the North and South Pacific increases. This, combined with the North Pacific becoming more radiogenic (seemingly because of nonconservative mixing of Nd) indicates that there is a fundamental transition in the North Pacific from surface ventilated deep-water formation to circulation dominated by sluggish diffusive mixing. There are two main results from The South Pacific becomes slowly less radiogenic, after 35Ma the gradient between the North and South Pacific increases. This indicates a fundamental transition in the North Pacific from surface ventilated deep-water formation to circulation dominated by sluggish diffusive mixing. Then I preform an assessment of the fidelity and reproducibility of the leaching procedure we use to isolate the Fe-Mn oxyhydroxide coatings.. Our procedure entails a two-step leach with a Hydroxylamine-Hydrochloride mixture: Leach 1 targets the Fe-Mn oxide coatings and is 1 hour, and Leach 2, is 24 hours designed to remove remaining Fe-Mn oxides from the detrital residue. In order to analyze leach reproducibility we performed replicate analysis of a USGS certified reference material (MAG-1) and an in house internal standard from the central Atlantic (INTL-STD-A). We also performed an assessment of the geochemistry of two sites (IODP Sites 1149 and U1370) from the Pacific. Finally we evaluate which geochemical phases are mobilized by each step of our sequential leaching procedure at Site 1149 and U1370. We utilize a novel multivariate statistical assessment of major, trace and rare earth elements to identify their sources and the sources relationship with the Nd and Pb isotopic composition.