Geochemical proxies

Dymond J, Suess E, Lyle M (1992) Barium in deep-sea sediment A geochemical proxy for paleoproductivity. Paleoceanography 7(2) 163-181... 

Meyers, P.A. (1997) Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes. Org. Geochem. 27, 213-250. 

A BRIEF HISTORY OF EARLY RESEARCH ON GEOCHEMICAL PROXIES OF TEMPERATURE... 

A Brief History of Early Research on Geochemical Proxies of Temperature... 

Two fundamental types of geochemical proxies are employed in the reconstruction of ancient depositional environments and geological processes. Broadly defined, these are elemental and compound concentration/accumulation data and stable isotopic data. Depending on the type of data used and the process under investigation, there are specific limitations that characterize each proxy. 

The chemical behavior of various minor and trace elements is relatively well characterized for particular redox conditions, and there has been significant effort directed at the development of geochemical proxies for paleo-oxygenation in black shale sequences (see reviews in Calvert and Pedersen, 1993 Arthur and Sageman, 1994 Jones and Manning, 1994 Wignall, 1994 Schieber et u/., 1998a,b). Elements... 

Anoxia was certainly a characteristic of the Late Devonian, but in the western US, based on geochemical proxies, anoxia ended 6 m below ( 100kyr before) the major F-F extinction. Bratton et al. (1999) discuss the possibility that this was a local phenomenon and that anoxia persisted through the F-F boundary elsewhere, but favor the alternative hypothesis that other sections suffered depositional hiatus or erosion of the latest Frasnian sediments. A positive excursion (in both carbonate and organic carbon) began in the Frasnian but continued well into the Famennian (Wang et al., 1996). A positive pyrite sulfur isotope excursion also occurred at this time. If these excursions indicate enhanced organic carbon and pyrite sulfur burial under widespread anoxic conditions, then it would seem that such conditions persisted well beyond the F-F boundary extinction. 

There are two main types of geochemical proxy used to identify former life. These are biomarker molecules and isotopic fractionations. Biomarker molecules are specific molecules preserved in sediments as molecular fossils, which are unambiguous indicators of their biological precursors. Some of the most successful... 

This volume begins with an overview by James Austin, highlighting some of the most important biological and geochemical proxies, and outlining their contribution to our understanding of the... 

Abstract Scientific observations of our oceans and climate go back no more than a couple of hundred years. Most of our information about the evolution of Earth s ocean-climate system relies instead on proxies - primarily measurements of sediment components that respond to changes in environmental parameters. This paper provides an overview of some of the most important biological and geochemical proxies and outlines their contribution to our understanding of the ocean-climate system. We also discuss some of the challenges that need to be overcome to obtain accurate records. These include better understanding of the controls on the mechanisms of biomineralization the impacts of post-depositional dissolution and diagenesis on primary proxy relationships proxy validation and analytical considerations. 

The incremental nature of shell growth in bivalves provides an opportunity for detailed temporal reconstruction of past environments (e.g. Weidman et al. 1994) and their widespread distribution from freshwater to deep-sea environments make them particularly useful for palaeoenviron-mental reconstructions. However, as Freitas et al. (2006) highlight, the potential of bivalves as useful geochemical proxies remains to be fuUy realized and these authors cite the limited number of calibration and validation studies completed to date. 

In this Section, we outline some of the challenges that need to be overcome in order to obtain accurate geochemical proxy records, and we describe some of the analytical tools that are available to us. 

Recent advances in analytical instrumentation and methodology promise to add many more geochemical proxies to the palaeoceanographer s toolbox. MC-ICP-MS has opened up a number of isotopic systems previously considered intractable, such as zinc (which shows potential as a tracer of nutrient utilization Vance et al. 2006) and germanium, whose isotopic composition has recently been measured in biogenic opal (Rouxel et al. 2006). Meanwhile improvements in gas source mass spectrometry techniques now permit precise measurement of the abundance of C- 0 bonds in biogenic carbonate. C- 0 abundance shows promise as a palaeothermometer which, unlike 8 0, is independent of the stable isotope composition of seawater (Ghosh et al. 2006). 

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