Tracer proxies

Many of the analytes of interest for solid phase chemical reference materials are the same as those in seawater, but the need for and the preparation of reference materials for suspended particulate matter and sediments is quite different. The low concentrations of many seawater species and the presence of the salt matrix create particular difficulties for seawater analyses. However while sediments frequently have higher component concentrations than seawater, they also have more complicated matrices that may require unique analytical methods. A number of particulate inorganic and organic materials are employed as paleoceano-graphic proxies, tracers of terrestrial and marine input to the sea, measures of carbon export from the surface waters to the deep sea, and tracers of food-web processes. Some of the most important analytes are discussed below as they relate to important oceanographic research questions. 

One other measurement technique that has been used to measure Kl over a shorter time period, and is thus more responsive to changes in wind velocity, is the controlled flux technique (Haupecker et al., 1995). This technique uses radiated energy that is turned into heat within a few microns under the water surface as a proxy tracer. The rate at which this heat diffuses into the water column is related to the liquid film coefficient for heat, and, through the Prandtl-Schmidt number analogy, for mass as well. One problem is that a theory for heat/mass transfer is required, and Danckwert s surface renewal theory may not apply to the low Prandtl numbers of heat transfer (Atmane et al., 2004). The controlled flux technique is close to being viable for short-period field measurements of the liquid film coefficient. 

Typical mass balance methods to measure the air-sea gas transfer have one major drawback the response time is of the order of hours to days, making a parameterisation with parameters such as wind forcing, wave field, or surface chemical enrichments nearly impossible. The controlled flux technique uses heat as a proxy tracer for gases to measure the air-sea gas transfer rate locally and with a temporal resolution of less than a minute. This method offers an entirely new approach to measure the air-sea gas fluxes in conjunction with investigation of the wave field, surface chemical enrichments and the surface micro turbulence at the water surface. The principle of this technique is very simple a heat flux is forced onto the water surface and the skin-bulk temperature difference across the thermal sublayer is measured. 

Using heat as a proxy tracer for gases not only delivers the transfer coefficient at high spatial and temporal resolution but also gives direct insight into the spatial structure of near surface turbulence enabling detailed investigations of the influence of surface chemical enrichments on the gas transfer velocity and the turbulent transport mechanisms. 

The conditions that need to be met for an element to be of use as a proxy tracer are that (1) it conserves a characteristic chemical or isotopic property when transferred from the water column into the sediment (2) the elements or their isotopic composition can be extracted from the sediment (3) the age of the sediment is known so that changes of the tracer over time can be reconstructed. 

One such proxy makes use of element ratios. For example, the ratio of Cd to Ca in foraminiferal tests is a proxy for the PO4 content of the past water mass in which the foraminifera formed and therefore provides information on the past thermohaline circulation. This tracer is explained in detail in the article on trace elements in foraminiferal tests. Similarly, the ratio of the intermediate uranium decay products Pa and °Th, measured in bulk sediment, may under certain conditions provide information on the advection of water masses in the overlying water column in the past see Cosmogenic Isotopes and Uranium-Thorium Series Isotopes in Ocean Profiles). Isotope ratios are ideally suited as long-term proxy tracers. Some of these isotope ratios are characteristic of certain seawater properties and... 

Haufiecker, H., Reinelt, S., and Jahne, B. 1995. Heat as a proxy tracer for gas exchange measurements in the field Principle and technical realization. In Air-Water Gas Transfer. B. Jahne and E. C. Monahan (Eds). Aeon Verlag, Hanau, Germany, 405. 

Henderson GM (2002b) New oceanic proxies for paleoclimate. Earth Planet Sci Lett 203 1-13 Henderson GM, Burton KW (1999) Using ( " U/ U) to assess diffusion rates of isotopic tracers in Mn cmsts. Earth Planet Sci Lett 170(3) 169-179... 

Zhu and MacDougall (1998) showed that calcium isotope composition of the shells was both species and temperamre dependent and that river water is depleted by 2%o relative to seawater. The temperature dependency of calcium isotope fractionation enables this tracer to be a potential paleotem-peramre proxy (Nagler et al., 2000). The seawater Cal" Ca secular variations are indicated by the results of De La Rocha and De Paolo (2000) for the last 160 Myr, and by the data of Zhu (1999) for the entire 3.4 Gyr of earth history. The latter indicate that, in analogy to strontium isotopes (Veizer and Compston, 1976), the Archean samples have C Ca ratios similar to the earth mantle, with the crustal-like values first appearing at the Archean/Proterozoic transition. However, in contrast to the strontium isotope trend, the calcium isotope ratios appear to dip towards mantle values also at —1.6 Gyr ago. 

All proxies of the temporal change in the chemical composition of seawater are complicated by artifacts. Some have been discarded because of difficulties in interpretation and may reappear as they are better rmderstood. Taken together, there are a few results about which we may be relatively confident. It is pretty clear that the chemical tracers and age of North Atlantic Deep Water indicate that it was less prominent in the Atlantic relative to Antarctic Bottom Water during the last glacial period. Cd Ca ratios indicate that the... 

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). 

JORISSEN, F. J., FONTANIER, C. THOMAS, E. 2007. Paleoceanographical proxies based on deep-sea benthic foraminiferal assemblage characteristics. Im Hillaire-Marcel, C. De Vernal, A. (eds) Proxies in Late Cenozoic Paleoceanography. Volume 1 - Methods in Late Cenozoic Paleoceanography Ill-Biological Tracers and Biomarkers. Elsevier, pp. 263-325. 

Zahn, R., Keir, R., 1994. Tracer-nutrient correlations in the upper ocean observational and box model constraints on the use of benthic foraminiferal and Cd/Ca as paleo-proxies for the intermediate-depth ocean. In Zahn, R., Pedersen, T.F., Kaminski, M.A., Labeyrie, L., (eds). Carbon cycling in the glacial ocean Constraints on the ocean s role in global change. Springer, Berlin, pp 195-223. 

Potential of He as a constant flux tracer. Marcantonio et al. s (1995, 1996, 2001) findings and interpretation open up the potential of IDP-derived extraterrestrial He as a constant flux proxy. In various recent studies, the He flux has been determined independently. The estimate of Marcantonio et al. (1996, 2001) for the HeEx flux in the equatorial Pacific (8.0 2.4x10 average Holocene... 

Although the coral-based paleoclimate records reconstructed to date are impressive, much work remains to be done. It is necessary to develop multiple tracer records from each coral record in order to establish a more comprehensive reconstruction of several concurrent climatic features. In addition, replication of long isotopic and elemental records from multiple sites is invaluable for establishing better signal precision and reproducibility. Coupled with fossil and deep-sea coral records, coral proxy records offer a comprehensive and effective means of reconstructing tropical paleo-climates. 

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