RESEARCH  

Thomas J. Algeo

Professor of Geology

University of Cincinnati

 

Development of Paleomarine Environmental Proxies

The environmental conditions in paleomarine systems can be interpreted from many types of proxies, but the use of trace-element data has been underdeveloped to date.  Commonly, the concentrations of trace metals such as molybdenum (Mo) are used to infer paleoredox conditions, with higher concentrations indicative of more reducing environments.  This relationship is valid in a general sense, as shown by studies of modern (above; Zheng et al., 2000) and ancient marine systems (below; Algeo and Maynard, 2004).  However, trace-metal concentrations in marine sediments are also strongly influenced by hydrographic factors, as shown by Algeo (2004), Algeo and Lyons (2006), and Algeo and Maynard (2008).

The concentrations of trace metals in seawater are nearly uniform globally for those metals having long residence times in seawater, e.g., Mo (~700 kyr) and U (~400 kyr).  However, the aqueous concentrations of these trace metals may deviate strongly from that of seawater in restricted marine systems with reducing deepwaters, in which trace metals are removed at relatively high rates.  As a consequence of such rapid removal, the concentrations of trace metals in the deep watermass will be drawn down to varying degrees, as in the modern Cariaco Basin (right; Algeo and Maynard, 2008).  The degree of drawdown of any given metal, such as Mo, varies among restricted basins, with strongly restricted basins such as the Black Sea and Framvaren Fjord showing greater drawdown than weakly restricted basins such as the Cariaco Basin and Saanich Inlet (far right; Algeo and Lyons, 2006; Algeo and Rowe, 2012).  

 

Trace-metal concentrations in marine sediments vary as a function of degree of deepwater drawdown, as discussed above.  This pattern is most readily observed by cross-plotting a trace metal such as Mo versus total organic carbon, or TOC.  Strong restriction results in lower Mo content per unit TOC than weak restriction in both modern (right; Algeo and Lyons, 2006; Algeo and Rowe, 2012) and ancient marine systems (far right; Algeo et al., 2007).  Because organic matter is the main host phase for trace metals in the sediment, total uptake "capacity" of the sediment correlates with TOC content.

The relationships between Mo, TOC, and watermass restriction in modern marine systems (above) are potentially useful for interpretation of watermass conditions in paleomarine systems.  A high Mo/TOC ratio (given as 'm', in units of 10^-4) is indicative of weak watermass restriction, whereas a low Mo/TOC ratio is indicative of strong watermass restriction.  The Mo/TOC ratio of ancient marine sediments can be used to estimate paleo-deepwater trace-metal concentrations (right) and renewal times (far right). 
 

 

The principles above can be used to reconstruct hydrographic conditions in paleomarine systems, such as the Devonian epeiric seas of North America (right; Algeo et al., 2007).  Other examples of this application include:

  • Coniacian-Santonian (März et al., 2009)

  • Cenomanian-Turonian (Hetzel et al., 2009)

  • Toarcian (McArthur et al., 2008)

  • Permian-Triassic (Xu et al., 2009)

  • Mississippian (Rowe et al., 2008)

 

 

References

Algeo, T.J., 2004. Can marine anoxic events draw down the trace-element inventory of seawater? Geology 32, 1057-1060.

Algeo, T.J., Lyons , T.W., 2006. Mo–total organic carbon covariation in modern anoxic marine environments: Implications for analysis of paleoredox and paleohydrographic conditions. Paleoceanography 21, PA1016, 23 pp.

Algeo, T.J., Maynard, J.B., 2004. Trace element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems. Chem. Geol. 206, 289-318.

Algeo, T.J., Maynard, J.B., 2008. Trace metal covariation as a guide to water-mass conditions in ancient anoxic marine environments. Geosphere 4(5), 872-887.

Algeo, T.J., Rowe, H., 2012. Paleoceanographic applications of trace-metal concentration data. Chemical Geology, v. 324-325, p. 6-18. doi:10.1016/j.chemgeo.2011.09.002.

Algeo, T.J., Lyons , T.W., Blakey, R.C., Over, D.J., 2007. Hydrographic conditions of the Devono-Carboniferous N orth American Seaway inferred from sedimentary Mo-TOC relationships. Palaeogeogr. Palaeoclimatol. Palaeoecol. 256, 204-230.

Hetzel, A., Böttcher, M.E., Wortmann, U.G., Brumsack, H.-J., 2009. Paleo-redox conditions during OAE 2 reflected in Demerara Rise sediment geochemistry (ODP Leg 207). Palaeogeogr. Palaeoclimatol. Palaeoecol. 273, 302-328.

März, C., Beckmann, B., Franke, C., Vogt, C., Wagner, T., Kasten, S., 2009. Geochemical environment of the Coniacian-Santonian western tropical Atlantic at Demerara Rise. Palaeogeogr. Palaeoclimatol. Palaeoecol. 273, 286-301.

McArthur, J.M., Algeo, T.J., van de Schootbrugge , B., Li, Q., Howarth, R.J., 2008. Basinal restriction, black shales, and the Early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography 23, PA4217, doi:10.1029/2008PA001607.

Rowe, H.D., Loucks, R.G., Ruppel , S.C. , Rimmer, S.M., 2008. Mississippian Barnett Formation, Fort Worth Basin , Texas : Bulk geochemical inferences and Mo–TOC constraints on the severity of hydrographic restriction. Chem. Geol. 257, 16-25.

Xu, G., Hannah, J.L., Stein, H.J., Bingen, B., Yang, G., Zimmerman, A., Weitschat, W., Mørk, A., Weiss, H.M., 2009. Re-Os geochronology of Arctic black shales to evaluate the Anisian-Ladinian boundary and global faunal correlations. Earth Planet. Sci. Lett. 288, 581-587.

 

Last updated 17 Sept 2012

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