ENVIRONMENTAL AND BIOLOGICAL VARIATION OF SR/CA AND BA/CA IN TERRESTRIAL AND MARINE SYSTEMS

The elemental ratios of strontium to calcium (Sr/Ca) and barium to calcium (Ba/Ca) are common proxies for trophic level in terrestrial systems. Previous research has demonstrated that Sr/Ca and Ba/Ca decrease with increasing trophic level as a result of biopurification, the process whereby calcium from the diet is preferentially incorporated into bioapatite while strontium and barium are preferentially excreted. Little research has been conducted to see if this relationship also holds for consumers in marine systems. This study compiled environmental and biological Sr/Ca and Ba/Ca data from published books, reports, and papers as well as original data with several goals: 1. demonstrate that the relationship between Sr/Ca and Ba/Ca holds in the marine system; 2. compare the environmental variation in terrestrial and marine systems; 3. show that global comparison in marine systems is feasible.

The environmental variation in Sr/Ca and Ba/Ca in marine systems (i.e. seawater) is much lower than what is observed in terrestrial systems (i.e. water, soil, and rock). The log(Sr/Ca) and log(Ba/Ca) values of marine environmental data range from -1.732 to -1.699 and -5.000 to -3.663, respectively, and from -4.058 to 0.131 and -4.661 to -0.565, respectively, for terrestrial environmental data. In terrestrial systems, biological data have a similar range of values to environmental data. However in marine systems, biological data have a much larger range with values both higher and lower than that of seawater, demonstrating that biopurification and bioaccumulation are integral parts of marine systems. Sr/Ca and Ba/Ca exhibit a similar relationship with trophic level in marine systems as they do in terrestrial systems, but the relationship in marine systems is stronger than in terrestrial systems. Sr/Ca values can be used to differentiate trophic groups with little overlap; however, the same is not true for Ba/Ca values. Bioaccumulation is an issue in marine systems and could be the reason why Ba/Ca values cannot separate even basic trophic groupings. Trends in biological Sr/Ca and Ba/Ca values from a variety of ocean basins are statistically indistinct, further supporting the utility of direct global comparisons of marine Sr/Ca and Ba/Ca values and suggests Sr/Ca ratios have great promise as a global marine trophic level proxy.