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The main goal of this project is to quantify the productivity and complexity of Cenozoic marine foodwebs through new geochemical proxies for foraging preferences (i.e., δ¹³C), trophic level (Sr/Ca, δ⁴⁴Ca) and habitat preferences (⁸⁷Sr/⁸⁶Sr, δ¹⁸O) of marine consumers. The sampling strategy has focused on major accumulations of marine fossil material deposited during periods of climate variation (e.g., Eocene/Oligocene; Oligocene/Miocene; Middle Miocene Climatic Optimum) and is separated into three research phases (see below).

Phase I: Cenozoic stable isotope ecology of marine herbivores. Enamel was sampled from sirenian fossil material to establish baseline environmental conditions of nearshore marine ecosystems through the Cenozoic. Stable isotope compositions of the tooth enamel were used as proxies for salinity (δ¹⁸O values) and primary producer productivity and composition (δ¹³C values). We sampled teeth from the Tethys-Mediterranean (TM) region (n = 57) and India (n = 13). Results were combined with data previously reported for the West Atlantic-Caribbean (WAC) region (n = 89) and Egypt and France (n = 13).

Results: Enamel δ¹³C values were high and consistent with a diet of marine vegetation, primarily seagrasses, for all fossil species of sirenians sampled. Small, but significant differences were detected between sirenian families and subfamilies, suggesting a mix of dietary specialists and generalists within communities. Enamel δ¹⁸O values varied significantly with time and latitude. Enamel δ¹⁸O values for Eocene specimens were significantly lower than those sampled from younger time intervals, a pattern that is consistent with changes in ocean δ¹⁸O values as continental ice sheets formed and expanded at the onset of the Oligocene. In addition, latitude was found to be negatively correlated with enamel δ¹⁸O values regardless of time periods. The magnitude of the difference between low (<35°) and high (>35°) latitudes increased after the Oligocene, which indicates the initiation of present-day latitudinal gradients in seawater oxygen isotope composition. Elevated δ¹³C and δ¹⁸O values in TM sirenians during the Messinian Salinity Crisis corroborate the hypothesis of ecophenotypic dwarfing in these animals. These results were published in Geology and were incorporated into a successful NSF Early CAREER grant proposal, which was funded in spring 2009.

Phase II: Cenozoic stable isotope ecology of higher trophic level consumers. Enamel was sampled from 175 specimens of Eocene archaeocetes (i.e., ancient whales) and Oligocene-Miocene neocetes (i.e., modern whales) from sites in the USA, India (Gujarat), and New Zealand. The samples were complemented by new material from Indoyus, the recently proposed sister taxon to early whales (Artiodactyla: Raoellidae) from the middle Eocene of northern India and Pakistan. Values for these specimens were combined with published data for sites in Egypt, India and Pakistan. These specimens were selected to determine 1) the earliest appearance of whales in marine ecosystems; 2) the range in marine habitats of whales; and 3) the trophic diversity of whales from the Paleogene and early Neogene.

Results: Enamel stable isotope values indicate substantial ecological, environmental and, particularly for early whales, evolutionary change through the Paleogene and early Neogene. For the raoellid Indohyus, enamel δ¹⁸O values were extremely low and suggest an early movement into the water by whales, possibly at the very start of the Eocene. Carbon isotope values for Indohyus, however, do not support an aquatic diet for this early whale relative, which may mean that food was not the initial motivation for whales to take to the water. These findings were reported in the December 2007 issue of Nature. Subsequent sampling of later whales shows evidence of marine foraging by the middle Eocene (ca. 46 Ma), but with a preference for shallow, nearshore habitats for whales from the Eocene and Oligocene. Carbon isotope values for Oligocene and Miocene neocetes are much higher than values obtained for living cetaceans, which suggest that these fossil species were spending more time in shallow, highly productive waters near shore.

Phase III: Geochemical proxies for marine trophic diversity in the Cenozoic. The calcium isotope (δ⁴⁴Ca) and elemental (Sr/Ca, Ba/Ca) compositions of bone and enamel were analyzed from several species of marine producers and consumers as potential proxies for trophic position with marine food webs. Approximately 200 specimens of algae, seagrass, sirenians, cetaceans, pinnipeds and mustelids were sampled from museum collections that primarily contained material for species found in the North Pacific and North Atlantic.
Results: Contrary to prior research, enamel and bone δ⁴⁴Ca values were not found to strongly correlate with trophic position even though a significant range in δ⁴⁴Ca valueas was detected (~3.0‰). Differences among species, especially cetaceans, most likely reflects a combination of trophic position, prey type and seawater consumption; species that consumed the greatest quantities of seawater yielded the highest δ⁴⁴Ca values whereas those that obtained most of their water from fats in their diet had the lowest δ⁴⁴Ca values. In contrast, bone and enamel Sr/Ca ratios were negatively correlated with trophic position through the process of biopurification. This relationship is evident regardless of specimen age or sampling location and could serve as robust proxy for trophic position that is independent morphology or phylogenetic constraints. However, initial analysis of fossil specimens suggests that preservation of original Sr and Ba content is difficult and so we are currently in the process of developing preparation methods to correct for the addition of these elements during fossilization and diagenesis.  Once completed,  these data will provide new temporal and spatial information on the productivity of Cenozoic marine foodwebs. So far, this project has supported one graduate student from Italy; one graduate student (Stephanie Peek) and three undergraduate students (Matt Kline, James Durnan, Paul Haselhorst) from the University of Wyoming; and three high school students (Anwar Batte, Bibiana Brame, Anjuli Tuck) from Denver, CO.