46903-G2
Quantification of Cenozoic Marine Primary Productivity through Geochemical Analysis of Marine Mammal Bioapatite
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 was 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: High enamel δ13C values (> -2.0‰) for the Paleogene indicate diets composed mostly of marine seagrasses. In contrast, enamel δ18O values varied greatly; sirenians from southern sites (i.e., Egypt, Libya) and Florida had significantly higher mean δ18O values than specimens from most European locations, suggesting large differences in salinity across the TM and between the TM and WAC. The range in enamel δ18O values increased from the Paleogene through the Neogene, but enamel δ13C values remained steady in the TM, indicating that changes in environmental conditions did not necessarily coincide with major changes in diet. In contrast, middle Miocene sirenians from the WAC had significantly lower δ13C (< -10.0‰) and δ18O (< 25.0‰) values, which are indicative of movement into freshwater. Elevated δ13C and δ18O values in TM sirenians during the Messinian Salinity Crisis corroborate the hypothesis of ecophenotypic dwarfing in these animals. These initial results are currently in press (Geology) and a larger proposal on this topic has been submitted for a NSF Early CAREER award..
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 significantly depleted in 18O relative to coeval terrestrial mammals. These low values are indicative of semiaquatic, freshwater habits 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 show 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 cetcaeans, which suggest that these fossil species were spending more time in shallow, highly productive waters near shore.
Phase III: The next phase of this project will involve analysis of the calcium and strontium isotope composition of these specimens. Carbon and oxygen isotope data have served as proxies for habitat and foraging information, and will be supplemented with information on trophic diversity through these additional geochemical analyses. Calcium and strontium isotope analyses will be conducted in collaboration with Drs. Carol Frost and Ken Frost from the University of Wyoming and Woods Hole Oceanographic Institute. These data will provide new temporal and spatial information on the productivity of Cenozoic marine foodwebs and will serve as pilot data for proposals to the January and July submissions to NSF Sedimentology and Paleontology. So far, this project has supported one graduate student from Italy; one graduate student (Stephanie Peek) and two undergraduate students (Matt Kline, James Durnan) from the University of Wyoming; and one high school student (Anwar Batte) from Denver, CO and this funding is expected to continue student support into spring and fall 2009.
