Seasonal and Annual Variability In Elemental and Isotopic Proxies of Past Ocean Conditions: Results From the Guaymas and Cariaco Basins

The isotopic and trace element composition of planktonic foraminiferal shells are controlled by the chemical and physical proprieties of the ambient seawater and can be used to reconstruct past oceanic conditions. These proxies are quantitatively defined and validated by analyzing how the geochemistry of modern foraminifera varies with changing environmental conditions. I have conducted geochemical and faunal analyses of planktonic foraminifera collected in time series sediment traps from the Guaymas Basin in the Gulf of California and the Cariaco Basin, Venezuela. In Cariaco Basin, the highest flux of planktonic foraminifera occurs during spring upwelling. In response to decreasing upwelling strength and increasing temperatures, the abundance of warm water species is increasing, while the cold water species are decreasing. On average 80% of the foraminiferal shells dissolve while settling through the water column from 230 to 1200m, although the assemblage was not significantly modified. The δ18O data from both locations reflect species habitat preferences with shallow dwelling species having the lowest δ18O and deeper dwelling species the highest. The surface dwelling Globigerinoides ruber most accurately records sea surface temperature (SST). Surface to thermocline temperature gradients can be reconstructed using the difference between the δ18O of surface and thermocline dwelling species. The Mg/Ca and Li/Ca ratios of planktonic foraminifera are known to be controlled by temperature, salinity, and [CO32-]. In Cariaco Basin, there are interspecies differences in both ratios, but only the Mg/Ca differences reflect the temperature gradient of the species preferred depth habitats indicating that Li/Ca is impacted by strong species specific vital effects. The B/Ca ratios in Cariaco Basin show a repeatable seasonal pattern with the highest values occurring in summer and the lowest during winter upwelling. 11B MAS NMR spectra reveal that most of the boron (~75 - 90%) is in a borate carbonate complex, while the rest is either borate or boric acid. In Cariaco Basin, the nutrient proxies, P/Ca, Cd/Ca, and Zn/Ca, show no inter-species variation and do not have a significant relationship with measured phosphate concentrations; however this may be a result of uniformly low phosphate concentrations in the mixed layer