Molecular stratigraphic evaluation of twentieth century El Nino events and plankton productivity in the Santa Barbara Basin, California, and the Guaymas Basin, Mexico

Sedimentary abundance profiles for several source-specific molecules derived from marine plankton were compiled. Abundance trends were compared with historical records of annual climate change and plankton productivity to test the ability of molecular constituents to accurately record evidence of short-term environmental fluctuations. Molecular trends were shown to reflect documented patterns of change, most notably those induced by the El Nino climatic phenomenon.; Varved sediments from the Santa Barbara Basin (SSB), California, and the Guaymas Basin (GB), Mexico, were sampled at annual to subannual resolution, organically extracted and lipid extracts analyzed by gas chromatography. Molecules of principal focus were long chain, di- and triunsaturated ketones (alkenones) which are primarily derived from the prymnesiophyte alga Emiliania huxleyi. This eurythermal alga tolerates temperature variability in its growth habitat apparently by synthesizing these compounds with an increasing number of double bonds as water temperatures decline. This relationship, defined as the alkenone unsaturation index {dollar}rm Usbsp{lcub}37{rcub}{lcub}K{rcub}{dollar} and calibrated to yield absolute sea surface temperatures, has been utilized previously in studies of glacial/interglacial climate change and here was used to evaluate the SBB molecular stratigraphic record of surface warming induced by El Nino. The alkenones were shown to faithfully record the occurrence of all major El Ninos over the span of the record (ca. 1914-1988). They did not, however, accurately reflect the relative magnitudes of these events, perhaps due to the effects of averaging of the molecular sedimentary signal or interannual variability in the depth or seasonal timing of maximum alkenone synthesis.; Abundance profiles for dinosterol, brassicasterol, cholesterol, and the alkenones were used to evaluate annual productivity trends for dinoflagellates, diatoms, zooplankton, and prymnesiophyte algae, respectively. In GB surface sediments, brassicasterol concentration trends clearly reflect the increased diatom abundances measured during the 1982/83 El Nino event. In the SBB, dinosterol concentration peaks generally coincide with documented extrabasinal dinoflagellate blooms, suggesting productivity responses to regional and local oceanographic conditions.; By demonstrating that SBB and GB molecular trends reflect historically documented El Nino events and plankton dynamics, this study confirms the utility of molecular stratigraphy in ultra high resolution paleoclimatic and paleoenvironmental investigations.