Late Neogene to Pleistocene palynology of ODP Leg 112 samples

The main objectives of this study are (1) to characterize the spatial and temporal variations in organic matter deposited in upwelling and related sediments (manifest in the palynoclast and organic-walled microplankton assemblages) and (2) to relate these variations to paleoenvironmental changes. A total of 40 samples from Holes 679D, 680B, 681B, 684B, 686B, and 687B were analyzed. Without exception, amorphogen dominates the palynoclast assemblages overwhelmingly. Influx of terrestrial particulate organic matter to the marine realm was extremely low. Levels of amorphogen swamp other palynoclast categories, and little significance can be attached to any variations observed. Microplankton dominate the palynomorph assemblages, with variable levels of subordinate foraminiferal test linings. Miospores are rare and are absent in most samples. Foraminiferal test linings are particularly abundant in the shallowest samples, which may reflect low surface-water paleotemperatures. Cysts of heterotrophic peridiniacean dinoflagellates (P-cysts) dominate the microplankton assemblages, with variable levels of cysts of autotrophic gonyaulacacean dinoflagellates (G-cysts). Samples dominated by P-cysts are derived largely from laminated, unbioturbated units deposited under the influence of strong upwelling. A lower abundance of P-cysts in some samples is restricted to unlaminated, bioturbated units deposited under oxygenated conditions. We conclude that the ratio of P-cysts to G-cysts is a useful indicator of variable upwelling strength. Detailed study of the variations in the microplankton assemblages offers one the greatest potential for palynological characteriztion and understanding of the upwelling system.

Dinoflagellate cyst assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes, and geochemical parameters of sediment core GeoB9508-5

In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.

Separating ITCZ- and ENSO-related rainfall changes in the Galápagos over the last 3 kyr using D/H ratios of multiple lipid biomarkers

We present a 3000-yr rainfall reconstruction from the Galápagos Islands that is based on paired biomarker records from the sediment of El Junco Lake. Located in the eastern equatorial Pacific, the climate of the Galápagos Islands is governed by movements of the Intertropical Convergence Zone (ITCZ) and the El Niño-Southern Oscillation (ENSO). We use a novel method for reconstructing past ENSO- and ITCZ-related rainfall changes through analysis of molecular and isotopic biomarker records representing several types of plants and algae that grow under differing climatic conditions. We propose that ?D values of dinosterol, a sterol produced by dinoflagellates, record changes in mean rainfall in El Junco Lake, while dD values of C34 botryococcene, a hydrocarbon unique to the green alga Botryococcus braunii, record changes in rainfall associated with moderate-to-strong El Niño events. We use these proxies to infer changes in mean rainfall and El Niño-related rainfall over the past 3000 yr. During periods in which the inferred change in El Niño-related rainfall opposed the change in mean rainfall, we infer changes in the amount of ITCZ-related rainfall. Simulations with an idealized isotope hydrology model of El Junco Lake help illustrate the interpretation of these proxy reconstructions. Opposing changes in El Niño- and ITCZ-related rainfall appear to account for several of the largest inferred hydrologic changes in El Junco Lake. We propose that these reconstructions can be used to infer changes in frequency and/or intensity of El Niño events and changes in the position of the ITCZ in the eastern equatorial Pacific over the past 3000 yr. Comparison with El Junco Lake sediment grain size records indicates general agreement of inferred rainfall changes over the late Holocene.

Biomass of nanoprotozooplankton in the Atlantic sector of the Southern Ocean

The dynamic of early spring nanoprotozoa was investigated in three characteristic water masses of the Southern Ocean: the Marginal Ice Zone, the intermediate waters of the Antarctic Circumpolar Current and the Polar Frontal Zone. Biomass and feeding activities of nanoprotozoa were measured, as well as the biomass of their potential prey-bacteria and phototrophic flagellates-on the 6°W meridian in the Southern Ocean along three repetitive transects between 47 and 60° South from October to November 1992. On average, nanoprotozooplankton biomass accounted for 77% of the combined biomass of bacteria and phototrophic flagellates, and was dominated by dinoflagellates and flagellates smaller than 5 µm. As a general trend, low protozoan biomass of 2 mg C/m**3 was typical of the ice covered area, while significantly higher biomasses culminating at 15 mg C/m**3 were recorded at the Polar Front. Biomasses of bacteria and total phytoplankton were distributed accordingly, with larger values at the Polar Front. Phototrophic flagellates did not show any geographical trend. No seasonal trend could be identified in the Marginal Ice Zone and in the intermediate waters of the Antarctic Circumpolar Current. On the other hand, at the Polar Front region a three-fold increase was observed within a 2-month period for nanoprotozooplankton biomass. Such a biomass increase was also detected for bacterioplankton and total phytoplankton biomass. Half-saturation constants and maximum specific ingestion of nanoprotozoan taxons feeding on bacteria and phototrophic flagellates were determined using the technique of fluorescent labelled bacteria (FLB) and algae (FLA) over a large range of prey concentrations. Maximum ingestion rates ranged between 0.002 and 0.015/h for bactivorous nanoprotozoa and heterotrophic flagellates larger than 5 µm feeding on phototrophic flagellates. The markedly high maximum ingestion rates of 0.4/h characterising nanophytoplankton ingestion by dinoflagellates evidenced the strong ability of dinoflagellates for feeding on nanophytoplankton. Daily ingestion rates were calculated from nanoprotozoan grazing parameters and carbon biomass of prey and predators. This indicated that nanoprotozoa ingestion of daily bacterioplankton and phytoplankton production in early spring ranged from 32 to 40%.