Diatoms in Paleocene sediments of the southwestern South Atlantic

The taxonomy and stratigraphy of pelagic Paleocene diatoms from ODP Sites 698, 700, and 702 and DSDP Site 524 in the South Atlantic and DSDP Site 214 in the Indian Ocean are presented, as well as paleogeographic and paleoecologic implications. Eleven new species and one new variety are described and one new combination is proposed: Coscinodiscus cruxii sp. nov. Grunowiella palaeocaenica var. alternans var. nov. Hemiaulusl beatus sp. nov. Hemiaulusl ciesielskii sp. nov. Hemiaulusl conicus sp. nov. Hemiaulus kristoffersenii sp. nov. Hemiaulus nocchiae sp. nov. Hemiaulusl oonkii sp. nov. Hemiaulusl velatus sp. nov. Triceratium gombosii sp. nov. Trochosira gracillima comb. nov. Trochosira marginata sp. nov. Trochosira radiata sp. nov. Hole 700B provides one of the most continuous diatomaceous Paleocene profiles known. Stratigraphic ranges of diatom species from this and other Southern Hemisphere sites are calibrated against calcareous microfossil zones. The first-appearance datums of Triceratium gombosii, Hemiaulus incurvus, and Triceratium mirabile in Paleocene deep-sea sediments are useful for regional stratigraphic correlations. Quantitative analysis of the biosiliceous microfossil groups (diatoms, silicoflagellates, radiolarians, and archaeomonadaceae) shows that preservation of diatoms is confined primarily to the upper Paleocene (planktonic foraminifer Zones P3 and P4 and calcareous nannofossil Zones upper NP5 to lower NP9). In the lower Paleocene only short intervals in Hole 700B are diatomaceous. A correlation between the degree of silica diagenesis and the calcium carbonate content of the sediment is not obvious. Diatom species analysis reflects changes in the paleoenvironment between island-related upwelling conditions with highly diverse and well-preserved diatom assemblages and less productive periods resulting in less wellpreserved diatom assemblages with a higher content of robust neritic diatoms.

Sediment geochemical evidence for an early-middle Gilbert (early Pliocene) productivity peak in the North Pacific Red Clay Province

Current attempts to understand climatic variability during the early to middle Pliocene require paleoceanographic information from the Pacific and Indian Oceans that may serve to test and/or constrain future circulation models. Ocean Drilling Program (ODP) Sites 885/886 are located in the central subarctic North Pacific at water depths exceeding 5700 m. Recent studies of rock magnetic properties suggest that the fine-grained Fe oxide component in sediment at Sites 885/886 experienced reductive dissolution during the early-middle Gilbert. Because such an interval in the North Pacific Red Clay Province suggests a maximum in the sedimentary flux of organic carbon and/or a minimum in bottom water dissolved O2 concentrations (and hence, a peak change in North Pacific oceanographic conditions), a geochemical investigation was conducted to test the hypothesis. Quaternary sediment at Hole 886B was subjected to an oxyhydroxide removal procedure, and chemical analyses indicate that bulk sediment concentrations of Fe and the Fe/Sc ratio decrease significantly upon reductive dissolution. Downcore chemical analyses of untreated sediment at Hole 886B demonstrate that similar depletions also occur across the proposed interval of reduced sediment. Downcore chemical analyses also indicate that a pronounced increase in the Ba/Sc ratio occurs across the interval. These results are consistent with an interpretation that abyssal sediment of the North Pacific experienced a decrease in redox conditions during the early-middle Gilbert, and that this change in oxidation state was related to a peak in paleoproductivity. If the zenith of late Miocene to middle Pliocene enhanced productivity observed at other Indo-Pacific divergence regions similarly can be constrained to the early-middle Gilbert, there exists an oceanographic boundary condition in which to test future models concerning Pliocene warmth.