Hans Schrader

Paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite

Abstract Rare earth element concentrations in biogenic apatite of conodonts, fish debris and inarticulate brachiopods were determined in over 200 samples from Cambrian to modern sediments. Tests for experimental bias caused by the chemicals used to separate fossils from the rock matrix and for interlaboratory, interexperiment or interspecies related variations clearly show that no resolvable fractionation of REE occurs. Incorporation of REE in biogenic apatite of Recent fish debris occurs near the sediment-water interface soon after deposition and reflects characteristics of seawater. The original REE signature apparently survives subsequent burial and diagenesis. Cerium variations in fish debris from modern environments are controlled by redox potential. Ce is fractionated by co-precipitation with metallic oxides under oxidizing conditions. This fractionation produces a negative Ce anomaly (Ce anom ) in seawater that is reflected in the sedimented fish debris. Conversely, no fractionation of Ce occurs under reducing conditions, resulting in Ce concentrations that are normal to enriched in anoxic seawater and fish debris deposited under local or basinwide anoxic conditions. Extrapolation of observations of the redox control of Ce anom in modern oceans to fossil apatite indicates that anoxic conditions were prevalent in lower Paleozoic and Lower Triassic oceans, whereas in the upper Paleozoic, the world ocean was generally oxidizing.

Late Pliocene onset of glaciation: ice-rafting and diatom stratigraphy of North Pacific DSDP cores

Abstract The revised diatom biostratigraphy for the North Pacific allows the timing of ice-rafting, as recorded in Deep Sea Drilling Project cores, to be determined. Significant ice-rafting began at the time of the Matuyama-Gauss reversal boundary, about 2.48 Ma. Other bio- and lithostratigraphic data from the Pacific, Atlantic, and Arctic Oceans, and the Black Sea all indicate that northern hemispherical cooling and the accumulation of continental ice began at 2.4–2.5 Ma. We interpret all these data to indicate northern hemisphere ice-cap formation at that time. If so, the closing of the Isthmus of Panama, approximately 3.2 m.y. ago, was not the immediate cause of the Plio-Pleistocene glaciations.

Diatom taphocoenoses in the coastal upwelling area off South West Africa

Abstract The diatom floral composition of 124 sediment samples from the South East Atlantic records the influence of coastal upwelling on sediment composition off South West Africa. Inner shelf samples between 19° and 24°S are rich in diatom valves and the patterns of diatom species distribution in these samples are related to the coastal upwelling process. Comparison with recent phytoplankton data shows that the sediment assemblages preserve many of the important species of the diatom biocoenoses, including Chaetoceros (resting spores), Delphineis karstenii, Thalassiosira eccentrica, and Thalassionema nitzschioides. Delphineis karstenii, a pioneer species in enriched coastal water, occurs nearshore and Chaetoreros resting spores are widespread, with highest relative abundance values in some more offshore samples. The abundance of the Thalassiosira eccentrica group and of Thalassionema nitzschioides in sediment samples in and near Walvis Bay reflects the recurrence of intense upwelling off this part of the coast. An abundance of large centric species has been reported in hydrological conditions characteristic of newly upwelled waters and, correspondingly, Actinocyclus octonarius and some large Coscinodiscus species occur in the sediments in nearshore patches or belts.

A 20-year varve record of siliceous phytoplankton variability in the central Gulf of California

Abstract Year-to-year variability in the structure of the preserved siliceous phytoplankton assemblage is reconstructed from a box core retrieved from the varved sediments along the Guaymas slope of the Gulf of California. A 20-year record from 1953 through 1972 is obtained from interpretation of the varve stratigraphy supported by 210 Pb and 228 Th dating. Ecological time series derived from microfossil analysis show a strong coherence with interannual sea-level anomalies indicating a response to large-scale climatic variability associated with the El Nino/Southern Oscillation phenomenon. El Nino periods are generally marked by increases in preserved abundances of the total siliceous assemblage and in particular by greater numbers of individuals within species whose distribution is limited to tropical and subtropical waters.

Biogenic and abiogenic components of laminated hemipelagic sediments in the central Gulf of California

Abstract Alternating light and dark submillimeter to millimeter thick layered sediments (rhythmites) are found along the slopes of the central and southern Gulf of California where a strong oxygen minimum zone intersects the sediment—water interface. Two short intervals of distinct alternations were sampled lamina by lamina from two cores in the central Gulf. Both cores are from the slope of the Guaymas Basin, one core is from the mainland side, the other core is from the Baja side. Each lamina sample was analyzed for its opal micro-floral assemblage, textural character, trace metal and bulk chemical (Cu, Mn, Fe, Ni, Zn, Ca, K, Al, SiO 2 and C-org) content, and mineralogy. On both sides of the gulf the light laminae contain more diatom frustules (up to 90%) and less terrigenous and organic material than the dark laminae. The sediment is predominantly silt sized. The dark laminae contain more terrigenous material which is primarily clay size ( μ m) on the mainland side and fine silt size (4–20 μm) on the Baja side. All chemical species analyzed, except SiO 2 , are always higher in the dark laminae and show a high positive correlation to organic carbon. SiO 2 is higher in the light laminae and shows a high negative correlation to organic carbon. Non-opaline material is a secondary constituent, primarily consisting of quartz, plagioclase and calcite. The μ m fraction is dominated by opal and quartz. Smectite, illite, and kaolinite are present in the laminae from the mainland side, but are almost absent on the Baja side. On both sides of the gulf the light laminae apparently represent sedimentation during the dry winter season of northwesterly winds, whereas the dark laminae represent the wet summer season of southeasterly winds.

Variation of upwelling/oceanic conditions during the latest Pleistocene through Holocene off the central Peruvian coast: A diatom record

Diatoms are well preserved in partly laminated Holocene and late Pleistocene sediments on the upper continental slope of central Peru. Accumulation rates of diatom species previously recognized as tropical (oceanic, warm water) and subtropical (neritic, cooler water, upwelling) changed markedly downcore in Kasten cores obtained at 11°15′S, 12°58′S, and 13°37′S latitude. Subtropical species (e.g., Thalassiosira eccentrica “group”, Thalassiothrix frauenfeldii, Chaetoceros spp. resting spores, Delphineis spp.) predominated in late Pleistocene sediments of the two southern cores. A mix of tropical and subtropical diatom assemblages occurred in latest Pleistocene sediments, while tropical species (e.g., Coscinodiscus nodulifer/radiatus, Pseudoeunotia doliolus, Thalassionema nitzschioides var. parva) predominated in the late Holocene sediments. In the northern core latest Pleistocene sediments also contained a mix of tropical and subtropical species. Similarly, late Holocene sediments contained predominately tropical species of diatoms. Neoglacial periods (200–400 yr B.P., 2000–2700 yr B.P.), however, were sometimes characterized by a greater abundance of subtropical species and punctuated periodically with blooms of Skeletonema costatum and Rhizosolenia shrubsolei. It is proposed that during times of Southern Hemisphere cooling, Peruvian coastal waters north of 12°S were subjected to episodic mixing with warmer water masses before being overwhelmed by tropical waters that may have either passed southward across a weakened Equatorial Front or arose from intensified offshore countercurrents. Coastal waters south of 12°S latitude were probably unaffected by the tropical influence. Rather, these waters may have undergone intensified upwelling during the last Glacial epoch.

Laminated Diatomaceous Sediments from the Guaymas Basin Slope (Central Gulf of California): 250,000-Year Climate Record

During Deep Sea Drilling Project-International Program of Ocean Drilling leg 64, December 1978 to January 1979, the initial test of the Deep Sea Drilling Projects hydraulic piston corer obtained an almost undisturbed section from a 152-meter hole into the sediments of the oxygen minimum zone at a depth of 655 meters along the Guaymas slope in the central Gulf of California. The section records variations in climate, productivity, and circulation for more than 250,000 years of Late Pleistocene to Holocene history with recordings of seasonal variations in these parameters in the laminated sections.

Late Quaternary history of continental climate and the Benguela Current off South West Africa

Abstract Late Quaternary sediments from the southwest African continental slope were analyzed for their coarse fraction, diatom and clay mineral content and composition. Changes in the flow pattern of the Benguela Current were inferred from the abundance and preservation of opaline microfossils and diatom species composition. The Benguela Current turned westward in the general latitude of 24°S during oxygen isotope stages 1 and 3; this flow pattern caused transportation of opaline microorganisms from the near shore area to the continental slope area or caused increased upwelling phenomena further offshore. During oxygen isotope stage 2, on the other hand, the westward deflection of the Benguela Current was moved further to the north. Two sources of terrigenous material were determined: one is local with supply from the Namib desert, the other one is distant with supply from the Orange River mouth; material from the Orange River mouth might have been transported to the north by the Benguela Current. The local terrigenous supply from the Namib desert increased during oxygen isotope stage 2 as evidenced by the increased amounts of terrigenous matter, by an increase of illite and by greater grain sizes. Occurrence of wood fibres in oxygen isotope stage 3 indicates a denser continental vegetation over the present Namib desert.

Holocene depositional evolution of a small active-margin estuary of the northwestern United States

Seismic reflection data (Uniboom) and C14-dated core samples from a small activemargin estuary, Alsea Bay, Oregon, indicate that Holocene sediment fills an ancestral river valley which has an axial depth of 55 m below present sea level. Holocene sediment in the estuarine basin overlies late Pleistocene and late Pliocene(?) valley-fill deposits indicating several sequences of valley down-cutting, submergence and deposition. Core samples of Holocene fill from six core sites in the estuary were analyzed for age, embayment salinity indicators, bulk density, grain-size distribution, source (mineralogy) composition and sedimentary structures. Deposition of Holocene fill has been dominated by lateral channel migration and overbank suspension deposition. Three stages of deposition during the Holocene transgression are indicated on the bases of sedimentary structures, grain-size distribution and embayment salinity. These stages represent periods of transition from: (1) a shallow fluvial environment (10 × 103–7.5 × 103 yrs B.P.), sedimentation rate between 0.4 and 0.7 cm yr−1; to (2) a deep-water estuarine environment (7.5 × 103–5 × 103 yrs B.P.), sedimentation rate approximately 1.1 cm yr−1; to (3) a shallow-water estuarine environment (5 × 103–0 yrs B.P.), sedimentation rate approximately 0.21 cm yr−1. A major decline in sedimentation rate at about 5 × 103 yrs B.P.. follows a decline in the rate of eustatic sea-level rise and a corresponding decrease in sediment accumulation rate. The Holocene fill is comprised of approximately 70% river sand (2.7 × 103 tonnes), 20% river silt and clay (7.5 × 107 tonnes) and 10% beach sand (2.9 × 107 tonnes). Comparisons of modern terrestrial-sediment supply rates (2.8 × 105–3.2 × 105 tonnes yr−1) and long-term retention rates of terrestrial sediment (3.45 × 104 tonnes yr−1, average for the last 1 × 104 yrs) imply that the Alsea Bay basin has probably trapped <15% of the terrestrial sediment supplied to it.

Marine planktic diatom correlation of lower Messinian deposits in the western Mediterranean

Abstract Samples from late Neogene Mediterranean outcrops were analysed for diatom content. The presence of certain species in the Tripoli Formation (Caltanissetta Basin, Sicily) and in the Beida Formation (Cheliff Basin, NW Algeria) permits correlation between these sections and the paleomagnetic reversal record. The occurrence ofThalassiosira praeconvexa, Thalassiosira miocenica, Thalassiosira convexa var.aspinosa andAsterolampra acutiloba indicates that the Tripoli and Beida formations were deposited over an interval ranging from the upper half of Paleomagnetic Epoch 6 into the lower half of Epoch 5 (5.6 to 5.9 Ma). Previous diatom correlations were too young. The occurrence ofNitzschia reinholdii andAsterolampra acutiloba in pre-evaporitic diatomites from the upper Turre Formation (SE Spanish Sorbas Basin) is used to place deposits at this site tentatively in Paleomagnetic Epoch 5 or 6.