Stable carbon and oxygen isotope record of benthic foraminifera from Paleocene-Eocene sediments

A major change in Cenozoic deep-sea benthic foraminifera occurred in the Atlantic, Indian, and Pacific oceans near the Paleocene/Eocene boundary. Benthic foraminiferal abundance changes began at about 61.5 Ma at Pacific Deep Sea Drilling Project (DSDP) Site 577. A major extinction event followed at 58-57 Ma (between Zones P6a and P6b), and a series of first appearances continued until circa 55.5 Ma (Zone P6c). These faunal changes occurred during a 6°C warming of Pacific bottom water and may indicate that the primary cause was changing temperature. Other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions. Comparison of benthic and planktonic delta13C records requires no change in the ratio of oceanic phosphorous to carbon during the late Paleocene to early Eocene, which weakens the case for (but does not disprove) a change in surface ocean productivity at this time. Interbasinal comparisons of benthic foraminiferal delta13C records document that water with high delta13C values filled the Cape Basin during the late Paleocene and possibly the early Eocene (circa 61-57 Ma), but apparently did not extend into the western basins of the Atlantic. This pattern suggests a supply of Antarctic source water for the Cape Basin and possible tectonic isolation of the western Atlantic basins during at least part of the late Paleocene. Carbon isotope comparisons show that bottom water supply to the Cape Basin was reduced in the early Eocene. Eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the Paleocene. These late Paleocene climatic changes (bottom water warming and decreased wind intensity) correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism, which may have contributed to climatic warming through increased supply of CO2.

(Table 1) Bulk and grain density, porosity, and p- and s-wave velocity characteristics of ODP Hole 163-990A sediments

Seismic velocities in rocks are influenced by the properties of the solid, the pore fluid, and the pore space. Cracks dramatically affect seismic velocities in rocks; their influence on the effective elastic moduli of rocks depends on their shape and concentration. Thin cracks (or fractures) substantially lower the moduli of a rock relative to the effect of spherical voids (or vesicles), and lower moduli are reflected by lower P- and S-wave velocities. The objective of this research is to determine the types and concentrations of cracks and their influence on the seismic properties of subaerially erupted basalts drilled from Hole 990A on the Southeast Greenland margin during Ocean Drilling Program Leg 163. Ellipsoidal cracks are used to model the voids in the rocks. The elastic moduli of the solid (grains) are also free parameters in the inverse modeling procedure. The apparent grain moduli reflect a weighted average of the moduli of the constituent minerals (e.g., plagioclase, augite, and clay minerals). The results indicate that (1) there is a strong relationship between P-wave velocity and porosity, suggesting a similarity of pore shape distributions, (2) the distribution of crack types within the massive, central region of aa flows from Hole 990A is independent of total porosity, (3) thin cracks are the first to be effectively sealed by alteration products, and (4) grain densities (an alteration index) and apparent grain moduli of the basalt samples are directly related.

TOC, TN, d15N isotopes and alkenone concentration in equatorial Pacific sediments

The largest increase in export production in the eastern Pacific of the last 5.3 Myr (million years) occurred between 2.2 and 1.6 Myr, a time of major climatic and oceanographic reorganization in the region. Here, we investigate the causes of this event using reconstructions of export production, nutrient supply and oceanic conditions across the Pliocene-Pleistocene in the eastern equatorial Pacific (EEP) for the last 3.2 Myr. Our results indicate that the export production peak corresponds to a cold interval marked by high nutrient supply relative to consumption, as revealed by the low bulk sedimentary 15N/14N (d15N) and alkenone-derived sea surface temperature (SST) values. This ?0.6 million year long episode of enhanced delivery of nutrients to the surface of the EEP was predominantly initiated through the upwelling of nutrient-enriched water sourced in high latitudes. In addition, this phenomenon was likely promoted by the regional intensification of upwelling in response to the development of intense Walker and Hadley atmospheric circulations. Increased nutrient consumption in the polar oceans and enhanced denitrification in the equatorial regions restrained nutrient supply and availability and terminated the high export production event.