Nannofossil assemblages and flux rates during the late Oligocene-early Miocene

This study investigates abundance variations in Noelaerhabdaceae assemblages during the late Oligocene-early Miocene at three subtropical sites in the Atlantic and Pacific oceans (DSDP Sites 516, 608 and 588). At these three sites, nannofossil assemblages were characterized by the successive high proportion of Cyclicargolithus, Dictyococcites and Reticulofenestra. Local paleoceanographic changes, such as the input of nutrient-poor water masses, might explain shifts in ecological prominence within the Noelaerhabdaceae at DSDP Site 516 (South Atlantic). But the similar timing of a decline in Cyclicargolithus at the three studied sites more likely corresponds to a global process. Here, we explore possible causes for this long-term taxonomic turnover. A global change in climate, associated with early Miocene glaciations, could have triggered a decline in fitness of the taxon Cyclicargolithus. The ecological niche made vacant because of the decrease in Cyclicargolithus could then have been exploited by Dictyococcites and Reticulofenestra that became prominent in the assemblages after 20.5 Ma. Alternatively, this global turnover might reflect a gradual evolutionary succession and be the result of other selection pressures, such as increased competition between Cyclicargolithus and Dictyococcites/Reticulofenestra. A diversification within Dictyococcites/Reticulofenestra, indicated by an expansion in the size variation within this group since ~ 20.5 Ma, may have contributed to the decreased fitness of Cyclicargolithus.

Sedimentation and sediment redistribution on the Cocos Ridge from two seismic acquisition cruises, NEMO-3 and MV1014

We use digital seismic reflection profiles within a 1° * 1° survey area on the Cocos Ridge (COCOS6N) to study the extent and timing of sedimentation and sediment redistribution on the Cocos Ridge. The survey was performed to understand how sediment focusing might affect paleoceanographic flux measurements in a region known for significant downslope transport. COCOS6N contains ODP Site 1241 to ground truth the seismic stratigraphy, and there is a seamount ridge along the base of the ridge that forms a basin (North Flank Basin) to trap sediments transported downslope. Using the Site 1241 seismic stratigraphy and densities extrapolated from wireline logging, we document mass accumulation rates (MARs) since 11.2 Ma. The average sediment thickness at COCOS6N is 196 m, ranging from outcropping basalt at the ridge crest to ~ 400 m at North Flank Basin depocenters. Despite significant sediment transport, the average sedimentation over the entire area is well correlated to sediment fluxes at Site 1241. A low mass accumulation rate (MAR) interval is associated with the 'Miocene carbonate crash' interval even though COCOS6N was at the equator at that time and relatively shallow. Highest MAR occurs within the late Miocene-early Pliocene biogenic bloom interval. Lowest average MAR is in the Pleistocene, as plate tectonic motions caused COCOS6N to leave the equatorial productivity zone. The Pliocene and Pleistocene also exhibit higher loss of sediment from the ridge crest and transport to North Flank Basin. Higher tidal energy on the ridge caused by tectonic movement toward the margin increased sediment focusing in the younger section.

(Table 1) Helium concentration and flux in DSDP Hole 94-607 samples

Most of the helium-3 in oceanic sediments conies from interplanetary dust particles (IDPs), and can therefore be used to infer the accretion rate of dust to the Earth through time (Ozima et al., 1984, doi:10.1038/311448a0; Takayanagi and Ozima, 1987, doi:10.1029/JB092iB12p12531; Farley, 1995, doi:10.1038/376153a0). 3He records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years, probably owing to cometary and asteroidal break-up events3. Muller and MacDonald have proposed (Muller and MacDonald, 1995, doi:10.1038/377107b0) that periodic changes in this accretion rate due to a previously unrecognized 100-kyr periodicity in the Earth's orbital inclination might account for the prominence of this frequency in climate records of the past million years (Imbrie et al., 1993, doi:10.1029/93PA02751). Here we report variations in the 3He flux to the sea floor that support this idea. We find that the flux recorded in rapidly accumulating Quaternary sediments from the Mid-Atlantic Ridge oscillates with a period of about 100 kyr. We cannot yet say, however, whether the 100-kyr climate cycle is a consequence of, a cause of, or an effect independent of these periodic changes in the rate of delivery of interplanetary dust to the sea floor.