Showing papers by "Frans Jorissen published in 1998"

Magnitudes of sea-level lowstands of the past 500,000 years

Abstract: Existing techniques for estimating natural fluctuations of sea level and global ice-volume from the recent geological past exploit fossil coral-reef terraces or oxygen-isotope records from benthic foraminifera. Fossil reefs reveal the magnitude of sea-level peaks (highstands) of the past million years, but fail to produce significant values for minima (lowstands) before the Last Glacial Maximum (LGM) about 20,000 years ago, a time at which sea level was about 120 m lower than it is today1,2,3,4. The isotope method provides a continuous sea-level record for the past 140,000 years (ref. 5) (calibrated with fossil-reef data6), but the realistic uncertainty in the sea-level estimates is around ±20 m. Here we present improved lowstand estimates—extending the record back to 500,000 years before present—using an independent method based on combining evidence of extreme high-salinity conditions in the glacial Red Sea with a simple hydraulic control model of water flow through the Strait of Bab-el-Mandab, which links the Red Sea to the open ocean. We find that the world can glaciate more intensely than during the LGM by up to an additional 20-m lowering of global sea-level. Such a 20-m difference is equivalent to a change in global ice-volume of the order of today's Greenland and West Antarctic ice-sheets.

Live benthic foraminiferal faunas off Cape Blanc, NW-Africa: Community structure and microhabitats

Abstract: Live (Rose-Bengal stained) benthic foraminifera were studied along a transect across the main area of organic matter deposition in the Cape Blanc upwelling region. The faunal analyses suggest that at the shallowest station (1200 m) the benthic ecosystem is permanently influenced by the upwelling, whereas at the deepest stations (3010 and 2530 m depth) the ocean bottom is subject to significant organic influxes only in summer. The vertical zonation of foraminiferal species in the sediment shows a close correspondence with the depth distribution of oxic respiration, nitrate and sulphate reduction. It is suggested that this linkage is caused by the presence of various stocks of anaerobic and sulphate- and nitrate-reducing bacteria. Deep infaunal foraminiferal species are thought to feed selectively, either on the bacterial stocks or on nutritious particles produced by bacterial degradation of more refractory organic matter. As such, foramininiferal microhabitats are only indirectly controlled by pore water oxygen concentrations.

Bathymetric distribution and microhabitat partitioning of live (Rose Bengal stained) benthic Foraminifera along a shelf to bathyal transect in the southern Adriatic Sea

Abstract: Rose Bengal stained foraminifera from box cores, collected in the southern Adriatic Sea along a transect ranging from 146 to 1,200 m water depth, have been studied. Total numbers of supposedly deposit-feeding foraminifera decrease in a fairly regular manner with increasing water depth and distance from land, probably as a consequence of a decreasing flux of organic matter. Suspension-feeding astrorhizid taxa have an irregular distribution, apparently not related to water depth. Highest numbers of stained foraminifera are invariably found at the sediment surface, whereas numbers decrease exponentially deeper in the sediment. Although most species have maximum densities near the sediment surface, a few of the rarer species are concentrated at deeper levels in the sediment. The vertical distribution patterns of a number of common species are variable with depth along the transect, apparently determined by several different environmental parameters.