MAGNITUDES OF SEA-LEVEL LOWSTANDS OF THE PAST 500,000 YEARS

Existing techniques for estimating natural fluctuations of sea level a nd global ice-volume from the recent geological past exploit fossil co ral-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 min ima (lowstands) before the Last Glacial Maximum (LGM) about 20,000 yea rs ago, a time at which sea level was about 120 m lower than it is tod ay(1-4). The isotope method provides a continuous sea-level record for the past 140,000 years (ref. 5) (calibrated with fossil-reef data(6)) , but the realistic uncertainty in the sea-level estimates is around /- 20 m. Here we present improved lowstand estimates - extending the r ecord back to 500,000 years before present - using an independent meth od based on combining evidence of extreme high-salinity conditions in the glacial Red Sea with a simple hydraulic control model of water flo w 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 du ring 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.