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.

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

Palaeolithic landscapes of Europe and environs, 150,000-25,000 years ago: An overview

[1] To develop a better understanding of the abrupt Dansgaard-Oeschger mode of climate change, it is essential that we establish whether the ice sheets are actively involved, as trigger or amplifier, or whether they merely respond in a passive manner. This requires careful assessment of the fundamental issues of magnitude and phasing of global ice volume fluctuations within marine isotope stage 3 (MIS 3), which to date remain enigmatic. We review recent advances in observational studies pertaining to these key issues and discuss the implications for modeling studies. Our aim is to construct a robust stratigraphic framework for the MIS 3 period regarding sea level variability, using the most up-to-date arguments available by combining insights from both modeling and observational approaches.

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Marine isotope stage 3 sea level fluctuations: Data synthesis and new outlook

Results are presented of a high-resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the Cariaco Basin in the southern Caribbean Sea. The Cariaco Basin is a small anoxic marine basin on the northern continental margin of Venezuela in an area today characterized by both seasonal trade wind-induced upwelling and pronounced dry and wet seasons. Our data indicate that large changes in the intensity of upwelling, and hence trade wind strength, occurred in this region during the last glacial-interglacial transition and throughout the Holocene. During the last glacial lowstand of sea level, the Cariaco Basin was effectively isolated from the open Caribbean along its northern margin by the then largely emergent Tortuga Bank. Oxic conditions existed in the deep Cariaco Basin at this time, and surface productivity was low. About 12,600 years ago, the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the Laurentide Ice Sheet into the Gulf of Mexico. Strong upwelling between 12,600 and about 10,000 years ago may be related to intensified trade winds resulting, in part, from cooler sea surface temperatures in the Caribbean and Gulf of Mexico. After about 10,000 years ago, upwelling intensity was reduced, though highly variable. A preliminary frequency domain analysis of the Holocene portion of the Cariaco Basin time series suggests that solar forcing may explain a significant component of the century-scale variability observed in the record of upwelling and trade wind strength.

A high-resolution late Quaternary upwelling record from the anoxic Cariaco Basin, Venezuela

In response to thermodynamic driving forces, the domains in microphase-separated block copolymers have distinct intermaterial dividing surfaces (IMDS). Of particular interest are bicontinuous and tricontinuous, triply periodic morphologies and their mathematical representations. Level surfaces are represented by functions F: R3 f R of points (x, y, z) 2 R3, which satisfy the equation F(x, y, z) ) t, where t is a constant. Level surfaces make attractive approximations of certain recently computed triply periodic constant mean curvature (cmc) surfaces and they are good starting surfaces to obtain cmc surfaces by mean curvature flow. The functions F(x, y, z) arise from the nonzero structure factors F(hkl) of a particular space group, such that the resulting surfaces are triply periodic and maintain the given symmetries. This approach applies to any space group and can, therefore, yield desired candidate morphologies for novel material structures defined by the IMDS. We present a technique for generating such level surfaces, give new examples, and discuss certain bicontinuous cubic IMDS in detail.

Triply Periodic Bicontinuous Cubic Microdomain Morphologies by Symmetries

Previous studies of the Red Sea demonstrated that glacial surface-and bottom-water salinities in the basin were significantly higher than at present. The very low abundance of planktonic foraminifera, the so-called 'aplanktonic zone', during the last glacial indicates that surface-water conditions approached or exceeded the tolerance limits of this plankton group1,2. Glacial sediments are also characterized by high concentrations of magnesian calcite3,4, dolomite3, inorganically precipitated aragonite4,5 and benthic foraminifera typical of hypersaline environments3,6. Additional evidence from oxygen isotope records of planktonic2,7 and benthic foraminifera8,9, as well as pteropods2, demonstrate that glacial–interglacial contrasts in the Red Sea have an amplitude much larger than typically observed in open ocean records. Here we use both oxygen isotope data and a 'fractional overmixing' model to estimate the impact of the most recent (18,000 yr BP) Pleistocene glacio-eustatic sea-level lowering on Red Sea salinity. We estimate that during the last glacial maximum, surface salinities in the central Red Sea were more than 10.0‰ higher than at present. Deep-water salinities were also higher during the last glaciation and remained unusually high through deglaciation. The combination of very high bottom salinities and the onset of pluvial conditions during deglaciation in the Red Sea region prevented ventilation of Red Sea bottom waters and resulted in the accumulation of organic-rich sediments.

Sharon M. Locke

Papers

Glacio-eustatic sea-level control on Red Sea salinity

Paleoceanographic record of the last glacial/interglacial cycle in the Red Sea and Gulf of Aden