Physical and biochemical aspects of the flow across the Mascarene Plateau in the Indian Ocean.

This paper presents results from a detailed hydrographic survey of the Mascarene Plateau and surrounding area undertaken by the RRS Charles Darwin in June-July 2002. We examine how the westward-flowing South Equatorial Current (SEC) crosses the plateau, and how the structure of the flow determines the supply of nutrients to the surface waters. We find that the flow of the SEC across the plateau is highly dependent on the complex structure of the banks which make up the plateau, and that a large part of the flow is channelled between the Saya de Malha and Nazareth Banks. Furthermore, the SEC forms a sharp boundary between subtropical water masses from further south, which are low in nutrients, and waters from further north, which are relatively nutrient rich. Overall, the SEC delivers relatively high levels of nutrients to the near-surface waters of the central and northern regions of the plateau, compared with the southern regions of the plateau. This is partly due to uplifting of density surfaces through Ekman suction on the northern side of the SEC, and partly due to the higher levels of nutrients on those density surfaces on the northern side of the SEC. This may drive increased production of phytoplankton in these areas, which would in turn be expected to fuel increased abundances of zooplankton and higher levels of the food chain.

Sea-level fluctuations during the last glacial cycle.

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution or higher. Records based on oxygen isotopes, however, contain uncertainties in the range of +/-30 m, or +/-1 degrees C in deep sea temperature. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth-and hence global sea level-over the past 470,000 years (470 kyr). Our reconstruction is accurate to within +/-12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr(-1), occurred, coincident with abrupt changes in climate.