K. Stansfield

Hydrothermal vent fields and chemosynthetic biota on the world's deepest seafloor spreading centre.

The Mid-Cayman spreading centre is an ultraslow-spreading ridge in the Caribbean Sea. Its extreme depth and geographic isolation from other mid-ocean ridges offer insights into the effects of pressure on hydrothermal venting, and the biogeography of vent fauna. Here we report the discovery of two hydrothermal vent fields on the Mid-Cayman spreading centre. The Von Damm Vent Field is located on the upper slopes of an oceanic core complex at a depth of 2,300 m. High-temperature venting in this off-axis setting suggests that the global incidence of vent fields may be underestimated. At a depth of 4,960 m on the Mid-Cayman spreading centre axis, the Beebe Vent Field emits copper-enriched fluids and a buoyant plume that rises 1,100 m, consistent with >400 °C venting from the worlds deepest known hydrothermal system. At both sites, a new morphospecies of alvinocaridid shrimp dominates faunal assemblages, which exhibit similarities to those of Mid-Atlantic vents.

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.

High-resolution observations of the path of the overflow from the Sicily Strait

High-resolution velocity and hydrographic measurements, including data acquired by the autonomous underwater vehicle AUTOSUB-2, are used to provide a detailed description of the path of the deep and intermediate overflow waters in the north-west region of the Sicily Strait. There are two branches of the overflow: one passing through the western sill and the other passing through the eastern sill. The western branch of the overflow is seen to circulate around the central bank separating the two sills and head back towards the eastern Mediterranean before turning northwards again. It then follows a path similar to that of the eastern branch but at a deeper level. Although weak, the western branch is still identifiable 60 km downstream of the sills and lies below the eastern branch of the overflow. The net transport is estimated to be 1 Sv which is consistent with other studies. Evidence of tidal and sub-tidal variability in the overflow is also presented