Deep water in the western Mediterranean: peculiar 1999 and 2000 characteristics, shelf formation hypothesis, variability since 1970 and geochemical inferences

Abstract

Time series of hydrological data acquired in the Ligurian Sea (western Mediterranean) over the 1993–2000 period allowed us to complement the monitoring of the multi-annual increases in temperature and salinity (3.4×10 −3 °C and 1.05×10 −3 per year, mean values at depth greater than 2000 m since 1959). They also revealed peculiarities of dense water characteristics. Following a 7-year period (1993–1998) with potential temperature and salinity continuously decreasing between 1000-m depth and the bottom (∼2700 m), the 1999 and 2000 data evidenced a positive anomaly, i.e., increases of potential temperature and salinity in the last hundred meters above the bottom. Records of currents and temperature on the continental slope of the Gulf of Lions, particularly in the Lacaze-Duthiers canyon, as well as wind and heat fluxes data indicated that cascading of dense water occurred in February 1999. The transport of turbid shelf water and re-suspended sediment might explain the near-bottom turbidity anomaly found concomitantly with the hydrological anomaly. Besides, hydrological data collected during summer 1999 in the Algero-Provençal basin showed a spreading of the anomalous bottom water, radiating from the Gulf of Lions at a mean speed of 3.5 cm s −1 over the whole basin. The analysis of historical data suggested that over the 1971–2000 period, four events of intense cascading of dense water from the Gulf of Lions' shelf (in 1971, 1980, 1988 and 1999) affected the hydrology of the basin and contributed to the formation of western Mediterranean deep water. Though these cascading events contribute little to the overall dense water formation, which mostly occurs off-shelf (MEDOC area), they probably have a rather important role in the transfer of matter.