Magnesium, calcium and strontium in waters of the southern Tasman Sea at the confluence of the Indian, Pacific and Southern Oceans

Abstract

Data are presented for magnesium (Mg), calcium (Ca) and strontium (Sr) analyses for surface samples and conductivity–temperature–depth profiles of waters along three main transects across the Subtropical Front (STF) and the Subantarctic Front (SAF) in a region between Tasmania and New Zealand down to 57°S. This area is at the confluence of the Indian, Pacific and Southern Oceans where the Subantarctic Mode Water (SAMW) is formed and which is characterized by well-mixed and thick (down to 1000 m) thermostads with uniform salinity and temperature. Such thermostads form at the end of winter, and two of such mode water-forming areas were sampled in October–November 1998. Magnesium and Ca are proving to be quasi-conservative elements down the water column, but there is systematic depletion for Mg compared with profiles measured in the North and South Atlantic Oceans. This offset is interpreted to be the result of hydrothermal activity in the region at the south-east Indian Ridge, which scavenges some Mg. The Circumpolar Deep Water (CDW) intersects this Ridge and upwells south of the SAF and eventually is transformed in the upper 1000 m of the water column into the SAMW. This explains the uniform Mg depletion throughout the entire depth profiles. Strontium, on the other hand, displays significant shifts near the ocean surface at many locations, and this is interpreted as the result of biogenic productivity, which engenders the precipitation of celestite microcrystals.