EVIDENCE OF CORROSIVENESS WATER MASSES IN THE SOUTH ATLANTIC

Over 24 core tops samples taken from water depths around 750m to 1950m in the slope of Campos Basin, Southeastern South Atlantic Ocean, were investigated for the benthic foraminifera assemblages. Four assemblages were grouped using a cluster analysis, which appear to have a close relation with the vertical stratification of water masses in the South Atlantic. Assemblage I is related to Antarctic Intermediate Waters (AAIW), assemblages II and III with Circumpolar Deep Waters (CDW) and assemblage IV with North Atlantic Deep Waters (NADW).

Dissolution is evident on the calcareous benthic foraminifera tests in all. A detailed investigation of those tests was carried out to evaluate the calcium carbonate dissolution rates at a micro scale.

A visual degree of dissolution was created to categorize any specimen taxonomically classified: 0 (no dissolution evidence was found); 1 (weak dissolution features, less than 5% of the test); 2 (medium dissolution level, some parts missing, species level, less than 25% of the test); 3 (Intense dissolution, more destructive, genera level, less than 50% of the test); 4 (very intense dissolution, obliterate diagnostic features, more than 60% reached the test).

Hoegludina elegans (d´Orbigny) is the only widespread species at the study area and has a test composed by aragonite, which is more susceptible to dissolution. For this species an interval with increase of dissolution was observed between 1000m to 1700m of water depth.

In an attempt to evaluate the dissolution, a dissolution index was created. For this evaluation, only calcareous benthic foraminifera were taken into account. As seen before, the same water depths of H. elegans have an increase of the dissolution, between 1000m to 1700m.

One hypothesis for the related dissolution seems to a specific bathymetric range is corrosiveness of deep-water masses. The interval with increase dissolution (1.000 to 1.700m) overlaps the flux of the Circumpolar Deep Waters, which are in fact the lower part of the Antarctic Intermediate Waters (AAIW). Chemical parameters from GEOSECS were used to evaluate the corrosiveness of the water masses. Alkalinity and CO₂ profiles shows an increase of those indexes at the same water depths, corroborating that Circumpolar Deep Waters are more corrosive than the others (NADW and AAIW).