Water Column Cycling of Cadmium in the North-Eastern and Subtropical Gyre Region of the Indian Ocean

The biogeochemical cycling of dissolved cadmium (dCd) in the ocean has been the subject of extensive research, owing to its nutrient-like behavior and resemblance to the macronutrient phosphate (PO₄). In this study, 34 complete vertical profiles were collected during the cruises GI01 and GI06, meridional and zonal transects, respectively, as a part of the GEOTRACES India program from the Indian Ocean to investigate the biogeochemical cycling of dCd and PO₄. The vertical profiles of dCd along the GI01 and GI06 transects show nutrient-like distribution, similar to that of the macronutrient phosphate. The N-S decreasing gradient in the surface dCd concentration suggests the combined role of riverine and atmospheric input to the Bay of Bengal (BoB), like dAl and dCo. Very low dCd content was observed in the surface waters of the oligotrophic subtropical gyre region. In the diatom-rich region of BoB, comparatively higher dCd was observed in the intermediate waters due to the regeneration of Cd-rich POC compared to the other regions. The intense oxygen minimum zone region of the BoB does not show any evidence of Cd loss from the water column through cadmium sulfide precipitation in the anoxic microenvironment. A negative Cd* (−0.25 nmol kg⁻¹) observed at the intermediate water depth in the southern sector of the Indian Ocean marks the advection of Cd-deficient Antarctic Intermediate Water (AAIW). The overall slope obtained from Cd-PO₄ cross plot in the Indian Ocean is ∼0.33 nmol μmol^−1, which is well correlated with the global ocean slope of 0.35 nmol μmol⁻¹.