Winter dissolved and particulate nickel concentrations along 30 deg E transect, Southern Ocean: GEOTRACES GIPr07 cruise

The data describes the depth profiles of seawater concentrations of dissolved nickel (dNi, 0.2 µm filtered seawater) and particulate nickel (pNi, collected on 0.45 µm pore size filter) along the 30 deg E transect in the Indian sector of Southern Ocean. 

The output data of an Optimum Multiparameter Analysis (OMP) is also included. The goal of the OMP was to calculate the water mass fractions at each discreet sample location. Six water masses were considered, Subantarctic Surface Water (SASW), Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW), Upper Circumpolar Deep Water (UCDW), Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW). Samples with depths <200 m and/or temperature >10°C were excluded from the analysis to avoid the influence of surface biological processes on nutrient concentrations. Water masses were defined by endmember compositions of temperature, salinity, oxygen, nitrate, phosphate and silicate sourced from property-property plots. A linear fit regression model was used to predict the endmember dNi concentrations of water masses, and then reconstruct the dNi concentrations by direct multiplication of water mass fractions with their endmember compositions. Therefore, the OMP can be used to assess whether the dNi distributions can be reconstructed by water mass mixing without invoking other processes. The O2 deficit (calculated as the difference between measured and predicted O2 concentrations) was used to estimate the amount of remineralization that occurred relative to the endmember composition based on the Redfield ratio. Assuming the O2 deficits were solely attributed to remineralization of sinking phytoplankton cells, the amount of remineralized dNi was estimated by extending the Redfield ratio to include Ni, and based on estimates of phytoplankton cellular pNi:pP quotas from the literature.The uncertainties of water mass contributions in the OMP analysis were estimated by summing the residuals of temperature, salinity, oxygen, and nutrients scaled by their weights.