Hydrographic, nutrient, and molecular dissolved organic matter data including thermogenic compounds from the Indian Ocean during R/V SONNE cruise SO305/2 (2024) (E‐POLIO‐T; M2Argo‐T)

Hydrographic, biogeochemical, and dissolved organic matter (DOM) molecular data were collected during the R/V SONNE expedition SO305/2 (16 July - 6 August 2024) along a transect from 5° N to 20° S in the Indian Ocean. Continuous surface measurements and CTD profiles provided calibrated temperature, salinity, and oxygen data across major water masses from the surface to deep layers. A total of 176 filtered seawater samples from surface transects and vertical profiles were analysed for inorganic nutrients (nitrate, nitrite, phosphate, silicate) using continuous flow analysis, and for dissolved organic carbon (DOC), total dissolved nitrogen (TDN) via high-temperature catalytic oxidation and dissolved organic nitrogen (DON). Polycyclic aromatic hydrocarbons (PAHs) and dissolved black carbon (DBC) were determined using solid-phase extraction and used as proxies for thermogenic organic matter. Ultrahigh-resolution molecular characterization of solid-phase extracted DOM was conducted using FT-ICR-MS (7 T, negative ESI), with molecular indices (e.g., AIMOD, IDEG, ITERR, IBIO and IPHOTO) calculated to assess compositional and source variability. The combined dataset provides a comprehensive view of hydrographic structure, nutrient dynamics, and the composition and dynamics of DOM pool and thermogenic DOM across contrasting Indian Ocean water masses.