Differential Drivers of Thermogenic and Bulk Dissolved Organic Matter in the Indian Ocean

Abstract The oceanic dissolved organic matter (DOM) pool contains a substantial fraction of recalcitrant carbon, with thermogenic organic compounds representing a chemically distinct, long‐lived component. However, the distribution and transformation of these compounds remain poorly constrained in the Indian Ocean, a key basin of the global overturning circulation that connects the Atlantic and Pacific Oceans through the Circumpolar Current. Here, we present the first data set that resolves dissolved black carbon (DBC) and polycyclic aromatic hydrocarbons (PAHs) distribution in the Indian Ocean along a surface transect and at 11 depth profiles between 5°N and 20°S. Combining the targeted analyses with molecular‐level DOM characterization, we assessed the impact of oxygen availability, water column structure, and monsoon‐driven circulation on thermogenic DOM distribution. Surface PAH concentrations were influenced by atmospheric inputs and oceanographic features, while DBC was more homogeneously distributed, with no distinct patterns, presumably reflecting various sources, transformation pathways and sinks. Below the mixed layer, PAH concentration decreased with increasing oxygen consumption, suggesting partial degradation or sorption to particles, while DBC concentrations remained constant, showing no correlation with apparent oxygen utilization (AOU). DOM molecular features such as the degradation index ( I DEG ) and the “Island of Stability” (IOS) increased with AOU, indicating degradation and selective preservation of recalcitrant molecules. Our results highlight the contrasting behaviors of thermogenic markers across biogeochemical gradients and emphasize the role of the Indian Ocean in contributing to the global refractory DOM reservoir by integrating both atmospheric and riverine thermogenic inputs into deep ocean.