Origin of dissolved organic sulfur in marine waters and the impact of abiotic sulfurization on its composition and fate

Abstract

Despite being the largest reservoir of organic sulfur in the ocean, the major source and reactivity of non-volatile dissolved organic sulfur (DOS) pool in the water column remain uncertain. Three experiments were conducted to fill these gaps. In the first one, sulfur isotopic composition (δ34S) of marine and estuarine organic matter, extracted with various methods, indicated that biotic assimilatory sulfate reduction (ASR) is the main formation pathway of DOS in marine waters and of particulate organic sulfur (POS) in oxic sediment. The abiotic sulfurization by sulfides and polysulfides (e.g., in sulfidic sediment) appeared a negligeable contributor to surface DOS but can nevertheless account for up to 39% of deep oceanic DOS. In contrast, abiotic sulfurization appears to be the dominant source of POS in anoxic sediments. In the two other experiments, the possible contribution of sedimentary porewater DOS to the overlaying water column and the stability of DOS in oxic water, before and after abiotic sulfurization, were assessed. Results indicated that the main sources of water-column and sedimentary DOS are different. Laboratory abiotic sulfurization decreased the stability of the dissolved organic matter (DOM) in oxic conditions but seems to increase the proportion of carboxyl-rich alicyclic molecules (CRAM). In addition, the S/C ratio of the DOM changed during degradation: DOS was immobilized in S-poor non‑sulfurized DOM, but DOS was selectively degraded in S-rich sulfurized DOM. The high degradation rate of porewater DOS could explain why it is not the main contributor to deepwater DOS. Biotic ASR appears as the principal source of both labile and non-labile DOS in marine waters. The preservation of DOS may not be related to sulfur speciation or origin but may be controlled by alterations targeting other elements and leaving behind more recalcitrant molecules containing sulfur atoms.