Characteristics and potential formation mechanisms of dissolved organic matter at the water-sediment interface and assessment of its release potential in the eastern pacific polymetallic nodule area.

Deep-sea mining in polymetallic nodule areas poses significant risks to the marine carbon sink. This study investigated the vertical distribution and three-dimensional fluorescence spectral characteristics of dissolved organic matter (DOM) in seawater and sediments within the Clarion-Clipperton Zone, as well as the sources and transformation pathways of DOM at the seawater-sediment interface and evaluated potential mining induced DOM release and impacts. Dissolved organic carbon (DOC) and the relative abundance of chromophoric dissolved organic matter (CDOM) decreased from the euphotic to aphotic layers but accumulated slightly in bottom waters, while DOC concentrations in porewater and CDOM abundance in sediments exceeded that in bottom water by orders of magnitude. Fluorescence analysis indicated that phytoplankton production and microbial reprocess dominated in the euphotic layer, whereas continuous microbial degradation and reworking on organic matter were the primary processes in the dysphotic, aphotic, bottom seawaters and sediments. However, the discontinuous CDOM profiles across the water column and sediment layers indicate a dominant origin from particulate organic matter reworked in situ by microbes. This is especially evident in the distinct discontinuity of CDOM signatures between the bottom seawater and surface sediment. Simulations estimated that mining disturbances could release 0.02-0.15 mg/L of sediment derived DOC into bottom water, where it was likely to persist in bottom water. It was suggested that deep-sea mining would induce a moderate and durable increase of DOM in the bottom water, by which substantially disrupting the organic matter distribution patterns of organic matter at the seawater-sediment interface.