Organomineral nanocomposite carbon burial during Oceanic Anoxic Event 2

[摘要] Organic carbon (OC) enrichment in sediments deposited during Oceanic AnoxicEvents (OAEs) is commonly attributed to elevated productivity and marineanoxia. We find that OC enrichment in the late Cenomanian aged OAE 2 at theDemerara Rise was controlled by the co-occurrence of anoxic bottom water,sufficient productivity to saturate available mineral surfaces, and variabledeposition of high surface area detrital smectite clay. Redox indicators showconsistently oxygen-depleted conditions, while a strong correlation betweenOC concentration and sediment mineral surface area (R² = 0.92) occursacross a range of total organic carbon (TOC) values from 9 to 33%. X-raydiffraction data indicate the intercalation of OC in smectite interlayers,while electron, synchrotron infrared and X-ray microscopy show an intimateassociation between clay minerals and OC, consistent with preservation of OCas organomineral nanocomposites and aggregates rather than discrete,μm-scale pelagic detritus. Since the consistent ratio between TOCand mineral surface area suggests that excess OC relative to surface area islost, we propose that it is the varying supply of smectite that best explainsvariable organic enrichment against a backdrop of continuous anoxia, which isconducive to generally high TOC during OAE 2 at the Demerara Rise. Smectiticclays are unique in their ability to form stable organomineral nanocompositesand aggregates that preserve organic matter, and are common weatheringproducts of continental volcanic deposits. An increased flux of smectitecoinciding with high carbon burial is consistent with evidence for widespreadvolcanism during OAE 2, so that organomineral carbon burial may represent apotential feedback to volcanic degassing of CO₂.

[学科分类] 地球化学与岩石