Effects of seasonal deposition-erosion cycle on sedimentary organic carbon remineralization and oxygen consumption in a large-river delta-front estuary.

ABSTRACT

Seasonal sediment deposition-erosion events are dominant drivers of particle-solute dynamics in large-river delta-front estuaries (LDEs), but their influence on elemental cycles is not yet fully understood. To better constrain the role of deposition-erosion events on elemental cycling in LDEs, benthic fluxes of dissolved inorganic carbon (DIC), oxygen, and pore-water solute profiles were measured over different seasons in the Changjiang LDE. Benthic DIC efflux (23.4 ± 6.0 mmol C m-2 d-1) was greater than oxygen influx (7.5 ± 2.0 mmol O2 m-2 d-1) in summer but less in winter (7.7 ± 1.2 mmol C m-2 d-1 and 10.1 ± 1.5 mmol O2 m-2 d-1, respectively). The additional oxygen consumption in sediments in winter was likely due to the oxidation of inorganic diagenetic reductive products (IDRP) (e.g., NH4+, Fe2+, and Mn2+) in deeper sediments exposed by erosion, which resulted in the development of an "oxygen debt". Sedimentary oxygen respiration accounted for at least 48 % of total oxygen consumption (oxygen consumption in both water column and sediment) in winter and was significantly greater than in summer (∼15 %); this highlighted the importance of winter sediment erosion in oxygen depletion. In addition to IDRP oxidation, the remineralization of resuspended sedimentary organic carbon in water column also contributed to the oxygen consumption. The global dataset on benthic DIC and oxygen fluxes provides evidence that the "oxygen debt" is likely to be widespread in LDEs, exerting a significant impact on global carbon and oxygen cycling.