AMOR-Bflux porewater and sediment data

Estuarine regions are generally considered a major source of atmospheric CO2 as a result of the high organic carbon (OC) mineralization rates in their water column and sediments. Yet, the intensity of anaerobic respiration processes in the sediments tempered by the reoxidation of reduced metabolites near the sediment-water interface controls the flux of benthic alkalinity. This alkalinity may partially buffer metabolic CO2 generated by benthic OC respiration in sediments. Thus sediments with high anaerobic respiration rates could contribute less to local acidification than previously thought. In this study, a benthic chamber was deployed in the Rhône River prodelta and the adjacent continental shelf (Gulf of Lions, NW Mediterranean) in late summer to assess the fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC) from the sediment. Concurrently, in situ O2 and pH microprofiles, voltammetric profiles and pore water composition were measured in surface sediments to identify the main biogeochemical processes controlling the net production of alkalinity in these sediments. Benthic TA and DIC fluxes to the water column, ranging between 14 and 74 mmol m-2 d-1 and 18 and 78 mmol m-2 d-1, respectively, were up to 8 times higher than DOU rates (10.4 ± 0.9 mmol m-2 d-1) close to the river mouth, but their intensity decreased offshore, as a result of the decline in OC inputs. In the zone close to the river mouth, pore water redox species indicated that TA and DIC were mainly produced by microbial sulfate and iron reduction. Despite the complete removal of sulfate from pore waters, dissolved sulfide concentrations were low and significant concentration of FeS were found indicating the precipitation and burial of iron sulfide minerals with an estimated burial flux of 12.5 mmol m-2 d-1 near the river mouth. By preventing reduced iron and sulfide reoxidation, the precipitation and burial of iron sulfide increases the alkalinity release from the sediments during the spring and summer months. Under these conditions, the sediment provides a net source of alkalinity to the bottom waters which mitigates the effect of the benthic DIC flux on the carbonate chemistry of coastal waters and weakens the partial pressure of CO2 increase in the bottom waters that would occur if DIC was produced only.

河口区域通常被认为是大气二氧化碳的主要来源之一，这是由于其水柱与沉积物中的有机碳（OC，organic carbon）矿化速率极高。然而，沉积物-水界面（sediment-water interface）附近还原代谢物的再氧化作用会调控沉积物内厌氧呼吸（anaerobic respiration）过程的强度，进而控制底栖碱度的跨界面通量。这类碱度可部分缓冲沉积物中底栖有机碳呼吸所产生的代谢性二氧化碳。因此，厌氧呼吸速率较高的沉积物，其对当地水体酸化的贡献可能比此前研究预想的更低。本研究于夏末在罗讷河前三角洲及邻近大陆架（西北地中海利翁湾）部署底栖腔室（benthic chamber），以测定沉积物向水体输出的总碱度（TA，total alkalinity）与溶解无机碳（DIC，dissolved inorganic carbon）通量。同时，在表层沉积物中开展了原位氧与pH微剖面、伏安剖面及孔隙水组成的测定，以识别调控该区域沉积物碱度净产生的核心生物地球化学过程。底栖TA与DIC向水体的通量范围分别为14~74 mmol·m⁻²·d⁻¹与18~78 mmol·m⁻²·d⁻¹；在河口附近区域，其通量最高可达溶解氧消耗率（DOU，dissolved oxygen uptake，10.4±0.9 mmol·m⁻²·d⁻¹）的8倍。但随着有机碳输入量的减少，向海方向的通量强度逐渐降低。在河口近岸区域，孔隙水氧化还原物种的分布特征表明，TA与DIC主要通过微生物硫酸盐还原与铁还原过程生成。尽管孔隙水中的硫酸盐已被完全消耗，但溶解态硫化物浓度极低，且检测到大量硫化亚铁（FeS），这说明铁硫化物矿物发生了沉淀与埋藏，河口附近的埋藏通量约为12.5 mmol·m⁻²·d⁻¹。铁硫化物的沉淀与埋藏可阻止还原态铁与硫化物的再氧化，进而在春夏季节提升沉积物向水体释放的碱度总量。在此条件下，沉积物为底层水提供了净碱度来源，可缓解底栖DIC通量对近岸海水碳酸盐化学的负面影响，并削弱仅由DIC产生时底层水二氧化碳分压的升高幅度。