Tidal and spatial variations of DI13C and aquatic chemistry in a temperate tidal basin during winter time

Here, the pelagic carbonate system and the d13C signature of dissolved inorganic carbonate (DIC) were investigated in a tidal basin of the southern North Sea, the Jade Bay, with respect to tidal cycles and a transect towards the North Sea in winter time (January and November, 2010). Physical parameters, major and trace elements, and nutrient concentrations were considered, too. Primary production and pelagic organic matter respiration were negligible during winter time. Both, the compositional variations on the transects as well as during the tidal cycles indicate the mixing of North Sea with fresh water. The combined spatial co-variations of different parameters indicate an introduction of fresh water that was enriched in DI12C, metabolites (e.g., ammonia), protons, and dissolved redox-sensitive elements (e.g., Mn2+). During the January campaign, the discharge via the flood gates was limited due to ice cover of the hinterland drainage ditches, allowing for an observation of tidal variations without significant mixing contributions from surface water discharges. Considering a binary mixing model with North Sea and fresh water as end-members, the extrapolated fresh water end-member composition for this campaign is estimated to contain about 3.8 mmol/kg DIC , and enhanced concentrations of NH4+, Mn2+, and protons compared to North Sea water. The fast temporal response of dissolved geochemical tracers on tidal variations in the Jade Bay indicates a continuous supply of a fresh water component. The measured composition of fresh waters entering the Jade Bay via flood gates (end of October, 2010) did not match the values estimated by the binary mixing model. Therefore, the overall fresh water component likely is a mixture between sources originating from flood gates and (in January) dominating submarine groundwater discharge entering the Jade Bay. This model is consistent with the results obtained during the November campaign, when a more important contribution from flood gates is expected and a more variable fresh water end-member is estimated. The co-variations of the concentrations and the stable carbon isotope composition of DIC are applied to evaluate possible superimposed sink-source-transformation processes in the coastal waters and a general co-variation scheme is suggested.本研究针对北海南部亚德湾（Jade Bay）的潮汐海湾，于2010年冬季（1月与11月）围绕潮汐周期及指向北海的断面，对浮游碳酸盐体系（pelagic carbonate system）与溶解无机碳酸盐（dissolved inorganic carbonate, DIC）的d13C同位素组成开展调查。研究同步考量了物理参数、常量与微量元素及营养盐浓度。冬季期间，初级生产力与浮游有机质呼吸作用均可忽略不计。断面及潮汐周期内的组成变化均表明，北海水与淡水发生混合作用。不同参数的综合空间协变关系显示，存在富含轻碳同位素溶解无机碳（DI12C）、代谢产物（如氨）、质子及溶解态氧化还原敏感元素（如二价锰离子Mn²+）的淡水输入。2010年1月航次中，因腹地排水沟结冰覆盖，通过潮闸的径流量受限，得以在无地表径流显著混合贡献的条件下观测潮汐变化。以北海水与淡水作为端元组分的二元混合模型估算结果显示，本次航次外推得到的淡水端元组分中，DIC浓度约为3.8 mmol/kg，且相较于北海水，氨离子（NH4+）、二价锰离子与质子的浓度均有所升高。亚德湾内溶解态地球化学示踪剂对潮汐变化的快速时间响应，表明淡水组分持续补给。2010年10月底通过潮闸输入亚德湾的实测淡水组分，与二元混合模型的估算值并不匹配。因此，整体淡水组分大概率由潮闸来源的淡水与1月航次中占主导的海底地下水排泄（submarine groundwater discharge）混合而成。该模型与11月航次的研究结果一致——彼时潮闸的贡献更为显著，且估算得到的淡水端元变异性更强。本研究借助DIC的浓度与稳定碳同位素组成的协变关系，评估了近岸海域中可能存在的叠加汇-源-转化过程，并提出了一般性的协变关系模式。