Estuarine carbonate chemistry predicts that thermodynamic equilibration during the mixing of freshwater with seawater will generate a carbon dioxide (CO₂) sink in the case of warm and poorly buffered tropical rivers. The São Francisco River estuary has historically become oligotrophic after the construction of a series of hydroelectric dams in its watershed, where organic matter and nutrients are retained. During two cruises in late winter (Aug. 2014) and early summer (Nov. 2015), dissolved inorganic carbon (DIC) and total alkalinity (TA) were found to increase linearly with salinity in the main estuarine channel, where water mixing required half a day, and showed nearly conservative behaviour. In the main channel, the water partial pressure of CO₂ (pCO₂) recorded at a 1-min frequency followed an asymmetric bell-shaped trend versus salinity, similar to the curve predicted by the thermodynamic conservative mixing of freshwater DIC and TA with seawater DIC and TA. The low (0–3) salinity region was always a source of atmospheric CO₂, where despite low chlorophyll concentrations, a pCO₂ diurnal change of approximately 60 ppmv suggested the occurrence of photosynthesis in summer. At salinities above 3, undersaturated pCO₂ values (down to 225 ppmv in winter and neap tides) and invasion of atmospheric CO₂ of 0.38–1.70 mmol m⁻¹ h⁻¹ occurred because of the predominating thermodynamics during estuarine mixing. In winter and neap tides, the higher river discharge, intense estuarine mixing, lower temperatures and limited tidal pumping resulted in observed pCO₂ differences from the theoretical conservative pCO₂ by less than 3 ppmv at salinities >3. Conversely, in summer and spring tides, the recorded pCO₂ values were on average + 43 ± 35 ppmv above the conservative mixing curve, when tidal pumping, CO₂ invasion and surface heating were more significant in the mixing zone but not sufficient to offset the thermodynamic uptake of atmospheric CO₂. By combining carbonate chemistry with estuarine mixing modelling and gas exchange calculations, we estimate that heating contributed to approximately 15% and gas exchange contributed to approximately 10% of the positive pCO₂ deviation from conservative mixing during summer. The remaining 75% of the deviation reached its maximum at ebb tides and within salinity ranges consistent with the occurrence of tidal pumping from marches and mangrove soils. Indeed, in the mangrove channel, water was supersaturated, with pCO₂ values of 976 ± 314 ppmv, while in the main channel, the highest positive pCO₂ deviations from conservative mixing (up to +100 ppmv for several hours) occurred at ebb tides. An important finding was that in São Francisco, the thermodynamic and biological processes compete with each other for CO₂ fluxes both at low salinities where evasion and autotrophy occur and at high salinities where invasion, heterotrophy and tidal pumping occur. Our study suggests that carbonate thermodynamics during mixing is a key process that has been overlooked in estuarine studies, although they can generate important air-water CO₂ exchange and significantly contribute to the carbon budget of estuaries and river plumes.

碳酸盐河口化学预测，在热带和缓冲性较差的热带河流中，淡水与海水混合过程中的热力学平衡会产生二氧化碳（CO ₂ ）汇。在流域建造了一系列水电大坝之后，圣弗朗西斯科河河口历来变得贫营养，其中保留了有机物和养分。在冬季后期（2014年8月）和夏季初夏（2015年11月）的两次航行中，发现主要河道中的溶解无机碳（DIC）和总碱度（TA）与盐度呈线性关系，其中混合水需要一半一天，并且表现出近乎保守的行为。在主通道中，CO ₂（p以1分钟的频率记录的CO ₂）遵循不对称的钟形趋势与盐度的关系，类似于淡水DIC和TA与海水DIC和TA的热力学保守混合所预测的曲线。低（0–3）盐度区域始终是大气CO ₂的来源，尽管叶绿素浓度较低，但p CO _2的日变化约为60 ppmv，表明夏季发生了光合作用。在高于3的盐度下，p CO _2的饱和度不足（冬季和潮汐时低至225 ppmv），大气中的CO ₂侵入量为0.38–1.70 mmol m ^-1  h ^-1发生这种现象的原因是在河口混合过程中主要的热力学。在冬季和潮汐潮中，较高的河流流量，剧烈的河口混合，较低的温度和有限的潮汐抽水导致在盐分> 3时观察到的p CO ₂与理论保守的p CO ₂的差异小于3 ppmv。相反，在夏季和春季的潮汐中，记录的p CO ₂值平均比保守混合曲线高+ 43±35 ppmv，当潮汐泵送，CO ₂入侵和表面加热在混合区中更为明显，但不足以抵消时大气中CO ₂的热力学吸收。通过将碳酸盐化学与河口混合模型和气体交换计算相结合，我们估计在夏季，加热导致约15％的正CO ₂偏离正向混合气体，而气体交换则引起约10％的正P CO ₂偏离。剩余的75％的偏差在退潮时达到最大值，并在盐度范围内，这与行军和红树林土壤中的潮汐泵吸现象一致。事实上，在信道红树，将水过饱和，用p CO ₂ 976±314 ppmv的的值，而在主信道的最高正p CO ₂退潮时发生了保守混合的偏差（持续几个小时达到+100 ppmv）。一个重要发现是，在圣弗朗西斯科，无论是在发生逃逸和自养的低盐度环境还是在发生入侵，异养和潮汐抽水的高盐度环境中，热力学过程和生物过程都相互争夺CO ₂通量。我们的研究表明，碳酸盐混合过程中的热力学是河口研究中忽略的一个关键过程，尽管它们可以产生重要的空气-水CO ₂交换并显着促进河口和河羽的碳收支。