Journal of Earth System Science ( IF 1.9 ) Pub Date : 2021-05-29 , DOI: 10.1007/s12040-021-01611-4 Prosenjit Ghosh , Leena Ramdas , Yogaraj Banerjee , S Thamizharasan , Sanchita Banerjee
Abstract
Seawater incursion and freshwater discharges into the tropical Mandovi Zuari (MZ) estuary is investigated here using stable oxygen isotope data on monthly water samples from locations spatially separated within the estuary. Surface water samples were analysed for δ18O and salinity relationship,
$$\delta^{18} O = 0.0843 \times S - 2.1,$$where freshwater end member is designated as −2.1‰; similar to the composition recorded for the rain water. We have estimated percentage of monthly freshwater fluxes into the estuary using mass balance equation adopting an isotopic value of seawater end member. Our estimates showed freshwater contribution to the estuary water during monsoon time was 63%, while in the post-monsoon season the fraction drops to 34.7%. The contribution of the freshwater registered a minimum value of 6.8% during the pre-monsoon season. Further, we analysed the seasonal growth band secreted by the mollusc from the same estuary to understand the potential of mollusc as a recorder of seasonal water composition. The δ18O of mollusc shell growth layers varied over a range between −4.3‰ and −2.1‰. We interpreted this as seasonal signal assuming the growth rate from the culture experiment. The δ18O of estuary water and observed temperature are used to simulate the isotopic composition of seasonal growth bands. The lighter δ18O value of −4.3‰ precipitated during the month of July 2010, which coincides with the time of low productivity (δ13C = −3.5‰). While the heavier δ18O (−2.1‰) is recorded in the growth layer generated during November 2010 defining the period of post-monsoon growth.
Research Highlights
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Here we presented our observation on the spatiotemporal variability of salinity and δ18Owater across a tropical estuary, Mandovi Zuary located at the western coast of India and fed by rivers originating from the peninsular region which receives rainfall during period of Indian summer monsoon.
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We designed a two-component mixing model for the estimation of freshwater fluxes at monthly time intervals and demonstrated its efficacy with the tide gauge data.
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The study highlighted the scope of isotope mass balance approach in estimating freshwater fluxes at seasonal time domain.
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Further, we demonstrated that the isotopic signature preserved in the growth bands of the bivalve shell can be useful proxy for the reconstruction of freshwater fluxes in an estuary and thus add a new tool for reconstruction of seasonal runoff.
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The isotope approach used here can also be extended to other estuaries in the peninsular India and also in other tropical continental settings where hydrological process is driven by seasonal reversal of wind direction.
中文翻译：
使用来自印度果阿邦热带 Mandovi Zuari 河口的软体动物估算季节性淡水通量
摘要
这里使用稳定氧同位素数据对来自河口内空间分离位置的每月水样进行调查，研究海水入侵和淡水排放到热带 Mandovi Zuari (MZ) 河口。对地表水样品进行δ 18 O 和盐度关系分析，
$$\delta^{18} O = 0.0843 \times S - 2.1,$$其中淡水端构件指定为-2.1‰；类似于为雨水记录的成分。我们使用采用海水端元同位素值的质量平衡方程估算了进入河口的每月淡水通量的百分比。我们的估计表明，季风期间淡水对河口水的贡献为 63%，而在季风后季节，这一比例下降到 34.7%。在季风前季节，淡水的贡献值最低为 6.8%。此外，我们分析了来自同一河口的软体动物分泌的季节性生长带，以了解软体动物作为季节性水成分记录器的潜力。该δ 18软体动物壳生长层的 O 在 -4.3‰ 和 -2.1‰ 之间的范围内变化。假设来自培养实验的增长率，我们将此解释为季节性信号。河口水的δ 18 O和观测温度用于模拟季节性生长带的同位素组成。较轻的δ 18 O值-4.3‰在2010年7月析出，恰逢生产力低下时期（δ 13 C = -3.5‰）。而较重的δ 18 O（-2.1‰）被记录在2010年11月期间产生的限定后季风生长期的生长层。
研究亮点
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在这里，我们展示了我们对位于印度西海岸的 Mandovi Zuary 热带河口的盐度和δ 18 O水的时空变异性的观察结果，该河口来自半岛地区的河流，在印度夏季风期间接收降雨。
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我们设计了一个双组分混合模型，用于估计每月时间间隔的淡水通量，并用潮汐测量仪数据证明了其有效性。
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该研究强调了同位素质量平衡方法在估计季节性时域淡水通量方面的范围。
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此外，我们证明了双壳贝壳生长带中保存的同位素特征可以作为重建河口淡水通量的有用代表，从而为重建季节性径流增加了一个新工具。
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这里使用的同位素方法也可以扩展到印度半岛的其他河口以及其他热带大陆环境，在这些环境中，水文过程是由风向的季节性反转驱动的。
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