Groundwater mixing in an alkaline paleolake: Eocene Green River Formation, Wyoming
Groundwater mixing in an alkaline paleolake: Eocene Green River Formation, Wyoming
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DOI：
10.1016/j.palaeo.2020.110038
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
M’bark Baddouh;A. Carroll;E. Jagniecki;B. Beard;T. Lowenstein;C. Johnson
中科院分区：
文献类型：
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作者：
M’bark Baddouh;A. Carroll;E. Jagniecki;B. Beard;T. Lowenstein;C. Johnson
布里杰盆地北部小梅萨地区的凝灰岩被解释为通过始新世戈西特湖海岸线附近的水下和陆上泉水记录了碳酸盐沉积。沉积相记录了整体海侵，最终形成了高度达 9 m、直径达 40 m 的土丘结构。土丘在 δ13C 和 δ18O 之间表现出很强的正线性协方差，定义斜率约为 1。类似的趋势也出现在许多其他古湖沉积物中，它们被解释为反映了蒸发、大气二氧化碳交换和有机物埋藏的变化。然而，本研究中的δ13C和δ18O也与87Sr/86Sr强烈共变，这一新发现与之前提出的机制不一致。我们得出结论，小台地同位素趋势反映了低 87Sr/86Sr、δ18O 和 δ13C 地下水与具有相反特征的湖水的混合。地下水中的低 87Sr/86Sr 可能是由于西侧塞维尔褶皱和冲断带内与海相碳酸盐岩地层的相互作用所致，而戈休特湖北部、东部和南部边界的前寒武纪核心隆起的排水导致了较高的湖水比。小台地碳酸盐岩相的放射性成因均低于盆地中心附近的任何时间等效相，这意味着湖中的水平和垂直梯度戈西尤特87Sr/86Sr。先前的研究表明，盆地中心沉积物中最低的 87Sr/86Sr 对应于湖泊高位。这项研究的结果支持这样的假设：塞维尔带地表径流和泉水散发的气候调节是戈西特湖岁差规模扩张和收缩的主要原因。更广泛地说，地下水排放可能是封闭古湖系统中 87Sr/86Sr 比率、δ13C 和 δ18O 之间协方差的一个重要但未被充分认识的因素。
Tufa in the Little Mesa area of the northern Bridger Basin has been interpreted to record carbonate deposition via subaqueous and subaerial springs emanating near the shoreline of Eocene Lake Gosiute. Sedimentary facies record an overall transgression, culminating with mound structures that reach up to 9 m in height and 40 m in diameter. Mounds exhibit a strong positive, linear covariance between δ13C and δ18O, defining a slope of ~1. Similar trends occur in many other paleolake deposits, where they are interpreted to reflect changes in evaporation, atmospheric CO2exchange, and organic matter burial. However, δ13C and δ18O in this study also covary strongly with87Sr/86Sr, a new finding that is inconsistent with previously proposed mechanisms. We conclude that Little Mesa isotopic trends reflect mixing of groundwater with low87Sr/86Sr, δ18O and δ13C and lake water with opposite characteristics. Low87Sr/86Sr in groundwater likely resulted from interaction with marine carbonate strata within the Sevier fold and thrust belt to the west, whereas drainage from Precambrian-cored uplifts that bounded Lake Gosiute to the north, east, and south was responsible for higher lake water ratios.Little Mesa carbonate facies are all less radiogenic than any time-equivalent facies near the center of the basin, implying horizontal and vertical gradients in Lake Gosiute87Sr/86Sr. Previous studies have shown that the lowest87Sr/86Sr in basin center deposits correspond to lake highstands. Results of this study support the hypothesis that climatic modulation of surface runoff and spring emanations from the Sevier belt were principally responsible for precessional-scale expansions and contractions of Lake Gosiute. More broadly, groundwater discharge may represent an important but underappreciated contributor to covariance between87Sr/86Sr ratios, δ13C and δ18O in closed paleolake systems.