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Groundwater mixing in an alkaline paleolake: Eocene Green River Formation, Wyoming
Palaeogeography, Palaeoclimatology, Palaeoecology ( IF 3 ) Pub Date : 2021-01-01 , DOI: 10.1016/j.palaeo.2020.110038
M'bark Baddouh , Alan R. Carroll , Elliot A. Jagniecki , Brian L. Beard , Tim K. Lowenstein , Clark M. Johnson

Abstract 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 CO2 exchange, and organic matter burial. However, δ13C and δ18O in this study also covary strongly with 87Sr/86Sr, a new finding that is inconsistent with previously proposed mechanisms. We conclude that Little Mesa isotopic trends reflect mixing of groundwater with low 87Sr/86Sr, δ18O and δ13C and lake water with opposite characteristics. Low 87Sr/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 Gosiute 87Sr/86Sr. Previous studies have shown that the lowest 87Sr/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 between 87Sr/86Sr ratios, δ13C and δ18O in closed paleolake systems.

中文翻译:

地下水在碱性古湖中混合:怀俄明州始新世绿河组

摘要 Bridger 盆地北部 Little Mesa 地区的 Tufa 已被解释为通过始新世 Gosiute 湖海岸线附近发出的水下和空中泉水记录碳酸盐沉积。沉积相记录了整体海侵,最终形成了高达 9 m 和直径 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,定义了 ~1 的斜率。在许多其他古湖泊沉积物中也出现了类似的趋势,它们被解释为反映了蒸发、大气 CO2 交换和有机物质埋藏的变化。然而,本研究中的 δ13C 和 δ18O 也与 87Sr/86Sr 强烈共变,这一新发现与先前提出的机制不一致。我们得出结论,Little Mesa 同位素趋势反映了地下水与低 87Sr/86Sr 的混合,δ18O和δ13C与湖水特征相反。地下水中的低 87Sr/86Sr 可能是由于与西面的 Sevier 褶皱和逆冲带内的海相碳酸盐岩地层相互作用造成的,而北、东和南以 Gosiute 湖为界的前寒武纪岩心隆起的排水是造成湖水较高的原因比率。Little Mesa 碳酸盐岩相比盆地中心附近的任何时间等效相都具有较低的放射成因,这意味着 Gosiute 湖 87Sr/86Sr 的水平和垂直梯度。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了以下假设:地表径流和来自 Sevier 带的泉水的气候调制是导致 Gosiute 湖的岁差尺度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泊系统中 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贡献者。
更新日期:2021-01-01
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