Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes
Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur, calcium, magnesium and clumped isotopes
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DOI：
10.1111/sed.12775
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发表时间：
2020-08
期刊：
影响因子：
3.5
通讯作者：
S. Murray;J. Higgins;C. Holmden;Chaojin Lu;P. Swart
中科院分区：
文献类型：
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作者：
S. Murray;J. Higgins;C. Holmden;Chaojin Lu;P. Swart
为了限制巴哈马新近纪碳酸盐岩中碳酸盐化的机制，并提高对浅水碳酸盐沉积物中化学地层示踪剂使用的理解，已在来自Clino岩心的碳酸盐化间隔中测量了δ 34 S、Δ47、δ 13 C、δ 18 O、δ44/40 Ca和δ 26 Mg值以及Sr浓度，在巴哈马的大巴哈马浅滩和另外两个岩心（Unda和圣萨尔瓦多）的边缘钻探。Unda和圣萨尔瓦多岩心具有大量的石化层段，其碳酸盐相关硫酸盐δ 34 S值与同期海水中发现的值相似，δ44/40 Ca、δ 26 Mg值、Sr含量和Δ47温度（25至30°C）表明流体缓冲系统中的石化作用相对较浅。相比之下，Clino岩心中的结晶化层段具有较高的碳酸盐相关硫酸盐δ 34 S值，表明在更多沉积物缓冲成岩系统中的结晶化作用，其中细菌硫酸盐还原使34 S中的残留SO 42-富集，与高沉积物Sr浓度和低δ44/40 Ca和高δ 26 Mg值一致。只有与硬地有关的斜轴岩芯中的碳酸盐岩具有与海水相似的碳酸盐相关δ 34 S值，表明在沉积间断期间海水对沉积物上层的持续冲刷。硬地面上δ44/40 Ca值更正值的变化支持了这一解释。无论是在开放的流体缓冲或封闭的沉积物缓冲成岩系统中形成的所有镁铁质岩都具有相似的δ 26 Mg值，表明HMC转化为白云石。在Clino中的结晶化间隔中，聚集的同位素衍生温度产生高于正常温度的温度，这可能表明通过细菌硫酸盐还原对与碳酸盐形成相关的白云石Δ47值的动力学同位素效应。这项研究的结果强调了应用多种地球化学代理来解开浅水碳酸盐沉积物的成岩历史的效用，并警告不要简单地解释这些地球化学代理中的地层变异性，以指示海水中这些元素的全球地球化学循环的变化。
In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow‐water carbonate sediments the δ34S, Δ47, δ13C, δ18O, δ44/40Ca and δ26Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ34S values similar to those found in contemporaneous seawater and δ44/40Ca, δ26Mg values, Sr contents and Δ47 temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluid‐buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ34S values indicating dolomitization in a more sediment‐buffered diagenetic system where bacterial sulphate reduction enriches the residual SO42- in 34S, consistent with high sediment Sr concentrations and low δ44/40Ca and high δ26Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ34S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ44/40Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid‐buffered or closed sediment‐buffered diagenetic system have similar δ26Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ47 values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow‐water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.