In this study, we present new data on the δ7Li values and Li/(Ca+Mg) ratios of carbonate cores from the Great Bahama Bank (Clino, Unda), a deep water core off of the bank top (ODP Leg 166 Site 1007), and the coralline Key Largo Limestone. We use these samples to evaluate the influence of meteoric diagenesis, marine burial diagenesis, and dolomitization on the Li isotope system in carbonates. We find that recrystallization of aragonite to low-Mg calcite in the presence of meteoric fluids results in a systematic decrease of the Li/(Ca+Mg) ratio in Clino, Unda and Key Largo samples, due to the lower Li/(Ca+Mg) ratio in meteoric fluids compared to seawater. For Li isotopes, we observe that the δ7Li of meteorically altered low-Mg calcite is +22.0±3.8‰ (n=28, 1σ), which is coincidentally similar to the original aragonite-rich sediments (+22±1‰ in the Bahamas, +18±1‰ in Key Largo), but with a larger variability (from +15 to +27‰). We interpret these features as reflecting the overprinting of primary Li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and/or global climatic conditions; in either case, meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition. In contrast, marine burial diagenesis and dolomitization of Clino and Unda sediments under “fluid-buffered” conditions result in Li isotope composition that is similar (+30.2±1.5‰, n=36, 1σ) to modern seawater (+31‰). For Site 1007, the δ7Li values range between +23 permil and +31 permil. We interpret this range as reflecting a combination of varying diagenesis style (fluid to sediment-buffered) and varying contribution of calcite derived from pelagic sediments, with distinct isotopic composition due to primary mineralogy. Altogether, our results show that diagenesis does not invalidate the use of bulk carbonates for deriving Li isotope paleo-records, but the reliability of past carbonates as recorders of seawater δ7Li values will depend on carefully characterizing their diagenetic history.

中文翻译：

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成岩作用对浅海相碳酸盐岩锂同位素比的影响

在这项研究中，我们提供了来自大巴哈马银行（克利诺，Unda）的碳酸盐岩岩心的 δ7Li 值和 Li/(Ca+Mg) 比率的新数据，该岩心是远离河岸顶部的深水岩心（ODP Leg 166 Site 1007 )，以及珊瑚礁 Key Largo Limestone。我们使用这些样品来评估大气成岩作用、海埋成岩作用和白云石化作用对碳酸盐岩中锂同位素系统的影响。我们发现，在大气流体存在的情况下，文石重结晶为低镁方解石会导致 Clino、Unda 和 Key Largo 样品中 Li/(Ca+Mg) 比的系统降低，因为 Li/(Ca+与海水相比，大气流体中的 Mg) 比率。对于锂同位素，我们观察到陨石蚀变低镁方解石的δ7Li为+22.0±3.8‰ (n=28, 1σ)，这与原始的富含文石沉积物（巴哈马 +22±1‰，基拉戈岛 +18±1‰）巧合地相似，但具有更大的可变性（从 +15 到 +27‰）。我们将这些特征解释为反映了陨石蚀变过程中原生锂的叠印，具有高度可变的同位素特征，可能受当地孔隙水和/或全球气候条件的组合控制；在任何一种情况下，大气成岩作用都会产生与海水成分无关的同位素特征。相比之下，“流体缓冲”条件下 Clino 和 Unda 沉积物的海埋成岩作用和白云石化导致 Li 同位素组成与现代海水 (+31‰) 相似 (+30.2±1.5‰, n=36, 1σ)。对于站点 1007，δ7Li 值介于 +23 permil 和 +31 permil 之间。我们将此范围解释为反映了不同的成岩方式（流体到沉积物缓冲）和来自远洋沉积物的方解石的不同贡献的组合，由于主要矿物学具有不同的同位素组成。总而言之，我们的结果表明，成岩作用并没有使使用大块碳酸盐来推导 Li 同位素古记录无效，但过去碳酸盐作为海水 δ7Li 值记录器的可靠性将取决于仔细描述它们的成岩历史。