Major and trace element concentrations and Ni isotope compositions of carbonates from the Great Bahama Bank and carbonate diagenesis model parameters

We aimed to evaluate the fidelity of shallow-water carbonates as records of the Ni isotope composition of surface seawater. We first must know how Ni isotopes initially partition upon incorporation into carbonates and how early marine diagenesis (i.e., meteoric or marine diagenesis, dolomitization) alters the primary Ni signature. We measured the Ni isotope compositions and major and trace element concentrations of primary and diagenetically altered shallow-water carbonates from the Great Bahama Bank, a modern carbonate platform. The carbonates representing primary deposition are from short cores (collection described in Hardisty et al. 2017) and carbonates representing different types of diagenesis are from ODP cores, Clino and Unda (collection described in Eberli et al. 1997). Major and trace element concentrations were measured using ICP-MS. Ni isotope compositions were measured using either a Neptune or Nu III MC-ICP-MS, and instrumental mass fractionation was corrected using the double-spike technique as described in Siebert et al. (2001). We also modeled a simple diagenetic process (aragonite to calcite neomorphism under an open system), following Banner and Hanson (1990), to determine how the Ni isotope composition of diagenetically altered carbonate changes with diagenetic indicators (e.g., C isotope compositions). Model parameters are provided. We found that primary carbonates are fractionated from coexisting seawater (1.7‰, based on similar latitude and similar depth seawater; Lemaitre et al. 2022) by ~0.4‰. Only carbonates that experienced marine diagenesis, either with minimal alteration or alteration under sediment-buffered conditions, appear to preserve the primary carbonate Ni isotope signature. Our modeling supports this interpretation. Such carbonates hold promise as records of the Ni isotope composition of seawater.本研究旨在评估浅水碳酸盐岩（shallow-water carbonates）作为表层海水（surface seawater）镍同位素组成（Ni isotope composition）记录的保真度。我们首先需明确镍同位素在被碳酸盐岩固存时的初始分馏特征，以及早期海洋成岩作用（early marine diagenesis，即大气成岩作用（meteoric diagenesis）、海洋成岩作用与白云岩化作用（dolomitization））如何改变原始镍同位素信号。我们测定了采自现代碳酸盐岩台地（modern carbonate platform）大巴哈马滩（Great Bahama Bank）的原生及经成岩作用改造的浅水碳酸盐岩的镍同位素组成与主量、微量元素含量。代表原生沉积的碳酸盐岩采自短岩芯（short cores，采样方法详见Hardisty等人2017年的研究），而代表不同成岩作用类型的碳酸盐岩则采自大洋钻探计划（Ocean Drilling Program, ODP）岩芯Clino与Unda（采样方法详见Eberli等人1997年的研究）。主量与微量元素含量通过电感耦合等离子体质谱法（ICP-MS）测定。镍同位素组成分别通过Neptune型或Nu III型多接收杯电感耦合等离子体质谱法（MC-ICP-MS）测定，仪器质量分馏采用双稀释剂技术（double-spike technique）校正，校正方法详见Siebert等人2001年的研究。我们还参照Banner与Hanson（1990）的方法构建了一个简单的成岩作用模型：开放体系（open system）下文石（aragonite）向方解石（calcite）的新生变形作用（neomorphism），以探究经成岩作用改造的碳酸盐岩的镍同位素组成如何随成岩指标（如碳同位素组成（C isotope compositions））发生变化。模型参数已完整提供。研究发现，原生碳酸盐岩与共存海水之间存在约0.4‰的同位素分馏；基于相近纬度与水深的海水数据，该分馏值为1.7‰（引自Lemaitre等人2022年的研究）。仅经历海洋成岩作用的碳酸盐岩——无论改造程度微弱，还是在沉积物缓冲条件（sediment-buffered conditions）下发生改造——似乎能够保留原始碳酸盐岩的镍同位素信号。我们的模型结果也支持这一解释，这类碳酸盐岩有望作为海水镍同位素组成的可靠记录载体。