Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera
Apparent preservation of primary foraminiferal Mg/Ca ratios and Mg-banding in recrystallized foraminifera
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DOI：
10.1130/g49984.1
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发表时间：
2022-04
期刊：
影响因子：
5.8
通讯作者：
P. Staudigel;E. John;B. Buse;C. Lear
中科院分区：
文献类型：
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作者：
P. Staudigel;E. John;B. Buse;C. Lear
有孔虫的微量元素和δ18O值被广泛用于重建新生代及新生代以后的海洋温度。以前对不同物理保存程度的有孔虫进行的地球化学评估工作表明，镁/钙和δ18O古温度计在重结晶测试中给出的值不同，浮游氧同位素的温度往往明显低于镁/钙比值。为了研究成岩作用中元素的活动性，我们在始新世莫洛兹维拉进行了微空间微量元素分析。元素图显示，在一定程度上，在织构再结晶测试中，痕量元素带很容易识别和保存。用反应扩散模型检验了镁带的保留和δ18O和Mg/Ca值的分离是否是扩散限制的“闭合系统”再结晶的结果。结果表明，在封闭体系中，内部特征(如镁带状)会在体相Mg/Ca组分变化之前消失，而体相δ_(18)O值的变化通常比Mg/Ca快。无论镁的分配系数是多少，都能观察到这一点，并表明浮游镁/钙代理比δ18O代理在成岩作用上更健壮。因此，该模型可以解释观察到的这两个代理的解耦现象。此外，测试内镁带的保存显示出在评价原始镁/钙比值的保存以及古温度重建的准确性方面的潜力。
Trace element and δ18O values of foraminifera are widely used to reconstruct oceanic temperatures throughout the Cenozoic and beyond. Previous work evaluating the geochemistry of foraminifera with differing degrees of physical preservation have shown that Mg/Ca and δ18O paleothermometers give discrepant values in recrystallized tests, with planktonic oxygen isotopes often yielding significantly lower temperatures than Mg/Ca ratios. To study the mobility of elements during diagenesis, we performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is readily identifiable and preserved, to an extent, in texturally recrystallized tests. A reaction-diffusion model was used to test whether the preservation of Mg-banding and the decoupling of δ18O and Mg/Ca values could be the result of diffusively limited “closed-system” recrystallization. Results show that, in a closed system, internal features (such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca composition, while the bulk δ18O value will typically change faster than Mg/Ca. This is observed regardless of what partitioning coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy is more diagenetically robust than the δ18O proxy. Thus, this model can explain the observed decoupling of these two proxies. Furthermore, the preservation of intra-test Mg-banding shows potential for use in evaluating the preservation of primary Mg/Ca values and hence the accuracy of paleotemperature reconstructions.