A predictive framework for micro-scale carbonate diagenesis: Towards more accurate reconstructions of global climate and environmental change

碳酸盐岩是地球历史上环境条件的理想记录者，因为它们在地球历史上形成于陆地和海洋的许多环境中。然而，在碳酸盐岩的漫长历史中，其可以是数百万年或更长时间，岩石可以被埋到许多公里深，这使岩石暴露于极端温度和/或使岩石与可以化学改变原始矿物的水接触。换句话说，碳酸盐矿物中地球环境条件的化学记录可以在各种条件下改变，因此测量的化学值可能无法反映预期的古代环境。研究人员利用当时存在于印度和亚洲之间的特提斯海中沉积的6000万年前的海洋岩石，证明了传统工具可能无法识别所有类型的化学蚀变。该团队的初步数据和观察结果表明，目前还不清楚碳酸盐岩如何在不留下可见痕迹的情况下发生化学变化。在这项研究中，PI将测试他们使用一种新型化学工具的能力，该工具通过使用其他化学工具确定碳酸盐岩的年龄来确定碳酸盐矿物形成的温度，并且对埋藏的变化也很敏感。这项技术可以证明是有价值的石油和天然气工业提供额外的工具，以评估产油岩石的热成熟度，否则难以进入的盆地，传统的工具无法使用。第二，PI将测试许多化学技术的能力，这些技术在极小的尺度上寻找变化，以更好地了解变化是如何发生的，以及变化留下了什么指纹。该项目第二部分的成果将成为地球科学家用于识别其他碳酸盐矿物的地球化学信号的新工具。这将改善基于碳酸盐矿物的现代和古代环境的重建。碳酸盐在沉积后极易受到生物、物理和化学变化的影响。碳酸盐“聚集”同位素测温法，T（三角洲帽47），允许更准确和完整地重建古代环境，通过提供碳酸盐地层温度的测量。高三角洲帽47值（低温）通常被视为指示原生矿物形成温度，因为通常认为在地球表面没有后期水介导的重结晶的情况下，结块（三角洲帽47）在埋藏期间会减少。从埋藏的古新世海洋碳酸盐的初步数据表明，这是一个过于简单化;相反，海洋碳酸盐显示出广泛的氧交换与非海洋18 O-贫化水和深埋温度（150摄氏度）的证据，但出现不变使用传统的成岩筛选指标（即光学岩相学）和产量明显低三角洲帽47衍生的温度。这些观察突出了光学未检测到的碳酸盐蚀变的同位素效应的知识缺口。PI将（a）测试与方解石U/Pb年龄配对的聚集同位素测量重建沉积盆地分析的共生序列的能力，以及（B）优化一套微分析技术以识别不同蚀变条件的产物，建立稳定矿物重结晶的地球化学、结构和同位素指纹的预测框架。"PI将绘制碳酸盐岩中的晶体学、元素和同位素异质性，以指导选择更高空间分辨率的三角洲帽47测量。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

Carbonate rocks are an ideal recorder of environmental conditions on our planet throughout Earth history because they have been forming throughout Earth's history across many environments on land and in the sea. However, throughout the long history of a carbonate rock, which can be millions of years or more, the rock can be buried to many kilometers depth, which exposes the rock to extreme temperatures and/or bring the rock into contact with water that can chemically alter the original minerals. In other words, the chemical records of Earth's environmental conditions within a carbonate mineral can alter under various conditions, and therefore the chemical values measured may not reflect the ancient environment as expected. Using 60 million year old marine rocks that were deposited in the Tethys Sea that existed at that time between India and Asia, the investigators have demonstrated that traditional tools may not identify all types of chemical alteration. The team's initial data and observations show that it is not yet understood how a carbonate rock can be chemically altered without leaving a visual trace of that alteration. In this study, the PIs will test their ability to use a novel chemical tool that determines the temperature at which a carbonate mineral forms by using other chemical tools that determine the age of that carbonate rock and are also sensititive to changes with burial. This technique could prove valuable for the oil and gas industry by providing additional tools for assessing the thermal maturity of oil-producing rocks in otherwise inaccessible basins where traditional tools cannot be used. Second, the PIs will test the capability of many chemical techniques that look for changes on extremely small scales to better learn how alteration happens and what fingerprints that alteration leaves behind. The result of the second part of this project will be a new tool for Earth scientists to use to identify altered geochemical signals from other carbonate minerals. This will improve reconstructions of modern and ancient environments based on carbonate minerals.Carbonates are highly susceptible to biological, physical, and chemical alteration after deposition. Carbonate 'clumped' isotope thermometry, T(delta cap 47), allows to more accurately and completely reconstruct ancient environments by providing measurements of carbonate formation temperature. High delta cap 47 values (low temperature) are often viewed as indicative of primary mineral formation temperature because clumping (delta cap 47) is typically thought to decrease during burial in the absence of later water-mediated recrystallization at Earth's surface. Preliminary data from buried Paleocene marine carbonates suggests this is an oversimplification; rather, marine carbonates show evidence of extensive oxygen exchange with non-marine 18O-depleted water and deep burial temperatures (150 degree C), but appear unaltered using conventional diagenetic screening metrics (i.e. optical petrography) and yield apparent low delta cap 47-derived temperatures. These observations highlight a knowledge gap in the isotopic effects of optically undetected carbonate alteration. The PIs will (a) test the capability of clumped isotope measurements paired with calcite U/Pb ages to reconstruct paragenetic sequences for sedimentary basin analysis and (b) optimize a suite of micro-analytical techniques to identify products of different alteration conditions, building a predictive framework for geochemical, textural, and isotopic fingerprints of 'stable mineral recrystallization.' The PIs will map crystallographic, elemental, and isotopic heterogeneity within carbonates to guide selection of higher spatial resolution delta cap 47 measurements.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.