How phosphate is incorporated into carbonate minerals and its dependence on crystal growth conditions

溶解的无机磷酸盐与碳酸盐矿物方解石和文石的相互作用对许多低温地球化学和古环境系统具有重要意义。磷酸盐是一种必需的，有时是生物限制性的营养物质，但在土壤和自然水域中，碳酸盐矿物的生长可以去除磷酸盐。磷酸盐被强烈地吸附在方解石和文石的表面，并在晶体生长过程中作为不相容的微量元素并入这些矿物中。这些过程将磷从环境中去除，使其无法被生物利用，但可能会产生溶解磷含量的记录。例如，高空间分辨率方解石洞穴沉积物中磷酸盐浓度的变化被用于古气候研究，以标记洞穴主题的年生长层和指示古水文条件。最近有人提出，某些深海珊瑚的P含量可以作为生长时海水中溶解磷酸盐浓度的代表。虽然已经有许多关于磷酸盐在碳酸盐矿物中的吸附和掺入的研究，但掺入物种的化学性质仍然不确定。最近，我们已经证明，固态31P核磁共振光谱可以用来确定在自然浓度下方解石中P的原位形态。在方解石洞穴中，磷酸盐通过至少三种不同的过程结合，包括至少两种不同的晶体包裹体。我们提出了一个实验方案来研究磷酸盐在方解石和文石中的掺入途径，以及它如何随着晶体生长条件的变化而变化。方解石和文石将在精心控制的条件下制备，这些条件包括不同的溶解磷酸盐、生长速率、pH值和其他与自然界降水有关的重要参数。磷酸盐在矿物相中的分布将通过固态核磁共振光谱和P k边缘XANES和光谱显微镜方法的组合来确定，这也将允许识别任何结晶磷酸盐相。将特别强调确定在何种条件下，溶液中的磷酸盐浓度可以从沉淀的矿物的P分布推断出来。实验计划将以与其他研究人员合作调查天然碳酸盐矿物为指导，包括方解石和文石洞穴沉积物以及文石深海珊瑚。知识价值：该项目将通过提供碳酸盐矿物中P的化学形态及其随晶体生长条件变化的信息，提供有关磷酸盐在普通矿物中结合的基本新知识。为了更好地利用痕量磷进行古环境重建，需要了解磷的结合机制。这些数据也将有助于构建更一般的模型，以纳入大不相容的氧离子在碳酸盐。实验数据将提供导致表面沉淀形成的条件的约束，目前人们对这种沉淀知之甚少，因为这种沉淀很难检测到。更广泛的影响：该项目的主要目标是为研究生的教育提供相关的研究机会，并在结构化的环境中引入本科生进行科学研究。该研究计划是专门为学生在材料表征和实验低温地球化学、矿物学和光谱学方面获得各种技能而设计的，这些技能适用于广泛的科学问题。这项研究将以一种为研究生提供指导经验的方式涉及本科生。该项目将通过支持固态核磁共振设备和支持为研究合作操作这些光谱仪的研究生，为地球化学的研究基础设施做出贡献。

The interaction of dissolved inorganic phosphate with the carbonate minerals calcite and aragonite is important to many low temperature geochemical and paleoenvironmental systems. Phosphate is an essential, sometimes bio-limiting nutrient, but in soils and natural waters growth of carbonate minerals can remove phosphate. Phosphate is sorbed strongly to the surface of calcite and aragonite and incorporated into these minerals during crystal growth as an incompatible trace element. These processes remove P from the environment, making it unavailable to organisms, but potentially creating a record of the dissolved P content. For example, variation in phosphate concentration in calcite cave deposits at high spatial resolution is used in paleoclimate studies to mark annual growth layers in speleothems and to indicate paleohydrologic conditions. Recently it has been proposed that the P content of certain deep-sea corals can serve as a proxy for the dissolved phosphate concentration in seawater at the time of growth. Although there have been many studies of the sorption and incorporation of phosphate in carbonate minerals, the chemical nature of the incorporated species remains uncertain. Recently, we have shown that solid-state 31P NMR spectroscopy can be used to determine the P speciation in calcite in situ at concentrations found in nature. In calcite speleothems phosphate is incorporated by at least three different processes, involving at least two distinct crystalline inclusions.We propose an experimental program to investigate the pathway for phosphate incorporation in calcite and aragonite and how it varies with crystal growth conditions. Calcite and aragonite will be prepared under carefully controlled conditions with varying dissolved phosphate, growth rate, pH, and other important parameters relevant to precipitation in nature. The distribution of phosphate in the mineral phase will be determined by a combination of solid-state NMR spectroscopy and P K-edge XANES and spectromicroscopic methods, which will also allow identification of any crystalline phosphate phases. Particular emphasis will be placed on determining the conditions under which the phosphate concentration of the solution can be inferred from the P distribution of the mineral which precipitates. The experimental program will be guided by investigation of natural carbonate minerals in collaboration with other researchers, including calcite and aragonite cave deposits and aragonitic deep-sea corals.Intellectual Merit: This project will provide fundamental new knowledge of phosphate incorporation in common minerals, by providing information on the chemical form of the P in carbonate minerals how it varies with crystal growth conditions. An understanding of the P incorporation mechanism is needed to promote informed use of trace P content for paleoenvironmental reconstruction. These data will also be useful for constructing more general models for incorporation of large incompatible oxyanions in carbonates. The experimental data will provide constraints on the conditions that lead to formation of surface precipitates, which is at present poorly understood because such precipitates are very difficult to detect.Broader Impacts: A primary goal of this project is to provide relevant research opportunities for the education of graduate students and to introduce undergraduate students to scientific research in a structured environment. The research plan is designed specifically for students to acquire a diverse set of skills in materials characterization and experimental low-temperature geochemistry, mineralogy, and spectroscopy that are applicable to a wide range of scientific problems. The research will involve undergraduate students in a manner that provides mentoring experience for a graduate student. This project will contribute to research infrastructure in geochemistry through support of solid-state NMR facilities and support for graduate students who operate these spectrometers for research collaborations.