Calibration of a Novel Nutrient Paleoproxy in the Southern Ocean

人类对地球气候的观察只跨越了地球历史的一个狭窄窗口。通过测量和解释海洋沉积物的化学成分，可以在人类观察之前了解地球气候变化的方式和原因。要准确解释这些记录--也称为“代理”记录，因为它们代表过去对环境条件的直接测量--首先需要根据现代环境中的直接观测对代理的行为进行适当的校准。该项目将根据对南大洋颗粒状有机物的化学成分的全面描述，开发一种新的海洋营养物质利用指标。海洋中有机物的形成和输出需要营养物质，消耗碳，并可能影响全球二氧化碳的海气平衡。新的替代物和定标提供了观察地球气候历史的新方法，并可能阐明海洋生态系统内的相互作用。生物碳泵的效率关键取决于海洋营养物质的可获得性和利用率，特别是在南大洋。然而，当前养分利用的古替代物之间的歧义和不一致使得重建过去的营养体系变得不确定。镉--一种已知的磷酸盐替代品--提供了一种方法，可以绕过现有营养替代品的许多已知限制，但在应用之前需要完整的同位素表征。在这个项目中，研究人员将使用多收集器电感耦合等离子体质谱来追踪有机质中记录的镉的稳定同位素组成。研究人员将在南大洋的许多表层营养体系中进行同位素测量，重点关注三个关键的地球化学转变：有机碳在海面的形成，其在海洋内部的转移和分解，以及其在海底埋藏期间的行为。因此，这种海面到海底的校准将全面确定一种新的营养物质的实用性和保真度，使人们能够更准确地了解过去地球气候与海洋生物地球化学的关系。

Human observations of Earth's climate span only a narrow window of Earth History. Understanding how and why Earth's climate changed before human observations can be overcome through the measurement and interpretation of the chemical composition of marine sediments. Accurate interpretation of these records -- also known as "proxy" records, because they stand in for direct measurements of environmental conditions in the past -- first requires that the behavior of the proxy be properly calibrated against direct observations in modern environments. This project will develop a novel proxy for marine nutrient utilization based on a comprehensive characterization of the chemical composition of particulate organic matter in the Southern Ocean. The formation and export of organic matter in the ocean requires nutrients, consumes carbon, and can influence the global air-sea balance of carbon dioxide. New proxies and calibrations offer novel ways of looking at Earth's climate history and can potentially illuminate interactions within marine ecosystems.The efficiency of the biological carbon pump depends critically on the availability and utilization of marine nutrients, particularly in the Southern Ocean. However, ambiguity in, and disagreement between current nutrient utilization paleoproxies renders reconstruction of past nutrient regimes uncertain. Cadmium -- a known proxy for phosphate -- offers a means to circumvent many of the known limitations of existing nutrient proxies, but requires a full isotopic characterization before its application. In this project, researchers will trace the stable isotopic composition of cadmium recorded in organic matter using multiple collector inductively coupled plasma mass spectrometry. The researchers will make isotopic measurements across numerous surface nutrient regimes in the Southern Ocean, with a focus on three critical geochemical transitions: the formation of organic carbon at the sea surface, its transit and decomposition through the ocean's interior, and its behavior during burial on the sea floor. This sea surface-to-sea floor calibration will thus comprehensively determine the utility and fidelity of a novel nutrient proxy, allowing a more accurate view of how Earth's climate was related to ocean biogeochemistry in the past.