The Speed, Signature, and Significance of Barium Transformations in Seawater

海洋中碳的生物循环包含了许多其他元素，有些是直接的（如生命所必需的营养物质），有些是间接的，因为它们在化学上参与了影响碳的过程。其中一种元素是钡（Ba）。矿物重晶石（硫酸钡）的颗粒已被发现形成与微生物消耗的有机物质在海洋的“暮光地带”。这些颗粒沉降到海底，它们在沉积物中的存在被用来推断过去海洋条件的变化。沉积物中重晶石的数量和重晶石中钡的同位素组成对发生在过渡带的过程都有潜在的敏感性。然而，关于海洋中钡循环的几个长期存在的问题仍然存在，这使得以钡为基础的替代记录的解释变得复杂。剩下的问题包括有多少钡从大洋中脊的热液地点进入海洋，是什么控制了水柱中重晶石的沉淀和溶解。这个项目试图用新的方法解决这些问题，计划在太平洋和南大洋进行三次研究探险。在此过程中，该项目将支持研究生、博士后研究员和初级研究员的教育、培训和职业发展。将从代表性不足的群体中招募本科生进行补充性的岸上实验。本提案旨在回答海洋学中基于钡的代用物的四个核心问题：新钡对海洋的主要输入是什么？它们的同位素组成是什么？在有机质再矿化过程中，是什么控制了上层重晶石的析出量？是什么影响了它的同位素组成？这些问题将通过以现场为中心的方法来解决：现场和船载示踪剂孵育实验，auv引导的Ba循环“热点”自适应采样，以及基于剖面的周围海洋特征测量。这种多管齐下的方法将用于研究：从海洋中最大的热液场——东南东太平洋凸起释放的Ba的通量和同位素组成，重点是低温喷口；与不同浮游植物组合相关的南大洋重晶石降水速率和特征；以及环境条件的重要性，例如低环境氧浓度，在确定热带东部太平洋重晶石降水效率方面的重要性。每个转换的重要性将被评估，这可能导致排除某些过程的重要性，或者确定可能形成新代理基础的新依赖关系。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

The biological cycling of carbon in the oceans entrains many other elements, some directly (like nutrients that are essential for life) and some indirectly, as they become chemically involved in the processes that are affecting carbon. One such element is barium (Ba). Particles of the mineral barite (barium sulfate) have been found to form in association with microbial consumption of organic material in the ocean’s “twilight zone.” These particles settle to the ocean floor, and their presence in sediments has been used to infer changes in the conditions in the ocean back in time. Both the amount of barite in sediments and the isotope composition of Ba in barite are potentially sensitive to processes occurring in the twilight zone. However, several long-standing questions remain about Ba cycling in the oceans, which complicates the interpretation of barium-based proxy records. Examples of remaining questions include how much barium enters the oceans at mid-ocean ridge hydrothermal sites, and what controls the precipitation and dissolution of barite in the water column. This project seeks to tackle these questions using new approaches, on three scheduled research expeditions in the Pacific and Southern Oceans. In doing so, this project will support the education, training, and career development of a graduate student, postdoctoral researcher, and junior investigator. Undergraduate students from underrepresented groups will be recruited to conduct complementary shore-based experiments. This proposal seeks to answer four questions central to the utility of barium-based proxies in oceanography: What are the major inputs of new Ba to the ocean? What are their isotopic compositions? What controls the amount of pelagic barite precipitated during the remineralization of organic matter? What influences its isotopic composition? These questions will be addressed using a field-centric approach combining: in situ and shipboard tracer-incubation experiments, AUV-led adaptive sampling of Ba cycling ‘hotpots’, and section-based surveying of the surrounding oceanographic features. This multi-pronged approach will be used to investigate: the flux and isotopic composition of Ba released from the largest hydrothermal fields in the ocean, the Southern East Pacific Rise, with a focus on low-temperature venting; rates and signatures of pelagic barite precipitation associated with different phytoplankton assemblages in the Southern Ocean; and, the importance of environmental conditions, such as low ambient oxygen concentrations, in setting the efficiency of barite precipitation in the Eastern Tropical Pacific. The significance of each transformation will be assessed, which may lead to ruling out the importance of certain processes, or identifying new dependencies that could form the basis of new proxies.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.