Collaborative Research: Field and Modeling Studies of the Magnitude and Variability of 234-Th Based Estimates of POC Export Flux and Remineralization in the Upper Ocean

0327721/OCE-327644/OCE-0327693世界海洋表层输出的生物颗粒有机碳(POC)通量的大小对约束全球碳循环模型至关重要，但仍存在争议。在过去的十年中，234被越来越多地用作示踪剂，通过将深度积分的234通量乘以POC/234下沉颗粒的比率来估计从真光区输出的POC。这项技术的准确性高度依赖于POC/234比和234通量的自然变化，然而这种变化对POC出口估计的意义仍然不确定。仅由于POC/234比率和用于估计第234次通量的程序的变化，来自第234个来源的POC出口通量的全球分布可能会变化2-10倍或更多。因此，尽管经过十多年的研究，我们对234来源的POC出口通量的大小和变异性以及主要的控制机制的理解还没有得到很好的限制。在这个项目领域，来自罗德岛大学、德克萨斯农工大学和佐治亚大学的研究人员将进行实地和模拟研究，以解决以下主要问题：基于234的POC出口通量估计从真光层到中层带的准确性有多高，控制234清除和出口的主要机制是什么，以及如何改进这种示踪技术？为了解决这个中心问题，他们提出了两个可行的假设。假设1：第234个来源的POC出口量的区域差异是由颗粒源/有机化学成分和食物网结构的差异决定的。假设2：颗粒-颗粒相互作用和有机碳和234的不同生物地球化学循环控制了POC/234比值，从而控制了234衍生的POC输出和再矿化的大小。研究战略是将新的和现有的沉积物捕捉器和上层海洋(~0-500米)粒度分级的第234、POC和特定有机化合物的测量与新开发的颗粒凝聚模型相结合，以验证、测试和进一步发展所提出的工作假设。样本将来自三个研究地点，其特点是食物网络结构和颗粒输出明显不同(地中海、北极、大西洋西北部)，旨在提供有关控制POC/234比率和POC输出和再矿化机制的最大信息。模型开发和测试是该项目的一个关键方面，用于指导和验证现场观测，并提供关于第234次估计上层海洋POC输出和再矿化的机制的信息。拟议研究的智力价值与一些国家和国际碳循环研究计划(JGOFS、眩晕、DYFAMED、CARBOCEAN)有关。具体地说，这项建议对以前和目前由NSF资助的使用(或已经使用)234来测量上层海洋的POC出口通量的项目具有广泛的智力影响。例如，JGOFS计划期间获得的POC通量可能需要修订，未来对中层海洋的调查可能需要考虑到这项工作的结果，这将取决于对进入该海洋区域的颗粒通量的准确确定。这一结果还可能对提高对全球海洋碳循环的了解的SCOR/IGBP海洋倡议产生影响。

ABSTRACTOCE-0327721 / OCE-327644 / OCE-0327693The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. Over the past decade, 234Th has been increasingly used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234Th flux by the POC/234Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/234Th ratio and 234Th flux, yet the significance of this variability to estimates of POC export remains uncertain. The global distribution of 234Th-derived POC export fluxes can vary by 2-10 times or more due solely to variability in the POC/234Th ratio and procedures used to estimate the 234Th flux. Thus, despite over a decade of research, our understanding of the magnitude and variability of 234Th-derived POC export fluxes, and the dominant controlling mechanisms, is not well constrained.In this project field, researchers at the University of Rhode Island, Texas A & M University, and University of Georgia will conduct field and modeling studies to address the following major question: How accurate are 234Th-based estimates of POC export flux from the euphotic to the mesopelagic zone, what are the dominant mechanisms controlling 234Th scavenging and export, and how can this tracer technique be improved? They have two working hypotheses designed to address this central question. Hypothesis 1: Regional variability in the magnitude of 234Th-derived POC export is determined by differences in particle source/organic chemical composition and food web structure. Hypothesis 2: Particle-particle interactions and differential biogeochemical recycling of organic C and 234Th controls POC/234Th ratios and hence the magnitude of 234Th-derived POC export and remineralization. The research strategy is to combine new and existing measurements of sediment trap and particle size-fractionated 234Th, POC and specific organic compounds in the upper ocean (~0-500 m) with newly developed particle coagulation models to validate, test and further develop the proposed working hypotheses. Samples will be collected from three study sites characterized by distinct differences in food-web structure and particle export (Mediterranean, Arctic, NW Atlantic) and designed to provide maximum information on the mechanisms controlling POC/234Th ratios and POC export and remineralization. Model development and testing is a key aspect of the project and serves to guide and validate the field observations and provide information on mechanisms of 234Th estimates upper ocean POC export and remineralization.The intellectual merit of the proposed research bears on a number of national and international carbon cycle research programs (JGOFS, VERTIGO, DYFAMED, CARBOCEAN). Specifically, this proposal has broad intellectual implications for previous and currently NSF funded projects that use (or have used) 234Th to measure POC export fluxes from the upper ocean. For example, POC fluxes obtained during the JGOFS program may need to be revised, and future investigations of the mesopelagic, which will depend on accurate determination of the particle flux entering this region of the ocean, may need to take the results of this work into account. The results could also have implications for the SCOR/IGBP OCEANS initiative of improved understanding of the global ocean carbon cycle.