Collaborative Research: Estimation of particle aggregation and disaggregation rates from the inversion of chemical tracer data

海洋生物碳泵是将大气中的二氧化碳（CO2）从海洋表层有效输送到海洋深处，从而去除大气中CO2的重要过程。这种传输发生在一个多步骤的过程中。首先，浮游植物在海洋表面的阳光照射下进行光合作用，吸收大气中的二氧化碳并将其固定为颗粒有机碳。浮游植物中含有的一部分有机碳被打包成更大的簇（聚集体），可以沉入深海。这些聚集体下沉得越深，其中所含的碳从大气中被清除的时间就越长。聚集体沉降的深度随时间和空间变化很大，很难预测。一般来说，较大的聚集体比较小的颗粒下沉得更快，因此下沉得更深。促进聚集成更大颗粒的过程应该增强生物泵，而促进分解（颗粒团的分解）和再生（有机碳的分解）的过程应该降低生物泵的强度和效率。因此，粒子聚集和分解率对于理解生物泵的可变性至关重要，但很难直接测量。该项目将使用化学示踪剂和建模方法来量化这些重要过程的速率。研究人员将把这种方法应用到各种海洋环境中，并首次大规模地量化海洋500米以上的这些速率。作为该项目的一部分，他们将与CalTeach项目对接，CalTeach是加州大学科学与数学计划的一部分，旨在将大学科学、数学和工程专业的学生安置在K-12教室里。许多CalTeach的实习生后来成为加州K-12科学教师。加州大学圣克鲁斯分校（UCSC）的两名本科生将作为实验室助理参加CalTeach项目，并为K-12教室开发一个关于碳循环和生物泵的动手教学模块。加州大学圣克鲁斯分校和伍兹霍尔海洋研究所的科学家们提出，通过对三种化学示踪剂的观测结果进行反演，即钍(Th)-234、造岩颗粒和分布在小悬浮颗粒和大下沉颗粒之间的颗粒有机碳（POC），来估计中大洋区的颗粒聚集和分解速率。钍的同位素由于其已知的源函数和粒子反应行为，长期以来一直被用来估计粒子动力学过程的速率。以前的工作已经表明，造石颗粒是一种惰性的、被动的粒子动力学示踪剂。研究人员将把钍和岩石颗粒的测量与POC的测量结合起来，以估计大范围海洋环境中的颗粒聚集和分解率。从化学示踪剂数据的反演中推断出的粒子循环速率的估计提供了一个关键的定量约束，可以与其他方法得出的速率进行比较。这项工作的主要目的是估计与示踪数据最一致的EXPORTS和GEOTRACES巡航的每个站点的深度变化（非）聚集率。这项工作还将产生POC再矿化速率和颗粒沉降速率的深度变化估计。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 carbon pump in the ocean is an important process by which atmospheric carbon dioxide (CO2) is effectively transported from the surface ocean to the deep ocean, and thereby removing CO2 from the atmosphere. This transport occurs in a multi-step process. First, phytoplankton carry out photosynthesis in the surface, sunlit waters of the ocean, taking up atmospheric CO2 and fixing it into particulate organic carbon. A portion of the organic carbon contained in the phytoplankton is packaged into larger clusters (aggregates) that can sink to the deep ocean. The deeper these aggregates sink, the longer the carbon contained in them is removed from the atmosphere. The depth to which aggregates sink varies greatly over time and space and are difficult to predict. In general, larger aggregates sink more quickly, and thus more deeply, than smaller particles. Processes that promote aggregation to larger particles should enhance the biological pump, and processes that promote disaggregation (breakdown of particle clusters) and regeneration (decomposition) of the organic carbon should decrease the strength and efficiency of the biological pump. Particle aggregation and disaggregation rates are thus crucial to understanding the variability of the biological pump, but are very difficult to measure directly. This project will use chemical tracers and a modeling approach to quantify the rates of these important processes. The investigators will apply the approach to a variety of oceanic environments and provide the first large-scale effort to quantify these rates in the upper 500 meters of the ocean. As part of this project, they will interface with the CalTeach program, which is a University of California Science and Math Initiative to place university science, math, and engineering majors in K-12 classrooms. Many of these CalTeach interns go on to become K-12 science teachers in California. Two undergraduate students enrolled in the CalTeach program at the University of California, Santa Cruz (UCSC) will participate as laboratory assistants and develop a hands-on teaching module on the carbon cycle and biological pump for K-12 classrooms. Scientists from the University of California at Santa Cruz and Woods Hole Oceanographic Institution propose to estimate the rates of particle aggregation and disaggregation in the mesopelagic zone through the inversion of observations of three chemical tracers, namely thorium (Th)-234, lithogenic particles, and particulate organic carbon (POC) distributed between small, suspended particles and large, sinking particles. The isotopes of Th have long been used to estimate rates of particle dynamics processes because of their known source function and particle-reactive behavior. Previous work has shown that lithogenic particles act as an inert, passive tracer of particle dynamics. The investigators will couple thorium and lithogenic particle measurements to measurements of POC to estimate particle aggregation and disaggregation rates from a wide range of oceanographic environments. Estimates of particle cycling rates deduced from the inversion of chemical tracer data provide a crucial quantitative constraint to which rates derived from other approaches can be compared. The main objective of this work is to estimate depth- varying (dis-) aggregation rates at each station of the EXPORTS and GEOTRACES cruises that are most consistent with the tracer data. This work will also produce depth-varying estimates of POC remineralization rates and particle sinking rates.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.