Award: OCE-1443577

Much of what is known about Earth's climate history is derived from proxies: variables that stand in for another property of interest. The abundances of barium and cadmium in marine sediments are widely used as proxies of past biological productivity and nutrient cycling, respectively. An important source of these metals to sediments is via marine particulate matter. However, the manner in which particles accumulate and retain these metals -- including any transformations that may occur during transit to the seafloor -- remains poorly constrained. The goal of this project was to interrogate the processes controlling the uptake and cycling of barium and cadmium by analyzing the isotopic composition of these metals in marine particles collected from the Southern Ocean. These analyses resulted in a number of discoveries. First, we revealed that the distribution of barium in the Atlantic Ocean is intimately connected to ocean circulation. Vertical, zonal, and meridional barium distributions were all found to be strongly influenced by physical mixing, indicating that local biogeochemical processes exerted only a minimal impact on basin-scale barium cycling. Second, we established that the barium isotopic composition of particles is set shallowly, in the upper 200 m of the water column. This finding contrasts with previous suggestions that particles source barium from deeper, mesopelagic depths. Conversely, cadmium isotope distributions exhibited significantly larger depth-dependent variability, which we attribute to a complex internal cycle that is the subject of a number of ongoing efforts. Third, we discovered that barite (a mineral that is particularly important in the cycling of barium) can precipitate and persist in environments that are extremely unfavorable to formation. Our results underscore the importance of organic matter respiration to marine barium cycling, and of particles in general since they serve as microscopic mediators for a number of biogeochemical transformations that are unfavorable at the macroscopic scale. Overall, these findings have facilitated the development of new -- and refined existing -- applications of barium- and cadmium-based proxies to study Earth's climate history. This project had a number of broader impacts beyond the development of new proxies for Southern Ocean biogeochemistry. Support from this award contributed to the development of an early-career PI and their research team, including a technician and a postdoctoral researcher. Additionally, four undergraduates were trained during the course of this research, three of whom are now enrolled in graduate programs in the US and overseas. Lastly, a number of outreach activities were conducted under the auspices of this project, including: partnering with a nonprofit conservation organization to develop environmental education programs, collaborating with a social sciences organization to disseminate best practices in climate science communication, and presenting findings from these activities to formal and informal science educators through regional education and international scientific conferences, respectively. Last Modified: 06/20/2019 Submitted by: Tristan Horner