Award: OCE-1736949

In this project we determined the concentration and isotopic composition of barium in Pacific Ocean seawater and particles collected along 152W from Alaska to Tahiti. These samples were collected in 2018 as part of the U.S. contribution to the GEOTRACES program, an international study of marine biogeochemical cycles of trace elements and their isotopes. Though it is not a nutrient itself, barium generally behaves like one in seawater, and it is well preserved in many types of marine sediment. These properties mean that barium is widely used as a tracer of ocean circulation and carbon export, particularly in Earth's past. This project had four main objectives: Establish the overall distribution of barium isotopes in the Pacific Ocean Quantify the relative importance of various internal processes in setting this distribution Characterize the magnitude and isotope composition of regional barium sources Test the utility of different types of sediment to record seawater barium isotope compositions. Our data from the section show that barium isotopes exhibit an anti-correlated pattern with dissolved barium concentrations: Wherever barium concentrations are low, barium isotope compositions are generally 'heavy;' wherever barium concentrations are high, barium isotopes are generally 'light.' The Pacific Ocean exhibits the largest surface-to-depth gradients in dissolved barium concentrations. Our data show that these large gradients are also reflected in barium isotope compositions. A statistical analysis of the water masses encountered along the section reveals that the major features of the dissolved barium distribution reflect conservative mixing between water masses with different barium concentrations. However, significant barium deficits and excesses are observed in North Pacific Intermediate Waters and close to the seafloor along the entire section, respectively. These anomalies indicate processes unique to barium, such as particle removal and benthic inputs. We quantified the composition of these and other barium inputs by surveying a suite of samples for their barium isotope composition, including sediment porewaters, waters influenced by shelf processes, and groundwater discharge. While these analyses revealed a wide range of processes influencing barium release to seawater, they exhibit a surprisingly narrow range of barium isotope compositions. Lastly, an assessment of core-top barites and deep-sea corals revealed that both types of deposit are reliable archives of marine barium isotope compositions. These results are highly promising from the perspective of reconstructing the ancient barium cycle using marine sediments. The results from this project - along with those from other nutrient-like elements - were also used to write a series of synthesis articles that were published in 2021. These articles highlight the progress that has been made in understanding marine trace element and isotope distributions over the past decade, and offer a roadmap for improving process-level understanding of trace element cycling in future studies. Last Modified: 06/29/2022 Submitted by: Tristan Horner