Insights Into Intermediate Ocean Barium Cycling From Deep-Sea Bamboo Coral Records on the California Margin

Dissolved barium (Ba_SW) has been linked to several biogeochemical processes such as the cycling and export of nutrients, organic carbon (C_org), and barite in surface and intermediate oceans. The dynamic nature of barium cycling in the water column has been demonstrated on short timescales (days-weeks) while sedimentary records have documented geologic-scale changes in barite preservation driven by export production. Our understanding of how inter-annual-decadal scale climate variability impacts these biogeochemical processes currently lacks robust instrumental and paleoceanographic records. Recent work has calibrated and demonstrated the reproducibility of a new Ba_SW proxy in California Current System (CCS) bamboo corals (Ba/Ca) using a coral depth transect spanning the CCS oxygen minimum zone (792-2055m water depth). New ‘reconnaissance’ radiocarbon data identifying the bomb ¹⁴C spike in coral proteinaceous nodes and sclerochronological analyses of calcitic internodes are used to assign chronologies to the CCS coral records. Century-long coral records from ~900-1500m record ~4-7 year long increases in Ba/Ca (~10-70 nmol/kg Ba_SW) at depths where rapid barite cycling occurs on day-weekly timescales. The Ba_SW peaks punctuate the coral records at different time periods and depths and do not coincide with inter-annual/decadal climate transitions (e.g. ENSO/PDO). Stable surface productivity and coral δ¹⁵N records indicate that C_org export from CCS surface waters has been relatively constant over the past century. Thus, the inter-annual scale Ba_SW peaks recorded by the 900-1500m corals more likely reflect periods of decreased barite formation (and/or increased dissolution) via reduced bacterial C_org respiration or barite saturation state. Paleoceanographic Ba_SW records and continued research on barium cycling in the modern ocean have the potential to elucidate the mechanisms linking intermediate water carbon and barium cycling, climate, and ocean oxygenation in the past.