G02_06

From the Southern Ocean to the Bering Sea: using radiocarbon to investigate dissolved organic carbon aging in the Pacific

Schlagenhauff S¹,  Bercovici S³, Grotheer H¹, Niggemann J³, Dittmar T³, Mollenhauer G^1,2

¹Alfred Wegener Institute, Bremerhaven, Germany, ²Marum Center for Marine Environmental Research and Department of Geosciences, Bremen, Germany, ³Institute for Chemistry and Biology of the Marine Environment, Oldenburg, Germany

Although marine dissolved organic matter (DOM) mainly forms from primary production at the surface, from sources with young Δ¹⁴C values, DOM from deep water samples is significantly older (4000-6300 years) and is either supplemented by an older source or persists on millennial timescales. Understanding how DOM endures in the ocean is key to predicting future DOM fluctuations. This project presents the Δ¹⁴C values of marine DOM from a Pacific transect collected on the RV Sonne in 2016 and 2017. The transect runs from 58.9°N along the 180°E longitudinal meridian to 52.1°S and provides an unprecedented dataset of Δ¹⁴C values of Pacific DOM. The samples were solid phase extracted and then isotopically analyzed on the MICADAS mini-carbon dating system. At the surface (upper 100 m), Δ¹⁴C values ranged from -302 ‰ in the subtropics to a more depleted value of -466 ‰ in the Southern Ocean, likely influenced by upwelling deep waters via the Antarctic Circumpolar Current. In deeper waters across the transect (3000 – 6000 m) the average Δ¹⁴C value was -548 ± 25 ‰ with a minimum value of -603 ‰ in the Bering Sea. The northern stations also revealed local areas of old DOM input to the ocean, likely from the Bering Sea. The Δ¹⁴C values generally align well with known water mass gradients across the Pacific and decrease with depth. These findings provide evidence for (1) a stable pool of aged DOM in the deep ocean and (2) localized processes adding autochthonous DOM to the system.