A Lagrangian View of Trace Elements and Isotopes in the North Pacific

Ocean time-series sites are influenced by both temporal variability, as in situ conditions change, as well as spatial variability, as water masses move across the fixed observation point. To remove the effect of spatial variability, this study made sub-daily Lagrangian observations of trace elements and isotopes (Al, Sc, Mn, Fe, Co, Ni, Cu, Zn, Cd, Pb, Th-232, and Th-230) in surface water over a 12-day period (July-August 2015) in the North Pacific near the Hawaii Ocean Time-series Station ALOHA. Additionally, a vertical profile in the upper 250 m was analyzed. This dataset is intercalibrated with GEOTRACES standards and provides a consistent baseline for trace element studies in the oligotrophic North Pacific. No diel changes in trace elements could be resolved, although day-to-day variations were resolved for some elements (Fe, Cu, and Zn), which may be related to organic matter cycling or ligand availability. Pb concentrations remained relatively constant during 1997-2015, presenting a change from previous decreases. Nutrient to trace element stoichiometric ratios were compared to those observed in phytoplankton as an indication of the extent of biological trace element utilization in this ecosystem, providing a basis for future ecological trace element studies. Key Points A suite of 11 trace elements was analyzed in one patch of surface water at twice-daily resolution for 12 days near Station ALOHA No diel patterns could be resolved; daily variations are suggestive of active biogeochemical cycling in this oligotrophic region Interannual variations, in particular for Pb, and nutrient-trace element stoichiometry with respect to prior observations was assessed