Department of Earth System Science

It has long been assumed that radiocarbon (∆14C) content of dissolved inorganic carbon (DIC) is equal to that of particulate organic carbon (POC) in surface seawater; however, little research has been conducted to explicitly test this assumption. Here, we report ∆14C measurements of surface POC samples and compare them with contemporaneous DIC ∆14C measurements from the northeast Pacific Ocean (Hwang et al. 2004; Druffel et al. 2010). Samples were collected from surface waters at Station M off California between 1995 and 2004. The POC ∆14C values decreased 3.2‰ per year from 1995 to 2004, similar to the decline observed in the DIC ∆14C values during the same period. Overall, our results show no statistical difference between POC and DIC ∆14C—consistent with the assumption that DIC and POC ∆14C values can generally be considered equivalent. However, significant variability was observed for POC ∆14C values during several fall/summer events, where POC ∆14C signatures were lower than DIC ∆14C values. An evaluation of 2 sample pretreatments also suggests that non-homogenized POC samples deviated less from average POC ∆14C values and more closely matched the DIC ∆14C average for the time series. The presence of seasonal POC/DIC ∆14C disagreements, combined with sample processing effects, suggest that infrequent contributions of allochthonous, older carbon may have originated from deeper in the water column, especially during periods when upwelling in this area was prominent.