Seasonal variability of particulate organic radiocarbon in the northeast Pacific Ocean

Abstract

We present Δ 14 C measurements of particulate organic carbon (POC) collected on four cruises at our time series site (station M) in the northeast Pacific Ocean. We observe a large gradient with depth in the suspended POC Δ 14 C values (124–160‰). These profiles display lower Δ 14 C values (by 20–30‰) in samples between 2500 m and the bottom during June 1992 and July 1993 than those during February and October 1992. Values of Δ 14 C in sinking POC from deep‐moored sediment trap collections suggest a semiannual trend that displays lower overall Δ 14 C in material collected during periods of high flux. A limited number of Δ 14 C measurements of small swimmers picked from the trap 650 m above bottom are similar to surface Δ 14 C measurements of dissolved inorganic carbon (DIC) and suspended POC, indicating a surface carbon source. Overall, we postulate that the major process causing lower Δ 14 C values of deep suspended and sinking POC is sorption (or biological incorporation) of “old” DOC onto particulate matter. There appears to be a higher ratio of DOC sorbed to sinking particulate matter at times of high flux (late spring and early fall) that can be thought of as a “stripping out” of DOC from the water column. The DIC Δ 14 C display a small seasonal variation in the surface waters and is not the sole source of the observed seasonality in the POC Δ 14 C signals.