Chlorophyll-a and pheopigments as tracers of labile organic carbon at the central equatorial Pacific seafloor

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High rates of primary production and fast sinking rates lead to the deposition of phytodetritus and highly reactive organic matter to the sediments of the central equatorial Pacific. These substances are responsible for driving important chemical fluxes and fueling benthic organisms. Chloropigments have proven useful as tracers of similar highly reactive organic carbon components in lacustrine and nearshore marine sediments. In this study we investigate the degradation of chlorophyll-a and pheopigments-a at four abyssal sites on the JGOFS equatorial Pacific transect along 140°W and explore the usefulness of these chloropigments as tracers of the most-reactive component of the deep-sea particulate organic carbon (POC) rain. First-order reaction rate constants ( k = 1–75 yr −1, half-life 3–250 d) and relative reactivities (chlorophyll-a > allomer and pheophorbide-a > pheophytin-a) derived from most of the sedimentary profiles are similar to those found in laboratory and other field (lake and coastal marine) studies. However, in some profiles, the rate constants determined by fitting data below 0.5 cm are slower by up to 3 orders of magnitude despite an apparent abundance of bacteria, macrofauna, and porewater oxygen. Model results assuming multi-G kinetics suggest that these chloropigments degrade as two components: one, which accounts for at least 99% of the degradation and 11–57% of the sediment inventory, degrades with a half-life of 4–120 days. The other component degrades with a half-life of up to 440 years. These results suggest that some otherwise labile POC may be protected and escape rapid degradation near the sediment-water interface. If phytodetritus is deposited continuously throughout the year, our model-calculated chlorophyll-a fluxes indicate that it could account for 25–100% of the annual POC flux at sites close to the equator.