Department of Earth System Science

Pyrophaeophorbide-a, a degradation product of chlorophyll-a, is predominantly formed by grazing processes in sediments as well as in the water column. Water column profiles of pyrophaeophorbide-a/suspended particulate organic carbon (SPOC) concentrations, at an abyssal site in the northeast (NE) Pacific (Sta M, 34°50′N, 123°00′W; 4100 m water depth), show low concentrations (0.01–0.1 ng/μg SPOC) at surface and mesopelagic depths, and increasing concentrations with closer proximity to the sea floor (0.05–0.6 ng/μg SPOC). However, in June 1992, the deep maximum of pyrophaeophorbide-a/SPOC in the water column of Sta M extended higher into the water column, as much as 1600 m above the bottom (mab) (2500 m water depth); in other seasons they only extended up to 650 mab (3450 m water depth).

Previous studies have demonstrated lateral transport of particulate matter from the continental shelf to the deep ocean off the coast of northern California. Recent work suggests that the benthic boundary layer (BBL) extends to 50 mab, based on sediment trap and transmissometry measurements (Smith, K.L., Kaukmann, R.S., Baldwin, R.J., 1994. Coupling of near-bottom pelagic and benthic processes at abyssal depths. Limnology and Oceanography 39, 1101–1118.), and that lateral transport is significant only during summer, which is consistent with our observations. A partial vertical profile of pyrophaeophorbide-a/SPOC from the north central (NC) Pacific provides some evidence that the deep maximum may be absent due to the distance of this site from the continental margin. Thus, the observed deep maximum of pyrophaeophorbide-a/SPOC at Sta M is likely due mainly to lateral transport from the continental slope rather than to local vertical resuspension in the BBL exclusively.

Pyrophaeophorbide-a concentrations in SPOC at Sta M were negatively correlated with Δ¹⁴C values of SPOC (SPOC samples from Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S.A. and Wolgast, D., 1996, Seasonal variability of particulate organic radiocarbon in the northeast Pacific Ocean. Journal of Geophysical Research 101, 20543–20552), further supporting our contention that pyrophaeophorbide-apeaks in the deep water column are derived from “older” resuspended sediments that were laterally transported from continental margin sediments. Our molecular biomarker (pyrophaeophorbide-a) data are reflective of a specific fraction of SPOC that likely remains in the water column for long periods of time or is derived from resuspended sediments.