PYROPHAEOPHORBIDE-A AS A TRACER OF SUSPENDED PARTICULATE ORGANIC-MATTER FROM THE NE PACIFIC CONTINENTAL-MARGIN

Pyrophaeophorbide-a, a degradation product of chlorophyll-a, is predom inantly formed by grazing processes in sediments as well as in the wat er column. Water column profiles of pyrophaeophorbide-a/suspended part iculate organic carbon (SPOC) concentrations, at an abyssal site in th e northeast (NE) Pacific (Sta M, 34 degrees 50'N, 123 degrees 00'W; 41 00 m water depth), show low concentrations (0.01-0.1 ng/mu g SPOC) at surface and mesopelagic depths, and increasing concentrations with clo ser proximity to the sea floor (0.05-0.6 ng/mu g SPOC). However, in Ju ne 1992, the deep maximum of pyrophaeophorbide-a/SPOC in the water col umn 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 onl y extended up to 650 mab (3450 m water depth). Previous studies have d emonstrated lateral transport of particulate matter from the continent al shelf to the deep ocean off the coast of northern California. Recen t work suggests that the benthic boundary layer (BBL) extends to 50 ma b, based on sediment trap and transmissometry measurements (Smith, K.L ., Kaukmann, R.S., Baldwin, R.J., 1994. Coupling of near-bottom pelagi c 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) Pacif ic provides some evidence that the deep maximum may be absent due to t he distance of this site from the continental margin. Thus, the observ ed deep maximum of pyrophaeophorbide-a/SPOC at Sta M is likely due mai nly to lateral transport from the continental slope rather than to loc al vertical resuspension in the BBL exclusively. Pyrophaeophorbide-a c oncentrations in SPOC at Sta M were negatively correlated with Delta(1 4)C values of SPOC (SPOC samples from Druffel, E.R.M., Bauer, J.E., Wi lliams, P.M., Griffin, S.A. and Wolgast, D., 1996, Seasonal variabilit y of particulate organic radio-carbon in the northeast Pacific Ocean. Journal of Geophysical Research 101, 20543-20552), further supporting our contention that pyrophaeophorbide-a peaks 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 S POC that likely remains in the water column for long periods of time o r is derived from resuspended sediments. (C) 1998 Elsevier Science Ltd . All rights reserved.