PHOSPHORUS DISTRIBUTION, C-N-P RATIOS, AND DELTA-C-13(OC) IN ARCTIC, TEMPERATE, AND TROPICAL COASTAL SEDIMENTS - TOOLS FOR CHARACTERIZING BULK SEDIMENTARY ORGANIC-MATTER

Onshore-offshore trends in phosphorus (P), organic carbon (OC), and to tal nitrogen (TN) concentration, P distribution, elemental organic C:N :P ratios, and stable carbon isotopic composition of OC (delta(13)C(OC )) of surficial sediments, are presented from three river-dominated co astal regimes: the Mackenzie River/Beaufort shelf in the Canadian Arct ic; the Mississippi Delta and Louisiana shelf in the temperate Gulf of Mexico; and the tropical Amazon shelf. These parameters, measured in surficial sediments from the three sites, are used to assess changes i n the importance of terrestrial and marine organic matter sources to s ediments as a function of distance from the locus of riverine discharg e.Trends in elemental ratio data from the Arctic transect, and a porti on of the Gulf of Mexico transects, can be explained in terms of a two -end-member mixture of terrestrial and marine phytodetritus. In the ar ctic transect, covariation of organic C:P ratios with delta(13)C(OC) i s consistent with a two-end-member mixture of terrestrial and marine o rganic matter. A similar relationship between C:P ratios and delta(13) C(OC) is not evident in Gulf of Mexico or Amazon shelf sediments. At t hese sites the two-end-member model fails to explain the data adequate ly. The most striking feature of the onshore-offshore trends in elemen tal OC:OP and OC:TN ratios is the occurrence of low ratios in sediment s from deep-water stations in the Gulf of Mexico and all stations on t he Amazon shelf, resulting from P and N enrichments exceeding the Redf ield Ratio for marine plankton. Three explanations are discussed to ac count for these low ratios: (1) the dominance of refractory OP- and ON -compounds in the residuum of degraded organic matter; (2) differentia l sorption of OP- and N-compounds, regardless of lability, on the surf aces of the fine-grained sediments characteristic of these sites; and (3) a dominance of bacterial biomass, or components derived from bacte rial biomass. Elemental ratios and delta(13)C(OC) signatures indicate that differential sorption may be most important in the deep-water Gul f of Mexico sites. In contrast, elemental ratios and delta(13)C(OC), o n the Amazon shelf are most consistent with a sedimentary organic matt er pool dominated by bacterial biomass, or derivative substances. (C) 1997 Elsevier Science B.V.