Phosphorus distribution, C:N:P ratios, and δ13Coc in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary organic matter

Abstract

Onshore-offshore trends in phosphorus (P), organic carbon (OC), and total nitrogen (TN) concentration, P distribution, elemental organic C:N:P ratios, and stable carbon isotopic composition of OC ( δ 13 C oc ) of surficial sediments, are presented from three river-dominated coastal regimes: the Mackenzie River/Beaufort shelf in the Canadian Arctic; 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 in the importance of terrestrial and marine organic matter sources to sediments as a function of distance from the locus of riverine discharge. Trends in elemental ratio data from the Arctic transect, and a portion of the Gulf of Mexico transects, can be explained in terms of a two-end-member mixture of terrestrial and marine phytodetritus. In the arctic transect, covariation of organic C:P ratios with δ 13 C oc is consistent with a two-end-member mixture of terrestrial and marine organic matter. A similar relationship between C:P ratios and δ 13 C oc is not evident in Gulf of Mexico or Amazon shelf sediments. At these sites the two-end-member model fails to explain the data adequately. The most striking feature of the onshore-offshore trends in elemental OC:OP and OC:TN ratios is the occurrence of low ratios in sediments from deep-water stations in the Gulf of Mexico and all stations on the Amazon shelf, resulting from P and N enrichments exceeding the Redfield Ratio for marine plankton. Three explanations are discussed to account for these low ratios: (1) the dominance of refractory OP- and ON-compounds in the residuum of degraded organic matter; (2) differential sorption of OP- and N-compounds, regardless of lability, on the surfaces of the fine-grained sediments characteristic of these sites; and (3) a dominance of bacterial biomass, or components derived from bacterial biomass. Elemental ratios and δ 13 C oc signatures indicate that differential sorption may be most important in the deep-water Gulf of Mexico sites. In contrast, elemental ratios and δ 13 C oc on the Amazon shelf are most consistent with a sedimentary organic matter pool dominated by bacterial biomass, or derivative substances.