CHEMICAL AND ISOTOPIC EVIDENCE FOR THE IN SITU ORIGIN OF MARINE HUMIC SUBSTANCES1,2

Abstract

Humic and fulvic acids were extracted from marine and nonmarine Recent sediments and from soils, These acids are shown to be major components of the organic matter from marine and nonmarine sediments-some marine sediments may contain 70% of their organic carbon in the hmnic and fulvic acid fraction.Marine and terrestrial humic acids have similar carbon and hydrogen content, but the former generally contain more sulfur and nitrogen.

Acidification signals in estuarine sediment cores

“…Analysis of sedimentary HM fi-om coastal and littoral environments shows that the precipitated HM is deposited in the sediment (Nissenbaum and Kaplan 1972). A crude idea about the magnitude of the acidification signal in the investigated estuary (Topdal Fjord, Fig.…”

“…The amount of HM relative to total sedimentary organic matl;er (sedimentary organic C, not reported here, multiplied bly 1 .S) was 11% in the surface sediments and 16% in the deepest sample. Nissenbaum and Kaplan (1972) showed that in marine sediments, the sum of humic and fulvic acids can constitute 4-68% of the sedimentary organic C. In recent sediments from the Blake-Bahamas basin, Tanner basin, Walvis Bay, and Guayamas basin, humic acids constitute 12.8,2.7, 13.7, and 15-29% of the total sedimentary organic C (Stuermer et al 1978;Simonett et al 1979). The proportion of HM (isolated at pH 2 following extraction with pyrophosphate/NaOH) in surface sediments (O-5 cm) from the Baltic Sea ranges from 3 to 20% of the total organic matter (Poutanen 1985).…”

“…Moore et al 1979;Mantoura and Woodward 1983), most laboratory experiments and field observations show that various fractions of dissolved organic matter in stream waters, designated as humic substances, precipitate from the water column in estuaries (Sholkovitz et al 1978;Hunter and Liss 1979;Fox 1984). The distribution of 13C/12C isotopes in sediments across continental margins (Nissenbaum and Kaplan 1972;Schultz and Calder 1976) and the difference in isotopic co.mposition of riverine HM vs. oceanic HM (Hedges et al 1992) provide supporting evidence for this idea. Thus, sediment cores from estuaries should provide a record of the content of humic substances in river water discharging to the estuarine environment.…”

The characterization of humic substances in seawater

“…Wakesman, 1933; King, 1971, Ishiwatari, 1971;Nissenbaum and Kaplan, 1972;Bordovsky, 1965;Degens et al, 1964;Jackson, 1975.) Humic substances constitute the largest fraction of the organic matter in soils and modern sediments.…”

“…Soil and sedimentary humic substances show differences in elemental composition, specific density, functional groups, and products produced upon oxidative or reductive degradation which suggest differences in structure and a more aromatic character for the soil humic substances (Ishiwatari, 1971; Nissenbaum and Kaplan, 1972;Huc and Durand, 1974;Rashid and King, 1970;Ishiwatari, 1969). These investigations have demonstrated that humic substances are produced by decomposition and resynthesis in both soils and sediments, however, differences in the sources of organic matter and the properties of the environment influence the structural features of the final products.…”