Radiocarbon and stable carbon isotope compositions of organic compound classes in sediments from the NE Pacific and Southern Oceans

Radiocarbon (Delta C-14) abundance and stable carbon isotope (delta C-13) compositions were measured for total lipid, total hydrolyzable amino acids (THAA), total carbohydrates (TCHO), and acid-insoluble organic fractions separated from phytoplankton, zooplankton, and sediment cores collected from two abyssal sites, one in the northeast (NE) Pacific Ocean and one in the Southern Ocean. These results are compared with those obtained for a separate core from the NE Pacific and reported by Wang et al. [Geochim. Cosmochim. Acta 62 (1998) 1365.]. An uncharacterized acid-insoluble fraction dominated (43-57%) the sediment TOC pool at both sites. Sediment collected from the NE Pacific exhibited higher TOC, TN. and higher Delta C-14 values and contained both labile (THAA and TCHO) and refractory (lipid and acid-insoluble) fractions. In contrast, sediment from the Southern Ocean had much lower TOC and Delta C-14 values, which indicated that organic matter was extremely refractory. Sedimentation rates calculated from C-14 ages of TOC for both sites indicate that the differences in the organic composition and Delta C-14 signatures of organic matter in the sediments likely resulted from the differences in production and deposition of organic matter to the sediment, and the diagenetic stages of sedimentary organic matter in the two oceans. Unlike Delta C-14, stable carbon isotopic (delta C-13) compositions of TOC in the sediments of the two oceans had similar values, which reflect not only the organic matter input from marine-derived sources but also the nature of degraded, predominately uncharacterized organic fraction in these sediments. The differences in delta C-13 values among the compound classes in sediments at both sites can be attributed to the carbon isotopic signatures in original sources during photosynthesis and associated environmental factors, Preferential degradation of organic matter and heterotrophic effects may also play important roles for the observed delta C-13 variations and these effects need to be further investigated. (C) 2001 Elsevier Science B.V. All rights reserved.