Transformation and aging of organic matter during degradation and preservation in marine sediment of the northern South China Sea

Abstract

Organic matter (OM) preservation in marine sediments is an important sink in the carbon cycle and the isotopic records of the preserved OM provide useful information on the sources, biodegradation, and transformation of organic carbon during early diagenesis. Here, we present radiocarbon data combined with stable carbon isotope measurements of total organic carbon (TOC) and four organic fractions, namely, total lipids, hydrolysable amino acids (THAAs), humic acids (HAs), and acid-base insoluble OM, separated from four sediment cores collected from the South China Sea (SCS), to reveal their sources and transformation. The total lipids, THAAs, HAs and acid-base insoluble OM fractions account for 1.1 ± 0.4%, 3.1 ± 1.6%, 12.7 ± 9.4% and 61.6 ± 10.0% of the TOC (the percentages of organic fraction in TOC), respectively, and the acid-base insoluble OM is the dominant fraction of TOC preserved in the sediments. Distinct differences in both 13C and 14C values are found among these organic fractions. The δ13C values are more depleted in total lipids and HAs (−23.1‰ to −30.2‰) than in THAAs and the acid-base insoluble OM (−15.3‰ to −20.6‰), which indicates that HAs could be polymerized from lipid-like materials. The acid-base insoluble OM has the lowest Δ14C values (−848‰ to −358‰), with the oldest 14C ages (mean of 7330 years before present) compared to total lipids (−574‰ to −253‰, 3860 years), THAAs (−331‰ to −66‰, 1527 years) and HAs (−569‰ to −168‰, 2916 years) in these sediment cores. The δ13C and Δ14C values of THAA in the sediment are similar to those of pore-water dissolved organic carbon (DOC), indicating that THAA is preferentially degraded and converted to DOC. These results reveal that OM derived from marine and terrestrial sources decompose selectively and contribute differently to the organic fractions during early diagenesis and preservation in marine sediment. The very aged acid-base insoluble fraction is likely formed as a complex of the most refractory OM during microbial degradation and alteration of TOC, representing a major mechanism of OM sequestration in marine sediments.