Organic matter (OM) preservation in marine sediments is an important sink in the carbon cycle and provides useful information on the sources, biodegradation, and transformation of organic carbon during early diagenesis. Here, we present the first radiocarbon data set 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 northern South China Sea (SCS). The total lipids, THAAs, HAs and acid-base insoluble OM fractions accounted for 1.1±0.4%, 3.1±1.6%, 12.7±9.4% and 61.6±10.0% of the TOC, respectively, and the acid-base insoluble OM was the dominant fraction of TOC preserved in the sediments. Distinct differences in both ¹³C and ¹⁴C values were 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 may 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) 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. These results revealed that OM derived from marine and terrestrial sources decomposed selectively and contributed differently to the organic fractions during early diagenesis and preservation in marine sediment. The very aged acid-base insoluble fraction was likely formed as a complex of the most refractory OM during microbial degradation and alteration of TOC and represents a major mechanism of OM sequestration in marine sediments.
 

14C,lipids,Humic acids