Sources and radiocarbon ages of organic carbon in different grain size fractions of Yellow River-transported particles and coastal sediments

Abstract

The elemental (TOC, TN and C/N) and carbon isotope (Δ14C and δ13C) compositions of organic matter were measured in different grain size fractions of particles transported by the Yellow River and surface sediments in the Bohai Sea and the Yellow Sea. In the riverine particle and sediment samples, high OC contents were associated with small grain size fractions consisting mainly of clay minerals. The δ13C and Δ14C values of the bulk riverine particulate organic carbon (POC) collected from the Lijin and Xiaolangdi sites were relatively constant but varied significantly (−21.9‰ to −26.0‰ and −325‰ to −620‰, respectively) among the different size fractions. In comparison, large spatial variations in δ13C (−20.6‰ to −24.5‰) and Δ14C (−188‰ to −646‰) values, which increased seaward due to the difference in source carbon, were found for bulk TOC Δ14C preserved in the surface sediments, but no significant differences were observed among the values in the different size fractions in most sediments. The different carbon isotopic values of the riverine POC and sedimentary TOC reflect differences in the sources, degradation and cycling time scales of the OC. The Yellow River exports very old (5220 ± 295 yrs) POC that is much older than the TOC (2457 ± 676 yrs) preserved in the surface sediments in the Bohai and Yellow seas. Calculations using a dual-isotope three end-member model indicate that pre-aged soil OC and ancient fossil OC represent major proportions (57 ± 16% and 30 ± 8%, respectively) of the riverine POC and that terrestrial biomass OC represents a minor proportion (13 ± 11%). The drainage environment of the river plays important roles in controlling the sources and ages of the riverine POC. In contrast, the TOC in the grain size fractions of the surface sediments in the Bohai and Yellow seas is dominated by marine-derived modern OC (47 ± 13%), followed by pre-aged soil OC (29 ± 9%) and ancient fossil OC (25 ± 14%). The ages of the TOC are determined mainly by the source input, rapid sedimentation, sediment minerology and decomposition of OC during early diagenesis in these large river-influenced marginal seas.