The production and remineralization of pelagic porewater dissolved organic carbon (DOC) are of great significance for its sequestration in deep-sea sediments and the circulation of oceanic DOC. Here, we present the results of concentration and carbon isotope (¹³C, ¹⁴C) measurements of solid phase organic (SOC), DOC and dissolved inorganic carbon (DIC) in the overlying water and multi-core sediments collected from the Philippine Sea. Our results indicate that selective degradation and sequestration of SOC and porewater DOC occurred in the given multi-cores. The SOC, dominated by surface photosynthesis, was decomposed in the overlying and sediment interface rapidly, making porewater DOC concentrations about ten-fold higher than that of overlying water. After deposition, SOC was degraded continuously and its D¹⁴C values were almost decay below 10-15 cm, making the residue a carbon sink. The degradation of SOC and porewater DOC had little effect on porewater DIC. Although porewater DOC aging with depth, its remineralization pathway showed different pattern compared to SOC. The results of dual-carbon isotope three end-member model revealed the transformation mechanism of OC components with different reactivities of porewater DOC. There was a conversion from labile fraction to refractory fraction in porewater DOC with depth and the refractory DOC was accumulated as a carbon sink in the sediments. Otherwise, the DOC escaping from sediment is a matrix of labile and refractory components, where almost all labile components are consumed at the interface, allowing the aged refractory DOC to participate in the oceanic DOC cycle.
 

deep-sea sediments,porewater DOC,refractory DOC,radiocarbon,stable carbon isotope