Isotopic composition and speciation of sedimentary nitrogen and carbon in the Okinawa Trough over the past 30ka

Total organic carbon to total nitrogen ratios (C/N) and their isotopic compositions ([delta]13CTOC and [delta]15NTN) are oft-applied proxies to discern terrigenous from marine-sourced organics and to unravel ancient environmental information. In high depositional Asian marginal seas, the inclusion of N-bearing minerals in the bulk sediment dilutes or masks signals sought after in [delta]15N and [delta]13C analyses, thus leading to incorrect and even contradictory interpretations. We used KOH-KOBr to separate operationally defined total organic matter into oxidizable (labile) and residual fractions for content and isotope measurements. In a sediment core in the Okinawa Trough, significant amounts of carbon and nitrogen existed in the residual phase, in which the C/N ratio was ~9, resembling most documented sedimentary bulk C/N ratios in the China marginal seas. The residual carbon, which was included in operationally defined total organic carbon (TOC), displayed a [delta]13C range (-22.7 to -18.9[per thousand], mean -20.7[per thousand]) similar to the oft-used marine end-member. After removing the interference from the residual fraction, we found that the temporal pattern of [delta]13CLOC (LOC denotes labile organic carbon) was more variable. The residual nitrogen content was associated with illite, suggesting a terrestrial origin. Additionally, [delta]15N in the residual fraction likely reflected the climatic controls (e.g., precipitation) on lithogenic source materials. Further studies are required to explore the controlling factors for carbon and nitrogen isotopic speciation and to retrieve the information locked in the residual fraction. Key Points New insights from isotopic speciation of sedimentary C and N Significant amount of IN hindered the application of sedimentary C/N in Asian marginal seas IN is associated with illite and its isotope reflects the climatic controls on lithogenic materials