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

Abstract

Total organic carbon to total nitrogen ratios (C/N) and their isotopic compositions (δ 13 C TOC and δ 15 N TN ) 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 δ 15 N and δ 13 C 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 δ 13 C range (À22.7 to À18.9‰, mean À20.7‰) similar to the oft-used marine end-member. After removing the interference from the residual fraction, we found that the temporal pattern of δ 13 C LOC (LOC denotes labile organic carbon) was more variable. The residual nitrogen content was associated with illite, suggesting a terrestrial origin. Additionally, δ 15 N 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.

Soil Carbon, Nitrogen, and Phosphorus Stoichiometry and Fractions in Blue Carbon Ecosystems: Implications for Carbon Accumulation in Allochthonous-Dominated Habitats

“…We found that 25.7 ± 4.2%, 44.4 ± 5.0%, and 31.9 ± 7.6% of the total N occurred as inorganic N (N fix ) fraction in the surface soils of mangroves, saltmarshes, and seagrasses, respectively. Additionally, the percentages of KOBr-resistant OC (OC res ), likely dominated by black carbon and some lithogenic kerogen, were consistent with the significant contribution of black carbon (9–25%) with ages of 4097–15907 BP in deltaic BCEs . The δ 13 C res values of mangroves and seagrasses were higher and lower, respectively, than the δ 13 C tot and δ 13 C KOBr values (Figure b, Table S4).…”

“…Although allochthonous particle deposition and mineral protection are key biogeochemical controls of OC accumulation in BCEs, we observed distinct mineral effects on N and P fractions and OC density (Figure ), suggesting different roles for minerals during transportation and deposition of these three elements. During transportation, large riverine allochthonous inputs promote the accumulation of recalcitrant N and P fractions because clay minerals (e.g., Illite) are the dominant carrier of inorganic N (N fix ) and many negatively charged P species are strongly adsorbed to positively charged mineral surfaces (e.g., amorphous Al/Fe oxides); , thus, they may dilute the relatively labile organic fractions, which together with autochthonous OC have proven to be important fractions contributing to OC preservation according to our model results. After deposition, the physical protection provided by minerals and aggregates, if it exists, and the formation of organomineral complexes , may facilitate accumulation of autochthonous OC mainly originating from plant–microbe interactive residuals or dissolved OC. , These results provide empirical evidence that terrestrial (riverine)-dominated BCEs are beneficial for preservation of refractory and mineral-bound organic matter and nutrients and increase the adaptive capacity of BCEs to cope with climate change.…”

“…The δ 13 C res values of mangroves and seagrasses were higher and lower, respectively, than the δ 13 C tot and δ 13 C KOBr values (Figure b, Table S4). The δ 13 C res values indicated terrestrial sources that were dominated by black carbon and lithogenic kerogen. − The δ 13 C tot and δ 13 C KOBr values were indistinguishable, indicating little diagenetic changes of δ 13 C KOBr since diagenesis may result in an enrichment of organic matter in 13 C during its degradation. The carbon isotopic compositions suggest a source-dominated signature rather than a diagenetic-dominated signature.…”

“…Previous studies have indicated a significant presence of inorganic N in marine sediments , and terrestrial soils, but this N fraction has never been explored in BCEs. We found that 25.7 ± 4.2%, 44.4 ± 5.0%, and 31.9 ± 7.6% of the total N occurred as inorganic N (N fix ) fraction in the surface soils of mangroves, saltmarshes, and seagrasses, respectively.…”

Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

“…Moreover, clay mineral evidence illustrates that the relative contributions of lithogenic particles from the adjacent land present a distinct variability from the last glaciation to the Holocene [e.g., Liu et al ., ]. Since the clay‐fixed N may carry the climate information from land and the occupation of clay‐fixed N in total N is relatively high in China's marginal seas [ Zheng et al ., ], it is conceivable that changes in climate and the sediment source region may have also led to the variability of down‐core δ 15 N sed .…”