Understanding the fate of terrestrial organic carbon (Corg) delivered to oceans by rivers is critical for constraining models of biogeochemical cycling and Earth surface evolution. Corg fate is dependent on both intrinsic characteristics (molecular structure, matrix) and the environmental conditions to which fluvial Corg is subjected. Three distinct patterns are evident on continental margins supplied by rivers: (a) high-energy, mobile muds with enhanced oxygen exposure and efficient metabolite exchange have very low preservation of both terrestrial and marine Corg (e.g., Amazon subaqueous delta); (b) low-energy facies with extreme accumulation have high Corg preservation (e.g., Ganges-Brahmaputra); and (c) small, mountainous river systems that sustain average accumulation rates but deliver a large fraction of low-reactivity, fossil Corg in episodic events have the highest preservation efficiencies. The global patterns of terrestrial Corg preservation reflect broadly different roles for passive and active margin systems in the sedimentary Corg cycle.

Burial of organic material in marine sediments represents a dominant natural mechanism of long-term carbon sequestration globally, but critical aspects of this carbon sink remain unresolved. Investigation of surface sediments led to the proposition that on average 10-20% of sedimentary organic carbon is stabilised and physically protected against microbial degradation through binding to reactive metal (e.g. iron and manganese) oxides. Here we examine the long-term efficiency of this rusty carbon sink by analysing the chemical composition of sediments and pore waters from four locations in the Barents Sea. Our findings show that the carbon-iron coupling persists below the uppermost, oxygenated sediment layer over thousands of years. We further propose that authigenic coprecipitation is not the dominant factor of the carbon-iron bounding in these Arctic shelf sediments and that a substantial fraction of the organic carbon is already bound to reactive iron prior deposition on the seafloor.

有机碳在海洋环境中的长期保存机制一直是海洋碳循环研究的重点,也是研究气候变化与全球碳循环之间作用和反馈的关键。据估算,表层海洋沉积物中约20%的有机碳是通过与氧化铁的结合而保存下来的,因此与氧化铁结合是有机碳长期保存的关键因素之一。研究表明,有机碳—氧化铁复合物的形成主要通过吸附和共沉淀这2种机制,共沉淀反应是有机碳与氧化铁在海洋环境中结合的主导机制。不同来源的有机物在发生与铁氧化物的共沉淀或吸附作用时是有选择性的,在大部分海洋环境中铁氧化物优先与海洋有机碳结合,但在河口三角洲区域,铁氧化物优先与陆源有机碳结合。大量的陆源输入,较高的初级生产和频繁的再悬浮活动使河口边缘海特别适于开展有机碳—氧化铁结合的相关研究,这也是今后研究的重点方向。

Understanding the mechanisms responsible for long-term storage of organic carbon (OC) in marine environment is important for studying the marine carbon cycling and predicting how the global carbon cycle will respond to climate change. It is estimated that more than 20% of the OC in marine sediments is associated with iron oxides and thus these complexes are one of the most important factors in the long-term storage of OC. The OC-iron oxide (OC-Fe) association can be formed through either adsorption or co-precipitation, but the dominant mechanism of OC-Fe association in marine environments is co-precipitation. The combination of OC from different sources with iron oxides is selective. Iron oxides preferentially combine with marine OC in most marine environments, but in estuarine delta regions they prefer terrestrial OC. Due to large inputs of terrestrial materials, high primary production and frequent re-suspension, estuarine and marginal seas are suitable sites for OC-Fe association studies, which should be emphasized in the future.

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding C dissolved organic matter (DOM) and Fe to soil slurries incubated under static or fluctuating O. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive Fe phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding Fe alone increases SOM mineralization by 8% following oxidation to Fe. Under O limitation, de novo reactive Fe phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.

To better understand the role of reactive Fe (FeR) in the preservation of sedimentary organic carbon (SOC) in estuarine sediments, we examined specific surface area, grain size composition, total OC (TOC), lignin phenols, FeR, FeR‐associated OC (Fe‐OC) and lignin phenols (Fe‐lignin), and δ13C of FeR‐associated OC (δ13CFe‐OC) in surface sediments of the Changjiang Estuary and adjacent shelf. An estimated 7.4 ± 3.5% of the OC was directly bound with FeR in the Changjiang Estuary and adjacent shelf. Unusually low TOC/specific surface area loadings and Fe‐OC/Fe ratios in mobile muds suggest that frequent physical reworking may reduce FeR binding with OC, with selective loss of marine OC. More depleted 13CFe‐OC relative to 13C of TOC (13Cbulk) in deltaic regions and mobile muds showed that FeR was largely associated with terrestrial OC, derived from extensive riverine OC and Fe inputs. A higher proportion of hematite in the mobile muds compared to the offshore samples indicated that Fe oxides are likely subjected to selective sorting and/or become mature during long‐term sediment transport. When considering the percentage of Fe‐OC to SOC and SOC burial rates in different marine environments (e.g., nondeltaic shelf, anoxic basins, slope, and deep sea), our findings suggest that about 15.6 ± 6.5% of SOC is directly bound to FeR on a global scale, which is lower than the previous estimation (~21.5%). This work further supports the notion of a Rusty Sink where, in this case, FeR plays an important role in the preservation and potential transport of terrestrial OC in the marine environment.

Factors influencing reactive Fe cycling and its protection of organic carbon (OC) in sediments are poorly understood. Here we comparatively study Fe speciation and Fe‐associated OC (Fe‐OC) in surface sediments of the Bohai Sea (BHS) and southern Yellow Sea (SYS), two seas with common sediment sources but different depositional regimes. Though significant sequestration of highly reactive Fe (FeHR) is expected in the estuarine system stream from the river‐dominated BHS, this pool is, however, slightly enriched in the BHS sediments relative to their source material. This reconfirms a previous speculation of sedimentary FeHR enrichment in semi‐protected settings. Relative to the BHS, the SYS sediments are depleted of FeHR, despite common sediment sources of these two areas. Estuarine pre‐enrichment and subsequent redistribution of FeHR in the BHS, and aging of Fe‐bearing authigenic clays during transport from the BHS to the SYS are potential mechanisms for the depletion. The fractions (fFe‐OC) of Fe‐OC in total OC in sediments of the two seas are at the lower end for soils and sediments, indicating Fe being a minor “rusty sink.” 13CFe‐OC fractionations indicate preferential sequestration of terrestrial OC by Fe oxides in the BHS, in contrast with preferential retention of marine OC in the SYS. Different fractionations of 13CFe‐OC in the two seas are a net result of selective adsorption of OC by Fe oxides and selective stabilization of OC during Fe reductive dissolution. Preferential sequestration of terrestrial OC may exert an important influence on distribution and compositions of OC buried in the river‐dominated system.