Lateral downslope transport and tentative sedimentary organic carbon box model in the southern Yap Trench,western Pacific Ocean

Sediment collapse and subsequent lateral downslope migration play important roles in shaping the habitats and regulating sedimentary organic carbon(SOC)cycling in hadal trenches.In this study,three sediment cores were collected using a human-occupied vehicle across the axis of the southern Yap Trench(SYT).The total organic carbon(TOC)and total nitrogen(TN)contents,δ13C,radiocarbon ages,specific surface areas,and grain size compositions of sediments from three cores were measured.We explored the influence of the lateral downslope transport on the dispersal of the sediments and established a tentative box model for the SOC balance.In the SYT,the surface TOC content decreased with water depth and was decoupled by the funneling effect of the V-shaped hadal trench.However,the sedimentation(0.0025 cm/a)and SOC accumulation rates(∼0.038 g/(m^(2)·a)(in terms of OC))were approximately 50%higher in the deeper hadal region than in the abyssal region(0.0016 cm/a and∼0.026 g/(m^(2)·a)(in terms of OC),respectively),indicating the occurrence of lateral downslope transport.The fluctuating variations in the prokaryotic abundances and the SOC accumulation rate suggest the periodic input of surficial sediments from the shallow region.The similar average TOC(0.31%–0.38%),TN(0.06%–0.07%)contents,and SOC compositions(terrestrial OC(11%–18%),marine phytoplanktonic OC(45%–53%),and microbial OC(32%–44%))of the three sites indicate that the lateral downslope transport has a significant mixing effect on the SOC composition.The output fluxes of the laterally transported SOC(0.44–0.56 g/(m^(2)·a)(in terms of OC))contributed approximately(47%–73%)of the total SOC input,and this proportion increased with water depth.The results of this study demonstrate the importance of lateral downslope transport in the spatial distribution and development of biomes.

Synchronous response of sedimentary organic carbon accumulation on the inner shelf of the East China Sea to the water impoundment of Three Gorges and Gezhouba Dams

Coastal seas,located between continents and the open ocean,are an important active carbon pool.The sedimentary total organic carbon(TOC)in these areas is a mixture of terrestrial and marine sources,and can be a powerful proxy for tracing natural processes and human activities.In this study,one fine-grained sediment core(DH5-1) from the inner shelf of the East China Sea was systematically analyzed for TOC and black carbon(BC) contents and TOC stable carbon isotope ratios(d13 C).By combining these data with 210~Pb dating,an improved carbon correction model and a two end-member mixing model,we reconstructed century-scale high-resolution sequences of corrected TOC,terrestrial TOC and marine TOC contents and identified two carbon depletion events in the DH5-1 record.The two events,shown as two minima in the TOC profiles,correspond temporally to 1985-1987 AD and 2003-2006 AD,which exactly matches the water impoundment of the Gezhouba Dam and Three Gorges Dam,respectively.In addition,the variations in TOC contents and δ^(13)C values before,during or after the minima demonstrate a relationship between the depletion events and water impoundment of the dams on the Changjiang River.The TOC reductions may represent synchronous responses of sedimentary TOC and resultant ecological ef fects on the inner shelf of the East China Sea to the water impoundment of the dams.These new TOC records reflect the interaction between natural and anthropogenic processes and,accordingly,provide a deep insight and important references for assessing marine ecological ef fects resulting from water impoundment of largescale dams.

Distribution, burial fluxes and carbon sink effect of sedimentary organic carbon in the eastern China seas

The ocean is the largest active carbon reservoir on Earth. Organic carbon(OC), as the primary species of carbon sequestration in the ocean, plays an important role in the global carbon cycle through its deposition and burial. In this study,sedimentary OC data from 5796 stations, together with relevant geochemical and sedimentological parameters in the Bohai Sea,Yellow Sea, and East China Sea(BYES) were used to summarize and elucidate the distribution and burial patterns of sedimentary OC, and assess carbon sink effect of sedimentary OC burial. The results show that the OC content in the sediments of the BYES ranges from 0.00% to 2.12%, with an average content of 0.47%±0.26%. OC content is significantly correlated with finegrained sediments, with an average OC content in mud areas being 39% higher than that in non-mud areas. Modern OC buried in the BYES are mainly deposited in 7 major mud areas, with a total sedimentary OC burial flux of approximately 8.20 Mt C yr^(–1).Among them, the burial flux of biospheric OC is 6.92 Mt C yr^(–1), equivalent to the OC consumption amount of silicate weathering of the 9 major river basins in the eastern China. In its natural state, the annually sequestered OC in the sediments of the eastern China seas is equivalent to 25.37 Mt of atmospheric CO_(2), indicating a significant carbon sink effect. The distribution and burial of terrigenous OC in the BYES are mainly influenced by the large river inputs and complex marine hydrodynamic environment,while human activities such as dam construction have significantly altered the OC burial in these coastal mud areas.

Sediment transport and carbon sequestration characteristics along mangrove fringed coasts

Mangroves play an important role in sequestering carbon and trapping sediments. However, the effectiveness of such functions is unclear due to the restriction of knowledge on the sedimentation process across the vegetation boundaries. To detect the effects of mangrove forests on sediment transportation and organic carbon sequestration, the granulometric and organic carbon characteristics of mangrove sediments were investigated from three vegetation zones of four typical mangrove habitats on the Leizhou Peninsula coast. Based on our results, sediment transport was often ＂environmentally sensitive＂ to the vegetation friction. A transition of the sediment transport mode from the mudflat zone to the interior/fringe zone was often detected from the cumulative frequency curve. The vegetation cover also assists the trapping of material, resulting in a significantly higher concentration of organic carbon in the interior surface sediments. However, the graphic parameters of core sediments reflected a highly temporal variability due to the sedimentation process at different locations. The sediment texture ranges widely from sand to mud, although the sedimentary environments are restricted within the same energy level along the fluvial-marine transition zone. Based on the PCA results, the large variation was mainly attributed to either the mean grain size features or the organic carbon features. A high correlation between the depth and δ13C value also indicated an increasing storage of mangrove-derived organic carbon with time.

Sedimentary and evolutionary characteristics of Sinian in the Tarim Basin

Analysis of Sinian outcrops in the peripheral regions of the Tarim Basin,drilling and seismic data within the Tarim Basin,and combined with previous studies,distribution of Sinian strata,tectonic setting,sedimentary system and lithofacies palaeogeography were well discussed.Results showed that three sedimentary systems were developed in the Sinian System of the Tarim Basin,i.e.the clastic sedimentary system,the clastic-carbonate mixed sedimentary system,and the carbonate sedimentary system,vertical sedimentary characteristics evolved from the clastic sedimentary system to the carbonate sedimentary system.These three sedimentary systems were corresponded to three transgression-regression cycles in the Sinian.The marine transgression during the initial period of the Early Sinian(ZSQI)resulted in development of the clastic sedimentary system in the lower part of the Lower Sinian;the transgression during the late period of the Early Sinian(ZSQII)led to formation of the clastic-carbonate mixed sedimentary system in the upper part of the Lower Sinian;the transgression during the initial period of the Late Sinian(ZSQIII)resulted in development of carbonate sedimentary system in the Upper Sinian.Three regressions resulted in formation of three unconformities,which were on the middle part of the Lower Sinian,the top of the Lower Sinian and the top of the Upper Sinian respectively.Due to breakup of Rodinia supercontinent in the Neoproterozoic,the Tarim Basin was in the continental rifting evolutionary process.In the Sinian,the basin experienced evolutionary processes of intra-cratonic depression and passive continental margin,thus characteristic of the lithofacies palaeogeography was characterized by uplifts in the south and depressions in the north.In the Early Sinian,the Tarim Basin developed the Bachu-Tazhong-Tadongnan uplift and the Tabei residual ancient land,and the shore-shelf sedimentary environment in the north as well as the shore-shelf-bathyal sedimentary environment in the southwest.In the Late Sinian,the Tabei residual ancient land disappeared;the basin developed the tidal flat sedimentary environment in the north and the neritic sedimentary environment of the passive continental margin in the southwest.

H_(2)O-induced sedimentary carbon migration from subducting slabs to the forearc mantle

Carbon in sedimentary carbonates dominates the global carbon input flux in subduction zones,the fate of which makes an impact on the global carbon cycle.At forearc depths,~32%of subducting water is released through slab dehydration and may greatly promote sedimentary carbon migration to the forearc mantle.However,it is controversial that considering the infiltration of external aqueous fluids,whether extremely limited or a significant portion of sedimentary carbon is liberated from subducting slabs in the forearc region.To explore to what extent hydrous fluids could facilitate carbon migration at forearc depths,hydrous carbonate-dominated sediment(1.14 wt.%H2O)-harzburgite reaction(layered)experiments have been performed at 1.5 GPa and 600–1000℃with various durations.For comparison,an anhydrous sediment-harzburgite reaction experiment was conducted to investigate the role of water on carbon migration.In hydrous experiments under subsolidus conditions(600–900℃),(1)a reaction zone comprised of clinopyroxene+dolomite forms at the sediment-harzburgite interface due to the metasomatic reaction;(2)the Ca#(100×Ca/[Ca+Mg+Fe],in molar)of calcite in the sediment layer drastically deceases when approaching the reaction zone;(3)newly formed dolomite and pargasite occur in the upper harzburgite layer.The above phenomena were not observed in the anhydrous experiment.Under a supersolidus condition(1000℃),a reaction zone composed of olivine+clinopyroxene+pargasite+CO_(2)formed as a result of hydrous carbonate melt-harzburgite interaction.The experiments demonstrate that aqueous fluids could significantly promote the chemical reaction and component exchange between sediments and mantle peridotite,and also induce subducting sedimentary carbon migration to the forearc mantle.It is estimated roughly that globally,~50%of subducting sedimentary carbon may be released at forearc depths.The carbon and water would be stabilized as carbonates(e.g.,dolomite)and hydrous minerals(e.g.,pargasite)in the forearc mantle,implying that the forearc mantle may be an important carbon reservoir.Our study explains the fate of a portion of carbon that is not returned to the atmosphere through arc volcanism.

The effect of iron on the preservation of organic carbon in marine sediments and its implications for carbon sequestration

Marine sediments are the most significant reservoir of organic carbon(OC)in Earth′s surface system.Iron,a crucial component of the marine biogeochemical cycle,has a considerable impact on marine ecology and carbon cycling.Understanding the effect of iron on the preservation of OC in marine sediments is essential for comprehending biogeochemical processes of carbon and climate change.This review summarizes the methods for characterizing the content and structure of iron-bound OC and explores the influencing mechanism of iron on OC preservation in marine sediments from two aspects:the selective preservation of OC by reactive iron minerals(iron oxides and iron sulfides)and iron redox processes.The selective preservation of sedimentary OC is influenced by different types of reactive iron minerals,OC reactivity,and functional groups.The iron redox process has dual effects on the preservation and degradation of OC.By considering sedimentary records of iron-bound OC across diverse marine environments,the role of iron in long-term preservation of OC and its significance for carbon sequestration are illustrated.Future research should focus on identifying effective methods for extracting reactive iron,the effect of diverse functional groups and marine sedimentary environments on the selective preservation of OC,and the mediation of microorganisms.Such work will help elucidate the influencing mechanisms of iron on the long-term burial and preservation of OC and explore its potential application in marine carbon sequestration to maximize its role in achieving carbon neutrality.