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Mangrove forests can help to mitigate climate change by storing a significant amount of carbon (C) in soils. Planted mangrove forests have been established to combat anthropogenic threats posed by climate change. However, the efficiency of planted forests in terms of soil organic carbon (SOC) storage and dynamics relative to that of natural forests is unclear. We assessed SOC and nutrient storage, SOC sources and drivers in a natural and a planted forest in southern Thailand. Although the planted forest stored more C and nutrients than the natural forest, the early-stage planted forest was not a strong sink relative to mudflat. Both forests were predominated by allochthonous organic C and nitrogen limited, with total nitrogen being a major driver of SOC in both cases. SOC showed a significant decline along land-to-sea and depth gradients as a result of soil texture, nutrient availability, and pH in the natural forest. • Fine soil and high nutrients in planted mangrove forest led to greater organic carbon storage compared to natural forest. • The early-stage planted mangrove forest did not outperform adjacent mudflat as a significant sink for carbon and nutrients. • Total nitrogen content controlled soil organic carbon content for both natural and planted mangrove forests. [ABSTRACT FROM AUTHOR]

Terrestrial sediment supply and its composition, aquatic productivity and coastal hydrodynamics together control the nature of organic matter (OM) that buried in ocean margins. Distinguishing their individual roles in river-dominated continental margins is challenging, but crucial for understanding (i) the regional carbon cycle in modern times and (ii) the OM burial-climate link on shorter timescales. Here we investigate the contents of total organic carbon (TOC) and total nitrogen (TN) and stable carbon isotopic composition of TOC (δ13C), along with grain size and calcium carbonate, in three short, 210Pb- and 137Cs-dated, sediment cores (DH1-1, DH3-3 and DH5-1) from the Yangtze distal mud belt in the East China Sea to delineate the spatiotemporal distribution, origin and burial flux of sedimentary OM for the last ca. 130 years. We also use selected inorganic elements (Si, Al, Ca, Na, K, Rb and Sr) and geochemical proxies derived from them (i.e. chemical index of alteration-CIA, molar Al/Si, molar K/Na, K/Rb and Rb/Sr) to elucidate the importance of detrital composition and chemical weathering on the supply, transport and burial of OM. Bulk organic geochemical and grain size data suggest a longer transport of fine sediments-laden OM, indicating the role of China Coastal Current on hydrodynamic sorting in the study area. Spatial distribution of Al/Si, K/Na, Rb/Sr and CIA in all cores corroborates the selective transport of fine-grained minerals from the Yangtze estuary to the distal mud belt. Less variable molar C/N ratios (5.6–7.7) and δ13C values (–22.4 to –21.2‰) in our cores and the results of δ13C mixing model indicate the predominance of marine OM burial over the last century in the study area. Relationships of TOC, δ13C and mean grain size and spatial evolution of δ13C along the river-mud belt continuum show that adsorption of marine OM on the Yangtze-derived fine sediments during their southward transport. Estimated mean burial fluxes of terrestrial and marine OC (0.73 ± 0.21 and 1.88 ± 0.29 Tg C yr−1) indicate that the Yangtze distal mud belt, despite covering ~18% of the shelf area, acts as a key OM sequestration center (>50%) in the East China Sea. Comparison of records of riverine OC burial and geochemical weathering and grain size proxies of our cores with East Asian winter monsoon index, and sediment and water discharges in the Yangtze River confirms that hydrodynamics associated with the winter monsoon winds-driven China Coastal Current predominantly controlled the sediment accumulation and OM burial along the mud belt before 1970s. Nonetheless, human activities especially the construction of Three Gorges Dam have reduced the burial flux of terrestrial OC from 0.93 to 0.54 Tg yr−1, significantly affecting the supply and burial of OM and thus the regional carbon cycle since 1970s. [ABSTRACT FROM AUTHOR]

The major source of particulate matter in the East China Sea (ECS) is the Changjiang (Yangtze) River. Sediment types, the geochemical indices of terrigenous and biogenic inputs (TOC, CaCO 3 and Sc), and biomarker indices such as the carbon preference index (CPI) of long-chain n-alkanes and the cinnamyl/vanillyl ratio (C/V) in surface sediments, all reveal that the influence of terrestrial material initially declines away from the mouth of the Changjiang River across the ECS continental shelf. However, the influence then strengthens from the middle ECS shelf toward the continental slope and the Okinawa Trough, because when the northeast winds prevail from September to April, the Changjiang River plume flows southwestward along the coast of China. Part of this flow turns eastward in the northern Taiwan Strait, and then joins the northeastwardly flowing Kuroshio to reach the Okinawa Trough. As the central ECS is bypassed, the sediments accumulated there are geologically older, carbonate-rich and organic-poor than those found off the coast of China and in the Okinawa Trough. Between June and August, when southwest winds prevail, the Changjiang plume generally flows northeastward. Yet, strong cyclonic currents that are generated by typhoons that pass through the ECS move the suspended particulates and the resuspended sediments southwestward from the coast of China. The turbid water then turns toward Taiwan in the northern Taiwan Strait and joins the Kuroshio, before reaching the Okinawa Trough. Again, young sediments are transported to the Okinawa Trough without passing through the central ECS. [ABSTRACT FROM AUTHOR]

Grain size is an important controlling factor of the chemical composition of sediments. To understand the role of grain size in the content of rare earth elements (REE) in tropical mountainous river sediments and to establish suitable provenance proxies, sediments collected from Pahang River and Kelantan River, Peninsular Malaysia, were determined for REE contents in six size fractions (<2 μm, 2–4 μm, 4–8 μm, 8–16 μm, 16–32 μm, and 32–63 μm). The results indicate that the total REE contents (∑REE) from these two rivers are weakly correlated with grain sizes. The normalized REE patterns of various grain-size fractions to <63 μm fractions revealed a flat pattern for light rare earth elements (LREE), but were gradually enriched in heavy rare earth elements (HREE) with increasing grain size. These results indicate that the grain size constraint on REE is very complex. The relationship between REE contents and mineral compositions of size-fractioned sediments further suggests that clay minerals cause high REE contents in clay components (<2 μm). The dilution effect of quartz and feldspars on REE is another controlling factor, leading to low REE content in medium-fine silt components (2–32 μm). In addition, heavy minerals, zircon, monazite and amphibole, play an important role in enriching REE in coarse silt components (32–63 μm), especially zircon as the major contributor to the enrichment of HREE with increasing grain size. By combining the content of REE, the normalized REE patterns and the REE characteristic parameters for different grain sizes, we find that the 4–8 μm fraction is sensitive to REE variation among the six size fractions investigated. Furthermore, the relationship between upper continental crust-normalized δEu and (Gd/Yb) is reliable for distinguishing the sediment characteristics of Pahang River and Kelantan River and can therefore be used as an effective indicator for identifying Malaysian river-sourced sediments in the southern South China Sea. • REE compositions in different grain-size fractionated sediments from two tropical mountainous rivers are different. • Mineral composition is the main controlling factor of the grain size effect on REE. • The 4–8 μm fraction represents REE variation sensitively among six size fractions investigated in these two rivers. • δEu UCC -(Gd/Yb) UCC discriminant diagram can be used to distinguish Malaysian river-sourced sediments in the southern SCS. [ABSTRACT FROM AUTHOR]

Large river systems accumulate, process, and transport huge quantities of organic matter (OM) from their catchments, part of which is exported to the ocean. Although this suite of processes comprises an important component of the global carbon cycle, integrated studies examining the nature and extent of OM processing on a basin-wide scale remain rare. Here, we provide an overview of provenance and composition of OM in suspended and deposited sediments within the Pearl River watershed in South China. We present new data on the organic carbon (OC) and total nitrogen (TN) contents, stable carbon and radiocarbon isotopic compositions of OC (δ13C and F m), as well as grain size distribution and mineral-specific surface area of Pearl River sediments. These results are combined with published data on suspended particulate matter (SPM) and soil profiles in the Pearl River watershed in order to determine the provenance and transformation of OM in this large subtropical fluvial system. We find that the low 14C contents, expressed as fraction modern (F m) values, in suspended (F m : 0.58–0.87) and sedimentary OM (F m : 0.38–0.82) are attributed to contributions from 14C-depleted soils, bedrock, as well as riverine primary productivity (R pp) that utilizes 14C-depleted sources of dissolved inorganic carbon (DIC). For SPM, soil OM (F m : 0.87 ± 0.13) is inferred to be the dominant fraction during the wet season, whereas the contributions of R pp (F m : 0.86 ± 0.04) and petrogenic OC (devoid of 14C) are enhanced during the dry season. This manifests itself in differences in OC soil , OC Rpp , and OC petro contributions in SPM between wet and dry seasons (1.05 ± 0.18 vs. 1.20 ± 0.50%, 0.16 ± 0.03 vs. 0.32 ± 0.15% and 0.21 ± 0.07 vs. 0.38 ± 0.19%). During erosion and transport, the most labile OM in the top soil is rapidly degraded, as indicated by a stronger contribution from soil CO 2 into riverine DIC during the wet season and flood event (39 ± 1% and 45 ± 3%) compared to the dry season (31 ± 2%). River sediments are primarily accumulated during the wet season when suspended sediment fluxes are high. Refractory deep soil OM (F m : 0.74 ± 0.07) dominate in these sediments, whereas moderately labile soil OM components are further degraded during settling and storage. This sedimentary OM is predominantly composed of aged soil (92 ± 4%), with a minor contribution from bedrock (7 ± 4%) and negligible input from R pp (0.2 ± 0%). The longitudinal changes in the composition of suspended and sedimentary OM are mainly controlled by input of R pp and ongoing degradation processes in the river system. The riverine particulate OC flux to the Pearl River estuary and ocean thus contains a mixture of soil, petrogenic and R pp OM, all exhibiting relatively low F m values. The riverine OM transformation and dynamics are important for the short-term carbon cycle, whereas the remaining signature and fate of the extensively processed, refractory OM has implications for the long-term carbon cycle. [ABSTRACT FROM AUTHOR]

Abstract Anaerobic oxidation of methane (AOM) coupled with sulfate reduction (SR) generates authigenic carbonate which has the unique carbon and oxygen isotope composition in sulfate methane transition zone (SMTZ). The carbon and oxygen isotope composition of carbonate in sediments, which can be used to revel the variable methane flux, is affected by the percentage of AOM-driven carbonate in sediments. To investigate the methane hydrate decomposition history in the southwest Taiwan Basin, we analyzed a 13 m-long sediment core for grain size, carbonate and elemental carbon contents and stable carbon and oxygen isotopes composition of carbonate (δ13C carb and δ18O carb). The AMS 14C dating results and carbonate content reveal that a turbidity current occurred at around 14 ka B.P., which reversed sediment accumulation between 455 and 885 cm in depth. Combining the chronological framework and δ13C carb values, we identified that the methane hydrate decomposition scale began to decrease at 14 ka B.P. Moreover, we discovered that three transiently enhanced methane hydrate decomposition events occurred from then on. Although the δ13C carb and δ18O carb variability are as expected, the low δ18O carb values are inconsistent with the previous theory which favors 18O-enriched in AOM-driven authigenic carbonate. Thus, we suggest that a large amount of 18O-depleted water which originally migrated from southwest Taiwan island was injected into surface sediment along with the eruption of mud volcanos. As the terminal electron accepter of AOM-SR reaction, the exogenous 18O-depleted water leads its special oxygen isotope to be inherited by the authigenic AOM-driven carbonate. The proposed theory provides new insights on the authigenic carbonate in methane hydrate area and the association between methane hydrates and mud volcanoes. Highlights • At least three enhanced methane hydrate decomposition events occurred during last 14 ka. • Methane hydrate decomposition associated with the mud volcano eruption in study area. • Mud volcano eruption injected large amount of 18O-depleted water into surface sediment. • The 18O-depleted water may come from Taiwan Island, as submarine groundwater discharge. [ABSTRACT FROM AUTHOR]

Variations in petrography, stable isotopes, reflectance, and luminescence along the central growth axis of a 14.5 cm stalagmite from Panigarh cave indicate cooler and slightly wetter conditions in the Himalayan foothills of northern India during the Little Ice Age (LIA), which lasted from ∼AD 1489–1889 based on deposition of calcite, and AD 1450–1820 based on rapid changes in δ 18 O values. Conditions were warmer and drier during the preceding Medieval Climate Anomaly (MCA) and also in the post-LIA periods, as evidenced by deposition of aragonite. A review of currently existing stalagmite and other proxy data from south and east Asia reveals a broad spatial pattern in precipitation over south and east Asia during the LIA, with northern areas showing generally increased precipitation and southern areas reduced precipitation. During the MCA and after the LIA, the records suggest this pattern was reversed. Weaker ISM during the LIA brought drought conditions to the core ISM area but triggered more monsoon ‘breaks’ that brought higher precipitation to the Himalayas. At the same time, the weaker ISM may also have pushed more depressions along the path of the southern winter jet which brought more winter precipitation to the Himalayas and therefore a LIA wetter in our study area. [ABSTRACT FROM AUTHOR]

As an integral part of the Earth's climate system, the Kuroshio Current (KC) plays a crucial role in shaping the regional oceanography and climate in the Northern Hemisphere. However, how the KC dynamics have varied over glacial-interglacial cycles is still under debate. The dynamic transfer and accumulation of submarine hydrothermal source materials by deep-reaching KC offer us a unique opportunity to examine the variations in dynamics of the KC. Here, we used novel proxies of sedimentary mercury (Hg) and antimony (Sb) in core MD01-2404 retrieved from the middle Okinawa Trough (OT) to reconstruct the evolution of the KC hydrodynamics over the last 92,000 years. We infer the enrichments of sedimentary Hg and Sb to signify hydrothermal input, which is delivered laterally to the study site by deep circulation in association with the KC, thus indicating the dynamics of KC. Overall, both the sedimentary Hg and Sb in core MD01-2404 indicate a persistent influence on the KC dynamics within the OT over the last glacial-interglacial cycles. Furthermore, our Hg and Sb proxies suggest a significantly weakened influence during the last deglaciation and last glacial period while a strengthened influence during the Holocene and late Marine Isotope Stage 5. Our studies imply that the orbital-scale dynamics of KC are controlled by tropical atmosphere-ocean interactions induced by sea surface temperature changes and regulated by the extratropical climate conditions. • We reconstructed the first 92,000-year records of sedimentary mercury and antimony from the middle Okinawa Trough. • The extent of hydrothermally-contributed enrichments of sedimentary Hg and Sb reflects the dynamics of Kuroshio Current. • Orbital-scale Kuroshio Current dynamics are controlled by both the tropical and extra-tropical climate. [ABSTRACT FROM AUTHOR]

To understand the modern sedimentation and land-sea interaction processes in the Sunda Shelf, we evaluated the rare earth element (REE) compositions, the total organic carbon (TOC) contents, grain sizes, and detrital minerals of 39 surface sediments, collected from the western Sunda Shelf. All of the sampled sediments are characterised by a higher content of light REEs (LREEs) relative to the content of heavy REEs (HREEs) with negative Eu anomalies. The total content of REE (ΣREE) in the sediments ranges from 7 μg/g to 193 μg/g, with an average of 119 μg/g. Due to the uneven spatial distribution of REEs in the study area, a Q-cluster analysis is applied to the mean grain size (Mz), ∑REE, δEu, δCe, (La/Yb) N , and (Gd/Yb) N data. On this basis, the study area can be classified into three geochemical provinces. The north to central zone in the study area is designated as province I, which has the highest REE content among the three provinces. This is a result of the finer grain size, dominance of siderite, and higher TOC contents of the sediments in province I. The REE content of province II in the southwest coastal zone is lower than that of province I due primarily to a coarser grain size and high quartz, biodetritus, and plagioclase contents. Covering the central coastal zone, province III has the lowest REE content among the three provinces because of the extremely coarse grain size and very high quartz and biodetritus contents. The upper continental crust (UCC)-normalised REE patterns differ markedly among the three provinces, thereby indicating different provenances and transport mechanisms. Based on the provenance discrimination plot and the UCC-normalised REE patterns of these three provinces and adjacent rivers, we infer that province I is largely fed by the Mekong and Kelantan rivers. The fine-grained sediments from these rivers are transported to north and central zone by the northeast monsoon current. The Pahang River is identified as the main sediment source of province II. The coarse-grained sediments from this river mostly accumulate near the estuary via hydrodynamic sorting, while finer-grained sediments are transported to the south by the southward coastal currents. The sediments in province III are inferred as being primarily the result of coastal erosion due to the strong wave energy in this area, as well as offshore and longshore sediment transport acting, in parallel to the coast line. Our study can serve as a reference for studying the diffusion and transport processes of mountainous river sediment and the evolution of the paleoclimate and paleoenvironment in tropical low-latitude regions. • Surface sediments of the western Sunda Shelf can be classified into three geochemical provinces. • Grain size and heavy minerals play important constraint roles on REE distribution. • Different geochemical provinces have their unique provenances. • Erosion, transport and deposition are mainly controlled by northeast monsoon current, coastal current and wave energy. [ABSTRACT FROM AUTHOR]

• Various types of soils and bedrocks contribute to variability of sedimentary OC composition in different rivers. • Biospheric OC contents are higher in sediments of northwestern rivers than that of southern rivers. • Marked differences in fluxes of biospheric and petrogenic OC emerge for different proxy constraints tracing OC provenance. Oceanic islands in the western Pacific export ∼35% of particulate organic carbon (OC) to the world ocean, but composition and age of OC varies dramatically among and within the islands. Understanding island-wide variation in sources and controls on the sedimentary OC in a series of rivers is therefore essential to assess the role of oceanic islands on the global carbon cycle. Stable and radioactive carbon isotopic compositions of sedimentary OC in 17 rivers across Taiwan indicate various proportions of biospheric OC derived from soil overlain by C3 and C4 vegetation and petrogenic OC from sedimentary and metamorphic rocks. Higher fractions of biospheric OC occur in sediments of northwestern rivers, while petrogenic OC is dominant in southern river sediments. Vegetation type, lithological setting, and sediment yield in river basins largely control island-wide OC compositions in river sediments. Considerable differences in export fluxes of biospheric and petrogenic OC based on different provenance proxies exist for individual rivers, highlighting the need for additional radiocarbon measurement (with high sample frequency) to better understand the role of oceanic islands in the global carbon cycle. [ABSTRACT FROM AUTHOR]