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Hyperpycnal flows are important agents for transporting detrital sediments from rivers to oceans. Previous studies often assumed that the deposits of flood‐controlled delta fronts would be dominated by graded sand with common hummocky cross‐stratification. This study documents, for the first time, hybrid event beds and plumite deposits inside the succession of a modern delta front. This delta in the Taiwan Strait is fed by the Choshui River, which is considered a highly efficient sediment transport system with individual floods with extremely high suspended sediment concentrations. The deltaic event beds recorded in the core were interpreted as hyperpycnal flow‐generated hybrid event beds and turbidites triggered by hypopycnal flows. Accelerator Mass Spectrometry 14C dating, grain‐size analysis and measurements of stable isotopic composition of organic matter were conducted to delineate the depositional process of the recognized beds. The internal subdivisions of the hybrid event beds were differentiated mainly based on sedimentary textures, including cohesive mud content, sand content and sorting. The disorganized portion (division H3) appears internally chaotic and contains large rafted substrate clasts but also displays an upward increase in dispersed cohesive mud from 35% (division H3a) to 50% (division H3b). In contrast, massive H1 divisions are characterized by much lower cohesive mud of ca 8%. The vertical arrangement between depositional facies allows the discrimination of three hybrid event bed types. The stable carbon isotopic composition of the organic matter reveals that the cohesive mud in each division of the hybrid event beds was sourced from marine substrate, rather than supplied by the original hyperpycnal flows. Therefore, the hybrid event beds are generated by energetic hyperpycnal flows, which can delaminate the muddy sea‐floor and incorporate large quantities of substrate fragments. The bulking of erosional hyperpycnal turbulent flow forces flow transformation and generates a more cohesive flow with a turbulent, dense head and trailing bipartite debris flow. The occurrence of hybrid event beds in modern shallow delta front settings is also an important novelty, implying a high density of hyperpycnal flows at the mouth of sediment‐laden rivers as well as the crucial impact of substrate entrainment on the development of hybrid event beds. [ABSTRACT FROM AUTHOR]

Tropical rivers deliver ∼60% of particulate organic carbon to the world ocean. However, compositions and loadings of sedimentary organic matter (OM) from tropical small mountainous rivers are largely unknown. Here, we provide an initial constrain on sources of sedimentary OM from 28 fluvial systems across Luzon in the Philippines by measuring total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C) and radiocarbon activity of TOC (expressed as fraction modern—Fm), as well as grain size and mineral surface area (SA) of sediments. Results indicate that sediments in Luzon rivers contain both contemporary and 14C‐depleted OM (Fm: 0.71–1.06, mean 0.97 ± 0.07) with a wide range of δ13C values (−28.3‰ to −17.7‰, −24.9 ± 2.2‰). This is attributed to the OM sources originated from modern surface soil and 14C‐depleted subsoil and deep soil vegetated by C3 and C4 plants, with mean fraction of C3 plant at 80% ± 11%. Minor input from bedrock may also contribute to the 14C‐depleted OM in sediments, accounting for 6% ± 6%. Sediments in most rivers are featured by low OC loadings (OC/SA ratio < 0.4 mg C m−2), owing either to a less OM input or intensive OM degradation. The estimated yields of particulate OC from Luzon vary between 3.2 and 3.7 t km−2 yr−1, which is higher than most tropical large rivers. Key Points: C3 and C4 plants underlain by modern surface soils represent the main contribution to organic matter (OM) found in Luzon river sedimentsDegraded OM in subsoils and deep soils or bedrock contribute to radiocarbon (14C)‐depleted OM in Luzon riversThe yields of particulate organic carbon from tropical small mountainous rivers on Luzon are much higher than that of other tropical large river systems [ABSTRACT FROM AUTHOR]

The East China Sea Shelf has an unusually wide and low gradient shelf, supplied from sediment‐charged rivers and large river delta systems, with bottom currents sweeping the sea floor and located in the path of strong typhoons. Sediment gravity flow deposits, including four hybrid event beds and a high density turbidite, are identified in a core from the mid‐shelf of the East China Sea. The hybrid event beds typically comprise three or two internal divisions from the base to the top: (i) H1, H3 and H5; or (ii) H3 and H5. Radiocarbon ages of the hybrid event beds were in the range of 3821 to 8526 yr bp. Based on correlation with surrounding cores, the hybrid events may have happened at any time between 1930 yr bp and 3890 yr bp. The δ13C values in hybrid event beds together with bathymetry data suggest local erosion on the shelf. The average δ13C value for the H1 division is similar to the H3 division in the hybrid event beds, implying that the organic matter in the H1 and H3 divisions may come from the same source area. Cross‐plots of upper continental crust normalized rare earth elements in the five units reveal that the sediment source of the four hybrid event beds and the turbidite was ultimately primarily from Korean rivers. Partial transformation from a moderate‐strength debris flow with the additional role of erosional bulking can explain occurrences of hybrid event beds on the East China Sea Shelf. The data indicate that hybrid sediment gravity flow deposits were sourced from intra‐shelf failures and subsequently transformed and deposited as hybrid event beds. The study shows that hybrid sediment gravity flows and turbidity currents may not necessarily indicate proximity to a major fluvial or deltaic system and that intra‐shelf sedimentation can be a sediment source. It is unlikely that the debris flows and turbidity currents were triggered by a hyperpycnal flow or tsunami, because both can carry continental and/or coastal signals which have not been recognized in the core. Typhoons are the probable triggering mechanism. [ABSTRACT FROM AUTHOR]

We investigate suspended particles collected from the upper mixed layer in the inner shelf of the southern East China Sea during autumn 2013 for carbon and nitrogen (POC and PN) contents and their isotope compositions (δ13C and δ15N) along with hydrographic parameters to understand the sources and dynamics of particulate organic matter (POM) in the study area. Results indicated that the extensive hydrological processes affect the biogeochemical composition of suspended POM, as revealed by the horizontally mixing POM and the spatial variation of δ15N and molar C/N ratio. Low C/N (2.4–6.5) and a weak correlation between POC and in situ chlorophyll fluorescence suggested that POM is dominated by the recently formed and well‐preserved planktonic OM. By fitting the linear correlation between δ13C and POC data with a photosynthetic fractionation model, we further disentangled dynamic controls of phytoplankton production and species diversity on δ13C variability (−24.3 to −21.3‰), emphasizing the constant effect of productivity‐derived POC on δ13C (0.02‰ per μg/L). The δ15N variability (2.3–7.4‰) is largely controlled by the mixing of isotopically different nitrogen sources, in which the importance of biological nitrogen fixation is unfolded based on the small δ15N and the negative correlation between δ15N of POM and seawater temperature. This implies that Kuroshio‐induced biological N fixation plays an important role in supporting the marine production in the East China Sea. These hydrologically driven δ13C and δ15N changes of marine productivity‐derived POM suggest that internal biophysical dynamics rather than terrestrial versus marine OM mixing largely control the C and N compositional variability in the shelf seas. Plain Language Summary: Mixing of water masses on the shallow continental shelf of the East China Sea is thought to impact biogeochemical dynamics of suspended particulate organic matter on seasonal basis, but the mechanisms involved remain unclear. In this study, we measured carbon and nitrogen elemental and isotopic composition of particulate organic matter in the upper mixed layer along with hydrographic parameters during autumn season when the upwelled Kuroshio water travels closer to the coast. Results indicated that particulate organic matter is dominated by two sources of plankton‐derived organic matter that mixed horizontally by hydrodynamics. In addition, the upwelled Kuroshio contribute to marine productivity significantly by inducing biological nitrogen fixation on the shelf; the region is otherwise N‐starved. These findings based on observational and analytical results can help marine biogeochemists understand how carbon and nitrogen isotopes varied seasonally by the impact of offshore current over the river‐dominated shallow shelf, although the influence of Kuroshio on marine fisheries is beyond the scope of our investigation. Key Points: Hydrological conditions largely control the biogeochemical composition of suspended particulate organic matter in the southern East China SeaParticulate organic matter in the upper mixed layer is mainly of marine origin, despite having wide ranges of δ13C and δ15N valuesKuroshio‐induced biological nitrogen fixation is important for marine production in the southern East China Sea [ABSTRACT FROM AUTHOR]

We measured bromine to total organic carbon (Br:TOC) ratios as an organic source indicator in terrestrial and marine settings of Taiwan, a tectonically active margin that differs from previously studied, passive margins. Terrestrial lake sediments, soils, and river bed and suspended sediments showed Br:TOC ratios ranging from 0.02 to 2.8 mg‐Br/g‐TOC. In ocean margin sediments, Br:TOC ratios ranged from terrigenous values up to 7.6 mg‐Br/g‐TOC. A two‐component Br:TOC mixing model for ocean margin sediments indicates roughly equal proportions of terrigenous and marine organic matter (FT and FM, respectively). Terrestrial δ13C values varied from −31.5 to −24.7‰, a range which is twice the difference between the isotopic terrigenous and marine end‐members used to calculate these proportions. FT and FM based on δ13C were similar to those using Br:TOC if averaged across all marine sediments but often differed at the level of individual samples. Depth profiles of amounts of these two forms of organic matter (OCterr and OCmar) using Br:TOC ratios show that OCmar burial is relatively constant while OCterr and hence TOC vary more strongly; this pattern suggests either episodic terrigenous carbon inputs or winnowing before burial. Variations in TOC:TN and δ13C are consistent with this control of TOC by terrigenous carbon. This study validates Br:TOC as a source indicator for terrigenous versus marine‐derived organic matter in sediments derived from mountainous areas in Oceania. As a single parameter source indicator it should have less error than δ13C due to less uncertainty of the terrestrial end‐member. Plain Language Summary: Ocean margins account for most burial of organic matter in Earth's sediments. This burial affects global cycles such as carbon, nitrogen, and oxygen. Both land and sea contribute organic matter to these margin sediments, varying in extent in different places and times. Here we explore the use of bromine as an indicator of land versus sea contribution in Taiwan—representing the sediment‐rich, southwest Pacific region of Oceania. We compare the use of bromine with the more commonly used stable carbon isotope approach, finding that each provides 50:50 estimates of land versus sea sources at the scale of averages. Nevertheless, there is less agreement at the scale of individual samples, probably due to wide ranges of carbon isotope compositions in terrestrial end‐members. Key Points: We report the first time Br measurements of terrestrial particulates and marine sediments in and off Taiwan and, by example, the important source areas of OceaniaBr:TOC ratios show less variability in terrigenous end‐members than δ13C and hence provide an advantage in binary mixing models to estimate organic matter sourcesBr:TOC ratios in marine sediments imply relatively constant inputs of marine organic matter coupled to episodic inputs of terrigenous organic matter [ABSTRACT FROM AUTHOR]

The stability of Earth's critical zone is intimately linked with erosion, weathering and vegetation type and density. Therefore, it affects global biogeochemical processes which in turn affect the global climate by absorbing and reflecting solar radiation, and by altering fluxes of heat, water vapour, carbon dioxide and other trace gases through various feedback mechanisms. However, there is a lack of knowledge about how Earth's critical zone processes have changed over time and their link with past monsoon variability, especially in Asia. The study of lake sediments, which contain a suite of inorganic elemental and isotopic proxies, may facilitate the understanding of the Earth's critical zone processes on millennial timescales. Here we reconstruct the history of erosion-weathering-vegetation interactions since ~14.7 ka using geochemical records from a radiocarbon-dated sediment core from Lake Gonghai in the monsoon-arid transitional zone of north China. Detrital (Al, Ti, K, Rb) and authigenic (Ca, Sr) elemental records reveal distinct, millennial-scale, late deglacial-Holocene erosion and weathering patterns and transitions with the former (latter) elements showing higher (lower) values in warm intervals and vice versa. Chemical Index of Alteration (CIA) molar, a humidity proxy, suggests low humidity during the late deglacial ~11.5-14.7 ka, high humidity during the early-mid Holocene ~11.5-3.2 ka and intermediate humidity during the late Holocene interval since ~3.2 ka. The results of cross-spectral analysis and comparison of our records with other climate reconstructions also suggest a pattern of orbitally-phased humidity changes in north China. Overall, our results provide evidence for the solar-forcing of Earth's surface processes in mid-latitude China under natural climatic conditions. [ABSTRACT FROM AUTHOR]

Abstract: Marine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate‐bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973‐3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon (DIC) and high total organic carbon (TOC) (mean = 0.46%) originated from terrigenous organic matter (mean δ13CTOC value of −23.6±) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon (IC), and the depleted δ13CIC, coupled with the increase of TOC and the decrease of δ13CTOC values owing to the methanotroph ‘Methanosarcinales/ANME’ in 430–840 cm. Environmental factors and archaeal communities in core 973‐3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate‐associated sediments. [ABSTRACT FROM AUTHOR]