Offshore relatively fresh groundwater (ORFG; <10 grams per liter) represents an underexplored frontier in addressing global freshwater scarcity, yet its origin, persistence, and vulnerability remain poorly constrained. Here, we reveal a vast climate-driven freshwater reservoir beneath the Pearl River estuary and adjacent shelf. Integrating hydrochemical profiling, isotopic tracing, and groundwater dating, we demonstrate that this ORFG is a relic of ancient meteoric recharge, emplaced during Late Pleistocene sea-level lowstands and preserved beneath the seafloor for tens of millennia. Secular equilibrium in offshore 226 Ra/ 230 Th activity ratios, consistent 14 C ages of 6 to 10 thousand years in onshore groundwater, and pronounced onshore-offshore contrasts in salinity and δ 18 O collectively confirm its fossil nature rather than slowly circulating modern groundwater. Numerical modeling further constrains the timescale of natural degradation, underscoring the system’s resilience. These findings establish ORFG as not only as a long-lived archive of paleohydrological cycles but also as a potential strategic freshwater reserve, with substantial implications for coastal water security in a changing climate.

Sandy beaches are an important passage for the transport of various forms of nitrogen from land to sea. However, export fluxes of these forms of nitrogen and the mechanisms controlling their transformation remain elusive. Using the Ra-224/Th-228 disequilibrium approach, we estimated export fluxes of dissolved inorganic carbon and dissolved inorganic nitrogen at three intertidal sandy beaches with distinct slopes along the southeast China's coast. We identified that sandy beaches are a hotspot of nitrogen loss in the coastal ecosystem, with nitrogen removal rates reaching up to 87.1 mmolN m(-2) d(-1) and removal efficiencies varying between 7% and 82%. Notably, nitrogen removal rates peaked at intermediate seawater percolation fluxes, reflecting the optimal balance of oxygen consumption, marine organic matter remineralization, and nitrate production for fueling denitrification in the beach's interior. In addition, total nitrogen removal increased with beach slopes. This is likely due to the fact that steeper beaches facilitate seawater to percolate more efficiently into the beach's interior and travel along a longer flow path before it drains out, thus allowing denitrification to prevail. Overall, our field observations reveal that instead of the surface "skin circulation," the "body circulation" system within an intertidal beach governs fluid transport and solute exchange between land and sea. We conclude that intertidal sandy beaches function as an efficient biogeochemical reactor, which attenuates anthropogenic nitrogen inputs to the coastal ocean.

Estuarine sediments are pivotal zones for iron (Fe) cycling, mediated by microbial communities and coupled to carbon, nitrogen, sulfur and phosphorus transformations. However, the microbial iron metabolic processes in estuarine sediments remain poorly characterized, particularly under hypoxia. This study compared metagenomes from the Oujiang River Estuary, an oxic estuary, and the Yangtze River Estuary, a seasonally hypoxic estuary, complemented by sediment core incubations to assess geochemical responses to deoxygenation. The taxonomic affiliations of iron metabolism-related genes in the oxic estuary were homogeneous with depth, dominated by Proteobacteria and Thermodesulfobacteriota. In contrast, the hypoxic estuary exhibited strong stratification, with the surface enriched in Proteobacteria and deeper horizons dominated by Chloroflexota and Candidatus Bathyarchaeota. The surface sediments of the hypoxic estuary at 0-8 centimeters below the seafloor showed a hotspot with co-enrichment of dissimilatory iron reduction (e.g., mtrABC) and iron oxidation genes (e.g., mtoA) relative to both deeper layers in the same estuary and the oxic estuary, consistent with elevated genetic potential for Fe redox turnover. This hotspot also harbored high-affinity Fe acquisition systems (siderophores, inorganic Fe transporters, and heme uptake), suggesting the potential for microbial competition for iron. Co-occurrence networks connecting Fe metabolism with carbon, nitrogen, sulfur and phosphorus cycling were more complex in the hypoxic estuary than in the oxic estuary, revealing strong associations between Fe acquisition/redox cycling and organic matter turnover. A 16-day incubation of sediment cores from the oxic estuary showed that short-term deoxygenation enhanced dissolved Fe, phosphate, and ammonium release. Overall, our results suggest that bottom-water hypoxia is associated with major shifts in microbial iron metabolism potential, with implications for iron-organic matter interactions and nutrient regeneration under coastal deoxygenation.

During the past decades, large enrichments of Ra-226 in coastal waters have been reported worldwide and were used to infer large submarine groundwater discharge into the sea. However, this recognition has not been widely appreciated outside the hydrology community because the exact location of discharge and its driving mechanisms remain largely unknown to date. Here, we report the first measurements of Ra-226/Th-230 disequilibria in sediment cores drilled from a large subterranean estuary in the Pearl River delta, China. In stark contrast to the surface Pearl River estuary, no significant deficit of Ra-226 relative to Th-230 was observed in aquifer sediments of the subterranean estuary except for the upper similar to 5 m, which corresponds roughly to the length scale of Ra-226 diffusion into the water column. Based on a transport-reaction model of Ra-226, we show that the subterranean estuary is stagnant and the replenishing rate of the groundwater must be <0.01 yr(-1). Furthermore, we show that Ra-226, dissolved inorganic carbon, and nutrients are mainly released from surface sediments of several meters, likely via local transport processes occurring at the sediment-water interface. We advocate that future studies aiming to resolve sources of the large Ra-226 enrichments in coastal waters should focus on the regeneration of Ra-226 in surface sediments.

The existence of offshore freshened groundwater (OFG) has been well recognised over the globe but studies on the chemistry of OFG are extremely limited due to the scarcity of dedicated drilling investigations. In this study, we integrate offshore hydrogeology, geochemical and isotopic tracers, and transport modelling to quantitatively evaluate the hydrochemical characteristics and persistence of an OFG system in the Pearl River Estuary and its adjacent shelf. Offshore drilling suggests the OFG system comprises a vast low-salinity groundwater body extending up to 180 km offshore, with a chloride concentration as low as only ~2.5% of that in seawater. The integrated analysis of porewater isotopic signals 18O and 2H with sediment radioisotope pair 226Ra/230Th indicates that the OFG in the Pearl River Estuary and its adjacent shelf is fossil groundwater, sequestered since the late Pleistocene during periods of low sea level. Geochemical modelling of conservative tracers further corroborates the system’s persistence, estimating its residence time at approximately 69–82 kyr. The significant reduction of major ions, along with the isolated status and long residence time of porewater in the estuarine-shelf sediment system, suggests distinctive redox conditions in the OFG systems compared to OFG-free sediments. The study underscores the profound role of OFG in influencing sub-seafloor biogeochemical cycles and ecosystems on local and global scales over extended timescales.

Sedimentary 231 Pa/ 230 Th has been used as a proxy for understanding changes in ocean circulation and productivity over the last glacial–interglacial cycle. Its application relies on the influence of meridional overturning circulation (MOC) and particle scavenging on the distribution of 231 Pa and 230 Th in the water column and sediments. While previous studies have addressed the role of MOC on the 230 Th and 231 Pa water profiles and sedimentary 231 Pa/ 230 Th in the Atlantic and Pacific Oceans, including the influence of boundary scavenging in the latter, the impact of these processes in the Indian Ocean remains unresolved. This study employs a two‐dimensional scavenging model with prescribed overturning schemes to simulate the latitudinal distribution of 230 Th and 231 Pa in the water column and sediments of the Indian Ocean. The water column profiles of both nuclides deviate from linearity, reflecting the influence of deep convection, advection, and upwelling controlled by MOC. Additionally, bottom scavenging within the nepheloid layer and boundary scavenging significantly depletes 231 Pa in the Madagascar Basin. The gradual decrease in sediment 231 Pa/ 230 Th below 1500 m in the main basins is primarily linked to MOC, while boundary scavenging contributes to systematically lowering the 231 Pa/ 230 Th. These findings point to the potential of sedimentary 231 Pa/ 230 Th as a proxy for studying the alteration of deep ocean circulation and particle flux in the Indian Ocean.

Seawater lithium isotopes (S7LiSW) are increasingly used as a proxy for continental silicate weathering and marine reverse weathering throughout Earth's history. However, the mechanisms driving S7LiSW evolution remain uncertain, partly due to insufficient constraints on modern Li sources and sinks. Among these, sediment diagenesis has long been identified as an important Li sink, yet direct and quantitative evaluations of global authigenic Li sink remains limited. Here, we present porewater data to explore the controls on porewater Li distribution and to directly quantify Li removal and isotopic fractionation in shelf sediments. We find that Li removal associated with marine authigenic clay (MAAC) formation in shelf sediments is not limited by Si(OH)4 because of its sufficient supply from the dissolution of terrigenous reactive aluminosilicates and biogenic silica. We show that irrigation-enhanced transport, coupled with active Li removal into authigenic clays in shelf sediments, result in a flux of 21 +/- 8 G mol year-1, suggesting that Li removal associated with MAAC formation in shelf sediments is the dominant Li sink from seawater. We further constrain an isotopic fractionation (Delta SW-MAAC=S7LiSW-S7LiMAAC) of 14.7-16.5 %o associated with this irrigation-enhanced shelf sediment sink. Our results highlight that authigenic Li removal into shelf sediments, complemented by a smaller contribution from seafloor basalt alteration, can closely balance the oceanic Li sources and have profound implications for understanding authigenic clay formation and Li cycle in the ancient ocean.

Dissolved 232Th and biologically-essential micronutrients, such as Fe, are simultaneously supplied to the open ocean mainly through the dissolution of dust aerosols. Both dust and associated Fe deposition fluxes are therefore able to be estimated by the long-lived Th isotopes (230Th and 232Th) in seawater in combination with the concentration and solubility of Th and Fe in aerosols. The vertical distribution of dissolved 230Th and 232Th in water columns and the solubilities of Fe and Th in aerosols were examined in the tropical western North Pacific during the GEOTRACES-China GP09 cruise. The solubilities of Fe ad Th in aerosols were 14.38 +/- 1.02 % and 12.22 +/- 2.46 %, respectively, giving a solubility ratio of SFe/Th of 1.24 +/- 0.34. Estimated dust deposition fluxes integrated over the upper 500 m of the water column ranged from 0.58 to 2.35 g m-2 yr-1 based on the vertical distribution of long-lived Th isotopes in seawater and measured aerosol Th solubility. The dust-borne Fe deposition flux in the tropical western North Pacific was further estimated as 4.02-12.55 mg m-2 yr-1, using the measured SFe/Th. Both estimated dust and dust-borne Fe deposition fluxes agree well with the dust deposition model. The spatial variability of surface dissolved Fe and primary production were predominantly driven by dust-borne Fe deposition as supported by their significant correlation. Dust-borne Fe input regulated the spatial distribution of Fe:N supply ratios, which, in turn, affected the growth of marine phytoplankton, notably diazotrophs, by stimulating N2-fixation. Quantification of dust-borne Fe fluxes may improve our understanding of the biogeochemical cycling of Fe and N2-fixation process in the tropical western North Pacific.

During the past decades, large enrichments of 226Ra in coastal waters have been reported worldwide. By means of elimination, these 226Ra enrichments were used to infer large submarine groundwater discharge from a hypothetical “subterranean estuary”. A critical assumption thereof is that regeneration of 226Ra on marine sediments contributes little to enrichments of this nuclide in the coastal ocean. In this study, we have measured 226Ra and 230Th activities in two ~ 30-meter-long sediment cores collected from the subaqueous delta of the Pearl River, China. Using this novel 226Ra/230Th tracer approach, we show that regeneration of 226Ra from surface sediments between 0 and 5 m dominated the total 226Ra flux out of the seabed. We have further demonstrated that the replenishing rate of the subterranean estuary must be < 0.01 yr-1. As a consequence, the total groundwater flux is at least 2 orders of magnitude lower than the river-water flux. More importantly, the fluxes of associated dissolved constituents are also orders of magnitude lower than the regenerated fluxes from the surface sediments. Thus, to acquire an unbiased understanding of coastal ocean chemistry, future studies should focus on solute exchange occurring at the sediment-water interface.

By taking advantage of recent analytical advances, we herein develop the Ra-226/Th-230 isotope systematics as a novel tool for quantifying nitrate and dissolved silicate fluxes across the sediment-water interface of the deep-ocean floor. Sediment cores were retrieved from the seabed between 4927 m and 5951 m in the North Pacific Ocean. Downcore profiles of Th-230 and both dissolved and total Ra-226 were measured using a high-sensitivity inductively coupled plasma mass spectrometer. At all study sites, a marked deficit of total Ra-226 with respect to Th-230 was observed between 0 and 20 cm, indicating active migration of soluble Ra-226 from the sediment into the overlying seawater. By constructing the mass balance of Ra-226 in the sediment column, the flux of dissolved Ra-226 across the sediment-water interface was estimated to range from 461 to 1320 dpm m(-2) yr(-1). When coupled to a diffusive transport model as developed by early investigators, these flux values of Ra-226 enabled us to calculate the flux of any dissolved constituent of interest by measuring their bottom water concentrations and pore water "saturation" concentrations. Based on the Ra-226/Th-230 disequilibrium approach, the derived fluxes vary between 4.1 and 10.5 mmol m(-2) yr(-1) for nitrate and between 11 and 49 mmol m(-2) yr(-1) for dissolved silicate. A compilation of nitrate and silicate fluxes from the seabed in the deep Pacific Ocean shows that these values are consistent with historical flux measurements based on the conventional core incubation method in the same study region. In addition, both nitrate and silicate fluxes exhibit a clear depth-dependent trend. Overall, our results suggest that sedimentary diagenetic alterations at the North Pacific Ocean floor below similar to 5000 m are efficient so that only < 2 % of the particulate organic carbon and < 12 % of the biogenic opal raining to the seafloor are ultimately preserved in the sediment.

Rhenium (Re) and uranium (U) are essential proxies in reconstructing past oceanic oxygenation evolution. However, their removal in continental shelf sediments, hotspots of early diagenesis, were previously treated as quantitatively unimportant sinks in the ocean. Here we examine the sedimentary reductive removal of Re and U and their coupling with organic carbon decomposition, utilizing the 224Ra/228Th disequilibria within the East China Sea shelf. We identified positive correlations between their removal fluxes and the rates of sediment oxygen consumption or organic carbon decomposition. These correlations enable an evaluation of global shelf reductive sinks that are comparable to (for Re) or higher than (~4-fold for U) previously established suboxic/anoxic sinks. These findings suggest potential imbalances in the modern budgets of Re and U, or perhaps a substantial underestimation of their sources. Our study thus highlights shelf sedimentary reductive removal as critical yet overlooked sinks for Re and U in the modern ocean.

Using a Ra-224/Th-228 disequilibrium approach, we demonstrate in this study that benthic fluxes of dissolved inorganic phosphorus (DIP) in the seasonally hypoxic Yangtze River Estuary were largely manipulated by two counteracting processes: the decomposition of sedimentary organic matter and adsorption of DIP onto iron (Fe) oxides. The decomposition rate of sedimentary organic matter rose exponentially with bottom water dissolved oxygen (DO) concentration multiplied by the amplification factor of sediment surface area (xi), a variable used to describe the intensity of bio-irrigation and physical reworking in the sediment deposit. In the summer of 2020, the Yangtze River catchment encountered the largest flood event in the past 20 years. As a result of enhanced physical reworking of the seabed, dissolved inorganic carbon (DIC) flux increased by approximately 4-fold as compared to the summer of 2019 (657 vs. 154 mmol m(-2) d(-1)). DIP flux exhibited similar inter-annual variations to DIC flux, indicating that changes in the decomposition rate of sedimentary organic matter were the main cause of the inter-annual differences in DIP flux (1.5 mmol m(-2) d(-1) in 2020 vs. 0.3 mmol m(-2) d(-1) in 2019). On the other hand, benthic dissolved Fe fluxes declined exponentially with rising DO concentration because of re-oxidation of Fe2+ in porewater into Fe oxides. As a consequence, approximately 90% of DIP sourced from the decomposition of sedimentary organic matter was retained within the sediment when the bottom water was well oxygenated (DO >125 mu mol l(-1)). Our results imply that regeneration of sedimentary P and Fe may be efficient only within a very narrow redox window where DO concentrations are below a threshold value of similar to 60 mu mol l(-1), but bio-irrigation or physical reworking is still active.

The intertidal zone is an important pathway for the transport of dissolved carbon across the land-ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 & PLUSMN; 0.1 to 1084 & PLUSMN; 53 L m-2 h-1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 & PLUSMN; 0.7 L m- 2 h-1. Advective porewater exchange was recog-nized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 & PLUSMN; 0.2 to 206 & PLUSMN; 9 mmol C m- 2 d-1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 & PLUSMN; 0.3 to 883 & PLUSMN; 43 mmol C m- 2 d- 1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h-1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 & PLUSMN; 4 and 206 & PLUSMN; 30 mmol C m- 2 d- 1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remi-neralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Coastal terrestrial groundwater discharge and porewater exchange are two different submarine groundwater discharge (SGD) pathways. The contribution of both pathways in dissolved carbon export to the coastal water remains almost unknown. Here, we investigated terrestrial groundwater discharge, porewater exchange, and dissolved carbon export in a tropical estuary (Moyangjiang, China) using stable water (& delta;2H and & delta;18O) and radioactive (222Rn, 224Ra, and 228Th) isotopes. The average terrestrial groundwater discharge was estimated to be (3.2 & PLUSMN;0.2) x 106 m3 d-1 by a three end-member mixing model of & delta;18O and salinity. The porewater exchange was estimated to be 11.3 & PLUSMN;0.14 cm d-1 by the 224Ra/228Th disequilibrium method. Accordingly, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) fluxes from terrestrial groundwater discharge were estimated to be (1.19 & PLUSMN;0.10) x 107 and (3.23 & PLUSMN;0.25) x 105 mol d-1, respectively. In comparison, when upscaling to the whole estuary, DIC and DOC export fluxes from porewater exchange were estimated to be (1.56 & PLUSMN;0.21) x 105 and (2.12 & PLUSMN;0.29) x 105 mol d-1, respectively. Terrestrial groundwater discharge derived DIC flux was two orders of magnitude greater than that from porewater exchange, and was three times greater than that from river discharge. Terrestrial groundwater discharge was recognized as a significant contributor to DIC export. Pore -water exchange played a disproportionately important role in the delivery of DOC to the ocean, and its contribution to DOC needed to be emphasized. Overall, both pathways contributed a large amount of dissolved carbon export to the coastal water, and play an important role in delivering terrestrial carbon across land-ocean interface. Evaluating the contributions of different pathways of SGD to terrestrial solutes will help to improve the accuracy of SGD associated terrestrial solute fluxes.

The Taiwan Strait is a coastal sea with dynamic hydrologic condition and dissolved organic matter (DOM) cycling. To better understand its phosphorus (P) transformation, phosphate and bulk dissolved organic phosphorus (DOP) concentrations were determined in the surface water of the Taiwan Strait during 2010 and 2011 summers. The bulk DOP was further fractionated into the low molecular weight (LMW) DOP and colloidal organic phosphorus (COP) with the application of the cross-flow ultrafiltration technique at a size cut-off of 10 kDa. The surface water in the Taiwan Strait generally showed higher bulk DOP concentration than phosphate excluding estuarine stations. This is similar to the surface waters in the subtropical gyres, implying a P-deficient condition in the Taiwan Strait. The LMW-DOP followed a constant permeation behavior in the ultrafiltration experiments, allowing the quantification of LMW-DOP and COP concentrations with the ultrafiltration permeation model. The average COP concentration was 0.044 & PLUSMN; 0.022 & mu;mol/L and 0.042 & PLUSMN; 0.027 & mu;mol/L during 2010 and 2011 summers, respectively. Accordingly, COP accounted for 35 & PLUSMN; 13% and 34 & PLUSMN; 16% of bulk DOP during 2010 and 2011 summers, respectively, suggesting a substantial occurrence of organic phosphorus in the colloidal fraction. The C/P ratio decreased from LMW-DOM to bulk DOM and to colloidal organic matter (COM), supporting the degradation pathway of bulk DOM from colloidal to LMW fractions and the lability of COP in the Taiwan Strait. The fractionation of C/P ratio between bulk DOM and COM in the Taiwan Strait, denoted by the fractionation factor (FC/D), was comparable to the open oceans, but significantly stronger than rivers and estuaries. It might indicate an autochthonous input and rapid turnover of bulk DOP pool in the Taiwan Strait in the context of low phosphate concentration. The FC/D also showed a spatial variability in the Taiwan Strait, high in the South China Sea Warm Current and upwelling occupied stations and low in inshore areas influenced by terrestrial input. This further supports the FC/D as an indicator of bulk DOM sources besides its application to decipher the diagenetic status of bulk DOM and biogeochemical cycles of bulk DOP in the coastal oceans.

We describe a new method suitable for the precise and accurate determination of Ra-226 in porewater and sediment samples using a single-collector sector field ICP-MS (ThermoFisher Element XR) equipped with an Apex-Q desolvation device and a high-sensitivity Jet-X interface. In combination with Th-230 measurements in parallel sediment samples, this method allows precise and accurate quantification of the Ra-226/Th-230 disequilibria in surface sediment cores, thereby enabling the use of this isotope pair as a tracer of solute transfer across the sediment-water interface in the deep ocean. The method integrates a step of isotope dilution with Ra-228 as an internal spike, a pre-concentration of Ra and Ba by MnO2 precipitation, and an efficient separation of Ra from other undesirable elements using a cation exchange resin and a Triskem Sr-spec resin. With the inclusion of one or two additional cation resin columns and the use of up to 16 bed-volumes of a lower molarity (1.7 M) HCl eluent, our procedure eliminates the complicated matrix effects persistently encountered in previous studies, and provides a highly purified solution suitable for Ra-226 measurement using an Element XR ICP-MS apparatus. Consequently, we are able to determine the activity of 226Ra in -20-50 ml of porewater or 100 mg of sediment with an internal precision of -1.0% and an accuracy of -99.2%. The precise measurements of porewater and solid phase Ra-226 in a sediment core from the North Pacific Ocean allowed the distribution coefficient (Kd) of Ra-226 to be constrained tightly within a range of 4700-11,600 ml g-1. Moreover, with the aid of a onedimensional exchange model, the combination of the Ra-226 and Th-230 measurements allowed us to estimate a Ra-226 flux of 1140 +/- 20 dpm m- 2 y-1 from the sediment core.

The use of Th-228 has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound Th-228 using a radium delayed coincidence counting system (RaDeCC). Herein we review the( 228)Th cycle in the marine environment and revisit the historical use of Th-228 as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from Th-228 and Pb-210 are presented for a micro-tidal salt marsh and a marginal sea environment. Th-228 and Pb-210 have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that Th-228 (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from Pb-210, reflecting different integration periods. Use of Th-228 is well-suited for shallow (<15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of Th-228 and Pb-210 can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment( 228)Th.

Deltaic systems are characterized by the highest sedimentation rates in the globe. Meanwhile, sedimentary organic matter therein can be efficiently decomposed so that these depositional systems may deviate substantially from the oft-quoted correlation between net sediment accumulation and preservation of organic matter. The exact mechanisms that cause such a deviation in any given case, however, remain poorly understood. In this study, we utilize a novel( 224)Ra/Th-228 disequilibrium method to examine sediment oxygen consumption and the release of diagenetic products of organic matter along the major mud wedge system in the inner shelf of the East China Sea. Our sampling campaign was carried out in two contrasting seasons: the summer when seasonal hypoxia was at its peak and physical conditions were relatively quiescent, and the winter when the water column was well oxygenated by intense winter mixing and underlying deposits were subjected to reworking. Unexpectedly, during summer 2017 when the seafloor received the annual maximum supply of organic matter, sediment oxygen consumption rates and benthic fluxes of NH4+ were relatively low, ranging from 6 to 59 mmol O-2 m(-2) d(-1) and from 1.6 to 13 mmol N m(-2) d(-1), respectively. In contrast, during winter 2018 sediment oxygen consumption rates and benthic fluxes of NH4+ surged to 44-690 mmol O-2 m(-2) d(-1) and 22-58 mmol N m(-2) d(-1), respectively. We have also identified an exponential relationship between amplification factor of sediment surface area and oxygen concentration in the bottom water. This relationship suggests that kinetic energy dissipation in the water column not only controlled air-sea exchange and seawater mixing, but also intensified sediment-water interaction. Importantly, sediment oxygen consumption rates (F-O2) in the mud wedge can be empirically described using a modified form of Michaelis-Menten kinetics, suggesting that F-O2 and the associated benthic consumption and production of chemicals are controlled by both the transport and reaction processes. We have further demonstrated that a large portion of the organic matter deposited over the seafloor in summer is likely decomposed in winter. Overall, this study highlights intense winter mixing as an important mechanism that causes the highly efficient decomposition of sedimentary organic matter in coastal seas. (C) 2021 Elsevier Ltd. All rights reserved.

A new method for measuring( 224)Ra in sediments is established by use of a pulsed ionization chamber (PIC). Based on the difference in their half-lives,Rn- 220 can be distinguished from Rn-222 so that one can easily measure Ra-224 and its parent( 228)Th in sediment samples. Results show that the measurement efficiency is 0.22 +/- 0.02 cpm/dpm, and is not readily affected by moisture. At an optimum airflow rate of 1.2 L/min, the precision of the( 224)Ra and Th-228 measurements is estimated at +/- 8%. Based on controlled experimental field application and a field application, we show that the Ra-224/Th-228 disequilibrium in sediments can be assessed and both 224Ra deficits and ingrowth to equilibrium may be quantified. The PIC device is simple to use, inexpensive and portable, making it especially suitable for determinations of sediment samples in the field.