Estuaries are action zones for organic carbon (OC) degradation and aging. These processes influence the nature of terrestrial OC (OCterr) export and the magnitude of OCterr burial in marginal seas, ...with important environmental implications such as CO2 release and hypoxia. In this study, we determined the contents and carbon isotopic compositions (13C and 14C) of bulk OC and fatty acids (FAs) as well as the sedimentological characteristics of suspended particulate matter (SPM) samples collected from two sites over four seasons and of surface sediment samples from three sites in the Pearl River estuary (PRE) to evaluate processes controlling OCterr degradation and aging along an estuarine gradient. We found that the abundance-weighted average C24–32FA 14C ages increased by an average of ∼1170 years for SPM and by an average of ∼3440 years in PR/PRE sediments, along the ∼60 km PRE transect. These increases in the FA age coincided with an 86% decrease in the corresponding mineral surface area-normalized FA loading along the sediment transport pathway, implying that selective degradation of labile and younger OC resulted in apparent OC aging. These measurements reveal an important shift in the nature of OC, with implications for biogeochemical cycling within estuaries and for regional environmental changes.
Large rivers connect the continents and the oceans, and corresponding material fluxes have a global impact on marine biogeochemistry. The Yellow River transports vast quantities of suspended ...sediments to the ocean, yet the nature of the particulate organic carbon (POC) carried by this system is not well known. The focus of this study is to characterize the sources, composition and age of suspended POC collected near the terminus of this river system, focusing on the abundance and carbon isotopic composition (13C and 14C) of specific biomarkers.
The concentrations of vascular plant wax lipids (long-chain (≥C24) n-alkanes, n-fatty acids) and POC co-varied with total suspended solid (TSS) concentrations, indicating that both were controlled by the overall terrestrial sediment flux. POC exhibited relatively uniform δ13C values (−23.8 to −24.2‰), and old radiocarbon ages (4000–4640 yr). However, different biomarkers exhibited a wide range of 14C ages. Short-chain (C16, C18) fatty acid 14C ages were variable but generally the youngest organic components (from 502 yr to modern), suggesting they reflect recently biosynthesized material. Lignin phenol 14C ages were also variable and relatively young (1070 yr to modern), suggesting rapid export of carbon from terrestrial primary production. In contrast, long-chain plant wax lipids display relatively uniform and significantly older 14C ages (1500–1800 yr), likely reflecting inputs of pre-aged, mineral-associated soil OC from the Yellow River drainage basin. Even-carbon-numbered n-alkanes yielded the oldest 14C ages (up to 26 000 yr), revealing the presence of fossil (petrogenic) OC.
Two isotopic mass balance approaches were explored to quantitively apportion different OC sources in Yellow River suspended sediments. Results indicate that the dominant component of POC (53–57%) is substantially pre-aged (1510–1770 yr), and likely sourced from the extensive loess-paleosol deposits outcropping within the drainage basin. Of the remaining POC, between 10 and 31% is fossil in origin (>26 000 yr), resulting from the physical erosion of ancient sedimentary rock and input of fossil fuel residues from anthropogenic activity, and 16–33% is modern carbon derived from terrestrial and aquatic productivity. These findings have implications both regarding the provenance and vintage of organic matter signatures emanating from the Yellow River basin and similar catchments containing extensive paleosol sequences, as well as for the reactivity and fate of this POC upon supply to adjacent marginal seas.
•Carbon isotopes were used to constrain organic carbon (OC) pools of the Yellow River.•The majority of POC is pre-aged soil OC emanating from loess-paleosol deposits.•Delivery of soil OC has broad implication for marginal sea biogeochemical cycling.
Mobilization of Arctic permafrost carbon is expected to increase with warming-induced thawing. However, this effect is challenging to assess due to the diverse processes controlling the release of ...various organic carbon (OC) pools from heterogeneous Arctic landscapes. Here, by radiocarbon dating various terrestrial OC components in fluvially and coastally integrated estuarine sediments, we present a unique framework for deconvolving the contrasting mobilization mechanisms of surface vs. deep (permafrost) carbon pools across the climosequence of the Eurasian Arctic. Vascular plant-derived lignin phenol ¹₄C contents reveal significant inputs of young carbon from surface sources whose delivery is dominantly controlled by river runoff. In contrast plant wax lipids predominantly trace ancient (permafrost) OC that is preferentially mobilized from discontinuous permafrost regions, where hydrological conduits penetrate deeper into soils and thermokarst erosion occurs more frequently. Because river runoff has significantly increased across the Eurasian Arctic in recent decades, we estimate from an isotopic mixing model that, in tandem with an increased transfer of young surface carbon, the proportion of mobilized terrestrial OC accounted for by ancient carbon has increased by 3-6% between 1985 and 2004. These findings suggest that although partly masked by surface carbon export, climate change-induced mobilization of old permafrost carbon is well underway in the Arctic.
Understanding the effects of hydrodynamic forcing on organic matter (OM) composition is important for assessment of organic carbon (OC) burial in marginal seas on regional and global scales. Here we ...examine the relationships between regional oceanographic conditions (bottom shear stress), and the physical characteristics (mineral surface area and grain size) and geochemical properties (OC content OC% and carbon isotope compositions 13C, 14C) of a large suite of surface sediments from the Chinese marginal seas to assess the influence of hydrodynamic processes on the fate of OM on shallow continental shelves. Our results suggest that 14C content is primarily controlled by organo‐mineral interactions and hydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (and bottom shear stress). Hydrodynamic processes influence 14C content due to both OC aging during lateral transport and accompanying selective degradation of OM associated with sediment (re) mobilization, these effects being superimposed on the original 14C characteristics of carbon source. Our observations support the hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev‐marine‐120709‐142717) and Leithold et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sediment translocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin settings than on active margins. On a global scale, this may influence the extent to which continental margins act as net carbon sources and sinks. Our findings thus suggest that hydrodynamic processes are important in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas.
Key Points
A survey of oceanographic conditions, as well as physical and geochemical properties of continental margin surface sediments, is presented
Hydrodynamic processes on shallow continental shelves emerge as an important control on the 14C contents of OC
General characteristics of OC in global active and passive continental margin sediments are shown
Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ...ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon (
C) ages on two groups of molecular tracers of plant-derived carbon-leaf-wax lipids and lignin phenols-from a globally distributed suite of rivers. We find significant negative relationships between the
C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil
C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change-induced perturbations of soil OC turnover and stocks.
The sedimentation of organic carbon in the Ulleung Basin, in the southwestern East Sea (Japan Sea) was investigated using radiocarbon and sterols. The accumulation rates of organic carbon and the ...contents of brassicasterol and dinosterol were higher on the slope than in the central basin, reflecting the surface water productivity, whereas cholesterol showed similar or higher contents in the central basin. The coprostanol concentration in surface sediments reflected the dispersion of sewage dumped in this region. The vertical distribution showed that the coprostanol concentration was the highest in the top 5-cm layer near the Korea Strait, close to one of the two dumping sites. A high coprostanol concentration was also found near the coast further north, where the content peaked at ~10 cm depth. The vertical distribution of coprostanol helped to estimate the sediment accumulation rate at sites where radiocarbon gradient was too small or the values were too variable.
•Sterols in sediments reflected phyto- and zoo-plankton abundance in surface water.•Coprostanol in surface sediments provided hints on dispersion of dumped sewage.•Vertical distribution of coprostanol reflected sewage dumping history.•Coprostanol from dumping helped to determine sediment accumulation rates.
Marginal seas are estimated to account for up to 90% of organic carbon (OC) burial in marine sediments, and thus play an important role in global carbon cycle. However, comprehensive assessments of ...carbon budgets for marginal sea systems are challenging due to their inherent complexity, with spatial and temporal variability in carbon inputs and dispersal processes. We examine the Bohai Sea and Yellow Sea (BS–YS) in order to further our understanding of sedimentary OC delivery, translocation and accumulation in a shallow marginal sea system. Bulk properties and the content and isotopic compositions (Δ14C, δ13C) of source-specific plant wax n-alkyl lipid biomarkers were determined for a suite of surficial sediment samples. Variable δ13C values (−25.1‰ to −28.5‰) and contemporary radiocarbon ages of short-chain n-fatty acids (FAs; C16, C18) reflect modern autochthonous marine and/or fresh terrestrial plant input. In contrast, extremely depleted Δ14C values (−932‰ to −979‰) of short-chain n-alkanes (C16, C18) suggest a predominant input from sedimentary rocks (petrogenic OC) or petroleum. Abundance-weighted average δ13C and Δ14C values of long-chain leaf wax lipids (C26+28+30n-FAs, C24+26+28n-alkanols, C27+29+31n-alkanes) are −29.1±1.1‰ to −30.2±0.3‰, and −286±150‰ to −442±119‰, respectively, illustrating that terrestrial OC delivery is dominated by pre-aged (∼3000–500014Cyrs) C3 vegetation sources.
A coupled carbon-isotopic mixing model, based on the bulk and compound-specific biomarker δ13C and Δ14C values, is used to partition the BS–YS sedimentary OC into three components that reflect both origins and transport processes. For all sampling sites, 31–64% is modern/contemporary OC, 24–49% is pre-aged terrestrial OC, and 7–26% is fossil OC, the latter likely derived from both physical erosion of ancient sedimentary rocks and fossil fuel sources. Pre-aged soil OC is most prominent in front of the modern and old Huanghe (Yellow River) delta (48% and 49%), and fossil OC is most significant north of the old Huanghe mouth (26%). Significant pre-aged soil contributions (33%) are also evident for sites further offshore, where transport and deposition of eolian dust supply may be important. For the three major deposition areas of the BS–YS system (Bohai Basin, sub-aqueous Huanghe delta and central south YS basin), we estimate that about 3.02Mt/yr of refractory, plant-derived pre-aged soil OC and 0.98Mt/yr of 14C-depleted fossil OC accumulates in surface sediments, corresponding to 35% and 11% of sediment TOC, respectively. Compared with estimates for fluxes from corresponding sources, the burial efficiency is close to 100% for pre-aged soil OC and 70% for fossil OC, implying efficient OC burial in delta and shelf environments. Re-burial of these two pools of terrigenous OC only affects carbon cycling on millennial and longer timescales respectively, and exerts little influence on the modern carbon cycle (<100yr). Carbon isotopic compositions of source specific biomarkers are a useful tool not only for constraining OC sources and transport vectors, but also for delineating their impact on the contemporary carbon cycling in marginal sea systems.
The fate of terrigenous organic carbon (OCterr) in the ocean remains an enigma for four decades. Hadal trenches, the deepest ocean realm (6–11 km deep), were recently proposed to be OC depocenters, ...but whether and how much OCterr was sequestrated there remain elusive. Here we conducted comprehensive analyses for four sediment cores from the New Britain Trench (NBT) close to Papua New Guinea to assess source, translocation and burial of OC. The bulk and molecular radiocarbon data suggest that the NBT landward slope and axis sediments mainly receive young and biogenic rather than petrogenic OC. The three-endmember mixing model based on Δ14C, δ13C and OC contents reveals that sediments of the NBT axis (8225 m) comprise relatively high OC contents (0.66 ± 0.08%), of which biogenic OCterr accounts for 62 ± 10%. The high proportion of biogenic OCterr was attributed to the selective translocation of OCterr-enriched coarse particles and rapid delivery of sediments supported by unique V-shape feature of the trench. In contrast, the sediment OC at the oceanward slope is primarily of a marine origin, suggesting that OCterr was efficiently trapped in the trench bottom. It is estimated that the burial rate is 2.75 ± 0.32 g C m−2 yr−1 for OC and 1.69 ± 0.41 g C m−2 yr−1 for OCterr in the NBT. Given a fact that many trenches are close to the landmasses, we propose that the hadal trenches may contribute significantly to the burial of OCterr in the ocean.
Burial of terrestrial organic carbon (OCterr) in marginal sea sediments is a key component of the carbon cycle, exerting long-term influence on atmospheric CO2 and climate. Assessment of the burial ...efficiency of OCterr is of key importance, yet remains poorly constrained due to current gaps in our knowledge of mechanistic controls, including the influence of organic matter composition and age on the fate of OCterr in marine sediments. We measured bulk characteristics (δ13C and Δ14C of OC; mineral surface area, SA; grain size; n = 98 samples) and biomarker carbon isotopic compositions (fatty acid δ13C and Δ14C; n = 11) of a suite of surface sediment samples from the Bohai Sea and Yellow Sea (BS-YS). Combining with published results, bulk OC (n = 234) and biomarker (n = 19) carbon isotopic data are used to examine spatial variability in the sources and ages of OCterr in this shallow marginal sea system. Biomarker carbon isotopic values are used to constrain endmember values, and a dual carbon isotope mixing model was applied to all bulk samples to develop a spatially explicit assessment of contributions of different pools of OCterr. Although highly spatially variable, pre-aged OC (OCpre-aged) and fossil OC (OCfossil) when combined (i.e., “non-modern OC”) on average accounted for 51 ± 10% of the OC in BS-YS surface sediments. This was equivalent to the proportion of OCterr (ave., 51 ± 14%) estimated from a δ13COC binary mixing model, suggesting OCterr in the mixed layer of BS-YS sediments is predominantly composed of millennial-aged OC. The burial potential of pool-specific OCterr was then evaluated through comparison of corresponding mineral SA-normalized loadings of surface sediments with those of Yellow River suspended particulate matter. Both the regression slope and the arithmetic mean value approaches reveal high burial potential for all terrestrial OC pools delivered by Yellow River, which is attributed to the aged and refractory nature of OC exported from the Yellow River. However, differing relationships of pool-specific OCterr loadings between BS-YS and Yellow River sediments imply contrasting fates. In particular, we find that the burial potential of OCpre-aged is consistently greater than that of OCfossil. This surprising observation suggests either enhanced degradation of OCfossil during lateral transport (possibly as a consequence of different mineral associations and hydrodynamic sorting effects), or additional sources for OCpre-aged, such as from small rivers or aerosol deposition. Overall, OC age, mineralogical composition and hydrological settings contribute to the complex patterns of OC residing in northern China marginal sea surface sediments. This novel molecular-isotopic approach reveals significant spatial variability in proportions, contents and burial potential among terrestrial OC pools, and underlines the importance of considering organic matter age and composition in understanding and constraining the fate of OCterr in marine sedimentary environments.
The “building blocks of life” are found nearly ubiquitously in the environment in the form of proteins, peptides, and single amino acids. To shed light on amino acid sources, cycling, and ...preservation in sedimentary environments, we present a method using high-pressure liquid chromatography to separate and isolate underivatized amino acids extracted from sediments to conduct compound-specific radiocarbon analysis. This method consists of three main steps including (1) amino acid extraction by hydrofluoric and hydrochloric acids followed by desalting, (2) liquid chromatographic isolation and purification using two complementary column chemistries, and (3) post-purification and measurement by accelerator mass spectrometry. The resulting blank of this procedure is estimated to contain 2.2±1.3 µgC with 0.25±0.09 Fm.
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