Temporal variability in composition and fluxes of Yellow River particulate organic matter

This study examines temporal variations of the abundance and carbon isotopic characteristics of particulate organic carbon (POC) and specific-source compounds in the context of hydrological variability in the Yellow River. The content and bulk carbon isotopic characteristics (13C and 14C) of POC were relatively uniform over the hydrologic (seasonal) cycle. We attribute these temporally invariant geochemical characteristics to the dominant contribution of loess material to the suspended particulate matter (SPM). In contrast, molecular-level signals revealed that hydrologic conditions exert a significant influence on the proportional contributions of petrogenic and especially fresh plant-derived OC, while pre-aged soil OC is mobilized via deeper erosion processes (e.g., gully erosion, mudslides) and is independent of hydrodynamics and surface runoff. A coupled biomarker-isotope mixing model was applied to estimate the time-varying supply of contemporary/modern biomass, pre-aged soil, and fossil OC components to Chinese marginal seas from the Yellow River. We found that natural (e.g., precipitation) and human-induced (e.g., water and sediment regulation) variations in hydrological regime strongly influence the flux with the magnitude of the corresponding annual fluxes of POC ranging between 0.343 ± 0.122 Mt yr−1 and 0.581 ± 0.213 Mt yr−1, but less strongly infleunce proportions of the different OC constituents. Inter-annual differences in pre-aged soil and fossil OC fluxes imply that extreme climate events (e.g., floods) modulate the exhumation and export of old carbon to the ocean, but the OC homogeneity in the pre-aged mineral soil-dominated watersheds facilitates robust predictions in terms of OC transport dynamics in the past (sediment cores) and in the future

Temporal deconvolution of vascular plant-derived fatty acids exported from terrestrial watersheds

Relatively little is known about the amount of time that lapses between the photosynthetic fixation of carbon by vascular land plants and its incorporation into the marine sedimentary record, yet the dynamics of terrestrial carbon sequestration have important implications for the carbon cycle. Vascular plant carbon may encounter multiple potential intermediate storage pools and transport trajectories, and the age of vascular plant carbon accumulating in marine sediments will reflect these different pre-depositional histories. Here, we examine down-core 14C profiles of higher plant leaf wax-derived fatty acids isolated from high fidelity sedimentary sequences spanning the so-called “bomb-spike”, and encompassing a ca. 60-degree latitudinal gradient from tropical (Cariaco Basin), temperate (Saanich Inlet), and polar (Mackenzie Delta) watersheds to constrain integrated vascular plant carbon storage/transport times (“residence times”). Using a modeling framework, we find that, in addition to a "young" (conditionally defined as < 50 y) carbon pool, an old pool of compounds comprises 49 to 78 % of the fractional contribution of organic carbon (OC) and exhibits variable ages reflective of the environmental setting. For the Mackenzie Delta sediments, we find a mean age of the old pool of 28 ky (±9.4, standard deviation), indicating extensive pre-aging in permafrost soils, whereas the old pools in Saanich Inlet and Cariaco Basin sediments are younger, 7.9 (±5.0) and 2.4 (±0.50) to 3.2 (±0.54) ky, respectively, indicating less protracted storage in terrestrial reservoirs. The "young" pool showed clear annual contributions for Saanich Inlet and Mackenzie Delta sediments (comprising 24% and 16% of this pool, respectively), likely reflecting episodic transport of OC from steep hillside slopes surrounding Saanich Inlet and annual spring flood deposition in the Mackenzie Delta, respectively. Contributions of 5-10 year old OC to the Cariaco Basin show a short delay of OC inflow, potentially related to transport time to the offshore basin. Modeling results also indicate that the Mackenzie Delta has an influx of young but decadal material (20-30 years of age), pointing to the presence of an intermediate reservoir. Overall, these results show that a significant fraction of vascular plant C undergoes pre-aging in terrestrial reservoirs prior to accumulation in deltaic and marine sediments. The age distribution, reflecting both storage and transport times, likely depends on landscape-specific factors such as local topography, hydrographic characteristics, and mean annual temperature of the catchment, all of which affect the degree of soil buildup and preservation. We show that catchment-specific carbon residence times across landscapes can vary by an order of magnitude, with important implications both for carbon cycle studies and for the interpretation of molecular terrestrial paleoclimate records preserved in sedimentary sequences

Influence of hydrodynamic processes on the fate of sedimentary organic matter on continental margins

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

14C and 13C characteristics of higher plant biomarkers in Washington margin surface sediments

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 105 (2013): 14-30, doi:10.1016/j.gca.2012.11.034.Plant wax lipids and lignin phenols are the two most common classes of molecular markers that are used to trace vascular plant-derived OM in the marine environment. However, their 13C and 14C compositions have not been directly compared, which can be used to constrain the flux and attenuation of terrestrial carbon in marine environment. In this study, we describe a revised method of isolating individual lignin phenols from complex sedimentary matrices for 14C analysis using high pressure liquid chromatography (HPLC) and compare this approach to a method utilizing preparative capillary gas chromatography (PCGC). We then examine in detail the 13C and 14C compositions of plant wax lipids and lignin phenols in sediments from the inner and mid shelf of the Washington margin that are influenced by discharge of the Columbia River. Plant wax lipids (including n-alkanes, n-alkanoic (fatty) acids, n-alkanols, and n-aldehydes) displayed significant variability in both δ13C (-28.3 to -37.5 ‰) and ∆14C values (-204 to +2 ‰), suggesting varied inputs and/or continental storage and transport histories. In contrast, lignin phenols exhibited similar δ13C values (between -30 to -34 ‰) and a relatively narrow range of ∆14C values (-45 to -150 ‰; HPLC-based mesurement) that were similar to, or younger than, bulk OM (-195 to -137 ‰). Moreover, lignin phenol 14C age correlated with the degradation characteristics of this terrestrial biopolymer in that vanillyl phenols were on average ~500 years older than syringyl and cinnamyl phenols that degrade faster in soils and sediments. The isotopic characteristics, abundance, and distribution of lignin phenols in sediments suggest that they serve as promising tracers of recently biosynthesized terrestrial OM during supply to, and dispersal within the marine environment. Lignin phenol 14C measurements may also provide useful constraints on the vascular plant end member in isotopic mixing models for carbon source apportionment, and for interpretation of sedimentary records of past vegetation dynamics. Key words: 14C and 13C composition, radiocarbon age, plant wax lipids, lignin phenols, Washington margin, marine carbon cycling, terrestrial organic matterGrants OCE-9907129, OCE-0137005, and OCE-0526268 (to TIE) from the National Science Foundation (NSF) supported this research

Diverse origins and pre-depositional histories of organic matter in contemporary Chinese marginal sea sediments

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 (∼3000–5000 14C yrs) 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 3.02 Mt/yr of refractory, plant-derived pre-aged soil OC and 0.98 Mt/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 (<100 yr). 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

Pre-aged soil organic carbon as a major component of the Yellow River suspended load: Regional significance and global relevance

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

Terrestrial Biomolecular Burial Efficiencies on Continental Margins

The Authors. The fate of terrestrial organic carbon (OCterr) exported from large rivers in marginal seas is an integral component of land-ocean-atmosphere carbon dynamics and influences on atmospheric CO2 concentrations on millennial and longer timescales. In this study, we employ a novel approach to constrain burial efficiencies for source-specific terrestrial biomolecules (long-chain n-alkanes and n-fatty acids) in two river-marginal sea systems. We find for the Pearl River-South China Sea system that 34 ± 19% and 11 ± 4% of n-alkanes and n-fatty acids, respectively, are preserved across the transport pathway from the river mouth to inner shelf. In contrast, terrestrial biomolecular burial efficiencies were markedly higher (64 ± 17% and 84 ± 30% of n-alkanes and n-fatty acids, respectively) in the Yellow River-Bohai Sea/Yellow Sea system. These findings reveal markedly different fates of OCterr in these two fluvial-marine systems, as well as sharp contrasts in OCterr reactivity within each system.ISSN:0148-0227ISSN:2169-8953ISSN:2169-896

Liquid Chromatographic Isolation of Individual Amino Acids Extracted From Sediments for Radiocarbon Analysis

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.ISSN:2296-774

Terrestrial Biomolecular Burial Efficiencies on Continental Margins

The Authors. The fate of terrestrial organic carbon (OCterr) exported from large rivers in marginal seas is an integral component of land-ocean-atmosphere carbon dynamics and influences on atmospheric CO2 concentrations on millennial and longer timescales. In this study, we employ a novel approach to constrain burial efficiencies for source-specific terrestrial biomolecules (long-chain n-alkanes and n-fatty acids) in two river-marginal sea systems. We find for the Pearl River-South China Sea system that 34 ± 19% and 11 ± 4% of n-alkanes and n-fatty acids, respectively, are preserved across the transport pathway from the river mouth to inner shelf. In contrast, terrestrial biomolecular burial efficiencies were markedly higher (64 ± 17% and 84 ± 30% of n-alkanes and n-fatty acids, respectively) in the Yellow River-Bohai Sea/Yellow Sea system. These findings reveal markedly different fates of OCterr in these two fluvial-marine systems, as well as sharp contrasts in OCterr reactivity within each system.ISSN:0148-0227ISSN:2169-8953ISSN:2169-896