Petrogenic Organic Carbon Research Articles

Effects of human activities on soil organic carbon redistribution at an agricultural watershed scale on the Chinese Loess Plateau

Human activity is thought to perturb the dynamic change of organic carbon (OC), but its impact on the transportation and redistribution of erosion-induced OC is still poorly understood. Here, we use multi-source field sampling data and long-term hydrological observation data to clarify the transportation and redistribution of OC in an agricultural watershed on the Loess Plateau. The endmember mixing model based on the radiocarbon isotopes (14C) and a budget equation were used to assess the composition and budget of erosion-induced OC in a 187 km2 watershed. The results showed that compared with the period of agricultural activities (1960−1969), soil conservation activities in 1970−1999 and 2000−2019 reduced soil erosion by 31.5 % and 75.4 %, respectively. Additionally, land use changes significantly reduced the mobilized OC, from 14,370 ± 1966 (1960−1969) to 3311 ± 431 Mg C yr−1 (2000−2019). Check dam construction led to the mobilized OC was effectively buried at a rate of 2125 ± 478 and 1420 ± 282 Mg C yr-1 in periods of 1970−1999 and 2000−2019, respectively. Further radiocarbon isotopes analysis shows that the land use changes and check dam construction effectively reduced the output of ancient petrogenic organic carbon (OCpetro) and young biospheric organic carbon (OCbio) from the watershed, which has a positive impact on the regulation of atmospheric CO2 level. Our results emphasize the positive effects of soil conservation activities on terrestrial carbon sequestration, which are of great significance to clarify the effects of soil erosion on the carbon cycle.

Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin

AbstractSoil erosion modulates the atmospheric CO2 level by affecting the redistribution of young biospheric organic carbon (OCbio) and ancient petrogenic organic carbon (OCpetro) in different order streams. However, the fate of soil organic carbon (SOC) in low‐order stream systems is still uncertain due to the complex influences of terrain, land uses, and anthropogenic disturbances. Here, we used the geochemical properties fingerprinting method and a radiocarbon‐based two‐member mixing model to clarify the source, budgets, and composition of SOC in a low‐order stream catchment in the Yellow River basin. The results showed that the primary source of SOC in the catchment was agricultural activity area, which accounted for 68.5%. After vegetation restoration, the SOC delivery rate and loss rate decreased from 0.38 ± 0.06 to 0.26 ± 0.04 and 0.19 ± 0.06 to 0.14 ± 0.04 t·ha−1·yr−1, respectively. The SOC mobilized by soil erosion in the source area was 1,085.8 ± 170.5 t from 1969 to 2015, and the SOC lost during the transport process was 552.9 ± 170.1 t. The lost carbon was dominated by OCbio, whereas the loss of OCpetro was minimal. Soil erosion can mobilize the OCpetro sequestered in terrestrial ecosystems and deliver it to a river system if it is not intercepted. The oxidation of OCpetro during long‐distance fluvial transport may represent a significant carbon source for atmospheric CO2 on a geological timescale. This study contributes to our understanding of the carbon sink/source issue and quantifies the SOC budgets in stream systems.

Island-wide variation in provenance of riverine sedimentary organic carbon: A case study from Taiwan

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.

Survival of graphitized petrogenic organic carbon through multiple erosional cycles

Graphite forms the endpoint for organic carbon metamorphism; it is extremely resilient to physical, biological and chemical degradation. Carbonaceous materials (CM) contained within sediments, collected across Taiwan and from the Gaoping submarine canyon, were analyzed using Raman spectroscopy to determine the crystallinity. This allowed the erosional and orogenic movements of petrogenic organic carbon (OCpetro) during the Taiwanese orogeny to be deduced. After automatically fitting and classifying spectra, the distribution of four groups of CM within the sediments provides evidence that many forms of OCpetro have survived at least one previous cycle of erosion, transport and burial before forming rocks in the Western Foothills of the island. There is extensive detrital graphite present in rocks that have not experienced high-grade metamorphism, and graphite flakes are also found in recently deposited marine sediments off Taiwan. The tectonic and geological history of the island shows that these graphite flakes must have survived at least three episodes of recycling. Therefore, transformation to graphite during burial and orogeny is a mechanism for stabilizing organic carbon over geological time, removing biospheric carbon from the active carbon cycle and protecting it from oxidation during future erosion events.

Perspectives on provenance and alteration of suspended and sedimentary organic matter in the subtropical Pearl River system, South China

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 (δ 13 C 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 14 C 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 14 C-depleted soils, bedrock, as well as riverine primary productivity ( R pp ) that utilizes 14 C-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 14 C) 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.

Carbon dioxide emissions by rock organic carbon oxidation and the net geochemical carbon budget of the Mackenzie River Basin

The exposure of organic carbon in rocks to oxidative weathering can release carbon dioxide (CO2) to the atmosphere and consume atmospheric oxygen. Alongside volcanism, metamorphism, and the weathering of carbonate minerals by sulfuric acid, this is a major source of atmospheric CO2 over million year timescales. The balance between CO2 release and CO2 drawdown by silicate weathering and organic carbon burial sets the net geochemical carbon budget during weathering and erosion. However, the rates of rock-derived organic carbon (petrogenic organic carbon, OCpetro) oxidation remain poorly constrained. Here, we use rhenium as a proxy to trace and quantify CO2 release by OCpetro oxidation in the Mackenzie River Basin, Canada, where the other carbon fluxes have been well constrained previously. River water and sediment samples were collected between 2009 and 2013 at gauging stations along the Mackenzie River and its main tributaries (Liard, Peel and Arctic Red). To assess rhenium inputs from silicate, sulfide and OCpetro mineral phases we normalize dissolved rhenium concentrations, [Re]diss, to sodium and sulfate ion concentrations. This approach suggests that >85 percent of [Re]diss is derived from OCpetro in the main river channels. [Re]diss and water discharge measurements are used to quantify dissolved Re yields. River sediments provide a measure of the Re to OCpetro ratio of materials undergoing weathering in the basin, and agree well with published rock samples. Dissolved Re yields are combined with river sediment [Re]/[OCpetro] ratios to estimate the CO2 emissions by OCpetro weathering. These are 0.45 +0.19/−0.11 metric tonnes of carbon, tC km−2 yr−1for the Mackenzie River at Tsiigehtchic (3.8 +1.5/−0.9 × 104 moles km−2 yr−1), and 0.94 +0.41/−0.26 tC km−2 yr−1, 0.78 +0.35/−0.21 tC km−2 yr−1 and 1.01 +0.42/−0.25 tC km−2 yr−1 for the Peel, Arctic Red and Liard catchments, respectively. When considered alongside published silicate and carbonate weathering rates and the sedimentary burial of biospheric organic carbon, these data suggest that the upper part of the Mackenzie River Basin presently acts as an atmospheric CO2 sink of ∼1 tC km−2 yr−1 (∼8 × 104 moles km−2 yr−1) as a result of the carbon transfers by weathering and erosion. During the Last Glacial Maximum, it is possible that the net geochemical carbon balance may have been very different: potential increases in CO2 emissions from oxidative weathering of OCpetro and carbonate minerals, coupled with reduced biospheric carbon burial, may have tipped the balance to a net source of CO2.

The isotopic composition and fluxes of particulate organic carbon exported from the eastern margin of the Tibetan Plateau

Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>104 years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POCbiosphere) is an important CO2 sink, while oxidation of organic carbon derived from sedimentary rocks (petrogenic, POCpetro) releases CO2 to the atmosphere. Over decadal to millennial timescales, the balance between POCbiosphere production and degradation affects atmospheric CO2 concentrations. To better constrain the controls on erosional carbon transfers, here we quantify POCbiosphere and POCpetro fluxes in a mountain range with relatively low runoff, the Longmen Shan, which drains the eastern margin of the Tibetan Plateau. We measure total organic carbon content ([OCtotal]) and the carbon isotopic compositions (13C/12C expressed as δ13C; 14C/12C expressed as fraction modern or Fmod) of organic matter in suspended sediments collected from six gauging stations on the Min Jiang, a tributary of the Yangtze River, from 2005 to 2012. We find that POCpetro has a large range of δ13C, from -26.2‰ to -13.2‰. This POCpetro mixes with POCbiosphere to set the δ13C of POC in river sediments. Binary mixing models reveal the possibility of aged POCbiosphere at two gauging stations which drain the high elevations of the eastern Tibetan Plateau, with modelled Fmod values of 0.82 ± 0.09 and 0.84 ± 0.08. This is consistent with prior suggestions of aged biospheric carbon being eroded from the Plateau. The annual POCpetro yields range from 0.04 ± 0.02 tC km-2 yr-1 to 1.69 ± 0.56 tC km-2 yr-1 across the five study catchments, with basin average yield that appears to be linked to catchment average slope as a likely proxy for erosion rate. Here, the variability in the petrogenic organic carbon content of rocks masks the signal of the weathering and oxidation of this rock-derived organic carbon. The annual POCbiosphere yields range from 0.21 ± 0.04 tC km-2 yr-1 to 3.33 ± 0.57 tC km-2 yr-1. These values are towards the lower end of those measured in mountain ranges around the world, which we suggest not only reflects the relatively low erosion rates of the Longman Shan, but also the low annual runoff (<1 m yr-1). Across this region, the river POCbiosphere discharge is related to the intensity of runoff events. Our data suggest that a wetter (and/or stormier) climate could increase the erosional export of POCbiosphere in this tectonically-active mountain range. Depending on the fate of POCbiosphere downstream in larger river systems, this could act as carbon-cycle climate feedback over geological timescales.

Petrogenic organic carbon retention in terrestrial basins: A case study from perialpine Lake Constance

Inland waters play a major role in the global carbon cycle, with particulate organic carbon (POC) burial in terrestrial wetlands surpassing that in ocean sediments. Lake Constance, the second largest lake at the periphery of the European Alps, receives POC sourced from both aquatic and terrestrial productivity as well as petrogenic OC (OCpetro) from bedrock erosion. Distinguishing POC inputs to lake sediments is key to assessing carbon flux and fate as reworked OCpetro represents neither a net sink of atmospheric CO2 nor source of O2. New stable and radiocarbon isotopic data indicate that 11 (9–12) Gg/yr of OCpetro is buried in Lake Constance with underlying sediments on average containing 0.3 (0.25–0.33) wt% OCpetro. Extrapolation of these results suggests that 27 TgOCpetro/yr (12–54 TgOC/yr) could be subject to temporary geological storage in lakes globally, which is comparable to estimates of 43−25+61 TgOCpetro/yr delivered to the ocean by rivers (Galy et al., 2015). More studies are needed to quantify OCpetro burial in inland sedimentary reservoirs in order to accurately account for atmospheric carbon sequestration in terrestrial basins.

On the geological and scientific legacy of petrogenic organic carbon

Weathering, erosion, and redeposition of exhumed rock-derived or "petrogenic" organic carbon (OC) co-occurs with the burial of biospheric OC within sediments, modulating atmospheric CO~2~ and O~2~ over geologic time. Disentangling the geochemical fingerprint of petrogenic OC from biospheric OC in sedimentary organic matter, as well as quantifying the influence of its remineralization and burial on atmospheric CO~2~/O~2~, has been the focus of numerous observational and geochemical modeling studies. In 1938, Matti Sauramo recognized that petrogenic OC is entrained in a "simple carbon" cycle operating alongside the "complicated" greater rest of the carbon cycle. Sauramo9s achievements were preceded by Charles Lyell9s thoughts on the subject a century earlier, and by observations of reworked palynomorphs in the modern environment made by palynologists in the 19^th^ Century. Towards the present, palynologists, organic petrologists, and geochemists have all made key advances, while their impact often did not radiate beyond their respective bodies of literature. This highlights the importance not only of further investigations focused on the continued pursuit of new information, but also on studies of the history of relevant disciplines in order to place new findings in appropriate context. Petrogenic OC cycling has emerged as a key process for constraining global carbon budgets, long-term biogeochemical cycles and associated variations in atmospheric chemistry. While petrogenic OC is now recognized as a significant component of bulk sedimentary OC in modern systems, its cycling throughout Earth9s history - including during pivotal episodes such as supercontinent amalgamation and late Proterozoic Snowball Earth events followed by greenhouse conditions - remains largely unexplored.

Contrasting Fates of Petrogenic and Biospheric Carbon in the South China Sea

AbstractA synthesis of published and newly acquired stable and radiocarbon isotope data from soil, river, and marine particulate organic carbon (OC) from the South China Sea drainage and sedimentary basin reveals that OC derived from bedrock‐erosion (petrogenic OC) and marine productivity comprises the major contributors to bulk OC in particulate matter reaching abyssal depths, while soil‐derived OC appears negligible. Aluminum‐radiocarbon relationships of sediments suggest that soil OC initially associated with detrital terrestrial minerals is lost and replaced by marine OC during transport beyond the continental shelf. We estimate that petrogenic OC sinking to a ~30,000 km2 region of the deep northeastern South China Sea accounts for 0.6% of global petrogenic OC burial. The basin‐wide OC isotope patterns coupled with sediment trap observations highlight both the spatial variabilities of OC components as they propagate from source to sedimentary sink and the significance of petrogenic OC to deep ocean sediments.

Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils

Lithospheric organic carbon ("petrogenic"; OCpetro) is oxidized during exhumation and subsequent erosion of mountain ranges. This process is a considerable source of carbon dioxide (CO2) to the atmosphere over geologic time scales, but the mechanisms that govern oxidation rates in mountain landscapes are poorly constrained. We demonstrate that, on average, 67 ± 11% of the OCpetro initially present in bedrock exhumed from the tropical, rapidly eroding Central Range of Taiwan is oxidized in soils, leading to CO2 emissions of 6.1 to 18.6 metric tons of carbon per square kilometer per year. The molecular and isotopic evolution of bulk OC and lipid biomarkers during soil formation reveals that OCpetro remineralization is microbially mediated. Rapid oxidation in mountain soils drives CO2 emission fluxes that increase with erosion rate, thereby counteracting CO2 drawdown by silicate weathering and biospheric OC burial.

Microbes eat rocks and leave carbon dioxide

Carbon Cycle![Figure][1] Microbes oxidize most of the petrogenic organic carbon in Taiwan's fast-eroding Central Range. CREDIT: WENILIOU/SHUTTERSTOCK The reaction of atmospheric carbon dioxide (CO2) with silicate rocks provides a carbon sink that helps counterbalance the release of CO2 by volcanic degassing. However, some types of rocks contain petrogenic organic carbon, the oxidation of which adds CO2 to the atmosphere, counteracting the drawdown by silicates. Hemingway et al. present evidence from the rapidly eroding Central Range of Taiwan showing that microbes oxidize roughly two-thirds of the petrogenic organic carbon there and that the rate of oxidation increases with the rate of erosion. Science , this issue p. [209][2] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aao6463

Biospheric and petrogenic organic carbon flux along southeast Alaska

Holocene fjords store ca. 11–12% of the total organic carbon (OC) buried in marine sediments with fjords along southeast (SE) Alaska possibly storing half of this OC (Smith et al., 2015). However, the respective burial of biospheric (OCbio) and petrogenic OC (OCpetro) remains poorly constrained, particularly across glaciated versus non-glaciated systems. Here, we use surface sediment samples to quantify the sources and burial of sedimentary OC along SE Alaska fjord-coastal systems, and conduct a latitudinal comparison across a suite of fjords and river-coastal systems with distinctive OC sources. Our results for SE Alaska show that surface sediments in northern fjords (north of Icy Strait) with headwater glaciers are dominated by OCpetro, in contrast to marine and terrestrially-derived fresh OC in non-glaciated southern fjords. Along the continental shelf of the Gulf of Alaska, terrestrial OC is exported from rivers. Using end-member mixing models, we determine that glaciated fjords have significantly higher burial rates of OCpetro (∼1.1×103 gOC m−2yr−1) than non-glaciated fjords and other coastal systems, making SE Alaska potentially the largest sink of OCpetro in North America. In contrast, non-glaciated fjords in SE Alaska are effective in burying marine OC (OCbio-mari) (13–82 g OC m−2yr−1). Globally, OC in fjord sediments are comprised of a mixture of OCpetro and fresh OCbio, in contrast to the pre-aged OC from floodplain river-coastal systems. We find that there may be a general latitudinal trend in the role of fjords in processing OC, where high-latitude temperate glacial fjords (e.g., Yakutat Bay, SE Alaska) rebury OCpetro and non-glacial mid-latitude fjords (e.g., Doubtful Sound, Fiordland) sequester CO2 from phytoplankton and/or temperate forests. Overall, we propose that fjords are effective in sequestering OCbio and re-burying OCpetro. Based on our study, we hypothesize that climate change will have a semi-predictable impact on fjords' OC cycling in the near future.

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 (∼3000–500014Cyrs) 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.

Spatial variations in geochemical characteristics of the modern Mackenzie Delta sedimentary system

The Mackenzie River in Canada is by far the largest riverine source of sediment and organic carbon (OC) to the Arctic Ocean. Therefore the transport, degradation and burial of OC along the land-to-ocean continuum for this riverine system is important to study both regionally and as a dominant representative of Arctic rivers. Here, we apply sedimentological (grain size, mineral surface area), and organic and inorganic geochemical techniques (%OC, δ13C-OC and Δ14C-OC, 143Nd/144Nd, δ2H and δ18O, major and trace elements) on particulate, bank, channel and lake surface sediments from the Mackenzie Delta, as well as on surface sediments from the Mackenzie shelf in the Beaufort Sea. Our data show a hydrodynamic sorting effect resulting in the accumulation of finer-grained sediments in lake and shelf deposits. A general decrease in organic carbon (OC) to mineral surface area ratios from river-to-sea furthermore suggests a loss of mineral-bound terrestrial OC during transport through the delta and deposition on the shelf. The net isotopic value of the terrestrial OC that is lost en route, derived from relationships between δ13C, OC and surface area, is −28.5‰ for δ13C and −417‰ for Δ14C. We calculated that OC burial efficiencies are around 55%, which are higher (∼20%) than other large river systems such as the Amazon. Old sedimentary OC ages, up to 12 14C-ky, suggest the delivery of both a petrogenic OC source (with an estimated contribution of 19±9%) as well as a pre-aged terrestrial OC source. We calculated the 14C-age of this pre-aged, biogenic, component to be about 6100yrs, or −501‰, which illustrates that terrestrial OC in the watershed can reside for millennia in soils before being released into the river. Surface sediments in lakes across the delta (n=20) showed large variability in %OC (0.92–5.7%) and δ13C (−30.7‰ to −23.5‰). High-closure lakes, flooding only at exceptionally high water levels, hold high sedimentary OC contents (>2.5%) and young biogenic OC with a terrestrial or an autochthonous source whereas no-closure lakes, permanently connected to a river channel, hold sediments with pre-aged, terrestrial OC. The intermediate low-closure lakes, flooding every year during peak discharge, display the largest variability in OC content, age and source, likely reflecting variability in for example the length of river–lake connections, the distance to sediment source and the number of intermediate settling basins. Bank, channel and suspended sediment show variable 143Nd/144Nd values, yet there is a gradual but distinct spatial transition in 143Nd/144Nd (nearly three ε units; from −11.4 to −13.9) in the detrital fraction of lake surface sediments from the western to the eastern delta. This reflects the input of younger Peel River catchment material in the west and input of older geological source material in the east, and suggests that lake sediments can be used to assess variability in source watershed patterns across the delta.

Global carbon export from the terrestrial biosphere controlled by erosion

Riverine export of particulate organic carbon (POC) to the ocean affects the atmospheric carbon inventory over a broad range of timescales. On geological timescales, the balance between sequestration of POC from the terrestrial biosphere and oxidation of rock-derived (petrogenic) organic carbon sets the magnitude of the atmospheric carbon and oxygen reservoirs. Over shorter timescales, variations in the rate of exchange between carbon reservoirs, such as soils and marine sediments, also modulate atmospheric carbon dioxide levels. The respective fluxes of biospheric and petrogenic organic carbon are poorly constrained, however, and mechanisms controlling POC export have remained elusive, limiting our ability to predict POC fluxes quantitatively as a result of climatic or tectonic changes. Here we estimate biospheric and petrogenic POC fluxes for a suite of river systems representative of the natural variability in catchment properties. We show that export yields of both biospheric and petrogenic POC are positively related to the yield of suspended sediment, revealing that POC export is mostly controlled by physical erosion. Using a global compilation of gauged suspended sediment flux, we derive separate estimates of global biospheric and petrogenic POC fluxes of 157(+74)(-50) and 43(+61)(-25) megatonnes of carbon per year, respectively. We find that biospheric POC export is primarily controlled by the capacity of rivers to mobilize and transport POC, and is largely insensitive to the magnitude of terrestrial primary production. Globally, physical erosion rates affect the rate of biospheric POC burial in marine sediments more strongly than carbon sequestration through silicate weathering. We conclude that burial of biospheric POC in marine sediments becomes the dominant long-term atmospheric carbon dioxide sink under enhanced physical erosion.

Direct measurement of riverine particulate organic carbon age structure

Carbon cycling studies focusing on transport and transformation of terrigenous carbon sources toward marine sedimentary sinks necessitate separation of particulate organic carbon (OC) derived from many different sources and integrated by river systems. Much progress has been made on isolating and characterizing young biologically‐formed OC that is still chemically intact, however quantification and characterization of old, refractory rock‐bound OC has remained troublesome. Quantification of both endmembers of riverine OC is important to constrain exchanges linking biologic and geologic carbon cycles and regulating atmospheric CO2 and O2. Here, we constrain petrogenic OC proportions in suspended sediment from the headwaters of the Ganges River in Nepal through direct measurement using ramped pyrolysis radiocarbon analysis. The unique results apportion the biospheric and petrogenic fractions of bulk particulate OC and characterize biospheric OC residence time. Compared to the same treatment of POC from the lower Mississippi‐Atchafalaya River system, contrast in age spectra of the Ganges tributary samples illustrates the difference between small mountainous river systems and large integrative ones in terms of the global carbon cycle.

Paleozoic multiple accretionary and collisional processes of the Beishan orogenic collage

The dissolved chemistry of rivers has been extensively studied to elucidate physical and climatic controls of chemical weathering at local to global spatial scales, as well as the impacts of chemical weathering on climate over short to geologic temporal scales. Within this effort, mixing models with Monte Carlo uncertainty propagation are a common tool for inverting measurements of dissolved river chemistry to distinguish among contributions from end-members with distinct elemental and/or isotopic compositions. However, the methods underlying prior river inversion models have typically been opaque. Here we present Mixing Elements ANd Dissolved Isotopes in Rivers (MEANDIR), a set of MATLAB scripts that enable highly customizable inversion of dissolved river chemistry with Monte Carlo propagation of uncertainty. First, we present an overview of the mathematics underlying MEANDIR. This overview includes, among other topics, derivation of equations for mass balance, implementation of chlorine critical values, construction of cost functions, normalization to the sum of dissolved variables, quantification of river sulfate sourced from pyrite oxidation, resolution of petrogenic organic carbon oxidation, representation of secondary phase formation with isotopic fractionation, and calculation of the impact of weathering on atmospheric carbon dioxide. Second, we apply MEANDIR to five previously published datasets to demonstrate the sensitivity of results to parameter choices. We invert data from two global compilations of river chemistry (Gaillardet and others, 1999; Burke and others, 2018), the major element chemistry and sulfate sulfur isotope ratios of rivers in the Peruvian Amazon (Torres and others, 2016), the major element chemistry of Icelandic rivers (Gíslason and others, 1996), and the major and trace element chemistry of water samples from the Mackenzie River (Horan and others, 2019). MEANDIR and its user guide are freely available online.

Oxidation of petrogenic organic carbon in the Amazon floodplain as a source of atmospheric CO2

The two long-term sources of atmospheric carbon are CO2 degassing from metamorphic and volcanic activity, and oxidation of organic carbon (OC) contained in sedimentary rocks, or petrogenic organic carbon (OCpetro). The latter fl ux is still poorly constrained. In this study, we report particulate organic carbon content and 14C activity measurements in Amazon River sediments, which allow for estimates of the OCpetro content of these sediments. A large decrease of OCpetro content in riverine sediments is observed from the outlet of the Andes to the mouth of the large tributaries. This loss reveals oxidation of OCpetro during transfer of sediments in the floodplain, and results in an escape of ~0.25 Mt C/yr to the atmosphere, which is on the same order of magnitude as the CO2 consumption by silicate weathering in the same area. Raman microspectroscopy investigations show that graphite is the most stable phase with respect to this oxidation process. These results emphasize the significance of OCpetrooxidation in large river floodplains in the global carbon cycle.

A new look at old carbon in active margin sediments

Recent studies suggest that as much as half of the organic carbon (OC) undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering. These estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils, freshwater sediments, and river deltas, and thus only minimally contributes to the extraneously old 14C ages of total organic matter often observed on adjacent shelves. Here we test this paradigm by measuring the Δ14C and δ13C values of individual higher plant wax fatty acids as well as the δ13C values of extractable alkanes isolated from the Eel River margin (California). The isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition. A coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated, with pre-aged terrestrial material instead composing a considerable proportion of all organic matter. If these findings are characteristic of other continental margins proximal to small mountainous rivers, then the importance of petrogenic OC burial in marine sediments may need to be reevaluated. © 2009 The Geological Society of America.