Particulate Organic Carbon Delivery Research Articles

How is particulate organic carbon transported through the river-fed submarine Congo Canyon to the deep sea?

Abstract. The transfer of carbon from land to the near-coastal ocean is increasingly being recognized in global carbon budgets. However, a more direct transfer of terrestrial organic carbon to the deep sea is comparatively overlooked. Among systems that connect coastal to deep-sea environments, the submarine Congo Canyon is of particular interest since the canyon head starts 30 km into the Congo River estuary, which delivers ∼7 % of the dissolved and particulate organic carbon from the world's rivers. However, sediment and particulate organic carbon transport mechanisms that operate in the Congo Canyon and submarine canyons more globally are poorly constrained compared to rivers because monitoring of deep-sea canyons remains challenging. Using a novel array of acoustic instruments, sediment traps, and cores, this study seeks to understand the hydrodynamic processes that control delivery of particulate organic carbon via the submarine Congo Canyon to the deep sea. We show that particulate organic carbon transport in the canyon axis is modulated by two processes. First, we observe periods where the canyon dynamics are dominated by tides, which induce a background oscillatory flow (speeds of up to 0.15 m s−1) through the water column, keeping muds in suspension, with a net upslope transport direction. Second, fast-moving (up to 8 m s−1) turbidity currents occur for 35 % of the time during monitoring periods and transport particulate organic carbon with mud and sand at an estimated transit flux that is more than 3 to 6 times the flux induced by tides. Organic carbon transported and deposited in the submarine canyon has a similar isotopic composition to organic carbon in the Congo River and in the deep-sea fan at 5 km of water depth. Episodic turbidity currents thus promote efficient transfer of river-derived particulate organic carbon in the Congo submarine fan, leading to some of the highest terrestrial carbon preservation rates observed in marine sediments globally.

Transport of particulate organic carbon in the lower Yellow River (Huanghe) as modulated by dam operation

The Yellow River has changed into a highly dam-controlled system over the recent decades. Based on gauging records of water-sediment discharge and particulate organic carbon (POC) datasets, this paper investigates the dam impacts on spatio-temporal variability of hydrology and POC transport. With a large part of suspended sediment and POC normally preserved within the Xiaolangdi (XLD) reservoir, erosion of riverbed downstream the reservoir has now become a primary source of sediment and POC to coastal ocean (172 Mt/yr for sediment and 0.56 Mt/yr for POC, respectively). Sediment accumulation and POC burial within the XLD reservoir are 246 Mt/yr and 0.60 Mt/yr, respectively, globally accounting for ∼10% and ∼1% of the retentions in reservoirs. Downstream the XLD reservoir, the averaged Δ 14 C values of POC was −451 ± 25‰, corresponding to a 14 C age of 4763 ± 366 yr BP, indicating that the POC in the lower reaches of the Yellow River was ∼2400 yr younger than that in the Loess Plateau region where most of the sediment and POC are sourced. Petrogenic POC fluxes upstream and downstream the XLD dam were 0.12 Mt/yr and 0.06 Mt/yr, respectively, clearly indicative of large-scale trapping of petrogenic POC by the XLD dam. At present, the XLD dam regulation intercepts this petrogenic POC from the Loess Plateau and enhances the downstream input of biospheric POC ( Fm = 0.85) to the coast. In contrast to the sediment and POC delivery of the rivers worldwide that are primarily dominated by tectonic, climatic and geographic settings, human activities, especially the dam regulation in the Yellow River, have significantly altered river hydrologic regime and overwhelmed the sediment and POC delivery within the river-coast continuum, which has profound impacts on carbon cycling in the coastal margin. Schematic diagram for POC and sediment budget within the Yellow River-Coast Continuum from 2002 to 2014 based on observed and reconstrcuted datasets. • The XLD reservoir acts as a sink of POC coupled with periodic rapid sediment accumulation. • Downstream riverbed erosion is a dominant source of POC to the coastal margin. • Dam regulation has substantially transformed the POC delivery during the WSRS.

Model study of organic carbon attenuation and oxygen mass transfer in persistent aggregate layers in the deep sea

A time series study from 1989 to 2017 indicates that, increasingly, carbon export to abyssal sediments in the California Current Ecosystem (CCE), 220 km west of the central California coast (Sta. M), occurs as rapidly sinking pulses of particulate organic carbon (POC). Nearly continuous sediment trap collections confirm that POC export to 3400 m increased significantly after 2011 with carbon attenuation (as fraction of POC remineralized) ranging from a low of 57% to a high of 77% during periods of pulsed flux. All of the major pulse events for the period resulted in the delivery of detrital aggregates that covered part or all of the sediment surface as an organic carbon rich layer at ~4000 m depth. However, the magnitude of the measured Sediment Community Oxygen Consumption (SCOC) did not increase proportionally to the organic carbon inventory change. Here, a model of oxygen consumption, informed by the time series data at Sta. M in the CCE, suggests that aggregate flocs constitute a major source for benthic carbon and a barrier to mass transport of oxygen, leading to reduced carbon attenuation in surface sediments. The correlation of POC delivery with aggregate coverage shows that the majority of POC delivery during pulse events is likely to consume all of the oxygen within the aggregates. Results indicate that an increasing fraction of POC reaches the abyssal sediments at Sta. M under conditions that support increased net burial of carbon.This finding is significant in the context of the fraction of the total POC flux that occurs as pulse events in this important upwelling region. During the period from 2011 to 2017, a third of the total POC flux resulted in aggregate flocs that covered a significant portion of the sediment surface (>47%). Following the 2015–2016 El Nino period the flocs became a dominant feature of the sediment surface. More than 80% of the POC flux to near bottom sediment traps in 2017 occurred coincident with aggregate floc cover of more than 90% of the sediments. The resultant physical barrier to oxygen transport and relatively low consumption of newly delivered carbon indicate conditions that are more typical of environments having low oxygen bottom waters.

Runoff, soil loss, and sources of particulate organic carbon delivered to streams by sugarcane and riparian areas: An isotopic approach

Soil erosion leads to land degradation and translocation of soil particles together with associated particulate organic carbon (POC) and nutrients, thereby influencing the global carbon cycle. In the present study, we estimated the contribution of POC delivered to a first-order stream from upslope sugarcane fields and a riparian forest in southeast Brazil. The results show that the amount of surface runoff and soil erosion generated in the riparian forest is significantly lower than in the upslope sugarcane field. However, the contribution of the forest to the total stream bed POC was above 70%, even though most sediments delivered to the stream originated from the upland sugarcane fields. The discrepancy between sediment and POC delivery from both land uses is a consequence of the presence of preferential runoff pathways from the agricultural fields, through the buffer strips, to the stream. This disconnection between the main sources of sediment and POC to the first-order stream is a potentially important mechanism influencing the transfer of POC from upslope areas to waterways. This mechanism should be considered in order to more reliably assess fluxes of OC from upslope areas to first-order streams in landscapes where arable land is separated from streams by a semi-natural buffer zone with permanent vegetation.

Hydrothermal Fe cycling and deep ocean organic carbon scavenging: Model-based evidence for significant POC supply to seafloor sediments

Submarine hydrothermal venting has recently been identified to have the potential to impact ocean biogeochemistry at the global scale. This is the case because processes active in hydrothermal plumes are so vigorous that the residence time of the ocean, with respect to cycling through hydrothermal plumes, is comparable to that of deep ocean mixing caused by thermohaline circulation. Recently, it has been argued that seafloor venting may provide a significant source of bio-essential Fe to the oceans as the result of a close coupling between Fe and organic carbon in hydrothermal plumes. But a complementary question remains to be addressed: does this same intimate Fe–Corg association in hydrothermal plumes cause any related impact to the global C cycle? To address this, SCOR-InterRidge Working Group 135 developed a modeling approach to synthesize site-specific field data from the East Pacific Rise 9°50′ N hydrothermal field, where the range of requisite data sets is most complete, and combine those inputs with global estimates for dissolved Fe inputs from venting to the oceans to establish a coherent model with which to investigate hydrothermal Corg cycling. The results place new constraints on submarine Fe vent fluxes worldwide, including an indication that the majority of Fe supplied to hydrothermal plumes should come from entrainment of diffuse flow. While this same entrainment is not predicted to enhance the supply of dissolved organic carbon to hydrothermal plumes by more than ∼10% over background values, what the model does indicate is that scavenging of carbon in association with Fe-rich hydrothermal plume particles should play a significant role in the delivery of particulate organic carbon to deep ocean sediments, worldwide.

Evidence for permafrost thaw and transport from an Alaskan North Slope watershed

Burial of organic carbon (OC) in marine sediments is one of the most important linkages between the short-term biologic carbon cycle and the long-term geologic carbon cycle. Yet much is still unknown about the fate of terrigenous OC in marine coastal margins. Here the delivery of particulate OC (POC) to the Colville River deltaic region in the Alaskan Beaufort Sea by particulates of varying densities is studied through the use of ramped temperature pyrolysis and radiocarbon analyses. The Colville River is the largest river in North America whose watershed is underlain completely by high Arctic permafrost tundra. A variety of sources of POC are considered, including terrestrial soils, Pleistocene-aged yedoma-like sediments, coastal peat erosion, and marine POC. We provide the first evidence that riverine POC from the Colville River contains old (Pleistocene-sourced) OC, suggesting ongoing thaw and mobilization of yedoma-like permafrost OC from this northern Alaskan watershed. Additionally, much of this OC appears to be fairly labile and therefore could be readily oxidized and returned to the atmosphere.

Sources of terrigenous inputs to surface sediments of the Colville River Delta and Simpson's Lagoon, Beaufort Sea, Alaska

AbstractThe provenance of sediments and particulate organic carbon (POC) in the Colville River delta and adjacent Simpson's Lagoon, in the Alaskan Beaufort Sea, was investigated using a variety of bulk and molecular techniques, including stable and radiocarbon isotopes, neodymium isotopes, algal pigments, and lignin‐phenols. Additionally, stable carbon isotopes and lignin‐phenols were analyzed on four different density fractions from sediments. The Colville River, the largest river in North America with a watershed exclusively located in the high‐Arctic tundra, was an important source of terrestrial POC to the western edge of the Lagoon, shown by extremely old radiocarbon ages (fraction modern of 0.165 ± 0.001 close to the river mouth up to 0.418 ± 0.002 farther away). Stations without northern protective barrier islands had large amounts of marine POC input, and evidence of benthic microalgae was found in one area of the Lagoon (chlorophyll‐a concentration 35.0 µg gOC−1 in the high‐algal biomass area compared to 1 to 7 µg gOC−1 outside of it). Stations in the middle and eastern end of the Lagoon showed significant sediment input from coastal erosion (input of peat‐indicating non‐lignin‐phenols 0.079 mg (100mgOC)−1 in the eastern lagoon compared to 0.022 mg (100mgOC)−1 near the Colville delta), and on the eastern end of the Lagoon there was evidence of input of Mackenzie River POC, shown with neodymium isotopes and also COP. POC inputs derived from rivers, coastal erosion, and marine productivity were quantified using a ternary mixing model and showed that fluvial supply and coastal erosion were the dominant carbon sources to the Lagoon. This constitutes the first study of POC delivery by the Colville River into a nearshore region and illustrates that continued warming of the high Arctic tundra will likely lead to increased riverine POC delivery to this region of the world.

New views on “old” carbon in the Amazon River: Insight from the source of organic carbon eroded from the Peruvian Andes

AbstractMountain rivers play a key role in the delivery of particulate organic carbon (POC) to large river systems and the ocean. Due to the extent of its drainage area and runoff, the Amazon River is one of Earth's most important biogeochemical systems. However, the source of POC eroded from the humid region of the Eastern Andes and the input of fossil POC from sedimentary rocks (POCfossil) remains poorly constrained. Here we collected suspended sediments from the Kosñipata River during flood events to better characterize Andean POC, measuring the nitrogen to organic carbon ratio (N/C), stable carbon isotopes (δ13Corg) and radiocarbon (Δ14Corg). Δ14Corg values ranged from −711‰ to −15‰, and significant linear trends between Δ14Corg, N/C and δ13Corg suggested that this reflects the mixing of POCfossil with very young organic matter (Δ14Corg ~ 50‰) from the terrestrial biosphere (POCnon‐fossil). Using N/C and Δ14Corg in an end‐member mixing analysis, we quantify the fraction of POCfossil (to within 0.1) and find that it contributes a constant proportion of the suspended sediment mass (0.37 ± 0.03%) and up to 80% of total POC. In contrast, the relative contribution of POCnon‐fossil was variable, being most important during the rising limb and peak discharges of flood events. The new data shed light on published measurements of “old” POC (low Δ14Corg) in Andean‐fed tributaries of the Amazon River, with their Δ14Corg and δ13Corg values consistent with variable addition of POCfossil. The findings suggest a greater persistence of Andean POC in the lowland Amazon than previously recognized.

The role of effective discharge in the ocean delivery of particulate organic carbon by small, mountainous river systems

Recent research has shown that small, mountainous river systems (SMRS) account for a significant fraction of the global flux of sediment and particulate organic carbon (POC) to the ocean. The enormous number of SMRS precludes intensive studies of the sort conducted on large systems, necessitating development of a conceptual framework that permits cross‐system comparison and scaling up. Herein, we introduce the geomorphic concept of effective discharge to the problem of source‐to‐sink POC transport. This idea recognizes that transport effectiveness is the product of discharge frequency and magnitude, wherein the latter is quantified as a power‐law relationship between discharge and load (the ‚rating curve’). An analytical solution for effective discharge (Qe) identifies two key variables: the standard deviation of the natural logarithm of discharge (Σq), and the rating exponent of constituent i (bi). Data from selected SMRS are used to show that for a given river Qe‐POC , Qe‐sediment, Qe for different POC constituents (e.g., POCfossil vs. POCmodern) differs in predictable ways, and Qe for a particular constituent can vary seasonally. When coupled with the idea that discharge peaks of small rivers may be coincident with specific oceanic conditions (e.g., large waves, wind from a certain direction) that determine dispersal and burial, these findings have potentially important implications for POC fate on continental margins. Future studies of POC transport in SMRS should exploit the conceptual framework provided herein and seek to identify how constituent‐specific effective discharges vary between rivers and respond to perturbations.

Particulate organic carbon sources and delivery to river channels in the Somerset Levels ECSFDI priority catchment, southwest UK

Soil erosion and sediment delivery represent an important transport pathway for particulate organic carbon (POC) in fluvial systems. Catchment management strategies designed to protect soil resources therefore require reliable information on the significance of key sediment sources for POC transfers. Within this context, a composite fingerprinting procedure, incorporating uncertainty and prior understanding of catchment processes, was used to investigate the significance of eroding pasture or cultivated topsoils, damaged road verges, channel banks/subsurface sources and point source discharges as POC sources and the associated net delivery to river channels across the Somerset Levels, southwest UK. Overall mean relative contributions of POC from the five individual sediment sources were estimated at 41 ± 2% (pasture topsoils), 18 ± 1% (STWs), 15 ± 2% (cultivated topsoils), 14 ± 1% (damaged road verges) and 12 ±2% (channel banks/subsurface sources). Respective estimates of net sediment‐associated POC delivery to watercourses, provided by integrating sediment source ascription 50th percentiles bounded by 95% confidence limits, organic carbon content, sediment yield ranges and sub‐catchment or land use areas, ranged between 2.4–48.5 kg ha−1 yr−1, 0–16.5 kg ha−1 yr−1, 1.7–119.2 kg ha−1 yr−1, 0–13.8 kg ha−1 yr−1 and 0–7.4 kg ha−1 yr−1.

Temporal dynamics of seston: A recurring nighttime peak and seasonal shifts in composition in a stream ecosystem

We measured the baseflow concentration and composition of seston and suspended particulate organic carbon (POC) over a 1-yr period in White Clay Creek, a third-order stream in the southeastern Pennsylvania Piedmont, to assess temporal variability in seston concentration and quality at seasonal and diel timescales. Each month, we sampled stream water under baseflow conditions every 1.5 h over a 24-h period and measured seston and POC concentrations, carbon composition, pigment content, and 13C isotopic ratios. Seston and POC concentrations exhibited a strong diel pattern; nighttime concentrations exceeded daytime concentrations by 80% and 43%, respectively. Suspended chlorophyll a concentrations did not exhibit a diel pattern. We attribute the diel pattern of seston concentration to bioturbation by the nocturnal stream community, including crayfish, amphibians, eels, and macroinvertebrates. Seasonally, carbon content of seston increased from 9% throughout most of the year to 15% during November, December, and March, while seston d13C was depleted in the late fall and enriched in early spring months relative to the rest of the year. Chlorophyll a and pheophytin a concentrations in seston peaked during the early spring. Seasonal patterns in seston and POC composition reflect cycles of autumnal leaf litter inputs and vernal algal production. Bioturbation and shifts in organic carbon inputs mediate changes in POC quality and fluxes, which affect the bioavailability of POC and ultimately influence rates of heterotrophic respiration. Particulate organic carbon (POC) provides energy for microorganisms and macroinvertebrates in headwater streams and, when transported in the suspended load (seston), supports heterotrophic metabolism in downstream reaches and rivers (Wipfli and Gregovich 2002; Mayorga et al. 2005). POC transport represents a longitudinal linkage between upstream autochthonous and allochthonous production and downstream ecosystems, where the magnitude of connection is controlled by the quality, quantity, and timing of POC delivery (Mayorga et al. 2005).

Feedbacks between oceanic redox states and marine productivity: A model perspective focused on benthic phosphorus cycling

A new model for the marine cycles of particulate organic carbon (POC), oxygen, nitrate, and phosphorus has been developed and applied to explore the controls and constraints on marine productivity and nutrient inventories. The coupled benthic‐pelagic model uses a new approach for the simulation of the reactive phosphorus turnover (Preac corresponding to the sum of organic P, authigenic P, and adsorbed phosphate) in marine sediments. The simulated POC/Preac burial ratio in shelf, slope and rise, and deep‐sea sediments increases under strongly reducing conditions in agreement with field observation. The model runs revealed that the spread of anoxia in bottom waters may enhance the productivity of the global ocean by one order of magnitude if sufficient nitrate is provided by N2‐fixation. Thus anoxic bottom waters promote eutrophic conditions and vice versa. Additional model runs showed that the productivity and nutrient inventory of the glacial ocean were probably enhanced due to the falling sea level. Marine regression induced a narrowing of the depositional areas on the continental shelves and thereby an increase in the fraction of POC exported to the deep ocean. The accelerated POC delivery, in turn, decreased the oxygen contents of the deep water and thus favored the release of phosphate from deep‐sea and rise sediments. Enhanced recycling of phosphate at the seafloor promoted further POC export in a positive feedback loop.