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Peter J. Talling - One of the best experts on this subject based on the ideXlab platform.
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controls on the formation of Turbidity Current channels associated with marine terminating glaciers and ice sheets
Marine Geology, 2019Co-Authors: Ed Pope, Alexandre Normandeau, Colm O Cofaigh, Chris R Stokes, Peter J. TallingAbstract:Abstract Submarine channels, and the sediment density flows which form them, act as conduits for the transport of sediment, macro-nutrients, fresher water and organic matter from the coast to the deep sea. These systems are therefore significant pathways for global sediment and carbon cycles. However, the conditions that permit or preclude submarine channel formation are poorly understood, especially when in association with marine-terminating glaciers. Here, using swath-bathymetric data from the inner shelf and fjords of northwest and southeast Greenland, we provide the first paper to analyse the controls on the formation of submarine channels offshore of numerous marine-terminating glaciers. These data reveal 37 submarine channels: 11 offshore of northwest Greenland and 26 offshore of southeast Greenland. The presence of channels is nearly always associated with: (1) a stable glacier front, as indicated by the association with either a moraine or grounding-zone wedge; and (2), a consistent seaward sloping gradient. In northwest Greenland, Turbidity Current channels are also more likely to be associated with larger glacier catchments with higher ice and meltwater fluxes which provide higher volumes of sediment delivery. However, the factors controlling the presence of channels in northwest and southeast Greenland are different, which suggest some complexity about predicting the occurrence of Turbidity Currents in glacier-influenced settings. Future work on tidewater glacier sediment delivery rates by different subglacial processes, and the role of grain size and catchment/regional geology is required to address uncertainties regarding the controls on channel formation.
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how Turbidity Current frequency and character varies down a fjord delta system combining direct monitoring deposits and seismic data
Sedimentology, 2019Co-Authors: Cooper Stacey, Peter J. Talling, John Hughes E Clarke, Philip R Hill, Randolph J Enkin, Gwyn D LinternAbstract:Submarine Turbidity Currents are one of the most important processes for moving sediment across our planet; they are hazardous to offshore infrastructure, deposit petroleum reservoirs worldwide, and may record tsunamigenic landslides. However, there are few studies that have monitored these submarine flows in action, and even fewer studies that have combined direct monitoring with longer‐term records from core and seismic data of deposits. This article provides one of the most complete studies yet of a Turbidity Current system. The aim here is to understand what controls changes in flow frequency and character along the turbidite system. The study area is a 12 km long delta‐fed fjord (Howe Sound) in British Columbia, Canada. Over 100 often powerful (up to 2 to 3 m sec−1) events occur each year in the highly‐active proximal channels, which extend for 1 to 2 km from the delta lip. About half of these events reach the lobes at the channel mouths. However, flow frequency decreases rapidly once these initially sand‐rich flows become unconfined, and only one to five flows run out across the mid‐slope each year. Many of these sand‐rich, channelized, delta‐sourced flows therefore dissipated over a few hundred metres, once unconfined, rather than eroding and igniting. Upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes, and also across the unconfined mid‐slope. These supercritical flows deposited thick sand beds in proximal channels and lobes, but thinner and finer beds on the unconfined mid‐slope. The distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years. This study shows how sand‐rich delta‐fed flows dissipate rapidly once they become unconfined, that supercritical flows dominate in both confined and unconfined settings, and how a second type of more hazardous, and much less frequent event is linked to a different scale of margin failure.
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newly recognized Turbidity Current structure can explain prolonged flushing of submarine canyons
Science Advances, 2017Co-Authors: Esther J Sumner, Maria Azpirozzabala, Cortis Cooper, Michael A Clare, D R Parsons, Peter J. Talling, Matthieu J.b. Cartigny, Stephen M. Simmons, Ed L. PopeAbstract:Seabed-hugging flows called Turbidity Currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of Turbidity Currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic Turbidity Currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for Turbidity Current structure that can explain why these are far more prolonged than all previously monitored oceanic Turbidity Currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these Turbidity Currents results from flow stretching and that this stretching is characteristic of mud-rich Turbidity Current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.
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implications of reduced Turbidity Current and landslide activity for the initial eocene thermal maximum evidence from two distal deep water sites
Earth and Planetary Science Letters, 2015Co-Authors: Michael A Clare, Peter J. Talling, James E HuntAbstract:Previous studies propose that submarine landslides and Turbidity Currents may become more likely due to future rapid global warming. Determining whether global warming increases likelihood assists in assessment of landslide-triggered tsunami hazards and risk to seafloor structures. Other studies propose that landslides helped to trigger past rapid climate change due to sudden release of gas hydrates. Two deep-water turbidite records show prolonged hiatuses in Turbidity Current activity during the Initial Eocene Thermal Maximum (IETM) at ∼55 Ma. The IETM represents a possible proxy for future anthropogenically-induced climate change. It is likely that our records mainly represent large and fast moving disintegrative submarine landslides. Statistical analysis of long term (>2.3 Myr) records shows that Turbidity Current frequency significantly decreased after the IETM. Our results indicate that rapid climate change does not necessarily cause increased Turbidity Current activity, and do not provide evidence for landslides as a primary trigger for the IETM.
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hybrid submarine flows comprising Turbidity Current and cohesive debris flow deposits theoretical and experimental analyses and generalized models
Geosphere, 2013Co-Authors: Peter J. TallingAbstract:Hybrid flows comprising both Turbidity Current and submarine debris flow are a significant departure from many previous influential models for submarine sediment density flows. Hybrid beds containing cohesive debrite and turbidite are common in distal depositional environments, as shown by detailed observations from more than 20 modern and ancient systems worldwide. Hybrid flows, and cohesive debris flows more generally, are best classified in terms of a continuum of decreasing cohesive debris flow strength. High-strength cohesive debris flows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure. They are typically confined to higher gradient continental slopes, but may occasionally form megabeds on basin plains, in both cases overlain by a thin turbidite. Intermediate-strength cohesive debris flows typically contain clasts, but their deposits may be <1 or 2 m thick on low-gradient fan fringes, and are encased in turbidite sand and mud. Clasts may be far-traveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding flow. Low-strength cohesive debris flows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fluid mud layers that do not support sand. Intermediate- and low-strength cohesive debrites are consistently absent in more proximal parts of submarine systems, where faster moving sediment-charged flows are more likely to be turbulent. Intermediate-strength debris flows can run out for long distances on low gradients without hydroplaning. Very low strength cohesive debris flows most likely form through late-stage transformations near the site of debrite deposition, and emplaced gently to avoid mixing with surrounding seawater. The location and geometry of cohesive debrites in hybrid beds are controlled strongly by seafloor morphology and small changes in gradient. Debrites occur as fringes around raised channel-levee ridges, or in the central and lowest parts of basin plains lacking such ridges. Small variations in mud fraction produce profound changes in cohesive strength, flow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude. Reduction in flow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fluids. Small amounts of sediment can dampen or extinguish turbulence, especially as flow decelerates, affecting how sediment is supported or deposited. This ensures that cohesive debris flows and hybrid flows have a rich variety of behaviors.
Gary Parker - One of the best experts on this subject based on the ideXlab platform.
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Turbidity Current with a roof success and failure of rans modeling for Turbidity Currents under strongly stratified conditions
Journal of Geophysical Research, 2013Co-Authors: Mariano I. Cantero, Alessandro Cantelli, Carlos Pirmez, Tzuhao Yeh, Gary ParkerAbstract:[1] Density underflows in general and Turbidity Currents in particular differ from rivers in that their governing equations do not allow a steady, streamwise uniform “normal” solution. This is due to the fact that density underflows entrain ambient fluid, thus creating a tendency for underflow discharge to increase downstream. Recently, however, a simplified configuration known as the “Turbidity Current with a roof” (TCR) has been proposed. The artifice of a roof allows for steady, uniform solutions for flows driven solely by gravity acting on suspended sediment. A recent application of direct numerical simulation (DNS) of the Navier-Stokes equations by Cantero et al. (2009) has revealed that increasing dimensionless sediment fall velocity increases flow stratification, resulting in a damping of the turbulence. When the dimensionless fall velocity is increased beyond a threshold value, near-bed turbulence collapses. Here we use the DNS results as a means of testing the ability of three Reynolds-averaged Navier-Stokes (RANS) models of turbulent flow to capture stratification effects in the TCR. Results showed that the Mellor-Yamada and quasi-equilibrium k-ϵ models are able to adequately capture the characteristics of the flow under conditions of relatively modest stratification, whereas the standard k-ϵ model is a relatively poor predictor of turbulence characteristics. As stratification strengthens, however, the deviation of all RANS models from the DNS results increases. All are incapable of predicting the collapse of near-bed turbulence predicted by DNS under conditions of strong stratification. This deficiency is likely due to the inability of RANS models to replace viscous dissipation of turbulent energy with transfer to internal waves under conditions of strong stratification. Within the limits of modest stratification, the quasi-equilibrium k-ϵ model is used to derive predictors of flow which can be incorporated into simpler, layer-averaged models of Turbidity Currents.
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Field-scale numerical modeling of breaching as a mechanism for generating continuous Turbidity Currents
Geosphere, 2011Co-Authors: Esther C. Eke, Enrica Viparelli, Gary ParkerAbstract:The term “breaching” refers to the slow, retrogressive failure of a steep subaqueous slope, so forming a nearly vertical Turbidity Current directed down the face. This mechanism, first identified by the dredging industry, has remained largely unexplored, and yet evidence exists to link breaching to the formation of sustained Turbidity Currents in the deep sea. In this paper we model a breach-generated Turbidity Current using a three-equation, layer-averaged model that has as its basis the governing equations for the conservation of momentum, water, and suspended sediment of the Turbidity Current. In the model, the Turbidity Current is divided into two regions joined at a migrating boundary: the breach face, treated as vertical, and a quasi-horizontal region sloping downdip. In this downstream region, the bed slope is much lower (but still nonzero), and is constructed by deposition from a quasi-horizontal Turbidity Current. The model is applied to establish the feasibility of a breach-generated Turbidity Current in a field setting, using a generic example based on the Monterey Submarine Canyon, offshore California, USA.
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Turbidity Current with a roof direct numerical simulation of self stratified turbulent channel flow driven by suspended sediment
Journal of Geophysical Research, 2009Co-Authors: Mariano I. Cantero, S Balachandar, Alessandro Cantelli, Carlos Pirmez, Gary ParkerAbstract:[1] In this work we present direct numerical simulations (DNS) of sediment-laden channel flows. In contrast to previous studies, where the flow has been driven by a constant, uniform pressure gradient, our flows are driven by the excess density imposed by suspended sediment. This configuration provides a simplified model of a Turbidity Current and is thus called the Turbidity Current with a roof configuration. Our calculations elucidate with DNS for the first time several fascinating features of sediment-laden flows, which may be summarized as follows. First, the presence of sediment breaks the symmetry of the flow because of a tendency to self-stratify. More specifically, this self-stratification is manifested in terms of a Reynolds-averaged suspended sediment concentration that declines in the upward normal direction and a Reynolds-averaged velocity profile with a maximum that is below the channel centerline. Second, this self-stratification damps the turbulence, particularly near the bottom wall. Two regimes are observed, one in which the flow remains turbulent but the level of turbulence is reduced and another in which the flow relaminarizes in a region near the bottom wall, i.e., bed. Third, the analysis allows the determination of a criterion for the break between these two regimes in terms of an appropriately defined dimensionless settling velocity. The results provide guidance for the improvement of Reynolds-averaged closures for turbulent flow in regard to stratification effects. Although the analysis reported here is not performed at the scale of large oceanic Turbidity Currents, which have sufficiently large Reynolds numbers to be inaccessible via DNS at this time, the implication of flow relaminarization is of considerable importance. Even a swift oceanic Turbidity Current which at some point crosses the threshold into the regime of relaminarization may lose the capacity to reentrain sediment that settles on the bed and thus may quickly die as it loses its driving force.
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conditions under which a supercritical Turbidity Current traverses an abrupt transition to vanishing bed slope without a hydraulic jump
Journal of Fluid Mechanics, 2007Co-Authors: Svetlana Kostic, Gary ParkerAbstract:Turbidity Currents act to sculpt the submarine environment through sediment erosion and deposition. A sufficiently swift Turbidity Current on a steep slope can be expected to be supercritical in the sense of the bulk Richardson number; a sufficiently tranquil Turbidity Current on a mild slope can be expected to be subcritical. The transition from supercritical to subcritical flow is accomplished through an internal hydraulic jump. Consider a steady Turbidity Current flowing from a steep canyon onto a milder fan, and then exiting the fan down another steep canyon. The flow might be expected to undergo a hydraulic jump to subcritical flow near the canyon–fan break, and then accelerate again to critical flow at the fan–canyon break downstream. The problem of locating the hydraulic jump is here termed the ‘jump problem’. Experiments with fine-grained sediment have confirmed the expected behaviour outlined above. Similar experiments with coarse-grained sediment suggest that if the deposition rate is sufficiently high, this ‘jump problem’ may have no solution with the expected behaviour, and in particular no solution with a hydraulic jump. In such cases, the flow either transits the length of the low-slope fan as a supercritical flow and shoots off the fan–canyon break without responding to it, or dissipates as a supercritical flow before exiting the fan. The analysis presented below confirms the existence of a range associated with rapid sediment deposition where no solution to the ‘jump problem’ can be found. The criterion for this range is stated in terms of an order-one dimensionless parameter involving the fall velocity of the sediment. The criterion is tested and confirmed against the experiments mentioned above. A sample field application is presented.
Mariano I. Cantero - One of the best experts on this subject based on the ideXlab platform.
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Entrainment in Temporally Evolving Turbidity Currents
2017Co-Authors: Jorge Salinas, Mrugesh Shringarpure, Mariano I. Cantero, Sadana BalachandarAbstract:Turbidity Currents are sediment laden shear flows that run along a sloping bed, often submerged beneath a deep layer of quiescent fluid, driven by the excess hydrostatic pressure due to the suspended sediments. Turbidity Currents are always turbulent since the suspended sediment particles that drive the flow cannot remain in suspension under laminar conditions. As the Turbidity Current travels downslope, the flow interacts with the bed at the bottom and with the ambient fluid layer at the top. Ambient fluid entrainment is a fascinating fluid mechanical phenomenon where quiescent ambient fluid is ingested into the Current to an active shear flow. As the Turbidity Current flows downstream over the sloping bed, under a deep ambient of clear fluid, clear ambient fluid is continuously entrained into the Turbidity Current and the thickness of the Current increases. In this work we study the entrainment mechanism taking place between the ambient fluid layer and the Turbidity Current by means of fully resolved direct numerical simulations. Entrainment is a function of both the local Richardson number, Ri, and the non-dimensional settling velocity of the sediments. Here we consider a model Turbidity Current that is homogeneous in the streamwise direction. Thus, the effect of entrainment of clear fluid at the top of the Turbidity Current results in a temporal growth of the Current height. With the assumption of streamwise homogeneity we investigate a non-stationary problem where the temporal growth of the height of the Turbidity Current is monitored in order to evaluate the role of entrainment of clear fluid.
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Turbidity Current with a roof success and failure of rans modeling for Turbidity Currents under strongly stratified conditions
Journal of Geophysical Research, 2013Co-Authors: Mariano I. Cantero, Alessandro Cantelli, Carlos Pirmez, Tzuhao Yeh, Gary ParkerAbstract:[1] Density underflows in general and Turbidity Currents in particular differ from rivers in that their governing equations do not allow a steady, streamwise uniform “normal” solution. This is due to the fact that density underflows entrain ambient fluid, thus creating a tendency for underflow discharge to increase downstream. Recently, however, a simplified configuration known as the “Turbidity Current with a roof” (TCR) has been proposed. The artifice of a roof allows for steady, uniform solutions for flows driven solely by gravity acting on suspended sediment. A recent application of direct numerical simulation (DNS) of the Navier-Stokes equations by Cantero et al. (2009) has revealed that increasing dimensionless sediment fall velocity increases flow stratification, resulting in a damping of the turbulence. When the dimensionless fall velocity is increased beyond a threshold value, near-bed turbulence collapses. Here we use the DNS results as a means of testing the ability of three Reynolds-averaged Navier-Stokes (RANS) models of turbulent flow to capture stratification effects in the TCR. Results showed that the Mellor-Yamada and quasi-equilibrium k-ϵ models are able to adequately capture the characteristics of the flow under conditions of relatively modest stratification, whereas the standard k-ϵ model is a relatively poor predictor of turbulence characteristics. As stratification strengthens, however, the deviation of all RANS models from the DNS results increases. All are incapable of predicting the collapse of near-bed turbulence predicted by DNS under conditions of strong stratification. This deficiency is likely due to the inability of RANS models to replace viscous dissipation of turbulent energy with transfer to internal waves under conditions of strong stratification. Within the limits of modest stratification, the quasi-equilibrium k-ϵ model is used to derive predictors of flow which can be incorporated into simpler, layer-averaged models of Turbidity Currents.
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Turbidity Current with a roof direct numerical simulation of self stratified turbulent channel flow driven by suspended sediment
Journal of Geophysical Research, 2009Co-Authors: Mariano I. Cantero, S Balachandar, Alessandro Cantelli, Carlos Pirmez, Gary ParkerAbstract:[1] In this work we present direct numerical simulations (DNS) of sediment-laden channel flows. In contrast to previous studies, where the flow has been driven by a constant, uniform pressure gradient, our flows are driven by the excess density imposed by suspended sediment. This configuration provides a simplified model of a Turbidity Current and is thus called the Turbidity Current with a roof configuration. Our calculations elucidate with DNS for the first time several fascinating features of sediment-laden flows, which may be summarized as follows. First, the presence of sediment breaks the symmetry of the flow because of a tendency to self-stratify. More specifically, this self-stratification is manifested in terms of a Reynolds-averaged suspended sediment concentration that declines in the upward normal direction and a Reynolds-averaged velocity profile with a maximum that is below the channel centerline. Second, this self-stratification damps the turbulence, particularly near the bottom wall. Two regimes are observed, one in which the flow remains turbulent but the level of turbulence is reduced and another in which the flow relaminarizes in a region near the bottom wall, i.e., bed. Third, the analysis allows the determination of a criterion for the break between these two regimes in terms of an appropriately defined dimensionless settling velocity. The results provide guidance for the improvement of Reynolds-averaged closures for turbulent flow in regard to stratification effects. Although the analysis reported here is not performed at the scale of large oceanic Turbidity Currents, which have sufficiently large Reynolds numbers to be inaccessible via DNS at this time, the implication of flow relaminarization is of considerable importance. Even a swift oceanic Turbidity Current which at some point crosses the threshold into the regime of relaminarization may lose the capacity to reentrain sediment that settles on the bed and thus may quickly die as it loses its driving force.
Mcghee C. A. - One of the best experts on this subject based on the ideXlab platform.
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Efficient preservation of young terrestrial organic carbon in sandy Turbidity-Current deposits
'Geological Society of America', 2020Co-Authors: Hage Sophie, Cartigny, Matthieu J B., Galy Valier, Acikalin Sanem, Clare, Michael A., Grocke, Darren R., Hilton, Robert G., Hunt, James E., Lintern D. Gwyn, Mcghee C. A.Abstract:© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hage, S., Galy, V. V., Cartigny, M. J. B., Acikalin, S., Clare, M. A., Grocke, D. R., Hilton, R. G., Hunt, J. E., Lintern, D. G., McGhee, C. A., Parsons, D. R., Stacey, C. D., Sumner, E. J., & Talling, P. J. Efficient preservation of young terrestrial organic carbon in sandy Turbidity-Current deposits. Geology, 48(9), (2020): 882-887, doi:10.1130/G47320.1.Burial of terrestrial biospheric particulate organic carbon in marine sediments removes CO2 from the atmosphere, regulating climate over geologic time scales. Rivers deliver terrestrial organic carbon to the sea, while Turbidity Currents transport river sediment further offshore. Previous studies have suggested that most organic carbon resides in muddy marine sediment. However, Turbidity Currents can carry a significant component of coarser sediment, which is commonly assumed to be organic carbon poor. Here, using data from a Canadian fjord, we show that young woody debris can be rapidly buried in sandy layers of Turbidity Current deposits (turbidites). These layers have organic carbon contents 10× higher than the overlying mud layer, and overall, woody debris makes up >70% of the organic carbon preserved in the deposits. Burial of woody debris in sands overlain by mud caps reduces their exposure to oxygen, increasing organic carbon burial efficiency. Sandy Turbidity Current channels are common in fjords and the deep sea; hence we suggest that previous global organic carbon burial budgets may have been underestimated.We thank C. Johnson, M. Lardie, A. Gagnon, A. McNichol, and the NOSAMS (National Ocean Sciences Accelerator Mass Spectrometry) team (Woods Hole Oceanographic Institution [WHOI], Massachusetts, USA) for their help with ramped oxidation system and isotopes. We thank the captain and crew of CCGS Vector. Support was provided by UK Natural Environment Research Council (NERC) grants NE/M007138/1 (to Cartigny) and NE/L013142/1 (to Talling), NE/P005780/1 and NE/P009190/1 (to Clare); a Royal Society Research Fellowship (to Cartigny); an International Association of Sedimentologists Postgraduate Grant and National Oceanography Centre Southampton–WHOI exchange program funds (to Hage); an independent study award from WHOI (to Galy); the Climate Linked Atlantic Sector Science (CLASS) program (NERC grant NE/R015953/1); and the European Research Council under the European Union’s Horizon 2020 research and innovation program (Grant 725955, to Parsons). We thank François Baudin, Xingqian Cui, editor James Schmitt, and three anonymous reviewers
Mcghee C.a. - One of the best experts on this subject based on the ideXlab platform.
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Efficient preservation of young terrestrial organic carbon in sandy Turbidity-Current deposits
'Geological Society of America', 2020Co-Authors: Hage S., Galy V.v., Cartigny M.j.b., Acikalin S., Gröcke D.r., Hilton R.g., Hunt J.e., Lintern D.g., Mcghee C.a.Abstract:Burial of terrestrial biospheric particulate organic carbon in marine sediments removes CO2 from the atmosphere, regulating climate over geologic time scales. Rivers deliver terrestrial organic carbon to the sea, while Turbidity Currents transport river sediment further offshore. Previous studies have suggested that most organic carbon resides in muddy marine sediment. However, Turbidity Currents can carry a significant component of coarser sediment, which is commonly assumed to be organic carbon poor. Here, using data from a Canadian fjord, we show that young woody debris can be rapidly buried in sandy layers of Turbidity Current deposits (turbidites). These layers have organic carbon contents 10× higher than the overlying mud layer, and overall, woody debris makes up >70% of the organic carbon preserved in the deposits. Burial of woody debris in sands overlain by mud caps reduces their exposure to oxygen, increasing organic carbon burial efficiency. Sandy Turbidity Current channels are common in fjords and the deep sea; hence we suggest that previous global organic carbon burial budgets may have been underestimated
