Li, X., T. S. Bianchi, Z. Yang, L. E. Osterman, M. A. Allison, S. F. DiMarco, and G. Yang, Historical trends of hypoxia in Changjiang River estuary: Applications of chemical biomarkers and microfossils. , J. Marine Systems, 86, 57-68, 2011, 2 citations, doi:10.1016/j.jmarsys.2011.02.003, #2340 
Over the past two decades China has become the largest global consumer of fertilizers, which has enhanced river nutrient fluxes and caused eutrophication and hypoxia in the Yangtze (Changjiang) large river delta-front estuary (LDE). In this study, we utilized plant pigments, lignin-phenols, stable isotopes (δ13C and δ15N) and foraminiferal microfossils in 210Pb dated cores to examine the history of hypoxia in the Changjiang LDE. Two sediment cores were collected onboard R/V Dong Fang Hong 2 using a stainless-steel box-corer; one at a water depth of 24.7 m on Jun. 15, 2006 and the other at 52 m on Nov. 20, 2007, both in the hypoxic region off the Changjiang LDE.
Nittrouer, J. A., D. Mohrig, M. A. Allison, and A.-P. B. Peyret, The lowermost Mississippi River: A mixed bedrock-alluvial channel, Sedimentology, 58, 1914-1934, 2011, doi:10.1111/j.1365-3091.2011.01245.x, #2419
In this study, the distribution of channel-bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high-intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio-deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach-scale distribution of channel-bed facies is related to river planform. From a broader perspective, the distribution of channel-bed facies is related to channel sinuosity - higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1.5:1 for a 45 km segment of the river that has a sinuosity of 1:76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1:21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock-alluvial channel. The analyses demonstrate that a mixed bedrock-alluvial channel boundary can be associated with low-gradient and sand-bed rivers near their marine outlet.
Nittrouer, J. A., D. Mohrig, and M. A. Allison, Punctuated sand transport in the lowermost Mississippi River, J. Geophys. Res., 116, F04025, 2011, doi:10.1029/2011JF002026, #2420
Measurements of sand flux and water flow in the Mississippi River are presented for a portion of the system 35-50 kilometers upstream from the head of its subaerial delta. These data are used to provide insight into how non-uniform flow conditions, present in the lower reaches of large alluvial rivers, affect the timing and magnitude of sand transport near the river outlet. Field surveys during both low and high water discharge include: (1) sequential digital bathymetric maps defining mobile river-bottom topography which were used to estimate bed-material flux, (2) multiple water-velocity profiles, and (3) multiple suspended-sediment profiles collected using a point-integrated sampler. These data show that total sand transport increases by two orders of magnitude over the measured range in water discharge (11,300 to 38,400 m3 s-1). During low-water discharge no sand is measured in suspension, and sand discharge via bedform migration is minimal. During high-water discharge 54% of the sand discharge is measured in suspension while 46% of the sand discharge is part of bedform migration. The component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment-transport algorithms, and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles. The estimates of boundary shear stress, using measurements of suspended sand transport, bedform transport and downstream oriented velocity profiles, are internally consistent, moreover, the analyses show that boundary shear stress increases by nearly ten-fold over the measured water-discharge range. We show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet. The hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux, and produce punctuated sand transport through the lowermost Mississippi River. Our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock-alluvial channel for the lowermost Mississippi River.
Sampere, T. P., T. S. Bianchi, and M. A. Allison, Historical changes in terrestrially derived organic carbon inputs to Louisiana continental margin sediments over the past 150 years, J. Geophys. Res., 116, G01016, 2011, doi:10.1029/2010JG001420, #2288
Major rivers (and associated deltaic environments) provide the dominant pathway for the input of terrestrial-derived organic carbon in sediments (TOCT) to the ocean. Natural watershed processes and land-use changes are important in dictating the amount and character of carbon being buried on continental margins. Seven core sites were occupied on the Louisiana continental margin aboard the R/V Pelican in July 2003 along two major sediment transport pathways south and west of the Mississippi River mouth. Lignin profiles in these age-dated cores (210Pb geochronology) indicate artificial reservoir retention as a primary control on organic carbon quantity and quality reaching the margin post-1950, whereas pre-1950 sediments may reflect soil erosion due to land clearing and farming practices. Lignin concentrations (range 0.2 to 1.7) also indicate that TOCT delivery rates/decay processes have probably remained relatively consistent from proximal to distal stations along transects. The down-core profile at the Canyon station seems to be temporally linked and connected to inner shelf deposition, suggestive of rapid cross-shelf transport. Sources of terrestrially derived organic carbon were reflective of mixed angiosperms over the last 150 years in cores west and south of the Mississippi River delta. The lignin-phenol vegetation index (LPVI) (range 130.0 to 510) proved to be a sensitive indicator of source changes in these sediments and eliminated some of the variability compared to C/V (range 0.01 to 0.4) and S/V (range 0.9 to 2.1) ratios. Stochastic events such as hurricanes and large river floods have a measurable, albeit ephemeral, effect on the shelf TOCT record. Burial of TOCT on the river-dominated Louisiana continental margin is largely driven by anthropogenic land-use alterations in the last 150 years. Land-use changes in the Mississippi River basin and river damming have likely affected carbon cycling and TOCT burial on the Louisiana continental margin over a large spatial extent as observed by similar trends in cores from across and along the margin.
Sheremet, A., S. Jarimillo, S.-F. Su, M. A. Allison, and K. T. Holland, Wave-mud interaction over the muddy Atchafalaya subaqueous clinoform, Louisiana, United States: Wave processes, J. Geophys. Res., 116, C06005, 2011, doi:10.1029/2010JC006644, #2418
Observations of wave and sediment processes collected at two locations on the Atchafalaya inner shelf show that wave dissipation in shallow, muddy environments is strongly coupled to bed-sediment reworking by waves. During an energetic wave event (2 m significant wave height in 5 m water depth), acoustic backscatter records suggest that sediment in the surficial bed layer evolves from consolidated mud through liquefaction, fluid mud formation, and hindered settling to gelled, under-consolidated mud. Net swell dissipation increases steadily during the storm from negligible prestorm values, consistent with bed softening, but shows no correlation with detectable fluid mud layers. Remarkably, the maximum dissipation rate occurs poststorm, when no fluid mud layers are present. In the waning stage of the storm, the contribution of different wave-forcing processes to wave dissipation is analyzed using an inverse modeling approach based on a nonlinear three-wave interaction model. Although wave-mud interaction dominates dissipative processes, nonlinear three-wave interactions control the shape of the frequency distribution of the dissipation rate. In the wake of the storm, the viscosity values predicted by the inverse modeling converge toward measured values characteristic for gelled mud in a trend that is consistent with a fluid mud entering dewatering and consolidation stages.
Allison, M. A., T. M. Dellapenna, E. S. Gordon, S. Mitra, and S. T. Petsch, Impact of Hurricane Katrina (2005) on shelf organic carbon burial and deltaic evolution, Geophys. Res. Lett., 37, L21605, 2010, doi:10.1029/2010GL044547, #2286
Sediment cores from the continental shelf adjacent to the Mississippi River delta immediately after the passage of Hurricane Katrina were used to examine the magnitude, and implications for the carbon budget, of sediment and particulate organic carbon (POC) remobilized by the storm on the river-dominated continental shelf. POC was sourced from incision of the innermost continental shelf (<25 m water depth) and from surge ebb advection from adjacent wetlands and shallow estuaries, and was re-deposited in deeper water on the shelf. This pulse of young (<1,600 yBP) labile POC, mixed with relict (>5000 yBP) POC eroded from the seafloor, has major implications for the remineralization versus burial of POC in deltas. The scale of erosional deflation of the shelf in water depths beyond seasonal wave-current conditions suggests that, over millennia, tropical cyclones may be responsible for partly removing prodeltaic strata from the geologic record in low-to-mid latitude deltas.
Allison, M. A., and E. A. Meselhe, The use of large water and sediment diversions in the lower Mississippi River (Louisiana) for coastal restoration, J. Hydrology, 387, 346-360, 2010, 2 citations, doi:10.1016/j.jhydrol.2010.04.001, #2289
This study examines the use of large sediment and water diversions in the lower Mississippi River (e.g., South Louisiana) as a tool for coastal restoration. Herein we provide a review, new analysis and synthesis of existing work, much of it previously only available in government reports, and integrate our recent research on the topic. We outline critical knowledge gaps that need to be addressed by the time that construction begins on any future large diversions. The focus of this study is on “river sideÃ¢â‚¬Âť issues and the policy considerations that arise from them.
Anthony, E. J., A. Gardel, N. Gratiot, C. Proisy, M. A. Allison, F. Dolique, and F. Fromard, The Amazon-influenced muddy coast of South America: A review of mud-bank-shoreline interactions, Earth Sci. Rev., 103, 99-121, 2010, doi:10.1016/j.earscirev.2010.09.008, #2342
The 1500 km-long coast of South America between the Amazon and the Orinoco river mouths is the world's muddiest. This is due to the huge suspended-sediment discharge of the Amazon River (106 - 754 tons yr+- 9%), part of which is transported alongshore as mud banks. Mud-bank formation is controlled by the physical oceanography of the continental shelf seaward of the Amazon River mouth, an initial seafloor storage area for much of the suspended sediment discharged from the river. In this area, rapid and sustained fluid-mud concentration and trapping are associated with fresh water-salt water interaction and estuarine front activity on the shelf due to the enormous Amazon water discharge (ca. 173,000 m3 s-1 at Obidos, 900 km upstream of the mouth). Fluid mud is transported shoreward and then along the coasts of the Guianas by a complex interaction of wave and tidal forcing, and wind-generated coastal currents. The mud banks, which may number up to 15 or more at any time, are up to 5 m-thick, 10 to 60 km-long, and 20 to 30 km-wide, and each may contain the equivalent mass of the annual mud supply of the Amazon. As the banks migrate alongshore, their interaction with waves results in complex and markedly fluctuating shorelines that are associated with space- and time-varying depositional -an- phases and erosional -inter-bank- phases. Bank zones are protected from wave attack as a result of wave-energy dampening by mud, and undergo significant, albeit temporary, coastal accretion accompanied by rapid mangrove colonization. The dampening of waves in bank areas as they propagate onshore is accompanied by the shoreward recycling of mud, commonly in the form of individual mud bars. These bars progressively undergo desiccation and consolidation, and thus constitute a major pathway for rapid and massive colonization by mangroves. Erosion by waves propagating across relatively mud-deficient shoreface zones in inter-bank areas can lead to muddy shoreline retreat rates of tens of metres to several kilometres over a few months to a few years, accompanied by massive removal of mangroves. Notwithstanding the higher incident wave energy on inter-bank shores, inter-bank shorefaces are permanently muddy due to the pervasive influence of the Amazon muddy discharge. Inter-bank and transitional bank-to-inter-bank phases are associated with both periodic sandy chenier formation and extreme forms of rotation of rare headland-bound sandy beaches.
Bianchi, T. S., S. F. DiMarco, J. H. Cowan, R. D. Hetland, P. Chapman, J. W. Jay, and M. A. Allison, The science of hypoxia in the Northern Gulf of Mexico: A review, Sci. Total Environ, 408, 1471-1484, 2010, 15 citations, doi:10.1016/j.scitotenv.2009.11.047, #2209
The Mississippi River is one of the world's 10 largest rivers, with average freshwater discharge into the northern Gulf of Mexico (GOM) of 380 km3 year− 1. In the northern GOM, anthropogenic nitrogen is primarily derived from agricultural fertilizer and delivered via the Mississippi River. The general consensus is that hypoxia in the northern Gulf of Mexico is caused primarily by algal production stimulated by excess nitrogen delivered from the Mississippi–Atchafalaya River Basin and seasonal vertical stratification of incoming stream flow and Gulf waters, which restricts replenishment of oxygen from the atmosphere.
In this paper, we review the controversial aspects of the largely nutrient-centric view of the hypoxic region, and introduce the role of non-riverine organic matter inputs as other oxygen-consuming mechanisms. Similarly, we discuss non-nutrient physically-controlled impacts of freshwater stratification as an alternative mechanism for controlling in part, the seasonality of hypoxia. We then explore why hypoxia in this dynamic river-dominated margin (RiOMar) is not comparable to many of the other traditional estuarine systems (e.g., Chesapeake Bay, Baltic Sea, and Long Island Sound). The presence of mobile muds and the proximity of the Mississippi Canyon are discussed as possible reasons for the amelioration of hypoxia (e.g., healthy fisheries) in this region. The most recent prediction of hypoxia area for 2009, using the current nutrient-centric models, failed due to the limited scope of these simple models and the complexity of this system. Predictive models should not be the main driver for management decisions. We postulate that a better management plan for this region can only be reached through a more comprehensive understanding of this RiOMar system—not just more information on river fluxes (e.g., nutrients) and coastal hypoxia monitoring programs.
Bianchi, T. S., M. A. Allison, P. Chapman, J. H. Cowan, M. J. Dagg, J. W. Day, S. F. DiMarco, R. D. Hetland, and R. Powell, New approaches to the Gulf hypoxia problem, Eos, Trans. Amer. Geophys. Un., 91, 173-174, 2010, #2341
Goff, J. A., M. A. Allison, and S. P. S. Gulick, Offshore transport of sediment during cyclonic storms: Hurricane Ike (2008), Texas Gulf Coast, USA, Geology, 38, 351-354, 2010, 3 citations, doi:10.1130/G30632.1, #2162
Extreme storms can have a large impact on coastal sediment budgets and the character of strata preserved in the geologic record. Here, we investigate the impact of a cyclonic storm-surge flood and ebb on sediment transport in a microtidal beach barrier–shelf system. Hurricane Ike made landfall on the Texas (United States) coast on 13 September 2008. The accompanying storm surge flooded Galveston Bay with up to 5 m of water above sea level. The surge flood and ebb preferentially flowed over a low-elevation, bay-fronting spit known as the Bolivar Peninsula, destroying buildings and eroding sediments. Surge waters also flowed through Bolivar Roads tidal inlet, the main passageway through the barrier system that separates the gulf and the bay. Bathymetry, Chirp data, and samples were collected in Bolivar Roads tidal inlet 9 to 10 d after the storm, and we compare them here to data collected 4 mo prior. Additional data were collected offshore of Bolivar Peninsula in October 2008. Our results document the dominance of the storm-surge ebb in forcing sediment transport through the inlet, which is not considered in models of beach-barrier evolution. Shoreface sands appear to have been incised by the storm, and advected with beach-barrier sediments sufficiently offshore by the storm-surge ebb that they cannot be reincorporated, indicating a significant loss to the barrier system's sediment budget as a result of a single storm.
Safak, I., A. Sheremet, M. A. Allison, and T.-J. Hsu, Bottom turbulence on the muddy Atchafalaya Shelf, Louisiana, USA, J. Geophys. Res., 115, C12019, 2010, 1 citation, doi:10.1029/2010JC006157, #2287
Wave, current, and sediment observations collected in approximately 5 m depth on the muddy Atchafalaya clinoform, LA, USA, are used to study the interaction between near-bed wave-induced turbulent flows and suspended sediment characteristics in a muddy environment. Low wave-bias estimates of near-bed Reynolds stresses are strongly correlated with flow accelerations and suspended sediment concentration, as previously observed on sandy beaches, where accelerations have been associated with bed fluidization and sediment transport. A detailed numerical analysis of the observations is performed, based on a uni-dimensional boundary layer model that accounts for the coupling between the fluid and the cohesive sediment phases. The numerical simulations suggest that sediment-induced stratification effects are of the same order of magnitude as turbulent dissipation, and thus play a significant role in the turbulent kinetic energy balance within the tidal boundary layer. However, inside the wave boundary layer, the ratio of stratification to shear-induced turbulence production (i.e., gradient Richardson number) decreases significantly, and shear-induced turbulence production dominates. For these observations, the vertical structures of currents and Reynolds stresses are relatively insensitive to the exact floc size.
Bianchi, T. S., and M. A. Allison, Large-river delta-front estuaries as natural "recorders" of global environmental change, Proc. National Acad. Sci., 106, 8085-8092, 2009, 27 citations, doi:10.1073/pnas.0812878106, #2115
Large-river delta-front estuaries (LDE) are important interfaces between continents and the oceans for material fluxes that have a global impact on marine biogeochemistry. In this article, we propose that more emphasis should be placed on LDE in future global climate change research. We will use some of the most anthropogenically altered LDE systems in the world, the Mississippi/Atchafalaya River and the Chinese rivers that enter the Yellow Sea (e.g., Huanghe and Changjiang) as case-studies, to posit that these systems are both “drivers” and “recorders” of natural and anthropogenic environmental change. Specifically, the processes in the LDE can influence (“drive”) the flux of particulate and dissolved materials from the continents to the global ocean that can have profound impact on issues such as coastal eutrophication and the development of hypoxic zones. LDE also record in their rapidly accumulating subaerial and subaqueous deltaic sediment deposits environmental changes such as continental-scale trends in climate and land-use in watersheds, frequency and magnitude of cyclonic storms, and sea-level change. The processes that control the transport and transformation of carbon in the active LDE and in the deltaic sediment deposit are also essential to our understanding of carbon sequestration and exchange with the world ocean—an important objective in global change research. U.S. efforts in global change science including the vital role of deltaic systems are emphasized in the North American Carbon Plan (www.carboncyclescience.gov).
Jaramillo, S., A. Sheremet, M. A. Allison, K. T. Holland, and A. H. Reed, Wave-mud interactions over the muddy Atchafalaya subaqueous clinoform, Lousiana, United States: Wave-supported sediment transport, J. Geophys. Res., 114, C04002, 2009, 12 citations, doi:10.1029/2008JC004821, #2007
Near-bottom fluid-mud layers were observed during two experiments conducted on the muddy Atchafalaya inner shelf (subaqueous clinoform), Louisiana, United States. On the face of the subaqueous delta (4–7 m water depth, first experiment) fluid-mud layers are produced by seafloor liquefaction and resuspension forced by swells associated with cold front passages, and supported by near-bed wave-induced turbulence. The layers are episodic (lifetime of 9–12 h), form prior to significant postfrontal settling of sediment in the overlying water column, and flow seaward (downslope) at about 5 cm/s. Farther westward on the delta front (finer grain size, with negligible sand or coarse silt content, second experiment), similar wave-supported fluid-mud layers are observed to last longer (>2 days), show weaker alongshore (westward) flow of about 1–3 cm/s. The results suggest a sequence of near-bed sediment transport processes, triggered by frontal swell activity (bed liquefaction, resuspension and advection, modulated by the bathymetric characteristics of the clinoform) that contribute to the formation of clinoform stratigraphy of muddy subaqueous deltas.
Mitra, S., J. J. Lalicata, M. A. Allison, and T. M. Dellapenna, The effects of Hurricane Katrina and Rita on seabed polycyclic aromatic hydrocarbon dynamics in the Gulf of Mexico, Marine Pollution Bull., 58, 851-857, 2009, 2 citations, doi:10.1016/j.marpolbul.2009.01.016, #2062
To assess the extent to which Hurricanes Katrina and Rita affected polycyclic aromatic hydrocarbons (PAH) in the Gulf of Mexico (GOM), sediment cores were analyzed in late 2005 from: a shallow shelf, a deeper shelf, and a marsh station. Sediment geochronology, fabric, and geochemistry show that the 2005 storms deposited 10 cm of sediment to the surface of a core at 5-12A. Bulk carbon geochemistry and PAH isomers in this top layer suggest that the source of sediment to the top portion of core 5-12A was from a relatively more marine area. Particulate PAHs in the marsh core (04 M) appeared unaffected by the storms while sediments in the core from Station 5-1B (deeper shelf) were affected minimally (some possible storm-derived deposition). Substantial amounts of PAH-laden particles may have been displaced from the seabed in shallow areas of the water column in the GOM by these 2005 storms.
Pereira, J. F., J. A. McCorquodale, E. A. Meselhe, I. Y. Georgiou, and M. A. Allison, Numerical simulation of bed material transport in the lower Mississippi River, J. Coastal Res., Spec. Issue, 56, 1449-1453, 2009, #2290
Reed, A. H., R. W. Faas, M. A. Allison, L. J. Calliari, K. T. Holland, S. E. O'Reilly, W. C. Vaughan, and A. Alves, Characterization of a mud deposit offshore of the Patos Lagoon, southern Brazil, Continental Shelf Res., 29, 597-608, 2009, 4 citations, doi:10.1016/j.csr.2009.02.001, #2210
Rapid deposition of mud on the beach along the shoreface of Rio Grande do Sul, Brazil dramatically influences the normal operations in the littoral zone. In the surf zone, fluid and suspended mud opposes water-wave movement and dissipates water-wave energy; on the beach, mud limits trafficability. As part of a multinational, multidisciplinary program to evaluate the influence of mud strength, density and viscosity on water-wave attenuation, sediments were evaluated in situ or collected for evaluation from an area offshore of Cassino Beach, slightly south of the Patos Lagoon mouth. Shear strength of deposited sediments ranged from 0.6 kPa at the seafloor to 3.4 kPa at not, vert, similar1 m below the seafloor. Mud sediments were also collected to simulate the in situ response of fluid mud to shear stresses. For this determination, rheological evaluations were made using a strain-controlled Couette viscometer on numerous remixed samples that ranged in density from 1.05 to 1.30 g/cm3. It was determined that this mud is a non-ideal Bingham material in that it has a true initial yield stress as well as a upper Bingham yield stress. Initial yield stress ranged from 0.59 to 2.62 Pa, upper Bingham yield stress ranged from 1.05 to 7.6 Pa. Apparent viscosity ranged from 0.02 to 4.7 Pa s with the highest viscosities occurring between the two yield stresses. Sediment strength in the remixed samples is 2 to 3 orders of magnitude lower than the horizontal shear strength of the sediment bed as determined by shear vane or predicted from penetrometer measurements. This difference is partially due to the fact that rheological evaluations are made on fully remixed sediments, whereas horizontal shear strength is determined within relatively undisturbed sediments. Similar values of viscosity and shear strength are comparable to those determined for mud in other coastal areas where fluid mud deposits occur.
Bianchi, T. S., S. F. DiMarco, M. A. Allison, P. Chapman, J. H. Cowan, R. D. Hetland, J. W. Morse, and G. T. Rowe, Controlling hypoxia on the U.S. Louisiana Shelf: Beyond the nutrient-centric view, Eos, Trans. Amer. Geophys. Un., 89, 236-237, 2008, doi:10.1029/2008EO260005, #2009
Galler, J. J., and M. A. Allison, Estuarine controls on fine-grained sediment storage in the lower Mississippi and Atchafalaya Rivers, Geol. Soc. Amer. Bull., 120, 386-398, 2008, 3 citations, doi:10.1130/B26060.1, #1964
Observations of the lower Mississippi River channel in 1999–2003 with side-scan and subbottom seismic profilers, multibeam bathymetry, core radiotracers (7Be, 137Cs, 210Pb, and 234Th), and water-column profiles of fluid and suspended sediment properties are utilized to constrain the timing, location, and intensity of the seasonal storage of fine-grained sediments in the estuarine channel reach below Venice, Louisiana. Unlike earlier studies that suggested little or no estuarine sedimentation was taking place in the river, the present study demonstrates that the river channel above the Head of Passes seasonally stores as much as 10% of the annual suspended sediment discharge during periods of falling-to-low Mississippi water discharge associated with salt stratification in the channel thalweg. Mud deposits are up to 3.2 m thick with 7Be-derived sediment deposition rates of up to 8.9 mm/day. Water-column profiling and 234Th/7Be ratios suggest that (1) particles are transported and settled mainly as flocs; and (2) riverine flocs are settling through the strong salt-wedge halocline and are protected from re-erosion by the salt stratification. With onset of the spring freshet and the corresponding seaward retreat of the saltwater wedge, the entire volume of fine-grained material stored in the lower river (except in low freshet years) is remobilized and transported to the Gulf of Mexico in the turbid freshwater plume. This tidal hysteresis strongly influences the timing of sediment supply to the ocean: high turbidities in the shelf plume during rising water discharge spikes of 1- to 2-week duration are the product of local channel floor remobilization as well as increased supply from the drainage basin. Pulsed resupply of sediments and the pore-water products of early diagenesis potentially have major implications for issues such as continental margin carbon budgets and coastal eutrophication. Companion surveys conducted in the lowermost 12 km reach of the Atchafalaya distributary of the Mississippi show no evidence of saltwater wedge intrusion at low discharge or seasonal fine-grained sediment storage, likely due to the shallow (7 m) river-mouth sill that impedes the upstream movement of Gulf water.
Mayer, L. M., L. L. Schick, and M. A. Allison, Input of nutritionally rich organic matter from the Mississippi River to the Louisiana coastal zone, Estuaries and Coasts, 31, 1052-1062, 2008, 1 citation, doi:10.1007/s12237-008-9080-5, #2061
Isotopes have often been used to discern riverine subsidies to coastal food chains, but there are few direct measurements of nutritional quality of riverine particulates. We tested for nutritionally enriched organic matter in the Mississippi River suspended sediment and evidence for its delivery to Louisiana coastal sediments by measuring enzymatically hydrolysable amino acids (EHAA). Riverine suspended sediments contained EHAA concentrations of up to 5 mg g−1, higher than reported in any coastal sediment. Pigment concentrations indicated that EHAA in some river samples were dominated by phytoplankton, but many samples contained significant non-algal EHAA. Coastal sediments showed EHAA concentrations lower than riverine sediments but still higher than most reported shelf values. Incubation of riverine sediment showed losses of 28–34% of their EHAA over 6 days, similar to differences found between riverine and coastal sediments. EHAA concentrations decreased more rapidly than total nitrogen, indicating the relative lability of this pool of material in the studied region. These EHAA-enriched materials provide fuel for various coastal biota whose composition likely depends on factors such as disturbance regimes.
Nittrouer, J. A., M. A. Allison, and R. Campanella, Evaluation of bedload transport for the lowermost Mississippi River:, J. Geophys. Res., 113, F03004 , 2008, 14 citations, doi:10.1029/2007JF000795, #1963
New methods of data collection and processing are developed to provide quantitative, reach-scale measurements of bedform transport mass within the tidally influenced Mississippi River. A multibeam swath profiler was used to collect daily bathymetry over a range of water discharges, and bed elevation changes induced by dune migration are measured. These values are coupled with bulk physical properties of the bed sediment to constrain mass flux, and annual bedform transport is estimated at 2.2 × 106 metric tons (MT). The total annual sand flux from the Mississippi River, calculated by combining measured bedform transport rates and suspended sediment flux, is estimated to be 20 × 106 MT. Survey data also provide information about the spatial distribution of dunes across the channel bottom. Straight reach segments are commonly mantled by dunes for the entire cross section, while bends are typically areas of focused scour devoid of bedforms. Presumably, any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach. Movement via suspension is therefore an important component for the downriver transport of bed materials in the lower Mississippi River.
Sampere, T. P., T. S. Bianchi, S. G. Wakeham, and M. A. Allison, Sources of organic matter in surface sediments of the Louisiana continental margin: Effects of major depositional/transport pathways and Hurricane Ivan, Continental Shelf Res., 28, 2472-2487, 2008, 16 citations, doi:10.1016/j.csr.2008.06.009, #2060
Lignin and pigment biomarkers were analyzed in surface sediments of the Louisiana Continental margin (LCM) to distinguish differences in the degradative state of sedimentary organic matter along and between two major depositional pathways (along shore and offshore to the Mississippi Canyon) from Southwest (SW) Pass in July 2003. Barataria Bay, an inter-distributary estuary, was also assessed as a potential source of terrestrial organic matter to the LCM. Sediment signatures taken along the same pathways after Hurricane Ivan (October 2004) were compared with the pre-Ivan signature to elucidate carbon dynamics after major hurricane events. Density fractions were investigated at key stages across the LCM. Mississippi Canyon sediments are a depocenter for labile and refractory organic matter derived from river and previously deposited shelf sediments. Barataria Bay material may be a contributing source of sedimentary organic matter in shallow shelf areas bordering the bay and is thus potentially important in carbon cycling in sediments of these shallow areas; however, our results show that organic matter inputs from the bay were likely rapidly decomposed and/or diluted. Hurricane Ivan mobilized sedimentary organic carbon (SOC) offshore and homogenized terrestrial sediment parameters and gradients. As observed through pigment concentrations sediments tended to equilibrate to a more steady-state condition within months of the disturbance. Insights from density fractions show that selective degradation and aggregation/flocculation processes were also very important processes during cross-shelf transport. Zooplankton grazing, largely on diatoms and other algae, was a shelf wide phenomenon, however, grazing products dominated the marine-derived SOC in margin sediments west of the birdsfoot delta indicated by the abundance of steryl chlorin esters (SCEs).
Wilson, C. A., and M. A. Allison, An equilibrium profile model for retreating marsh shorelines in southeast Lousiana, Estuarine, Coastal and Shelf Sci., 80, 483-494, 2008, 10 citations, doi:10.1016/j.ecss.2008.09.004, #2008
Louisiana's coastal marshes are experiencing the highest wetland loss rates in the U.S., in part due to subsidence-driven relative sea-level rise. These marshes are also vulnerable to the erosive power of wave attack: 1) on the marsh edge adjacent to open-water bodies, and 2) after the marsh platform is submerged. Marsh shorelines in Barataria Bay, Breton Sound, and the active Balize delta of southeastern Louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932. Vibracore transects of marsh and adjacent bay surface sediments (to 2 m depth) were analyzed using geotechnical, stratigraphic, and radiochemical (137-Cs and 210-Pb) methods, and the subaerial-to-subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements. Results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1.5 m are reached. This is observed even in interior pond regions, but the shoreface elevation profiles are a function of fetch: exposed open bay sites display greater incision (depth and rate) of the marsh platform than protected interior bay or pond sites. Core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds, covering the incised marsh deposits unconformably with estuarine shelly muds. 137-Cs and excess 210-Pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss. The consistency of the cross-shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast Louisiana: platform stratigraphy and resistance to erosion have a limited effect on profile shape. This equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process. On average, 1.5 m3 of sediment are yielded per m shoreline length annually from both Barataria Bay and Breton Sound. Due to the highly organic nature of the eroded sediment (30%), this supply of organic-rich material could significantly impact estuarine productivity and hypoxia on the Louisiana continental shelf.
Allison, M. A., T. S. Bianchi, B. A. McKee, and T. P. Sampere, Carbon burial on river-dominated continental shelves: Impact of historical changes in sediment loading adjacent to the Mississippi River, Geophys. Res. Lett., 34, L01606, 2007, 12 citations, doi:10.1029/2006GL028362, #2010
Seabed cores collected on the continental shelf adjacent to the Mississippi River show a direct temporal correlation between decreases in mass accumulation rate (factor of 2–3) and suspended sediment loads in the river. This mid 20th century decline is not apparent shelf-wide due to sediment focusing and biological seabed mixing. Total organic carbon diagenetic loss rate across this sediment age interval is relatively uninterrupted when corrected for the non-steady state mass flux. This suggests that organic carbon burial rates in oxic bottom water settings on river-dominated continental margins are directly proportional to lithogenic flux. Variations in OM remineralization rates due to changes in the composition (marine vs. terrestrial) of the particulate OM flux at the sediment surface are a second-order effect that cannot be distinguished in the bulk carbon sediment record at these oxic sites; although they may significantly alter the OM degradation-induced CO2 flux to the overlying water column.
Allison, M. A., T. M. Dellapenna, M. A. Goni, and A. Sheremet, Impact of hurricanes Katrina and Lili on the inner shelf of the Mississippi-Atchafalaya delta, Proc. Int. Conf. Coastal Sediments, 7, ISBN0-7844-0926-9, 2007, #2011
Bianchi, T. S., J. J. Galler, and M. A. Allison, Hydrodynamic sorting and transport of terrestrially derived organic carbon in sediments of the Mississippi and Atchafalaya Rivers, Estuarine, Coastal and Shelf Sci., 73, 211-222, 2007, 23 citations, doi:10.1016/j.ecss.2007.01.004, #2013
Over the course of two years, four cruises were conducted at varying levels of discharge in the lower Mississippi and Atchafalaya Rivers (MR and AR) where grab samples were collected from sand- and mud-dominated sediments. The tetramethylammonium hydroxide (TMAH) thermochemolysis method was used to determine sources of terrestrially derived organic carbon (OC) in these two sediment types, to examine the effects of hydrodynamic sorting on lignin sources in river sediments.
Goni, M. A., Y. Alleau, E. R. Corbett, J. P. Walsh, D. Mallinson, M. A. Allison, E. S. Gordon, S. T. Petsch, and T. M. Dellapenna, The effects of hurricanes Katrina and Rita on the seabed of the Louisiana Shelf, Sedimentary Record, 5 (1), 4-9, 2007, #2012
The legacy of hurricanes Katrina and Rita on land has been one of human devastation and long-term damage to the infrastructure of communities along the northern Gulf of Mexico. In addition, these hurricanes had major impacts on offshore regions of the shelf and slope. A multi-institution, rapid-response effort investigated the immediate effects of the storms on the seabed off the Louisiana coast. These studies revealed intense reworking of surface sediment layers during the storm passage and re-deposition of materials following the hurricanes over a broad area of the shelf and slope. The pattern of deposition varied significantly along the region between the Mississippi and Atchafalaya rivers, depending on both the characteristic of the shelf and the paths of the storms. Geochemical tracers indicate the origin of the materials in the post-hurricane layers was predominantly local sediments mobilized by the intense wave activity during the storms. The combined impact of the hurricanes was a massive disturbance of benthic communities throughout the region, including marked erosion of the seabed in the shallower regions of the shelf and elevated deposition of sediments in the deeper regions. The total amounts of sediment, carbon and nitrogen re-deposited following the storm far exceeded the combined annual inputs of these materials by the Mississippi/Atchafalaya Rivers. The characterization of these storm deposits provides an opportunity to investigate the history of hurricane activity in the recent past based on the sedimentary record preserved in this region.
Mayer, L. M., L. L. Schick, M. A. Allison, K. C. Ruttenberg, and S. J. Bentley, Marine vs. terrigenous organic matter in Louisiana coastal sediments: The uses of bromine:organic carbon ratios, Marine Chem., 107, 244-254, 2007, 9 citations, doi:10.1016/j.marchem.2007.07.007, #2014
Recent data on the sources of organic carbon buried in the ocean have emphasized the probable importance of terrigenous organic matter in burial budgets of deltaic depocenters. The many markers used to assess relative importance of marine vs. terrestrial sources each have ambiguities. We use the ratio of bromine to organic carbon (Br:OC) as a source indicator for organic matter in the Mississippi delta. Progressive increases in bromine concentrations from the river to the slope indicate increasing content of marine-derived organic matter. Quantitative estimates of marine vs. terrigenous organic matter using Br:OC ratios in a two-endmember mixing model are consistent with recent estimates using a combination of three other source markers [Gordon, E.S., Goñi, M.A. 2003. Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico. Geochim. Cosmochim. Acta, 67:2359–2375]. The Br:OC vs. δ13C relationship indicates seaward increase in δ13C without proportionate incorporation of marine organic matter, consistent with recent arguments that isotopically depleted terrestrial detritus derived from C3 plants is separated from C4-derived terrigenous organic matter during transport. Decreasing Br:OC ratios downcore at many sites that have significant amounts of marine organic matter indicate that the marine organic matter is preferentially lost during burial diagenesis. This preferential loss constrains the contribution of organic matter burial in deltaic environments to global removal of Br.
Thieler, E. R., B. Butman, W. C. Schwab, M. A. Allison, N. W. Driscoll, J. P. Donnelly, and E. Uchupi, A catastrophic meltwater flood event and the formation of the Hudson Shelf Valley, Palaeogeography, Palaeoclimatology, Palaeoecology, 246, 120-136, 2007, 10 citations, doi:10.1016/j.palaeo.2006.10.030, #2015
The Hudson Shelf Valley (HSV) is the largest physiographic feature on the U.S. mid-Atlantic continental shelf. The 150-km long valley is the submerged extension of the ancestral Hudson River Valley that connects to the Hudson Canyon. Unlike other incised valleys on the mid-Atlantic shelf, it has not been infilled with sediment during the Holocene. Analyses of multibeam bathymetry, acoustic backscatter intensity, and high-resolution seismic reflection profiles reveal morphologic and stratigraphic evidence for a catastrophic meltwater flood event that formed the modern HSV. The valley and its distal deposits record a discrete flood event that carved 15-m high banks, formed a 120-km2 field of 3- to 6-m high bedforms, and deposited a subaqueous delta on the outer shelf. The HSV is inferred to have been carved initially by precipitation and meltwater runoff during the advance of the Laurentide Ice Sheet, and later by the drainage of early proglacial lakes through stable spillways. A flood resulting from the failure of the terminal moraine dam at the Narrows between Staten Island and Long Island, New York, allowed glacial lakes in the Hudson and Ontario basins to drain across the continental shelf. Water level changes in the Hudson River basin associated with the catastrophic drainage of glacial lakes Iroquois, Vermont, and Albany around 11,450 14C year BP ( 13,350 cal BP) may have precipitated dam failure at the Narrows. This 3200 km3 discharge of freshwater entered the North Atlantic proximal to the Gulf Stream and may have affected thermohaline circulation at the onset of the Intra-Allerød Cold Period. Based on bedform characteristics and fluvial morphology in the HSV, the maximum freshwater flux during the flood event is estimated to be 0.46 Sv for a duration of 80 days.