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Robert A Wheatcroft - One of the best experts on this subject based on the ideXlab platform.
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early diagenesis of recently Deposited organic matter a 9 yr time series study of a Flood Deposit
Geochimica et Cosmochimica Acta, 2012Co-Authors: Tommaso Tesi, Robert A Wheatcroft, Miguel A Goni, Leonardo Langone, S Miserocchi, L BertottiAbstract:Abstract In Fall 2000, the Po River (Italy) experienced a 100-yr return period Flood that resulted in a 1–25 cm-thick Deposit in the adjacent prodelta (10–25 m water depth). In the following years, numerous post-Depositional perturbations occurred including bioturbation, reworking by waves with heights exceeding 5 m, as well as periods of extremely high and low sediment supply. Cores collected in the central prodelta after the Fall 2000 Flood and over the following 9 yr, allowed characterization of the event-strata in their initial state and documentation of their subsequent evolution. Sedimentological characteristics were investigated using X-radiographs and sediment texture analyses, whereas the composition of sedimentary organic matter (OM) was studied via bulk and biomarker analyses, including organic carbon (OC), total nitrogen (TN), carbon stable isotope composition (δ13C), lignin phenols, cutin-products, p-hydroxy benzenes, benzoic acids, dicarboxylic acids, and fatty acids. The 9-yr time-series analysis indicated that roughly the lower half of the original event bed was preserved in the sediment record. Conversely, the upper half of the Deposit experienced significant alterations including bioturbation, addition of new material, as well as coarsening. Comparison of the recently Deposited material with 9-yr old preserved strata represented a unique natural laboratory to investigate the diagenesis of sedimentary OM in a non-steady system. Bulk data indicated that OC and TN were degraded at similar rates (loss ∼17%) whereas biomarkers exhibited a broad spectrum of reactivities (loss from ∼6% to ∼60%) indicating selective preservation during early diagenesis. Given the relevance of episodic sedimentation in several margins, this study has demonstrated the utility of event-response and time-series sampling of the seabed for understanding the early diagenesis in non-steady conditions.
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the large scale distribution and internal geometry of the fall 2000 po river Flood Deposit evidence from digital x radiography
Continental Shelf Research, 2006Co-Authors: Robert A Wheatcroft, Andrew W Stevens, L Hunt, T G MilliganAbstract:Abstract Event-response coring on the Po River prodelta (northern Adriatic Sea) coupled with shipboard digital X-radiography, resistivity profiling, and grain-size analyses permitted documentation of the initial distribution and physical properties of the October 2000 Flood Deposit. The digital X-radiography system comprises a constant-potential X-ray source and an amorphous silicon imager with an active area of 29×42 cm and 12-bit depth resolution. Objective image segmentation algorithms based on bulk density (brightness), layer contacts (edge detection) and small-scale texture (fabric) were used to identify the Flood Deposit. Results indicate that the Deposit formed in water depths of 6–29 m immediately adjacent to the three main distributary mouths of the Po (Pila, Tolle and Gnocca/Goro). Maximal thickness was 36 cm at a 20-m site off the main mouth (Pila), but many other sites had thicknesses >20 cm. The Po Flood Deposit has a complex internal stratigraphy, with multiple layers, a diverse suite of physical sedimentary structures (e.g., laminations, ripple cross bedding, lenticular bedding, soft-sediment deformation structures), and dramatic changes in grain size that imply rapid Deposition and fluctuations in energy during emplacement. Based on the Flood Deposit volume and well-constrained measurements of Deposit bulk density the mass of the Flood Deposit was estimated to be 16×10 9 kg, which is about two-thirds of the estimated suspended sediment load delivered by the river during the event. The locus of Deposition, overall thickness, and stratigraphic complexity of the Flood Deposit can best be explained by the relatively long sediment throughput times of the Po River, whereby sediment is delivered to the ocean during a range of conditions (i.e., the storm responsible for the precipitation is long gone), the majority of which are reflective of the fair-weather condition. Sediment is therefore Deposited proximal to the river mouths, where it can form thick, but stratigraphically complex Deposits. In contrast, Floods of small rivers such as the Eel (northern California) are coupled to storm conditions, which lead to high levels of sediment dispersion.
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The use of 7Be to identify event and seasonal sedimentation near the Po River delta, Adriatic Sea
Marine Geology, 2005Co-Authors: C. M. Palinkas, Robert A Wheatcroft, Charles A. Nittrouer, L. LangoneAbstract:In October 2000, a major Flood event of the Po River occurred. The resulting seabed Deposit was initially sampled in December 2000, and most sites were reoccupied on subsequent cruises over a 3-yr period. Using the maximum penetration depth of the short-lived radioisotope 7Be (half-life 53.3 d), the Flood Deposit was found to be up to 15 cm thick. Individual depocenters of thick strata were located immediately adjacent to the distributaries at the river mouth in relatively shallow water (
D.r. Corbett - One of the best experts on this subject based on the ideXlab platform.
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characterization of a Flood associated Deposit on the waipaoa river shelf using radioisotopes and terrigenous organic matter abundance and composition
Continental Shelf Research, 2014Co-Authors: Tara A Kniskern, Courtney K. Harris, Siddhartha Mitra, Alan R Orpin, J P Walsh, D.r. CorbettAbstract:Abstract An ephemeral oceanic-Flood Deposit adjacent to a well-studied small mountainous river (SMR), the Waipaoa River in northeastern New Zealand, was characterized using multiple proxies, including radioisotopes ( 234 Th, 7 Be, and 210 Pb), bulk organic carbon abundance and isotopic signature (%OC, δ 13 C), as well as a biomarker of terrigenous organic matter (lignin). Field sampling was conducted within two weeks after a 1-in-8 year Flood that occurred between January 30 and February 6, 2010. Geochemical analyses indicated that initial Deposition of fresh riverine material extended alongshore to the north and south from the river mouth. A comparison of prior- and post-Flood 7 Be inventories revealed that Flood sediments were widely dispersed between 20 and 70 m water depth, accounting for 50–80% of the estimated Flood load. Surface (0–2 cm) isotopic carbon values increased with distance from Poverty Bay, positively correlating with total 210 Pb activities, potentially reflecting increasing marine influence with water depth. Abundances of sedimentary organic carbon (OC) were 0.18–0.76% dry weight, and the total nitrogen varied from 0.02 to 0.13%. Stable isotope signatures of carbon (δ 13 C OC ), nitrogen (δ 15 N), and lignin abundances (λ 6 ) throughout the study area ranged from −23.6 to −27.7‰, 1.9 to 5.3‰, and 0.93 to 9.0 mg 100 mg OC −1 , respectively. The spatial distribution pattern of terrigenous organic matter (OM) abundance and interclass ratios (indicative of freshness of organic matter) varied along and across-shelf. Lignin abundances were high and interclass ratios were low in the southern depocenter and inner shelf areas, suggesting that this zone had recently received vascular-plant enriched OM, minimally altered by shelf-bed mixing processes. In contrast, sediments in the northern depocenter and outer shelf also contained elevated amounts of terrigenous sedimentary OM, but this material was generally lower in lignin abundance and had higher interclass ratios (greater degradation). Collectively, these results suggest that the Flood-derived sediment and fresh terrigenous OM were mostly constrained between 20 and 70 m water depth, with enhanced Deposition overlapping the tectonic-controlled depocenters located to the northeast and southeast of Poverty Bay.
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Seabed erodibility variations on the Louisiana continental shelf before and after the 2011 Mississippi River Flood
Estuarine Coastal and Shelf Science, 2014Co-Authors: Kehui Xu, D.r. Corbett, John P. Walsh, D. Young, Kevin B. Briggs, Grace M. Cartwright, Carl T. Friedrichs, Courtney K. Harris, Rangley C. Mickey, Siddhartha MitraAbstract:Erodibility is critical to the sediment resuspension process but has not been measured systematically in large river-dominated muddy continental shelves before. During early summer of 2011, the Mississippi River experienced a major Flood event. This Flood provided a unique opportunity to examine how shelf seabed erodibility responded to a large river Flood, and the ultimate fate of Flood Deposition is important to geological and biogeochemical processes (e.g., stratal formation, carbon sequestration). A total of 106 sediment cores were collected on the Louisiana shelf during five cruises in 2010 and 2011, and a new dataset was used to evaluate the response of the seabed to the recent conditions. The localized Flood Deposit was mainly within tens of kilometers of river sources, and little sediment accumulated on the middle Louisiana shelf. Seabed erodibility was measured using a dual-core Gust Erosion Microcosm System. The erodibility of sediment collected in April 2011 exceeded that for August 2010 and August 2011. The springtime increase in erodibility seemed to be related to the recent presence of energetic waves that mobilized the seabed. Erodibility was highest on the inner shelf southwest of Atchafalaya Bay, intermediate on the middle shelf, lowest in the Mississippi Canyon, and highly variable on the Mississippi subaqueous delta. These spatial patterns were influenced by proximity to river sources, Flood-Deposit thicknesses, intensity of wave-driven bed stresses, and bioturbation. The Flood-Deposit thickness itself, however, was not sufficient to explain all the spatial variations of erodibility after the peak of the Mississippi Flood. Comparing values to published data, the depth-varying erodibility on the Louisiana shelf was close to the “low erodibility” level for the York River of Virginia, and similar to the data collected from Baltimore Harbor in Maryland and the main stem of upper Chesapeake Bay. Our findings promote understanding of the resuspension of fluffy organic-rich layer at the water–sediment interface, which influences sediment oxygen demand on the Louisiana shelf. This dataset is also valuable to observational and modeling studies of large river sediment dispersal systems worldwide.
Miguel A Goni - One of the best experts on this subject based on the ideXlab platform.
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early diagenesis of recently Deposited organic matter a 9 yr time series study of a Flood Deposit
Geochimica et Cosmochimica Acta, 2012Co-Authors: Tommaso Tesi, Robert A Wheatcroft, Miguel A Goni, Leonardo Langone, S Miserocchi, L BertottiAbstract:Abstract In Fall 2000, the Po River (Italy) experienced a 100-yr return period Flood that resulted in a 1–25 cm-thick Deposit in the adjacent prodelta (10–25 m water depth). In the following years, numerous post-Depositional perturbations occurred including bioturbation, reworking by waves with heights exceeding 5 m, as well as periods of extremely high and low sediment supply. Cores collected in the central prodelta after the Fall 2000 Flood and over the following 9 yr, allowed characterization of the event-strata in their initial state and documentation of their subsequent evolution. Sedimentological characteristics were investigated using X-radiographs and sediment texture analyses, whereas the composition of sedimentary organic matter (OM) was studied via bulk and biomarker analyses, including organic carbon (OC), total nitrogen (TN), carbon stable isotope composition (δ13C), lignin phenols, cutin-products, p-hydroxy benzenes, benzoic acids, dicarboxylic acids, and fatty acids. The 9-yr time-series analysis indicated that roughly the lower half of the original event bed was preserved in the sediment record. Conversely, the upper half of the Deposit experienced significant alterations including bioturbation, addition of new material, as well as coarsening. Comparison of the recently Deposited material with 9-yr old preserved strata represented a unique natural laboratory to investigate the diagenesis of sedimentary OM in a non-steady system. Bulk data indicated that OC and TN were degraded at similar rates (loss ∼17%) whereas biomarkers exhibited a broad spectrum of reactivities (loss from ∼6% to ∼60%) indicating selective preservation during early diagenesis. Given the relevance of episodic sedimentation in several margins, this study has demonstrated the utility of event-response and time-series sampling of the seabed for understanding the early diagenesis in non-steady conditions.
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development and reworking of a seasonal Flood Deposit on the inner continental shelf off the atchafalaya river
Continental Shelf Research, 2000Co-Authors: Mead A Allison, G C Kineke, Elizabeth S Gordon, Miguel A GoniAbstract:Abstract Sediment cores and water column measurements of suspended sediment and flow conditions were taken on the continental shelf off the Atchafalaya River in Louisiana to examine the development and reworking of a seabed Flood layer with seasonal variations in river discharge and hydrodynamics. Five stations in water depths of 5–23 m were occupied on the Atchafalaya inner shelf on four cruises from October 1997 to March 1999 representing a range of freshwater input and wave energy conditions. Downcore profiles of the short half-life (53 d) cosmogenic radiotracer 7Be showed a three to fivefold increase in seabed inventory and an increase in depth of penetration during the 1998 high Atchafalaya discharge period (April) at two inshore stations (5–7 m water depth). X-radiograph evidence of the absence of biological mixing at these sites suggests that the 7Be data is recording the Deposition of a 1–3 cm thick annual Flood Deposit. The organic carbon contents and stable carbon isotopic compositions of this Flood Deposit are distinct and reflect the increased terrestrial influence of the riverine sediment flux. 210Pb and 137Cs sediment profiles indicate that this seasonal Deposit is two to six times the long-term (e.g., decadal) accumulation at these sites. Passage of cold fronts on 3–7 d timescales interrupts the formation of these Flood Deposits, particularly during the rising to early high discharge period (December–March). The depth of sediment resuspension landward of 10 m water depth during these events may reach 1 cm and decreases offshore. Offshore stations (∼20 m water depth) show only a small increase in Deposition during the high Atchafalaya discharge period. Redistribution of sediment from shallower parts of the shelf during the remainder of the year is likely a major supplier to these areas. A station east of the Atchafalaya mouth exhibits no correlation with discharge and no long-term accumulation, indicating minimal influence from the Mississippi discharge 150 km to the east.
T G Milligan - One of the best experts on this subject based on the ideXlab platform.
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the large scale distribution and internal geometry of the fall 2000 po river Flood Deposit evidence from digital x radiography
Continental Shelf Research, 2006Co-Authors: Robert A Wheatcroft, Andrew W Stevens, L Hunt, T G MilliganAbstract:Abstract Event-response coring on the Po River prodelta (northern Adriatic Sea) coupled with shipboard digital X-radiography, resistivity profiling, and grain-size analyses permitted documentation of the initial distribution and physical properties of the October 2000 Flood Deposit. The digital X-radiography system comprises a constant-potential X-ray source and an amorphous silicon imager with an active area of 29×42 cm and 12-bit depth resolution. Objective image segmentation algorithms based on bulk density (brightness), layer contacts (edge detection) and small-scale texture (fabric) were used to identify the Flood Deposit. Results indicate that the Deposit formed in water depths of 6–29 m immediately adjacent to the three main distributary mouths of the Po (Pila, Tolle and Gnocca/Goro). Maximal thickness was 36 cm at a 20-m site off the main mouth (Pila), but many other sites had thicknesses >20 cm. The Po Flood Deposit has a complex internal stratigraphy, with multiple layers, a diverse suite of physical sedimentary structures (e.g., laminations, ripple cross bedding, lenticular bedding, soft-sediment deformation structures), and dramatic changes in grain size that imply rapid Deposition and fluctuations in energy during emplacement. Based on the Flood Deposit volume and well-constrained measurements of Deposit bulk density the mass of the Flood Deposit was estimated to be 16×10 9 kg, which is about two-thirds of the estimated suspended sediment load delivered by the river during the event. The locus of Deposition, overall thickness, and stratigraphic complexity of the Flood Deposit can best be explained by the relatively long sediment throughput times of the Po River, whereby sediment is delivered to the ocean during a range of conditions (i.e., the storm responsible for the precipitation is long gone), the majority of which are reflective of the fair-weather condition. Sediment is therefore Deposited proximal to the river mouths, where it can form thick, but stratigraphically complex Deposits. In contrast, Floods of small rivers such as the Eel (northern California) are coupled to storm conditions, which lead to high levels of sediment dispersion.
Siddhartha Mitra - One of the best experts on this subject based on the ideXlab platform.
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characterization of a Flood associated Deposit on the waipaoa river shelf using radioisotopes and terrigenous organic matter abundance and composition
Continental Shelf Research, 2014Co-Authors: Tara A Kniskern, Courtney K. Harris, Siddhartha Mitra, Alan R Orpin, J P Walsh, D.r. CorbettAbstract:Abstract An ephemeral oceanic-Flood Deposit adjacent to a well-studied small mountainous river (SMR), the Waipaoa River in northeastern New Zealand, was characterized using multiple proxies, including radioisotopes ( 234 Th, 7 Be, and 210 Pb), bulk organic carbon abundance and isotopic signature (%OC, δ 13 C), as well as a biomarker of terrigenous organic matter (lignin). Field sampling was conducted within two weeks after a 1-in-8 year Flood that occurred between January 30 and February 6, 2010. Geochemical analyses indicated that initial Deposition of fresh riverine material extended alongshore to the north and south from the river mouth. A comparison of prior- and post-Flood 7 Be inventories revealed that Flood sediments were widely dispersed between 20 and 70 m water depth, accounting for 50–80% of the estimated Flood load. Surface (0–2 cm) isotopic carbon values increased with distance from Poverty Bay, positively correlating with total 210 Pb activities, potentially reflecting increasing marine influence with water depth. Abundances of sedimentary organic carbon (OC) were 0.18–0.76% dry weight, and the total nitrogen varied from 0.02 to 0.13%. Stable isotope signatures of carbon (δ 13 C OC ), nitrogen (δ 15 N), and lignin abundances (λ 6 ) throughout the study area ranged from −23.6 to −27.7‰, 1.9 to 5.3‰, and 0.93 to 9.0 mg 100 mg OC −1 , respectively. The spatial distribution pattern of terrigenous organic matter (OM) abundance and interclass ratios (indicative of freshness of organic matter) varied along and across-shelf. Lignin abundances were high and interclass ratios were low in the southern depocenter and inner shelf areas, suggesting that this zone had recently received vascular-plant enriched OM, minimally altered by shelf-bed mixing processes. In contrast, sediments in the northern depocenter and outer shelf also contained elevated amounts of terrigenous sedimentary OM, but this material was generally lower in lignin abundance and had higher interclass ratios (greater degradation). Collectively, these results suggest that the Flood-derived sediment and fresh terrigenous OM were mostly constrained between 20 and 70 m water depth, with enhanced Deposition overlapping the tectonic-controlled depocenters located to the northeast and southeast of Poverty Bay.
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Seabed erodibility variations on the Louisiana continental shelf before and after the 2011 Mississippi River Flood
Estuarine Coastal and Shelf Science, 2014Co-Authors: Kehui Xu, D.r. Corbett, John P. Walsh, D. Young, Kevin B. Briggs, Grace M. Cartwright, Carl T. Friedrichs, Courtney K. Harris, Rangley C. Mickey, Siddhartha MitraAbstract:Erodibility is critical to the sediment resuspension process but has not been measured systematically in large river-dominated muddy continental shelves before. During early summer of 2011, the Mississippi River experienced a major Flood event. This Flood provided a unique opportunity to examine how shelf seabed erodibility responded to a large river Flood, and the ultimate fate of Flood Deposition is important to geological and biogeochemical processes (e.g., stratal formation, carbon sequestration). A total of 106 sediment cores were collected on the Louisiana shelf during five cruises in 2010 and 2011, and a new dataset was used to evaluate the response of the seabed to the recent conditions. The localized Flood Deposit was mainly within tens of kilometers of river sources, and little sediment accumulated on the middle Louisiana shelf. Seabed erodibility was measured using a dual-core Gust Erosion Microcosm System. The erodibility of sediment collected in April 2011 exceeded that for August 2010 and August 2011. The springtime increase in erodibility seemed to be related to the recent presence of energetic waves that mobilized the seabed. Erodibility was highest on the inner shelf southwest of Atchafalaya Bay, intermediate on the middle shelf, lowest in the Mississippi Canyon, and highly variable on the Mississippi subaqueous delta. These spatial patterns were influenced by proximity to river sources, Flood-Deposit thicknesses, intensity of wave-driven bed stresses, and bioturbation. The Flood-Deposit thickness itself, however, was not sufficient to explain all the spatial variations of erodibility after the peak of the Mississippi Flood. Comparing values to published data, the depth-varying erodibility on the Louisiana shelf was close to the “low erodibility” level for the York River of Virginia, and similar to the data collected from Baltimore Harbor in Maryland and the main stem of upper Chesapeake Bay. Our findings promote understanding of the resuspension of fluffy organic-rich layer at the water–sediment interface, which influences sediment oxygen demand on the Louisiana shelf. This dataset is also valuable to observational and modeling studies of large river sediment dispersal systems worldwide.