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Walter A. Barnhardt - One of the best experts on this subject based on the ideXlab platform.
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Pockmarks in Passamaquoddy Bay, New Brunswick, Canada
Geological Society London Memoirs, 2016Co-Authors: C. L. Legere, Joseph T. Kelley, Walter A. Barnhardt, Brian D. Andrews, J.e. Hughes Clarke, Daniel F. BelknapAbstract:Pockmarks are seafloor depressions associated with fluid escape (Judd & Hovland 2007). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin (Ussler et al. 2003; Rogers et al. 2006; Wildish et al. 2008). In North America, shallow-water Pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of Pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence Pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, Pockmarks can be large (>200 m diameter) and number in the thousands. Over 4500 Pockmarks litter the seafloor of Passamaquoddy Bay. These Pockmarks are within 5 km of shore in water depths ranging from 7 to 80 m (Fig. 1a). Occurring either as discrete depressions or in linear chains, Pockmarks are generally circular with concave sidewall …
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Shallow stratigraphic control on Pockmark distribution in north temperate estuaries
Marine Geology, 2012Co-Authors: Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Christine Legere, John E. Hughes ClarkeAbstract:Abstract Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with Pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for Pockmark formation are not obvious. Nor is it known why Pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for Pockmark formation; and 2) the region's physiography controls Pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine Pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to Pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the Pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5–10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 Pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r2 = 0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine Pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for Pockmark formation in nearshore coastal environments.
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More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for Pockmark field longevity
Geo-Marine Letters, 2011Co-Authors: Laura L. Brothers, Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Melissa Landon MaynardAbstract:Mechanisms and timescales responsible for Pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore Pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a Pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in Pockmark frequency or distribution over a 9-year period (1999–2008). In that time Pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that Pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore ^137Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the Pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to Pockmark formation and failure do not occur frequently within this field, and (2) Pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although Pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within Pockmarks induced by near-bed currents.
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Automated feature extraction and spatial organization of seafloor Pockmarks, Belfast Bay, Maine, USA
Geomorphology, 2010Co-Authors: Brian D. Andrews, Walter A. BarnhardtAbstract:article i nfo Seafloor Pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion, but few numerical analyses of their morphology and spatial distribution of Pockmarks exist. We introduce a quantitative definition of Pockmark morphology and, based on this definition, propose a three-step geomorphometric method to identify and extract Pockmarks from high-resolution swath bathymetry. We apply this GIS-implemented approach to 25 km 2 of bathymetry collected in the Belfast Bay, Maine USA Pockmark field. Our model extracted 1767 Pockmarks and found a linear Pockmark depth-to-diameter ratio for Pockmarks field-wide. Mean Pockmark depth is 7.6 m and mean diameter is 84.8 m. Pockmark distribution is non-random, and nearly half of the field's Pockmarks occur in chains. The most prominent chains are oriented semi-normal to the steepest gradient in Holocene sediment thickness. A descriptive model yields field-wide spatial statistics indicating that Pockmarks are distributed in non-random clusters. Results enable quantitative comparison of Pockmarks in fields worldwide as well as similar concave features, such as impact craters, dolines, or salt pools. Published by Elsevier B.V.
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shallow water Pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay
Marine Geology, 2006Co-Authors: Jeffrey N Rogers, Joseph T. Kelley, Daniel F. Belknap, Allen Gontz, Walter A. BarnhardtAbstract:Abstract A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km2 in nearshore muddy embayments. Many of these embayments also contain Pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack Pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources. Intensive mapping of the Belfast Bay Pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine Pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of Pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for Pockmark formation in areas lacking gas-curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field's margins.
Joseph T. Kelley - One of the best experts on this subject based on the ideXlab platform.
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Pockmarks in Passamaquoddy Bay, New Brunswick, Canada
Geological Society London Memoirs, 2016Co-Authors: C. L. Legere, Joseph T. Kelley, Walter A. Barnhardt, Brian D. Andrews, J.e. Hughes Clarke, Daniel F. BelknapAbstract:Pockmarks are seafloor depressions associated with fluid escape (Judd & Hovland 2007). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin (Ussler et al. 2003; Rogers et al. 2006; Wildish et al. 2008). In North America, shallow-water Pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of Pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence Pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, Pockmarks can be large (>200 m diameter) and number in the thousands. Over 4500 Pockmarks litter the seafloor of Passamaquoddy Bay. These Pockmarks are within 5 km of shore in water depths ranging from 7 to 80 m (Fig. 1a). Occurring either as discrete depressions or in linear chains, Pockmarks are generally circular with concave sidewall …
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Shallow stratigraphic control on Pockmark distribution in north temperate estuaries
Marine Geology, 2012Co-Authors: Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Christine Legere, John E. Hughes ClarkeAbstract:Abstract Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with Pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for Pockmark formation are not obvious. Nor is it known why Pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for Pockmark formation; and 2) the region's physiography controls Pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine Pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to Pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the Pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5–10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 Pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r2 = 0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine Pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for Pockmark formation in nearshore coastal environments.
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More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for Pockmark field longevity
Geo-Marine Letters, 2011Co-Authors: Laura L. Brothers, Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Melissa Landon MaynardAbstract:Mechanisms and timescales responsible for Pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore Pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a Pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in Pockmark frequency or distribution over a 9-year period (1999–2008). In that time Pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that Pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore ^137Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the Pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to Pockmark formation and failure do not occur frequently within this field, and (2) Pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although Pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within Pockmarks induced by near-bed currents.
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shallow water Pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay
Marine Geology, 2006Co-Authors: Jeffrey N Rogers, Joseph T. Kelley, Daniel F. Belknap, Allen Gontz, Walter A. BarnhardtAbstract:Abstract A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km2 in nearshore muddy embayments. Many of these embayments also contain Pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack Pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources. Intensive mapping of the Belfast Bay Pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine Pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of Pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for Pockmark formation in areas lacking gas-curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field's margins.
Daniel F. Belknap - One of the best experts on this subject based on the ideXlab platform.
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Pockmarks in Passamaquoddy Bay, New Brunswick, Canada
Geological Society London Memoirs, 2016Co-Authors: C. L. Legere, Joseph T. Kelley, Walter A. Barnhardt, Brian D. Andrews, J.e. Hughes Clarke, Daniel F. BelknapAbstract:Pockmarks are seafloor depressions associated with fluid escape (Judd & Hovland 2007). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin (Ussler et al. 2003; Rogers et al. 2006; Wildish et al. 2008). In North America, shallow-water Pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of Pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence Pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, Pockmarks can be large (>200 m diameter) and number in the thousands. Over 4500 Pockmarks litter the seafloor of Passamaquoddy Bay. These Pockmarks are within 5 km of shore in water depths ranging from 7 to 80 m (Fig. 1a). Occurring either as discrete depressions or in linear chains, Pockmarks are generally circular with concave sidewall …
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Shallow stratigraphic control on Pockmark distribution in north temperate estuaries
Marine Geology, 2012Co-Authors: Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Christine Legere, John E. Hughes ClarkeAbstract:Abstract Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with Pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for Pockmark formation are not obvious. Nor is it known why Pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for Pockmark formation; and 2) the region's physiography controls Pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine Pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to Pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the Pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5–10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 Pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r2 = 0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine Pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for Pockmark formation in nearshore coastal environments.
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More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for Pockmark field longevity
Geo-Marine Letters, 2011Co-Authors: Laura L. Brothers, Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Melissa Landon MaynardAbstract:Mechanisms and timescales responsible for Pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore Pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a Pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in Pockmark frequency or distribution over a 9-year period (1999–2008). In that time Pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that Pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore ^137Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the Pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to Pockmark formation and failure do not occur frequently within this field, and (2) Pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although Pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within Pockmarks induced by near-bed currents.
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shallow water Pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay
Marine Geology, 2006Co-Authors: Jeffrey N Rogers, Joseph T. Kelley, Daniel F. Belknap, Allen Gontz, Walter A. BarnhardtAbstract:Abstract A systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western Gulf of Maine has identified more than 70 biogenic natural gas deposits, occupying 311 km2 in nearshore muddy embayments. Many of these embayments also contain Pockmark fields, with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment. Pockmarks, hemispherically shaped depressions of various size and depths, formed through fluid escape of gas and/or pore water, are sometimes found within or outside gas fields, although many gas fields lack Pockmarks altogether. Although the origin of the natural gas remains unclear, if coastal environments at times of lower sea level were similar to the present, numerous lake, wetland, valley fill and estuarine sources of organic-rich material may have formed on the inner shelf. If these deposits survived transgression and remain buried, they are potential gas sources. Intensive mapping of the Belfast Bay Pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a Gulf of Maine Pockmark field with a corresponding 3-dimensional geological model generated from seismic data. Statistical analysis of Pockmark geometry, gas deposit loci, and subsurface evidence for gas-enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for Pockmark formation in areas lacking gas-curtain seismic reflections. The coarse-grained transgressive ravinement unconformity between Pleistocene glacial-marine mud and Holocene mud may act as a conduit for distributing methane to the field's margins.
Brian D. Andrews - One of the best experts on this subject based on the ideXlab platform.
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Pockmarks in Passamaquoddy Bay, New Brunswick, Canada
Geological Society London Memoirs, 2016Co-Authors: C. L. Legere, Joseph T. Kelley, Walter A. Barnhardt, Brian D. Andrews, J.e. Hughes Clarke, Daniel F. BelknapAbstract:Pockmarks are seafloor depressions associated with fluid escape (Judd & Hovland 2007). They proliferate in the muddy seafloors of coastal Gulf of Maine and Bay of Fundy, where they are associated with shallow natural gas likely of biogenic origin (Ussler et al. 2003; Rogers et al. 2006; Wildish et al. 2008). In North America, shallow-water Pockmark fields are not reported south of Long Island Sound, despite the abundance of gassy, muddy estuaries. The absence of Pockmarks south of the limit of North American glaciation suggests that local and regional heterogeneities, possibly related to glacial or sea-level history or bedrock geology, influence Pockmark field distribution. In shallow-water embayments, such as Passamaquoddy Bay, New Brunswick, Pockmarks can be large (>200 m diameter) and number in the thousands. Over 4500 Pockmarks litter the seafloor of Passamaquoddy Bay. These Pockmarks are within 5 km of shore in water depths ranging from 7 to 80 m (Fig. 1a). Occurring either as discrete depressions or in linear chains, Pockmarks are generally circular with concave sidewall …
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Shallow stratigraphic control on Pockmark distribution in north temperate estuaries
Marine Geology, 2012Co-Authors: Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Christine Legere, John E. Hughes ClarkeAbstract:Abstract Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with Pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for Pockmark formation are not obvious. Nor is it known why Pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for Pockmark formation; and 2) the region's physiography controls Pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine Pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to Pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the Pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5–10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 Pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r2 = 0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine Pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for Pockmark formation in nearshore coastal environments.
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More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for Pockmark field longevity
Geo-Marine Letters, 2011Co-Authors: Laura L. Brothers, Joseph T. Kelley, Walter A. Barnhardt, Daniel F. Belknap, Brian D. Andrews, Melissa Landon MaynardAbstract:Mechanisms and timescales responsible for Pockmark formation and maintenance remain uncertain, especially in areas lacking extensive thermogenic fluid deposits (e.g., previously glaciated estuaries). This study characterizes seafloor activity in the Belfast Bay, Maine nearshore Pockmark field using (1) three swath bathymetry datasets collected between 1999 and 2008, complemented by analyses of shallow box-core samples for radionuclide activity and undrained shear strength, and (2) historical bathymetric data (report and smooth sheets from 1872, 1947, 1948). In addition, because repeat swath bathymetry surveys are an emerging data source, we present a selected literature review of recent studies using such datasets for seafloor change analysis. This study is the first to apply the method to a Pockmark field, and characterizes macro-scale (>5 m) evolution of tens of square kilometers of highly irregular seafloor. Presence/absence analysis yielded no change in Pockmark frequency or distribution over a 9-year period (1999–2008). In that time Pockmarks did not detectably enlarge, truncate, elongate, or combine. Historical data indicate that Pockmark chains already existed in the 19th century. Despite the lack of macroscopic changes in the field, near-bed undrained shear-strength values of less than 7 kPa and scattered downcore ^137Cs signatures indicate a highly disturbed setting. Integrating these findings with independent geophysical and geochemical observations made in the Pockmark field, it can be concluded that (1) large-scale sediment resuspension and dispersion related to Pockmark formation and failure do not occur frequently within this field, and (2) Pockmarks can persevere in a dynamic estuarine setting that exhibits minimal modern fluid venting. Although Pockmarks are conventionally thought to be long-lived features maintained by a combination of fluid venting and minimal sediment accumulation, this suggests that other mechanisms may be equally active in maintaining such irregular seafloor morphology. One such mechanism could be upwelling within Pockmarks induced by near-bed currents.
-
Automated feature extraction and spatial organization of seafloor Pockmarks, Belfast Bay, Maine, USA
Geomorphology, 2010Co-Authors: Brian D. Andrews, Walter A. BarnhardtAbstract:article i nfo Seafloor Pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion, but few numerical analyses of their morphology and spatial distribution of Pockmarks exist. We introduce a quantitative definition of Pockmark morphology and, based on this definition, propose a three-step geomorphometric method to identify and extract Pockmarks from high-resolution swath bathymetry. We apply this GIS-implemented approach to 25 km 2 of bathymetry collected in the Belfast Bay, Maine USA Pockmark field. Our model extracted 1767 Pockmarks and found a linear Pockmark depth-to-diameter ratio for Pockmarks field-wide. Mean Pockmark depth is 7.6 m and mean diameter is 84.8 m. Pockmark distribution is non-random, and nearly half of the field's Pockmarks occur in chains. The most prominent chains are oriented semi-normal to the steepest gradient in Holocene sediment thickness. A descriptive model yields field-wide spatial statistics indicating that Pockmarks are distributed in non-random clusters. Results enable quantitative comparison of Pockmarks in fields worldwide as well as similar concave features, such as impact craters, dolines, or salt pools. Published by Elsevier B.V.
Marco Taviani - One of the best experts on this subject based on the ideXlab platform.
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Diversity and Distribution of Prokaryotes within a Shallow-Water Pockmark Field
Frontiers in microbiology, 2016Co-Authors: Donato Giovannelli, Giuseppe D’errico, Federica Fiorentino, Daniele Fattorini, Francesco Regoli, Lorenzo Angeletti, Tatjana Bakran-petricioli, Costantino Vetriani, Mustafa Yücel, Marco TavianiAbstract:Pockmarks are crater-like depression on the seafloor associated with hydrocarbon ascent through muddy sediments in continental shelves around the world. In this study, we examine the diversity and distribution of benthic microbial communities at shallow-water Pockmarks adjacent to the Middle Adriatic Ridge. We integrate microbial diversity data with characterization of local hydrocarbons concentrations and sediment geochemistry. Our results suggest these Pockmarks are enriched in sedimentary hydrocarbons, and host a microbial community dominated by Bacteria, even in deeper sediment layers. Pockmark sediments showed higher prokaryotic abundance and biomass than surrounding sediments, potentially due to the increased availability of organic matter and higher concentrations of hydrocarbons linked to Pockmark activity. Prokaryotic diversity analyses showed that the microbial communities of these shallow-water Pockmarks are unique, and comprised phylotypes associated with the cycling of sulfur and nitrate compounds, as well as numerous know hydrocarbon degraders. Altogether, this study suggests that shallow-water Pockmark habitats enhance the diversity of the benthic prokaryotic biosphere by providing specialized environmental niches.