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  • Open Access English
    Authors: 
    Tim Greenfield; Robert S. White; Tom Winder; Thorbjörg Ágústsdóttir;
    Country: United Kingdom

    A large seismic network deployed in the Icelandic highlands recorded more than 100,000 earthquakes from 2009 to 2015. We develop a local magnitude scale, appropriate for use in central Iceland, which is similar to the scale used by the Iceland Meteorological Office. Using this large catalogue of earthquakes, we analyze the spatial and temporal changes in seismicity rates and b-values. In microearthquakes recorded from the usually ductile lower crust we find that b-values are high, reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt. In contrast, b-values in the upper crust are variable. Low b-values, indicative of a high stress environment, are observed during seismic swarms such as those around Mt. Herðubreið and around Bárðarbunga caldera. A persistently seismically active area around a geothermal area within Askja caldera has a b-value around 1 but has a strong annual cycle of seismicity. We attribute the annual cycle to varying load from the snow cover modulating the seismicity. Seismicity driven by the intrusion of a large dyke has a b-value well above 1, driven by the high pore fluid pressures and thermal gradients around the dyke. NERC, Shell

  • Open Access English
    Authors: 
    Zong-Pei Jiang; Toby Tyrrell; David J. Hydes; Minhan Dai; Susan E. Hartman;
    Countries: United Kingdom, France

    The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non-SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity-normalized alkalinity with that of the open ocean end-member. Non-SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end-member can only be reliably derived from the y intercept of TA-S regression (TA(S0)) in river-dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TAS0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TAS0 can result from non-SDC TA removal at the low salinity end (relative to the salinity of the oceanic end-member) and/or non-SDC TA addition at high salinities (as occurs in the Mediterranean Sea).

  • Open Access English
    Authors: 
    Katharine M. Grant; Gerald R. Dickens;
    Country: United Kingdom

    Biogenic components of sediment accumulated at high rates beneath frontal zones of the Indian and Pacific oceans during the late Miocene and early Pliocene. The δ13C of bulk and foraminiferal carbonate also decreased during this time interval. Although the two observations may be causally linked, and signify a major perturbation in global biogeochemical cycling, no site beneath a frontal zone has independent records of export production and δ13C on multiple carbonate phases across the critical interval of interest. Deep Sea Drilling Project (DSDP) site 590 lies beneath the Tasman Front (TF), an eddy-generating jetstream in the southwest Pacific Ocean. To complement previous δ13C records of planktic and benthic foraminifera at this location, late Neogene records of CaCO3 mass accumulation rate (MAR), Ca/Ti, Ba/Ti, Al/Ti, and of bulk carbonate and foraminiferal δ13C were constructed at site 590. The δ13C records include bulk sediment, bulk sediment fractions (<63 µm and 5–25 µm), and the planktic foraminifera Globigerina bulloides, Globigerinoides sacculifer (with and without sac), and Orbulina universa. Using current time scales, CaCO3 MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are two to three times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease also occurs in all δ13C records. All evidence indicates that enhanced export production – the ‘biogenic bloom’ – extended to the southwest Pacific Ocean between ca. 9 and 3.8 Ma, and this phenomenon is coupled with changes in δ13C – the ‘Chron C3AR carbon shift’. However, CaCO3 MARs peak ca. 5 Ma whereas elemental ratios are highest ca. 6.5 Ma; foraminiferal δ13C starts to decrease ca. 8 Ma whereas bulk carbonate δ13C begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the TF, perhaps signifying a link between changes in ocean–atmosphere circulation change and widespread primary productivity.

  • Open Access English
    Authors: 
    J. Robinson; Ekaterina Popova; Meric Srokosz; Andrew Yool;
    Country: United Kingdom

    Iron limitation of primary productivity prevails across much of the Southern Ocean but there are exceptions; in particular, the phytoplankton blooms associated with the Kerguelen Plateau, Crozet Islands and South Georgia. These blooms occur annually, fertilized by iron and nutrient-rich shelf waters that are transported downstream from the islands. Here we use a highresolution (1/12°) ocean general circulation model and Lagrangian particle tracking to investigate whether inter-annual variability in the potential lateral advection of iron, could explain the inter-annual variability in the spatial extent of the blooms. Comparison with ocean color data, 1998 to 2007, suggests that iron fertilization via advection can explain the extent of each island's annual bloom, but only the inter-annual variability of the Crozet bloom. The area that could potentially be fertilized by iron from Kerguelen was much larger than the bloom, suggesting that there is another primary limiting factor, potentially silicate, that controls the inter-annual variability of bloom spatial extent. For South Georgia, there are differences in the year-to-year timing of advection and consequently fertilization, but no clear explanation of the inter-annual variability observed in the bloom's spatial extent has been identified. The model results suggest that the Kerguelen and Crozet blooms are terminated by nutrient exhaustion, probably iron and or silicate, whereas the deepening of the mixed layer in winter terminates the South Georgia bloom. Therefore, iron fertilization via lateral advection alone can explain the annual variability of the Crozet bloom, but not fully that of the Kerguelen and South Georgia blooms. This article is protected by copyright. All rights reserved.

  • Open Access English
    Authors: 
    Xiaowei Feng; Susan Gourvenec; David White;
    Country: United Kingdom

    Abstract Floating structures are often secured in position with a taut mooring system and suction caissons. Large seabed trenches have been observed adjacent to some suction caisson anchors with taut-line mooring systems. The trenches may jeopardise the geotechnical capacity of the caissons and in turn the stationkeeping of the floating structures. Finite-element method is employed to examine the geotechnical capacity of suction caissons in a trenching seabed. The results show that the reduction in the geotechnical capacity becomes more significant with increasing trench width due to the loss of soil support and a change in failure mechanism as the caisson rotates into the trench. For a given trench width, the reduction in capacity becomes more significant as the load inclination angle to the horizontal decreases. However, the shape of the normalised failure envelopes for combined vertical and horizontal load is insensitive to trench width. A strategy to design for inevitable trenching by moving the padeye shallower to reduce the depth of trench formation is not straightforward. The gain from a shallower trench may often be outweighed by the reduction in capacity from rotation of the caisson at failure for loading angles typical of taut moorings.

  • Open Access English
    Authors: 
    Timothy A. Minshull; Nicky White; R. A. Edwards; Donna J. Shillington; C. L. Scott; A. Demirer; M. Shaw-Champion; S. M. Jones; M. Erduran; T. Besevli; +4 more
    Country: United Kingdom

    Rifted continental margins are formed by progressive extension of the lithosphere. The development of these margins plays an integral role in the plate tectonic cycle, and an understanding of the extensional process underpins much hydrocarbon exploration. A key issue is whether the lithosphere extends uniformly, or whether extension varies with depth. Crustal extension may be determined using seismic techniques. Lithospheric extension may be inferred from the waterloaded subsidence history, determined from the pattern of sedimentation during and after rifting. Unfortunately, however, many rifted margins are sediment-starved, so the subsidence history is poorly known. To test whether extension varies between the crust and the mantle, a major seismic experiment was conducted in February–March 2005 in the eastern Black Sea Basin (Figure 1), a deep basin where the subsidence history is recorded by a thick, post-rift sedimentary sequence. The seismic data from the experiment indicate the presence of a thick, low-velocity zone, possibly representing overpressured sediments. They also indicate that the basement and Moho in the center of the basin are both several kilometers shallower than previously inferred. These initial observations may have considerable impact on thermal models of the petroleum system in the basin. Understanding the thermal history of potential source rocks is key to reducing hydrocarbon exploration risk. The experiment, which involved collaboration between university groups in the United Kingdom, Ireland, and Turkey, and BP and Turkish Petroleum (TPAO), formed part of a larger project that also is using deep seismic reflection and other geophysical data held by the industry partners to determine the subsidence history and hence the strain evolution of the basin.

  • Open Access
    Authors: 
    Yao Qian; David Heslop; Andrew P. Roberts; Pengxiang Hu; Xiang Zhao; Yan Liu; Jinhua Li; Katharine M. Grant; Eelco J. Rohling;
    Publisher: American Geophysical Union (AGU)
    Country: United Kingdom

    Periodic and marked redox changes in eastern Mediterranean marine sediments drive environmental and diagenetic changes to which magnetic minerals are sensitive. Magnetic property changes, therefore, provide useful indications of paleoceanographic conditions during and after periods of organic-rich sediment (sapropel) deposition. Magnetic properties of eastern Mediterranean sediments at room temperature have been studied for decades; however, few studies have considered low-temperature magnetic properties. Here, we investigate the low-temperature (10–300 K) magnetic properties of different eastern Mediterranean sediment types combined with room temperature (∼300 K) magnetic properties, transmission electron microscopy, and calibrated X-ray fluorescence elemental data to illustrate the valuable information that can be obtained from low-temperature magnetic analysis of sediments. Our low-temperature magnetic results suggest that magnetite magnetofossils and superparamagnetic (SP) particles occur widely in eastern Mediterranean sediments. SP particle contents are highest in diagenetically reduced intervals associated with sapropels. In contrast, magnetite magnetofossils are most abundant in oxidation fronts at the tops of sapropels, where strong redox gradients formed, but are also widespread throughout other sedimentary intervals that have not been subjected to extensive reductive diagenesis. Moreover, the surfaces of magnetite particles are maghemitized (i.e., partially oxidized) in oxidation fronts at the tops of sapropels, and in other oxic sediment intervals. Our results demonstrate the value of LT magnetic measurements for quantifying diverse sedimentary magnetic signals of interest in environmental magnetism when studying paleoceanographic and paleoenvironmental processes.

  • Open Access English
    Authors: 
    Chris Wilson; Chris W. Hughes; Jeffrey R. Blundell;
    Country: United Kingdom
    Project: UKRI | MESoscale Ocean eddies an... (NE/K005928/1)

    We use ensemble runs of a three-layer, quasigeostrophic idealized Southern Ocean model to explore the roles of forced and intrinsic variability in response to a linear increase of wind stress imposed over a 30-year period. We find no increase of eastward circumpolar volume transport in response to the increased wind stress. A large part of the resulting time series can be explained by a response in which the eddy kinetic energy is linearly proportional to the wind stress with a possible time lag, but no statistically significant lag is found. However, this simple relationship is not the whole story: several intrinsic timescales also influence the response. We find an e-folding timescale for growth of small perturbations of 1-2 weeks. The energy budget for intrinsic variability at periods shorter than a year is dominated by exchange between kinetic and potential energy. At longer timescales, we find an intrinsic mode with period in the region of 15 years, which is dominated by changes in potential energy and frictional dissipation in a manner consistent with that seen by Hogg and Blundell [2006]. A similar mode influences the response to changing wind stress. This influence, robust to perturbations, is different from the supposed linear relationship between wind stress and eddy kinetic energy, and persists for 5-10 years in this model, suggestive of a forced oscillatory mode with period of around 15 years. If present in the real ocean, such a mode would imply a degree of predictability of Southern Ocean dynamics on multi-year timescales.

  • Open Access
    Authors: 
    Jeffrey A. Hawkes; Pamela E. Rossel; Aron Stubbins; David A. Butterfield; Douglas P. Connelly; Eric P. Achterberg; Andrea Koschinsky; Valérie Chavagnac; Christian T. Hansen; Wolfgang Bach; +1 more
    Publisher: Springer Science and Business Media LLC
    Countries: United Kingdom, United Kingdom, Germany
    Project: EC | ABYSS (294757)

    Oceanic dissolved organic carbon (DOC) is an important carbon pool, similar in magnitude to atmospheric CO2, but the fate of its oldest forms is not well understood1, 2. Hot hydrothermal circulation may facilitate the degradation of otherwise un-reactive dissolved organic matter, playing an important role in the long-term global carbon cycle. The oldest, most recalcitrant forms of DOC, which make up most of oceanic DOC, can be recovered by solid-phase extraction. Here we present measurements of solid-phase extractable DOC from samples collected between 2009 and 2013 at seven vent sites in the Atlantic, Pacific and Southern oceans, along with magnesium concentrations, a conservative tracer of water circulation through hydrothermal systems. We find that magnesium and solid-phase extractable DOC concentrations are correlated, suggesting that solid-phase extractable DOC is almost entirely lost from solution through mineralization or deposition during circulation through hydrothermal vents with fluid temperatures of 212–401 °C. In laboratory experiments, where we heated samples to 380 °C for four days, we found a similar removal efficiency. We conclude that thermal degradation alone can account for the loss of solid-phase extractable DOC in natural hydrothermal systems, and that its maximum lifetime is constrained by the timescale of hydrothermal cycling, at about 40 million years3.

  • Open Access
    Authors: 
    Zheng-Tong Xie; Ian P. Castro;
    Publisher: Elsevier BV
    Country: United Kingdom

    Large-eddy simulations (LES) with our recently developed inflow approach (Xie &Castro, 2008a) have been used for flow and dispersion within a genuine city area -the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Plin Central London. Numerical results up to second-order statistics are reported fora computational domain of 1.2km (streamwise) x 0.8km (lateral) x 0.2km (in fullscale), with a resolution down to approximately one meter in space and one secondin time. They are in reasonable agreement with the experimental data. Such a comprehensiveurban geometry is often, as here, composed of staggered, aligned, squarearrays of blocks with non-uniform height and non-uniform base, street canyons andintersections. Both the integrative and local effect of flow and dispersion to thesegeometrical patterns were investigated. For example, it was found that the peaksof spatially averaged urms, vrms, wrms and < u0w0 > occurred neither at the meanheight nor at the maximum height, but at the height of large and tall buildings. Itwas also found that the mean and fluctuating concentrations in the near-source fieldis highly dependent on the source location and the local geometry pattern, whereasin the far field (e.g. >0.1km) they are not. In summary, it is demonstrated thatfull-scale resolution of around one meter is sufficient to yield accurate prediction ofthe flow and mean dispersion characteristics and to provide reasonable estimationof concentration fluctuations

Advanced search in
Research products
arrow_drop_down
Searching FieldsTerms
Any field
arrow_drop_down
includes
arrow_drop_down
Include:
1,681 Research products, page 1 of 169
  • Open Access English
    Authors: 
    Tim Greenfield; Robert S. White; Tom Winder; Thorbjörg Ágústsdóttir;
    Country: United Kingdom

    A large seismic network deployed in the Icelandic highlands recorded more than 100,000 earthquakes from 2009 to 2015. We develop a local magnitude scale, appropriate for use in central Iceland, which is similar to the scale used by the Iceland Meteorological Office. Using this large catalogue of earthquakes, we analyze the spatial and temporal changes in seismicity rates and b-values. In microearthquakes recorded from the usually ductile lower crust we find that b-values are high, reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt. In contrast, b-values in the upper crust are variable. Low b-values, indicative of a high stress environment, are observed during seismic swarms such as those around Mt. Herðubreið and around Bárðarbunga caldera. A persistently seismically active area around a geothermal area within Askja caldera has a b-value around 1 but has a strong annual cycle of seismicity. We attribute the annual cycle to varying load from the snow cover modulating the seismicity. Seismicity driven by the intrusion of a large dyke has a b-value well above 1, driven by the high pore fluid pressures and thermal gradients around the dyke. NERC, Shell

  • Open Access English
    Authors: 
    Zong-Pei Jiang; Toby Tyrrell; David J. Hydes; Minhan Dai; Susan E. Hartman;
    Countries: United Kingdom, France

    The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non-SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity-normalized alkalinity with that of the open ocean end-member. Non-SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end-member can only be reliably derived from the y intercept of TA-S regression (TA(S0)) in river-dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TAS0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TAS0 can result from non-SDC TA removal at the low salinity end (relative to the salinity of the oceanic end-member) and/or non-SDC TA addition at high salinities (as occurs in the Mediterranean Sea).

  • Open Access English
    Authors: 
    Katharine M. Grant; Gerald R. Dickens;
    Country: United Kingdom

    Biogenic components of sediment accumulated at high rates beneath frontal zones of the Indian and Pacific oceans during the late Miocene and early Pliocene. The δ13C of bulk and foraminiferal carbonate also decreased during this time interval. Although the two observations may be causally linked, and signify a major perturbation in global biogeochemical cycling, no site beneath a frontal zone has independent records of export production and δ13C on multiple carbonate phases across the critical interval of interest. Deep Sea Drilling Project (DSDP) site 590 lies beneath the Tasman Front (TF), an eddy-generating jetstream in the southwest Pacific Ocean. To complement previous δ13C records of planktic and benthic foraminifera at this location, late Neogene records of CaCO3 mass accumulation rate (MAR), Ca/Ti, Ba/Ti, Al/Ti, and of bulk carbonate and foraminiferal δ13C were constructed at site 590. The δ13C records include bulk sediment, bulk sediment fractions (<63 µm and 5–25 µm), and the planktic foraminifera Globigerina bulloides, Globigerinoides sacculifer (with and without sac), and Orbulina universa. Using current time scales, CaCO3 MARs, Ca/Ti, Al/Ti and Ba/Ti ratios are two to three times higher in upper Miocene and lower Pliocene sediment relative to overlying and underlying units. A significant decrease also occurs in all δ13C records. All evidence indicates that enhanced export production – the ‘biogenic bloom’ – extended to the southwest Pacific Ocean between ca. 9 and 3.8 Ma, and this phenomenon is coupled with changes in δ13C – the ‘Chron C3AR carbon shift’. However, CaCO3 MARs peak ca. 5 Ma whereas elemental ratios are highest ca. 6.5 Ma; foraminiferal δ13C starts to decrease ca. 8 Ma whereas bulk carbonate δ13C begins to drop ca. 5.6 Ma. Temporal discrepancies between the records can be explained by changes in the upwelling regime at the TF, perhaps signifying a link between changes in ocean–atmosphere circulation change and widespread primary productivity.

  • Open Access English
    Authors: 
    J. Robinson; Ekaterina Popova; Meric Srokosz; Andrew Yool;
    Country: United Kingdom

    Iron limitation of primary productivity prevails across much of the Southern Ocean but there are exceptions; in particular, the phytoplankton blooms associated with the Kerguelen Plateau, Crozet Islands and South Georgia. These blooms occur annually, fertilized by iron and nutrient-rich shelf waters that are transported downstream from the islands. Here we use a highresolution (1/12°) ocean general circulation model and Lagrangian particle tracking to investigate whether inter-annual variability in the potential lateral advection of iron, could explain the inter-annual variability in the spatial extent of the blooms. Comparison with ocean color data, 1998 to 2007, suggests that iron fertilization via advection can explain the extent of each island's annual bloom, but only the inter-annual variability of the Crozet bloom. The area that could potentially be fertilized by iron from Kerguelen was much larger than the bloom, suggesting that there is another primary limiting factor, potentially silicate, that controls the inter-annual variability of bloom spatial extent. For South Georgia, there are differences in the year-to-year timing of advection and consequently fertilization, but no clear explanation of the inter-annual variability observed in the bloom's spatial extent has been identified. The model results suggest that the Kerguelen and Crozet blooms are terminated by nutrient exhaustion, probably iron and or silicate, whereas the deepening of the mixed layer in winter terminates the South Georgia bloom. Therefore, iron fertilization via lateral advection alone can explain the annual variability of the Crozet bloom, but not fully that of the Kerguelen and South Georgia blooms. This article is protected by copyright. All rights reserved.

  • Open Access English
    Authors: 
    Xiaowei Feng; Susan Gourvenec; David White;
    Country: United Kingdom

    Abstract Floating structures are often secured in position with a taut mooring system and suction caissons. Large seabed trenches have been observed adjacent to some suction caisson anchors with taut-line mooring systems. The trenches may jeopardise the geotechnical capacity of the caissons and in turn the stationkeeping of the floating structures. Finite-element method is employed to examine the geotechnical capacity of suction caissons in a trenching seabed. The results show that the reduction in the geotechnical capacity becomes more significant with increasing trench width due to the loss of soil support and a change in failure mechanism as the caisson rotates into the trench. For a given trench width, the reduction in capacity becomes more significant as the load inclination angle to the horizontal decreases. However, the shape of the normalised failure envelopes for combined vertical and horizontal load is insensitive to trench width. A strategy to design for inevitable trenching by moving the padeye shallower to reduce the depth of trench formation is not straightforward. The gain from a shallower trench may often be outweighed by the reduction in capacity from rotation of the caisson at failure for loading angles typical of taut moorings.

  • Open Access English
    Authors: 
    Timothy A. Minshull; Nicky White; R. A. Edwards; Donna J. Shillington; C. L. Scott; A. Demirer; M. Shaw-Champion; S. M. Jones; M. Erduran; T. Besevli; +4 more
    Country: United Kingdom

    Rifted continental margins are formed by progressive extension of the lithosphere. The development of these margins plays an integral role in the plate tectonic cycle, and an understanding of the extensional process underpins much hydrocarbon exploration. A key issue is whether the lithosphere extends uniformly, or whether extension varies with depth. Crustal extension may be determined using seismic techniques. Lithospheric extension may be inferred from the waterloaded subsidence history, determined from the pattern of sedimentation during and after rifting. Unfortunately, however, many rifted margins are sediment-starved, so the subsidence history is poorly known. To test whether extension varies between the crust and the mantle, a major seismic experiment was conducted in February–March 2005 in the eastern Black Sea Basin (Figure 1), a deep basin where the subsidence history is recorded by a thick, post-rift sedimentary sequence. The seismic data from the experiment indicate the presence of a thick, low-velocity zone, possibly representing overpressured sediments. They also indicate that the basement and Moho in the center of the basin are both several kilometers shallower than previously inferred. These initial observations may have considerable impact on thermal models of the petroleum system in the basin. Understanding the thermal history of potential source rocks is key to reducing hydrocarbon exploration risk. The experiment, which involved collaboration between university groups in the United Kingdom, Ireland, and Turkey, and BP and Turkish Petroleum (TPAO), formed part of a larger project that also is using deep seismic reflection and other geophysical data held by the industry partners to determine the subsidence history and hence the strain evolution of the basin.

  • Open Access
    Authors: 
    Yao Qian; David Heslop; Andrew P. Roberts; Pengxiang Hu; Xiang Zhao; Yan Liu; Jinhua Li; Katharine M. Grant; Eelco J. Rohling;
    Publisher: American Geophysical Union (AGU)
    Country: United Kingdom

    Periodic and marked redox changes in eastern Mediterranean marine sediments drive environmental and diagenetic changes to which magnetic minerals are sensitive. Magnetic property changes, therefore, provide useful indications of paleoceanographic conditions during and after periods of organic-rich sediment (sapropel) deposition. Magnetic properties of eastern Mediterranean sediments at room temperature have been studied for decades; however, few studies have considered low-temperature magnetic properties. Here, we investigate the low-temperature (10–300 K) magnetic properties of different eastern Mediterranean sediment types combined with room temperature (∼300 K) magnetic properties, transmission electron microscopy, and calibrated X-ray fluorescence elemental data to illustrate the valuable information that can be obtained from low-temperature magnetic analysis of sediments. Our low-temperature magnetic results suggest that magnetite magnetofossils and superparamagnetic (SP) particles occur widely in eastern Mediterranean sediments. SP particle contents are highest in diagenetically reduced intervals associated with sapropels. In contrast, magnetite magnetofossils are most abundant in oxidation fronts at the tops of sapropels, where strong redox gradients formed, but are also widespread throughout other sedimentary intervals that have not been subjected to extensive reductive diagenesis. Moreover, the surfaces of magnetite particles are maghemitized (i.e., partially oxidized) in oxidation fronts at the tops of sapropels, and in other oxic sediment intervals. Our results demonstrate the value of LT magnetic measurements for quantifying diverse sedimentary magnetic signals of interest in environmental magnetism when studying paleoceanographic and paleoenvironmental processes.

  • Open Access English
    Authors: 
    Chris Wilson; Chris W. Hughes; Jeffrey R. Blundell;
    Country: United Kingdom
    Project: UKRI | MESoscale Ocean eddies an... (NE/K005928/1)

    We use ensemble runs of a three-layer, quasigeostrophic idealized Southern Ocean model to explore the roles of forced and intrinsic variability in response to a linear increase of wind stress imposed over a 30-year period. We find no increase of eastward circumpolar volume transport in response to the increased wind stress. A large part of the resulting time series can be explained by a response in which the eddy kinetic energy is linearly proportional to the wind stress with a possible time lag, but no statistically significant lag is found. However, this simple relationship is not the whole story: several intrinsic timescales also influence the response. We find an e-folding timescale for growth of small perturbations of 1-2 weeks. The energy budget for intrinsic variability at periods shorter than a year is dominated by exchange between kinetic and potential energy. At longer timescales, we find an intrinsic mode with period in the region of 15 years, which is dominated by changes in potential energy and frictional dissipation in a manner consistent with that seen by Hogg and Blundell [2006]. A similar mode influences the response to changing wind stress. This influence, robust to perturbations, is different from the supposed linear relationship between wind stress and eddy kinetic energy, and persists for 5-10 years in this model, suggestive of a forced oscillatory mode with period of around 15 years. If present in the real ocean, such a mode would imply a degree of predictability of Southern Ocean dynamics on multi-year timescales.

  • Open Access
    Authors: 
    Jeffrey A. Hawkes; Pamela E. Rossel; Aron Stubbins; David A. Butterfield; Douglas P. Connelly; Eric P. Achterberg; Andrea Koschinsky; Valérie Chavagnac; Christian T. Hansen; Wolfgang Bach; +1 more
    Publisher: Springer Science and Business Media LLC
    Countries: United Kingdom, United Kingdom, Germany
    Project: EC | ABYSS (294757)

    Oceanic dissolved organic carbon (DOC) is an important carbon pool, similar in magnitude to atmospheric CO2, but the fate of its oldest forms is not well understood1, 2. Hot hydrothermal circulation may facilitate the degradation of otherwise un-reactive dissolved organic matter, playing an important role in the long-term global carbon cycle. The oldest, most recalcitrant forms of DOC, which make up most of oceanic DOC, can be recovered by solid-phase extraction. Here we present measurements of solid-phase extractable DOC from samples collected between 2009 and 2013 at seven vent sites in the Atlantic, Pacific and Southern oceans, along with magnesium concentrations, a conservative tracer of water circulation through hydrothermal systems. We find that magnesium and solid-phase extractable DOC concentrations are correlated, suggesting that solid-phase extractable DOC is almost entirely lost from solution through mineralization or deposition during circulation through hydrothermal vents with fluid temperatures of 212–401 °C. In laboratory experiments, where we heated samples to 380 °C for four days, we found a similar removal efficiency. We conclude that thermal degradation alone can account for the loss of solid-phase extractable DOC in natural hydrothermal systems, and that its maximum lifetime is constrained by the timescale of hydrothermal cycling, at about 40 million years3.

  • Open Access
    Authors: 
    Zheng-Tong Xie; Ian P. Castro;
    Publisher: Elsevier BV
    Country: United Kingdom

    Large-eddy simulations (LES) with our recently developed inflow approach (Xie &Castro, 2008a) have been used for flow and dispersion within a genuine city area -the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Plin Central London. Numerical results up to second-order statistics are reported fora computational domain of 1.2km (streamwise) x 0.8km (lateral) x 0.2km (in fullscale), with a resolution down to approximately one meter in space and one secondin time. They are in reasonable agreement with the experimental data. Such a comprehensiveurban geometry is often, as here, composed of staggered, aligned, squarearrays of blocks with non-uniform height and non-uniform base, street canyons andintersections. Both the integrative and local effect of flow and dispersion to thesegeometrical patterns were investigated. For example, it was found that the peaksof spatially averaged urms, vrms, wrms and < u0w0 > occurred neither at the meanheight nor at the maximum height, but at the height of large and tall buildings. Itwas also found that the mean and fluctuating concentrations in the near-source fieldis highly dependent on the source location and the local geometry pattern, whereasin the far field (e.g. >0.1km) they are not. In summary, it is demonstrated thatfull-scale resolution of around one meter is sufficient to yield accurate prediction ofthe flow and mean dispersion characteristics and to provide reasonable estimationof concentration fluctuations

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