Earth and Ocean Sciences (Scholarly Articles)

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  • Publication
    Discovery and recovery of an exceptional accumulation of carboniferous echinoids at Hook Head, Wexford
    (Royal Irish Academy, 2022-09-30) Álvarez-Armada, Nidia; Murray, John; Gatley, Sarah; Ciborowski, T. Jake R.; Parkes, Matthew A.
    Geoconservation is a burgeoning area of activity within the Earth sciences and the preservation of significant fossil finds remains crucial for the advancement of palaeontological knowledge and protection of geoheritage. Here, we report on the discovery of an unusually large and remarkably well-preserved cluster of Carboniferous echinoids at Hook Head in County Wexford, Ireland. The fossil accumulation was located in the upper part of the Ballysteen Limestone Formation, which is Tournaisian in age and records sedimentation on a mixed carbonate-siliciclastic shelf/ramp. The fossil record of Paleozoic echinoids is generally poorly sampled; however, the fossil cluster from Hook Head includes at least 250 partially imbricated echinoid specimens in close association and preserves features such as the spines, Aristotle's lantern and peristomal plates. The fossil-bearing surface was precariously located on a coastal outcrop and at serious risk of being removed by storm activity, prompting an emergency rescue operation. The successful recovery of this important fossil find, which included securing official permission for extraction of the slab and its lodgement with the National Museum of Ireland, represents something of a first for Irish geological conservation.
  • Publication
    Arsenic in groundwater in south west Ireland: Occurrence, controls, and hydrochemistry
    (Frontiers Media, 2018-12-18) McGrory, Ellen; Holian, Emma; Alvarez-Iglesias, Alberto; Bargary, Norma; McGillicuddy, Eoin J.; Henry, Tiernan; Daly, Eve; Morrison, Liam; Department of Communications, Energy and Natural Resources, Ireland; Higher Edycation Authority
    Globally numerous regions have been identified with elevated arsenic within groundwater which can result in potential adverse health risks. In Ireland, a previous national-scale research assessment of groundwater identified isolated clusters of elevated arsenic and indicated that lithology was a major controlling factor on arsenic in groundwater. Complementary comparisons of national-scale and regional-scale groundwater assessments of arsenic are lacking in Europe when compared to other global regions. The aims of this study were to demonstrate the value of a regional-scale groundwater hydrochemistry dataset with an existing national-scale approach, describe anomalies that can become the focus of attention for public health and economic reasons, and to provide a wider context for arsenic in groundwater within Ireland and Europe. Regional-scale data using 470 locations comprising 1,493 analyses using several hydrochemical parameters (arsenic, pH, conductivity, iron, manganese, sodium, potassium, calcium, magnesium, and total hardness) in south west Ireland were integrated with geological, hydrogeological, and land use datasets. Statistical analysis was performed using a combination of methods including score tests of geological groups using an empirical cumulative distribution function plot in addition to spatial analysis. Results revealed that hydrochemical parameters exhibited different spatial clusters, which was generally associated with lithology. Arsenic was elevated in sandstone derived bedrock. Weak correlation of arsenic with other hydrochemical parameters were observed and redox-sensitive elements like manganese and iron showed a greater diversity in spatial occurrence. This study has shown that the variation of hydrochemical parameters are controlled by regional geology. Finally, the paper focuses on anomalies identified by concentrations of individual ions or statistical associations in the context of, for example, historical mineral exploration and mining in the area and also discusses whether groundwater chemistry sampling on this scale can assist in future mineral exploration, as well as guiding the future development of high quality public and private water supplies.
  • Publication
    Damaged beyond repair? Characterising the damage zone of a fault late in its interseismic cycle, the Alpine Fault, New Zealand
    (Elsevier, 2016-07-25) Williams, Jack N.; Toy, Virginia G.; Massiot, Cécile; McNamara, David D.; Wang, Ting; |~|
    X-ray computed tomography (CT) scans of drill-core, recovered from the first phase of the Deep Fault Drilling Project (DFDP-1) through New Zealand's Alpine Fault, provide an excellent opportunity to study the damage zone of a plate-bounding continental scale fault, late in its interseismic cycle. Documentation of the intermediate-macro scale damage zone structures observed in the CT images show that there is no increase in the density of these structures towards the fault's principal slip zones (PSZs), at least within the interval sampled, which is 30 m above and below the PSZs. This is in agreement with independent analysis using borehole televiewer data. Instead, we conclude the density of damage zone structures to correspond to lithology. We find that 72% of fractures are fully healed, by a combination of clays, calcite and quartz, with an additional 24% partially healed. This fracture healing is consistent with the Alpine Fault's late interseismic state, and the fact that the interval of damage zone sampled coincides with an alteration zone, an interval of extensive fluid-rock interaction. These fractures do not impose a reduction of P-wave velocity, as measured by wireline methods. Outside the alteration zone there is indirect evidence of less extensive fracture healing.
  • Publication
    Quantitative geometric description of fracture systems in an andesite lava flow using terrestrial laser scanner data
    (Elsevier, 2017-06-03) Massiot, Cécile; Nicol, Andrew; Townend, John; McNamara, David D.; Garcia-Sellés, David; Conway, Chris E.; Archibald, Garth; |~|
    Permeability hosted in andesitic lava flows is dominantly controlled by fracture systems, with geometries that are often poorly constrained. This paper explores the fracture system geometry of an andesitic lava flow formed during its emplacement and cooling over gentle paleo-topography, on the active Ruapehu volcano, New Zealand. The fracture system comprises column-forming and platy fractures within the blocky interior of the lava flow, bounded by autobreccias partially observed at the base and top of the outcrop. We use a terrestrial laser scanner (TLS) dataset to extract column-forming fractures directly from the point-cloud shape over an outcrop area of ∼3090 m2. Fracture processing is validated using manual scanlines and high-resolution panoramic photographs. Column-forming fractures are either steeply or gently dipping with no preferred strike orientation. Geometric analysis of fractures derived from the TLS, in combination with virtual scanlines and trace maps, reveals that: (1) steeply dipping column-forming fracture lengths follow a scale-dependent exponential or log-normal distribution rather than a scale-independent power-law; (2) fracture intensities (combining density and size) vary throughout the blocky zone but have similar mean values up and along the lava flow; and (3) the areal fracture intensity is higher in the autobreccia than in the blocky zone. The inter-connected fracture network has a connected porosity of ∼0.5 % that promote fluid flow vertically and laterally within the blocky zone, and is partially connected to the autobreccias. Autobreccias may act either as lateral permeability connections or barriers in reservoirs, depending on burial and alteration history. A discrete fracture network model generated from these geometrical parameters yields a highly connected fracture network, consistent with outcrop observations.
  • Publication
    Calcite sealing in a fractured geothermal reservoir: Insights from combined EBSD and chemistry mapping
    (Elsevier, 2016-05-10) McNamara, David D.; Lister, Aaron; Prior, Dave J.; |~|
    Fractures play an important role as fluid flow pathways in geothermal resources hosted in indurated greywacke basement of the Taupo Volcanic Zone, New Zealand, including the Kawerau Geothermal Field. Over time, the permeability of such geothermal reservoirs can be degraded by fracture sealing as minerals deposit out of transported geothermal fluids. Calcite is one such fracture sealing mineral. This study, for the first time, utilises combined data from electron backscatter diffraction and chemical mapping to characterise calcite vein fill morphologies, and gain insight into the mechanisms of calcite fracture sealing in the Kawerau Geothermal Field. Two calcite sealing mechanisms are identified 1) asymmetrical syntaxial growth of calcite, inferred by the presence of single, twinned, calcite crystals spanning the entire fracture width, and 2) 3D, interlocking growth of bladed vein calcite into free space as determined from chemical and crystallographic orientation mapping. This study also identifies other potential uses of combined EBSD and chemical mapping to understand geothermal field evolution including, potentially informing on levels of fluid supersaturation froni the study of calcite lattice distortion, and providing information on a reservoir's history of stress, strain, and deformation through investigation of calcite crystal deformation and twinning patterns. (C) 2016 Elsevier B.V. All rights reserved.
  • Publication
    A review of the Rotokawa Geothermal Field, New Zealand
    (Elsevier, 2015-07-29) McNamara, David Daniel; Sewell, Steven; Buscarlet, Etienne; Wallis, Irene C.; |~|
    The Rotokawa Geothermal Field of New Zealand has seen significant development over the last 20 years and has been the study site for new and innovative geological and geothermal research. This includes the one of the first direct data acquisition and characterisation of subsurface structure and stress properties via borehole image logs, a robust study of the magneto-telluric and micro-seismicity of the field, and the establishment of a comprehensive mode of the field's variable fluid chemistry. This paper reviews published material on the geology, geophysics, and fluid chemistry of this field, as well as summarises the development history and sustainability of this nationally important energy resource. (c) 2015 Elsevier Ltd. All rights reserved.
  • Publication
    Processing and analysis of high temperature geothermal acoustic borehole image logs in the Taupo Volcanic Zone, New Zealand
    (Elsevier, 2014-06-19) Massiot, Cécile; McNamara, David D.; Lewis, B.; |~|
    Acoustic borehole televiewer (BHTV) logs provide direct observations of lithology, structure and in-situ stress in reservoirs, essential for successful well targeting and field management. Analyses of BHTV logs acquired in twenty-three high temperature ( 288C) geothermal wells in the Taupo Volcanic Zone, New Zealand, resulted in the modification of BHTV processing techniques and the creation of a descriptive feature classification for hydrothermally altered, volcano-sedimentary-basement type reservoirs lacking other complementary information common in hydrocarbons or lower temperature geothermal systems. Lithological characteristics observed on these BHTV logs are presented, alongside an assessment of the reliability of the structural measurements and image interpretation.
  • Publication
    Heterogeneity of structure and stress in the Rotokawa Geothermal Field, New Zealand
    (American Geophysical Union (AGU), 2015-02-12) McNamara, David D.; Massiot, Cécile; Lewis, Brandon; Wallis, Irene C.; |~|
    Geometric characterization of a geothermal reservoir's structures, and their relation to stress field orientation, is vital for resource development. Subsurface structure and stress field orientations of the Rotokawa Geothermal Field, New Zealand, have been studied, for the first time, using observations obtained from analysis of three acoustic borehole televiewer logs. While an overall NE-SW fracture strike exists, heterogeneity in fracture dip orientation is evident. Dominant dip direction changes from well to well due to proximity to variously oriented, graben-bounding faults. Fracture orientation heterogeneity also occurs within individual wells, where fractures clusters within certain depth intervals have antithetic dip directions to the well's dominant fracture dip direction. These patterns are consistent with expected antithetic faulting in extensional environments. A general S-Hmax orientation of NE-SW is determined from induced features on borehole walls. However, numerous localized azimuthal variations from this trend are evident, constituting stress field orientation heterogeneity. These variations are attributed to slip on fracture planes evidenced by changes in the azimuth of drilling-induced tensile fractures either side of a natural fracture. Correlation of observed fracture properties and patterns to well permeability indicators reveal that fractures play a role in fluid flow in the Rotokawa geothermal reservoir. Permeable zones commonly contain wide aperture fractures and high fracture densities which have a dominant NE-SW strike orientation and NW dip direction. Studies of this kind, which show strong interdependency of structure and stress field properties, are essential to understand fluid flow in geothermal reservoirs where structural permeability dominates.
  • Publication
    Late-interseismic state of a continental plate-bounding fault: Petrophysical results from DFDP-1 wireline logging and core analysis, Alpine Fault, New Zealand
    (American Geophysical Union (AGU), 2013) Townend, John; Sutherland, R.; Toy, V. G.; Eccles, J. D.; Boulton, C.; Cox, S. C.; McNamara, David D.; |~|
    We present a geophysical characterization at 0.1-100 m scales of a major plate-bounding continental fault in a late-interseismic state. The Alpine Fault produces M-W approximate to 8 earthquakes every 200-400 years and last ruptured in 1717 AD. Wireline geophysical logs and rock cores extending from one side of the Alpine Fault to the other were acquired in two boreholes drilled in 2011 at Gaunt Creek during the first phase of the Deep Fault Drilling Project (DFDP-1). These data document ambient conditions under which the next Alpine Fault earthquake will occur. Principal component analysis of the wireline logging data reveals that >80% of the variance is accounted for by electrical, acoustic, and natural gamma properties, and preliminary multivariate classification enables the lithologies of sections of missing core to be reconstructed from geophysical measurements. The fault zone exhibits systematic variations in properties consistent with common processes of progressive alteration and comminution near the principal slip zone, superimposed on different protolith compositions. Our observations imply that the fault zone has the opposite sense of elastic asymmetry at 0.1-100 m scales to that of the crustal-scale orogen imaged by remote geophysical methods. On the basis of the fault-zone scale asymmetry, the bimaterial interface model of preferred earthquake rupture directions implies a northeastward direction of preferred Alpine Fault rupture. On-going characterization of the structural and hydraulic architecture of the Alpine Fault will improve our understanding of the relationship between in situ conditions, earthquake rupture processes, and the hazards posed by future earthquakes.
  • Publication
    Fabrics produced mimetically during static metamorphism in retrogressed eclogites from the Zermatt-Saas zone, Western Italian Alps
    (Elsevier, 2012-09-12) McNamara, David D.; Wheeler, J.; Pearce, M.; Prior, D.J.; |~|
    Lattice preferred orientations (LPOs) are commonly interpreted to form by dislocation creep. Consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable, especially if those minerals show a shape fabric. Here we show that LPOs can occur through mimicry of a pre-existing LPO, so they formed statically, not during deformation. Omphacite and glaucophane LPOs occur in eclogite facies rocks from the Zermatt-Saas Unit of the Northwest Italian Alps. Barroisite grew during greenschist facies retrogression and has an LPO controlled significantly by the eclogite fades omphacite and glaucophane LPOs, rather than directly by deformation. Using spatially resolved lattice orientation data from the three key minerals, collected using electron backscatter diffraction, we deploy a new technique of interphase misorientation distribution analysis to prove this. Barroisite LPO develops by mimicry of omphacite (via a particular lattice orientation relationship) and by direct topotactic and epitactic replacement of glaucophane. LPO in turn influenced anisotropic grain growth, resulting in a barroisite grain shape fabric. Thus regional retrogression during exhumation of the Zermatt-Saas high-pressure rocks was, in large part, static, rather than dynamic as previously interpreted. In general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation, in both crust and mantle. (C) 2012 Elsevier Ltd. All rights reserved.
  • Publication
    An energy-driven geologist
    (The New Zealand Association of Scientists, 2013) McNamara, David D.; |~|
    [No abstract available]
  • Publication
    Identifying the sources and sinks of CDOM/FDOM across the Mauritanian Shelf and their potential role in the decomposition of superoxide (O2- )
    (Frontiers Media, 2016-08-03) Heller, Maija I.; Wuttig, Kathrin; Croot, Peter L.; |~|1267880|~|
    Superoxide (O−2O2-) is a short lived reactive oxygen species (ROS) formed in seawater by photochemical or biological sources, it is important in the redox cycling of trace elements and organic matter in the ocean. The photoproduction of O−2O2- is now thought to involve reactions between O2 and reactive reducing (radical) intermediates formed from dissolved organic matter (DOM) via intramolecular reactions between excited singlet state donors and ground-state acceptors (Zhang et al., 2012). In seawater the main pathways identified for the decomposition of O−2O2- into H2O2 and O2, involve reactions with Cu, Mn, and DOM. In productive regions of the ocean, the reaction between DOM and O−2O2- can be a significant sink for O−2O2-. Thus, DOM is a key component of both the formation and decomposition of O−2O2- and formation of H2O2. In the present work we examined the relationships between O−2O2- decay rates and parameters associated with chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) by using the thermal O−2O2- source SOTS-1. Filtered samples (0.2 μm) were run both in the presence, and absence, of the metal chelator diethylenetriaminepentaacetic acid (DTPA) to determine the contribution from DOM. Samples were collected along a transect across the continental shelf of the Mauritanian continental shelf during a period of upwelling. In this region we found that reactions with DOM, are a significant sink for O−2O2- in the Mauritanian Upwelling, constituting on average 58 ± 13% of the O−2O2- loss rates. Superoxide reactivity with organic matter showed no clear correlation with bulk CDOM or FDOM properties (as assessed by PARAFAC analysis) suggesting that future work should concentrate at the functional group level to clearly elucidate which molecular species are involved as bulk properties represent a wide spread of chemical moieties with different O−2O2- reactivities. Analysis of FDOM parameters indicates that many of the markers used previously for terrestrial sources of DOM and FDOM are called into question as marine sources exist. In particular recent work (Rico et al., 2013) indicates that algal species may also produce syringic, vanillic, and cinnamic acids, which had previously been ascribed solely to terrestrial vegetation.