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  • Articles  (77)
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  • 1
    Electronic Resource
    Electronic Resource
    Seismic and hydrothermal evidence for a cracking event on the East Pacific Rise crest at 9° 50′ N (1998)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 396 (1998), S. 159-161 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Interaction between the hydrothermal system and the axial magma chamber at a mid-ocean ridge spreading centre takes place in a boundary layer of crust that separates circulating sea water from basaltic melt. The nature of heat flow through this region is critical because it determines the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/24146
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  • 2
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    Long-baseline acoustic navigation for under-ice autonomous underwater vehicle operations (2009)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Journal of Field Robotics 25 (2008): 861-879, doi:10.1002/rob.20250.
    Description: The recent Arctic GAkkel Vents Expedition (AGAVE) to the Arctic Ocean’s Gakkel Ridge (July/August 2007) aboard the Swedish ice-breaker I/B Oden employed autonomous underwater vehicles (AUVs) for water-column and ocean bottom surveys. These surveys were unique among AUV operations to date in requiring georeferenced navigation in proximity to the seafloor beneath permanent and moving ice cover. We report results for long-baseline (LBL) acoustic navigation during autonomous under-ice surveys near the seafloor and adaptation of the LBL concept for several typical operational situations including navigation in proximity to the ship during vehicle recoveries. Fixed seafloor transponders were free-fall deployed from the ship for deep positioning. The ship’s helicopter collected acoustic travel times from several locations to geo-reference the transponders’ locations, subject to the availability of openings in the ice. Two shallow beacons suspended from the ship provided near-surface spherical navigation in ship-relative coordinates. During routine recoveries, we used this system to navigate the vehicles into open water near the ship before commanding them to surface. In cases where a vehicle was impaired, its position was still determined acoustically through some combination of its acoustic modem, the fixed seafloor transponders, the ship-deployed transponders, and an on-board backup relay transponder. The techniques employed included ranging adapted for a moving origin and hyperbolic navigation.
    Description: Major funding for AGAVE was provided by the National Science Foundation (PP-0425838, ATM- 0428122), the National Aeronautics and Space Administration (Z601701), and the Woods Hole Oceanographic Institution.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
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    Tidal modulation of Sr/Ca ratios in a Pacific reef coral (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 31 (2004): L16310, doi:10.1029/2004GL020600.
    Description: The strontium-to-calcium ratio (Sr/Ca) of reef coral skeleton is an important tool for reconstructing past sea surface temperatures (SSTs). However, the accuracy of paleoSSTs derived from fossil coral Sr/Ca is challenged by evidence that physiological processes influence skeletal chemistry. Here we show that water level variations from tidal forcing are correlated with changes in coral Sr/Ca that cannot be accounted for by changes in SST. Ion microprobe measurements of Sr/Ca ratios in a Pacific Porites lutea reveal high-frequency variations at periods of ~6, ~10, and ~25 days. The relationship between Sr/Ca and temperature on these short timescales does not follow trends observed at longer periods, indicating that an additional forcing is required to explain our observations. We demonstrate that Sr/Ca is correlated with both tidal water level variations and SST, and that their contributions to the Sr/Ca content of the skeleton vary as a function of period. We propose that water level influences Sr/Ca indirectly via modulation of photosynthetically-active radiation (PAR) that drives large changes in zooxanthellate photosynthesis.
    Description: This research was supported by WHOI Ocean Life Institute grant 25051316 to ALC; NSF grants EAR-9628749 and EAR-9904400 to the WHOI Northeast National Ion Microprobe Facility; DAMD 17-93-J-3052 supported ALC’s fieldwork on JA.
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Microearthquake patterns following the 1998 eruption of Axial Volcano, Juan de Fuca Ridge : mechanical relaxation and thermal strain (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B01101, doi:10.1029/2003JB002499.
    Description: Ocean bottom seismic networks deployed following the 1998 eruption of Axial seamount reveal an evolving pattern of microearthquake activity associated with subsurface magmatism and thermal strain. Seismicity rates decay steadily over 15 months of observation (February 8, 1998, to April 30, 1999), consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event. Immediately after the eruption, seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped. A small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption. These events, which are characterized by slip on nearly vertical faults in the shallow crust, extend about 6 km from the southeast corner of the caldera and overlie a mid-crustal low-velocity zone. After this episode, seismicity rates remain low until the end of the observation period, 455 days after the eruption. Shallow (~0.7 km depth) events, consistent with thermal contraction and volume changes of ~2 × 10−3 m3 in ~5 m3 sources, are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera. Microearthquakes observed during the last 70 days of observation are distributed around the central caldera, most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption.
    Description: Sohn, Webb, and the field program were supported by NSF grant OCE 97- 11700. Barclay was supported in part by the Woods Hole Oceanographic Institution.
    Keywords: Microearthquakes ; Hydrothermal ; Magmatism
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Heterogeneous and asymmetric crustal accretion : new constraints from multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (36°15'N) (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 2994–3014, doi:10.1002/2015GC005743.
    Description: At slow-spreading mid-ocean ridges, crustal accretion style can vary significantly along and across ridge segments. In magma-poor regions, seafloor spreading can be accommodated largely by tectonic processes, however, the internal structure and formation mechanism of such highly tectonized crust are not fully understood. We analyze multibeam bathymetry and potential field data from the Rainbow area of the Mid-Atlantic Ridge (35°40'N–36°40'N), a section of the ridge that shows diverse accretion styles. We identify volcanic, tectonized and sedimented terrain and measure exposed fault area to estimate the tectonic strain, T, and the fraction of magmatic accretion, M. Estimated T values range from 0.2–0.4 on ridge segments to 0.6-0.8 at the Rainbow nontransform discontinuity (NTD). At segment ends T is asymmetric, reflecting asymmetries in accretion rate, topography and faulting between inside and outside offset corners. Detachment faults have formed preferentially at inside corners, where tectonic strain is higher. We identify at least two oceanic core complexes on the fossil trace of the NTD, in addition to the Rainbow massif, which occupies the offset today. A gravity high and low magnetization suggest that the Rainbow massif, which hosts a high-temperature hydrothermal system, was uplifted by a west dipping detachment fault. Asymmetric plate ages indicate localization of tectonic strain at the inside corners and migration of the detachment toward and across the ridge axis, which may have caused emplacement of magma into the footwall. Hydrothermal circulation and heat extraction is possibly favored by increased permeability generated by fracturing of the footwall and deep-penetrating second-generation faults.
    Description: NSF Grant Numbers: OCE-0961151 , OCE-0961680
    Description: 2016-03-13
    Keywords: Oceanic detachment faults ; Geomorphology ; Crustal accretion ; Slow-spreading ridges ; Nontransform discontinuities ; Gravity anomalies
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Segment-scale variations in seafloor volcanic and tectonic processes from multibeam sonar imaging, Mid-Atlantic Ridge Rainbow region (35°45′–36°35′N) (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 3560–3579, doi:10.1002/2016GC006433.
    Description: Along-axis variations in melt supply and thermal structure can lead to significant variations in the mode of crustal accretion at mid-ocean ridges. We examine variations in seafloor volcanic and tectonic processes on the scale of individual ridge segments in a region of the slow spreading Mid-Atlantic Ridge (35°45′–36°35′N) centered on the Rainbow nontransform discontinuity (NTD). We use multibeam sonar backscatter amplitude data, taking advantage of multifold and multidirectional coverage from the MARINER geophysical study to create a gridded compilation of seafloor reflectivity, and interpret the sonar image within the context of other data to examine seafloor properties and identify volcanic flow fields and tectonic features. Along the spreading segments, differences in volcanic productivity, faulting, eruption style, and frequency correlate with inferred magma supply. Regions of low magma supply are associated with more widely spaced faults, and larger volcanic flow fields that are more easily identified in the backscatter image. Identified flow fields with the highest backscatter occur near the ends of ridge segments. Their relatively smooth topography contrasts with the more hummocky, cone-dominated terrain that dominates most of the neovolcanic zone. Patches of seafloor with high, moderately high, and low backscatter intensity across the Rainbow massif are spatially correlated with observations of basalt, gabbro and serpentinized peridotite, and sediment, respectively. Large detachment faults have repeatedly formed along the inside corners of the Rainbow NTD, producing a series of oceanic core complexes along the wake of the NTD. A new detachment fault is currently forming in the ridge segment just north of the now inactive Rainbow massif.
    Description: National Science Foundation Grant Numbers: OCE-0961151, OCE-0961680
    Description: 2017-03-07
    Keywords: Mid-ocean ridges ; Oceanic core complex ; Rainbow massif ; Backscatter
    Repository Name: Woods Hole Open Access Server
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  • 7
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    Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean (2009)
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    Publication Date: 2022-05-25
    Description: Author Posting. © Nature Publishing Group, 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 453 (2008): 1236-1238, doi:10.1038/nature07075.
    Description: Roughly 60% of the Earth’s outer surface is comprised of oceanic crust formed by volcanic processes at mid-ocean ridges (MORs). Although only a small fraction of this vast volcanic terrain has been visually surveyed and/or sampled, the available evidence suggests that explosive eruptions are rare on MORs, particularly at depths below the critical point for steam (3000 m). A pyroclastic deposit has never been observed on the seafloor below 3000 m, presumably because the volatile content of mid-ocean ridge basalts is generally too low to produce the gas fractions required to fragment a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel Ridge in the Arctic Basin at 85°E, to acquire the first-ever photographic images of ‘zero-age’ volcanic terrain on this remote, ice-covered MOR. Our imagery reveals that the axial valley at 4000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large area greater than 10 km2. At least 13.5 wt% CO2 is required to fragment magma at these depths, which is ~10x greater than the highest values measured to-date in a MOR basalt. These observations raise important questions regarding the accumulation and discharge of magmatic volatiles at ultra-slow spreading rates on the Gakkel Ridge (6- 14 mm yr-1, full-rate), and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global MOR volcanic system.
    Description: This research was funded by the National Aeronautics and Space Administration, the National Science Foundation, and the Woods Hole Oceanographic Institution.
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  • 8
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    Fine-scale seismic structure of the shallow volcanic crust on the East Pacific Rise at 9°50′N (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B12104, doi:10.1029/2004JB003152.
    Description: We use a combination of body wave and interface wave observations from an on-bottom seismic refraction survey to constrain the fine-scale seismic structure of the upper crust in a ∼3 × 3 km field area centered on the East Pacific Rise at 9°50′N. We detonated 18 explosive shots (18 sources) in a circular pattern (1.5 km radius) on the rise axis and recorded seismic arrivals with eight ocean bottom seismometers (eight receivers). We observed 30–40 Hz compressional body waves from all shots (144 P waves) and 1–3 Hz Stoneley (interface) waves on a subset of source-receiver pairs (58 interface waves). Using a station correction inversion, we find that roughly half of the variance in the P wave first-arrival times results from lateral variations in the thickness of the surface low-velocity layer (SLVL), a layer of extremely porous lava and basalt breccia with an average P wave velocity of 2.2 km s−1. The SLVL thickness increases from 〈20 m along the axial summit trough (AST) to ∼120 m at near-axis lava depocenters, which are not symmetric about the rise axis. Depocenters are located ∼0.5 km to the west and ∼1.5 km to the east of the rise axis. Tomographic inversion of the Stoneley wave first arrivals reveals that shear velocities in the SLVL covary with the layer thickness, exhibiting a similar asymmetric pattern, with shear velocities increasing from ∼320 m s−1 near the AST to ∼520 m s−1 at the near-axis depocenters. Our analysis demonstrates that the seismic characteristics of the extrusive layer near the rise axis are related primarily to volcanic features and processes. The thickness and velocity of the SLVL are low on the axis and within channel networks that deliver lava flows away from the axis and then increase rapidly at the distal ends of the channels where the lavas are deposited. We find that azimuthal anisotropy exerts only a weak influence on our P wave first-arrival times, which we model as weak (4%) seismic azimuthal anisotropy in the upper dikes with a fast axis oriented N23°–32°W. We find no evidence for seismic azimuthal anisotropy in the extrusive layer.
    Keywords: Volcanic ; Seismic ; Structure
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Analysis and modeling of hydrothermal plume data acquired from the 85°E segment of the Gakkel Ridge (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 115 (2010): C06028, doi:10.1029/2009JC005776.
    Description: We use data from a CTD plume-mapping campaign conducted during the Arctic Gakkel Vents (AGAVE) expedition in 2007 to constrain the nature of hydrothermal processes on the Gakkel Ridge at 85°E. Thermal and redox potential (Eh) anomalies were detected in two discrete depth intervals: 2400–2800 m (Interval 1) and 3000–3800 m (Interval 2). The spatial and temporal patterns of the signals indicate that the Interval 1 anomalies were most likely generated by a single large, high-temperature (T 〉 100°C) vent field located on the fault terraces that form the NE axial valley wall. In contrast, the Interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley, many of which may be sites of diffuse, low-temperature (T 〈 10°C) discharge. Numerical simulations of turbulent plumes rising in a weakly stratified Arctic Ocean water column indicate that the high-temperature field on the axial valley wall has a thermal power of ∼1.8 GW, similar to the Trans-Atlantic Geotraverse and Rainbow fields in the Atlantic Ocean, whereas the sites on the axial valley floor have values ranging from 5 to 110 MW.
    Description: Thiswork was funded by the NSF Office of Polar Programs, Tellus—The Centre of Earth Systems Science at theUniversity of Gothenburg, and the Woods Hole Oceanographic Institution.
    Keywords: Hydrothermal plumes ; Gakkel Ridge ; Hydrothermal vents
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  • 10
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    Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system (2017)
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    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geology 45 (2017): 451-454, doi:10.1130/G38795.1.
    Description: Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P-wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ∼2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.
    Description: This research was funded by National Science Foundation (NSF) grants OCE-0961680 and OCE-0961151.
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