Sharon Walker - Academia.edu (original) (raw)

Papers by Sharon Walker

Frontiers in Marine Science, 2019

The distribution of hydrothermal venting reveals important clues about the presence of magma in s... more The distribution of hydrothermal venting reveals important clues about the presence of magma in submarine settings. The NE Lau Basin in the southwest Pacific Ocean is a complex back-arc region of widespread hydrothermal activity. It includes spreading ridges, arc volcanoes, and intra-plate volcanoes that provide a perhaps unique laboratory for studying interactions between hydrothermal activity and magma sources. Since 2004, multiple cruises have explored the water column of the NE Lau Basin. Here, we use these data to identify and characterize 43 active hydrothermal sites by means of optical, temperature, and chemical tracers in plumes discharged by each site. Seventeen of 20 prominent volcanic edifices dispersed among the Tofua arc, spreading ridges, and plate interiors host active hydrothermal sites. Fourteen apparently discharge high-temperature fluids, including a multi-year submarine eruption at the intraplate volcano W Mata. The 430 km of spreading ridges host 31 active sites, one an eruption event in 2008. Our data show that the relationship between site spatial density (sites/100 km of ridge crest) and ridge spreading rate (8-42 mm/year) in the NE Lau Basin follows the same linear trend as previously established for the faster-spreading (40-90 mm/year) ridges in the central Lau Basin. The lower site density in the NE Lau Basin compared to the southern Lau is consistent with recent plate reconstructions that more than halved earlier estimates of ∼50-100 mm/year spreading rates in the NE Lau Basin. Combined data from the spreading ridges throughout the entire Lau back-arc basin demonstrates that hydrothermal sites, normalized to spreading rate, are ∼10× more common than expected based on existing mid-ocean ridge data. This increase documents the ability of meticulous exploration, using both turbidity and chemical sensors, to more fully describe the true hydrothermal population of a spreading ridge, compared to conventional techniques. It further reveals that the Lau back-arc basin, benefiting from both ridge and arc magma sources, supports an exceptionally high population of ridge and intra-plate hydrothermal sites.

Journal of Volcanology and Geothermal Research, 2016

A new technique for measuring conductive heat flux in a lake was adapted from the marine environm... more A new technique for measuring conductive heat flux in a lake was adapted from the marine environment to allow for multiple measurements to be made in areas where bottom sediment cover is sparse, or even absent. This thermal blanket technique, pioneered in the deep ocean for use in volcanic mid-ocean rift environments, was recently used in the geothermally active Lake Rotomahana, New Zealand. Heat flow from the lake floor propagates into the 0.5 m diameter blanket and establishes a thermal gradient across the known blanket thickness and thereby provides an estimate of the conductive heat flux of the underlying terrain. This approach allows conductive heat flux to be measured over a spatially dense set of stations in a relatively short period of time. We used 10 blankets and deployed them for 1 day each to complete 110 stations over an 11-day program in the 6 × 3 km lake. Results show that Lake Rotomahana has a total conductive heat flux of about 47 MW averaging 6 W/m 2 over the geothermally active lake. The western half of the lake has two main areas of high heat flux; 1) a high heat flux area averaging 21.3 W/m 2 along the western shoreline, which is likely the location of the pre-existing geothermal system that fed the famous Pink Terraces, mostly destroyed during the 1886 eruption 2) a region southwest of Patiti Island with a heat flux averaging 13.1 W/m 2 that appears to be related to the explosive rift that formed the lake in the 1886 Tarawera eruption. A small rise in bottom water temperature over the survey period of 0.01°C/day suggests the total thermal output of the lake is~112-132 MW and when compared to the conductive heat output suggests that 18-42% of the total thermal energy is by conductive heat transfer.

During the 2005-06 GalAPAGoS expedition, we conducted nested sonar, plume, and camera surveys alo... more During the 2005-06 GalAPAGoS expedition, we conducted nested sonar, plume, and camera surveys along a 300 n.m.-long portion of the Galapagos Spreading Center (GSC) where the ridge intersects the Galapagos hotspot between 94.5° and 89.5°W. Hydrothermal plumes were located by placing a variety of sensors on the clump weight of the DSL-120, which was towed approximately 100 m above the seafloor. These sensors included the vents in situ analyzer (nee SUAVE), which measured Fe, Mn, and pH; a redox potential (Eh) sensor; an optical backscatter sensor; a METs methane analyzer; and a CTD. These sensors were uploaded in real time allowing us to monitor for plumes during the tow. In addition, the tow line of the DSL-120 carried a vertical array of optical backscatter sensors (MAPRs). Almost all of the plumes found were identified solely from the data uploaded to the ship in real time and were confirmed from the vertical MAPR array. The MAPR array provided information on the rise height and ve...

Economic Geology, 2012

Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ 3 He, pH, di... more Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ 3 He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ~3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable δ 3 He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land.

Economic Geology

Brothers volcano is arguably the most well-studied submarine arc volcano on Earth. Between 1996, ... more Brothers volcano is arguably the most well-studied submarine arc volcano on Earth. Between 1996, when massive sulfides were first recovered by dredging, and 2018, when International Ocean Discovery Program (IODP) Expedition 376 recovered cores from as deep as 453 m below the sea floor at two chemically distinct hydrothermal upflow zones, over 60 conductivity-temperature-depth (CTD) vertical casts and tow-yo operations mapped hydrothermal plumes over and around the edifice by employing hydrothermal tracer-specific sensors. These surveys started in 1999 and were completed during nine separate expeditions at one- to three-year intervals, except for a six-year gap between 2011 and 2017. Hydrothermal plume distributions over this two-decade period show variability in the intensity and vertical rise height of plumes from the four main vent fields (Upper Cone, Lower Cone, NW Caldera, and Upper Caldera, with the latter not discovered until 2017). Upper Cone plumes were more intense than all...

This data set was acquired with a Sea-Bird SBE-49 FastCAT CTD and APS 1540 Magnetometer on the AU... more This data set was acquired with a Sea-Bird SBE-49 FastCAT CTD and APS 1540 Magnetometer on the AUV Sentry during Falkor expedition FK151121 conducted in 2015 (Chief Scientist: Dr. Joseph Resing). These data files are of Text File (ASCII) format and include Pressure, Temperature, and Conductivity data that have not been processed. Data were acquired as part of the project(s): Hydrothermal Hunt at Mariana. Funding was provided by NOAA-OER-2016-003 and SOI-FK151121.

The ISME Journal, 2022

Hydrothermal plumes transport reduced chemical species and metals into the open ocean. Despite th... more Hydrothermal plumes transport reduced chemical species and metals into the open ocean. Despite their considerable spatial scale and impact on biogeochemical cycles, niche differentiation of abundant microbial clades is poorly understood. Here, we analyzed the microbial ecology of two bathy- (Brothers volcano; BrV-cone and northwest caldera; NWC) and a mesopelagic (Macauley volcano; McV) plumes on the Kermadec intra-oceanic arc in the South Pacific Ocean. The microbial community structure, determined by a combination of 16S rRNA gene, fluorescence in situ hybridization and metagenome analysis, was similar to the communities observed in other sulfur-rich plumes. This includes a dominance of the vent characteristic SUP05 clade (up to 22% in McV and 51% in BrV). In each of the three plumes analyzed, the community was dominated by a different yet uncultivated chemoautotrophic SUP05 species, here, provisionally named, Candidatus Thioglobus vadi (McV), Candidatus Thioglobus vulcanius (BrV-...

This data set was acquired with a NOAA-PMEL MAPR device mounted on AUV Sentry during Falkor exped... more This data set was acquired with a NOAA-PMEL MAPR device mounted on AUV Sentry during Falkor expedition FK151121 conducted in 2015 (Chief Scientist: Dr. Joseph Resing). These data files are of Text File (ASCII) format and include Conductivity, Pressure, Temperature, and Optical Backscatter data that have not been processed. Data were acquired as part of the project(s): Hydrothermal Hunt at Mariana. Funding was provided by NOAA-OER-2016-003 and SOI-FK151121.

Deep Sea Research Part I: Oceanographic Research Papers

Journal of Volcanology and Geothermal Research, 2015

Autonomous underwater vehicles were used to conduct a high-resolution water column survey of Lake... more Autonomous underwater vehicles were used to conduct a high-resolution water column survey of Lake Rotomahana using temperature, pH, turbidity, and oxidation-reduction potential (ORP) to identify active hydrothermal discharge zones within the lake. Five areas with active sublacustrine venting were identified: (1) the area of the historic Pink Terraces; (2) adjacent to the western shoreline subaerial "Steaming Cliffs," boiling springs and geyser; (3) along the northern shoreline to the east of the Pink Terrace site; (4) the newly discovered Patiti hydrothermal system along the south margin of the 1886 Tarawera eruption rift zone; and (5) a location in the east basin (northeast of Patiti Island). The Pink Terrace hydrothermal system was active prior to the 1886 eruption of Mount Tarawera, but venting along the western shoreline, in the east basin, and the Patiti hydrothermal system appear to have been initiated in the aftermath of the eruption, similar to Waimangu Valley to the southwest. Different combinations of turbidity, pH anomalies (both positive and negative), and ORP responses suggest vent fluid compositions vary over short distances within the lake. The seasonal period of stratification limits vertical transport of heat to the surface layer and the hypolimnion temperature of Lake Rotomahana consequently increases with an average warming rate of~0.010°C/day due to both convective hydrothermal discharge and conductive geothermal heating. A sudden temperature increase occurred during our 2011 survey and was likely the response to an earthquake swarm just 11 days prior.

Oceanography

Deep-sea volcanic eruptions create unique chemical and biological linkages between the subsurface... more Deep-sea volcanic eruptions create unique chemical and biological linkages between the subsurface lithosphere and the oceanic hydrosphere.

Frontiers in Marine Science

NW Rota-1 is an actively erupting submarine volcano along the Mariana Arc, rising some 2500 m fro... more NW Rota-1 is an actively erupting submarine volcano along the Mariana Arc, rising some 2500 m from the local seafloor. Eruptions at Brimstone Pit, located about 30 m below the summit on the south side of the volcano at 550 m water depth, have been directly observed since 2004. Water column surveys (using CTD-O vertical cast and tow methods) in

President of the Board of the Monarch School, conducting a live interaction from the studio on bo... more President of the Board of the Monarch School, conducting a live interaction from the studio on board Nautilus with 7 th and 8 th grade students at Monarch, a K-12 school for children affected by homelessness in San diego, California.

Marine Geology, 1988

... grained particles including sulfides and sul-fates, Fe-oxyhydroxides, Fe~Si phases, Ca~Si pha... more ... grained particles including sulfides and sul-fates, Fe-oxyhydroxides, Fe~Si phases, Ca~Si phases, and many others (Haymon and Kastner, 1981; Leinen ... for samples on the (a) 191~m aperture and (b) 5011m aperture, c. Replicate Coulter analyses on the 19 tim aperture for ...

Geochemistry, Geophysics, Geosystems, 2014

Multiple geological processes affect the distribution of hydrothermal venting along a midocean ri... more Multiple geological processes affect the distribution of hydrothermal venting along a midocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65-71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77 Eand99 E to examine the spatial density of hydrothermal venting relative to regional and segment-scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation-reduction potential along 27 first and second-order ridge segments on and adjacent to the Amsterdam-St. Paul (ASP) Plateau to map p h , the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma-thickened hot spot plateau is almost entirely suppressed (p h 5 0.02). Conversely, the combined p h (0.34) from all other segments follows the global trend of p h versus spreading rate. Off the ASP Plateau, multisegment trends in p h track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between p h and cross-axis inflation for first-order segments shows that different magmatic budgets among first-order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among second-order segments. Eighty-five percent of the plumes occur in eight clusters totaling 350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.