Gustafson Chloe

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Gustafson
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Chloe
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  • Article
    Bayesian joint inversion of controlled source electromagnetic and magnetotelluric data to image freshwater aquifer offshore New Jersey
    (Oxford University Press, 2019-06-03) Blatter, Daniel ; Key, Kerry ; Ray, Anandaroop ; Gustafson, Chloe ; Evans, Rob L.
    Joint inversion of multiple electromagnetic data sets, such as controlled source electromagnetic and magnetotelluric data, has the potential to significantly reduce uncertainty in the inverted electrical resistivity when the two data sets contain complementary information about the subsurface. However, evaluating quantitatively the model uncertainty reduction is made difficult by the fact that conventional inversion methods—using gradients and model regularization—typically produce just one model, with no associated estimate of model parameter uncertainty. Bayesian inverse methods can provide quantitative estimates of inverted model parameter uncertainty by generating an ensemble of models, sampled proportional to data fit. The resulting posterior distribution represents a combination of a priori assumptions about the model parameters and information contained in field data. Bayesian inversion is therefore able to quantify the impact of jointly inverting multiple data sets by using the statistical information contained in the posterior distribution. We illustrate, for synthetic data generated from a simple 1-D model, the shape of parameter space compatible with controlled source electromagnetic and magnetotelluric data, separately and jointly. We also demonstrate that when data sets contain complementary information about the model, the region of parameter space compatible with the joint data set is less than or equal to the intersection of the regions compatible with the individual data sets. We adapt a trans-dimensional Markov chain Monte Carlo algorithm for jointly inverting multiple electromagnetic data sets for 1-D earth models and apply it to surface-towed controlled source electromagnetic and magnetotelluric data collected offshore New Jersey, USA, to evaluate the extent of a low salinity aquifer within the continental shelf. Our inversion results identify a region of high resistivity of varying depth and thickness in the upper 500 m of the continental shelf, corroborating results from a previous study that used regularized, gradient-based inversion methods. We evaluate the joint model parameter uncertainty in comparison to the uncertainty obtained from the individual data sets and demonstrate quantitatively that joint inversion offers reduced uncertainty. In addition, we show how the Bayesian model ensemble can subsequently be used to derive uncertainty estimates of pore water salinity within the low salinity aquifer.
  • Article
    Aquifer systems extending far offshore on the US Atlantic margin.
    (Nature Research, 2019-06-18) Gustafson, Chloe ; Key, Kerry ; Evans, Rob L.
    Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.
  • Article
    Scientific access into Mercer Subglacial Lake: scientific objectives, drilling operations and initial observations
    (Cambridge University Press, 2021-01-08) Priscu, John C. ; Kalin, Jonas ; Winans, John ; Campbell, Timothy ; Siegfried, Matthew R. ; Skidmore, Mark ; Dore, John E. ; Leventer, Amy ; Harwood, David M. ; Duling, Dennis ; Zook, Robert ; Burnett, Justin ; Gibson, Dar ; Krula, Edward ; Mironov, Anatoly ; McManis, James ; Roberts, Graham ; Rosenheim, Brad E. ; Christner, Brent C. ; Kasic, Kathy ; Fricker, Helen A. ; Lyons, W. Berry ; Barker, Joel ; Bowling, Mark ; Collins, Billy ; Davis, Christina ; Gagnon, Alan R. ; Gardner, Christopher B. ; Gustafson, Chloe ; Kim, Ok-Sun ; Li, Wei ; Michaud, Alex ; Patterson, Molly O. ; Tranter, Martyn ; Venturelli, Ryan ; Vick-Majors, Trista ; Elsworth, Cooper
    The Subglacial Antarctic Lakes Scientific Access (SALSA) Project accessed Mercer Subglacial Lake using environmentally clean hot-water drilling to examine interactions among ice, water, sediment, rock, microbes and carbon reservoirs within the lake water column and underlying sediments. A ~0.4 m diameter borehole was melted through 1087 m of ice and maintained over ~10 days, allowing observation of ice properties and collection of water and sediment with various tools. Over this period, SALSA collected: 60 L of lake water and 10 L of deep borehole water; microbes >0.2 μm in diameter from in situ filtration of ~100 L of lake water; 10 multicores 0.32–0.49 m long; 1.0 and 1.76 m long gravity cores; three conductivity–temperature–depth profiles of borehole and lake water; five discrete depth current meter measurements in the lake and images of ice, the lake water–ice interface and lake sediments. Temperature and conductivity data showed the hydrodynamic character of water mixing between the borehole and lake after entry. Models simulating melting of the ~6 m thick basal accreted ice layer imply that debris fall-out through the ~15 m water column to the lake sediments from borehole melting had little effect on the stratigraphy of surficial sediment cores.
  • Article
    Constraints on the timing and extent of deglacial grounding line retreat in West Antarctica
    (American Geophysical Union, 2023-04-26) Venturelli, Ryan A. ; Boehman, Brenna ; Davis, Christina ; Hawkings, Jon R. ; Johnston, Sarah E. ; Gustafson, Chloe D. ; Michaud, Alexander B. ; Mosbeux, Cyrille ; Siegfried, Matthew R. ; Vick‐Majors, Trista J. ; Galy, Valier ; Spencer, Robert G. M. ; Warny, Sophie ; Christner, Brent C. ; Fricker, Helen A. ; Harwood, David M. ; Leventer, Amy ; Priscu, John C. ; Rosenheim, Brad E.
    Projections of Antarctica's contribution to future sea level rise are associated with significant uncertainty, in part because the observational record is too short to capture long‐term processes necessary to estimate ice mass changes over societally relevant timescales. Records of grounding line retreat from the geologic past offer an opportunity to extend our observations of these processes beyond the modern record and to gain a more comprehensive understanding of ice‐sheet change. Here, we present constraints on the timing and inland extent of deglacial grounding line retreat in the southern Ross Sea, Antarctica, obtained via direct sampling of a subglacial lake located 150 km inland from the modern grounding line and beneath >1 km of ice. Isotopic measurements of water and sediment from the lake enabled us to evaluate how the subglacial microbial community accessed radiocarbon‐bearing organic carbon for energy, as well as where it transferred carbon metabolically. Using radiocarbon as a natural tracer, we found that sedimentary organic carbon was microbially translocated to dissolved carbon pools in the subglacial hydrologic system during the 4.5‐year period of water accumulation prior to our sampling. This finding indicates that the grounding line along the Siple Coast of West Antarctica retreated more than 250 km inland during the mid‐Holocene (6.3 ± 1.0 ka), prior to re‐advancing to its modern position.