Search for: All records

Large-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterize the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new Sif Island. The island comprises granite, zircon U-Pb dated to ~177–174 Ma and characterized by initial ɛ_Nd,⁸⁷Sr/⁸⁶Sr and ɛ_Hfisotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block's boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low-temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100–90 Ma and then remained close to the Earth's surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous.

 

Marine sediments, obtained from cores and captures from deep sea and continental shelf sites of West Antarctica, contain rich records of latest Miocene to Present glacial and deglacial processes and conditions at the margin of the West Antarctic ice sheet (WAIS). The materials we are investigating were recovered from a) Resolution Drift on the Amundsen Sea continental rise (water depths >3900m), b)the continental shelf in the Amundsen Sea, Wrigley Gulf, and Sultzberger Bay (water depths <1000m). Resolution Drift cores were drilled by IODP Expedition 379 (Gohl et al., doi:10.14379/iodp.proc.379.2021) in sediments dominated by compacted clay and silty clay, with conglomeratic intervals of ice-rafted detritus (IRD) and downslope deposits. The shelf sediments were recovered by piston core, trigger core, and Smith McIntyre Grab (SMG) during USA research cruises of the RVIB Nathaniel B Palmer (1999, 2000, 2007) and USCGC Glacier (1983). The shelf samples are non-compacted clay, containing abundant cobbles, pebbles and biogenic fragments. Our research focuses upon rock clasts, detrital apatite and zircon, felsic volcanic tephra, and micro-manganese nodules separated from marine and glaciomarine clay. The rock clasts and detrital minerals represent samples of continental crust that we characterise according to rock type, petrology, geochemistry, and geo-thermochronology [U-Pb, (U-Th)/He, and fission track methods]. These characteristics illuminate solid Earth processes, including the development of subglacial topography . We compared clasts’ petrology and age data to the exposed onshore geology and thermochronology of bedrock, and determined that ≥90% of clasts likely originated in West Antarctica. Therefore the materials can be used to assign roughness, erodibility, and heat production factors for subglacial bedrock, which constitute boundary conditions used by ice sheet modelers. Rhyolite ash and fragments provide new evidence for explosive eruptions (dated ca. 2.55 to 2.92 Ma; feldspar 40Ar/39Ar) delivered to sea as airfall, IRD, and possible subglacial water transport. Silicic eruptions produce ash and aerosols that may screen solar energy, and provide bio-available nutrients that produce phytoplankton blooms leading to sequestration of carbon. The rhyolite dates coincide with the end of a Pliocene warm period recorded in IODP379 cores (Gille-Petzoldt et al., 10.3389/feart.2022.976703). Our work in progress seeks to obtain higher resolution geochronology in order to determine whether silicic continental volcanism occurred in response to ice unloading due to deglaciation (cf. Lin et al., 10.5194/cp-18-485-2022) and whether erupted products contributed to latest Pliocene significant cooling and WAIS re-glaciation. Another distinctive sediment constituent is micro-manganese nodules of unusual form. Whereas typical micro-MN nodules are dark, formed of concentric layers, this form is pale in color, ‘barbell’ shaped, and transparent in transmitted light. Scanning electron microscopy shows these to be microcrystalline Mn-oxide with embedded grains of quartz and feldspar, which likely served as seed material. Mn-oxides form by authigenesis at/near the seafloor surface, requiring high oxygen concentrations in the bottom water and low sedimentation rates, generally associated with the end of glacials/during interglacials (Hillenbrand et al. 2021, 10.1029/2021GL093103). Work is in progress to determine whether Mn oxides formed through passive accretion upon seed grains or microbially-mediated precipitation from Mn-oxyhydroxides or colloids, of possible relevance for coastal carbon budgets. https://doi.org/10.5194/egusphere-egu23-9728 

Abstract A dataset to describe exposed bedrock and surficial geology of Antarctica has been constructed by the GeoMAP Action Group of the Scientific Committee on Antarctic Research (SCAR) and GNS Science. Our group captured existing geological map data into a geographic information system (GIS), refined its spatial reliability, harmonised classification, and improved representation of glacial sequences and geomorphology, thereby creating a comprehensive and coherent representation of Antarctic geology. A total of 99,080 polygons were unified for depicting geology at 1:250,000 scale, but locally there are some areas with higher spatial resolution. Geological unit definition is based on a mixed chronostratigraphic- and lithostratigraphic-based classification. Description of rock and moraine polygons employs the international Geoscience Markup Language (GeoSciML) data protocols to provide attribute-rich and queryable information, including bibliographic links to 589 source maps and scientific literature. GeoMAP is the first detailed geological map dataset covering all of Antarctica. It depicts ‘known geology’ of rock exposures rather than ‘interpreted’ sub-ice features and is suitable for continent-wide perspectives and cross-discipline interrogation. 

Brittle faults in the southern Ford Ranges of Marie Byrd Land, West Antarctica, provide unique opportunity to study fluid-rock interactions in the West Antarctic Rift System and the role of crustal fluids during regional-scale faulting. This fault array contains steep, NNW-striking, normal-oblique slip faults and sub-vertical WNW-ESE strike-oblique faults. \ Faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak display strongly aligned tourmaline, indicating syntectonic mineralization; surfaces in one location feature distinctive mirror surfaces, suggestive of formation during seismic slip. Tourmaline has been demonstrated to resist chemical and isotopic re-equilibration during even high-temperature metamorphism, and to maintain a record of conditions during formation, therefore oxygen isotope compositions of tourmaline and quartz pairs may elucidate crustal conditions (e.g. temperatures and fluid-rock ratios) and fluids sources. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces; host rocks are tourmaline-free. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰; ∆Qtz-Trm values between 1.3 and 2.0 may reflect an inability of quartz to equilibrate during tourmaline crystallization. Equilibrium between quartz and tourmaline would suggest temperatures of formation in excess of 550°C. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. Geometric and kinematic relationships suggest a link to deeper level shears hosting muscovite, and sillimanite with garnet. On-going investigation includes O isotope analyses of these shears, as well as analysis of H and B isotopes in tourmaline, which will better characterize the relationship between the deeper crustal shears with the brittle fault array, and the fluid sources and metasomatic processes of regional fault systems. Furthermore, temporal constraints on tourmaline mineralization will establish whether faulting is associated with Cretaceous intracontinental extension of the West Antarctic rift system (Siddoway 2008) or a crustal response to Neogene mantle delamination beneath the South Pole region (Shen et al 2018). 

Outcrops of brittle faults are rare in Marie Byrd Land, West Antarctica, because fault damage zones commonly undergo enhanced erosion and form bedrock troughs occupied by glacier ice. Where exposures do exist, faults yield information about regional strain in the West Antarctic Rift System (WARS) and may host minerals that contain a record of the temperature and chemistry of fluids during regional-scale faulting. In MBL’s southern Ford Ranges, bordering Ross Sea, a distinctive fault array was sampled that hosts tourmaline and quartz, a mineral-pair that can provide temperature and composition of fault-associated fluids, using 18O. Host rocks are tourmaline-free. At three separate sites, fault surfaces display strongly aligned tourmaline, suggesting that mineralization occurred during tectonism. One site features highly polished, or mirrored, surfaces, a characteristic that may indicate tourmaline precipitation during seismic slip. The orientation and kinematics of the high angle faults are NNW-striking: normal-slip, and WNW-ESE striking: right-lateral strike-slip. The timing of mineralization is yet to be determined, but viable possibilities are that the faults formed during broad intracontinental extension during formation of Ross Embayment in the Cretaceous, or during development of deep, narrow basins beneath the RIS grounding zone, in the Neogene (newly detected, see Tankersley et al., this meeting). Once formed, tourmaline is resistant to chemical and isotopic re-equilibration, and therefore can retain a record of its conditions during formation. We used oxygen isotope compositions of tourmaline and quartz pairs to investigate temperatures, fluid-rock ratios, and fluid sources, with bearing on fault-localized flux of fluids and geothermal heat. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces, as well as quartz separated from host rock in the same hand samples. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. ∆Qtz-Trm values fall between 1.3 and 2.0, and 18O of quartz in faults closely resembles 18O of host rock quartz. We tentatively determine the water oxygen isotope ratio as greater than ~7.7 ‰. Plutonic-metamorphic associations in the immediate region, and comparisons with similar faults elsewhere (i.e. Isola d’Elba, Italy), suggest temperatures as high as 500°C for the fluids that circulated into the faults. The data are interpreted to show that brittle faults provided pathways for hot fluids derived from mid-crustal processes to make their way to shallow crustal depths. 18O values indicate magmatic and/or metamorphic fluid sources, with minor to no introduction of meteoric fluids. Tourmaline-quartz pairs did not attain equilibrium, likely due to tourmaline’s rapid crystallization. On-going investigation includes analysis of H and B isotopes in tourmaline, which will better characterize the relationship between fault-hosted and mid-crustal fluids. 

Bedrock topography is a key boundary condition for ice sheet modeling, and determining changes in subglacial topography through time can provide insight into the timing of ice sheet development, the magnitude of glacial erosion, and the co-development of glaciers and glacial topography. West Antarctica hosts an unusually high geothermal gradient supported by hot, low-viscosity mantle which likely enhanced the lithospheric response to West Antarctic Ice Sheet (WAIS) cycles of growth and increased the sensitivity of thermochronometers to landscape evolution on million-year timescales. Thus, a valuable record of glacial landscape change might be recovered from apatite fission track [AFT 80-130°C range] and (U-Th)/He [AHe; 50-90°C] dating, provided that landscape evolution can be distinguished from tectonic signals, including the effects of faults. This study utilizes AFT-AHe thermochronology and thermo-kinematic Pecube modeling to investigate interactions between the hot geotherm, glacial erosion, and inferred crustal structures in the Ford Ranges and the DeVicq Glacier trough in western and central Marie Byrd Land (MBL), respectively. The Ford Ranges host glacial troughs (up to 3km relief) dissecting a low-relief erosional surface. Previous work suggests a majority of bedrock exhumation and cooling occurred at/by 80 Ma. However, new data hint at renewed exhumation linked to glacial incision since WAIS formation at 34 or 20 Ma. Prior (U-Th)/He zircon dates from exposures of crystalline bedrock span 90 – 67 Ma. New AHe bedrock dates are 41 to 26 Ma, while two glacial erratics (presumed to be eroded from walls or floor of glacial troughs) yielded AHe dates of 37 Ma and 16 Ma. Initial modeling results suggest a tectonic boundary between the Ford Ranges and Edward VII Peninsula separating regions with distinct exhumation histories. The boundary may cause differential WAIS incision at 34 or 20 Ma, a possibility being investigated with new models. The DeVicq Glacier trough (>3.5km relief) coincides with a prominent crustal lineament but lacks temperature-time information compared to other regions. The crustal structure may have accommodated motion between elevated central MBL and the subdued crust of the Ford Ranges. Here, owing to the lack of onshore non-volcanic bedrock exposure, we have employed AHe and AFT dating of glacial sediment marine core samples offshore of the DeVicq Glacier to investigate the timing and rates of exhumation of the bedrock carved by the DeVicq trough, with initial results revealing detrital AHe ages as young as 24 Ma. Our new Pecube models test a series of thermal, tectonic, and landscape evolution scenarios against a suite of thermochronologic data, allowing us to assess the timing of glacial incision and WAIS initiation in the Ford Ranges, and to seek evidence of an inferred tectonic boundary at DeVicq Trough. Modeling efforts will be aided by new AHe and AFT analyses from ongoing work. These models combine topographic, tectonic, thermal, and key thermochronologic datasets to produce new insight into the unique cryosphere-lithosphere interactions affecting landscape change in West Antarctica. 

An enormous reserve of information about the subglacial bedrock, tectonic and topographic evolution of Marie Byrd Land (MBL) exists within glaciomarine sediments of the Amundsen Sea shelf, slope and deep sea, and MBL marine shelf. Investigators of the NSF ICI-Hot and NSF Linchpin projects partnered with Arizona Laserchron Center to provide course-based undergraduate research experiences (CUREs) for from groups who do not ordinarily find access points to Antarctic science. Our courses enlist BIPOC and gender-expansive undergraduates in studies of ice-rafted debris (IRD) and bedrock samples, in order to impart skills, train in the use of research instrumentation, help students to develop confidence in their scientific abilities, and collaboratively address WAIS research questions at an early academic stage. CUREs afford benefits to graduate researchers and postdoctoral scientists, also, who join in as instructional faculty: CUREs allow GRs and PDs to engage in teaching that closely ties to their active research, yet provides practical experience to strengthen the academic portfolio (Cascella & Jez, 2018). Team members also develop art-science initiatives that engage students and community members who may not ordinarily engage with science, forging connections that make science relatable. Re-casting science topics through art centers personal connections and humanizes science, to promote understanding that goes beyond the purely analytical. Academic research shows that diverse undergraduates gain markedly from the convergence of art and science, and from involvement in collaborative research conducted within a CURE cohort, rather than as an individualized experience (e.g. Shanahan et al. 2022). The CUREs are offered as regular courses for credit, making access equitable via course enrollment. The course designation carries a legitimacy that is sought by students who balance academics with part-time employment. Course information is disseminated via STEM Bridge programs and/or an academic advising hub that reaches students from groups that are insufficiently represented within STEM and cryosphere science. CURE investigation of Amundsen Sea and WAIS problems is worthy objective because: 1) A variety of sample preparation, geochemical methods, and scientific best-practices can be imparted, while educating students about Antarctica’s geological configuration and role in the Earth climate system. 2) Individual projects that are narrowly defined can readily scaffold into collaborative science at the time of data synthesis and interpretation. 3) There is a high likelihood of scientific discovery that contributes to grant objectives. 4) Enrolled students will experience ambiguity and instrumentation setbacks alongside their faculty and instructors, and will likely have an opportunity to withstand/overcome challenges in a manner that trains students in complex problem solving and imparts resilience (St John et al., 2019). Based on our experiences, we consider CUREs as a means to create more inclusive and equitable spaces for learning to do research, and a basis for a broadening future WAIS community. Our groups have yet to assess student learning gains and STEM entry in a robust way, but we can report that two presenters at WAIS 2022 came from our 2021 CURE, and four polar science graduate researchers gained experience via CURE teaching. Data obtained by CURE students is contributing to our NSF projects’ aims to obtain isotope, age, and petrogenetic criteria with bearing on the subglacial bedrock geology, tectonic and landscape evolution, and ice sheet history of MBL. Cited and recommended works: Cascella & Jez, 2018, doi: 10.1021/acs.jchemed.7b00705 Gentile et al., 2017, doi: 10.17226/24622 Shanahan et al. 2022, https://www.cur.org/assets/1/23/01-01_TOC_SPUR_Winter21.pdf Shortlidge & Brownell, 2016, doi: 10.1128/jmbe.v17i3.1103 St. John et al. 2019, EOS, doi: 10.1029/2019EO127285. 

IODP Expedition 379 to the Amundsen Sea continental rise recovered latest Miocene-Holocene sediments from two sites on a drift in water depths >3900m. Sediments that are dominated by clay and silty clay host pebbles and cobbles of ice-rafted detritus (IRD) (Gohl et al. 2021, doi: 10.14379/iodp.proc.379.2021). Cobble-sized dropstones also appear as fall-in, in the top of cores recovered from sediments >5.3 Ma. The principle means to deposit abundant IRD and sparse dropstones in deep sea sediment is through melting of icebergs released by Antarctic ice-sheet calving events. We use petrological and age characteristics of clasts from the Exp379 sites to fingerprint their bedrock provenance to extend knowledge of subglacial bedrock, and with the intention to illuminate changes in icesheet extent between 7 – 3 Ma that lend credence to forecasts of extensive future change. Mapped onshore geology shows pronounced distinctions in bedrock age between tectonic provinces of West or East Antarctica (e.g. Cox et al. 2020, doi:10.21420/7SH7-6K05; Jordan et al. 2020, doi.org/10.1038/s43017-019-0013-6). This allows us to use geochronology and thermochronology of rock clasts and minerals for tracing their provenance, and hence ascertain whether IRD deposited at the 379 drillsites originated from proximal or distal Antarctic sources. We here report zircon and apatite U-Pb dates from several sand samples and dropstones taken from latest Miocene, early Pliocene, and Plio-Pleistocene-boundary sediments. Additional Hf isotope data, and apatite fission track and 40Ar/39Ar Kfeldspar ages for some of the same samples help to strengthen provenance interpretations. The study revealed three distinct zircon age populations at ca. 100, 175, and 250 Ma. Using Kolmogorov-Smirnov (K-S) statistical tests to compare our new igneous and detrital zircon (DZ) U-Pb results with previously published data, we found strong similarities to West Antarctic bedrock, but low correspondence to prospective sources in East Antarctica, implying a role for icebergs calved from the West Antarctic Ice Sheet (WAIS). The ~100 Ma age resembles plutonic ages from Marie Byrd Land and islands in Pine Island Bay. The ~250 and 175 Ma populations match published mineral dates from shelf sediments in the eastern Amundsen Sea Embayment as well as granite ages from the Antarctic Peninsula and the Ellsworth-Whitmore Mountains (EWM). The different derivation of coarse sediment sources requires changes in iceberg origin through the latest Miocene, early Pliocene, and Plio/Pleistocene, likely the result of changes in WAIS extent. One unique dropstone recovered from Exp379 Site U1533B is green quartz arenite, which yielded mostly 500-625 Ma detrital zircons. In visual appearance and dominant U-Pb age population, it resembles a sandstone dropstone recovered from Exp382 Site U1536 in the Scotia Sea (Hemming et al. 2020, https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/357276). K-S tests yield high values (P ≥ 0.6), suggesting a common provenance for both dropstones recovered from late Miocene to Pliocene sediments, despite the 3270 km distance separating the sites. Comparisons to published data, in progress, narrow the group of potential on-land sources to exposures in the EWM or isolated ranges at far south latitudes in the Antarctic interior. If both dropstones originated from the same source area, they signify that dramatic shifts in the WAIS grounding line position do occur, along with periodic opening of a seaway connecting the Amundsen and Weddell Seas. 

The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.