Collaborative Research: Ice sheet erosional interaction with hot geotherm in West Antarctica
Sediment records off the coast of Marie Byrd Land (MBL), Antarctica suggest frequent and dramatic changes in the size of the West Antarctic Ice Sheet (WAIS) over short (tens of thousands of years) and long (millions of years) time frames in the past. WAIS currently overrides much of MBL and covers the rugged and scoured bedrock landscape. The ice sheet carved narrow linear troughs that reach depths of two to three thousand meters below sea level as outlet glaciers flowed from the interior of the continent to the oceans. As a result, large volumes of fragmented continental bedrock were carried out to the seabed. The glaciers cut downward into a region of crystalline rocks (i.e. granite) that display a significant temperature change as a function of rock depth. The strong geothermal gradient in the bedrock is favorable for determining when the bedrock became exhumed, or "uncovered" by action of the overriding icesheet or other processes. Our approach takes advantage of a reference horizon, or paleogeotherm, established when high-T mineral thermochronometers across Marie Byrd Land (MBL) cooled from temperatures of >800° C to 300° C, due to rapid regional extension at ~100 Ma . The event imparted a signature through which the subsequent Cenozoic landscape history can be explored: MBL's elevated geothermal gradient, sustained during the Cenozoic, created favorable conditions for sensitive apatite and zircon low-T thermochronometers to record bedrock cooling related to glacial incision. Analyzing the chemistry of minerals (zircon and apatite) within fragments of eroded rock will reveal the rate and timing of the bedrock erosion and development of topography in West Antarctica. This collaborative project addresses the following questions: When did the land become high enough for a large ice sheet to form? What was the regional pre-glacial topography? Under what climate conditions, and at what point in the growth of an ice sheet, did glaciers begin to cut sharply into bedrock to form the narrow troughs that flow seaward? The research will lead to greater understanding of past Antarctic ice sheet fluctuations and identify precise timing of glacial incisionm which will clarify the onset of WAIS glacier incision and assess the evolution of Cenozoic paleo-topography. The collaborative project provides training for one graduate and 8 undergraduate students in STEM. These students, together with PIs, will refine West Antarctic ice sheet history and obtain results that pertain to the international societal response to contemporary ice sheet change and its global consequences. The methods used for the research include: •Low-temperature (T) thermochronology and Pecube 3-D thermo-kinematic modeling, applied to the timing and characterizatio episodes of glacial erosional incision. •Single-grain double- and triple-dating of zircon and apatite, to determine the detailed crustal thermal evolution of the region, enabling the research team to identify the comparative topographic influences on glaciation versus bedrock uplift induced by Eocene to present tectonism/magmatism. Students and PIs employed state-of-the-art analytical facilities in Arizona and Minnesota, expanding the geo- and thermochronologic history of MBL from bedrock samples and offshore sedimentary deposits. The temperature and time data we acquired will provide constraints on paleotopography, isostasy, and the thermal evolution of MBL that will be modeled in 3D using Pecube model simulations. Within hot crust, less incision is required to expose bedrock containing the distinct thermochronometric profile; a prediction we are testing through use of inverse Pecube 3-D models of the thermal field through which bedrock and detrital samples cooled. Using results from Pecube, the ICI-Hot team will examine time-varying topography formed in response to changes in erosion rates, topographic relief, geothermal gradient and/or flexural isostatic rigidity. These effects are manifestations of dynamic processes in the WAIS, including ice sheet loading, ice volume fluctuations, relative motion upon crustal faults, and magmatism-related elevation increase across the MBL dome. The project makes use of pre-existing sample collections housed at the US Polar Rock Repository, IODP's Gulf Coast Core Repository, and the OSU Marine and Geology Repository. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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