Glacial and Geomorphic History of Grinnell Glacier Valley, Glacier National Park, Montana: Evidence From Geochemistry and Mass-balance
Abstract
Sedimentary deposits in proglacial lakes provide high-resolution records of past climate and landscape processes. However, few studies have used geochemical evidence to understand the evolution of bedrock sources of proglacial lake sediments, and therefore the geomorphic and glaciologic history of a valley. Sediment cores collected from Lake Josephine, eastern Glacier National Park, were analyzed to assess the geochemical variability during the past 16,000 years. These sediments were likely solely derived from the Proterozoic Belt Supergroup and mafic sills, comprising five lithologic end-members (quartzite, shale, limestone, dolostone, and basalt) that occur at different elevations in the enclosed Grinnell Glacier valley. Bedrock near the top of the valley (1975-2580 m elevation) consists of quartzite, shale, limestone, and dolostone intruded by basaltic sills, whereas exposures at lower elevations (1490-1975 m) are composed of shale and quartzite. Major and trace element analyses of 55 samples from the 13-meter composite core indicate that lake sediments are enriched in CaO (5.39-10.93 wt. %) near the base (late Pleistocene) and become more enriched in Al2O3 (9.87-14.54 wt. %) near the top (Holocene), consistent with previous work (e.g., Schactman et al., 2015). Mass-balance calculations indicate that Pleistocene sediments in the core can be modeled as mostly shale (20-44 %), quartzite (8-15 %), and dolostone (40-58 %), with minor limestone (0-6 %) and basalt (2.08-5.47 %); the later three lithologies are exposed near the headwall region of the valley. In contrast, Holocene sediments are modeled as mixtures of shale (55-78 %), quartzite (9-22 %), and dolostone (6-27 %); these sediments cannot be modeled successfully when limestone or basalt are included as potential end-members. The modeled decreasing proportion of valley headwall lithologies in younger sediments suggests reduced sediment production and/or transport from the glacier to the Lake Josephine from the late Pleistocene to Holocene, consistent with a diminished ice volume and an increasing number of alpine lakes acting as sediment sinks along the valley. These results suggest that under ideal conditions, geochemical evidence from pro-glacial lake sediments may provide unique information about climate change and geomorphic evolution.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP43B2332L
- Keywords:
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- 1616 Climate variability;
- GLOBAL CHANGEDE: 1631 Land/atmosphere interactions;
- GLOBAL CHANGEDE: 1637 Regional climate change;
- GLOBAL CHANGEDE: 1694 Instruments and techniques;
- GLOBAL CHANGE