Geology topics

G. Balco

Publications and source records attributed to G. Balco.

3 recordsLinked to original sources

USGS2017

Glacioisostasy and Lake-Level Change at Moosehead Lake, Maine

Reconstructions of glacioisostatic rebound based on relative sea level in Maine and adjacent Canada do not agree well with existing geophysical models. In order to understand these discrepancies better, we investigated the lake-level history of 40-km-long Moosehead Lake in northwestern Maine. Glacioisostasy has affected the level of Moosehead Lake since deglaciation ca. 12,500 14C yr B.P. Lowstand features at the southeastern end and an abandoned outlet at the northwestern end of the lake indicate that the lake basin was tilted down to the northwest, toward the retreating ice sheet, by 0.7 m/km at 10,000 14C yr B.P. Water level then rose rapidly in the southeastern end of the lake, and the northwestern outlet was abandoned, indicating rapid relaxation of landscape tilt. Lowstand features at the northwestern end of the lake suggest that the lake basin was tilted to the southeast at ca. 8750 14C yr B.P., possibly as the result of a migrating isostatic forebulge. After 8000 14C yr B.P., water level at the southeastern end was again below present lake level and rose gradually thereafter. We found no evidence suggesting that postglacial climate change significantly affected lake level. The rebound history inferred from lake-level data is consistent with previous interpretations of nearby relative sealevel data, which indicate a significantly steeper and faster-moving ice-proximal depression and ice-distal forebulge than geophysical models predict. ?? 1998 University of Washington.

Quaternary Research↗

USGS2007

Regolith transport in the Dry Valleys of Antarctica

The stability of ground surface and preservation of landforms that record past events and environments is of great importance as the geologic and climatic history is evaluated in the Dry Valleys of Antarctica. Currently little is known about the regolith transport that tends to eradicate and confound this record and regolith transport is itself an environmental indicator. Based on analyses of repeat photographs, soil traps, and pebble transport distances, it was found that there is a large spatial variation in topographic diffusivities at least in the annual basis and that counter intuitively the highest topographic diffusivities are found in the alpine valleys that are located farther inland from the coast where the lowest topographic diffusivities were recorded. An average topographic diffusivity for the Dry Valleys was determined to be 10M -5 –10 -4 m 2 /yr. This average topographic diffusivity is surprisingly large, equaling or bordering the smallest values from elsewhere on Earth

Open-File Report↗

USGS2005

Dating Plio-Pleistocene glacial sediments using the cosmic-ray-produced radionuclides 10Be and 26Al

We use the cosmic-ray-produced radionuclides 26 Al and 10 Be to date Plio-Pleistocene glacial sediment sequences. These two nuclides are produced in quartz at a fixed ratio, but have different decay constants. If a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic-ray flux, the 26 Al/ 10 Be ratio is related to the duration of burial. We first attempted to date pre-Wisconsinan tills by measuring 26 Al and 10 Be in fluvial sediments beneath them and applying the method of “burial dating,” which previous authors have used to date river sediment carried into caves. This method, however, requires simplifying assumptions about the 26 Al and 10 Be concentrations in the sediment at the time of burial. We show that these assumptions are not valid for river sediment in glaciated regions. 26 Al and 10 Be analyses of such sediment do not provide accurate ages for these tills, although they do yield limiting ages in some cases. We overcome this difficulty by instead measuring 26 Al and 10 Be in quartz from paleosols that are buried by tills. We use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried, as well as the duration of burial. This technique provides a widely applicable improvement on other means of dating Plio-Pleistocene terrestrial glacial sediments, as well as a framework for applying cosmogenic-nuclide dating techniques in complicated stratigraphic settings. We apply it to pre-Wisconsinan glacial sediment sequences in southwest Minnesota and eastern South Dakota. Pre-Wisconsinan tills underlying the Minnesota River Valley were deposited 0.5 to 1.5 Ma, and tills beneath the Prairie Coteau in eastern South Dakota and adjacent Minnesota were deposited 1 to 2 Ma.

American Journal of Science↗