Evidence for seasonal controls on the transportation of loess in Matanuska Valley, southern Alaska
Loess is windblown silt. It is an important surficial deposit that can be centimeters to tens of meters thick, and it forms the parent material for soils on many continents. During the last glacial period, about 25,000-12,000 years ago, loess was deposited over wide areas of midcontinental North America, Alaska, Asia, Europe and South America.
Because it is deposited directly by the wind and preserved in place, spatial changes in loess thickness and grain size characteristics can provide important information about past wind directions that can aid our understanding of past atmospheric circulation. In regions such as the midcontinent of North America, however, what is not well understood is the seasonal timing of loess deposition.
Loess is derived largely from continental glaciers that are no longer found in North America; thus, analogs must be sought in places where glaciers occur today, such as Alaska. Loess is a widespread surficial deposit in Alaska, and loess accretion occurs today in some regions, such as the Matanuska Valley.
The source of loess in the Matanuska Valley has been debated for more than seven decades, with the Knik River and the Matanuska River, both to the east of the Valley, being the leading candidates and the Susitna River, to the west of the Valley, as a less favorable source.
USGS researchers, in a newly published paper, report new stratigraphic, mineralogical, and geochemical data that test the competing hypotheses of these river sources. Loess stratigraphic data are consistent with previous studies that show that a source or sources lay to the east of the Matanuska Valley, which rules out the Susitna River. Knik and Matanuska River silts can be distinguished using geochemical “fingerprinters” that match the silt in a loess deposit with its source material. Using these geochemical tools, the researchers determined that Matanuska Valley loess falls clearly within the range of values found in Matanuska River silt.
Dust storms from the Matanuska River are most common in autumn, when river discharge is at a minimum and silt-rich point bars are exposed, wind speed from the north is beginning to increase after a low-velocity period in summer, snow depth is still minimal, and soil temperatures are still above freezing. Thus, seasonal changes in climate and hydrology emerge as critical factors in the timing of windblown silt transport and loess accretion in southern Alaska.
These findings could be applicable to understanding seasonal controls on last-glacial-age loess accretion in Europe, Asia, South America, and midcontinental North America.
The paper, “Geochemical evidence for seasonal controls on the transportation of Holocene loess, Matanuska Valley, southern Alaska, USA”, was published in Aeolian Research.