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Cosmic Airburst on Developing Allerød Substrates (Soils) in the Western Alps, Mt. Viso Area

Mahaney W. C., West A., Milan A., Krinsley D. H., Somelar P., Schwartz S., Milner M. W., Allen C. C. R.

Studia Quaternaria

2018

Vol. 35, Nr 1

3--23

Although much has been written about a cosmic impact event in the Western Alps of the Mt. Viso area, the event closely tied with the Younger Dryas Boundary (YDB) of 12.8 ka and onset of the Younger Dryas (YD), the affected land surface is considered to contain a similar black mat suite of sediment found on three continents. While work elsewhere has focused on recovered sediment from lake and ice cores, buried lacustrine/alluvial records, and surface glacial and paraglacial records, no one has traced a mountain morphosequence of deposits with the objective of investigating initial weathering/soil morphogenesis that occurred in ice recessional deposits up to the YDB when the surface was subjected to intense heat, presumably, as hypothesized by Mahaney et al. (2016a) from a cosmic airburst. With the land surface rapidly free of ice following glacial retreat during the Bølling-Allerød interstadial, weathering processes ~13.5 to 12.8 ka led to weathering and soil morphogenesis in a slow progression as the land surface became free of ice. To determine the exposed land character in the mid- to late-Allerød, it is possible to utilize an inverted stratigraphic soil morphogenesis working backward in time, from known post-Little Ice Age (LIA) (i.e. time-zero) through LIA (~0.45 to ~0.10 ka), to at least the middle Neoglacial (~2 ka), to answer several questions. What were the likely soil profile states in existence at the end of the Allerød just prior to the cosmic impact/airburst (YDB)? Assuming these immature weathered regolith sections of the Late Allerød approximated the <1 ka old profiles seen today, and assuming the land surface was subjected to a hypothesized instant temperature burst from ambient to ~2200oC at ~12.8 ka, what would be the expected effect on the resident sediment? To test the mid-LG (YDB) to YD relationship we analyzed the paleosols in both suites of deposits – mid-LG to – to test that the airburst grains are restricted to Late Allerød paleosols and using relative-age-determination criteria, that the overlapping YD to mid-LG moraines are closely related in time. These are some of the questions about the black mat that we seek to answer with reference to sites in the upper Guil and Po rivers of the Mt. Viso area.

Weathering rind age assignment of Neoglacial deposits in the Okstindan Mountains, northern Norway

Mahaney W. C.

Studia Quaternaria

2016

Vol. 33, Nr 2

131--138

Previous soil stratigraphic analysis of soil morphogenesis in the Okstindan Mountains established a Late Neoglacial soil evolutionary sequence based on historically monitored and radiocarbon-dated moraine positions over the last ~3.0 cal yr BP. Thus pedon evolution ranged from C/Cu®Ah/Cox/Cu®pedostratigraphic succession of Ah/C/Cu/Lb/ Cub/Ahb/Coxb/Cub profiles with a maximum rind weathering time of ~1.0 kyr. Following successive retreat phases of Neoglacial ice, weathering rind development continued apace on moraines, each rind population recording weathering time following successive glacier stillstands. The age of the youngest deposits falls within the period 1900–1910 AD, or the last 100 yrs, with variable moraine positions all documented by historical depictions of the position of the Austre Okstindan glacial lobe prior retreating to its present position. The next older group of deposits is considered to have been emplaced near the end of the LIA or around ~1800 AD, with time of rind development set at 200 yr, possibly older. The oldest moraine set within the late Neoglacial sequence lies atop a pedostratigraphic column, the uppermost soil radiocarbon dated at ~1.0 yr BP. Given the range of mean rind development across this threshold of deposits, from 0.22 ±0.03 mm in the inner group, 0.66 ±0.07mm in the middle group, to 1.38 ±0.15 mm in the outer, older group, it is clear that finite measurements at several sites within a suite of deposits, some dated by radiocarbon, can evenly discriminate between deposits in a glacial succession.