Late Quaternary tephrostratigraphy, Ahklun Mountains, SW Alaska

Abstract

Radiocarbon-dated sediment cores from six lakes in the Ahklun Mountains, south-western Alaska, were used to interpolate the ages of late Quaternary tephra beds ranging in age from 25.4 to 0.4 ka. The lakes are located downwind of the Aleutian Arc and Alaska Peninsula volcanoes in the northern Bristol Bay area between 159 degrees and 161 degrees W at around 60 degrees N. Sedimentation-rate age models for each lake were based on a published spline-fit procedure that uses Monte Carlo simulation to determine age model uncertainty. In all, 62 C-14 ages were used to construct the six age models, including 23 ages presented here for the first time. The age model from Lone Spruce Pond is based on 18 ages, and is currently the best-resolved Holocene age model available from the region, with an average 2 sigma age uncertainty of about +/- 109 years over the past 14.5 ka. The sedimentary sequence from Lone Spruce Pond contains seven tephra beds, more than previously found in any other lake in the area. Of the 26 radiocarbon-dated tephra beds at the six lakes and from a soil pit, seven are correlated between two or more sites based on their ages. The major-element geochemistry of glass shards from most of these tephra beds supports the age-based correlations. The remaining tephra beds appear to be present at only one site based on their unique geochemistry or age. The 5.8 ka tephra is similar to the widespread Aniakchak tephra [3.7 +/- 0.2 (1 sigma) ka], but can be distinguished conclusively based on its trace-element geochemistry. The 3.1 and 0.4 ka tephras have glass major-and trace-element geochemical compositions indistinguishable from prominent Aniakchak tephra, and might represent redeposited beds. Only two tephra beds are found in all lakes: the Aniakchak tephra (3.7 +/- 0.2 ka) and Tephra B (6.1 +/- 0.3 ka). The tephra beds can be used as chronostratigraphic markers for other sedimentary sequences in the region, including cores from Cascade and Sunday lakes, which were previously undated and were analyzed in this study to correlate with the new regional tephrostratigraphy. Copyright (C) 2012 John Wiley & Sons, Ltd.Peer reviewe

The last deglaciation of Alaska

“…There, Kaufman et al (2003) dated lacustrine sediments related to the LGM ice extent of a major outlet glacier that flowed down the Goodnews River valley. The LGM sediment unit, dated by a macrofossil-based 14 C age-depth model that was updated in Kaufman et al (2012), spans from 23.9 to 19.4 cal ka. Closer to the ice cap center in that same valley, a 14 C age from a sediment section constrains one recessional moraine to be older than ~20.4 cal ka, and a recessional moraine farther upvalley to be younger than ~20.4 cal ka (Manley et al, 2001).…”

Tephra without Borders: Far-Reaching Clues into Past Explosive Eruptions

“…(3) As well as pinpointing diagnostic features for different volcanic regions and tectonic settings, the tephrochronological community also faces a challenge when different eruptions of the same source volcano produce identical glass compositions (e.g., Siani et al, 2004;Santacroce et al, 2008;Smith et al, 2011a;Kaufman et al, 2012;Lane et al, 2012;Wutke et al, 2015). These eruptions may be separated by hundreds or even thousands of years, but if these are close in age, it may cause erroneous identification and mis-correlation of a tephra.…”

A 14,500-year record of landscape change from Okpilak Lake, northeastern Brooks Range, northern Alaska

“…The silt and clay layers that occur throughout the Okpilak Lake sediment core potentially represent contributions of minerogenic sediments by one or more processes, including tephra fall, re-deposition of sediments from the littoral zone during periods of reduced water depth, mass wasting in the watershed, or fluvial deposition from the Okpilak River. Given that the estimated ages of the inorganic layers do not appear to be coincident with those of tephras studied in southern, southwestern, and western Alaska (Beget et al 1992;Carson et al 2002;de Fontaine et al 2007;Kaufman et al 2012), along with the absence of identified tephras in previous work on lake-sediment records in the Brooks Range and Arctic Foothills (Anderson 1985(Anderson , 1988Brubaker et al 1983;Oswald et al 1999;Anderson et al 2001;Oswald et al 2003b;Higuera et al 2009;Abbott et al 2010), we attribute these sedimentary features to a combination of the aforementioned local processes.…”

“…The sediments alternate between gyttja and inorganic silt and clay (see ''Results'') and we developed an agedepth model assuming that most of the inorganic units denote episodes of rapid accumulation associated with one or more processes (see ''Discussion''). The typical means of developing an age-depth model in these circumstances would be to subtract the thickness of the inorganic layers and model the age-depth relationship using adjusted depths (Kaufman et al 2012). However, that approach requires the classification of discrete, instantaneously deposited core segments, which in the case of the Okpilak Lake record is not straightforward.…”