A late–Middle Pleistocene (Marine Isotope Stage 6) vegetated surface buried by Old Crow tephra at the Palisades, interior Alaska

Abstract

A 40 cm thick primary bed of Old Crow tephra (131 ± 11 ka), an important stratigraphic marker in eastern Beringia, directly overlies a vegetated surface at Palisades West, on the Yukon River in central Alaska. Analyses of insect, bryophyte, and vascular plant macrofossils from the buried surface and underlying organic-rich silt suggest the local presence of an aquatic environment and mesic shrub-tundra at the time of tephra deposition. Autochthonous plant and insect macrofossils from peat directly overlying Old Crow tephra suggest similar aquatic habitats and hydric to mesic tundra environments, though pollen counts indicate a substantial herbaceous component to the regional tundra vegetation. Trace amounts of arboreal pollen in sediments associated with the tephra probably reflect reworking from older deposits, rather than the local presence of trees. The revised glass fission-track age for Old Crow tephra places its deposition closer to the time of the last interglaciation than earlier age determinations, but stratigraphy and paleoecology of sites with Old Crow tephra indicate a late Marine Isotope Stage 6 age. Regional permafrost degradation and associated thaw slumping are responsible for the close stratigraphic and paleoecological relations between Old Crow tephra and last interglacial deposits at some sites in eastern Beringia.

Introduction

The late Cenozoic sediments of unglaciated Yukon and Alaska host numerous tephra beds, sourced from volcanoes in the Aleutian arc and Wrangell Mountains, that have been used as chronostratigraphic markers for over half a century (e.g. Taber, 1943, Péwé, 1975, Preece et al., 2000, Jensen et al., 2008). The development of a detailed tephrostratigraphic framework in the region, in concert with advances in the geochemical characterization (e.g. Pearce et al., 2004) and geochronology of tephra (e.g. Berger, 2003, Westgate et al., 2007), has facilitated regional correlation of terrestrial sedimentary and paleoenvironmental records well beyond the limit of radiocarbon dating (e.g. Westgate et al., 1990, Hamilton and Brigham-Grette, 1991, Begét, 2001).

Old Crow tephra is the most widespread of the many known beds from this region. Its distribution extends from the Seward Peninsula of northwestern Alaska, across many localities in southern and interior Alaska, to central and northern Yukon (Westgate et al., 1983). The regional importance of Old Crow tephra lies in its consistent stratigraphic position below woody, organic-rich sediments that are thought to be associated with the last interglaciation sensu stricto (Marine Isotope Stage, MIS, 5e). The age of Old Crow tephra is commonly cited as 140 ± 10 ka, based on a weighted mean (1σ uncertainty) of six glass fission-track ages (Westgate et al., 1990). However, its age has been refined recently. Péwé et al. (2009) obtained a weighted-mean age of 131 ± 11 ka (1σ uncertainty), based partially on inclusion of a new age determination on a coarse-grained sample of Old Crow tephra from Togiak Bay, Alaska (Kaufman et al., 2001). Using a new, more precise age estimate for the Moldavite tektite glass, the age monitor in glass fission-track dating (Westgate et al., 2007), gives an even younger mean age of 124 ± 10 ka for Old Crow tephra (J.A. Westgate, personal communication, Nov. 2009). These three age estimates overlap at 1σ, and are consistent with thermoluminescence (Berger, 2003) and infrared-stimulated luminescence (Auclair et al., 2007) ages on bracketing loess in the Fairbanks region.

The new glass fission-track age of Old Crow tephra revives questions concerning the nature of the temporal relation of Old Crow tephra to the last interglaciation (e.g. McDowell and Edwards, 2001, Muhs et al., 2001). This problem is particularly acute at the type locality for the Eva Interglaciation Forest Bed in the Fairbanks region, where thermokarst-related slope processes may have reworked the tephra into, and locally above, last interglacial organic-rich sediments (Péwé et al., 1997, Muhs et al., 2001). The tephra and its associated sediments have been the subject of numerous paleoecological studies across eastern Beringia (e.g. Schweger and Matthews, 1985, Matthews et al., 1990, Hamilton and Brigham-Grette, 1991, Waythomas et al., 1993, Muhs et al., 2001), yet a coherent regional interpretation of the paleoenvironmental setting at the time of Old Crow tephra deposition remains elusive because of apparent contradictory climate and ecological signals among sites (McDowell and Edwards, 2001). Moreover, despite its regional distribution, we are not aware of any published records of in situ vegetation directly underlying Old Crow tephra, which could provide paleoecological data from the moment of tephra deposition and help refine its chronostratigraphic significance.

In this paper, we report the presence of a vegetated surface buried by Old Crow tephra at the Palisades, a series of bluffs on the Yukon River in interior Alaska. We compare multiple paleoecological indicators (bryophyte, pollen, vascular plant, and insect assemblages) from the vegetated surface and associated sediments. Our multiproxy data from the vegetated surface indicate that this important chronostratigraphic marker dates to late-MIS 6.