Southeast Alaska’s Mount Edgecumbe Volcanic Field (MEVF) lies along the transform boundary that separates the North American and Pacific plates. The MEVF is situated approximately 20 km from Sitka, a town with more than 8,000 residents, and understanding the history of MEVF volcanism is therefore critical for regional geohazard assessments. Reconstructions of MEVF activity can also aid in determining when humans first arrived to the northwest Pacific coast of North America (Potter et al., 2017; Braje et al., 2020). For example, the Tlingit name for Mt. Edgecumbe, L’úx, translates to “flash” or “blinking,” suggesting that the volcano was active when humans settled in the region (Bunten, 2015). Moreover, areas of Southeast Alaska, including portions of the MEVF, have been identified as potential ice-free refugia during the Last Glacial Maximum (LGM; Carrara et al., 2007). Unraveling the eruptive history of the MEVF can thus play a role in assessing both the extent and timing of ice-free terrain available for human occupation during the late Pleistocene. Studies of Quaternary MEVF activity indicate that eruptions began as early as ~600 ka and continued until the mid-Holocene (Riehle et al., 1989). Tephrochronological evidence from deposits in the volcanic field reveals a period of intense explosive volcanism between ~14.6 and 13.1 ka (Riehle et al., 1992b; Begét and Motyka, 1998; Addison et al., 2010; Praetorius et al., 2016), possibly related to crustal flexure caused by retreat of the Cordilleran Ice Sheet following the LGM (Praetorius et al., 2016). The chronology of effusive activity in the MEVF, particularly during the postglacial period, is less well understood (Riehle et al., 1989).

In this presentation, we will focus on a basalt flow exposed on the eastern shores of the MEVF. This flow was mapped by Riehle et al. (1989) as potentially postglacial in age, but K-Ar ages derived from this unit are inconclusive. The basalt flow is overlain by wave-eroded pyroclastic flow deposits and a ~13.2 ka dacitic tephra (Riehle et al., 1989; Begét and Motyka, 1998). Surface features of the eastern MEVF lava flow such as tumuli and ropy structures are indicative of subaerial emplacement and cooling, and appear to rule out overriding by warm-based, erosive glacier ice. It has also been reported that in some locations the basalt encases erratics deposited during the most recent glaciation (Riehle, 1996). These lines of evidence may suggest that the basalt erupted during the late Pleistocene, sometime between deglaciation at ~15 ka (Walcott et al., 2021) and the deposition of the dacite tephra at ~13.2 ka. Alternatively, a pre- or syn-LGM eruption and subsequent exposure in an unglaciated area could also explain preservation of the delicate flow surface features that we observed in the field, and would indicate that effusive basaltic volcanism was confined to earlier stages of the MEVF’s activity (Riehle et al., 1992a). K-Ar dates on other MEVF basalt flows range from ~600-300 ka (Riehle et al., 1989), but the stratigraphic relationship between these units and the eastern MEVF basalt is unclear. Thus, both the emplacement age of the eastern MEVF basalt and the duration of surface exposure before ~13.2 ka remain an open question.

To determine the duration of eastern MEVF basalt exposure and assess if this region was ice-free during the LGM, we collected two surface samples for ³⁶Cl dating in summer 2019. Apparent exposure ages for these samples, which lie at 3 and 8 m asl, respectively, are 7.0 ± 0.6 and 7.1 ± 0.6 ka. To help explain these apparent ages, we turn to a new, locally-constrained relative sea-level curve for the MEVF and surrounding region (Baichtal et al., in review). The curve suggests sea-level was below our sampling sites before ~8.3 ka and reached a maximum elevation of 10 m asl at ~7.9 ka. This rise in relative sea-level is attributed to the migration and collapse of a peripheral crustal forebulge associated with the retreating Cordilleran Ice Sheet (Lesnek et al., 2020; Baichtal et al., in review). Between ~7.9 ka and the present, relative sea-level has fallen steadily at a rate of ~0.1 cm a^-1. Given this relative sea-level history, we therefore might expect both submergence/burial and exposure of our sampling sites during the Holocene. In the simplest scenario, the eastern MEVF basalt was emplaced sometime after regional deglaciation, immediately before the deposition of pyroclastic material and dacite tephra at ~13.2 ka. These deposits would have shielded the surface from cosmic radiation. Wave action during the sea-level transgression then eroded the overlying deposits, exposing the basalt flow at ~7 ka. This postglacial eruption scenario accounts for the fresh appearance of the flow surfaces, the short duration of apparent exposure, and the inclusion of glacial erratics in the flow. However, our ³⁶Cl ages alone do not rule out an earlier eruption of the eastern MEVF basalt. For example, a scenario where the basalt was emplaced in an ice-free refugium during the local LGM at ~17 ka, exposed for ~3 kyr before burial by pyroclastic deposits and tephra, and then re-exposed due to erosion of the overlying material at ~4 ka is also consistent with our results given the uncertainty in our ³⁶Cl ages and the relative sea-level curve. A mapping and ³⁶Cl dating campaign planned for summer 2021 should allow us to determine which of these two scenarios is more likely, and to better reconstruct the dynamic late Pleistocene environment of coastal Southeast Alaska.