THE ~7000 B.P. BLACK NOSE PUMICE, A POTENTIALLY WIDESPREAD TEPHRA FROM ANIAKCHAK VOLCANO WITH ECOLOGICAL AND POSSIBLE CULTURAL IMPACT ON THE ALASKA PENINSULA

Aniakchak is a historically active caldera volcano of the Alaska–Aleutian arc within Aniakchak National Monument and Preserve 670 km southwest of Anchorage. The Black Nose Pumice (BNP), informally named for caldera rim exposures by Black Nose peak, is the product of the most explosive and voluminous silicic eruptions of Aniakchak known prior to the ~3430 ¹⁴C yr B.P. Aniakchak II caldera-forming event. We apply the name Black Nose Pumice to Plinian fall deposits (total thickness ≥25 m) and intraplinian ignimbrite north of The Gates and elsewhere near the caldera rim. Lower BNP consists of highly inflated crystal-poor buff-colored rhyodacite pumice (~69% SiO₂). Postglacial lava under, and ignimbrite within the BNP below the caldera rim north of The Gates are compositionally identical to lower BNP fall. The upper BNP fall consists of brown dacite pumice (~67.5% SiO₂) with greater phenocryst content than lower BNP. Basal upper BNP fall is partly welded and forms a dark-gray band in caldera rim exposures. A northeast flank postglacial dacite lava flow is similar in composition to upper BNP. Both lower and upper units are less evolved than Aniakchak II rhyodacite, which also is distinctive in carrying hornblende phenocrysts.

The age of the lower BNP is constrained by tephrochronology for a 45-cm-thick pumice bed ~40 km southeast of the caldera at Cabin Bluff near Aniakchak Bay. New electron microprobe analyses of glass, magnetite, and ilmenite in pumice from the caldera rim and Cabin Bluff show that this bed correlates with lower BNP. Fe–Ti oxides yield preeruption temperatures of ~940–950 °C and fO₂ ~Ni–NiO for both localities. Peat from beneath the Cabin Bluff pumice bed gave a radiocarbon age of 7,350±70 ¹⁴C yr B.P., whereas soil beneath the same bed 600 m distant gave 6,760±60 ¹⁴C yr B.P. (VanderHoek 2009 Ph.D. thesis), leading us to suggest an age of ~7,000 ¹⁴C yr B.P. for the lower Black Nose Pumice. Pollen study at Cabin Bluff by R.E. Nelson (in VanderHoek 2009) indicates grasses dominated in revegetation following the BNP eruption, with lesser sage, birch, and sedges, in contrast to the modern Pacific coast assemblage dominated by alder and fern. The BNP eruption strongly depressed biological productivity of the region and appears to have caused a long-term hiatus in human occupation of the central Alaska Peninsula.