The 3.6 ka Aniakchak tephra in the Arctic Ocean: a constraint on the
Holocene radiocarbon reservoir age in the Chukchi Sea
Christof Pearce1,2, Aron Varhelyi1, Stefan Wastegård3, Francesco Muschitiello1,4,5, Natalia Barrientos1, Matt O'Regan1, Thomas Cronin6, Laura Gemery6, Igor Semiletov7,8, Jan Backman1, and Martin Jakobsson11Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden 2Department of Geoscience, Aarhus University, 8000 Aarhus, Denmark 3Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, 10691 Stockholm, Sweden 4Lamont-Doherty Earth Observatory, Columbia University, Palisades NY 10964-8000, New York City, NY, USA 5Uni Research Climate, Bjerknes Centre for Climate Research, 5007 Bergen, Norway 6US Geological Survey, Virginia 20192, USA 7Tomsk National Research Polytechnic University, Tomsk, Russia 8Russian Academy of Sciences, Far Eastern Branch, Pacific Oceanological Institute, Vladivostok, Russia
Received: 03 Nov 2016 – Accepted: 29 Nov 2016 – Published: 29 Nov 2016
Abstract. The caldera-forming eruption of the Aniakchak volcano in the Aleutian Range on the Alaskan Peninsula at 3.6 cal ka BP, was one of the largest Holocene eruptions worldwide. The resulting ash is found as a visible sediment layer in several Alaskan sites and as a cryptotephra on Newfoundland and Greenland. This large geographic distribution combined with the fact that the eruption is relatively well constrained in time using radiocarbon dating of lake sediments and annual layer counts in ice cores, makes it an excellent stratigraphic marker for dating and correlating mid – late Holocene sediment and paleoclimate records. This study presents the outcome of a targeted search for the Aniakchak tephra in a marine sediment core from the Arctic Ocean, namely Core SWERUS-L2-2-PC1 (2PC), raised from 72 m water depth in Herald Canyon, western Chukchi Sea. High concentrations of tephra shards, with a geochemical signature matching that of Aniakchak ash, were observed between 550 and 711 cm core depth. Since the primary input of volcanic ash is through atmospheric transport, and assuming that bioturbation can account for mixing up to ca 10 cm of the marine sediment deposited at the coring site, the broad signal is interpreted as sustained reworking at the sediment source input. The isochron is therefore placed at the base of the sudden increase in tephra concentrations rather than at the maximum concentration. This interpretation of major reworking is strengthened by analysis of grain size distribution which points to ice rafting as an important secondary transport mechanism of volcanic ash. Combined with radiocarbon dates on mollusks in the same sediment core, the volcanic marker is used to calculate a marine radiocarbon reservoir age offset ΔR = 477 ± 60 years. This relatively high value may be explained by the major influence of typically ''carbon-old'' Pacific waters and it agrees well with recent estimates of ΔR along the northwest Alaskan coast, possibly indicating stable oceanographic conditions during the second half of the Holocene. Our use of a volcanic absolute age marker to obtain the marine reservoir age offset, is the first of its kind in the Arctic Ocean and provides an important framework for improving chronologies and correlating marine sediment archives in this region. Core 2PC has a high sediment accumulation rate averaging 200 cm/kyr throughout the last 4000 years, and the chronology presented here provides a solid base for high resolution reconstructions of late Holocene climate and ocean variability in the Chukchi Sea.
Pearce, C., Varhelyi, A., Wastegård, S., Muschitiello, F., Barrientos, N., O'Regan, M., Cronin, T., Gemery, L., Semiletov, I., Backman, J., and Jakobsson, M.: The 3.6 ka Aniakchak tephra in the Arctic Ocean: a constraint on the
Holocene radiocarbon reservoir age in the Chukchi Sea, Clim. Past Discuss., doi:10.5194/cp-2016-112, in review, 2016.