Both social and engineering factors must be further explored and scrutinized across the globe—as should notions of justice related to flooding impacts and responses.
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Edited by John W. Williams, University of Wisconsin–Madison, Madison, WI, and accepted by the Editorial Board October 28, 2014 (received for review August 20, 2014)
Significance
New radiocarbon (14C) dates on American mastodon (Mammut americanum) fossils in Alaska and Yukon suggest this species suffered local extirpation before terminal Pleistocene climate changes or human colonization. Mastodons occupied high latitudes during the Last Interglacial (∼125,000–75,000 y ago) when forests were established. Ecological changes during the Wisconsinan glaciation (∼75,000 y ago) led to habitat loss and population collapse. Thereafter, mastodons were limited to areas south of the continental ice sheets, where they ultimately died out ∼10,000 14C years B.P. Extirpation of mastodons and some other megafaunal species in high latitudes was thus independent of their later extinction south of the ice. Rigorous pretreatment was crucial to removing contamination from fossils that originally yielded erroneously “young” 14C dates.
Abstract
Existing radiocarbon (14C) dates on American mastodon (Mammut americanum) fossils from eastern Beringia (Alaska and Yukon) have been interpreted as evidence they inhabited the Arctic and Subarctic during Pleistocene full-glacial times (∼18,000 14C years B.P.). However, this chronology is inconsistent with inferred habitat preferences of mastodons and correlative paleoecological evidence. To establish a last appearance date (LAD) for M. americanum regionally, we obtained 53 new 14C dates on 36 fossils, including specimens with previously published dates. Using collagen ultrafiltration and single amino acid (hydroxyproline) methods, these specimens consistently date to beyond or near the ∼50,000 y B.P. limit of 14C dating. Some erroneously “young” 14C dates are due to contamination by exogenous carbon from natural sources and conservation treatments used in museums. We suggest mastodons inhabited the high latitudes only during warm intervals, particularly the Last Interglacial [Marine Isotope Stage (MIS) 5] when boreal forests existed regionally. Our 14C dataset suggests that mastodons were extirpated from eastern Beringia during the MIS 4 glacial interval (∼75,000 y ago), following the ecological shift from boreal forest to steppe tundra. Mastodons thereafter became restricted to areas south of the continental ice sheets, where they suffered complete extinction ∼10,000 14C years B.P. Mastodons were already absent from eastern Beringia several tens of millennia before the first humans crossed the Bering Isthmus or the onset of climate changes during the terminal Pleistocene. Local extirpations of mastodons and other megafaunal populations in eastern Beringia were asynchrononous and independent of their final extinction south of the continental ice sheets.
Footnotes
- ↵1To whom correspondence should be addressed. Email: grant.zazula{at}gov.yk.ca.
Author contributions: G.D.Z., R.D.E.M., and J.Z.M. designed research; G.D.Z., R.D.E.M., J.Z.M., A.V.R., F.B., H.G.M., S.N.-C., and J.R.S. performed research; G.D.Z., R.D.E.M., J.Z.M., A.V.R., F.B., P.S.D., P.G., C.R.H., G.W.L.H., M.L.K., F.J.L., D.H.M., H.G.M., S.N.-C., and J.R.S. analyzed data; G.D.Z., R.D.E.M., J.Z.M., and A.V.R. wrote the paper; and P.S.D., P.G., C.R.H., M.L.K., D.H.M., and H.G.M. provided data.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.W.W. is a guest editor invited by the Editorial Board.
See Commentary on page 18405.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1416072111/-/DCSupplemental.
Arctic mastodon extirpation
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