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476 results on '"Southon, John R."'

151. A preliminary determination of the absolute 14C/12C ratio of OX-I

Author

Roberts, Mark L., Southon, John R., Roberts, Mark L., and Southon, John R.

Abstract

A preliminary determination of the absolute 14C/12C ratio of the oxalic acid I standard (NBS SRM 4990B) has been made. Using an accelerator mass spectrometry (AMS) system, a known number of radiocarbon ions were implanted in a thin copper foil. The foil was then combusted with the addition of 14C-free carrier material. This resulting gas (which had a known 14C/12C ratio) was graphitized and compared to OX-I material. This comparison yielded an absolute 14C/12C ratio of OX-I. The absolute 14C/12C ratio of OX-I, coupled with knowledge of the specific activity of OX-1, provides an alternative determination of the 14C half-life.

Published
2007

152. Bison phylogeography constrains dispersal and viability of the Ice Free Corridor in western Canada.

Author

Heintzman, Peter D., Froese, Duane, Ives, John W., Soares, André E. R., Zazula, Grant D., Letts, Brandon, Andrews, Thomas D., Driver, Jonathan C., Hall, Elizabeth, Hare, P. Gregory, Jassi, Christopher N., MacKay, Glen, Southon, John R., Stiller, Mathias, Woywitka, Robin, Suchard, Marc A., and Shapir, Beth

Subjects
BISON, PHYLOGEOGRAPHY, SPECIES distribution, MITOCHONDRIAL DNA, PLEISTOCENE paleoecology, RADIOCARBON dating, FOSSIL bovidae, BIOGEOGRAPHY
Abstract

The Ice Free Corridor has been invoked as a route for Pleistocene human and animal dispersals between eastern Beringia and more southerly areas of North America. Despite the significance of the corridor, there are limited data for when and how this corridor was used. Hypothetical uses of the corridor include: the first expansion of humans from Beringia into the Americas, northward postglacial expansions of fluted point technologies into Beringia, and continued use of the corridor as a contact route between the north and south. Here, we use radiocarbon dates and ancient mitochondrial DNA from late Pleistocene bison fossils to determine the chronology for when the corridor was open and viable for biotic dispersals. The corridor was closed after ~23,000 until 13,400 calendar years ago (cal y BP), after which we find the first evidence, to our knowledge, that bison used this route to disperse from the south, and by 13,000 y from the north. Our chronology supports a habitable and traversable corridor by at least 13,000 cal y BP, just before the first appearance of Clovis technology in interior North America, and indicates that the corridor would not have been available for significantly earlier southward human dispersal. Following the opening of the corridor, multiple dispersals of human groups between Beringia and interior North America may have continued throughout the latest Pleistocene and early Holocene. Our results highlight the utility of phylogeographic analyses to test hypotheses about paleoecological history and the viability of dispersal routes over time. [ABSTRACT FROM AUTHOR]

Published
2016
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154. Widespread collapse of the Ross Ice Shelf during the late Holocene.

Author

Yusuke Yokoyama, Anderson, John B., Masako Yamane, Simkins, Lauren M., Yosuke Miyairi, Takahiro Yamazaki, Mamito Koizumi, Hisami Suga, Kusahara, Kazuya, Prothro, Lindsay, Hiroyasu Hasumi, Southon, John R., and Naohiko Ohkouchi

Subjects
CLIMATE change, ABRUPT climate change, MARINE sediments, ICE shelves, ICE sheets
Abstract

The stability of modern ice shelves is threatened by atmospheric and oceanic warming. The geologic record of formerly glaciated continental shelves provides a window into the past of how ice shelves responded to a warming climate. Fields of deep (-560 m), linear iceberg furrows on the outer, western Ross Sea continental shelf record an early post-Last Glacial Maximum episode of ice-shelf collapse that was followed by continuous retreat of the grounding line for ~200 km. Runaway grounding line conditions culminated once the ice became pinned on shallow banks in the western Ross Sea. This early episode of ice-shelf collapse is not observed in the eastern Ross Sea, where more episodic grounding line retreat took place. More widespread (~280,000 km2) retreat of the ancestral Ross Ice Shelf occurred during the late Holocene. This event is recorded in sediment cores by a shift from terrigenous glacimarine mud to diatomaceous open-marine sediment as well as an increase in radiogenic beryllium (10Be) concentrations. The timing of ice-shelf breakup is constrained by compound specific radiocarbon ages, the first application of this technique systematically applied to Antarctic marine sediments. Breakup initiated around 5 ka, with the ice shelf reaching its current configuration ~1.5 ka. In the eastern Ross Sea, the ice shelf retreated up to 100 km in about a thousand years. Threedimensional thermodynamic ice-shelf/ocean modeling results and comparison with ice-core records indicate that ice-shelf breakup resulted from combined atmospheric warming and warm ocean currents impinging onto the continental shelf. [ABSTRACT FROM AUTHOR]

Published
2016
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155. Comment on 'Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/234U/238U and 14C dates on pristine corals' by R.G. Fairbanks et al. (Quaternary Science Reviews 24 (2005) 1781-1796), and 'Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals' by T.-C. Chiu et al. (Quaternary Science Reviews 24 (2005) 1797-1808).

Author

Reimer, Paula J., Baillie, Mike G. L., Bard, Edouard, Beck, J. Warren, Blackwell, Paul G., Buck, Caitlin E., Burr, George S., Edwards, R. Lawrence, Friedrich, Michael, Guilderson, Thomas P., Hogg, Alan G., Hughen, Konrad A., Kromer, Bernd, McCormac, Gerry, Manning, Sturt, Reimer, Ron W., Southon, John R., Stuiver, Minze, van der Plicht, Johannes, Weyhenmeyer, Constanze E., Reimer, Paula J., Baillie, Mike G. L., Bard, Edouard, Beck, J. Warren, Blackwell, Paul G., Buck, Caitlin E., Burr, George S., Edwards, R. Lawrence, Friedrich, Michael, Guilderson, Thomas P., Hogg, Alan G., Hughen, Konrad A., Kromer, Bernd, McCormac, Gerry, Manning, Sturt, Reimer, Ron W., Southon, John R., Stuiver, Minze, van der Plicht, Johannes, and Weyhenmeyer, Constanze E.

Abstract

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Quaternary Science Reviews 25 (2006): 855-862, doi:10.1016/j.quascirev.2005.09.009., A recently published radiocarbon calibration curve extending to 50,000 cal BP (Fairbanks et al. 2005) is purportedly superior to that generated by the IntCal working group beyond the end of the tree-ring data at 12,400 cal BP (Reimer et al. 2004). This claim is based, in part, on different diagenetic screening criteria and pretreatment for coral samples (Fairbanks et al. 2005; Chiu et al. 2005) which do not stand up under careful scrutiny. Also at issue is the conversion of the coral-based calibration curve to an atmospheric curve where large inter-annual variability in the sea-surface age reservoir age has been observed in the southwest Pacific where one of two sets of corals used were sampled. In addition we comment on the seemingly ad hoc statistical methods utilized by Fairbanks et al. (2005) to construct their curve. We recognize the value of the Fairbanks et al. (2005) coral radiocarbon data set, but reassert the need for multiple, independently derived data to provide confirmation and validation to all radiocarbon calibration data. This is especially important before 26,000 cal BP when lower sea-levels of the last glacial maximum exposed most coral samples to alteration by fresh water.

Published
2006

156. Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks

Author

Miller, Gifford H., primary, Geirsdóttir, Áslaug, additional, Zhong, Yafang, additional, Larsen, Darren J., additional, Otto-Bliesner, Bette L., additional, Holland, Marika M., additional, Bailey, David A., additional, Refsnider, Kurt A., additional, Lehman, Scott J., additional, Southon, John R., additional, Anderson, Chance, additional, Björnsson, Helgi, additional, and Thordarson, Thorvaldur, additional

Published
2012
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165. A new look at old carbon in active margin sediments

Author

Drenzek, Nicholas J., primary, Hughen, Konrad A., additional, Montluçon, Daniel B., additional, Southon, John R., additional, dos Santos, Guaciara M., additional, Druffel, Ellen R.M., additional, Giosan, Liviu, additional, and Eglinton, Timothy I., additional

Published
2009
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171. Comment on “Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/234U/238U and 14C dates on pristine corals” by R.G. Fairbanks et al. (Quaternary Science Reviews 24 (2005) 1781–1796)☆☆doi of original article: 10.1016/j.quascirev.2005.04.007 and “Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals” by T.-C. Chiu et al. (Quaternary Science Reviews 24 (2005) 1797–1808)☆☆☆☆doi of original article: 10.1016/j.quascirev.2005.04.002

Author

Reimer, Paula J., primary, Baillie, Mike G.L., additional, Bard, Edouard, additional, Warren Beck, J., additional, Blackwell, Paul G., additional, Buck, Caitlin E., additional, Burr, George S., additional, Lawrence Edwards, R., additional, Friedrich, Michael, additional, Guilderson, Thomas P., additional, Hogg, Alan G., additional, Hughen, Konrad A., additional, Kromer, Bernd, additional, McCormac, Gerry, additional, Manning, Sturt, additional, Reimer, Ron W., additional, Southon, John R., additional, Stuiver, Minze, additional, van der Plicht, Johannes, additional, and Weyhenmeyer, Constanze E., additional

Published
2006
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174. Marine04 Marine Radiocarbon Age Calibration, 0–26 Cal Kyr Bp

Author

Hughen, Konrad A, primary, Baillie, Mike G L, additional, Bard, Edouard, additional, Warren Beck, J, additional, Bertrand, Chanda J H, additional, Blackwell, Paul G, additional, Buck, Caitlin E, additional, Burr, George S, additional, Cutler, Kirsten B, additional, Damon, Paul E, additional, Edwards, Richard L, additional, Fairbanks, Richard G, additional, Friedrich, Michael, additional, Guilderson, Thomas P, additional, Kromer, Bernd, additional, McCormac, Gerry, additional, Manning, Sturt, additional, Bronk Ramsey, Christopher, additional, Reimer, Paula J, additional, Reimer, Ron W, additional, Remmele, Sabine, additional, Southon, John R, additional, Stuiver, Minze, additional, Talamo, Sahra, additional, Taylor, F W, additional, VAN der Plicht, Johannes, additional, and Weyhenmeyer, Constanze E, additional

Published
2004
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176. Comment on “Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired 230Th/ 234U/ 238U and 14C dates on pristine corals” by R.G. Fairbanks et al. (Quaternary Science Reviews 24 (2005) 1781–1796) and “Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals” by T.-C. Chiu et al. (Quaternary Science Reviews 24 (2005) 1797–1808)

Author

Reimer, Paula J., Baillie, Mike G.L., Bard, Edouard, Warren Beck, J., Blackwell, Paul G., Buck, Caitlin E., Burr, George S., Lawrence Edwards, R., Friedrich, Michael, Guilderson, Thomas P., Hogg, Alan G., Hughen, Konrad A., Kromer, Bernd, McCormac, Gerry, Manning, Sturt, Reimer, Ron W., Southon, John R., Stuiver, Minze, van der Plicht, Johannes, and Weyhenmeyer, Constanze E.

Published
2006
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180. Preliminary Report of the First Workshop of the Intcal04 Radiocarbon Calibration/Comparison Working Group

Author

Reimer, Paula J, primary, Hughen, Konrad A, additional, Guilderson, Thomas P, additional, McCormac, Gerry, additional, Baillie, Mike G L, additional, Bard, Edouard, additional, Barratt, Phillip, additional, Warren Beck, J, additional, Buck, Caitlin E, additional, Damon, Paul E, additional, Friedrich, Michael, additional, Kromer, Bernd, additional, Bronk Ramsey, Christopher, additional, Reimer, Ron W, additional, Remmele, Sabine, additional, Southon, John R, additional, Stuiver, Minze, additional, and VAN der Plicht, Johannes, additional

Published
2002
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189. American mastodon extirpation in the Arctic and Subarctic predates human colonization and terminal Pleistocene climate change.

Author

Zazula, Grant D., MacPhee, Ross D. E., Metcalfe, Jessica Z., Reyes, Alberto V., Brock, Fiona, Druckenmiller, Patrick S., Groves, Pamela, Harington, C. Richard, Hodgins, Gregory W. L., Kunz, Michael L., Longstaffe, Fred J., Mann, Daniel H., McDonald, H. Gregory, Nalawade-Chavan, Shweta, and Southon, John R.

Subjects
MASTODONS, CARBON isotopes, PLEISTOCENE Epoch, CLIMATE change, ULTRAFILTRATION
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. [ABSTRACT FROM AUTHOR]

Published
2014
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