Guest shoot-down by David Middleton
In my previous two posts on uniformitarian impact craters, we examined the pitfalls of drawing cartoons on Google Earth images without ever looking at the geology and how the Carolina Bays are as antithetical to impact features as any dents in the ground possibly could be. Judging by some of the comments, this seems to have given some readers the opinion that I am an opponent of the Younger Dryas Impact Hypothesis (YDIH), despite repeatedly stating that I find some of the evidence compelling.
For the record, I have not drawn any conclusions about whether or not a significant impact event or events may have occurred coincident with the Younger Dryas glacial stadial. However, I do have firmly negative opinions about science fiction-derived and sloppy science-derived evidence for the YDIH and other similarly derived impact hypotheses.
This will be another long post. If the subject matter doesn’t interest you… Sorry.
This post will deal primarily with one issue in the latest YDIH paper (Wolbach et al,. 2018): A purported increase in atmospheric CO2 during the Younger Dryas as evidence of “extraordinary biomass-burning.” While I will discuss other aspects of the Younger Dryas and YDIH, my main focus will be on atmospheric CO2 and CH4.
Real Clear Science
I have the greatest respect for Ross Pomeroy, Chief Editor of RealClearScience (RCS), as I do for RCS’s founding editor Alex Berezow. Mr. Pomeroy recently wrote a very thoughtful article about Wolbach et al., 2018:
A Cometary Upheaval in the History of North America
By Ross Pomeroy
May 10, 2018
Around 12,900 years ago, Earth’s climate abruptly changed, but the upheaval was particularly felt in North America. Over the span of just a decade, temperatures fell between 3.6 and 10.8 degrees Fahrenheit on average. Glaciers which had been gradually receding through Canada reversed course and crept southward. The air dried, and droughts became frequent. Megafauna like mammoths, camels, and giant bears couldn’t adapt to the sudden changes and died out in droves, their extinction abetted by hungry humans on the hunt.
Dozens, and perhaps hundreds, of scientists from all across the world now ascribe to the notion that some sort of cometary impact triggered the Younger Dryas cooling and extinctions.
But critics have countered their claims every step of the way, questioning the dating techniques used, disputing the origin of the nanodiamonds, and pointing out that no large impact craters linked to the Younger Dryas have yet been found. Their fierce skepticism is justified. Supporters of a Younger Dryas impact hypothesis are making a bold claim. It is their responsibility to back it up.
And back it up they have. In the past few years, researchers have published dozens of peer-reviewed scientific papers to support a comet impact. Just a few months ago, they released their most exhaustive research yet – two studies of 129 lake cores taken from around the world showing that 12,900 years ago there was a spike in sediment charcoal levels, a clear sign of burning on a massive scale. Moreover, ice cores clearly indicate a ten percent spike in global carbon dioxide levels at the same time! It seems something cataclysmic happened just before the Younger Dryas that sent all sorts of carbon into the atmosphere.
While scientists on each side of this issue have stood opposed, debating for their preferred theories, they have been counterintuitively working together. Every point and counterpoint has forced the other side to refine their ideas and search for new evidence. Indeed if the impact hypothesis does finally win out, it will take its place as the leading theory largely thanks to its most ardent critics.
This is scientific debate at its best.
I agree wholeheartedly with Mr. Pomeroy here:
Indeed if the impact hypothesis does finally win out, it will take its place as the leading theory largely thanks to its most ardent critics.
Genuine scientific debate is good for science. It strengthens valid hypotheses. However, as Holliday et al., 2016 demonstrated, the YDIH has a long way to go.
The Younger Dryas Impact Hypothesis (YDIH) states that North America was devastated by some sort of extraterrestrial event ~12,800 calendar years before present. Two fundamental questions persist in the debate over the YDIH: Can the results of analyses for purported impact indicators be reproduced? And are the indicators unique to the lower YD boundary (YDB), i.e., ~12.8k cal yrs BP? A test reported here presents the results of analyses that address these questions. Two different labs analyzed identical splits of samples collected at, above, and below the ~12.8ka zone at the Lubbock Lake archaeological site (LL) in northwest Texas.
The results of analyses of blind samples collected at the Lubbock Lake site to test the YD impact hypothesis produced no evidence of an extraterrestrial impact at the YDB. The results from one lab show no peak in magnetic grains nor in magnetic microspheres but data from another lab shows significantly elevated levels of purported impact indicators (magnetic microspherules and nanodiamonds) at <11,500 cal yrs BP, well over 1000 years later than the YDB. These results are consistent with a growing body of data that shows that claimed impact indicators are found in deposits both older and younger than the YDB.
To move forward and better understand what happened at the YDB and also to understand the meaning of purported impact markers, stratigraphic sections with continuous records of sedimentation through the late Pleistocene and Holocene must be sampled throughout at close intervals and dated using high precision methods. To date, only Bement et al  report such an approach and their data show peaks in possible impact indicators above and below the YDB. Further, agreed upon criteria must be established for microspherule identification; otherwise counts of spheres are pointless. Complete stratigraphic descriptions of sampled sections also are needed, indicating sediment lithologies, weathering zones, soil morphology, and erosional unconformities. Few are available from among the dozens of sites with claimed impact proxies but are critical for evaluating the depositional context of impact proxies and the interpretation of numerical dates.
The YDIH needs a lot more uniformitarian geology (AKA science) before it will be widely accepted as anything more than a novelty. Although there does seem to be a growing body of evidence to support a platinum group elements (PGE) anomaly:
In addition, the discoveries by Dr. Chris Moore and his colleagues could provide support for some scientists’ claims that a large number of Paleo-Indians from the Clovis culture also died off around that time because of a comet strike.
“It may have been an impact without any impact, but my suspicion is that it probably did have an effect because it is coincident with a major ecological calamity,” said Moore, outreach coordinator for the Savannah River Archeological Research Program, which is based at the Savannah River Site.
Overhunting of megafauna by humans and natural changes in the climate and the environment also could have been factors, he added.
“As things usually are, it probably was a combination of events,” Moore said. “The debate has raged in paleoanthropology and archaeology. The comet thing is sort of a new twist on it, and some scientists absolutely do not buy it.”
Moore’s team wrote a paper about its work that was published by Scientific Reports earlier this year.
In 2013, researchers from Harvard University revealed that they had detected higher-than-normal concentrations of platinum in ice core samples from Greenland that coincided with the start of Younger Dryas.
The scientists said the likely source of the precious metal was a “sub-kilometer iron meteorite.”
Moore and his colleagues wanted to see if they could find the same platinum anomalies in sediment samples from other places.
“Platinum is rare in the Earth’s crust, but it’s common in asteroids and comets,” Moore said. “The only way it could have gotten into the ice was through atmospheric input, which means it fell out of the atmosphere.”
Moore’s team analyzed sediment samples that were about the same age as the Greenland ice core samples from 11 different sites in the United States.
One of the locations was on Santa Rosa Island off the coast of California. Five were in South Carolina, two were in North Carolina and there was one each in Arizona, New Mexico and Ohio.
Nearly all of the sediment samples contained abnormally high levels of platinum.
One of Dr. Moore’s recent publications fairly conclusively demonstrated that the Carolina Bays were not impact features. Other evidence for the YDIH does not constitute evidence that the Carolina Bays are impact features… And the Caroline Bays not being impact features doesn’t shoot down the YDIH. Dr. Moore clearly has a favorable view of the YDIH despite delivering a clean kill to the notion that the Carolina Bays are impact features. If the YDIH is substantially confirmed, it will be through science and scientific debate, not science fiction and cartoons.
While I generally agreed with Mr. Pomeroy’s article, this sentence set my internal alarm bell off:
Moreover, ice cores clearly indicate a ten percent spike in global carbon dioxide levels at the same time!
Having read dozens, if not hundreds, of papers on ice core and plant stomata-derived CO2 histories of the Late Pleistocene and Holocene over the past decade, this statement didn’t sound reasonable.
A Little Background on the Younger Dryas glacial stadial
The Younger Dryas is also known as Glacial Stadial 1 (GS-1). Glacial stadials are the cold phases of glacial stages, some are referred to as Heinrich events. Interstadials (AKA Dansgaard-Oeshger events) are the warm phases of glacial stages. Some interstadials of the most recent Pleistocene glacial stage warmed to nearly interglacial temperatures.
As Late Pleistocene glacial stadials go, the Younger Dryas was not particularly anomalous. If there was a climatology anomaly, it was the warmth of the preceding Bølling-Allerød interstadial (sometimes subdivided).
Late Quaternary Extinctions
While the Younger Dryas and the transition from the last Pleistocene glacial stage to the current interglacial stage (the Holocene) may have been unremarkable from a climatology perspective, about 90 genera of megafauna (mammals weighing at least 44 kg) became extinct. While not classified as a mass extinction event, the Late Quaternary extinctions were significant and have been difficult to explain.
The Late Pleistocene-Early Holocene megafauna extinctions took place over 10’s of thousands of years at different times on different continents (Koch & Barnosky, 2006).
“Australia lost 14 of its 16 genera of Pleistocene mammalian megafauna along with
all megafaunal reptiles” (Koch & Barnosky, 2006). By 40,000 years ago, Australia had already lost more than 90% its larger species (Prideaux et al., 2010).
North American (Ranchlabrean) extinctions appear to have occurred much later and possibly in at least two phases. While 16 of 35 Rancholabrean extinctions took place during the terminal Pleistocene (~2,000 yr period coincident with the Younger Dryas, the other 19 genera disappeared from the North American fossil record thousands of years earlier (Faith & Surovell, 2009).
Disagreement over the chronology of North American late Pleistocene extinctions stems largely from an incomplete fossil record. Of the 35 genera to disappear from North America, only 16 can be shown to have survived to between 12,000 and 10,000 radiocarbon years B.P. Those 16 genera known from the terminal Pleistocene have been observed to be better represented in the fossil record than those that are not (8, 18, 22). This raises the possibility that the remaining 19 genera have not been dated to the terminal Pleistocene because of their rarity in the fossil record (27). Terminal Pleistocene dates also possibly are lacking for some genera because they did not survive to that time. If so, then this would imply a more complex causality than that supposed by extinction hypotheses requiring a high degree of simultaneity (e.g., overkill or extraterrestrial impact).
2,000 years is a geological blink of the eye. Something catastrophic may happened in North America during the terminal Pleistocene. This was also when the Folsom culture replaced the Clovis culture. A bolide is certainly a possibility.
So… This takes us to Wolbach et al., 2018.
Extraordinary Biomass-Burning… Maybe, Maybe Not
Extraordinary Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas Cosmic Impact ∼12,800 Years Ago.
1. Ice Cores and Glaciers
The Younger Dryas boundary (YDB) cosmic-impact hypothesis is based on considerable evidence that Earth collided with fragments of a disintegrating ≥100-km-diameter comet, the remnants of which persist within the inner solar system ∼12,800 y later. Evidence suggests that the YDB cosmic impact triggered an “impact winter” and the subsequent Younger Dryas (YD) climate episode, biomass burning, late Pleistocene megafaunal extinctions, and human cultural shifts and population declines. The cosmic impact deposited anomalously high concentrations of platinum over much of the Northern Hemisphere, as recorded at 26 YDB sites at the YD onset, including the Greenland Ice Sheet Project 2 ice core, in which platinum deposition spans ∼21 y (∼12,836–12,815 cal BP). The YD onset also exhibits increased dust concentrations, synchronous with the onset of a remarkably high peak in ammonium, a biomass-burning aerosol. In four ice-core sequences from Greenland, Antarctica, and Russia, similar anomalous peaks in other combustion aerosols occur, including nitrate, oxalate, acetate, and formate, reflecting one of the largest biomass-burning episodes in more than 120,000 y. In support of widespread wildfires, the perturbations in CO2 records from Taylor Glacier, Antarctica, suggest that biomass burning at the YD onset may have consumed ∼10 million km2 , or ∼9% of Earth’s terrestrial biomass. The ice record is consistent with YDB impact theory that extensive impact-related biomass burning triggered the abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous YD climate episode.
The Taylor Glacier data are listed as Basuka et al., 2016 in the NOAA paleoclimatology library. Brook et al., 2015 was one of several papers produced as a result of the NSF grant. The full text of Basuka et al., 2016 is available from PNAS:
We extracted atmospheric gases from large (400–500 g) samples taken from surface outcrops of ancient ice at Taylor Glacier, Antarctica, at an average temporal resolution of 165 y between 20 and 10 ka, and subcentury resolution during rapid change events.
A recent high-resolution record from the West Antarctic Ice Sheet (WAIS) Divide ice core demonstrated that rapid increases in CO2 of about 12 ppm at both the onset of the BA (14.6 ka) and end of the YD (11.5 ka) occurred exactly coincident with abrupt increases in CH4 and Northern Hemisphere (NH) temperature (13).
A resolution of 165 years means that CO2 shifts lasting less than 165 years cannot be resolved. They note that the higher resolution WAIS Divide core puts the CO2 spikes at the onset of the Bølling-Allerød interstadial and termination of the Younger Dryas stadial, citing Marcott et al., 2014…
The WDC methane record shows in detail the abrupt changes at the onset of the Bølling– Allerød and Younger Dryas stadials and the start of the Holocene. We observe a smaller, but prominent, methane excursion at 16.3 kyr ago not previously reported from other records (Fig. 1). Our record also resolves the beginning of the deglacial methane rise at 17.8 kyr ago (Extended Data Table 1).
The WDC CO2 record demonstrates that CO2 varied in three distinct modes during the deglaciation. The first mode is relatively gradual change (,10 p.p.m. kyr21): such changes in CO2 began at 18.1 and 13.0 kyr ago and were broadly coincident with a reduction in the strength of the Atlantic meridional overturning circulation (AMOC; Fig. 2i), a cold North Atlantic2 and warming in the Southern Hemisphere2. The second mode is rapid increase: 10–15 p.p.m. increases in CO2 occurred in three short (100–200 yr) intervals at 16.3, 14.8 and 11.7 kyr ago, the latter two at times of rapid resumption of the AMOC and warming in the Northern Hemisphere. The rapid changes at 14.8 and 11.7 kyr ago were first noted at EDC9,14, but the magnitude, duration and timing are now more fully resolved because of the unique site conditions at WDC.
The highest resolution Antarctic ice cores covering the Younger Dryas are the WAIS Divide cores. This cores clearly demonstrates abrupt spikes in atmospheric CO2 at the onset of the Bølling-Allerød interstadial and the termination of the Younger Dryas stadial. These spikes punctuate the secular rise from the last glacial maximum (LGM) to the Holocene. WDC also clearly demonstrates a sharp drop in CH4 during the Younger Dryas.
Furthermore, the Antarctic equivalent of the Younger Dryas cooling event (Antarctic Cold Reversal) preceded the Younger Dryas by about 1,000 years. Antarctica was warming throughout the Younger Dryas. The rise in atmospheric CO2 in Antarctic ice cores during the Younger Dryas is consistent with the secular rise in CO2 during deglaciation, most likely due to warming of the southern oceans.
Some time ago CO2 chronologies from Greenland ice cores were cast aside because they consistently indicated higher and more variable CO2 levels during the Late Quaternary. It was assumed that in situ chemical reactions from volcanic dust were the cause of these anomalies. However, CO2 chronologies from Greenland ice cores clearly demonstrate a sharp, short-duration decline during the Younger Dryas. Antarctic ice cores of comparable age cannot resolve such short duration shifts in atmospheric CO2.
And here’s the cherry on top of the ice cream sundae…
Stomata chronologies clearly demonstrate an abrupt spike in in atmospheric CO2 during the Bølling-Allerød interstadial (GI-1), followed by a sharp decline during the Younger Dryas stadial (GS-1). The secular rise from the last glacial maximum (LGM) to the Holocene (H) coupled with the smoothing effect of the gas age distribution in the Antarctic ice core creates a false impression of rising CO2 during the Younger Dryas.
Clearly there’s no evidence of an anomalous rise in atmospheric CO2 during the Younger Dryas. Equally troubling for the extraordinary biomass-burning episode is the collapse of atmospheric CH4 during the Younger Dryas.
The current rate of biomass-burning contributes 5-15% of the methane budget.
Resent estimates indicate that biomass burning contributes between about 20 to about 60 Teragrams per year of carbon in the form of methane to the atmosphere. This represents only 5 to 15% of the global annual emissions of methane.
The rapid burning of 9% of Earth’s terrestrial biomass should have left a mark in the methane record.
Oddly enough, biomass burning did leave a mark in the methane record. Unfortunately, it’s the wrong kind of mark for the YDIH.
Biome and Fire Regime Changes Caused by Megafauna Extinction.
Arguably, the most surprising feature of our records (Fig. 1) is the pronounced difference in absolute levels for both δ13CH4 and δD(CH4) for the Holocene compared with MIS 5.5 and MIS 11.3 and for the LGM compared with MIS 6 and MIS 12. Shifts of ∼2–3‰ for δ13CH4and 10–18‰ for δD(CH4) toward higher numbers are found, with no obvious difference in CH4 mixing ratio between these time slices (Table S1) (42). Straightforward explanations for similar [CH4] accompanied by shifted isotope records require changes in the source signatures or changes in emission strength of a source with strong leverage. To our knowledge, no general isotope shifts of that size have been described in precursor materials for methanogenesis before MIS 2. It is also unlikely that the source strength or signature of GEM (Geologic Emissions of Methane) or biogenic emissions changed markedly compared with previous glacial/interglacial cycles. In fact, GEM is expected to change in response to sea level or ice sheet extent, but the two parameters remain within a similar range for all glacials and all interstadials considered in this study. One possibility to reconcile the observations is CH4emission changes related to changes in biomes and fire regimes, because BB is a CH4source strongly enriched in 13C and D (13, 22, 65). BB (Biomass-Burning) is an ancient and persistent feature throughout the geologic record (113), and there is evidence of net changes in fire regimes as a consequence of the megafauna extinction that was presumably caused by rapid climate changes in combination with human interference in the course of the last glacial (refs. 114⇓⇓⇓⇓–119 and references therein). The review by Johnson (120) on the timing of the arrival of humans on different continents and the ecological consequences of megafauna extinction supports the idea that increased fire frequency was caused by increased vegetation density and the accumulation of plant material not consumed by herbivores. For example, records from Australia of charcoal, different plant pollen types, and spores of the fungus Sporormiella are used by Rule et al. (121) to indicate large herbivore activity and conclude that megafauna extinction caused increased fire activity after 41 ka BP. Furthermore, these Australian records show that fires were common during the Holocene but much less frequent in the previous interglacial. We note that responses might be different in other parts of the globe (122⇓–124) and that, today, Australia accounts for only roughly 6% of global fire carbon emissions (125). However, other authors reported similar observations of fire activity changes on other continents (126, 127), but a global synthesis is not available yet.
The period from the Last Glacial Maximum (LGM) through the Holocene (~25,000 yrs) exhibits elevated biomass-burning (BB) markers. Unfortunately for the YDIH, “BB is a CH4 source strongly enriched in 13C,” not a CH4 sink depleted in 13C (Melton et al., 2012).
The abrupt warming across the Younger Dryas termination (∼11,600 yr before present) was marked by a large increase in the global atmospheric methane mixing ratio. The debate over sources responsible for the rise in methane centers on the roles of global wetlands, marine gas hydrates, and thermokarst lakes. We present a new, higher-precision methane stable carbon isotope ratio (δ13CH4) dataset from ice sampled at Pakitsoq, Greenland that shows distinct ˚13C enrichment associated with this rise.
- There was no anomalous rise in atmospheric CO2 associated with the Younger Dryas glacial stadial. CO2 concentrations sharply declined in the northern hemisphere.
- The key biomass-burning marker, elevated 13C enriched atmospheric CH4, is totally lacking. The Younger Dryas exhibits a global decline in CH4.
If anyone is troubled by the general lack of sarcastic smart@$$ comments in this post, I apologize. By the time I finished it up, I realized that I hadn’t liberally applied my usually dose of sracasm and pop culture references. This is probably due to having seen Avengers: Infinity War this past weekend (two thumbs up) and I am still emotionally drained (/SARC).
Alley, R.B. 2000. “The Younger Dryas cold interval as viewed from central Greenland.”
Quaternary Science Reviews 19:213-226.
Anklin, M., J. Schwander, B. Stauffer, J. Tschumi, A. Fuchs, J. M. Barnola, and D. Raynaud (1997), “CO2record between 40 and 8 kyr B.P. from the Greenland Ice Core Project ice core,” J. Geophys. Res., 102(C12), 26539–26545, doi: 10.1029/97JC00182.
Bauska, Thomas K., Daniel Baggenstos, Edward J. Brook, Alan C. Mix, Shaun A. Marcott, Vasilii V. Petrenko, Hinrich Schaefer, Jeffrey P. Severinghaus, James E. Lee. “Carbon dioxide isotopes during the deglaciation.” Proceedings of the National Academy of Sciences Mar 2016, 113 (13) 3465-3470; DOI: 10.1073/pnas.1513868113
Blunier, T., J. Schwander, B. Stauffer, T. Stocker, A. Dällenbach, A. Indermühle, J. Tschumi, J. Chappellaz, D. Raynaud, J.‐M. Barnola. “Timing of the Antarctic cold reversal and the atmospheric CO2 increase with respect to the Younger Dryas Event.” GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 21, PAGES 2683-268. 1 November 1997 https://doi.org/10.1029/97GL02658
Bock, Michael, Jochen Schmitt, Jonas Beck, Barbara Seth, Jérôme Chappellaz, Hubertus Fischer. “Ice core isotope data constrain methane emissions.” Proceedings of the National Academy of Sciences Jul 2017, 114 (29) E5778-E5786; DOI: 10.1073/pnas.1613883114
Faith, J. Tyler, Todd A. Surovell. “Synchronous extinction of North America’s Pleistocene mammals.” Proceedings of the National Academy of Sciences Dec 2009, 106 (49) 20641-20645; DOI: 10.1073/pnas.0908153106
Hoesel, Annelies & Hoek, W.Z. & Pennock, G.M. & Drury, M.R.. (2014). “The Younger Dryas impact hypothesis: A critical review.” Quaternary Science Reviews. 83. 95–114. 10.1016/j.quascirev.2013.10.033. (Abstract only).
Holliday, V., Surovell, T, Johnson, E (2016) “A Blind Test of the Younger Dryas Impact Hypothesis.” PLoS ONE 11(7): e0155470. https://doi.org/10.1371/journal.pone.0155470
Koch, Paul L. and Anthony D. Barnosky. “Late Quaternary Extinctions: State of the Debate.” Annu. Rev. Ecol. Evol. Syst. 2006. 37:215–50
Levine, J. S. (1990), “Global biomass burning: Atmospheric, climatic and biospheric implications,” Eos Trans. AGU, 71(37), 1075–1077, doi: 10.1029/90EO00289. (Abstract only).
Levine, J., W. Cofer, III, AND J. Pinto. (2002) “BIOMASS BURNING AND THE PRODUCTION OF METHANE.” U.S. Environmental Protection Agency, Washington, D.C., EPA/600/A-92/221 (NTIS PB93119824). (Abstract only).
Marcott, Shaun A., Thomas K. Bauska, Christo Buizert, Eric J. Steig, Julia L. Rosen, Kurt M. Cuffey, T. J. Fudge, et al. “Centennial-Scale Changes in the Global Carbon Cycle during the Last Deglaciation.” Nature 514, no. 7524 (2014): 616–19. doi:10.1038/NATURE13799.
Melton, R., J & Schaefer, Hinrich & J. Whiticar, M. (2011). “Enrichment in 13C of atmospheric CH4 during the Younger Dryas termination.” Climate of The Past Discussions. 7. 10.5194/cpd-7-3287-2011.
Moore, Christopher & Brooks, Mark & Mallinson, David & Parham, Peter & Ivester, Andrew & K. Feathers, James. (2016). “The Quaternary evolution of Herndon Bay, a Carolina Bay on the Coastal Plain of North Carolina (USA): implications for paleoclimate and oriented lake genesis.” Southeastern Geology. 51. 145-171.
Moore, Christopher & West, Allen & Lecompte, Malcolm & Brooks, Mark & Randolph, I & Goodyear, Albert & Ferguson, Terry & Ivester, Andrew & K Feathers, James & P Kennett, James & Tankersley, Kenneth & Adedeji, A.V & E Bunch, Ted. (2017). “Widespread platinum anomaly documented at the Younger Dryas onset in North American sedimentary sequences.” Nature: Scientific Reports. 7:44031. 1-9. 10.1038/srep44031.
Prideaux, Gavin J., Grant A. Gully, Aidan M. C. Couzens, Linda K. Ayliffe, Nathan R. Jankowski, Zenobia Jacobs, Richard G. Roberts, John C. Hellstrom, Michael K. Gagan, Lindsay M. Hatcher. “Timing and dynamics of Late Pleistocene mammal extinctions in southwestern Australia.” Proceedings of the National Academy of Sciences Dec 2010, 107 (51) 22157-22162; DOI: 10.1073/pnas.1011073107
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May 14, 2018 at 04:28PM