I'm still working on the last glacial transition. The other week I drafted this comment:
Any questions?Some remarks on
"Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling"
by R. B. Firestone et al. 2007
Firestone et al. 2007 link evidence of an unidentified extraterrestrial event (ETE) to the onset of the Younger Dryas cooling which may have contributed to the megafauna extinction. We do not contest the evidence or occurrence of this extraterrestrial event, but the proposed consequences meet a considerable challenge when confronted with other evidence. We intend to demonstrate that the timing for the onset of the Younger Dryas is at odds with the bulk of the evidence. Furthermore, comparison of isotope records of multiple proxies show the same characteristics for the termination of the Dansgaard Oeschger events as well as the Bølling-Allerød events, suggesting that no foreign cause is required to explain the termination of the latter. Finally, on a global scale, the extinction of the megafauna happened rather gradually between 18,000 and 4,000 years ago in North America, but many endemic species did not die out from the event.
The dating gap of the onset of the Younger Dryas exceeds the error margin
Based on two series of carbon dates which mark the end of the Clovis stratum, Firestone et al. establish the dating of the ETE at 12.9 +/- 0.1 ka Cal BP. This date is cross-checked with the onset of the Younger Dryas in the Greenland GISP-2 ice core marked by a sharp drop of water isotopes.
These carbon date series, averaging 10,890 14C years and 10,940 14C years BP, would calibrate to 12.87 and 12.88 ka Cal BP using the INTCAL04 calibration table (Reimer et al., 2004) while the article mentions 12.92 and 12.93 ka Cal BP, which is likely based on the previous INTCAL98 calibration table (Stuiver et al., 1998).
We compare the isotope records of the major Greenland ice cores for the period of the onset of the Younger Dryas marked by a sudden drop in d18O isotope values of about 4-5 mil.
Insert fig1 here
d18O records of the main Greenland ice cores during the onset of the Younger Dryas. Vertical lines show approximately the neutral average isotope value to denote the onset of the Younger Dryas. Datasets are obtained from NOAA (Grootes, P.M., and M. Stuiver. 1997, Johnson et al., 1997, NGRIP members 2004) Note that the “present” base for NGRIP is 2000AD, this was converted back to the standard “present” of 1950AD. Also double isotope values per date have been averaged for smoothing.
NGRIP and GRIP both suggest that the Bølling Allerød to Younger Dryas transition is between 12,700 and 12,650 calendar years BP. This conflicts with the GISPII chronology which suggests around 12,850 years ago for the beginning of the Younger Dryas. Although the difference is small, there is still the larger part of the last Allerød spike in between. Hence, the date of 12,900 years ago of Firestone et al., 2007 is off by a sufficient margin to miss that spike. Therefore, it would be advisable to crosscheck this boundary with other, independent, high resolution chronologies.
The most accurate dating may be found by counting lake varves and correlating these to several well-dated tephra layers (Zolitschka et al., 2000). The records of the Meerfelder maar (Lücke and Brauer, 2004), Lake Gosciaz, (Goslar et al., 1995), the Ammersee (von Grafenstein et al., 1995) closely follow the GRIP ice core chronology. Hence, high resolution records independently reproduce an onset of the isotope Younger Dryas at around 12,675 +/- 25 varve years BP, (see http://www.gfz-potsdam.de/pb3/pg33/projects/eifelmaar/index.html ) which is more than two sigma outside the error range of the date of the ETE which seems too large to links these two events.
The nature of the onset of the Younger Dryas appears to be identical to the termination of Dansgaard Oeschger events.
The Bølling Allerød events and Dansgaard Oeschger events show up in multiple isotope proxies of the ice cores and ocean drilling project (ODP) cores. The most compelling comparison can be made using deuterium excess of the GISP ice core (Masson-Delmotte et al., 2005). Deuterium excess is a very sensitive proxy and the similarity between the Dansgaard Oeschger events and the Bølling Allerød is striking. Hence, the chance is remote that an ETE triggered other events that resulted in exactly the same deuterium excess fingerprint at the termination of the multiple Dansgaard Oeschger events. Instead, this matching fingerprint strongly suggests that all of these events share the same as-yet-unknown cause or causes for its onset and termination, found in irregular millennial scale cycle changes in moisture source. A good candidate may be changing flows of the Thermohaline Current.
The extent of megafaunal extinction exceed single location and single date
The global megafauna extinction during the Late Pleistocene appears to have accelerated significantly in Alaska around 15 ka Cal BP (Guthrie 2003) with the disappearance of horses, and may have terminated around 4,000 Cal BP with the definite extinction of the woolly mammoth on Wrangel Island (Vartanyan et al., 1995). Channel Islands (Agenbroad, 1998), and Pribilof islands (Crossen, 2005). Furthermore, the extinctions also deviated from species to species. Ground sloths (Steadman et al 2005) disappeared in the Americas asynchronously, spanning the Younger Dryas. Meanwhile, Straight-tusked elephants and woolly mammoths disappeared in Europe, clearly before the onset of the Younger Dryas (Stuart 2005). In contrast, American mastodons (Miller 1987, Polaco et al., 2001 ) and the Irish Elk (Stuart et al., 2004) survived on the Eurasian continent until well into the Holocene. Also, modern species like the American bison, elk and deer survived the ETE, which suggest that the impact on the species may have been limited, and given the extent of the extinction, its contribution to the mass extinction event may be overrated.
It is clear that the evidence of Firestone et al., 2007 suggest unusual occurrences in the terminal phase of the Allerod event. It may very well explain the sudden disappearance of the Clovis but the assumed link with the Younger Dryas and the megafauna extinction may be too ambitious. The ETE precedes the actual onset of the isotope Younger Dryas considering the bulk of the high resolution evidence. Furthermore, the records of multiple proxies, especially the deuterium excess of the GRIP ice core strongly suggest identical causes for the onset and termination of all Dansgaard Oeschger events as well as the Bolling Allerod events. However, there is only one ETE. Finally, on a global scale, the extinction of the megafauna happened rather gradually before and after the ETE, while some local megafauna species appear to have survived it. Therefore linking the ETE to the Younger Dryas and the megafauna extinction appears to be unsupportable.
Agenbroad, L. 1998. Pygmy (Dwarf) Mammoths of the Channel Islands of California. Mammoth Site of Hot Springs, SD, Inc.
Crossen K.J. 2005 GSA Meeting Salt Lake City Abstracts with Programs, Vol. 37, No. 7, p. 463 (http://gsa.confex.com/gsa/2005AM/finalprogram/abstract_97313.htm )
Firestone R.B. et al 2007; PNAS 104/ no 41 pp 16016-16021
Goslar et al. 1995. Nature, 377: 414-417.
Grafenstein U von, et al, 1995 Science 284, 1654-1657.
Grootes, P.M., and M. Stuiver. 1997. Journal of Geophysical Research 102:26455-26470
Guthrie, R.D 2003 Nature 426, 169-171 (13 Nov)
Johnsen, S.J et al 1997. Journal of Geophysical Research 102:26397-26410.
Lücke, A. Brauer A., 2004. Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 211, Issues 1- 2, 19 August.
Masson-Delmotte et al 2005. Science 1 July 2005: Vol. 309. no. 5731, pp. 118 - 121
Miller W.E. 1987 Journal of Paleontology, Vol. 61, No. 1, pp. 168-183
North Greenland Ice Core Project members. 2004 Data Contribution Series # 2004-059. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
Reimer P, et al 2004. Radiocarbon (Volume 46, nr 3).
Polaco et al 2001, proceedings first international congress of the World of Elephants 2001 Rome pp 237 - 242
Steadman DW et al 2005; PNAS August 16, vol. 102 no. 33 11763–11768
Stuart, A.J., 2005. Quaternary International, Volumes 126-128, 2005, Pages 171-177
Stuart A.J et al 2004 Nature 431, 684-689, 7 October
Stuiver M, et al 1998. Radiocarbon 40(3):1041–83.
Vartanyan, S.L., et al 1995. Radiocarbon 37: pp.1-6.
Zolitschka, B., et al 2000 Geology, 28/9, 783-786.