Highlights
· Number of Bering Land Bridges and dates shown graphically and tabulated for the majority of the Pleistocene
Example
·
The Bering Land Bridge is believed to have
formed between 9 and 20 times during the Pleistocene (Hopkins1 and Pielou2
respectively)
·
That 4 waves of mammalian immigration had
occurred over the land bridge, once during early Miocene, again at the
Pliocene-Pleistocene boundary and probably at least twice more during
Pleistocene. Furthermore the climate on the land bridge appeared to be
temperate, humid and forested in the first two episodes, temperate grasslands
in the mid Pleistocene and only in late Pleistocene was the climate
significantly colder being represented by steppe tundra, ‘alpine desert’ plant
communities. Additionally there was some back migration from the Americas to
Asia.
The main question remaining is
therefore: At what dates during the Pleistocene was the land bridge above sea
level and therefore available for the migration of animals and most importantly
humans into the Americas?
Not unexpectedly much research
has attempted to address this question. Whilst the first opening of the Bering
seaway in the Miocene, and its final reopening at the end last glacial maximum
(LGM) has been relatively well dated by, for example, but not limited to, Marincovich
and Gladenkov3 and Elias et. al.4 respectively.
Direct evidence of past sea
levels and hence the emergence and submergence of the Bering Land Bridge is
however, rather scant and indirect.
For this post I reviewed a range
of evidence regarding sea level across the Pleistocene including papers by Brigham-Grette
et. al.5, Roeda, et. al.6, Goodfriend et. al.7
and Scherer et. al.8 amongst many others. None of the papers I could
find and/or access fully gave enough evidence to infer a chronology for Bering
Land Bridge formation.
Another line of evidence to infer
land bridge existence at a particular epoch is the mammalian fauna migrations.
If anything these are even more fragmentary than the sea level evidence. As an
example, in a review of the work of Reppening on arvicoline, rodents migrations
and species, Bell and Jass9 had 68 references spread across 22
authors! And that’s just a small section of the rodents! To provide a synthesis
of the evidence of mammalian migrations to the Americas would therefore, be a
monumental undertaking, and beyond the scope of this post.
General references, however state
the rough timings of two important species migrations to the Americas, in “When
Did Columbian Mammoths Come to North America?”10 The author states:
“The ancestors of Columbian mammoths lived in Asia and came to North
America about 1.8 million years ago across the Bering land bridge (see the map
below). This land bridge was between Russia and Alaska. The Columbian mammoth
moved throughout the United States and parts of Mexico. They never went south
of Mexico.
The woolly mammoth also came to North America from Asia across the
Bering land bridge. They started coming to North America 100,000 years ago and
stayed in the north, remaining in Alaska and Canada.”
Image credit: Children’s
Discovery Museum of San Jose10
Original caption reads: Migration
patterns of Columbian and Woolly Mammoths.
Noteworthy is the agreement of
these dates with those of Reppening11 of nearly 50 years ago.
Some recent research12
however, indicates early dates of entry for other proboscids museum “staff were working at the late Miocene-age
(7.0-4.5 million years old) locality just 20 minutes from the university in
eastern-most Tennessee and came across some tusk fragments! ..the long, straight tusk – it will be over
2.5 m (8 ft) long when complete. It is not highly curved such as the mammoth
tusks we have here in The Mammoth Site.
Following the tusks back into the excavation wall, staff members Shawn
Haugrud and Brian Compton located the skull and lower jaws. Further excavation
allowed them to discover the articulated neck vertebrae…and it keeps
going. There is more of the animal but
under lots of in-situ sediments yet to be excavated! Look at the teeth. Note the cusps, ridges, and valleys – these are
buno-lophodont teeth. These are not like
the flat-grinding teeth of our mammoths or today’s elephants. We are not sure yet of a detailed
identification but we do know that these teeth are from a mastodont”
[Mastodon early relative].
Image credit: Mammoth Site of Hot Springs South Dakota12
Original caption reads: Here is a close-up, side view of the upper cheek teeth. Note the cusps, lophs, and valleys on these mastodont teeth. These teeth do not grind grasses and sedges as do mammoths but chomp up woody plants while browsing in woodlands and forests.
Image credit: Mammoth Site of Hot
Springs South Dakota12
Original caption reads: One of
four adult tusk fragments so far recovered that belong to a large mastodont
from the late Miocene at the Gray Fossil Site, eastern Tennessee.Lastly the final mammalian migrations - including that of humans - occurred, according to a recent paper13 up until the last glacial cycle c. 11000 year ago.
Image credit: Meiri et. al.
(2014)13
Original caption reads: Figure 1.
(a) Bayesian phase-modelled timing of the late-glacial colonization of Alaska
and Yukon by brown bears, cave lions, moose, wapiti and humans. The
distributions are start boundaries. (b) Finite radiocarbon dates of wapiti
occupying northeast Siberia plotted against NorthGRIP ẟ 18O data.
Other lines of investigation,
particularly ocean sediment cores could give direct evidence of these events.
As a relatively recent report14 states of the Bering strait region:
“This is the only area on Earth where the circulation between ocean
basins has been blocked and a migration corridor between continental landmasses
has been opened by falling sea levels of the Pliocene and Pleistocene epochs,
yet scientific drilling for the purpose of paleoclimate analysis has never been
conducted in the Bering Strait region. ..In order to address unresolved
questions regarding global ocean circulation and rapid climate changes, and to
permit reconstruction of the flora, fauna, and climate of the Bering Land Bridge,
basinal features that contain both marine and terrestrial lacustrine sediment
must be targeted. ..Norton and Hope basins are most proximal to the Bering
Strait, constituent records of Pleistocene and Holocene transgressions and
regressions from any of the nine basins (Fig. 1) would serve to constrain
temporal estimates of the opening and closing of the Bering Strait..”
Despite all these decades of
work, by a myriad of scientists, using multiple lines of evidence, direct dates
for the timings of emergence and submergence of the Bering Land Bridges STILL haven’t
been arrived at!
There is however ONE method that can estimate sea levels
globally and hence allow us to derive dates for the emergence and submersion of
the Bering Land Bridge.
The method is based the Marine Isotope Stage data. A good
description from Wikipedia:
“Marine isotope stages (MIS), marine oxygen-isotope stages, or oxygen
isotope stages (OIS), are alternating warm and cool periods in the Earth's
paleoclimate, deduced from oxygen isotope data reflecting changes in
temperature derived from data from deep sea core samples. Working backwards
from the present, which is MIS 1 in the scale, stages with even numbers have
high levels of oxygen-18 and represent cold glacial periods, while the
odd-numbered stages are troughs in the oxygen-18 figures, representing warm
interglacial intervals. The data are derived from pollen and foraminifera
(plankton) remains in drilled marine sediment cores, sapropels, and other data
that reflect historic climate; these are called proxies.
The MIS timescale was developed from the pioneering work of Cesare
Emiliani in the 1950s, and is now widely used in archaeology and other fields
to express dating in the Quaternary period (the last 2.6 million years), as
well as providing the fullest and best data for that period for
paleoclimatology or the study of the early climate of the earth, representing the
standard to which we correlate other Quaternary climate records. Emiliani's
work in turn depended on Harold Urey's prediction in a paper of 1947 that the
ratio between oxygen-18 and oxygen-16 isotopes in calcite, the main chemical component
of the shells and other hard parts of a wide range of marine organisms, should
vary depending on the prevailing water temperature in which the calcite was
formed.”
This data can be used to estimate
sea levels as explained by Lambek et. al16: “The isotope ratio ẟ18O={(18O/16O)sample/(18O/16O)standard,
expressed as parts per thousand, is therefore believed to be an indicator of
global ice volume — low values indicate small ice volumes and hence globally
warm conditions, and high values imply large ice sheets and low temperatures..”
[and by extension global sea levels].
Climate scientists have therefore
combined many of oxygen isotope ratio data sets from deep sea cores to obtain
values for global ice volume and thus infer eustatic global sea levels. Some
authors have produced graphs of sea level verses time for considerable past epochs.
Three of the best these are Siddal17, Compton18 and
Lambeck et. al.16.
If we combine this with Hopkins
assertion that ”Sea level would have to
fall only 46 meters below its present position to expose a narrow land
connection between Chukota and Alaska by way of St. Lawrence Island; a
reduction to -50 metres would expose a second narrow connection north of the
Bering Strait..” (ref. 1 p460).
We can superimpose a -50m below
present, line on their graphs and thus find the dates and numbers of Bering
Land Bridges during the latter (1.8Mya to present) when modern humans or
previous species of Homo such as Homo neanderthalis or Homo erectus s.l. could
conceivably have crossed into the Americas. See graphs below.
Fig 1. Bering Land Bridges
Present to 800Ky BP. Adapted from Siddal17
Fig 2. Bering Land Bridges 800-1800Ky
BP. Adapted from Compton18
Fig 3. Bering Land Bridges Present to 150Ky BP. Adapted from
Lambeck16
I have summarised the data in table form for ease of
viewing/use enjoy!
There are some profound implications of these data, which I
will expand on in a future post.
Note Bering Land Bridges 1-6 were generated using the more detailed Fig 3 to show finer scaling of Land Bridge emergences whilst Land Bridges 7-39 were drawn from Figs. 1 and 2, consequently Land Bridges 7 and 8 show some overlap. This is not an error, just a result of the finer scaling of Fig 3 vs Fig 1.
References
1. Hopkins, D.M. 1967. The Cenozoic history of Beringia—A
Synthesis., in The Bering Land Bridge Hopkins, D.M, ed. Stanford University
Press.
2. Pielou, E.C. 1991.
After the Ice Age: The Return of Life to Glaciated North America.
University of Chicago Press, Chicago
3. Louie Marincovich, Jr & Andrey Yu. Gladenkov. 1999. Evidence
for an early opening of the Bering Strait. Nature 397, 149-151
4. Elias, Scott A. et al. 1996. Life and times of the Bering
land bridge
Nature 382, 60 - 63 doi:10.1038/382060a0
5. Brigham-Grette, J., and Hopkins, D.M., Benson, S.L.,
Heiser, P., Ivanov, V.F., Basilyan, A., and Pushkar, V., 1995, Coastal records
of Pleistocene Glacial and Sea level events on Chukotka Peninsula, northeast
Siberia: A new interpretation, Current Research in the Pleistocene --Special
issue on Beringia, v.11.
6. Roeda, Murray A. et. al. 2013. Evidence
for an Early Pleistocene glaciation in the Okanagan Valley, southern British
Columbia. Canadian Journal of Earth Sciences v. 51 no. 2
p. 125-141
7. Goodfriend, G. A., J. Brigham-Grette, and G. H. Miller,
1996, Enhanced age resolution of the Marine Quaternary Record in the Arctic
using Aspartic Acid Racemization dating of Bivalve shells, Quaterary
Research,v. 45, 176-187.
8. Scherer, Reed P. et. al. 1998. Pleistocene Collapse of
the West Antarctic Ice Sheet. Science Vol. 281 no. 5373 pp. 82-85. DOI: 10.1126/science.281.5373.82
9. Bell, Christopher J., and Jass, Christopher N. 2011.
Polyphyly, paraphyly, provinciality, and the promise of intercontinental
correlation: Charles Repenning’s contributions to the study of arvicoline rodent
evolution and biochronology. Palaeontologia Electronica Vol. 14, Issue 3;
18A:15p;
palaeo-electronica.org/2011_3/28_bell/index.html
10. From the Children’s Discovery Museum of San Jose.
Retrieved from;
11. Reppening, C. 1967 Palearctic-Nearctic Mamalian
Dispersal in the Late Cenozoic. In The Bering Land Bridge Hopkins, D.M, ed.
Stanford University Press.
12. Dr Jim Mead. 2015 A new HUGE discovery…and it is so OLD!
From the Mammoth Site of Hot Springs South Dakota. Retrieved from:
13. Meiri M et al. 2014 Faunal record identifies Bering
isthmus conditions as constraint to end-Pleistocene migration to the New World.
Proc. R. Soc. B 281: 20132167.
14. The Bering Strait, Rapid Climate Change, and Land Bridge
Paleoecology Final Report of the JOI/USSSP/IARC Workshop Held in Fairbanks,
Alaska on June 20-22, 2005. Eds. Fowell, S and D. Scholl (Stanford University
and USGS)
15. Marine Isotope Stage;
16. Lambek et. al. 2002. Links between climate and sea
levels for the past three million years. Nature VOL 419 p199-206
17. M. Siddall, J. Chappell, E.-K. Potter, Eustatic Sea
Level During Past Interglacials, in The Climate of Past Interglacials F.
Sirocko, M. Claussen, T. Litt and M.F. Sanchez-Goni, Eds Elsevier 2006.,
18. Compton, John S. 2011. Pleistocene
sea-level fluctuations and human evolution on the southern coastal plain of
South Africa. Quaternary Science Reviews 30 506-527
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