Desert Speleothem Reveal Climatic Migration

[L Roebuck]

Migration of Early Humans From Africa Aided By Wet Weather

Science Daily — The African origin of early modern humans 200,000--150,000 years ago is now well documented, with archaeological data suggesting that a major migration from tropical east Africa to the Levant took place between 130,000 and 100,000 years ago via the presently hyper-arid Saharan-Arabian desert.

This migration was dependent on the occurrence of wetter climate in the region. Whereas there is good evidence that the southern and central Saharan-Arabian desert experienced increased monsoon precipitation during this period, no unequivocal evidence has been found for a corresponding rainfall increase in the northern part of the migration corridor, including the Sinai-Negev land bridge between Africa and Asia.

Passage through this "bottleneck" region would have been dependent on the development of suitable climate conditions.

Vaks et al. present a reconstruction of paleoclimate in the Negev Desert based on absolute uranium series dating of carbonate cave deposits (speleothems). Speleothems only form when rainwater enters the groundwater system and vegetation grows above a cave.

Full Article

[kenredux]

It is my understanding that the two major human migrations occurred during Interglacial periods. The first during the last Interglacial -- out of Africa and into Europe and Asia; and the second during this Interglacial -- out of Eurasia and into the Americas.

Rainfall is much greater during Interglacials where high levels of slow moving groundwater dissolve our southeastern caves. The last Interglacial is usually dated at about 130,000 to 100,000 years before today which fits nicely within their speleothems dates.

It follow that the caves in the flat laying limestones of the Cumberland Plateau can be Inter-glacially dated by their relative position within the geologic column. For example, groupings of caves that occur at base level in the Cumberland Plateau would be said to have formed between 130,000 and 100,000 years ago.

[Brian Roebuck]

kenredux said:

It is my understanding that the two major human migrations occurred during Interglacial periods. The first during the last Interglacial -- out of Africa and into Europe and Asia; and the second during this Interglacial -- out of Eurasia and into the Americas.

Rainfall is much greater during Interglacials where high levels of slow moving groundwater dissolve our southeastern caves. The last Interglacial is usually dated at about 130,000 to 100,000 years before today which fits nicely within their speleothems dates.

It follow that the caves in the flat laying limestones of the Cumberland Plateau can be Inter-glacially dated by their relative position within the geologic column. For example, groupings of caves that occur at base level in the Cumberland Plateau would be said to have formed between 130,000 and 100,000 years ago.

kenredux,

It is interesting that these data seem to correlate so well. I am not aware of any efforts to date the speleothems of Cumberland Plateau caves however. Are there any studies being done here (or have they already done them)? That would be a very nice piece of the puzzle to have available - caves around the world that all date to approximately the same period when water levels in them provided speleothem growth opportunities.

Since the interglacial period was a global event this should become evident (with perhaps some exceptions where dry conditions may have prevailed). Since I have not really looked into this maybe you have more information that you can share with us on the subject. I for one am full of questions but no real answers.

Thanks for your thought provoking post!

[kenredux]

And thank you Dr. Beaner, for your interest in the paleo reconstruction of speleological events in the Cumberlands.
To me this is a subject of keen interest.

In answer to your question concerning speleothem dating in southeastern caves, No, I don't know of any measured radiation results. But Monday I'll see if I can find out the results of a thesis study by a student at the University of Alabama who was relatively dating wet periods by measuring the darker bands in speleothems.

I'll check back with you next week..

[Brian Roebuck]

Sounds good.

I look forward to hearing the results.

[kenredux]

Lewis Dean sent me this email yesterday and called this morning and OKed posting it here intact. Unfortunately the paleo- rainfall study is ongoing and any conclusions might not be available for a long while. Feel free to phone Mr.Dean if you have any further questions concerning this study.

DeSoto Caverns Provide UA Geologists Insight into Global Warming
uanews.ua.edu/anews2004/dec04/caverns120204.htm

Caverns Provide Geologists Insight Into Global Warming
research.ua.edu/archive2004/caverns.html

Caves, Ocean Floor Give Geologists Weather Reports from Thousands of Years Ago
research.ua.edu/archive2002/secrets1.html

If you desire additional information, please feel free to contact us.

Lewis S. Dean, Geologist
Library and Geologic Investigations Program
Geological Survey of Alabama
P.O. Box 869999
420 Hackberry Ln.
Tuscaloosa, AL 35486
205-247-3548

[Brian Roebuck]

Thanks Ken,

Good articles all!

[kenredux]

Good God, Yall! They're poking holes in stalagmites everywhere.
Well at least the Georgia Tech folks, Kim Cobb and Jud, know where to drill. If you just gotta drill, the caves of a tropical paradise like Borneo sounds like fun. or maybe not.

Cave records provide clues to climate change
www.physorg.com ^ | 09/26/2007 |
Georgia Institute of Technology

When Georgia Tech Assistant Professor Kim Cobb and graduate student Jud Partin wanted to understand the mechanisms that drove the abrupt climate change events that occurred thousands of years ago, they didn't drill for ice cores from the glaciers of Greenland or the icy plains of Antarctica, as is customary for paleoclimatolgists. Instead, they went underground.

Growing inside the caves of the tropical Pacific island of Borneo are some of the keys to understanding how the Earth's climate suddenly changed - several times - over the last 25,000 years. By analyzing stalagmites, the pilar-like rock formations that stem from the ground in caves, they were able to produce a high-resolution and continuous record of the climate over this equatorial rainforest.

"These stalagmites are, in essence, tropical ice cores forming over thousands of years," said Partin. "Each layer of the rock contains important chemical traces that help us determine what was going on in the climate thousands of years ago, much like the ice cores drilled from Greenland or Antarctica."

The tropical Pacific currently plays a powerful role in shaping year-to-year climate variations around the globe (as evidenced by the number of weather patterns influenced by the Pacific's El Nino), but its role in past climate change is less understood. Partin and Cobb's results suggest that the tropical Pacific played a much more active role in some of the abrupt climate change events of Earth's past than was once thought and may even have played a leading role in some of these changes.

Polar ice cores reveal that the Northern Hemisphere and the Southern Hemisphere each have their own distinct patterns of abrupt climate change; the tropical Pacific may provide the mechanistic link between the two systems. Understanding how the climate changes occurred and what they looked like is important to helping scientists put into context the current trends in today's climate. They published their findings in the Sept 27, 2007, issue of the journal Nature.

The research team collected stalagmites from the Gunung Buda cave system in Borneo in 2003, 2005 and 2006. Analyzing three stalagmites from two separate caves allowed the pair to create a near-continuous record of the climate from 25,000 years ago to the present. While this study is not the first to use stalagmites to examine climate over this time period, it is the first to do so in the tropical Pacific. Typically, in these types of studies, only one stalagmite is analyzed, but Partin and Cobb compared their three stalagmite records to isolate shared climate-related signals.

Stalagmites are formed as rain water, mixed with calcium carbonate and other elements, makes its way through the ground and onto the cave floor. As this solution drips over time, it hardens in layers, creating a column of rock.

Partin and Cobb cut open each stalagmite and took 1,300 measurements of their chemical content to determine the relative moisture of the climate at various periods in history starting from the oldest layers at the bottom to the present at the top. They dated the rocks by analyzing the radioactive decay of uranium and thorium, and determined the amount of precipitation at given times by measuring the ratio of oxygen isotopes.

"Our records contain signatures of both Northern and Southern Hemisphere climate influences as the Earth emerged from the last ice age, which makes sense given its equatorial location,” said Cobb. “However, tropical Pacific climate was not a simple linear combination of high-latitude climate events. It reflects the complexity of mechanisms linking high and low latitude climate.”

For example, Partin and Cobb's records suggest that the tropical Pacific began drying about 20,000 years ago and that this trend may have pre-conditioned the North Atlantic for an abrupt climate change event that occurred about 16,500 years ago, known as the Heinrich 1 event.

"In addition, the Borneo records indicate that the tropical Pacific began to get wetter before the North Atlantic recovered from the Heinrich 1 event 14,000 years ago. Perhaps the tropical Pacific is again driving that trend," said Partin.

[Azurerana]

kenredux said:

It is my understanding that the two major human migrations occurred during Interglacial periods. The first during the last Interglacial -- out of Africa and into Europe and Asia; and the second during this Interglacial -- out of Eurasia and into the Americas.

Hmm.. . depends how you define 'interglacial'. There have been hundreds of interglacials-- the old model of 4 Pleistocene ice ages has been largely superseded to a graph looking something looking more like a seismic signature or brainwaves.

Rainfall is much greater during Interglacials where high levels of slow moving groundwater dissolve our southeastern caves. The last Interglacial is usually dated at about 130,000 to 100,000 years before today which fits nicely within their speleothems dates.

I wonder about this statement. Rainfall would surely be greater due to the temperature, but would total precipitation? All that glacial ice and snow had to come from somewhere...What temperature records were there for the Cumberland Plateau for that time? There seems to be some controversy whether the Cumberland was ever actually glaciated or if it were just on the glacial fringe. This makes a fair amount of difference in terms of geomorphic change (land sculpting.)

Also, the concept that you need slowly moving groundwater to dissolve caves is suspect. Sure, that is one explanation. But perhaps you should investigate J Harlen Bretz and the Channelled Scablands of central and easter Washington and the collapse of Lake Missoula. (There is good book called Cataclysm on the Columbia which does a good job of recounting this. Or, for those looking for a more entertaining version, the plot of Ice Age II -- the movie-- was lifted pretty much straight from Bretz.)

It follow that the caves in the flat laying limestones of the Cumberland Plateau can be Inter-glacially dated by their relative position within the geologic column. For example, groupings of caves that occur at base level in the Cumberland Plateau would be said to have formed between 130,000 and 100,000 years ago.

I'm not sure that I would follow this. How do you know this is when the caves were formed? From stal data? Stal data puts a minimum age on a cave, not a maximum age. In fact, considering that caves usually have to reach a certain level of 'dryness' before aerial stal can form.

Anybody else looked at the data from Missouri?

www.sciencemag.org/cgi/content/abstract/282/5395/1871

[kenredux]

Hmm.. . depends how you define 'interglacial'. There have been hundreds of interglacials-- the old model of 4 Pleistocene ice ages has been largely superseded to a graph looking something looking more like a seismic signature or brainwaves.

I wonder about this statement. Rainfall would surely be greater due to the temperature, but would total precipitation? All that glacial ice and snow had to come from somewhere...What temperature records were there for the Cumberland Plateau for that time? There seems to be some controversy whether the Cumberland was ever actually glaciated or if it were just on the glacial fringe. This makes a fair amount of difference in terms of geomorphic change (land sculpting.)

Also, the concept that you need slowly moving groundwater to dissolve caves is suspect. Sure, that is one explanation. But perhaps you should investigate J Harlen Bretz and the Channeled Scablands of central and easter Washington and the collapse of Lake Missoula. (There is good book called Cataclysm on the Columbia which does a good job of recounting this. Or, for those looking for a more entertaining version, the plot of Ice Age II -- the movie-- was lifted pretty much straight from Bretz.)


I'm not sure that I would follow this. How do you know this is when the caves were formed? From stal data? Stal data puts a minimum age on a cave, not a maximum age. In fact, considering that caves usually have to reach a certain level of 'dryness' before aerial stal can form.

Anybody else looked at the data from Missouri?

www.sciencemag.org/cgi/content/abstract/282/5395/1871

I dunno Geo, what I've read of the distinctions between glacial periods that last about a hundred thousand years give or take several thousand, and the intervening interglacial periods which last from ten to thirty thousand years, notwithstanding many temperature fluctuations within their respective cycles, are clearly demarcated.

And it seems (if I may) that your Missouri results are reflective of slight local climate variations and (to me at least) the testing measurements seem neat but obscure.

And no, Geo, I never said that the Cumberlands were glaciated, just the opposite; the accumulated snow in the highlands would normally melt in the summer and feed great downcutting rivers entrenching their way to the Gulf.