The Lake Malawi Sediment Chronometer and the Toba Super Eruption

The site of one of the largest volcanic eruptions in earth’s history, the effects of the Toba Super Eruption have been of considerable interest to archaeologists and anthropologists because of its potential impacts on past human populations.   New research exploring the Toba Super Eruption (Sumatra) has been published since I explored the implications of the Toba eruption on human origins last year (See: The Toba Super Eruption: A Non-Flood Catastrophe – The Artifacts Say Yes! and The Toba Super Eruption and Polar Ice Cores).   There is an important paper released in the last week that argues that modern humans were not present in Asia at the time of the eruption and I will try to comment on that in the near future but I read another paper that reports on the extent of the impacts of the Toba eruption in Africa  and it is that paper that I want to discuss today.

Christine Lane and her co-authors reported in PNAS (reference below) in May 2013 their study of sediment from Lake Malawi in Malawi, Africa that extends the known range of ash dispersed from the Toba super eruption.  They examined the chemical composition and surface shapes of microscopic volcanic glass shards found in sediments 28 meters (89 feet) below the lake floor of Lake Malawi in Malawi, African.  Their results reveal strong evidence that this volcanic glass was derived from the Toba super eruption. From these results we can infer then that the top 28 meters of sediments settled to floor of this lake AFTER the Toba volcano blew its top, spewing thick ash over Southeast Asian into India.  These researchers have provided the best evidence to-date that small amounts of this ash reached all the way to central African approximately 7300 kilometers away from the blast site in present day Sumatra.  The authors’ investigating these sediments found that unlike India and Southeast Asia, the climate of central Africa was not severely affected by this ash fall. They say this because the found the composition of the sediments below and above the layer of ash was not significantly different from one another.

Example of finely-laminated, diatom-rich silty-clay, deposited when Lake Malawi was very deep.  These are typical for much of the hundreds of meters of sediments that were cored by the Lake Malawi drilling project. Image courtesy of the Lake Malawi Drilling Project.

Example of finely-laminated, diatom-rich silty-clay, deposited when Lake Malawi was very deep. These are typical for much of the hundreds of meters of sediments that were cored by the Lake Malawi drilling project.
Image courtesy of the Lake Malawi Drilling Project.

Comparison of material emitted for a number of famous volcanic eruptions. All the ones that emitted 100 cubic kilometers or more and considered super-eruptions.    Toba has erupted more than once but the one shown is the youngest.   Image credit: USGS - http://volcanoes.usgs.gov/images/pglossary/eruptionsize.php

Comparison of material emitted for a number of famous volcanic eruptions. All the ones that emitted 100 cubic kilometers or more and considered super-eruptions. Toba has erupted more than once but the one shown (the largest circle!) is the youngest.   Mt. St. Helens in 1980 is the small dark green circle. Image credit: USGS – http://volcanoes.usgs.gov/images/pglossary/eruptionsize.php

If you are like me at all  you probably find it amazing that a volcano could be so explosive that is results in glass shards deposited 7300 kilometers away.  Remember, the Toba super eruption was one of the most massive in earth’s history being at least 3000 times the size of Mt. St Helens in 1980.  However, it wasn’t the effects of this eruption that drew my particular interest in this paper, but rather a simple figure about the column of sediments itself. That figure showed the relationship of the ash layer 28 meters below the lake floor and the age of the sediments as measured by C14 dating.  The paper notes that estimated rate of sediment deposition (ie. how fast sediments are accumulating on the lake floor) to have been around 0.03 cm/year for much of the history of the lake.  That might not sound like very much sediment, and it isn’t, but this core was taken near the middle of the lake  (see image below) many kilometers from the shore and 10s of kilometers from any significant water input source that would be bringing in sediments.  These estimates of sedimentation are made by taking the C14 radiocarbon dates of positions along the core and dividing by the distance between the sample positions.  For example, if sediment at 10 cm and 20 cm were dated to be 1000 years old and 2000 years old then the estimated average sedimentation rate would be 10cm/1000 years or 0.01 cm/year.  In our example, the Toba ash was found 28 meters (2800 centimeters) below the most recent sediments to have fallen to the lake floor.  The Toba super eruption has been dated to 74,000 years ago by multiple methods from multiple locations.  If this ash layer is, by inference, the same age then a crude estimate of sedimentation rate would be 2800 cm/74,000 years = 0.038 cm/year.

University of Rhode Island graduate student Meghan Paulson recovers a 50,000-year-old lake sediment core aboard the drilling barge Viphya, during the Lake Malawi Drilling Project in 2005. In the background are the Livingstone Mountains.  Photo by C.A. Scholz

University of Rhode Island graduate student Meghan Paulson recovers a 50,000-year-old lake sediment core aboard the drilling barge Viphya, during the Lake Malawi Drilling Project in 2005. In the background are the Livingstone Mountains. Photo by C.A. Scholz

Generalized cartoon graph of the relationship between the age of sediments and their depths in cores from the middle of lake Malawi in Africa.  The slope of the line drawn through the radiometric dates predicts the rate of sediment deposition each year.  Following the line down through where the Toba ash is found in the column finds that the Toba ash is predicted to be 70 to 80 thousand years old which is in the range it has been dated in other locations.

Generalized cartoon graph of the relationship between the age of sediments and their depths in cores from the middle of lake Malawi in Africa. The slope of the line drawn through the radiometric dates predicts the rate of sediment deposition each year. Following the line down through where the Toba ash is found in the column finds that the Toba ash is predicted to be 70 to 80 thousand years old which is in the range it has been dated in other locations.

So how does this crude estimate based on distance divided by radiometric data stack up to other estimates of sedimentation?  Quite well it seems.  The current rate of sedimentation in this part of Lake Malawi is around 0.03 to 0.04 cm/year.  C14 dates have been taken from multiple positions along the core and in each case dividing the distance between those positions and the dates derived yields values of 0.03 to 0.04 cm/year.  Some variation would be expected because the climate has changed in this area from arid times to wetter times which would change the amount of sediment input into the lake. But the overall picture is one where the rate of current sediment accumulation has been relatively constant over a very long time.  This Toba ash layer is 28 meters (89 feet) deep and the sediments in the core provide no evidence of any large sudden influx of sediments but rather is fairly uniform except for many very thin layers of ash from volcanic eruptions in central Africa.

I’m going to risk being overly repetitive but I want to stress the significance of the dating of these sediments and ash layers.  The Toba volcanic eruption has been dated many times from many locations and has come up as being 74,000 years old.  Now ash identified as being from this particular volcanic explosion has been found at 28 meter below the surface.  Estimates of sedimentation rate, based on radiometric dating, that were already known for this location in the lake PREDICT that the Toba ash should be found about at this depth in the sediment column and that is where it was found.  My cartoon figure to the right shows how this works.  C14 dating cant’ date items more than around 50,000 years old but dates from the layers above 28 meters can be used to predict the depth at which the sediments should be 74,000 years old.  And that is just where this Toba ash layer is found.

It is just this sort of independent confirmation of predictions that lends further support to the validity of radiometric dating for dating events in earth’s history.   There are hundreds of places on earth where similar types of data have been collected and in each case the simplest explanation for the data is that the sediments underlying these lakes have taken a long time to accumulate.  Young earth creationists have proposed an alternative hypothesis: that there was very rapid accumulation of sediment several thousand years ago and then a rapid slowing of the sedimentation rate to the point that we see today.   But, so far they have been unable to produce physical evidence to support this hypothesis.  This Malawi sediment core is another example. The radiometric dates result in a graph that shows fairly straight line relationship of depth and age which strongly supports a relatively constant input of sediments.   For YECs to fit the 30 meters of sediments into their 4000 year timescale they would need sedimentation rates to be hundreds of times higher than it is measured in the present day.

What further complicates and discounts the young earth creation hypothesis is that that Toba ash is a full 28 meters (89 feet) below the current lake floor and that Toba ash fall happened after hominids had already dispersed across Africa and southeastern Asia (see my article, The Toba Super Eruption: A Non-Flood Catastrophe)  The YECs think that all records of human remains and artifacts must represent a time after the Tower of Babel dispersion which happened hundreds of years after the Flood.  Logically then this entire 28 meters of sediment must have formed after a global flood and during a time when YEC already see the earth’s geology having settled down quite a bit. They don’t have any source for this very fast sediment deposition above the Toba ash layer.

To top it off, these 30 meters of the core section under this lake is really just the tip of iceberg. The Lake Malawi drilling project produced cores of lakes sediments that reached over 500 meters below the lake bottom. Those cores reveal the presence of freshwater lake for the entire period of the sediment accumulation.  So that Toba ash is really just in the very top of the sediment layers and yet for us today, the Toba super eruption seems to be an event that must have happened a long time ago since the ash layer it produced is buried under many meters of sediments.   Furthermore, all of these lake sediments are thousands and thousands of feet sedimentary and metamorphic rocks that make up the crust of the earth. According to creationists these rocks were laid down during the global flood an only after that flood did this lake form, it was only after its formation that the hundreds of meters of lake sediments could have formed.   The compressed timeline that creationists have constructed for all these events once again stretches all reasonable bounds of credibility. The Toba super eruption is a tangible, well verified, cataclysmic event.  No model of Earth history can ignore the reality of the event or the effects of it.

Christine S. Lane, Ben T. Chorn, and Thomas C. JohnsonAsh from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka PNAS 2013 110 (20) 8025-8029; published ahead of print April 29, 2013,doi:10.1073/pnas.1301474110

Comments

  1. The current rate of sedimentation in this part of Lake Malawi is around 3-4 cm/year.

    I think you meant 0.03-0.04 cm/year consistent with historical data?

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  2. I think you mean 7300 km, not miles.

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  1. […] in more depth in this article on The Toba Super Eruption and Polar Ice Cores and this one on The Lake Malawi Sediment Chronometer and the Toba Super Eruption [3]. How is this not […]

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  2. […] mean, I hate to rant on a bit, but look at this, for example. The Lake Malawi Sediment Chronometer and the Toba Super Eruption – Naturalis Historia Scientists date the eruption to be 74,000 years ago. When scientists go down and look at the […]

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