The Lake Malawi Sediment Chronometer and the Toba Super Eruption

One of the largest volcanic eruptions in earth’s history, the Toba super eruption has been a special interest to anthropologists and climatologists because of its potential impacts on past human populations.   I have explored the implications of the Toba eruption on human history in previous posts (See: The Toba Super Eruption: A Global Catastrophe that Young-earth Creationists Ignore and The Toba Super Eruption, Polar Ice Cores and Climate Change).   Here we look at published research on the impacts of the Toba eruption in Africa and the implications for the young-earth model of earth’s history.

Dr. Christine Lane and her co-authors reported (see references) on sediments extracted from beneath Lake Malawi in Malawi, Africa.  By examining the chemical composition and surface shapes of microscopic volcanic glass shards found in those sediments they were able to determine that at 28 meters (89 feet) below the lake floor there is preserved ash from the Toba eruption in Indonesia. 

From these results we can reasonably infer that the top 28 meters (89 feet) of sediment were deposited onto the floor of this lake after the Toba volcano blew its top, spewing ash over Southeast Asian into India.  This is 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 scientists investigating these sediments discovered that unlike in India and Southeast Asia and polar regions of the earth, the climate of central Africa was not severely affected by this ash fall.  This determined this from observations of the composition of the sediments below and above the layer of ash which were not significantly different from one another indicating a similar deposition environment.

If you are like me you probably find it amazing that a volcano could be so explosive that glass shards could be deposited 7300 kilometers away.  But the Toba super eruption was one of the most massive in earth’s history producing at least 2800 times the material thrown into the atmosphere as Mt. St Helens in 1980.

However, what caught my attention in this paper was a simple figure about the column of sediments below Lake Malawi. That figure showed the relationship of the ash layer 28 meters (89 feet) below the lake floor and the age of the sediments above it measured by Carbon 14 dating.  Using the C14 dating and modern observations the authors estimate the rate of sediment deposition (ie. how fast sediments are accumulating on the lake floor) to have been about 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 sediment core was pulled up from near the center of this large lake (see image below) many kilometers from the shore and tens of kilometers from any significant stream or river source that would contribute new sediment to the lake basin.  The estimate of long-term sedimentation rate was made by obtaining radiocarbon (C14) dates from multiple positions along the core and dividing those dates by the distance between the sample positions.  For example, if sediment at 10 cm and 20 cm were C14 dated to be 1000 and 2000 years old then the estimated average sedimentation rate would be 10 cm/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 several methods at multiple locations.  The ash layer found in this sediment core has all the characteristics of ash released by the Toba eruption and therefore it should be the same age as all other ash from the same eruption.   If we assign a date of 74,000 years to the sediment core at the 28 meter mark then the average sedimentation rate would be 2800 cm/74,000 years = 0.038 cm/year.

So how does this crude estimate based on distance divided by radiometric data stack up to other estimates of sedimentation?  Rather well.

The current rate of sedimentation has been measured in this part of Lake Malawi and is between 0.03 to 0.04 cm/year.  So, here we have an example were modern measurements yield very similar rates as estimates made from C14 dates taken from multiple positions along the core.  The sedimentation rate is rather consistent throughout the core no matter which points are used to make the calculation.

Some variation would be expected because the climate has changed in this area from arid to moister conditions which would alter the amount of sediment input into the lake. But the overall picture painted by the sediment core which represents the actual history of sediments in the lake is one of relatively constant sediment accumulation over a very long period of time.  The 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 thin layers of ash which record numerous other volcanic eruptions in central Africa.

I’m going to risk being overly repetitive because I want to stress the significance of the dating of the sediments and ash layers.  The Toba volcanic eruption has been dated from multiple locations to be approximately 74,000 years old.  Now ash from this same volcanic explosion has been found identified 28 meter below the bottom of a lake in Africa.

Estimates of sedimentation rates based on radiometric dates from the sediment core below this lake predicted 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 above shows how this works.  C14 dating can only effectively date material that is 50,000 years old or less and so the portion of the core that contains the Toba ash is not able to be dated by this technique and as predicted yielded no reliable C14 date (just an indication that it is more than 50,000 years old).  But C14 dating did yield dates from sediments above 28 meters which could 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 was found.

It is just this sort of independent confirmation of predictions that lends further support to the consensus view of earth’s history.   There are hundreds of places on earth where similar types of data have been collected and in each case the simplest interpretation of that data is that the sediments underlying these lakes have taken a long time to accumulate.

This chronology is unacceptable to young-earth creationists who believe the earth is only 6000 years old and most geological features including this lake and all its sediments are no more than 4350 years old.

Sediments cores such as this and many others in lakes around the world present a serious challenge to that restricted chronology.  In response, young earth creationists have proposed an alternative hypothesis for sediment cores that must be post-flood deposits.  They propose that there was very rapid accumulation of sediments shortly after after the Flood and during a 2-3 century-long biblical ice age followed by a rapid slowing of the sedimentation rate to the point that we see today.  This is similar to their explanation for plate tectonics in which the plates used to be moving very fast but have only very recently slowed to their current extremely slow pace.

So far they have been unable to produce physical evidence to support this hypothesis.  The Malawi sediment core is another example of evidence that doesn’t fit this model. 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 and atomic decay of nuclei to have slowed greatly over time in direct correlation with reduced sedimentation rates.

What further complicates and contradicts 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)  Most YECs believe 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 are left with no source (i.e. post flood erosion or Ice Age deposits) for this very fast sediment deposition above the Toba ash layer.

These 30 meters of sediment from Lake Malawi are another in a series of significant challenges to the YEC chronology.  But those 30 meters are just the tip of the iceberg of problems that sediment cores are for YECs. The Lake Malawi drilling project produced cores of lakes sediments that reached nearly 400 meters below the lake bottom.  Those cores contain evidence for the existence of a freshwater lake for the entire period that this sediment accumulated.  Now we see that the Toba eruption ash is situated in the uppermost portion of the sediments below this lake.  Scientists studying this lake talk about these sediments representing 1.3 million years of history! YECs must fit all of this sediment into less than 4350 years.

The Toba ash is buried under 28 meters of sediment that must have been deposited after the Toba eruption but it sits upon over 350 (over 1100 feet!) of sediment that was deposited before the Toba eruption.  In the YEC timeline of earth’s history the lake sits in a large rift valley. The volcanic and sedimentary rock that separated to form the valley all must have been deposited during a global flood. The lake presumably formed after the flood and the 1500+ feet of sediments under this freshwater lake that sit on to of the original volcanic and sedimentary basement rock must have been deposited since the end of a global flood only 4350 years ago.  This massively compressed timeline that young-earth proponents 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 and yet ignore it is just what YECs have been doing for quite some time.

References

Lane, Christine S., Ben T. Chorn, and Thomas C. Johnson. “Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka.” Proceedings of the National Academy of Sciences 110, no. 20 (2013): 8025-8029.

Scholz, C. A., A. S. Cohen, T. C. Johnson, J. King, M. R. Talbot, and E. T. Brown. “Scientific drilling in the Great Rift Valley: the 2005 Lake Malawi Scientific Drilling Project—an overview of the past 145,000 years of climate variability in Southern Hemisphere East Africa.” Palaeogeography, Palaeoclimatology, Palaeoecology 303, no. 1-4 (2011): 3-19.

Scholz, Christopher A., Andrew S. Cohen, Thomas C. Johnson, John W. King, and Kathryn Moran. “The 2005 Lake Malawi scientific drilling project.” (2006).