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Climate Change

New insights into old events using marine sediment

Article: could gradual changes in Holocene Saharan landscape have caused the observed abrupt shift in North Atlantic dust deposition? Sabine Egerer, Martin Clausen, Christian Reick, Tanja Stanelle. EPSL, 2017.

 

Introduction 

Whether on a cruise ship or using satellites, we scientists spend a lot of time looking at what the ocean looks like today. Just as importantly, though, we can use the ocean to study what happened in the past, and use that to predict what might happen in the future. There are many ways to do this, but one widely used source of information is sediment core records. For a brief history of how and why we get sediment cores from all over the seafloor, check out this article. In short, we can look at changes in things like mud type and chemistry to determine when and how the sediment got buried, and sometimes even where it came from. There are whole fields of science dedicated to studying what happens to sediment during and after it hits the seafloor. Today, let’s look at an example of how sediment records can tell us what happened far above the seafloor.

 

Back up to around 6,000 years ago. The climate is a bit different than it is today. Today, the Sahara in North Africa is a desert – hot, dry, and inhabited by only the toughest of life. Around 16,000 to 6,000 years ago, though, during the African Humid Period, North Africa was lush and rainy, with sprawling grasslands. Lakes, animals, and people dotted what is called the “Green Sahara”. However, around 6,000 years ago, this all changed. Over the next thousand years or so, the landscape changed to the desert we recognize today. This change was recorded in sediments.

http://althistory.wikia.com/wiki/File:Green_Sahara.png

Figure 1. A map of climate zones in Africa during the African Warm Period

Saharan dust gets blown into the Atlantic Ocean, where it is deposited as sediment that is recognizably Saharan. Typically, the fewer plants to cover the ground, the more dust gets blown into the ocean. When the African Humid Period ended, the amount of sediment that got buried dramatically increased, making an abrupt transition if you are looking down a sediment section like you would a timeline.

So, based on sediment records, it looks like the African Humid Period ended and the Saharan desert established very quickly. To go from observations in sediments and other type of climate records, scientists will use reconstructive models. In the past 20 years, various models have been made that either support a very fast transition from a lush, green Saharan grassland to a Saharan desert, or a more gradual transition.

Figure 2. Example of lake levels in Africa during the African Humid Period, around 9,000 years ago. From Tierney et al., 2011.

Recently, scientists from Germany and Switzerland asked if a gradual transition from green to desert could cause a dramatic shift in the amount of dust blown from the Sahara to the ocean. To do this, they used models that include both dust transport and the climate.

The experiment: The scientists used a model called ECHAM-HAM, and compared modern day results to other models to make sure the model was working well for their region. They then looked at two different scenarios, and estimated the amount of dust that would be deposited at specific ocean locations in each case. These locations were where sediment cores had been taken, and gave the researchers something to compare their estimates to. If the scientists’ estimates were close to the actual amount of dust in the sediment, then the scenario they were exploring is more likely.

The scientists were exploring two scenarios that had to do with the idea that even a gradual process can cross a threshold, which could result in an abrupt change in another process, such as dust deposition. The scenarios here were:

  1. The Vegetation Fraction Threshold, in which you must lose a certain amount of vegetation (plants) covering the ground before you get large amounts of dust blown to the ocean.
  2. Latitudinal Threshold: Plants and lakes must shift a certain amount southward before large amounts of dust deposition happen.

Results: In scenario number 1, the amount of plants and lake covering the Sahara were decreased slowly. This did not cause an abrupt change in the amount of dust deposited in sediment. In scenario number 2, the location of the plants and lakes were modeled to move southwards over time. Like scenario 1, this did not cause an abrupt increase in dust deposited. Both gradual scenarios appeared to have gradual increases in dust deposition to the ocean.

By NASA - Cropped from Image:Africa satellite plane.jpg., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1654153

Figure 3. A satellite image of current day Saharan desert.

Conclusions 

The scientists who did this study concluded that there must have been an abrupt change from a Sahara with green grasslands and lakes to the desert we see today. The scientists do make the distinction, however, that this is not the same throughout the region. The sediment from locations further south show a more gradual increase in the amount of dust deposited during the transition from a lush and desert Sahara.

This is just one of many examples of how things in the ocean, like sediments, can be used to tell us about the past! Things environment can be cleverly used as ‘proxies’ for finding out past climate events, determining what life in the past was like, and more!

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