It ‘s all about calibration!

Your climate reconstructions are only as good as your calibrations!

One of the most important goals of our research expedition is to reconstruct climate and hydrological conditions on the South American continent over the past 60,000 years. To reconstruct the past climate, we measure specific chemical components in sediments that record climate conditions. In order to be sure our climate reconstructions are accurate we are always testing or calibrating our chemical tools in the modern environment. To calibrate means that we relate modern conditions to the variation in our measured chemical components measured from modern water column and surface sediment samples. In this way we develop a mathematical relationship between temperature and variations in our chemical components that can, in turn be used to reconstruct past temperature when layers of the sediment are analyzed for the specific chemical components.

The red points indicate the sampling points of the transect north and south of the main Amazon river out flow.

The new method that we want to apply will help us to understand continental temperature. To learn about the temperature of the continents the out flow of the Amazon River from the South American continent is a perfect study site, because the river transports particles from all over the Amazon basin into the ocean. To extensively study the transport of the particles into the ocean we took samples during two transect from deep water sites on the continental slope at 3100 m to very shallow water sites on the continental shelf at 35 m. One transect was north and one transect was south of Amazon River.

So how can we deduce the temperature of the continent from river water and sediment?

Some of the molecules that are transported from the land into the sea, contain the secret in their chemical structure. The molecules are membrane lipids (called branched glycerol dialkyl glycerol tetraether or short GDGT) from soil bacteria. All bacteria have a cell wall and a membrane that protect them.

Like we are putting more cloth on when it is colder, bacteria are also able to adapt to temperature. They do this by changing the chemical structure of their membrane lipids. The structure changes so that the membranes are tighter when it is warmer and looser when it is cold.

Scheme of a bacterial cell

Once a method is calibrated and tested in several environments it can be used to determine the temperatures of the past times by looking at deeper sediment layers. Utilizing our calibration study from the water column and surface sediments we will be able to accurately reconstruct South American land temperatures over the past 60,000 years.

Example of a branched GDGT

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