I put some music on and sat down to write this blog about what I do and the next thing I know the Rolling Stones came on. With the classic lyrics: “Pleased to meet you, Hope you guess my name, But what’s puzzling you, Is the nature of my game”. How perfect I thought. So what is the nature of my game? I suppose the shortest answer is “Quaternary loess deposits”.
For those who don’t know the Quaternary is the most recent geological period covering the last 2.6 million years (Figure 1). We live in the Quaternary period. As for loess… it is a silt sized mineral dust, associated with aeolian (wind related) deposition and transport that is found in very characteristic sequences (Figure 2) and provides a record of past climate change. The yellowy dust bands are deposited during glacial periods (cold), creating loess units. During the interglacial (warm) phases reddish soil, palaeosol, develops on top of/within the loess unit (Figure 2). This process repeats numerous times, creating very thick and extensive sequences. For example, the Chinese Loess Plateau covers an area of over 440,000km2 (that’s more than the size of Germany or the UK and Ireland combined) and can be as thick as 300m (about 69 double decker buses). There the record of numerous loess-palaeosol cycles (glacial-interglacial cycles) extends as far back in time as 22 million years1– well beyond the Quaternary. That is a lot of dust in one place if you think about it… But loess is found on almost every continent and actually covers 10% of all land surfaces2.
What I am interested in, is the source of all that material. Where did it originate from? How was it created? How was it transported? And when did it all happen?
The study area for my project is Central Europe, along the Danube River (Figure 3). Although not as extensive as Chinese loess, it still stretches back 1 million years3, though I am concentrating on the last glacial-interglacial period (125,000 years). There are several hypotheses regarding loess provenance, which include the Fennoscandinavian Ice-Sheet (the Ice Sheet that extended all the way from Scandinavia down to Central Poland), smaller glaciers in the Alps and the Carpathians, local smaller mountain chains and eroded sediments from the Carpathian (Pannonian) Basin. I am focusing on individual silt grains, rather than bulk sediments. The reason behind this is simple–it can be difficult to obtain clear signals from bulk samples especially if sediment was mixed and originates from multiple areas. In essence, if multiple sources are involved, a mixing before deposition is likely to produce values that are an average of the different source areas. On the other hand, a single grain can only come from one source, significantly increasing the potential accuracy.
Establishing the timing of deposition and any potential provenance shifts will form the second part of my project. To develop a chronology, I will be using Optically Stimulated Luminescence dating, which measures the period which has passed since the last time the grains were exposed to light. That is to say, the time since the grains were deposited and buried by more sediment.
“So… apart from scientific curiosity, why is it important? Why should we understand this?” These are the questions the science community gets asked a lot lately. There are usually big picture and smaller scale answers. From the big picture perspective, dust is an important component of the Earth system. It affects how much heat gets trapped (or reflected) in the atmosphere, naturally fertilises oceans, and influences cloud formation, plus it interacts with many other systems. What we don’t know is the extent of its travels in the atmosphere, the quantities and how it is transported. The Intergovernmental Panel on Climate Change4 views atmospheric dust as a substantial uncertainty in future climate models, so identifying its past and present behaviour is important not only for understanding how the system works but also providing information that will allow accurate modelling future climate. Loess offers an opportunity to investigate past emissions and transport and so it provides essential information to test the robustness of current models and therefore can reduce their uncertainties.
On the smaller scale, the source information is key to deciphering information about past environments recorded in loess, including, understanding which regions become active dust emitters during drier periods, what mechanism(s) produce(s) these huge quantities of silt, as well as better interpretation of the palaeoclimatic information recorded in loess proxies (click here for details on proxies), such as grain-size. Grain size is often used to infer past aridity or windiness, however without source knowledge, we do not know what controls these proxies. Are they purely climatic? Or do we have other factors, such as sediment availability? And that’s even without mentioning the understanding of regional and continental climatic patterns, wind directions, understanding the timing of changes as well as any regional climatic leads and lags.
I am very excited for the next 3 years of this project because, there is so much we can learn from dust. So watch my dust…
1. Guo, Z. T. et al. Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China. Nature 416, 159–163 (2002).
2. Pésci, M. Loess is not just the accumulation of dust. Quat. Int. 7/8, 1–21 (1990).
3. Marković, S. B. et al. The last million years recorded at the Stari Slankamen (Northern Serbia) loess-palaeosol sequence: revised chronostratigraphy and long-term environmental trends. Quat. Sci. Rev. 30, 1142–1154 (2011).
4. IPCC. 7: Clouds and Aerosols BT – Climate Change 2013: The Physical Science Basis. Climate Change 2013: The Physical Science Basis (2013).