Linking climate, humans and abrupt vegetation changes
Northwest Africa and Southeast Australia are regions which are particularly vulnerable to climate change. In her thesis, Raquel Lopes dos Santos of the Royal Netherlands Institute for Sea Research, investigate organic compounds in marine sediment cores, in order to reconstruct past environmental conditions in these areas. She found that vegetation changes were large and abrupt over the past 150,000 years in these regions and were caused by climate change as well as indirectly by humans. Lopes dos Santos will defend her thesis at the Utrecht University, on 17 December, 2012.
Raquel studied marine sediments covering the last 150,000 years collected from the seafloor off the coast of Northwest Africa as well as the coast of Southeast Australia. Strong off-shore winds transported large volumes of dust from the Sahara and Sahel to the coast of Northwest Africa while the largest river system of Australia, the Murray-Darling rivers, transported material to the coast of Southeast Australia. Mixed in with the dust and river materials are plant leaf waxes, which are transported long distances to the coastal ocean, where they are deposited on the seafloor. Over thousands of years, layers of sediment accumulate on the seafloor, each layer containing evidence of past environmental conditions from these areas.
Based on the analysis of plant leaf waxes, it was possible to determine the relative importance of the so-called C3 and C4 vegetation, which differ in their adaptation to temperature and precipitation. During three discrete periods, ca. 120,000-110,000 years, 50,000-45,000 and 10,000-8,000 years ago, substantially more C3 plants, probably trees, grew in the Sahara/Sahel region indicating significantly wetter conditions than at present. The two oldest periods exactly coincide with times when the earliest humans were migrating out of East Africa. At these times, the wetter conditions in central North Africa likely enabled humans to cross this normally inhospitable region, allowing them to migrate into other continents.
The plant leaf waxes of Southeast Australia revealed an extensive period (68,000-31,000 years) of generally high C4 plant abundance that is punctuated by a sharp increase in C3 vegetation at ~43,000 years. This sharp increase in C3 vegetation lasted ~5,000 years and directly followed a well-known period in which the large animals in Australia became extinct due to the arrivals of humans on the continent. This extinction allowed more C3 vegetation, normally consumed by the larger animals, to grow and expand. Together with the expansion of C3 vegetation, evidence was found for a large number of vegetation fires likely because the C3 vegetation is more prone to fire than C4 vegetation.
Raquel Lopes dos Santos (1979) was born in Cape Verde. She got a bachelor degree in Biology from the University of Coimbra (Portugal) in 2002 and a master degree in Environmental sciences from the UNESCO-IHE (The Netherlands) in 2008. Since 2008 she worked at NIOZ Texel at his PhD research.
The project was performed at NIOZ Texel and was funded through a VICI grant to Stefan Schouten by the Netherlands Organization for Scientific Research (NWO-ALW).
Photo 1: Retrieval of a marine sediment core.
Photo 2: Example of typical vegetation occurring in South East Australia.