African Lake Hold Clues to Ancient Climate History
SU scientists publish two papers in Nature journals
Climate signatures preserved in a 380-meter-deep sediment core from Lake Malawi, obtained during a Syracuse University-led drilling operation, captured past conditions in the Malawi basin region of Africa. Contrary to prevailing models of the area, Syracuse researchers and colleagues found that the basin has not become progressively drier over time.
“Our Lake Malawi study observed a long-term trend of overall wetter conditions over the past 1.3 million years, despite periods of profound drought occurring over briefer, ten-thousand-year intervals within the record,” says Earth Sciences Professor Christopher Scholz, co-author of the new study and the lead-investigator of the Malawi Scientific Drilling Project.
The study, published online Aug. 10 in Nature, takes advantage of numerous climatic and ecological markers preserved in the sediment of Lake Malawi. Syracuse University researchers, including Scholz, were instrumental in the initial drilling of the sediment cores on Lake Malawi in east Africa, which contains a record of geological events over the past 1.3 million years.
The SU team also led a drilling project on Lake Bosumtwi in west Africa to obtain a sediment core covering 1.1 million years of lake history. Their article reporting on the relationship between fire and carbon dioxide from Lake Bosumtwi sediments appeared in Nature-Scientific Reports, on July 18.
The source and quality of the Lake Malawi core may explain why the researchers’ results are in contrast to the “progressively drier” model of the African climate. Scholz explains that the old model is based on climate indicators in the northern hemisphere (Malawi is in the southern hemisphere), either from oceanic sediment or rock outcroppings that represent short stretches of time.
“The new samples come from sediment in a lake within the southern hemisphere of the continent, rather than from a distant part of the ocean,” Scholz says.
Tiny organisms preserved in the core enabled determination of past temperature. The creatures, called crenarchaea, produce biomarkers that essentially record lake surface temperatures. By lining up the age of a section of the core and the crenarchaea-thermometer records, Scholz and colleagues could track temperature over time.
The team also tracked calcium levels, which reflect area wetness, and isotopic signatures of leaf wax, which gives insight to vegetation levels near the lake. Taken together, the data show that the area has generally progressed from drier grasslands, to wetter, more wooded forests.
The core held additional surprises for the researchers in terms of what controls climate in the Lake Malawi basin. “We expected the climate ‘forcing’ in the lowest latitudes to be controlled by a special wobble in the earth’s orbit around the sun, known as precession. Instead, it turns out the temporal wetness variability is more complex in the region, which is roughly 10 degrees south of the equator,” Scholz says. “The climate is influenced by fluctuations in Indian Ocean surface water temperatures, which in turn are linked to ice age-influenced temperature changes at high latitudes.”
Climate data in this area is also relevant to our evolution. “This paper shows that there was considerable spatial variability in African climate during times of migrations and the emergence of key ancestors,” Scholz says.
The Lake Malawi core has already exposed drastic fluctuations in lake level, which may have played a role in cichlid fish evolution, and the relationship between fire, carbon dioxide and climate change in the area. This has stoked Scholz’s curiosity such that he hopes to drill a similar sediment core in another African lake, Tanganyika.
“A key goal is to extend our understanding much further back in time. Beyond Lake Malawi, there is an extremely rich record of past climate and environmental change preserved in the sedimentary deposits of the giant lakes of the rift valley,” Scholz says.