DeepCHALLA goes to Nairobi, Kenya and….Lake Challa!…by Heather Moorhouse and Erin Martin-Jones

A handful of members from the UK DeepCHALLA team

This July, four scientists working on the Lancaster-BGS-Cambridge joint led DeepCHALLA project attended the African Quaternary Environments, Ecology and Humans (AFQUA) conference hosted at the National Museum of Kenya in Nairobi. This meeting brings together researchers who study the Quaternary period (last 2.6 million years) and are interested in past climate, ecosystem and ecological change, as well as human evolution across the entire African continent…

East Africa, is home to the East African Rift (EAR) Valley, one of the most extensive active rifts on Earth. The EAR valley represents the formation of a new ocean, created by two slowly moving diverging continental plates. This has resulted in volcanic and seismic activity, as well as producing some of the world’s most dynamic and unique ecosystems including the EAR lakes. These lakes are some of the oldest, deepest and largest in the world. Thus, these lakes have sediment records millions to hundreds of thousands of years old, capturing long-term changes in their local and regional environment. In addition, past eruptions from volcanoes along the EAR emitted ash that not only is relatively easy to date but provided excellent preservation of the remains of our human ancestors and the megafauna they hunted. This resulted in the region being termed “the cradle of mankind”, globally important archaeological sites which have advanced our understanding on the evolution of our own and other species. During the AFQUA conference, attendees were lucky to visit such globally unique ecological and archaeological sites.

The international group of scientists working on DeepCHALLA are investigating ~250,000 years of environmental change using sediments retrieved from the bottom of Lake Challa, a steep-sided crater lake on the Kenyan, Tanzanian border, close to Mt Kilimanjaro. Whilst technically not considered an EAR lake, Challa’s creation is a result of the volcanic activity caused by rifting.  Presentations and workshops were conducted by all four of the UK-based scientists working on the DeepCHALLA record, and involved how to produce reliable radiocarbon chronologies by Dr. Maarten Blaauw, Queens University Belfast and understanding the patterns and drivers of fires in Africa by Dr Daniele Colombaroli from Royal Holloway alongside others. Heather and Erin ran a workshop on how lake sediments can be used to understand natural hazards.

Lake Challa

Erin is investigating how volcanoes throughout the EAR system have behaved in the past, in order to provide an indication of the potential for future eruptions. Kenya and Tanzania are home to 28 volcanoes which are suspected to have been active over the last 10,000 years, however historical and geological evidence for the timing and size of past eruptions remains minimal. The workshop explored how we can also use lake sediments to chronicle the timing and magnitude of past eruptions.  Through time, lakes capture and preserve volcanic ash (tephra) horizons in their stratigraphically-resolved sediments, providing a picture of past volcanism that is frequently more complete than that preserved in outcrop. The geochemical fingerprint of glassy particles comprising each tephra acts allows it to be traced back to the source volcano and can be used to map out tephra dispersal, and dates on sediment sequences can be built into Bayesian age models to understanding the timing of past eruptions. Erin used the near-continous and well-dated Challa record as an example, finding previously unrecognised eruptions from cinder cones in the Kilimanjaro region over the last ~250,000 years.  Such knowledge on past volcanism is crucial to developing an understanding of the potential for hazards in this rapidly developing area, and is part of a wider effort to shed light on the volcanic record throughout East Africa using lake sediments at the Cambridge Tephra Laboratory.  

Alongside Erin, Heather spoke about how we can use fossilised algae from photosynthesisers (microscopic to larger plants) in lake sediments to understand climate and human impacts on lake ecological communities. Like many lakes globally, lakes in East Africa and those across the continent have been subject to climatic variability and pollution from the intensification of human activity and growing human populations Understanding the timing and magnitude of ecological change can help pinpoint impacts and causes of environmental modifications and ultimately guide where management should focus.

Erin enjoying the volcaniclastic deposits of the dried up river bed found
 in the catchment of Lake Challa

Post-conference, Erin and Heather were lucky enough to go on a scientific pilgrimage to Lake Challa itself, having missed the opportunity of helping with the drilling campaign in early 2017 as theit jobs had not yet begun. They walked along the top of the steep crater wall of the lake and Erin was excited to see how past volcanism had impacted the landscape at and around Lake Challa, including the thick reworked tephra accumulations in an ephemeral river bed and the numerous, and now vegetated volcanic craters. Whilst it was the dry season in Tanzania, and cool 24°C, they braved the water to take some water and rock scrape samples. These will tell them what phytoplankton or microscopic photosynthesisers are growing in the lake currently, and can be used to help interpret the historical changes documented in the lake sediment deposits.

This blog was written by Dr Heather Moorhouse, Diatom Isotope Research Technician working at Lancaster University, alongside the stable isotope facility at the British Geological Survey and Dr Catherine (Erin) Martin-Jones at the University of Cambridge.