50,000 years of Environmental Change in West Tropical Africa

Tropical forests provide three vital ‘ecosystem services’ to the Earth, they: i) contain c. 40% of the terrestrial carbon stock, ii) store c. 50% of global biodiversity, and iii) feedback into global climate and carbon cycles. In addition, tropical forests are thought to have been actively absorbing atmospheric carbon dioxide over recent decades and consequently may be mitigating the impact of ongoing human induced global climate change. The services provided by tropical ecosystems are now threatened by human land use practices and projected future climate change. However, due to the complex nature of tropical ecosystems it is unclear how vegetation will respond to changes in global climate conditions. To provide an empirical insight into the response of tropical vegetation to global climate change it is necessary to learn lessons from the past by exploring the fossil record. Lake sediments are the ideal source for fossils to provide evidence of terrestrial vegetation response to past global climate change. The identification of fossil pollen grains trapped within lake sediments is a tried and tested way of establishing past terrestrial vegetation change. Determining the types of plant represented in the fossil pollen record at any particular point in time provides a good indication of the vegetation that surrounded that lake during sediment deposition. In this paper we present a new c. 50,000 year fossil pollen record from Lake Bosumtwi (Ghana; 06o 30’N, 01o 25’ W; c. 100 m above sea level). Lake Bosumtwi is today located within the Guineo-Congolian rainforest close to the ecotone. The seasonal migration of the Inter Tropical Convergence Zone (ITCZ) passes over Bosumtwi and consequently vegetation is likely to be sensitive to any changes in the ITCZ position and the associated monsoon. Sediments recovered from Lake Bosumtwi in 2004 by the Intercontinental Drilling Program provide an opportunity to investigate tropical vegetation response to climate change at a high resolution. Sub-millennial resolution fossil pollen data from Bosumtwi can be divided into three distinct pollen assemblage zones through the last 50,000 years. The oldest zone (c. 51,000-28,000 years ago) is dominated by grasses but has some lowland tropical (Moraceae/Urticaceae c. 11 %) and montane (Olea c. 8 %). woodland elements. The second zone (c. 28,000-11,700 years ago) is again dominated by grasses with few other associated elements besides obligate aquatic Cyperaceae and Typha. The youngest zone (c. 11,700 years ago to present) is the most palynologically diverse and contains tropical forest elements >50 % (including Moraceae/Urticaceae, Alchornea and Celtis). Within the youngest zone there is significant compositional change with pollen characteristic of the surface sediments only being established c.1000-2000 years ago. The three pollen zones in Lake Bosumtwi correspond directly to Marine Isotope Stages 3-1 suggesting that global temperature change was a key factor in driving change within lowland west tropical African vegetation.