Continuous 1.3-million-year record of East African hydroclimate, and implications for patterns of evolution and biodiversity.

The transport of moisture in the tropics is a critical process for the global energy budget and on geologic timescales, has markedly influenced continental landscapes, migratory pathways, and biological evolution. Here we present a continuous, first-of-its-kind 1.3-My record of continental hydroclimate and lake-level variability derived from drill core data from Lake Malawi, East Africa (9-15° S). Over the Quaternary, we observe dramatic shifts in effective moisture, resulting in large-scale changes in one of the world's largest lakes and most diverse freshwater ecosystems. Results show evidence for 24 lake level drops of more than 200 m during the Late Quaternary, including 15 lowstands when water levels were more than 400 m lower than modern. A dramatic shift is observed at the Mid-Pleistocene Transition (MPT), consistent with far-field climate forcing, which separates vastly different hydroclimate regimes before and after ∼800,000 years ago. Before 800 ka, lake levels were lower, indicating a climate drier than today, and water levels changed frequently. Following the MPT high-amplitude lake level variations dominate the record. From 800 to 100 ka, a deep, often overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of extreme drought. Periods of high lake level are observed during times of high eccentricity. The extreme hydroclimate variability exerted a profound influence on the Lake Malawi endemic cichlid fish species flock; the geographically extensive habitat reconfiguration provided novel ecological opportunities, enabling new populations to differentiate rapidly to distinct species.

Microbially influenced lacustrine carbonates: A comparison of Late Quaternary Lahontan tufa and modern thrombolite from Fayetteville Green Lake, NY.

Carbonate microbialites in lakes can serve as valuable indicators of past environments, so long as the biogenicity and depositional setting of the microbialite can be accurately determined. Late Pleistocene to Early Holocene frondose draping tufa deposits from Winnemucca Dry Lake (Nevada, USA), a subbasin of pluvial Lake Lahontan, were examined in outcrop, petrographically, and geochemically to determine whether microbially induced precipitation is a dominant control on deposition. These observations were compared to modern, actively accumulating microbialites from Fayetteville Green Lake (New York, USA) using similar methods. In addition, preserved microbial DNA was extracted from the Lahontan tufa and sequenced to provide a more complete picture of the microbial communities. Tufas are texturally and geochemically similar to modern thrombolitic microbialites from Fayetteville Green Lake, and the stable isotopic composition of organic C, N, inorganic C, and O supports deposition associated with a lacustrine microbial mat environment dominated by photosynthetic processes. DNA extraction and sequencing indicate that photosynthetic microbial builders were present during tufa deposition, primarily Chloroflexi and Proteobacteria with minor abundances of Cyanobacteria and Acidobacteria. Based on the sequencing results, the depositional environment of the tufas can be constrained to the photic zone of the lake, contrasting with some previous interpretations that put tufa formation in deeper waters. Additionally, the presence of a number of mesothermophilic phyla, including Deinococcus-Thermus, indicates that thermal groundwater may have played a role in tufa deposition at sites not previously associated with groundwater influx. The interpretation of frondose tufas as microbially influenced deposits provides new context to interpretations of lake level and past environments in the Lahontan lake basins.

CO2 and fire influence tropical ecosystem stability in response to climate change.

Interactions between climate, fire and CO2 are believed to play a crucial role in controlling the distributions of tropical woodlands and savannas, but our understanding of these processes is limited by the paucity of data from undisturbed tropical ecosystems. Here we use a 28,000-year integrated record of vegetation, climate and fire from West Africa to examine the role of these interactions on tropical ecosystem stability. We find that increased aridity between 28-15 kyr B.P. led to the widespread expansion of tropical grasslands, but that frequent fires and low CO2 played a crucial role in stabilizing these ecosystems, even as humidity changed. This resulted in an unstable ecosystem state, which transitioned abruptly from grassland to woodlands as gradual changes in CO2 and fire shifted the balance in favor of woody plants. Since then, high atmospheric CO2 has stabilized tropical forests by promoting woody plant growth, despite increased aridity. Our results indicate that the interactions between climate, CO2 and fire can make tropical ecosystems more resilient to change, but that these systems are dynamically unstable and potentially susceptible to abrupt shifts between woodland and grassland dominated states in the future.

Ecological consequences of early Late Pleistocene megadroughts in tropical Africa.

Extremely arid conditions in tropical Africa occurred in several discrete episodes between 135 and 90 ka, as demonstrated by lake core and seismic records from multiple basins [Scholz CA, Johnson TC, Cohen AS, King JW, Peck J, Overpeck JT, Talbot MR, Brown ET, Kalindekafe L, Amoako PYO, et al. (2007) Proc Natl Acad Sci USA 104:16416-16421]. This resulted in extraordinarily low lake levels, even in Africa's deepest lakes. On the basis of well dated paleoecological records from Lake Malawi, which reflect both local and regional conditions, we show that this aridity had severe consequences for terrestrial and aquatic ecosystems. During the most arid phase, there was extremely low pollen production and limited charred-particle deposition, indicating insufficient vegetation to maintain substantial fires, and the Lake Malawi watershed experienced cool, semidesert conditions (<400 mm/yr precipitation). Fossil and sedimentological data show that Lake Malawi itself, currently 706 m deep, was reduced to an approximately 125 m deep saline, alkaline, well mixed lake. This episode of aridity was far more extreme than any experienced in the Afrotropics during the Last Glacial Maximum (approximately 35-15 ka). Aridity diminished after 95 ka, lake levels rose erratically, and salinity/alkalinity declined, reaching near-modern conditions after 60 ka. This record of lake levels and changing limnological conditions provides a framework for interpreting the evolution of the Lake Malawi fish and invertebrate species flocks. Moreover, this record, coupled with other regional records of early Late Pleistocene aridity, places new constraints on models of Afrotropical biogeographic refugia and early modern human population expansion into and out of tropical Africa.

East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins.

The environmental backdrop to the evolution and spread of early Homo sapiens in East Africa is known mainly from isolated outcrops and distant marine sediment cores. Here we present results from new scientific drill cores from Lake Malawi, the first long and continuous, high-fidelity records of tropical climate change from the continent itself. Our record shows periods of severe aridity between 135 and 75 thousand years (kyr) ago, when the lake's water volume was reduced by at least 95%. Surprisingly, these intervals of pronounced tropical African aridity in the early late-Pleistocene were much more severe than the Last Glacial Maximum (LGM), the period previously recognized as one of the most arid of the Quaternary. From these cores and from records from Lakes Tanganyika (East Africa) and Bosumtwi (West Africa), we document a major rise in water levels and a shift to more humid conditions over much of tropical Africa after approximately 70 kyr ago. This transition to wetter, more stable conditions coincides with diminished orbital eccentricity, and a reduction in precession-dominated climatic extremes. The observed climate mode switch to decreased environmental variability is consistent with terrestrial and marine records from in and around tropical Africa, but our records provide evidence for dramatically wetter conditions after 70 kyr ago. Such climate change may have stimulated the expansion and migrations of early modern human populations.