Erik T. Brown

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 lakes 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 ≈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.

History and timing of human impact on Lake Victoria, East Africa

Lake Victoria, the largest tropical lake in the world, suffers from severe eutrophication and the probable extinction of up to half of its 500+ species of endemic cichlid fishes. The continuing degradation of Lake Victorias ecological functions has serious long–term consequences for the ecosystem services it provides, and may threaten social welfare in the countries bordering its shores. Evaluation of recent ecological changes in the context of aquatic food–web alterations, catchment disturbance and natural ecosystem variability has been hampered by the scarcity of historical monitoring data. Here, we present high–resolution palaeolimnological data, which show that increases in phytoplankton production developed from the 1930s onwards, which parallels human–population growth and agricultural activity in the Lake Victoria drainage basin. Dominance of bloom–forming cyanobacteria since the late 1980s coincided with a relative decline in diatom growth, which can be attributed to the seasonal depletion of dissolved silica resulting from 50 years of enhanced diatom growth and burial. Eutrophication–induced loss of deep–water oxygen started in the early 1960s, and may have contributed to the 1980s collapse of indigenous fish stocks by eliminating suitable habitat for certain deep–water cichlids. Conservation of Lake Victoria as a functioning ecosystem is contingent upon large–scale implementation of improved land–use practices.

Quaternary Climate Change and the Formation of River Terraces across Growing Anticlines on the North Flank of the Tien Shan, China

Nested stream terraces, warped upward over actively growing anticlines along the north flank of the Tien Shan in western China, appear to record alternating phases of valley widening and incision. Differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines. We infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr. The crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm/a. The maximum heights of 25 and 35 ( ± 10) m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr ( ± 10 kyr). Hence, they suggest that flood plains, which were abandoned to form the terraces, developed adjacent to active stream beds during the last glacial period, when climates were relatively cold and dry. We presume that they were incised during deglacial periods when discharges and stream power increased. Apparent durations of exposure, obtained from

Slip rates of the Karakorum fault, Ladakh, India, determined using cosmic ray exposure dating of debris flows and moraines

^{10}Be

The Holocene paleolimnology of Lake Issyk-Kul, Kyrgyzstan: trace element and stable isotope composition of ostracodes

in quartz cobbles lying on the surface of the lower terrace from one anticline, concur with abandonment and deep (~150 m) incision of the flood plain during the last global deglaciation (ca. 20 to 13 kyr B.P.). A minimum carbon-14 date of 33.9 kyr B.P. from deposits on the lowest terrace sequence from the other anticline, however, implies that such abandonment and incision of this flood plain occurred before the most recent global glacial maximum, about 20 kyr B.P. We infer that incision of this second anticlines floodplain began during an earlier deglacial epoch within the last glacial period (between about 70 and 20 kyr, and perhaps near 35 kyr B.P.).

Crustal Shortening on the Margins of the Tien Shan, Xinjiang, China

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 Africas 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 ≈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 (≈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.

Extended megadroughts in the southwestern United States during Pleistocene interglacials

most recent major glacial advance occurred at 90 ± 15 ka. This is consistent with the inference of others that alpine glaciers in this region have not necessarily expanded in concert with Northern Hemisphere continental ice sheets. If features, including lateral moraines, that Liu inferred to have been offset 300–350 m by the Karakorum fault date from the same period, they too imply a slip rate of 3–4 mm yr � 1 . This slip rate is comparable to rates of extension across grabens within Tibet. With recent evidence that slip along the Altyn Tagh fault occurs at � 10 mm yr � 1 , our rate suggests that slip along the boundaries of Tibet is not significantly more rapid than extension within the plateau. Hence, plate tectonics, in the strictest sense, ought not be applied to Tibet, because Tibet does not behave as a rigid plate. INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 1035 Geochemistry: Geochronology; 9320 Information Related to Geographic Region: Asia; 8107 Tectonophysics: Continental neotectonics; KEYWORDS: Karakorum Fault, slip rate, cosmic ray exposure age, beryllium 10

Constraints on age, erosion, and uplift of Neogene glacial deposits in the Transantarctic Mountains determined from in situ cosmogenic 10Be and 26Al

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.