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Continuous 1.3-Million-Year Record of East African Hydroclimate, and Implications for Patterns of Evolution and Biodiversity

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Continuous 1.3-Million-Year Record of East African Hydroclimate, and Implications for Patterns of Evolution and Biodiversity Continuous 1.3-million-year record of East African hydroclimate, and implications for patterns of evolution and biodiversity Robert P. Lyonsa,1, Christopher A. Scholza,2, Andrew S. Cohenb, John W. Kingc, Erik T. Brownd, Sarah J. Ivorye, Thomas C. Johnsond, Alan L. Deinof, Peter N. Reinthalg, Michael M. McGlueh, and Margaret W. Blomeb,3 aDepartment of Earth Sciences, Syracuse University, Syracuse, NY 13244; bDepartment of Geosciences, University of Arizona, Tucson, AZ 85721; cGraduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882; dLarge Lakes Observatory and Department of Earth and Environmental Sciences, University of Minnesota, Duluth, MN 55812; eInstitute at Brown for the Study of the Environment and Society, Brown University, Providence, RI 02912; fBerkeley Geochronology Center, Berkeley, CA 94709; gDepartment of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721; and hDepartment of Earth and Environmental Sciences, University of Kentucky, Lexington, KY 40506 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved October 29, 2015 (received for review June 30, 2015) The transport of moisture in the tropics is a critical process for the Lake Malawi and Its Catchment global energy budget and on geologic timescales, has markedly Lake Malawi (Nyasa) is one of the world’s largest and oldest lakes, influenced continental landscapes, migratory pathways, and bi- and is situated at the southern end of the East African Rift Sys- ological evolution. Here we present a continuous, first-of-its-kind tem. The hydrologically open, freshwater ecosystem spans 6° of 1.3-My record of continental hydroclimate and lake-level variability latitude (9–15° S), and has a length of ∼580 km and a maximum derived from drill core data from Lake Malawi, East Africa (9–15° S). depth of 700 m (Fig. 1). Lake morphometry is a consequence of Over the Quaternary, we observe dramatic shifts in effective mois- crustal subsidence induced by slip along large border faults that ture, resulting in large-scale changes in one of the world’s largest define the three main structural segments in the Malawi rift (11, lakes and most diverse freshwater ecosystems. Results show evi- 12) (Fig. 1). Steep slopes adjacent to border faults characterize the dence for 24 lake level drops of more than 200 m during the Late asymmetric central and northern basins (∼700 and ∼600 m water Quaternary, including 15 lowstands when water levels were more depth, respectively), whereas the southern part of the lake is an than 400 m lower than modern. A dramatic shift is observed at the extended shallow-dipping ramp, with maximum water depth of Mid-Pleistocene Transition (MPT), consistent with far-field climate ∼400 m. Rocky shorelines interspersed with pocket beaches are forcing, which separates vastly different hydroclimate regimes be- prominent along the coasts, especially along border fault margins. fore and after ∼800,000 years ago. Before 800 ka, lake levels were Lake Malawi is home to >1,000 species of mainly endemic lower, indicating a climate drier than today, and water levels cichlid fishes (13), more than any other lake in the world, as well changed frequently. Following the MPT high-amplitude lake level as numerous endemic invertebrates, and its sediment record is a variations dominate the record. From 800 to 100 ka, a deep, often proven climate archive (3, 14). Because of its anoxic hypolimnion, overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of ex- Significance treme 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 Lake Malawi is one of the world s oldest and deepest lakes, with > flock; the geographically extensive habitat reconfiguration provided 1,000 species of endemic cichlid fish; its water bottom anoxia novel ecological opportunities, enabling new populations to differ- prevents bioturbation of deep-water sediments, which preserve entiate rapidly to distinct species. exceptional paleoclimate signals. The Lake Malawi Drilling Proj- ect recovered the first continuous 1.3-My record of past climates of the African interior. These sediments show that the catch- Lake Malawi | tropical paleoclimatology | East African rift | cichlid fish | quaternary ment experienced 24 dry periods over that time, when lake levels dropped more than 200 m. After ∼800,000 years ago, the lake was commonly deeper and overflowing, indicating wetter nsolation forcing of tropical convection and shifts in the In- conditions, but lowstand intervals became more prolonged and Itertropical Convergence Zone (ITCZ) are considered prin- extreme. These changes promoted the evolution of the endemic cipal mechanisms driving tropical climate variability on cichlid fishes, through shifting of habitats, and through isolation geologic time scales (1–5). However, instrumental records show and restriction of populations. that the ITCZ over land is poorly characterized compared with the oceans (Fig. 1), and the transport of oceanic moisture to the Author contributions: C.A.S., A.S.C., J.W.K., and T.C.J. designed research; R.P.L., C.A.S., A.S.C., J.W.K., E.T.B., S.J.I., T.C.J., P.N.R., M.M.M., and M.W.B. performed research; J.W.K., E.T.B., and hinterlands is complex, because topographic barriers redirect A.L.D. contributed new reagents/analytic tools; R.P.L., C.A.S., A.S.C., J.W.K., E.T.B., S.J.I., T.C.J., winds and block advection to the continental interiors (6). To A.L.D., M.M.M., and M.W.B. analyzed data; and R.P.L., C.A.S., A.S.C., T.C.J., and P.N.R. wrote document moisture transport onto the continents over geo- the paper. logical time scales, terrestrial records of tropical paleoclimate The authors declare no conflict of interest. with the length and continuity of ocean drilling records are This article is a PNAS Direct Submission. required. Continental drilling in long-lived tropical lakes pro- Freely available online through the PNAS open access option. vides continuous, high-resolution paleoclimate records that Data deposition: The paleoclimate proxy data and geochronology data have been de- posited with the NOAA paleoclimatology database of the National Centers for Environ- extend well past the last glacial maximum (7, 8), and augment mental Information: https://www.ncdc.noaa.gov/paleo/study/19424. shorter, well-dated records from outcrops. Lakes of great an- 1Present address: Chevron Corporation, Houston, TX 77002. — tiquity also contain numerous endemic species notably, cich- 2To whom correspondence should be addressed. Email: [email protected]. — lid fishes whose modern assemblages evolved in concert with 3Present address: BP L48 Onshore, Houston, TX 77079. changing climates, and which figure prominently into models of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. speciation and diversification (9, 10). 1073/pnas.1512864112/-/DCSupplemental. 15568–15573 | PNAS | December 22, 2015 | vol. 112 | no. 51 www.pnas.org/cgi/doi/10.1073/pnas.1512864112 Downloaded by guest on September 25, 2021 Fig. 1. (A and B) Continental TRMM 2B31 [combined TRMM microwave imager (TMI) and precipitation radar (PR)] precipitation estimates from 1998 to 2009 for January and July (SI Appendix) across Africa. Drill site 1 from the Lake Malawi Drilling Project and the interpreted ITCZ and CAB are labeled. (C) Bathymetric map of Lake Malawi (maximum water depths of north and central basin are ∼600 and ∼700 m, respectively), with digital elevation model of the region surrounding the lake. The lake surface elevation is 474 m, and maximum relief on map is ∼2,600 m. Red circle is central basin drill site. Yellow circle is location of Rungwe volcanoes, the presumptive source of the tephras dated in the core. Main border faults, which accommodate most of the basin sub- sidence, are shown in red. finely laminated lacustrine sediments below 200 m water depth are and alkaline lakes. Terrestrial paleoclimate indicators, includ- exquisitely preserved (3), permitting studies of long-term high- ing charcoal recovered from the last 150,000 y, indicate drier resolution climate change. The watershed experiences a single conditions relative to modern, coincident with the severe low- rainy season, as the ITCZ passes over the lake during the austral stands (18). Hydrologic modeling of the catchment’s water budget summer (Fig. 1). Moisture transport over tropical Africa is con- indicates that a precipitation rate 50% of modern sustains equi- trolled by the migration of the ITCZ and Congo Air Boundary librium lake level during −200-m lowstands (assuming modern (CAB; Fig. 1) (15, 16). As a result of its southern tropical position, bathymetry) and a precipitation rate 39% of modern sustains − EARTH, ATMOSPHERIC, elongated North/South configuration, and expansive catchment 600-m lake levels (19); these simulations require major atmo- AND PLANETARY SCIENCES (∼128,000 km2), Lake Malawi is ideally positioned to record long- spheric reconfigurations during each of the 24 arid intervals and term, continental-scale paleoclimate signals. lowstands of more than 200 m (Fig. 2). Patterns of environmental variability shift markedly at 800– Hydroclimate Proxy Analyses 900 ka (Fig. 2), indicating that the Mid-Pleistocene Transition Drill cores from the central basin of Lake Malawi extend 380 m (MPT) (20) was an important climate transformation in central below lake floor at a water depth of 590 m, a site dominated by and southern tropical Africa, as well as north of the equator (21, ECOLOGY hemipelagic deposits sensitive to environmental variability (8, 17). 22) (Fig. 2). This transition marks the boundary between two Site survey and geochronological data demonstrate that sampled distinct hydroclimate regimes within the rift valley, likely related sediments represent continuous sedimentation over the last to changes in global climate cyclicity (20).
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