Scientific Drilling in the East African Rift Lakes: a Strategic Planning Workshop
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Directory of Open Access Journals :RUNVKRS5HSRUWV Scientific Drilling in the East African Rift Lakes: A Strategic Planning Workshop by James M. Russell, Andrew S. Cohen, Thomas C. Johnson, and Christopher A. Scholz doi:10.2204/iodp.sd.14.08.2012 Introduction and Key Science Themes Science Foundation (NSF) and Past Global Changes (PAGES) project sponsored “Continental Drilling in the The East African Rift lakes offer unparalleled opportuni- East African Rift Lakes”, a workshop attended by thirty-six ties to investigate fundamental climate, environmental, bio- African, European, and U.S. scientists and hosted by Brown logical, and geological processes through deep coring into University in Providence, Rhode Island, U.S.A. on 14–16 the lake bed. Their sediments hold signals of the evolution November 2011. of tropical rainfall, temperatures, and winds across 20q of latitude on both sides of the equator from the Miocene to the Recent years have witnessed significant developments present. Fossil material in these basins chronicles the devel- in multidisciplinary sciences in the African rift. There are opment of the East African landscapes in which our own ongoing initiatives to integrate paleoenvironmental and species evolved, and records the explosive evolutionary paleoanthropological data to understand the environmental radiation of literally thousands of species of fish, snails, and context of hominid evolution. These include the International other aquatic organisms endemic to East Africa’s lakes. Continental Scientific Drilling Program (ICDP)- and Drilling these sediments will also provide insights into the NSF-sponsored Hominid Sites and Paleolakes Drilling tectonic processes that shape the largest active continental Project (HSPDP, http://www.icdp-online.org/front_content. rift system on Earth today. php?idcat=1225); new initiatives to investigate the fundamen- tal processes underpinning rift initiation and evolution (e.g., The past decade has witnessed major advances in our GeoPRISMS, http://www.geoprisms.org/); and new obser- efforts to obtain long climate records from the East African vational and climate modeling efforts to understand East lakes, highlighted by the successful scientific drilling of African climate history. Workshop participants reviewed Lake Malawi in 2005 (Scholz et al., 2007). Since 2005, newly past studies and current initiatives to define scientific goals acquired and ongoing geophysical surveys by industrial and priorities in the critical areas of East African paleocli- and academic scientists have provided us with much of the mate, rifting processes, and ecosystem evolution. We then requisite site survey information to propose scientific drill- reviewed the stratigraphic architecture and environmental ing projects in some of the premier target lakes in East history of Lake Turkana, Lake Albert, and Lake Tanganyika Africa, most notably Lake Turkana, Lake Albert, and Lake to evaluate their potential for addressing key scientific ques- Tanganyika (Fig. 1). To strategically plan for the next decade tions in order to develop a strategic plan for drilling projects of scientific drilling in East Africa’s lakes, the U.S. National in the East African lakes. Plio-Pleistocene East African 25˚E 30˚E 35˚E 40˚E Climate Lake Tanganyika Lake Turkana 5˚N 100 km 50 km Decades of research have provided us with a Isobaths (m) Isobaths (m) relatively coherent picture of climate change at 80+ 1400+ 0˚ 1250 60 the high latitudes during late Cenozoic time, 1000 40 including iconic marine sedimentary records 750 20 500 0 and ice cores that document glacial-interglacial 250 5˚S 100 variability and abrupt, millennial climate change (Fig. 2). Unfortunately, our understanding of Lake Albert 10˚S tropical climate systems, and particularly conti- nental tropical climate, lags far behind our knowledge of the high latitudes. Only recently 15˚S 50 km we thought that African climate marched largely to the beat of the northern high-latitude ice Figure 1. Elevation map of East Africa with bathymetries of Lake Tanganyika, Lake sheets, yet drill cores recovered by the Lake Turkana, and Lake Albert. The inset in the central figure shows the general location of the East African Rift (blue box). Isobaths for Albert and Turkana follow the same color Malawi drilling project revealed the existence of scheme (right). major droughts in Southeast Africa, far exceed- Scientific Drilling, No. 14, September 2012 49 :RUNVKRS5HSRUWV ing the aridity experienced during the Last Glacial lighted that such records will allow us, for the first time, to Maximum, prior to 75,000 years ago (Scholz et al., 2007). address various questions: These droughts appear to have been forced by eccentricity modulation of orbital precession, and they possibly affected t What are the dynamics of late Miocene-Pleistocene the dispersal of anatomically modern humans both within (last 7 Myr) African climate as a consequence of the and out of Africa (Blome et al., 2012). Still, the Malawi drill mid-Pliocene termination of a permanent El Niño, cores cover only the latter part of the Pleistocene and record ocean circulation change with the closure of the climate history at the southernmost end of the African rift— Indonesian seaway, the onset, intensification, and a climate setting that differs considerably from the rest of changes in the periodicity of Northern Hemisphere the rift to the north. Recent modeling studies have suggest- Glaciation, and the development of and interactions ed strong linkages between Plio-Pleistocene changes in the with East African Rift topography? sea-surface temperature structure of the tropical Pacific and t What is the sensitivity and spatial variability of East Indian Oceans and changes in East African rainfall (Brierley African hydrology and temperature to orbital-scale and Fedorov, 2010), potentially affecting the evolution of our climate forcings? Are tropical air temperature and species. These new perspectives call into question many of hydrologic change coupled through time, and what the fundamental mechanisms and processes causing African are their sensitivities to radiative forcing from green- climate variability, including the following. Are mega- house gases and seasonal insolation? droughts present at sites to the north, and are they antiphased in their timing relative to those in Malawi as predicted by the t How do the rates and amplitudes of East African cli- precessional insolation forcing model? How consistent is the mate change on millennial to decadal time scales relationship between precessional orbital forcing and East vary as a function of mean climate state? What are African climate through the Plio-Pleistocene? the mechanisms underpinning this time scale of variability? Numerous outcrop and marine sediment records from tropical East Africa suggest that East African climate has Fundamentally, our goal is to understand the Plio- responded to a complex and interrelated set of climate forc- Pleistocene climate evolution of Africa at a resolution com- ings. These include orbitally-driven parable to the records of climate from changes in insolation, changes in the high latitudes (Fig. 2), providing key Age (kyr) new insights into the dynamics of trop- global ice volume and atmos- 0 1000 2000 3000 4000 5000 pheric greenhouse gas concentra- ical continental rainfall variability and 2.5 Global Ice Volume tions, long-term reorganizations of sensitivity. ‰) 3 the principal modes of sea-surface O ( 18 3.5 temperature variability such as G Rift Initiation and Evolution 4 El Niño–Southern Oscillation 4.5 (ENSO), and tectonically-driven Benthic The general notion is that the East 5 changes in oceanic and atmo- African Rift formed as heat flow from the convecting mantle weakened the litho- spheric circulation (Scholz et al., 0 100 200 300 400 500 600 700 800 900 1000 2007; Tierney et al., 2008; Brierley 3 sphere and allowed the African Plate to 100-kyr glacial cycles and Fedorov, 2010). Understanding 3.5 stretch and fracture along pre-existing the governing mechanisms and weaknesses in the lithosphere (Ebinger, O (‰) 4 18 sensitivity of African climate to G 1989). This, coupled with the introduc- 4.5 these phenomena will require long tion of magma in at least some parts of 5 the rift, led to rapid deformation and and continuous climate records Benthic extending into at least the Pliocene 5.5 localization of strain along pre-existing to observe East African climate lithospheric structures. However, these 0 20406080100120 responses across multiple events simple conceptual models fail to explain -33 Greenland Temperatures and time scales. Moreover, the the rates and complexity of rifting pro- cesses, and the relative roles of magma- Pliocene offers the most recent O (‰) -37 18 extended warm period, when at- G tism, volatiles, and pre-existing struc- tures in the rifting process are very mospheric greenhouse gases -41 approached values similar to those poorly known. Both magmatism and Greenland Greenland reactivation of pre-existing structures we will witness in the near future, -45 can help to explain rift initiation and and when continental configura- Figure 2. Climate change at different time scales tions were similar to those today as indicated by marine foraminiferal benthic lithospheric stretching (Dunbar and (Committee on the Earth System oxygen isotopic data (Liesecki and Raymo, 2005) Sawyer, 1989; Buck, 2004), given the rel- and oxygen isotopic reconstructions