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Onset of the African Humid Period by 13.9 Kyr BP at Kabua Gorge

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Onset of the African Humid Period by 13.9 Kyr BP at Kabua Gorge HOL0010.1177/0959683619831415The HoloceneBeck et al. 831415research-article2019 Research paper The Holocene 1 –9 Onset of the African Humid Period © The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions by 13.9 kyr BP at Kabua Gorge, DOI:https://doi.org/10.1177/0959683619831415 10.1177/0959683619831415 Turkana Basin, Kenya journals.sagepub.com/home/hol Catherine C Beck,1 Craig S Feibel,2,3 James D Wright2 and Richard A Mortlock2 Abstract The shift toward wetter climatic conditions during the African Humid Period (AHP) transformed previously marginal habitats into environments conducive to human exploitation. The Turkana Basin provides critical evidence for a dynamic climate throughout the AHP (~15–5 kyr BP), as Lake Turkana rose ~100 m multiple times to overflow through an outlet to the Nile drainage system. New data from West Turkana outcrops of the late-Pleistocene to early- Holocene Galana Boi Formation complement and extend previously established lake-level curves. Three lacustrine highstand sequences, characterized by laminated silty clays with ostracods and molluscs, were identified and dated using AMS radiocarbon on molluscs and charcoal. This study records the earliest evidence from the Turkana Basin for the onset of AHP by at least 13.9 kyr BP. In addition, a depositional hiatus corresponds to the Younger Dryas (YD), reflecting the Turkana Basin’s response to global climatic forcing. The record from Kabua Gorge holds additional significance as it characterized the time period leading up to Holocene climatic stability. This study contributes to the paleoclimatic context of the AHP and YD during which significant human adaptation and cultural change occurred. Keywords African humid period, Galana Boi, ostracods, paleoclimate, paleoenvironment, Turkana Received 3 April 2017; revised manuscript accepted 23 December 2018 Introduction Leakey, 2011), the oldest stone tools described to date (Harmand et al., 2015), and evidence of early human warfare (Mirazon Lahr East African climate et al., 2016). Since much of the past 4 myr has been characterized Climate variability has a major impact on environments and the by a high degree of climate variability (deMenocal, 1995), recon- ecosystems that depend upon it. The African Humid Period (AHP), structing the late-Pleistocene to Holocene paleoenvironmental spanning 15–5 kyr BP, is a significant event for East African cli- response to paleoclimatic events in the basin informs broader mate (deMenocal et al., 2000; Shanahan et al., 2015; Tierney and interpretations of Plio-Pleistocene records such as the Hominin deMenocal, 2013). Increased solar insolation because of Milanko- Sites and Paleolakes Drilling Project West Turkana Kaitio vitch forcing enhanced the intensity of the African monsoons, (HSPDP-WTK) core (Cohen et al., 2016). which simple climate models predict would have increased rainfall by 17–50% over the continent (Kutzbach and Liu, 1997). During the AHP, the typically endorheic Lake Turkana spilled into the Geologic setting Nile drainage basin through an outlet in the northwest of the pres- The Turkana Basin, located in the Eastern Branch of the East ent-day Omo River (Butzer et al., 1972; Garcin et al., 2012; Mor- African Rift System (EARS) of northwestern Kenya and southern rissey and Scholz, 2014). In contrast to the increased moisture of Ethiopia, contains sedimentary strata spanning more than 4 mil- the AHP, the Younger Dryas (YD), a ~1-kyr cold interval in the lion years (Figure 1). Throughout the basin’s history, lacustrine northern hemisphere from 12.7 to 11.7 kyr BP (Rasmussen et al., systems have waxed and waned due to the combined influences of 2006), corresponded to increased aridity in East Africa (Berke tectonics and climate (Feibel, 2011). The Galana Boi Formation et al., 2014; Talbot et al., 2007). As a one of the largest catchments records the most recent phase of sedimentation in the Turkana in the rift system, it is imperative to understand the timing and Basin. The outcrop stratigraphy of the Galana Boi Formation was magnitude of base-level changes at Lake Turkana in response to the global climatic forcing like the AHP and the YD. These events have implications for resource availability for anatomically mod- 1Geosciences Department, Hamilton College, USA ern humans living in the basin and region (Shea and Hildebrand, 2Department of Earth and Planetary Sciences, Rutgers University, USA 3 2010; Whitworth, 1965). In addition, understanding lake-level Department of Anthropology, Rutgers University, USA fluctuations through this time period provides an important analog Corresponding author: for the Plio-Pleistocene record. The Turkana Basin preserves a Catherine C Beck, Geosciences Department, Hamilton College, 198 long geological record (Feibel, 2011) with a rich fossil fauna College Hill Road, Clinton, NY 13323, USA. assemblage, including many key hominin specimens (Wood and Email: [email protected] 2 The Holocene 00(0) c) a) 10° N Charcoal Slumping Blue Nile Diatomite Galana Boi Lake Formation Ziway- Shalla Beach ETHIOPIA River Road 1 Lothidok Hills River Road 2 Omo Basalt SOUTH Lake Abaya 1 km SUDAN Lake Chamo b) Lake Turkana Kalokol 5° N River Lake Chew Lothidok Bahir Kalokol5° N Ferguson’s Hills Gulf Lake Turkana Rivers (modern and ancient) KENYA Kabua Gorge Modern Lakes Lake Suguta Late Pleistocene- Early Holocene lakes Lake Baringo 0° 10 km Overflows 32° E 34° E 36° E 38° E Figure 1. (a) Lakes of the Kenyan and Ethiopian parts of the EARS, highlighting hydrologic connections between basins in the past. (b) Map of the Kalokol region, the Lothidok Hills basalt (dark gray), and Kabua Gorge showing, (c) locations where stratigraphic sections were measured, outcrops investigated, and other significant samples were taken. The basalts of the Lothidok Hills are indicated in black and the channel of the modern Kalokol River is shaded in gray for emphasis; Figure 1a is modified from Garcin et al. (2012). first described from the eastern margin of Lake Turkana by Von- Basin to the south (Melnick et al., 2012), much of the Turkana dra et al. (1971) and further developed by others (Harvey and Basin has remained tectonically quiescent over the past 200,000 Grove, 1982; Owen et al., 1982). The Galana Boi Beds, as they years when subsidence was concentrated in the center of the basin were originally called, were later promoted to formation status (Feibel, 2011). (Owen and Renaut, 1986); however, no type section was defined due to the spatial variability of these deposits and the badland topography which limits the availability of long, continuous sec- Methods tions (Owen and Renaut, 1986). In the type locality on the eastern Fieldwork side of Lake Turkana, the Galana Boi Formation is characterized Fieldwork conducted in the summers of 2011–2015 at Kabua by diatomaceous clayey silts that contain horizontal to wispy Gorge focused on the following activities: (1) measuring sections laminations and cross-bedded quartzofeldspathic sands. through the Galana Boi Formation to facilitate comparisons with Kabua Gorge is positioned to the west of Lake Turkana, ~12 other Holocene sedimentary archives from cores and outcrops; km southwest of the town of Kalokol (Figure 1). The locality is (2) mapping outcrops in the environs of Kabua Gorge to provide ideally situated at an elevation to record deposition during high- a spatial, as well as temporal, setting for deposition; and (3) col- stand events. In this location, the Kalokol River forms a gorge lecting bulk sediment samples for subsequent analyses, including through Miocene volcanics of the Lothidok Hills (Whitworth, AMS radiocarbon dating and isotopic analysis (δ13C and δ18O) of 1965). Late-Pleistocene to Holocene sediments of the Galana Boi ostracods. Formation rest unconformably on Pliocene Lonyumum Member deposits of the Nachukui Formation or directly on Miocene vol- canics. Based on field observations and mapping, the Galana Boi Radiocarbon dating section at Kabua Gorge is not rotated and there are no faults with Six AMS radiocarbon dates were generated from the sections, four significant offset (>1 m) in the study area. Extensive investiga- from molluscs (Melanoides tuberculata) and two from charcoal. tion of the Galana Boi Formation and the laterally equivalent Three shell dates came from the base of each of the lacustrine Member IV of the Kibbish Formation in southern Ethiopia intervals at River Road 1 (RR1) (Figure 1). A fourth shell date (Brown and Fuller, 2008) found no evidence of significant fault- came from a smaller exposure 1.58 km north of RR1 (Charcoal ing in these strata. While the Turkana Basin remains tectonically section) and allowed for a paired charcoal–mollusc date to esti- active and flexural rebound occurs in volcanically active areas in mate potential reservoir effects. 14C ages are reported in calibrated the center of the basin such as South Turkana and the Suguta years before present (cal yr BP) using the IntCal13 database. Beck et al. 3 Table 1. AMS radiocarbon dates from Kabua Gorge. Lab ID (beta) Location Material δ13C (‰) Conventional 14C 2-sigma calibrated result Median age Elevation dates (BP) (cal yr BP) (cal yr BP) (m) 402648 Charcoal section Charcoal –26 9390 +/– 30 10695–10560 10630 420 402649 Charcoal section <4 Mollusc –3.3 9640 +/– 30 11175–11070, 10955–10865, 10990 420 cm below charcoal 10850–10805 3775179 Slumping section Charcoal –24.4 9540 +/– 40 11085–10920, 10890–10705 10900 418 396299 River Road 1 Mollusc –4.1 10130 +/– 40 11970–11610, 11520–11510 11740 445 396300 River Road 1 Mollusc –2.4 11160 +/– 40 13090–12990 13040 442 396301 River Road 1 Mollusc –4.4 12010 +/– 40 13980–13755 13870 438 All measurements were made at Beta Analytic Radiocarbon Dating; conventional C14 ages are converted to calendar age using Calib 7.1 (IntCal13) and reported uncertainties represent 2-sigma (68% probability). In cases with bimodal or trimodal distributions, the median age was used. No reservoir age correction is applied to dates and all elevations are reported as recorded in the field using GPS.
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