Reply to the comment on ``Environmental change and ...

Quaternary Science Reviews xxx (2016) 1e4

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Correspondence

Reply to the comment on “Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years. Quaternary Science Reviews 129: 333e340” We welcome the comment by Wright and Forman (2016) on our paper as a useful opportunity to stimulate the discussion around interdisciplinary comparative approaches using palaeoenvironmental and archaeological data. The authors of the comment question our interpretation of the archaeological record, and the concept of a Holocene refugium in the southwestern Ethiopian highlands. We would like to take this opportunity to explain our conclusions once again, in particular since some of the points raised by Wright and Forman (2016) show that they might have misunderstood key scientific concepts of our work as we try to show in the following paragraphs. We respond here to the four main issues highlighted in their comment: (1) “The treatment of the radiocarbon dates available for the Turkana Basin is selective and does not include several key sites.” e Our analysis is deliberatively selective because we aimed to test the hypothesis that two ecological zones, around adjacent lake margins (Turkana and Ziway-Shala lakes) and the southwestern Ethiopian highlands, could have been favoured sites of human occupation during times of climatic stress during the last 20,000 years. Wright and Forman (2016) included a map, which is misleading as it suggests that all archaeological key sites in the Turkana basin have been ignored by Foerster et al. (2015). In fact, we cite 31 radiocarbon dates from different well documented archaeological key sites at the Turkana shores (as one of the hypothetical retreat areas), plus 6 from the Ziway-Shala lakes, and 26 dates from the southwestern Ethiopian highlands, as evidence for human occupation in these locations (e.g. Owen et al., 1982; Barthelme, 1985; Johnson et al., 1991; Brown and Fuller, 2008; Garcin et al., 2012; Hildebrand and Grillo, 2012; Wright et al., 2015). Our paper clearly discusses in some detail the limitations of using radiocarbon date frequencies and distributions for this purpose. As indicated (Foerster et al., 2015, p.334), applying a critical review to the reliability of all radiocarbon ages leads inevitably to the exclusion of dates on bone apatite due to the documented very large age uncertainties for those ages (especially critical for age determinations that were measured before decisive progress was made in the field of AMS bone dating; Hedges and Van Klinken, 1992;

DOI of original article: http://dx.doi.org/10.1016/j.quascirev.2016.01.031.

Brock et al., 2007; Geyh et al., 1998; Cherinsky, 2009). We agree with the authors of the comment, however, that we could have listed these older dates in a table and shown the dates in Fig. 2 (in Foerster et al., 2015, p. 336) with a symbol such as done for “ … the green star signifies culture-related evidence of occupation that is not clearly datable” (Foerster et al., 2015). Applying the same criteria as for the previously stacked archaeological data, we have now taken this opportunity to include data recently published by Wright et al. (2015), and update the used archaeological data set as shown in Fig. 1 and Table 1. (2) “Depositional and taphonomic processes are not considered as factors in archaeological site preservation, thereby implying that a complete archaeological record is available for comparative purposes.” e We have not implied that a complete archaeological record is available. We clearly signal the limitations of our approach: “Due to the incompleteness of the archaeological data set, the results are of course very preliminary and hypothetical, but could be an important starting point for further research in this field” (Foerster et al., 2015, p.334). The archaeological record is the result of continuous processes that are influenced by natural and anthropogenic forces. Thus, the archaeological record can never be complete. Wright and Forman (2016) seem to criticize an established approach to formulating hypotheses on the dynamics of settlement and migration patterns or populations, usually employed when more sophisticated approaches to reconstructing human activity are not available (e.g. Deacon, 1974; Deacon and Deacon, 1999; Richter, 1991; Shennan et al., 2013; Manning and Timpson, 2014). Wright and Forman (2016) argue that the preservation of archaeological sites in the Turkana Basin is generally very low, mostly attributed to Holocene water level fluctuations, apparently believing that we overlooked this important fact. We clearly discuss “… whether the level of paleolake Turkana has fluctuated repeatedly by 50 m during this interval or it may have fallen gradually by 20 m between ~10.8 and 10 ka BP” (Foerster et al., 2015, p.337). Furthermore, we suggested that the “Apparent break in the occupation record after ~9.2 ka could be explained by the highly fluctuating lake levels, simply washing away all archaeological evidence” (Foerster et al., 2015, p.337). The extensive work on the lake level reconstructions by Bloszies et al. (2015) certainly shows the complexity of the water

http://dx.doi.org/10.1016/j.quascirev.2016.04.003 0277-3791/© 2016 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Foerster, V., et al.Reply to the comment on “Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years. Quaternary Science Reviews 129: 333e340”, Quaternary Science Reviews (2016), http://dx.doi.org/10.1016/j.quascirev.2016.04.003

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Correspondence / Quaternary Science Reviews xxx (2016) 1e4

Fig. 1. Updated comparison of (A) the 20 ka Chew Bahir climatic record (K content as a proxy for aridity) and the variations with the earth's precession (Berger and Loutre, 1991) with (B) settlement in the SW Ethiopian Highlands and around lake margins (Turkana and Ziway-Shala lakes). Climatic events: AHP e African Humid Period (~15e5 ka BP), YD e Younger Dryas (~12.8e11.6 ka BP), OD e Older Dryas (around 14 ka BP), H1 - Heinrich event 1 (around 16 ka BP), LGM e Last Glacial Maximum (~24e18 ka BP). During the AHP, several pronounced dry spells occur, modulating the wet phase; the gradual Holocene aridification (gradually coloured orange bar) is punctuated by arid events on a decadal timescale (Trauth et al., 2015). Settlement activities in both potential refugia are indicated by radiocarbon frequency of archaeological finds, as listed in Suppl. Table 1 (Foerster et al., 2015). The lower radiocarbon frequency in grey comprises six further reliable radiocarbon dates on archaeological finds from sites around Lake Turkana as indicated in Wright et al. (2015). Cultural innovation is indicated by first documented wavy-line pottery (pot symbol) and the introduction of pastoralism (cow symbol); red or green colours refer to SW highlands or lake margins respectively. The green star signifies culture-related evidence of occupation that is not clearly datable. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Table 1 Update to the supplementary Table 1 in Foerster et al. (2015), including conventional radiocarbon dates from archaeological sites in the Turkana basin, as published by Wright et al. (2015). The ages were additionally included in the updated archaeological data set in Fig. 1, marked in grey. Conventional radiocarbon ages were converted to calendar € ris, 2008). EMH ¼ Early Middle Holocene; LH ¼ Late Holocene. years using the IntCal13 data set (Reimer et al., 2013) and CalPal (Weninger and Jo Site

Cultural complex

Lab. number

Sample material

Date BP

GcJh1 Apeget 1 GeJi11 Lothagam fishing GeJi11 Lothagam fishing FwJh14 Kaerdit FwJh16 Abelete Akoit GeJk13 Adakurake

LH EMH EMH N N N

UCLA-2124K UCLA-2124A N-812 ISGS-A3050 SNU13-390 ISGS-A1341

Charcoal Charcoal Charcoal OES Charcoal OES

1800 6300 6200 1365 3160 2575

± ± ± ± ± ±

Reference 300 800 130 25 40 20

Robbins, 1980: 98e105, Robbins, 1984 Robbins and Lynch, 1978: 619, Robbins, 1974; Yamasaki 1972 Robbins and Lynch 1978: 619, Robbins, 1974; Yamasaki, 1972 Soper unpublished Soper unpublished Wright and Forman, 2011



level history of Lake Turkana, (as acknowledged by us in citing their work), and shows that low site preservation may be responsible for gaps in the archaeological record. Inevitably the archaeological record is selective for sites that are at or above current lake level, and selective against sites occupied during lowstand intervals, either because they are below current lake level, or because they were destroyed by wave action during subsequent transgressions. Since the level of Lake Turkana is currently at a historic low, it seems a reasonable assumption that much of the evidence for early occupation of the lake margins should be apparent in the terrain around the lake. Nevertheless, the Turkana lake margin certainly represents a key site for the occupation dynamics during a highly variable climate and should not be left out in this comparative discussion. (3) “The term “refugium” is used idiosyncratically to connote population expansion into highland regions during arid periods, though there is a lack of evidence for a concurrent population bottleneck in the lowlands.” Our use of the term refugium is entirely consistent with wider ecological usage. We stated that, “ … the term refugium is used here for areas that might have permitted the survival of human populations during arid phases” (Foerster et al., 2015, p.334). Although the term refugium originates in biological sciences it is today broadly applied in a wider sense as temporal or spatial protection from disturbances or in this sense with largely “continued moisture availability” (Tzedakis et al., 2002, p.2044; Keppel et al., 2012). We need to emphasize that the study design was not about genetics (bottleneck populations), nor would the chosen time frame of 20 ka BP been suitable for that. Instead, Foerster et al. (2015) deliberately follow an interdisciplinary approach and we understand refugium as the commonly used concept of where people could have migrated during extreme dry periods that we interpreted as climatic stress situations. We therefore included both the moister southwestern Ethiopian Highlands and the margins of the adjacent big lakes in the lowlands that have been documented to contain water even during the long droughts and should have permitted survival of humans during climatic extremes due to the continued moisture availability. We agree that refugium in the current context implies population restriction in unfavourable (non-refugial) areas during arid intervals, which is implicit in the hypothesis that we set out to test. We also agree that additional archaeological data from the region are needed for a fully comprehensive analysis as the data that are currently available do not show whether, and to what extent, people survived in the arid lowlands during droughts or where else they might have gone during intervals of pronounced aridity. However, this was explicitly not the aim of our work, instead we were interested in short-term migration and occupation patterns in hypothesized retreat areas. Therefore the suggestion to include sites in the dry lowlands (Ele Bor) to the data sets of two hypothetical refugia (precipitation-rich highlands and lake shores of the MER and the Turkana Basin) reflects that the concept of comparing settlement activities in such retreat areas and a record of climatic transitions on different timescales might have been miscomprehended. (4) “Unsupported inferences are made on human population dynamics without an anthropological foundation to support how foragers and pastoralists used landscapes in both the past and the present.” e We welcome the qualifications

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made by Wright and Forman (2016) who point out that anthropological studies demonstrate the capacity for human survival under the harshest ecological conditions. Nevertheless, it is not unreasonable to assume that populations, which seek out and occupy more favourable habitats in times of climatic stress are more likely to prosper. Ethnographic analogies are also somewhat hypothetical; it is questionable whether humans really acted in the same way two thousand years ago as they do today. Despite these and the other issues highlighted by Wright and Forman (2016), we feel that our study makes a strong contribution to understanding the drivers of human population dynamics and migration, than traditional approaches of descriptive archaeology. Despite the fact that human decision-making within certain environmental boundaries plays an important though incalculable role, our analysis tends to support the hypothesis that ecologically favourable zones of lake marginal and precipitation rich montane habitats (refugia) were preferentially occupied during intervals of climatic stress (Fig. 1). Our work relies on the continuous palaeoclimatic record of the Chew Bahir cores, because a comparable core record is not currently available from Lake Turkana. However, the detailed, although complex, shoreline evidence for fluctuating levels of Lake Turkana, provided by Forman et al. (2014), Bloszies et al. (2015), Wright et al. (2015) and others, offers a welcome opportunity for a further test of human responses to climatic change in the region. References Barthelme, J.W., 1985. Fisher-hunters and neolithic pastoralists in east Turkana, Kenya. In: British Archaeological Reports International Series, vol. 254. Oxford, UK. Berger, A., Loutre, M.-F., 1991. Insolation values for the climate of the last 10 million years. Quat. Sci. Rev 10, 297e317. Bloszies, C., Forman, S.L., Wright, D.K., 2015. Water level history for Lake Turkana, Kenya in the past 15,000 years and a variable transition from the African humid period to Holocene aridity. Glob. Planet. Change 132, 64e76. http://dx.doi.org/ 10.1016/j.gloplacha.2015.06.006. Brock, F., Bronk Ramsey, C., Higham, T., 2007. Quality assurance of ultrafiltered bone dating. Radiocarbon 49, 187e192. Brown, E.T., Fuller, C.H., 2008. Stratigraphy and tephra of the Kibish formation, southwestern Ethiopia. J. Hum. Evol. 55, 366e403. Cherinsky, A., 2009. Can weg et a good radiocarbon age from “bad bone“? Determining the reliability of radiocarbon age from bioapatite. Radiocarbon 51, 647e655. Deacon, J., 1974. Patterning in the radiocarbon dates for the Wilton/Smithfield complex in southern Africa. South Afr. Archaeol. Bull. 28, 1e18. Deacon, H.J., Deacon, J., 1999. Human Beginnings in South Africa. David Philip Publishers, Cape Town and Johannesburg, p. 28. Foerster, V., Vogelsang, R., Junginger, A., Asrat, A., Lamb, H.F., Schaebitz, F., Trauth, M.H., 2015. Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years. Quat. Sci. Rev. 129, 333e340. Forman, S.L., Wright, D.K., Bloszies, C., 2014. Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity. Quat. Sci. Rev 97, 84e101. Garcin, Y., Melnick, D., Strecker, M.R., Olago, D., Tiercelin, J.-J., 2012. East African mid-Holocene wetedry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift. Earth Planet. Sci. Lett. 331e332, 322e334. Geyh, M.A., Schotterer, U., Grosjean, M., 1998. Temporal changes of the (super 14) C reservoir effect in lakes. Radiocarbon 40 (2), 921e931. Hedges, R.E.M., Van Klinken, G.J., 1992. A Review of current approaches in the pretreatment of bone for radiocarbon dating by AMS. Radiocarbon 34, 279e291. Hildebrand, E., Grillo, K., 2012. Early herders and monumental sites in eastern Africa: new radiocarbon dates. Antiquity 86, 338e352. Johnson, T.C., Halfman, J.D., Showers, W.J., 1991. Paleoclimate of the past 4000 years at Lake Turkana, Kenya, based on the isotopic composition of authigenic calcite. Palaeogeogr. Palaeoclimatol. Palaeoecol. 85, 189e198. Keppel, G., Van Niel, K.P., Wardell-Johnson, G.W., Yates, C.J., Byrne, M., Mucina, L., Schut, A.G.T., Hopper, S.D., Franklin, S.E., 2012. Refugia: identifying and understanding safe havens for biodiversity under climate change. Glob. Ecol. Biogeogr. 21, 393e404. Manning, K., Timpson, A., 2014. The demographic response to Holocene climate


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Verena Foerster* University of Potsdam, Institute of Earth and Environmental Science, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany Ralf Vogelsang University of Cologne, Institute of Prehistoric Archaeology, BernhardFeilchenfeld-Str. 11, 50969 Cologne, Germany Annett Junginger Eberhard Karls UniversitVat Tuebingen, Department of Earth Sciences, Senckenberg Center for Human Evolution and €lderlinstrasse 12, 72074 Palaeoenvironment (HEP-Tuebingen), Ho Tübingen, Germany Asfawossen Asrat Addis Ababa University, School of Earth Sciences, P. O. Box 1176, Addis Ababa, Ethiopia Henry F. Lamb Aberystwyth University, Department of Geography and Earth Sciences, Aberystwyth SY23 3DB, UK Frank Schaebitz University of Cologne, Seminar for Geography and Education, Gronewaldstrasse 2, 50931 Cologne, Germany Martin H. Trauth University of Potsdam, Institute of Earth and Environmental Science, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany * Corresponding author. E-mail address: [email protected] (V. Foerster).

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