Elsevier

Quaternary Science Reviews

Volume 202, 15 December 2018, Pages 197-210
Quaternary Science Reviews

Paleoclimate change in Ethiopia around the last interglacial derived from annually-resolved stalagmite evidence

https://doi.org/10.1016/j.quascirev.2018.06.016Get rights and content

Highlights

  • Stalagmite GM1 discontinuously deposited at ∼129 ka, 120 ka, an undated growth phase, and ∼108 ka.

  • Multi-proxy analysis necessary to identify climatic from karstic signal.

  • Low δ18O at ∼129 ka and ∼108 ka attributed to wetter climatic conditions.

  • The first annually-resolved stalagmite record of the last interglacial from Ethiopia.

  • The earliest Modern Human dispersal out of Africa occurred during humid climate conditions.

Abstract

Oxygen and carbon (δ18Ο/δ13C) isotope, growth rate and trace element data are reported for a U-Th dated, annually-laminated stalagmite, GM1 from Goda Mea Cave, Ethiopia. The stalagmite grew intermittently around the last interglacial. The proxy records are used to develop a conceptual growth model of the stalagmite and to assess its potential for revealing a climate signal in this climatically sensitive northeastern African region during an important period in the evolution of Homo sapiens and dispersal of Anatomically Modern Humans out of Africa. Speleothem deposition is of short-duration occurring at ∼129 ka, ∼120 ka, in an undated growth phase, and at ∼108 ka; probably due to tectonic activity. δ18Ο composition is very stable within growth phases (1σ variability < 0.76‰), as are Mg/Ca, Sr/Ca and Ba/Ca, all indicative of well-mixed source-waters. A shift to positive δ18Ο values and increased variability in Mg/Ca, Sr/Ca and Ba/Ca prior to growth hiatuses is observed, indicating a loss of the well-mixed water source prior to growth cessation. Mean δ18Ο composition (−3.82 to −7.77‰) is lower than published modern and Holocene stalagmites from the region. Geochemical data, statistical analyses, and a conceptual model of stalagmite growth, demonstrate that climatic conditions recorded by GM1 were wetter than the Holocene. The ∼129 ka growth phase particularly presents an annual record of the relative Intertropical Convergence Zone (ITCZ) position. The GM1 record, the oldest high-resolution continental climate record from Ethiopia so far published, presents evidence that any early human migrations which occurred during MIS 5 are likely to have occurred during a wet event in northeast Africa.

Introduction

In Ethiopia, stalagmites provide high-resolution records of past climate and environment (Asrat et al., 2007; Baker et al., 2007, 2010). Fast-growing, annually-laminated stalagmites are ubiquitous, due to the strong seasonality of rainfall and the water balance in Ethiopia. Regular laminae, visible in hand section, can provide precise annual chronology. Annual growth rates of these stalagmites, determined from the thickness of an annual lamina, is at the upper range of those observed in stalagmites (typically ∼0.5 mm/yr). This is due to the optimal climatic conditions (high temperature and rainfall) for limestone dissolution and re-precipitation. This rapid growth facilitates the high-resolution sampling of stalagmite calcite.

In Ethiopia, the real advantage of using speleothems to provide a paleoclimate proxy record is that they contain information on past rainfall variability in the region. Several major air streams and convergence zones influence the current climate pattern in northeast Africa, whose effects are often compounded by such regional factors as topography and the proximity to the oceans (e.g., Nicholson, 1996). The relatively dry north-easterly and south-easterly monsoons and the humid and moisture-laden (rainfall generating), westerly and south-westerly air flow of the Congo air stream, generally dominate the regional wind and pressure patterns. The Intertropical Convergence Zone (ITCZ) and the Congo Air Boundary (CAB) separate these major air streams. The passage of the ITCZ (Fig. 1a) dominantly determines the rainy seasons in Ethiopia, while the topography (highland barriers separated by a rift zone) modulates the local rainfall distribution. Accordingly, Ethiopian climate has two rainy seasons, one from the northward passage of the ITCZ, called locally the ‘big rains’ (between June and September), which is reliable and whose maxima migrates with the position of the ITCZ. A second rainy season, the ‘small rains’, is less consistent and occurs between March and May with maxima in April. Dryland farming, including subsistence farming, leads to a high dependency on rains in both seasons. Failure of the ‘small’ rains is common and has occurred in recent years in 2013/2014 and 2015/2016, particularly in the southeastern Ethiopian lowlands bordering the current study area. The climate dynamical cause of the failure of the ‘small’ rains, and how this varies over time, is still poorly understood.

In addition, reliable, high-resolution climate records beyond the Holocene are scarce in the northeastern African region, one of the major candidates for the origin of Homo sapiens and a gateway to the “out of Africa” migration of our species during the late Pleistocene. The influence of climate on the dispersal of Anatomically Modern Humans from northeastern Africa particularly during the period ∼120 to ∼50 ka has been a subject of intense discussion (e.g., Tierney et al., 2017 and references therein; Lamb et al., 2018). Recent discovery of Homo sapiens fossils dated to 177 to 194 ka in the Misliya cave in Israel (Hershkovitz et al., 2018) indicates that the “out of Africa” migration episodes have started earlier than the previously thought period of migration (∼120–50 ka). Discussions on influence of climate on human dispersal often rely on marine climate records from the Indian Ocean and Mediterranean Sea. The recently published Lake Tana record from the northwestern Ethiopian highland, largely covering the last ∼150 ka (Lamb et al., 2018) is the only continental record available. In this paper, we present a high-resolution continental climate record from an Ethiopian stalagmite (GM1) that grew intermittently around the last interglacial, which is very pertinent to this discussion. Though it is not a continuous record over the whole period of the last interglacial, the growth phases of GM1 are dated at particularly important periods of the MIS 5. The GM1 record, the oldest high-resolution climate record so far published from Ethiopia and continental eastern Africa, is therefore very significant in an area where any kind of reliable continental climate records from this period are scarce.

These annual-resolution records of δ18O, δ13C, trace elements and growth rate are from the Goda Mea Cave in Ethiopia (Fig. 1b). A combination of U-Th dates and lamina counting are used to identify the timing of the growth phases. Samples milled at annual resolution were analysed for δ18O and δ13C, and at decadal resolution for trace elements. Variogram, autocorrelation and spectral analyses of the geochemical and growth rate time series are used to develop a conceptual model for the hydrology of the waters feeding the speleothem. The time series of δ18O, δ13C, trace elements and annual growth rate are then interpreted, with comparison to the published modern and Holocene stalagmites from the region and globally. Such high resolution, multi-proxy approach has been proved useful in reconstructing annual, in some cases seasonal, rainfall (e.g., Johnson et al., 2006).

Section snippets

Site description

Goda Mea Cave was explored and surveyed in 2007 and a full description can be found in Gunn et al. (2009). The cave is entered from a collapse doline and after about 30 m there is a large flowstone deposit that almost fills the passage. A crawl beneath opens into a NE-SW oriented rift passage that is initially some 5 m wide by 1 m high but increases downstream to 10–15 m by 7 m. The cave ends in a 90m x 40m x 20m high chamber formed by upwards stoping as evidenced by abundant breakdown. Above

Lamina

Lamina were counted (in duplicate) and a total of 1356 lamina were identified with a mean lamina thickness of 0.44 ± 0.14 mm (ranging between 0.19 and 1.12 mm). This lamina thickness compares well to those reported for stalagmites in previous studies in the region (Ach-1, mean = 0.53 ± 0.26 mm, Bero-1 = 0.45 ± 0.23 mm, Asfa-3 = 0.32 ± 0.11 mm; Merc-1 = 0.29 ± 0.04 mm; Asrat et al., 2007; Baker et al., 2007; Baker et al., 2010). In these stalagmites, the visible laminae have been demonstrated to

Holocene stalagmite records

Previous cave research in Ethiopia has included limited cave drip water and climate monitoring during sampling expeditions to the Mechara region of Ethiopia between 2004 and 2008 (Asrat et al., 2008) (Fig. 1), and the analysis of modern and Holocene stalagmite samples. Modern calibration studies of stalagmite δ18O of carbonate shows evidence of climate sensitivity, despite deposition out-of-equilibrium (Baker et al., 2007, 2010). The latter is potentially due to both rapid degassing and

Conclusions

Stalagmite GM1 was deposited discontinuously around the time of the last interglacial, at ∼129 ka, 120 ka, an undated growth phase, and ∼108 ka. Variogram analysis of growth rate shows a low range (20.5 years), some flickering (−0.33) and good information content (56%), indicative of a stalagmite fed by a karst water store of limited volume. Oxygen and carbon isotopes and trace elements generally have low variability, indicative of a second, well-mixed water source feeding the stalagmite.

Acknowledgements

Stable isotope analyses were funded by NERC Isotope Geosciences Facility (IP-1099-0509) to AB, AA, CNJ and MJL. Fieldwork to the Mechara caves and subsequent sample preparation (lamina counting, drilling) was supported by START-PACOM grant to AA, a Royal Society grant to AA and AB, and a Leverhulme Trust International Network led by AB. U-Th analyses were supported by a UNSW Early Career Researcher grant to CNJ. The School of Earth Sciences of the Addis Ababa University supported and

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