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The Holocene Lake-Evaporation History of the Afro-Alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, Based on Δ18o Records of Sugar Biomarker and Diatoms

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The Holocene Lake-Evaporation History of the Afro-Alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, Based on Δ18o Records of Sugar Biomarker and Diatoms Quaternary Research (2021), 1–14 doi:10.1017/qua.2021.26 Research Article The Holocene lake-evaporation history of the afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on δ18O records of sugar biomarker and diatoms Lucas Bittnera,b* , Graciela Gil-Romerac,d, Dai Gradye, Henry F. Lambe,f, Eva Lorenza, Mikaela Weinerg, Hanno Meyerg, Tobias Brommb, Bruno Glaserb and Michael Zecha aHeisenberg Chair of Physical Geography with focus on paleoenvironmental research, Institute of Geography, Technische Universität Dresden, 01069 Dresden, Germany; bInstitute of Agronomy and Nutritional Sciences, Soil Biogeochemistry, Martin-Luther-University Halle-Wittenberg, 06108 Halle (Saale), Germany; cDepartment of Ecology, Philipps-Marburg University, 35037 Marburg, Germany; dDepartment of Geo-environmental Processes and Global Change, Pyrenean Institute of Ecology, CSIC, 50059 Zaragoza, Spain; eDepartment of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK; fDepartment of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland and gAlfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Polar Terrestrial Environmental Systems, TelegrafenbergA45, 14473 Potsdam, Germany Abstract In eastern Africa, there are few long, high-quality records of environmental change at high altitudes, inhibiting a broader understanding of regional climate change. We investigated a Holocene lacustrine sediment archive from Lake Garba Guracha, Bale Mountains, Ethiopia, (3,950 m asl), and reconstructed high-altitude lake evaporation history using δ18O records derived from the analysis of compound-specific δ18 δ18 ‰ ‰ sugar biomarkers and diatoms. The Odiatom and Ofuc records are clearly correlated and reveal similar ranges (7.9 and 7.1 , respec- tively). The lowest δ18O values occurred between 10–7 cal ka BP and were followed by a continuous shift towards more positive δ18O values. δ18 δ18 Due to the aquatic origin of the sugar biomarker and similar trends of Odiatom, we suggest that our lacustrine Ofuc record reflects δ18 ‘ ’ ‘ ’ Olake water. Therefore, without completely excluding the influence of the amount-effect and the source-effect , we interpret our record to reflect primarily the precipitation-to-evaporation ratio (P/E). We conclude that precipitation increased at the beginning of the Holocene, δ18 leading to an overflowing lake between ca. 10 and ca. 8 cal ka BP, indicated by low Olake water values, which are interpreted as reduced evaporative enrichment. This is followed by a continuous trend towards drier conditions, indicating at least a seasonally closed lake system. Keywords: Paleolimnology, Lake level, Evaporation, Oxygen isotopes, δ18O, Sugar biomarkers, Diatoms (Received 8 October 2020; accepted 19 March 2021) INTRODUCTION masses and Congo Basin air masses shifts to the east (Costa et al., 2014). The climate of eastern Africa is driven by the position of the In the past, in addition to the effects described above, an tropical rain belt. Increasing north hemisphere insolation during increased land-ocean temperature gradient caused strengthening northern hemisphere summer forces the tropical rain belt and of the West African Monsoon (WAM) and ISM. The interaction ITCZ northward and the Congo Air Boundary (CAB) migrates of monsoon intensity and eastward migration of the CAB may eastward (Nakamura, 1968; Hills, 1979; Davies et al., 1985). The have been responsible for enhanced moisture in the region, eastward extent of CAB movement is related to the seasonal thereby generating a water level rise in eastern African lakes changes of Indian Summer Monsoon (ISM) strength (Junginger et al., 2014; Lezine et al., 2014). Additionally, increased (Camberlin, 1997). Moreover, an increased W-E atmospheric lake levels may have been driven by a northward shift in the pressure gradient between Africa and India during the northern mean position of the tropical rain belt during the early hemisphere summer enhances the ISM (Wagner et al., 2018). Holocene northern hemisphere summer insolation maximum The enhanced ISM redirects Indian Ocean air masses towards (Gasse, 2000). These processes led to a more pluvial early–mid India preventing them from penetrating deeply into eastern Holocene (12–5 cal ka BP), termed the African Humid Period Africa. Therefore, the convergence zone of Indian Ocean air (AHP) (deMenocal et al., 2000), which was particularly intense in North Africa and extended south to 10°S in eastern Africa (Gasse, 2000). While the general mechanisms for the orbitally *Corresponding author email address: [email protected] forced AHP are well understood, the spatial and temporal patterns Cite this article: Bittner L et al (2021). The Holocene lake-evaporation history of the are intensively debated. afro-alpine Lake Garba Guracha in the Bale Mountains, Ethiopia, based on δ18O records of sugar biomarker and diatoms. Quaternary Research 1–14. https://doi.org/10.1017/ Due to the complexity of atmospheric circulation in eastern qua.2021.26 Africa, it is not surprising that the reconstructed timing of the © University of Washington. Published by Cambridge University Press, 2021. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 27 Sep 2021 at 16:35:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/qua.2021.26 2 L. Bittner et al. AHP differs across sites. The termination of the AHP has been interpretations of the proxy from the archive itself. Our main described as rapid (Collins et al., 2017), as synchronous and aims are: (1) to determine the allochthonous versus autochtho- abrupt (Tierney and deMenocal, 2013), or asynchronous and nous source of the analyzed sugar biomarkers, (2) to use δ18 δ18 gradual (Foerster et al., 2012; Costa et al., 2014; van der Lubbe Odiatom analyses to test and corroborate the Ofuc results, et al., 2017). However, the inferred timing of the AHP termination (3) to describe the hydrological history of high-altitude Garba may depend largely on the analyzed proxy (Castañeda et al., 2016) Guracha, and (4) to consider the implications of our data for and the type of archive. Furthermore, in addition to a variable cli- understanding regional paleoclimate. mate, the scarce and patchy paleoenvironmental records of Northern and eastern Africa supply insufficient data for compre- hensive climate reconstruction. The hydrological history of east- REGIONAL SETTING ern Africa has been reconstructed mainly from low-elevation sites. Study area High-altitude lakes have proved to be excellent sensors of envi- ronmental change (Catalan et al., 2006, 2013). They incorporate Garba Guracha is situated in the Bale Mountains of the Bale-Arsi information about changes in the catchment, including varying Massif, located east of the Main Ethiopian Rift (Fig. 1). The erosion, often without significant anthropogenic disturbance or Sanetti Plateau is the highest plateau in the Bale Mountains at ∼ ∼ desiccation (Arnaud et al., 2016). In eastern Africa, high-altitude an altitude between 3800 and 4200 m asl, encompassing an 2 paleolimnological research has been carried out at Lake Ashenge area of 600 km , bordered by a steep escarpment to the south (2442 m asl; Marshall et al., 2009), Lake Dendi (2840 m asl; (Osmaston et al., 2005). Deep valleys incised by northward Wagner et al., 2018), Sacred Lake (2350 m asl on Mount Kenya; descending rivers characterize the northern slopes. The volcanic Barker et al., 2001, 2011; Street-Perrott et al., 2008; Loomis plateau is formed of solidified horizontal lava consisting of alkali et al., 2015, 2017), and Lake Garba Guracha (3950 m asl on the basalt, trachyte, and tuffs with rhyolites, overlying older volcanic Bale Mountains; Umer et al., 2007; Tiercelin et al., 2008; Bittner material (Uhlig and Uhlig, 1991; Williams, 2016). During the Last et al., 2020). Glacial Maximum, the plateau and the valleys were locally glaci- Paleolimnological research has advanced in recent decades by ated (Osmaston et al., 2005; Ossendorf et al., 2019; Groos et al., the use of stable isotope ratios as environmental indicators. The 2020, 2021). The glacial cirque Garba Guracha (6.875781°N, oxygen isotope composition (δ18O) of biogenic compounds and 39.878075°E) (Fig. 2b) was first mentioned by Werdecker autochthonous carbonates has been established as a valuable pale- (1962) and was described in detail by Umer et al. (2007) and oclimate proxy because the value of δ18O in water, which is incor- Tiercelin et al. (2008). It is located at 3950 m asl, has a maximum 2 porated into biogenic and autochthonous compounds, depends water depth of 6 m, a very small watershed (0.15 km ), and ∼ on fractionation processes linked to temperature, air mass source extends to 500 x 300 m in area (Fig. 2c). The catchment bedrock and trajectory, and global ice volume. is carbonate-poor (Löffler, 1978; Uhlig and Uhlig, 1991). The lake Oxygen isotope composition has been determined for several has an outlet during the rainy season. A marshy alluvial plain fed δ18 by several springs extends to the south of the lake. Quaternary records in eastern Africa: for example, Oice cores δ18 (Thompson et al., 2002), Ocarbonate
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