Towards an understanding of climate proxy formation in the Chew Bahir basin, southern Ethiopian Rift

Highlights

• μXRF-derived proxy K was linked to authigenic illitization of smectites under more arid conditions.

• Inverse correlation of K flux with precipitation as illitization is controlled by evaporation.

• The hydrochemistry of palaeolake Chew Bahir controls the degree of authigenic mineral alteration.

• Increased alkalinity and salinity enhance illitization (salinity) and analcime formation (pH).

• K-fixation in smectites could be enhanced by changes in octahedral occupancy of clay minerals.

Abstract

Deciphering paleoclimate from lake sediments is a challenge due to the complex relationship between climate parameters and sediment composition. Here we show the links between potassium (K) concentrations in the sediments of the Chew Bahir basin in the Southern Ethiopian Rift and fluctuations in the catchment precipitation/evaporation balance. Our micro-X-ray fluorescence and X-ray diffraction results suggest that the most likely process linking climate with potassium concentrations is the authigenic illitization of smectites during episodes of higher alkalinity and salinity in the closed-basin lake, due to a drier climate. Whole-rock and clay size fraction analyses suggest that illitization of the Chew Bahir clay minerals with increasing evaporation is enhanced by octahedral Al-to-Mg substitution in the clay minerals, with the resulting layer charge increase facilitating potassium-fixation. Linking mineralogy with geochemistry shows the links between hydroclimatic control, process and formation of the Chew Bahir K patterns, in the context of well-known and widely documented eastern African climate fluctuations over the last 45,000 years. These results indicate characteristic mineral alteration patterns associated with orbitally controlled wet-dry cycles such as the African Humid Period (~15–5 ka) or high-latitude controlled climate events such as the Younger Dryas (~12.8–11.6 ka) chronozone. Determining the impact of authigenic mineral alteration on the Chew Bahir records enables the interpretation of the previously established μXRF-derived aridity proxy K and provides a better paleohydrological understanding of complex climate proxy formation.

Introduction

The relationship between climate and sediment composition in lake cores is nonlinear due to differential and incongruent weathering and to variations in the erosion and dissolution, transport, sedimentation, and precipitation of minerals within the catchment (Muhs et al., 2001; Phillips, 2006; Porter et al., 2007; Carré et al., 2012). The complexity of this relationship makes it difficult to decipher climatic information in sediment cores as it may result in different time-lags between the responses of different climate proxies. However, these differences provide valuable information on the dynamics of the source-to-sink sedimentation system and its controlling factors. For example, differential weathering of less resistant and more resistant minerals in magmatic source-rocks is dependent on the local climatic conditions of the source area and may therefore in itself be useful as a climate proxy (Nesbitt and Young, 1984; Muhs et al., 2001; Navarre-Sitchler and Brantley, 2007; Moses et al., 2014).

The increasing availability of micro-X-ray fluorescence (μXRF) scanners has led to an increase in the use of major and trace element concentrations (often without any quantitative calibration) as climate proxies (Peterson et al., 2000; Jaccard et al., 2005; Yancheva et al., 2007; Martínez-Garcia et al., 2010; Foerster et al., 2012, Foerster et al., 2014). The physicochemical processes linking climate with element concentrations, however, are not sufficiently well understood. Investigations on short (<20 m) pilot cores taken from the Chew Bahir basin in 2009/2010 have shown that potassium concentrations are sensitive to rapid variations in climate, with high potassium concentrations being associated with a drier climate (Foerster et al., 2012) (Fig. 1, Fig. 2). The reasons for this inverse correlation between K concentration and precipitation within the catchment remain a matter of intense debate (Foerster et al., 2014), as the processes linking climate with chemical weathering, transport and sedimentation within the Chew Bahir basin are largely unknown (Foerster et al., 2012). Following comprehensive multi-proxy analyses, core lithology investigations, and radiocarbon-based chronology it is evident, however, that the mineralogical results presented herein correlate well with previously identified and well documented climatic phases, including the African Humid Period, Younger Dryas and Last Glacial Maximum) in the sediments of the Chew Bahir basin (Foerster et al., 2012, Foerster et al., 2014, Foerster et al., 2015; Trauth et al., 2015).

Differential weathering of relatively resistant potassium feldspar (KAlSi₃O₈) and less resistant mica (e.g., muscovite KAl₂[AlSi₃O₁₀(OH)₂], biotite K(Mg,Fe²⁺,Mn²⁺)₃[(OH,F)₂|(Al,Fe³⁺,Ti³⁺)Si₃O₁₀]), or feldspathic glass in volcanic terrain may have occurred in the source area of the sediment, causing dramatic distortions in the amplitude and phase of the K influx to the sediments at the sink in relation to climatic variations (Pawar et al., 2008; Sak et al., 2010; Locsey et al., 2012; Foerster et al., 2012). Furthermore, incongruent weathering of K-feldspar and mica complicates the interpretation of sediment composition as some weathering products, such as detrital illite (K_0,65Al_2,0Al_0,65Si_3,35O₁₀(OH)₂) or kaolinite (Al₄[(OH)₈|Si₄O₁₀), may be transported in a solid state while others may be transported in solution and then precipitate as, for example, secondary illite, smectite, authigenic K-feldspars or zeolites, within the sediment (Eugster and Jones, 1979; Singer and Stoffers, 1980; Hay and Kyser, 2001; Trauth et al., 2001; Stroncik and Schmincke, 2002; Meunier and Velde, 2004; Mees et al., 2005). Other possible carriers of K are smectites, which result from the weathering of volcanic material in the catchment and are transported into the Chew Bahir basin (Navarre-Sitchler and Brantley, 2007; Velbel and Losiak, 2008; Navarre-Sitchler et al., 2009; Sak et al., 2010; McHenry et al., 2011; Ehlmann et al., 2012), where they may also be converted to authigenic illites (Singer and Stoffers, 1980; Deconinck et al., 1988; Hay and Kyser, 2001; Huggett and Cuadros, 2005).

Solutes usually reach the lake quite quickly and can therefore form proxies that are in phase with the climate variability whereas, following their erosion, minerals in a solid state (and their aggregates) may be temporarily stored along the way during transport, eventually reaching the terminal lake with a considerable time delay relative to the climatic variations in the catchment area that they reflect (Velde and Meunier, 2008; Pawar et al., 2008; Sak et al., 2010; Locsey et al., 2012; Bösche, 2012; Foerster et al., 2012). In hydrologically closed basins, solutes build up in concentration, potentially precipitating into solid phases when concentrations are sufficiently high (Deocampo and Jones, 2014). On the way from source to sink both sediments and solutes are subjected to complex redox processes (involving, e.g., Fe³⁺ or Mn⁴⁺) and dissolution/precipitation processes (involving, e.g., Ca²⁺ or Sr²⁺), further complicating the interpretation of geochemical climate proxies in sediment cores (Bösche, 2012; Foerster et al., 2012). The sediments and the climate information contained therein may also be subjected to numerous post-depositional processes such as compaction, recrystallization, dissolution, redeposition, and diagenesis (Kasten et al., 2003), as well as the influence of benthic organisms within the sediment (Trauth, 2013).

The interpretation of chemical proxies therefore requires a careful investigation of the processes linking climate with sediment composition. Although findings from different basins are not necessarily transferable, our results are expected to reveal general connections between climatic parameters, surface processes, and sediment compositions that will also be valuable for the interpretation of other climate archives. Three of these cores (CB-01, CB-03 and CB-05), which were collected in a pilot study for the Hominin Sites and Paleolakes Drilling Project (HSPDP; HSPDP-CHB deep coring site in Fig.3) (Cohen et al., 2016; Campisano et al., 2017), have been described in previous publications (Foerster et al., 2012, Foerster et al., 2014, Foerster et al., 2015, Foerster et al., 2016; Trauth et al., 2015, Trauth et al., 2018). Within the current age control, K content correlates with well documented transitions in climatic history, including the delta D leaf wax records from Lake Tanganyika and Lake Shalla (Tierney et al., 2008, Tierney et al., 2011), Lake Albert (Berke et al., 2014), marine records of terrigenous dust flux (deMenocal et al., 2000), delta-N-15 record from the Arabian sea (Altabet et al., 2002), Asian cave stable isotope records (Wang et al., 2001), and the Greenland ice core records (NGRIP, 2004).

In this study we seek to ascertain the mineralogical controls on the K record that has been shown to so robustly follow those previously established phases of climatic change, such as the onset and termination of the African Humid Period (~15–5 ka), the pronounced dry phase correlating with the Younger Dryas chronozone or the impacts of the Dansgaard-Oeschger (D-O) cycles and the Heinrich events of the northern hemisphere high latitudes on eastern African moisture availability (e.g. Junginger and Trauth, 2013; Tierney and deMenocal, 2013; Shanahan et al., 2015; Brown et al., 2007; Lamb et al., 2007).

We present herein our μXRF records in comparison with a new comprehensive XRD data set, which we have used to determine the relative importance of authigenic mineral alteration to the sediment composition in the Chew Bahir basin. Above, we will illustrate how sensitive the degree of authigenic transformation in especially clay minerals has recorded even subtle shifts in the hydrochemistry of paleolake and porewaters, therewith representing a robust proxy for different chemical environments, controlled by climatic change (e.g. Moore and Reynolds, 1997; Velde and Meunier, 2008). This will in turn improve our understanding of the role of different processes involved in proxy formation, which is crucial for reliable interpretation of the Chew Bahir sediments as climate archives, especially since biological indicators are not represented (Foerster et al., 2012; Deocampo et al., 2010) throughout the entire core.