Whole-rock geochemistry tracks precessional control of Pleistocene lake salinity at Olduvai Gorge, Tanzania: A record of authigenic clays

D.M. Deocampo1, P.A. Berry1, E.J. Beverly1, G.M. Ashley2, and R.E. Jarrett1 1Department of Geosciences, Georgia State University, Atlanta, Georgia 30302, USA 2Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA

ABSTRACT The paleoclimatic framework of human evolution is central to the study of human origins. Olduvai Gorge, Tanzania, has a rich paleontological record, including four hominin species. We analyzed 177 mudstone samples from a lacustrine section, supported by a new high-resolution geochronology, to track changes in paleolake water chemistry. Six peaks in Al2O3/MgO indicate freshening events occur- ring over a generally saline and alkaline environmental background. Five of the six events occurred at peak climatic precession, correlating with December insolation at 20°S. The southern monsoon therefore may have been more effective at delivering moisture to East Africa than today. Peak amplitude of geochemical variation preceded the ca. 1.84 Ma eccentricity maximum by ~20 k.y., suggesting that eccentric- ity modulation was unexpectedly weak in this interval, or that other factors affected the water balance or geochemical record. This study demonstrates that authigenic clay geochemistry determined through whole-rock analyses can preserve a record of astronomically forced environmental change, and documents paleohydrologic changes in Figure 1. A: Field loca- tion map (Tanzania) from tropical Africa at a time when zonal Walker circulation increasingly Ashley (2007), with lacus- affected global climate, new stone technologies emerged, and the trine plain for bed I and genus Homo spread beyond Africa. bed II times, defined by the estimated expanded and contracted lake expanse. INTRODUCTION CB—central basin. B: Old- Late Cenozoic environmental fluctuations in East Africa are thought to uvai Gorge stratigraphy, be critical to terrestrial ecosystem evolution, the rise of hominins, and the following Hay (1976) and development of stone technology by our ancestors (Potts, 1996; deMeno- Deino (2012), with chro- cal, 2011). New efforts are underway to generate high-resolution lake core nostratigraphic markers used by this study (tuff records through scientific drilling in several East Africa basins (Cohen I-F = 1.803 ± 0.002 Ma; tuff et al., 2016). Over the past two decades, models relating environmen- I-B = 1.848 ± 0.003 Ma; tuff tal change and hominin evolution (e.g., Behrensmeyer, 2006; Potts and I-A = 1.92 Ma). Reported Faith, 2015) have been increasingly testable through new continental errors for I-F and I-B are 1σ of unit ages; age of I-A proxy data sets. is based on interpolation In shallow, alkaline, oxidizing lakes of the eastern East African Rift, from other units with 1σ sediments are commonly devoid of biotic proxies such as diatoms, ostra- values of 0.003 Ma and cods, and pollen; many records are composed predominantly of silici- 0.006 Ma (Deino, 2012). clastic mud. When such lake basins have elevated alkalinity and silica content, Mg enrichment of authigenic clays can occur, providing a proxy of paleosalinity (Deocampo, 2015). Despite the uniformity of both grain size and physical appearance of these lake sediments, chemical compo- sition can vary widely. When other proxies are unavailable, authigenic archaeological sites (e.g., Hay, 1976; Ashley et al., 2010). Central Basin clays can provide the only proxy data on lake geochemistry, and therefore clays (Fig. 1A) can record limnological fluctuations through composi- basin paleohydrology. tion and relative abundances of authigenic clay mineral phases (Hay and The Pleistocene Olduvai Basin in northern Tanzania (Fig. 1) was a Kyser, 2001; Hover and Ashley, 2003). saline, alkaline lake basin with low detrital input that allowed authigenic This study tests whether Olduvai mudstone geochemistry records envi- silicates to accumulate (Hay, 1976). Olduvai Gorge is well known for its ronmental change in the basin, and whether that environmental change spectacular record of hominin fossils and stone tools (Leakey, 1971) and relates to orbital variation. In particular, we test the hypothesis that humid high-resolution paleoenvironmental reconstructions associated with many events in the basin are indicated by Mg depletion in authigenic clays.

GEOLOGY, July 2017; v. 45; no. 7; p. 1–4 | doi:10.1130/G38950.1 | Published online XX Month 2017 ©GEOLOGY 2017 Geological | Volume Society 45 | ofNumber America. 7 For| www.gsapubs.org permission to copy, contact [email protected]. 1 The close link between clay chemistry and bulk composition has been powders were analyzed by X-ray diffraction from 5° to 75° 2θ for min- shown for these mudstones (Deocampo and Tactikos, 2010), providing eralogy (Deocampo, 2015). a means to assess authigenic clay geochemistry for a large sample set. These results will add to a growing multiproxy record that includes lake- RESULTS margin lithostratigraphy (Ashley, 2007; Beverly et al., 2014) and stable Lithologies are dominantly mudstone, but a number of tuffs were isotopes (Hay and Kyser, 2001; Magill et al., 2013a). identified and excluded from analysis, as were dolostones, cherts, and

mudstones containing dolomite. Al2O3/MgO ratios ranged from ~2.5 to GEOLOGIC SETTING ~0.2 (Fig. 2) and rise and fall sharply, quickly returning to background Olduvai Gorge is incised into the Serengeti Plain of East Africa (Fig. low values. From ca. 1.81 Ma onward, the later part of the record has

1A), on the western flank of the East African Rift (Hay, 1976). Exposures gradually increasing Al2O3/MgO values that do not return to the low val- are mostly of volcaniclastic rocks from the Ngorongoro Volcanic High- ues observed lower in the section. lands to the east, and uplifted western metamorphic rocks. The extensional Although there is no strong correlation, Mg enrichment occurs in basin was endorheic for all its history. intervals identified by Magill et al. (2013a) as having leaf lipid stable Authigenic and diagenetic minerals including clay minerals, zeolites, isotopic compositions documenting expanded grasslands (Fig. 2). Leaf K-feldspar, chert, calcite, and dolomite precipitated in a complex diage- lipid indications of expanded forests, however, are not clearly related to netic history dominate central basin muds (Hay and Kyser, 2001). Pollen, intervals of Al enrichment. diatoms, and calcareous fossils are rare, but Magill et al. (2013a) showed Al-rich samples have 060 peaks ~61.4° 2θ (1.509Å), whereas Mg- through organic stable isotopes that grassland and woodland proportions rich samples have peaks ~61.0° 2θ (1.517Å). Figure 3A shows summed covaried with orbital precession. diffractograms for selected samples with Al2O3/MgO <0.5 (n = 9) and >5 (n = 11), effectively yielding an average X-ray diffractogram for each CLIMATIC SETTING end member. East African moisture is driven by the twice-yearly passing of the Intertropical Convergence Zone (Johnson and Odada, 1996). Continental DISCUSSION insolation controls monsoon intensity via the land-sea thermal gradient. Insolation changes due to orbital variations are therefore thought to affect Sedimentary Geochemistry continental hydrology. The connection between precession-driven inso- Changes in lake hydrochemistry had a significant impact on sedi- lation and precipitation is well known for the late Quaternary, although mentary geochemistry. Al-rich peaks correspond to intervals enriched hydroclimatic records show complexity beyond simple orbital control with dioctahedral smectites similar to those from relatively fresh (Lyons et al., 2015). As it does today, the Ngorongoro Volcanic Highlands formed a major rain shadow on the Serengeti for Indian Ocean moisture during the Qua- freshening events 6543 21 0.06 Climatic Precessioncession 0.04 0.05 ternary. Fossil leaf lipid biomarkers suggest that the Olduvai Basin mean Eccentricity 0.02 A 0.03 0 annual precipitation fluctuated between ~250 mm and 700 mm (Magill si n -0.02 0.01 et al. 2013b), considerably less than the ~1000 mm/yr modern precipita- -0.04 -0.06 -0.01 520 tion at Ngorongoro. 20°S DecDe 20°NN AprilApril 500 -2 480 B 460

W m 440 420 LACUSTRINE AUTHIGENIC CLAY MINERALS 400 380 Typically, Al-rich or Fe-rich smectites or kaolin-group minerals form 2.02.5 2.5

ScaleSca 2 2.0 Al

fresh changchange 2 through upland silicate hydrolysis, releasing alkalinity and silica (Deo- 1.5 O 3

/MgO 1.5 /MgO 1.5 /MgO 3 3 saline O O 1 1.0 2 campo and Jones, 2013). In hydrologically restricted settings, evaporative 2 1.0 Al Al 0.5 C 0.5 brines may develop; detrital clays can then interact with saline fluids, lead- 0.50 0 -20 ing to mineral precipitation. When Ca-carbonate precipitates, Mg/Ca ratios 31 C grassland 13 -25 rise in the residual fluids, so silicates formed later precipitate in Mg-rich -30 conditions, sometimes even producing pure Mg silicates (Deocampo, 2015). -35 D woodland -40 Leaf Lipid As clays alter in saline water, octahedral cation changes can increase 55 0.30 0.25 45 ODP 7721 ODP 967 layer charge (i.e., Mg uptake), and smectite can convert to illite (Deo- 0.20 IRM 35 0.15 rrigenous 0.10 campo, 2015). Low-temperature illitization can occur through repeated 25 0.05

% Te E 15 0 desiccation cycles or through interaction with K-rich brines. Models sug- 1.78 1.80 1.82 1.84 1.86 1.88 1.90 1.92 gest that octahedral layer charge increase is decoupled from K fixation, Age (Ma) although it can facilitate illitization (Deocampo et al., 2009). In saline settings, the clay mineral octahedral composition is therefore of even Figure 2. A: Precession (εsinω) and eccentricity (ε) factors following Laskar et al. (2004). B: Insolation curves for the northern Indian Ocean greater paleolimnological importance than simple smectite:illite ratios. monsoon at Ngorongoro Crater (Tanzania) in April, and the southern monsoon in December, following Laskar et al. (2004). C: Mudstone METHODS compositions; note change in scale ca. 1.86 Ma to show lower ampli- Approximately 40 m of bed I and lowermost bed II outcrop at locality tude fluctuations in younger sediment. Vertical gray bars indicate range of freshening events (1–6) identified by peaks in Al O /MgO. D: Stable 80 was freshly scraped, and ~2-cm-thick samples were collected every 2 3 carbon isotopic signature of leaf lipids in Olduvai Gorge muds (from ~20 cm and stored in polyethylene bags. Geochronology is based on linear Magill et al., 2013a). Age model was adjusted from Magill et al. (2013a) interpolation between dated tuffs (I-A, I-B, and I-F), assuming constant to reflect the latest chronology (Deino, 2012). Higher values indicate sedimentation rates of non-tuff lithologies, following Hay and Kyser more grassland, lower values indicate more woodland. E: Terrigenous (2001), using a new 40Ar/39Ar and paleomagnetic chronology provided sediment flux (left scale) to the Arabian Sea (purple: Ocean Drilling Program, ODP Site 721, from deMenocal and Bloemendahl, 1995), and by Deino (2012). 177 bulk samples were air-dried, powdered, fused with isothermal remanent magnetization (IRM, right scale), which is a proxy Li2B4O7, and analyzed by X-ray fluorescence. For data analysis, a five- for Saharan hematite dust to the Mediterranean Sea (green: ODP Site point running average was applied through the section. Randomly oriented 967, from Larrasoana et al., 2003).

2 www.gsapubs.org | Volume 45 | Number 7 | GEOLOGY paleo–Lake Olduvai basin (Fig. 2C). Major freshenings occurred ca. 1.91, 1.89, 1.87, and 1.85 Ma (events 1, 2, 3, and 4); 2 weaker events occurred ca. 1.84 and 1.82 Ma (events 5 and 6).

The gradual increase in Al2O3/MgO ratios in muds from 1.81 to 1.75 Ma also suggests lake freshening. This trend corresponds to increased moisture in the basin soon after emplacement of tuff I-F ca. 1.80 Ma, with a major lake expansion indicated by lake margin lithologies (Hay, 1976; Ashley, 2007; Deino, 2012). It is also recorded as Al-enrichment in <0.1 µm clays at a site ~700 m southwest of locality 80 (Deocampo, 2015). Correspondence of various lines of evidence indicating this major change in hydrology underscores the basin-wide ecological significance of this event.

East Africa Paleoclimate All freshening events except event 4 (1.85 Ma) correlate with peaks in climatic precession (Fig. 2C). Event 4 clearly was a significant hydrologic episode, although its timing is not coincident with the climatic forcing that appears to drive the others. It is possible that a volcanic or tectonic event influenced hydrologic inflow into the basin, perhaps related to fault activation at the time (Ashley et al., 2016). The more gradual freshening ca. 1.81 Ma is also associated with precession, although it apparently persisted beyond the peak. To test for a relationship between lake salinity and the land-sea thermal gradient drivers of the Indian Ocean monsoon, we compared the paleo- Figure 3. A: X-ray diffractogram of randomly oriented bulk samples, hydrologic record with calculated insolation for 20°N and 20°S in April highlighting the 060 peak, which is related to the octahedral Mg con- and December, respectively, following Laskar et al. (2004; Fig. 2B). The tent of authigenic Mg-rich clays (Deocampo, 2015; cps—counts per heaviest rains today at Ngorongoro peak in April, i.e., the long rains of second). Dominant peaks at 1.509Å and 1.517Å correspond to Al-rich the northeasterly monsoon, the intensity of which correlates with North- and Mg-rich clays, respectively. Each pattern is the sum of scans of end-member samples as indicated, producing an average diffracto- ern Hemisphere insolation. The Olduvai record, however, relates best to gram for each group. B: Correlation between bulk composition and that December insolation at 20°S, which is linearly in phase with precession of <0.2 µm clays extracted from Olduvai Gorge (Tanzania) lacustrine (Figs. 2A and 2B). This suggests that the southeasterly monsoon, which muds (data from Deocampo and Tactikos, 2010). in modern times peaks in December, played a more important role than previously recognized. No relationships were found between clay geo- chemistry and equatorial insolation curves. paleoenvironments of the eastern lake margin associated with distal allu- Freshening event intensity decreases upsection as eccentricity reached vial fan groundwater seeps (Cuthbert and Ashley, 2014) (Fig. 1). In the its peak ca. 1.84 Ma (Fig. 2A); after ca. 1.86 Ma, the amplitude of envi- central basin, the Al-rich clays represent minimally altered detrital input, ronmental change was much more subdued. This is surprising, because a suggesting relatively fresh waters. Mg-rich deposits in most of the sec- number of early Pleistocene modeling and field studies suggest that high tion, in contrast, represent elevated salinity that is required to produce eccentricity is associated with the greatest amplitude of environmental trioctahedral (or intermediate) components (Hover and Ashley, 2003; change (Deino et al., 2006; Scholz et al., 2007; Deino, 2012; Lyons et Deocampo et al., 2009). al., 2015). Records from beyond East Africa also hint at complexity. The Differences in the 060 peak positions between the Al-rich and Mg-rich Arabian Sea terrigenous dust record (Ocean Drilling Program, ODP Site populations confirm the relationship between composition and clay min- 721) suggests that the Horn of Africa underwent high-amplitude fluctua- eralogy (Fig. 3A), consistent with high-resolution transmission electron tions during the high eccentricity interval (deMenocal and Bloemendahl, microscopy observations (Deocampo et al., 2009). Such variability in 1995), but terrigenous flux from the Nile Basin (including the Ethiopian the ultrafine clays is a primary control on the bulk composition (Fig. 3B). Plateau) was stable at that time (Fig. 2E; ODP Site 967; Larrasoana et The dominance of authigenic silicates ensures that Mg partitioning al., 2003). New multiproxy records from basins across the region will between carbonate and siliceous phases favors silicates. This is likely provide new perspectives on this long-standing problem in Quaternary due to the long-term persistence of high alkalinity, causing incoming Ca African paleoclimatology (Cohen et al., 2016). to precipitate as lake-margin carbonates. Dolostones are present, but the timing of their deposition is not well understood. For this reason, they Implications for Hominin Evolution were excluded from the analysis here; Mg-calcite was not detected in any Current models of hominin evolution link the amplitude of environ- samples, so MgO attributable to non-silicate phases is minor. mental change with evolution in the earliest Pleistocene, including events such as the dispersal of Homo erectus beyond Africa ca. 1.85 Ma (Potts, Paleohydrology 2013). At Olduvai, environmental change amplitude was high prior to ca.

The basin history, indicated by the generally Mg-rich lacustrine clay 1.86 Ma, with less severe fluctuations over the next ~50 k.y. Al2O3 / MgO minerals (mean Al2O3/MgO = 0.8, σ = 0.6), is one of hydrologic closure ratios prior to 1.86 Ma have a standard deviation of 1.38 (n = 48), whereas and elevated salinity and alkalinity, consistent with earlier reconstructions after 1.86 Ma this falls to 0.48 (n = 80). Although the time series is not (Hay and Kyser, 2001). Volcanism and tectonism likely affected Olduvai long enough to capture the full return to higher amplitude environmental hydrology. The Ngorongoro Highlands were taller than today, perhaps change, the lake expanded substantially ca. 1.81 Ma, and significant trans- allowing only the greatest of monsoon incursions to reach Olduvai. On gression-regression cycles are recorded on the lake margin (Ashley, 2007; this generally saline background, six episodes of increased Al2O3/MgO Deocampo and Tactikos, 2010). This pattern correlates with Mediterra- were observed and interpreted as representing freshening events in the nean sapropels (Hilgen et al., 1999) and Arabian Sea dust flux (deMenocal

GEOLOGY | Volume 45 | Number 7 | www.gsapubs.org 3 and Bloemendahl, 1995), both of which record a low-amplitude interval combined data-model approach, in Vrba, E., et al., eds., Paleoclimate and embedded within a longer term period of high-amplitude environmen- evolution, with emphasis on human origins: New Haven, Connecticut, Yale tal change. It is difficult to test for causal links between environmental University Press, p. 262–288. Deocampo, D.M., 2015, Authigenic clays in lacustrine mudstones, in Egenhoff, S., changes and hominin dispersal, but it is clear that this time interval had et al., eds., Paying attention to mudstones: Priceless!: Geological Society of significant precession-driven hydroclimatic change. At the local scale America Special Paper 515, p. 49–64, doi:​10​.1130​/2015​.2515​(03). on which natural selection occurs, these changes had strong effects on Deocampo, D.M., and Jones, B.F., 2013, Geochemistry of saline lakes, in Drever, hominin habitats at Olduvai and throughout the region (Behrensmeyer, J.I., ed., Surface and groundwater, weathering and soils: Treatise on Geochem- istry, Volume 7 (second edition): Amsterdam, Elsevier, p. 437–469, doi:10​ ​ 2006; Potts, 2013). Many paleoecological parameters likely changed in .1016​/B0​-08​-043751​-6​/05083​-0. sympathy with lake salinity, such as freshwater availability, plant com- Deocampo, D.M., and Tactikos, J.C., 2010, Geochemical gradients and artifact position and distribution, and habitat fragmentation. mass densities on the lowermost Bed II eastern Lake Margin (~1.8 Ma), Oldu- vai Gorge, Tanzania: Quaternary Research, v. 74, p. 411–423, doi:​10​.1016​ CONCLUSIONS /j​.yqres​.2010​.09​.004. Deocampo, D.M., Cuadros, J., Wing-Dudek, T., Olives, J., and Amouric, M., 2009, Mudstone bulk compositions record the effects of authigenic clay Saline lake diagenesis as revealed by coupled mineralogy and geochemistry of mineral neoformation in paleo–Lake Olduvai, with Al-rich muds accu- multiple ultrafine clay phases: Pliocene Olduvai Gorge, Tanzania: American mulating during humid times and Mg-rich muds accumulating during Journal of Science, v. 309, p. 834–868, doi:​10​.2475​/09​.2009​.03. arid times. Five of six Pleistocene freshening events correlate well with Hay, R.L., 1976, Geology of the Olduvai Gorge: Berkeley, University of Califor- nia Press, 203 p. calculated orbital precession. This adds to a growing body of literature Hay, R.L., and Kyser, T.K., 2001, Chemical sedimentology and paleoenvironmental finding precession-dominated orbital cyclicity in Pleistocene Sub-Saharan history of Lake Olduvai, a Pliocene lake in northern Tanzania: Geological Africa. Issues that remain unresolved include: (1) the highest amplitude Society of America Bulletin, v. 113, p. 1505–1521, doi:​10​.1130​/0016​-7606​ of environmental change preceded the 1.84 Ma eccentricity maximum by (2001)113​<1505:​CSAPHO>2​.0​.CO;2. ~20 k.y., contrary to expectations; and (2) episodes of freshening are best Hilgen, F.J., et al., 1999, Present status of the astronomical (polarity) time-scale for the Mediterranean late Neogene: Royal Society of London Philosophi- correlated to Southern Hemisphere insolation (20°S) during December, cal Transactions, ser. A, v. 357, p. 1931–1947, doi:​10​.1098​/rsta​.1999​.0408. a time that today is associated with the short southern monsoon. These Hover, V.C., and Ashley, G.M., 2003, Geochemical signatures of paleodepositional hydroclimatic changes have great implications for the environmental and diagenetic environments: A STEM/AEM study of authigenic clay miner- context of hominin evolution. als from an arid rift basin, Olduvai Gorge, Tanzania: Clays and Clay Minerals, v. 51, p. 231–251, doi:​10​.1346​/CCMN​.2003​.0510301. Johnson, T.C., and Odada, E.O., eds., 1996, The limnology, climatology, and ACKNOWLEDGMENTS palaeoclimatology of the East African lakes: Toronto, Canada, Gordon and We thank the Tanzania Commission for Science and Technology (COSTECH) and Breach, 456 p. the Department of Antiquities for permissions to R. Blumenschine and F. Masao. Larrasoana, J.C., Roberts, A.P., Rohling, E.J., Winklhofer, M., and Wehausen, R., R.W. Renaut and G.M. Ashley collected all samples. 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