Journal of Human Evolution 116 (2018) 75e94

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Journal of Human Evolution

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Subdecadal phytolith and charcoal records from Lake Malawi, East Africa imply minimal effects on human evolution from the ~74 ka Toba supereruption

* Chad L. Yost a, , Lily J. Jackson b, Jeffery R. Stone c, Andrew S. Cohen a a Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, USA b Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712, USA c Department of Earth and Environmental Systems, Indiana State University, Terre Haute, IN 47809, USA article info abstract

Article history: The temporal proximity of the ~74 ka Toba supereruption to a putative 100e50 ka human population Received 23 June 2017 bottleneck is the basis for the volcanic winter/weak Garden of Eden hypothesis, which states that the Accepted 21 November 2017 eruption caused a 6-year-long global volcanic winter and reduced the effective population of anatomi- cally modern humans (AMH) to fewer than 10,000 individuals. To test this hypothesis, we sampled two cores collected from Lake Malawi with cryptotephra previously fingerprinted to the Toba supereruption. Keywords: Phytolith and charcoal samples were continuously collected at ~3e4 mm (~8e9 yr) intervals above and Toba below the Toba cryptotephra position, with no stratigraphic breaks. For samples synchronous or prox- Supereruption Lake Malawi imal to the Toba interval, we found no change in low elevation tree cover, or in cool climate C3 and warm Phytoliths season C4 xerophytic and mesophytic grass abundance that is outside of normal variability. A spike in Bottleneck locally derived charcoal and xerophytic C4 grasses immediately after the Toba eruption indicates reduced Homo sapiens precipitation and die-off of at least some afromontane vegetation, but does not signal volcanic winter conditions. A review of Toba tuff petrological and melt inclusion studies suggest a Tambora-like 50 to 100 Mt SO2 atmospheric injection. However, most Toba climate models use SO2 values that are one to two orders of magnitude higher, thereby significantly overestimating the amount of cooling. A review of recent genetic studies finds no support for a genetic bottleneck at or near ~74 ka. Based on these previous studies and our new paleoenvironmental data, we find no support for the Toba catastrophe hypothesis and conclude that the Toba supereruption did not 1) produce a 6-year-long volcanic winter in eastern Africa, 2) cause a genetic bottleneck among African AMH populations, or 3) bring humanity to the brink of extinction. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction known volcanic eruption of at least the last 2 Ma. There are two competing 40Ar/39Ar age estimates for the eruption, 75 ± 0.9 ka The magnitude of the Indonesian Mount Toba supereruption at from Mark et al. (2014) and 73.88 ± 0.32 ka from Storey et al. (2012). ~74 ka and its temporal proximity to a Late Pleistocene human These ages differ due to alternative ages used for the Alder Creek population bottleneck and to various models of anatomically sanidine dating standard (Roberts et al., 2013), but overlap using 2s modern human (AMH) dispersal out of Africa have made this uncertainties. Based on ejected mass, the Toba supereruption was eruption the subject of much debate (Oppenheimer, 2002; two orders of magnitude greater than the 1815 Tambora eruption, Ambrose, 2003; Gathorne-Hardy and Harcourt-Smith, 2003; Has- which has been linked with the ‘Year Without a Summer’ of 1816, a lam and Petraglia, 2010; Balter, 2010; Williams et al., 2010; Wil- period of widespread cooling and anomalous rainfall in Northern liams, 2012; Mark et al., 2013; Roberts et al., 2013; Lane et al., Hemisphere continental regions that may have been linked to 2013b; Haslam, 2014). The youngest Toba eruption is the largest weakening of the Asian and African monsoons (Wegmann et al., 2014; Luterbacher and Pfister, 2015). Ash from the youngest Toba eruption, the youngest Toba Tuff (YTT), has been identified thou- * Corresponding author. sands of km from the Toba caldera in marine cores from the Indian E-mail address: [email protected] (C.L. Yost). https://doi.org/10.1016/j.jhevol.2017.11.005 0047-2484/© 2017 Elsevier Ltd. All rights reserved. 76 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

Ocean, the Arabian Sea, and the South China Sea (Williams, 2012). rapidly during a freeze event, and that chilling events resulting in From the southernmost lake in the East African Rift Valley, Lane air temperatures of 10e15 C for a few days would have severely et al. (2013a) chemically matched a cryptotephra layer in Lake damaged tropical plants. Simulations of nuclear winter cooling Malawi cores MAL05-2A and MAL05-1C to the Toba supereruption, effects on a grassland ecosystem indicate a 3e9 C decrease for one extending the range of the ash fall to 7300 km. An ash fallout year would lead to a reduction in grassland NPP from 9 to 42%, thickness model estimates that between 0.1 and 0.5 cm of Toba ash respectively; after year two, NPP values would be at 13e51% of was deposited in the Lake Malawi region, with modeled estimates normal (Harwell, 1984). Rampino and Ambrose (2000) also sug- of 2e3 cm of deposition in northern Tanzania and Kenya (Costa gested that the accumulation of dead woody material from an et al., 2014). Thus, the Toba supereruption isochron is likely to initial die-off and modeled reductions in precipitation post-Toba become an important marker in dating key events in human eruption might have led to increased wildfire activity. They evolution. concluded that severe drought in the tropical rainforest belt and in Here we test the hypothesis that the Toba supereruption at monsoon regions, and significant reductions in plants and animal ~74 ka caused an environmental catastrophe in East Africa capable populations, especially in the tropics, would have resulted in a of severely reducing human populations and causing a genetic global ecological disaster and population crashes of various or- bottleneck by using plant opal phytoliths and charcoal extracted ganisms. Such a scenario is now commonly referred to as the Toba from continuously sampled Lake Malawi sediment cores that catastrophe hypothesis. contain the Toba supereruption isochron at subdecadal resolution. 1.1.2. Summary of recent Toba supereruption modeling and We compare simulated climate and vegetation outcomes of recent geologic evidence There is considerable uncertainty as to what Toba supereruption models such as the magnitude and duration of effects the Toba supereruption had on regional and global climate. the cooling, changes in precipitation, grass and tree cover, and This is due, in part, to a lack of high fidelity climate records that wildfire activity to our record of plant micro remains from a region contain the YTT, which would provide an unambiguous chrono- in East Africa where AMH lived before and after the Toba logical tie point and boundary marker for periods before and after supereruption. the eruption. In a speleothem stable isotope study from New Mexico (USA), Polyak et al. (2017:843) claimed their data support 1.1. The Toba supereruption the hypothesis that the Toba supereruption caused “far-reaching climate change” and possibly triggered the Greenland Stadial 20 1.1.1. The Toba catastrophe hypothesis Ambrose (1998) formally cold event. However, the absence of an unambiguous Toba proposed the hypothesis that there is a connection between the marker in their speleothem record precludes the ability to youngest Toba supereruption and an effective human population confidently identify causal relationships for events separated in decline to fewer than 10,000 individuals sometime between 50 time by intervals significantly less than the chronological and 100 ka. He suggested that the Toba eruption caused six years uncertainties of the proxy records themselves. Our high-fidelity of volcanic winter followed by 1000 years of the coldest, driest Lake Malawi cores with the YTT position securely identified allow climate of the late Quaternary (Greenland Stadial 20), and that us to unambiguously examine the distal effects of the Toba this event caused low net primary productivity (NPP) and famine. supereruption in East Africa. The development of the Toba catastrophe hypothesis can be Uncertainty as to the climatic effects of the Toba supereruption traced back to the weak Garden of Eden hypothesis (weak GOE), also arises as a result of limited information about the eruption's which is supported by early genomic studies of human intensity, height of plume, and the sulfur yield. It is this last vari- mitochondrial DNA. The weak GOE posits that, ~100 ka, AMH able, the amount of sulfur injected into the atmosphere, that is the spread into separate regions from a restricted source with either main determinant of global climatic consequences (Oppenheimer, an early rapid expansion and subsequent population bottlenecks 2002; Timmreck et al., 2012). Volcanic activity releases either sul- or early modest population growth and slow expansion fur dioxide (SO2) or hydrogen sulfide (H2S), which is mostly con- (Harpending et al., 1993). Around 50 ka, these dispersed verted into sulfuric acid (H2SO4) and other sulfate aerosols through populations, which were genetically isolated from each other, photochemical reactions with water vapor in the atmosphere experienced dramatic population growth and expanded with the (Scaillet et al., 1998). Selecting a SO2 atmospheric injection load for support of more modern technologies (Harpending et al., 1993). climate modeling is typically based on petrological evidence and ice By combining elements of the weak GOE with the Toba core sulfate records, and often expressed in terms relative to the supereruption, Ambrose (1998) proposed the volcanic winter/ SO2 release from the 1991 Mt. Pinatubo eruption. Early modeling of weak GOE model, where rapidly reduced populations induced Toba volcanic forcing by Rampino and Self (1992), using an founder effects on genetic diversity and were a catalyst for approximately 33 times Pinatubo SO2 injection, produced a global subsequent technological innovations and migrations. cooling of 3e5 C. Oppenheimer (2002) used the estimated annual Rampino and Ambrose (2000) further refined the volcanic sulfate loading from the Zielinski et al. (1996) high resolution sul- winter/weak GOE hypothesis, incorporating atmospheric modeling fate depositional record from the Greenland Ice Sheet Project II of volcanic forcing and possible ecological and environmental ef- (GISP2) ice core to calculate an annual global cooling of 0.9e1.3 C fects of the Toba eruption at low latitudes into their discussion. over a 6e7.5 year period. Jones et al. (2005), using a coupled at- They proposed that the injection of volcanic aerosols after an mosphere ocean general circulation model and a 100 times Pina- eruption would have had two major effects on plants, a reduction of tubo scaling, modeled a maximum cooling of 10.7 C below normal light levels due to high-atmospheric opacity and rapid cooling. globally, and 17 C below normal for Africa. Using two models and a Light level reductions following the Toba eruption may have ranged range of sulfur injections from 33 to 900 times Pinatubo, Robock between ~75% sunlight transmitted, such as the dim-sun conditions et al. (2009) produced average global cooling values between 8 that followed the 1815 Tambora eruption, to ~10% sunlight trans- and 17 C, but under all of their simulations there was significant mitted (Rampino and Ambrose, 2000). For grasses, a 10% scenario climate recovery within a decade. They observed that radiative would reduce photosynthesis by ~85% (van Keulen et al., 1975). forcing lasted 4e7 years, which agrees well with the ~6 years of Rampino and Ambrose (2000) summarized several studies on the H2SO4 deposition in the GISP2 ice core (Zielinski et al., 1996). They effects of rapid cooling in tropical forests and proposed that concluded that the modeled cooling could have produced great essentially all above-ground plant tissues would have been killed stress on humans and their environment, but that it would have C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 77 been concentrated within a few dark, cold, and dry years imme- 4e7 year forcing modeled by Robock et al. (2009). The diately following the eruption. mechanism for this residence time is based on the limited Timmreck et al. (2012), using a complete treatment of strato- availability of OH radicals for SO2 oxidation, which delays the spheric aerosol formation and growth, and 100 times Pinatubo SO2 formation of sulfate aerosols for extremely large eruptions (Bekki injection, modeled a global cooling of ~3.5 C(5C over the con- et al., 1996; Timmreck et al., 2012). When the low and high tinents) for the first 3 years after the eruption, and after 13 years, estimates of the GISP2 sulfate spike loading are distributed stabilizing to within natural variability. They also focused their annually over a 7.5-year period, values of 61 (4 times Pinatubo) model runs on four regions of human habitation during the Toba and 305 Mt SO2 (18 times Pinatubo) are derived, respectively eruption, Southeast Asia, India, East Africa, and South Africa. For (Table 1, Fig. 1). East Africa, Timmreck et al. (2012) simulated a temperature Petrological approaches to determining the amount of degassed response of 4.5 C cooling (5.3 C for Southern Africa), with sulfur from the Toba eruption produce estimates with some degree monthly mean daily minimum temperatures remaining above the of uncertainty. However, they may provide some constraints on the frost point of the control run. After 5 years, temperatures return to magnitude of sulfur release (Oppenheimer, 2011). An early petro- the range of natural variability. Modeled precipitation for East Af- logical study by Scaillet et al. (1998) on Toba tuff fluid/melt parti- rica shows a strong decrease for post-Toba year 1, and high pre- tioning estimated a 60 Mt SO2 injection, which is four times cipitation for years 3e5, peaking in year 4. Vegetation modeling by Pinatubo and one to two orders of magnitude less than the loading Timmreck et al. (2012) indicates that tree cover in equatorial Africa used in many Toba supereruption climate models. They concluded decreased by 20e80% and was replaced by grasses. For East Africa, that explosive eruptions of very large magnitude but involving total tree cover was reduced by 20% and grass cover only marginally reduced and cool silicic magmas like the Toba eruption release only receded, resulting in an increase of bare ground from 20 to 40%. minor amounts of sulfur, resulting in negligible long-term atmo- After 25 years, bare ground reduces to pre-Toba values and grasses spheric effects. A Toba tuff melt inclusion study by Chesner and expand to values higher than before the eruption. In conclusion, Luhr (2010) estimated that 196 to 392 Mt SO2 would have been Timmreck et al. (2012:42) pointed out that temperature changes injected if all sulfur in the magmatic melt was exolved. However, after a Toba supereruption (using 100 times Pinatubo SO2) might based on the work of Scaillet et al. (1998), they suggested it is most have created “thermal discomfort” but were unlikely to have been a likely that a maximum of 25% sulfur was exolved from the melt, challenge to humans, and that large-scale reductions in tree cover yielding an injection of 59e98 Mt SO2, which is three to six times would have had a marginal effect on modern human nutrition. Pinatubo and roughly equal to or slightly greater than the 1815 It should be noted that the sulfate spike in the GISP2 ice core eruption of Mount Tambora. Thus, despite the fact that Toba ejected attributed by Zielinski et al. (1996) to Toba during the Greenland at least 56 times more dense rock equivalent that Tambora, the Interstadial 20 (GI-20) to Greenland Stadial 20 (GS-20) transition Toba SO2 injection may have been approximately equal to Tambora. has not been unequivocally matched to the Toba supereruption, For climate modeling, this highlights the inherent uncertainty in and ash from Toba has yet to be recovered from any ice core. In a scaling the SO2 injection of the Toba eruption based on the relative direct linking study of Greenland and Antarctic ice cores, Svensson size of the Pinatubo eruption. et al. (2013) identified four potential Toba sulfate peaks that Recent high-resolution analysis of Lake Malawi sediments occurred in ice records from both poles within a 400-year time straddling the Toba cryptotephra position also indicated limited span, peaks T4 and T3 that occurred within GI-20, and peaks T2 and cooling associated with the Toba eruption. A TEX86 paleotemper- T1 that occurred during the transition from GI-20 to GS-20. The T1 ature reconstruction by Lane et al. (2013a) observed only ~1.5 Cof spike is best matched with the Zielinski et al. (1996) Toba- cooling following the Toba eruption. They also conducted X-ray attributed sulfate spike, but Svensson et al. (2013) concluded that fluorescence scans at 200-mm intervals (subannual resolution), and the strong acidity spike in the North Greenland Ice Core Project core the Fe/Ti ratio indicated no thermally driven overturn of the water (NGRIP) at T2, and strong acidity spikes in various Antarctic cores column post-Toba. By analyzing the remains of climatically sensi- for T1eT4 make any one, or even more than one, a possible match tive aquatic organisms in Lake Malawi at subdecadal resolution, with the Toba eruption. However, marine records containing the Jackson et al. (2015) observed no evidence for significantly Toba ash layer indicate its position during the transition from GI-20 enhanced mixing or ecosystem disturbance that would be antici- to GS-20 (Kudrass et al., 2001; Schulz et al., 2002), providing sup- pated for a Toba-induced volcanic winter. Both Lane et al. (2013a) port for attributing either the T1 (attributed to Toba in Zielinski and Jackson et al. (2015) concluded that significant cooling (a et al., 1996) or T2 acidity spikes to the Toba supereruption. ‘volcanic winter’) was unlikely to have taken place in East Africa. 1.1.3. A less impactful Toba eruption There is increasing geologic However, these studies are primarily recording limnological ecol- evidence to indicate that the Toba eruption was not as impactful on ogy and geophysical conditions within Lake Malawi and not the climate or humans as most climate modeling studies would imply, terrestrial ecology and habitat structure that would have been part and that the commonly used 100 times Pinatubo SO2 injection of the human landscape. With our current study, we aim to resolve scenario may be overestimating Toba erupted aerosols by one or this part of the post-Toba paleoecological record in East Africa by two orders of magnitude (Table 1, Fig. 1). Zielinski et al. (1996) using phytoliths and charcoal. measured sulfate concentrations in a large sulfate spike in the GISP2 ice core attributed to the Toba eruption. The sulfate spike 1.2. Phytolith analysis was measured in five samples that span 6e7.5 years, peaking in year 3 post-eruption, and yielding a total mass loading of 460 to Phytoliths are microscopic opal silica infillings and casts of plant 2300 Mt SO2 (27e135 times Pinatubo). The range and magnitude cells (Piperno, 2006; Blinnikov, 2013). They are similar in size to of the values reported by Zielinski et al. (1996) includes various pollen but, unlike pollen, can be produced in different tissues multipliers to account for annual ice layer thinning, the latitude throughout a plant. When plant material decays or is burned, of the eruption, and assumptions about atmospheric parameters. phytoliths are released and incorporated into soils and sediments. Thus, there is a level of uncertainty not captured by the reported Phytoliths exposed to fire can change appearance from translucent range of values. The ~6 year residence time for the Toba aerosols to opaque and be used to reconstruct wildfire frequency (Parr, is in agreement with Bekki et al. (1996), who modeled a 6 year 2006; Cordova et al., 2011). Phytoliths generally represent a local- sulfate aerosol forcing that peaks in year 3 post-eruption, and the ized vegetation signal, as they are not easily transported long 78 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

Table 1

Summary of modeled Toba supereruption cooling and SO2 loadings used in climate models and estimates obtained from petrological and ice core studies.

Data source Climate model SO2 model Max. avg. global Pinatubo eruption loadings (Mt)a and [E. Africa] equivalencyb temp. anomaly (C)

Rampino and Self (1992) Devine et al. (1984) 653 4.0 38 5 0.31 c Oppenheimer (2002) DT ¼5.9 10 Ms (low est.) 91 0.9 5 5 0.31 c DT ¼5.9 10 Ms (high est.) 328 1.3 19 Jones et al. (2005) HadCM3 1400 [17] 10.0 100 Robock et al. (2009) CCSM3.0 2000 10.0 100 ModelE 670 8.0 33 2000 12.0 100 6000 16.0 300 18,000 18.0 900 Timmreck et al. (2012) MAECHAM5/HAM þ MPI-ESM 1700 [4.5] 3.5 100 53 [1.3] 0.8 3

Data source Toba supereruption sulfur Est. SO2 Max. global temp. Pinatubo eruption injection estimate method loading (Mt)a anomaly (C) equivalencyb

Scaillet et al. (1998) Tuff fluid/melt partitioning 60 4 c Zielinski et al. (1996) GISP2 H2SO4 conc.: Total (low) 460 27 c GISP2 H2SO4 conc.: Total (high) 2300 135 Annual over 7.5 yrs (low) 61 4 Annual over 7.5 yrs (high) 305 18 Chesner and Luhr (2010) Melt inclusion; all S exolved (low) 196 12 Melt inclusion; all S exolved (high) 392 23 25% S exolved (low) 52 3 25% S exolved (high) 98 6 Metzner et al. (2014) Modern eruption: 1991 Pinatubo 17 0.4 1 Raible et al. (2016) Modern eruption: 1815 Tambora 53e58 1.1 3

a SO2 (sulfur dioxide) mass was calculated when originally reported as either H2SO4 (sulfuric acid) or S (sulfur). b Pinatubo equivalency as reported by the study. When not stated, equivalency calculated from 17 Mt SO2 value. c Lowest and highest values from Zielinski et al. (1996) calculated from 5 GISP2 samples within the YTT H2SO4. Spike that spans 6e7.5 years. The temperature estimates reported by Oppenheimer (2002) are derived from annual SO2 loadings, not a single mass injection.

20,000 -20 Climate models Geologic estimates Historic 18,000 eruptions -18

16,000 Atm. SO2 injection -16 Global cooling modeled 14,000 -14 Global cooling estimated 12,000 -12 2 10,000 -10 Mt SO 8,000 -8

6,000 -6 Maximum cooling (°C)

4,000 -4

2,000 -2

0 0 x Pinatubo 3598 10110 100 33 100 300 900 100 3 4 148 12 23 3 6 3

09) 09) on

Jones et al. (2005) Scaillet et al. (1998) OppenheimerOppenheimer (2002)Robock (2002)Robock et al.Robock (2009)et al.Robock (2009)et al.Robock (2009)et al. (20 et al. (20ZielinskiZielinski et al. (1996) et al.Chesner (1996)Chesner et al.Chesner (2010)et Chesneral. (2010)et al. (2010)et al. (2010) TimmreckTimmreck et al. (2012)et al. (2012) 1815 Tambora1991 Pinatuboerupti eruption Rampino and Self (1992)

Figure 1. Atmospheric SO2 injections used in volcanic-forced climate models and estimated from geologic sources to determine maximum global cooling from the Toba super- eruption. SO2 mass is expressed numerically along the x-axis as equivalency to the 1991 Pinatubo eruption SO2 release. Global cooling estimates for the geologic-sourced SO2 injection values were calculated from the Timmreck et al. (2012) values using an exponential best fit. The two Oppenheimer (2002) calculations are low and high annual estimates based on SO2 values from Zielinski et al. (1996). The first Robock et al. (2009) calculation uses the CCSM3.0 climate model. The next four calculations are derived from the ModelE climate model and only vary by the SO2 value used. The Timmreck et al. (2012) calculations only vary by the SO2 value used. The two Zielinski et al. (1996) calculations are low and high annual estimates averaged over 7.5 years. The Chesner and Luhr (2010) calculations are couplets of low and high estimates for 100% and 25% sulfur exolved, respectively. C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 79 distances. However, wind and water movement energetic enough Because one species of grass can make multiple types of phy- to transport silt-sized particles can redeposit phytoliths from their toliths and the same morphotype can sometimes be produced by place of formation (Fredlund and Tieszen, 1994). Phytolith con- different grass taxa, there are few phytolith morphotypes un- centrations in lake sediments often vary between 104 to 106 phy- equivocally diagnostic of a grass subfamily or photosynthetic toliths per cm3 (Yost et al., 2013; Aleman et al., 2014). Phytoliths in pathway (Barboni and Bremond, 2009). However, the dominant lake sediments are primarily derived from near-shore habitats association of particular phytolith morphotypes with specific sub- (Yost et al., 2013; Aleman et al., 2014), but sediments in lake basins families and/or photosynthetic pathways allows for the generalized surrounded by high relief like the African Rift lakes are expected to taxonomic interpretations used here (Table 2). receive an increased proportion of their phytolith assemblages The overwhelming majority of grasses in East Africa are mem- from the surrounding watershed via runoff and stream inputs. Ash bers of the Pooideae, Panicoideae or Chloridoideae subfamilies from fire may also be a significant source of phytoliths in lake (Clayton, 1970, 1974; Tieszen et al., 1979; Barboni and Bremond, sediments (Aleman et al., 2014). The phytolith record from Lake 2009). Pooideae grasses are hydrophytic or mesophytic C3 Malawi sediments likely reflects a weighted average of the water- grasses, and their abundance is negatively correlated with shed vegetation mosaic closest to each core location. increasing temperature (Paruelo and Lauenroth, 1996). Grasses that utilize the C3 carbon fixation metabolic pathway are optimized for 1.2.1. Climate reconstructions using grass phytoliths Grasses are low to moderate temperatures, atmospheric CO2 enrichment, and annuals, biennials or perennials that typically live for just a few variable light intensity. At low latitudes, C3 grasses are typically years (Lauenroth and Adler, 2008), and their composition on the restricted to higher elevations with decreased temperatures and landscape and along elevation gradients can change relatively increased moisture, shady forested habitats or wetlands (Tieszen quickly in response to changes in temperature and precipitation. et al., 1979; Epstein et al., 1997; Cordova, 2013). The majority of This is potentially important in regard to detecting the effects of Panicoideae grasses are mesophytic C4 grasses that utilize the fi volcanic eruptions on climate, as these environmental forcings NADP-ME carbon xation metabolic pathway. Panicoideae grasses can persist for as little as one to several years and may not always are typical of tall grass prairie and savanna, and their abundance is be detrimental to longer-lived and more robust plant taxa. positively correlated with increasing summer precipitation (Taub,

Table 2 Phytolith morphotypes used in this study for analysis and interpretation.

Morphotypea Anatomical origin Taxonomic interpretationb Interpretive index usec Plant functional type or group

d Rondeleangular keel Leaf/Culm/Inflor. Phalaris spp. Ic, D/P C3 Grass Rondelekeeled Leaf/Culm/Inflor. Pooideae Ic, D/P C3 Grass Trapeziform sinuate Leaf/Culm/Inflor. Pooideae Ic, D/P C3 Grass d Plateaued saddle Leaf/Culm/Inflor. Phragmites spp. Ic, D/P C3 Grass d Very tall saddle Leaf/Culm/Inflor. Bambusoideae Ic, D/P C3 Grass e Bilobate Leaf/Culm/Inflor. Panicoideae Ic, D/P , Iph C4 Mesophytic grass e Cross Leaf/Culm/Inflor. Panicoideae Ic, D/P , Iph C4 Mesophytic grass e Polylobate Leaf/Culm/Inflor. Panicoideae Ic, D/P , Iph C4 Mesophytic grass Saddle Leaf/Culm/Inflor. Chloridoideae Ic, D/P , Iph C4 Xerophytic grass Bulliformecuneiform Leaf Poaceae Grasseindeterminate Dendriform Inflorescence Poaceae Grasseindeterminate Multicellular long cell fragment Leaf/Culm/Inflor. Poaceae Grasseindeterminate Substomatal & stomatal complex Leaf Poaceae Grasseindeterminate Rondelehorned Leaf/Culm/Inflor. Poaceae Ic, D/P Grasseindeterminate Rondelepyramidal Leaf/Culm/Inflor. Poaceae Ic, D/P Grasseindeterminate Rondeleconical Leaf/Culm/Inflor. Poaceae Ic, D/P Grasseindeterminate Elongate psilate Leaf/Culm/Inflor. Poaceae and Cyperaceae Grass/Sedge Elongate echinate Leaf/Culm/Inflor. Poaceae and Cyperaceae Grass/Sedge Trichome Leaf/Culm/Inflor. Poaceae and Cyperaceae Grass/Sedge Cone cells w/papilla & satellites Leaf/Culm/Inflor. Cyperaceae Sedge Polygonal cells w/central papilla Achene (seed) Cyperaceae Sedge Blocky w/dark infilling Leaf/Culm Cyperaceae Sedge Thin w/ridgesd Culm Cyperaceae Sedge Irreg. w/tubular projectionsd Root/Rhizome Cyperaceae Sedge Perforate decorated Leaf Podostemaceae Herbaceous Prismatic domed cylinder Seed Commelina spp. Herbaceous Opaque plate w/perforations Achene (seed) Asteraceae Herbaceous Spherical psilate Various Ferns, monocots, dicots Various plants Elongate w/helical thickenings Vascular tissue Various plants Various plants Elongate decorated Tracheary tissue Woody/Arboreal Woody/Arboreal Elongate facetate Sclereid tissue Woody/Arboreal Woody/Arboreal Polyhedral facetate Sclerenchyma Woody/Arboreal Woody/Arboreal Wavy margin plate Leaf epidermis Woody/Arboreal Woody/Arboreal Spherical decorated Woody tissue Woody/Arboreal D/P Woody/Arboreal Spherical echinate Various tissue Arecaceae (palms) Woody/Arboreal

a References used for morphotype names and taxonomic interpretations are listed in Methods subsection 2.3. b Because of issues with taxonomic multiplicity and redundancy in grass phytolith morphotype production, there are few to no morphotypes unequivocally diagnostic of grass subfamilies Pooideae, Panicoideae, Chloridoideae, and others. However, the dominant association of particular morphotypes with specific photosynthetic pathways and/ or ecological requirements allows for the generalized taxonomic interpretations used here. c See Methods subsection 2.4 for index definitions and calculations. d See Supplementary Online Material (SOM) for illustrations and descriptions of these morphotypes. e In the highlands of East Africa some Panicoideae grasses utilize the C3 photosynthetic pathway (see Bremond et al., 2008). 80 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

2000; Cabido et al., 2008). The Chloridoideae are typically xero- October. Average daily temperatures are controlled by the varying phytic C4 grasses that utilize either the NAD-ME or PCK metabolic topography and range from 22 to 27 C in the summer months pathway, and their abundance is negatively correlated with (Ngongondo et al., 2015). In the dry austral winter months between increasing precipitation (Taub, 2000; Cabido et al., 2008). Chlor- May and August, average daily temperatures drop to around 18 C idoideae grasses are optimized for stressfully hot, arid and saline (Ngongondo et al., 2015). Mesic environments dominate the sur- environments that include short grass prairie and savanna, scrub- rounding watershed, which receives highly seasonal rainfall of lands, desert, beach, dune and marine shoreline habitats (Tieszen 500e2500 mm per year (Ngongondo et al., 2015). Areas sur- et al., 1979; Epstein et al., 1997). In summary, C3 grass abundance rounding the northernmost part of the lake rise abruptly from the is positively correlated with decreasing temperature and increasing lake surface at 478 m above sea level (masl) towards the north and soil moisture, C4 mesophytic grass abundance is positively corre- west, reaching elevations of 3000 masl in the Rungwe Highlands lated with increasing summer precipitation, and C4 xerophytic and Nyika Plateau. Recently mapped and refined vegetation com- grass abundance is negatively correlated with increasing summer munities (Lillesø et al., 2011; van Breugel et al., 2015) illustrate the precipitation. elevation enhanced mosaic of afromontane forest and grassland, It should be noted that, with the exception of palms (Arecaceae; wet and dry miombo woodlands, North Zambezian woodland, and Albert et al., 2009), arboreal taxa are poor phytolith producers, so edaphic grasslands that surround the northern half of Lake Malawi direct comparisons of grass phytolith to arboreal phytolith per- (Fig. 2). Previous pollen-based paleovegetation reconstructions centages are often not indicative of actual grass versus arboreal have revealed significant and sometimes rapid vegetation response coverage on the landscape. However, general interpretations of to changes in precipitation and temperature over the last 800 ka in increasing or decreasing grass versus arboreal coverage are valid. the Lake Malawi watershed (Beuning et al., 2011; Ivory et al., 2012, 1.2.2. Detecting effects of the Toba supereruption using 2016). It should be noted that these pollen studies were conducted phytoliths The existence of volcanic winter conditions in East Af- at millennial-scale temporal resolutions and cannot be used to rica after the Toba supereruption should be detectable in the phy- study vegetation change associated with the Toba supereruption. tolith record. Since phytoliths form primarily in epidermal and vascular tissues throughout the plants they are derived from, a 2. Methods massive vegetation kill from freezing or prolonged exposure of Afrotropical vegetation to chilling events of 10e15 C for a few days 2.1. Drill cores would release a pulse of phytoliths into lake sediments as the plant material decays. Woody material would take longer to decompose, The cores studied in this project were retrieved in 2005 by the but leaves would decay rapidly. Either way, a spike in phytolith Lake Malawi Drilling Project (Scholz et al., 2006). Core MAL05-2A- concentrations above background variability is expected in 10H-2, hereafter referred to as core 2A, was retrieved from the continuously sampled intervals with subdecadal resolution. Even- northern basin in a region subject to upwelling variability. Core tually, dead plant material would burn, and a spike in burned MAL05-1Ce8H-1, hereafter referred to as core 1C, was retrieved phytoliths and charcoal would also be expected, especially since from the central basin near the deepest part of the lake. Lane et al. decreased precipitation is expected immediately after an eruption. (2013a) identified Toba supereruption cryptotephra at a depth of A decrease in temperatures of ~4 C(Timmreck et al., 2012) and 26.752e26.772 m below lake floor (mblf) in core 2A, and at a depth reduced light (dim-sun to overcast) from stratospheric aerosols of 28.080e28.100 mblf in core 1C. Jackson et al. (2015) observed a (Robock et al., 2009) for 4e5 years would likely increase the turbidite ~1 cm below the core 2A Toba interval which may relative abundance of C3 grasses at the expense of C4 grasses. This confound a small portion of the pre-Toba record; however, core 1C scenario would be reflected by an increase in trapeziform sinuate is excellently preserved with no turbidites adjacent to the Toba phytoliths and a decrease in saddle and lobate phytoliths. horizon and is laminated throughout the Toba interval. Multidecadal reductions in tree cover and increases in grass cover, as modeled by Timmreck et al. (2012), would be reflected 2.2. Phytolith, charcoal, and cryptotephra extractions by a decrease in globular granulate (arboreal) phytoliths and an increase in short- and long-cell grass phytoliths. Burned grass Samples were collected from 15 intervals below and from 30 phytolith percentages and charcoal concentrations would also intervals above the Toba cryptotephra position in Lake Malawi rise, as most grasslands are naturally fire-prone ecosystems. cores 2A and 1C. The ~3e4 mm intervals were sampled continu- ously, with no stratigraphic breaks between adjacent samples. We 1.3. Study site assume linear sedimentation rates in the study interval based on the radiocarbon-dated portion of the cores published in Scholz Modern Lake Malawi is a large (29,500 km2), deep (700 m), et al. (2007). This gives us a temporal resolution of ~8e9 years long-lived (>7 Ma) and stratified (anoxic below 250 m) African Rift per sample, with the exception of the samples within the turbidite lake located near the southern extent of the intertropical conver- zone in core 2A, which is assumed to have been deposited much gence zone (ITCZ; Cohen et al., 2007). The long sedimentary record more rapidly, possibly in less than 1 year as a discrete event and well-studied and diverse ecosystems of Lake Malawi make it an (Jackson et al., 2015). Small aliquots from each sample were ideal repository for records of long- and short-term climatic fluc- removed for diatom and water content analysis. The remainder of tuations (Ivory et al., 2016; Johnson et al., 2016). Lake Malawi each sample was weighed, disaggregated in distilled water, and inherently responds to large-scale phenomenon more readily than screen-washed using 45 mm stainless steel sieves. The screen con- ‘local’ phenomenon because it integrates information from a broad tents were examined for key ecologically sensitive aquatic organ- region of over 4 latitude; local events rarely have a substantial isms as part of a companion study that included the diatom and enough impact without a large-scale driver that might affect the water content analysis (Jackson et al., 2015) and for charcoal (this entire region (Stone et al., 2011). Thus, the ability to extrapolate study). Contents passing through the 45-mm sieves were retained paleoenvironmental results to a broader scale is an intrinsic aspect for phytolith, microcharcoal, and cryptotephra analysis. Ideally, of the lake. sediment sieved through a 250 mm screen would have been dedi- The Lake Malawi region has an austral summer rainy season cated exclusively to phytolith analysis; however, limited core ma- between November and April and a dry season between May and terial necessitated working with the 45-mm fraction. Most C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 81

33° E 34° E 35° E Modern Natural Vegetation Wd Bd Wn/g Fa/Fb of the Northern Lake Wn/g 1 Wd/Wc Malawi Region Somalia-Masai Acacia-Commiphora bushland (Bd) 9° S Wmw Wmd Acacia-Commiphora deciduous wooded grassland + Combretum wooded ∆ K Gm/F grassland (Wd/Wc) Mt. Rungwe i p e Wmw Gm/F Lake n Colophospermum mopane woodland g Masoko e and scrub woodland (Wo) So r n e Miombo woodland on hills and gw Wd e R. L R rocky outcrops (Wmr) iv a in Fa/Fb Drier miombo woodland (Wmd) g n 2 s g i R. t Wetter miombo woodland (Wmw) liz o a n e b e North Zambezian woodland + M M edaphic grassland (Wn/g) 3 t n . 4 10° S s Wmw Edaphic grassland on drainage-impeded R Core 2A

u

r soils (wd)

u k 5 Zanzibar-Inhambane lowland u Ruhuhu River R Wmr . 6 rainforest (Fo) N Wmd Wmw 7 Wn/g Afromontane rainforest (Fa/Fb) Gm/F Montane grassland and afromontane a Wmd ik forest mosaic (Gm/F) y au N te la P er Major watershed boundaries iv R u Gm/F Drill core locations ur 11° S uk Fa/Fb R Matengo Selected Paleolithic archaeological uth # So Highlands localities Wo Core 1C

Kasitu R. Africa Gm/F Lake Detailed vegetation data south of this Fo Africa Malawi line is unavailable Lake Tanganyika 10 11 12° S Lake Malawi 12

8 02550 02550 9 Km Km

Figure 2. Map of the potential natural vegetation of the northern Lake Malawi region, with locations of Lake Malawi core sites used in this study, and locations of selected Middle Stone Age (MSA) Paleolithic archaeological sites (see Table 3). GIS layers from Potential Natural Vegetation Map of Eastern Africa Version 2.0 (van Breugel et al., 2015). taxonomically significant phytoliths are well under 50 mm in size, However, for the removal of organic matter, Blockley et al. (2005) so very little information was lost by working with the 0e45 mm found that strong acids can alter the geochemical signature of fraction. shards, and strong bases can dissolve some shards. Thus, crypto- It should be noted that the extraction procedure below is opti- tephra recovered from a phytolith extraction may not be suitable mized for phytolith recovery; however, it is also effective for re- for chemical fingerprinting, and shards with a density much greater covery of microcharcoal and cryptotephra particles. In a controlled than 2.3 g/cm3 may not be recovered. Due to the chemically and comparison of microcharcoal extraction methods, density separa- mechanically rigorous phytolith extraction method used here, and tion using a heavy liquid set at 2.2 g/mL and chemical removal of differences in particle densities, there may have been some loss of organic matter produced a significantly higher microcharcoal yield microcharcoal and cryptotephra in absolute terms, but changes in than other common methods (Turner et al., 2008). Cryptotephra their relative abundances over time are considered to be robust. extraction methods are very similar to phytolith extraction Average starting dry weights for the sediment samples were methods, as they both rely on a heavy liquid density 0.2673 g for core 2A, and 0.2748 g for core 1C. Ideally, 2 g or 1 cm3 of separationd2.5 g/mL for cryptotephra and 2.3 g/mL for phytoliths. sediment would have been analyzed, but multiproxy sampling of 82 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 the cores for several studies have left very little sediment for us to Cryptotephra was identified with the aid of modern reference im- work with. Exact weights and calculations can be viewed in the ages from Heiken (1974) and Myrbo et al. (2011). Some Cyperaceae supplemental data associated with Jackson et al. (2015). For each phytolith morphotypes are strikingly similar to cryptotephra (SOM sample, sediment was placed in a 400 mL beaker with 10 mL 37% Fig. S6), and extra care was taken to avoid any misidentifications. hydrochloric acid (HCl) and 75 mL 70% nitric acid (HNO3), and heated for 1 h at 80 C to oxidize organics and remove the acid 2.4. Phytolith calculations soluble fraction. Samples were rinsed five times with reverse osmosis deionized (RODI) water to neutral pH, and then 5 mL of 10% Percent relative abundance calculations were based on the total potassium hydroxide (KOH) was added to remove the base soluble phytolith count and concentrations were calculated from the organic fraction. After 10 min the samples were rinsed to neutral pH microsphere counts and the starting dry weight. The plots of C3 with RODI water. To remove clay-sized particles and to reduce the versus C4 grass phytoliths were based only on grass silica short cells diatom load, 5% sodium hexametaphosphate was mixed into each (GSSC) attributed to those plant functional types (Table 2). The 400 mL sample beaker and allowed to settle by gravity for 1 h. climatic (Ic), tree cover (D/P), and aridity (Iph) phytolith indices Samples were then decanted to remove any clay-sized particles and were calculated as described in Bremond et al. (2008) using the diatoms still in suspension. This step was repeated until the su- specific morphotypes listed in Table 2. The climatic index (Ic) is the pernatant was clear after 1 h of settling time (typically 4e5 times). ratio of C3 Pooideae versus all GSSC phytoliths, which is essentially The samples were then transferred to 50 mL centrifuge tubes and a measure of C3 versus C4 grasses. Ic values increase linearly with dried under vacuum in a desiccator. The dried samples were mixed increasing elevation, thus tracking a combination of decreasing with 5 mL of lithium metatungstate (LMT) heavy liquid, density temperature and increasing precipitation (Bremond et al., 2008). 2.3 g/mL, and centrifuged for 10 min at 1500 rpm to separate the The tree cover index (D/P) is a ratio of globular granulate versus all phytolith fraction, which will float, from the denser inorganic GSSC phytoliths, and is best used for tracking low elevation semi- mineral fraction. Each sample was rinsed five times with RODI water deciduous forest change (e.g., open versus closed forests). The to remove the LMT heavy liquid and then spiked with a 0.5 mL aridity index (Iph) is the ratio of saddle phytoliths versus aliquot containing 25,000 ± 8% microspheres (10e20 mm diameter) saddle þ bilobate þ cross þ polylobate phytoliths, which is for phytolith, microcharcoal, and cryptotephra concentration cal- essentially a measure of C4 xerophytic versus C4 mesophytic culations. The samples were then rinsed twice with 99% ethyl grasses. This index works best for grasslands such as the discrim- alcohol and transferred with alcohol to 1.5 mL vials for storage. ination between Sahelian versus Sudanian savannas (Bremond et al., 2005), or the expansion and contraction of riparian grass 2.3. Microscopy and phytolith, microcharcoal, and cryptotephra communities (Novello et al., 2012), but is less robust at higher el- identifications evations where C3 grasses are dominant (Bremond et al., 2008).

For microscopy, subsamples were mounted on slides using 3. Results Cargill Type-A optical immersion oil to allow phytolith rotation and each cover glass was sealed with fingernail polish. Phytolith, Phytoliths were exceptionally well preserved for all samples in microcharcoal, and cryptotephra counting was conducted with a both cores. Phytolith concentrations were high; however, diatom transmitted light microscope at a magnification of 500, with a (Aulacoseira spp.) concentrations were much higher, making phy- target goal of 200 phytoliths counted (range 127e400; tolith counting laborious. Phytolith concentrations in core 2A var- Supplementary Online Material [SOM] Tables S1, S2). High diatom ied between 5 104 and 34 104 per g dry sediment (Figs. 5, 7g), concentrations in five samples from core 2A made counts of 200 and between 22 104 and 108 104 per g dry sediment for core 1C difficult to obtain, so counts were stopped after 4 h. (Figs. 6, 8g). These concentration values are typical for lake sedi- Phytoliths were observed in three dimensions and typically ments (Yost et al., 2013; Aleman et al., 2014; Nurse et al., 2017). described using Madella et al. (2005), with the addition of Grass (Poaceae) and sedge (Cyperaceae) phytoliths dominated the anatomical or taxonomic descriptors when known. Phytoliths were phytolith records for both cores. Average percent lowland arboreal classified using a generalized approach, resulting in just 35 indi- phytoliths for core 1C is 31% versus 15% for Core 2A. Average vidual morphotypes (Table 2, Figs. 3, 4). Grass phytolith classifica- percent C3 grass phytoliths for core 1C is 3.3% versus 8.2% for Core tion was based primarily on Twiss et al. (1969), Fredlund and 2A. Average percent sedge phytoliths for core 1C is 5.6% versus 8.5% Tieszen (1994), and one of the authors' (C.L.Y.) phytolith reference for core 2A. collection at the University of Arizona. East African grass phytolith Because the strongest effects of volcanism on climate are typi- environmental and taxonomic significance was guided by the work cally felt within the first or second year after an eruption, an of Barboni et al. (2007), Barboni and Bremond (2009), and attempt to better constrain the location of the Toba eruption was Mercader et al. (2010). Non-grass phytolith identification and conducted by counting cryptotephra in the phytolith samples. Lane classification, especially for arboreal and woody morphotypes, et al. (2013a) used two adjacent 1 cm sample intervals to locate the relied primarily on the review and reference work of Kondo and Toba cryptotephra in each core, and we analyzed 7 samples within Peason (1981), Runge (1999), Piperno (2006), Mercader et al. that same 2 cm interval. In core 2A, samples 26.762 and 26.759 mblf (2009b), Neumann et al. (2009) and Collura and Neumann (2017). had the highest cryptotephra concentrations within the Lane et al. Phytoliths identified from several specific plants and plant parts (2013a) Toba interval (Fig. 7h). In core 1C, sample 28.0955 mblf was that are either not represented or underrepresented in the existing the first sample with an elevated cryptotephra spike just before the phytolith literature are illustrated and discussed in the SOM highest cryptotephra concentration at 28.0925 mblf (Fig. 8h). (Figs. S1eS6; Tables S3eS5). Microcharcoal was identified using the diagnostic characteris- 3.1. Core 2A tics described by Turner et al. (2008), with particular emphasis on avoiding naturally dark plant matter and insect cuticles in the Within the phytolith-extracted Toba cryptotephra zone and microcharcoal counts. Because of the 45 mm screen size used to immediately following the cryptotephra peak, there is a significant separate the Jackson et al. (2015) material from this study, micro- spike in phytolith concentration at 26.759 mblf that appears to be a charcoal was counted in two size fractions (<45 mm and >45 mm). singular event or a series of events that took place within this one C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 83

a b c g h

d e f i j k l m p t q n o r s u v w x y z

a´ b´ c´ d´

Figure 3. Phytolith micrographs of important grass (Poaceae) morphotypes recovered from Lake Malawi sediments. Morphotype order approximately follows the phytolith diagram order (Figs. 5 and 6). a) Rondel with angular keel derived from the C3 genus Phalaris.bec) Modern reference Phalaris phytoliths (SOM Fig. S4). d) Rondel with straight keel typical of C3 subfamily Pooideae grasses. e) Plateau saddle phytolith typical of C3 genus Phragmites (SOM Fig. S3). f) Pyramidal rondel typical of C3 grasses but also produced by some C4 grasses. geh) Rondel with keel and basal sinus typical of C3 grasses in side (g) and in top (h) views. iej) Cylindrical rondel typical of C3 and C4 grasses in side (i) and in top (j) views. kem) Range of trapeziform sinuate morphotypes typical of C3 Pooideae grasses. n) Very tall saddle (VTS) considered diagnostic for bamboo (SOM Fig. S1) and most likely derived from the C3 bamboo Y. alpina. o) Burned VTS saddle phytolith. p) Bilobate phytolith diagnostic of the typically C4 Panicoideae subfamily. q) Burned bilobate phytolith. r) Polylobate phytolith typical of the Panicoideae subfamily. s) Cross phytolith typical of the Panicoideae subfamily. t) Saddle phytolith typical of the C4 Chloridoideae subfamily. u) Burned saddle phytolith. v) Bulliform phytolith diagnostic of grass leaves. w) Dendritic long-cell phytolith diagnostic of grass inflorescence tissue. x) Burned grass leaf fragment with bilobates visible. y) Burned grass leaf fragment with stoma and long cells visible. z) Unburned fragment of grass leaf epidermis with papilose long cells. a’eb’) Silicified cf. substomatal cavity phytoliths. c’) cf. Substomatal phytolith attached to stoma guard cells (black arrow). d’) cf. Substomatal phytolith attached to interstomatal cell (black arrow). White scale bar at bottom right equals 10 mm.

~8e9 year sample interval (Figs. 5, 7g). Although other phytolith perturbation. The Iph, or aridity index (Fig. 7d), has a moderately concentration spikes are evident earlier and later in the record, this high peak within the Toba cryptotephra zone that is synchronous is the highest phytolith concentration for the entire 2A core with the local and regional charcoal peak, but is not synchronous examined here. At the same level, there is a prominent spike in with the first occurrence of the Toba cryptotephra and may be regionally sourced (<45 mm particle size) and locally sourced unrelated to the eruption. Other visible trends in the phytolith re- (>45 mm particle size) charcoal concentrations indicative of an in- cord that may be Toba related include the abrupt reduction in the crease in both regional and local fires (Fig. 7f). Interestingly, there is ‘very tall saddle’ phytolith morphotype attributed here to mountain no corresponding spike in the percent burned phytoliths, sug- bamboo (Yushania alpina, formerly Arundinaria alpina; see SOM gesting that much of the burned material may have been from Fig. S1 and Table S3 for identification rationale) near the center of woody non-phytolith producing taxa, and that the phytolith con- the Toba zone (Fig. 5), and indicating drier conditions at higher centration spike was due to decomposed rather than burned leaves elevations. Also, a spike in sedge phytoliths at the first occurrence of and grasses. Spikes in phytolith and charcoal concentrations are not Toba cryptotephra suggests progradation at the north end of Lake as pronounced in core 1C (Figs. 6, 8f), suggesting that this was more Malawi due to increased sediment yield from fire activity. There of a high elevation (>1500 m) afromontane forest zone also appear to be some long-term trends unrelated to the Toba 84 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

a

b c d

efg hi j k l

mnopqr

stuvwx

Figure 4. Phytolith micrographs of important sedge (Cyperaceae), other herbaceous, and arboreal/woody tissue morphotypes recovered from Lake Malawi sediments. Mor- photype order approximately follows the phytolith diagram order (Figs. 5 and 6). a) Elongate echinate. b) Elongate psilate. c) Bulliform cell. d) Trichome (prickle). The mor- photypes in aed are produced by grasses and sedges. eef) Polygonal sedge achene epidermis cells with central papilla. g) Sequence of sedge (Cyperaceae) achene epidermis cone cells with satellites around the papilla. hei) Thin facetate plates with raised ridges from sedge culms (SOM Fig. S6). j) Irregular with tubular projections from sedge (Cyperaceae) roots (SOM Fig. S5). k) Fragment of a perforate decorated plate from riverweed (Podostemaceae) leaves. l) Fragment of a perforate plate from composite family (Asteraceae) achenes. m) Prismatic dome cylinder with decorated stalk from Commelina seeds. neo) Commelina seed phytolith in side (n) and in top (o) views. peq) Elongate decorated arboreal tracheary element phytoliths. r) Elongate facetate arboreal sclereid phytolith, possibly burned. seu) Globular granulate phytoliths from woody arboreal tissue. v) Globular echinate phytolith from palm (Arecaceae) leaf/bark/fruit tissue. w) Burned globular echinate (palm) phytolith. x) Wavy margin plate from arboreal leaf tissue. White scale bar at bottom right equals 10 mm. eruption that may be forced by Northern Hemisphere (NH) cooling first occurrence of the Toba cryptotephra and two relatively high during the transition from GI-20 to GS-20 (Fig. 7i). A decline in values of local charcoal within the Toba cryptotephra zone (Fig. 8f), phytoliths from arboreal taxa (Fig. 5), a steady rise in the percent- indicating that core 1C is picking up the same fire event that was age of burned phytoliths that later transitions to high-amplitude prominent in the core 2A Toba cryptotephra zone. The Iph aridity variability (Figs. 5, 7f), and a decrease in the D/P (tree cover) in- index (Fig. 8d), which is mainly measuring C4 xerophytic grasses, dex were all initiated well before the Toba eruption. peaks concurrently with the first occurrence of the Toba crypto- tephra and then declines as quickly as 8e9 years later, in agreement 3.2. Core 1C with modeled (Iles et al., 2013) and observed (Zhuo et al., 2014) significant decreases in precipitation lasting 1e4 years after an Within the Toba cryptotephra zone for core 1C, there is no sig- eruption. nificant abrupt change in phytolith morphotype percentages that is Long-term directional trends are more muted in core 1C. greater than the variability observed in other parts of the core. There is an increase in local and regional wildfire events in the There is a small peak in phytolith concentration in two samples upper portion of the record, but there is no apparent trend to- (~16e18 yr) after the first occurrence of Toba cryptotephra; how- wards increasing aridity based on the Iph index (Fig. 8d). How- ever, this value is within the range of variability seen throughout ever, the highest Iph index values occur in the upper portion of the record (Figs. 6, 8g). There is a regional charcoal peak with the the record, suggesting an increase in the severity of aridity C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 85

C4 MESIC GRASSES C4 XERIC GRASSES GRASS/SEDGE SEDGE (CYPERACEAE) HERBACEOUS C3 GRASSES VARIOUS GRASSES ARBOREAL

plants .)

s ry sed e epidermis spp. pidermis al tissue elina e e ots, dicots ASTERACEAE AE (Palms) spp. spp. E TYPES Comm af/stem Phalaris asses, some sedges dge epidermis ENTRATION (per g d Phragmites ate: Arboreal sclerenchyma l (5-45 um) oba RACEAE T ICOIDEAE rk infill: Le E a r with ridges (TWR): sheath, leaf, culm AN with tubular projections: rhizome granulate: Woody arbore VARIOUS GRASSES te w/helicalt ethickenings: facetate: Arboreal Xylem-various sclereid L CYP L ARBOREAL OLITH CONC riform: Grass inflorescence T TAL GRASS/SEDGE Volcanic glass (cryptotephra) richome: OGrass/se paque plate w/perforations: CORE 2A MBLFRondel-keeled:TrapeziformTOTAL POOIDEAE sinuate: POOIDEAE POOIDEAEBilobate: PANICOIDEAECross:Polylobate: PANICOIDEAETOTAL PANICOIDEAE P Saddle: CHLORIDOIDEAEBulliform:Dend mostlyEpidermis-long grSubstomatalRondel-horned cells:Rondel-pyramidal & stomatal Rondel-conicalGrassTOTAL leaf/inflor. complex Elongate-psilate: typesElongate-echinate:T Grass/sedgeT Grass/sedg epidermis BlockyThin w/d tabulaIrregularTOTA Perforate decorated:SphericalO psilate: ElongaPODOSTEMACEAEElongate ferns,Elonga monoc decorated:PolyhedralGlobular Arboreal facet Globular trachearyTOTA echinate: element% ARECAC Fungal% spores% Microcharcoa % Burned phytolithsPHY MICROSPHERESPHYTOLITHS COUNTED COUNTED Rondel-angular keel: PlateauedVery tallsaddle: saddle: Bambusoideae ConePolygonal cells w/papilla cells &w/central satellites: papilla: Epidermis AchenPrismatic domed cylinder: ~yrs pre-/post- 26.688 212 200 26.692 112 200 26.695 72 200 +200 yr 26.698 78 200 26.700 155 202 26.702 77 200 26.704 74 200 26.707 168 200 26.710 101 200 26.712 141 200 26.714 60 200 26.716 109 200 26.718 205 170 26.721 173 200 26.724 156 200 +100 yr 26.726 102 200 26.729 225 200 26.733 207 200 26.737 26.741 143 200 26.745 219 200 26.749 172 127 26.752 221 183 Toba 26.755 149 200 Zone 26.759 243 200 26.762 242 178 0 yr 26.765 Toba 349 200 26.767 cryptotephra 230 155 26.770 1st occ. 227 200 26.774 113 200 26.778 70 200 26.782 143 200 -50 yr 26.786 123 200 26.790 53 200 26.793 50 200 26.797 Turbidite 55 200 26.801 59 200 26.805 26.809 125 200 26.813 138 200 26.817 139 200 26.821 110 200 -100 yr 26.825 66 200 26.829 100 200 26.833 109 200 % 20 20 40 20 40 20 20 20 20 20 40 20 20 20 20 20 40 60 80 100 20 100 200 300x 103

Figure 5. Core 2A relative abundance diagram for phytoliths, charcoal, and other microfossil remains. All percentages are based on the total phytolith count. Dark gray vertical shading is 5 exaggeration. The upper horizontal gray shaded area is the Toba cryptotephra zone, and the dashed line represents the position of first occurrence of Toba cryp- totephra. The lower gray vertical shading represents a turbidite, which was only present in core 2A.

C XERIC GRASSES C MESIC GRASSES 4 SEDGE (CYPERACEAE) HERBACEOUS C3 GRASSES 4 VARIOUS GRASSES GRASS/SEDGE ARBOREAL

.)

ry sed EAE r. spp.icots Achene epidermis cheary element lex types AE (Palms) spp. spp. p S papilla: Commelina al sclereid l com TYPES l E NTRATION (per g d Phalaris sses, some sedges ArborealWoody leaf arborealepidermis tissue Phragmites DOIDEAE (Cryptotephra) ated: Arboreal tra COUNTED COIDEAE COIDEAE S I I rk in-fill: Leaf/stem H CHLORI al facetate: Arboreal te w/helicalte decor thickenings: Xylem-various plants ARBOREAL L PAN bular with ridges (TWR):ate sheath, decorated: leaf, PODOSTEMAC culm OLIT AL VARIOUS GRASSE ta AL CYPERACEAE T OTAL CORE 1C Rondel-angularMBLF Rondel-keeled:Trapeziform keel: TOTALPOOIDEAE sinuate: POOIDEAEPlateaued POOIDEAEVery saddle: tallBilobate: saddle: BAMBUSOIDEAEPANICOIDEAECross:Polylobate: PAN TOTA PANICOIDEAESaddle:Bulliform: Dendriform: mostlyEpidermis-longSubstomatal graGrassRondel-horned inflorescence cells:Rondel-pyramidal & stomata Rondel-conicalGrassTO leaf/infloElongate-psilate:Elongate-echinate:Trichome: Grass/sedgeTOTAL Grass/sedge Grass/sedge GRASS/SEDGepidermisCone epidermisPolygonal epidermiscellsBlocky w/papillaThin cells w/da &w/centra satellites:IrregularTOT Epidermiswith tubularPerforPrismatic projections:Spherical Elongadomed rhizomes psilate:Elonga cylindar:Elongate ferns,Polyhedr monocots, facetate:WavyGlobular margin Arbore d plate: granulate: Globular T echinate: ARECACE% Fungal% Chrysophyte spores% Volcanic% Microcharcoal cysts glass (5-45 %um) Burned phytolithsPHYTOLITH CONCE MICROSPHERESPHYT ~ yrsCOUNTED pre-/post-Toba 28.0040 41 200 28.0085 33 200 28.0115 24 200 28.0145 45 200 28.0175 28 200 +200 yrs 28.0205 42 200 28.0235 52 200 28.0265 39 200 28.0295 42 200 28.0340 54 200 28.0400 51 200 28.0445 67 200 28.0475 60 200 28.0510 69 349 28.0555 25 200 28.0595 35 200 +100 yrs 28.0625 34 200 28.0655 41 200 28.0685 28 200 28.0715 56 322 28.0745 61 300 28.0775 116 340 28.0805 54 310 28.0835 62 373 28.0865 79 321 28.0895 60 341 Toba 28.0925 135 360 Zone 28.0955 57 368 0 yr 28.0985 68 400 28.1015 Toba 53 354 28.1045 71 328 28.1075 crypto- 52 334 28.1105 tephra 54 354 28.1135 48 322 28.1165 1st occ. 46 302 28.1195 22 200 28.1225 37 200 28.1255 26 200 28.1285 19 200 28.1315 15 200 28.1355 26 200 -100 yrs 28.1415 89 200 28.1475 49 200 28.1525 23 200 28.1565 39 200 % 20 40 20 40 20 20 20 20 20 20 20 40 20 40 20 40 60 20 500 1000x 103

Figure 6. Core 1C relative abundance diagram for phytoliths, charcoal, and other microfossil remains. All percentages are based on the total phytolith count. Dark gray vertical shading is 5 exaggeration. The upper horizontal gray shaded area is the Toba cryptotephra zone, and the dashed line represents the position of first occurrence of Toba cryptotephra. 86 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

Figure 7. Summary phytolith, charcoal, and cryptotephra plots for core 2A. The red shaded area is the Toba cryptotephra zone defined by an elevated abundance of cryptotephra attributed to the Youngest Toba Tuff (YTT). This zone is within the 2 cm interval where Lane et al. (2013a) chemically identified YTT cryptotephra, and further refines the position of first occurrence for core 2A (dashed blue line). a) Relative abundance of grass silica short cell (GSSC) phytoliths ascribed to a specific functional type. b) Climatic index (Ic) is the ratio of C3 Pooideae versus all GSSC phytoliths, which is essentially a measure of C3 versus C4 grasses. c) Tree cover index (D/P ) is a ratio of globular granulate versus all GSSC phytoliths. d) Aridity index (Iph) is the ratio of saddle phytoliths versus saddle þ bilobate þ cross þ polylobate phytoliths, which is essentially a measure of C4 xerophytic versus C4 mesophytic grasses. e) Percent burned phytoliths calculated from the total phytolith sum. f) Charcoal concentrations for <45 mm (local and regional fires) and >45 mm size fractions (local fires). 18 g) Phytolith concentrations. h) Cryptotephra concentrations from this study. i) Greenland Ice Sheet Project 2 d O values from Grootes and Stuiver (1997), and location of the H2SO4 spike attributed to the Toba supereruption from Zielinski et al. (1996). Greyed-out portions of the d18O record are outside the timeframe of our analysis.

Figure 8. Summary phytolith, charcoal, and cryptotephra plots for core 1C. The red shaded area is the Toba cryptotephra zone defined by an elevated abundance of cryptotephra attributed to the Youngest Toba Tuff (YTT). This zone is within the 2 cm interval where Lane et al. (2013a) chemically identified YTT cryptotephra, and further refines the position of first occurrence for core 1C (dashed blue line). See Figure 7 caption for individual plot descriptions. C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 87 events when they do occur. There is a small decrease in grass using plant leaf wax dD and TEX86 biomarkers from lakes Tanga- phytolith concentrations, indicating that grass densities nyika and Malawi, showed that during millennial-scale NH cooling decreased slightly and fire frequency increased over the course events, aridity increases in East Africa. They proposed that cold sea of the record. It is interesting to note the large and sustained surface temperatures (SSTs) during millennial-scale stadials reduce peak in regional charcoal concentrations at the bottom of the 1C the amount of winter monsoon moisture transported into East record that are not always accompanied by a peak in locally Africa from the western Indian Ocean. With our records positioned sourced charcoal. The converse is true for core 2A, suggesting a at the beginning of a NH cooling event, we would expect to see period of high fire frequency ~200 km north of core 1C in the some type of long-term directional aridity signal. Rungwe Highlands for this part of the record. Core 2A, which receives runoff from high elevation afromontane forests and grasslands, may capture part of a NH-forced aridity 4. Discussion signal, as evidenced by a decline in phytoliths from arboreal taxa and mountain bamboo, a decrease in the phytolith derived D/P Because the two cores are separated by approximately 100 km (tree cover) index, a rise in percent burned phytoliths that then and are adjacent to different elevation gradients comprised of transitions to high-amplitude variability, and an increase in sedge different dominant vegetation types, cores 2A and 1C were not cone cell phytoliths suggesting marsh expansion at the north end of expected to record the same vegetation signal. Core 2A is more Lake Malawi due to lower lake levels or progradation of river deltas likely to receive a sedge and C3 grass vegetation signal from both due to post-fire sedimentation. The high concentrations of locally the high elevation afromontane grasslands and the edaphic grass- sourced charcoal at the beginning of the 2A record are likely land and freshwater swamp at the north end of the lake. Also, derived from the Rungwe Highlands and suggest replacement of afromontane arboreal taxa are poor phytolith producers. By afromontane forest with mostly C3 and C4 mesophytic Panicoideae contrast, core 1C is adjacent to lower elevation areas dominated by grasses, which would explain the paucity of burned phytoliths, as miombo and North Zambezian woodlands with taxa that are much woody afromontane forest taxa produce few to no phytoliths better producers of arboreal (e.g., globular granulate) phytoliths. (Barboni et al., 2007). Presently, there is less than 5% afromontane Thus, these two core locations give us an opportunity to examine forest in the Rungwe Highlands, and what is left is confined to fire- whether the Toba eruption had differing effects on lowland versus sheltered settings (Lillesø et al., 2011). Transitions between forested highland vegetation. and grassland-dominated regions of the highlands has likely been With sedge and C3 grass phytoliths much higher for core 2A, and maintained by fire for thousands of years, as evidenced by the high with lowland arboreal phytolith values for core 1C double that for level of endemism in afromontane plants (Lillesø et al., 2011). Core core 2A, there is little doubt that core 2A phytoliths at the northern 1C, which receives runoff from lower elevations and generally more end of Lake Malawi are recording more of a mesic highland vege- arid-adapted vegetation types, shows no obvious trends of tation signal and core 1C phytoliths more of an arid lowland increasing aridity across the entire record, but it does exhibit more vegetation signal. Core 1C grass phytolith concentrations on variability in mesic versus xeric grasses and an increase in locally average were three times higher than for core 2A, which is likely derived charcoal in the upper portion of the record. In summary, it due to the fact that core 1C is exposed to a much larger area of the appears that NH cooling during the transition from GI-20 to GS-20 Lake Malawi watershed where grasses are a major or dominant is correlated with decreasing tree cover, increasing aridity, component of the landscape. increasing grassland fire frequency, and increasing climate vari- Bremond et al. (2008) found that for Lake Masoko, Mt. Rungwe, ability overall. and Mt. Kenya, Ic phytolith index values increased linearly with increasing elevation (r2 ¼ 0.92), thus tracking a combination of decreasing temperature and increasing precipitation. For the Mt. 4.2. Mount Tambora eruption as an aerosol injection analog for the Rungwe and Lake Masoko regions, they found that the average Ic Toba supereruption value for semi deciduous miombo woodland soils was 2.9% (range 0e8%), for afromontane forest soils it was 37.1%, and for afro- Because of its similarity in location and likely similarity in the montane grassland soils it was 46.4%. The average Ic value for all of amount of atmospheric SO2 injection (Table 1, Fig. 1), the Indone- the core 2A samples is 22%, and the average value for all of the core sian Mount Tambora eruption of April 1815 may be a good analog 1C samples is 10%. This result again indicates that core 2A is for the Toba supereruption from an aerosol perspective. The 1815 recording more of a highland vegetation signal than core 1C. Tambora eruption is linked to the 1816 ‘Year Without a Summer’, The dashed line in Figures 5e8 represents the first occurrence of when Europe and parts of North America experienced anomalously the YTT cryptotephra, and the spike in cryptotephra following the cold and wet conditions (Stothers, 1984; Luterbacher and Pfister, first occurrence in both cores represents a peak in cryptotephra 2015). Although climate records from Africa during this time have transport from the surrounding landscape to the lake. By better not been found or reported on yet, ships' logs from the Indian constraining the Toba cryptotephra position, we can now compare Ocean recorded a temperature drop of 0.6 C from average for 1816 any pre- and post-Toba vegetation perturbations and parse them (Raible et al., 2016). Although not discussed in detail, Timmreck from longer-term climate variability trends. et al. (2012) conducted a Toba supereruption model run with a Tambora-like (~3 times Pinatubo) SO2 injection, which is more in 4.1. NH cooling and increased aridity in the Lake Malawi region line with the previously discussed Toba tuff petrological study by Scaillet et al. (1998) and the Toba tuff melt inclusion study by The 6-year-long H2SO4 spike in the GISP2 ice core places the Chesner and Luhr (2010). Using the Tambora-like SO2 loadings, they Toba supereruption towards the base of a rapid d18O descent from modeled a maximum global cooling of 0.8 ± 0.3 C that persisted a GI-20 to GS-20, and this poses a potential challenge in separating little over five years, and regional cooling for East Africa of Toba-forced from NH-forced vegetation change. Khodri et al. (2017) ~1.3 ± 1.0 C that persisted for approximately three years. Although showed that low latitude explosive volcanic eruptions can trigger not specifically discussed by Timmreck et al. (2012), the significant an El Nino~ by cooling much of Africa and reducing monsoon pre- decreases in precipitation for years 1e3 post-Toba using a 100 times cipitation in tropical Africa during the year after the eruption; Pinatubo forcing would have been muted using the Tambora-like however, this would be a short-term forcing. Tierney et al. (2008), SO2 loadings, but this is not a simple linear relationship. 88 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94

4.3. Limited climatic response to the Toba supereruption in the Lake showed that when the volcanic models are run under glacial Malawi region background conditions, the effect of the Toba eruption on climate is significantly reduced. Thus, future Toba eruption model runs The strongest evidence for a Toba supereruption forced vege- should include glacial (stadial) or transition to glacial background tation response is the prominent spike in phytolith concentration at conditions. 26.759 mblf in core 2A, which is coeval with a prominent spike in local and regionally sourced charcoal concentrations. It should be 4.4. Evidence for human occupation near Lake Malawi before and noted that there is one other local charcoal spike of similar after the Toba eruption magnitude ~200 years after the Toba eruption, and a regional charcoal spike of similar magnitude ~100 years before the Toba Archaeological surveys are yielding an increasing number of eruption, so the local charcoal spike within the Toba cryptotephra Middle Stone Age (MSA) sites that demonstrate the presence of zone is not unique (Fig. 7f). There is a corresponding charcoal spike AMH in the greater Lake Malawi region for at least the last 100,000 in the 1C core; however, it is well within normal background years (Table 3, Fig. 2); however, there is a paucity of sites that un- charcoal values for the core. Core 1C did yield a spike in C4 xero- equivocally straddle the Toba supereruption. This may result from fi phytic grasses concomitant with the rst occurrence of Toba the fact that at the time of the Toba eruption, Lake Malawi was cryptotephra, indicating an increase in aridity and a rise in the ~20e40 m shallower than today (Lyons et al., 2015; Johnson et al., abundance of more drought adapted grasses in the lower elevations 2016), thus near shore sites at the time of Toba are likely sub- of the basin. A reduction in bamboo phytoliths that starts within merged. In addition, many sites that could contain MSA compo- the Toba cryptotephra zone is a strong indication for higher nents contemporaneous with the Toba eruption await further elevation afromontane vegetation perturbation. Mountain bamboo excavation and chronological control (Thompson et al., 2016). The is sometimes a dominant grass in afromontane forest openings closest well-dated MSA site that straddles the Toba eruption is the between 2200 and 2500 m elevation on Mt. Rungwe (Bremond cave site of Ngalue, located ~42 km east of the southern basin of et al., 2008). Lake Malawi (Mercader et al., 2009a). This site yielded evidence of Although there had not been a charcoal concentration peak of multiple human occupations of unknown duration that span from similar magnitude since ~100 years before and ~200 years after the 105 to 42 ka. Toba eruption, the fact that similar charcoal peaks have occurred Another reason for the paucity of MSA sites that date to ~74 ka is within the analysis period precludes the labeling of the peak as a that AMH density in Africa was likely very low at the time of the ‘ ’ smoking-gun . However, the charcoal peak within the Toba cryp- Toba eruption. Using full-genome sequence data, Kidd et al. (2012) totephra zone occurred in only a single sample, and stands out from estimated an African human effective population (Ne)1 peak of adjacent samples that exhibit some of the lowest charcoal con- 16,000 at ~150 ka that then declined to ~10,000 at around the 74 ka centrations of the entire record. The other prominent charcoal Toba eruption, and continued to decline to ~5000 by 30 ka. Li and spikes tend to be associated with multidecadal periods of increased Durbin (2011) estimated similar Ne values that peak at 16,100 fi fi re activity. Thus, this re event appears to have taken place within around 150 ka but had diminished to ~13,000 by the time of the fi a ~100-year period of relatively low re activity. Toba eruption. Our combined phytolith and charcoal data from the two cores Since these genetic based estimates of population would likely suggest that the Toba eruption had a much greater effect on higher be distributed over all habitable areas of Africa, population den- elevation afromontane forest and grassland vegetation than on sities would have been very low. In a survey of 32 global hunter- lower elevation woodlands, and that even a moderate drop in gatherer populations, mean band size (residential unit) averages temperatures coupled with nighttime lows, especially at eleva- to 28 individuals (Hill et al., 2011; Henn et al., 2012). The Baka tions above 1500 m, was enough of a shock to kill cold sensitive hunter-gatherers of Cameroon have a population density of 0.47 plants. This, coupled with the likelihood of a year or two of people per km2 (Verdu et al., 2010), and the population density of decreased precipitation, produced a woody debris fuel load that western Pygmies in the Mbaiki area of the Central African Republic fi fl eventually burned. Severe re would have brie y increased is even lower at 0.14 people per km2 (Henn et al., 2012). Human sediment yield and marsh progradation, which is supported by a density modeling by Timmermann and Friedrich (2016) indicates spike in sedge phytoliths. For lower elevations, xerophytic C4 that much of eastern Africa could have supported up to 0.28 in- grasses increased in abundance at the expense of mesophytic C4 dividuals per km2 at the time of the Toba eruption. grasses. Increased precipitation modeled for years 3e5post-Toba, even if reduced in intensity, transported phytoliths released from decomposed and burned plant material, charcoal, topsoil, and 4.5. The effects of the Toba eruption on plant resources and human landfall Toba tephra into Lake Malawi sediments. Given our ~8e9 survival year continuous sampling interval, it is not surprising that all of fi the evidence for this scenario was deposited within two of our Our ndings indicate that the Toba eruption had a limited ef- sampled intervals. fect on plant resources within the Lake Malawi watershed. It ap- Had the Toba supereruption occurred during one of the warm pears that higher elevation afromontane forests were most Greenland interstadials (DansgaardeOeschger [DeO] events) and impacted and lower elevation grassy woodlands were largely not midway between the transition from a warm GI-20 to a cold unaffected. Rapid cooling followed by arid conditions may have GS-20, vegetation might have been more profoundly affected, as a killed some proportion of cold and moisture sensitive vegetation significant proportion of the cold and moisture sensitive vegetation above 1500 m, which subsequently burned and was replaced by C3 may have already been replaced by the time of the Toba eruption. and C4 grasses and sedges. The highly patchy mosaic of vegetation Millennial-scale NH cooling is linked to increased aridity in East Africa (Tierney et al., 2008), so drought sensitive vegetation may 1 have already been gradually dying, decomposing and burning prior Estimates for Ne use assumed mutation rates, generation time, and other pa- rameters (Li and Durbin, 2011), and include mostly modern population genomics to the Toba eruption. Most volcanic-forced climate models used for that have experienced significant turnover and mixing. Thus, Ne estimates are investigating the Toba eruption use either present-day or pre- considered preliminary and will likely change as additional ancient DNA becomes industrial background climate parameters. Timmreck et al. (2012) available. C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 89

Table 3 Selected Paleolithic localities and sites from the Lake Malawi region.

Map# Locality/site name Proximity to modern lake Location (Lat, Long) Chronology (age range) Reference

1 Songwe River Valley ~150 km NW 8.716667, 33.116667a MSA, LSA, Iron Age Willoughby and Sipe, 2002 2 Kafula Ridge West ~7 km W 9.7632186, 33.818381 MSA Thompson et al., 2010 3 Mwanganda's Village ~6 km W 9.940956, 33.890729 MSA (~42e22 ka) Wright et al., 2014 3 Airport Site ~5 km W 9.954392, 33.893016 MSA Thompson et al., 2014 3 CHA-II ~5 km W 9.955, 33.892 MSA (~65e17 ka) Wright et al., 2017 4 Ruasho Catchment ~5 km W 10.007420, 33.935695a MSA Thompson et al., 2014 5 Remero Catchment ~5 km W 10.160397, 33.966707a MSA Thompson et al., 2014 6 Ngara-I ~3 km W 10.239652, 34.083460 MSA Thompson et al., 2016 6 White whale ~4 km W 10.275133, 34.034740 MSA Thompson et al., 2016 6 Wovwe Fork ~13 km W 10.335643, 34.104536 MSA Thompson et al., 2016 7 Vintukuthu ~6 km W 10.414440, 34.195838 MSA Thompson et al., 2016 8 Misse ~15 km E 12.342667, 34.850552a MSA, LSA Gonçalves et al., 2016 9 Lunguíce ~12 km E 12.475373, 34.808078a ESA, MSA Gonçalves et al., 2016 10 Kalambo Falls ~300 km NW 8.591947, 31.242603 Acheulean, MSA, LSA, Iron Age Duller et al., 2015 11 Isimila ~250 km NE 7.896667, 35.603333 Acheulean, MSA, LSA Cole and Kleindienst, 1974 12 Mikuyu Lakeshore 12.724067, 34.825083 MSA (~29 ka) Mercader et al., 2012 12 Mvumu Lakeshore 12.734983, 34.81915 MSA (~29 ka) Mercader et al., 2012 12 Ngalue ~42 km E 12.858617, 35.198367 MSA (cave; 105e42 ka e multiple habitations) Mercader et al., 2009a

Abbreviations: ESA ¼ Early Stone Age; MSA ¼ Middle Stone Age; LSA ¼ Late Stone Age. a Approximate location. seen today in the Lake Malawi region (Fig. 2) likely existed there at and 50 ka. What is clear is that OOA dispersal events were complex 74 ka, but may have been comprised of different combinations of and involved interbreeding between AMH and archaic humans. taxa in the past, as plants migrated along elevational gradients in The strongest fossil evidence for the presence of AMH outside of response to changes in precipitation (Beuning et al., 2011; Ivory Africa pre-Toba is from the Levant sites of Skhul (~120e90 ka) and et al., 2012, 2016). The high level of endemism in afromontane Qafzeh (~100e90 ka; Groucutt et al., 2015). Stone tool assemblages plants today (Lillesø et al., 2011) suggests species continuity in the found with AMH at these sites disappeared by ~75e70 ka (Shea, Lake Malawi region for thousands of years or longer. There is no 2008). This apparent extinction of AMH in the Levant has been evidence that net primary productivity was significantly reduced, linked by some to rapid climatic deterioration associated with the especially in the lower elevation woodlands. Topographic Toba eruption and the onset of the Marine Oxygen Isotope Stage 4 complexity and plant species richness within the Lake Malawi (MIS 4) cooling event (Shea, 2008; Petraglia et al., 2010). While watershed would have allowed humans, browsers, and grazers some have considered the early Levant occupation to be an ulti- ample resources for subsistence during periods of abrupt climate mately ‘failed’ OOA event, cranial comparisons suggest that the change, and for plants to migrate along elevational gradients in Skhul and Qafzeh fossils are related to the ancestral population of response to long term climate change. With the exception of any Australians (Schillaci, 2008; Reyes-Centeno et al., 2015). There is afromontane trees that produce edible fruits and nuts, lower fossil evidence from southern China that dates to 120e80 ka (Liu elevation trees, grasses and sedges that can produce edible fruits, et al., 2017); however, there are questions about the reliability of seeds, starchy roots, and fresh shoots were either unaffected or the age determinations (Michel et al., 2016). AMH teeth recovered actually increased (sedges) in relative abundance. Within the from Lida Ajer cave on Sumatra, located just 300 km southeast of southern watershed of Lake Malawi, the MSA cave site at Ngalue the Toba caldera, have been dated to 73e63 ka (Westaway et al., (Mozambique) yielded grinding tools dominated by grass seed 2017), suggesting a possible pre-Toba AMH exit from Africa. Other starch, but also some starches from nuts, roots and other plant pre-Toba sites in the Arabian Peninsula (Armitage et al., 2011) and tissues as early as 105 ka (Mercader, 2009). Many of those starches India (Petraglia et al., 2007; Haslam et al., 2010) indicate likely AMH were linked to plants that occur today in modern miombo occupations based on the presence of Middle Paleolithic stone as- woodlands from Mozambique, and demonstrate the utilization of semblages with affinities to MSA assemblages in Africa, but these both above ground and underground edible plant tissues. Given sites are considered equivocal by some since they lack direct AMH the low levels of vegetation perturbation associated with the Toba fossil evidence (Balter, 2010). eruption and the likelihood for low human population densities, it Genetic evidence for the timing of OOA, for both single and seems unlikely that edible resources would have been made un- multiple migration models, includes both pre- and post-Toba available or limiting to humans living in the Lake Malawi region divergence times when the span of uncertainty is considered and beyond as a consequence of the Toba eruption. (Lippold et al., 2014; Karmin et al., 2015; Pagani et al., 2015). Analysis of ancient mitochondrial DNA places OOA at 79 ka with an 4.6. Pre- or post-Toba AMH migration out of Africa uncertainty range of 63e95 ka (Fu et al., 2013). Tassi et al. (2015) provided support for an early wave of migration along the south- Whether AMH dispersed out-of-Africa (OOA) before or after the ern route to Oceania, with Australo-Melanesian divergence times Toba supereruption has implications on the magnitude and loca- dating to pre-Toba even when including the uncertainty ranges. tion of any reductions in human populations. However, a Tambora- More recently, Pagani et al. (2016) estimated the timing of an early like climate forcing would likely have resulted in dramatic popu- OOA wave at ~120 ka. lation reductions only within a few to several hundred km from the Timmermann and Friedrich (2016), using a numerical human eruption. As summarized by Nielsen et al. (2017), there remains dispersal model forced by spatiotemporal estimates of climate and persuasive evidence for both sides of several leading OOA hy- sea level changes over the past 125 ka, identified three favorable potheses, including support for both single and multiple dispersals pre-Toba OOA migration windows at >125e120 ka, 106e94 ka, and OOA, which may have taken place along northern or southern (or 89e73 ka, and two later windows at 59e47 ka, and 45e29 ka. The both) dispersal routes, sometime or multiple times between 100 modeled data are in good agreement with pre-Toba paleoclimate 90 C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 proxy data indicating peak humid periods in North Africa and the floresiensis in Indonesia after the Toba supereruption (Malaspinas Levant from ~135 to 115 ka and again from 100 to 75 ka (Blome et al., 2016; Sutikna et al., 2016) indicate that archaic hominin et al., 2012), in the Horn of Africa from 130 to 80 ka and again populations were able to survive the eruption. from 57 to 50 ka (Tierney et al., 2017), and along the ‘Tabuk Corridor’ between the Levant and the Arabian interior from 137 to 4.7. A falsified Toba catastrophe hypothesis 114 ka and again from 85 to 71 ka (Breeze et al., 2016). 4.6.1. Scenario 1: Out of Africa post-Toba There is evidence based Since the publication of Ambrose (1998), the Toba supereruption on large modern eruptions to suggest that the Toba supereruption and its proposed 6-year-long volcanic winter continues to be cited may have caused a southward migration of the Intertropical repeatedly, particularly in introductory paragraphs, as the natural Convergence Zone (ITCZ), resulting in reduced rainfall at low lati- catastrophe that brought humanity to the brink of extinction (hu- tudes of the NH and a general weakening of the African monsoon man populations reduced to 10,000 individuals). Recent studies system (Baldini et al., 2015). Climate models previously discussed in have clearly shown that volcanic winter conditions never occurred the Introduction indicate a significant decrease in precipitation in in East Africa after the eruption (Lane et al., 2013a; Jackson et al., Africa for years 1 and 2 after the eruption, and our data do 2015), and we have shown that there was a very limited vegeta- indicate a brief aridity event immediately after the Toba eruption. tion perturbation in the Southern Rift Valley of East Africa after the However, given that the likelihood that the Toba supereruption, eruption. Further, we demonstrated the overestimation of SO2 in- from an aerosol forcing perspective, was on the same order of jections in Toba supereruption climate model simulations by one or magnitude as the 1815 Tambora eruption that produced ~1.1 C two orders of magnitude. This overestimation includes the early global cooling, human populations in Africa would have passed models of Rampino and Self (1992) that helped to build the volcanic through this period of moderate and short-term cooling and winter model proposed in Ambrose (1998). The hypothesis that aridity relatively unscathed. A human genomic study by Mallick Toba triggered the 1000-year GS-20 cold period is also unlikely to et al. (2016) yielded no evidence of an African population be correct given that rapid cooling in the NH actually started a few bottleneck that coincided with the Toba supereruption. This is not hundred years before the Toba eruption, not to mention the fact to say that populations further away from the equator at latitudes that modeling by Robock et al. (2009) using a 900 Pinatubo SO2 with more moisture and temperature variability would not have injection failed to initiate NH glaciation. been negatively affected to some degree. However, a small Numerous genetic analyses have not detected a bottleneck that population loss within one generation would not have left a coincides with the Toba eruption. In fact, if the source population detectable reduction in genetic diversity (Rogers and Jorde, 1995). for the OOA expansion suffered a severe bottleneck, there should be 4.6.2. Scenario 2: Out of Africa pre-Toba The argument for a pre- a poorer linear fit to the decline of heterozygosity with distance Toba (~74 ka) dispersal out of Africa is still valid and perhaps from Africa (Henn et al., 2012). With the advancement of whole gaining ground (Tassi et al., 2015; Pagani et al., 2016; Clarkson genome sequencing, the once elusive 100e50 ka Late Pleistocene et al., 2017). With an optimal climatic window from ~130 to 80 ka human genetic bottleneck is now converging on ~50 ka (Lippold (Blome et al., 2012; Tierney et al., 2017), small populations of et al., 2014; Karmin et al., 2015; Malaspinas et al., 2016) and is AMH could have been scattered along a route from Arabia to being attributed to an OOA founder effect bottleneck (Mallick et al., India, Southeast Asia, and Indonesia. The 1815 Tambora eruption 2016) instead of a population reduction bottleneck. Studies produced 5e90 cm of ashfall and dark sky conditions up to focusing on reconstructing population histories are identifying a 500 km away, and a 1 m high tsunami was reported over possible population reducing bottleneck between ~150 and ~130 ka 1000 km away (Stothers, 1984). Although likely similar in the (Li and Durbin, 2011; Kidd et al., 2012), which coincides with the amount of aerosol injected into the atmosphere, the Toba penultimate ice ace during MIS 6. However, the peak in Neat eruption ejected ~100 times more magma than Tambora, so the ~150 ka could have also arisen from increased genetic diversity due Toba ashfall and tsunami effects would have been significantly to population structure involving separation and admixture (Li and more impactful than Tambora, as evidenced by the deposition of Durbin, 2011), which is reasonable to expect during a cooler and ~5 cm of ashfall over much of India (Costa et al., 2014) and up to drier MIS 6 climate in Africa. The hypothesis that human pop- several meters of reworked ash in alluvial sediments along the ulations were reduced to 10,000 individuals after the Toba eruption east coast of India (Oppenheimer, 2002). Jones (2012) is currently unsupported, as AMH populations were always rela- hypothesized that the Toba eruption had highly varied impacts tively low, started to decline around 150 ka, and continued to on hominin populations throughout India, with localized decrease until ~30 ka (see Discussion above). As paleoenvir- population extinctions in some areas and population survival and onmental, archaeological, and genetic research continues to accu- continuity in other areas. Several studies examining Middle mulate, it is becoming increasingly hard to find evidence in favor of Paleolithic occupations attributed to AMH in India (Haslam et al., the Toba catastrophe hypothesis. 2010; Clarkson et al., 2012) reported cultural continuity before and after the Toba eruption. However, tribal populations from the 5. Conclusions eastern coast of India predominantly exhibit unique M haplogroups and lack N subgroups, consistent with local drift Our phytolith and charcoal analysis of continuously sampled resulting from near extinction (Oppenheimer, 2012). Interestingly, lake sediments of subdecadal resolution for ~100 years before and at least 2% of the modern Papuan genome reflects an earlier, >200 years after the Toba supereruption found no evidence of a otherwise extinct, OOA dispersal (Pagani et al., 2016). volcanic winter event or of a major vegetation perturbation for the There is little doubt that AMH living in relatively close proximity Lake Malawi watershed that would have been catastrophic to hu- to the Toba eruption would have been severely impacted, possibly man survival. These results are in agreement with recent paleo- leading to the disappearance of early OOA genetic lineages in parts temperature Lane et al. (2013a) and paleolimnological (Jackson of Indonesia and a genetic bottleneck in India. However, pre-Toba et al., 2015) studies that came to similar conclusions but did not AMH populations in much of South Asia, as well as those in Af- address terrestrial paleoecological conditions in Africa around the rica and elsewhere would likely have experienced little to no Toba eruption. A review of previously published Toba tuff melt population change associated with the Toba supereruption. After inclusion studies and the GISP2 ice core H2SO4 record suggest a 50 all, the persistence of Denisovans in southern Asia and Homo to 100 Mt SO2 atmospheric injection from the eruption, which is C.L. Yost et al. / Journal of Human Evolution 116 (2018) 75e94 91 equivalent to a 3 to 6 times Pinatubo (Tambora-like) eruption; Ambrose, S.H., 2003. Did the super-eruption of Toba cause a human population however, most Toba-forced climate models use SO injections that bottleneck? 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