Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka

Christine S. Lane; Ben T. Chorn; Thomas C. Johnson

doi:10.1073/pnas.1301474110

New Research In

Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved March 15, 2013 (received for review January 23, 2013)

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Toba supereruption: Age and impact on East African ecosystems - June 21, 2013

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Subdecadal climate record of the Toba eruption
- Jun 21, 2013

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Fig. 1.
Bathymetric map of Lake Malawi and overview map of YTT distribution, showing sites mentioned in the text. (A) Bathymetric map of Lake Malawi showing central GLAD7-MAL05-1 and northern GLAD7-MAL05-2 core locations (25). (B) Occurrences of the YTT across the Indian Ocean region, southern Asia, and the South China Sea (1, 3, 5–8, 12, 23). Lake Malawi, the Rungwe Volcanic Province, and other volcanoes mentioned in the text are marked.
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Fig. 2.
The position of the YTT cryptotephra layer in MAL05-1C and sediment composition data. (A) The distribution of YTT glass shards in GLAD-MAL05-1C. The number of volcanic glass shards (shards/gram of sediment) was counted in 10-cm (gray bars) then 1-cm (black bars) depth samples. A peak in concentration is visible at 28.10 MBLF. (B) The YTT cryptotephra layer is not visible in a SEM image (backscatter electron mode) of a thin section from the core between 28.04 and 28.11 MBLF; however, the image confirms the lack of bioturbation or disturbance in the core at this depth and is evidence for anoxic bottom water conditions at this time. The white layer above 28.06 MBLF is a turbidite layer not related to the YTT. (C) Scanning XRF data across the same interval also shows no significant perturbations in either composition or coherence of the sediment. Gray shading denotes the 2 cm of sediment containing YTT shards, 28.10–28.08 MBLF.
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Fig. 3.
Chemical and visual characteristics of YTT glass shards. (A) SEM image of glass shards from the YTT layer in MAL05-1C. Note the dominance of platy, bubble wall shards as well as one with some elongate vesicles (Right). (B) Chemical correlation of glass shards from cryptotephra layer in MAL05-1C (28.10 MBLF) with both proximal (Sumatra) and distal (India) deposits of the YTT (29). Glass compositions of the YTT (open squares) are clearly distinct from tephra of the Rungwe Volcanic Province (26) and of the Olkaria (27) and Eburru (28) volcanoes, Kenya, which have produced rhyolitic eruptions during the last 150 ka._. Uncertainty for data generated in this study, calculated from 2 SD precision of secondary standard analyses, is contained within the YTT symbols.
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Fig. 4.
The revised age-depth model for MAL05-1C. A revised Bayesian age-depth model for the upper 30 m of MAL05-1C based on 15 radiocarbon dates (24) and the age of the YTT: 75 ± 0.9 ka B.P. (4). Gray shading denotes modeled 95% confidence intervals. Beyond 30 m depth, no secure independent age estimates are available and a tentative age–depth relationship is projected to 40 m assuming linear sedimentation (dashed lines). The previous MAL05-1C age model (24) is shown for comparison. For details of model construction, see Materials and Methods.

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Table 1.

Chemical composition of YTT glass shards in Lake Malawi, compared with proximal samples from the Toba caldera in Sumatra, Indonesia and YTT ash from the Indian archaeological site of Jwalapuram (29)

Sample	SiO₂	TiO₂	Al₂O₃	FeO	MnO	MgO	CaO	Na₂O	K₂O	Total^*
Cryptotephra layer, MAL05-1C 28.10 MBLF
Average (n = 18)	77.24	0.05	12.41	0.84	0.07	0.05	0.77	2.95	5.61	100
2 SD	1.79	0.04	0.31	0.14	0.08	0.06	0.19	0.37	0.45
Cryptotephra layer, MAL05-2a 26.78 MBLF
Average (n = 9)	77.22	0.04	12.32	0.84	0.07	0.05	0.78	3.2	5.47	100
2 SD	0.47	0.05	0.27	0.11	0.05	0.03	0.21	0.26	0.26
YTT, Toba caldera
Average (n = 118)	77.24	0.06	12.54	0.85	0.07	0.05	0.78	3.10	5.20	100
2 SD	0.7	0.06	0.39	0.24	0.09	0.04	0.21	0.34	0.29
YTT, Jwalapuram, India
Average (n = 113)	77.36	0.06	12.49	0.87	0.07	0.07	0.75	3.25	4.93	100
2 SD	0.28	0.02	0.2	0.09	0.04	0.1	0.78	0.16	0.16

↵* All data normalized to water free compositions. Table S1 contains the full dataset and associated secondary standard analyses. For details of analytical conditions, see Materials and Methods.