Supplementary Table 6. Results of 40Ar/39Ar dating of single crystals of anorthoclase from pumice clasts in the Moiti Tuff, Moiti Member, Koobi Fora Formation, Turkana, northern Kenya	

Sample	
36Ar/39Ar
x 10-4	
37Ar/39Ar
x 10-3	
40Ar/39Ar	
K/Ca	
40Ar*/39ArK
± c.v.	
40Ar* %	
39Ar
10-14 mol	
Calculated age
Ma ± 1 s.d.	
									
93-1081B anorthoclase; J = 1.195E-3 ± 0.30%; level 12, ANU4 	
	   2	40.157 ±  1.82%	6.704 ±  1.89%	3.067 ± 0.14%	79	1.858 ± 1.19%	60.6	1.93	4.002 ± 0.049	
	  [3	3.073 ±  4.13%	10.625 ±  1.60%	2.043 ± 0.14%	50	1.931 ± 0.24%	94.5	2.94	4.159 ± 0.016]	
	   4	2.213 ±  5.98%	15.621 ±  1.68%	1.936 ± 0.13%	34	1.849 ± 0.25%	95.5	2.85	3.983 ± 0.016	
	   5	1.716 ±  9.53%	15.302 ±  1.23%	1.931 ± 0.13%	34	1.860 ± 0.29%	96.3	3.23	4.005 ± 0.017	
	   6	3.605 ±  3.91%	6.898 ±  1.54%	1.979 ± 0.14%	76	1.850 ± 0.27%	93.5	2.72	3.985 ± 0.016	
	   7	5.493 ±  4.15%	106.210 ±  0.68%	2.038 ± 0.20%	5	1.864 ± 0.42%	91.5	2.31	4.014 ± 0.021	
	   8	2.463 ±  9.55%	8.275 ±  2.78%	1.930 ± 0.26%	64	1.835 ± 0.47%	95.1	1.64	3.953 ± 0.022	
	   9	2.151 ± 10.28%	9.748 ±  0.69%	1.950 ± 0.14%	54	1.865 ± 0.38%	95.6	1.43	4.017 ± 0.019	
	10	3.967 ±  6.33%	5.490 ±  5.35%	1.986 ± 0.16%	96	1.846 ± 0.44%	93.0	1.37	3.976 ± 0.021	
	11	4.746 ±  6.72%	10.608 ±  1.76%	2.003 ± 0.15%	50	1.841 ± 0.54%	91.9	1.37	3.964 ± 0.024	
    12	2.407 ±  4.45%	14.897 ± 10.11%	1.951 ± 0.18%	35	1.859 ± 0.25%	95.3	8.96	4.004 ± 0.016	
	Arithmetic mean K/Ca (n = 10)	      53 ± 27	Arithmetic mean age (n =10)	3.990 ± 0.022	
					              Weighted mean age (n =10)
	3.992 ± 0.013	
93-1081C anorthoclase; J = 1.188E-3 ± 0.30%; level 13, ANU4	
	  1	4.145 ± 12.19%	-	1.993 ± 0.25%	-	1.849 ± 0.85%	92.8	0.89	3.959 ± 0.036	
	  2	12.698 ±  2.81%	5.450 ± 46.14%	2.260 ± 0.22%	97	1.864 ± 0.63%	82.5	1.55	3.991 ± 0.028	
	  3	7.274 ±  5.75%	8.950 ± 26.45%	2.090 ± 0.22%	59	1.855 ± 0.71%	88.7	1.16	3.971 ± 0.031	
	  4	13.910 ±  3.38%	-	2.280 ± 0.15%	-	1.847 ± 0.77%	81.0	0.81	3.955 ± 0.033	
	  5	2.728 ± 14.07%	-	1.958 ± 0.18%	-	1.855 ± 0.64%	94.8	0.81	3.974 ± 0.028	
	 [6	3.150 ± 20.08%	3.523 ± 70.65%	2.027 ± 0.17%	149	1.913 ± 0.99%	94.4	0.79	4.096 ± 0.042]	
	  7	2.087 ±  9.72%	5.390 ± 14.80%	1.944 ± 0.17%	98	1.861 ± 0.37%	95.7	2.97	3.986 ± 0.019	
	  8	2.621 ± 18.52%	1.184 ± 99.0%	1.957 ± 0.16%	445	1.858 ± 0.79%	94.9	1.79	3.980 ± 0.034	
	 [9	2.813 ±  5.02%	1.482 ± 99.5%	1.996 ± 0.22%	355	1.892 ± 0.32%	94.8	1.60	4.051 ± 0.018]	
	10	8.288 ±  3.63%	12.838 ± 21.13%	2.098 ± 0.27%	41	1.833 ± 0.57%	87.4	1.01	3.925 ± 0.025	
	Arithmetic mean K/Ca (n = 5)	148 ± 168	Arithmetic mean age (n = 8)	3.968 ± 0.021	
					                       Weighted mean age (n = 8)	3.971 ± 0.015	
									
93-1081A anorthoclase; J = 1.190E-3 ± 0.30%; level 10, ANU4	
	   1	7.782 ±  5.49%	8.663 ±  1.98%	2.091 ± 0.29%	61	1.837 ± 0.76%	87.9	1.14	3.940 ± 0.032	
	   2	11.962 ±  2.63%	7.151 ±  5.31%	2.187 ± 0.31%	74	1.810 ± 0.64%	82.7	1.20	3.882 ± 0.027	
	   3	5.935 ±  5.92%	5.118 ±  2.24%	2.048 ± 0.30%	103	1.848 ± 0.65%	90.3	1.04	3.964 ± 0.028	
	   4	5.656 ±  4.00%	6.345 ±  1.01%	2.040 ± 0.21%	83	1.849 ± 0.43%	90.6	1.48	3.966 ± 0.021	
	   5	3.734 ±  6.24%	8.484 ±  2.25%	1.974 ± 0.47%	62	1.840 ± 0.63%	93.2	1.63	3.946 ± 0.027	
	   6	4.030 ±  4.98%	6.220 ±  2.08%	2.005 ± 0.15%	85	1.862 ± 0.36%	92.9	2.05	3.993 ± 0.019	
	   7	3.245 ±  5.43%	6.966 ±  3.60%	1.982 ± 0.13%	76	1.862 ± 0.31%	94.0	2.33	3.993 ± 0.017	
	   8	13.452 ±  3.52%	6.946 ±  1.31%	2.269 ± 0.25%	76	1.848 ± 0.82%	81.4	1.50	3.963 ± 0.034	
       9	14.445 ±  2.63%	6.346 ±  2.54%	2.272 ± 0.16%	83	1.822 ± 0.65%	80.2	1.10	3.907 ± 0.028	
	Arithmetic mean K/Ca (n = 9)	78 ± 13	Arithmetic mean age (n = 9) Weighted mean  age (n = 9)  	3.950 ± 0.037
3.966 ± 0.014	

 = 5.543 x 10-10 a-1	Sensitivity 2.5 (± ~15%) x 10 –17 mol/mV      Bracketed results excluded from age calculation
(36Ar/37Ar)Ca = 3.50 x 10-4	                     (39Ar/37Ar)Ca = 7.86 x 10-4                  (40Ar/39Ar)K =  0.022 to 0.025 depending on position
40Ar* - radiogenic argon		39ArK – K-derived 39Ar	 Mass of feldspar used in each analysis in range 0.7 to 2.8 mg
Irradiated for 48 hours in X33 or X34, HIFAR nuclear reactor
Fluence monitor: 92-176 sanidine from Fish Canyon Tuff; reference age 28.1 Ma
All errors are one standard deviation. Uncertainty on the 40Ar*/39ArK ratio is the analytical error. This error is combined quadratically with the estimated error of J to yield the quoted error for each age