Supplementary Table 8. Results of laser fusion 40Ar/39Ar dating of anorthoclase crystals from small pumice clasts in the Tulu Bor Tuff, Tulu Bor 

Member, Koobi Fora Formation,  northern Kenya. 00-294 (K00-6128), Area 206, Allia Bay, 3˚39.92’N, 36˚15.0’E (WGS84), 00-301A, B 
(K00-6149), Area 207, Il Toroto, 3˚43.72’N, 36˚24.65’E (WGS84)

Sample 	36Ar/39Ar
x 10-4	37Ar/39Ar
x 10-1	40Ar/39Ar	K/Ca	40Ar*/39ArK
± c.v.	% 40Ar*	39Ar
10-15 mol	Calculated age
Ma ± 1 s.d.	
									
00-294 anorthoclase; J = 4.269E–4 ± 0.40%; level 3, ANU 80	
	1	3.042 ± 33.44%	1.037 ± 1.66%	4.717 ± 0.16%	5.1	4.594 ± 0.68%	97.4	4.27	3.535± 0.028	
	3	28.475 ±  5.51%	5.725 ± 0.93%	5.459 ± 0.26%	0.9	4.635 ± 1.05%	84.9	2.03	3.567 ± 0.040	
	4	6.981 ± 10.71%	2.706 ± 0.58%	4.857 ± 0.22%	1.9	4.636 ± 0.53%	95.4	4.83	3.567 ± 0.024	
	5	4.396 ±  7.88%	2.074 ± 1.27%	4.679 ± 0.26%	2.5	4.528 ± 0.35%	96.8	3.27	3.484 ± 0.018	
	6	5.236 ± 11.87%	1.395 ± 1.03%	4.736 ± 0.21%	3.8	4.552 ± 0.46%	96.1	4.10	3.503 ± 0.021	
	7	4.732 ± 20.09%	1.833 ± 0.97%	4.738 ± 0.26%	2.9	4.574 ± 0.67%	96.5	2.44	3.519 ± 0.027	
	8	13.325 ± 17.23%	5.587 ± 1.06%	4.846 ± 0.31%	0.9	4.469 ± 1.56%	92.2	1.78	3.438 ± 0.055	
	9	7.179 ± 12.15%	1.737 ± 0.87%	4.771 ± 0.30%	3.0	4.534 ± 0.65%	95.0	3.11	3.489 ± 0.027	
	10	10.020 ± 11.92%	2.053 ± 0.82%	4.915 ± 0.17%	2.6	4.597 ± 0.79%	93.5	3.11	3.537 ± 0.031	
	11	9.120 ±  4.60%	3.078 ± 1.13%	4.786 ± 0.26%	1.7	4.506 ± 0.39%	94.1	2.34	3.467 ± 0.019	
	12	94.582 ±  2.39%	37.114 ± 0.54%	7.120 ± 0.30%	0.1	4.679 ± 1.50%	65.5	1.58	3.600 ± 0.056	
	13	10.739 ±  3.66%	1.735 ± 0.87%	4.981 ± 0.28%	3.0	4.639 ± 0.39%	93.1	3.88	3.570 ± 0.020	
	14	78.911 ±  1.68%	6.854 ± 0.44%	6.879 ± 0.16%	0.8	4.577 ± 0.89%	66.5	4.08	3.522 ± 0.034	
	15	13.024 ±  1.88%	2.062 ± 1.53%	5.050 ± 0.16%	2.6	4.644 ± 0.23%	91.9	5.81	3.573 ± 0.016	
	16	47.378 ±  2.16%	35.548 ± 0.50%	5.541 ± 0.32%	0.1	4.478 ± 0.78%	80.6	1.18	3.445 ± 0.030	
	17	17.605 ±  7.04%	7.619 ± 0.62%	4.989 ± 0.20%	0.7	4.507 ± 0.84%	90.3	2.60	3.468 ± 0.032	
	18	6.854 ±  2.62%	3.170 ± 0.67%	4.772 ± 0.20%	1.7	4.560 ± 0.24%	95.5	5.31	3.508 ± 0.016	
	                                  Arithmetic mean K/Ca (n = 17)	2.0 ± 1.3	          Arithmetic mean age (n = 17)
            Weighted mean age (n = 17)	3.517 ± 0.048
3.524 ± 0.015	
									
00-301A anorthoclase; J = 4.268E–4 ± 0.40%; level 4, ANU 80	
	1	10.745 ±  3.98%	0.508 ± 1.42%	4.875 ± 0.18%	10.4	4.520 ± 0.34%	92.7	6.95	3.477 ± 0.018	
	2	5.818 ±  3.60%	0.678 ± 1.57%	4.703 ± 0.14%	7.8	4.496 ± 0.20%	95.6	6.32	3.458 ± 0.016	
	3	7.315 ±  1.46%	0.648 ± 1.73%	4.831 ± 0.15%	8.1	4.579 ± 0.17%	94.8	7.45	3.522 ± 0.015	
	4	10.027 ±  2.37%	0.697 ± 1.47%	4.803 ± 0.15%	7.6	4.472 ± 0.23%	93.1	7.27	3.440 ± 0.016	
	5	12.534 ±  1.19%	0.633 ± 0.86%	4.932 ± 0.15%	8.3	4.526 ± 0.20%	91.8	10.75	3.482 ± 0.015	
	6	9.150 ±  2.70%	0.631 ± 0.56%	4.855 ± 0.30%	8.3	4.549 ± 0.36%	93.7	6.60	3.500 ± 0.019	
	7	4.764 ±  8.94%	0.609 ± 1.47%	4.779 ± 0.18%	8.6	4.603 ± 0.33%	96.3	5.79	3.541 ± 0.018	
	8	9.122 ±  5.42%	0.418 ± 2.60%	4.701 ± 0.27%	12.6	4.394 ± 0.44%	93.5	3.56	3.380 ± 0.020	
	9	10.716 ±  1.70%	0.462 ± 2.05%	4.843 ± 0.22%	11.4	4.489 ± 0.26%	92.7	5.82	3.453 ± 0.016	
	10	10.140 ±  3.17%	0.670 ± 1.59%	4.913 ± 0.16%	7.9	4.578 ± 0.27%	93.2	5.89	3.522 ± 0.017	
	11	12.608 ±  1.81%	0.611 ± 2.18%	4.831 ± 0.22%	8.6	4.423 ± 0.29%	91.5	4.28	3.402 ± 0.017	
	12	11.232 ±  1.53%	0.536 ± 1.94%	4.840 ± 0.21%	9.8	4.472 ± 0.25%	92.4	5.15	3.440 ± 0.016	
	13	11.971 ±  2.36%	0.695 ± 0.36%	4.918 ± 0.16%	7.6	4.529 ± 0.25%	92.1	12.86	3.484 ± 0.016	
	                Arithmetic mean K/Ca (n = 13)	9.0 ± 1.6	Arithmetic mean age (n = 13)
Weighted mean age (n = 13)	3.469 ± 0.047
3.475 ± 0.014	
									
00-301B anorthoclase; J = 4.267E–4 ± 0.40%; level 6, ANU 80	
	1	2.753 ± 31.02%	0.637 ± 1.65%	4.605 ± 0.17%	8.3	4.489 ± 0.59%	97.5	3.78	3.452 ± 0.025	
	2	4.134 ± 27.83%	0.695 ± 1.42%	4.648 ± 0.19%	7.6	4.492 ± 0.78%	96.6	3.73	3.454 ± 0.030	
	3	6.654 ± 12.09%	0.573 ± 2.00%	4.717 ± 0.21%	9.2	4.485 ± 0.57%	95.1	3.11	3.449 ± 0.024	
	4	7.109 ±  4.79%	0.622 ± 1.34%	4.707 ± 0.15%	8.5	4.462 ± 0.28%	94.8	7.95	3.432 ± 0.017	
	5	6.214 ± 10.67%	0.577 ± 1.76%	4.684 ± 0.21%	9.1	4.465 ± 0.49%	95.3	3.88	3.434 ± 0.022	
	6	3.900 ± 29.13%	0.454 ± 2.70%	4.691 ± 0.21%	11.6	4.539 ± 0.77%	96.8	3.98	3.491 ± 0.030	
	7	2.692 ± 29.48%	0.632 ± 2.20%	4.609 ± 0.21%	8.3	4.495 ± 0.56%	97.5	3.25	3.457 ± 0.024	
	8	12.026 ±  7.05%	0.497 ± 1.74%	4.834 ± 0.22%	10.6	4.443 ± 0.61%	91.9	4.34	3.417 ± 0.025	
	9	7.512 ±  3.00%	0.471 ± 2.01%	4.672 ± 0.17%	11.2	4.414 ± 0.23%	94.5	4.79	3.395 ± 0.016	
	10	7.342 ±  7.59%	0.594 ± 1.23%	4.707 ± 0.23%	8.9	4.455 ± 0.44%	94.6	4.78	3.426 ± 0.020	
	11	6.840 ± 10.86%	0.542 ± 1.59%	4.708 ± 0.21%	9.7	4.470 ± 0.54%	95.0	3.90	3.438 ± 0.023	
	12	4.415 ± 12.53%	0.517 ± 1.11%	4.658 ± 0.18%	10.2	4.491 ± 0.41%	96.4	6.72	3.454 ± 0.020	
	13	3.595 ± 25.67%	0.674 ± 1.52%	4.621 ± 0.14%	7.8	4.481 ± 0.63%	97.0	3.64	3.446 ± 0.026	
	14	8.492 ± 11.03%	0.523 ± 2.61%	4.782 ± 0.20%	10.1	4.495 ± 0.65%	94.0	3.35	3.457 ± 0.026	
	15	8.902 ±  8.62%	0.535 ± 1.75%	4.721 ± 0.14%	9.8	4.422 ± 0.54%	93.7	3.65	3.401 ± 0.023	
	16	5.994 ±  5.57%	0.535 ± 2.33%	4.668 ± 0.23%	9.8	4.455 ± 0.33%	95.4	3.70	3.426 ± 0.018	
	17	3.535 ± 14.80%	0.486 ± 2.08%	4.589 ± 0.23%	10.8	4.448 ± 0.42%	96.9	3.85	3.421 ± 0.020	
	Arithmetic mean K/Ca (n = 17)	9.5 ± 1.2	  Arithmetic mean age (n = 17)
     Weighted mean age (n = 17)	3.438 ± 0.023
3.427 ± 0.014	

l = 5.543 x 10-10 a-1 		Sensitivity 2.5 (± ~15%) x 10-17 mol/mV
(36Ar/37Ar)Ca = 3.50 x 10-4     (39Ar/37Ar)Ca = 7.86 x 10-4     (40Ar/39Ar)K = 0.0435 (00-294); 0.0424 (00-301A); 0.0412 (00-301B)
40Ar* – radiogenic 40Ar		39ArK – K-derived 39Ar	Irradiated 24 hours in X33 or X34, HIFAR reactor, end day 200, 2001
Mass of alkali feldspar used in each analysis in range: 00-294 – 0.7 to 1.25 mg; 00-301A – 0.7 to 3.3 mg; 00-301B – 0.5 to 1.6 mg
Fluence monitor: 92-176 sanidine from Fish Canyon Tuff; K-Ar reference age utilized: 28.1 Ma
For sample 00-294 analyses were done on single crystals except for analyses 6 and 8 to 12 for which 2 crystals were used; analyses 15 and 16, 3 crystals were used and for analyses 13, 14, 17 and 18, 4 crystals were used. For sample 00-301A analyses on single crystals, except for analysis 12, done on 2 crystals.  For sample 00-301B, analyses 1 to 7 were done on single crystals, analyses 8 to 17 were done on 2 crystals
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