This is what the paper says:
" We can infer the date of admixture by modeling the decay of LD between ancestral chunks, which decreases more rapidly the longer ago admixture occurred." I didn't find anything more specific.
http://www.cell.com/cms/attachment/2036746823/2051577514/mmc1.pdf
From page 23 of the supplement, this is what they see as the "challenges" (among others) for their program:
3.even in theory our approach finds it challenging to distinguish distinct continuous “pulses” of admixture and continuous migration over some time frame, because of the difficulty of separating exponential mixtures [S? If the time frame were narrow, we expect to infer a single admixture time within the range of migration dates. Where we infer two admixture dates, in particular with the same source groups, the exponential decay signal could also be consistent with more continuous migration, and so we conservatively refer to this as admixture at multiple dates".
Oh, I forgot to mention this doozy of a comment, after all these graphs and percentages etc:
"When we compare the relative differences between pre- and post-admixture groups, we observe no appreciable difference between them, suggesting that admixture has not had a significant impact on genetic variation in West Eurasia (
Figure 4G)."
HUH? Then what was this all about, and how valid were their "admixture" numbers?