"Mike Baillie is Professor of Palaeoecology at Queens University, Belfast, Northern Ireland. He is an authority on tree rings and their use in dating ancient events (every year, a tree adds a "ring" to its trunk as it grows - good years are represented by thick rings while bad years are represented by thin rings).
He conducted a complete (and continuous) review of annual global tree growth patterns over the last 5,000 years and found that there were five major environmental shocks that were witnessed worldwide. These shocks were reflected in the ring widths being very thin. Wanting to know more, he turned to human historical records, and found that the years in question (between 2354 and 2345 BC, 1628 and 1623 BC, 1159 and 1141 BC, 208 and 204 BC, and AD 536 and 545) all corresponded with "dark ages" in civilisation. "
Your post does not imply that there aren’t still bodies of floating tree-ring data that have yet to be anchored.