Analyses of rock varnish samples from latest Pleistocene alluvial-fan surfaces in Las Vegas Valley, southern Nevada, reveal replicable lamination patterns that are characterized by low-Mn orange surface layers and high-Mn dark basal layers. Radiocarbon dating from beneath the sampled alluvial-fan surfaces suggests that the Mn-rich basal layers accumulated during a short wet phase 10-11 C-14 ka when extensive black mars were deposited throughout the region, and paleolake records in the Great Basin also indicate wet conditions during this time period. In contrast, the Mn-poor orange surface layers formed under relatively dry conditions in the Holocene. Thus, these varnish microlaminations are connected with environmental fluctuations that appear to be related to climate change. Evidence from Las Vegas Valley, together with that from Death Valley and the Mojave Desert, suggests that the deposition of these Mn-rich dark basal layers in rock varnish likely corresponded in time to the terminal Pleistocene Younger Dryas-aged wet event in the Great Basin. (C) 2000 Elsevier Science B.V. All rights reserved.
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