Many of the largest wildfires in US history burned in recent decades, and climate change explains much of the increase in area burned. The frequency of extreme wildfire weather will increase with continued warming, but many uncertainties still exist about future fire regimes, including how the risk of large fires will persist as vegetation changes. Past fire-climate relationships provide an opportunity to constrain the related uncertainties, and reveal widespread burning across large regions of western North America during past warm intervals. Whether such episodes also burned large portions of individual landscapes has been difficult to determine, however, because uncertainties with the ages of past fires and limited spatial resolution often prohibit specific estimates of past area burned. Accounting for these challenges in a subalpine landscape in Colorado, we estimated century-scale fire synchroneity across 12 lakesediment charcoal records spanning the past 2,000 y. The percentage of sites burned only deviated from the historic range of variability during the Medieval Climate Anomaly (MCA) between 1,200 and 850 y B.P., when temperatures were similar to recent decades. Between 1,130 and 1,030 y B.P., 83% (median estimate) of our sites burned when temperatures increased ∼0.5°C relative to the preceding centuries. Lake-based fire rotation during the MCA decreased to an estimated 120 y, representing a 260% higher rate of burning than during the period of dendroecological sampling (360 to −60 y B.P.). Increased burning, however, did not persist throughout the MCA. Burning declined abruptly before temperatures cooled, indicating possible fuel limitations to continued burning.wildfire | climate change | Medieval Climate Anomaly

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Upper Cretaceous nonmarine strata in southern New Mexico mark the final phase of Late Cretaceous (Cenomanian-Campanian) sedimentation associated with the Sevier orogeny. Presented here are U-Pb detrital zircon ages, sandstone modal composition, and paleocurrent measurements from the Dakota Sandstone, Tres Hermanos Formation, and Crevasse Canyon Formation (Lower Member and Ash Canyon Member). The Dakota Sandstone is dominated by monocrystalline quartz (84%) with minor volcanic and metamorphic lithic grains (15%) with rare occurrences of feldspar (1%). Paleoflow measurements show east-directed (108°) to northeast-directed flow (50°). Detrital zircon age peaks occur at 1732, 1651, 1416, 1050, 626, 412, 230, and 103 Ma. The calculated rage of maximum depositional ages (MDAs) for the Dakota Sandstone is 103-104 Ma. The Tres Hermanos Formation is composed primarily of monocrystalline quartz (63%) with volcanic and metamorphic lithic grains (27%) and minor feldspar (10%). Paleoflow measurements reflect east-directed (93-109°) and southeast-directed (166°) flow. Peak detrital zircon ages occur at 1709, 1420, 1085, 169, and 94 Ma. MDAs for the Tres Hermanos range from 93-96 Ma. The Lower Member of the Crevasse Canyon Formation is composed of monocrystalline quartz (50%) along with volcanic and metamorphic lithic grains (41%) and minor abundance of feldspar (9%). Paleoflow measurements show primarily east-directed flow (100°). Detrital zircon age peaks occur at 1702, 1420, 1067, 167, and 91 Ma. MDAs for the Lower Member of the range from 91-92 Ma. The Ash Canyon Member of the Crevasse Canyon Formation is composed of monocrystalline quartz (48%) with volcanic and metamorphic lithic grains (46%) and minor occurrences of feldspar (6%). Paleoflow trends show a east-to southeast-directed flow (108-118°). Peak detrital zircon ages were determined to be 1682, 1415, 1108, 169 and 90 Ma. MDAs for the Ash Canyon Member range from 80-91 Ma. Precambrian to Paleozoic zircons overlap in age with the Yavapai, Mazatzal, Granite-Rhyolite, and Grenville provinces (and age-equivalent ~1.0 Ga rocks). Neoproterozoic, Early Paleozoic, and some Mesoproterozoic-age detritus was originally derived from Appalachian-Ouachita sources and transported to parts of the southwestern U.S. (e.g., Mesozoic eolianites of the Colorado plateau). Second-order recycling of Mesozoic eolianites has been reported from Lower Cretaceous strata of the Bisbee Rift which exhibit U-Pb detrital zircon spectra that very similar to Mesozoic strata of the Four Corners region. Permo-Triassic age detritus overlap in age with granitoid rocks of the Cordilleran magmatic arc that outcrop in California and Arizona. Jurassic to Cretaceous age zircons overlap with the mid-Mesozoic Cordilleran magmatic arc and the Sierra Nevada batholith. Based on the provenance trends summarized above, a sediment dispersal model is favored where the Dakota Sandstone was derived largely from recycled Lower Cretaceous strata of the Bisbee Rift of southeastern Arizona and southwestern New Mexico (pres...