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ABSTRACT The Carrizo Plain, the only closed basin in California’s Southern Coast Ranges, preserves landforms and deposits that record both climate change and tectonic activity. An extensive system of clay dunes documents the elevations of late Pleistocene and Holocene pans. Clay dune elevations, drowned shorelines, eroded anticlinal ridges, and zones of perturbed soil chemistry provide evidence of two lake levels higher than today’s (currently 581 m above sea level [masl]), one at ~591 masl at ca. 20 ka and another at ~585 masl that existed at ca. 10 ka, based on optically stimulated luminescence (OSL) dates on clay dune sediment. Two cores from the abandoned floor of the lake provide additional evidence of a long-lived lake in the Carrizo Plain during the late Pleistocene. The longer of the two cores (~42 m) was sampled for palynology, environmental magnetism, and scanning electron microscope–petrography. The magnetic susceptibility signal contains two notable features corresponding to sedimentary materials consistent with reducing conditions. The higher of these features occurs near the surface, and the lower occurs at ~18 m depth. A 14C date on charcoal from the upper reduced zone places the top of this zone at no older than 22.6–20.9 cal ka. This date is consistent with the OSL date on geomorphic features associated with a highstand above ~591 masl. Assuming that reducing conditions correspond to at least a few meters’ depth of relatively fresh water, the new 14C date suggests that the upper reduced zone represents a marine isotope stage (MIS) 2 pluvial maximum lake in the Carrizo Plain. Pollen and ostracodes from the reduced sediments indicate a wetter and cooler climate than today. These conditions would have been capable of sustaining a lake with water much less saline than that of the modern lake. The timing of the oldest documented highstand (no later than 20 ka) is consistent with a modified jet stream migration model and is not consistent with a tropical incursion model. Northeast-to-southwest asymmetry across the lake floor may be consistent with southwestward tilting driven by Coast Range shortening normal to the San Andreas fault, as is seen throughout the region.
The discovery of a unique organic deposit in a dry cave on the Colorado Plateau, southern Utah, permits the first comparison of the physical characteristics and the diet of the dung of the extinct mammoths from the arid Southwest, North America, with that of mammoths from Siberia and northern China, the only other known locations of such remains. The deposit buried beneath sand and rockfall is composed primarily of mammoth dung, estimated at over 300 m 3 . Radiocarbon dates on dung boluses indicate that the mammoths frequented the cave between approximately 14,700 and 11,000 yr B.P. (the range of ages at 2σ). The desiccated boluses, measuring approximately 230 × 170 × 85 mm, are nearly identical in size to dung from extant elephants. The largest contents in the dung are stalks measuring 60 × 4.5 mm. Grasses and sedges dominated the diet, although woody species were commonly eaten.
Abstract The impact of Indigenous populations on historical fire regimes has been controversial and beset by mismatches in the geographic scale of paleofire reconstructions and the scale of land-use behaviors. It is often assumed that anthropogenic burning is linearly related to population density and not different cultural practices. Here we take an off-site geoarchaeology strategy to reconstruct variability in historical fire regimes (<1000 years ago) at geographic scales that match the archaeological, ethnohistorical, and oral tradition evidence for variability in the intensity of Indigenous land use by two different cultural groups (Ancestral Pueblo and Western Apache). We use multiple, independent proxies from three localities in ponderosa pine ( Pinus ponderosa ) forests in east-Central Arizona to reconstruct fire regime variability during four phases of cultural use of different intensities. Elevated charcoal with domesticate pollen ( Zea spp.) but otherwise unchanged forest pollen assemblages characterized intensive land use by Ancestral Pueblo people during an early phase, suggesting fire use to support agricultural activities. By contrast, a phase of intensive pre-reservation Western Apache land use corresponded to little change in charcoal, but had elevated ash-derived phosphorus and elevated grass and ruderal pollen suggestive of enhanced burning in fine fuels to promote economically important wild plants.
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