| dc.description.abstract | Lacustrine systems capture high-resolution records of continental climate, including information on paleotemperatures, hydroclimate, and paleofire activity. An increasing number of studies have reported paleoclimate and paleofire records from Holocene and upper Pleistocene sedimentary archives across various regions of the arid western U.S., shedding light on possible future scenarios of linked climate-fire regimes attendant with continued climate change; however, records from older intervals are rare to nonexistent. Here, we report on an investigation of paleoclimate and paleofire from an upland lake of the early Pleistocene that records accumulation prior to the Mid-Pleistocene Transition. We recovered an exceptionally long (147 m) core from an early Pleistocene (~1.4-1.3 Ma) lake buried ~200 m subsurface in Unaweep Canyon, a high-relief, upland system of the Uncompahgre Plateau in western Colorado that hosts a thick sediment fill. The age (~1.4-1.3 Ma) is based on cosmogenic-nuclide dating of a previous, correlative section. The sediment comprises predominantly mass flow event beds such as turbidites, with grain sizes ranging from sand to mud. Over the ~147 m of the lacustrine section, prominent color variations occur: two intervals (20 - 32 m thick) exhibiting mostly red and ochre (oxidized) colors alternating with two intervals (35 - 53 m thick) exhibiting mostly drab olive to dark gray (reduced) colors. These color variations correspond to changes in palynological assemblages that indicate two cold-warm variations—inferred glacial-interglacial cycles—with glacials represented by high values of Picea and Abies pollen and generally oxidized colors, and interglacials represented by high values of Artemisia and Pinus pollen and generally drab colors. Climatic conditions are further explored through the variance of magnetic susceptibility (MS), Zr, Ti/Al and Mg/Al ratios between the cycles: glacials exhibit higher averages of MS, Zr, Ti/Al and Mg/Al, suggesting increased detrital input and runoff, interpreted to record wetter conditions. Charcoal accumulation rates vary between the glacial-interglacial hemicycles, with higher concentrations in warmer (interglacial) conditions. Additionally, mean daily insolation patterns were calculated for the general Unaweep region, demonstrating that higher charcoal accumulation rates correspond with times of higher insolation. Overall, the UDR 1A core records that interglacials at the beginning of the mid-Pleistocene transition record warmer and drier conditions that were accompanied by increased frequency of fire occurrence and intensity. | en_US |
