Late Glacial and Holocene record of climatic change in the southern Rocky Mountains from sediments in San Luis Lake, Colorado, USA

Abstract

in hydroclimate and widely distributed on land (Street and Grove, 1979). Oxygen isotope record of Owens Lake documents the existence of millennial-scale hydrologic oscillations during the last glacial termination (Benson et al., 1997). Holocene records from Walker and Pyramid Lakes indicate decadal to centennial-scale hydroclimatic variability in the Sierra Nevada (Benson et al., 2002; Yuan et al., 2004). Stable isotope records from alpine lakes over the Colorado Rocky Mountains suggest that the climate shifted from a greater summer precipitation in the early Holocene to a winter-dominated precipitation regime in the mid-Holocene (Friedman et al., 1988; Anderson, 2011; Anderson, 2012). These studies provide us important isotopic data on the past changes in climate over the American Southwest. However, lake-based δ 18 O records spanning the entire Late Glacial and Holocene transition (LGHT) are scarce in this region. Here we describe our efforts to reconstruct climatic and environmental changes in the southern Rocky Mountains during the Late Glacial and Holocene. We chose San Luis Lake mainly because it sits at the southern edge of the annual latitudinal range of the Pacific winter storm tracks and on the northern limit of the North American monsoon (NAM) rainfall regime (Metcalfe et al., 1997; Barron et al., 2012; Ellis and Barton, 2012). Changes in either winter or summer precipitation regime may alter the lake's hydrology, isotope geochemistry and depositional environment. Past work on this basin has showed that deposits from certain locations may be appropriate for paleoclimate reconstructions (Shafer, 1989; Rogers et al., 1992; De Lanois, 1993). We collected sediment cores from San Luis Lake and carried out a series of physical and chemical analyses, including magnetic susceptibility, grain size, total inorganic carbon (TIC), Mg/Ca, δ 18 O, and δ 13 C, to indicate climatic and environmental changes over the last 16.5 ka. Results from radiometric dating analyses (C-14, Pb-210, Cs-137 and Ra-226) are obtained for age control. Our sediment record from San Luis Lake documented a detailed history of climatic and environmental changes in the southern Rocky Mountains since the last glacial maximum. We compared our record from San Luis Lake with existing paleoclimate data from Colorado and elsewhere in the American Southwest and distant regions to gain novel insights into the changes in hydroclimate over the southern Rocky Mountains during the Late Glacial and Holocene.

Deglaciation of the Colorado Rocky Mountains following the Last Glacial Maximum

“…The sedimentary record from proglacial Sky Pond in the northern Front Range (Fig. 2, Site R; Menounos and Reasoner, 1997), and several pollen studies (Reasoner and Jordy, 2000;Jimenez-Moreno et al, 2011;Briles et al, 2012) do suggest Younger Dryas cooling in the Colorado Rocky Mountains, as does a study of playa-lacustrine sediment (Yuan et al, 2013) in the San Luis Valley of southern Colorado (Fig. 2).…”

A multi-proxy record of hydroclimate, vegetation, fire, and post-settlement impacts for a subalpine plateau, central Rocky Mountains, U.S.A

“…During the early Holocene, higher δ 18 O values and smaller differences (2‰) indicate greater rainfall proportions of the total precipitation amounts and lower overall effective moisture totals, consistent with generally lower alpine lake levels during that time (Shuman et al, 2009;Thompson et al, 1993;Toney and Anderson, 2006;Yuan et al, 2013). Bison Lake δ 18 O declines after ~3500 cal.…”

Monazite chemical weathering, rare earth element behavior, and paleoglaciohydrology since the last glacial maximum for the Loch Vale watershed, Colorado, USA

“…Yuan et al (2013) place the Pleistocene-Holocene boundary in the Colorado Rocky Mountains at 10.5 ka, reflecting the largest climatic change in the past 100 ka. The largest sedimentological, physical, and geochemical transitions in our data set occur at ~10.5 ka, corresponding to a depth of ~179 cm.…”

“…From the beginning of the YDC at 13.2 ka (Menounos and Reasoner, 1997; Reasoner and Jodry, 2000) to the Pleistocene-Holocene boundary at ~10.5 ka (Benedict, 1973; Yuan et al, 2013), REE concentrations monotonically increase (Fig. 7).…”