Carrie Morrill

Ecotone shift and major droughts during the mid-late Holocene in the central Tibetan Plateau.

A well-dated pollen record from a large lake located on the meadow-steppe ecotone provides a history of ecotone shift in response to monsoonal climate changes over the last 6000 years in the central Tibetan Plateau. The pollen record indicates that the ecotone shifted eastward during 6000-4900, 4400-3900, and 2800-1600 cal. yr BP when steppes occupied this region, whereas it shifted westward during the other intervals when the steppes were replaced by meadows. The quantitative reconstruction of paleoclimate derived from the pollen record shows that monsoon precipitation fluctuated around the present level over the last 6000 years in the central Tibetan Plateau. Three major drought episodes of 5600-4900, 4400-3900, and 2800-2400 cal. yr BP are detected by pollen signals and lake sediments. Comparison of our record with other climatic proxy data from the Tibetan Plateau and other monsoonal regions shows that these episodes are three major centennial-scale monsoon weakening events.

Elevation or alteration? Evaluation of isotopic constraints on paleoaltitudes surrounding the Eocene Green River Basin

Low oxygen isotope values (∼−16‰, relative to Peedee belemnite standard) obtained from microbial carbonates of the Green River Formation have been interpreted as evidence for snowmelt and high elevations surrounding this early Eocene lake basin. However, low values from microbial precipitates could also represent a diagenetic overprint. We investigate these alternate hypotheses by measuring the oxygen isotope composition of altered and unaltered freshwater bivalves from the basin. We analyzed subannual samples from 3 individuals and bulk samples from 54 individuals. Subannual samples exhibit clear seasonal fluctuations in oxygen isotope values. These fluctuations are large enough to require seasonal shifts in temperature and/or the oxygen isotope value of the lake water. However, the lowest value of any unaltered sample (either bulk or high resolution) corresponds to lake-water δ 18 O of ∼−12‰ (relative to standard mean ocean water) and is not low enough to require the addition of snowmelt to the lake. A few bulk samples exhibit very low oxygen isotope values, which would seem to suggest snowmelt. However, these samples also show clear evidence of calcite in X-ray diffraction patterns, demonstrating the presence of diagenetic precipitates. Given that (1) diagenetic alteration is a plausible explanation of very low δ 18 O values, (2) alteration has not been examined in microbial carbonates, and (3) unaltered aragonitic bivalves provide no independent evidence for low δ 18 O values for lake waters, we conclude that robust isotopic evidence for high elevations surrounding the Green River Basin during the Eocene is currently lacking.

Orbital forcing and Eocene continental temperatures

Abstract The ability to define terrestrial paleotemperatures is a key issue for understanding past climate states and the processes that produced them. Paleotemperatures are defined, ideally, by proxy data interpretations, and supported by theoretical modeling results that provide physical explanations for the temperatures. Such explanations are especially critical for paleoclimates substantially warmer than present, for which we have no modern or recent examples. Model results and geologic data describing continental mean annual temperatures for the warm Early Eocene Earth as well as other time periods are generally in agreement. However, there are persistent discrepancies between climate model results and proxy data interpretations for temperature seasonality estimates. In this paper we test the idea that orbital forcing and resulting distributions of solar radiation played a role in determining continental interior temperatures during the Eocene. Specification of extreme values of orbital parameters (within the range of calculated Pleistocene values) produces a wide range of continental temperatures. These include cooler Northern Hemisphere continental summer temperatures and milder winter temperatures by up to 30%, and an annual temperature range reduced by 75%, relative to temperatures produced at the same location with modern orbital configuration. Temperature responses to the specified orbital forcing are most evident at middle and high latitudes. The results produced with one specification of extreme orbital configurations are more similar to proxy data interpretations than any previous results of these temperature parameters. We suggest that orbital variation and resulting insolation responses must be included in paleoclimate modeling studies that aim to explain proxy data and paleoclimate conditions for specific times in Earths history.

Modeling orbital forcing of lake level change: Lake Gosiute (Eocene), North America

Abstract The sedimentary record of Lake Gosiute, a lake that existed in southwestern Wyoming during the Eocene, contains evidence for lake level fluctuations thought to be caused by the earths precession cycle. However, it is not clear how the effects of these orbital variations were transferred through the climate system and into the sedimentary record. We carry out a series of experiments using a general circulation model (GCM), a lake energy balance model and a lake water balance model to better understand the processes by which these orbital variations could have altered lake evaporation, on-lake precipitation and runoff from the lakes catchment. GCM simulations indicate significant differences in surface incident shortwave radiation between the two end-members of the precession cycle. These differences cause lake evaporation to be ∼25% higher when perihelion occurs at the summer solstice. GCM simulations also indicate significant seasonal changes in the amount of precipitation between the two end-members, but no change in the annual mean precipitation. Preliminary experiments with a lake water balance model show that local effects such as changes in vegetation, in snowmelt runoff, or in the area of mudflats surrounding the lake could have a large impact on lake level. However, more data need to be collected to determine the importance of these effects. Our results challenge previous interpretations of paleoclimate that were based on geologic data and simple assumptions regarding the effects of orbital variations on the water balance of the lake. In particular, we find that (1) changes in shortwave radiation may have been more important than changes in temperature or moisture in causing lake level fluctuations and (2) changes in catchment and lake characteristics should be further examined. In order to make an accurate reconstruction of past climatic change from a lake level record, climate system processes and local non-climatic variables must be considered explicitly.

Is the Last Glacial Maximum a reverse analog for future hydroclimate changes in the Americas

Future hydroclimate change is expected to generally follow a wet-get-wetter, dry-get-drier (WWDD) pattern, yet key uncertainties remain regionally and over land. It has been previously hypothesized that lake levels of the Last Glacial Maximum (LGM) could map a reverse analog to future hydroclimate changes due to reduction of CO2 levels at this time. Potential complications to this approach include, however, the confounding effects of factors such as the Laurentide Ice Sheet and lake evaporation changes. Using the ensemble output of six coupled climate models, lake energy and water balance models, an atmospheric moisture budget analysis, and additional CO2 sensitivity experiments, we assess the effectiveness of the LGM as a reverse analog for future hydroclimate changes for a transect from the drylands of North America to southern South America. The model ensemble successfully simulates the general pattern of lower tropical lake levels and higher extratropical lake levels at LGM, matching 82% of the lake proxy records. The greatest model-data mismatch occurs in tropical and extratropical South America, potentially as a result of underestimated changes in temperature and surface evaporation. Thermodynamic processes of the mean circulation best explain the direction of lake changes observed in the proxy record, particularly in the tropics and Pacific coasts of the extratropics, and produce a WWDD pattern. CO2 forcing alone cannot account for LGM lake level changes, however, as the enhanced cooling from the Laurentide ice sheet appears necessary to generate LGM dry anomalies in the tropics and to deepen anomalies in the extratropics. LGM performance as a reverse analog is regionally dependent as anti-correlation between LGM and future Pu2009−u2009E is not uniformly observed across the study domain.