COLLABORATIVE RESEARCH: Laminated soil carbonate rinds as a tool for investigating late Quaternary climate-vegetation links

点击翻译按钮获取中文摘要

Drylands such as those found in the western United States are characterized by high precipitation variability and seasonally-high temperatures. Climate change is likely to affect both, increasing the chances of drought in the future. Plants are the base of the food chain, so it is important to understand how they respond to climate variability. This research seeks to further develop a new way of creating detailed histories of past vegetation, soil temperature, and soil water evaporation. These histories, which can go back tens of thousands of years, are preserved in a mineral, calcium carbonate (CaCO3), that forms in soils. These minerals grow incrementally, century after century, on the undersides of rocks to form 'laminated soil carbonate rinds'. Prevailing vegetation composition, temperature, and degree of soil water evaporation are recorded in the carbon and oxygen chemistry (isotopes) that make up each layer. Carbonate rinds from several locations in the Capitol Reef / Boulder Mountain region of southern Utah will provide detailed records extending back approximately 40,000 years. Isotopic records from these rinds will be compared to other records regionally and globally to get a better understanding of the causes of climate and vegetation change in southern Utah. The research will answer questions such as: How do plant distributions change in response to changes in soil temperature and evaporation? How does climate change influence plant distributions at different elevations? How comparable are the records from one location to another? In addition to the new knowledge about plant responses to climate change, this study will benefit the national geoscience workforce through (1) training a new generation of geoscience professional at the undergraduate, graduate, and postdoctoral levels (2) providing continuing education for national park staff, and (3) bringing immersive scientific outreach experiences to under-served high school students.Laminated soil carbonate rinds form on the undersides of soil clasts (ranging from pebbles to boulders) in many dryland regions worldwide. On long-lived, stable geomorphic surfaces, rinds can grow for 10s to 100s of kiloyears, and can present ordered, continuous, and detailed (century-scale) stratigraphies of past conditions, as recorded by stable isotopes. The long-lived geomorphic surface and laminated rind are analogous to a cave and speleothem; laminated rinds are by extension a promising, but underutilized, critical zone archive recording past climate and vegetation processes. In his previous work, the investigators developed a single rind-based climate and vegetation record from the Capitol Reef region of southern Utah spanning 5 – 35 ka. They showed how Secondary Ion Mass Spectrometry (SIMS)-based delta 13C and delta 18O records, along with temperature information provided by clumped isotope thermometry, can be used to reconstruct changes in the fraction of C4 vegetation on the ancient landscape, delta 18O-soil water, and past soil temperature. In the present work, they address important questions related to replicability on local to regional scales and, within the context of existing Quaternary records from the western United States, demonstrate the investigative power of this critical zone archive for paleoclimate studies: Does each rind record the story of just one soil, or can records be extrapolated laterally? How does rind location within the soil (shallow versus deep) affect the record? How do records change across an elevation transect? Are triple oxygen isotopes in rind carbonate sensitive to evaporation? To answer these questions, they will focus on a core set of four rinds spanning an elevation transect from 1750 to 2500 m on the Johnson Mesa and Torrey Bench geomorphic surfaces (paleo-debris flows) near Capitol Reef National Park, Utah. Six additional rinds will be used to investigate a wider geographic range, and to investigate the effects of soil depth. They will develop a detailed radiocarbon age model for each profile, and analyze rind delta13C and delta18O via high spatial-resolution SIMS (~10 µm spot diameter; centennial resolution). Clumped and triple oxygen isotope profiles will be generated in higher resolution (ca. 2-3 kyr/sample) for the core rinds, and for key intervals (e.g. recent, mid-Holocene, LGM) for the six additional profiles. They will make paleoclimate inferences based on modern datasets of soil temperature, pCO2, soil moisture, and rainfall. The proposed research will result in several detailed climate records with novel insight into ancient dryland critical zones, a better understanding of what kind of information the records provide, and a guide for how to best apply the stable surface – laminated rind approach elsewhere. Broader societal impacts of the work include the training of undergraduate, graduate, and postdoctoral students in the geosciences, continuing education for Capitol Reef National Park staff in local geology and geomorphology, and a Friends of the Pleistocene field trip focusing on this region. They will also develop a new soils, CO2, and climate module for the University of Michigan's Earth Camp program, which provides promising high school students from underserved communities in Detroit with week-long science camp experiences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.