Active Research Areas
Neogene Climate Change in the Atacama Desert, Chile
Currently there is little agreement regarding the timing for the initiation of hyperarid climatic conditions in the Atacama Desert of northern Chile. Some argue that the Atacama became the hyperarid desert that you see today around 15 million years ago, and that this desiccation is the result of Andean uplift to high elevations. Currently the Andes block most of the moisture coming from the Amazon Basin. Others, however, argue that the onset of hyperaridity in the Atacama commenced around 4 million years ago and is related to global climate cooling and not Andean uplift.
In order to identify the timing of extreme desiccation in the Atacama, and infer the cause of this for this climate change, our research group is working on Miocene paleosols and groundwater deposits in the Calama Basin. Pedogenic salts such as soil carbonate, soil sulfate, and soil halite and nitrate, are extremely sensitive to precipitation values between <5mm/yr up to ~25mm/yr. The Barros Arana paleosol (below) indicates the transition to hyperarid conditions in the Atacama during the Mid-Miocene.
Barros Arana Paleosol
in the Calama Basin in northern Chile
Soil Development in the Atacama Desert
Soil development in the Atacama is unique on Earth because it is one of the few places where soils contain large concentrations of nitrate, as well as significant concentrations of perchlorate, iodate, and phosphate. These anions are derived mainly from photochemical reactions in the atmosphere, and their accumulation in the Atacama is the result of both the extreme aridity and the stability of landscape surfaces. The only other place on Earth that has comparable soils are the dry valleys of Antarctica.
Our research group is actively working to understand the nature and rate of pedogenesis in the hyperarid core of the Atacama. Understanding modern soil forming processes in the Atacama is essential if we are to use fossil paleosols to reconstruct climate change. Below is a photograph of a nitrate soil in the Baquadano Valley that is overlain by Pleistocene alluvium. This soil contains ~10% nitrate. The salt fracture in the center of the photograph, which formed in response to expansion and contraction of the salts, contains ~13% nitrate.
Nitrate soil with salt
fracture in the Baquedano Nitrate District, Chile
In-stream wetlands and their response to Climate Change
In arid lands many perennial streams are supported by ground-water discharge. These streams only rarely (a few times a year) experience discharge events from overland flow. As a result, the channel bottoms of these streams are covered by a dense network of hydrophilic and phreatophytic vegetation (see photo below), which we refer to as in-stream wetlands. This dense vegetation armors the stream bottom and prevents it from being eroded during occasional discharge events. Most perennial streams in northern Chile contain in-stream wetlands.
My initial work on in-stream wetlands in northern Chile focused on using wetland terraces to reconstruct periods of enhanced ground-water discharge and wetter climatic periods. Mapping and dating wetlands from a variety of environments and correlating time-stratigraphic units from these different regions allowed us to demonstrate a climatic control. Now my interest is mainly in the response time and sensitivity of these systems to climatic change, and the proper identification of these deposits. In-stream wetland deposits are often misinterpreted as lake deposits.
I am also working on in-stream wetland deposits in western Jordan with graduate student Emily Winer. In-stream wetland deposits are common within drainages that flow into the Jordan Valley, and can be used to reconstruct late Quaternary paleohydrology in western Jordan.
In-stream wetland deposits at Rio Salado and Mid-Holocene
wetland terrace
Wetland deposits in
western Jordan at Wadi Feydan, near the town of Gregra
Potential problems of using soil carbonate for PCO2 and Paleoelevation reconstructions
The isotopic values of carbon and oxygen in soil carbonate have been used to reconstruct climate, the abundance of C3/C4 vegetation, PCO2, and paleoelevation since Cerling and Quade identified the controls on the isotopic composition of soil carbonate in the 1980's. However, the extent to which diagenesis has influenced the isotopic composition of soil carbonate, especially in older rocks, is not well known.
Our research group is currently working on Paleozoic rocks in the Appalachians to address the question of diagenetic alteration in pedogenic and lacustrine carbonate. We are studying rocks that contain alternating sequences of marine limestones, freshwater limestones, and calcic vertisols. Each of these units has different Sr isotopic signatures, and therefore we are using Sr, C and O isotopic values to understand the diagenetic alteration of these rocks and determine their viability for reconstructing paleo PCO2 values.
Calcic Vertisol Paleosol
in the Pennsylvanian Conemaugh Formation, Ohio
Geoarchaeology: Radiocarbon dating mortars and plasters
In historic Near Eastern Archaeology (Iron Age - Islamic periods), the age of archaeological structures is generally determined by ceramics and coins that are within the foundation of buildings, or which directly overlie the structure in question. This method, however, only provides broad age constraints. We are working with dating organic inclusions within mortar and plaster so that we can directly determine the age of archaeological structures. Moreover, this method allows ages to be determined on any structure with plaster, even those founded directly on bedrock, including cisterns, tombs, roads and aqueducts. Also, by directly dating the plaster, we can determine the phasing and reuse of structures that were in use for long periods of time.
We had great success with radiocarbon dating organic inclusions in mortar and plaster at the site of Khirbet Qana in northern Israel. Now we are trying to determine how common organic inclusions are within mortar and plaster in the Near East, and we are trying to date structures of known age to determine the accuracy and resolution of the technique. By addressing these two research questions we will be able to determine the viability of using this technique to address various research questions in Near Eastern Archaeology.
Plaster samples with
organic inclusions from Khirbet Qana, Israel
Radiocarbon dating of North American terrestrial gastropods
The primary objective of this collaborative research project, with Jeff Pigati (USGS) and Jeff Nekola (University of New Mexico) is to determine the suitability of various North American terrestrial gastropod shells for radiocarbon dating. Although geologists have been dating gastropod shells for over forty years, there has always been a level of uncertainty as to the accuracy of these ages because many terrestrial gastropods ingest limestone and therefore 14C analyses of shell material yield ages that are too old. However, some recent studies suggest that limestone ingestion is species dependant, and that some gastropod taxa do not ingest limestone and therefore are potentially capable of yielding reliable radiocarbon ages. The objectives of this project is to: 1) determine which species of North American terrestrial gastropods do not ingest limestone, and 2) determine if gastropod shells remain a closed system with respect to carbon over time. The first phase of this project entailed determining the 14C content of modern gastropod shells to determine which taxa do and do not ingest limestone. We have determined the 14C content of over 200 modern gastropod shells so far. The seocnd phase of the project focuses on collecting gastropods from well-dated exposures to see if the 14C ages of gastropods match the expected ages, and thus are indicative of closed system behavior with respect to carbon.
Examples of North American terrestrial gastropods (photo by Jeff Nekola).
Possible Research Topics
Potential research topics for graduate students include various aspects of any of the above research projects.
Current Graduate Students
Joel Prellwitz , M.S. (expected, 2007) - Soil development in the hyperarid core of the Atacama Desert.
Craig Tully, M.S. (expected, 2008) - In-stream wetlands and their response to climate change.
Emily Winer, M.S. (expected, 2007) - Late Quaternary paleohydrology of western Jordan.