My research is on monsoonal ecosystems and the changes that are brought about by tectonic, volcanic, and climatic perturbations. This has led me to study the East African Rift System as well as Triassic formations of North America.
Triassic sediments of the Pangaean supercontinent preserve a remarkable record of monsoonal rainfall. The Pangaean monsoon was special as compared to today since there was no polar ice to speak of, obviously no anthropogenesis, and changes to the Pangaean monsoon were influenced by large-scale processes of continental drift, which do not really affect modern monsoons—at least on the timescales that civilization has persisted. Therefore, I consider the monsoon of Pangaea as an ideal foil to the Plio-Pleistocene monsoon of East Africa. This has shaped my perspectives on the causes of climate change during the evolution of our human ancestors as well as climate changes experienced by contemporary people. Delicate ecosystems and billions of people cohabitate under monsoonal climates, and we understand very little about how monsoons will respond to a future Earth that is different from today.
Stratigraphy is a fundamental tool for studying the past. I use this to interpret sequences of environmental changes, successions of fossil fauna, and the amount of time represented by geologic records. A particular focus of mine is paleomagnetic stratigraphy, which is used for dating layers as well as correlating layers form vastly different geographic areas of the world. Paleomagnetism necessitates a fundamental understanding of geophysical processes as they apply to the magnetic remanence preserved by a rock or sediment unit. Study of the magnetic field of Earth begins with the geodynamo and ends with the interactions of electrons within an iron-oxide mineral. My studies lie closer to the iron-oxide end of the spectrum, particularly the conditions of occurrence and magnetic properties of magnetite and hematite in terrestrial sedimentary environments.
I integrate spectrophotometry with traditional paleomagnetic methods. NSF has funded the work and I have established an iron-oxide lab that has published on the Plio-Pleistocene soils of Kenya and the Late Triassic Chinle Formation of Arizona. Identifying minerals in these ancient soils helps to understand the formation history of rocks, diagenetic alterations, sources of the detritus, and the environment of accumulation. It also aids in paleomagnetic stratigraphy as the spectrophotometry provides information on what minerals may be carrying magnetic signals. The spectrophotometry is a more expedient technique, as compare to rock magnetic methods, and measures concentrations of minerals within soils and rock/sediment, which is information difficult to obtain from rock magnetism alone. The ultimate goal is to develop spectrophotometry methods that use mineral concentrations (g/kg) in ancient soils to quantify mean annual precipitation for ancient monsoons.
An outstanding topic that I address is how widespread across Pangaea were shifts to the ancient monsoon and what were the shifts caused by. We are well aware of several perturbations to atmospheric pCO2 during the Late Triassic. This is especially well documented from the Newark-Hartford basins of New Jersey and Connecticut. It is not well known if these atmosphere changes caused rainfall changes, and there is a lack of understanding if paleoclimate shifts in western equatorial Pangaea were synchronous with what has been observed from the Newark-Hartford records. This is a problem I have been addressing as part of the Colorado Plateau Coring Project, collaborating with Paul Olsen and Dennis Kent on the paleomagnetic stratigraphy of the Chinle Formation. My paleomagnetic and spectrophotometry data demonstrate a wholesale shift in the monsoon coinciding with the Adamanian-Revueltian tetrapod faunal event. Moreover, spectral analyses of the spectrophotometry data has resolved periodicities that indicate some of the rainfall variations are due to orbital climate forcing, related to the 405,000-year eccentricity-insolation cycle. We are now working to correlate changes in the Chinle paleoenvironmental record to what has been established by the Newark-Hartford records, in addition to submitting a new proposal for phase two of the Colorado Plateau Coring Project.
Surprisingly we may know more about the broad pattern of the Pangaean monsoon as compared to the East Africa monsoon. A short time ago it was proposed that El Niño may explain most of the inter-annual variability of modern East African rainfall. Now it seems that the rainy seasons are more complex than this. Similarly, there is uncertainty about how the Plio-Pleistocene monsoon may have developed through the beginning of Quaternary ice ages, through the onset of Walker circulation, and during the late Cenozoic drawdown of atmospheric pCO2. This has sparked much debate about observed changes in paleoenvironments of ancient savanna ecosystems, and the implications for the archeological and fossil records of humans. My NSF project addresses this by sampling the ancient Vertisols sequences of the Turkana Basin in northwest Kenya. We know very little about iron-oxide minerals of modern or ancient Vertisols from tropical latitudes. Hence, our understanding of the magnetic properties of these soils is poor and their use a paleomagnetic records is understudied. My work on this problem demonstrates that these ancient Vertisols differ from modern expectations according to their magnetite composition and water table depths during dry monsoon seasons.
To further the implications of my work, I am exploring the possibility of retrieving seasonal rainfall information from the paleosols and iron oxides of the Late Triassic redbeds of the Newark Basin. The Newark Basin has detailed records of precessional insolation forcing of water level in tropical Pangaean lakes. Orbital precession and monsoonal seasonality are strongly linked. Therefore, a high-resolution seasonal rainfall record may be within these redbed sequences. Spectrophotometry and paleomagnetic studies of the hematite minerals may help to design a method that can resolve sub-Milankovitch climate variations from the paleosols of the Newark Basin and East Africa.
Lepre's geology lab 