I research the influence of external climate drivers and boundary conditions on regional ecosystems and hydroclimate. I am interested in how human responses to temporal and spatial environmental variations in the past can inform our understanding of the sensitivity of vulnerable regions to modern global warming. My work has focused primarily on Africa, where many regions are highly water- and resource-stressed according to the IPCC, yet the continent is historically under-studied and we lack full understanding of environmental sensitivity to natural and human-induced climate changes. Understanding past climate change and natural oscillations will provide further characterization of vulnerability to a changing world. Additionally, increasingly more hominin fossil and archaeological finds constrain the history of our human ancestors, but comparisons of evolutionary change with the human environment remains poorly understood due to the lack of high-resolution, geochemical climate and vegetation records.
I utilize the novel geochemical proxies of leaf wax biomarker hydrogen and carbon isotopes to quantitatively reproduce past hydroclimate and vegetation. Plants produce waxes to prevent evaporation from and physical damage to their leaves. Leaf waxes are hydrocarbon chains that incorporate a hydrogen isotopic signature from the water they use to photosynthesize, and thus reflect rainfall amount in the tropics. Carbon isotope signatures from the same leaf wax compounds are determined by the plant type, i.e. trees vs. grasses. These geochemical signals of climate are obtained by extracting the leaf waxes from sediment, performing liquid chromatography separations, and running isolated compounds on an isotope ratio mass spectrometer (IRMS) coupled to a gas chromatograph (GC) to measure the hydrogen and carbon isotope ratios. With these paleoclimate records of rainfall and environment, I perform time series and statistical analyses to understand how environments changed and what drove climate through the Oligocene, Miocene, Pliocene, and Pleistocene.
North African climate from eastern Med sapropels
Sapropels are dark, organic-rich layers in sediment that form in the Mediterranean Sea when intense North African rainfall flows from the Nile River basin and provides a freshwater cap to limit oxygenation of the deep sea. These layers have been used in astrochronological reconstructions to demonstrate their faithful response to precession and eccentricity cycles. Dr. Cassy Rose demonstrated that measuring leaf wax isotopes preserved in the ODP 966 and 967 cores from the Eastern Mediterranean Sea was a wonderful technique to quantify past climate to understand the strength of precession minima and maxima.
For my postdoc research, I have measured the hydrogen isotopic makeup of leaf waxes from each of the sapropel layers over the past 4.5 million years. This, along with a precise astronomically tuned age model, allows us to better understand long term climate change, as well as eccentricity’s role in amplifying precession-driven monsoon systems.
West African hydroclimate of the Cenozoic
Hydroclimate in the western Sahel is controlled by the West African Monsoon (WAM), bringing heavy rains to the region during the summer months. Modest changes in this system over the historical record have had outsized impacts on ecosystems and resources. Prior studies have documented the remarkable sensitivity of this system to low-latitude orbital insolation changes as well as high-latitude temperature changes throughout the Neogene. We explore the Cenozoic history of WAM hydroclimate sensitivity to orbital forcing as well as long-term, secular trends and transitions. Land surface changes, particularly the expansion of C4 grasslands, which has been dated in this region to ~10 Ma, could have a dramatic effect on tropical hydroclimate by way of various feedbacks. However, our understanding of the potential impacts on tropical monsoonal hydroclimate from major boundary regime changes is poor because we lack quantitative, high resolution records from the tropics that span the Cenozoic. I will measure new orbitally resolved hydrogen isotope records from leaf waxes (δDwax) preserved in ODP Site 959 marine sediment core, located in the Gulf of Guinea. This sediment core archives ~25 myr of fairly constant sedimentation, and contains distinct intervals of color and chemical oscillations that we have targeted for variability analyses. We compare these results with Plio-Pleistocene δDwax records from the same region to quantify how the monsoon responded to orbital forcings in differing global and regional boundary conditions throughout the Cenozoic. These will be the first orbitally resolved δDwax records from Africa prior to the Pliocene, and hence provide a new deeper-time perspective on hydroclimate variability in a region with uncertainty surrounding its response to future global warming.
Climate thresholds, environmental feedbacks, and human evolution
This newly funded project in collaboration with Matt Grove at University of Liverpool aims to improve our ability to examine high-resolution climate fluctuations to better understand climate thresholds, feedbacks, and variability on human generational timescales. Reconstructing climate with conservative geochemical proxies is crucial in this endeavor, yet analytical constraints have hindered our ability to generate the long, high-resolution records required for the robust testing of hypotheses linking paleoclimate to hominin evolution. The Turkana Basin in Kenya is the ideal location to study the interplay of climate change across timescales and proxies because of its importance to the field of paleoanthropology and the ample previous work on a drill core reconstructing climate on both orbital and centennial timescales in the early Pleistocene. We will explore high-resolution hydroclimate and vegetation to maximize the potential of the application of neural network modeling to geochemical records. This combined approach will allow us to train sophisticated nonlinear models with empirical data and use those models to generate synthetic high-resolution (centennial-scale) records over many orbital cycles to understand lead/lag relationships, climate thresholds and feedbacks, and subsequently apply these findings to test hominin evolutionary theories. This powerful combination of geochemical and computational methods may prove to be an important tool that can be utilized in a wide variety of time periods, locations, and geological archives.
Hominin Sites and Paleolakes Drilling Project
My dissertation work was part of HSPDP, an international effort to use lacustrine deep drill cores to reconstruct Plio-Pleistocene climate and answer pressing questions on the role of environmental change in driving human evolution. Lead by both paleoclimatologists and paleoanthropologists, the project successfully obtained six cores from East African basins famous for hominin fossils and archaeological artifacts.
As a member of the organic geochemistry team in HSPDP, I worked to produce leaf wax biomarker hydrogen and carbon isotope records from many of these lake cores. I have studied everything from long term million-year climate trends, to millennial-scale hydroclimate fluctuation. I implemented time series analysis tools to pick up statistically significant changes in environmental variability, periodicity, and trend, linking East African climate to global-scale climate transitions. I currently have two papers published, one in review, and two in preparation that have come out of my dissertation work. Please view my CV or contact me with any questions or to request the PDF.
In addition to the biomarker isotope work as part of HSPDP, I have aided in the production of age models, time series analyses of grain size and other lithological indicators, and GDGT work.
Dr. Rachel Lupien 