Insights into North African Climate Variability Over the Last 1.1 Million Years from Dust Fluxes and Leaf Wax Isotopes

The Sahel region of North Africa experienced dramatic swings in precipitation patterns over the latter half of the 20th century, with devastating impacts on food security during the droughts of the 1970s and 1980s. Looking ahead, projections of future climate change in the Sahel show little agreement, with some climate models predicting significantly wetter conditions in the 21st century and others suggesting drying, leaving great uncertainty as to the impacts of climate change on this sensitive region. This project aims to improve understanding of controls on North African climate by reconstructing regional climate changes over the last 1 million years using deep-sea sediments located off the west African coast. This long-term sampling of climate variability will reveal the region's response during multiple ice age cycles, including periods when the high latitudes of the Northern Hemisphere were substantially warmer and colder than at present and periods when North Africa was much wetter and drier than at present. By systematically documenting North Africa's response during this range of climatic conditions, the results will provide important new opportunities to test climate models' ability to accurately represent past climate variability in the region. The project will employ recently developed tools to reconstruct windblown dust emissions from the Sahara desert (a tracer of winds and aridity), the carbon isotopic composition of fossil leaf molecules in the sediment (a tracer of vegetation on the continent), and the hydrogen isotopic composition of the same leaf molecules (a tracer of monsoon strength.) This dataset will enable a better understanding of what has driven climate change in the past, and what this will mean for future change in North Africa. The project will also include public outreach projects to offer education about the impact of climate change in North Africa, and it will provide training and career development for a female postdoctoral researcher and multiple undergraduate researchers.

The present understanding of African climate evolution over the Plio-Pleistocene is based in large part on a benchmark set of marine records of the accumulation rate of windblown dust that provide some of the only continuous data spanning thousand to million year timescales. Now, 25 years after the initial publication of these records, recent advances in hydroclimate reconstruction offer the opportunity for a new generation of insights into North African climate. Constant flux proxies (230Th, 3He) allow fundamental improvements in the accuracy of fluxes by minimizing the effects of lateral advection and age model uncertainties; as a result, dust fluxes can be compared through time and space with much greater confidence. Additionally, the carbon and hydrogen isotope composition of terrestrial leaf waxes (n-alkanes) have been established as hydrological proxies that can offer independent insights into monsoon precipitation and vegetation.

At present, these methods have received only limited application to periods prior to the last glacial cycle, precluding rigorous exploration of the drivers, timing and amplitude of North African climate change over the Pleistocene. This project will use measurements of dust flux (determined by 230Th- and 3He-normalization) and leaf wax isotopes (dD, d13C) to reconstruct changes in the mean state and variability of African climate over the last 1.1 Ma. This interval includes periods with broad variations in orbital parameters, ice volume, and high-latitude temperatures, including maximum high latitude warmth during so-called "super-interglacials", low-amplitude ice age cycles during the "luke-warm" interglacials, and times of especially high and low eccentricity. By pairing high-fidelity dust fluxes with complementary data reflecting land cover and hydrology, the project will offer detailed insights into the nature of past changes in this water-stressed monsoonal region and will test specific hypotheses relating to the roles of high-latitude temperatures and local insolation in driving past hydrological changes. The project will offer new opportunities for testing climate models used to forecast future changes in the region by providing robust records of the response of North African climate to a wide range of forcings and boundary conditions. This dataset will also enable improved testing of hypotheses relating changes in the mean state and variability of African climate to human evolution.