Weathering and climate: New insights from the marine records of Li, Nd and Os

Since 65 million years ago, there have been huge changes in Earth's climate. Back then, the Earth was a 'greenhouse'; the Arctic was devoid of permanent ice and supported luxuriant forests, while the low latitudes were seasonally arid. Today, the Earth experiences 'icehouse' conditions, characterized by extensive permanent polar ice sheets. It isn't clear what caused the transition from a greenhouse to an icehouse world, but there is growing evidence that amongst the primary controls is the concentration of atmospheric carbon dioxide, the principal greenhouse gas. Weathering of continental silicate rocks regulates levels of atmospheric carbon dioxide because it leads to drawdown of CO2. Silicate weathering rates are controlled primarily by temperature and rainfall, creating a dynamic link between weathering and climate: for example, higher global temperatures accelerate weathering of silicates causing drawdown of atmospheric CO2, which in turn leads to cooler temperatures through reduced greenhouse warming. Such 'feedback' processes are important for regulating Earth's climate system. On long timescales (>100,000 years), silicate weathering also responds to tectonically-driven changes in continental topography; for example, uplift and erosion of the Himalaya-Tibet plateau is believed to be associated with intensified silicate weathering and long-term cooling over the past 40 million years or so. Thus far most of our information about how silicate weathering rates have changed over the past 65 million years has come from the temporal record of the strontium (Sr) isotopic composition of the oceans. However, in recent years this proxy has been widely criticized because shifts in Sr isotopes may reflect changes in the Sr isotopic composition of the rocks being weathered, rather than a simple change in the silicate weathering flux. Furthermore, the Sr isotopic composition of seawater changes very slowly (on timescales of >100,000 years), so the Sr isotope record cannot capture the short-term events that can influence Earth's long-term climate evolution. At the Open University, we have been developing new ways of tracing past changes in silicate weathering. Our preliminary studies show that seawater lithium (Li) concentrations respond to long-term changes in the silicate weathering flux, while seawater Li isotope ratios reflect weathering intensity. Meanwhile, the osmium (Os) isotopic composition of seawater tells us about short-term changes in the weathering flux and also the Os isotopic composition of the rocks that are weathered. Finally, the neodymium (Nd) isotopic composition of seawater responds rapidly to changes in the isotopic composition of the continental source. Thus, considered together, the temporal records of seawater Li concentrations and Li, Os and Nd isotopes can provide valuable information as to past changes in weathering rates and processes, on both long and short timescales. The aim of this proposal is to generate records of seawater Li concentrations and Li, Os and Nd isotopes for the past 65 million years. We will then integrate these records with those of global temperature, ice volume etc, in order to better understand the links between weathering and climate.