Controls on stalagmite geochemistry

Stalagmites and other carbonates deposited in caves provide a potentially powerful record of past climate. Stalagmites have a wider geographical dispersion than lakes or ice cores and provide an ideal terrestrial complement to marine sediment cores. Stalagmites have additional advantages in that they can be very accurately and precisely dated, and that they suffer no sedimentary mixing so can provide very high resolution geochemical records. These advantages have led to a burgeoning interest in reconstruction of climate from stalagmites in the last decade - a trend that looks set to continue. There is, however, a big problem with such stalagmite paleoclimate research. This is that we cannot yet reliably turn geochemical measurements in stalagmites into quantitative information about the past climate. In some locations, stable-isotope data provides qualitative information about change, but we desperately need to develop better understanding of these and other geochemical proxies so we can reliably use them to reconstruct the past. The work proposed here will provide understanding of stalagmite paleoclimate proxies through a series of laboratory experiments mimicking the cave environment in which stalagmites grow. We have built a laboratory apparatus that allows super-saturated waters with high CO2 contents to drop onto glass-plates in closely controlled conditions and to degas to form calcite in a manner identical to that seen in the cave environment. We have demonstrated the success of this apparatus and used it to assess the role of temperature and drip-rate in controlling stalagmite geochemistry. Here we propose to replicate these experiments, and to go beyond them to also understand the role of variables such as pCO2, solution saturation, and humidity in controlling stalagmite geochemistry. We will characterize the samples grown in this way both for their chemistry and for their crystallographic features, and apply some simple models to develop a significantly better understanding of trace-metal and stable isotopes incorporation into stalagmites, under conditions of both thermodynamic equilibrium and kinetic fractionation. This work will have direct implications for the interpretation of existing and new stalagmite records, with perhaps the clearest reward coming in the interpretation of high-resolution climate records. We will also apply some new geochemical tools which have seen little previous application to the cave environment. The clumping of minor isotopes within molecules (such as the carbonate ion) has been shown to be temperature dependant, providing a potentially powerful paleothermometer in caves, but one that is unfortunately complicated by kinetic effects. Our laboratory samples will help, via a collaboration with Yale University, to understand the uses and pitfalls of this clumped-isotope paleothermometer. We will also measure some relatively unexplored isotope systems such as Ca, Li, Sr, and Mg isotopes to assess their use as paleoproxies. Finally, we will assess, by adding microbes to our experiments, the possibility that life plays a role in the precipitation and chemistry of stalagmites. Such cave carbonates are normally thought to grow inorganically, but very recent culturing and sequencing work has uncovered a diverse microbial assemblage on stalagmite surfaces, with some species known to have a role in carbonate precipitation in other environments. We will include microbial strains found in the natural cave environment in our experiments to assess the importance of life for growth of cave carbonates. In total, the outcome of these laboratory experiments will be a much improved understanding of the geochemistry of stalagmites, significantly advancing their usefulness as archives of past climate, and therefore providing new insights into the magnitude, timing, and processes of climate change on the continents.