Controls on stalagmite geochemistry

石笋和其他碳酸盐沉积在洞穴中，为过去的气候提供了潜在的有力记录。石笋比湖泊或冰芯具有更广泛的地理分布，为海洋沉积物芯提供了理想的陆地补充。石笋还有另外的优点，它们可以非常精确地确定年代，而且它们没有沉积混合，因此可以提供非常高分辨率的地球化学记录。在过去的十年里，这些优势使得人们对利用石笋重建气候产生了浓厚的兴趣——这一趋势似乎将继续下去。然而，这种石笋古气候研究存在一个大问题。这就是我们还不能可靠地将石笋中的地球化学测量结果转化为关于过去气候的定量信息。在一些地区，稳定同位素数据提供了关于变化的定性信息，但我们迫切需要更好地了解这些和其他地球化学指标，以便我们能够可靠地利用它们来重建过去。本文提出的工作将通过一系列模拟石笋生长的洞穴环境的实验室实验，提供对石笋古气候代用物的理解。我们已经建立了一个实验室设备，可以让高二氧化碳含量的过饱和水在严格控制的条件下滴到玻璃板上，并以与洞穴环境相同的方式脱气形成方解石。我们已经证明了该装置的成功，并使用它来评估温度和滴速在控制石笋地球化学中的作用。在这里，我们建议重复这些实验，并在此基础上进一步了解二氧化碳分压、溶液饱和度和湿度等变量在控制石笋地球化学中的作用。我们将对以这种方式生长的样品进行化学和晶体学特征的表征，并应用一些简单的模型来更好地理解在热力学平衡和动力学分馏条件下石笋中微量金属和稳定同位素的掺入。这项工作将对现有的和新的石笋记录的解释产生直接的影响，也许最明显的回报是对高分辨率气候记录的解释。我们还将应用一些以前很少应用的新的地球化学工具来研究洞穴环境。分子内少量同位素的聚集（如碳酸盐离子）已被证明与温度有关，这为洞穴提供了一种潜在的强大的古温度计，但不幸的是，动力学效应使其变得复杂。通过与耶鲁大学的合作，我们的实验室样本将有助于了解这种块状同位素古温度计的用途和缺陷。我们还将测量一些相对未开发的同位素系统，如Ca、Li、Sr和Mg同位素，以评估它们作为古代用物的用途。最后，我们将通过在实验中加入微生物来评估生命在石笋的沉淀和化学过程中发挥作用的可能性。这种洞穴碳酸盐通常被认为是无机生长的，但最近的培养和测序工作发现了石笋表面上多种多样的微生物组合，其中一些物种已知在其他环境中的碳酸盐沉淀中起作用。我们将在实验中纳入在自然洞穴环境中发现的微生物菌株，以评估生命对洞穴碳酸盐生长的重要性。总的来说，这些实验室实验的结果将大大提高对石笋地球化学的理解，显著提高它们作为过去气候档案的实用性，从而为大陆气候变化的幅度、时间和过程提供新的见解。

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.