Controls on stalagmite geochemistry

洞穴中沉积的石笋和其他碳酸盐为过去的气候提供了潜在的强有力的记录。与湖泊或冰芯相比，石笋在地理上的分布范围更广，是海洋沉积物岩心的理想陆地补充。石笋还有另一个优点，那就是它们可以非常准确地确定年代，而且它们不会遭受沉积混合，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，这些优势导致了人们对利用石笋重建气候的兴趣日益高涨--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很大的问题。这就是说，我们还不能可靠地将石笋中的地球化学测量转化为关于过去气候的定量信息。在一些地方，稳定同位素数据提供了有关变化的定性信息，但我们迫切需要更好地理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重建过去。这里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来提供对石笋古气候指标的了解。我们建造了一个实验室设备，允许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，并以与洞穴环境中看到的相同的方式形成方解石。我们已经证明了这个装置的成功，并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，并超越它们，也了解变量，如二氧化碳，溶液饱和度，和湿度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的化学和结晶学特征进行表征，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微量金属和稳定同位素在石笋中的结合情况。这项工作将对现有和新的石笋记录的解释产生直接影响，也许最明显的回报来自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到洞穴环境中的新的地球化学工具。分子(如碳酸盐离子)中少量同位素的聚集已被证明与温度有关，这在洞穴中提供了一个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。我们的实验室样本将通过与耶鲁大学的合作，帮助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被探索的同位素系统，如钙、锂、锶和镁同位素，以评估它们作为古代用品的用途。最后，我们将通过在我们的实验中添加微生物来评估生命在石笋的沉淀和化学中发挥作用的可能性。这种洞穴碳酸盐通常被认为是以无机方式生长的，但最近的培养和测序工作在石笋表面发现了一个多样化的微生物组合，其中一些物种已知在其他环境中对碳酸盐沉淀起到了作用。我们将把在自然洞穴环境中发现的微生物菌株纳入我们的实验，以评估生命对洞穴碳酸盐生长的重要性。总而言之，这些实验室实验的结果将大大改善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有用性，从而为了解大陆气候变化的规模、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.