Collaborative Research: Developing Continental Paleoclimate Records from Soils Using Pedogenic Carbonate

NSF的支持将使研究人员能够完成他们认为应该详细了解过去十万年中北美大陆气候的第一个关键阶段。这个记录来自怀俄明州风河流域的干旱土壤。绝大多数高分辨率（约百年至千年的时间窗）过去的气候数据来自海洋沉积物，化学记录提供了海洋温度和体积随时间变化的信息。虽然这些记录对于了解地球气候如何变化以及过去的温暖时期如何告诉我们地球如何应对当前温室气体增加非常重要，但这些记录并没有直接告诉我们大陆上发生了什么，以及陆地上的气候模式如何随着全球海洋变化而变化。可以说，陆地上只有一个“高分辨率”的陆基古气候记录：所谓的魔鬼洞石灰岩堆积，它是由加州/内华达州边界沙漠中的地下水形成的。石灰岩中的氧同位素提供了南部大盆地在多个冰期和间冰期气候中陆地温度的详细变化。到目前为止，这仍然可以说是地球上最详细的陆地记录，并且形成于一个非常独特的地质环境中，在许多其他地方都没有发现。在过去的二十年里，研究人员一直在研究风河流域土壤中碳酸钙的生长环。在这段时间内，在微观细节上研究这些年轮的能力发生了革命：激光可以在微米尺度上挥发矿物，新一代质谱仪可以确定矿物的年龄及其氧同位素组成（这是该地区温度和降雨来源的指南）。他们已经能够从现场收集多个样本，并通过众多的选择标准，已经发现了连续超过十万年时间尺度的年轮。其中六个已经确定了日期，一个已经被送到威斯康星州麦迪逊的NSF西姆斯同位素设施进行初步的氧同位素分析。氧同位素数据与样本年龄相关联时，表明这些薄（约2厘米或更少）的年轮跨越近20万年，并且氧同位素模式可能与魔鬼洞的时间相当。初步记录显示了与魔鬼洞的相似之处和不同之处（这可能是因为两个地点之间的距离），这表明了对中部大陆降水来源的见解。此外，由于沙漠（和含碳酸盐的土壤）覆盖了地球的20%，因此将这种技术扩展到各种各样的地点似乎是有希望的，为最近几次冰川循环中地球陆地表面的气候历史提供了新的见解。该项目要求资金支持一名高级博士生一年，以（1）两次前往威斯康星州的西姆斯设施，收集剩余的过时叠层的碳和氧同位素剖面，（2）就工作结果撰写研究论文，（3）将结果用作请求长期支持的基础，以将记录扩展到其他地点。

This NSF support will allow the investigators to complete the first critical phase of what they think should be a detailed view into the climate of the mid North American continent over the past hundred thousand years. This record comes from arid soils of the Wind River Basin in Wyoming. The vast majority of high resolution (~ hundred to thousand year time windows) past climate data comes from oceanic sediments, and the chemical record provides information on the temperature and volume of the oceans over time. While this record is very important to understand how the Earth's climate has varied, and how past warm periods can inform us how the Earth might respond to present greenhouse gas increases, these records do not tell us directly what was happening on the continents, and how climate patterns on land changed with global ocean changes. There is arguably only one "high resolution" land-based paleoclimate record on land: the so-called Devil's Hole limestone accumulation that formed by ground water in the desert of the California/Nevada border. The oxygen isotopes in the limestone provide detailed changes in the land temperature of the southern Great Basin over multiple glacial and interglacial climates. To date, this remains arguably the most detailed land record on Earth, and is formed in a very unique geological setting that is not found in many other locations. For the past two decades, the investigators have been studying the growth rings of calcium carbonate in Wind River Basin soils. During this time interval, there has been a revolution in the ability to study these growth rings in microscopic detail: lasers can volatilize the minerals at a micron scale and new generations of mass spectrometers can both determine the age of the mineral and its oxygen isotope composition (which is a guide to the temperature and the source of rainfall in the region). They have been able to collect multiple samples from the field, and through numerous selection criteria, have found growth rings that are continuous over hundred thousand year time scales. Six of these have been dated, and one has been sent to the NSF SIMS isotope facility in Madison WI for preliminary oxygen isotope analyses. The oxygen isotope data, when linked with the sample age, indicate that these thin (~ 2 cm or less) growth rings span nearly two hundred thousand years, and that the oxygen isotope pattern is likely temporally comparable to that of Devil's Hole. The preliminary record shows both similarities and dissimilarities to Devil's Hole (as might be expected for the distance between the two locations), suggesting insights into the precipitation sources of the mid-continent. Additionally, since desert (and carbonate bearing soils) cover ~20% of the Earth, the extension of this technique to a wide variety of locations seems promising, opening new insights into the climate history of the Earth's land surface over the last few glacial cycles. This project requests the funds to support an advanced PhD student for one year to (1) make two trips to the SIMS facility in Wisconsin to collect carbon and oxygen isotope profiles on the remaining dated laminations, (2) write a research paper on the results of the work, and (3) use the results as the basis for a request for longer term support to expand the record to other locations.