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

美国国家科学基金会的支持将使研究人员能够完成第一个关键阶段，他们认为这应该是对过去十万年间北美大陆中部气候的详细看法。这一记录来自怀俄明州风河盆地的干旱土壤。绝大多数高分辨率（百至千年时间窗）的过去气候数据来自海洋沉积物，化学记录提供了关于海洋随时间变化的温度和体积的信息。虽然这些记录对于了解地球气候如何变化，以及过去的温暖期如何告诉我们地球对当前温室气体增加的反应非常重要，但这些记录并不能直接告诉我们大陆上发生了什么，也不能告诉我们陆地上的气候模式如何随着全球海洋的变化而变化。可以说，陆地上只有一个“高分辨率”的陆地古气候记录：所谓的魔鬼洞石灰石堆积，它是由加利福尼亚州/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大盆地南部在多个冰期和间冰期气候下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，它是在一个非常独特的地质环境中形成的，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。在此期间，在微观细节上研究这些生长环的能力方面发生了一场革命：激光可以在微米尺度上挥发矿物，新一代的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，并通过许多选择标准，已经找到了超过十万年时间尺度连续的生长环。其中六个已经确定了年代，一个已经送到威斯康星州麦迪逊的NSF SIMS同位素设施进行初步的氧同位素分析。氧同位素数据，当与样品年龄联系起来时，表明这些薄的（~ 2厘米或更小）生长环跨越了近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（由于两个地点之间的距离，这是可以预料到的），这表明人们对中部大陆降水来源的了解。此外，由于沙漠（和含碳酸盐的土壤）覆盖了约20%的地球，将这种技术扩展到各种各样的地方似乎很有希望，为过去几个冰川循环期间地球陆地表面的气候历史开辟了新的见解。该项目要求资助一名高级博士生一年的时间，以(1)两次前往威斯康星州的SIMS设施收集剩余年代薄片的碳和氧同位素剖面，(2)就工作结果撰写研究论文，(3)将结果作为申请长期支持的基础，将记录扩展到其他地方。

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.