COLLABORATIVE RESEARCH: Dating Quaternary Deposits and Soils with TIMS U-Series on Pedogenic Carbonate and Silica

9725836查德威克基金被要求进一步开发和应用TIMS（热电离质谱）u系列测年成壤碳酸盐和二氧化硅。该研究将通过在两个气候和岩性不同的地区进行详细的工作，试图了解u系列日期在化学/矿物学特征上的潜力和局限性。两个地区将被研究，使用的技术是建立在私家侦探最近独立研究结果的基础上的。Kyle Canyon（内华达州）提供了一系列从全新世到780 ka的碳酸盐碎屑冲积矿床和形态面，而Wind River Basin（怀俄明州）提供了一系列从全新世到早更新世的花岗岩碎屑冲积矿床。在这两个地区，第四纪地质和土壤学已经被本提案的一位或多位研究者进行了很好的研究。这是三个机构之间的合作建议：伯克利地质年代学中心的Ken Ludwig和Warren Sharp将负责使用新的TIMS设备和BGC的清洁实验室进行采样和铀系列分析；加州大学伯克利分校的Ron Amundson和加州大学圣巴巴拉分校的Oliver Chadwick将提供第四纪地质学和土壤学的实地和实验室专业知识，以及对研究地点的熟悉程度。所有被点名的研究人员都将参与成土碳酸盐和二氧化硅的化学和微观表征，阿蒙森将负责稳定同位素的工作和解释。Ludwig和J.B. Paces最近在内华达州的Crater Flat进行的一项研究中采用了单样品/单日期的方法，其中对单个碳酸盐/二氧化硅外壳的亚毫米层状进行了微采样，以避免碎屑污染，该方法将应用于两个地点沉积物的仔细表征的碎屑外壳。探讨了该方法的局限性，包括碎屑230Th校正的可靠性，开放系统的结构和地球化学指标，以及将该方法扩展到早更新世使用234U/238U的实用性。全新世碎屑壳的分析将特别有趣，因为这些样品将提供对碎屑沉积与可测定的碳酸盐或碳酸盐/硅壳堆积之间可能的时间滞后的理解。了解这一时间差（以及它在不同气候和岩性背景下的变化）对于使用最内层碎屑壳的u系列日期来确定第四纪沉积物本身的年代至关重要。微采样技术的一些发展将需要探索样本大小和空间分辨率的限制，但这可以用相对适度的设备来完成。实验室的挑战主要涉及小样本230分析，其中一个P.I.他有丰富的经验。测年研究的结果将用于了解凯尔峡谷的侵蚀和沉积的历史和时间，以及风河盆地的冰川事件。有限数量的稳定同位素分析将用于评估这些环作为古气候记录的有用性，以及检查更新世中期和早期环的开放系统历史。

9725836 Chadwick Funds are requested to further develop and apply TIMS (Thermal Ionization Mass Spectrometry) U-series dating to pedogenic carbonate and silica. The research will seek to understand the potential and limitations of U-series dates on chemically/mineralogically-characterized, micro-sampled clast rinds by detailed work in two climatically and lithologically distinct areas. Two localities will be studied, using techniques that build on the results of recent, independent studies by the P.I.'s. Kyle Canyon (Nevada) provides a sequence of carbonate-clast alluvial deposits and morphologic surfaces ranging from Holocene to 780 ka, whereas the Wind River Basin (Wyoming) provides a sequence of granitic-clast dominated alluvial deposits from Holocene to early Pleistocene age. In both localities, the Quaternary geology and pedology have been well studied by one or more of the investigators of this proposal. This is a collaborative proposal among 3 institutions: Ken Ludwig and Warren Sharp at the Berkeley Geochronology Center will be responsible for sampling and uranium-series analyses using the new TIMS facility and clean lab at BGC; Ron Amundson at U.C. Berkeley and Oliver Chadwick at U.C. Santa Barbara will provide field and laboratory expertise in Quaternary geology and pedology, as well as familiarity with the study localities. All of the named investigators will participate in the chemical and microscopic characterization of the pedogenic carbonate and silica, and Amundson will be responsible for the stable-isotopic work and interpretation. The single-sample/single-date approach developed by Ludwig and J.B. Paces for a recent study at Crater Flat, Nevada, in which sub-millimeter laminae of individual carbonate/silica rinds are micro-sampled to avoid detrital contamination, will be applied to carefully-characterized clast rinds of the deposits in both localities. The limitations of the approach will be explored including the reliability of the detrital 230Th corrections, the textural and geochemical indicators of open systems, and the practicality of extending the method into the Early Pleistocene using 234U/238U. Analyses of Holocene clast-rinds will be of special interest, as these samples will provide an understanding of the likely time-lag between clast deposition and the accumulation of datable carbonate or carbonate/silica rinds. Knowledge of this time lag (and its variability in different climatic and lithologic contexts) will be essential in using U-series dates of innermost clast-rinds to date the Quaternary deposits themselves. Some development of micro-sampling techniques will be required to explore the limits of sample size and spatial resolution, but this can be done with relatively modest equipment. The laboratory challenges involve mainly those of small-sample 230Th analyses, in which one of the P.I.'s has extensive experience. The results of the dating studies will be applied to understanding the history and timing of erosion and sedimentation in Kyle Canyon, and of glacial events in the Wind River Basin. A limited number of stable-isotope analyses will be used to evaluate the usefulness of the rinds as a paleoclimate record, as well as to check for open-system histories of the mid-and early Pleistocene rinds.