Collaborative Research: Constraining the Temporal Resolution and Geochronology of Holocene Sediment Records from the Antarctic Continental Shelf

该奖项由极地项目办公室的南极地质和地球物理项目提供，支持研究南极沉积物中的古环境代理记录并改进确定记录年龄的方法的项目。南极大陆边缘栖息盆地的海相沉积物岩心为该地区晚第四纪的环境条件提供了重要的代用记录。这些沉积物岩心通常缺乏碳酸盐微化石，主要通过使用加速器质谱法测量的散装有机碳的碳-14 （14C）含量来确定年代。然而，沉积物-水界面有机物的非零放射性碳年龄（所谓的碳储层问题）需要对下核14C年龄进行“修正”，这可能高达1000 - 3000年。此外，底栖生物对上部沉积物柱的扰动影响了沉积物岩心中保存的替代记录，从而限制了这些沉积物记录的时间分辨率。对于南极大陆架的大部分地区来说，地表沉积物的放射性碳年龄和底栖生物混合的深度尚不确定。该项目将对大块有机质的碳同位素组成和无支撑铅-210的活度进行高分辨率下核分析，以确定几个南极陆架盆地表面沉积物的放射性碳年龄和沉积物混合层的深度。这项工作将检查在Scholleart Drift （Gerlache海峡中部，南极半岛西部），Vega Drift（威德尔海西北部，南极半岛东部）和Nielsen Basin和Iceberg Alley （MacRobertson Shelf，南极洲东部）收集的Kasten岩芯。之所以选择这些研究区域，是因为：(1)这些样本收集的数据将直接有利于从这些地点恢复的高质量长沉积物岩心的持续研究；(2)这些地点在地表水的储集年龄、沉积环境的能量、底栖生物群的范围和类型以及靠近再悬浮和残留有机物质的潜在来源方面提供了一系列对比环境。这将有助于评估这些因素对南极大陆架表面沉积物放射性碳年龄的相对重要性。放射化学测定将利用加速器质谱法进行碳同位素测量，利用高灵敏度伽马谱法分析铀系列核素，这两种技术以前已成功地用于南极陆架沉积物。南极大陆架是海洋深水形成、季节性海冰形成和海洋初级生产过程的重要区域，这些过程很可能受到未来气候变化的强烈影响。预测该地区将如何受到未来气候变化的影响和调节的能力，严重依赖于对过去气候变化期间南极边缘状况的了解。该项目将在这方面作出重大贡献，提供准确破译和解释南极大陆架全新世沉积物岩心记录所需的关键数据。此外，通过提高我们对沉积环境和底栖生物群落对放射性碳年代学和南极沉积物记录分辨率的影响的理解，这项工作将有助于选择未来可能提供高质量古环境记录的地点。该项目还将在多个层面上对教育做出贡献，包括本科生和研究生的研究以及基于网络的公共宣传。

This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate paleoenvironmental proxy records in Antarctic sediments and improve approaches to determine the age of the records. Marine sediment cores from perched basins on the Antarctic continental margin have provided important proxy records of environmental conditions in this region during the late Quaternary. Typically lacking in carbonate microfossils, these sediment cores have primarily been dated by using the carbon-14 (14C) content of bulk organic carbon, measured by accelerator mass spectrometry. However, the non-zero radiocarbon age of organic matter at the sediment-water interface (the so-called carbon reservoir problem) necessitates a "correction" to down-core 14C ages, which may be as much as 1,000-3,000 years. In addition, bioturbation of the upper sediment column by benthic organisms serves to smear the proxy records preserved in sediment cores, thus limiting the temporal resolution of such sediment records. For most of the Antarctic shelf, the radiocarbon age of surface sediments and the depth of mixing by benthic organisms are not known with any certainty.This project will undertake high-resolution down-core analyses of the carbon isotopic composition of bulk organic matter and the activity of unsupported lead-210 in order to establish the radiocarbon age of surface sediments and the depth of the sediment mixed-layer in several Antarctic shelf basins. The work will examine Kasten cores collected in the Scholleart Drift (central Gerlache Strait, west Antarctic Peninsula), the Vega Drift (northwestern Weddell Sea, east Antarctic Peninsula), and the Nielsen Basin and Iceberg Alley (MacRobertson Shelf, East Antarctica). These study areas have been selected because (1) the data collected for these samples will be of direct benefit to ongoing studies of high-quality long sediment cores recovered from these sites, and (2) these sites provide a range of contrasting environments in terms of reservoir age of surface waters, energy of depositional environment, extent and type of benthic biota, and proximity to potential sources of re-suspended and relict organic material, which will allow evaluation of the relative importance of these factors for the radiocarbon age of surface sediments on the Antarctic shelf. The radiochemical determinations will make use of accelerator mass spectrometry for carbon isotope measurements, and high-sensitivity gamma spectrometry for analysis of uranium-series nuclides, two techniques that have been previously used with success for Antarctic shelf sediments. The Antarctic continental shelves are important regions of oceanic deep-water formation, seasonal sea-ice formation, and marine primary production - processes that are likely to be strongly influenced by future climatic change. The ability to predict how this region will be affected by, and modulate, future climate change is critically dependent on an understanding of conditions on the Antarctic margin during past climatic variations. This project will make a significant contribution in this regard, by providing key data that are required to accurately decipher and interpret Holocene sediment-core records from the Antarctic continental shelf. In addition, by improving our understanding of the effects of depositional environment and benthic community on the radiocarbon chronology and resolution of Antarctic sediment records, this work will assist in the selection of future sites that are likely to provide high-quality paleoenvironmental records. This project will also make educational contributions at several levels, including undergraduate and graduate research as well as web-based public outreach.