Erosion Beneath the Laurentide Ice Sheet, and its Role in Pleistocene Ice Age Dynamics

劳伦太冰盖下方的侵蚀及其在更新世冰河时代动力学中的作用

• 批准号: 0207844
• 负责人: John Stone
• 金额: $ 21.74万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-08-15 至 2005-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0207844&HistoricalAwards=false
• 关键词: Erosion; Beneath; Laurentide; Ice; Sheet

EROSION BENEATH THE LAURENTIDE ICE SHEET, AND ITS ROLE INPLEISTOCENE ICE AGE DYNAMICSSubglacial erosion and sediment dynamics influence the size, stability, and climatic sensitivity of large ice sheets. These processes may regulate large-scale surging behavior, initiating rapid shifts in climate and sea level (MacAyeal, 1993a,b), and perhaps dictate the periodicity of the Quaternary ice ages (Clark and Pollard, 1998). We intuitivly associate the scoured landscapes of the northern continents with subglacial erosion, yet estimates of the rates, timing and spatial pattern of erosion by the Pleistocene ice sheets are conflicting or ambiguous. Subglacial erosion is difficult to study because: (i) The processes involved take place beneath large ice sheets. (ii) As in all eroding landscapes, the record of change is continually effaced as the surface is removed, and (iii) Although thick deposits of Pleistocene glacial sediment, which contain information about erosional conditions, survive around the margins of former ice sheets, they are patchily preserved and difficult to date.We propose to study the history of erosion by the Laurentide ice sheet, using thecosmogenic isotopes 10 Be and 26 Al. First, we will date glacial sedimentary sequences at the southern margin of the former ice sheet using the technique of "burial dating" (Granger et al., 1997). Cosmogenic 10 Be and 26 Al are produced within the crystal structure of sedimentary quartz grains while the sediment is exposed to cosmic radiation near the Earth's surface. Once the sediment is buried, these isotopes decay at different rates and their ratio provides a measure of the burial time. We have used this technique in trial measurements on glaciofluvial sands interbedded with till, providing stratigraphically consistent mid-Pleistocene bracketing ages on previously undated glacial deposits. Second, we will use atmospherically-produced 10 Be, which accumulates in soils and is highly concentrated near the surface of deeply weathered terrains, to investigate the contribution of pre-glacial regolith to Laurentide tills. We presume the deep regolith that covered the Canadian Shield prior to the Quaternary must have contained an enormous inventory of 10 Be, far exceeding what could have accumulated on the craton during Quaternary interglacial periods. By measuring the 10 Be content of sediments eroded and deposited by the Laurentide ice sheet throughout the Pleistocene, we will track the removal of pre-glacial regolith and evaluate the ice sheet's response to the onset of hard-bed conditions. Our initial measurements show that mid-Pleistocene tills contain enormous concentrations of 'meteoric' 10 Be. Concentrations in Wisconsin till are two orders of magnitude lower. It appears that the Laurentide ice sheet was 'mining' a cover of heavily-weathered surficial material in the middle Pleistocene, but that this source has now been largely exhausted.We will apply these methods to well-studied sequences of till and interbedded glaciofluvial sediments in southwestern Minnesota, and early Pleistocene tills bracketed by well-dated ash layers in drill cores from South Dakota and Nebraska. This combination will provide samples of material eroded by the Laurentide Ice Sheet throughout the Pleistocene. The field area in southwestern Minnesota provides access to at least nine separate till units in outcrop and drill core. Extensive prior work in the area, and easy access to both surface and subsurface samples make it ideal for this study.

劳伦潮冰盖下的侵蚀及其在更新世冰期沉积物中的作用冰下侵蚀和沉积动力学影响着大冰盖的大小、稳定性和气候敏感性。这些过程可能会调节大规模的涌动行为，引发气候和海平面的快速变化（MacAyeal，1993 a，B），并可能决定第四纪冰期的周期性（Clark和Pollard，1998）。我们直观地将北方大陆的冲刷景观与冰下侵蚀联系起来，但对更新世冰盖侵蚀的速率、时间和空间模式的估计是相互矛盾或模糊的。冰下侵蚀很难研究，因为：（i）所涉及的过程发生在大冰盖之下。(ii)在所有的侵蚀景观，变化的记录是不断抹去的表面被删除，和（iii）虽然更新世冰川沉积物的厚存款，其中包含有关侵蚀条件的信息，生存在边缘的前冰盖，他们是零星保存和难以确定日期。首先，我们将使用“埋藏年代测定”技术（格兰杰等人，1997年）。宇宙成因的10 Be和26 Al是在沉积石英颗粒的晶体结构中产生的，而沉积物暴露于地球表面附近的宇宙辐射。一旦沉积物被掩埋，这些同位素以不同的速度衰变，它们的比例提供了埋藏时间的测量。我们使用这种技术对与冰碛互层的冰川河流砂进行了试验测量，为之前未注明日期的冰川沉积物提供了地层上一致的中更新世括号年龄。其次，我们将使用大气产生的10 Be，它积累在土壤中，是高度集中的深度风化的地形表面附近，调查的贡献，冰前风化层Laurentide冰碛。我们推测深风化层，覆盖加拿大盾之前，第四纪必须包含一个巨大的库存10 Be，远远超过可能积累的克拉通在第四纪间冰期。通过测量沉积物的10 Be含量的侵蚀和沉积的Laurentide冰盖在整个更新世，我们将跟踪冰川前风化层的去除和评估冰盖的硬床条件的发病的反应。我们的初步测量结果表明，中更新世冰碛物含有巨大的浓度的“流星”10 Be。威斯康星州的浓度要低两个数量级。看来，Laurentide冰盖是“采矿”的覆盖层的严重风化的表层物质在中更新世，但这种来源现在已经基本耗尽。我们将这些方法应用到研究序列的冰碛和层间冰川河流沉积物在明尼苏达州西南部，和早更新世冰碛包围的年代确定的灰层在钻孔岩芯从南达科他州和内布拉斯加州。这种组合将提供整个更新世被劳伦泰德冰盖侵蚀的物质样本。明尼苏达州西南部的油田区域提供了至少九个独立的露头和岩心耕作单元。在该地区进行的大量前期工作，以及容易获得的地表和地下样品使其成为这项研究的理想选择。