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

Laurentide冰盖下的侵蚀及其在冰期动态冰下侵蚀和沉积物动力学中的作用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，1993a，b)，并可能决定第四纪冰期的周期性(Clark和Pollard，1998)。我们直观地将北部大陆的冲刷地貌与冰下侵蚀联系在一起，但对更新世冰盖侵蚀的速度、时间和空间模式的估计是相互矛盾或模棱两可的。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)与所有侵蚀地貌一样，随着地表的去除，变化的记录不断被抹去，以及(Iii)虽然更新世冰川沉积物的厚厚沉积物在以前的冰盖边缘保留了下来，其中包含有关侵蚀条件的信息，但它们保存得很零星，很难确定日期。首先，我们将使用“埋葬测年”技术测定前冰盖南缘的冰川沉积序列(Granger等人，1997年)。宇宙成因的10Be和26Al是在沉积石英颗粒的晶体结构中产生的，而沉积物则暴露在地球表面附近的宇宙辐射中。一旦沉淀物被埋藏，这些同位素以不同的速度衰减，它们的比率提供了埋葬时间的衡量标准。我们已经使用这项技术在冰河砂岩和冰川互层上进行了试验测量，提供了先前未确定日期的冰川沉积的中更新世地层一致的年龄。其次，我们将使用大气产生的10Be，它积累在土壤中，高度集中在深度风化的地形表面附近，以调查冰前风化层对Laurentide耕作的贡献。我们推测，在第四纪之前，覆盖在加拿大盾上的深层风化层一定包含了10Be的巨大库存，远远超过了第四纪间冰期克拉通可能积累的储量。通过测量整个更新世Laurentide冰盖侵蚀和沉积的沉积物中10Be的含量，我们将跟踪冰前冰盖的移动，并评估冰盖对硬床条件开始的响应。我们的初步测量结果显示，中更新世冰盖含有大量的“流星”10Be。威斯康星州的收割机浓度要低两个数量级。在中更新世，Laurentide冰盖似乎正在开采一层严重风化的地表物质，但现在这一来源已经基本枯竭。我们将把这些方法应用于明尼苏达州西南部经过充分研究的冰川和互层冰川沉积物序列，以及南达科他州和内布拉斯加州钻芯中被准确年代的火山灰层包围的早更新世冰盖。这种组合将提供在整个更新世被劳伦蒂冰盖侵蚀的物质样本。明尼苏达州西南部的田野区域提供了至少九个独立的露头和钻芯耕作单位。在该地区进行了大量的前期工作，而且很容易获得地表和地下样品，这使其成为本研究的理想选择。

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.