北美五大湖区晚更新世劳伦泰德冰盖演化年代学综述及石英光释光测年的初步尝试

北美五大湖区的安大略湖北岸Don Valley Brickyard、Scarborough Bluffs、Bowmanville Bluffs剖面共同构成了北美东北部最长也是最厚的陆地第四纪沉积记录, 较完整地记录了晚更新世劳伦泰德冰盖(the Laurentide Ice Sheet)的演化. 晚更新世劳伦泰德冰盖演化的重建有赖于这些经典剖面中重要沉积地层单元的准确年代学控制. 传统的地层年代学主要是依靠少量¹⁴C年代, 将主要的混杂堆积单元(diamicton)解释为气候变冷环境下的冰川扩张, 并与指示全球冰量变化的深海氧同位素曲线一一比对建立起来的. 这样建立起来的年代学存在很大的不确定性. 20世纪80-90年代的少量热释光年代也不相吻合, 最近的13个长石红外释光定年则只集中于Bowmanville Bluffs的一个分层, 并未建立整个剖面的地层年代学, 使这些经典沉积剖面的年代学一直没有得到系统的建立. 应用石英光释光SAR-SGC法测试了Bowmanville Bluffs剖面Glaciofluvial Sand单元的2个冰水沉积样品, 年代结果分别为(41.6±3.8) ka、(48.1±4.4) ka, 分析表明这一年代结果偏老, 石英颗粒可能晒褪不完全. 由于大测片无法识别晒褪不完全的颗粒, 因此, 测试更多的剖面序列的光释光年代并尝试采用粗颗粒小测片或单颗粒技术解决样品颗粒晒褪不完全的问题将是必要的.

Late Pleistocene chronology of the Laurentide Ice Sheet in the Great Lakes regions of North America: a review and new Quartz OSL dating

The three sections, Don Valley Brickyard, Scarborough Bluffs and Bomanville Bluffs on north-shore of Lake Ontario, are the longest and thickest Quaternary terrestrial sediments in northeastern North America and a relatively complete record of the evolution of the Laurentide Ice Sheet since late Pleistocene. Reconstruction of the evolution has to be depended on the accurate absolute dates of the key units from these reference sections. The established stratigraphic chronology framework has been based mainly on limited ¹⁴C and luminescence (TL and IRSL using multiple-mineral technique) ages, diamictons interpreted as till representing ice advance that was controlled by climate change, and comparison with marine oxygen isotope stages. However, there is great uncertainty because well-preserved organic materials were difficult to find in glacial sediments and ¹⁴C dating range is limited to the last 50 ka. The thermoluminescence ages reported in the 1980s and 1990s are conflicting. Feldspar infrared stimulated luminescence ages reported recently focused on a sublayer of Bowanville Bluffs section only and with no ages for other units. Thus, the stratigraphic chronology of these classic sections remains unclear. Quartz optical stimulated luminescence, that has advantages over ¹⁴C, feldspar TL and feldspar IRSL dating by directly defining the timing of deposition and quartz is more rapidly bleached than feldspar, has been widely applied to a wide range of Quaternary sediments as a result of rapid technical developments. Here a review of the Late Pleistocene chronology is tried to provide for the Laurentide Ice Sheet in the Great Lakes of North America, and also two quartz OSL ages for a key unit stratigraphy. The two ages were dated using SAR-SGC protocol (using large aliquot) on two samples from Glaciofluvial Sand unit of Bowmanville Bluffs section. The results are (41.6±3.8) ka and (48.1±4.4) ka, indicating it deposited in MIS 3. Further OSL dating using small aliquot or single grain are required to refine the chronology.