A detailed lake-level reconstruction shows evidence for two abrupt lake drawdowns in the late-stage history of the eastern Lake Agassiz-Ojibway basin

Glacial Lake Ojibway occupied large areas in northeastern Ontario and northwestern Quebec during the last deglaciation. Paleogeographic reconstructions depict Lake Ojibway and Lake Agassiz sharing a common water plane shortly before the final drainage of this lake similar to 8200 years ago. However, the predrainage surface elevation and extent of this large coalesced lake is based on scattered raised shorelines in the Ojibway basin and poorly constrained geomorphological considerations, which convey large uncertainties in lake-level reconstructions and the attendant meltwater volume estimates used to assess the impact of this freshwater discharge. Here, we address this issue by using 3098 elevation measurements of shorelines mapped on a LiDAR digital terrain model covering a key area in the Ojibway basin. Our reconstruction is based on a GIS model integrating the paleotopography, which reinforces correlation of shorelines and the identification of lake levels. The spatial distribution of shoreline-elevation data points shows well-defined clusters of shorelines aligned on distinct elevation ranges that delineate three widespread lake levels, which are separated by two intervals with atypically low number of shorelines reflecting abrupt lake drawdowns. This reconstruction considerably refines the sequence in the eastern (Ojibway) basin and shows evidence for a two-step drainage of Lake Agassiz-Ojibway. The results provide constraints on the magnitude of the lake-level drops and indicate that the pre-drainage surface-elevation of Lake Agassiz-Ojibway of earlier reconstructions was likely overestimated. Accordingly, the configuration and structure of this lake stage should be re-evaluated. To that effect, the new shoreline record provides an important framework for assessing late-stage changes in lake levels across the Agassiz-Ojibway basin, which is a critical step towards the production of realistic models depicting the drainage of this large lake. (C) 2020 Elsevier Ltd. All rights reserved.

在末次冰消期，奥吉布瓦冰湖占据了安大略省东北部和魁北克省西北部的大片区域。古地理重建显示，在大约8200年前该湖最终排水之前不久，奥吉布瓦湖和阿加西湖共享一个共同的水面。然而，这个大型合并湖在排水前的湖面高程和范围是基于奥吉布瓦盆地中零散的上升岸线以及受约束较差的地貌因素，这在湖面重建以及用于评估这种淡水排放影响的相关融水量估算中带来了很大的不确定性。在此，我们通过使用在覆盖奥吉布瓦盆地一个关键区域的激光雷达数字地形模型上绘制的3098个岸线高程测量值来解决这一问题。我们的重建基于一个整合了古地形的地理信息系统模型，该模型加强了岸线的相关性以及对湖面高程的识别。岸线 - 高程数据点的空间分布显示，在不同高程范围内排列着定义明确的岸线群，这些岸线群勾勒出三个广泛分布的湖面高程，它们被两个岸线数量异常少的区间隔开，反映了湖面的突然下降。这种重建极大地细化了东部（奥吉布瓦）盆地的序列，并显示出阿加西 - 奥吉布瓦湖分两步排水的证据。研究结果对湖面下降的幅度提供了限制，并表明早期重建中阿加西 - 奥吉布瓦湖排水前的湖面高程可能被高估了。因此，应该重新评估这个湖阶段的形态和结构。为此，新的岸线记录为评估阿加西 - 奥吉布瓦盆地湖面的后期变化提供了一个重要框架，这是构建描绘这个大型湖泊排水的真实模型的关键一步。（C）2020爱思唯尔有限公司。保留所有权利。