SAWTOOTH VARVE THICKNESS PATTERNS RECORD RE-ORGANIZATION OF MELTWATER PATHWAYS ALONG THE LAURENTIDE ICE SHEET

Determining the pathway routing from pro-glacial lakes is challenging. We present here a distinct, sawtooth varve thickness pattern that may contribute to decoding the signature of meltwater diversions. The sawtooth pattern is characterized by abrupt thickening followed by decreasing varve thickness for one or more decades after the initial event. Such sawtooth-pattern varve series are found in sediments from many large glacial lakes (including the Baltic Ice Lake, Agassiz, Duluth, Hitchcock, Lind, and Ojibway). Multiple mechanisms have been proposed to explain this pattern, including the rapid onset of ice sheet melting (which slows through time), and abrupt lake drawdown (followed by erosion of sediment along lake margins).

Here we propose an alternative mechanism based on five example events from three different glacial lakes (Agassiz, Ojibway, and Superior). In each of these cases, geochemical evidence (scanning XRF, δ¹⁸O, and/or REE) show that a change in the provenance of the sediment is associated with the sawtooth pattern. In two of these cases, lake levels appear to have increased after the event. We suggest that these sawtooth patterns could represent the new routing of meltwater and sediment that has a sudden onset, but decreases over a few decades. The ultimate test of this model would be to find a correlation between upstream and downstream glacial lake basins, whereby the upstream basin shows lowering at the time the downstream basin shows the sawtooth pattern.