Impact of ice sheet dynamics and short-lived climate oscillations on the late Quaternary stratigraphy and paleo-environments of the Gulf of St. Lawrence off Prince Edward Island (Canada)

The last glacial period hugely affected the modern landscape of high latitude regions of the Northern Hemisphere, as continents and surrounding shelves were covered by massive ice sheets. Deglaciation of these ice masses during the late Pleistocene resulted in isostatic rebounds in response to unloading. In many regions the initial uplift changed to isostatic subsidence in response to the movement of the collapsing forebulge margin and hydro-isostatic loading. As a consequence, many modern coastal regions nowadays still experience subsidence, which in many cases is accompanied by a rise in relative sea level (RSL). The timing and impact of a retreating ice sheet on the continental shelf environments, and the potential effects on local seal level is, however, still poorly understood in many regions of North America. In addition, the effects of short-lived climatic events on ice dynamics such as the Younger Dryas are still debated. In this study we provide new information on the dynamics of deglaciation and sedimentation along the outer margin of the Laurentide Ice Sheet in the southern Gulf of St. Lawrence off Prince Edward Island, located on the eastern Canadian Continental Shelf. Here, many questions remain as to the sedimentary processes following the Last Glacial Maximum (LGM) and, in particular, the influence of the Younger Dryas on the deglaciation and sedimentation. Using information from sub-bottom profiles, sediment cores and multibeam bathymetry from the Gulf of St. Lawrence, we were able to determine geological processes that affected sedimentation and used this new information to reconstruct the deglaciation history and RSL changes since the LGM. This study demonstrates that the bedrock morphology was shaped during the last glacial period, with the Younger Dryas cooling event having a particularly significant impact on sedimentation processes in the southern Gulf of St. Lawrence. Paleo-basins and channels, likely glacially excavated, were completely filled with late Pleistocene glaciolacustrine to marine sediments. Most of these sediments were deposited during the early Younger Dryas, with sedimentation rates as high as 1 cm a-1, which were likely a consequence of readvancing ice masses. We further observe an erosional truncation on top of the Younger Dryas sediment package, which presumably indicates a drop in RSL before the Holocene. The new paleo-environmental reconstruction of the region not only sheds light on the potential impact of short-lived climatic events during deglaciation but also reduces uncertainties in our knowledge about past sea level changes.

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