Post-glacial development of marine conditions on the Scotian Shelf inferred from pore water profiles and reaction-transport modeling

The Scotian Shelf on the northwest Atlantic Margin is located at the confluence of two important components of the Atlantic Meridional Overturning Circulation (AMOC). The southward flowing Labrador Current supplies cold, oxygen rich waters and the northward flowing Gulf Stream delivers warm, nutrient rich waters low in O₂. Their mixing allows the establishment of a productive marine ecosystem. The relative influence of the current systems is governed by northern hemispheric climate patterns, such as the overall AMOC strength and the North Atlantic Oscillation mode. However, the exact atmospheric and oceanographic mechanisms are still under debate. Due to this knowledge gap regarding the climate-bioproductivity feedback, a deeper insight into the biogeochemical evolution of the region since the Holocene is an important aspect for understanding North Atlantic climate and circulation.

On the Scotian Shelf, glacially eroded basins are separated from the open ocean by shallower sills on the outer shelf. Using solid phase and pore water geochemical data from three eight- to twelve-metre-long sediment cores, in combination with reaction-transport modelling, we reconstructed carbon and sulfur cycling at the seafloor along the Scotian Shelf since the last deglaciation. Chloride profiles imply that the basins were filled with freshwater during the earliest phase of the deglaciation. Due to the absence of sulfate reduction in freshwater sediments, reactive Fe oxides escaped pyritization during deposition of the deepest sediment layers. Between 14 and 8 ka BP, a combination of eustatic sea-level rise and isostatic adjustment led to marine transgression and the establishment of fully marine conditions on the shelf, accompanied by increased organic matter deposition and burial. Modelled anaerobic oxidation of methane coupled to reduction of iron oxide minerals in deeper sediment layers in the present day alludes to a geochemical fingerprint of the formerly prevailing freshwater conditions in the shelf basins.  

Our data and model outcomes allow us to pinpoint the timing of marine transgression for three individual basins along the Scotian Shelf and reconstruct the corresponding evolution of contemporary biogeochemical conditions. We conclude that the diagenetic conditions in Scotia Shelf sediments evolved in a similar manner to those described previously for marginal seas with restricted exchange with the open ocean, such as the Baltic Sea.