Recent changes in bottom water oxygenation and temperature in the Gulf of St. Lawrence: Micropaleontological and geochemical evidence

Micropaleontological and geochemical analyses of a sediment core collected in the Laurentian Trough of the Gulf of St. Lawrence were carried out to reconstruct temporal variations in pelagic productivity and benthic environmental conditions. Dinoflagellate cyst assemblages reveal relatively stable pelagic productivity over the last two centuries. Similarly, geochemical (organic C, C-org : N) and isotopic (delta C-13(org), delta N-15) data reveal that organic matter fluxes to the seafloor have been relatively constant over the same period. In contrast, significant changes are recorded in the benthic foraminifer assemblages. A sediment surface peak in the abundance of Cassidulina laevigata and Brizalina subaenariensis is consistent with the recent record of oxygen depletion in the bottom water. A decrease in the relative abundance of Nonionellina labradorica, concomitant with a relatively higher occurrence of Oridorsalis umbonatus in the upper part of the core, reflects a significant warming of the bottom water. Changes in bottom-water properties are further constrained by a negative trend of the delta O-18 in Bulimina exilis carbonate shells over the last century, corresponding to a warming of about 2 degrees C. These results strongly suggest that the recent oxygen depletion in the bottom waters of the Gulf of St. Lawrence is due to changes in water masses that have led to increased bottom-water temperatures and, to some extent, a resultant increase in organic matter respiration rates.