The changing composition of the Gulf of St. Lawrence inflow waters observed from transient tracer measurements

The deep waters of the Gulf of St. Lawrence (GSL) have experienced a significant reduction in dissolved oxygen content during the past decades. One widely documented driver of this deoxygenation is a change in the composition of the deep inflowing water that ventilates the Gulf. This deep water is known to consist of a mix of warmer, less-oxygenated North Atlantic Central Waters (NACW) and cooler, more-oxygenated Labrador Current Waters (LCW), with prior studies inferring a shift towards increased NACW contribution. However, this compositional change has only ever been inferred indirectly from physical and biogeochemical measurements via the use of inverse methods such as water mass analysis. In this study, we present results from the first spatially-comprehensive deep water transient tracer surveys in the GSL, allowing us to directly map mean age estimates and use these to infer recent changes in the composition of regional deep waters. The results reveal an unexpected age distribution, with 'older' deep waters present near the Gulf's entrance, whereas 'younger' water is found further inshore, contrary to the expected estuarine circulation pattern, which transports deep water inland (increasing age along the flow path). This implies a gradual increase in the proportion of NACW from inshore areas towards the Gulf's entrance and provides direct evidence that the shift towards NACW dominated deep waters is ongoing as of 2022, contrary to earlier predictions of the complete disappearance of the younger, well-oxygenated LCW.