Transient tracer observations in the Gulf of St. Lawrence reveal shift from younger to older inflow waters

Abstract

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, which consists of a mixture of warmer, less-oxygenated North Atlantic Central Waters (NACW) and cooler, more-oxygenated Labrador Current Waters (LCW). While previous studies have inferred changes in this inflow from hydrographic and biogeochemical observations, direct constraints on deep-water ventilation timescales have been lacking. Here, we present the first spatially-comprehensive measurements of transient tracers (SF6 and CFC-12) in the GSL, allowing direct estimates of tracer-derived mean ages and ventilation timescales throughout the deep layer. Mean ages increase systematically with density and exhibit a pronounced along-channel gradient, with younger waters (30-40 years) in the Lower St. Lawrence Estuary and older waters (50-60 years) toward Cabot Strait. This west-east increase in ventilation age is more gradual than corresponding changes in temperature and salinity, indicating that tracer-derived ages respond differently to variations in source water composition than hydrographic properties. The observed gradual age structure is consistent with a greater influence of older, earlier ventilated NACW in the eastern Gulf, while younger ages inland reflect a continued contribution of LCW. These results demonstrate that, as of 2022, the shift toward longer deep-water ventilation timescales, and associated changes in inflow properties toward higher NACW composition, is ongoing rather than having reached a steady end state, contrary to earlier predictions.