Dissolved and particulate 231Pa and 230Th in the Atlantic Ocean: constraints on intermediate/deep water age, boundary scavenging, and 231Pa/230Th fractionation

Abstract

231Pa and 230Th concentrations were determined in filtered seawater and suspended particulate matter collected from the Labrador Sea and the Equatorial and South Atlantic to constrain their application as tracers of intermediate/deep water age and Atlantic thermohaline circulation. Distributions of total 231Pa and 230Th indicate the influence of recently formed North Atlantic Deep Water, as evidenced by nearly invariant concentrations below ∼1000 m in the Labrador Sea and increasing 231Pa and 230Th concentrations as deep waters progress southward from northern source regions. Application of a scavenging–mixing model to both tracer distributions indicates an intermediate/deep water age of 12 yr in the Labrador Sea and a ∼30–140 yr transit time to the low-latitude stations. We attribute a striking increase in total 230Th in the Labrador Sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the Labrador Sea since 1993. The temporal change in the 230Th age of these waters is consistent with the 6 yr time interval between the observations. The average particulate 231Pa/230Th activity ratio in the Labrador Sea and low-latitude deep waters is 0.057±0.003, significantly below the 231Pa/230Th production ratio (0.093) and in agreement with excess 231Pa/230Th ratios in Holocene sediments (0.060±0.004) and sediment trap material (0.034±0.012) from the Atlantic and model simulations. This observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the Atlantic. A latitudinal dependence in particle fractionation of these tracers is also evident, with elevated fractionation factors (FTh/Pa) observed near the Equator and South Atlantic gyre (∼11) compared to low values in the Labrador Sea (∼3) and Southern Ocean (∼2). There also exists a depth dependence in FTh/Pa, characterized by low values in surface waters, a broad mid-depth maximum, and decreasing values towards the sea-floor. The latitudinal and depth variations in FTh/Pa are suggested to reflect differences in the chemical composition of marine particles.