Fine-scale spatial and interannual cadmium isotope variability in the subarctic northeast Pacific

Abstract We present dissolved cadmium (Cd) concentrations, [Cd], and stable isotope compositions, e 112 / 110 Cd , in high-resolution depth profiles from five stations along the Line P transect in the subarctic northeast Pacific Ocean. In addition to profiles collected in 2012, subsurface isopycnal samples and surface samples were collected in 2013 and 2014 respectively, providing both temporal and spatial coverage. Surface waters are characterized by Cd depletion relative to phosphate ( PO 4 3 − ) compared to deepwater Cd:PO 4 3 − , and high inferred remineralization ratios in the nutricline ( 0.45 nmol μmol − 1 ) are observed, consistent with Cd enrichment relative to phosphorus (P) in surface-derived biogenic particles. The correlation between Cd and PO 4 3 − weakens at depths where oxygen is highly depleted as shown by local minima in dissolved [Cd] and the tracer Cd ⁎ . The decoupling, which is driven by a deficit of Cd relative to PO 4 3 − , appears consistent with the recent hypothesis of dissolved Cd removal in oxygen-depleted regions by insoluble metal sulfide formation. Dissolved e 112 / 110 Cd indicates a biologically driven fractionation in surface waters with more positive (heavy) values in the upper water column and lower (light) values in deeper waters. The highest e 112 / 110 Cd observed in our sample set ( 5.19 ± 0.23 ) is comparable to observations from the Southern Ocean but is significantly lighter than maximum reported surface values from the subtropical North Pacific of e 112 / 110 Cd ≥ 15 . A global compilation of low [Cd] surface water shows similar differences in maximum e 112 / 110 Cd . A surface water intercalibration should be prioritized to help determine if these differences at low [Cd] reflect true physical or biological variability or are due to analytical artefacts. Surface samples from the 2012 sampling campaign fit a closed-system Rayleigh fractionation model; however, surface waters sampled in 2014 had much lower [Cd] with relatively constant e 112 / 110 Cd that cannot be explained by a closed-system Rayleigh model. These results correspond with a warm water surface anomaly found along Line P in 2014 and demonstrate that there is interannual variability in the biogeochemical cycling of Cd and its isotopes in the subarctic North Pacific. In contrast to other ocean basins where vertical variability in e 112 / 110 Cd is observed at depth, deep and intermediate waters in the North Pacific have a near-uniform e 112 / 110 Cd value (mean of 1.14 ± 0.37 , n = 43 , 2SD) representative of nearly all samples at or below 1000 m depth. Imprinted upon this nearly homogeneous intermediate and deep North Pacific e 112 / 110 Cd signature are fine-scale spatial trends, with heavier values observed toward the coastal end of Line P than the oceanic end at intermediate depths, and with slightly heavier values in subtropical North Pacific deep water compared to the subarctic North Pacific. The nearly constant Cd isotopic composition of North Pacific deep waters is consistent with the inflow of Circumpolar Deep Water at depth in the Pacific basin, along with deep remineralization, and supports the potential of e 112 / 110 Cd as a tracer of global deepwater circulation.