4.3.1 Implications of water mass [Cr] accumulation trends on Cr–macronutrient coupling in the global ocean
Widely observed correlations between [Cr] and macronutrient concentrations suggest shared biogeochemical controls (e.g. Campbell & Yeats, 1981; Cranston, 1983; Jeandel & Minster, 1987; Rickli et al., 2019; Nasemann et al., 2020). Recent studies of other metal–macronutrient associations have highlighted the importance of three dimensional processes, rather than vertical-controlled simplifications, with pre-formed relationships (e.g. Vance et al., 2017), scavenging, and variable metal:macronutrient uptake ratios across taxa and nutrient regimes (Quay et al., 2015; Ohnemus et al., 2019) driving globally-correlated distributions.
While correlations between [Cr] and PO4 exist on different spatial scales (e.g. Cranston, 1983; Rickli et al., 2019), biogenic Cr fluxes are low relative to other nutrient-type metals and macronutrients as apparent in smaller surface [Cr] depletions, lower surface-to-deep [Cr] gradients, reduced [Cr] accumulation in global deep water, and [Cr] accumulation in subsurface waters that is independent from organic matter respiration (sections 4.1-4.3, Figure 5). This means that the observed associations between [Cr] and PO4 probably reflect mixing between distinct oceanic end-members (see Rickli et al., 2019) rather than coupled vertically-driven processes (e.g. particle sinking and regeneration). Therefore end-member pre-formed Cr:macronutrient ratios would shape much of the Cr–macronutrient relationships in the ocean interior, while end-members impacted by biological uptake at their formation sites, and water mass-driven Cr:macronutrient associations could result in variable regional relationships.
A prominent benthic source of dissolved Cr provides a mechanistic explanation for [Cr] and Si(OH)4 coupling by vertical processes in addition to water mass mixing. However, deep water data demonstrate that [Cr] and Si(OH)4 can decouple (Figures 4-5), and local differences in Cr–Si(OH)4slopes (Jeandel & Minster, 1987) suggest the relationship between Cr and Si(OH)4 may reflect a combination of advection and mixing of waters with different pre-formed Cr:Si(OH)4ratios, Cr and Si(OH)4 drawdown by phytoplankton in surface waters, and the impact of sediment type and location on benthic dissolved Cr and Si(OH)4 fluxes (e.g. elevated [Cr] below OMZs, Figures 4-5). Based on these variable controls on Cr:macronutrient ratios, and recognizing that intermediate and deep water circulation likely plays a central role, interpretations of Cr–macronutrient trends within depth profiles should be made with caution.