5. An updated global ocean Cr biogeochemical cycle
The foundations for understanding oceanic δ53Cr distributions were presented by Scheiderich et al. (2015), who first demonstrated a tight coupling between [Cr] and δ53Cr. Building on the [Cr] literature, Scheiderich et al. (2015) hypothesized that the controls on δ53Cr were the same as those known for [Cr]: reduction and removal in OMZs and Cr export with biogenic particle flux along with regeneration from biogenic material. Subsequent δ53Cr research has attempted to assess these mechanisms and their associated fractionations to identify their roles in the global Cr and δ53Cr cycle. We combine these initial hypotheses and recent advancements with our new data to improve our mechanistic understanding of Cr cycling in the global ocean and highlight remaining uncertainties. This set of mechanistic controls define how internal oceanic processes regulate δ53Cr, forming the global relationship (Figure 7 panel A), and will help to guide paleoceanographic applications of marine δ53Cr.
Here, we have demonstrated that Cr release from biogenic particles, either in the water column or as a benthic flux, can explain the [Cr]-rich side of the array (Figure 7 panels B and C) consistent with previous work on biological uptake shaping the [Cr]-depleted side of the δ53Cr–[Cr] array (Figure 7 Panel B; Goring-Harford et al., 2018; Janssen et al., 2020). In OMZs, dissolved Cr can be scavenged in the water column (Moos et al., 2020) and at the sediment surface (Moos et al., 2020; Nasemann et al., 2020), a process that largely follows the global array (Figure 7 Panel D). Mixing will generally act to homogenize process- and source-induced variability, with the specific effect depending on the signatures of mixing water masses (Figure 7 panel E, see also section S.4 and Figure S3; Rickli et al., 2019; this study). Hydrothermal circulation may also impact [Cr] and δ53Cr in the modern ocean and paleoceanographic interpretations (Holmden et al., 2016), but remains largely unconstrained at present (Figure 7 panel F).
Coastal environments are more likely to deviate from the global δ53Cr–[Cr] array due to localized influences, including shelf sources (Goring-Harford et al., 2018) (Figure 7 Panel C), more quantitative removal in shelf OMZs (Nasemann et al., 2020, Figure 7 Panel D); dilution from meltwater (Scheiderich et al., 2015, Figure 7 Panel E), and local riverine inputs (Figure 7 Panel F). Consequently, reconstructions of seawater δ53Cr from coastally-sourced marine sediment records, including continental margins, should be approached with caution as these records are likely not reflective of global ocean conditions for δ53Cr and/or [Cr].
Taken together, the available body of oceanic data indicate that Cr and δ53Cr distributions are controlled by biological uptake and scavenging onto sinking particles, with local enhanced reduction and scavenging in OMZs, and regeneration from particles in the water column and/or oxic sediments. These signals are then transported and mixed through ocean circulation. Consequently, δ53Cr records in marine-origin sediments should reflect the combination of these processes, with both export productivity and OMZ reduction contributing to Cr accumulation in sediments and with temporal δ53Cr records reflecting modification of these processes over time and with changes in global climate. Therefore, δ53Cr records are not exclusively reflecting changes in O2 availability. Additional research is needed to understand early sediment diagenesis, as indicated by elevated pore water [Cr], and the extent to which this may alter sediment Cr signals.