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.