4.1 Regeneration incubation experiments
Three incubation experiments to probe for broad, differential effects on
the regeneration of trace elements were conducted at the same location
in the subantarctic Southern Ocean (Figure 1) within 10 days of each
other, with varying experimental design (collection depth and incubation
temperatures) and natural variability in the initial particulate regime
and elemental concentrations. Only one incubation (Inc. 3) generated
significant differences in [Cr] and δ53Cr by the
final time point. We focus on this incubation here, with results from
Inc. 1 and 2 provided in the supplemental material (Section S1, Figures
S1, Tables S3-S4). Initial particulate P (pP), Mn (pMn) and Fe (pFe)
from all three incubations are shown in Table S1. Flowcam imaging data
indicate that particles in all three incubations were small (mostly
~2 µm, nearly all < 6 µm), with a high
abundance of flagellates (data not shown), while regional dust
deposition is very low (Mahowald et al., 2005), confirming a
predominantly biogenic particulate composition.
There was minimal to no net regeneration of organic-associated
macronutrients as indicated by the lack of macronutrient variability (N
and P) (Figure 2, Table S2). Chromium concentrations, [Cr(III)] and
δ53Cr remained stable over the first three days of the
treatments. However, [Cr] increased by an average of 0.46 ± 0.14 and
0.56 ± 0.18 nmol kg-1 (1 SD of the triplicates) by the
final sampling for the ambient and cold treatments, respectively (Figure
2), demonstrating that Cr release was independent from organic carbon
respiration. Small increases in [Cr(III)] were also observed by the
end of the incubation, though the increases in [Cr] were
approximately seven times larger. Particulate Cr was probably primarily
Cr(III) – Cr(III) dominates the Cr adsorbed to biogenic particles (e.g.
Semeniuk et al., 2016) and the Cr content of calcium carbonate, hosting
Cr(VI), is very low (Remmelzwaal et al., 2019). Therefore, due to the
small changes in [Cr(III)], particulate Cr was either oxidatively
released or rapidly oxidized after release.
Particulate Mn oxides are known to oxidize Cr(III), facilitating
particulate Cr(III) dissolution (Oze et al., 2007), and incubated
particles were enriched in Mn beyond typical cellular quotas (Table S1,
Twining & Baines, 2013), indicating the presence of Mn oxides.
Therefore, Cr oxidation coupled to the reduction of Mn oxides could
explain dissolved Cr release without N and P regeneration or increases
in [Cr(III)]. Indeed, the timing of the release of Cr in Inc. 3
matches a decrease in pMn (Figure 2). Differences between the three
incubations can also be explained by Mn oxides. Incubations from 150 m
had much higher pMn than those from shallower depths (Inc. 1, 100 m &
Inc. 2; Table S1), following trends in oceanic pMn distributions
(Ohnemus et al., 2019), and explaining the lack of Cr release in
shallower treatments. Particulate Fe in Inc. 1 was higher than in Inc. 3
(Table S1), indicating more Fe-rich mixed oxides, which may impact Cr
oxidation and explain differences between these incubations. The
enriched incubation particle concentrations, possibly in combination
with the formation of particle aggregates, would help to increase the
probability of oxidative Cr release by particulate Mn oxides relative to
ambient seawater conditions. Consequently, oxidative particulate Cr
release driven by reduction with Mn oxides would be less likely in
natural seawater, suggesting a deeper regeneration cycle for Cr than
organic matter respiration influenced by more Mn-rich deeper particles
(Ohnemus et al., 2019) and marine sediments.
The increase in [Cr] was accompanied by a statistically significant
decrease in δ53Cr in four of six samples (Figure 2,
Table S2). δ53Cr decreases were not significant for
the remaining two samples, which is consistent with the small increase
in [Cr] insufficient to significantly lower δ53Cr.
Inter-replicate variability is likely controlled by small initial
differences in biological communities and particle compositions in each
cubitainer, hence real natural variability, rather than analytical
uncertainty, given differences in [Cr] between triplicates were well
outside of analytical uncertainty (Figure 2, Table S2). This release of
low δ53Cr from particles provides evidence for
predictions that Cr adsorbed onto biogenic particles is isotopically
light (Scheiderich et al., 2015; Semeniuk et al., 2016; Janssen et al.,
2020).
A strong linear relationship is observed between δ53Cr
and ln[Cr] in the regeneration incubation samples
(r2 = 0.89) with an implied enrichment factor (ε) =
-0.66 ‰. This enrichment factor matches the global
δ53Cr–[Cr] array (global ε ≈ -0.70 ‰), and the
data plot along the global array (Figure 2). This supports release from
biogenic particles as an important process in driving global [Cr]
and δ53Cr distributions. While proposed earlier
(Scheiderich et al., 2015), previous studies have highlighted that
biogenic Cr accumulation was lacking strong support in oceanic depth
profiles (Scheiderich et al., 2015; Goring-Harford et al., 2018; Moos &
Boyle, 2019; Rickli et al., 2019). This study provides the first direct
evidence that Cr release from biogenic particles acts as a control on
δ53Cr across intermediate and deep waters, and
ultimately also supports the inverse of this process – the adsorption
of Cr onto biogenic particles in the surface ocean – as a control of
δ53Cr and [Cr] (Scheiderich et al., 2015; Semeniuk
et al., 2016; Goring-Harford et al., 2018; Janssen et al., 2020).