4.2 Benthic Cr supply: the flux of Cr to bottom waters from pore
waters
Pore waters were sampled from calcareous sediments
(~3350 m water depth) collected from a shallower feature
punctuating the abyssal plain (~4000 to 5000 m) in the
Tasman Basin (Figure 1). Regional sedimentation rates are on the order
of 1-2 cm kyr-1 (Cochran & Osmond, 1976). Organic
matter is around 1% at the sediment surface, though upper sediments
remain oxic and organic carbon content decreases with depth (Table 1,
Table S7). Surface sediment Mn/Al (~0.016) and Fe/Al
(~0.53) are higher than upper continental crust values
(0.009 and 0.48 respectively, Rudnick & Gao, 2003; Table S7),
suggesting slight authigenic oxide enrichment, though Fe-Mn oxides are
not a dominant phase. Sediment Mn/Al begins to decrease below 11 cm,
suggesting oxic conditions in the upper 10 cm and more reducing
conditions below this depth.
Pore water dissolved [Cr] shows a shallow sub-surface maximum (0-1
cm below the sea floor) of 47.4 nmol kg-1,
approximately an order of magnitude greater than local bottom waters
(4.81 nmol kg-1, Figure 3), and pore water [Cr]
generally decreases exponentially with depth. The lowest pore water
concentrations are observed in the deepest samples (6.7 nmol
kg-1 at 9-10 cm depth), which remain elevated relative
to bottom water. This upper pore water Cr maximum is consistent with
observations by Shaw et al. (1990). In contrast to our observations of
maximum pore water [Cr] at the sediment surface, Shaw et al. (1990)
observed Cr removal in the uppermost oxic sediments, above the
near-surface pore water [Cr] maximum. The absence of Cr removal in
our uppermost oxic sediments may reflect the strong differences in bulk
sediment composition between the lithogenic-dominated sites in Shaw et
al. (1990) and our carbonate-dominated site.
A near-surface pore water maximum in [Cr] suggests pore waters may
act as a diffusive source of dissolved Cr to the ocean. Bottom waters
composed of CDW are enriched in dissolved Cr and are isotopically
distinct ([Cr] = 4.81 nmol kg-1,
δ53Cr = 0.76 ± 0.03 ‰) from CDW at nearby stations not
in contact with sediments ([Cr] = 3.95 nmol kg-1,
δ53Cr = 0.84 ± 0.03 ‰, 2SD, n = 6 samples ≥ 3000 m at
stations TS8 and PS2, Table S9). Given the gradient of [Cr] from the
pore water to the overlying bottom water, the diffusive flux of
dissolved Cr can be estimated using Equation 1
\(\text{Flux}_{\text{Cr}}\ =\ D_{s}\frac{\text{ΔC}}{\text{Δz}}\)Equation 1
where Ds is the Cr diffusion coefficient
corrected for temperature and tortuosity (supplemental material, see
also Abbott et al., 2015), giving an estimated benthic flux of
~3.2 nmol Cr cm-2yr-1.
To contextualize this, if global oxic sediments were characterized by a
benthic source of similar magnitude to that observed at our site, the
global benthic Cr flux would be comparable to or larger than riverine
inputs, currently believed to be the dominate Cr source (Bonnand et al.,
2013; supplemental material), consistent with box model estimates
(Jeandel & Minster, 1987). While quantitative estimates of global
benthic Cr fluxes await greater data availability across diverse
sediment types, our data identify that benthic sources are at least
locally important and support previous studies that: (1) have argued for
benthic Cr fluxes based on globally distributed elevated deep water
[Cr] (Cranston, 1983; Murray et al., 1983; Jeandel & Minster,
1987), and (2) have invoked elevated pore water [Cr] to explain Cr
enrichments in planktonic foraminifera in sediments compared to water
column samples (Remmelzwaal et al., 2019).
We use the bulk sediment composition to examine potential sources of Cr
to pore waters by applying mass balance calculations based on the
average composition of the upper 4 cm, reported Cr concentrations of
these phases, and regional sedimentation rates (Table 1). These
calculations suggest that neither lithogenic nor carbonate-hosted Cr can
account for the Cr flux out of these carbonate-rich and detrital-poor
sediments. Nor is the Cr content of surface phytoplankton
(~1-21 ppm, Martin & Knauer, 1973) high enough to
explain observations (Table 1). Therefore, we suggest a combination of
mechanisms causing Cr enrichments in particles delivered to sediments
can explain the observed pore water data. First, respiration may result
in a relative Cr enrichment in particles reaching the seafloor because
the release of Cr from particles appears decoupled from organic matter
respiration (see sections 4.1, 4.3). At the same time, scavenging of
particle-reactive Cr(III) may increase the Cr content on biogenic and
non-biogenic particles. Regardless of which process(es) are involved,
the Cr released from these sediments must largely be derived from the
water column based on mass balance and is incorporated into the
sediments as particle-adsorbed Cr that has been scavenged onto particle
surfaces in the upper ocean (Connelly et al., 2006; Semeniuk et al.,
2016; Janssen et al., 2020), and at intermediate depths (see section
4.3), while detrital Cr may be more inert. Mn oxides in surface
sediments may also facilitate oxidative release of Cr(III) coupled to Mn
reduction.
To better constrain the origin of dissolved Cr in pore water we
calculate its δ53Cr from mass balance, using observed
bottom and deep water [Cr] and δ53Cr (bottom
water: [Cr] = 4.81 nmol kg-1,
δ53Cr = 0.76 ± 0.03 ‰; deep water: ([Cr] = 3.95
nmol kg-1, δ53Cr = 0.84 ± 0.03 ‰,
2SD; supplemental material). Pore water δ53Cr is
estimated at 0.34 ± 0.25 ‰ (2 SEM propagated error), which is heavier
than silicate earth (-0.124 ± 0.101 ‰, Schoenberg et al., 2008). While
the closeness of these ranges indicates that some lithogenic component
may contribute to the pore water flux, mass balance calculations,
combined with the likely refractory nature of lithogenic Cr (e.g. Bauer
et al., 2019), indicate that it is unlikely that lithogenic Cr
contributes significantly to the pore water flux. In contrast, pore
water δ53Cr is lighter than dissolved
δ53Cr in the upper 1000 m (δ53Cr ≈
1.1-0.9 ‰) by a similar amount as the enrichment factor predicted by the
global δ53Cr–[Cr] array (ε ≈ -0.7 ‰). In other
words, removal of dissolved Cr to particles in the upper 1000 m with an
enrichment factor following the global array would result in
δ53Crparticulate similar to the
calculated δ53Crpore water.
Although the exact origin of pore water Cr is hard to discern and may
include biogenic, scavenged, and lithogenic sediment components, our
mass balance calculations highlight the importance of internal processes
acting to redistribute Cr within the water column, transferring
dissolved Cr from upper waters to deep waters. Therefore, the benthic
flux may act more as an attenuation of the Cr sink term associated with
particle export than an entirely new Cr source. Pore water data from
diverse sediment types are needed to determine whether a truly new
contribution from lithogenic material may be regionally important in
more lithogenic-rich sediments.