4.3 Accumulation of Cr in intermediate and deep water masses
To investigate the implications of the mechanistic insight from our
incubation and pore water findings on the global ocean, a compilation of
globally-distributed [Cr] data is presented along with
macronutrients and apparent oxygen utilization (AOU, a quantification of
the O2 used for organic matter respiration) following
the northward advection of water masses originating in the Southern
Ocean (Figure 4). Intermediate and deep waters are split into three
water mass ranges based on hydrographic properties: AAIW, UCDW/PDW and
LCDW (Figure S2, Tables S9-S10).
Deeper water masses (UCDW and LCDW) show relatively uniform [Cr],
AOU and macronutrients at their southern origin, and concentrations
increase with northward transport (Figure 4). However, key differences
emerge between tracers primarily reflecting the respiration of organic
matter (AOU, PO4), phytoplankton frustule-associated
Si(OH)4, and [Cr]. AOU and PO4consistently show latitudinal maxima in the upper deep water mass
(UCDP/PDW) relative to LCDW, in agreement with global distributions of
PO4, NO3 and O2throughout the ocean (e.g. Schlitzer et al., 2018). Similar behaviour of
Cr results in broadly correlated Cr–AOU and Cr–PO4distributions (Figure 5), where correlations weaken as [Cr]
increases. However, Si(OH)4 shows maxima in LCDW, and
[Cr] in LCDW is comparable to or higher than in UCDW. These deeper
maxima, reflecting an apparent deeper regeneration cycle, may reflect
globally important benthic sources as has been shown for Si (Treguer &
de la Rocha, 2013), consistent with our calculated pore water fluxes and
earlier global Cr cycle models (Jeandel & Minster, 1987).
Chromium–Si(OH)4 distributions show a stronger
correlation (r2 = 0.58, n = 65) than Cr–AOU
(r2 = 0.44, n = 71) and Cr–PO4(r2 = 0.52, n = 67) (Figure 5), supporting Cr release
from biogenic material as being mechanistically independent from organic
matter respiration.
The absolute maxima in deep water [Cr] are found below the ETSP OMZ,
where accumulated Cr represents up to ~45% of total
deep water [Cr] based on Southern Ocean end members. While suboxic
sediments are a net sink term (e.g. Moos et al., 2020; Nasemann et al.,
2020), these deep water [Cr] enrichments may reflect the proximity
to a benthic source from deeper oxic sediments (section 4.2, see also
Figure 1). Deep ETNP samples are also enriched relative to the
subtropical South Pacific and subarctic North Pacific (Figure 4),
suggesting a connection to the intense OMZs overlying these deep
[Cr] enrichments (Murray et al., 1983; Rue et al., 1997; Moos et
al., 2020). Mechanistically, the enhanced export of particulate Cr to
depth from water column removal combined with the elevated Mn present
below these OMZs (e.g. Murray et al., 1983) would facilitate the
oxidative release of Cr from particles in the water column or from oxic
pore waters. This Cr-specific enrichment process would cause deep waters
below intense OMZs to deviate from correlations between Cr and
macronutrients, which is confirmed by the resulting strengthening of the
Cr correlation with Si (r2 = 0.73) when removing
samples below the ETSP OMZ (correlations with PO4(r2 = 0.56) and AOU (r2 = 0.41)
remain similar; Figure 5).
Intermediate waters are associated with high carbon respiration with
northward advection (Figure 4). Southern waters with properties similar
to newly-formed AAIW show the highest [Cr], and concentrations are
generally stable or decrease with northward transport, indicating
[Cr] is decoupled from organic matter respiration. A general
decrease in [Cr] northward is found for Atlantic AAIW accompanied by
increasing salinity (r2 = 0.54). This trend likely
reflects mixing of AAIW with Cr-poor but more saline thermocline waters
and NADW (Figure 6, Rickli et al. 2019). However, the weakness of the
correlation likely suggests more complicated mixing of multiple water
masses, variable scavenging of Cr in intermediate waters, or both.