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.