Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific

Abstract

A hydrographic data set collected in the region and season of Subantarctic Mode Water and Antarctic Intermediate Water (SAMW and AAIW) formation in the southeastern Pacific allows us to estimate the preformed properties of surface water detrained into these water masses from deep mixed layers north of the Subantarctic Front and Antarctic Surface Water south of the front. Using 10 measured seawater properties, we estimate: the fractions of SAMW/AAIW that originate as surface source waters, as well as fractions that mix into these water masses from subtropical thermocline water above and Upper Circumpolar Deep Water below the subducted SAMW/AAIW; ages associated with the detrained surface water; and remineralization and dissolution rates and ratios. The mixing patterns imply that cabbeling can account for ∼0.005–0.03 kg m−3 of additional density in AAIW, and ∼0–0.02 kg m−3 in SAMW. We estimate a shallow depth (∼300–700 m, above the aragonite saturation horizon) calcium carbonate dissolution rate of 0.4 ± 0.2 µmol CaCO3 kg−1 yr−1, a phosphate remineralization rate of 0.031 ± 0.009 µmol P kg−1 yr−1, and remineralization ratios of P:N:–O2:Corg of 1:(15.5 ± 0.6):(143 ± 10):(104 ± 22) for SAMW/AAIW. Our shallow depth calcium carbonate dissolution rate is comparable to previous estimates for our region. Our –O2:P ratio is smaller than many global averages. Our model suggests neglecting diapycnal mixing of preformed phosphate has likely biased previous estimates of –O2:P and Corg:P high, but that the Corg:P ratio bias may have been counteracted by a second bias in previous studies from neglecting anthropogenic carbon gradients.

Episodic Southern Ocean Heat Loss and Its Mixed Layer Impacts Revealed by the Farthest South Multiyear Surface Flux Mooring

“…The large range of Argo float winter MLDs (100 m to 500 m) in years with deep MLDs (2015 and 2017) shows spatial heterogeneity. The potential temperature was between 5.25 and 5.5 ∘ C, near the upper limit for Southeast Pacific SAMW (Carter et al, 2014;supporting information Figure S3). Flanking mooring A CTDs show that the 2015 winter mixed layers exhibit the density of Southeast Pacific SAMW (26.99-27.02 g/cm 3 ) described by Holte et al (2012) and Carter et al (2014) from mid-August through early October with MLDs exceeding 500 m (Figure 3g).…”

“…In each year, MLD estimates from both Argo float and mooring data show a gradual mixed layer deepening from summer through early winter; however, 2015 and 2017 have much deeper winter mixed layers than 2016 (Figure 3f ). Flanking mooring A CTDs show that the 2015 winter mixed layers exhibit the density of Southeast Pacific SAMW (26.99-27.02 g/cm 3 ) described by Holte et al (2012) and Carter et al (2014) from mid-August through early October with MLDs exceeding 500 m (Figure 3g). Even so, the overall MLDs were substantially shallower in winter 2016 compared to winter 2015 in the broader Southeast Pacific (supporting information Figure S2).…”

“…The distribution is left skewed and has a 0.25 quantile of −72 W/m 2 , a median of −36 W/m 2 , and a 0.75 quantile of −14 W/m 2 ; this contrasts the net heat flux which is approximately normally distributed. Gray shading indicates when flanking mooring A and Argo float MLDs were >250 m. (g) Daily average potential density from flanking mooring A with a black contour at 26.99 kg/m 3 , the lower limit for SAMW from Carter et al (2014). loss exceeding 3 below the mean, < −193 W/m 2 (see supporting information Table S2). The decorrelation time scale is 3 to 4 days for each heat flux component reflecting the time scale of synoptic variability associated with episodic heat loss events.…”

“…There are 29 days that meet this criterion all located in the negative tail of the turbulent heat flux probability density function in supporting information Figure S1; seven of these days have daily mean turbulent heat + Q SH ) with 2 and 3 heat loss events (fluxes more than 2 standard deviations below the mean) labeled with orange points, while the red points and lines indicate heat loss events with fluxes more than 3 standard deviations below the mean; (b) relative wind speed; (c) northward (black) and eastward (blue) wind components; (d) SST minus air temperature, (e) sea surface specific humidity minus air specific humidity, and (f ) MLDs from Argo floats (black) and the profiler mooring (blue for reference depths of 12 m from the surface mooring and cyan for reference depths of 30 m from flanking mooring A). Gray shading indicates when flanking mooring A and Argo float MLDs were >250 m. (g) Daily average potential density from flanking mooring A with a black contour at 26.99 kg/m 3 , the lower limit for SAMW from Carter et al (2014). loss exceeding 3 below the mean, < −193 W/m 2 (see supporting information Table S2). Of these seven 3heat loss events, five occurred in 2015 despite the large gap in winter data, while only two events occurred in 2016.…”

Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean

“…They form a coherent group of stations, which we will discuss first. The AAIW sampled at stations S2 and S3 is formed by subduction of AASW that itself results from the upwelling of UCDW (24,27,28). The δ 56 DFe minimum found in the AAIW of S2 and S3 may therefore originate from the dilution of the S4 UCDW δ 56 DFe minimum through mixing with AASW.…”