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

Abstract

AbstractA 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.