Apparent Periodic and Long-Term Changes in AAIW and UCDW Properties at Fixed Depths in the Southwest Pacific, With Indications of a Regime Shift in the 1930s

Metal/calcium ratios in two long-lived deep-sea gorgonian corals (Lepidisis and Corallium spp.) in the Southwest Pacific evidence periodic decadal variability at depths that correspond to Antarctic Intermediate Water (AAIW) and shallow Upper Circumpolar Deep Water, and a shift in the mid-1930s to late-1930s in mean ambient temperatures, barium/silicate concentrations and possibly pH, the rate at which these properties change over time, and the relationship between temperatures at fixed depth and the Southern Annular Mode (SAM). The decadal periodicity, which is evident in other biological indices in the study area, can be accounted for by water mass heave on the order of 100-150 m, which is consistent with observed scales of variability in the AAIW. The proximate and ultimate causes of the midcentury shifts are unclear, but could be related to suggested mid-20th century changes in climate parameters globally and, more specifically, in the subpolar SW Pacific. Plain Language Summary The dynamics of the Southern Ocean play a critical role in global climate and oceanography, but sparse historical instrumental data make it difficult to assess the significance of recently observed changes. We help fill this information gap by analyzing apparent proxies for ambient temperature, barium/silicate concentrations, and possibly pH in the carbonate skeletons of a pair of very long-lived deep-sea corals. The data strongly suggest a previously unreported similar to 23-year periodicity in two Southern Ocean water masses (Antarctic Intermediate Water and Upper Circumpolar Deep Water), which is likely to be the result of vertical shifts in water mass distributions. The long-term data also suggest the water masses cooled, became less acidic and had declining barium/silicate concentrations from at least the early 1700s until the early to mid-1900s, after which the sign of the long-term trends reversed. The causes of the long-term trends and their reversal are not clear, but may be related to changes to climate parameters globally, but more specifically to those in the SE Pacific, where Antarctic Intermediate Water is formed. Key Points Three environmental proxies were examined in skeletal composition of long-lived deep-water gorgonian corals in the SW Pacific The time series indicate coherent ca. 23-year periodicity at depths corresponding to the AAIW and UCDW Long-term trends in all three proxies reversed sign in the early to mid-1900s, along with their relationship with the SAM

Automated summary

The study found periodic decadal variability in metal/calcium ratios in deep-sea gorgonian corals in the Southwest Pacific, likely caused by vertical shifts in water mass distributions. Additionally, long-term trends in three environmental proxies reversed in the early to mid-1900s.