Shifting Depth Distributions of Deep‐Sea Corals in the Southwest Pacific: Implications for Deglacial Dynamics of the Southern Ocean.

We compare depth and temporal distributions of sub‐fossil assemblages of two cold‐water scleractinian corals on seamounts in the Southwest Pacific to help define the temporal variations of water mass properties in the Southern Ocean (SO) during deglaciation. Peaks in the deep‐water abundance of the two species complement one another, with Desmophyllum dianthus peaking around the Antarctic Cold Reversal (ACR), and Solenosmilia variabilis briefly during the late Heinrich Stadial 1 (HS1) and during the Younger Dryas (YD). Environmental tolerances of the two species and the geochemistry of S. variabilis carbonate skeletons suggest that their secular distributions reflect complementary effects of temperature (higher at Antarctic Intermediate Water/Upper Circumpolar Deep Water depths during the YD and late HS1) and surface productivity (lower during the YD and HS1). Higher temperatures at depth we interpret as evidence of increased Zonal West Wind (ZWW)‐driven Ekman pumping during the late HS1 and YD, whereas coeval low surface production reflects poleward expansion of sub‐tropical water masses as a result of correlated poleward shifts of the ZWW belt and the Intertropical Convergence Zone. More broadly, a continuous deep coral population in the southwest Pacific that spans two species and three deglacial periods (HS1, ACR and the YD) and an early Holocene shift in coral distribution from deeper to shallower habitats appear to reflect large‐scale changes during deglaciation in SO temperature profiles and productivity. Plain Language Summary: The dynamics of the Southern Ocean are critical for understanding the development of global climate after the last glaciation, but information on the transition is fragmentary and often sparse. We help fill this gap by examining the environmental tolerances and distribution on sea mounts in the Southwest Pacific of two species of cold‐water corals to help better define oceanic conditions during deglaciation. The two species largely alternate in abundance, which suggests alternating periods of warm, but relatively unproductive conditions during the late Heinrich Stadial 1 and the Younger Dryas and a colder, but more productive intervening Antarctic Cold Reversal. These combinations appear to derive from increased wind‐driven upwelling of deep water in the Southern Ocean during the late HS1 and YD, but a coeval poleward shift in low productivity sub‐tropical surface water, driven by correlated shifts in the latitudes of the Southern Ocean zonal wind belt and the tropical Intertropical Convergence Zone. Key Points: Deep populations of two cold‐water corals alternate across the late Heinrich Stadial 1, the Antarctic Cold Reversal and the Younger DryasThe alternation reflects interleaving periods of high temperature/low productivity (HS1 and YD) and low temperature/high productivity (ACR)Drivers appear to be enhanced wind‐driven upwelling of warm Circumpolar Deep Water and coeval poleward shifts in low nutrient surface waters [ABSTRACT FROM AUTHOR]