Deep-water carbonate ion shifts during the last glacial termination in the Bay of Plenty, New Zealand

Abstract:

It has been suggested that a greater amount of CO₂ was stored in the deep sea during glacial periods via changes in biologic pump efficiency and increased uptake by a more alkaline ocean, characterized by carbonate ion concentrations higher than today. Reconstructing past ocean carbonate ion will enable better quantification of the relative roles of different CO₂ storage mechanisms. Here, we present records of deep water carbonate chemistry since the Last Glacial Maximum derived from sediment cores located in New Zealand's Bay of Plenty. Today, Bay of Plenty mid- to deep waters consist of a relatively fresh Antarctic Intermediate Water overlying southern-sourced Circumpolar Deep Water, which in turn is underlain by CO₂-enriched deep water, partially derived from the North Pacific. We reconstruct past changes in bottom water carbonate chemistry from the trace element and stable isotopic composition recorded in shells of the epibenthic foraminifer Cibicidoides wuellerstorfi. The boron to calcium ratio (B/Ca) in these shells indicates that during the last glacial maximum (LGM), deep water ΔCO₃^2- was slightly less than the modern value of ~ 20 µmol/kg. When combined with shifts in carbon isotopes, these results imply changes in respired CO₂ storage, with low-ΔCO₃^2-, low-δ¹³C waters dominating during the LGM and higher-ΔCO₃^2-, higher-δ¹³C waters prevailing in the Holocene. During the transition between LGM and Holocene, ΔCO₃^2- shifts rapidly in the mid-depth record (~1,600 m), rising from ~5 µmol/kg at about 18 ka to a peak of 30 µmol/kg at 16 ka. To determine whether these sharp changes are due to vertical shifts in water mass boundaries or rapid changes in source water formation regions, shallower and deeper cores (~1,200 m and ~3,000 m) from the same region are also being evaluated.