Deep-water carbonate concentrations in the southwest Pacific

We have compiled carbonate chemistry and sedimentary CaCO3% data for the deep-waters (>1500 m water depth) of the southwest (SW) Pacific region. The complex topography in the SW Pacific influences the deep-water circulation and affects the carbonate ion concentration ([CO32-]), and the associated calcite saturation horizon (CSH where Ωcalcite =1). The Tasman Basin and the southeast (SE) New Zealand region have the deepest CSH at ~3100 m, primarily influenced by middle and lower Circumpolar Deep Waters (m or lCPDW), while to the northeast of New Zealand the CSH is ~2800 m, due to the corrosive influence of the old North Pacific deep waters (NPDW) on the upper CPDW (uCPDW). The carbonate compensation depth (CCD; defined by a sedimentary CaCO3 content of <20%), also varies between the basins in the SW Pacific. The CCD is ~4600 m to the SE New Zealand, but only ~4000 m to the NE New Zealand. The CaCO3 content of the sediment, however, can be influenced by a number of different factors other than dissolution, therefore we suggest using the water chemistry to estimate the CCD. The depth difference between the CSH and CCD (ΔZCSH-CCD), however, varies considerably in this region and globally. The global ΔZCSH-CCD appears to expand with increasing age of the deep-water, resulting from a shoaling of the CSH. In contrast the depth of the chemical lysocline (Ωcalcite = 0.8) is less variable globally and is relatively similar, or close, to the CCD determined from the sedimentary CaCO3%. Geochemical definitions of the CCD, however, cannot be used to determine changes in the paleo-CCD. Given the range of factors that influence the sedimentary CaCO3%, an independent dissolution proxy, such as the foraminifera fragmentation % (>40% = foraminiferal lysocline) is required to define a depth where significant CaCO3 dissolution has occurred back through time. The current foraminiferal lysocline for the SW Pacific region ranges from 3100-3500 m, which is predictably just slightly deeper than the CSH. This compilation of sediment and water chemistry data provide a CaCO3 dataset for the present SW Pacific for comparison with glacial/interglacial CaCO3 variations in deep-water sediment cores, and to monitor future changes in [CO32-] and dissolution of sedimentary CaCO3 resulting from increasing anthropogenic CO2.

Bostock H. C., Hayward B. W., Neil H. L., Currie K. I. & Dunbar G. B., in press. Deep-water carbonate concentrations in the southwest Pacific. Deep-Sea Research (Part I, Oceanographic Research Papers)Article (subscription required).

 


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