Deep-water carbonate concentrations in the southwest Pacific Helen C. Bostock a,n , Bruce W. Hayward b , Helen L. Neil a , Kim I. Currie c , Gavin B. Dunbar d a National Institute of Water and Atmospheric Research, Greta Point, Wellington, New Zealand b Geomarine Research, 49 Swainston Rd., St Johns, Auckland, New Zealand c National Institute of Water and Atmospheric Research, University of Otago, Dunedin, New Zealand d Antarctic Research Centre, Victoria University Wellington, Wellington, New Zealand article info Article history: Received 16 August 2010 Received in revised form 24 November 2010 Accepted 30 November 2010 Available online 5 December 2010 Keywords: Southwest pacific Circumpolar deep water (CPDW) Dissolved inorganic carbon Calcite saturation horizon (CSH) Carbonate content (CaCO 3 %) Carbonate compensation depth (CCD) Foraminiferal fragmentation % abstract We have compiled carbonate chemistry and sedimentary CaCO 3 % data for the deep-waters ( 41500 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 CO 3 2 h i   , and the associated calcite saturation horizon (CSH, where X calcite ¼1). 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 CaCO 3 content of o20%), 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 CaCO 3 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 (DZ CSHCCD ), however, varies considerably in this region and globally. The global DZ CSHCCD appears to expand with increase in age of the deep-water, resulting from a shoaling of the CSH. In contrast the depth of the chemical lysocline (X calcite ¼0.8) is less variable globally and is relatively similar, or close, to the CCD determined from the sedimentary CaCO 3 %. Geochemical definitions of the CCD, however, cannot be used to determine changes in the paleo-CCD. For the given range of factors that influence the sedimentary CaCO 3 %, an independent dissolution proxy, such as the foraminifera fragmentation % ( 440% ¼foraminiferal lysocline) is required to define a depth where significant CaCO 3 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 provides a CaCO 3 dataset for the present SW Pacific for comparison with glacial/interglacial CaCO 3 variations in deep-water sediment cores, and to monitor future changes in [CO 3 2 ] and dissolution of sedimentary CaCO 3 resulting from increasing anthropogenic CO 2 . & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Carbon, primarily in the form of CO 2 , is exchanged between the geosphere, biosphere, atmosphere and oceans. Over millions of years the geosphere plays an important role in modulating the atmospheric CO 2 concentration. However on intermediate time- scales of thousands of years, oceans play the primary role in regulating and storing CO 2 (e.g. Toggweiler et al., 2006). Central to this regulation is the precipitation, burial and dissolution of carbonate sediment (Archer, 1996). In the last century the oceans have absorbed between 30–40% of anthropogenically produced CO 2 . This has resulted in the average pH of the surface oceans decreasing by 0.1 unit (Caldeira and Wickett, 2003) and a reduction in the carbonate ion concentration CO 3 2 h i   . As the CO 3 2 h i decreases the waters become under- saturated with respect to calcium carbonate (CaCO 3 ), which starts to dissolve. The dissolution of CaCO 3 minerals will play an increasing role in buffering changes in the ocean chemistry and the ability of the oceans to take up more atmospheric CO 2 (Broecker and Takahashi, 1977; Archer, 1996; Archer et al., 1998; Ridgwell and Zeebe, 2005; Ridgwell and Hargreaves, 2007; Goodwin and Ridgwell, 2010). Minerals formed from calcium carbonate are unusual because they become increasingly soluble at lower temperatures and higher pressures; thus, dissolution increases with depth in the ocean. A range of terms and definitions are commonly used in the literature and are summarised below and illustrated in Fig. 1. The depth at which the ocean becomes undersaturated with respect to CaCO 3 minerals is termed the ‘‘lysocline’’ or ‘‘saturation horizon’’. This Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2010.11.010 n Corresponding author. Tel.: + 64 43860371. E-mail address: h.bostock@niwa.co.nz (H.C. Bostock). Deep-Sea Research I 58 (2011) 72–85