Characterising the intermediate depth waters of the Pacific Ocean using d 13 C and other geochemical tracers Helen C. Bostock a,c,n , Bradley N. Opdyke b , Michael J.M. Williams c a Department of Earth and Marine Sciences, The Australian National University, Canberra ACT 0200, Australia b Research School of Earth Sciences, The Australian National University, Canberra ACT 0200, Australia c National Institute of Water and Atmospheric Research, Greta Point, Wellington, New Zealand article info Article history: Received 12 June 2009 Received in revised form 12 February 2010 Accepted 21 April 2010 Available online 28 April 2010 Keywords: Pacific Ocean Intermediate depth waters AAIW NPIW Equatorial intermediate waters Geochemical tracers d 13 C d 14 C Benthic foraminifera abstract Evidence from geochemical tracers (salinity, oxygen, silicate, nutrients, alkalinity, dissolved inorganic carbon (DIC), carbon isotopes (d 13 C DIC ) and radiocarbon (D 14 C)) collected during the Pacific Ocean World Ocean Circulation Experiment (WOCE) voyages (P10, P15, P17 and P19) indicate there are three main water types at intermediate depths in the Pacific Ocean; North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW) and Equatorial Pacific Intermediate Waters (EqPIW). We support previous suggestions of EqPIW as a separate equatorial intermediate depth water as it displays a distinct geochemical signature characterised by low salinity, low oxygen, high nutrients and low D 14 C (older radiocarbon). Using the geochemical properties of the different intermediate depth waters, we have mapped out their distribution in the main Pacific Basin. From the calculated pre-formed d 13 C air–sea conservative tracer, it is evident that EqPIW is a combination of AAIW parental waters, while quasi-conservative geochemical tracers, such as radiocarbon, also indicate mixing with old upwelling Pacific Deep Waters (PDW). The EqPIW also displays a latitudinal asymmetry in non-conservative geochemical tracers and can be further split into North (NEqPIW) and South (SEqPIW) separated at 21N. The reason for this asymmetry is caused by higher surface diatom production in the north driven by higher silicate concentrations. The d 13 C signature measured in benthic foraminifera, Cibicidoides spp. (d 13 C Cib ), from four core tops bathed in AAIW, SEqPIW and NPIW, reflects that of the overlying intermediate depth waters. The d 13 C Cib from these cores show similarities and variations down-core that highlight changes in mixing over the last 30,000 yr BP. The reduced offset between the d 13 C Cib of AAIW and SEqPIW during the last glacial indicates that AAIW might have had an increased influence in the eastern equatorial Pacific (EEP) region at this time. Additional intermediate depth cores and other paleo-geochemical proxies such as Cd/Ca and radiocarbon are required from the broader Pacific Ocean to further understand changes in intermediate depth water formation, circulation and mixing over glacial/interglacial cycles. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Intermediate depth waters are found between 500 and 1000 m below sea level in the subtropical regions of the world oceans. The present day global volume of intermediate depth waters is estimated to be 0.385  10 18 m 3 (Talley, 1999) and Antarctic Intermediate Water (AAIW) in the Pacific Ocean makes up 450% of the volume of intermediate depth waters. Intermediate depth waters are important for ventilating the intermediate depths of the ocean and are considered to play a major role in the global thermohaline circulation, by balancing the export of North Atlantic Deep Water (NADW; Schmitz, 1995; Saenko et al., 2003). AAIW is also currently absorbing a considerable amount of the increasing anthropogenic CO 2 and storing it in the intermediate depths of the ocean (Sabine et al., 2004). Stable carbon isotopes analysed on foraminifera (d 13 C foram ) from sediment cores are commonly used as a paleoproxy for ocean circulation changes (e.g. Duplessy et al., 1988), although d 13 C is also influenced by variations in biological productivity (Kroopnick, 1985) and air–sea exchange processes (Mook et al., 1974). d 13 C foram and trace element analyses on benthic foramini- fera and authigenic oxides from sedimentary marine cores in the Atlantic Ocean have suggested that enhanced northward flow of AAIW balanced the decline in NADW production during the last glacial and deglaciation (Oppo and Horowitz, 2000; Zahn and Stuber, 2002; Rickaby and Elderfield, 2005; Pahnke et al., 2008). In the Pacific Ocean, d 13 C foram variations in sediment cores have been used to suggest that AAIW may have played an important role at the onset of the deglaciation ( 18 kyr), acting ARTICLE IN PRESS 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.2010.04.005 n Corresponding author at: National Institute of Water and Atmospheric Research, Greta Point, Wellington, New Zealand. Tel.: + 64 43860371. E-mail address: h.bostock@niwa.co.nz (H.C. Bostock). Deep-Sea Research I 57 (2010) 847–859