Geochemically tracing the intermediate and surface waters in the Tasman Sea, southwest Pacific

Abstract

The relatively understudied intermediate waters of the world have been implicated as an important part of the global ocean circulation. This thesis discusses the intermediate waters of the Pacific over space and time. Initially, by using geochemical tracers to look at the present distribution, sources and mixing of the water masses. Secondly, by using oxygen and carbon isotopes from sediment cores to study changes in Antarctic Intermediate Waters (AAIW) over the late Quaternary in the north Tasman Sea. The geochemical tracers highlight the presence of three separate intermediate water masses in the Pacific: North Pacific Intermediate Water (NPIW), AAIW and Equatorial Intermediate Water (EqIW). The EqIW has previously been considered an extension of intermediate water masses to the north or south. The unique geochemical characteristics of EqIW indicate, however, that it cannot be formed by direct mixing of the NPIW and AAIW. Geochemical tracers suggest instead that EqIW must also include mixing with nutrient rich, oxygen deficient, old Pacific Deep Water (PDW). The AAIW distribution and circulation shows a primary formation region in the southeast Pacific. The high oxygen, low salinity signature of this southeast Pacific source is altered during its circulation of the subtropical gyre. This is the result of aging during circulation and mixing with other intermediate waters entering the region. A second source region is in the eastern equatorial Pacific (EEP), where mixing occurs with the old, nutrient rich EqIW. Mixing between the EqIW and AAIW is evident as far south as 30◦S. The data also suggests a tongue of EqIW, overlying and mixing with the upper layers of the AAIW flows south adjacent to the South American coast. A third source of AAIW enters the south Tasman Sea directly from the Southern Ocean. This third source, has little influence on the main subtropical gyre. However, it contributes to the geochemistry of the intermediate waters in the Tasman Sea and Coral Sea. The uniformity of the AAIW in the Tasman Sea and Coral Sea advocates for a separate recirculating gyre in the southwest Pacific. The main exit of AAIW from the south Pacific is either through mixing to form the EqIW along the equator, or south to the east of the Tonga-Kermadoc Ridge following the subtropical gyre circulation. This present day distribution and chemistry of the intermediate water masses allows interpretation of past conditions.