Abstract
Abstract. Using ocean carbon data from global datasets, we have developed several multiple linear regression (MLR) algorithms to estimate alkalinity and dissolved inorganic carbon (DIC) in the intermediate and deep waters of the Southern Hemisphere (south of 25° S) from only hydrographic data (temperature, salinity and dissolved oxygen). A Monte Carlo experiment was used to identify a potential density (σθ) of 27.5 as an optimal break point between the two regimes with different MLR algorithms. The algorithms provide a good estimate of DIC (R2=0.98) and alkalinity (R2=0.91), and excellent agreement for aragonite and calcite saturation states (R2=0.99). Combining the algorithms with the CSIRO Atlas of Regional Seas (CARS), we have mapped the calcite saturation horizon (CSH) and aragonite saturation horizon (ASH) for the Southern Ocean at a spatial resolution of 0.5°. These maps are more detailed and more consistent with the oceanography than the previously gridded GLODAP data. The high-resolution ASH map reveals a dramatic circumpolar shoaling at the polar front. North of 40° S the CSH is deepest in the Atlantic (~ 4000 m) and shallower in the Pacific Ocean (~ 2750 m), while the CSH sits between 3200 and 3400 m in the Indian Ocean. The uptake of anthropogenic carbon by the ocean will alter the relationships between DIC and hydrographic data in the intermediate and deep waters over time. Thus continued sampling will be required, and the MLR algorithms will need to be adjusted in the future to account for these changes.
Highlights
Our understanding of the carbonate concentrations and saturation in the oceans has been considerably advanced by the collection of large global datasets such as GEOSECS (1960– 1970s), World Ocean Circulation Experiment (WOCE, 1990s; Lamb et al, 2002; Key et al, 2004) and more recently CLIVAR/CO2 Repeat Hydrography programme (2000s; Tanhua et al, 2008; Sabine et al, 2010)
There may be temporal variability in these parameters due to seasonal (Feely et al, 1988; Juranek et al, 2009, 2011; McNeil, 2010; McNeil et al, 2011; Alin et al, 2012) or interannual variations caused by phenomena like El Niño Southern Oscillation (ENSO; McNeil, 2010) or the Pacific Decadal Oscillation (PDO; Kim et al, 2010), which would not have been captured by the oneoff or decadal repeat transects across the global oceans
We focus on the intermediate and deep waters and the depth of the calcite (CSH) and aragonite saturation horizons (ASHs), as our interest is in understanding how carbonate ion concentrations of bottom waters, which interact with the seabed, affect the distribution of deep water carbonate organisms and sediments
Summary
Our understanding of the carbonate concentrations and saturation in the oceans has been considerably advanced by the collection of large global datasets such as GEOSECS (1960– 1970s), World Ocean Circulation Experiment (WOCE, 1990s; Lamb et al, 2002; Key et al, 2004) and more recently CLIVAR/CO2 Repeat Hydrography programme (2000s; Tanhua et al, 2008; Sabine et al, 2010). There are still large gaps, with many areas of the globe that have had little sampling for carbonate parameters Some of these regions have significant topographic features, such as plateaux and ridges that produce complex currents. If it were possible to use the detailed hydrographic data to estimate the carbonate parameters in areas where there has been limited sampling for alkalinity and dissolved inorganic carbon (DIC), we could potentially provide detailed carbonate saturation estimates for all areas of the global ocean.
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