Abstract
Abstract. Predicting when surface waters of the Ross Sea and Southern Ocean will become undersaturated with respect to biogenic carbonate minerals is challenging in part due to the lack of baseline high-resolution carbon system data. Here we present ~ 1700 surface total alkalinity measurements from the Ross Sea and along a transect between the Ross Sea and southern Chile from the austral autumn (February–March 2013). We calculate the saturation state of aragonite (ΩAr) and calcite (Ω Ca) using measured total alkalinity and pCO2. In the Ross Sea and south of the Polar Front, variability in carbonate saturation state (Ω) is mainly driven by algal photosynthesis. Freshwater dilution and calcification have minimal influence on Ω variability. We estimate an early spring surface water ΩAr value of ~ 1.2 for the Ross Sea using a total alkalinity–salinity relationship and historical pCO2 measurements. Our results suggest that the Ross Sea is not likely to become undersaturated with respect to aragonite until the year 2070.
Highlights
Atmospheric CO2 concentrations have increased by 40 % since preindustrial times to ∼ 400 ppm today and could reach 936 ppm by the year 2100 (IPCC AR5 WG1, 2013)
Our study demonstrates the possibility of setting up underway total alkalinity (TA) measurement systems
Our system was relatively unattended, carbon system crosschecks and comparisons between hydrocast and underway data indicate that our measurements were accurate
Summary
Atmospheric CO2 concentrations have increased by 40 % since preindustrial times to ∼ 400 ppm today and could reach 936 ppm by the year 2100 (IPCC AR5 WG1, 2013). DeJong et al.: Carbonate saturation state in the Ross Sea and Southern Ocean respect to aragonite by 2050 and be fully undersaturated by 2100 (Orr et al, 2005; Feely et al, 2009). Orr et al (2005) found that the shell of a subarctic pteropod started to dissolve within 48 h when placed in waters with the level of aragonite saturation expected to occur in the Southern Ocean by 2100. A model by McNeil et al (2010) suggests that winter surface waters in the Ross Sea will become undersaturated with respect to aragonite by the year 2045 since sea ice, upwelling of deep water, and short residence times prevent these surface waters from reaching equilibrium with the atmosphere. After establishing a relationship between salinity and TA, we use the Lamont Doherty Earth Observatory (LDEO) pCO2 database (Takahashi et al, 2009) (available at http://www. ldeo.columbia.edu/res/pi/CO2) to provide an independent estimate of Ross Sea surface water Ar in early spring
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