Ocean acidification in the Arabian Sea and the Red Sea – factors controlling pH

The CO2 increase in the ocean due to uptake of anthropogenic CO2 and the companying lowering of ocean pH is of major concern. In this study we investigated the variability of CO2 system parameters, focusing particularly on the pH and how it changes with changes in other parameters like: temperature (T), salinity (S), total dissolved inorganic carbon (CT ), and total alkalinity (AT). For Arabian Sea the data from the United States Joint Global Ocean Flux Study (US{JGOFS) in 1995 were used. For the Red Sea data from the Geochemical Ocean Section Study (GEOSECS) in 1977 and the Mer Rouge (MEROU) cruises in June and October 1982 were used.

The seasonal and spatial variations in pH and therefore also for calcium carbonate saturation (Ar for aragonite and Ca for calcite) are controlled by biological and physical processes that in turn are driven by the in uence of monsoonal seasons. In winter season the surface average pH, Ar, and Ca in the Arabian Sea were 8.070.01, 3.90.1 and 5.90.2, respectively. A relatively high biological
production, due to the winter cooling and mixing caused by the northeast monsoonal winds increases the pH. During summer season, Southwestern monsoonal winds caused upwelling along the coast of Oman, resulting in extremely low pH values (7.9) and lower saturation for aragonite (Ar 2.36) and for calcite (Ca 3.62). Because of the strong change in pH, this area might serve as a natural laboratory for studies of ocean acidication.

For comparison, in the Red Sea, the surface average pH was 8.10.02 during winter with higher values in the north due to lower temperatures and high A(T) and C(T). The Ar and Ca were around 4.120.02 and 6.20.15, respectively, with highest values in the central part of the basin caused by higher temperatures. Summer surface pH was 8.070.03, with higher values in the north and the south due to relatively low temperature. In the central of the Red Sea, pH was low due to the convergence (high temperature). The  Ar and Ca were averaged to 4.60.3 and 6.950.35, respectively, with higher values in the south and north. This is attributed to the high biological productivity in the south and the high temperature in the center of the Red Sea.

The vertical distributions of Ar, and Ca showed that the Arabian Sea is undersaturated with respect to aragonite below 600 m and calcite below 3500 m, whereas the Red Sea is supersaturated throughout the water column. In both seas pH was higher in the surface layers due to the consumption of CO2 by photosynthesis, but decreased rapidly in subsurface waters due to the release of CO2 by respiration processes. Between about 100 and 1500 m in the Arabian Sea pH is nearly constant due to the counteracting eects of decreasing temperatures and oxidation of the organic matter. The temperature effect on pH is about 0.015 units per 1C both in the Arabian Sea and Red Sea. Thus, the 0.5C warming reported for the Arabian Sea between 1904 and 1994, theoretically would result in a pH reduction of about 0.007, but the temporal coverage of the available data is unfortunately too short to verify this.

Omer W. M. M., 2016. Ocean acidification in the Arabian Sea and the Red Sea – factors controlling pH. MSc thesis, University of Bergen, 72 p. Thesis.

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