The Inorganic carbon cycle in the Red Sea. Item Type Theses and Dissertations Authors Ali, Elsheikh Bashir Publisher University of Bergen, Geophysical Institute Download date 01/10/2021 23:37:35 Item License http://creativecommons.org/licenses/by-nc/3.0/ Link to Item http://hdl.handle.net/1834/5076 Master Thesis in Chemical Oceanography THE INORGANIC CARBON CYCLE IN THE RED SEA Elsheikh Bashir Ali June 2008 Acknowledgements First of all, I would like to express my gratitude to my advisors, Truls Johannessen, Abdirahman Omar, and Ingunn Skjelvan, who guided this thesis and helped me whenever I needed them. Thanks for all the extra time you have spent on this thesis and for always keeping your door open. Special thanks go to Eva Falck and Knut Barthel for their valuable advices on this thesis. Many thanks go to Nicolas Metzl for letting me use the Red sea data. Without it this thesis would not be possible. Great thanks to Dr. Abdelgadir Dafalla Elhag for encourage me to do this master study and help me to organize the data collection in summer 2007. Thanks a lot to all friends who help me during the data collection in Port Sudan this summer, especially Institute of Marine Research staff & Faculty of Marine Sciences and Fisheries staff at Red Sea University, and Fishers Administration Office staff. I would like to express my deepest gratitude for all those who supported me during my studies in Norway. In particular, all the people at Geophysical Institute, my friends in Bergen, my neighbors in Fantoft, and all the Sudanese living in Bergen. I would like to thank the Norwegian State Educational Loan Fund for granting me a scholarship throughout my master studies. Last but not least, I would like to thank my family for their love, encouragement and prayers. ii Abstract The inorganic carbon cycle in the Red Sea has been examined based on various datasets from six different years during the period 1977 and 2007. The study has been performed parameter wise, and the processes biological production/remineralization (soft tissue and hard parts), calcium carbonate sedimentation, air-sea gas exchange, and evaporation/precipitation have been considered. The surface water was relatively warm in the central part of the Red Sea due to wind convergence, and colder towards the south and north due to influence of relative cold Gulf of Aden water and net evaporation, respectively. The surface salinity increased all the way from the south towards the north, due to evaporation, and this explained the major part of the northwards increasing concentration in both surface AT and DIC. The surface AT was, in addition slightly influenced by biological production. Air-sea CO2 exchange was believed to influence the surface DIC, however, this effect seemed to be hidden in the random error of the observed data. For the deep water, the AT concentration was mainly influenced by calcium carbonate sedimentation, while remineralization/respiration could explain the major part of the DIC variations observed. fCO2 was positively correlated to the sea surface temperature. During the period 1977 - 2007, the fCO2 of the water increased at a similar rate as the atmospheric fCO2, however, while the atmospheric CO2 increase had obvious reasons, the oceanic increase most likely was a result of an increase in surface temperature. Air-sea CO2 flux was calculated, and the Red Sea was in general a small source for atmospheric carbon for all years except 1991 and 1992, when the southern parts of the ocean appeared as a large source and the northern part had turned into a small sink for atmospheric carbon. This particular situation in the north was connected to strong NNW wind and subsequent upwelling in the north during these two years. iii Contents Acknowledgements ……………………………………………………………………………ii Abstract ……………………………………………………………………………………….iii Contents ………………………………………………………………………………………iv 1. Scientific background ………………………………………………………………………1 2. Description of the Red Sea…………………………………………………………………..2 2.1 Hydrography of the Red Sea……………………………………………………………6 2.1.1 Water masses and exchanges with adjacent seas………………………………...6 2.1.2 Currents and tides………………………………………………………………...8 2.2 Physico-chemical properties………………………………………………………….12 3. The marine inorganic carbon system………………………………………………………16 3.1 Measurement variables for the marine carbon cycle…………………………………..18 3.2 The main processes in the marine inorganic carbon cycle…………………………….22 3.2.1 Solubility pump………………………………………………………………....22 3.2.2 Biological pump………………………………………………………………...24 3.3 Air-sea CO2 exchange………………………………………………………………...26 3.3.1 Model approaches of air-sea gas exchange………………………………….....27 3.3.2 Gas transfer relationships………………………………………………………30 3.4 The effect of climate change on important process: anthropogenic CO2 and ocean acidification…………………………………………………………………………...32 4. Data and methods……………………………………………………………………….....37 4.1 Datasets and analytical methods……………………………………………………....37 4.1.1 GEOSECS data (1977 data)……………………………………………….........37 4.1.2 MEROU cruise data (1982 data)………………………………………….........37 iv 4.1.3 MINERVE cruises data (1991, 1992, and 1999 data)…………………………..40 4.1.4 2007 data………………………………………………………………………..41 4.1.5 World Ocean Atlas 2005 data (WOA05)…………………………………….....42 4.2 Data analysis…………………………………………………………………………..42 4.2.1 Atmospheric fCO2………………………………………………………………42 4.2.2 Air-sea CO2 flux………………………………………………………………..43 5. Results and discussion…………………………………………………………………......45 5.1 Total alkalinity………………………………………………………………………...48 5.2 Total carbon, nutrients, and AOU……………………………………………………..53 5.2.1 DIC and nutrients, surface distribution………………………………………...53 5.2.2 DIC and nutrients, vertical distribution………………………………………...55 5.3 fCO2 values…………………………………………………………………………....60 5.4 fCO2 values………………………………………………………………………….64 5.5 Air-sea CO2 fluxes……………………………………………………………….……66 6. Summary and future work…………………………………………………………………69 7. References………………………………………………………………………………….72 v 1. Scientific background The aim of this thesis has been to study the distribution of inorganic carbon cycle parameters in the Red Sea area and how they vary in time and space naturally and as a function of increasing atmospheric CO2 levels. Data from six cruises have constituted the general analytical background covering a period of 30 years (between 1977 and 2007). The main motivation for selecting this area is the special features found here; it is an area for CO2 release to the atmosphere, it is an important calcification area, and an area for warm deep water formation. Further, the Red Sea has vulnerable coast line and coral reefs and it is one of the most important repositories of biodiversity in the world. The carbon chemistry influences and is influenced by most of above mentioned features of the Red Sea. 1 2. Description of the Red Sea The Red Sea is an important international water body that separates the northeastern Africa from the Arabian Peninsula (Fig. 2.1). It has since ancient times been a source of food for the coastal population and a maritime, trading, and cultural route. The Red Sea in general comprises the most unique coastal and marine environments. It is one of the most important repositories of biodiversity in the world. Its relative isolation and varying physical conditions have given rise to an extraordinary range of ecosystems and biological diversity. The most notable features of the sea are complex coral reefs systems and their associated fauna and flora including several important endangered species (e.g. sea grass beds, salt pans, mangroves and salt marches). The common and local Arabic name for this sea is Al Bahar Al Ahmar. The Red Sea has been formed as a part of the African Rift System. It lies between arid land, desert and semi-desert. It extends from the Strait of Bab Al Mandab at 12.5°N to Ras (cape) Mohammed at the southern part of Sinai Peninsula at 28°N. At this point the Red Sea splits into two branches, one directed northeastwards and called the Gulf of Aqaba and the other directed to the northwest and called the Gulf of Suez (Table 2.1). The Red Sea’s only link to the open ocean in the south is through the Strait of Bab Al Mandab (Bab means door in Arabic) into the Gulf of Aden and the Indian Ocean. The strait of Bab Al Mandab is a shallow and narrow channel. The shallowest section of Bab Al Mandab Strait consist Hanish sill which is located 150 km to the north of the narrowest passage near Perim Island (Fig. 2.1) (Table 2.1). The Red Sea exchanges its water masses with the Arabian Sea and the Indian Ocean via the Gulf of Aden. This transport is number of magnitudes larger than the transport through the Suez Canal in the north, which connects the Mediterranean Sea with the Gulf of Suez and the Red Sea (Sofianos and Johns, 2002). Irregular, eroded escarpments face seaward from the uplifted rift shoulders that flank the Red Sea. The rift shoulders average 1000−3000 m in elevation and expose Late Precambrian granitic, metamorphic, and volcanic rocks. The total width of the rift 2 Sinai Peninsul a Fig. 2.1 Map showing the location and bathymetry of the Red Sea, arrows indicate prevalent wind directions for summer (thick arrows) and winter (thin arrows). Bab Al Mandab Strait is indicated in the figure. 3 occupied by the Red Sea increases from about 250 km in the north to 500 km in the south. The Red Sea consists of narrow marginal shelves and coastal plains and a broad main trough with depths ranging from about 400 to 1200 m. South of 20°N on the eastern side and 17°N on the western side, shallow reefs and carbonate banks nearly fill the entire main trough. Carbonate reefs are situated along the entire length of the Red Sea, but are confined to near shore areas further north. From 15°N to 20°N, the main trough is bisected by a narrow (60 km wide) axial trough with very rough bottom topography and depths greater than 2000 m (Cochran, 2002). Table 2.1 The Red Sea in numbers.