Foraminiferal Calcium Carbonate Pump Response to Temporal Changes of Carbon dioxide System in the Sundarban Mangrove Environment, NE Coast of Bay of Bengal, India

Rising atmospheric CO2 to changes in seawater carbonate chemistry, which could affect marine biogenic calcification of the foraminifera. This study reports the production/ dissolution of foraminifera in relation to the air- water exchange of CO2 from mangrove water at the land-ocean boundary condition of Sundarban. Globigerina sp. and Ammonia beccarii were found to be most abundant amongst the foraminifera occurring in the 0-2 cm layer of the mangrove sediment coloumn. Other species were found to be Nominella stella, Milammina fusca, Protelphidium sp., Elphidium rugulosum, Tipotrocha sp., Discorbis sp., Rotalia sp., Tentularia sp. etc. Salinity of the water during the study period (Sept ´06 to Feb ¢07) varied between 15 and 28 psu and temperature, between 24.5 and 30°C. Air-Water flux of CO2 varied between - 4.8 and 101.9 mmol m-2h-1. Tidal water was supersaturated with respect to calcite (37 - 1243%). Foraminifera samples were classified into 7-9 size mode fractions. The observed radius (r) range was found to be between 7.47 and 52.45 mm. Number (N) / Volume (V) size distribution dN or dV/dlogr of the total foraminifera samples were observed either in trimodal or bimodal. Volume-size distribution with respect to depth showed prominent peak in the size range between 10-20 mm, 22- 42 mm and > 42 mm. However, the maximum volume of dV/dlogr (peak height) was found to be decreased from 1.1´ 109 mm3cm -2 at 0-2cm layer of the sediment column to 9.1´ 108 mm3cm-2 at 2-4cm and to 4.37´108 mm3 cm 2 at 4-6cm layer of the sediment column indicating dissolution. However, maximum values of dV/dlogr in the radius range 20-40mm at 0-2cm layer of the sediment column was found to be increased from 1.14 ´ 107 mm3cm-2 in September to 1.106x 10 9 mm3cm-2 in November indicating production while partial pressure of CO2 in the tidal water was found considerably decreased from 659 matm in September to 553 matm in November. Even though foraminifera showed calcium carbonate production in the surface sediment layer (0- 2cm), refered to as the calcium carbonate pump, water still showed emission flux to the atmosphere but at reduced rate.