Dynamics of sea-ice biogeochemistry in the coastal Antarctica during transition from summer to winter

The seasonality of carbon dioxide partial pressure （pC02）, air-sea CO2 fluxes and associated environ- mental parameters were investigated in the Antarctic coastal waters. The in-situ survey was carried out from the austral summer till the onset of winter （January 2012, February 2010 and March 2009） in the Enderby Basin. Rapid decrease in pCO2 was evident under the sea-ice cover in January, when both water column and sea-ice algal activity resulted in the removal of nutrients and dissolved inorganic carbon （DIC） and increase in pH. The major highlight of this study is the shift in the dominant biogeochemica/ factors from summer to early winter. Nutrient limitation （low Si/N）, sea-ice cover, low photosynthetically active radiation （PAR）, deep mixed layer and high upwelling velocity contributed towards higher pCO2 during March （early winter）. CO2 fluxes suggest that the Enderby Basin acts as a strong CO2 sink during January （-81 mmol m-2 d 1）, however it acts as a weak sink of CO2 with -2.4 and -1.7 mmol m 2 d-1 during February and March, respectively. The present work, concludes that sea ice plays a dual role towards climate change, by decreasing sea surface pCO2 in summer and enhancing in early winter. Our observations emphasize the need to address seasonal sea-ice driven CO2 flux dynamics in assessing Antarctic contributions to the global oceanic CO2 budget.

Contrasting Productivity and Redox Potential in Arabian Sea and Bay of Bengal

Understanding the past and present changes is critical for evaluating the future climatic changes. In order to understand the paleoproductivity and depositional environments of Northern In- dian Ocean, two sediment cores were collected, one each from the Arabian Sea （lat. 16°51.40′N and long. 71°54.37′E, water depth 803 m） and the Bay of Bengal （lat. 13°05.35′N and long. 91°28.21′E, wa- ter depth 3 054 m）. The surface seawater samples indicate higherpCO2 values in Arabian Sea as com- pared to the Bay of Bengal. The sediment organic carbon variations along with sedimentological and other geochemical parameters were studied. Sediment organic carbon varied from 0.5%--4.7% and 0.3%-1.22% in Arabian Sea and the Bay of Bengal, respectively. In Arabian Sea, low productivity, oxic conditions and less intense southwest monsoon prevailed during the deglacial period, whereas productivity has increased from last 16 kyr to the modern age. In the Bay of Bengal, organic carbon decreased from the Last Glacial Maxima （LGM） to the modern age, indicating higher productivity in the past as compared to modern age. Fe was associated with organic carbon in the Bay of Bengal and increased during LGM, showing similar trend to that of organic carbon, indicating that Fe may be the limiting factor for the growth of phytoplankton in the Bay of Bengal in the modern age. In the Bay of Bengal, Mn is enriched during modern age and is depleted during LGM, whereas chromium showed the opposite trend indicating anoxic conditions during the LGM, whereas in Arabian Sea the trends are opposite to the Bay of Bengal.

Satellite remote sensing of the island mass effect on the Sub-Antarctic Kerguelen Plateau, Southern Ocean

The presence of the Kerguelen Plateau and surrounding bathymetric features has a strong influence on the persistently eastward flowing Antarctic Circumpolar Current （ACC）, resulting in enhancement of surface chlorophyll-a （Chl-α） in the downstream section of the plateau along the polar front （PF）. The phenomenon is reported in this paper as the island mass effect （IME）. Analysis of climatological Chl-a datasets from Aqua- Moderate Resolution Imaging Spectroradiometer （Aqua- MODIS） and Sea-viewing Wide Field-of-view Sensor （SeaWiFS） shows distinct bloomy plumes （Chl-α 〉 0.5 mg/m3） during austral spring-summer spreading as far as -1800 km offshore up to 98°E along the downstream of the north Kerguelen Plateau （NKP）. Similar IME phenomena is apparent over the south Kerguelen Plateau （SKP） with the phytoplankton bloom extending up to 96.7°E, along the southern boundary of ACC. The IME phenomena are pronounced only during austral spring-summer period with the availability of light and sedimentary source of iron from shallow plateau to sea surface that fertilizes the mixed layer. The NKP bloom peaks with a maximum areal extent of 1.315 million km2 during December, and the SKP bloom peaks during January with a time lag of one month. The blooms exist for at least 4 months of a year and are significant both as the base of regional food web and for regulating the biogeochemical cycle in the Southern Ocean. Even though the surface water above the Kerguelen Plateau is rich in Chl-a, an exception of an oligotrophic condition dominated between NKP and SKP due to apparent intrusion of iron limited low phytoplankton regime waters from the Enderby basin through the north- eastward Fawn Trough Current.