Impact of monsoon-driven surface ocean processes on a coral off Port Blair on the Andaman Islands and their link to North Atlantic climate variations Tim Rixen a, ⁎, Purvaja Ramachandran b , Laura Lehnhoff a , Dorothee Dasbach a , Birgit Gaye c , Brigitte Urban d , Ramesh Ramachandran b , Venugopalan Ittekkot a a Leibniz-Zentrum für Marine Tropenökologie, Fahrenheitstr. 6, D-28359 Bremen, Germany b Institute for Ocean Management, Anna University, Chennai 600 025, India c Institut für Biogeochemie und Meereschemie, Universität Hamburg, Bundesstraße 55, D-20146 Hamburg, Germany d Leuphana University Lüneburg, Faculty of Environmental Science and Engineering, Herbert-Meyer-Str. 7, D-29556 Suderburg, Germany abstract article info Article history: Received 1 October 2009 Accepted 18 September 2010 Available online 24 September 2010 Keywords: Bay of Bengal monsoon corals Andaman Islands marine nitrogen cycle North Atlantic climate variations are reflected in sedimentary records from the northern Indian Ocean in which two basins, the Arabian Sea and the Bay of Bengal, are strongly affected by the monsoon. Contrary to the Bay of Bengal the Arabian Sea plays an important role in the global marine nitrogen cycle. In its mid-water oxygen minimum zone (OMZ) bioavailable fixed nitrogen is reduced to nitrogen gas (NO 3 - - N N 2 ), whereas oxygen concentrations are slightly above the threshold of nitrate reduction in the OMZ of the Bay of Bengal. A coral colony (Porites lutea) growing south of Port Blair on the Andaman Islands in the Bay of Bengal was studied for its response to changes in the monsoon system and its link to temperature changes in the North Atlantic Ocean, between 1975 and 2006. Its linear extension rates, δ 13 C and δ 18 O values measured within the coral skeleton reveal a strong seasonality, which seems to be caused by the monsoon-driven reversal of the surface ocean circulation. The sampling site appears to be influenced by low salinity Bay of Bengal Water during the NE monsoon (boreal winter) and by the high salinity Arabian Sea Water during the SW monsoon in summer. The high salinity Arabian Sea Water circulates along with the Summer Monsoon Current (S-MC) from the Arabia Sea into the Bay of Bengal. Decreasing δ 18 O and reconstructed salinity values correlate to the increasing SSTs in the North Atlantic Ocean indicating a reduced influence of the S-MC at the sampling site in the course of northern hemispheric warming. During such periods oxygen depletion became stronger in the OMZ of the Arabian Sea as indicated by the sedimentary records. A reduced propagation of oxygen-depleted high salinity Arabian Sea Water into the Bay of Bengal could be a mechanism maintaining oxygen concentration above the threshold of nitrate reduction in the OMZ of the Bay of Bengal in times of global warming. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Fertility of the ocean, and the sequestration of CO 2 from the atmosphere by marine organisms is strongly influenced by the marine nitrogen cycle which is mainly driven by nitrate reduction and nitrogen fixation (N 2 – N 2NH 3 )(Dugdale and Goering, 1967; McElroy, 1983; Brandes and Devol, 2002; Deutsch et al., 2007). The Arabian Sea in the northern Indian Ocean plays an important role in the global nitrogen cycle because ~30% of the global water-column nitrate reduction occurs in its OMZ (Naqvi, 1987; Bange et al., 2000; Codispoti et al., 2001). The Bay of Bengal also reveals a pronounced OMZ which is strongly influenced by the propagation of oxygen-depleted, high salinity Arabian Sea Water into the Bay of Bengal (Rao et al., 1994). Since oxygen concentrations are slightly higher in Bay of Bengal than in the Arabian Sea mid-water nitrate reduction, which is generally assumed to occur at oxygen concentrations b ~3–5 μM, is so far absent in the Bay of Bengal (Rao et al., 1994; Codispoti et al., 2001). On centennial to glacial interglacial time scales nitrate reduction rates in the Arabian Sea were very sensitive to northern hemispheric climate variations and increased during times of northern hemispheric warming (Altabet et al., 1995; Suthhof et al., 2001; Agnihotri et al., 2008). Temperature reconstructions (Levitus et al., 2000; Smith et al., 2008; Banzon et al., 2010) in line with records derived from century old coral colonies indicate a general warming of the entire tropical/ subtropical oceans during the last ~ 100 to 150 years (Cole et al., 2000; Pfeiffer et al., 2006; Grottoli and Eakin, 2007). A declining Eurasian snow cover, increasing wind stress over the Arabian Sea in line with other satellite-derived information, suggests an intensification of the Asian summer monsoon and the monsoon-driven upwelling in the Global and Planetary Change 75 (2011) 1–13 ⁎ Corresponding author. E-mail address: trixen@uni-bremen.de (T. Rixen). 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2010.09.005 Contents lists available at ScienceDirect Global and Planetary Change journal homepage: www.elsevier.com/locate/gloplacha