Suhas Shetye

First direct measurements of N2 fixation during a Trichodesmium bloom in the eastern Arabian Sea

We report the first direct estimates of N 2 fixation rates measured during the spring, 2009 using the 15N 2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N 2 fixation rates vary from 0.1 to 34 mmol N m -2d -1 after correcting for the isotopic under-equilibrium with dissolved air in the samples. These higher N 2 fixation rates are consistent with higher chlorophyll a and low I´ 15N of natural particulate organic nitrogen. Our estimates of N 2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.

Occurrence of Nitrogen Fixing Cyanobacterium Trichodesmium under Elevated pCO2 Conditions in the Western Bay of Bengal

Recent studies on the diazotrophic cyanobacterium Trichodesmium showed that increasing CO2 partial pressure (pCO2) enhances N2 fixation and growth. We studied the in situ and satellite-derived environmental parameters within and outside a Trichodesmium bloom in the western coastal Bay of Bengal (BoB) during the spring intermonsoon 2009. Here we show that the single most important nitrogen fixer in today’s ocean, Trichodesmium erythraeum, is strongly abundant in high (≥300 μatm) pCO2 concentrations. N : P ratios almost doubled (~10) at high pCO2 region. This could enhance the productivity of N-limited BoB and increase the biological carbon sequestration. We also report presence of an oxygen minimum zone at Thamnapatnam. Earlier studies have been carried out using lab cultures, showing the increase in growth rate of T. erythraeum under elevated pCO2 conditions, but to our knowledge, this study is the first to report that in natural environment also T. erythraeum prefers blooming in high pCO2 concentrations. The observed CO2 sensitivity of T. erythraeum could thereby provide a strong negative feedback to rising atmospheric CO2 but would also drive towards phosphorus limitation in a future high CO2 world.

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 Indian 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, water depth 3 054 m). The surface seawater samples indicate higher pCO2 values in Arabian Sea as compared 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.

Latitudinal shifts in the Polar Front in Indian sector of the Southern Ocean: evidences from silicoflagellate assemblage

We used silicoflagellate assemblage records to describe the polar frontal variability over the last 48kyr in the Indian sector of the Southern Ocean. The studied core was collected onboard ORV Sagar Nidhi from within the Polar frontal zone (PFZ) during the 4th Indian Scientific Expedition to Southern Ocean. The Polar front is dominated by silica-rich sediments (diatoms and silicoflagellates). Silicoflagellates were dominated by Distephanus speculum and Dictyocha fibula species. The biostratigraphic record of these silicoflagellates was used qualitatively to examine past changes in polar frontal variability in the Southern Ocean. Warming is indicated by an increase (decrease) in Dictyocha sp. (Distephanus sp.) from the LGM to the Holocene. Dictyocha sp. abundance indicates warmer temperatures during 43–45 kyr and is nearly synchronous with the warming event recorded in an Antarctic ice core. Dictyocha/Distephanus ratio also suggests a northern and southern shift in the polar front during LGM and 43–45 kyr respectively. The southward displacement of the frontal system is linked to an increase in sea surface temperature as evidenced from the δ18O Byrd Antarctic ice core data and solar insolation data. The low dust flux, higher δ18O and absence of an upwelling indicator diatom, Thallasionema nitzchoides during the Antarctic warming event also suggest stronger thermal stratification during the Antarctic warming event as compared to LGM. The present study would improve our understanding of the frontal variability under future warming scenarios.