Rajesh Agnihotri

Seasonal Anoxia Over the Western Indian Continental Shelf

The eastern Arabian Sea contains the only eastern-boundary-type upwelling environment in the entire Indian Ocean, albeit on a seasonal basis. During the southwest monsoon, when the surface current flows equatorward, upwelling brings oxygen-poor, nutrient-rich subsurface waters to the Indian continental shelf that turn anoxic (sulfate-reducing) by late summer due to exhaustion of oxygen and nitrate by heterotrophic microorganisms. This natural oxygen-deficient system, by far the worlds largest in the coastal ocean, is apparently more intense now than it was three decades ago. This is consistent with the sedimentary record, which indicates that productivity over the past few decades has been the highest ever in the last seven centuries. However, a trend of ongoing intensification is not seen in the data collected during the last 10 years at a coastal time series station off Goa. These data, nevertheless, show considerable interannual variability with the most severe anoxia having occurred in 2001, adversely impacting local fisheries (especially demersal fish catch). While increased nutrient loading to the coastal zone largely through atmospheric deposition of nitrogen most likely occurred in the last few decades, contributing to a shift to fully anoxic conditions, the observed interannual variability suggests that subtle changes in local hydrography/circulation could also modulate coastal anoxia in the region.

8000-year monsoonal record from Himalaya revealing reinforcement of tropical and global climate systems since mid-Holocene

We provide the first continuous Indian Summer Monsoon (ISM) climate record for the higher Himalayas (Kedarnath, India) by analyzing a 14C-dated peat sequence covering the last ~8000 years, with ~50 years temporal resolution. The ISM variability inferred using various proxies reveal striking similarity with the Greenland ice core (GISP2) temperature record and rapid denitrification changes recorded in the sediments off Peru. The Kedarnath record provides compelling evidence for a reorganization of the global climate system taking place at ~5.5 ka BP possibly after sea level stabilization and the advent of inter-annual climate variability governed by the modern ENSO phenomenon. The ISM record also captures warm-wet and cold-dry conditions during the Medieval Climate Anomaly and Little Ice Age, respectively.

Anthropogenic Climate Change: Observed Facts, Projected Vulnerabilities and Knowledge Gaps

Rising temperatures on earth especially during the later half of the twentieth century have been largely attributed to increasing greenhouse gases (GHG) levels in atmosphere, due to worldwide growth in fossil fuel consumptions by the industrialized nations. Though there are several other offspring of industrial revolution other than GHGs, e.g., increased atmospheric water vapor, aerosols, which also may affect terrestrial climate in either way on shorter timescales, understanding their net role in terms of climate change on regional to global scale remains limited (IPCC 2007). In aggregate, though anticipated anthropogenic climate changes have generated immense curiosity, concerns and lucid debates among intellectuals, scientific experts as reported by Rockstorm et al. (Nature 461:472–475, 2009) broadly agree that deeper understanding is required on how these anthropogenic drivers mutually interact among themselves and most importantly with natural drivers in terms of their net climatic impact. In this communication, we intend to put forward some of the established facts of anthropogenic climate change, projected vulnerabilities of immediate concern and important knowledge gaps that are yet to be explored and understood.

Is δ15N of sedimentary organic matter a good proxy for paleodenitrification in coastal waters of the eastern Arabian Sea

We compared recently published sedimentary records of δ 15 N from several coastal areas affected by both natural and anthropogenically produced shallow hypoxia with the objective of testing this as a proxy for denitrification in coastal settings. We examined the eastern boundary systems of continental shelves off western India and Peru, which appear to be experiencing intensification of bottom-water oxygen depletion, most likely as a consequence of intensification of eastern boundary coastal upwelling over the last few decades. In systems that are significantly affected by an enhanced inventory of nutrients from organic matter in soils due to continental erosion following colonial land clearing (e.g., Chesapeake Bay), fertilizer and wastewater runoff (e.g., western Indian shelf and Long Island Sound), the productivity increase is largely local and induced by anthropogenic activity. The western Indian shelf thus experiences a double effect, being both an upwelling zone and prone to nutrient enrichment from land. While in other regions of both natural and anthropogenic bottom-water hypoxia, sedimentary δ 15 N has undergone significant increases over the Anthropocene; in the eastern Arabian Sea, an opposite trend is noticed despite historical water-column measurements revealing a contemporaneous intensification of denitrification. Plausible causes are discussed here in detail, which led us to conclude that the sedimentary δ 15 N may not always work as a reliable proxy of denitrification in coastal regions.

Nitrogen isotope composition of particulate organic matter (POM) in Lake Kinneret, Israel

Natural variations in the ratio of the 2 stable isotopes of nitrogen (15N and 14N) of particulate organic matter (POM) in aquatic environments reflects and integrates the influence of biogeochemical processes in the water column and/or sediments, an d make them valuable in s itu indicators o f N cycling in ecosystems (PETERSON & FRY 1987, KENDALL et al. 2001, ALTABET 2006). The 015N POM is influenced by the isotopic composition of disso1ved inorganic N (DIN) sources for phytoplankton, isotopic fractionation during partial utilization of these sources, as well as transfer to higher trophic levels. Increased ÕN for sinking and decomposed POM has been found in estuarine and marine ecosystems (ALTABET & MceARTHY 1985) forests and soils (NADELHOFFER & FRY 1994). IfN2 fixatian is significant, however, nitrogen will be added to the system with a relatively low 015N between 0 and -2%o (MAHAFFEY et al. 2005). There are relative few studies of lake N isotope biogeochemistry despite its potential for elucidation ofN cycle dynamics. In surface waters of Lake Lugano, Italy, no consistent relation between the o15N of POM, the o15N of nitrate, and nitrate concentration was found (LEHMANN et al. 2004). The appearance of cyanobacteria in Lake Kinneret, Israel, has increased since 1994, Israel, particularly the filamentous nitrogen-fixing Aphanizomenon ovalisporum, and Cylindrospermopsis cuspis in summer-autumn (HADAS et al. 1999, ZoHARY 2004), which may add to new production and may be imprinted in the o15N ofPOM during summer-autumn. We followed the temporal and depth-related fluctuations in the 015N ofPOM during an annual cycle to explore whether this isotope ratio could contribute to a better understanding of nitrogen cycling in Lake Kinneret, Israel.