Nitrogen isotopic studies in the suboxic Arabian Sea

Measurements of¹⁵N/¹⁴N in dissolved molecular nitrogen (N₂), nitrate (NO ₃ ⁻ ) and nitrous oxide (N₂O) and¹⁸O/¹⁶O in N₂O [expressed as δ¹⁵N and δ¹⁸O, relative to atmospheric N₂ and oxygen (O₂), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest and most intense O₂ minima observed in the open ocean. Microbially-mediated reduction of NO ₃ ⁻ to N₂ (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ¹⁵N of NO ₃ ⁻ increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ¹⁵N of N₂ concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ¹⁵N of NO ₃ ⁻ is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen fixation. Isotopic analysis of N₂O reveals extremely large enrichments of both¹⁵N and¹⁸O within the OMZ, presumably due to the preferential reduction of lighter N₂O to N₂. However, isotopically light N₂O is observed to accumulate in high concentrations above the OMZ indicating that the N₂O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the southwest coast of India, where some of the highest concentrations of N₂O ever found at the sea surface are observed, show moderate depletion of¹⁵N, but slight enrichment of¹⁸O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly¹⁸O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or sinks of N₂O in the atmosphere. Also, the N₂O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling between the two processes as an important mechanism of N₂O production.     

Behaviour of boron,calcium and magnesium in a polluted estuary

Behaviour of industrially added boron in the well mixed Ambika river estuary of South Gujarat, India has been studied along with that of calcium and magnesium. The ratios of Ca : Cl and Mg : Cl at varying chlorinities were always within the normal ranges found in river water at one end and seawater at the other end. B : Cl exceeded the river water end limit at the ebb tide at an upstream station. Percentage addition or removal of boron, calcium and magnesium was calculated using the concept of the theoretical dilution line. The external input raised the boron concentration by 263% in the vicinity of the industrial discharge. A correlation coefficient of ?0·82 between chlorinity and percentage addition or removal of boron supports the theory of boron incorporation in sediments in amounts proportional to salinity. Inverse relationship between percentage addition or removal of boron and suspended solids was observed. Behaviour of calcium and magnesium was in line with the replacement of Ca²⁺ ions occupying the majority of ion-exchange sites in riverine clays by Mg²⁺, Na⁺ and K⁺ during their first encounter with seawater.