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The Arabian Sea is a productive basin where seasonal upwelling and convective mixing result in high surface nutrient concentrations and widespread algal blooms. The factors controlling primary productivity in the Arabian Sea are of interest because the region contains an intense oxygen minimum zone (OMZ) that is a major sink for nitrate in the ocean. A survey of iron (Fe) distribution and redox chemistry was carried out in 2007 to assess its role in Arabian Sea biogeochemistry, including an investigation of the biological response to Fe additions. Results show that Fe is strongly enriched in the eastern Arabian Sea, associated with the OMZ. Much of the Fe within the OMZ is present as Fe(II), which enhances the residence time and accumulation of Fe. In contrast, Fe concentrations are lower in the western Arabian Sea, separated from the Fe-rich OMZ by a water mass boundary. Consequently, low surface values are associated with monsoon-driven upwelling off the Omani coast. Incubations revealed that primary production during the southwest monsoon was strongly limited by Fe over much of the study area. Incubation of surface waters with Fe resulted in rapid growth of Phaeocystis and up to sixfold increase in chlorophyll. This is the first demonstration of Fe limitation in the Arabian Sea, and the first high resolution zonal survey of dissolved Fe and Fe(II) across the basin. Our findings suggest that models developed to predict the response of the Arabian Sea biogeochemistry to global warming need to consider effects on Fe inputs.

Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light-brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod-shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P 2 O 5 content of the phosphorite is 29%, and the CO 2 content of the CFA is about 4.5%. The rare-earth element (REE) abundance (ΣREE=2.02 μg g -1 ) is lower than in other modern phosphorites. The 87 Sr/ 86 Sr ratio and eNd value of this sample are 0.70921 and -9.9 respectively. The 14 C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U-series method (100 ka). The second type, phosphorite 2, comprises a friable, light-brown nodule consisting of CFA as the only major mineral, with a CO 2 content of the CFA of 4.5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P 2 O 5 content is 33%, and REE contents (ΣREE = 0.18 μg g -1 ) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low-energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P 2 O 5 content of the phosphorites are found to be dependent on the nature of the initial substrates and physico-chemical conditions during phosphatization. The CO 2 content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment-water interface may control REE concentrations in phosphorites.

The δ15N composition of nitrate and N2 gas was measured in the eastern tropical North Pacific (ETNP) and central Arabian Sea (AS) suboxic regions. The δ15N of nitrate increased from 6‰ at 2,500 m to 15‰ at 250–350 m in both regions, while the δ15N of N2 concurrently decreased from 0.6‰ to 0.25‰. The denitrification isotopic fractionation factor (edenit) for each region was estimated using both advection-reaction and diffusion-reaction models. Values for edenit in the ETNP ranged from 25 ± 2 (advection-reaction) to 30 ± 3 (diffusion-reaction). Values for edenit in the central AS varied from 22 ± 3 (advection-diffusion) to 25 ± 4 (reaction-diffusion) using a starting nitrate isotopic composition of 6‰ but were indistinguishable from calculated values from the ETNP when an initial value of 5‰ was employed. Based upon the model results, an average global edenit of 27 ± 3 is proposed for marine suboxic water columns. Isotopic enrichment of nitrate in oxic waters beneath the active denitrification regions was observed and indicates the presence of significant cross-isopycnal ventilation at depth. The isotopic composition of nitrate decreased above 250 m to −80 m, and this pattern is hypothesized to be caused by the input of isotopically light nitrogen from nitrogen fixation in the euphotic zone. A simple isotopic mass balance indicates that a significant percentage of primary productivity in the central AS may be fueled by nitrogen fixation.