Arabian Sea oxygen deficient
zone (ODZ) evolution
Arabian Sea winter algal blooms: Image source - www.nasa.gov
The loss of biologically available nitrogen (N) from the ocean occurs through denitrification. Marine denitrification occurs under oxygen-limited conditions, both in the sediment and ODZs in the subsurface water column . The Arabian Sea hosts one of the 3 major ODZs of the ocean (the other two ODZs are in the Equatorial Pacific). During denitrification, ocean nitrate is converted to nitrogen gas, which escapes to the atmosphere. Isotope fractionation during denitrification preferentially removes the lighter N isotope (14-N), causing elevated 15N/14N ratios in the residual waters.
Glacial-interglacial variability of Arabian Sea denitrification strength
In the 1990s, scientists measured stable N isotope ratios in marine sediments to study denitrification variability within the ODZs through geologic time. The findings implied that during glacial times, denitrification strength was lower in both Equatorial Pacific and Arabian Sea ODZs. However, later studies revealed that bulk sediment N isotope measurements are heavily altered by diagenesis and contaminant organic matter.
In an ongoing project at Princeton, I measure N isotope ratios of organic matter preserved within the calcite shells of planktonic foraminifera (a single-celled zooplankton). Foraminifera-bound organic matter (FBOM) are protected from diagenesis and contaminant N, preserving past environmental signals across several millions of years. Therefore, Isotope measurements made on FBOM may provide information otherwise inaccessible through the bulk sediment.
A scanning electron microscope (SEM) image of Globigerinoides ruber, a common planktonic foraminifera in the modern ocean.
source- joidesresolution.org