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New light on mass fish deaths

Anil Gupta in his lab

A new study might provide new clues into mass fish mortality in the depths of the northwestern Arabian Sea. It suggests that these deaths could be a result of oxygen reduction caused by remineralisation or oxidization of organic carbon that leaves little or no oxygen for fish to breathe1. The study contradicts earlier ones that linked the changes in the deep waters of the northwest Arabian Sea to outflow from the Red Sea and the Persian Gulf.

Geologists studying the dietary habits of some marine protozoans (foraminifera) in the sediments off the Oman Margin in the northwestern Arabian Sea found that there was higher organic carbon production in the surface ocean but not on the sea floor at the depth of 800 meter.

They analyzed deep-sea fossil census records of benthic foraminifera and their isotope values from the sediments for the past 12,000 years — in the vicinity of the Holocene, an interval of great human interest.

The findings suggested that though there was significant production of organic carbon in the surface waters during the early Holocene (10,000 to 8,000 years ago) at times of intense summer monsoon, little or no organic carbon was deposited in the deep-sea sediments.

"We relate this to the remineralization (oxidation) of organic matter by the invasion of a mixture of high oxygen waters — the North Atlantic Deep Water (of Arctic origin) and the Circumpolar Water (of Antarctic origin)," says lead researcher Anil Kumar Gupta.

The findings suggest that the variability in benthic ecosystem in the deep Arabian Sea is not only driven by the upwelling induced by summer monsoon and the organic matter flux but also by changes in deepwater ventilation.

Gupta says that intense summer monsoon circulation may not always result in an oxygen-poor condition with higher organic carbon flux in the northwest Arabian Sea, which has earlier been cited as the reason for mass fish deaths. "Instead our data reflects that remineralization of organic matter at sites within the core of the 'oxygen minimum zone' in northwestern Arabian Sea with no oxygen replenishment could be the culprit," he says.

The type of organic matter also plays an important role in controlling the oxygenation of the deep ocean, he adds.

The data could be used by climate modelers who estimate carbon dioxide budgets of deep seas or geochemists using stable isotope studies.



  1. Gupta, A. K. et al. Benthic foraminiferal faunal and isotopic changes as recorded in Holocene sediments of the northwest Indian Ocean. Paleoceonography (2008) doi: 10.1029/2007PA001546

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