Goa, December 8, 2021

As a part of the GEOTRACES-INDIA initiative, a major oceanographic programme supported by the Ministry of Earth Sciences, Govt. of India, dissolved iron cycling in the Arabian Sea and subtropical gyre region of the Indian Ocean has been studied. The study has made several breakthroughs in the field of micronutrients and trace metal biogeochemistry in the Northern Indian Ocean, particularly in the Arabian Sea region. Iron is an essential micronutrient for microorganisms in seawater and its scanty availability limits primary production in almost ~30% of the global ocean including part of the Arabian Sea. Iron is required by phytoplankton and plants to transfer electrons during photosynthesis, to make chlorophyll and to utilize macronutrient such as nitrogen.

 Iron is so meagre in the ocean that even a bit of rust from the iron made ship in the ocean throws measurements significantly off. To avoid any such contaminations, scientists sampled the seawater with specially designed and installed metal free sampling gear and processed seawater samples in a metal free clean laboratory installed onboard ocean research vessel. The instruments used to detect iron onboard or at shore-based laboratory are highly sensitive to detect minor fluctuation of the order of nano-molar in seawater.

The new study published in the Geochimica et Cosmochimica Acta (Chinni and Singh, 2022) journal highlights the discovery of very high dissolved iron concentrations emanating from the hydrothermal vents in the water column over the Carlsberg Ridge in the Southern Arabian Sea that can be a hotspot for other trace metals too. It is surprising to observe hydrothermal sourced Fe in the shallow water depth. Another very important finding of this study is the dominant supply of dissolved iron from the continental shelf/slope sediments in the Arabian Sea region while atmospheric soluble Fe dust flux plays a secondary role, which was considered the main source of Fe prior to this study. Perennial oxygen deficient intermediate waters of the Arabian Sea source significant amount of iron.

Dissolved Fe in the Arabian Sea supplied by various sources are getting removed equally by biotic and abiotic removal processes. The integrated dissolved iron in the sun-lit region is double in the Arabian Sea compared to the subtropical Indian Ocean which is responsible for the higher primary productivity in the Arabian Sea.

This study reveals that a large part of the Arabian Sea in the south and in the northwest are iron deficient leading to lower productivity. The intrusion of Southern Ocean waters and upwelling of Fe deficient deep water alongside the Murray Ridge both likely cause Fe limitation in the Arabian Sea.

In a current alarming situation of global warming and climate change our scientific results insight a solution to mitigate the abundance of greenhouse gas, CO₂. Our findings suggest that seeding the Fe deficit waters of the Arabian Sea with an ample amount of Fe can enhance the biological pump and can increase the capacity to sequester atmospheric CO₂ through primary production (conversion of atmospheric CO₂ to organic carbon). This process will diminish the atmospheric CO₂ levels introduced into the biosphere by anthropogenic activities which will also take care of Ocean acidification problem. If this climate change problem is left unattended, decrease in pH of the surface oceans will lead to ocean acidification. Geoengineering such as Fe seeding in the Arabian Sea will enhance biological productivity leading to increase in fish production thereby stimulating the fishery-based economy of almost ~14 million dependent people.

Chinni Venkatesh and Singh Sunil Kumar (2022) Dissolved iron cycling in the Arabian Sea and sub-tropical gyre region of the Indian Ocean, Geochimica et Cosmochiomica Acta 317, 325-348.

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