For 8500 years, monsoons affecting the growth of marine plants in the Arabian Sea: study

Between 8,500 and 6,000 years before present, there were strong summer monsoons, while the monsoon remained in its weak phase for the next 4,000 years, according to the study published in the journal Quaternary International.

The monsoon is the lifeline of India and a good monsoon essentially stimulates the agrarian economy. India has two types of monsoon – the summer monsoon between June and September and the northeast monsoon between October and December.

In this study, researchers from IIT Kharagpur, Wadia Institute of Himalayan Geology, Vellore Institute of Technology and Bharathidasan University attempted to understand the response of some marine microorganisms to the changing monsoon, from about 8500 years before 1950. wind reversals during the summer and winter seasons, the most important being the circulation of summer monsoon winds.

For the study, the researchers studied the oxygen and carbon isotope ratios of fossil shells and calculated the organic carbon and calcium carbonate content of 100 foraminifera sediment samples. They are single-celled microorganisms that grow on the seabed and also float on the surface of seawater.

The most dominant planctic species, including Globigerina bulloides, were also studied. They are sensitive to changes in the marine environment, such as seawater temperature, pollution levels and others, but are able to retain organic carbon. Paleoclimatic and paleooceanographic variabilities are obtained from these shells on secular and millennial time scales.

Scientists noted that for some years plankton like Globigerina bulloides bloomed abundantly in the Arabian Sea. Unlike some periods between 6000 years and 2000 years before the present, their flowering was noticeably lower than normal. Senior geologist Anil K Gupta of IIT-Kharagpur linked this behavior to monsoon winds and their intensities.

He said: “When the monsoon is strong, the associated monsoon winds are also strong, causing significant mixing of surface ocean waters. In turn, this results in cold, nutrient-rich water from the thermocline layer reaching the upper levels of the ocean. Thus, planktons like Globigerina bulloides bloom and increase in population – a direct response to the arrival of nutrients at the surface.

“We thus linked the strong monsoon winds to a rich population of plankton,” he added.

Also, even though there is higher food production on the ocean surface, it eventually gets deposited on the ocean floor, thus benefiting even the marine zoobenthos that grows on the ocean floor. , Gupta added.

“Since the lifespan of these organisms is limited between days and weeks, nutrient signals are well captured by them and preserved even after they die and leave behind fossil shells,” he said.

The situation was dramatically different when monsoon winds were weak, Gupta said. “The presence of plankton was almost insignificant due to the lack of nutrients for their growth,” he said.