New Delhi: As world leaders and climate activists prepare to gather in Dubai in the United Arab Emirates (UAE) for COP28 (28th Conference of the Parties of the United Nations Framework Convention on Climate Change) this month, the urgency to incorporate evidence-based strategies in the fight against climate change is more apparent than ever.
Oceans play a crucial role in supporting life on Earth and have a profound impact on the planet’s ecosystems, climate and overall biodiversity. Oceans absorb, store and redistribute heat from the sun, helping to regulate the Earth’s climate. The ocean's ability to retain and release heat moderates temperature extremes, influencing weather patterns and stabilising the global climate. Oceans significantly influence weather systems and precipitation patterns. The evaporation of water from the ocean surface contributes to the formation of clouds and precipitation, influencing rainfall and weather conditions across the globe.
But, more than the surface of the oceans, it is the study of oceanic sediments that not only offers a window into the earth’s climate history, but also equips experts with the knowledge needed to make informed decisions about the future of our planet. Oceanic sediments are valuable archives that contain important clues about past climate changes. By analysing these sediments, scientists can reconstruct historical climate conditions and gain insights into the earth’s climate system.
The study of oceanic sediments is crucial for understanding past climate changes and can provide valuable information to help predict future climate change. Oceanic sediments serve as archives of past environmental conditions. By analysing sediment cores, scientists can reconstruct past climate variations, including changes in temperature, precipitation and ocean circulation.
Among the sources of oceanic sediments provided about climate change are foraminifera and isotopes. Foraminifera are microscopic marine organisms that have shells composed of calcium carbonate. The isotopic composition of their shells, specifically the ratio of oxygen isotopes, can provide information about past sea surface temperatures. Changes in these isotopic ratios in sediments can help scientists reconstruct temperature variations over time.
“Foraminifera species are usually less than 1 mm in size except for a few,” Rajiv Nigam, former Head of Geology and Marine Archaeology at the National Institute of Oceanography, told ETV Bharat. “They are extremely sensitive to climate change. They are exclusively marine organisms." In 2022, Nigam became the first Indian to be awarded the prestigious Joseph A Cushman Award for his outstanding contributions to the field of foraminifera.
Nigam said that the presence or absence of foraminifera decides many things. He gave the example of Lothal, an ancient civilisation in today’s Gujarat, where, during an excavation in 1954, a very big tank-like body was found. “Then it was debated whether it was a dockyard or an irrigation tank,” he explained. “Then foraminifera were found in the tank. That tilted the debate in favour of it being a dockyard. It turned out that it was the world’s oldest-ever dockyard. It was filled with marine water.”
Nigam explained that based on this, scientists constructed a sea-level curve. It was found that 6,000 years ago, the sea level was higher than it is today. That was the time Lothal was connected to the sea through an estuary. The isotopic composition of shells of the foraminifera, specifically the ratio of oxygen isotopes, can provide information about past sea surface temperatures. Changes in these isotopic ratios in sediments can help scientists reconstruct temperature variations over time. “Sea level decided the destiny of many ancient coastal towns,” Nigam explained. “Sea level changes because of major climatic change.”
Changes in the composition of sediments, including the types of minerals present and their distribution, can provide information about past environmental conditions. “Sediments comprise living and non-living components,” Nigam said. “Minerals are non-living components. Remnants of past life or fossils are living components. Fossils are more susceptible to climate change.”
Sediment cores extracted from the ocean floor provide a chronological record of deposition. Distinct layers in these cores can correspond to different climatic periods. By analysing the composition and characteristics of these layers, scientists can identify shifts in climate and environmental conditions. Sediments can also record changes in the earth’s magnetic field over time. Magnetic susceptibility measurements on sediment cores can help date and correlate sediment layers, providing a chronological framework for climate reconstructions. According to a study published in Researchgate.net, magnetic susceptibility has been widely used for paleoclimatic and paleoenvironmental studies. However, there exist ambiguities when accurately interpreting this mineral magnetic proxy.
“Study of the magnetic susceptibility of the sediments is useful in understanding long-time climate changes,” Nigam explained. Among other clues that oceanic sediments provide regarding climate change are carbonate preservation, fossilised pollen and biomarkers. The preservation of carbonate minerals in sediments is influenced by factors such as ocean acidity and temperature. Changes in carbonate preservation can indicate variations in ocean chemistry and help reconstruct past climate conditions.
Pollen grains found in marine sediments can provide information about past vegetation and climate on land. Changes in the types and abundance of pollen can indicate shifts in temperature, precipitation, and vegetation cover. Organic molecules derived from biological sources, known as biomarkers, can be preserved in sediments. By analysing these biomarkers, scientists can reconstruct past conditions such as sea surface temperatures, productivity, and the presence of specific organisms.
Nigam, however, said that because of anthropogenic changes, the climate is likely to change. Those changes which are the result of human activities are known as anthropogenic changes. They affect the ecosystem by habitat destruction. People living in coastal areas have to be very careful, according to Nigam.
At the same time, Nigam pointed out that climate change happened even when there was no human habitation on earth. “There is natural climate variability because of changes in incoming energy from the sun,” he said, adding it is necessary to know the reasons for this. Nigam also said that by linking past climate changes, future climate changes can be predicted. “The need of the hour is to study high-resolution past climate changes during the last few thousand years,” he said.
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