Evidence for high annual rainfall 66 million years ago found
- July 9, 2023
- Posted by: OptimizeIAS Team
- Category: DPN Topics
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Evidence for high annual rainfall 66 million years ago found
Subject :Geography
Section Physical geography
Context:
- A team of scientists from IIT Kharagpur and Academia Sinica, Taipei has found evidence of very high annual rainfall during the catastrophic volcanism that formed the Deccan Traps in India about 66 million years back.
Details of the study:
- New techniques used:
- They used a new technique — Nanoscale Secondary Ion Mass Spectrometry — to analyse three isotopes of oxygen (Oxygen-16,17, and 18) in fossil trees of the Cretaceous period and measure the isotopic composition of the lake water derived from rainfall.
- Findings:
- The depleted values of the oxygen isotopes suggest a higher tropical rainfall (1,600 mm per year) in India during the terminal Cretaceous period.
- The increase in rainfall and its waning in the early Palaeocene closely follows the change in palaeo-atmospheric (paleo carbon dioxide) suggesting a possible underlying link.
- The available records of the atmospheric carbon dioxide concentration and temperature over both land and ocean during the time of the Deccan Trap eruption were analysed.
- Deccan trap lavas were erupting spewing huge amounts of carbon dioxide thus increasing the then atmospheric carbon dioxide concentration to as high as 1,000 ppm (parts per million).
- Except for the arid/semi‐arid regions, the modern annual rainfall over large parts of peninsular India on average is about 1,000‐1,200 mm.
- Our data suggested that these fossil trees recorded 1,800‐1,900 mm of rainfall per year.
- This is exactly what the IPCC predicts in case of a future extreme 4-degree C warming of the planet.
Impact of rising CO2 concentrations:
- Fossil fuel emission has increased carbon dioxide from a pre‐industrial level of 280 ppm to about 420 ppm in 2023.
- Climate models suggest that a doubling of carbon dioxide will intensify the atmospheric circulation and consequently the rainfall.
- The 2023 AR6 report by the Intergovernmental Panel on Climate Change (IPCC) warns if the carbon dioxide emission and global warming continue unabated, annual wettest day precipitation will increase manifold across all continents.
- Monsoon rainfall associated with tropical cyclones over India will also increase by 40%.
What are Deccan Traps?
- A thick succession of (3200 metres) late Cretaceous basaltic lava flows known as the Deccan Traps covers around 500 000 square kilometres of peninsular India. This basaltic lava soil has gradually eroded due to erosion, resulting in the formation of a Deccan trap.
- The Deccan Traps is an area of black dirt on the Peninsular Plateau. The rocks are igneous and have volcanic origins. Black dirt has been produced as a result of the erosion of these rocks over time.
Distribution of Deccan Traps:
- The majority of these volcanic rocks are found in Gujarat’s Kutch and Kathiawar regions, as well as in Andhra Pradesh, Madhya Pradesh, and Maharashtra.
- Its thickness varies depending on whether the surface is flat or sloppy, being midway between deep and shallow.
- These (the Deccan Traps) might be considered Peninsular India’s most widespread geological formations after the Archean Rocks.
How are Deccan traps formed?
- Cretaceous vulcanicity, the development of many linear cracks in the earth’s crust, is what causes the formation of decan traps. This eruption did not produce any volcanic domes since it was of the quiet kind.
- This incident started 66.25 million years ago (End of the Cretaceous period) when India was near present placed Madagascar and during its movement north, it passed over Reunion Hotspot. Presently this hotspot is dormant in nature.
- From a general study of the Deccan Traps, the following broad conclusions can be drawn about their mode of eruption:
- They erupted in a sub-aerial environment. Their relationship with the older rocks suggests this.
- Eruptions of the lava took place along linear fissures, forming fissure-type of eruptions and not from any central cone-type volcanoes. This is explained by numerous flows and their remarkably consistent horizontal attitude.
- The lava when erupted was in a highly liquid form, which is indicated by the long distances over which the individual flows could spread. This fact also suggests that the lava might have been super-heated to reach that state of mobility.
- A large number of dykes that have been found occurring intersecting many lava flows might be quite younger than those flows, but some such dykes might have also been the feeder dykes for the volcanic eruptions.
- The lava flows, especially after the beginning and before the close of the activity, passed through definite periods of no volcanic activity. During such periods streams and lakes appeared on the cooled and congealed volcanic landforms here and there and both animal and plant life domiciled these areas.
- This is indicated by the occurrence of inter-trappean layers found in the lower and upper flows and their fossil contents.
Characteristics of Deccan Trap:
- They develop in semi-arid regions that are coated with basalt.
- The soil is almost devoid of humus and is black due to the presence of titanium salt in the lava land that makes up the Deccan trap. They have a lot of lime in them.
- Particularly in the lowlands and river valleys where it is deep and clayey, the soil holds moisture well and is extremely productive.
- The construction of traps is very constant and regular; they are layered and the strata are practically horizontal everywhere, with an average dip (angle of inclination) of 5°–10°. This is similar to a homogenous mineralogical and chemical composition.
- The thickness of the individual strata, known as lava flows, can range from 3 metres to 30 metres or even more.
- The area of the flows is frequently much larger than the layer thickness, sometimes even reaching 100 km.
- The Inter-Trappean Beds, which are non-volcanic sedimentary levels, frequently interrupt the consecutive flows. These show deposits in rivers and lakes with shallow water that developed during the lulls in the age’s volcanic activity.
- There are several excellent fossils of animals and plants that may have survived during those times when there was no volcanic activity to be found in the inter-trappean strata. The age of the Traps has been interpreted using these fossils.