Between fifteen and six million years ago, a decrease in oceanic crust formation may have led to a deepening of ocean basins, causing sea levels to fall by 26 to 32 metres.
Nowadays, rising sea levels are mainly attributed to global warming, which causes ice caps and glaciers to melt and water to expand thermally. However, over longer timescales, spanning millions of years, the dynamics of tectonic plates also alter the shape and depth of ocean basins, thereby influencing global sea level.
Published at the end of February on the AGU scientific platform, a study conducted by Colleen A. Dalton and three of her colleagues demonstrates the significant impact on sea levels of a phase of slowing seafloor expansion that occurred between 15 and 6 million years ago.
The researchers based their findings on previous studies that had revealed a 35% decrease in oceanic crust production during this period. This drastic reduction was mainly due to the formation of new rocks at ocean ridges, which gradually pushed the older rocks towards subduction zones.
Reduced temperature and rate of oceanic crust disappearance
This phenomenon resulted in a gradual deepening of the ocean basins, leading to a drop in sea level. To quantify this variation, scientists took several parameters into account, including the age and surface area of the oceanic crust and its rate of destruction. Their calculations agreed that this general slowdown in the expansion of the seabed would have caused a drop in sea level estimated at between 26 and 32 metres.
At the same time, scientists examined the impact of this slowdown on the heat flow from the Earth’s mantle. Normally, the Earth’s internal heat escapes through the oceanic crust, particularly at ridges where fresh rock is in direct contact with seawater. Between 15 and 6 million years ago, this heat dissipation is thought to have decreased by an average of 8%, with a more marked reduction of up to 35% at ocean ridges. This drop could have had significant consequences on ocean chemistry, altering hydrothermal interactions and the dissolution of minerals in seawater.
Submarine volcanic activity slowed down
The production of oceanic crust is accompanied by submarine volcanic activity that releases carbon dioxide (CO₂) into the atmosphere. Previous work, involving some of the researchers in this new study, suggests that this decline in production may have led to a decrease in CO₂ emissions, contributing to global climate cooling.
However, lower temperatures promote the expansion of ice caps by trapping more water in the form of ice on the continents, which reduces the volume of water in the oceans and accentuates the drop in sea level. According to the authors of the report, the thermal contraction of the oceans combined with the expansion of ice caps may have caused an additional sea level drop of more than 60 metres during this period.
Although the geological record of these sea level changes is limited, the study’s findings are consistent with sequential stratigraphy data obtained from coastal sedimentary deposits, particularly in New Jersey and off the coast of Nova Scotia.
Source link
