by Elizabeth Dengler Thursday, September 6, 2018
Daily tides are driven primarily by Earth’s rotation and the gravitational force of the moon on oceans. However, in a new study in Geophysical Research Letters, researchers suggest that tidal magnitudes are also influenced, on longer timescales, by the size and shape of the ocean basins, and are therefore driven by plate tectonics.
Plate tectonics gives rise to the formation of supercontinents — massive aggregations of continental lithosphere — which form and break apart in cycles that last about 400 million to 500 million years. With the breakup of the last supercontinent, Pangea, about 180 million years ago, and the projected formation of a new supercontinent, known as Aurica, in about 200 million years, Earth is currently in the middle of a supercontinent cycle.
Because the size and shape of ocean basins impact ocean circulation and tides, researchers led by Mattias Green, a physical oceanographer at Bangor University in England, hypothesized that tides may be linked to the supercontinent cycle in a so-called supertidal cycle.
Current tides, particularly those in the North Atlantic, are very large, Green’s team noted because of tidal resonance, which occurs when ocean basins and continental shelves reinforce and amplify the natural oscillation of tides as they sweep back and forth across oceans. “So the tides are larger at present because the continents are configured the way they are.”
To model Earth’s future oceanic tides, the researchers used predictions of continental configurations for the next 250 million years, through when Aurica is predicted to form. Ocean basin size was the main factor considered in the modeling, but the team also accounted for the moon’s gravitational pull on the oceans, Earth’s axial tilt, and simplified ocean bathymetries for future plate tectonic reconstructions. Simplification of these fine details does affect the team’s modeling, notes David Waltham, a mathematical geologist at the Royal Holloway University of London, who was not involved in the study. But the simplifications used likely do not change the overall results, he adds.
Green and his colleagues reported that global tides are likely to increase over the next 50 million years “due to an enhanced tide in the North Atlantic and Pacific at 25 million years, followed by a very large Pacific tide at 50 million years.” A new ocean basin, the Pan-Asian Ocean, is projected to open in about 100 million years, contributing to large tides that persist 150 million years into the future. At that time, tidal resonance in the Atlantic briefly returns, resulting in another tidal maximum. However, once Aurica begins assembling, a little after 150 million years from now, global tides decline in size. By the time Aurica is fully formed, 250 million years from now, global tides are small, with only small embayments showing locally large tides. According to the study, tides are largest when the continental configuration results in ocean basins that contribute to tidal wave resonance, and smallest when supercontinents exist.
Though the timescales of this study likely extend far beyond that of humanity, studies like this help develop a better understanding of Earth’s tides and how they change through time, Waltham says. Additionally, understanding how tides operated during previous supercontinent cycles can aid in the search for natural resources like oil and gas deposits, he says. “The strength of tidal currents will affect the sorting of sediments; whether the sediments are very pure sands, which are better for holding oil or gas, or if they have other things mixed in,” Waltham explains. “So it allows us to better understand which places are good to explore and which are not.”
The team has made progress in understanding how the supertidal cycle is linked to the supercontinent cycle, but much remains unknown, including how these tidal relationships fit into climate cycles, Green says. Hopefully, understanding continental configurations and tidal strengths will impact the development of climate models, he adds.
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