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How the Atlantic Ocean has become an integral part of global traffic at a climate tipping point

The collection of deep marine sediments, which are valuable archives of ocean circulation and past climates. Credit: Department of Earth Sciences

A team of scientists, led by Dr. Sietske Batenburg of the Department of Earth Sciences of the University of Oxford, in close collaboration with German and British institutions, discovered that the exchanges in water between the North Atlantic and South had become considerably more important years ago.

The scientists made this discovery when they compared the isotopic signatures of neodymium from deep water sediment samples from both Atlantic regions. Their article "Major intensification of the reverse circulation of the Atlantic at the beginning of paleogenic heat" was published today in Nature Communications, reveals that the more vigorous circulation associated with an increase in atmospheric CO2 has led to a climate tipping point. As a result of a more even distribution of heat on the Earth, a long term cooling phase is over and the world is engaged in a new greenhouse period.

Neodymium (Nd) isotopes are used as tracers for water bodies and their mixtures. Surface water acquires the Nd-isotope signature of the surrounding landmasses crossing the rivers and dust carried by the wind. When the surface waters flow to form a mass in deep waters, they carry with them their specific signature of the isotope Nd. When a mass of deep water crosses the ocean and mixes with other water masses, its isotopic signature of Nd is incorporated into the sediments. Deep water sediments are valuable records of ocean circulation and past climates.

The story revealed in this article begins at the end of the Cretaceous period (ending 66 million years ago), when the world was between two greenhouse states. The climate has been cooling for tens of millions of years since the Middle Cretaceous cretaceous peak about 90 million years ago. Despite the long-term cooling, temperatures and sea level at the end of the Cretaceous period were higher than today.

Sietske Batenburg said: "Our study is the first to establish how and when a deep water connection was formed.There are 59 million years ago, the Atlantic Ocean was really part of the global thermohaline circulation, the flow connecting four of the five major oceans. "

Deep water sediments are valuable records of ocean circulation and past climates. Credit: Department of Earth Sciences

The Atlantic Ocean was still young and the basins of the North and South Atlantic were shallower and narrower than today. The equatorial bridge between South America and Africa allowed a shallow bond with surface waters only during most of the Upper Cretaceous period. Active volcanism has formed mountains and underwater plateaus that have blocked the circulation in deep water. In the South Atlantic, the Walvis Ridge Barrier is formed over an active volcanic hot spot. This ridge was partially above sea level and formed a barrier for the circulation of deep water masses.

As the Atlantic Ocean continued to open, the oceanic crust cooled down and subsided. The basins became deeper and wider, and the underwater plateaus and ridges were sunk with crust. At one point, the deep waters of the Southern Ocean could cross the Walvis Ridge to the north and fill the deepest parts of the Atlantic basins.

59 million years ago, the North Atlantic isotope signatures of the North Atlantic and the South Atlantic were remarkably similar. This may indicate that a deep water mass, probably originating in the south, has crossed the Atlantic Ocean and filled the basin with depths at intermediate depths. The improvement of deep-water exchanges, combined with the increase in atmospheric CO2, may have allowed a more efficient distribution of heat on the planet.

This study shows that to understand the role of ocean circulation in past greenhouse climates, it is important to understand the different roles of geography and climate.

Isotopic ratio of neodymium. Credit: Department of Earth Sciences

The current rate of climate change due to CO2 emissions from human activities far exceeds the rate of warming observed in past greenhouse climates. The study of ocean circulation during the last greenhouse interval of the geological past could provide clues to the future evolution of ocean circulation and the distribution of heat on the planet by ocean currents.

This research is the result of international collaboration with Goethe University Frankfurt; the Ruprecht-Karls University of Heidelberg; the GEOMAR Helmholtz Ocean Research Center in Kiel; the Federal Institute of Geosciences and Natural Resources Hannover; the Royal Holloway University of London and the University of Oxford.

Sediments for this study were all collected from long oceanic drill cores. The International Ocean Discovery Program (IODP) coordinates scientific expeditions to drill the seabed to recover these sediments and store sediment cores so that they are accessible to the wider scientific community.

Explore further:
Prehistoric greenhouse data from the ocean floor could predict the future of the Earth

Journal reference:
Nature Communications

Provided by:
University of Oxford

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