A team of scientists, led by Dr. Sietske Batenburg of the University of Oxford at Earth Sciences University, in close cooperation with German and British institutions, found that the water exchange between the North Atlantic and the South Atlantic was significantly higher than the fifties nine million years ago.
Scientists made this finding when comparing the isotope neodymium signatures of deepwater sediment samples from the two Atlantic regions. Their document "Massive intensification of the reversal of the Atlantic circulatory circle upon the occurrence of paleogenetic greenhouse heat", published today Nature Communications, reveals that more vigorous circulation, along with the increase in atmospheric CO2, has led to a climatic point. With a more even distribution of heat over the Earth, the long-term cooling phase continued, and the world was in a new greenhouse period.
Neodymium (Nd) isotopes are used as an indicator of water masses and their mixing. Surface waters acquire a Nd-isotope signature from surrounding earth masses through rivers and dust from the wind. When surface water sinks to form a deep-water mass, they carry a special Nd-isotope signature. As the deep-sea mass flows through the ocean and is mixed with other water masses, its Nd-isotope signature is included in the sediment. Deep-sea sediments are valuable archives 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 greenhouses. The climate has cooled for tens of millions of years from the highest conditions in the midst of Creta, about 90 million years ago. Despite long-term cooling, temperatures and sea level at the end of the Critical Period were higher than the current day.
Dr. Sietske Batenburg says, "Our study is the first to find out how and when a deepwater connection is formed." 59 million years ago, the Atlantic Ocean really became part of the world's thermochine circulation linking four of the five major oceans.
The Atlantic Ocean was still young, and the northern and southern Atlantic basins were smaller and narrower than today. The Equatorial Portal between South America and Africa allowed only a shallow connection with surface water for most of the end of the Cretaceous. Active volcanism forms submarine mountains and plateaus that block the flow of deep water. In the southern Atlantic, the Walvis Ridge barrier has formed above an active volcanic hotspot. This ridge is partly above sea level and forms a barrier to the flow of deep-sea masses.
As the Atlantic Ocean continued to open, the ocean crust cooled and weakened. Pools became deeper and wider, underwater plateaus and ridges sank together with bark. At some point the deep waters of the South Ocean managed to cross north through Walvis Hill and fill the deeper parts of the Atlantic basins.
Over 59 million years ago, Nd-isotopic signatures from the North and South Atlantic Ocean were remarkably similar. This may mean that a deepwater mass, probably from the South, has sneaked through the Atlantic and filled the pool from deep to intermediate depths. Improved deep-water exchange, along with an increase in atmospheric CO2, can allow a more efficient heat distribution across the planet.
This study shows that, in order to understand the role of ocean circulation in past greenhouse climates, it is important to understand the different roles of geography and climate.
The current rate of climate change from CO2 emissions from human activity far exceeds the rate of warming during the past greenhouse climate. Studying the ocean circulation during the newest greenhouse interval in the geological past may give guidance on how ocean circulation can develop in the future and how the heat will be distributed to the planet by ocean currents.
This research is the result of international collaboration with Goethe-University Frankfurt; Rupert-Carls University in Heidelberg; GEOMAR Helmholtz Center for Ocean Studies Kiel; The Federal Institute for Geological Sciences and Natural Resources in Hannover; King's Holloway University in London and the University of Oxford.
The sediments for this study are taken from long ocean drilling cores. The International Ocean Oriented Program (IODP) coordinates scientific expeditions to drill the ocean floor to recover these sludge, and preserves sludge cores so that they are available to the entire scientific community.
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