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Climate shift: Currents play vital role

Antarctica (global-adventures.us): Thirty-eight million years ago, tropical jungles thrived in what are now the cornfields of the American Midwest and furry marsupials wandered temperate forests in what’s now the frozen Antarctic. The temperature differences of that era, referred to as the late Eocene, between the equator and Antarctica were half what they’re today.

Drillship Joides ResolutionA debate has been ongoing within the scientific community about what changes in our global climate system led to such a serious shift from the more tropical, greenhouse climate of the Eocene to modern and far cooler climates. New research results published within the journal Science, led by Rensselaer polytechnic (RPI) scientist Miriam Katz, are providing a number of the strongest evidence so far that the Antarctic Circumpolar Current (ACC) played a key role within the shift.

“What we’ve found is that the evolution of the Antarctic Circumpolar Current influenced global ocean circulation much before previous studies have shown,” said Katz. “This finding is especially significant because it places the impact of initial shallow ACC circulation within the same interval when the climate began its long-term shift to cooler temperatures.”

There has been a debate over the past 40 years on what role the Antarctic Circumpolar Current had within the past cooling trend.

“These climate changes are one among the foremost significant shifts in Earth’s history, from early Cenozoic ‘greenhouse’ climates to the mid- to late Cenozoic ‘icehouse’ that saw repeated massive glaciations of the polar regions,” said Candace Major, program director within the National Science Foundation’s (NSF) Division of Ocean Sciences. “The work by Katz and colleagues is that the first to demonstrate that the essential structure of currents related to modern ocean circulation has existed for the past 33 million years,” said Major.

Previous research had placed the event of the deep Antarctic Circumpolar Current at depths greater than 1.24 miles (2,000 meters) within the late Oligocene, approximately 23-25 million years ago. That’s well after the worldwide cooling pattern had been established. The new research placed the worldwide impact of the ACC at approximately 30 million years ago, when it had been still just a shallow current.

Oceans and global temperatures are closely linked. Warmer ocean waters end in warmer air temperatures and the other way around . within the more tropical environs of the Eocene, ocean circulation was weaker and currents more diffuse. As a result, heat was more evenly distributed round the world. That resulted in fairly mild ocean temperatures worldwide.

Today, ocean temperatures vary considerably and redistribute warm and cold water round the globe. “As the worldwide ocean currents were formed and strengthened, the redistribution of warmth likely played a big role within the overall cooling of the world ,” Katz said.

Often mentioned because the “mixmaster” of the ocean, the ACC thermally isolates Antarctica by preventing the nice and cozy surface waters of subtropical gyres from passing through. the present redirects a number of that warm water heater toward the North Atlantic , creating Antarctic Intermediate Water. This blocking of warmth enabled the formation and preservation of the Antarctic ice sheets, consistent with Katz. The circumpolar circulation was liable for the event of the fashionable four-layer current and warmth distribution system, the research paper concludes.

Katz checked out the uptake of several elements’ isotopes, or variants, within the fossil skeletons of small planktonic organisms found in ocean sediments. Using the drillship, the fossil organisms, referred to as benthic foraminifera, were mentioned from beneath the sea-floor in cores of sediments. The foraminifera incorporated certain elements and isotopes, reflecting environmental conditions at the time. By analyzing the ratios of those elements and isotopes, researchers were ready to reconstruct past environmental conditions.

Analysis of those isotopes showed the earliest evidence for Antarctic Intermediate Waters, which circulates as a consequence of the ACC. This finding is that the first evidence of the consequences of shallow ACC formation. The results place the ACC’s global impact much closer to the time when Antarctica separated from South America, creating a gateway. It had previously been thought that currents moving through this gateway couldn’t be strong enough at such shallow depths to affect global ocean circulation.

The picture shows an aerial view of the drillship Joides Resolution, the workhorse of the Integrated Ocean Drilling Program.

By Bernd F. Laeschke


Chief Editor at Global Adventures Magazine

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