Ocean 'Mixmaster' Moves Global Climate
The temperature differences of that era, known as the late Eocene, between the equator and Antarctica were half what they are today.
A debate has been ongoing in the scientific community about what changes in our global climate system led to such a major shift from the more tropical, greenhouse climate of the Eocene to modern and much cooler climates.
New research results published in this week's issue of the journal Science, led by Rensselaer Polytechnic Institute scientist Miriam Katz, are providing some of the strongest evidence to date that the Antarctic Circumpolar Current (ACC) played a key role in the shift.
"What we have found is that the evolution of the Antarctic Circumpolar Current influenced global ocean circulation much earlier than previous studies have shown," said Katz. "This finding is particularly significant because it places the impact of initial shallow ACC circulation in 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 in the past cooling trend.
"These climate changes are one of the most 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 in the National Science Foundation's (NSF) Division of Ocean Sciences.
"The work by Katz and colleagues is the first to demonstrate that the basic structure of currents associated with modern ocean circulation has existed for the past 33 million years," said Major.
Previous research had placed the development of the deep ACC--greater than 2,000 meters water depth--in the late Oligocene, approximately 23-25 million years ago.
That's well after the global cooling pattern had been established. Katz and colleagues have placed the global impact of the ACC at approximately 30 million years ago, when it was still just a shallow current.
"As the global ocean currents were formed and strengthened, the redistribution of heat likely played a significant role in the overall cooling of the Earth," Katz said.
No current is more major than the ACC, scientists believe. Often referred to as the "mixmaster" of the ocean, the ACC thermally isolates Antarctica by preventing the warm surface waters of subtropical gyres from passing through. The ACC instead redirects some of that warm water back toward the north Atlantic, creating Antarctic Intermediate Water. This blocking of heat enabled the formation and preservation of the Antarctic ice sheets, according to Katz.
The circumpolar circulation, Katz concludes, was responsible for the development of the modern four-layer ocean current and heat distribution system.
Analysis of these isotopes showed the earliest evidence for Antarctic Intermediate Waters, which circulates as a consequence of the ACC. This finding is the first evidence of the effects 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 could not be strong enough at such shallow depths to affect global ocean circulation.
"By reconstructing the climates of the past, we can explore Earth system responses to current climate change," Katz said.