Clues to an ancient upside-down ocean.
Picture an ice cream cake covered with a pool of hot fudge sauce. this is the way the ocean is organized, with buoyant warm water floating atop cold, denser layers. Scientists have considered such an arrangement a permament feature of the ocean, but new evidence from the seafloor might turn this image on its head for certain times in the geologic past.
Researchers from the University of California, Santa Barbara, say their findings hint that 40 million years ago, warm, salt-rich water filled the deep ocean while cooler water covered the upper layers. "People have thought about the possibility of warm, saline deep water for years," says James P. Kennett. "These are the first data hinting that we may be onto something important."
Kennett and Lowell D. Stott found evidence for an upside-down ocean in sediment cores drilled from the Maud Rise near Antarctica. They measured the relative amounts of different oxygen isotopes locked within the calcium carbonate shells of tiny animals that fall to the seafloor after death. Oxygen isotope ratios indicate the water temperature during the animal's life.
Isotope ratios at two sites with different depths suggest the lower site was significantly warmer than the upper site at least as far back as the middle Eocene period, 46 million years ago. After many fluctuations, the ocean flipped to a more modern style around 28 million years ago, the researchers proposed last week at the meeting of the International Geological Congress in Washington, D.C.
From these data, Kennett and Stott infer that the entire ocean circulation ran backwards during the Eocene. Today, the bottom water forms at the poles, where air and ice cool the surface water and make it dense enough to sink. The cold bottom currents flow toward the equator.
The researchers say the deep water of the Eocene ocean formed not at the poles but primarily in the northern mid-latitudes. during this period, a large ocean called the Tethys separated Eurasia from Africa and Arabia. With its high evaporation rates and scant precipitation, the shallow Tethys generated warm water so dense with salt that it sank to form the bottom layer, say Kennett and Stott. Ancient Antarctica, much warmer than today, spawned the cool water that filled in the upper layer of the ocean, according to their theory. The reverse circulation would have broken down when plate tectonics began pushing Africa and Arabia toward Eurasia, closing off the Tethys. Turning off the tap of warm water flowing to the poles would have helped cool Antarctica.
Kennett and Stott's interpretation may be flawed, argues Gerta Keller of Princeton (N.J.) University. She says investigators studying the same cores think the chemistry of the calcium carbonate from the Maud Rise may have changed over the eons--a finding that would limit the mineral's veracity as an indicator of past temperatures.
Many scientists are warming to the idea that bottom water once formed in the Tethys region. The question is whether this water was warmer or colder than the upper layers. Even if the Maud Rise isotope ratios represent true records, the case for an upside-down ocean will require more data from other sites.