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Paleoclimatology
Warm winds in the Arctic

About 50 million years ago, redwood forests flourished on Axel Heiberg land high in the Canadian Arctic. The existence of the warm Paleocene-Eocene climate that enabled well developed redwood forests to flourish at such high latitudes has puzzled scientists for years. A recent study has offered a bold hypothesis for what circulation patterns could have produced such a warm climate only a few degrees latitude from the North Pole. Published in the January GSA Today, the hypothesis has continued to stir up controversy in the following months.

“It’s been puzzling how you get that kind of a lush ecosystem that far north,” says lead author Hope Jahren of Johns Hopkins University. “But everyone has agreed that it involves climate conditions unlike anything we see today.”

Jahren and Leonel Silveira Lobo Sternberg, of the University of Miami, analyzed oxygen isotopes from 40 million-year-old fossil redwood (Metasequoia) logs, in which the wood has not been mineralized, to examine the ancient climate. In their paper, Jahren and Sternberg report finding an isotope signature depleted in oxygen-18, a heavy natural isotope of oxygen.

Hope Jahren, pictured here on Axel Heiberg land in the Canadian Arctic, holds the fossil of a redwood, which grew about 50 millions years ago just a few degrees latitude from the North Pole. Image by William Hagopian/JHU

They interpreted this isotope ratio to mean that the rainwater falling on the forest more than 40 million years ago had traveled across large swaths of land. By the time the air masses reached Axel Heiberg, the heavier isotopes in the rainwater would have been left behind as a result of repeated precipitation and evaporation events over the continental surface. Jahren and Sternberg assert that the source of much of the precipitation initially came from tropical waters off the coast of what is now Mexico.

“As the air mass moves over the ocean, [isotope depletion is] occurring, but there is replenishment,” Jahren says. “The only way to get that kind of depletion of heavier isotopes is by moving over land. The only pathway that takes you across a large amount of land is by the straight north-south route over North America.”

Jahren and Sternberg also propose that the polar front, which tends to inhibit northward movement of more southerly air masses, had disappeared in the absence of a polar icecap to anchor it. This atmospheric circulation pattern is significantly different from what has been observed, as well as from what typically occurs in climate models.

Gavin Schmidt, a paleoclimatologist with NASA’s Goddard Institute of Space Studies who uses oxygen isotopes in his attempts to reconstruct past climates, says the circulation could not have been north-south.

“Model simulations of atmospheric circulations during this period have been done many times,” Schmidt says, “and although there remain many uncertainties in the simulations and in interpreting the data, there has been no suggestion that the circulation would not be predominantly east-west — as it is today — with transports to the higher latitudes controlled by mid-latitude storm systems and the like.”

“We do not see these sorts of patterns in the modern or in any model results of past climates,” says Lisa Sloan, a paleoclimate researcher at the University of California, Santa Cruz. “I’d prefer to see the authors consider the isotopic composition of water on a trajectory from the mid-latitude Pacific.”

Sloan and one of her graduate students, Jacob Sewall, published a study in a issue of Geophysical Research Letters last year that suggested that a well developed Arctic low pressure system could have helped reduce Arctic ice cover. This system would have produced warmer conditions in the mid-latitude regions of the continents, resulting in the warm climate conditions indicated by Metasequoia and other fossil evidence.

At the same time, the interpretion of the isotopic ratios in the fossils is up for debate.

Jahren and Sternberg based their interpretation of the isotope ratios on an analysis of precipitation patterns in modern-day Africa and South America, an interpretation suggesting that the Axel Heiberg isotopes were depleted in heavy oxygen. But Karen Bice, a scientist at Woods Hole Oceanographic Institution, says that current heavy oxygen istotope concentration in rainwater in Thule, Greenland, is lower than what Jahren and Sternberg derived from the Axel Heiberg Metasequoia.

Bice says that recent modeling work by a research group led by Jean Jouzel of the Laboratoire des Sciences du Climat et de l’Environment in France indicates that the ratios of heavy isotopes of oxygen to light in precipitation falling at high latitudes would be greater in a warmer climate. Jahren and Sternberg’s findings seem to support Jouzel’s hyphothesis, Bice says. “To me that is what is new and interesting about Jahren and Sternberg’s results.”

David Lawrence
Geotimes contributing writer



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