February Geology and GSA Today media highlights

The first continuous high-resolution temperature record from the North American Great Plains grasslands documents unexpected trends during the past 12,000 years. Stable carbon isotopes of buried soil organic carbons contain an ecological record of the warm-season grass productivity of ancient landscapes, which according to modern analogs is directly related to mean July temperatures. This method shows that temperatures, which increased dramatically to near modern temperatures from 10,500 to 9600 years ago, were as much as 4 °C cooler than present during the well-known Younger Dryas. Two additional warm excursions were identified at 5000 and 1500 years ago when temperatures were as much as 1 °C warmer than present. Remarkably, the cooler intervals corresponded with ice rafting events in the North Atlantic, glacier meltwater pulse, and the movement of the intertropical convergence zone into the far northwestern Gulf of Mexico in response to 1500 solar irradiance cycles. The last warm-cool couplet corresponds to the Medieval Warm Period and Little Ice Age, respectively. These data offer for the first time a reliable record of temperature in a previously unknown region for calibration of general circulation models in the Great Plains, where fossil pollen preservation is rare.

The changes in North American atmospheric circulation patterns indicated by wood cellulose
Xiahong Feng, Dartmouth College, Earth Science, Hanover, NH 03755, USA; et al. Pages 163-166.

Today, the prevailing wind in the mid-latitude of North America (40–50°N) is westerly. In this zone of general atmospheric circulation patterns, most marine moisture is brought to the continent either from the western coast by westerly winds, or from both western and eastern coasts by extra-tropical storms. Using oxygen and hydrogen isotopic compositions of cellulose extracted from ancient wood, Feng et al. provide evidence that during the Last Glacial Period (14 to 36 thousand years ago), the prevailing wind in this zone was easterly, and marine moisture came predominantly from the eastern coast. The isotopic evidence comes in two interrelated observations. One is the relationship between ratios of deuterium to hydrogen and oxygen-18 to oxygen-16 in cellulose; the other is the variation of isotopic ratios in cellulose across the continent (as function of longitude). Feng et al. interpret the prevailing easterlies to be a result of intensification and enlargement of the northern circumpolar vortex under the powerful influence of the Laurentide Ice Sheet. Under this circulation regime, the jet stream shifted southward, carrying extratropical storm systems to the south of their current latitudes. As a result, the Pacific Northwest received much less marine moisture from the Pacific.