MID-TO-LATE HOLOCENE PACKRAT MIDDEN DATA FROM THE SHIVWITS PLATEAU RECORD AN EASTWARD CONTRACTION OF THE NORTH AMERICAN MONSOON

The North American Monsoon (NAM) brings precipitation to southwestern North America between July and September. The NAM evolved in strength and extent during the Holocene, in response to changes in North Hemisphere insolation and associated sea-surface temperature changes. Today, the strong moisture surges associated with the NAM are concentrated in areas east of 114˚W, the longitude of St. George, Utah. However, in the early Holocene, summer precipitation was considerably stronger than it is today west of 114˚W, influencing Mojave Desert plant and animal communities in southeastern California and southern Nevada. In the mid-Holocene, cooler sea-surface temperatures in the Gulf of California are presumed to have shifted the bulk of NAM precipitation east of 114˚W, out of southern California and southern Nevada, and more or less into its present range. However, the position of the western boundary of the strong NAM belt in mid-Holocene through late-Holocene time is poorly constrained.

In this study we use pollen and macrofloral data from a packrat midden located on the Shivwits Plateau, close to 114˚W, to constrain the position of the NAM-related precipitation during mid-to-late Holocene time. The midden is located in Belle’s Cave, at an elevation of approximately 1,650 m (5,400 feet) within Grand Canyon-Parashant National Monument. We sampled this midden under permit from the National Park Service. The midden is only 20 cm thick, and we sampled the bottom and top. AMS radiocarbon dating yielded a 2-sigma calibrated age of 4830 to 4570 BP for the bottom of the midden and a 2-sigma calibrated age of 2150 to 1990 BP for the top. Thus the bottom of the midden was constructed in mid-Holocene time, approximately 4,700 years ago, and the top of the midden was constructed in the late Holocene, approximately 2,000 year ago.

The pollen and macrofloral data indicate that conspicuously drier conditions prevailed at this site at about 2,000 BP, in the late Holocene, than had been the case at 4,700 BP, in the mid-Holocene. Other studies have shown that sea-surface temperatures in the Gulf of Mexico were decreasing from late mid-Holocene into early late-Holocene time. We interpret our packrat midden data to record a contraction of the strength and extent of the NAM across the 114th meridian between 4,700 and 2,000 BP.