LATE HOLOCENE EASTWARD EXPANSION OF THE MOJAVE DESERT IS RECORDED IN PACKRAT MIDDENS FROM NORTHWESTERN ARIZONA

The North American Monsoon (NAM) brings precipitation to southwestern North America between July and September. The NAM evolved in strength during the Holocene, in response to changes in Northern 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 communities. 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 southeastern California and southern Nevada, and more or less into its present range, but the position of the eastern boundary of the strong NAM belt in mid-Holocene through late-Holocene time is poorly constrained.

In this study I use pollen and macrofloral data from packrat middens located 50 km apart at different elevations within Grand Canyon-Parashant National Monument to better constrain the position of the NAM during mid-to-late Holocene time. Both middens are close to 114°W. One is at an elevation of 1,100 m (3,600 ft) in the Grand Wash Cliffs (GWC) area, and the other is 50 km farther north, on the Shivwits Plateau, at an elevation of 1,650 m (5,400 ft). The middens were sampled under permit from the National Park Service.

The Shivwits Plateau midden yielded a mid-Holocene date (about 4,700 BP) and also a late-Holocene date (about 2,000 BP), while the GWC midden yielded a Late Pleistocene date (about 16,000 BP) and a late Holocene date (about 2,000 BP). Pollen and macrofloral data from both middens indicate that conspicuously drier conditions prevailed at both sites in the late Holocene than are recorded in the mid-Holocene by data from the Shivwits Plateau midden. Other studies have shown that sea-surface temperatures in the Gulf of California were decreasing from the mid-Holocene into early late-Holocene time. I interpret these packrat midden data to record a contraction in the strength and extent of the NAM across the 114°W meridian between 4,700 and 2,000 BP, and a corresponding expansion of the Mojave Desert.