Snowy Winters and Dry Summers Prevailed in Southwestern North America during the Late Pleistocene

Ice Age climates spawned dramatically altered weather patterns compared to those of the present day. The result of those different weather patterns is evident in how changed Southwestern North America has become since then. During Ice Ages Southwestern North America was a land of vast lakes, abundant springs, and widespread wetlands. There even was a lake in Death Valley, California where it almost never rains today. There were especially large lakes in Utah, Nevada, and central Oregon–areas that today are quite arid. Scientists debate the source of the greater precipitation that occurred then. Some think the source was summer rains coming from fronts originating in the tropics, while most believe the polar jet stream carried moisture from the North Pacific that fell as heavy snows during winter. A new study of carbon and oxygen isotope ratios in tooth enamel from Pleistocene mammals supports the latter scenario.

Scientists analyzed 39 teeth from mammoth, bison, horse, and camel excavated from the Tule Spring Fossil Bed National Monument in Nevada. They can determine how precipitation was delivered based on the ratios of carbon and oxygen isotopes in the teeth because the animals ate the plants that absorbed the water, and the animals directly drank it. Most of the precipitation in the region came from heavy snows, and the lakes refilled every spring and early summer from snow melt. They believe summers were relatively dry, and lakes began to evaporate until seasonal snowfall. Mammoths, bison, and horses ate a lot of the fresh grass that grew tall on water from snowmelt. Horses may have eaten more grass here during Ice Ages than they do today. But camels browsed on saltbush (Atriplex sp.). The presence of this species indicates dry summers and arid localities within the lush landscape. Scientists think glaciers to the north of the region split the polar jet stream, and the lower stream carried moisture from the North Pacific, causing winter precipitation. Lake levels were highest during the Last Glacial Maximum following Heinrich Events that occurred when ice dams melted, and massive pulses of freshwater studded with ice bergs flooded into the oceans. Moisture in earth’s atmosphere increased following Heinrich events.

Map of Southwestern North America during the Late Pleistocene. Meltwater from much snowier winters caused the formation of giant lakes in the region then. From the below reference by Munroe and Laabs.

Beth Zaiken’s depiction of wildlife in Nevada during the last Ice Age. Vegetation was much lusher than it is today due to higher annual precipitation.

When glaciers retreated at the end of the Ice Age, the polar jet stream recombined and began to flow to the north. Winter snowfall was greatly reduced, and the lakes gradually evaporated. The Great Salt Lake of Utah is a remnant of a much larger freshwater lake that existed during Ice Ages.

The abundant wetlands and lakes of the region hosted many species of birds that today breed in the Arctic during summer. These species could not live in the Arctic during the Ice Ages because their present-day ranges were under miles of glacial ice. Their breeding ranges shifted to the Southwest. See also:

https://markgelbart.wordpress.com/2017/04/09/ice-age-western-lakes-and-altered-bird-migrations/

References:

Kohn, M. et. al.

“Seasonality of Precipitation in the Southwestern U.S. during the Late Pleistocene Inferred from Stable Isotopes in Herbivore Tooth Enamel”

Quaternary Science Review 290 November 2022

Munroe, J.; and B. Laabs

“Temporal Correspondence Between Pluvial Lake High Stands in Southwestern U.S. and Heinrich Event 1”

Journal of Quaternary Science 28 (11) 2013