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Home , Late Pleistocene, Younger Dryas

Late Glacial to Early Oscillations in the American Southwest Kenneth Cole, USGS Southwest Biological Science Center Flagstaff, AZ; [email protected]

View of the Grand Canyon North Rim (2500 m) from a mid elevation (1500 m) on the South Rim.

1 Most of the earliest conceptions of the versus Holocene plant zonation consisted of lower Pleistocene zones and higher Holocene zones without much information on how they shifted from one to the other.

2 15,000 -old packrat midden in a Grand Canyon .

Steven’s Woodrat (Neotoma stevensii) poses with Kirsten Ironside.

3 Lower Colorado River Elevational Zonation

Treeline Snowline 3000 Forest

Treeline Modified From: Ponderosa - Fir Forest Cole, K. L. 1990. Reconstruction Spruce Forest of past desert vegetation along the Colorado River using packrat 2000 middens. Palaeogeography, Palaeoclimatology, and Pinyon-Juniper Woodland Limber Pine, Fir Forest Palaeoecology 76: 349-366.

Blackbrush - Sagebrush Juniper - Sagebrush Elevation (m) Desert Woodland

1000 Juniper - Ash Juniper - Blackbrush Brittle Bush - Woodland Woodland Creosote Bush Desert

Joshua Tree - Brittle Bush - Creosote Bush Desert Sea Level Brittle Bush - Creosote Bush - Catclaw

0 5000 10000 15000 20000 Radiocarbon age (yr B.P.) K. Cole, 1995

This diagram shows a species individualistic approach made possible from plant macrofossils in packrat middens. There was still little information on the rapid Pleistocene to Holocene shift although intermediate plant assemblages were identified.

4 Carbon 13 values of packrat pellets from 92 fossil middens from the Grand Canyon, AZ

Adapted From: Cole, K. L. and S. T. Arundel, 2005. Carbon 13 isotopes from fossil packrat pellets and elevational movements of Utah Agave reveal cold period in Grand Canyon, Arizona. Geology 33: 713-716.

More recent data is coming from archives of previously collected midden assemblages that can now be used in the application of new techniques providing that there has been adequate funding for storage and curation of older collections (an unusual occurrence).

5 Holocene YD B/A

Holocene YD

B/A

Adapted From: Cole and Arundel, 2005, Geology 33: 713-716.

This diagram shows the climate oscillations from the warm Bolling/Allerod to the cold Younger Dryas, followed by the rapid temperature increase as the Holocene started. These changes are shown in the shifts of carbon isotopes and well as the upper elevational limit of Utah Agave. This limit (blue line) is controlled by winter minimum temperature which was 8 degrees colder than today during the Younger Dryas. These rapid shifts were invisible until new AMS Radiocarbon analyses made possible better dating control.

6 Eastern Grand Canyon

Potato Lake

7 0 = Younger Dryas

Allerod?

Plant macrofossil analysis from Potato Lake also suggests a warm Allerod Period.

8 ODP Core 1019

Data from ocean cores are in agrement with the terrestrial data and far more detailed.

9 Adapted From: Barron, J.A., Huesser, L., Herbert, T., and Lyle, M., 2003, High-resolution climatic evolution of coastal northern during the past 16,000 : , v. 18, p. 1020–1035. Grootes, P.M., and Stuiver, M., 1997, Oxygen 18/16 variability in snow and with 103- to 105-year time resolution: Journal of Geophysical Research, v. 102, p. 26,455–26,470.

This ocean core (top) suggests a 4 degree (C) cooling of mean annual temperature in the Younger Dryas. If winter temperatures were 8 degrees colder, yet annual only 4 degrees, then summer temperatures were only a degree or two colder as shown in some studies.

10 Bolling/ Younger Early Full Glacial Allerod Dryas Holocene

The disparity in summer versus winter solar insolation caused this difference between summer and winter temperatures in the Younger Dryas.

11 Carbon 13 in Grand Canyon Packrat Pellets

Concentration of fossil Agave in Grand Canyon Packrat Middens

ODP1019 Alkenone derived Barron et al. 2003 mean annual sea surface temperature from CA / OR border. GISP2 Stuiver et al. 1997 Oxygen 18 from Greenland

All of these data show a rapid warming of at least 4 degrees C around 11,600 years ago at the end of the Younger Dryas. This is similar in rate and magnitude to the change expected to starting now due to global warming.

12 YD

Modified from: Cole, K., 1985, Past rates of change, species richness, and a model of Vegetational Inertia in the Grand Canyon, Arizona. American Naturalist 125:289-303.

The previous shifts in climate caused lower species diversity of trees and shrubs as shown in these older poorly dated middens. Current analyses are underway to improve these data.

13 Global Warming and the rapid invasions of weedy species are related to the relatively slow response of trees and shrubs to warming as seen at the end of the Younger Dryas.

14 The end.

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