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Kenneth Cole, USGS Southwest Biological Science CenterFlagstaff, AZ; ken_cole@usgs.gov

Late Glacial to Early Holocene Climate Oscillations in the American Southwest

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

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Most of the earliest conceptions of the Pleistocene 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.

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Steven’s Woodrat (Neotomastevensii) poses with Kirsten Ironside.

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

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Lower Colorado River Elevational Zonation

Radiocarbon age (yr B.P.)

0 5000 10000 15000 20000

Ele

vati

on

(m

)

Sea Level

1000

2000

3000

Snowline

Treeline

Spruce Forest

Limber Pine, Fir Forest

Juniper - Sagebrush Woodland

Juniper - Blackbrush Woodland

Joshua Tree - Brittle Bush - Creosote Bush Desert

Ponderosa Pine -Fir Forest

Pinyon-JuniperWoodland

Blackbrush -SagebrushDesert

Brittle Bush -Creosote BushDesert

Juniper - Ash Woodland

Spruce Forest

Treeline

Brittle Bush - Creosote Bush - Catclaw

K. Cole, 1995

Modified From:

Cole, K. L. 1990. Reconstruction of past desert vegetation along the Colorado River using packrat middens. Palaeogeography, Palaeoclimatology, and Palaeoecology 76: 349-366.

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.

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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 Younger Dryas 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).

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B/A

B/A

YD

YD

Holocene

Holocene

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

This diagram shows the late Pleistocene 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.

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Eastern Grand Canyon

Potato Lake

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Heinrich Event 0= Younger Dryas

Allerod?

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

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ODP Core 1019

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

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Adapted From:Barron, J.A., Huesser, L., Herbert, T., and Lyle, M., 2003, High-resolution climatic evolution of coastal northern California during the past 16,000 years: Paleoceanography, v. 18, p. 1020–1035.

Grootes, P.M., and Stuiver, M., 1997, Oxygen 18/16 variability in Greenland snow and ice 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 pollen studies.

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Full GlacialYounger Dryas

Bolling/Allerod

EarlyHolocene

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

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Carbon 13 in Grand Canyon Packrat Pellets

Concentration of fossil Agave in Grand Canyon Packrat Middens

Alkenone derived mean annual sea surface temperature from CA / OR border.

Oxygen 18 from Greenland Ice Core

GISP2Stuiver et al. 1997

ODP1019 Barron et al. 2003

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.

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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.

YD

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.

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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.

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The end.