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Great Basin Naturalist

Abstract

Cores obtained in 1978 from Diamond Pond, Diamond Craters, Harney County, Oregon, as part of the Steens Mountain Prehistory Project, provide a record of vegetation change on the sagebrush/shadscale ecotone and of local and perhaps regional water tables. Pollen, macrofossils, sediments, and charcoal from these radiocarbon-dated cores were analyzed. Varying abundance of juniper, grass, sagebrush, and greasewood pollen, and of aquatic to littoral plant macrofossils reflects changing regional effective moisture and local water table since 6000 B. P.

Eleven dates spanning 5200 radiocarbon years and four regionally correlated volcanic ashes establish the dating of seven periods of different moisture regimes:

1. Greasewood and saltbush pollen dominance before 5400 B. P. indicates shadscale desert. Rapid accumulation of alternating silts and medium sands lacking aquatic plant macrofossils and pollen reflects periods of ephemeral ponds with water table 17 m below the present level and considerable erosion of maar slopes.

2. Increasing sagebrush pollen from 5400 to 4000 B. P. indicates sagebrush expansion into shadscale desert. Scirpus, Rumex, Ceratophyllum, and Polygonum persicaria macrofossils and finely laminated clayey silts evidence perennial pond.

3. From 4000 to 2000 B. P. abundant juniper and grass pollen reflects extensive juniper grasslands (juniper seeds from trees growing nearby fell into the pond during this period). Rising charcoal values indicate greater importance of fire. Deepest late-Holocene pond ca 3700 B. P. corresponds with postulated intensive human occupation of northern Great Basin marsh and lake locales.

4. Between 2000 and 1400 B. P. increased sagebrush pollen mirrors reduced effective moisture and reexpanding sagebrush steppe. More abundant Scirpus and Rumex macrofossils evidence shallow pond.

5. From 1400 to 900 B. P. more numerous grass pollen indicates returning greater effective moisture resulting in deeper water with abundant Potamogeton.

6. About 500 B. P. increased greasewood and saltbush pollen evidences drought. Ruppia seeds and pollen and the mollusk Musculium indicate shallow, brackish water.

7. Abundant juniper and grass pollen reflects moister conditions between 300 and 150 B. P. Numerous Ceratophyllum fruits indicate deeper, freshened water. Since the mid-1800s man and changing climate have encouraged sagebrush reexpansion. Increased Scirpus macrofossils indicate shallower water.

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