Presentation on theme: "Bull Creek Paleoenvironment: The Pollen and Archaeoclimate Models R.A. Varney and Linda Scott Cummings PaleoResearch Institute Golden, Colorado."— Presentation transcript:
Bull Creek Paleoenvironment: The Pollen and Archaeoclimate Models R.A. Varney and Linda Scott Cummings PaleoResearch Institute Golden, Colorado
PaleoResearch Institute has had the pleasure of working on four separate, but related, pollen, phytolith and archaeoclimate modeling projects at the Bull Creek Project area over the last ~5 years. Our project 04-09: 34BV167 BCI 34 stratigraphic pollen samples- surface to 3.51 meters below surface. 14 C dates from 6200 ± 90 from the Akb2 (~50 cmbs) to 11070 ±60 from the 2Akb8 (~3mbs). The results of this project were reported at the Billings Plain conference in 2004. Five pollen zones reflecting, from most recent to oldest, dry grassland, lush grassland, shrub land, open forest, and steppe. Our project 06-34: 34BV181 BCII 10 stratigraphic pollen samples. 14 C date from this project 12,300 ± 80 fo the middle of the clay loam. Probable steppe signature in top sample, followed by steppe with very rapid sediment aggradation through the next three samples down, and then what appears to be an anomalously monospecific rabbit brush signature to the bottom of the column. Our project 07-25: 34BV166, 34BV181, 34BV182, 34WO69 6 pollen and 9 phytolith samples. Put the sites into the paleoenvironmental context- not included in this presentation. Our project 09-04: 34BV176 BCIII five stratigraphic pollen samples from 445 to 491 cmbs. Center sample from 464-473 cmbs returned a 14C age of 13210 ± 80. Pollen signature suggests riparian vegetation in a mixed pine forest.
Presentation Format Revisit initial pollen diagram presented at Plains 2004: introduce types, zones, and reasons we divided into those zones. Present current pollen diagram, show new zones. Compare new zones to archaeoclimate model annual water balance from Beaver Oklahoma. Present regional compiled archaeoclimate models, discuss inferences and implications of the regional scale perspective. Conclusion and Supposition.
Comment on the Correlation of the Beaver Model and the Pollen Zones Forest-Riparian: High moisture, stable. Time of transition uncertain, but likely. Rabbitbrush: High amplitude changes. Beginning time uncertain, but likely, ending time uncertain. Possible unconformity. Rapid aggradation: Rapid fluctuations. Beginning reasonably firmly dated, ending uncertain, but likely. Steppe: Younger Dryas, Beginning uncertain, but likely, ending reasonable assuming no unconformity. Open Forest: Unstable, generally declining, but considerably wetter than modern beginning firmly dated, ending firmly dated. Shrubland: Significant decline near beginning, then wildly unstable. Beginning firmly dated, ending uncertain, possibly later than indicated. Lush Grassland: Rapid or steady drying depending on timing of beginning, then steady drying. Beginning uncertain possibly later, ending firmly dated assuming no unconformity. Dry grassland: Stably dry with slight fluctuations. Beginning firmly dated. Probable unconformities within zone.
The models are annual precipitation minus evaporation: in other words, water balance for the year. Temperature alone is a poor indicator of climate conditions over this time and area. Precipitation alone is a poor indicator of climate, though not as poor as temperature. Water balance shows the modeled water regularly available to plants, and as such is a good indicator of vegetation density. Only annual water balance is presented, any modeled seasonal changes in water balance, while potentially critical to understanding vegetation, are not yet completed.
The pattern through time in the next few slides exhibits what is expected: A general drying trend. I will flip through these fairly quickly, as the temporal trend is what we expect, the regional distribution pattern is used for inferences, and that changes little through the period.
What do the models suggest? 1.Bull Creek is positioned near a climatic boundary between a relatively more moist zone with a southern boundary paralleling the southern Colorado and Kansas borders and the relatively drier zone to the south. Water balance becomes sharply less dry northward from this boundary. 2.Water balance becomes less dry eastward along a line paralleling the Texas/Oklahoma North/South Border. This is due to the decreasing effect of the rain- shadow from the mountainous areas in eastern NM. While not as pronounced as the northern climatic boundary, it is not very far east until you encounter mosaic prairie today. A less dry regime could have allowed the mosaic prairie to have been located even closer that we now encounter. 3.Bull Creek, in the past, may have been positioned near the intersection of three climate zones, with differing vegetation and dependent fauna, permitting a wealth of resources over a limited distance.
Conclusions And Suppositions Conclusions Eight pollen zones are evident from the Bull Creek area. From top to bottom: Dry Grassland >6200 RCYBP – modern Lush but decreasing Grassland >8670 - >6200 RCYBP Shrub land with plentiful grass ~10400 - >8670 RCYBP Forest parkland ~11070 - ~10400 RCYBP Steppe with more stable surface >11070 - ~11070 RCYBP Possible steppe with rapid aggradation <12300 -<11070 RCYBP An anomalous rabbitbrush-dominated vegetation >12300 RCYBP Forest-Riparian ~13210 RCYBP
Conclusions And Supposition Supposition The Bull Creek area was located near the boundary of three climate zones, permitting access to different resources over a limited area.