Bull Creek Paleoenvironment: The Pollen and Archaeoclimate
Models R.A. Varney and Linda Scott Cummings PaleoResearch Institute
Golden, Colorado Slide 2 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. Slide 3 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. Slide 4 Bull Creek Slide 5 8670+90 Forest Scrub
Grassland lush Steppe Grassland dry Slide 6 Dry Grassland Lush
Grassland Shrubland Open Forest Steppe? Rapid aggrading Rabbitbrush
Forest/ Riparian Slide 7 Dry Grassland Lush Grassland Shrubland
Forest/Parkland Rapid Aggradation ?Rabbit brush/ No Analog
Forest/Riparian 6200 RCYBP 7660 RCYBP 8670 RCYBP 9850 RCYBP 10400
RCYBP 10850 RCYBP 11070 RCYBP 12300 RCYBP 13210 RCYBP Steppe? Slide
8 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. Slide 9 Regional Archaeoclimate Models
The numerical output from 18 models in the region of SE Colorado,
SW Kansas, NE New Mexico, and the panhandles of Texas and Oklahoma.
Data are prepared for visual presentation using Golden software
Surfer 8 . These are MODELS, and as such, should not be taken as
literal representations. The models are useful tools to generate
ideas and hypotheses. In all cases where we have compared the
models to proxy data, the models support the indications of the
proxy data. In my mind this lends a tenuous credence to the method
of archaeoclimate modeling. Models were prepared for the closest
calendar time available to the 14 C dates at BCI, BCII, and BCIII.
Slide 10 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. Slide
11 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. Slide 12 13020 RCYBP Slide 13 12315
RCYBP Slide 14 11110 RCYBP Slide 15 10850 RCYBP Slide 16 10400
RCYBP Slide 17 9935 RCYBP Slide 18 9735 RCYBP Slide 19 8675 RCYBP
Slide 20 7600 RCYBP Slide 21 6200 RCYBP Slide 22 Slide 23 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. Slide 24
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
