Sangamonian(?) and Wisconsinan Paleoenvironments in Yellowstone National Park

Sangamonian(?) and Wisconsinan paleoenvironments in Yellowstone National Park

RICHARD G. BAKER Department of Geology, University of Iowa, Iowa City, Iowa 52242

ABSTRACT This paper summarizes five pollen sequences, three of which contained plant macrofossils (see Fig. 1 for locations). The Grassy Lake Res- Pollen and plant macrofossil records from Yellowstone National ervoir pollen sequence was interpreted as a warm interstadial event (Baker Park, if combined with dated glacial events, provide a paleoenviron- and Richmond, 1978). In this paper, the pollen sequence is compared with mental record of much of the last glacial-interglacial cycle. Bull Lake plant macrofossils collected at the same time from the same section. Sec- glaciation has been dated at -140,000 yr B.P. Section EP-6 records a tion EP-6 was interpreted from initial pollen counts as a late-glacial to late-glacial to full interglacial sequence which is correlated with the full-interglacial sequence (Baker, 1981). The complete pollen sequence is Sangamonian interglacial and estimated to be 127,000 yr old at the presented here, along with a pollen concentration diagram. In addition, the peak warm period. A prevailing Pseudotsuga-Pinus flexilis-Pinus section was resampled, and the pollen and plant macrofossils from the ponderosa forest suggests that climate was considerably warmer than interglacial portion of the sequence are examined in detail. A second any in the Holocene. Section EP-5 is somewhat younger, probably section, EP-S, is stratigraphically above EP-6, and pollen and macrofossils late Sangamonian, and shows a forest dominated by Picea, Abies, from it are presented. Reconnaissance palynology of a younger, cold inter- Pseudotsuga, and a haploxylon Pinus. Slightly cooler conditions than stadial section is also briefly discussed. those of the present are indicated. Pollen sequences from Yellowstone National Park have outlined The warmest phase of the Grassy Lake Reservoir section is con- vegetational changes in late glacial and Holocene time (Baker, 1970,1976; sidered to be -82,000 yr old and records a warm interstadial cycle Waddington and Wright, 1974; Gennett, 1977). Plant macrofossil anal- beginning with tundra. The wanning sequence is indicated by change yses have provided more detailed understanding of these changes (Baker, to a Picea-A bies-Pinus albicaulis forest and then to a Pinus contorta 1976). Pollen sequences from southern Montana (Mehringer and others, forest. The cycle ends with forest destruction and a return to open 1977; Brant, 1980) and some plant-macrofossil work (R. C. Bright, 1980, (tundra?) and, presumably, cold conditions. written commun.) from Montana support the general trends for the region. Extremely low values of arboreal pollen indicate that tundra This late Quaternary work provides a basis for understanding the earlier vegetation and cold conditions continued from-70,00 0 to -50,000 yr pollen sequences. B.P. One cool and, apparently, short interstadial interrupted this extended period of tundra shortly after 68,000 B.P. when a very open Vegetation and Climate parkland of Picea-A bies-Pinus albicaulis appeared. No records of environment are available between -50,000 and The vegetation of the Yellowstone Park region is grassland or steppe 30,000 yr B.P. The Pinedale icecap apparently expanded -30,000 yr at low elevations (below -1,700 m), a series of overlapping forest series at B.P. and lasted until -14,000 yr B.P. Late-glacial Picea-A bies-Pinus middle and upper elevations (1,700 to 3,000 m), and tundra on the high albicaulis parklands gave way to Pinus contorta forests that have mountaintops (>3,000 m) (Steele and others, 1983; Arno, 1979). The prevailed with minor variation for -10,000 yr. forest zones are broadly controlled by elevation and include (base to top) Pinus flexilis, Pseudotsuga, Pinus contorta, Picea engelmannii, Abies lasio- INTRODUCTION carpa, and Pinus albicaulis (Fig. 2). Topographic aspect, soil types, and other factors cause considerable variability in the elevation of each habitat Pollen sequences of pre-late Wisconsin or Sangamon age are known type, although the general pattern is widely established. Pinus ponderosa from few sites in the western United States. Heusser (1977) has developed does not grow locally in the Yellowstone Park area, but in central Mon- a generalized sequence of pollen assemblages back through the last inter- tana, it forms a habitat type below the Pinus flexilis type along the glacial period for western Washington, using a number of different sec- grassland-forest border (Arno, 1979). tions. Based on long cores from Clear Lake, Adam and West (1983) have Climatic stations in Yellowstone Park provide some data on the produced a continuous pollen sequence from Sangamon to present in climate associated with three of the habitat types (Fig. 2). Mean monthly central California. Few Rocky Mountain pollen records, however, are precipitation curves show that precipitation is rather evenly distributed older than Pinedale (late Wisconsin). Yellowstone National Park is per- throughout the year in this region. Mean monthly temperatures are lower haps unique in having many Wisconsin and older sections that contain each month at high-altitude sites as compared to low-altitude sites. Mean pollen and plant macrofossils (Baker and Richmond, 1978; Baker, 1981). annual temperature also varies inversely with altitude. Mean annual pre- The purposes of this paper are (1) to document vegetational sequences cipitation commonly increases with altitude, but Yellowstone National from early Wisconsin and probable Sangamon sites by comparing pollen Park has substantial variation of precipitation geographically (highest and plant macrofossil sequences and (2) to present a firstapproximatio n of values in the southwest corner and lowest along the northern border) so the environmental history of Yellowstone National Park during an esti- that the pattern is not uniform with elevation. The cause for this variation mated 140,000 yr. is uncertain, but it is not, apparently, caused by major air-mass boundaries

Geological Society of America Bulletin, v. 97, p. 717-736,13 figs., 1 table, June 1986.

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Figure 1. Map of Yellowstone National Park showing ssunple locations and arias men- tioned in text. Modified from Baker arid Rich- mond (1978).

440J

0 10 20 30 KILOMETERS L- _JI I _J

in the region (Miichell, 1976). It may be that the southwest corner of the park has been glaciated several times (Richmond, 1970, 1976), iind the park receives maximum effect from westerly and southwesterly winds last major icecaps apparently occurred -140,000 and 20,000 to 30,000 yr blowing across the relatively low Snake River Plain (Mitchell,1976) and B.P. (Pierce and others, 1976; Pierce, 1979). suddenly cooling adiabatically as a result of the orographic effect of the Yellowstone Plateau. Methods The geology of Yellowstone National Park is fairly well known (Christiansen and Blank, 1972; Smedes and Prostka, 1972; Ruppel, 1972; The large sections exposed along stream cuts were measured using a Love and Keefer, 1975; Pierce, 1979). Volcanic and glacial geologic fea- steel tape, and samples were collected using shovels and knives. The slope tures predominate. Sedimentary rocks are exposed in the northwest and angles were measured with a Brunton compass, and the tape measure- south-central border areas of the park (U.S. Geological Survey, 1972). ments were corrected to vertical. Measurements of dipping units were Eocene volcanic and volcaniclastic rocks compose the Absaroka Range corrected to true thicknesses. Sediments were placed in plastic boxes or along the eastern border. Quaternary rhyolites from early to late Quater- zip-lock plastic bags and stored at room temperature. nary volcanic eruptions and related volcanic events cover central areas in Sediments were processed for pollen using a treatment modified from the park (Christiansen and Blank, 1972). The latest eruptive event took Faegri and Iversen (1975). Samples used for pollen concentration were place -70,000 yr ago and resulted in the Pitchstone Plateau flow. The immersed in water and their volume measured in a graduated centrifuge

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tube. A tablet containing 16,180 ± 1,500 gTains oi Eucalyptus pollen was GRASSY LAKE RESERVOIR added to these samples. All sediments were treated with KOH, HCL, HF, and acetolysis solution; rinsed in tertiary butyl alcohol; and mounted in The sample site at Grassy Lake Reservoir (Fig. 1) exposes lake sedi- silicon oil. In the spiked samples, Eucalyptus pollen grains were counted ments underlying Pinedale Till (Richmond, 1973, sec. 2; Baker and Rich- along with a minimum of 300 indigenous grains for calculation of pollen mond, 1978), and, in nearby sections, these lake sediments interfinger with concentration (Maher, 1972). Pollen was identified using the University of Bull Lake Till. Obsidian hydration dates indicate that the Bull Lake Till Iowa Pollen Reference Collection. was deposited before 130,000 yr B.P. near West Yellowstone, Montana About 200 ml of sediment for plant macrofossils analysis were (Pierce and others, 1976). Pieces of the Pitchstone Plateau rhyolite flow, washed through 35- and 105-mesh screens using a gentle stream of water which crops out "up-glacier" from Grassy Lake Reservoir, occur in the from a shower nozzle. The fossils were hand picked and then stored in overlying Pinedale Till but not in the lake sediments nor in Bull Lake Till glycerin. Seeds, fruits, leaves, bracts, and other macrofossils were identified by comparison with the Seed Collection of the Geology Department, University of Iowa. Elevations [-10,000 ft.

(2970 m.)

•9000 (2743)

8000 (2438) t/> w 3 0. 6000 C 1 PINUS FLEXILIS SERIES (1829) 1Û¡ . grassland and sagebrush

B LAKE YELLOWSTONE, WY. 2366m. WEST YELLOWSTONE, MT. 2031m. YELLOWSTONE PARK, WY. 1902m. mm. 60

J F M A M

Figure 2. Vegetational series in the Yellowstone National Park area and associated climatic data. Solid arrows indicate altitudes where the tree is important in the vegetation; dashed lines indicate where the tree is present. Upper curve in climatic data is precipitation; lower curve is temperature. The two numbers in the centers of the graphs are mean annual temperature (°C) and mean annual precipitation (mm), respectively. A, B, and C show the elevation and vegetational association of the climatic stations on the cross section.

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(Baker and Richmond, 1978). This flow has a K-Ar age of -70,000 yr prevents a more detailed interpretation of the upland vegetation. The rarity B.P. (Richmond, 1973). The lake sediments are therefore between of fossils and the types present may have several possible explanations, but, -70,000 and -130,000 yr old. essentially, either the seeds and fruits were not available or they were not Pollen analysis for the Grassy Lake Reservoir section (Fig. 3) has delivered to the sample site. provided a framew ork for interpreting the vegetational history of that area The interpretation of the regional vegetation remains that of a tundra (Baker and Richmond, 1978; Baker, 1981). Four zones were described as or tundra-like environment. No macrofossils of plants limited to tundra follows. Zone GL-4, at the base, is an Artemisia-Bistorta assemblage zone environments were found, but Selaginella densa indicates open ground (Baker and Richmond, 1978), suggesting cold climate and tundra-like and is most common in the tundra. Its spores are most abundant in conditions. Zone GL-3, a Picea-Abies assemblage zone, was interpreted as late-glacial pollen zones in the several post-Pinedale sequences available a cool climate dominated by Picea-Abies forest. Zone GL-2, a Pinus (Baker, 1976; Waddington and Wright, 1974). Selaginella selaginoides is contorta assemblag e zone, was predominantly diploxylon Pinus pollen and also a common tundra species, but it can also occur below treeline, and, in probably represented a relatively warm climate and a Pinus contorta this sequence, it occurs more abundantly higher in the sequence with forest forest. Zone GL-1, at the top, is an Isòetes-Ambrosia assemblage zone and species. The presence of Isoetes bolanderi and charophyte oospores at the was tentatively interpreted as a return to collier conditions with trees base of the section and the absence of shallow-water aquatic plants suggest diminishing in imfiortance. Plant macrofossil zones do not exactly match that the lake was relatively clear and deep during its early stage:;. The the pollen zones and are shown as M-l through M-4 (Figs. 4A and 4B). disseminules of Juncus and Carex probably represent the moist environ- The basal-pla nt-macrofossil zone (M-4) is characterized especially by ments along the shoreline. Juncus and Selaginella densa; its boundaries correspond exactly with The overlying macrofossil zone M-3 is characterized by the presence those of the basal pollen zone (Figs. 3, 4A, and 4B). The macrofossil of Betula, Picea, and Abies macrofossils (Figs. 4A and 4B). The upper content of this zone is low and commonly consists of seeds of aquatic boundary of this zone is slightly lower than that of zone GL-3, although plants; tiny seeds c

20 40 60 SUM = A P+NAP ITAXON 'SUM + TAXON

Figure 3. Pollen percentage diagram for Grassy Lake Reservoir.

zones GL-3 and M-3. Fruits of cf. Betula are not common in the sequence, changes in the local and regional vegetation. It seems likely that these but they occur with low percentages of birch pollen in zones GL-3 and conifers were absent during the time zone GL-4 and M-4 sediments were M-3 and at the top of zones GL-4 and M-4. Equally low and persistent deposited and that they immigrated and became relatively abundant dur- pollen percentages in the rest of zone GL-4 are not accompanied by fruits. ing deposition of zone GL-3 and M-3 sediments. Betula pollen was present Selaginella densa microspores are most abundant in zones GL-4 and throughout zone GL-4, but its macrofossils do not occur until the top of GL-3, whereas the megaspores are most abundant in zones M-3 and M-2. zone M-4, where pollen and fruits occur together. The wings of the fruits S. densa microspores are rare in zone GL-2. In the case of S. selaginoides, were not preserved, making species identification impossible, but B. glan- megaspores were present, but, curiously, no microspores were found. dulosa is most likely based on the very similar post-Pinedale sequence in Carex fruits of the biconvex type, Juncus seeds, and Selaginella Yellowstone Park where both pollen and well-preserved fruits of that densa megaspores increase in occurrence markedly from the bottom to species were found in a similar assemblage (Baker, 1976). Betula was the top of zone M-3. Carex pollen is included with Cyperaceae pollen, and probably present regionally but not locally until sediments in zones P-3 its pollen percentages change little throughout the entire sequence. Juncus and M-3 began to accumulate. pollen is not preserved in sediments. Selaginella densa microspores show a Picea and Abies needles occur slightly above the level of the birch small increase in percentages corresponding to the megafossil increase. fruits and slightly above the level of pollen peaks of those two genera (Figs. The interpretation of zone GL-3 based on the pollen was that it 3 and 4). Pollen should reach the sites before the macrofossils because it is marked the invasion of the area by Picea, Abies, and Pinus albicaulis. This transported further. In addition, Pinus albicaulis needles occur in the upper view is well supported by the good correlation between pollen and needles part of the zone, also just above the peak in pollen percentages. It appears of Pinus, Picea, and Abies. The pollen changes probably reflect real from both the pollen and macrofossils that Picea engelmannii was the first tree to reach the site, followed by Abies lasiocarpa and Pinus albicaulis. In addition, the first bud scales of Salix sp. and Populus cf. P. tremuloides Grassy Lake Reservoir reached the site during this same period. At the top of this zone, Pinus contorta-type needles firstappea r coincident with the rise in diploxylon (P. Plant Macrofossils(l) / Trees and Shru bs ~y ^/Upland Herbs ^ / o// / r/ V fr- / i) A Ä •Ji* •? c c

M I

M 2

M 3

M 4

I—I—I—I—I—I—I—I—I—I—I—I—I—I—r-i—I—I—I—i—i—I—I—I—r-i—I—I—n—i—i—i—i—i—i—i—i—i—i—i—m—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i 10 30 10 30 50 10 20 10 30 50 10 30 10 40 80 200 500 Note scale changes at dotted lines

Figure 4A. Plant macrofossil diagram for Grassy Lake Reservoir, trees, shrubs, and upland herbs. Dashed vertical lines on some curves mark changes in scale.

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contorta-type) pollen percentages. Apparently the regional treeline rose Abies lasiocarpa climax series and one phase of the Picea climax series. across the elevatior. of Grassy Lake Reservoir when zone M-3 sediments Apparently some open meadows were present close to the former lake. were deposited, and increasingly dense and diverse forests grew in the area. The aquatic environment also changed at the collecting locality. A Fruits and seeds of upland herbs also occur in the record in zone M-3 great increase in probable shoreline species, such as Juncus and Car ex, (Fig. 4A). These include a few seeds of Potentilla spp., cf. Gaultheria suggests that the shore was closer to the site during zone M-3 time than in humifusa, and, possibly, Hypericum cf. formosum and Viola. Several spe- zone M-4 time. Water may have remained relatively deep and clear, cies of Potentilla sesm to be present, based on the wide range of fruit size because aquatic-plant seeds and fruits are not still present in zones 3L-3 and shape. PotentiUa is a difficult genus to work with in the Rocky Moun- and M-3. tains, and the fruits are not particularly distinctive. Variation in size is Zone M-2 Ms slightly below the corresponding pollen zone GL-2. greater in the fossil fruits than in the 19 species in our reference collection. Zone M-2 is defined by the co-occurrence of needles of Pinus albicaulis, P. Species of this genus grow in a wide range of habitats in the Rocky contorta, Picea engelmannii, and Abies lasiocarpa. The macrofossil sum is Mountain area, but all except P. palustris (which does have distinctive higher than in any other zone, and many other types become abundant. fruits) grow in upland habitats, most commonly in meadows, and not These include bud scales of Salix and Populus and seeds or fruits of commonly in forests (Weber, 1976). Gaultherui humijusa grows in cool Potentilla, Viola, cf. Gaultheria humifusa, and other upland herbs. Mega- forests and on moist, sloping meadows (Hitchcock and Cronquist, 1973; spores of Selaginella densa, another upland plant, continue to be particu- Weber, 1976). Pfister and others (1977) reported G. humifusa mainly in larly abundant. Aquatic and wetland macrofossils also become very abundant, especially Isoetes, Carex spp., Juncus spp., and Urtica. 1 Pollen characteristic of macrofossil zone M-2 shows the maximum of Grassy Lake Reservoir ! tree pollen and includes the broad maximum of diploxylon pine pollen. Plant Macrofossils(2) Good correspondence again is present between pollen and macrofossil

10 30 50 10 30 10 30 50 10 40 80 160 10 30 10 40 100 400 700

Note scale changes at dotted lines

Figure 4B. Plant macrofossil diagram for Grassy Lake Reservoir, semiaquatic and aquatic plants and plants of uncertain ecological habitats. Dashed vertical lines on some curves mark changes in scale.

curves, including Pinus contorta-type needles and diploxylon pine pollen, themselves became more organic than previously, and a rich aquatic flora Salix bud scales and pollen, Potentilla fruitsan d Rosaceae pollen, and became established. Plants commonly found in deep water, such as Pota- Gramineae fruits and pollen (Figs. 3, 4A, and 4B). The following imper- mogetón gramineus, Sparganium angustifolium, and charophytes, are fect matches show pollen and macrofossils occurring together, but not with present, although a number which are more characteristic of shallow water the peaks of each matching: Pinus albicaulis-type needles with haploxylon are more prominent. These include Ranunculus aquatilis, Sagittaria, Calli- pollen, Picea and Abies needles and pollen, Selaginella densa and Isoetes triche, and Typha (Weber, 1976; Muenscher, 1944). Marsh or wetland megaspores and microspores, Eleocharis and Carex fruitswit h Cyperaceae plants are also well represented by Typha, Rorippa islándico, Lycopus pollen, and Sparganium fruits and pollen (not shown). americana, Eleocharis palustris, Selaginella selaginoides, and Urtica The interpretation again strongly supports that suggested by the (Hitchcock and Cronquist, 1973). Apparently the lake became shallow as pollen evidence alone (Baker and Richmond, 1978). The forest was prob- sediments filled it. This fillingma y have been complete in nearshore areas, ably a mixture of Picea engelmannii, Abies lasiocarpa, Pinus albicaulis,convertin g them to wetlands. The lake probably changed from a some- and Pinus contorta. In addition, Populus tremuloides and P. balsamiferawha t oligotrophy lake in its earlier stages toward a more eutrophic lake can now be suggested as forest components. Salix probably grew along the during the time zone M-2 sediments were deposited, judging from the lake shore. The appearance of abundant Potentilla fruitsnea r the top of increase in organic sediments and the more extensive aquatic flora. this zone may suggest that the forest was changing from very dense to Zone M-l is defined by the high numbers of Potentilla and Urtica partially open with interspersed meadows. fruits (Figs. 4A and 4B). The base of zone M-l is lower in the section than Substantial changes in the aquatic and sub-aquatic habitats occurred is the base of pollen zone GL-1. The general trend of macrofossils in zone between the deposition of zone M-2 and M-3 sediments. The sediments M-l is a decrease in arboreal types, which finally disappear at the top of the zone. The general abundance of macrofossils is somewhat lower than in the preceding zone, and it also decreases upward. The correspondence of pollen with macrofossils is reasonably good in zone M-l. For example, Grassy Lake Reservoir Potentilla fruits and Rosaceae pollen continue to occur together, as do Plant Macrofossils(2)—continued

y'Semiaquatics ^ / Aquatics ~J ^/Uncertain ^

s M / $ § •-f r? £

J* f / é¡é¡ ßßgff^ £ i DEPTH IN ^ METERtf M&j J? Zones

M I

0.5-'

M 2

1.0- • i

1.5- M 3

2.0-

M 4 2.5-

J 3.0 i—i—i—i—i—i—r- -1—m—i—r- I—r—i—i—i—i—i—i—i—i—i—i—i—r—i—i—I—i—i i i—i—i—i—i—i—r—i—i—i—r-i—i—i—i—i—i—i—i—i 10 30 10 40 80 160 10 30 10 40 80 160 Note scale changes at dotted lines

Figure 4B. (Continued).

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Gramineae fruits and pollen and Eleocharis and Care:t fruitswit h Cypera- BEAVERDAM CREEK SECTIONS ceae pollen. The peak in Isoetes microspores docs not match the maximum occurrence of menaspores, although it does coirespond with a secondary The Beaverdam Creek sections are large stream cuts as much a> 60 m peak. high and 100 m long. Slumping changes the exposures from year to year, The pollen zone GL-1 was previously interpreted to represent a cool- covering parts of some sections and exposing previously covered sejsments ing period, and the vegetation became more open until Pinedale glaciers of other sections. Two sequences of lake sediments (one lake silts and one advanced across the site (Baker and Richmond, 1978). The possibility of deltaic) are present (Richmond and Pierce, 1972). The upper secion of warmer or drier conditions leading to a more open environment could not, lake sediments and its enclosed pollen sequence underlie Pinedale Till however, be ruled out. A rise in pollen percentages of Chenopodiineae, (Fig. 5) and are described by Baker and Richmond (section EP-8,1978). Artemisia, Ambrosia, and Gramineae (Fig. 3) could be interpreted as an The underlying deposits, including the lower lake-sediment sec uence, indication of wanner climate and steppe-like conditions. All but Ambro- are also discussed by Baker and Richmond (1978). The upper lake sedi- sia, however, occur in relatively high percentages during late Pinedale ments overlie a thin, sandy peat overlain and infiltrated by a thin, white, time, and all are common in surface samples from modern tundra in the volcanic ash bed (Fig. 5) of Yellowstone Park pétrographie affinity. This area (Baker, 1976; R. G. Baker, unpub. data). The plant macrofossils ash bed is believed to represent the last major eruptive phase in what is support the decrease and probable demise of trees near the site when the now Yellowstone National Park, the extrusion of the Pitchstone Plateau upper sediments of zone M-l were deposited. The upland plants that are flow (Fig. 1), dated at -70,000 yr old; the peat was collected by Rich- represented at the top of zone M-l are mostly plants of open environ- mond (1973; Baker and Richmond, 1978) and dated at 68,000 +2200 ments, such as Potentilla, Descurainia cf. pinnata, and Selaginella densa. - 1700 yr B.P. (GrN-7332) by radiocarbon dating (Grootes, 1977). Un- None is a tundra or steppe indicator, and they do not clearly represent derlying the ash is a gravel unit, and, in one exposure, Richmond found either cooler or warmer-drier conditions. An element of uncertainty, thus, that two gravels were present and that the upper gravel was channeled into still remains. I believe that the original interpretation of colder climate and the lower (Baker and Richmond, 1978). The gravels overlie the lower a return to tundra conditions is most likely. lake-sediment sequence (Fig. S), which is a deltaic unit which has foreset The aquatic and semi-aquatic plants indicate a continuation of the beds of organic sands and silts. The age of the lower lake sediments must deep-water, shallow-water, and wetland environments of the previous be >70,000 yr. zone. Eleockaris palustris, Typha, and most Alismataceae occur presently at low to middle elevations (Porter, 1962-1972), from the plains up to Section EP-6-1 areas of Pinus coniorta forest (Fig. 2). Potamogeton alpinus, P. praebngus, and Isoetes bolanderi, however, live at or above middle elevations in Section EP-6-1 of deltaic lake sediments was sampled by Richmond areas of Pinus contorta or Picea-Abies forests. The aquatic and wetland and W. F. Harvey, Jr., in 1977.1 analyzed the samples (Fig. 6), and a brief plants, thus give conflicting information as to direction of climatic change. account of the palynology is published (Baker, 1981). Pollen concentra- In summary, the plant macrofossil record tends to reinforce many of tion diagrams for the section (Figs. 7A and 7B), however, have not been the interpretations based on the pollen record. It has provided species previously published. Pollen concentration (a measure of both pollen pro- identifications for many taxa, has added a number of taxa unrecorded in duction and sedimentation rate) is not as useful in paleoecology ai pollen the pollen sequence, and has indicated that several taxa were present accumulation rate ("influx," which is a measure of production). Pollen locally at the site. It has not relieved the uncertainty of the interpretation in concentration data, nonetheless, remove the constraints of percenfcige dia- zones GL-1 and M-l. grams, and they are useful in determining whether individual fluctuations are real or caused by changes in percentages of other pollen types. Legend Four zones were designated for the pollen sequence. The lowermost TILL (zone 4) is the Juniperus-hcrb zone, characterized by relatively low values GRAVEL of Pinus and relatively high values oi Picea, Juniperus, Artemisia, Grami- _o EP-8 neae, and Cyperaceae (Figs. 6, 7A, and 7B). The Pinus is mostly of the . <0 EP-7 SAND iv „ • » t° < haploxylon type. Total pollen concentration is low (mostly <15,000 Vi"? LAKE: SILTS • ° o B grains/cc), and only Juniperus is commonly higher in this zone than VOLCANIC ASH elsewhere (Fig. 7A). Pinus has the highest concentration of any taxon at DELTAIC DEPOSITS J _ o» -5,000 grains/cc, although it is much lower in this zone than in any other. EP-5 Other taxa that have relatively high concentrations are Picea, Artemisia, _ ~ IHP-6 Compositae, Gramineae, and Cyperaceae. < In zone 3, Pinus, Picea, and Abies make up the dominant asse mblage O 10 (Figs. 6 and 7 A). Pinus pollen risest o 60%-70%, and haploxylon types are only slightly more common than are diploxylon types. Total pollen con-

9• *• # 9O « centration jumps markedly to as much as 60,000 grains/cc and fluctuates m * « » »« • O». . • 0 7 ' / < greatly, although always at values greater than those in zone 4. Concentra- . • • • * • O ' • * tions of Pinus, Picea, and Abies pollen reach maxima at values as large as e o • o ' 41,500, 7,000, and 4,500, respectively. Concentrations of Artemisia, Che- nopodiineae, Sarcobatus, Ambrosia, Gramineae, and Cyperaceae pollen lo.e km| are also relatively large in this zone. I0"6 kml |0.3 Km) In zone 2, haploxylon Pinus pollen becomes more predominant (Fig. 6). Total Pinus pollen percentages drop. Pseudotsuga pollen begins to Figure 5. Generalized sediment sequences along Beaverdam occur regularly in the upper half of the zone. Pollen concentrât ons are Creek. commonly somewhat lower and more stable in zone 2. Pinus pollen

concentrations vary between -15,000-20,000 grains/cc, and Picea and concentrations may support this interpretation (although they could result Abies concentrations drop to <1,000 grains/cc. Pseudotsuga pollen con- from rapid sedimentation), because pollen influx was commonly low dur- centrations rise throughout the zone, and Chenopodiineae concentrations ing late Pinedale time as well (Waddington and Wright, 1974). The vege- reach maximum levels in the zone. Pollen of other herb taxa remains tation suggested by this pollen zone is open parklands with scattered Picea relatively constant or decreases slightly from the previous zone. engelmannii and Pinus albicaulis. The Juniperus is probably J. communis, In zone 1, Pseudotsuga pollen dominates, reaching -11% (Fig. 6). based on its identification from plant macrofossils in the apparently similar Pinus percentages and haploxylon-diploxylon ratios remain nearly con- late-glacial community. Areas between the scattered trees were probably stant. Picea and Abies pollen percentages reach low levels (commonly covered with meadow and, perhaps, tundra species. The climate of this <3%), Chenopodiineae percentages decline upward (from -7% to <3%), time is considered to be cold, and this sequence probably represents Dli- and most other taxa do not change markedly from zone 2. Arboreal pollen noian(?) late-glacial leading into interglacial conditions. Zone 3 shows a increases to a peak of -85% near the top of the zone. Pollen concentration very abrupt change to a forested environment. The zone boundary occurs remains at levels similar to those of zone 2. Pseudotsuga pollen concentra- where the sample collection was offset along an apparent bedding plane. tions increase to maxima of nearly 3,000 grains/cc near the top of zone 1. The abrupt change in pollen content suggests that a hiatus may be present. Interpretations of this sequence are based here on both the pollen The forests represented by the zone-3 pollen assemblage were proba- percentage and concentration diagrams. Zone 4 closely resembles the bly dominated by Pinus albicaulis, P. contorta, Picea engelmannii, and Pinedale late-glacial sequence in Yellowstone National Park (Baker, 1976; Abies lasiocarpa. The equal mix of diploxylon and haploxylon pine types Waddington and Wright, 1974). The relatively low percentage oi Pinus suggests that both types were present, and their occurrence with spruce and pollen, predominantly of the haploxylon type; the relatively high percen- fir suggests that the forests were similar to modern forests in the region tages oi Picea, Juniperus, Artemisia, Compositae (Tubuliflorae and Liguli- today at slightly higher altitudes. The modern surface sample from this site florae), Gramineae, and Cyperaceae; and the rather consistent appearance is heavily dominated by diploxylon Pinus pollen, with small percentages of of Rosaceae (not shown), Caryophyllaceae (not shown), and SelagineUa Picea and Abies (Baker, 1981). The forest was probably relatively closed, dertsa all are characteristic of late Pinedale pollen spectra. The low pollen although herb concentrations are commonly as large or larger than those of the preceding zone. The herb percentages probably are smaller because of the swamping effect of the large arboreal pollen concentrations. Some open areas must have existed, however, to have produced the increased Beaverdam Creek : EP-6-1 herb-pollen concentrations. Likewise, Juniperus percentages drop to very Pollen Percentage Diagram

* / g/ £ POLLEN METER ZONES J

SUM + TAXON

Figure 6. Pollen percentage diagram for section EP-6-1.

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low levels, but concentrations remain close to values in the underlying Beaverdam Creek: EP-6-1 zone. It is, therefore, likely that Juniperus communis persisted in the com- Pollen Concentration Diagram munity. Climate must have become considerably warmer compared to Depth that represented by zone 4. .<4 Zones Zone 2 is perhaps the most difficult to interpret. Three hypotheses are presented. (1) The increase in haploxylon pine pollen could mean an increase in Pinus albicaulis in the forest, implying cool, dry conditions. (2) It could mean an increase in Pinus flexilis, the pine which currently grows at the lower treeline in the central Rocky Mountains and commonly at lower elevations than does Pinus contorta This interpretation would suggest a warm climate. It does, however, grow at upper treeline in the southern Rocky Mountains. (3) A pine not presently growing in the area may have been present when zone 2 sediments were deposited. Pinus monticola, a pine of the Pacific Northwest, now extends into western Montana (Little, 1971) and might have extended into this area. P. long- aeva, which presently ranges north to north-central Utah, might also have been present. Other haploxylon pines now occur no closer than California or the Midwest and are considered unlikely. Pseudotsuga or Larix pollen occurs with the haploxylon pine in the upper half of this zone, and the simplest interpretation is that the vegetation was a Pinus flexilis-Pseudo- tsuga menziesii forest. That vegetation is present below the Pinus contorta forest in this region (Fig. 2), and hypothesis 2 fits best with the modern biogeography and ecology. Picea and Abies were probably less abundant in the forests represented by zone 2 as compared to zone 3, arid Pseudotsuga became an ~i ' i 1 i 1 i ' i 1 i ' r increasingly important forest tree. The amoun t of unforested area present O 10 20 0I230I230I0I23 at that time is difficult to determine. The increase in Chenopodiineae Pollen concentration (lOOO's of grains/cc) percentages and concentrations could indicate that more areas were covered by this open-ground group of lowland plants. Selaginella densa Figure 7A. Pollen concentration diagram for section EP-6-1, becomes very uncommon in this same zone. It grows on open ground at trees. middle and upper elevations in Wyoming at present (Porter, 1962-1972), and it may have ¡given way to various Chenopodiineae as plants of lower elevation migrat<;d up and onto the Yellowstone Plateau. Other open- percentages and concentrations at this time. Herb and shrub pollen com- ground plants, including Artemisia, Sarcobatus, Ambrosia, Compositae: monly declined in percentages and concentrations in this zone fis com- Tubuliflorae, and Gramineae, remain at about the same percentages as in pared to zone 3. the previous zone. Open ground apparently persisted in the area as the Pseudotsuga forests presently grow mostly at lower elevations around climate became warmer and drier. the edge of the park (Fig. 1). It is present on the dry upland ridge of the Zone 1 shows an extended period of dominance by Pseudotsuga and peninsula between the arms of Yellowstone Lake ~ 10 km to the northwest has some of the highest values of this genus in the Rocky Mountain pollen of Beaverdam Creek, but only 1 of 4 surface samples within 6 km of the record. Higher viilues of Pseudotsuga pollen occur in modern (Heusser, site recorded Pseudotsuga pollen, and that sample contained <1%. The 1978) and Holocene (Barnosky, 1981; Tsukada, 1982) sediments in the presence of such forests in central areas in the park, as suggested by the Pacific Northwest but a different variety of the tree grows there than in the pollen of zone 1, indicates that climate was warmer than the modern Rocky Mountains. P. menziesii var. menziesii, the northwestern variety, climate and warmer than any climate within Holocene time (Baker.. 1981). forms dense forests of very large trees. P. menziesii var. glauca, the Rocky Mountain varietj', is a small tree of dry habitats, and it apparently pro- Section EP-6-2 duces much less pollen. Rocky Mountain surface samples seldom contain >5% Pseudotsuga pollen (Baker, 1976; Mack and others, 1978). Even The prospect of an interglacial section suggested by zone 1 in section samples from closed Pseudotsuga forests east of the Cascade Mountains EP-6-1 led to a visit to the site with G. M. Richmond in 1978 to collect seldom contain >10% douglas fir pollen (Mack and others, 1978). Fur- additional pollen and plant macrofossil samples. The lower pari: of the thermore, dispersal distance of Pseudotsuga pollen is small. Tsukada west-dipping deltaic sequence at the east side of the cut, from which (1982) shows throretically that 90% of Pseudotsuga pollen is deposited Richmond had previously collected samples, was covered, and a new within 800 m of the source forest. The high percentages at EP-6-1 strongly section overlapping and probably younger than the top of the original suggest that a closed douglas-fir forest existed at the site. The possibility section was exposed on the west side of the cut. This new exposure, that Larix, rather than Pseudotsuga, is represented by this pollen type is designated EP-6-2, was sampled for pollen and plant macrofossils (Figs. 8, ruled out by macrofossil identifications (see below). 9A, and 9B). Inspection of the pollen diagram (Fig. 8) suggests that only The Pinus in this zone ranges from ~2/3 haploxylon in the bottom zone 1 fromsectio n EP-6-1 is represented in EP-6-2. Fortunately, this zone part of the zone to 7/8 haploxylon near the top. As previously argued, the represents the significant interglacial component of the section. species is most likely Pinus flexilis. The most likely diploxylon species was The plant macrofossils are concentrated in a few silt beds (for thought to be P. contorta prior to macrofossil analysis of EP-6-2, and it still example, 6.75 and 6.SS m) rich in coarse organic material. Other silt beds may be a contributor to the pollen rain. Picea and Abies remain at low contain less obvious disseminated organic materials, and the inter bedded

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Beaverdam Creek: EP-6-1 Pollen Concentration Diagram

Zones

Figure 7B. Pollen concentration diagram for section EP-6-1, total pollen and herbs.

1 1 1 ' 1 ' 1 • i 1 ' 1 1 ' 1 ' 1 1 1 1 1 1 I 1 1 10 20 30 40 50 0 12 3 4 0 0 0 0 0 0 2 3 4 Pol Ieri concentration (1000's of qrains/cc) —

sands are barren. This variation in organic material and in numbers of The pollen and plant macrofossil assemblages are considered to be all fossils from one bed to another probably reflects changes in local deposi- in one zone which is more or less equivalent to zone 1 in section EP-6-1. tional environments rather than changes in vegetation. The variety of The interpretation of the vegetation is refined from the reconstruction fossils within an individual bed, however, certainly reflects the surrounding based on pollen from the EP-6-1 sequence. Needle identifications show vegetation. that Pseudotsuga, not Larix, was the dominant forest tree, that the The pollen sequence is dominated by arboreal pollen, and haploxylon haploxylon Pinus was P. flexilis,an d that one of the diploxylon pines was and diploxylon Pinus and Pseudotsuga pollen are most important (Fig. 8). P. ponderosa. Pseudotsuga does grow in open stands in the northeastern Small percentages of Picea and Abies are present along with traces of part of Yellowstone Park, and in a small stand on the peninsula between Betula, Salix, Juniperus, Quercus, Alnus, and Populus pollen. Artemisia isth e south and southeast arms of Yellowstone Lake, although it is common the most abundant of the non-arboreal pollen (NAP), with significant as a forest tree only at lower elevations (Figs. 2 and 5). It is widely amounts of Ambrosia, Compositae (Tubuliflorae and Liguliflorae), distributed in closed forests in the low valleys east of the Absaroka Moun- Sarcobatus, Chenopodiineae, Gramineae, and Cyperaceae. Occasional tains that form the eastern border of the park. Pinus flexilisca n occur as grains of Selagirtella densa, Polypodiaceae, and Isoetes are also present, high as 2,500 m in Yellowstone National Park and as high as 3,400 m in although they are not shown in the figures. Colorado, but it is important in the vegetation only at low elevations in The plant macrofossils are dominated by Pseudotsuga among the Montana and northwestern Wyoming (Table 1) (Steele and others, 1983; trees; Isoetes among the wetland plants; and Carex, Juncus, and Urtica Pfister and others, 1977). It grows on some of the driest sites capable of among the semiaquatics (Figs. 9A and 9B). Other important trees include supporting trees (Steele and others, 1983). It forms extensive stands on the a haploxylon Pinus, Pinus ponderosa, Picea, Abies lasiocarpa, Betula east side of the Absaroka Mountains between 1,830 and 2,285 m (Table occidentalis type, Salix, Populus balsamifera type, and Populus tremu- 1). Pinus ponderosa is limited to low elevations in the central Rocky loides. In the three levels that contained sufficiently well preserved needles, Mountains. In many places in Montana, the P. ponderosa series is the the haploxylon pine was P. flexilis. The variety of herbaceous plants is lowest forest zone encountered above the grasslands, and it also endures relatively small, but an interesting group has a number of weedy plants, dry conditions better than do most other conifers (Pfister and others, 1977). including several species of Chenopodium and Rumex and a Helianthus sp. In central Montana, where P. flexilis and P. ponderosa occur together, the A number of aquatic and semiaquatic plants are also present. P. ponderosa series lies at lower elevations than the P.flexilis series

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(Arno, 1979). In Montana, the P. ponderosa series commonly occurs land, and aquatic plants of lower elevations are represented, suggesting below -1,680 m, and the maximum elevation is -1,980 m (Pfister and that many habitats experienced the warmer conditions. The sjcond others, 1977). Other plants of commonly lower elevations are Betula hypothesis is favored for several reasons. First, the recent movements are occidentalis, Urtica dioica, Rumex maritimus, Chenopodium glaucum, in different directions, up within historic time and down sometime in the Cyperus aristatus, Eleocharis palustris, Scirpus acutus/validus, Typha,pas t 1,000 yr. Second, the actively moving area has been in the zone of Potamogeton filifomis, and Ranunculus pensylvanicus (Table 1). Clearly, ring fractures in the old caldera. Section EP-6 is outside of the caldera and a vegetational series including closed forest which is now typical of near a point which Pelton and Smith (1982) used as a reference bench much lower elevations grew along the east edge: of ancestral Yellowstone mark (one which had not likely moved). Third, the park was glaciated Lake when sediments of section EP-6-2 were deposited. Furthermore, 140,000 yr ago (Pierce and others, 1976) in much the same way it was topographic considerations indicate that this forest would have been during the Pinedale glaciation. Bull Lake glaciers were only slightly larger growing on north-facing slopes as well as at lake level. than were Pinedale glaciers, suggesting that elevations have not changed One hypothesis that would explain this plaint distribution is uplift of greatly during that time. In fact, by this reasoning, the area of the parle may the area since defosition of the sediments. What is now Yellowstone have been slightly higher 140,000 yr ago and, perhaps, also in the time National Park has clearly been tectonically active in the past 2 million yr. immediately following the 140,000-yr-old glaciation. The record is, there- Evidence of recent movements indicates that such movements are still fore, interpreted, although tentatively, as interglacial, with a climate suffi- going on. Pelton and Smith (1982) and Hamilton (1984) showed that ciently warmer than present to allow plants to grow at elevations 400 to uplift of as much as 14 mm/yr has occurred between 1923 and 1975 1,000 m higher. within a zone of ring fractures in the 0.6-million-yr-old caldera. W. L. Reconstruction of past communities remains somewhat tentative be- Hamilton (1984, oral commun.) has found submerged beaches 10 m cause the deltaic sequence could have received sediment from anywhere in below modern lake level, and this submergence occurred within the past the drainage basin, which presently ranges in elevation from 2,400 m 1,000 yr. A second hypothesis is that climate at that time was considerably (where the deltaic beds are exposed) to 3,350 m. It seems likely that the warmer than at present, allowing plants to grow at elevations 400 to plants above their present elevational limits would have been growing at 1,000 m higher than they do now. A variety of forest, open-ground, wet- the lowest altitudes possible in the area, namely, -2,400 m near the lake. The subalpine conifers Picea (probably P. engelmannii) and Abies lasiocarpa could also have been growing near the lake. They would have been close, however, to their present lower altitudinal limits of 2,740 m for Beaverdam Creek : EP-6-2 A. lasiocarpa and 2,440 m for P. engelmannii (Porter, 1962-1972). It Pollen Percentage Diagram seems more likely that they grew at higher elevations in the drainage aasin.

"ft™ POLLEN / METTER ZONES <3 NAP / c^V*^ 10.8tv- TBI H Barren sands 6.8-

6.0-;

Q> 5.2- C O N 4.4-i

4 Earren sands 4 0-

-0.8-

-1.6-Ì

r 1 > i •11111 ' i •1 ' i1 % 0 20 40 60 80 • =

Figure 8. Pollen percentage diagram for section EP-6-2.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/6/717/3445121/i0016-7606-97-6-717.pdf by guest on 29 September 2021 Beaverdam Creek : EP-6-2 Beaverdam Creek : EP-6-2 Plant Macrofossils(l) Plant Macrofossils(l)-continued Trees a n d sh r u b s Upland Her bs 7/ Aquatics 7 / / oi? / // O / S> ê // f £ O/ J/r / // Jt«/ DEPTH

6.8- 6.8-

6.0- 6.0-

5.2- 5.2-

-0.8- -0.8-

-1.6- -1.6-

-I—I—I—I—I—I—I—r—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—r~i—I—I—I—r—i—i—i 0 5 10 30 10 40 100 220 I—I—I—I—1—I—r -1—i—I—I—I—r—I—I—I—(—I—I—I—i—I—I—I—I—I—I—r-1—I—I—I—»—I—I—r 10 30 Note scale changes at dotted ¡¡nes Note scale change at dotted line Figure 9A. Plant macrofossil diagram for section EP-6-2, trees, shrubs, upland herbs, and aquatic plants. Dashed vertical lines on some curves mark changes in scale.

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TABLE 1. PRESENT ALTITUDES IN WYOMING AND VICINITY OF SELECTED PLANTS FOUND IN Needles can sometimes be seen in transit in modern streams, suggesting SEDIMENTS OF SECTION EP-6-2 that the fossil needles may have been derived from higher elevation, car-

ried downstream, and deposited in the delta. Plant Maximum Normal Optimal The vegetation near the site is interpreted as a Pseudotsuga menziesii- elevation (m) range (m) development (m)

Pinus flexilis-Pinus ponderosa forest with associated Populus tremuloides Pinusflexilis 2,500* 1,525-3,050+ 1,830-2^)0* and P. balsamifera. Some open areas with Artemisia, Chenopodium glau-Pinta ponderosa 1,980$ 790-1,650* Pseudotsuga meruiesii 2,135-2,590+ 1,980-2,3 »5 cum, Urtica dioica, Potentilla spp., and Selaginella densa were likely toBetula occidentalts 1,675-1,980* Populus balsami/era 1,220-2,440+ have been present in drier areas and in meadows. Riparian vegetation Populus tremuloides 1,830-3,050+ along the stream or lake shore probably included Betula occidental, Salix Sambucussp. 1,830-2^90* Unica dioica 2,590t spp., Eleocharis palustris, Cyperus aristatus, Carex spp., Juncus spp., Ro-Rumex maritima 1,930* 1,065-2^90+ Chenopodium glaucum 2,290+ Ranunculus pensylvanlcus rippa, and Mentha arvensis. A number of aqua tic species may have lived 1,070-1370+ Betula occidentalis middle elevations+ in ancestral Lake Yellowstone, in oxbow lakes on the floodplain, or in the Cyperus aristatus low elevations+ Eleocharis palustris stream. These include Potamogeton spp., Naias Jlexilis, Sparganium, low to middle elevations+ Isoetes bolanderi, Scirpus acutus or validus, and Typha. Scirpus vaUdus/acuaa low to middle elevations+ The sequenc; contains few, if any, plants: that do not occur in the Typha spp. low devations+ Rocky Mountain area at present, but some are not now known from Potamogeton fiHformis low to middle elevatiom+

Note: elevation of the site is 2,400 m. •Yellowstone National Park (D. G. Despain, 1981, written commun.). twyoming (Porter, 1962-1972). Beaverdam Creek : EP-6-2 ^Southern Montana (Pfister and others, 1977). Plant Macrofossils(2) Semi aquati Uncertai n 7

/ / Sb f . X?" q-^-A,-o £ 6-.V? DEPTH ^/f.Tá ïî» # o*

<5"

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 10 30 10 30 70 10 40 100 10 40 80 Note scale changes at dotted lines

Figure 9B. Plant macrofossil diagram for section EP-6-2, semiaquatic plants and plants of uncertain ecological habitats. Dashed vertical lines on some curves mark changes in scale.

Wyoming. Naias flexilis, an easily overlooked, submerged aquatic plant, Section EP-5 is not reported from Wyoming (Porter, 1962-1972) or Yellowstone National Park (Despain, 1975). It is a widespread aquatic plant, how- Section EP-5 is -0.3 km downstream from EP-6 on Beaverdam ever, and it is present in western Montana (Booth, 1950). Potamogeton Creek (Fig. 1). The section appears to be a continuation of the same foreset obtusifolius has its main distribution in the northeastern United States beds that occur in the EP-6 deltaic sequence, and it is stratigraphically (Muenscher, 1944) but it also occurs in Montana. Cyperus aristatus occurs above (but probably only slightly younger than) the EP-6 section. The in southeastern Wyoming (Porter, 1962-1972) and in Montana (Booth, stratigraphy of the two sections is similar (Fig. 5), and the sands and silts of 1950) but not in northwestern Wyoming. Perhaps the most interesting the foreset deltaic beds were sampled as at section EP-6. The uneven distributional change is that in Pinus ponderosa. It does not occur in sample intervals reflect the distribution of organic silt beds; intervening northwestern Wyoming or southwestern Montana (Little, 1971). Its near- beds are medium to coarse inorganic sands. est occurrences are a small band on the north edge of the Beartooth Range No substantial changes occur in the sequence at EP-5, so the se- 100 km north of Beaverdam Creek (and separated from it by the Bear- quence was treated as a single zone. The entire sequence is dominated by tooth Mountains) and an extensive area on the southern and eastern sides pollen and needles of Pinus (mainly haploxylon type), Picea, and Abies of the Bighorn Mountains >200 km to the east. These changes in plant (Figs. 10 and 11). Pseudotsuga pollen percentages are lower than those in distributions are not surprising in light of the age of the samples; in fact, it section EP-6-2, but they are consistently 1% to 3%. Other arboreal pollen is surprising that there are not more such changes. The antiquity of the types are minimal, and only Artemisia of the nonarboreal pollen is of Yellowstone and Rocky Mountain floras, however, is apparently rather consequence, averaging -15%. great. Richmond and others (1978) found that Pliocene pollen spectra Picea and Abies are the dominant trees that are represented by from what is now Yellowstone National Park were little different from macrofossils. Pseudotsuga needles are abundant in the extremely organic modern spectra. Barnosky (1984) described pollen from Miocene sedi- layer at 0.65 to 0.70 m and sparsely present at the top, and haploxylon mentary rocks near Jackson Hole a few kilometres south of Yellowstone pine needles are sparsely present throughout the section. A Pinus contorta- Park as essentially modern in aspect, and Leopold (1967) found that 90% type needle was present at one level. Betula fruits are present at several of the genera from Oligocene pollen floras in the region are present in the levels, and, at two levels, preservation was sufficiently to identify these as modern Rocky Mountain flora. B. cf. glandulosa. Bud scales of Salix, Populus cf. balsami/era, and P. cf. tremuloides were abundant at 0.65 to 0.70 m. Upland herbs and aquatic taxa are poorly represented in section EP-5. Rubus, Potentilla spp., and Selaginella densa are the only upland

Beaverdam Creek : EP-5 Pollen Percentage Diagram

Figure 10. Pollen percentage diagram for section EP-5.

J 2oT"sUlvr=rÄP+NAP| TAX ON SUM+TAXON

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Beaverdam Creek : EP-5 Plant Macrofossils

Figure 11. Plant macrofossil dia- Trees and Shrubs ~~"Z^Upla id/ gram for section EP-5. Dashed vertical // herb*/ lines on some curves mark changes in .W "7 scale. J / yS>S ¿Pi«? / .oVi? a â/s e> e> IN / f9¿ .OW jf r METER

1.0

0,5

—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r—i 10 60 110 10 30 10 30 10 60 110 Note scale changes at dotted lines

plants present, and Isóetes, Potamogetón, and charophytes are the only disappearance of the many taxa of low elevations recorded in the macro- aquatic plants. Weiland plants are well represented, both in abundance fossils of section EP-6-2. and diversity. Juncus and Carex are most common, but Typha, Scirpus, Urtica, Polygonum hydropiper, Rorippa, Elatine, Saxífraga argüía, Selagi-YELLOWSTONE AND MOUNTAIN SECTION nella selaginoides, and Callitriche all represent this group. Correspondence between pollen and plant macrofossils is again rea- A very large section more than 40 m high and 100 m long crops out sonably good. The relatively continuous needle record of haploxylon at the confluence of Mountain Creek (called "Monument Creek" on some Pinus, Picea, Abies, and Pseudotsuga match well with their pollen records. older maps) and the Yellowstone River (Fig. 1). It is designated as "section Diploxylon Pinus pollen is much less abundant, and only one needle of TOP-2" by G. M. Richmond (1984, written commun.) and is "section 2" Pinus contorta type was recorded. On the other hand, Betula fruits are of Richmond and Pierce (1972). In the section, laminated lake sediments more consistently present than is Betula pollen. Populus and Salix are underlie Pinedale Till. The section was sampled by G. M. Richmond and sporadic in both pollen and macrofossil remains. Emil Feuz in 1970, and I processed and analyzed the 16 samples from The forest was likely dominated by Picea, Abies, Pseudotsuga, and a 30 m of section. The resulting sketchy pollen diagram is shown in haploxylon Pinus. Needle preservation was not good enough to determine Figure 12. Pinus species, but the increased importance of Picea and Abies in both the Gradual changes occur on the diagram, although most are not pollen and macrofoiisil records suggests that perhaps Pinus albicaulis, the substantial, and the sequence is considered as a unit until more closely pine of higher elevarions, may have been present Populus balsamifera, P. spaced sampling is done. Artemisia pollen dominates throughout the entire tremubides, and Sa!ix were also probably present at the site. The scarcity section. Pollen of other predominantly herbaceous groups, like the Tubuli- of open-ground plants suggests that forests were fairly dense with few florae, Gramineae, and Cyperaceae, is relatively abundant as well. IHnus openings. The local habitat was apparently a marshy area on the delta. pollen percentages are extremely low throughout the section, and, al- Few aquatic plants are present, although the river and the lake could not though no single level had sufficient subgeneric identifications to plot, the have been far away. sum of all levels was almost entirely haploxylon type. Picea pollen is The climate indicated by section EP-5 is pro bably cooler than that of present continuously only in the upper half of the section. Both pine and the EP-6 sections. This climatic interpretation is suggested by the increased spruce percentages increase slightly upward in the section. Abies pollen is importance of Picea and Abies, the decrease of Pseudotsuga, and the nearly absent.

Beaverdam Creek : EP-5 Plant Macrofossils—continued

y^Aquatics/^" Sem ¡aquatics ^7/Uncertain Figure 11. {Continued). .4 J? J 4" J / P.e / if / Ol -Ö 5» c ê / / - /- g Ms « -<5 > O . / J&œm,* METER

h 1.5-

1.0-

0.5'

o-I UI I I I I—I I I I I I I I I I I I I I 10 40 Note scale change at dotted line

This pollen sequence is interpreted as representing cold, nearly glacial 1973) and in Greenland icecap cores (Dansgaard and others, 1971), as climate and tundra vegetation. Spores and pollen of some of the groups well as with sea-level changes, for example, from Barbados (Bender and previously found to be common in modern tundra and in tundra inferred others, 1979). Correlation of continental and oceanic records is relatively from fossil tundra profiles include Selaginella densa, Phlox type, Oxyria good (Kukla, 1977). type, Caryophyllaceae, and Rosaceae. These types, combined with the This paleoenvironmental history of the Yellowstone area begins with extremely low pine-pollen values, strongly suggest tundra conditions. the Bull Lake glaciation. An icecap covered what is now Yellowstone The pollen profile most strongly resembles that at section EP-8 along National Park, dated at -140,000 yr using obsidian-hydration techniques Beaverdam Creek (Baker and Richmond, 1978). There, a short interstadial (Pierce and others, 1976). This cold episode has been correlated (Pierce interval was present near the base, but the wide sampling interval could and others, 1976) with the latter part of Oxygen Isotope Stage 6 of easily have missed such a change in the Yellowstone and Mountain sec- Shackleton and Opdyke (1973) and with the Illinoian glaciation. No tion. Both sections underlie Pinedale Till, and both are thick sequences of vegetational records reflect this glacial interval, but the climate is assumed gray lake sediments that probably were part of an ancestral Yellowstone to have been cold. Lake. They are remnants in connected valley systems and are separated by Sections EP-6-1 and EP-6-2 indicate an environment warmer than <12 km. Direct tracing of units, however, is not possible. Correlation of that of the Holocene and known to be considerably older than 70,000 yr. these two sequences should be regarded as a working hypothesis. These sections, containing records of Pseudotsuga forests, with Pinus ponderosa and other plants above their present altitudinal limits, are con- DISCUSSION sidered to be Sangamon interglacial in age. The type section of the Sangamon in Illinois is not dated, but pollen records from Illinois (Griiger, By using stratigraphic position, radiometric and obsidian-hydration 1972; King, 1984) indicate that the climate was warmer than that of the dating, and correlation with other pollen and climatic sequences, the Holocene during presumed Sangamon time in what is now Illinois. Both pollen and plant macrofossil sequences in Yellowstone National Park can pollen records and oxygen isotope curves from the same marine section off be arranged to give a tentative environmental history for the past 140,000 the coast of Washington also indicated climate warmer than any Holocene yr (Fig. 13). This record can then be compared with the paleoclimatic climate during Oxygen Isotope Stage 5e (Heusser and Shackleton, 1979). record from oxygen isotopes in deep-sea cores (Shackleton and Opdyke, The warm peak of this climatic episode probably dates at —127,000 yr old

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(CLIMAP, 1984). This correlation remains tenative; the warmth-loving The Grassy Lake Reservoir sequence indicates a cold-to-warm-to- plants could have been growing there because the region was lower due to cold interstadial sequence with maximum temperatures only slightly tectonic movements. cooler than at present (Baker and Richmond, 1978). The pollen sequence The EP-5 pollen and plant-macrofossil sequences are dominated by indicates that conditions were not as warm as those indicated by the EP-6 haploxylon Pinus, Picea, and Abies. These taxa, along with Betula cf. profile, which is -200 m higher in elevation. This sequence lies ¡¡trati- glandulosa and the disappearance of the warmth-loving species of section graphically above the 140,000-yr-old Bull Lake Till, and it was apparently EP-6, suggest a cooler climate than that of EP-6. The continued presence deposited before the extrusion of the Pitchstone Plateau rhyolite flow, of Pseudotsuga doss not fit this interpretation. Three hypotheses could which is -70,000 yr old. It probably correlates with either the 105,000- or explain this seemir g anomaly. (1) Some trees of Pseudotsuga may have the 82,000-yr high stand of sea level (Bender and others, 1979) and with survived for a time in warm microhabitats after the climate became cooler. either Isotope Stage 5a or 5c (Shackleton and Opdyke, 1973). Climate (2) The flora may te disharmonious; all of the species that lived together at was apparently not quite as warm as present conditions during either of that time may not coexist at any one place at present. This concept is those periods. It is tentatively correlated with the 82,000-yr interval , Iso- common in vertebrate paleontology (see Lundelius and others, 1983, for tope Stage 5a. example). Specific climatic conditions may have prevailed that would Section EP-8, with its extended period' of very low Pinus and high allow for such occurrences. (3) The climatic tolerances of Pseudotsuga nonarboreal pollen, represents a long cold period, as discussed by 1 Baker may have changed since the deposition of these deposits. I have provision- and Richmond (1978). Slightly above the base, a slight rise in percentages ally accepted hypothesis 1 as correct, because most elements of the flora do of Pinus, Picea, and Abies indicates that the cold stadial conditions were presently occur together and indicate cool conditions. The cool episode interrupted by a short-lived, slightly warmer period. Near the base of the represented by EF'-5 is younger than are the EP-6 sequences and is sequence is an ash (Fig. 5) thought to be 70,000 yr old, and the associated probably part of the same depositional episode. It is tentatively correlated peat colleged by G. M. Richmond was dated at 68,000 +2,200 -1,700 yr with Oxygen Isotope Stage 5d and, if the correlation is true, would be B.P. (GrN-7332) by radiocarbon dating (Grootes, 1977). The section is, between 105,000 and 120,000 yr old. Unfortunately, there were no out- thus, younger than -70,000 yr B.P. A section at Solution Creek, ana- crops between sections EP-6 and EP-5 to connect these sequences. lyzed by J. P. Bradbury (Richmond and Bradbury, 1982), is likewise characterized by very low Pinus and high nonarboreal pollen percentages. This pollen sequence has been interpreted as representing widespread tundra vegetation, and it has a radiocarbon age of 54,000 ± 550 yr B.P. Yellowstone and Mountain Pollen Percentage Diagram

Figure 12. Pollen percentage diagram for Yellowstone and Mountain section.

GLACIATION: TIME CLIMATE VEGETATION % ol lull glacial SOURCE -IOPYRS. 100 — 50-r— 0 lodgepole pine- cool spruce Buckbean fen lodgepole pine warm parkland cool Baker,1976 Richmond, none very cold 20 — 1969

40 — Figure 13. Suggested environmental history of Yellow- stone National Park for the past ? - ? -?-?-?- Solution Cr. 140,000 yr. Richmond & Bradbury, 60 — tundra cold 1982 EP-8 Baker and ßa_rkTan

100 —

sqruce -7i r- pj ne_ co oT EP-5

120 douglas fir very warnr EP-6 Baker mixed pine warm and spruce-fir- cool Richmond, _ _whjtebark pine_ 1978 tunBra I I Pierce 140 none very cold et al. 1976

(GrN-8140) (Richmond and Bradbury, 1982) near the top of the section. Presumably, climates were not cold enough for glaciers to form, and The sequence resembles the EP-8 profile, although it is a much smaller climatic records elsewhere suggest that this period was probably cool to section. These two sections are placed as part of the same cold interval cold as well. (Fig. 13). The section at Yellowstone and Mountain Creek (section TOP-2) is similar and may also represent this time period. ACKNOWLEDGMENTS No pollen sections are presently known from -50,000 to 14,000 yr in Yellowstone National Park. Pierce and others (1976) suggested that I am grateful to the U.S. Geological Survey for funding Project what is now Yellowstone Park was covered by Pinedale ice from -30,000 9530-02530, Interglacial Climates of the Northern Rocky Mountains to 14,000 yr B.P. In the Colorado Front Range, the Pinedale glaciation (G. M. Richmond, U.S. Geological Survey, Project Chief). I thank Don occurred from 22,500 to ~ 12,500 yr B.P. (Madole and others, 1984; Legg Despain (National Park Service) for providing information on the distribu- and Baker, 1980). Few parts of what is now Yellowstone National Park tion of several plant species in Yellowstone National Park; he and Lon were exposed above the icecap for at least 10,000 to perhaps 16,000 yr or Drake (University of Iowa) also helped sample some of the sections. I more during the Pinedale glaciation (Pierce, 1979). Nothing is known especially thank G. M. Richmond for help in all phases of the project and about the 20,000+ yr prior to glaciation (50,000 to 30,000 yr B.P.). for his faith that it would eventually be finished.

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