Late quaternary floodplain sedimentation along the Pomme de Terre River,southern Missouri

New cross sections and dates from along the Pomme de Terre River clarify the complex local history of valley development and floodplain sedimentation. The observed history begins with a series of ancient bedrock strath terraces that record past bedrock valley positions at 15.5 to more than 58 m above the modern bedrock floor. Each strath is capped by 1–2 m of channel gravel and sand permeated by red clay. Sometime previous to ca. 140,000 yr B.P., a much lower bedrock valley only about 5–6 m above the modern level was excavated. By 140,000 yr B.P., accumulation of red and gray mottled silty clay had commenced, and had reached to 8.5 m above the modern floodplain before 48,900 ± 900 ¹⁴C yr B.P. Sometime between ca. 49,000 and 45,000 ¹⁴C yr B.P., erosion caused abandonment of an oxbow meander, and lowered the bedrock valley to about its present depth. Younger yellowish-red and gray mottled silty clay alluvium then began accumulating. This mid-Wisconsinan fill reached to 2.5 m above the modern floodplain sometime before 31,800 ± 1340 ¹⁴C yr B.P., at which time another erosional phase was in progress. A late Wisconsinan olive clay accumulated between 27,480 ± 1950 and ca. 23,000 ¹⁴C yr B.P., followed by approximate stability until 13,550 ± 400 ¹⁴C yr B.P. After stability, an erosional episode began, but by 10,200 ± 330 ¹⁴C yr B.P., deposition of a distinctive brown clayey silt was underway. This early Holocene fill reached to about the same level as the mid-Wisconsinan fill by 8100 ± 140 ¹⁴C yr B.P. Erosion occurred between this date and 7490 ± 170 ¹⁴C yr B.P., but the former floodplain level was rapidly reattained, and was apparently stable until ca. 5000 ¹⁴C yr B.P. Finally, erosional unconformities and 17 dates from the brown clayey silt, and from younger grayish-brown silty sand underlying the modern floodplain, record subsequent episodes of floodplain erosion at ca. 5000, 2900, 1500 and 350 ¹⁴C yr B.P. The timing of Pomme de Terre floodplain sedimentary regimes, characterized by net aggradation, erosion, or stability, may have been controlled by climate. In particular, both periods of stability appear to have been coeval to times of strongly zonal upper atmospheric circulation. Intensified zonal circulation would have resulted in less frequent large floods and an increased dominance by floods of small to moderate size. In contrast, there are no obvious parallels to be drawn between this local alluvial history and sea level or glacial outwash induced baselevel changes.     

The impact of fluvial processes and landscape evolution on archaeological sites and settlement patterns along the San Xavier reach of the Santa Cruz River,Arizona

The Santa Cruz arroyo in the Tucson Basin, Arizona, has undergone major environmental changes over the last 8000 years. the Holocene stratigraphy along a 15 km segment of the arroyo, known as the San Xavier reach, which traverses the San Xavier Indian Reservation was investigated in detail. the Holocene alluvial sequence reveals that aggradation occurred until 8000 yr B.P. within a braided stream, was followed by a major period of channel erosion and widening from 8000 to 5500 yr B.P., which, in turn, was followed by vertical aggradation of the floodplain and five short periods of channel cutting. Broad climatic changes are correlated with major changes in the fluvial regime and landscape. However, the cycles of arroyo cutting and filling, during the semiarid climate of the last 2500 years, were probably the result of the creation of unstable internal geomorphic conditions, flooding, and human impacts on the floodplain. the alluvial history of the San Xavier reach has had a pronounced effect on the preservation and completeness of the archaeological record. Periods of erosion have created absences or gaps in the archaeological record and deep burial has removed some of the preserved archaeological remains from view. Environmental changes on the floodplain also influenced late prehistoric Hohokam settlement and subsistence patterns in the San Xavier region.     

An 8000-year record of vegetation, climate, and human disturbance from the Sierra de Apaneca, El Salvador

An ∼8000-cal-yr stratigraphic record of vegetation change from the Sierra de Apaneca, El Salvador, documents a mid-Holocene warm phase, followed by late Holocene cooling. Pollen evidence reveals that during the mid-Holocene (∼8000-5500 cal yr B.P.) lowland tropical plant taxa were growing at elevations ∼200-250 m higher than at present, suggesting conditions about 1.0°C warmer than those prevailing today. Cloud forest genera (Liquidambar, Juglans, Alnus, Ulmus) were also more abundant in the mid-Holocene, indicating greater cloud cover during the dry season. A gradual cooling and drying trend began by ∼5500 cal yr B.P., culminating in the modern forest composition by ∼3500 cal yr B.P. A rise in pollen from weedy plant taxa associated with agriculture occurred ∼5000 cal yr B.P., and pollen from Zea first appeared in the record at ∼4440 cal yr B.P. Human impacts on local vegetation remained high throughout the late Holocene, but decreased abruptly following the Tierra Blanca Joven (TBJ) eruption of Volcán Ilopango at ∼1520 cal yr B.P. The past 1500 years are marked by higher lake levels and periodic depositions of exogenous inorganic sediments, perhaps indicating increased climatic variability.     

Quaternary stratigraphy and geoarchaeology of the colorado and concho rivers,west texas

Geoarchaeological investigations in an area surrounding the confluence of the upper Colorado and Concho Rivers, Edwards Plateau of West Texas, have produced a detailed landscape evolution model which provides a framework for discussion of the influences of geomorphic processes on the development, preservation, and visibility of the archaeological record. Field mapping within the study area has differentiated six allostrati-graphic units of fluvial origin in both valleys, as well as extensive eolian sand sheets along the Colorado River. Early to middle Pleistocene terrace remnants cap many upland areas, whereas two distinct late Pleistocene terrace surfaces are widespread within the study area at somewhat lower elevations. Fluvial activity during the time period of human occupation is represented by an extensive Holocene terrace and underlying valley fill, which is up to 11 m in thickness. Valley fill sediments can be subdivided into allostratigraphic units of early to middle Holocene (ca. 10,000–5000 yr B.P.) and late Holocene age (ca. 4600–1000 yr B.P.), which are separated by a buried soil profile. The modern incised channels and very narrow floodplains represent the last millennium. Eolian sand sheets of early to middle Holocene age overlie limestone- and shale-dominated uplands, Pleistocene terraces, and in some cases the Holocene valley fill along the Colorado River. Pleistocene terraces have been stable features in the landscape and available for settlement through the time period of human occupation. Archaeological materials of all ages occur at the surface, and the record preserved in individual sites range from that associated with discrete periods of activity to longer-term palimpsests that represent repeated use over millennia. Sites within early to middle Holocene and late Holocene fills represent short-term utilization of constructional floodplains during the Paleoindian through early Archaic and middle to late Archaic time periods respectively. By contrast, those that occur along the buried soil profile developed in the early to middle Holocene fill record middle to late Archaic cultural activity on stable terrace surfaces, and represent relatively discrete periods of activity to longer-term palimpsests that represent repeated use over the 3000–4000 years of subaerial exposure. Late Prehistoric sites occur as palimpsests on soils developed in late Holocene alluvium or stratified within modern floodplain facies. Paleoindian through Late Prehistoric sites occur stratified within eolian sand sheets or along the unconformity with subjacent fluvial deposits. The landscape evolution model from the upper Colorado and Concho Rivers is similar to that developed for other major valley axes of the Edwards Plateau. This model may be regionally applicable, and can be used to interpret the geomorphic setting and natural formation processes for already known sites, as well as provide an organizational framework for systematic surface and subsurface survey for new archaeological records. 0 1992 John Wiley & Sons, Inc.     

Late Quaternary Floodplain History of the Brazos River in East-Central Texas

The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.     

Hunter-gatherers in the nashville basin: Archaeological and geological evidence for variability in prehistoric land use

Results of intensive archaeological surveys conducted within two environmentally contrastive settings in the Nashville Basin of Tennessee demonstrate that prehistoric human activity was much more extensive in the inner Basin where the Duck River floodplain was flanked by patchy upland vegetation relative to the outer Basin where upland vegetation was more homogeneous. Vertebrate remains from caves and rockshelters show that the inner Basin supported such patchy upland vegetation throughout the known period of hunter-gatherer occupation of the area. There is close correspondance between periods when forest openings were most prevalent (i.e., Late Pleistocene/Early Holocene and Mid-Holocene) and times when prehistoric human occupation of the inner Basin was most intensive (Paleoindian and Middle Archaic). Comparisons of major artifact categories from systematic collections of various landform surfaces show that the T2 terrace was the most extensively utilized by prehistoric hunter-gatherers. the T3/valley slope, the uplands, and the T1 produced progressively lower artifact densities. Results of systematic backhoe trenching indicate that substantial Holocene aggradation accounts for the seemingly minor use of the T1.     

Holocene Lakes from Ramlat as-Sab'atayn (Yemen) Illustrate the Impact of Monsoon Activity in Southern Arabia

Paleoecology and paleohydrology of the Ramlat as-Sab'atayn (Southern Arabia) are reconstructed from a comparative study including sedimentology, mineralogy, stable isotope ratios of carbonates, and palynology of lacustrine sediments recovered from the al-Hawa depression. The section dates from 8700 to 7200 yr B.P. and records an early phase of flooding followed by distinct lacustrine development from 7800 to 7200 yr B.P., coeval with maximum activity of the Indian monsoon. Comparison of the pollen record with modern pollen deposition suggests that regional vegetation was then already of desert type and was related to strong seasonal trade winds.     

Evidence for multiple, episodic, mid-Holocene Hypsithermal recorded in two soil profiles along an alluvial floodplain catena, southeastern Tennessee, USA

Floodplain soil-paleosol successions are valuable archives for reconstructing Pleistocene-Holocene climate changes but have been relatively unstudied in the southern Appalachian region. Two soil profiles on a small floodplain in southeastern Tennessee, USA were described and sampled in detail using both pedological and geological approaches, including stable carbon isotope analysis of soil organic matter (SOM). Correlation between the 2 profiles was constrained by uncalibrated AMS ¹⁴C ages of bulk humates, and using SOM δ¹³C values, both mobile and immobile elements. Four distinct 2.5-4‰ shifts towards less negative δ¹³C values for SOM suggest ∼ 300-yr cyclicity and transient warmer and drier climate events, with either water-stressed C₃ vegetation or as much as 35% C₄ plants present during the mid-Holocene. These postulated multi-episodic drier climate conditions have never before been documented in the southern Appalachian region and are tentatively correlated with mid-Holocene warming and drying in the eastern US, the nearly time-equivalent mid-Holocene events documented in Texas, the US High Plains and in the Gulf of Mexico. High rates of floodplain sediment accumulation (0.5-3 mm/yr), high clay content and maintenance of poorly drained soil conditions favor preservation of high-resolution climate archives in floodplain deposits by inhibiting oxidation and translocation of organic C.     

Postglacial Vegetation and Climate of the Cascade Range, Central Oregon

Pollen data from two sites provide information on the postglacial vegetation and climate history of the Cascade Range. Indian Prairie in the western Cascade Range was colonized by subalpine forests of Pinus, Picea, and Tsuga and open meadows prior to ca. 12,400 ¹⁴C yr B.P. The treeline lay 500 to 1000 m below its modern elevation and conditions were cooler than at present. From ca. 12,400 to ca. 9950 ¹⁴C yr B.P. Abies became important and the forest resembled that presently found at middle elevations in the western Cascade Range. The pollen record implies a rise in treeline and warmer conditions than before. From ca. 10,000 to 4000-4500 ¹⁴C yr B.P., conditions that were warmer and effectively drier than today led to the establishment of a closed forest composed of Pseudotsuga , Abies, and, at lower elevations, Quercus and Corylus . During this period, Gold Lake Bog in the High Cascades was surrounded by closed forest of Pinus and Abies. The early-Holocene pollen assemblages at both Indian Prairie and Gold Lake Bog lack modern analogues, and it is likely that greater-than-present summer radiation fostered unique climatic conditions and vegetation associations at middle and high elevations. In the late Holocene, beginning ca. 4000-4500 ¹⁴C yr B.P., cooler and more humid conditions prevailed and the modern vegetation was established. A comparison of these sites with others in the Pacific Northwest suggests that major patterns of vegetational change at individual sites were a response to large-scale changes in the climate system that affected the entire region.     

A 23,000-yr pollen record from Lake Euramoo, Wet Tropics of NE Queensland, Australia

A new extended pollen and charcoal record is presented from Lake Euramoo, Wet Tropics World Heritage rainforest of northeast Queensland, Australia. The 8.4-m sediment core taken from the center of Lake Euramoo incorporates a complete record of vegetation change and fire history spanning the period from 23,000 cal yr B.P. to present. The pollen record is divided into five significant zones; 23,000–16,800 cal yr B.P., dry sclerophyll woodland; 16,800–8600 cal yr B.P., wet sclerophyll woodland with marginal rainforest in protected pockets; 8600–5000 cal yr B.P., warm temperate rainforest; 5000–70 cal yr B.P., dry subtropical rainforest; 70 cal yr B.P.–AD 1999, degraded dry subtropical rainforest with increasing influence of invasive species and fire.The process of rainforest development appears to be at least partly controlled by orbital forcing (precession), though more local environmental variables and human activity are also significant factors. This new record provides the opportunity to explore the relationship between fire, drought and rainforest dynamics in a significant World Heritage rainforest region.     

Late Quaternary vegetation in the Southwestern Columbia Basin,Washington

A 33,000-yr pollen record from Carp Lake provides information on the vegetation history of the forest/steppe border in the southwestern Columbia Basin. The site is located in the Pinus ponderosa Zone but through much of late Quaternary time the area was probably treeless. Pollen assemblages in sediments dating from 33,000 to 23,500 yr B.P. suggest a period of temperate climate and steppe coinciding with the end of the Olympia Interglaciation. The Fraser Glaciation (ca. 25,000–10,000 yr B.P.) was a period of periglacial steppe or tundra vegetation and conditions too dry and cold to support forests at low altitudes. Aridity is also inferred from the low level of the lake between 21,000 and 8500 yr B.P., and especially after about 13,500 yr B.P. About 10,000 yr B.P. Chenopodiineae and other temperate taxa spread locally, providing palynological evidence for a shift from cold, dry to warm, dry conditions. Pine woodland developed at the site with the onset of humid conditions at 8500 yr B.P.; further cooling is suggested at 4000 yr B.P., when Pseudotsuga and Abies were established locally.     

An 11 000-year record of vegetation and environment from Lago di Martignano,Latium, Italy

A pollen diagram from Lago di Martignano, a maar lake in central Italy, provides an 11000-year record of vegetation and environment change. The earliest pollen spectra are dominated by Artemisia and Gramineae, representing late glacial steppe vegetation typical of the Mediterranean region. Broad-leaved forests were established by ca. 11 000 yr BP. Although Quercus initially dominated their canopy, a wide range of other mesophyllous trees were also present. Pollen values for sclerophyllous tree and shrub taxa characteristic of Mediterranean woodlands and scrub are initially low (<10%). After ca. 7000 yr BP, however, they begin to increase and rise to a peak of >40% of total land pollen at ca. 6700 yr BP, with Olea europaea the single most abundant taxon. Human influence upon the vegetation only becomes significant somewhat after this peak, with progressive clearance of woodland and expansion of herbaceous communities. Castanea sativa and luglans regia pollen is recorded consistently from the beginning of the rise in pollen values for taxa characteristic of Mediterranean scrub communities. Pollen values for arable crops increase progressively after ca. 5500 yr BP, following the peak pollen values for taxa characteristic of Mediterranean scrub vegetation. Late glacial and Holocene climate changes have been complex in this region, with the present character of the climate developing only during the last millennium. Rates of change of pollen spectra peak during this period.     

Late glacial and postglacial pollen and plant macrofossils from Lake West Okoboji, Northwestern Iowa

Pollen and plant macrofossils preserved in lake sediment from Lake West Okoboji, Dickinson County, Iowa, indicate how the vegetation of that area changed during the late glacial and postglacial. A closed coniferous forest, dominated by spruce and larch trees, produced the Picea-Larix pollen assemblage zone. Fir trees were a minor constituent of this forest; pine trees were probably absent. Black ash trees increased in abundance at Lake West Okoboji and by 13,500 yr ago were an important constituent of the forest. The sediment accumulation rate and the pollen influx were low throughout this time. Birch and alder pollen peaked in abundance approximately 11,800 yr ago. Pollen influx increased rapidly as birch and alder replaced coniferous trees on the uplands. A deciduous forest, containing abundant oak and elm trees, replaced the birch-alder-coniferous forest. This forest inhabited northwestern Iowa from approximately 11,000 to 9000 yr B.P. Nonarboreal species became prevalent between approximately 9000 and 7700 yr B.P. as prairie began to replace deciduous forest on the uplands. Charred remains of Amorpha canescens and other upland species attest to the presence of prairie fires as an aid in establishing prairie and destroying the forest. The pollen influx declined. The warmest, driest part of the postglacial occurred in northwestern Iowa from approximately 7700 to 3200 yr ago. Lake level fell 9 to 10 m, and prairie extended to the edge of the lake. Wet-ground weeds inhabited areas near lake level which were alternately flooded, then dry. Pollen influx was approximately 100 grains/cm²/yr during the driest time in this dry interval.Deciduous trees, particularly oaks, returned after approximately 3200 yr B.P. Prairie continued to occupy the uplands but trees were more common in the lowlying wet areas. Settlement by Europeans in northwestern Iowa about 1865 is marked by an increase in weed pollen. Macrofossil deposition changed in 1910 in response to the stabilization of lake level.     

Environmental history since 11,000 C yr B.P. of the northeastern Pampas, Argentina, from alluvial sequences of the Luján River

Sedimentological, malacological, and pollen analyses from ¹⁴C-dated alluvial sections from the Luján River provide a detailed record of environmental changes during the Holocene in the northeastern Pampas of Argentina. From 11,200 to 9000 ¹⁴C yr B.P., both sedimentary and biological components suggest that the depositional environment was eutrophic, alkaline, and freshwater to brackish shallow water bodies without significant water circulation. During this time, bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid, suggesting an increase in precipitation. Short-term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies, as reduction of the volume alternated with periods of flooding. The beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one. From 9000 and 7000 ¹⁴C yr B.P., mesotrophic, alkaline, brackish, probably anoxic swamps existed. Between 7000 and 3000 ¹⁴C yr B.P., anoxic calcareous swamps were formed, with subaerial exposure and development of the Puesto Berrondo Soil (3500-2900 ¹⁴C yr B.P.). A trend to a reduction of water bodies is recorded from 9000 to ca. 3000 ¹⁴C yr B.P., with a significant reduction after ca. 7000 ¹⁴C yr B.P. A shift to subhumid-dry climate after 7000 ¹⁴C yr B.P. appears to be the main cause. During this time, an additional external forcing toward higher groundwater levels was caused by Holocene marine transgression causing changes in the water bodies levels. The climate became drier during the late Holocene (ca. 3000 yr B.P.), when clastic sedimentation increased, under subhumid-dry conditions. Flood events increased in frequency during this time. From ca. A.D. 1790 to present, the pollen record reflects widespread disturbance of the vegetation during the European settlement.     

Vegetation history of the English chalklands: a mid‐Holocene pollen sequence from the Caburn,East Sussex

A pollen diagram has been produced from the base of the Caburn (East Sussex) that provides a temporally and spatially precise record of vegetation change on the English chalklands during the mid‐Holocene (ca. 7100 to ca. 3800 cal. yr BP). During this period the slopes above the site appear to have been well‐wooded, with vegetation analogous to modern Fraxinus–Acer–Mercurialis communities in which Tilia was also a prominent constituent. However, scrub and grassland taxa such as Juniperus communis, Cornus sanguinea and Plantago lanceolata are also regularly recorded along with, from ca. 6000 cal. yr BP onwards, species specific to Chalk grassland (e.g. Sanguisorba minor). This supports suggestions that elements of Chalk grassland persisted in lowland England through the Holocene. Such communities are most likely to have occupied the steepest slopes, although the processes that maintained them are unclear. Human interference with vegetation close to the site may have begun as early as ca. 6350 cal. yr BP and initially involved a woodland management practice such as coppicing. From the primary Ulmus decline (ca. 5700 cal. yr BP) onwards, phases of limited clearance accompanied by cereal cultivation occurred. Taxus baccata was an important component of the woodland which regenerated between these phases. Copyright © 2000 John Wiley & Sons, Ltd.     

Late Holocene δC and pollen records of paleosalinity from tidal marshes in the San Francisco Bay estuary, California

Records of stable carbon isotopes (δ¹³C) are presented from cores collected from four San Francisco Bay marshes and used as a proxy for changes in estuary salinity. The δ¹³C value of organic marsh sediments are a reflection of the relative proportion of C₃ vs. C₄ plants occupying the surface, and can thus be used as a proxy for vegetation change on the marsh surface. The four marshes included in this study are located along a natural salinity gradient that exists in the San Francisco Bay, and records of vegetation change at all four sites can be used to infer changes in overall estuary paleosalinity. The δ¹³C values complement pollen data from the same marsh sites producing a paleoclimate record for the late Holocene period in the San Francisco Bay estuary. The data indicate that there have been periods of higher-than-average salinity in the Bay estuary (reduced fresh water inflow), including 1600-1300 cal yr B.P., 1000-800 cal yr B.P., 300-200 cal yr B.P., and ca. A.D. 1950 to the present. Periods of lower-than-average salinity (increased fresh water inflow) occurred before 2000 cal yr B.P., from 1300 to 1200 cal yr B.P. and ca. 150 cal yr B.P. to A.D. 1950. A comparison of the timing of these events with records from the California coast, watershed, and beyond the larger drainage of the Bay reveals that the paleosalinity variations reflected regional precipitation.     

Development of a tidal marsh in a New England river valley

A model for the geomorphic and vegetation development of a river valley tidal marsh in southern New England (Connecticut) is based on both the species composition of roots and rhizomes and on the mineralogic sediments preserved in peat. The maximum depth of salt marsh peat is 3.8 m and in the deepest areas this can overlie up to 1.9 m of fresh to brackish water peat. Based on a radiocarbon date of 3670±140 yr before the present (B.P.) for basal peat at a depth of 4.0 m, vertical accretion rates have averaged ca. 1.1 mm yr^?1. Salt marsh formation began in response to rising sea level 3800–4000 yr B.P., as brackish marshes, dominated by bulrush (Scirpus sp.), replaced freshwater wetlands along stream and river channels. Gradually salt marsh vegetation developed over submerging brackish marshes, adjacent uplands, and accreting tidal flats. By 3000 yr B.P. the lower estuary was tidal, with sufficient salinity for salt marsh to dominate most wetlands. Spikegrass (Distichlis spicata) was an important early colonizer in salt marsh formation and its role in marsh development has not been documented previously. Blackgrass (Juncus gerardi), currently a typical upper border species, appears in the peat record relatively recently, perhaps within the last few centuries. In contrast, reed (Phragmites australis) has been present for at least 3500 yr. The dominance of reed along the upper border today, however, appears to be a relatively recent phenomenon.     

Middle Wisconsin stratigraphy and climatic reconstruction,southern Vancouver Island,British Columbia

Interbedded, organic-rich terrestrial and marine sediments exposed along the eastern coastal lowland of Vancouver Island contain an almost continuous record of middle Wisconsin vegetation and climate. The record has been interpreted largely from palynostratigraphic studies at three sites and supported by a study of modern pollen spectra from the three major biogeoclimatic zones of the extant vegetation. Radiocarbon dates from a variety of organic materials in the middle Wisconsin beds reveal that the fossil pollen spectra span an interval ranging from approximately 21,000 yr B.P. to more than 51,000 yr B.P. The spectra are divided into eight major pollen zones encompassing the Olympia Interglaciation and early Fraser Glaciation geologicclimate units of the Pacific Northwest. The Olympia Interglaciation extended from before 51,000 yr B.P. to ca. 29,000 yr B.P. and was characterized by a climate similar to present. During the early Fraser Glaciation, from 29,000 years ago to approximately 21,000 yr B.P., climate deteriorated until tundra like conditions prevailed. These pollen sequences are correlative with those of coastal British Columbia and partly with those from Olympic Peninsula, but apparently are not comparable with events in the Puget Lowland.     

Paleoecological and archaeological implications of a late Pleistocene/Early holocene record of vegetation and climate from the pacific coastal plain of panama

A phytolith record from Monte Oscuro, a crater lake located 10 m above sea level on the Pacific coastal plain of Panama, shows that during the Late Pleistocene the lake bed was dry and savanna-like vegetation expanded at the expense of tropical deciduous forest, the modern potential vegetation. A significant reduction of precipitation below current levels was almost certainly required to effect the changes observed. Core sediment characteristics indicate that permanent inundation of the Monte Oscuro basin with water occurred at about 10,500 ¹⁴C yr B.P. Pollen and phytolith records show that deciduous tropical forest expanded into the lake’s watershed during the early Holocene. Significant burning of the vegetation and increases of weedy plants at ca. 7500 to 7000 ¹⁴C yr B.P. indicate disturbance, which most likely resulted from early human occupation of the seasonal tropical forest near Monte Oscuro and the development of slash-and-burn methods of cultivation.