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1.
Accelerator mass spectrometer radiocarbon ages of the Roxana Silt (loess) along the Upper Mississippi Valley of Wisconsin and Minnesota indicate that loess sedimentation of the Roxana Silt occurred between about 55,000 and 27,000 14 C yr B.P. However, due to local environmental controls, the basal age at any given site may range from 55,000 to 35,000 14C yr B.P. The radiocarbon ages presented here are in agreement with previous radiocarbon ages for the Roxana Silt in its type area of west-central Illinois, but indicate that long-term sedimentation rates along the bluffline of the Upper Mississippi Valley were very slow (4-8 cm/1000 yr) compared to long-term sedimentation rates along the bluffline of the type area (40-70 cm/1000 yr). Comparison of radiocarbon ages for midcontinent middle Wisconsinan loess deposits indicates that sedimentation along the Mississippi River valley may have preceded loess sedimentation along the Missouri River valley by as much as 20,000 yr or that basal ages for middle Wisconsinan loess along the Missouri Valley are erroneously young. The bracketing ages for the Upper Mississippi Valley Roxana Silt indicate that the Mississippi River valley was receiving outwash sedimentation between 55,000 and 27,000 14C yr B.P.  相似文献   

2.
Matrix-supported diamicton and uniform to laminated, silty, fine-grained sediment deposited from about 42,500 to 27,600 cal yr B.P. under slackwater conditions nearly filled two caves in southwestern Illinois. At some point, most of the sediment was flushed from the caves and from about 22,700 to 4000 cal yr B.P., floods deposited a drape of sandy and silty sediment on remnant slackwater successions, cobbly alluvium, and bedrock (especially from 7700 to 4000 cal yr B.P.). Clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly Petersburg Silt, a smectite- and illite-rich proglacial lacustrine unit present in the overlying Illinois Episode glacial succession. Today, remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and, in at least two locations, cover a cobble-mantled strath terrace 1.3 to 1.5 m above active stream channels. Slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits. Slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes (in addition to sinkhole formation) and enter caves; however, the finer organics, some of them comminuted during transport into the caves, become part of the cave alluvium. This finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods. Twenty-four 14C ages on humified organic matter provide chronologic control. The δ13C values of the organic matter reflect the proportion of C4-type to C3-type vegetation growing in and around swallets and sinkholes at the time of redeposition. Drought-tolerant C4-type vegetation was more prevalent relative to C3-type vegetation from 42,500 to 31,200 cal yr B.P. compared to conditions from 28,800 cal yr B.P. to the present. The δ13C values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols.  相似文献   

3.
The Paso Otero Locality is a cluster of archaeological sites within the middle Río Quequén Grande basin located in the northern coastal plain of Argentina. The valley fill is Holocene alluvial, eolian, and palustrine sediment, including the top of the Guerrero Member (∼10,000 14C yr B.P.), upper Río Salado Member (∼3000 14C yr B.P.), and lower La Postrera Formation (∼2400 14C yr B.P.). Regional soils include the Puesto Callejón Viejo (10,000 to 9400 14C yr B.P.) and the Puesto Berrondo (∼4800 14C yr B.P.). Radiocarbon sampling of buried A‐horizons on both sides of the river produced 17 dates considered reliable. The geoarchaeological information allows exploration of the implications for the formation of the archaeological record. Similar geological processes of differential intensity have resulted in contexts of different archaeological resolution and integrity. The Paso Otero Locality provides both a local and regional view of late Quaternary events and processes for the middle basin of the Río Quequén Grande.  相似文献   

4.
The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of 230Th/234U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with 14C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.  相似文献   

5.
The Nolan site (16MA201), 14C dated 5200–4800 cal yr B.P. and located in the Tensas Basin of northeastern Louisiana, is the only recorded Middle Archaic mound site in the alluvial valley of the Mississippi River. Alluvial deposition has buried the Nolan site under 3–4 m of Holocene sediment, prohibiting traditional excavation of the site. Because data are unattainable by other means, soil coring and subsequent stratigraphic and sedimentological analyses permit reconstruction of the natural and cultural depositional history of the Nolan site. The sedimentary characteristics of basal deposits within cores suggest the presence of an Arkansas River paleochannel immediately adjacent to the site. Chronostratigraphic data show this channel was no longer active by ca. 5200 cal yr B.P. Contrary to existing models, the Arkansas River Meander Belt 4 and the Mississippi River Meander Belt 4 are not the same age. Microartifact and losson‐ignition analyses of sediment identify natural versus cultural strata and permit the identification of artificial constructions—including four earthen mounds and one earthen ridge—at the Nolan site. Overbank sediments attributed to a mapped Mississippi River Stage 4 meander belt are dated ca. 4800–3800 cal yr B.P. This age is considerably younger than previous estimates and demonstrates the existing chronological models for Mississippi River meander belts must be carefully assessed. Core analyses also reveal flood‐related crevasse splays deposited throughout the Tensas Basin after the occupation of the Nolan site. These deposits serve as relative chronological indicators and aid in stratigraphic assessments of the Nolan site. Reconstruction of the earthworks and their stratigraphic context reveals one of the largest and earliest Middle Archaic mound sites in North America. © 2006 Wiley Periodicals, Inc.  相似文献   

6.
This study evaluates the accuracy of optically stimulated luminescence to date well-preserved strandline sequences at Manistique/Thompson bay (Lake Michigan), and Tahquamenon and Grand Traverse Bays (Lake Superior) that span the past ∼4500 yr. The single aliquot regeneration (SAR) method is applied to produce absolute ages for littoral and eolian sediments. SAR ages are compared against AMS and conventional 14C ages on swale organics. Modern littoral and eolian sediments yield SAR ages <100 yr indicating near, if not complete, solar resetting of luminescence prior to deposition. Beach ridges that yield SAR ages <2000 yr show general agreement with corresponding 14C ages on swale organics. Significant variability in 14C ages >2000 cal yr B.P. complicates comparison to SAR ages at all sites. However, a SAR age of 4280 ± 390 yr (UIC913) on ridge77 at Tahquamenon Bay is consistent with regional regression from the high lake level of the Nipissing II phase ca. 4500 cal yr B.P. SAR ages indicate a decrease in ridge formation rate after ∼1500 yr ago, likely reflecting separation of Lake Superior from lakes Huron and Michigan. This study shows that SAR is a credible alternative to 14C methods for dating littoral and eolian landforms in Great Lakes and other coastal strandplains where 14C methods prove problematic.  相似文献   

7.
Remains identified as those of a woolly mammoth (Mammuthus primigenius) dated at 12,200 ± 55 14C yr B.P. were recovered while excavating in a complex sequence of glaciomarine sediments in Scarborough, Maine, USA. The mammoth was found in the top meter of a fossiliferous unit of mud and sand laminites. These sediments were deposited during a marine regressive phase following the transgression that accompanied northward retreat of the margin of the Laurentide ice sheet. A Portlandia arctica valve from the underlying transgressive unit provides a minimum age of 14,820 ± 105 14C yr B.P. for local deglaciation. The mammoth, an adult female, died in midwinter with no evidence of human involvement. Tusk growth rates and oxygen-isotope variation over the last few years of life record low seasonality. The mammoth was transported to the site as a partial carcass by the late-glacial proto-Saco River. It sank in a near-shore setting, was subjected to additional disarticulation and scattering of elements, and was finally buried in sediments reworked by the shallowing sea.  相似文献   

8.
Sediment cores with new radiocarbon dates from the southern outlet of glacial Lake Agassiz indicate that meltwater delivery to the Mississippi valley was disrupted at 10,800 14C yr B.P. and the outlet was abandoned by 9400 14C yr B.P. These findings confirm the timing of generally accepted terminations of the Lockhart and Emerson Phases of Lake Agassiz. Additionally, the radiocarbon chronology indicates that the spillway was fully formed by 10,800 14C yr B.P. and that the occupancy in late-Emerson time was likely short-lived with minimal spillway erosion.  相似文献   

9.
Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past ∼23,000 14C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600 14C yr B.P. and the Tres Cruces Tephra of 8500 14C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 14C yr B.P. to ca. 11,600 14C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000 14C yr B.P., and at ca. 12,600 14C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000 14C yr B.P. After 10,000 14C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 14C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 14C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles.  相似文献   

10.
Seismic stratigraphy, sedimentary facies, pollen stratigraphy, diatom-inferred salinity, stable isotope (δ18O and δ13C), and chemical composition (Sr/Ca and Mg/Ca) of authigenic carbonates from Moon Lake cores provide a congruent Holocene record of effective moisture for the eastern Northern Great Plains. Between 11,700 and 950014C yr B.P., the climate was cool and moist. A gradual decrease in effective moisture occurred between 9500 and 710014C yr B.P. A change at about 710014C yr B.P. inaugurated the most arid period during the Holocene. Between 7100 and 400014C yr B.P., three arid phases occurred at 6600–620014C yr B.P., 5400–520014C yr B.P., and 4800–460014C yr B.P. Effective moisture generally increased after 400014C yr B.P., but periods of low effective moisture occurred between 2900–280014C yr B.P. and 1200–80014C yr B.P. The data also suggest high climatic variability during the last few centuries. Despite the overall congruence, the biological (diatom), sedimentological, isotopic, and chemical proxies were occassionally out of phase. At these times the evaporative process was not the only control of lake-water chemical and isotopic composition.  相似文献   

11.
Eolian landforms are widespread alongside proglacial valley-sandurs in West Greenland and comprise low-relief sand sheets, climbing dunes, and upland loess. Sedimentary facies mainly reflect distance to outwash-source zones and the influence of vegetation cover. The sediments show stratification types typical for poorly to moderately vegetated sand-sheets, alternately laminated silt/peat sequences, and unstratified loess. Twenty-five accelerator mass spectrometry 14C dates provide the basis for the chronostratigraphy of the inland eolian deposits. 14C dates from interstratified sand-sheets suggest that the bulk of eolian sands were deposited prior to 3400 cal yr B.P. and after 550 cal yr B.P. This two-phase formation for the inland dunes most likely reflects local changes in proglacial floodplain development and meltwater rerouting associated with a significant recession of the Greenland ice sheet during the mid Holocene climate optimum. Subsequent floodplain regeneration and renewed sand-sheet formation after 550 cal yr B.P. followed when the ice margin readvanced to its present position. In contrast, atmospheric deposition of regionally derived silt in upland peat mires has been continuous since at least 4750 cal yr B.P. Silt influx data demonstrate a strongly episodic history of the intensity of eolian activity over the past five millennia, which tentatively reflects alternating periods of (winter) aridity associated with the variable incursion of maritime air masses over the interior ice-free areas of West Greenland.  相似文献   

12.
Palynological analysis of a core from the Atlantic rain forest region in Brazil provides unprecedented insight into late Quaternary vegetational and climate dynamics within this southern tropical lowland. The 576-cm-long sediment core is from a former beach-ridge “valley,” located 3 km inland from the Atlantic Ocean. Radio-carbon dates suggest that sediment deposition began prior to 35,000 14C yr B.P. Between ca. 37,500 and ca. 27,500 14C yr B.P. and during the last glacial maximum (LGM; ca. 27,500 to ca. 14,500 14C yr B.P.), the coastal rain forest was replaced by grassland and patches of cold-adapted forest. Tropical trees, such as Alchornea, Moraceae/Urticaceae, and Arecaceae, were almost completely absent during the LGM. Furthermore, their distributions were shifted at least 750 km further north, suggesting a cooling between 3°C and 7°C and a strengthening of Antarctic cold fronts during full-glacial times. A depauperate tropical rain forest developed as part of a successional sequence after ca. 12,300 14C yr B.P. There is no evidence that Araucaria trees occurred in the Atlantic lowland during glacial times. The rain forest was disturbed by marine incursions during the early Holocene period until ca. 6100 14C yr B.P., as indicated by the presence of microforaminifera. A closed Atlantic rain forest then developed at the study site.  相似文献   

13.
In the west-central part of Lago Argentino, the Puerto Bandera moraines are clearly detached from longer, more prominent moraines of the last glaciation and from shorter and smaller Neoglacial moraines. Scientists have long speculated about the age of the Puerto Bandera moraines. Detailed geomorphologic studies in the western area of Lago Argentino, including stratigraphic profiles at Bahía del Quemado in the northern branch (Brazo Norte), indicate that the Puerto Bandera moraines were deposited by three pulses of ice. Each of the three pulses is represented by single moraine ridges and belts of tightly arranged ridges. The timing of the three glacier advances was established by radiocarbon dating, including data published by John Mercer. The oldest moraine system, formed during the Puerto Bandera I substade, was deposited ca. 13,000 14C yr B.P. Moraines of the Puerto Bandera II substade were deposited ca. 11,000 14C yr B.P. The youngest moraine system was deposited during a minor readvance, shortly before 10,390 C14 yr B.P., and thus appears to have occurred some time during the European Younger Dryas interval. After this third substade, the ice tongues retreated into the interior branches of Lago Argentino and have remained there since. Evidence found at Bahía del Quemado, together with data provided by other authors, attests to a significant climatic change by the middle Holocene, which we believe occurred during the Herminita advance, the first Holocene glacial readvance recognized within the area.  相似文献   

14.
This study presents a multiproxy record of Holocene environmental change in the region East of the Pechora Delta. A peat plateau profile (Ortino II) is analyzed for plant macrofossils, sediment type, loss on ignition, and radiocarbon dating. A paleosol profile (Ortino III) is described and radiocarbon dated. A previously published peat plateau profile (Ortino I) was analyzed for pollen and conifer stomata, loss on ignition, and radiocarbon dating. The interpretation of the latter site is reassessed in view of new evidence. Spruce immigrated to the study area at about 8900 14C yr B.P. Peatland development started at approximately the same time. During the Early Holocene Hypsithermal taiga forests occupied most of the present East-European tundra and peatlands were permafrost free. Cooling started after 5000 14C yr B.P., resulting in a retreat of forests and permafrost aggradation. Remaining forests disappeared from the study area around 3000 14C yr B.P., coinciding with more permafrost aggradation. The retreat of forests resulted in landscape instability and the redistribution of sand by eolian activity. The displacement of the Arctic forest line and permafrost zones indicates a warming of at least 2–3°C in mean July and annual temperatures during the Early Holocene. At least two cooling periods can be recognized for the second half of the Holocene, starting at about 4800 and 3000 14C yr B.P.  相似文献   

15.
Deposits of the Tamanduá River contain evidence for four major paleohydrologic stages in the last 33,000 years. A wet period between 33,000 and 20,00014C yr B.P. produced a high water table that allowed organic-rich deposition in the Tamanduá valley. A dry interval 17,000-10,00014C yr B.P. produced sandy deposits of braided channels and alluvial fans. River aggradation during this period probably resulted from a high sediment load promoted by intense slope erosion and from flash floods. A wet period after 10,000 and before 600014C yr B.P. was marked by reduced slope erosion and by high discharge that led to erosion of the valley fill. During that time forests developed widely in Brazil. A drier climate after 600014C yr B.P. caused a reduction of discharge but allowed a high water table to be maintained.  相似文献   

16.
A high-resolution pollen record from a 5-m-long sediment core from the closed-lake basin Laguna Piusbi in the southern Colombian Pacific lowlands of Chocó, dated by 11 AMS14C dates that range from ca. 7670 to 22014C yr B.P., represents the first Holocene record from the Chocó rain forest area. The interval between 7600 and 610014C yr B.P. (500–265 cm), composed of sandy clays that accumulated during the initial phase of lake formation, is almost barren of pollen. Fungal spores and the presence of herbs and disturbance taxa suggest the basin was at least temporarily inundated and the vegetation was open. The closed lake basin might have formed during an earthquake, probably about 440014C yr B.P. From the interval of about 600014C yr B.P. onwards, 200 different pollen and spore types were identified in the core, illustrating a diverse floristic composition of the local rain forest. Main taxa are Moraceae/Urticaceae,Cecropia,Melastomataceae/Combretaceae,Acalypha, Alchornea,Fabaceae,Mimosa, Piper, Protium, Sloanea, Euterpe/Geonoma, Socratea,andWettinia.Little change took place during that time interval. Compared to the pollen records from the rain forests of the Colombian Amazon basin and adjacent savannas, the Chocó rain forest ecosystem has been very stable during the late Holocene. Paleoindians probably lived there at least since 346014C yr B.P. Evidence of agricultural activity, shown by cultivation ofZea maissurrounding the lake, spans the last 1710 yr. Past and present very moist climate and little human influence are important factors in maintaining the stable ecosystem and high biodiversity of the Chocó rain forest.  相似文献   

17.
The Savanna Terrace, composed of alternating red and gray clayey sediments of late Wisconsinan age, can be found in five states along the upper Mississippi valley from Pepin County, Wisconsin, to Jackson County, Illinois. The terrace is the highest glaciofluvial-lacustrine deposit without a loess cover in the upper Mississippi valley. Chemical, physical, and mineralogical data show that two different sources provided sediment. The red clay is believed to have come from Lake Superior sources, while the gray clay is believed to have come from sources farther west. Large-scale flood events from glacial Lakes Agassiz, Grantsburg, and Superior were probably the main contributors of the sediments. The red clay in the terrace is similar in composition to red glaciolacustrine sediment found in eastern and northern Wisconsin. It also is mineralogically similar to the Hinckley Sandstone and the Fond du Lac Formation, which occur under and around Lake Superior. Radiocarbon dates obtained from the lower Illinois valley indicate that the terrace sediments were deposited sometime between about 13,100 and 9500 yr ago. Soils developed on the terrace are variable in their physical, chemical, and mineralogical properties, which reflect the composition of the clayey sediments.  相似文献   

18.
Although many of the current hypotheses to explain the origin and distribution of the Amazon biodiversity has been based directly or indirectly on geological data, the reconstruction of the geological history of the Amazon region is still inadequate to analyze its relationship with the biodiversity. This work has the main goal to characterize the sedimentary successions formed in the Brazilian Amazon in the Neogene-Quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap. Radar image interpretation, sedimentological studies, and radiocarbon dating allowed the mapping of Plio-Pleistocene to Holocene units along the Solimões-Amazonas River, Brazil. This integrated work led to the characterization of five sedimentary successions overlying Miocene deposits of the Solimões/Pebas Formation, which include the following: Içá Formation (Plio-Pleistocene), deposits Q1 (37,400-43,700 14C yr B.P.), deposits Q2 (27,200 14C yr B.P.), deposits Q3 (6730-2480 14C yr B.P.), and deposits Q4 (280-130 14C yr B.P.). These deposits occur mostly to the west of Manaus, forming NW-SE elongated belts that are progressively younger from SW to NE, indicating a subsiding basin with a depocenter that migrated to the NE. The reconstruction of the depositional history is consistent with significant changes in the landscapes. Hence, the closure of a large lake system at the end of the Miocene gave rise to the development of a Plio-Pleistocene fluvial system. This was yet very distinct from the modern drainage, with shallow, energetic, highly migrating, braided to anastomosed channels having an overall northeast outlet. This fluvial system formed probably under climatic conditions relatively drier than today's. During the early Pleistocene, there was pronounced erosion, followed by a renewed depositional phase ca. 40,000 14C yr B.P., with the development of prograding lobes and/or crevasse splays associated with a lake system (i.e., fan-delta) and/or fluvial flood plain areas. After a period of erosion, a fluvial system with eastward draining channels started to develop at around 27,000 14C yr B.P. The fluvial channels were overflooded in mid-Holocene time. This flooding is attributed to an increased period of humidity, with a peak between 5000 and 2500 14C yr B.P. The data presented herein support that, rather than being a monotonous area, the Amazonia was a place with frequent changes in landscape throughout the Neogene-Quaternary, probably as a result of climatic and tectonic factors. We hypothesize that these changes in the physical environment stressed the biota, resulting in speciation and thus had a great impact on modern biodiversity.  相似文献   

19.
Major Holocene monsoon changes in continental Southeast Asia are reconstructed from analysis of 14C-dated changes in pollen and organic/inorganic carbon in sediment cores taken from permanent, closed-basin, volcanic lakes in Ratanakiri Province, northeastern Cambodia. Analysis focuses on the nature and timing of monsoon changes, inferred from changes in vegetation and lake conditions. These data provide the first well-dated palynological record, covering most of the Holocene and continuous up to the present, from a terrestrial site in mainland Southeast Asia. The record from a 15-m core retrieved from Kara Lake, representing the last 9300 years, shows that the late Glacial conditions ended about 8500 14C yr B.P., more than 1000 years later than sites in southwest China. Summer monsoon intensity increased over the period ca. 8400–5300 14C yr B.P., similar to most other sites in the Asian monsoon region. A subsequent expansion of secondary forests at the expense of dense semievergreen forests suggest a drier climate leading to more frequent fire disturbance. After ca. 3500 14C yr B.P. disturbance frequency may have increased further with increasing seasonality. From ca. 2500 14C yr B.P. to the present, dense forest has recovered in a mosaic with annually burned dry forest, but climate may not be the main control on local vegetation dynamics in the late Holocene.  相似文献   

20.
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.  相似文献   

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