Showing papers in "The Holocene in 1998"

High-resolution palaeoclimatic records for the last millennium: Interpretation, integration and comparison with General Circulation Model control-run temperatures

Abstract: Palaeoclimatology provides our only means of assessing climatic variations before the beginning of instrumental records. The various proxy variables used, however, have a number of limitations which must be adequately addressed and understood. Besides their obvious spatial and seasonal limitations, different proxies are also potentially limited in their ability to represent climatic variations over a range of different timescales. Simple correlations with instrumental data over the period since AD 1881 give some guide to which are the better proxies, indicating that coral- and ice-core-based reconstructions are poorer than tree-ring and historical ones. However, the quality of many proxy time series can deteriorate during earlier times. Suggestions are made for assessing proxy quality over longer periods than the last century by intercomparing neighbouring proxies and, by comparisons with less temporally resolved proxies such as borehole temperatures. We have averaged 17 temperature reconstructions (representing various seasons of the year), all extending back at least to the mid-seventeenth century, to form two annually resolved hemispheric series (NH10 and SH7). Over the 1901-91 period, NH10 has 36% variance in common with average NH summer (June to August) temperatures and 70% on decadal timescales. SH7 has 16% variance in common with average SH summer (December to February) temperatures and 49% on decadal timescales, markedly poorer than the reconstructed NH series. The coldest year of the millennium over the NH is AD 1601, the coldest decade 1691-1700 and the seventeenth is the coldest century. A Principal Components Analysis (PCA) is performed on yearly values for the 17 reconstructions over the period AD 1660-1970. The correlation between PC1 and NH10 is 0.92, even though PC1 explains only 13.6% of the total variance of all 17 series. Similar PCA is performed on thousand-year-long General Circulation Model (GCM) data from the Geophysical Fluid Dynamics Laboratory (GFDL) and the Hadley Centre (HADCM2), sampling these for the same locations and seasons as the proxy data. For GFDL, the correlation between its PC1 and its NH10 is 0,89, while for HADCM2 the PCs group markedly differently. Cross-spectral analyses are performed on the proxy data and the GFDL model data at two different frequency bands (0.02 and 0.03 cycles per year). Both analyses suggest that there is no large-scale coherency in the series on these timescales. This implies that if the proxy data are meaningful, it should be relatively straightforward to detect a coherent near-global anthropogenic signal in surface temperature data.

Synchronous Holocene climatic oscillations recorded on the Swiss Plateau and at timberline in the Alps

Abstract: Eight synchronous pre-Roman cold phases were found at 9600–9200, 8600–8150, 7550–6900, 6600– 6200, 5350–4900, 4600–4400, 3500–3200 and 2600–2350 radiocarbon years BP by reconstructing past climate at two sites on the Swiss Plateau and at timberline in the Alps. The cooling events during the early-and mid-Holocene represent temperature values similar to today, and apparently the onset of cooling events represents a deviation from today's mean annual temperature of about 1°C and is triggered at a 1000-year periodicity. At Wallisellen-Langachermoos (440 m), a former oligotrophic lake near Zurich, the correlation between sum mertime lake levels and the seed production of the amphi-Atlantic aquatic plantNajas flexilis was used to reconstruct lake levels over a 3000-year period during the first part of the Holocene. At Lake Seedorf on the western Swiss Plateau (609 m) the sedimentological, palynological and macrofossil record revealed fluctuations of lake levels for the complete Holocene. From Lago Basso in the...

A reconstruction of the North Atlantic Oscillation using tree-ring chronologies from North America and Europe

Abstract: Tree-ring records, six from eastern North America and four from northwestern Europe, are used to develop the first reconstruction of the winter North Atlantic Oscillation (NAO) The reconstructed series spans the interval AD 1701-1980 and explains 41%, of the variance in the NAO over the AD 1874-1980 calibration period The reconstruction also captures the spectral properties of this index, suggesting that the oscillatory character of the NAO is a long-term feature of the North Atlantic climate system

Relationships between charcoal particles in air and sediments in west-central Siberia:

Abstract: Production and size of charred particles determine transport and deposition in lakes. Lack of such data is a principal obstacle to interpretation of past fire from charcoal profiles. Our two-part a...

Pollen and charcoal in lake sediments compared with historically documented forest fires in southern Switzerland since AD 1920

Abstract: Charcoal in unlaminated sediments dated by 210Pb was analysed by the pollen-slide and thin-section methods. The results were compared with the number and area of forest fires on different spatial scales in the area around Lago di Origlio as listed in the wildfire database of southern Switzerland since AD 1920. The influx of the number of charcoal particles > 75 µm2 in pollen slides correlates well with the number of annual forest fires recorded within a distance of 20-50 km from the coring site. Hence a size-class distinction or an area measurement by image analysis may not be absolutely necessary for the reconstruction of regional fire history. A regression equation was computed and tested against an independent data set. Its use makes it possible to estimate the charcoal area influx (or concentration) from the particle number influx (or concentration). Local fires within a radius of 2 km around the coring site correlate well with the area influx of charcoal particles estimated by the thin-section method measuring the area of charcoal particles larger than 20 000 µm2 or longer than 50 µm. Pollen percentages and influx values suggest that intensive agriculture and Castanea sativa cultivation were reduced 30-40 years ago, followed by an increase of forest area and a development to more natural woodlands. The traditional Castanea sativa cultivation was characterized by a complete use of the biomass produced, so abandonment of chestnut led to an increasing accumulation of dead biomass, thereby raising the fire risk. On the other hand, the pollen record of the regional vegetation does not show any clear response to the increase of fire frequency during the last three decades in this area.

Quantitative estimates of water tables and soil moisture in Holocene peatlands from testate amoebae

Abstract: Changes in surface wetness on Holocene ombrotrophic mires have principally been estimated from plant macrofossils and humification. Testate amoebae (Protozoa: Rhizopoda) provide an additional technique and have the potential to provide improved quantitative estimates of water-table depths and soil moisture. The relationship between hydrology and testate amoebae assemblages from 163 samples on nine British mires is explored using canonical correspondence analysis (CCA). Mean annual water-table depth and percentage soil moisture are two of the most important environmental variables related to the distribution of testate amoebae within peat. Transfer functions for these variables are developed using four underlying models; weighted aver aging (WA), tolerance downweighted weighted averaging (WA-Tol), weighted averaging partial least squares (WA-PLS) and partial least squares (PLS). In ‘jack-knifed’ validation, WA produced the lowest prediction errors for water table, but was outperformed by WA-Tol for percent...

A method for the preparation and quantification of microscopic charcoal from terrestrial and lacustrine sediment cores

Abstract: Existing methods of microscopic charcoal analysis are briefly reviewed. A simple but effective alter-native method for preparing and quantifying fossil microscopic charcoal in sediment samples from terrestrial and lacustrine cores is outlined. Weak hydrogen peroxide (6%f/o H202) is used to partially digest and bleach organic material in sediment samples. Charcoal particles are not bleached by the hydrogen peroxide and are thus easily identified when counted under a binocular dissecting microscope. The sample preparation procedure is extremely gentle on charcoal particles, much more so than the majority of existing methods, ensuring that little particle fragmentation occurs during preparation.

Experimental production and analysis of microscopic charcoal from wood, leaves and grasses

Abstract: Study of microscopic charcoal from lake sediments has led to a greater understanding of past veg etation, climate and fire ecology. We investigated the potential of charcoal morphology as an indicator of vegetation type. Grasses, leaves and wood were burned under controlled conditions in the laboratory, and we used a dissecting scope, video camera, and image-capture software to image-sieved (125-μm screen) micro-scopic charcoal. Charcoal from grasses was significantly longer (562 μm) and had a greater length:width ratio (3.62) than charcoal derived from leaves (380 μm; 1.91) or wood (348 μm; 2.13). Length:width ratios of mixtures of grass and leaf charcoal were intermediate (50:50 mixture; 2.36) between ratios for grass or leaf charcoal alone, and charcoal yield (on a weight basis) declined as a function of combustion temperature. While a number of issues may complicate the application of these results to the field, the results do suggest that length:width ratios can be used as an indicator of vegetation ...

The carbon isotope record in soils along a forest-cerrado ecosystem transect: implications for vegetation changes in the Rondonia state, southwestern Brazilian Amazon region:

Abstract: This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savannah (cerrado), a tropical semideciduous forest (cerradao), a forest tran- sition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. I4C data of total soil organic matter and charcoal indicate that the organic matter in these soils is at least Holocene in age. The forest and forest transition sites are characterized by 6I3C soil depth profiles gener- ated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradao have experienced significant vegetation changes during the Holocene. The st3C data (-3o%o to -27%0) obtained in the deepest part of the profile at the cerradao site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C, plant influence is reflected in the "C-enriched values (-19%0 to -16.0%) at both sites during the middle Holocene. The recent part of the I3C record shows a clear expansion of C, vegetation, particularly at the cerrad20 site. The regressionkxpansion of the forest and savannah vegetation documented at the cerradao and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas ,between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Ama- ,'zen region.

A calendar age estimate of the Younger Dryas-Holocene boundary at Kråkenes, western Norway

Abstract: The Younger Dryas/Holocene transition (YD/H) in the sediments of Krakenes Lake, western Norway, is well marked both lithologically and palaeobiologically at 756.5 cm in the investigated core. A ser ...

Evidence for abrupt climatic change in northern Scotland between 3900 and 3500 calendar years BP

Abstract: Palaeohydrological and palaeoecological investigations from five locations in northern Scotland reveal a distinct and large-scale shift to wetter climatic conditions which may reflect a major transition in climate from the midto late Holocene. Radiocarbon age estimates place this transition between about 3900 and 3500 cal. BP, although the transition appears to have occurred abruptly, possibly over a decadal to century timescale. This event appears to be synchronous with changes inferred for other regions, suggesting that it reflects a continental-scale, or possibly even global-scale, change in climate. Broad correlation with deep-sea sediment records suggests that the transition may reflect colder sea-surface temperatures in the North Atlantic Ocean.

The recent eutrophication of Baldeggersee (Switzerland) as assessed by fossil diatom assemblages

Abstract: Diatom analyses with an annual resolution were carried out on varves of the hypertrophic Baldeg- gersee (Central Swiss Plateau) for the timespan ad 1885 to 1993. They reveal seven major changes in the dominant planktonic diatoms. As a result of progressive nutrient enrichment, Baldeggersee changed in the 1910s from a Cyclotella to a Tabellaria fenestrata dominated assemblage, and eventually in the 1950s to a Stephanodiscus parvusdominated diatom assemblage. The timing and direction of diatom-assemblage changes in the varved sediment compare well with sedimentological and limnological observations. Partitioning of the variance in the diatom data revealed that TP is a stronger explanatory variable than temperature for these changes. A diatom-inferred total phosphorus (TP) reconstruction indicates three major steps in eutrophication, occurring at 1909, the mid-1950s and the mid-1970s. Comparison with TP measurements in the water column demonstrates that the diatom-TP inference model used is able to hindcast past TP concentrations reliably. The major steps in eutrophication led to decreases in diatom diversity and also resulted in a progressive increase of calcite grain-size. The lake restoration programme established since 1982 shows no direct impact on the composition of the diatom assemblages. However, the decrease in phosphorus loads since the mid-1970s is reflected in the diatom assemblages and in decreasing diatom-inferred TP concentrations.

Palaeohydrological variation in a tropical river catchment: a reconstruction using fluorescent bands in corals of the Great Barrier Reef, Australia

Abstract: Massive, long-lived corals in inshore waters of the Great Barrier Reef contain yellow-green fluorescent bands. These bands are due to terrestrial humic and fulvic compounds incorporated into the coral skeleton during high river flow events. Fluorescence measurements are presented for two colonies of Porites spp. from locations in the path of the Burdekin River floodwaters the major river in north Queensland draining into the Coral Sea. The records extend from AD 1737 to 1980 and 1644 to 1986, respectively. The two independent coral records show a high degree of similarity. The two series are combined and used to reconstruct Burdekin River runoff for the period AD 1644 to 1980. The regression model accounts for 83% of the annual (water year) variability of Burdekin River flow and is verified over independent data. The 337-year reconstruction thus increases by threefold the length of record for considering interannual to decadal climate variations in northeast Australia. Instrumental and reconstructed Burde...

Winter air temperature variations in western Europe during the Early and High Middle Ages (AD 750–1300):

Abstract: In this paper an attempt is made to reconstruct air temperature variations in winter (December, January and February) from 2500 documentary data over the period ad 750–1300 for a region comprising the Benelux countries, eastern France, western Germany, Switzerland and northern Italy. Anomalous (warm and cold) winters were classified on the basis of proxy information on frost, freezing of water bodies, duration of snowcover and untimely activity of vegetation using semiquantitative indices. For the most severe winters during the ‘Medieval Warm Period’ (MWP) as well as for the outstanding warm and dry winter ad 1289/90, possible analogue cases from the last 300 years are considered, analysed, synoptically interpreted and compared with each other. It is concluded that severe winters were somewhat less frequent and less extreme during the MWP, ad 900–1300, than in the ninth century and from 1300 to 1900. Mean air temperatures for 30 year. periods were estimated from linear regression models including indices ...

Vegetation dynamics and human impact in the Sierra de Guadarrama, Central System, Spain

Abstract: Analyses of pollen, non-pollen microfossils and carbonaceous particles from a valley mire in the Lozoya Valley in the Sierra de Guadarrama, which forms part of the Spanish Central System, are presented. The sedimentary sequence, which covers the last 8500 years, provides one of the oldest records in the central sector of this mountain chain. The core displays evidence of well developed Pinus sylvestris forests coupled with the presence of taxa such as Castanea or Olea in the early Holocene, highlighting the value of near-tree-line montane enclaves in the reconstruction of vegetation dynamics. The results show that the Lozoya Valley was an important refugial area for thermophilous taxa and that taxa like Olea and Castanea were present in pre-Roman times in this part of the Central System. In contrast to other areas of Spain and elsewhere in Europe where Quercus replaces Pinus in the early Holocene, evidence is presented of a natural pine belt in the Iberian continental montane system throughout the Holocen...

A 5000-year record of agriculture and tropical forest clearance in the Tuxtlas, Veracruz, Mexico

Abstract: We present pollen and microscopic charcoal evidence from securely dated lake sediments, located in the Tuxtlas region of Veracruz, Mexico. Two periods of agricultural activity are recognized. The earliest phase reflects a low level of forest clearance and agriculture. The data provides a minimum date of 4830 cal. BP for the introduction of maize agriculture to the region. The second phase indicates intensive and prolonged human activity within the tropical forest. Commencing approximately 2600 years ago and ending a thousand years later, this episode reflects Pre-Classic to Middle Classic agricultural intensification. Coincident with this phase, we find evidence for drier climatic conditions. Subsequent to each episode of disturbance the forest regenerated within approximately 300 years.

Identifying forest stand types using pollen from forest hollows

Abstract: Methods of interpreting pollen assemblages in sediment were examined using surface samples from 66 small forest hollows in Michigan and Wisconsin. All canopy trees in the surrounding 50 m were meas...

Tree-ring based reconstructions of northern Patagonia precipitation since AD 1600

Abstract: Long-term reconstructions (400 years) of seasonal and annual precipitation variations were developed for northern Patagonia east of the Andes using a new set of 16 tree ring-width chronologies from Austrocedrus chilensis (D.Don) Endl. Reconstructions, which capture between 41 and 50% of the precipitation variance, show that the twentieth century contains the most extreme long periods of wetness and dryness in the past 400 years. Since about AD 1910, the reconstructions are also characterized by an increase in interannual variability and one of the highest rates of extreme events within the last 400 years. A prominent oscillation on the order of 2-2.1 years in length has been identified in the reconstructions using spectral analysis. Quasi-Biennial Oscillations have been shown to be very marked in some circulation indices of the Southern Hemisphere. Although significant oscillations within the preferred frequency domain of El Nino-Southern Oscillation (ENSO) are present in the reconstructions, no clear trod consistent responses to ENSO have been observed. Correlations of reconstructions with mean sea-level pressure around South America for the interval AD 1912-1984, reveal the influence of subtropical and high-latitude features of the atmospheric circulation on precipitation variations in northern Patagonia. Droughts result from an intensification of the subtropical Pacific anticyclone off the Chilean coast and the deepness of the circum-Antarctic trough over the South Orkney-Antarctic Peninsula sector. Mean sea-level pressure reconstructions for the South Atlantic sector of the Southern Oceans were used to evaluate the temporal stability of the relationships between northern Patagonia precipitation and high-latitude climatic variations since AD 1750. The influence of high-latitude circulation on precipitation appears to be more significant during the twentieth century, which in turn may respond to an intensification of wavenumbers 3 on the mean planetary wave structure over the Southern Hemisphere. Recent increase of precipitation variability in northern Patagonia may reflect stronger interactions between middle- and high-latitude atmospheric circulation in the Southern Hemisphere during the twentieth century.

Climatic signals recorded in snow avalanche-dominated colluvium in western Norway: depositional facies successions and pollen records

Abstract: This paper is focused on the palaeoclimatic significance of snow-avalanche (snow-flow) activity and palaeobotanical data derived frompostglacial colluvium. It is based on a series of case studies from western Norway, with emphasis on sedimentary facies and radiocarbon dating of the deposits. The present data demonstrate that the colluvial sedimentary successions bear important palaeoclimatic signals related to extreme weather events, and the record of snow avalanches reveals, in particular, a detailed pattern of winter climatic conditions (snow fall rates and the frequency of winter storms). The Late Weichselian record shows some snow avalanching around 12 300–12 000 BP, suggesting occurrence of major winter storms at that time. An overwhelming amount of data demonstrates the dramatic increase in snow-flow processes during the Younger Dryas chronozone, documenting that this phase was characterized by severe winter storms with distinct snowfall rates. The Holocene record of snow avalanching shows in genera...

Stable carbon isotope ratios of latewood cellulose in Pinus sylvestris from northern Finland: variability and signal-strength:

Abstract: Stable carbon-isotope ratios were measured on the latewood cellulose of 25 Pinus sylvestris trees at five sites from the Arctic Circle to the northern limit of pine in Finland. A correlation-based analysis of variance was used to define within and between-site signal strengths. Expressed Population Signals (EPS) from samples of five trees are very high (0.89 to 0.95). Combining 15 trees from the three northernmost sites yields an EPS of 0.97. Despite the very strong common signal, different trees yield absolute δ13C values offset by >2%, which is similar in magnitude to the variability within individual series that includes the effect of climate. Samples of. 10 trees are necessary to obtain mean absolute values with confidence limits <1% wide (p < 0.05). The common practice of pooling four cores from four trees to yield absolute δ13C values is thus inadequate for palaeoclimate research. Because the δ13C values are homeoscedastic they can be indexed by differencing from the mean of each tree series, retain...

The use of testate amoebae in studies of sea-level change: a case study from the Taf Estuary, south Wales, UK:

Abstract: Micropalaeontological techniques play an important role in high-resolution studies of sea-level change. Salt-marsh foraminifera are among the most valuable groups of sea-level indicators as their d...

Cosmogenic Cl-36 dating of postglacial landsliding at The Storr, Isle of Skye, Scotland

Abstract: Major postglacial rock slope failures are a common feature of the Scottish Highlands and other mountainous areas that were deglaciated at the end of the Pleistocene, but evaluation of the causes and triggers of failure has been hindered by a lack of reliable dating evidence. We report the result of a pilot study designed to establish the absolute age of a large postglacial rotational rockslide at The Storr on the Isle of Skye, Scotland, using 36Cl surface exposure dating. Exposure ages of 6.3 ± 0.7 cal. ka BP and 6.6 ± 0.8 cal. ka BP were obtained for rock samples from two separate landslide blocks, giving an overall age estimate of 6.5 ± 0.5 cal. ka BP for rock slope failure at this site. This date is consistent with AMS radiocarbon dating of windblown sand derived from the failure scarp, and with previous inferences (based on relative dating evidence) concerning an early-Holocene age for most rock slope failures in the Scottish Highlands. The long time lag (. 7 ka) between deglaciation and failure sugge...

Genesis and environmental history of Lagoa Santa, southeastern Brazil:

Abstract: Geomorphologic studies and palynological analysis of a core in the middle of Lagoa Santa, a lake in central Brazil (19°38'S; 43°54'W), have provided palaeoenvironmental information on the region an...

Radiocarbon age of the Kaharoa Tephra, a key marker for late-Holocene stratigraphy and archaeology in New Zealand

Abstract: The Kaharoa eruption, the most recent rhyolitic volcanic event in New Zealand, resulted in the deposition of the compositionally distinctive Kaharoa Tephra over at least 30 000 km2 of northern and eastern North Island. The tephra forms an isochronous marker bed for various late-Holocene stratigraphic and palaeoen vironmental studies but is particularly important for evaluating the chronology of New Zealand's notably short prehistory because it was erupted within the last millennium, close to the time of first Polynesian colonization. We derive a definitive radiocarbon age, previously ambiguous, for the Kaharoa Tephra of 665615 BP using cluster analysis of 22 radiocarbon ages relevant to the Kaharoa eruptive episode. The error-weighted mean age obtained on unscreened ages, minus outliers, is supported by statistically identical ages obtained from three sets of screened ages selected to minimize the effects of inbuilt age or contamination. Based on the intercepts method and 1 sigma range of Northern Hemisph...

Holocene sea-level change and coastal evolution in the Humber estuary, eastern England: an assessment of rapid coastal change

Abstract: New stratigraphic data collected from six sites in the Humber estuary establish a record of Holocene relative sea-level (RSL) change, and enable testing of four possible causes of rapid coastal change: sea-level rise, changes in sedimentation, storm-surge history, and human impact. Mean high water of spring tides (MHWST) in the Humber rose from c. 9 m OD at 7500 cal. yrs BP to 0 m OD by 4000 cal. yrs BP, at an average long-term rate of c. 3.9 mm yr-1. After this, the rate of rise gradually decreased to c. 1 mm yr’. Discrete episodes of rapid RSL rise are not identified although their absence may reflect limited data availability. However, we do observe two episodes of rapid coastal change in the Humber estuary. The first occurs between c. 3200 and 1900 cal. yrs BP, as marine conditions expand to their Holocene maximum and then contract. This pattern of coastal development differs from that in the East Anglian Fenlands, suggesting local processes control sedimentation at one or both of these sites. The sec...

Early-Holocene limnological and climatic variability in the Northern Great Plains:

Abstract: Information on the timing and direction of climatic and environmental change on a millennial scale exists for many regions of North America, whereas little is known about decadal-to centennial-scale variability. Here we present a high-resolution analysis of diatom-inferred salinity from a site in the Northern Great Plains to reconstruct multidecadal-and centennial-scale climatic patterns during the early Holocene. The diatominferred salinity indicates a transition from fresh to highly saline conditions between c. 13 400 and 7700 cal. yr BP, which suggests a major shift in climate from wet to dry conditions. The overall trend toward increasing salinity is interrupted by several freshwater intervals between c. 9800 and 7950 cal. yr BP, which may be the result of an increase in the frequency of monsoonal flow from the Gulf of Mexico. The early Holocene is considered to be a time of rapid change in climate and vegetation within the Holocene. Although rates of change in the Moon Lake diatom assemblages were hi...

Postglacial trends in palynological richness in the northern Fennoscandian tree-line area and their ecological interpretation:

Abstract: The northern Fennoscandian tree-line area consists of fairly distinct vegetation zones. The pollen stratigraphies of two boreal sites and two tundra sites were studied and the palynological data an...

The role of relative sea-level rise and changing sediment supply on Holocene gravel barrier development: the example of Porlock, Somerset, UK

Abstract: The Holocene development of the coastal system at Porlock has been driven by variations in the rate of relative sea-level rise providing a long-term control on longshore sediment supply, which in t...

Evaluating late-Holocene relative sea-level change in the Somerset Levels, southwest Britain

Abstract: Holocene relative sea-level (RSL) change is investigated at Nyland Hill (Somerset Levels). The lithostratigraphy comprises turfa peat underlying marine clay, both onlapping a sloping Upper Palaeozoic basement. The altitude of the peat-clay contact ranges from 2.42 to 4.52 m OD, becoming lower down basement slope, yet 14C dates of three samples taken laterally along the contact at different altitudes yield similar ages (3640-3330, 3715-3460, 3725-3465 cal. yrs BP), suggesting either very rapid (‘instantaneous’) RSL rise or sediment compaction. Biostratigraphic data indicate a gradual transition to marine conditions across the peat-clay contact. Therefore, the height difference is interpreted as evidence for significant compaction. We consider 4.64 m OD the minimum pre-compaction altitude, indicating maximum observed compaction of 2.22 m. The clay surface, reclaimed during Roman occupation, represents an anthropogenically induced negative tendency and is dated chemostratigraphically to 1776 ± 46, using a da...

Dynamics of montane forest in central Africa during the late Holocene: a pollen-based record from western Uganda

Abstract: According to the results from a number of palaeoecology-based studies, montane forest appears to have been present across a broad range of altitudes in western Uganda throughout much of the Holocene (before increased levels of human impact over the past 2000 years or so resulted in extensive clearance). Critically for pollen-based studies of vegetation history, clearance has limited investigations into the late- Holocene history of montane forest, and thus the impact of small amplitude, sub-Milankovich variations in climate and low levels of human activity. Mubwindi Swamp in the Rukiga Highlands of southwest Uganda is today surrounded by dense stands of moist lower montane forest. The uppermost 5 m of organic-rich deposits from Mubwindi Swamp are dated to the last 2100 years, and thus provide an opportunity, so far unique for Uganda, to record variations in the composition of lower montane forest during the late Holocene. Results of analyses of pollen indicate that lower montane forest has been present in the catchment for Mubwindi Swamp throughout the last 2100 years. A transition to a more open, and possibly drier, form of forest is apparent from around 710 BP. It seems likely that this transition was at least partly driven by climate. There is an increased occurrence of pollen from plants presently associated with areas of degraded forest from around 200 BP and a decrease in pollen from important sources of timber. These changes could be interpreted as representing slightly increased humidity. However, in view of the nature of these changes, a low level of human impact was probably the most important causal factor. The gazetting of Bwindi-Impenetrable Forest as a Forest Reserve in the early 1930s may have facilitated a recovery of timber trees during the present century.