Paoay Lake,northern Luzon,the Philippines: a record of Holocene environmental change

The last 7000 years of environmental history for Paoay Lake and its surrounding landscape is examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating. Basal sediments contain shells of Cerithiidae and the saline‐tolerant diatom Diploneis indicating that this was an estuarine environment before becoming a freshwater lake after 6000 bp . Pollen analysis shows that submontane forests, characterized by Pinus pollen, underwent a major disturbance around 5000 years ago, recovering to previous levels by 1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although the highest levels occur in the earlier part of the record, between 6500 and 5000 years ago. An aspect of the project was to examine whether there is evidence of land clearance and agricultural development in the region during the late Holocene. While a clear signal of human impact in the record remains equivocal, there appears to be a correspondence between submontane forest decline and mid‐Holocene ocean data that depict warmer and possibly drier conditions for the region. The study highlights the vulnerability of these montane forests to forecasts of a warmer and drier climate in the near future.     

Late Holocene variability in pelagic fish scales and dinoflagellate cysts along the west coast of Vancouver Island, NE Pacific Ocean

Fish stocks and dinoflagellates are essential components of the marine food chain. Sediment cores from a predominantly anoxic basin in Effingham Inlet, Vancouver Island, British Columbia, archive a late Holocene (500–5300 years BP) record of paleoproductivity in the North American Coastal Upwelling Domain (CUD). We present evidence that late Holocene changes in the dinoflagellate cyst assemblages, sedimentary record, and fish stocks in the northeastern Pacific Ocean fluctuated, at least partially, in accordance with regional and global climate cycles.Principal components analysis (PCA), and trend, wavelet and spectral analyses were used to identify relationships, cycles and trends in sediment grey-scale values, and the abundances of fish scales and dinoflagellate cysts on centennial to millennial time scales. Most observed cycles fluctuated in intensity over time, particularly following transition of the regional climate to a higher rainfall phase that impacted coastal oceanic dynamics 3400 ± 150 years ago. Correlation of the marine paleoproductivity records observed in Effingham Inlet with solar influenced climate proxy cycles observed in the North Atlantic region indicates that solar forcing at different scales might have influenced the climate in the northeast Pacific as well. In particular an 1100- to 1400-year cycle in regional climate is well represented in the fish productivity proxy and sedimentological record. It was also observed that colder water, high-productivity, Selenopemphix nephroides and anchovy-dominated “Anchovy Regime” ecosystems alternate with warmer water, herring-dominated “Herring Regime” ecosystems at millennial time scales. The fish scale record preserved in Effingham Inlet indicates that the NE Pacific is now in transition from an ‘anchovy-’ to a ‘herring’-dominated regime.     

Holocene pollen diagrams from Greece

Pollen diagrams are presented from three sites in Greece. Two cores from the Strymon Delta in Macedonia are thought to span parts of the period from Classical to Turkish times. A dated core from Kleonai in the Peloponnese has good pollen preservation in the early Neolithic, Bronze Age and Roman periods. A peat bog at Asi Gonia in the White Mountains of Crete has a continuous pollen record for the last 1500 years. The palynological evidence for human impact in various historic periods is assessed in the light of archaeological evidence. The problems of interpretation of pollen cores from Greece are discussed and the complex inter‐relationship between human impact and environmental factors is emphasised.     

Holocene environments in the central Sahara

Palynological investigations of corings in the sebkhas of Taoudenni (N-Mali) and Segedim (N-Niger), archaeological excavations in the Acacus Mts. (SW-Libya) and charcoal records in the central Ténéré (Niger) give evidence for a northward shift of the desert-savanna boundary to 22°–20° N during the middle Holocene. Between Niger and S-Libya there was a ecological gradient from the sudanian, sahelian and saharan savannas to a denser saharan desert vegetation. After a transition phase between 6000 and 4000 BP the saharan desert vegetation was finally established in the Taoudenni and Segedim region and this degraded from ca. 2000 BP to its present condition.During the middle Holocene the central Sahara had a monsoonal summer rain climate with an effective rainfall of 250–300 mm per year near the desert-savanna boundary (ca. 22° N). Interaction between the monsoon and the atlantic cyclones also allowed rainfall in other periods of the year.     

A critical assessment of standard processing methods for the preparation of palynological samples

Standard processing techniques for the isolation of organic walled dinoflagellate cysts from geological samples are examined, with particular attention to the size and type of sieve mesh used. Variations within the ‘standard’ processing techniques used by different laboratories are identified, and an assessment of the retention capacities of meshes of different sizes and different materials is carried out. Some dinoflagellate cysts and large numbers of Lycopodium spores, used for the calculations of absolute abundance data, were found to pass through 20 μm meshes. This is due to a combination of factors including: the diagonal aperture diameter of a 20 μm mesh measuring over 28 μm; the three-dimensional properties of different mesh weaves (nylon and polyester); and the non-spherical shape of the particles. Experiments demonstrate that the maximum mesh size that should be used in palynological processing is 15 μm. Nylon mesh is more practical to use than polyester as processing time is reduced, but nylon is degraded by contact with acid solutions. Meshes with apertures < 15 μm may be used, though this may be impractical for large samples containing significant quantities of fine siliciclastic or organic material.     

A 6500-year pollen record from the Polistovo-Lovatskaya Mire System (northwest European Russia). Vegetation dynamics and signs of human impact

New pollen and plant macrofossil evidence from the Polistovo-Lovatskaya Mire System is presented. The results show that local vegetation of the mire system was affected by various factors such as climate, hydrogeology and autochthonous processes in the peat bog. An important palaeoecological event took place around 6500 cal bp and led to a dramatic increase of paludification processes and lateral expansion of the mire. Forests of spruce and broad-leaved species in various combinations represented primary vegetation of uplands; they began to degrade around 1500 years ago and disappeared in historical times (400–500 cal bp). First signs of arable farming in the area occurred around 4200 cal bp, regular and mild human pressure established about 1000 years cal bp, and a significant impact at the regional level became evident around 400 years ago.     

A modern pollen–climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains

Aim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E. Methods We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (P_ann; 31–1022 mm), mean annual temperature (T_ann; −6.5 to 1 °C), and mean July temperature (T_July; 2.6–19.7 °C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for P_ann, T_ann and T_July were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results The root mean square errors of prediction (RMSEP) were 104 mm (P_ann), 1.18 °C (T_ann) and 1.17 °C (T_July). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (P_ann), 15.7% (T_ann) and 11.9% (T_July). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern P_ann 480 mm; T_ann−1 °C). T_ann and P_ann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 °C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 °C greater than present). Main conclusions The modern pollen–climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.     

Orbital signatures in a late Miocene dinoflagellate record from Crete (Greece)

A high-resolution palynological study of the cyclically bedded Faneromeni section (upper Tortonian-lower Messinian) on Crete (Greece) is presented. This study aims to recognize orbitally-driven variations in the palynological record and to validate the age model based on the astronomical calibration of the sedimentary cycles. Four palynology-based environmental proxies were utilised using interpretations of fossil dinoflagellate associations based on modern ecological characteristics. Cross-spectral analysis between the proxy records and astronomical target curve, the 65°N summer insolation, yielded in most cases significant spectral power and coherence in the precession and/or obliquity frequency bands. Precession-controlled variations in the proxy records are related to lithology and indicate that maxima in continental input and minima in sea surface salinity coincide with sapropel formation. The influence of obliquity is most clearly reflected in the index of continental versus marine palynomorphs (S-D). The absence of a distinct time lag relative to obliquity indicates that the 41-kyr component in continental input is controlled by oscillations in regional Mediterranean climate rather than by glacial cyclicity. Phase relations in the different astronomical frequency bands of the spectrum, as compared with the Mediterranean Pliocene, essentially confirm the validity of the Miocene astronomical time scale. Finally, a major non-cyclic change in the palynological assemblage at 6.68 Ma indicates enhanced salinity and decreased river discharge. This shift coincides with a significant drop in sedimentation rate informally termed the “Early Messinian Sediment starvation Event”.     

Pollen in a perfect trap: the palynological record in Miocene gypsum

Gypsum deposits formed during the middle Miocene Badenian Salinity Crisis (BSC) contain perfectly preserved palynomorphs with fossilised cytoplasms. The nature of the cytoplasm indicates that plasmolysis in a condensed brine solution took place prior to fossilisation. Two forms of plasmolysis are present: concave in zonoporate grains and convex in saccate, inaperturate, pantoporate and colporate specimens. The composition of the palynomorph assemblage is typical for the Miocene European lowland, with plants representing swamp, riparian, mesic and upland communities. Among the dominant taxa are: Pinus/Picea, Taxodium/Glyptostrobus, Ericaceae, Cathaya, Engelhardia and Quercus. The percentages of pollen grains with preserved content are very high for some taxa (Engelhardia, Ericaceae, Fagus, Liquidambar, Potamogeton, Quercus, Taxodium/Glyptostrobus and the Tricolporopollenites pseudocingulum group) and constitute a valuable indicator of the proximity of source vegetation to the depositional basin. Analysis of the pollens shows that, despite the salinity crisis, swamp/riparian and mesic plant communities surrounded the sedimentary basin and the climate remained humid.     

A 7000-year pollen record from the Amazon lowlands,Ecuador

The longest continuous Amazonian palynological record (ca 7010 yrs B.P. to present) from Lake Ayauchⁱ, Ecuador, reveals species-by-species abundance changes during a period of climatic change. Pollen influx from a wet tropical rain forest was found to be high, 1×10⁴–10⁵ grains cm^-2 yr^-1, although mature forest taxa were poorly represented. Horizons of laminated sediments and weathered gyttja, dated to ca 4200–3150 B.P., evidence a period of reduced net water availability. During this period Ficus, Alchornea and Palmae pollen representation appears to decline, although there is no evidence of a major forest compositional change. The lake was reduced to a shallow, possibly seasonal, pool. Zea cultivation was recorded between ca 2850 B.P., (the earliest paleoecological record to date in the Amazon basin) and ca 800 B.P. It is suggested that Zea was cultivated on exposed lake sediment within the crater at times of low water levels. The abandonment of Zea cultivation may have been due to rising water levels or social unrest.     

Vegetation and climatic history of southwest Africa: A marine palynological record of the last 300,000 years

A continuous palynological record from the marine core GeoB1016-3 from the Angola Basin reveals the regional vegetation and climate history of the last 300 ka. Pollen and spores found at the studied site have their source areas in the different vegetation zones of the adjacent part of the West African continent. Those vegetation zones comprise tropical rain forest, coastal mangrove swamp, Miombo woodland, dry forest, Afromontane forest, desert and semi-desert. The main pollen transport agent is the southeast trade wind system. Ocean currents also partly play a role in transporting pollen and spores. During the interglacial periods, ocean currents also transported palynomorphs southward. During the glacial periods, increased trade winds are indicated by high influx of pollen and spores and high pollen percentages of Poaceae and taxa from desert and semidesert vegetations. Reconstruction of the geographical position of palaeo-vegetation zones shows that the northern boundary of the Namib Desert did not move north of 12°S during the last 300 ka. This implies that northward shifts of the Angola-Benguela Front did not pass the latitude of 12°S.     

Vegetational stability and rare species from Early Holocene Lewis

Summary

Strata of complex geomorphological origin at Skigersta in the northeast of the Isle of Lewis include organic sediments which have provided a detailed pollen profile and two Early Holocene radiocarbon dates. These two sources of evidence suggest that a stable, pre-woodland, species-rich grass and sedge landscape may have existed for at least one thousand radiocarbon years (10,250 to 9250 BP). Included in the pollen taxa are Koenigia islandica and Polemonium caeruleum, neither of which is now native to Lewis. The former was present in the Middle Devensian (c. 30 kyr BP), perhaps hinting at survival in a local refugium during the Late Devensian glaciation.     

A Holocene environmental record from the southern Yangtze River delta, eastern China

The Holocene sedimentary record of core ZX-1, recovered west of mid-Holocene Chenier ridges on the monsoon-controlled Southern Yangtze delta, eastern China, consists of lagoon, salt marsh, upper tidal flat, and limnic facies, reflecting low-energy depositional environments. Holocene deposits mainly originated from the surrounding Tai Lake drainage basin. The temporal variation of most geochemical element percentages corresponds with the climatic phases inferred from the pollen record, i.e., the relatively low values of SiO₂, Na₂O, CaO and high values of Al₂O₃, K₂O, MgO, Fe₂O₃, FeO, FeO + Fe₂O₃ and TiO₂ generally concur with the warm and humid climate, and vice versa. Three geochemical indices − Al₂O₃/Na₂O, K₂O/Na₂O, and CaO/K₂O − are found to be sensitive to past precipitation in the monsoon-controlled southern Yangtze River delta. Based on fine sediment analysis of core ZX-1, the samples deposited under a dry climate with a mean annual precipitation (MAP) < 500–900 mm tend to have Al₂O₃/Na₂O values less than 12.2, and the samples deposited under a moist climate with an MAP 1000–1800 mm mostly have Al₂O₃/Na₂O values higher than 12.2. Similarly, the K₂O/Na₂O boundary value is 2.0. However, for CaO/K₂O, the dry climate sediments likely show values higher than 0.9, and those wet climate ones generally have values less than 0.9. This geochemical response suggests the potential application of these indices in the interpretation of palaeoclimate variation of monsoon-controlled eastern China.

Holocene climatic variation history is reconstructed for the southern Yangtze delta. From 8000 to 7000 yr BP, regional climate demonstrated frequent fluctuations, with warm and wet periods (8000–7700; 7500–7200 yr BP) alternating with cool and less humid periods (7700–7500; 7200–7000 yr BP). From 7000 to 6000 yr BP, the climate was relatively warm and humid. Since then, it had turned cool and dry, climaxing in an intense cold event around 4000 yr BP. After the cold event, it became warm and humid until around 2500 yr BP.     

An example of Holocene vegetation stability from Camerons Lagoon,a near treeline site on the Central Plateau,Tasmania

Abstract A discontinuous record of vegetation over the past 7500 years was obtained through pollen analysis of pond sediments from an extensive treeless plain on the relatively dry Central Plateau of Tasmania. The results demonstrate continuity of treeless conditions, which probably persisted from the Pleistocene throughout the Holocene, up to the present day. Some changes to the structure of the grassland were observed, especially over the past 200 years. Analysis of carbonized particles showed that fires had been infrequent over the period examined. It seems that this area, close to the altitudinal tree limit, has remained a natural grassland, and the hypothesis of Jackson (1973), that unreliable summer conditions may be a major factor in maintaining open conditions, is supported. Parts of the Central Plateau may thus preserve plant communities with some of the floristic elements and structure of widespread Bassian grasslands of the Late Pleistocene.     

Holocene climatic changes in the Western Mediterranean, from south-east France to south-east Spain

Holocene climatic changes along coastal regions from south-east France to south-east Spain were studied using pollen ratios. Comparing modern pollen rain, vegetation and climate along selected transects from the Atlantic Ocean to the Mediterranean, we obtained threshold values of two different ratios corresponding to the different climatic conditions along the transects. These pollen ratios and threshold values were employed to characterize the Holocene climatic changes from nine Mediterranean coastal sites. The results were compared with data from marine and continental pollen sequences distributed in the western Mediterranean basin, and with additional regional data independent of human activity: lake-level fluctuations, alpine glacier advance and retreat chronology, ¹⁴C anomaly and cooling phases in Eastern France and Central Europe. The role of anthropogenic activities and climate on the changes in vegetation is discussed. Six major changes in vegetation cover were identified. They correspond to aridification phases that occurred around 9500–9000 yr BP (10 900–9700 cal BP), 7500–7000 yr BP (8400–7600 cal BP), 4500–4000 yr BP (5300–4200 cal BP), 3700–3300 yr BP (4300–3400 cal BP), 2600–1900 yr BP (2850–1730 cal BP) and 1300–1000 yr BP (1300–750 cal BP). These arid episodes were regional responses to more global climatic changes and determined the changes in the vegetation cover. Humans undoubtedly enhanced the vegetation changes, but none the less had to adapt to these new climatic conditions.     

A review of the chronostratigraphical ages of Middle Triassic to Late Jurassic dinoflagellate cyst biozones of the North West Shelf of Australia

The chronostratigraphical ages of the 20 dinoflagellate cyst zones and one dinoflagellate cyst assemblage for the Middle Triassic (Ladinian) to the Jurassic-Cretaceous transition of the North West Shelf of Australia are comprehensively reviewed. Evidence from macro- and micropalaeontology, palynology and strontium isotopes made available after the establishment of these biozones in the 1980s has been used to reassess the ages of this important zonal scheme and to calibrate it to the international stratigraphical stages. The Shublikodinium Superzone is renamed herein as the Rhaetogonyaulax Superzone, and based on conodont evidence is determined to span the Ladinian to Early Sinemurian. This is significantly shorter in duration than was originally envisaged (Late Anisian to Late Pliensbachian). The Luehndea Assemblage is a low diversity dinoflagellate cyst association which marks a eustatic rise; it is subdivided into two subzones. It is of latest Pliensbachian to Early Toarcian age, based largely on palynological evidence. The Bajocian to earliest Oxfordian Pareodinia ceratophora Superzone represents the inception of a continuous Mesozoic-Cenozoic dinoflagellate cyst record in Australia. It comprises seven zones, which are considered to be slightly older than originally interpreted. The overlying Pyxidiella Superzone is characterised by diverse dinoflagellate cyst associations. It is Early Oxfordian to Kimmeridgian in age, and comprises three zones. The bases of the Wanaea spectabilis and Wanaea clathrata zones are reinterpreted as being slightly older than originally proposed. The superjacent Fromea cylindrica Superzone is Tithonian to earliest Valanginian and modified ages are indicated for four of the nine zones. This unit is dominated by endemic dinoflagellate cysts, reflecting a global trend towards provincialism at this time due to a regressive eustatic regime.     

西北干旱区石羊河流域全新世早期植被与环境演化

西北干旱区石羊河流域终闾湖泊边缘三角城剖面湖相沉积物（10.0-6.3ka.B.P.）高分辨率（每样平均40－50a左右）孢粉分析揭示该流域全新世早期植被与气候环境变化过程是：全新世初期（10－9.8ka.B.P.）,温度、湿度开始上升,山上针叶林发育,该期持续较短时间后,温度、湿度下降（9.8-9.2ka.B.P.）,山上森林萎缩,山下荒漠范围扩大;此后是一个持续时间较长、波动的温度、湿度上升、植被发育状况逐步好转的过程（9.2-7.75ka.B.P.）;随后又是短暂的气候冷干、植被亚化阶段(7.75-7.25ka.B.P.）和一个相对持续时间较长,植被发育较好的暖湿期（7.25-6.3ka.B.P.）。石河流域全新世早期气候环境变化具有较强的不稳定性,每个相对暧湿期和冷干期中都有多个次一级的冷干、暖湿波动,植被也相应地随之变化。