Martyn Waller

Estimates of‘relative pollen productivity’ and‘relevant source area of pollen’ for major tree taxa in two Norfolk (UK) woodlands

Surface sample pollen assemblages and vegetation data collected from two East Anglian fen carr sites with adjacent dry woodland belts are analysed to estimate the relevant source area of pollen (RSAP) and the relative pollen productivity (RPP) values for the major canopy trees. The‘relevant source area of pollen’ is found to be on the order of 50-150 m, comparable with but slightly greater than estimates for forest hollow contexts in dry woodlands. Estimates of pollen productivity relative to Quercus are then compared with published values from south Sweden. Betula and Corylus values are similar, but some values estimated for taxa characteristic of wetter habitats, and therefore competitively advantaged in the fen carr system (Alnus, Salix and Fraxinus), are substantially higher at one or both sites. The results suggest that palaeoecological records from fen carr systems should be interpreted as reflecting predominantly local vegetation signals once the tree canopy is established.

The Holocene vegetational history of the Nigerian Sahel based on multiple pollen profiles

Four Holocene pollen diagrams are presented from interdune depressions in the Manga Grasslands (northeastern Nigeria near 13°N). These sequences are interpreted using modern pollen spectra, statistical analyses and groupings based on phytogeographical affinity (Long Distance, Sahelian, Sudanian, Guinean). The roles of climate change and anthropogenic activity in the vegetational history of the central part of Sahel are evaluated. Particular attention is given to the separation of regional (Manga Grasslands) from local (individual depressions) vegetation trends. Humid conditions in the early and mid-Holocene (from 10,000 to ca. 3300 yr B.P.) enabled the establishment of, and sustained, swamp forest vegetation in the interdune depressions. The main taxa (Alchornea, Syzygium and Uapaca) have Guinean affinities. The surrounding dunefields consisted of open savanna with Sahelian and Sudanian (e.g., Combretaceae, Detarium) elements present. Drier conditions ca. 3300 yr B.P. produced abrupt changes in pollen stratigraphy and led to the establishment of the modern vegetation of the Manga Grasslands. Although occupied since at least 3700 yr B.P., there is little evidence of human activity in the pollen diagrams. The nomadic pastoralism practiced by the human occupants of the Manga Grasslands may be palynologically undetectable. Although the Holocene vegetational history of the Manga Grasslands appears to have been primarily controlled by climate, caution should be exercised before drawing climatic inferences from these pollen sequences. For the early and mid-Holocene differences of ca. 1000 years exist in the timing of vegetation changes between the individual depressions. While water levels in the depressions are likely to be coupled to climate, the vegetation response appears to be strongly influenced by local conditions (in particular variations in the depth of depressions and so the relative height of water table). In addition, the Guinean swamp forest vegetation of the early and mid-Holocene is unlikely to be representative of vegetation trends at this latitude, but rather developed extrazonally as a result of the particular topographic/hydrological conditions prevailing in the Manga Grasslands. The problems of lags between climate and vegetation change and the presence of extrazonal vegetation, experienced in the Manga Grasslands, are likely to be common to other Sahelian pollen sites. The palynological information presently available for this zone is deemed insufficient for detailed subcontinental scale reconstructions of vegetation and climate to be attempted.

Drought and dust deposition in the West African Sahel: a 5500-year record from Kajemarum Oasis, northeastern Nigeria

A high-resolution, multiproxy palaeolimnological record from the Manga Grasslands, northeastern Nigeria, spanning the last 5500 calendar years, reveals the episodic deterioration in Sahelian climate as significant biogeophysical thresholds were crossed. Desert-dust deposition began to increase 4700 cal. BP. Rainfall during the summer-monsoon season declined permanently after 4100 cal. BP. A further significant change in atmospheric circulation, giving rise to multidecadal to centennial-scale droughts and enhanced dust deposition, occurred 1500 cal. BP. Hence, the post-1968 Sahel drought is not unique. The prolonged arid episode that occurred around 1200–1000 cal. BP in Ethiopia, the Sahel and tropical Mexico may have been linked to an abrupt cooling event in the North Atlantic and to a cluster of intense El Niño-Southern Oscillation events in the Eastern Equatorial Pacific.

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

Holocene coastal sedimentation in the Eastern English Channel: New data from the Romney Marsh region, United Kingdom

Abstract Multiproxy analysis (grain size, mineral magnetics, pollen, diatoms and radiocarbon dates) of deep (20 m+) cores collected from the Rye area of the Romney Marsh region are used to reconstruct coastal evolution between ca. 7000 and 2000 yr “conventional radiocarbon” B.P. A rapid phase of sea-level rise and tidal flat sedimentation occurred between ca. 7000 and 6000 “conventional radiocarbon” yr B.P., after which peat accumulation took place as a coastal barrier of sand and gravel extended eastward across Rye Bay and the rate of sea-level rise fell. Marine conditions returned to these areas at ca. 3000-2000 yr B.P. Erosion and landward migration of the barrier accompanied this return, heralding a change in the stability of the barrier and its ability to afford protection to back-barrier areas of the Romney Marsh region. Comparison with other sites in the central and eastern English Channel and the Thames estuary suggests that the prolonged removal of marine conditions from the study site accompanying barrier establishment reflects an ample sediment supply and the large volume of material within the barrier, which would have provided considerable scope for internal re-organisation when faced with changing coastal processes.

Holocene landscape evolution of the Manga Grasslands, NE Nigeria: evidence from palaeolimnology and dune chronology

This paper presents a synthesis of results from investigations into palaeolimnology and dune chronology in the Manga Grasslands and adjacent areas of NE Nigeria, in order to reconstruct the evolution of this semi-arid landscape since the late glacial. A marked wet phase gave way to a fall in lake levels during the late glacial. Dune emplacement was active at this time to the south of the Manga Grasslands, but seems to have been absent from the grasslands themselves. Wet conditions prevailed during much of the early to mid-Holocene, accompanied by periods of significant barchanoid dune reactivation. A marked deterioration in climate and vegetation commenced around 4.1 ka BP, leading to the formation of the present-day semi-arid landscape. The data suggest that there have been major changes in humidity in this part of Subsaharan West Africa during the late glacial to Holocene and that the relationship between changes in precipitation and aeolian activity is complex.

Late Holocene palaeolimnology of Bal Lake, Northern Nigeria, a multidisciplinary study

Abstract A multidisciplinary study of lake sediments from Bal Lake, a small saline lake in the Sahel zone of northeastern Nigeria, provides evidence for changing precipitation/evaporation ratios over the last 1000 cal. years. Climatic and environmental changes over this period have been reconstructed using evidence from ostracod faunal assemblages and shell chemistry, sediment properties and palynology and the sequences dated by a combination of 137 Cs, 210 Pb and AMS radiocarbon methods. The results testify to major hydrological changes in the Sahel over the last millennium. The end of a major drought phase that is seen in other records from the Manga Grasslands is dated to around 950 AD. This was followed by a prolonged freshwater phase of the lake from the 12th to the 16th century AD which points to wetter conditions. A period of higher salinity and hence drier and more variable conditions from the 1700s to the mid-1900s was followed by a pronounced drying trend into the 20th century.

Paludification and pollen representation: the influence of wetland size on Tilia representation in pollen diagrams

The representation of taxa with poorly dispersed pollen grains in pollen diagrams is strongly influenced by basin size. Waterlogging and peat formation (paludification) can, by leading to an expansion in wetland area, produce changes in the representation of dry-land taxa. This phenomenon appears to be responsible for reductions in Tilia pollen at a number of sites across lowland England, reductions which formerly may have been wrongly attributed to anthropogenic activity. Distinguishing between these processes can be problematic as both are likely to produce increases in herb pollen.

Interpretation of radiocarbon dates from the upper surface of late Holocene peat layers in coastal lowlands

Marine/brackish clastic sediments replace freshwater peats in the stratigraphic column of many coastal lowland areas bordering the North Sea during the late Holocene. Radiocarbon dates are routinely used to provide a chronology for this shift. We examine the assumptions underpinning this approach. The results of investigations from 13 sites in the Rye area of Romney Marsh, southeast England, are reported. Dates from apparently gradational contacts of a highly humified, laterally persistent, peat layer range from 3170-2840 cal. yr BP to 1290-1050 cal. yr BP. Multiple inundations or prolonged gradual inundation are nevertheless rejected, as discrete post-peat bodies of sediment are absent and because peat growth appears to have slowed-down or ceased at many sites in advance of inundation. Additionally in the Rye area, sharp contacts are widespread and the pollen assemblages rarely indicate the occurrence of transitional plant communities. A review of the dating evidence from other coastal lowland regions reveals that multiple dating of the upper surface of peat beds invariably produces diachronous results. As a consequence time transgressive processes feature prominently as causal mechanisms underlying this shift. However, many of the dating difficulties recognized in the Rye area appear to apply to other regions. We conclude that radiocarbon dates from the upper surface of peat layers should in most instances only be regarded as limiting ages for the deposition of the overlying clastic sediments. New chronologies need to be built without a priori assumptions as to the underlying processes, ideally through the direct dating of the clastic sediments.

The Holocene lithostratigraphy of Fenland, eastern England; a review and suggestions for redefinition

The Fenland basin is filled with unconsolidated Holocene marine and freshwater deposits. Stratigraphic studies of the basin date back to 1877. This paper reviews the various lithostrati- graphic schemes which have been proposed. Particular attention is paid to the presently accepted lithostratigraphy. Examples and a case study of a newly surveyed area are used to highlight its failings. Many of the difficulties experienced in Fenland are common to problems of lithostratigraphic classification in coastal lowland environments, as is demonstrated by reference to recent debate in the southeastern North Sea. In Fenland, as a result of the shortcomings of the various schemes, it is proposed that a new lithostratigraphy with formal stratotypes is devised. Suggestions are made as to the form this new stratigraphy could take. In the meantime, an informal lithostratigraphy should be adopted which has no regional or chronostratigraphic connotations.