Palaeoenvironmental Research

Palaeoenvironmental Sampling

Information on the nature, timing and rates of past environmental change provides a valuable long term perspective for understanding current and future impacts of climate change. Physical, chemical and biological analysis of sedimentary archives help us to build up a picture of both regional climatic variability and local environmental change. The CHERISH project is focusing on sediment sequences which have the potential to provide records of past storm activity, extending back over thousands of years, such as coastal peat bogs, back-barrier lagoons and dune systems.

Sand accumulation on top of peat at Ynyslas, Ceredigion.Sand accumulation on top of peat at Ynyslas, Ceredigion.

Field sampling (peat bogs and lagoons): Cores will be obtained from coastal peat bogs which may be subject to marine influence from sea spray and deposition of windblown sand and silt particles, which would be increased during periods of storm activity. Cores will be retrieved from the deepest point to retrieve the highest resolution record possible, following an initial survey. Further sampling will be undertaken along a transect towards the sea. Cores will also be obtained from back-barrier lagoons, where evidence of over-washing of beach material and marine incursions during storm events may be recorded in the sediment stratigraphy.

Field sampling (dunes): Where required, ground penetrating radar (GPR) surveys will be carried out on transects across a dune system, to identify the stratigraphy of the underlying sediments and suitable sampling sites, such as those which reveal evidence of multiple phases of dune activity. As individual dunes can be many metres thick, the best opportunity to retrieve such sequences is in the troughs between dune ridges.

Three main coring techniques are used to retrieve sediments for CHERISH palaeoenvironmental research:

Russian Corer
A Russian corer is ideal for sampling peats and soft, organic clays. It is not suitable for coarser, sandy sediments. A semi-circular core chamber usually 50cm or 100cm long, which rotates around a flat plate, is pushed manually into the sediment to the required depth. The sampler is then rotated 180 degrees to cut a sample. Core samples are transferred to plastic guttering. Samples from subsequent depths are retrieved with the addition of coring rods.

Russian corer is ideal for sampling peats and soft, organic clays

Livingstone Corer
A Livingstone corer is used on shallow lagoons and lakes. A securely anchored coring platform is essential and casing tube is used to guide the corer down the same hole for each drive. The Livingstone corer is piston operated, usually with a 100cm core barrel. The piston is set at the base of the core barrel and the corer is deployed to the appropriate depth using metal extension rods. Once at the sampling depth, the piston is tied off and the corer is pushed in to the required depth past the piston, which will end up at the top of the core barrel. As the corer is pulled back up to the platform, the piston will keep the sediment sample within the core barrel. Core samples are extruded into guttering either by hand or by using a hand winch. Some compression of sediment samples can occur during this process.

 Livingstone corer is used to core shallow lagoons and lakes

Percussion Corer
A percussion corer is suitable for drier, compacted sites which are impossible to core manually. It can be used to retrieve coarser, unconsolidated materials and is able to penetrate through gravel and cobbles. This system will be used to obtain core sequences from inter-dune settings. The percussion corer is essentially a jack-hammer with a core barrel attachment. Cores up to c. 5 metres depth can be retrieved in 1 metre sections with an open barrel and sub-sampled in the field, or with a closed, lined core barrel and transported intact to the laboratory.

Percussion corer is suitable for drier, compacted sites which are impossible to core manually

Laboratory Analysis

XRF Core ScannerXRF Core Scanner

All sediment cores are wrapped in plastic and kept in cold storage at Aberystwyth University. For Livingstone and Percussion samples, cores are split into archive and working halves. Initial core description (ICD) is the first phase of analysis, which involves visual description of the stratigraphic units and an initial assessment of their organic and inorganic components through microscopic investigation of prepared slides of untreated material.

Prior to any destructive sampling, all cores are scanned using an Itrax® X-ray Fluorescence core scanner. An optical image and X-radiograph from each core section will be obtained to aid stratigraphic descriptions. Radiographic images are particularly useful for identifying changes in density, that might be caused by deposition of a mineral layer (e.g. sand, volcanic ash) within a generally more organic sequence. The XRF scanner generates a high-resolution geochemical record of elements in the range from aluminium (Al) to uranium (U), with measurements typically every 200 microns (every 1/5th of a millimetre). Further information on applications of XRF scanning in palaeolimnology is provided in Davies et al (2015). A range of palaeoenvironmental analyses will be undertaken on core samples, the exact combination depending on individual site context and priorities, but may include:

Physical properties

The physical properties of lake, mire and peat bog sediments contain valuable information about the nature, source and depositional environment in which they accumulate. A wide range of laboratory techniques are available to quantify and classify sedimentary deposits. For example, the organic content of sediments can be quantified using a technique called loss-on-ignition, which involves weighing bulk sediment samples, then burning off the organic matter in a furnace and re-weighing the sample.

The size of mineral particles which make up the inorganic component is an important diagnostic feature in determining the source and mode of deposition of sediments, and can be measured accurately using laser diffraction analysis. Generally the larger the grain size the more energy is required to transport it and any changes in grain size might indicate a change in the source or mode of deposition. Thin layers of silts or sand within a coastal lagoon or wetland sediment might indicate that material has been washed into the site or blown in during stormy conditions. The relative thickness of such layers can provide information about the source of the material and the intensity or duration of an event, whereas event frequency might be inferred from the number of depositional layers.

Chemical indicators

A rapid assessment of the chemical composition of sediment cores can be made using the Itrax XRF core scanner. High resolution profiles of elements such as titanium and silica can detect the location and nature of mineral layers in sediment cores. Elements which are associated with marine conditions or sea spray (such as the halogen, bromine) can be measured and potentially used to identify periods of increased storminess in our core records. Itrax data, combined with x-radiographs of cores provide information on which sections of cores should be targeted for more detailed analysis. Further geochemical analysis of trace elements will be carried out on sub-samples from selected cores using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).

Biological indicators

Diatoms
Diatoms are microscopic (0.02 mm to 0.5 mm), single-celled algae that inhabit all water bodies - fresh, brackish or marine. They have a hard cell wall made from silica. Diatoms are an essential part of the aquatic food web as they are consumed by zooplankton, insects, fish and even whales. There are thought to be thousands of different species of diatoms, and, like higher plant forms, many have narrow, well-defined ecological tolerances. Diatoms have a short life-cycle, lasting days or weeks which means they respond rapidly and directly to environmental change. Some forms are free-floating existing singly in the plankton of ponds lakes and oceans, or joining together to form long chains; whilst others are attached to rock surfaces, mud substrates or aquatic vegetation.

The hard silica cell wall of diatoms is composed of 2 valves that fit together in the manner of a ‘pillbox.’ Variations in the morphology and ornamentation of the silica cell wall are used to distinguish between the different species. The silica cell wall is extremely resistant to decay and is often well preserved in sediments. Diatoms are particularly good indicators of changes in the salinity of water and can be used to identify transitions between marine, brackish and freshwater environments.

Scanning electron Microscope (SEM) image of DiatomsScanning electron Microscope (SEM) image of Diatoms

Pollen
Pollen Analysis is perhaps the best known and widely used palaeoenvironmental technique and is essentially employed to investigate and reconstruct vegetation histories and changes brought about through natural and human induced environmental change. Pollen spores, containing the male gametes of plants, are transferred by wind currents, insects, birds and animals to the female part of the flower and initiate fertilisation. Pollen spores are generally produced in vast quantities to increase the likelihood of pollination and they can carried over wide areas, settling out onto the surfaces of soils, bogs, and lake. These tiny spores usually 0.02 to 0.08 mm in size are extremely resistant to decay thanks to their waxy outer layer called sporopollenin - the chemical structure of which is still largely unknown. Structural and morphological features on the surface of the grains are used to identify the source plant, potentially indicating the family or genus.

Preservation of pollen grains is greatest in peat bogs, lake and pond sediments since the anaerobic conditions limit microbial attack, desiccation and disturbance by biological activity. Samples taken from cores of sediment extracted from these environments are treated with a combination of very strong acids and oxidisers to remove minerogenic and organic detritus. The isolated grains are then stained to enhance their surface detail, before being counted under a light microscope.

Pollen production, dispersal, source, deposition and preservation are important, complicating factors that need to be taken into consideration when interpreting pollen percentage changes within a sedimentary sequence. However, pollen analysis can be reliably employed to reconstruct vegetation changes on local or regional scales, changes through time, major climatic changes, and the influence of human activity on the landscape.

Ostracods
Ostracods are small bivalved crustaceans ranging in size from 0.2 to 2mm in length that have lived on Earth for nearly 500 million years, surviving 5 mass extinction phases. They occupy most aquatic habitats from marine through to freshwater, and are found in the warm tropical waters as well as the cold Polar Regions. Their shell is made from calcium carbonate; the size, shape and morphology of which is used to distinguish between different species. When preserved in sediments, ostracods can be extremely useful indicators of changes in temperature, salinity and water depth and even water quality. They have been studied in coastal lagoons to reconstruct salinity changes and the impacts of past storm events.

Foraminifera
Foraminifera are soft-bodied, single-celled organism enclosed within a shell of calcite or aragonite. They range in size from around 0.4 mm up to about 100 mm and while most species favour marine or brackish environments, a small number have adapted to life in freshwater. Their affinity to marine habitats has been used to investigate past changes in sea levels over a range of timescales.