SEDIMENT DESCRIPTION

SEDIMENT DESCRIPTION

Coring

Coring at Blundell s Flat

SEDIMENT DESCRIPTION Core Splitting Cores should be split into two halves using materials and procedures that will minimize disturbance and contamination, especially that related to organic geochemistry and sediment magnetic properties. In general, one half of the core should be designated as the working half, and the other preserved as an archive half Photography Basic photography of split cores sediments documents changes of sediment color and texture that are often important indications of changes in the rates and processes of sedimentation in lakes. These changes also aid in correlation between cores from a single lake RODBELL, D. et al. 1999. Science 283, 516-520.

SEDIMENT DESCRIPTION X-ray Images X-ray images (commonly contact prints at a scale of 1:1) of whole or split core are commonly useful for observing bedding, laminations, and other sedimentary structures, even when the core is visually homogeneous. Gray-scale Measurements and Multi-spectral Scanning Various kinds of lightreflection scanning have proven useful for some sediments. Gray-scale profiles can be made from photographs of the core or thin sections. MOY, C. et al. 2002. Nature 420, 162-165.

SEDIMENT DESCRIPTION Thin Sections Thin sections are valuable for a variety of lithological studies such as varve counting or characterising fine laminations. Most methods include freeze-drying and impregnation of the sediment with resin. Visual Descriptions A standard protocol should be adopted for core description, including lithological and sedimentological classifications and procedures. At a minimum, all classification terms must be defined. Munsell colors and Troels-Smith symbols (Troels-Smith, 1955) are recommend. Troels-Smith symbols Munsell colour chart

SEDIMENT DESCRIPTION Particle size (Wentworth 1922)

SEDIMENT DESCRIPTION Nomenclature of sand, silt and clay mixtures Field estimates of sorting

SEDIMENT DESCRIPTION Characteristics of modified Troels-Smith system after Kershaw (1997). Kershaw, A.P. 1997. Quaternary Australasia 15, 63-68.

SEDIMENT DESCRIPTION Characteristics of modified Troels-Smith system after Kershaw (1997). Kershaw, A.P. 1997. Quaternary Australasia 15, 63-68.

SEDIMENT DESCRIPTION Example of a field sediment description form Kershaw, A.P. 1997. Quaternary Australasia 15, 63-68.

SEDIMENT DESCRIPTION Sediment description diagram Kershaw, A.P. 1997. Quaternary Australasia 15, 63-68.

Tidal-marsh lithofacies sequences in three adjacent core Alan R. Nelson, Andrew C. Asquith, and Wendy C. Grant (2004) Great earthquakes and tsunamis of the past 2000 years at the Salmon River estuary, central Oregon coast, USA. Bulletin of the Seismological Society of America, 94(4):1276-1292.

ANALYTICAL METHODS - Physical/Chemical

ANALYTICAL METHODS Physical/Chemical Bulk Sampling Several considerations are important for bulk sampling of cores for analytical work. The condition of samples required for each analysis must be considered foremost. In general, core material is never sufficient for all analytical needs, so conservation of material and efficiency should be considered. For example, samples taken in plastic cubes for magnetic properties and remanence analyses are often suitable for a variety of other analyses once the magnetic measurements are made. Finally, samples for some analyses should be taken at the same depths in the core as those for other analyses in order to derive the maximum information from the combined analyses.

ANALYTICAL METHODS Physical/Chemical Bulk density, Water Content Calculations of fluxes for any sediment constituent require measurements of sediment density. Samples should be carefully taken at appropriate intervals using standard volumes (e.g., syr-inges or paleomagnetic cubes). Wet volume density can be determined by simply weighing the sample container, subtracting the weight of the container, and dividing by the volume. Samples can then be slowly air dried (at up to about 60 C) and re-weighed. This measurement allows the dry volume density to be calculated (g/cc) and, with comparison to the wet volume density, allows the weight percentage of water in the original sample to be calculated.

ANALYTICAL METHODS Physical/Chemical Granulometry Sediment textures and grain-size distributions should be described in standard sedimentological terminology and size classes. In some cases, texture and grain size can be interpreted directly in terms of climatic variables, such as wind speed. The sand fraction is usually determined by shaking through a series of nested sieves. Material finer than 0.063 mm is commonly determined by use of a Sedigraph or by automatic particle counters based on light or resistivity. It can also be determined by pipette methods, which has the advantage of retaining size fractions for further analysis.

ANALYTICAL METHODS Physical/Chemical Organic Carbon (LOI) The amount of organic carbon in sediments is a fundamental property that is a function of autochthonous and allochthonous organic production, bacterial decay, and the rate of clastic sediment input. As measures of productivity, organic carbon is critical for paleoecological studies. Loss of weight on ignition (LOI) is the simplest method, although constituents other than carbon may contribute to weight loss. LOI can be determined by simply weighing the dried sample + crucible, subtracting the weight of the crucible, then igniting for 4 hours at 550 C, allowing to cool and re-weighing. LOI = (dry sample weight-ignited sample weight)/dry sample weight Wet-chemical methods are also used for both carbon and nitrogen.

ANALYTICAL METHODS Physical/Chemical Sediment Magnetic Properties Measurements designed to reveal changes in the concentration of magnetic minerals, their grain size, and their mineralogic composition. In addition, magnetic properties often provide a strong basis for local to regional core-to-core correlations. Magnetic property measurements have the advantages of being relatively cheap and simple to perform, largely non-destructive, and can be carried out on either a constant volume or a mass specific basis. The Bartington MS2 Meter and dual frequency susceptibility sensor

ANALYTICAL METHODS Physical/Chemical Chemical Analysis Elemental composition has been used to identify distinct layers such as volcanic ash. In addition, variations in elemental composition of autochthonous and allochthonous sediment fractions have been used to document soil erosion and development, vegetation change, and limnological conditions. example: Humification using visible light spectrophotometry. Used to determine degree of humification of peats and is considered a proxy for wet to dry conditions. Turney, C. et al. 2004. Nature 428, 306 310.

High-Tech Tech: : ODP Ship JOIDES Resolution

Low-Tech Tech: : Rubber raft Lake Euramoo sediment analysis Stratigraphy 14 210 C/ Pb Chronology Loss on Ignition (black, %) Magnetic Susceptiblility (grey, χ) 0 100 1600 210 Pb 1700 Depth below water surface (cm) 1800 1900 2000 2100 2200 2300 2400 0 500 Dark brown organic detritus Coarse detritus and wood Black lake muds with light brown bands Grey-green silty clays

SUMMARY SEDIMENT DESCRIPTION Core Splitting Photography X-ray Images Gray-scale Measurements and Multi-spectral Scanning Thin Sections Visual Descriptions ANALYTICAL METHODS physical/chemical Bulk Sampling Bulk density, Water Content Granulometry Organic Carbon (LOI) Sediment Magnetic Properties Chemical Analysis eg. Humification

ENVIRONMENTAL MAGNETISM

What is environmental magnetism? Main types of environmental magnetic measurements Measurement Sample Properties Objectives Palaeomagnetic Magnetic Fabric Made upon oriented blocks of 'undisturbed' rock and sediment samples. Made upon oriented blocks of 'undisturbed' sediment samples. Detection of changes in the Earth's magnetic field recorded within the geological record. The geological timing of these changes can be established, allowing palaeomagnetic sequences to be used as a relative dating technique. Like other fabric techniques, detection of the direction of flow of the transporting medium responsible for deposition of the sediment. Mineral Magnetic Samples of rock or sediment do not need to be oriented or undisturbed. Attempts to identify the concentrations and types of magnetic minerals within a sample (analogous to XRD or heavy mineral analysis). No interest in the orientation of grains.

Why are iron minerals interesting in an environmental context? Persistant: The iron oxides which dominate the magnetic properties of most soils, sediments and rocks are both robust (persistent) and yet sensitive to a whole range of environmental processes. Although often present in very small amounts, they are rarely totally absent and, even in small amounts, can play an important role in the chemical behaviour of the material and dominate its colour. Sensitivity: Mineral magnetic instrumentation is relatively cheap and easy to use. However, and much more importantly, it is very sensitive. For example, in terms of detecting the presence of different iron oxide assemblages within a sample, magnetic measurements can be several orders of magnitude more sensitive the other forms of analysis such as X- ray diffraction. Non-destructive: Most magnetic measurements are non-destructive. That is, magnetic measurements do not preclude other subsequent forms of analysis on the same samples.

Main types of magnetic behaviour displayed by rock forming minerals Type of magnetic behaviour Example minerals Diamagnetic Paramagnetic Quartz, Feldspar, Calcite, Water Olivine, Pyroxene, Garnet, Biotite Ferromagnetic group Ferri-magnetic Canted anti-ferromagnetic Magnetite, Maghaemite Haematite, Goethite

Types of mineral magnetic measurements Type Properties Example 'In-field' Remanence Sample is placed in a small, artificial magnetic field (larger than the Earth's natural magnetic field) and its response measured. Sample is placed into a large magnetic field (usually a series of gradually increasing field sizes are used) and then removed from that field. The magnetic response of the sample is then measured (that is, the amount of 'remanence' the sample has of the field). Magnetic susceptibility (χ). Frequency dependent susceptibility (χ fd ). Saturation Isothermal Remanent Magnetisation (SIRM). Anhysteretic Remanent Magnetisation (χ ARM ).

Papua New Guinea Crater Mt

Crater Mountain Wildlife Management Area

Crater Mountain Wildlife Management Area

Crater Mountain Wildlife Management Area

Crater Mountain Wildlife Management Area Magnetic Susceptibility (volumetric) from three cores Volume Susceptibility (k) Volume Susceptibility (k) Volume Susceptibility (k) 1000 500 0 500 0 500 0 200 Depth below water surface (cm) 400 600 210 Pb samples 14 AMS C samples Lake Muds Tephra 800