Chronology considerations

There are several different methods which can be applied to assign a chronology. The chosen method depends upon the time frame which the record is within and the dateable material available. Methods include radioisotopes, luminescence and chronostratigraphic markers including tephra and pollution. Each of the available methods have their own problems and errors associated with them and interpolation of dates may also increase errors further (Gehrels, 1999).

The principle limitation of reconstructed records is the coarsely constrained chronology which produces large uncertainties in the timing of changes (Edwards and Horton, 2006). The principal problems with dating include uneven temporal and spatial data distribution and the differing magnitude of associated errors, which results in variable age control (Edwards and Horton, 2006).

63 Radiocarbon

Past studies reconstructing sea levels using traditional radiocarbon methods were relatively imprecise, primarily due to uncertainties involved with conventional dating of large peat samples using radiocarbon dating (Belknap and Kraft, 1977). As accurate sea-level records are based upon precise ages, more detailed chronologies had to be utilised (Edwards and Horton, 2006). Accelerator Mass Spectrometry (AMS) radiocarbon ages on plant macrofossil fragments embedded within the sediment have provided more precise ages for SLIPs (Gehrels et al., 1992) and provide the chronology for most sediment sequences. In studies where the sediments are several thousands of years old, AMS radiocarbon may provide a suitable chronology alone, if enough accurate dates can be ascertained (e.g. Zong and Horton, 1999).

AMS radiocarbon dating has several disadvantages, including the limited availability of suitable organic deposits resulting in poorly constrained chronologies (Edwards and Horton, 2006). Where plant macrofossils are not available, AMS radiocarbon dating of calcareous foraminifera may be used and offers the potential for increasing the temporal precision of the resulting relative sea-level records (Horton et al., 2000). AMS radiocarbon dating also has the disadvantage of having large errors associated with them. The method in general may also produce erroneous dates. It may also be difficult to date the last few centuries due to the levelling of the calibration curve (Stuiver et al., 1998) which results in the production of a wide range of ages (Gehrels et al., 2006).

More recently, 14C bomb spike calibration has been used to provide high resolution

chronologies spanning the last few centuries, overcoming the problem associated with the levelling of the calibration curve. The approach has been successfully applied to peat sequences (e.g. Shotyk et al., 2003; Garnett and Stevenson, 2004) and saltmarsh sediments (e.g. Marshall et al., 2007). However, as with other dating techniques, independent age markers may be needed.

Luminescence

For sediments older than 300 years, AMS radiocarbon and luminescence are the most appropriate techniques which can be employed, for example, Horton and Edwards (2005) based their chronology on radiocarbon ages as well as infrared stimulated luminescence ages (IRSL) for sediments aged between 2000 and 7000 BP. However the use of IRSL dates may have large age uncertainties (Edwards and Horton, 2006). Luminescence has not been

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used on younger sediments as it was thought that there would be incomplete resetting of the signal before or during deposition and weak luminescence signals (Madsen et al., 2005). However, recent studies have investigated the use of optically stimulated luminescence (OSL) on younger sediments and determined that it may now be possible to obtain OSL dates on a scale of decades to a few hundred years (Madsen et al., 2005).

Metal Pollution

Estuaries are predominantly areas of deposition and act as an important sink for sediments and the metals associated with them, natural or otherwise (Ridgway and Shimmield, 2002; Spencer et al., 2003). Estuaries are often areas of port, industrial and urban development which results in large amounts of contaminants entering the surrounding environment (Ridgway and Shimmield, 2002).

Anthropogenically derived contaminants can enter the sediments directly through effluent discharge as well as through deposition from atmospheric pollution. They are transferred from solution to sediments by adsorption onto suspended particulate matter and deposited with relatively short lag times (Spencer et al., 2003). The supply of industrial pollution in association with incremental sedimentation can provide saltmarshes and mudflats with a stratigraphic record of metal pollution (Berry and Plater, 1998). Concentrations may then reflect the pollution deposited in the environment at the time of deposition (Cundy et al., 2003). Increases in the production of the contaminant will lead to increased input to the saltmarsh surface. It is, therefore, possible to relate peaks in concentrations to past changes in the industrial history of the area which may allow a chronology to be produced for the profile in the form of ‘event dating’ (Spencer et al., 2003). In order to use pollution to provide a chronology; knowledge of the distribution of contaminated material, an understanding of the processes which influence its accumulation and knowledge of the industrial activities in the area are all required.

65 Radionuclides

210_Pb

Younger sediments are often dated using the radioisotope 210Pb. This method can be used

to date sediments which are up to 120 years old, and is a common technique used in saltmarsh sediments.

210

Pb is a natural radioactive isotope formed from the 238U decay series (Anderson et al.,

2006). 238U which is present in the Earth’s crust decays to 226Ra and further to 222Rn which

escapes into the atmosphere and then decays to 210Pb (Anderson et al., 2006). Precipitation

brings 210Pb to the Earth’s surface where it adheres to fine-grained material and organic

matter (Anderson et al., 2006). 210Pb covers all surfaces which are exposed to the

atmosphere including sediments, this forms the ‘unsupported ‘part of the 210Pb. 238U also

decays from the mineral grains within the sediment also producing 210Pb, known as

‘supported’ activity. The supported activity is then subtracted from the total content to give the ‘unsupported’ only or ‘excess’ content which is used in the analysis (Anderson et

al., 2006). As 210Pb has a half-life 22.3 years, it is possible by analysing the exponential

decay of the content of the isotope with depth to date the sediment for last 120 years (approximately five half-lives) (Anderson et al., 2006).

When using 210Pb it is important that an independent age control to validate the chronology

is also used (e.g. Koide et al., 1973). However, according to Smith (2001) it is common to find that independent tracers are not used and it is assumed that no post-depositional mixing or single-particle sedimentation has taken place. Appleby and Oldfield (1992) and

Smith (2001) stress that independent validation of 210Pb chronologies must be an integral

part of the overall methodology. 137Cs and 241Am, pollution markers, varves and tephra

could all be used for this purpose. A disadvantage of 210Pb is that it may be subject to the

same diagenetic remobilisation processes as Pb, such as Fe and Mn cycling (Cundy and Croudace, 1996) and this may produce inaccurate estimates of sediment ages (Gubala et al., 1990).

137_{Cs and} 241_Am

The use of radionuclides can also be used in event dating to provide a chronology, and has been successfully applied in estuarine sediments around the country including the Mersey

Estuary (e.g. Fox et al., 1999). The radionuclide 137Cs (half-life 30 years) and 241Pu (half life

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generated radioactive nuclides that were released primarily by fallout from weapons

testing and the Chernobyl reactor accident (Faure et al., 2005).137Cs and 241Am can

therefore be used to assign the dates 1963 from the peak in weapons testing and 1986

from Chernobyl (Michel et al., 2001). The 241Am from fresh nuclear weapons test debris is

essentially zero (Krey et al., 1976) and its presence in older deposits is through in-growth

from 241Pu.

These radionuclides are particularly useful chronostratigraphic markers as they have very specific time frame in which it has been released into the environment. Weapons testing began in 1952 and peaked in 1963 when a ban was then introduced and resulted in a rapid decline in deposition of the radionuclide (Carpenter et al., 1987). Identification of peaks in

137

Cs in sediment records potentially enables the determination of these dates (e.g. Aston and Stanners, 1979; Delaune et al., 1978; Andersen et al., 2000). However, another input of radionuclides to the North West of the England, is the discharge from the nuclear fuel reprocessing facility at Sellafield, Cumbria. This provides the main delivery of increased concentrations of radionuclides to the Irish Sea and therefore to sediments on the west

coast of the UK and the east coast of Ireland. Discharge of radionuclidesfrom Sellafield

were at its peak in the mid-1970s which can provide a chronological marker for these sediments (Fox et al., 1999).

The use of Sellafield-derived radionuclides has an advantage over fallout sources as the radionuclides from nuclear reactors are disposed of directly into the coastal waters and are therefore rapidly transported into nearby estuaries, compared with the accumulation from fallout which may be longer (Aston and Stanners, 1979). It is important to note, however, that Irish Sea saltmarsh sediments preserve a record of the time-integrated discharge of Sellafield-derived radionuclides and not an annual record, due to intense mixing of contaminated offshore sediment before deposition in saltmarshes (Harvey et al., 2007).

Considerations of using pollution and radionuclide event dating

The input of pollution to sediments is not the only factor determining the concentrations of heavy metals and radionuclides within the profile. It is important to understand the other processes effecting the metals and radioisotopes in the sediment as it can affect the interpretation of the profile. Other factors which can complicate the interpretation include: bioturbation and mixing of the sediments, grain size, composition, diagenesis, post- depositional mobility, organic matter content, varying sedimentation rate and erosion of

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the sediment record (Valette-Silver, 1993). Therefore, a false impression of contamination may be recorded in the profile.

In order for pollution to be used to provide accurate chronologies there are several requirements that must be fulfilled: 1) since deposition of the marker horizon, sediment accumulation must be at a uniform rate; 2) little post-depositional mixing should have taken place or should be included in the model; 3) and little variation in the sediment characteristics which may affect the distributions (Morris et al., 2000).

Sediment characteristics which may affect the concentrations include the particle size which has a marked effect on the concentrations of pollutants per unit sediment mass (Aston et al., 1985). The finest sediments have a greater potential for trace metal binding than coarser grained particles due to their large surface area to volume ratios and the high density of metal binding sites potentially available (Kersten and Forstner, 1986; Schoer, 1985; Horowitz et al., 1989; Loring and Prosi, 1986) Therefore, significant changes in grain

size in the profile could lead to features in the concentration of metals and 137Cs and 241Am

(Sharma et al., 1987) which do not represent real changes in contamination levels but a change in grain size (Fox et al., 1999).

The same is true for organic matter content which has strong affinities with some metals as

well as 137Cs and 241Am, therefore, increases in organic matter may result in increases in

concentrations. Heavy metals may become associated with organic matter during initial deposition (Salomons and Forstner, 1984), then later may be redistributed by early aerobic degradation of organic matter. This may result in a gradual decrease in metal concentration with depth (Allen, 1990; Valette-Silver, 1993).

Organic matter plays an important role in forming complexes with heavy metals, as well as retaining heavy metals in an exchangeable form. The complexes are different for each metal and they also affect each metal differently. For example, Cu is bound and rendered unavailable mainly through the formation of organic complexes (Kirkham, 1977), while Cd is retained in an exchangeable form and is more readily available (Haghiri, 1974).

Post-depositional changes from diagenesis and remobilisation of metals and radionuclides, as well as the reworking of sediments themselves also cause problems. Mn and Fe are the most affected by diagenesis. Surface enrichment of Mn is common and related to redox cycling in the sediment (Spencer et al., 2003) with early-diagenetic remobilisation and reprecipitation in the oxic zone (Spencer et al., 2003). The reduction of Fe and Mn ions

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results in the mobilisation of these metals and diffusion to oxic surface sediments where they are reprecipitated as oxides or occasionally carbonates (Farmer and Lovell, 1984). Other metals which may be affected by mobilisation include Cu, Pb and Zn which commonly co-precipitate with sulphides below the redox-cline and therefore may show enrichment at depth (Croudace and Cundy, 1995; Zwolsman et al., 1993). Fuller (1977) found that in acidic soils (pH 4.2-6.6) the elements Cd, Ni, and Zn are highly mobile and Pb practically immobile, in neutral to alkaline (pH 6.7-7.8), Cr is highly mobile, Cd and Zn are moderately mobile and Ni is immobile.

137

Cs has also been found to be mobile and in many cases the value of it as a chronological marker has been significantly reduced (Appleby et al., 1991). However, many sequential

observations of 137Cs over 20 years have found the persistence of the 137Cs peak

corresponding to the mid-1970s regardless of the remobilisation (Pulford et al., 1998).

241

Am which has been found to exhibit less post-depositional re-dissolution (Appleby et al., 1991).

Post-depositional mixing may occur and may also alter the vertical profiles of the radionuclides. This includes the bioturbation of sediments by benthic fauna and the mixing of sediment by physical reworking (Aston and Stanners, 1979). In addition other processes which may cause problems include delays between delivery and sedimentation, as this may lead to an under-estimate of sediment accumulation rates (Milan et al., 1995).

Although there are many potential problems which could affect the sediment record, Cundy et al. (2003) found that saltmarsh cores from even the most heavily disturbed estuarine sites can provide useful information on variations in historical contaminant input. Saltmarshes are generally less susceptible to post-depositional disturbance and reworking of sediments as they are stabilised, and have a dense root system (Cundy et al., 1997) as well as anoxic conditions at depth which inhibit bioturbation (McCaffrey and Thomas, 1980). However, for sediments which have been vigorously mixed or reworked, large-scale compositional variations are present or where significant early-diagenic remobilisation has taken place, only general information on the scale of contamination can be observed (Cundy et al., 2003). Care should be taken in understanding the processes which may have occurred and affected the sediment profile whilst making interpretations.

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