58
Sand-size fraction was obtained by wet sieving each 1-cm-thick sample through
2 mm and 63-micron sieves to remove large organic fragments and fine-grained
sediments, respectively. Sand-size fraction (retained in the 63-micron sieve) was
then oven dried at 50 °C and weighed to calculate the sand to mud ratio. Results are
presented as a percentage of the total dry weight.
2.5 Proxies to Date Human Intervention
2.5.1 Lead-210
Recent sediments (last 100–120 years) can often be dated through the short-lived
isotope
210
Pb (half-life 22.3 years), which is a natural radionuclide from the
238
U
decay series (Appleby 2001). Its parent isotope
226
Ra in sediments decays into
210
Pb,
through the emission of
222
Rn. Part of this
222
Rn escapes into the atmosphere and
decays into
210
Pb, which is then deposited in the sediment. Therefore,
210
Pb appears
in excess (
210
Pb xs or unsupported, from atmospheric deposition) in the sediment
respect to the supported
210
Pb. Supported
210
Pb derives from in situ decay of its parent radionuclide
226
Ra in secular equilibrium (Appleby 2001). The
210
Pb xs activity in
the sediment can then be used to estimate the age (Cochran and Masque 2005).
In the cores described here, the total
210
Pb activity was determined by alpha spectrometry of its granddaughter
210
Po in secular equilibrium (Sanchez-Cabeza et al.
1998). The
226
Ra activity was determined in a coaxial high-purity Ge detector (EG&G
Ortec) by the gamma emission of
214
Pb (351 keV) in secular equilibrium. The
210
Pb xs
activity was calculated by subtracting the
226
Ra activity from the total
210
Pb activity.
Finally, the Constant Flux model (CF model: Sanchez-Cabeza and Ruiz-Fernández
2012; also known as the CRS model: Appleby and Oldfield 1978) was applied to the
Carasa core for age determination. Other cores (e.g. Lastra; Fig. 2.6) were analyzed
following slightly different procedures (see García-Artola et al. 2016).
2.5.2 Cesium-137
In order to corroborate the accuracy of
210
Pb-derived ages, the artificial radionuclide
137
Cs (half-life 30 years) is commonly used (Andersen et al. 2011).
137
Cs is related
to nuclear activity and provides three main chronohorizons: (1) the beginning of
atmospheric-nuclear-weapon-testing record in 1954 (2) the peak of nuclear detonations in 1963; and (3) the Chernobyl accident in 1986 (Ritchie and McHenry 1990).
Spain is far from major nuclear facility discharges in Europe (e.g. Sellafield in
Great Britain and La Hague in France), as well as from the Chernobyl plume.
Therefore, the peak of
137
Cs, determined by gamma spectrometry (661 keV), is
related to the maximum in nuclear activity in 1963 (Fig. 2.6). We discard the use of
the oldest chronohorizon, indicative of 1954, because agricultural activities and the
associated sediment disturbance occurred until the 1950s in most locations.
A. García-Artola et al.
Sand-size fraction was obtained by wet sieving each 1-cm-thick sample through
2 mm and 63-micron sieves to remove large organic fragments and fine-grained
sediments, respectively. Sand-size fraction (retained in the 63-micron sieve) was
then oven dried at 50 °C and weighed to calculate the sand to mud ratio. Results are
presented as a percentage of the total dry weight.
2.5 Proxies to Date Human Intervention
2.5.1 Lead-210
Recent sediments (last 100–120 years) can often be dated through the short-lived
isotope
210
Pb (half-life 22.3 years), which is a natural radionuclide from the
238
U
decay series (Appleby 2001). Its parent isotope
226
Ra in sediments decays into
210
Pb,
through the emission of
222
Rn. Part of this
222
Rn escapes into the atmosphere and
decays into
210
Pb, which is then deposited in the sediment. Therefore,
210
Pb appears
in excess (
210
Pb xs or unsupported, from atmospheric deposition) in the sediment
respect to the supported
210
Pb. Supported
210
Pb derives from in situ decay of its parent radionuclide
226
Ra in secular equilibrium (Appleby 2001). The
210
Pb xs activity in
the sediment can then be used to estimate the age (Cochran and Masque 2005).
In the cores described here, the total
210
Pb activity was determined by alpha spectrometry of its granddaughter
210
Po in secular equilibrium (Sanchez-Cabeza et al.
1998). The
226
Ra activity was determined in a coaxial high-purity Ge detector (EG&G
Ortec) by the gamma emission of
214
Pb (351 keV) in secular equilibrium. The
210
Pb xs
activity was calculated by subtracting the
226
Ra activity from the total
210
Pb activity.
Finally, the Constant Flux model (CF model: Sanchez-Cabeza and Ruiz-Fernández
2012; also known as the CRS model: Appleby and Oldfield 1978) was applied to the
Carasa core for age determination. Other cores (e.g. Lastra; Fig. 2.6) were analyzed
following slightly different procedures (see García-Artola et al. 2016).
2.5.2 Cesium-137
In order to corroborate the accuracy of
210
Pb-derived ages, the artificial radionuclide
137
Cs (half-life 30 years) is commonly used (Andersen et al. 2011).
137
Cs is related
to nuclear activity and provides three main chronohorizons: (1) the beginning of
atmospheric-nuclear-weapon-testing record in 1954 (2) the peak of nuclear detonations in 1963; and (3) the Chernobyl accident in 1986 (Ritchie and McHenry 1990).
Spain is far from major nuclear facility discharges in Europe (e.g. Sellafield in
Great Britain and La Hague in France), as well as from the Chernobyl plume.
Therefore, the peak of
137
Cs, determined by gamma spectrometry (661 keV), is
related to the maximum in nuclear activity in 1963 (Fig. 2.6). We discard the use of
the oldest chronohorizon, indicative of 1954, because agricultural activities and the
associated sediment disturbance occurred until the 1950s in most locations.
A. García-Artola et al.