Quaternary Geochronology 4 (2009) 378–390 Contents lists available at ScienceDirect Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo Review Radiocarbon: A chronological tool for the recent past Quan Hua* Australian Nuclear Science and Technology Organisation (ANSTO), PMB 1, Menai, New South Wales 2234, Australia a r t i c l e i n f o a b s t r a c t Article history: Received 2 April 2007 Received in revised form 3 March 2009 Accepted 6 March 2009 Available online 2 April 2009 The past few hundred years have seen large fluctuations in atmospheric 14C concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age, and the Spörer and Maunder solar activity minima. In addition, however, changes in human activity since the middle of the 19th century have released 14C-free CO2 to the atmosphere. Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapon testing resulted in a dramatic increase in the concentration of 14C in the atmosphere. This was followed by a significant decrease in atmospheric 14C as restrictions on nuclear weapon testing began to take effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The large fluctuations in atmospheric 14C that occurred prior to 1955 mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. This difficulty may be overcome by obtaining a series of 14C dates from a sequence and either wigglematching these dates to a radiocarbon calibration curve or using additional information on dated materials and their surrounding environment to narrow the calibrated age ranges associated with each 14 C date. For the period since 1955 (the bomb-pulse period), significant differences in atmospheric 14C levels between consecutive years offer the possibility of dating recent samples with a resolution of from one to a few years. These approaches to dating the recent past are illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms and speleothems. Ó 2009 Elsevier Ltd. All rights reserved. Keywords: Radiocarbon dating Chronology Recent past Carbon-14 wiggle-matching Bomb-pulse carbon-14 dating 1. Introduction The past few centuries have been characterised by dramatic and significant environmental changes. Sub-millennial scale climatic variations have resulted in a shift from the Little Ice Age, which began around the 14th century and may have continued to the mid19th century, to the warm episode of the last half century. Human activity has also contributed to changes in the Earth’s environment via land clearing, urbanisation and industrialisation, especially since 1850. Studies of these changes require a precise and accurate chronological framework, to which radiocarbon dating can contribute. Sixty years after the discovery of radiocarbon dating by W.F. Libby (1946), the method provides one of the most reliable and well-established means of dating the Holocene and Late Pleistocene. This is indicated by the worldwide existence of over 150 radiocarbon laboratories (Anon., 2008) that deliver tens of thousands of radiocarbon dates every year (Geyh, 2005). After a short description of basic radiocarbon dating, this paper discusses the features, potential and limitations of the method for dating the past few hundred years. Examples of the use of the technique in * Tel.: þ61 2 9717 3671; fax: þ61 2 9717 9265. E-mail address: qhx@ansto.gov.au 1871-1014/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2009.03.006 constructing chronologies of recent environmental archives and dating recent materials are presented. 2. Radiocarbon dating method 2.1. Principles Carbon has three natural isotopes: 12C and 13C, with relative abundances of w98.9% and w1.1%, and 14C or radiocarbon, which occurs only in minute amounts (w1.2  1010% in the troposphere, for example) (Olsson, 1968). Carbon-12 and 13C are stable isotopes, while 14C is radioactive. Radiocarbon is produced continuously in the atmosphere by the interaction of the secondary neutron flux from cosmic rays with atmospheric 14N, following the reaction 14 N þ n (neutron) / 14C þ p (proton). About 55% of 14C is formed in the lower stratosphere and 45% in the upper troposphere (Gäggeler, 1995). Following its production, 14C is oxidised to produce 14CO2, which is quickly dispersed throughout the atmosphere. The 14C is then transferred to other carbon reservoirs, such as the biosphere and oceans, via photosynthesis and air-sea exchange of CO2 respectively. Living organisms take up radiocarbon through the food chain and via metabolic processes. This provides a supply of 14 C that compensates for the decay of the existing 14C in the organism, establishing an equilibrium between the 14C Q. Hua / Quaternary Geochronology 4 (2009) 378–390 379 concentration in living organisms and that of the atmosphere. When an organism dies, this supply is cut off and the 14C concentration of the organism starts to decrease by radioactive decay at a rate given by the radiocarbon half-life. This rate is independent of other physical and environmental factors. The time t elapsed since the organism was originally formed can be determined from: bones (Hedges and van Klinken, 1992), pollen extracted from peats and lake sediments (Brown et al., 1989, 1992), macrofossils from lake sediments (Goslar et al., 2000; Kitagawa and van der Plicht, 2000), foraminifera from marine sediments (Broecker et al., 1990; Hughen et al., 2000) and specific skeletal components of carbonate sediments (Woodroffe et al., 1999, 2007).   T1=2 NðtÞ t ¼  ln No ln 2 2.4. Radiocarbon conventional ages (1) where T1/2 is the radiocarbon half-life, No is the original 14C concentration in the organism and N(t) is its residual 14C concentration at time t. 2.2. Contamination In general, any organism containing carbon and that once lived in equilibrium with atmospheric 14C can be dated by the radiocarbon method. Typical material for radiocarbon dating includes wood, charcoal and bones. Before samples are processed for dating, any contaminants (carbon-containing materials that do not belong to the original sample) must be removed, otherwise incorrect ages may be determined. According to Hedges (1992), the removal of contamination, known as the pretreatment step, can be carried out using two strategies. The first involves the physical and chemical removal of contaminants such as soils and sediments from the surrounding environment, roots and rootlets that may have penetrated from higher up the sequence, dissolved carbonates carried by groundwater, and humic acids derived from decomposed organic materials in the upper part of the sequence (Olsson, 1979; Mook and Streurman, 1983). This approach is usually employed in the case of samples of charcoal and wood. The second strategy involves the extraction of a specific contamination-free component from the sample, such as collagen from bones (Longin, 1971; Hedges and van Klinken, 1992) and alpha-cellulose from woods (Head, 1979; Hoper et al., 1998; Hua et al., 2004a; Anchukaitis et al., 2008). 2.3. Measurement methods Two different methods have been used for the measurement of 14 C concentration in a sample: decay counting and accelerator mass spectrometry (AMS). Since 14C is radioactive, emitting b particles with a maximum energy of about 156 keV, 14C can be measured by detecting these particles. This decay counting or radiometric method involves measuring 14C by either gas proportional or liquid scintillation counters (Taylor, 1987). In the case of gas proportional counters, pretreated samples are converted to CO2, while liquid scintillation counters employ benzene synthesised from the samples. By contrast, rather than counting the b particles resulting from 14C decay, whose rate is controlled by the long 5730 year halflife of 14C (Godwin, 1962), the AMS method counts 14C atoms directly (relative to those of the stable carbon isotopes 13C and 12C in the samples). Compared to the radiometric method, AMS has advantages in terms of measurement time (from tens of minutes to a few hours for AMS compared to a few days for the radiometric method) and the quantity of material required for dating (0.1–2 mg of carbon for AMS compared to 0.5–2 g or more of carbon for the radiometric method) (Tuniz et al., 1998; Jull and Burr, 2006). For AMS, pretreated samples are converted to CO2 and then graphite. After three decades of development, samples containing as little as 10–20 mg of carbon can now be reliably analysed by AMS (Hua et al., 2004b; de Jong et al., 2004; Santos et al., 2007; Smith et al., 2007; Petrenko et al., 2008). The ability to analyse small samples using AMS techniques has opened up opportunities for radiocarbon dating of new materials such as specific amino acids extracted from Radiocarbon ages are reported in years before present (BP), where ‘present’ is conventionally defined as AD 1950. In radiocarbon age calculations, the atmospheric 14C concentration in 1950, a hypothetical value, is conventionally set at 100 percent Modern Carbon (pMC) (Stuiver and Polach, 1977) or 1 fraction modern carbon (F) (Donahue et al., 1990; McNichol et al., 2001; Reimer et al., 2004a). As isotopic fractionation (differentiation against heavier isotopes) occurs in nature, for example during photosynthesis and air-sea exchange of CO2, different carbon materials have different d13C values (Hoefs, 1987). In addition, the depletion in 14C relative to 12C as a result of fractionation is approximately twice the depletion in 13C relative to 12C (Craig, 1954). Measured 14C concentrations must therefore be corrected for isotopic fractionation using d13C. Conventionally, this is achieved by the normalisation of measured 14C values from measured d13C to d13C ¼ 25& PDB (an average d13C value for C3 plants) before age calculations are performed. To simplify the calculation of radiocarbon ages, atmospheric 14C concentration is assumed to be constant through time, with the implication that all living terrestrial materials have an initial 14C concentration of F ¼ 1. From equation (1), the conventional radiocarbon age of a sample S is defined as: t ¼  T1=2 lnðFS Þ ln 2 (2) or t ¼ 8033 lnðFS Þ (3) 14 where FS is the C concentration in sample S in fraction modern carbon after correction to d13C ¼ 25& PDB. This value is determined by measuring sample S against 14C standard reference materials such as oxalic acid I (Olsson, 1970), oxalic acid II (Stuiver, 1983) or ANU-sucrose (Polach, 1979). T1/2 is the Libby half-life of radiocarbon of 5568 years. This half-life is approximately 3% shorter than the correct half-life of 5730  40 years (Godwin, 1962). The discrepancy between the two half-lives is corrected during the radiocarbon calibration process. Ages up to about 50 000 years (w9 to 10 half-lives of 14C) can be determined by radiocarbon dating. 2.5. Calibration It is well known that the 14C concentration of the atmosphere has not been constant in the past (Reimer et al., 2004b). Variations in atmospheric 14C concentrations are mainly due to variations in the rate of radiocarbon production in the atmosphere, caused by changes in the Earth’s magnetic field, variability in solar activity and changes in the carbon cycle (Taylor, 1987; Damon and Sonett, 1991). Long-term (103–104 years) fluctuations in atmospheric 14C are the result of changes in the Earth’s magnetic field. Short- and medium-term (101–102 years) variations in atmospheric 14C are mainly due to variability in solar activity, although centennial-scale variability may also be due to changes in geomagnetic field intensity (St-Onge et al., 2003). The result is that radiocarbon and calendar ages are not identical, and the former ages have to be 380 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 converted to the latter using a calibration curve, which describes the atmospheric 14C concentration in the past measured in precisely and independently dated materials. The current internationally-ratified calibration curve IntCal04 covers the past 26 000 calendar years (cal) BP (Reimer et al., 2004b). This curve is based on dendrochronologically-dated tree rings for the period 0–12 400 cal BP. For the remaining period 12 400–26 000 cal BP, the curve is derived from independently dated marine samples such as foraminifera and corals, with an assumption of a constant marine reservoir effect for each sampling site. Beyond the IntCal04 timescale, there are several published age calibration data sets. These are derived from independently dated materials such as corals (Fairbanks et al., 2005), foraminifera in marine sediments (Hughen et al., 2006), speleothems (Beck et al., 2001) and terrestrial macrofossils in varved sediments (Kitagawa and van der Plicht, 2000). However, these data sets are not in good agreement beyond 26 000 cal BP (van der Plicht et al., 2004) either because the archives from which the data sets are derived have their own problems and/or because the data sets are based on a simple assumption of a constant radiocarbon reservoir effect through time (van der Plicht, 2002). Calibration of 14C ages is usually undertaken using a computer program. Several calibration programs are available on-line. These include CALIB (http://radiocarbon.pa.qub.ac.uk/), OxCal (http://c14. arch.ox.ac.uk/embed.php%3FFile¼oxcal.html) and CalPal (http:// www.calpal.de/). Additional calibration programs can be found on the Radiocarbon journal website at http://www.radiocarbon.org/ Info/index.html. 2.6. Reservoir effects The deep ocean has a much lower 14C content than that of the atmosphere. This is because deep ocean waters experience long periods when they are not in contact with the atmosphere (the residence time of carbon in the deep ocean is w800 years (Broecker, 2000)). During this time the 14C content of deep ocean waters is depleted by radioactive decay. As a result, materials drawing carbon from deep ocean reservoirs may have lower initial 14 C concentrations than contemporaneous materials of terrestrial origin. This may result in their appearing older than contemporaneous terrestrial materials. In the case of the surface ocean, by contrast, interaction with both the atmosphere and the deep ocean means that surface waters have 14C concentrations that are intermediate between these two reservoirs. Organisms that live in the surface ocean, such as shells, corals and planktonic foraminifera, therefore appear younger than contemporaneous deep ocean materials, but older than contemporaneous terrestrial samples. The offset between surface ocean and terrestrial samples is known as the marine reservoir age (R). To calibrate a radiocarbon date for a surface ocean sample, the IntCal04 curve can be used with a known value of R. Alternatively, the current internationally-ratified marine calibration curve, Marine04 (Hughen et al., 2004), can be used, with a known value of regional offset from the global marine model age for that sample, defined as DR. The latter method is generally preferred and an on-line database of DR for different regions is available (Reimer and Reimer, 2001). For age calibration, the DR and R of a location are usually assumed to be constant through time (Stuiver et al., 1986). However, recent studies have reported variations of several hundred to a couple of thousand years in these values during the Late-glacial (for the southwest Pacific (Sikes et al., 2000), the Mediterranean (Siani et al., 2001), northern and equatorial Atlantic (Kromer et al., 2004; Bondevik et al., 2006; Cao et al., 2007; Sarnthein et al., 2007), and northern and tropical Pacific (Sarnthein et al., 2007)) and the Holocene (for the tropical Pacific (Yu et al., 2007; McGregor et al., 2008)). These variations are due to changes in ocean circulation and the carbon cycles associated with climatic changes. Temporal variations in DR and R values should therefore be considered when calibrating 14C ages of marine samples from these regions. Aquatic plant fragments, freshwater shells and lake sediments are also influenced by reservoir effects. These materials can appear older than contemporaneous terrestrial samples because a portion of the carbon in lakes comes from depleted 14C-carbon sources, such as dissolved inorganic carbon from groundwater and carbonates from limestone (Deevey et al., 1954). This reservoir age lies between several hundred to more than a thousand years (Colman et al., 2000; Zoppi et al., 2001) and can vary significantly with time (Geyh et al., 1998). Radiocarbon dates from these materials must therefore be corrected for any reservoir effect before being calibrated using the IntCal04 curve. 3. Radiocarbon dating of the recent past: features, potential and limitations During the past few hundred years the carbon cycle has experienced both human disturbance and natural variation. The natural variations are largely a product of climatic change (for example, the Little Ice Age from approximately the 14th to the mid-19th centuries) and changes in solar activity (for example, the Spörer, Maunder and Dalton minima). Disturbances due to human activities include anthropogenic CO2 perturbation resulting from the combustion of fossil fuel, changes in land use since the middle 19th century and 14C disturbances due to atmospheric nuclear explosions beginning in 1945. These variations and disturbances have led to changes in atmospheric 14C concentration through time. 3.1. Radiocarbon dating during the period before the onset of bomb 14C Natural short- and medium-term variations in atmospheric 14C are largely attributable to solar variability, especially to changes in the magnetic field strength of the solar wind. These alter the magnitude of deflection (the shielding effect) of galactic cosmic rays travelling towards the Earth, resulting in variations in secondary neutron flux and in the production rate of 14C in the atmosphere (Stuiver and Quay, 1980). As a result, during intervals of high solar activity, shielding of the Earth’s atmosphere from cosmic rays increases, which causes a decrease in 14C production. By contrast, during periods of low solar activity, the shielding effect decreases, leading to an increase in 14C production. Climatic change also contributes to short- and medium-term variations in atmospheric 14C via the redistribution of 14C between carbon reservoirs (mainly the atmosphere and oceans) due to changes in air–sea exchange of CO2, variations in deep water formation and changes in the ocean’s thermohaline circulation (THC) (Stocker and Wright, 1996; Broecker, 1997; Broecker et al., 1999; Clark et al., 2002). A weaker THC results in higher atmospheric 14C as less 14C from the surface ocean/atmosphere is carried away to the deep ocean, while a stronger THC results in lower atmospheric 14C. The centuries before the first appearance of bomb 14C in the mid-20th century are characterised by large fluctuations in atmospheric 14C concentration. These are depicted in Fig. 1. Increases in atmospheric 14C, which are centred on AD 1500, 1700 and 1815, are largely the product of the Spörer, Maunder and Dalton solar activity minima respectively, although a small portion of these increases may be attributed to changes in the carbon cycle associated with climatic change during the LIA (Stuiver and Quay, 1980; Damon and Sonett, 1991; Bard et al., 1997). The large decrease in atmospheric 14 C after c. AD 1900 is mainly due to the continuous release of 14 C-free CO2 to the atmosphere as a consequence of the combustion 381 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 a Year (AD) 1500 1600 1700 IntCal04 calibration curve Gaussian distribution of uncalibrated 14C age Probability distribution of calibrated ages 600 1800 1900 500 Radiocarbon Age (BP) Radiocarbon Age (BP) 1400 600 500 400 300 400 300 200 100 150 ± 40 BP 0 200 100 1400 1500 1600 1700 1800 1900 Year (AD) b Fig. 2. Calibration of a single radiocarbon date of 150  40 years BP using the IntCal04 calibration curve (shown at a 1s range). The calibrated age of this sample encompasses five possible age ranges from AD 1660 to 1950 (indicated by the grey boxes). 20 0 S M D -10 -20 LIA -30 1400 1500 1600 1700 1800 1900 Year (AD) Fig. 1. Atmospheric 14C in the period AD 1400–1950. (a) The IntCal04 curve (Reimer et al., 2004b) plotted at a 1s range. (b) Atmospheric D14C, derived from the IntCal04 data, plotted at a 1s range. D14C is the age- and fractionation-corrected deviation from the hypothetical value of atmospheric 14C in 1950 (Stuiver and Polach, 1977). S ¼ Spörer minimum, M ¼ Maunder minimum, D ¼ Dalton minimum and LIA ¼ the Little Ice Age. of fossil fuels since AD 1850. This is known as the Suess effect (Suess, 1955). It is worth noting that the combustion of fossil fuels in the period c. AD 1850–1900 was too small to cause an obvious decrease in atmospheric 14C (Marland et al., 2008). These 14C variations result in large fluctuations (wiggles) in the IntCal04 curve for recent periods, particularly from AD 1650 to 1950 (Fig. 1a). Any attempt to determine the calibrated age of a sample with a 14C age of few hundred years may thus yield several possible age ranges. As an illustration, a single sample with a conventional radiocarbon age of 150  40 years BP is shown in Fig. 2. The calibrated age of this sample contains five possible age ranges from AD 1660 to 1950, indicated by grey boxes. Even if the precision of the age were improved, the range of calibrated ages would not change. This provides a limit to the capacity of radiocarbon methods for dating the recent past when only a single sample is dated. In other words, the existence of large wiggles in the calibration curve during the last few centuries markedly decreases the precision of single radiocarbon dates. In order to avoid this problem and to obtain more precise calibrated ages, two approaches have been applied, both using a series of 14C dates instead of a single date. The first approach is known as 14 C wiggle-matching. This is illustrated in Fig. 3. A series of samples, each separated by known time-spans (ti), is dated by radiocarbon. These radiocarbon dates form a block of wiggles that can be compared with those in the radiocarbon calibration curve. This block of 14C dates is shifted along the x-axis. When the total difference between these radiocarbon dates and the calibration curve reaches a minimum, the best fit of the two data sets is approached and a more precise calibrated age associated with each single date in the series is obtained (Pearson, 1986; Bronk Ramsey et al., 2001; Geyh, 2005). Radiocarbon wiggle-matching between a series of 14C dates and a radiocarbon calibration curve can be performed using the OxCal calibration program (Bronk Ramsey, 2001). The 14C wiggle-matching method has been used to date peat profiles on the assumption that the peat accumulated in a piecewise linear fashion (Blaauw and Christen, 2005; Yeloff et al., 2006). The second approach also uses multiple 14C dates in sequence, but additional information on dated materials and their surrounding environment (for example, changes in peat composition) is required to narrow the calibrated age ranges associated with each 14 C date (Turetsky et al., 2004; Goslar et al., 2005; Yeloff et al., 2006). In this way, more precise calibrated ages can be achieved. Examples of these approaches are illustrated in Section 4. There are small differences in the natural atmospheric 14C concentration between the Northern and Southern Hemispheres. These are known as inter-hemispheric 14C offsets. The Southern Hemisphere has a larger surface ocean area than the Northern Hemisphere (w60% compared to w40%) with greater wind velocities. As a result, more 14C in the southern troposphere is transported to the oceans through air-sea exchange of CO2 and more 600 Radiocarbon Age (BP) ∆14C (o/oo) 10 500 400 300 200 100 t1 0 1400 1500 1600 1700 t2 t3 1800 t4 1900 Year (AD) Fig. 3. Radiocarbon dating by the wiggle-matching method. The solid dots represent a series of radiocarbon dates separately by known time intervals (ti, with i ¼ 1–4). The error bars are 1s. The dates form a block of wiggles (dashed grey lines) that may be moved along the x-axis to achieve the best fit with the 1s range of the IntCal04 curve (solid black lines). Q. Hua / Quaternary Geochronology 4 (2009) 378–390 3.2. Bomb-pulse radiocarbon dating As a result of hundreds of atmospheric nuclear detonations almost entirely in the Northern Hemisphere in the late 1950s and early 1960s, there was a dramatic increase in the concentration of 14 C in the atmosphere. Atmospheric 14C reached a maximum in the Northern Hemisphere in AD 1963–1964, almost double its prebomb level (Fig. 5). Since then, atmospheric 14C concentrations have decreased due to rapid exchange between the atmosphere and other carbon reservoirs (mainly the biosphere and oceans). Although several atmospheric nuclear bomb tests were carried out in the period AD 1945–1951, these were too small to increase atmospheric 14C (Hua et al., 1999) and the take-off in atmospheric 14 C began only in AD 1955 with the injection of moderate amounts of 14C into the atmosphere in association with atmospheric nuclear bomb tests during AD 1952–1954. For the period since AD 1955, significant differences in atmospheric 14C levels between consecutive years offer the possibility of dating recent terrestrial samples with a resolution of from one to a few years. Bomb-pulse 14C dating, which is based on the 14C concentration in materials at the time of their formation, therefore differs from conventional radiocarbon dating, which is based on the residual 14C concentration in dated samples due to radioactive decay. For the bomb period, measured 14 C concentration is usually reported in pMC or F rather than as radiocarbon ages, and a bomb 14C curve is employed to convert the measured pMC or F values to calendar years. A simple illustration of bomb-pulse 14C dating is shown in Fig. 5, in which a sample S with a 14C concentration of FS possesses two possible calendar age ranges T1 and T2. Hua and Barbetti (2004) recently reviewed all available atmospheric 14C data for the bomb period from AD 1955 onwards and provided a comprehensive compilation of tropospheric bomb 14C concentration for use in bomb-pulse 14C dating. They compiled four zonal data sets of tropospheric bomb 14C data at (mostly) monthly 700 400 350 C Age (BP) 600 500 300 250 200 14 Radiocarbon Age (BP) 14 C-depleted CO2 from the oceans is transported to the southern troposphere. Natural 14C levels in the southern troposphere are therefore usually lower than those in the northern troposphere, and the radiocarbon ages of terrestrial materials in the Southern Hemisphere for a particular period of time are usually older than those in the Northern Hemisphere. The calibration curves for the northern (IntCal04) (Reimer et al., 2004b) and southern (SHCal04) (McCormac et al., 2004) temperate regions for the past few hundred years are shown in Fig. 4. For the period AD 1500–1920, the SHCal04 radiocarbon ages are older than their IntCal04 counterparts with a difference varying from 3 to 63 14C years. However, for the period from AD 1920 to 1955, the SHCal04 radiocarbon ages are almost equal to or younger than their Northern Hemisphere counterparts, with a maximum 14C offset of 35 years in AD 1940. This is due to a weaker dilution effect of 14C-free CO2 from the combustion of fossil fuels in the southern troposphere compared to that in the northern troposphere (the primary source of this anthropogenic CO2 is in the Northern Hemisphere) (McCormac et al., 1998; Stuiver and Braziunas, 1998). There is currently no radiocarbon calibration curve that may be applied to tropical regions. A decadal 14C data set derived from dendrochronologicallydated tree rings from northern Thailand for the period AD 1620– 1780 (Hua et al., 2004a) is shown in the inset diagram of Fig. 4 in comparison with IntCal04 and SHCal04. The Thai data differ little from IntCal04 for the period of steep decrease in 14C age (AD 1630– 1710), but are similar to SHCal04 for the period AD 1720–1780. A simple calibration data set for the tropics based on average values of IntCal04 and SHCal04 may therefore not be ideal, especially for the use of radiocarbon to reconstruct high-precision tropical chronologies. 150 400 100 1600 1650 300 1700 1750 1800 Year (AD) 200 IntCal04 SHCal04 N. Thailand (17°N, 102°E) 100 1400 1500 1600 1700 1800 1900 Year (AD) Fig. 4. Radiocarbon calibration curves for the period AD 1400–1950. IntCal04 and SHCal04 are representative of northern and southern temperate regions respectively. In the inset diagram these records are compared with decadal 14C data from tropical northern Thailand. The error bars for the Thai data are 1s. resolution (three in the Northern Hemisphere and one in the Southern Hemisphere), which are depicted in Fig. 6. Significant differences in 14C between the four zones are evident for the early bomb period from the late 1950s to the late 1960s. As almost all the sources of bomb 14C were located in the Northern Hemisphere, the distribution of bomb 14C during this period reflects the major zones of atmospheric circulation and their boundaries as excess 14C was transferred southwards from the northern high-latitudes (Hua and Barbetti, 2004, 2007). Since AD 1980, the atmospheric 14C level of the Southern Hemisphere has been slightly higher than that of the Northern Hemisphere. This is a result of greater contamination by 14 C-free anthropogenic CO2 in the Northern Hemisphere. Several calibration programs have been devised for bomb-pulse 14C dating. These include CALIBomb (http://radiocarbon.pa.qub.ac.uk/) (Stuiver and Reimer, 1993) and OxCal v4.0 (http://c14.arch.ox.ac.uk/ embed.php%3FFile¼oxcal.html) (Bronk Ramsey, 2001). The bombpulse 14C method has been used to date recent skeletons for forensic studies (Wild et al., 1998), to establish the age of drugs and wines (Zoppi et al., 2004), and to date human cells and teeth for biological studies (Spalding et al., 2005; Bhardwaj et al., 2006). Further applications of this dating method are described in the next section. 2.0 Fraction modern carbon (F) 382 Bomb curve for Northern Hemisphere zone 1 F value measured in sample S Probability distribution of calibrated dates 1.8 1.6 Fs 1.4 1.2 1.0 1950 T1 1960 T2 1970 1980 1990 2000 Year (AD) Fig. 5. Atmospheric 14C in Northern Hemisphere zone 1 for the period AD 1955–2000 (Hua and Barbetti, 2004). For an F value measured in a terrestrial sample S (FS), bomb 14 C delivers two possible calendar dates (T1 and T2), indicated by the grey boxes. 383 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 4. Examples of radiocarbon dating of the recent past 4.1. Dating of peat profiles The radiocarbon dating of recent peat profiles has been undertaken by a number of investigators (Gallagher et al., 2001; Goodsite et al., 2001; Mauquoy et al., 2002; Blaauw et al., 2003; Donders et al., 2004; Charman and Garnett, 2005; van der Linden and van Geel, 2006). A comprehensive review of dating recent peats was carried out by Turetsky et al. (2004), who discussed several dating methods, including 14C and the use of chronostratigraphic age markers. They concluded that radiocarbon dating of recent peat profiles may offer accurate and precise results at decadal-scale resolution for the few hundred years before the onset of bomb 14C and at 1–2 year resolution during the bomb period. Here, two examples of radiocarbon dating of recent peats using different age– depth models are presented. Yeloff et al. (2006) radiocarbon-dated a number of peat profiles in northwest Europe. For each core a series of radiocarbon dates on terrestrial plant remains, composed of seeds and Sphagnum stems and branches, were obtained at 1 cm intervals. The authors used two different age models based on Bayesian statistics to estimate peat accumulation rates: 14C wiggle-matching using the Bpeat Fraction modern carbon (F) a program (Blaauw and Christen, 2005) and multiple dates from a stratigraphic sequence using BCal (Buck et al., 1999). Bpeat assumes piece-wise linear accumulation of peat deposits, while BCal uses constraints in the chronological ordering of dates (deeper samples are older than shallower samples) to reduce the calibrated age ranges of individual 14C dates. The authors dated five northern European ombrotrophic peat bogs. The accumulation rates at each site derived using the two methods were generally similar. A comparison of the rates estimated using the two methods for the top part (30–50 cm depth) of a peat core from Lille Vildmose (Denmark) is shown in Fig. 7. Goslar et al. (2005) dated a number of peat profiles in Europe spanning the past 400 years. At each site, high sampling resolution was achieved by collecting materials at 3–5 mm intervals. In most cases Sphagnum was used for 14C analysis. The maximum values of bomb 14C in peat profiles varied between sites, and in all cases were lower than the atmospheric bomb peak value. This implies that the peat sections contain a mixture of 14C assimilated over a period longer than the resolution of atmospheric records. This period was estimated for each site and taken into account when an age–depth model was built. Goslar et al. used multiple dates from a stratigraphic sequence and lithological information (accumulation rates may change when lithological conditions change) to constrain their 2.0 NH zone 1 NH zone 2 NH zone 3 SH zone 1.8 1.6 1.4 1.2 1.0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year (AD) b Fig. 6. (a) Regional tropospheric 14C curves for the period AD 1955–2001 for four different zones (Northern Hemispheric zones 1–3 and Southern Hemispheric zone). (b) The four zones into which the tropospheric 14C data have been grouped (Hua and Barbetti, 2004). 384 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 cal BP 500 30 400 300 200 200 100 0 200 100 100 b d 100 80 Sphagnum Cuspidatum 300 Sphagnum magellanicum 400 Sphagnum section Cuspidata 500 60 Sphagnum tenellum 300 Monocots undiff. cal BP 14 C Age (BP) 400 20 Depth (cm) 35 40 45 50 a c Fig. 7. Age–depth models for the top part (30–50 cm depth) of a peat core from Lille Vildmose in Denmark. (a) The age–depth model based on Bpeat wiggle-matching. The dark solid line indicates the best fit with the IntCal04 curve. The grey bars indicate 95% confidence intervals. (b) The section of the IntCal04 calibration curve corresponding to the period during which the peat was deposited. (c) The age–depth model based on BCal (solid line). The hollow histograms represent the probability distributions of individual calibrated ages calculated using CALIB 5.0. The solid histograms depict the possible calibrated age ranges of individual samples determined by BCal. (d) The main plant macrofossil components in the peat sequence expressed as percentages. The reduction in the accumulation rate above 40 cm inferred by the BCal age model coincides with a change in peat composition from Sphagnum section Cuspidata/Sphagnum cupsidatum to Sphagnum magellanicum (Yeloff et al., 2006). age–depth model. Fig. 8 illustrates an age–depth model for peat core Mauntschas-03 from southeast Switzerland and its evolution after three stages of 14C dating. The results show that the denser the sequence of 14C dates, the more accurate the age–depth model. Although many studies have reported good results from the 14C dating of peats, the method is not free of problems. Charman and Garnett (2005) dated two peat profiles from Butterburn Flow in England. They found that many of the ages were older than expected and that the oldest ages occurred at the top of the profiles. This may have been because the most recent peat samples had been affected by inputs of older carbon from industrial emissions such as coal burning and the fallout of particulates. These contaminants may have included spheroidal carbonaceous particles, which would not have been removed completely during sample pretreatment as a result of their bonding to Sphagnum leaves (they may be retained within the pores on the cells: see Charman and Garnett (2005) and references therein). This implies that peats from areas close to industrial activity should not be used for radiocarbon dating. 4.2. Dating of lake and salt marsh sediments Accurate radiocarbon dating of lake and salt marsh sediments is not simple due to the presence in such deposits of organic and inorganic carbon materials from various sources (McGeehin et al., 2004). Dating is complicated by reservoir effects (in the case of aquatic plant macrofossils and inorganic materials) and by the reworking of older material into recent sediments. In order to determine which materials from modern lake sediments are most suitable for 14C dating, Davidson et al. (2004) dated a range of organic materials from a core from Sky Lake, Mississippi, USA. They found twigs (representing 1–2 years of growth) of local plant species to be more reliable for 14C dating than either fine (>250 mm) organic debris or wood fragments (from large branches or tree trunks). The 14C content of the twigs showed a clear bomb signal that was always higher than that of either the fine organic debris or the wood fragments at the same depths (Fig. 9a). They therefore used twigs to date two cores from the same lake (Fig. 9b). The results showed a good match between the 14C content of the twigs and that in nearby tree rings for the bomb period. Prior to this, there was a generally decreasing trend in 14C concentration with depth. Although a few data points from one of the cores did not match the bomb curve very well and variations are evident in the pre-bomb data, perhaps the result of minor bioturbation or the occasional influx of reworked materials from the surrounding forest (Davidson et al., 2004), this method shows considerable promise for dating recent lake sediments. Marshall et al. (2007) constructed a chronology of deposition for a 76 cm sequence of salt marsh sediments from Poole Harbour in southern England. They radiocarbon-dated fragments of grass stem, which lay horizontally in the clayey sediment matrix. This sampling strategy was used to minimise the chance of younger roots being selected for dating. As grass stems are fragile, the use of these samples for dating also minimised the possibility that the materials had been reworked. The 14C values of the grass fragments from the top 26.5 cm of the sequence followed both the rising and the falling limbs of the pulse in bomb 14C. Chronological ordering (deeper samples are older than shallower samples) and independent time markers (obtained from the analysis of pollen and spheroidal carbonaceous particles) were used as constraints to eliminate unlikely calibrated age ranges of individual 14C ages, allowing the construction of a reliable chronology of deposition. Another approach to dating lake sediments involves the direct analysis of the deposit. McGeehin et al. (2004) extracted humin (<63 mm) from two lake sediment cores from Grenada Lake, northcentral Mississippi, USA. Humin samples were oxidised to CO2 using a stepped-combustion method, with samples combusted at 400 C and then 900 C. In all cases, the bomb 14C values of the low temperature (400 C) fractions were much higher than those of the high temperature (900 C) fractions, indicating an improvement in dating of sediment by combusting at low temperature. The authors argued that the combustion of sediments at low temperature could reduce the contribution of reworked carbon bound to clay minerals. Q. Hua / Quaternary Geochronology 4 (2009) 378–390 385 However, the bomb 14C values of the low temperature fractions were still significantly lower than those of the atmosphere, indicating a reservoir effect problem in direct dating of sediments. 4.3. Dating of tree rings Bomb radiocarbon has been used to validate the annual nature of distinct growth zones or rings in some species of tropical and temperate trees and mangroves (Worbes and Junk, 1989; Fichtler et al., 2003; Menezes et al., 2003; Biondi et al., 2007). The 14C concentration in wood reflects the atmospheric 14C at the time it was formed. By measuring 14C concentrations in the individual growth rings of a tree and comparing them with a suitable atmospheric bomb curve, the dates of their growth may be determined. Comparing these growth years with those estimated from ring counts makes it possible to establish whether the tree produces annual rings. As shown in Fig. 5, each value of 14C for the period after AD 1955 gives two possible (calendar) time windows: one on the rising limb and the other on the falling limb of the bomb curve. To overcome this problem, at least two single growth rings in each tree need to be analysed. The most suitable period for this kind of application is from AD 1958 to 1970, when the differences in atmospheric 14C between consecutive years are highest and the bomb 14C method delivers the greatest temporal resolution (Fig. 6a). Alpha-cellulose extracted from wood should be used for 14 C analysis in order to obtain a reliable determination of 14C concentration in single growth rings (Hua et al., 1999). An example of this kind of application is shown in Fig. 10. This compares the 14C concentration in the rings of a dendrochronologically-dated specimen of Triplochiton scleroxylon from Cameroon and an atmospheric 14 C record from the Southern Hemisphere. The good agreement between the two data sets indicates that T. scleroxylon has produced annual rings for the past 50 years (Worbes et al., 2003). If the annual nature of growth rings is validated for the bomb period, it may be reasonably assumed that this is also true for earlier times (Menezes et al., 2003). Bomb 14C is also a useful tool to complement the standard techniques of dendrochronology in species in which annual rings are not always clearly defined. Hua et al. (2003) determined the 14C content of 27 consecutive annual rings of a section of Pinus radiata (DRF 021) from Armidale, northern New South Wales, Australia. Some 14C values of single rings from DRF 021 (based on ring counts) were not in agreement with atmospheric 14C records at similar latitudes, suggesting the possibility of two false rings and thus two mis-identified rings in the preliminary count for this section. This possibility was supported by a better ring-width correlation between the revised DFR 021 count and other P. radiata chronologies in the study region. For the pre-bomb period, precise dating of single tree rings is problematic because of the large fluctuations in atmospheric 14C over the past few hundred years (see Section 3.1). The 14C wigglematching method has successfully been used to date precisely Holocene floating tree-ring sequences that cannot be cross-dated Fig. 8. Age–depth modelling of peat core Mauntschass-03 from southeast Switzerland showing the effect of the increase in the density of 14C dating (Goslar et al., 2005). The probability distributions of the calibrated ages of individual samples down the sequence are shown with grey silhouettes. The small rectangle in the upper right-hand corner of each figure represents the date of collection of the section (depth ¼ 0 cm). The arrow in the lower left-hand corner of each figure represents the depth of a sample whose calendar age lies beyond the range of the diagram. The smooth lines passing through the maxima of the probability distributions represent the most probable age– depth curves. The dashed lines represent the uncertainties of each age–depth model. The vertical bars on the right-hand side show changes in peat accumulation rate, as inferred from the lithology of the sequence. Stage 1 (top panel): seven samples were dated. These yielded two possible age–depth curves. Stage 2 (middle panel): 10 samples were dated. These produced a single age–depth curve. Stage 3 (bottom panel): 12 samples were dated. These generated an age–depth model with much lower uncertainty. 386 a Q. Hua / Quaternary Geochronology 4 (2009) 378–390 different periods: AD 100–250 (a plateau in the calibration curve), AD 900–1100 (a part of the calibration curve characterised by a steep decrease in 14C age) and 1750–1650 BC (a portion of the calibration curve dominated by large wiggles). They reported that 5–10 radiocarbon dates on 10-ring samples at precisions of 25–30 14 C years were sufficient to achieve a precision of less than 25 years (95% confidence level) for the wiggle-matching method. This method may therefore be applied to the precise dating of tree rings for the pre-bomb period. 0 20 Depth (cm) 40 60 80 100 120 140 Wood fragments Fine organic debris Twigs 160 180 0.8 1.0 1.2 1.4 1.6 4.4. Estimation of the ages of marine samples 1.8 Fraction modern carbon (F) 0 2000 1990 1980 1970 1960 1950 20 Depth (cm) 40 Year (AD) b 60 80 100 120 140 Core 3 twigs Core 4 twigs Tree rings 160 180 0.8 1.0 1.2 1.4 1.6 1.8 Fraction modern carbon (F) Fig. 9. (a) The downcore variation in the 14C concentration in wood fragments, fine organic debris and twigs from Core 3, Sky Lake, Mississippi, USA. (b) The 14C concentration in twigs from Cores 3 and 4, Sky Lake, Mississippi, USA compared with that in nearby tree rings of known age. The error bars (1s) associated with the 14C data are equal to or smaller than the size of the data symbols (Davidson et al., 2004). by standard dendrochronological techniques (van der Plicht et al., 1995; Kromer et al., 2001; Barbetti et al., 2004; Galimberti et al., 2004; Kuzmin et al., 2004; Nakamura et al., 2007). Dating precision using the 14C wiggle-matching method depends upon the shape of the radiocarbon calibration curve at the period concerned, the number of 14C dates available and the precisions associated with the 14C dates. Galimberti et al. (2004) investigated cases at three When atmospheric 14C concentration increased in the mid1950s as a result of atmospheric nuclear weapon tests, 14C levels in the surface ocean also increased. This is because excess radiocarbon from the atmosphere was incorporated in the upper ocean by the air-sea exchange of CO2. However, the distribution of bomb 14C in the surface ocean differs from the simple zonal distribution of atmospheric 14C illustrated in Fig. 6. This is because levels of 14C in the surface ocean are mainly controlled by local and regional patterns of ocean circulation. These include horizontal advection and vertical movements such as ocean upwelling (Druffel, 1997; Gagan et al., 2000; Hua et al., 2005; Grottoli and Eakin, 2007). The latter process may bring 14C-depleted waters from the deeper ocean to the surface, producing areas of lower surface ocean 14C levels. As a result, 14C levels in the surface ocean have local and regional characteristics such as those depicted in Fig. 11 for the Pacific Ocean. In addition, due to the dampening effect of the oceans, the magnitude of increases in 14C in the surface ocean after the mid-1950s was much smaller than that in the atmosphere (the surface waters of the Pacific Ocean experienced an increase of w0.12 to 0.24 F from pre-bomb to maximum bomb values (Fig. 11) compared with an increase of w0.6 to 2 F in the troposphere (Fig. 6)). Surface ocean 14C also increased more slowly than atmospheric 14C, with the oceanic 14C reaching its maximum w10 years later than atmospheric 14C (Druffel and Suess, 1983; Nydal and Gislefoss, 1996). This occurred because excess 14C in the atmosphere after the atmospheric 14C bomb peak (AD 1963–1965) continued to be transferred to the oceans. These issues lead to difficulties in accurately dating marine samples using bomb 14C. Nevertheless, bomb 14C may be used to estimate the ages of marine 1.18 Fraction modern carbon (F) Fraction modern carbon (F) 1.8 Wellington, New Zealand 42°S, 175°E Triplochiton scleroxylon 1.6 1.4 1.2 1.10 1.06 1.02 1960 1970 1980 1990 2000 Year (AD) Fig. 10. The 14C concentration of ring-count dated growth rings of Triplochiton scleroxylon from Cameroon (Worbes et al., 2003) plotted on the atmospheric 14C curve from Wellington, New Zealand for the period AD 1957–1992 (Manning and Melhuish, 1994). Okinawa 26°N, 128°E Hawaiian Is. 24°N, 166°W Tarawa 1°N, 172°E Panama 8°N, 82°W Fiji 18°S, 179°E Heron Is. 23°S, 152°E 0.98 0.94 0.90 1950 1.0 1950 1.14 1955 1960 1965 1970 1975 1980 1985 Year (AD) Fig. 11. The concentration of 14C in the surface waters of the Pacific Ocean for the period AD 1950–1984 recorded in corals. The data are from Konishi et al. (1982) for Okinawa, Druffel (1987) and Druffel et al. (2001) for Hawaii and Panama, Toggweiler et al. (1991) for Tarawa and Fiji, and Druffel and Griffin (1995) for Heron Island. The error bars are 1s. The chronologies of these corals are based on counting growth bands backwards from the dates of collection. 387 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 4.5. Time markers for speleothems Time series of 14C in modern speleothems have been reported by Genty et al. (1998) and Genty and Massault (1999). The authors used annually laminated stalagmites from two caves in Belgium and France for 14C measurements. Their results are illustrated in Fig. 13. The two records show a clear increase in 14C resulting from Fraction modern carbon (F) 1.15 1.10 1.05 1.00 0.95 Pagrus auratus ototliths, New Zealand Centroberyx affinis otoliths, NSW, Australia 0.90 1945 1955 1965 1975 1985 1.16 Fraction modern carbon (F) materials formed after AD 1960. Such estimates are most reliable for materials formed in the period from c. AD 1960 to the mid1970s, when there was a significant increase in surface ocean 14C. Kalish (1995) and Kalish et al. (1996) measured the 14C content of fish otoliths and used a ‘regional’ surface ocean 14C bomb curve to estimate fish birth dates and consequently fish ages. An example of this approach is shown in Fig. 12. The birth dates of specimens of Centroberyx affinis caught off the coast of New South Wales (Australia) were first established by counting annual increments visible in otolith thin sections. These dates were then checked by comparing the 14C content of the first annual otolith increment with a surface ocean 14C bomb curve. The curve was constructed from 14C values measured in otoliths of known age from the species Pagrus auratus taken off the coast of New Zealand, on the assumption that the 14C levels of surface waters in the two regions were similar. Kalish (1995) reported that 13 out of 16 presumed birth dates for C. affinis fell within the 95% confidence limits of the ‘bomb-14C calibration curves’ based on the New Zealand P. auratus data. This method has also been successfully applied to validate shark ages by analysing 14C in growth bands of shark vertebrae (Campana et al., 2002; Ardizzone et al., 2006). Another application of this dating method was reported by Frantz et al. (2000). They estimated growth rates for the rhodolith Lithothamnium crassiusculum, a free-living calcareous red alga from the southern Gulf of California, by measuring a dense series of samples from the rhodolith for 14C and matching them with a surface ocean 14C bomb curve for the Galapagos, which is located not far from the southern Gulf of California. Using this method the authors reported an average growth rate of 0.6 mm a1 for the rhodolith, suggesting that large L. crassiusculum with radii in excess of 6 cm may live over 100 years. As rhodoliths occupy extensive areas of the world’s oceans, ranging from polar deeps to tropical shallows, they may have the potential to provide proxies for past ocean conditions (Frantz et al., 2000). Han-stm5, Han-sur-Lesse cave Fau-stm14, La Faurie 1.12 1.08 1.04 1.00 0.96 0.92 0.88 1940 1950 1960 1970 1980 1990 2000 Year (AD) Fig. 13. The pattern of bomb 14C in annually laminated stalagmites from the caves of Han-sur-Lesse, Belgium (Genty et al., 1998) and La Faurie, France (Genty and Massault, 1999). The stalagmites were dated by counting annual layers (represented by a couplet composed of a white porous lamination and a dark compact lamination). The horizontal error bars encompass the one to two years of laminae sampled for each 14C analysis. The vertical error bars associated with the 14C data are 1s. the input of excess 14C from atmospheric nuclear bomb tests. However, the magnitude of the increase is smaller than that found in the atmosphere and differs between the two cave systems. Possible sources of the carbon in speleothems include (1) limestone carbon containing no measurable 14C and (2) soil CO2 derived from plant root respiration, whose 14C concentration is similar to that of the atmosphere, and from organic matter decomposition with a turnover time varying from decades to thousands of years (Genty et al., 1998). The maximum value of bomb 14C in a particular speleothem is therefore dependent on the contribution of dead carbon from limestone, and on the proportion and 14C content of soil organic matter incorporated into the speleothem. To quantify the total dead carbon in a speleothem from the two sources, the Dead Carbon Fraction (DCF) is usually calculated: DCF ¼   Fspel 1 Fatm (4) where Fspel and Fatm are the measured 14C concentration in a speleothem and the contemporaneous atmospheric 14C concentration respectively, both expressed as fraction modern carbon. The higher the DCF, the lower the maximum value of bomb 14C found in the speleothem. Although the bomb 14C profiles in the speleothems show different maxima in the two cave systems (Fig. 13), the timing of their onset is no more than 1–2 years after the start of the rise in atmospheric 14C in 1955 (Genty and Massault, 1999). This feature can be used as a time marker for modern speleothems, especially when they cannot be precisely dated by the Th/U method due to low uranium concentration (<1 mg g1) and insufficient 230Th. This method has successfully been applied to help to build chronologies for young speleothems from Gibraltar (Mattey et al., 2008) and from New South Wales in Australia (Hodge et al., 2007). 1995 Birthdate (Year AD) Fig. 12. The 14C concentrations of the first annual increment of Centroberyx affinis otoliths from New South Wales, Australia plotted against their otolith-count derived birth dates (Kalish, 1995). These are compared with the 14C concentrations of Pagrus auratus otoliths of known age from New Zealand (Kalish, 1993). The horizontal error bars associated with the presumed birth dates are 1s. The vertical error bars associated with the 14C data are 1s. 5. Summary Over the last few hundred years, atmospheric 14C has been characterised by large fluctuations caused by variations in solar activity (the Spörer, Maunder and Dalton minima, for example) and climatic changes (such as those of the Little Ice Age). The injection of 14C-free anthropogenic CO2 into the atmosphere since the 388 Q. Hua / Quaternary Geochronology 4 (2009) 378–390 Industrial Revolution and the dramatic increase in atmospheric 14C due to atmospheric nuclear detonations starting in AD 1955 have also had massive impacts on 14C levels. The large fluctuations in atmospheric 14C that took place before the onset of bomb 14C mean that a single 14C date may possess several possible calibrated age ranges, making the 14C dating method imprecise for that period. This problem may be overcome by measuring a series of 14C dates from a sequence and locating their most likely positions on a calibration curve using either the 14C wiggle-matching method or additional information on dated materials and their surrounding environment. For the period from 1955 onwards, atmospheric 14C levels differ significantly from year to year, offering the possibility of dating modern terrestrial samples with a resolution of from one to a few years. The method of analysing a series of 14C dates from a sequence has been successfully applied to the precise dating of recent peat profiles. The most reliable components of the peats for dating are the leaves and twigs of Sphagnum. However, recent peats from regions close to industrial activity may not be suitable for radiocarbon dating as spheroidal carbonaceous particles from industrial sources attached to Sphagnum leaves may not be completely removed during sample pretreatment (Charman and Garnett, 2005). The method of analysing multiple 14C dates from a sequence has also been used for dating recent lake and salt marsh sediments from the pre- and post-bomb periods. Accurate radiocarbon dating of such sediments is not simple due to the presence of organic and inorganic carbon from various sources (McGeehin et al., 2004). The most reliable materials for dating young sediments are short-lived macrofossils of local species, such as small twigs and grass stems (Davidson et al., 2004; Marshall et al., 2007). Direct dating of lake and salt marsh sediments is still problematic because of reservoir effects (McGeehin et al., 2004). Given the potentially complex array of carbon found in modern organic and inorganic sediments, it is helpful if alternative dating methods such as 137Cs, 210Pb and 241Am are used to corroborate the 14C chronologies (Gallagher et al., 2001; Davidson et al., 2004; McGeehin et al., 2004; Turetsky et al., 2004; Marshall et al., 2007). Bomb 14C has successfully been used to validate the annual nature of distinct growth zones or rings of some species of tropical and temperate trees and mangroves (Worbes and Junk, 1989; Fichtler et al., 2003; Menezes et al., 2003; Biondi et al., 2007). This provides a useful complement to the standard techniques of dendrochronology in species where annual rings are not always clearly defined (Hua et al., 2003). Precise dating of tree rings for the pre-bomb period may be achieved by obtaining a series of 14C dates and applying the wiggle-matching method. Bomb 14C has also been used to estimate the ages of modern marine materials. It has been employed, for example, to verify fish birth dates (Kalish, 1995; Kalish et al., 1996; Campana et al., 2002; Ardizzone et al., 2006) and to determine the growth rates of young rhodoliths (Frantz et al., 2000). For young speleothems, which cannot be precisely dated by the Th/U method due to low uranium concentration (<1 mg g1) and insufficient 230Th, the timing of the onset of bomb 14C may be used as a time marker (Hodge et al., 2007; Mattey et al., 2008). In addition, if the DCF of a speleothem is almost constant over time, which can be indicated by its d13C values (Genty et al., 2001), a series of 14C dates from a sequence can potentially be employed to construct a reasonable age–depth model for the speleothem for the pre-bomb period. Acknowledgements The author would like to thank Dan Yeloff and Tomasz Goslar for their generous provision of Figs 7 and 8 respectively. Stephen Gale and two anonymous reviewers provided critical and constructive comments, which greatly improved the manuscript. Editorial handling by: S.J. Gale References Anchukaitis, K.J., Evans, M.N., Lange, T., Smith, D.R., Leavitt, S.W., Schrag, D.P., 2008. 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