Radiocarbon dating glacial landforms (Cryospheric Geomorphology)

By Bethan Davies – Last updated 16/04/2023 tagged cryospheric geomorphology, dating

This article is edited and drawn from:

Davies, B.J., 2021. Dating Glacial Landforms II: Radiometric Techniques, in: Haritashya, U. (Ed.), Treatise in Geomorphology (Second edition). Cryospheric Geomorphology. Elsevier, pp. 249-280. (link)

Introduction to radiocarbon dating

Radiocarbon dating has been around since the 1940s (Libby, 1961, 1955), and has transformed our understanding of the timing of events and rates of change. It is one of the most widely applied techniques for dating Quaternary environments.

Over Quaternary timescales, radiocarbon dating is widely applied and relies on the principal of radioactive decay. There are many reviews (e.g. Alves et al., 2018; Briner, 2011; Burr, 2013; Hatté and Jull, 2013; Jull, 2018; Lowe and Walker, 2014). It has been widely applied to understand deglacial chronologies in North America (Dalton et al., 2020; Dyke et al., 2003), Patagonia (Denton et al., 1999; Moreno et al., 2015; Strelin et al., 2011), Europe (Bateman et al., 2018; Hughes et al., 2011; Livingstone et al., 2015), and Antarctica (Larter et al., 2014; Ó Cofaigh et al., 2014).

Formation of radiocarbon

Meteoric radiocarbon (¹⁴C) is formed in our atmosphere by geomagnetic and solar modulation of cosmic rays, and variations in the carbon cycle. Natural radiocarbon forms in the Earth’s stratosphere through the interaction of ¹⁴N and neutrons produced by cosmic rays (Guilderson et al., 2005). The newly formed ¹⁴C is oxidized to ¹⁴CO₂ where it enters the biosphere. Radiocarbon dating relies on the assumption that organic or inorganic materials were in equilibrium with the production of ¹⁴C in the atmosphere (Jull, 2018), and that the ¹⁴C in the organism will decay, converting ¹⁴C back to ¹⁴N through beta decay, following the death of the organism.

The physics of decay and origin of carbon 14 for the radiocarbon dating 1: Formation of Carbon-14. 2: Decay of Carbon-14. 3: The “equal” equation is for living organisms, and the unequal one is for non-living ones, in which the C-14 then decays (hence the 2). From: Wikimedia Commons

Radiocarbon half life

Through this process, radiocarbon has a half-life of 5,568 years (Alves et al., 2018). Because of this relatively short half-life, radiocarbon dating has a useable range of ~300 to ~50,000 years (Briner, 2011; Guilderson et al., 2005). ¹⁴C ages do not equate directly with calendar years, because ¹⁴C concentration in the atmosphere varies through time due to changes in the production rate (Burr, 2013), and so require calibration with incremental datasets such as tree rings or corals (Reimer et al., 2013). Typical analytical uncertainties are ~2 to 5%, although calibration adds further uncertainty.

Describing radiocarbon ages

Convention dictates that uncalibrated ages are referred to as ¹⁴C ka BP (radiocarbon age in thousands of years before 1950 AD) and calibrated ages as cal. ka BP (calibrated age in thousands of years before 1950 AD) (Alves et al., 2018; Reimer et al., 2013).

Common tools for calibration include OxCal (Bronk Ramsey, 2009) or Calib (Stuiver et al., 2009). Calibrated ages are commonly presented as median ages, with an uncertainty to 1 or 2 sigma (σ). Ages should be presented in publications with all raw data needed for calibration to be updated by later researchers when new calibration curves are published.

Dating terrestrial landforms using Radiocarbon

Glacial environments are usually pretty barren places. However, dating terrestrial glacial landforms using radiocarbon presupposes that organic material is available. In temperate terrestrial environments, this could include basal organic sediments in bogs, lakes and mires either in kettle holes associated directly with moraines, or samples from sites inside and outside ice limits, bracketing those moraine sequences (Figure 1). It could include marine or other organic sediments beneath the moraine that have been overridden (Hjort et al., 1997; McCabe et al., 2007) or material reworked into the moraine (Denton et al., 1999; Luckman et al., 2017; Strelin et al., 2011).

In many cases, radiocarbon dating can be used in conjunction with other techniques, such as varve chronology, dendrochronology, lichenometry and cosmogenic nuclide dating, depending on the situation. Relating the time of organism death to the timing of landform generation requires careful stratigraphic work, and a clear understanding of whether the organic matter provides a minimum or maximum age for the glacier variability.

Radiocarbon ages are typically derived from bulk samples, microfossils samples requiring microscopy to extract, or macrofossil samples that can be visually identified and sampled (Small et al., 2017). Sedimentary sequences should have multiple radiocarbon ages taken in stratigraphic order, so that problematic materials yielding age reversals or consistent offsets can be identified and removed.

In a sediment core, multiple up-core radiocarbon ages are required to confirm that sediments are in stratigraphic order and have not undergone significant disturbance. When multiple radiocarbon ages are available for the same site, the oldest date is typically most useful for dating past glaciation, if the dates are a stratigraphic sequence (Dalton et al., 2020) (Figure 1).

Dating moraines

This approach to radiocarbon dating is widely used in palaeoenvironmental studies, but its application to dating terrestrial glacial landforms can be more challenging. In this realm, radiocarbon dating is most frequently applied to moraines, by dating sediments that comprise the moraine, sediments below and within (providing a minimum age) and above (providing a maximum age) the moraine (Figure 1).

Careful stratigraphic work is required to ascertain whether the organic matter in question provides a minimum or maximum age for landform formation, and usually requires large datasets to produce robust chronologies (e.g., Denton et al., 1999; Moreno et al., 2015).

Figure 1. Cartoon illustrating techniques for dating terrestrial moraines with radiocarbon. From Davies, 2021

Geological uncertainty in radiocarbon dating

There are three key sources of geological uncertainty in radiocarbon ages: calibration to calendar years (see below), laboratory contamination, and site-specific geological problems (Lowe and Walker, 2000; Small et al., 2017). Site-specific geological issues include processes, other than radioactive decay, that influence the ¹⁴C /¹²C ratio within an organism (before or after death), or processes that result in the age of the sample not properly reflecting the age of the sedimentary archive. The ¹⁴C /¹²C ratio can be affected by chemical processes such as isotopic fractionation, recrystallization, contamination, or reservoir effects (Small et al., 2017).

The geological context (transport and deposition) of the sediment can affect its geological context, and thus the relative age of the organism and the sedimentary archive. This can result in aging or rejuvenation (Hatté and Jull, 2013), caused by anomalously low (aging) or high (rejuvenation) ¹⁴C content in the original carbon.

Dating trees and wood

Radiocarbon dating of woody material may be challenging because the radiocarbon age may be hundreds of years older than expected, especially for long-lived trees (Hatté and Jull, 2013). The tree adds wood to the outside of the trunk every year, so for older trees, the outside may have a radiocarbon age hundreds of years younger than the heartwood. Bulk woody material reworked into moraines may lack the associated data required to inform the dating strategy. It would be best to use small twigs if possible, as they can integrate at most five years and so should give more precise ages.

Dating bogs, mires and lakes

For bogs, mires, lake sediments etc., some studies use an age-depth model through multiple ages in a sediment core to establish an estimate for basal age (a minimum age for the onset of deglaciation at that location). This can increase confidence in the basal age, if radiocarbon ages are present in stratigraphic order.

Oldest ages (at the bottom of the core) give an indication of the timing of the onset of organic sedimentation, but we add the significant caveat that such ages may over- or under-estimate the true onset of deglaciation given factors such as detrital contamination or undated core sections. From these environments, terrestrial plant macrofossils should be targeted for dating.

Radiocarbon dating of freshwater environments

The ¹⁴C of lakes and bog waters is often depleted, resulting in an artificial aging of the waters with wide spatial variation. Hard water can affect the dating of macrofossils from freshwater environments. The radiocarbon age of fresh water (or organisms living in the water) in contact with calcium carbonate rich rocks (such as limestone) can be increased by dissolved carbonate in the water (Hatté and Jull, 2013). The limestone contains no ¹⁴C (radiocarbon), so it acts to dilute the concentration of ¹⁴C in the incoming CO₂ in the water. This can affect freshwater aquatic taxa, meaning that they have anomalous ages. Algal macrofossils should, therefore, be avoided.

Freshwater taxa (ostracods, algaes, aquatic mosses) are generally not suitable for radiocarbon dating (Dalton et al., 2020; Hatté and Jull, 2013), since algae and aquatic mosses build carbon from dissolved inorganic carbon in lakes and bog waters. This, therefore, reflects the ¹⁴C :¹²C ratios of the water from which they grew. These aquatic taxa are therefore vulnerable to the hard water effect. They are also vulnerable to dissolved carbonate from surrounding rocks, the residence time of the bog or lake, and other factors (Hatté and Jull, 2013).

Calibration of terrestrial radiocarbon ages

Radiocarbon dating of terrestrial samples assumes that organic or inorganic materials were in equilibrium with the production of ¹⁴C in the atmosphere (Jull, 2018). After death, the plant or animal is removed from this equilibrium, and so the level of ¹⁴C should decay, allowing the time of death to be calculated. Terrestrial radiocarbon ages then require subsequent calibration before they can be related to calendar years because the value of ¹⁴C in the atmosphere can vary with time (Jull, 2018). Calibration using incrementally and independently dated tree rings now extends back to 13.9 cal. Ka BP (Reimer et al., 2013; Small et al., 2017); corals, speleothems, floating tree ring chronologies and lacustrine and marine sediments have extended it back further to 55,000 cal. Years BP (Fairbanks et al., 2005; Reimer et al., 2020).

There is an offset between the Northern and Southern hemispheres, with Southern Hemisphere samples being ~40 years older, due to a higher sea-air ¹⁴CO₂ flux from the larger Southern Hemisphere oceans (Hogg et al., 2013). In these studies, the value of ¹⁴C through time is calibrated against tree-rings of a known age from the appropriate hemisphere (Hogg et al., 2013; McCormac et al., 2004), with a standard offset applied beyond the range of dendrochronological methods. Radiocarbon ages, therefore, require calibration using the latest datasets (Bronk Ramsey, 2009; Hogg et al., 2013; Reimer et al., 2020, 2009).

Calibration uncertainties are, therefore, controlled by the accuracy of the calibration curve (Small et al., 2017). The use of standardized calibration curves ensures that uncertainties are consistent within a dataset. Commonly used calibration curves include SHCAL13 for terrestrial samples from the Southern Hemisphere (Hogg et al., 2013), and IntCal20 and Marine20 curves for terrestrial and marine samples from the Northern Hemisphere respectively (Heaton et al., 2020; Reimer et al., 2020, 2013). Commonly used software for calibration includes OxCal (Bronk Ramsey, 2013) and CALIB (Stuiver et al., 2009).

The calibration curves have several ‘age plateaus’ caused by variations in atmospheric ¹⁴C content. In these plateaus, the ¹⁴C/12C ratio falls to a rate equal to that of radiocarbon decay (Guilderson et al., 2005). The utility of radiocarbon dating during these plateaus is very limited. There are two plateaus associated with the Younger Dryas (11,900 to 13,000 cal. years BP), which have made it challenging to determine synchronicity of this event globally (Guilderson et al., 2005; Muscheler et al., 2008). The Hallstatt Plateau is another flattening of the calibration curve, that homogenises calibration outputs across a 300 year interval from 2400 to 2700 cal. years BP (Burley et al., 2018). These wiggles and plateaus can cause radiocarbon ages to have several plausible calibrated ages (Small et al., 2017). A careful sampling strategy with multiple ages is required to improve the calibration here. Additionally, using independent dating tools such as tephrochronology can be beneficial.

Figure 2. Radiocarbon calibration curve, with some key periods highlighted. Modified and adapted from multiple sources (Burley et al., 2018; Fairbanks et al., 2005; Guilderson et al., 2005; Reimer et al., 2013; Small et al., 2017). From Davies, 2021.

Dating submarine landforms using radiocarbon

Radiocarbon dating of submarine glacial sediments has been widely applied on continental shelves in order to establish the timing and rates of ice-sheet retreat (Bentley et al., 2011; Bradwell et al., 2019; Davies et al., 2012; Heroy and Anderson, 2007; Pudsey et al., 2006; Smith et al., 2014). This is particularly widely applied in Antarctica, where limited ice-free areas on land, combined with good access from large research ships and a well-surveyed continental shelf, make this technique particularly practical. Here, the chronology is largely derived from radiocarbon dating of bulk organic carbon or marine micro- and macro-fossils in marine sediment cores from the continental shelf and slope (Davies et al., 2012). It can be combined with multibeam swath bathymetry and seismic surveys that provide detailed images of the surface geomorphology as well as a seismic stratigraphy.

In marine settings, especially in quiescent locations on the continental shelf, a drape of organic sedimentation overlying glacial sediments and landforms (such as grounding zone wedges or morainal banks) offers the opportunity to provide a maximum age for the timing of ice recession (Kilfeather et al., 2011; Ó Cofaigh et al., 2014). Marine carbonates (single or broken bivalves) and benthic foraminifera are typically targeted for dating (Graham et al., 2017; Graham and Smith, 2012; Ó Cofaigh et al., 2019). Alternatively, ages may be obtained from bulk organic carbon in acid insoluble organic (AIO) residues (e.g., McKay et al., 2008).

Here, the target sediments for dating are the “Transitional Glaciomarine Sediments” (Figure 3). These transitional glaciomarine sediments immediately over subglacial till reflect the recession of the grounding line from this point. In the ideal case, the core bottoms out in deglacial sediment, and the first samples at the boundary to glaciomarine sediments provide a minimum age for deglaciation (Ó Cofaigh et al., 2019).

Figure 3. Cartoon illustrating the different facies on glaciated continental shelves such as around Antarctica, and the sampling patterns for radiocarbon dating. This method can be used in conjunction with tephrochronology if visible tephra or cryptotephra layers are present in the sediment core. In this scenario, a retreating ice sheet has deposited morainal banks or grounding zone wedges at locations where the ice margin stabilised. Subglacial diamicton (till) is deposited across the ocean floor, overlain by proximal glaciomarine sediments (transitional glaciomarine sediments) and then distal glaciomarine sediments. Reworked shells within the till can provide minimum bracketing ages for the ice advance, and in situ shells and microfossils from the transitional glaciomarine sediments provide a maximum bracketing age for the till and a minimum age for the timing of deglaciation. From Davies, 2021.

Marine reservoir effect

Radiocarbon dating of marine materials requires correction for a global marine reservoir effect (MRE), which varies spatially and temporally in response to changes in oceanic and atmospheric circulation and ventilation between the ocean and the atmosphere (Alves et al., 2018; Bondevik et al., 2006; Heaton et al., 2020; Ortlieb et al., 2011). Surface-ocean environments are typically depleted of ¹⁴C compared with the atmosphere (Heaton et al., 2020). Because oceanic carbon is not in isotopic equilibrium with the atmospheric carbon reservoir, radiocarbon ages from marine materials provide older apparent ages than terrestrial counterparts. Deep ocean masses with low radiocarbon concentrations may yield ages older by several hundred years. Global marine reservoir values have been estimated for the last 22,000 years at a decadal resolution, with a current MRE value of 400 years (Hughen et al., 2004; Ortlieb et al., 2011). This globally averaged MRE is included in the Marine20 radiocarbon calibration curve (Heaton et al., 2020; Reimer et al., 2013).

However, marine correction varies regionally, especially in high-latitude coastal zones (see Dalton et al., 2020; Hall et al., 2010; Ó Cofaigh et al., 2014). This requires the worldwide quantification of the local parameter ΔR, which is the local variation from the global average MRE (Alves et al., 2018). Whilst the MRE changes over time, for any specific location, ΔR is assumed to be constant over time (Heaton et al., 2020). Pre-bomb estimates for ΔR across a wide range of locations is reported by (Reimer and Reimer, 2001) and a database is maintained online at http://calib.org/marine.

In Antarctica, present-day marine species have radiocarbon ‘ages’ of ~1200 ± 200 cal. years (Ingólfsson, 2004; Sterken et al., 2012; Verleyen et al., 2011) (i.e. a ΔR of 800 ± 200 years taking into account the global MRE of 400 years). In coastal tropical regions, such as the western coast of Chile and Peru, the upwelling of deep 14C-depleted waters to the surface results in high regional reservoir effects. In Chile, the modern ΔR value has been calculated as 190 ± 40 years (Stuiver and Braziunas, 1993), but was updated by Ortlieb et al. (2011) to 253 ± 207 years during the Twentieth Century. However, this ΔR value fluctuated over the Holocene (Table 1). In many places, radiocarbon sampling of the surficial sediments provides a ‘core-top’ age that is used to correct stratigraphically down-core ages for the marine reservoir (Figure 3) (Andrews et al., 1999; McKay et al., 2008).

Table 1. Example of ΔR values along the Chile-Peru coastline, from Ortlieb et al., 2011.

Quality assurance protocols

High-quality radiocarbon ages dating glacial landforms should be taken from known and uncontaminated sample material.
For bulk ages, the organic content should be more than 5% when measured on loss-on-ignition (LOI). Bulk samples (including gyttja, organic silt, carbonate clasts) are considered lower quality than individual samples from plant macrofossils.
Uncalibrated ages should be presented with full errors to enable recalibration with modern calibration curves.
In cores, high-quality ages would have multiple and stratigraphically consistent ages.
The δ13C content should be published. High-quality ages would have appropriate δ13C values (-25‰ to -32‰ for terrestrial plants; -15‰ for marine plants; -0‰ for marine carbonates) (Lowe and Walker, 2014).
High-quality ages would be within the ranges of calibration datasets (Hogg et al., 2013; Reimer et al., 2013).
For marine radiocarbon ages, the ΔR should be well understood, justified and provided.
Radiocarbon ages should not be derived from freshwater taxa (ostracods, freshwater algaes, aquatic mosses).

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