Reservoir effect or C14 dating of marine organisms

Carbon-14 dating is an essential method in archaeology. It can be used to determine the age of organic artifacts (wood, charcoal, bones, etc.).. When it comes to marine organisms, this method faces unique challenges due to the reservoir effect, a phenomenon that makes it difficult to interpret the results. In this article, we explain this concept, the correction methods used by scientists, and the limitations of C14 dating in marine environments.

The reservoir effect in C14 dating of marine organisms

Origins and mechanisms of the reservoir effect

The reservoir effect is a variation in carbon-14 (C14) concentration between terrestrial and marine organisms. This difference poses unique challenges for dating marine samples. Unlike atmospheric carbon, which is uniformly distributed and rapidly recycled, seawater has significantly lower C14 levels. This C14 deficit can be explained by several factors.

All First of all, carbon exchange between the atmosphere and the ocean is a slow process, due to the large mass of the oceans and the low solubility of carbon dioxide in water. In addition, ocean currents and the stratification of water masses result in a circulation of carbon that varies with depth and latitude. On the other hand, the ocean contains significant quantities of ancient carbon, particularly in the form of dissolved carbonate that has not been in recent contact with the atmosphere. This ancient carbon can have several origins, such as the dissolution of carbonate rocks or underwater volcanism, contributing to the apparent "aging" of marine organisms.

Consequences of the reservoir effect: the reservoir age

The consequences of the reservoir effect lead to "ageing aging The consequences of the reservoir effect lead to an "aging" of the age of marine organisms when they are carbon-14 dated. Marine organisms integrate less C14 than their terrestrial counterparts, due to the reduced levels of C14 in seawater. In fact, marine organisms start with a radiocarbon "clock" that is already out of sync with that of terrestrial organisms at the time of their death.

This difference, known as the "reservoir age", averages 400 years for surface ocean waters. However, reservoir age is not constant and can vary according to various geographical and environmental factors, such as water depth, proximity to freshwater sources, and the geochemical composition of water masses. For example, waters in estuaries or deltas show significant variations in carbon-14 concentrations due to mixing between fresh and marine waters, which impacts reservoir age. Similarly, C14 concentration varies according to latitude: polar waters often have higher reservoir ages due to slower circulation and low exchange with the atmosphere.

Scientists correct reservoir effect

Calibration methods with reference samples

Scientists rely on reference samples to compensate for shifts caused by the reservoir effect and thus obtain more accurate dating. One frequently used method is to compare results obtained on marine shells whose year of death is known. These shells from well-dated archaeological contexts or recent collections are used to calibrate the ages measured in C14. Using this reference point, researchers can adjust the results obtained for samples from the same region or of the same type, taking into account local variations in reservoir age.

Another key repository in this calibration process is the Marine 2020 Reservoir Database. This resource compiles data on reservoir ages observed around the world and provides corrective factors specific to different geographical locations. Scientists can then cross-reference the data from the sample under study with the information in this database, in order to apply an appropriate correction, taking into account the environmental and geographical particularities of the area concerned. This approach makes it possible to refine dating and reduce the margins of error associated with the reservoir effect.

Use of isotope ratio mass spectrometry (IRMS)

Isotope ratio mass spectrometry (IRMS) is an advanced technique used to analyze stable isotopes of carbon and nitrogen in samples. This technique is particularly useful for differentiating between sources of organic matter, identifying whether a sample comes from a terrestrial or marine environment. Isotope analysis therefore determines whether a correction for reservoir effect should be applied or not.

Using IRMS, archaeological dating laboratories can also refine the necessary corrections. For example, stable carbon isotopes can be used to detect the specific isotopic signatures of different carbon sources, such as those derived from marine or terrestrial photosynthesis. This process is essential for samples of uncertain origin, such as objects carved from organic materials of mixed or undetermined origin. Researchers can apply more precise corrections, reducing the uncertainties associated with the reservoir effect thanks to this improved knowledge of the isotopic composition of such samples.

Outlook for the reservoir effect

Geochemical models and reservoir age estimation

Another promising analytical method for improving age estimates of marine samples is the use of advanced geochemical models. These models integrate data on ocean circulation, atmospheric exchange, and spatial and temporal variations in ocean carbon-14 concentrations. These models can be used to calculate region-specific reservoir ages, adjusted for variables such as latitude, depth and ocean currents, taking into account the complex dynamics of oceanography. 

The use of geochemical models offers greater precision in estimating reservoir ages by modeling the physical and chemical processes that influence the distribution of carbon 14 in the oceans. These take into account elements such as carbonate dissolution, circulation of deep water masses and ocean-atmosphere interactions. Researchers obtain more reliable estimates of the age of marine samples by applying these models, even in complex environments such as estuaries and deltas.

Carbon-14 dating of marine organisms is a complex yet essential field in archaeology. It requires precise methods to correct for reservoir effects. While limitations remain, technological and methodological advances are continually improving the accuracy of these dates.

The scientists at CIRAM laboratories offer this type of analysis and are at your disposal to guide you through the dating process. If you would like to carry out a dating study, you can request a study to benefit from our expertise and obtain precise answers to your archaeological questions.

Why use C14 to date bones?

Carbon-14, or radiocarbon, dating is the most widely known dating method, but it is above all the most relevant technique for dating organic materials, particularly bones. Carbon-14 dating was developed in the 1940s and, like most dating methods, is based on radioactivity.

The quantity of carbon is stable for living beings, because on the one hand, carbon 14 disintegrates, but on the other hand, it is reintegrated by respiration or photosynthesis. This is why C14 dating will date the death of the individual or plant, and the remaining quantity of C14 will enable us to assess the date of death.

Collagen extraction and dating

Bones are very good chronological markers in an archaeological dig, as they are closely linked to the stratigraphy in which they are found. A bone is composed of a mineral part, bioapatite, and an organic part, collagen. Collagen is the most suitable fraction, and is normally used for radiocarbon dating.

The first step in the dating process is to extract the collagen. To this end, they are treated with cold hydrochloric acid (HCl, 1 M) for 24 h, in order to eliminate any surface contamination and partially deteriorate the mineral part of the bone, thus making collagen extraction more efficient. Samples are then treated with sodium hydroxide (0.1 M) at room temperature and again with cold hydrochloric acid, to avoid absorption of atmospheric carbon dioxide. After washing with demineralized water, the samples are boiled to dissolve and recover the collagen.

The extracted collagen undergoes combustion at 920°C and is transformed into gas. During this stage, an initial check of the C/N ratio is carried out using an elemental analyzer (Elementar Vario ISOTOPE Select). This is an essential quality control step.
A C/N value between 2.9 and 3.6 indicates that the calogen is well preserved and will provide reliable dating. If the C/N ratio is outside this range, C14 dating of the collagen will not be possible. In this case, it will be necessary to use the mineral part of the bone and date the bioapatite.

Next, stable isotopes of carbon and nitrogen will be analyzed by IRMS. These values will provide information on the diet of the individuals. At the same time, carbon dioxide from combustion is separated from other residues using a zeolite trap. This carbon dioxide is then catalytically converted into graphite using an automated system (AGE 3, Ion Plus).

C14 dating and calibration

In order to validate our analytical protocols, it is essential to first check the precision of our measurements, as well as their reproducibility. To do this, we analyze international standards whose values are known and recognized. These values enable us to assess our uncertainties, around 0.5 pMC, and 0.1 to 0.2‰ for δ¹³C and δ¹5N. Real-time verification of measured values for standards enables us to identify and resolve any problems associated with pollution, graphitization and measurements.

Finally, the various carbon isotopes are separated by gas pedal mass spectrometry (AMS). Then, the 14C concentration was determined by simultaneously comparing 14C, 13C and 12C measurements with those contained in international standards (oxalic acid, standard CO2, charcoal). The conventional carbon-14 age was calculated using the method described by Stuiver and Polach. It takes into account correction for isotopic fractionation.

The results are calibrated using OxCal v4.4 software. The measurement taken is expressed in two different ways: part of Modern Carbon (or pMC) and conventional age. Conventional age is expressed in years before 1950 (BP standing for before present), which is the reference year. Age is expressed to one standard deviation. The dating intervals reflect a two-sigma distribution, i.e. 95.4% of all solutions. The dated event can be found in any interval, regardless of the probability distribution, which is given for information only.