CARBON-14 SYSTEMATICS

Because of its half-life of 5,730 years and its involvement in the carbon cycle, ¹⁴C is a uniquely valuable "clock" for studying carbonate deposition and the transition from the last glacial period to the present interglacial.

Source - ¹⁴C is produced in the upper atmosphere as a result of incoming cosmic rays (high energy protons). These protons collide with atmospheric components, generating lower energy neutrons in the process. Some of these neutrons, in turn, collide with the nuclei of ¹⁴N atoms:

• ¹⁴N (7p + 7n) + n = ¹⁴C (6p + 8n) + p

Decay - dN/dt = lambda * N where lambda is decay constant

• If N = N₀ at t = 0, then N = N₀ exp(-lambda*t)

  For N/N₀ = 0.5, t(1/2) = ln 2 / lambda

Decay occurs by the nuclear reaction:

• ¹⁴C - e = ¹⁴N

Until recently, detection has required the measurement of the electrons released by this decay. Because of the small numbers of disintegrations per hour, this requires extended counting times (days) to get accurate dates, and careful shielding to minimize background counts.

Currently, accelerator mass spectrometry (AMS), which detects the ¹⁴C atoms by their mass, rather than their radioactive decay, is the preferred detection method due to its sensitivity and the small amount of sample required (a thousandfold less than for beta counting). However, the most accurate and precise ¹⁴C activities still come from beta counting.

Problems - The conversion of ¹⁴C activities (or concentrations) into ages is complicated by the following phenomena:

• Variations in N₀.

  Will be addressed later.

• Differences between ¹⁴C/C in the atmosphere and the same ratio in HCO₃^- in natural waters. The ratio in HCO₃^- is always less because of "reservoir effects." On land, these are caused by the dissolution of "dead" (ancient, non-radioactive) CaCO₃, whereas in the ocean, decay due to the overturn time of the oceans (of order 1000 years) lowers the proportion of ¹⁴C.

  Thus, all ¹⁴C data derived from HCO₃^- or CO₃^-2 or oceanic organic carbon must be corrected in order to yield meaningful ages..

  For surface tropical and temperate waters, contemporary shells and corals have ¹⁴C ages of 400 ± 200 years.

• Contamination by young carbon. Because of the exponential decrease in ¹⁴C with time, a given fraction of modern contamination is 16x as serious for a 34,000 year old sample (which has decayed for 6 half lives) as it is for an 11,000 year old one (which has decayed for only 2 half lives). For 45,000 year old samples, even 0.5% contamination seriously degrades the result, which is why this is close to the upper limit of the applicability of this technique.

Methods to correct/verify ¹⁴C ages

• Tree rings. By careful cross correlation, a continuous record of tree rings from German oak trees and logs has been created for the past 11,900 years. By measuring the ¹⁴C activities of rings of known age, a curve relating calendar to ¹⁴C years has been built up (e.g. Figure C14-1).

Figure C14-1. Corrections to be added to ¹⁴C dates to convert them to calendar dates, based on tree-ring analyses.

• Lake varves. Annual layers (varves), particularly in lake sediments, can often be correlated within and even between lakes. The lakes must be anoxic and the varves characteristic enough to allow for correlation. Few varve sequences are continuous to the present, so must be calibrated by counting varves between ¹⁴C-dated levels (to ensure that the layers are, in fact, annual), and must then be spliced into the tree-ring sequence on the basis of ¹⁴C activity of included wood fragments. Questions have been raised as to the possibility of detecting all hiatuses in varve records
  
• Cariaco Basin varves. Sediments from the Cariaco Basin (an anoxic basin on the margin off Venezuela) contain varved sediments spanning the last glacial-interglacial boundary (Peterson et al., Hughen et al., Science (2000) 290: 1947, 1951). The surface water "reservoir age" of 420 years matches the open Atlantic and the ¹⁴C record from the foram G. bulloides from the varves can be correlated almost perfectly with the tree ring data from 10,200 to 11,900 years BP (Figure C14-2).

Figure C14-2.Calendar and ¹⁴C ages for German pine data (red) and G. bulloides from Cariaco Basin core PL07-58PC (black) which have been aligned by sliding the varve data within a moving 1370 year window to maximize the correlation between the two data sets. YD = Younger Dryas, PB = Preboreal.

In addition, the Cariaco varve sequence extends the record to almost 15,000 years, providing a detailed chronology and ¹⁴C record covering the end of the last glacial to the Preboreal (Figure C14-3).

Figure C14-3. Calendar versus ¹⁴C ages for Cariaco Basin varves compared to pine, coral, and Japanese lake varve data (see Hughen et al. op. cit.). GL = Glacial, B/A = Bolling/Allerod, YD = Younger Dryas, PB = Preboreal.

• Ice cores. As snow accumulates on ice caps, it develops an icy crust each summer. This crust persists as a zone of coarser crystals in the ice that forms as the snow is compressed by later years' accumulations. The annual layers can be detected visually in young ice, and are apparent from the oxygen isotope record even in old ice (Figure C14-4). The annual layers have been counted back to about 40,000 years with an error of about 200 years.

Figure C14-4. Annual layers in an ice core revealed by detailed oxygen isotope profiles. Both the thickness of the annual layers and the amplitude of the oxygen isotope signal decrease with increasing age.

• The layers in ice cores cannot be dated directly by ¹⁴C because:
  • The amount of trapped CO₂ is too small, even for AMS analysis.
  • ¹⁴C is created within the ice by cosmic ray bombardment of oxygen nuclei
  • The ice contains ancient CaCO₃ dust that adds "dead" carbon to the record

  However, the dates of rapid climatic fluctuations during the last deglaciation (Figure C14-5) can be determined and compared with ¹⁴C dates of the same events in other records.

Figure C14-5. Dated events during the transition from the last ice age to the present interglacial. The names are derived from outcrops in Europe and appear frequently in the literature. H-1 and H-2 are the first two Heinrich events (enhanced ice-rafted debris in the North Atlantic) which will be discussed in a subsequent lecture.

• Corals dated by ²³⁰Th. Corals take up U in almost the seawater U/Ca ratio, but are totally free of Th, one isotope of which is a decay product (daughter) of ²³⁴U. The decay sequence:

  ²³⁸U (t_1/2 = 4.47 billion years) - ²³⁴U (t_1/2 = 244,000 years) - ²³⁰Th (t_1/2 = 77,000 years) provides a way for dating corals (by measuring the ingrowth of ²³⁰Th) as well as checking on the extent to which the coral has been closed to U and Th gain or loss since its formation (by measuring the age-corrected ²³⁴U/²³⁸U activity ratio to see if it deviates from the seawater value of 1.15).

  ²³⁰Th dates are not as precise as ¹⁴C dates, but are more accurate because the systematics of the system are much simpler. The ²³⁰Th dates on corals overlap with the tree-ring series, and show the same discrepancies between calendar and ¹⁴C years (Figure C14-6).

Figure C14-6. Discrepancy between calendar and ¹⁴C ages for the past 25,000 years based on tree ring analyses (open circles) and ²³⁰Th dating of corals (solid circles with error bars).

The difference between the ²³⁰Th and ¹⁴C dates of corals are consistent with the ¹⁴C corrections inferred from the ice-core dating of the Bolling-Allerod warm interval and Younger Dryas cool interval (the correction increasing from about 1 ky at 11,000 years to 2 ky at 14,000 years). Scandinavian varve data suggest the opposite trend, raising questions as to the continuity of the varve sequence.

The difference between Th/U and uncorrected¹⁴C ages create significant differences in the history of the transition from the last glacial to the present interglacial (e.g. Figure C14-7).

Figure C14-7. Chronology of the rise of sea level from the last glacial to the present based on uncorrected ¹⁴C and Th/U dating of corals. The difference in sea level can be as much as 35 m during the interval of rapid rise.

Why do ¹⁴C years not correspond to calendar years? Two possible causes have been proposed:

• 1. Changing reservoir effects. If the deep waters of the ocean were to be more effectively isolated from the surface waters, the ¹⁴C system would be biased towards the kinds of young ages seen for the deglaciation events, for example. However, such a change would increase the apparent ¹⁴C age discrepancy between contemporaneous planktonic and benthic foram tests in deep sea cores. While small glacial-interglacial differences are seen (Figure C14-8) they are nowhere near large enough to create the ¹⁴C errors observed.

Figure C14-8. Relative ¹⁴C/C ratios in the atmosphere and deep and surface ocean waters today and during the last glacial. The data require that the atmospheric ¹⁴C/C ratio during the last glacial be about 1.4 times its 1850 value.

• 2. Changes in the source term. Although the source of cosmic rays is unlikely to have fluctuated on the scale required to produce the observed variations in ¹⁴C production, the number striking the atmosphere is affected by variations in the intensity of the earth's magnetic field. Paleointensities can be recovered from the magnetic properties of dated lavas and pot sherds, both of which recorded the intensity of the ambient field as they cooled through their Curie points. A compilation of the inferred changes expressed in terms of the predicted effect on ¹⁴C formation (Figure C14-9) shows fair agreement with the coral data. More work is needed to validate the reliability of both data sets.

Figure C14-9. Perturbations in ¹⁴C ages predicted from the reconstructed record of the intensity of the earth's magnetic field, compared to the results from 230-Th-dated corals (solid squares with error bars).

There are still questions as to the appropriate corrections prior to about 15,000 years, but the younger corrections, which cover the most recent deglaciation, are well established.

Uses - ¹⁴C, because it adds a time component not provided by isotopic and trace element tracers, has been used extensively for dating carbonate-bearing sediments and for constraining models of vertical and horizontal mixing in the oceans.

Atmospheric nuclear testing, particularly during the 1960s, added ¹⁴C that swamped the natural input. The origin and magnitude of this "spike" is well known, so it can be used to measure the rate and locations of penetration of atmospheric carbon dioxide into the upper ocean, an important parameter in assessing the role of the ocean in taking up greenhouse CO2.

The burning of fossil fuels, which adds "dead" carbon to the atmosphere, has the opposite effect, by reducing its ¹⁴C/C ratio. This "Suess" effect began before the turn of the century. Hence, ¹⁴C activities are commonly compared to the 1850 atmospheric ¹⁴C/C ratio.