Thomas F. Braziunas

High-precision radiocarbon age calibration for terrestrial and marine samples

Single-year and decadal radiocarbon tree-ring ages are tabulated and discussed in terms of 14 C age calibration. The single-year data form the basis of a detailed 14 C age calibration curve for the cal ad 1510–1954 interval (“cal” denotes calibrated). The Seattle decadal data set (back to 11,617 cal BP, with 0 BP = ad 1950) is a component of the integrated decadal INTCAL98 14 C age curve (Stuiver et al. 1998). Atmospheric 14 C ages can be transformed into 14 C ages of the global ocean using a carbon reservoir model. INTCAL98 14 C ages, used for these calculations, yield global ocean 14 C ages differing slightly from previously published ones (Stuiver and Braziunas 1993b). We include discussions of offsets, error multipliers, regional 14 C age differences and marine 14 C age response to oceanic and atmospheric forcing.

Modeling Atmospheric 14 C Influences and 14 C Ages of Marine Samples to 10,000 BC

The detailed radiocarbon age vs. calibrated (cal) age studies of tree rings reported in this Calibration Issue provide a unique data set for precise 14 C age calibration of materials formed in isotopic equilibrium with atmospheric CO 2 . The situation is more complex for organisms formed in other reservoirs, such as lakes and oceans. Here the initial specific 14 C activity may differ from that of the contemporaneous atmosphere. The measured remaining 14 C activity of samples formed in such reservoirs not only reflects 14 C decay (related to sample age) but also the reservoir 14 C activity. As the measured sample 14 C activity figures into the calculation of a conventional 14 C age (Stuiver & Polach 1977), apparent 14 C age differences occur when contemporaneously grown samples of different reservoirs are dated.

Radiocarbon age calibration of marine samples back to 9000 cal yr BP.

Calibration curves spanning several millennia are now available in this special issue of R adiocarbon . These curves, nearly all derived from the 14 C age determinations of wood samples, are to be used for the age conversion of samples that were formed through use of atmospheric CO 2 . When samples are formed in reservoirs (eg, lakes and oceans) that differ in specific 14 C content from the atmosphere, an age adjustment is needed because a conventional 14 C age, although taking into account 14 C (and 13 C) fractionation, does not correct for the difference in specific 14 C activity (Stuiver & Polach, 1977). The 14 C ages of samples grown in these environments are too old, and a reservoir age correction has to be applied. This phenomenon has been referred to as the reservoir effect (Stuiver & Polach, 1977).

Climatic, solar, oceanic, and geomagnetic influences on late-glacial and holocene atmospheric 14C12C change

Late-glacial and Holocene 14C12C ratios of atmospheric CO2 vary in magnitude from a few per mil for annual/decadal pertubations to more than 10% for events lasting millennia. A data set illuminating 10- to 104-yr variability refines our understanding of oceanic (climatic) versus geomagnetic or solar forcing of atmospheric 14C12C ratios. Most of the variance in the Holocene atmospheric 14C12C record can be attributed to the geomagnetic (millennia time scale) and solar (century time scale) influence on the flux of primary cosmic rays entering the atmosphere. Attributing the observed atmospheric 14C12C changes to climate alone leads to ocean circulation and/or global wind speed changes incompatible with proxy records. Climate-(ocean-)related 14C redistribution between carbon reservoirs, while evidently playing a minor role during the Holocene, may have perturbed atmospheric 14C12C ratios measurably during the late-glacial Younger Dryas event. First-order corrections to the radiocarbon time scale (12,000–30,000 14C yr B.P.) are calculated from adjusted lake-sediment and tree-ring records and from geomagnetically defined model 14C histories. Paleosunspot numbers (100–9700 cal yr B.P.) are derived from the relationship of model 14C production rates to sunspot observations. The spectral interpretation of the 14C12C atmospheric record favors higher than average solar activity levels for the next century. Minimal evidence was found for a sun-weather relationship.

Anthropogenic and solar components of hemispheric 14C

A variable solar (helio-magnetic) modulation of the cosmic ray flux causes atmospheric 14C to change on a decadal to century timescale. Ocean-atmosphere exchange rates and atmospheric circulation patterns play a role in Northern-Southern Hemispheric 14C offsets and regional 14C differences that are significant for radiocarbon dating. Time dependent radiocarbon age differences, relative to Washington, were determined for Alaska (A.D. 1884–1932), South Chile (A.D. 1850–1952 and A.D. 1670–1722), Tasmania (A.D. 1895–1950) and Siberia (A.D. 1545–1715). Twentieth century fossil fuel CO2, lacking 14C and mostly released in the Northern Hemisphere (N), has entered the Southern Hemisphere (S) by atmospheric N-S exchange. Regional 14C time series show that initial (19th century) positive N-S 14C offsets switch to negative values by AD 1940. First order carbon reservoir modeling predicts such crossovers.

The Solar Component of the Atmospheric 14C Record

Heliomagnetic modulation of the cosmic ray flux causes variations in annual 14C production during the 11 yr solar cycle. Sunspot, auroral, and 10Be records suggest that century type 14C oscillations are also primarily influenced by the sun rather than climate. The history of decadal and bi-decadal 14C production rates during the past 9700 years, as derived from the 14C content in tree-rings using a carbon reservoir model, indicates at least two recurring patterns of solar change. Increases in 14C production rate that peak at 30% and last ca. 280 years (“Sporer types”) are repeated 8 times at irregular intervals while similar fluctuations that last only ca. 200 years (“Maunder types”) are found 9 times. The decadal and bi-decadal records of atmospheric Δ14C and model-derived 14C production rates, with long-term trend removed, are displayed in 1000 yr intervals.