Sun, Ocean, Nuclear Bombs, and Fossil Fuels: Radiocarbon Variations and Implications for High-Resolution Dating

Abstract

Radiocarbon, or 14 C, is a radiometric dating method ideally suited for providing a chronological framework in archaeology and geosciences for timescales spanning the last 50,000 years. 14 C is easily detectable in most common natural organic materials and has a half-life (5,730 ± 40 years) relevant to these timescales. 14 C produced from large-scale detonations of nuclear bombs between the 1950s and the early 1960s can be used for dating modern organic materials formed after the 1950s. Often these studies demand highresolution chronology to resolve ages within a few decades to less than a few years. Despite developments in modern, high-precision 14 C analytical methods, the applicability of 14 C in high-resolution chronology is limited by short-term variations in atmospheric 14 C in the past. This article reviews the roles of the principal natural drivers (e.g., solar magnetic activity and ocean circulation) and the anthropogenic perturbations (e.g., fossil fuel CO 2 and 14 C from nuclear and thermonuclear bombs) that are responsible for short-term 14 C variations in the environment. Methods and challenges of high-resolution 14 C dating are discussed.

Abundant pre-industrial carbon detected in Canadian Arctic headwaters: implications for the permafrost carbon feedback

“…Old C (defined here as C assimilated into plant material from the atmosphere and sequestered in organic-rich soils prior to the 1950s) has proven notoriously difficult to detect in aquatic systems due to the overwhelming presence of 'modern' C (assimilated since 1950). Post-1950s C has a significantly enriched 14 C signature as a result of atmospheric nuclear weapons testing (Dutta 2016). This 14 C enrichment has severely hindered attempts to provide high resolution dating of C assimilated over the past ∼200 years, especially when the material is derived from multiple years (Evans et al 2014).…”

IntCal, SHCal, or a Mixed Curve? Choosing a14C Calibration Curve for Archaeological and Paleoenvironmental Records from Tropical South America

“…This difference has been attributed to the fact that the Southern Hemisphere has more of the world’s ocean surface area: 61%, in contrast with 39% in the Northern Hemisphere (Braziunas et al 1995; Stuiver and Braziunas 1998:331). Air–sea exchange of CO 2 injects relatively old carbon from the surface ocean into the atmosphere (see overviews in Dutta 2016; Turnbull et al 2016), an effect that is strongest in the Southern Ocean surrounding Antarctica (Anderson et al 2009; Franke et al 2008; Rodgers et al 2011). Here, strong winds induce upwelling of “old” deep water to the surface and into the atmosphere, to be subsequently mixed northward throughout the Southern Hemisphere (Krakauer et al 2006; Rodgers et al 2011).…”

Global Peak in Atmospheric Radiocarbon Provides a Potential Definition for the Onset of the Anthropocene Epoch in 1965

“…Direct fallout of radioactive particles (such as Pu) can be influenced by proximity to nuclear weapon test sites and environmental processes, with relatively low concentrations in the Southern Hemisphere at detection limits and/or the local expression of ‘early’ 1950s tests reaching levels comparable to a decade later 24 . In contrast, the so-called ‘bomb’ radiocarbon was primarily injected into the stratosphere 25 , forming 14 CO 2 that was transferred down into the troposphere through the late Northern Hemisphere spring exchange of air masses and subsequently mixed through zonal and meridional atmospheric circulation, ensuring global distribution 26 . As a consequence, the atmospheric 14 C content (or ∆ 14 C) approximately doubled in the Northern Hemisphere to form a peak centred on 1964 but encompassing a broader period, CE 1962 to 1967 (Figure S1 ; ref.…”

“… 27 ). Following the 1963 Partial Test Ban Treaty and later agreements, atmospheric ∆ 14 C levels have decreased through air-sea exchange 28 , fixation by the biosphere 29 and dilution by fossil fuel emissions 26 . In contrast to radionuclide particles, the relatively slow removal of 14 C from the troposphere 25 raises the possibility that the preservation of an atmospheric signal in the geological record may be detected synchronously around the world.…”

“…In contrast to radionuclide particles, the relatively slow removal of 14 C from the troposphere 25 raises the possibility that the preservation of an atmospheric signal in the geological record may be detected synchronously around the world. Although excess 14 C associated with thermonuclear testing is preserved in marine sequences (corals and sediments) 30 , the transfer is governed by a number of different factors including local rates of exchange, diffusivity and ocean circulation 26 , 31 , resulting in spatial and temporal differences in the expression of the bomb peak. The longest series of atmospheric 14 CO 2 observations commenced in the Southern Hemisphere in CE 1954 at Lower Hutt, Wellington (New Zealand) 32 , recording the global expression of the bomb peak.…”