Mysteriously high Δ<sup>14</sup>C of the glacial atmosphere: influence of <sup>14</sup>C production and carbon cycle changes

Abstract

<jats:p>Abstract. Despite intense focus on the ∼190 ‰ drop in atmospheric Δ14C during Heinrich Stadial 1 at ∼17.4–14.6 ka, the specific mechanisms responsible for the apparent Δ14C excess in the glacial atmosphere have received considerably less attention. The computationally efficient Bern3D Earth system model of intermediate complexity, designed for long-term climate simulations, allows us to address a very fundamental but still elusive question concerning the atmospheric Δ14C record: how can we explain the persistence of relatively high Δ14C values during the millennia after the Laschamp event? Large uncertainties in the pre-Holocene 14C production rate, as well as in the older portion of the Δ14C record, complicate our qualitative and quantitative interpretation of the glacial Δ14C elevation. Here we begin with sensitivity experiments that investigate the controls on atmospheric Δ14C in idealized settings. We show that the interaction with the ocean sediments may be much more important to the simulation of Δ14C than had been previously thought. In order to provide a bounded estimate of glacial Δ14C change, the Bern3D model was integrated with five available estimates of the 14C production rate as well as reconstructed and hypothetical paleoclimate forcing. Model results demonstrate that none of the available reconstructions of past changes in 14C production can reproduce the elevated Δ14C levels during the last glacial. In order to increase atmospheric Δ14C to glacial levels, a drastic reduction of air–sea exchange efficiency in the polar regions must be assumed, though discrepancies remain for the portion of the record younger than ∼33 ka. We end with an illustration of how the 14C production rate would have had to evolve to be consistent with the Δ14C record by combining an atmospheric radiocarbon budget with the Bern3D model. The overall conclusion is that the remaining discrepancies with respect to glacial Δ14C may be linked to an underestimation of 14C production and/or a biased-high reconstruction of Δ14C over the time period of interest. Alternatively, we appear to still be missing an important carbon cycle process for atmospheric Δ14C. </jats:p>

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

“…A long‐standing overarching challenge remains to fully reconcile past atmospheric radiocarbon variability with reconstructed changes in radiocarbon production rates, and evolving marine radiocarbon activity (Dinauer et al., 2020). This challenge provides a stringent test, and an excellent training ground, for our understanding of global carbon cycle evolution over the last glacial cycle, as it requires accurate and consistent descriptions of radionuclide production, ocean ventilation, terrestrial biomass evolution, and marine sedimentation.…”

“…Paired calendar‐ and radiocarbon ages obtained for for example, coral, speleothem, and plant macrofossil samples, indicate that atmospheric $${{\Delta}}^{14}\mathrm{C}$$ was ∼400‰ higher at the LGM (see Figure 4) (Reimer et al., 2020). As discussed further below, passive cosmogenic nuclide fluxes recorded in ice cores, and paleomagnetic field intensity reconstructions, further indicate that increased radiocarbon production at (and leading up to) the LGM can only partially account for the observed increased atmospheric radiocarbon activity (e.g., Bard, 1998; Broecker & Barker, 2007; Channell et al., 2018; Dinauer et al., 2020; Hain et al., 2014; Hughen, Lehman, et al., 2004; Köhler et al., 2005; Marchitto et al., 2007; Skinner et al., 2010). This would suggest a significant change in the partitioning of radiocarbon between the various surface carbon reservoirs, including in particular an increase in the mean residence time of carbon in the ocean (e.g., Bard, 1998; Muscheler et al., 2004).…”

“…(2015) suggest that biology and mass‐dependent fractionation may actually have quite a significant effect in simulations, both on the distribution of radiocarbon in the ocean‐atmosphere system (biotic runs were linked to an overall more radiocarbon depleted ocean), and on the equilibration time for model runs. One area where neglecting mass‐dependent fractionation corrections will definitely have a significant effect, is in the calculation of global radiocarbon inventories (e.g., Dinauer et al., 2020), for which “raw” uncorrected ratios must be used. The reason for this is that the very large marine carbon reservoir will appear to include less radiocarbon if it is “corrected” to what it would be if it obtained its carbon from the atmosphere through photosynthesis, as illustrated by a comparison of measured‐ and modeled bomb‐ 14 C inventories (Bard et al., 1988).…”

Ideas and perspectives: Emerging contours of a dynamic exogenous kerogen cycle

“…In addition to kerogen oxidation at glacial terminations, the presented hypothesis carries other important implications, including the closure of the exogenous kerogen cycle during glacial periods (Fig. 2) potentially contributing to relatively high atmospheric 14 C signatures, a problem highlighted by Dinauer et al (2020), as a reduced 14 C-free CO2 flux would reduce the dilution of the atmosphere's cosmogenic 14 C. Overall, increased reburial efficiency of kerogen can account for several tens of PgC over millennial timescales entirely bypassing actively circulating carbon pools on Earth's surface (Fig. 1).…”

Ideas and perspectives: Emerging contours of a dynamic exogenous kerogen cycle

“…In addition to kerogen oxidation at glacial terminations, the hypothesis presented carries other important implications, including the closure of the exogenous kerogen cycle during glacial periods (Fig. 2) potentially contributing to relatively high atmospheric 14 C signatures, a problem highlighted by Dinauer et al (2020), as a reduced 14 C-free CO 2 flux would reduce the dilution of the atmosphere's cosmogenic 14 C. Overall, increased reburial efficiency of kerogen can account for several tens of petagrams of carbon over millennial timescales, entirely bypassing actively circulating carbon pools on Earth's surface (Fig. 1).…”