Radiocarbon as a Dating Tool and Tracer in Paleoceanography

Abstract

Radiocarbon is an extremely useful carbon cycle tracer and radiometric dating tool. Here, we review the main principles and challenges involved in the use of radiocarbon in paleoceanography. First, we present a conceptual framework in which there are three possible uses of a radiocarbon measurement: (a) to obtain a calendar age interval, or a fossil entity's age; (b) to obtain an estimate of a carbon reservoir's past radiocarbon activity; or (c) to compare the relative radiocarbon activities of two contemporary carbon reservoirs. We discuss the analysis of marine fossil material, the generation of an atmospheric reference curve, and the interpretation of marine radiocarbon “ventilation metrics” in relation to this reference curve. It is emphasized that marine radiocarbon integrates the influences of: changing radiocarbon production; air‐sea gas exchange effects at the sea surface; transport times within the ocean interior; and the mixing of water parcels with different transit times from the sea surface, and different sea‐surface sources. These controls are what make radiocarbon such a powerful paleoceanographic tracer, though the difficulty of disentangling them is what makes marine radiocarbon dating and tracer studies so challenging. We discuss the implementation of radiocarbon in numerical models, and explore the theory linking ocean‐atmosphere partitioning of radiocarbon and CO2. Finally, we review existing records of marine radiocarbon variability over the last ∼25,000 years, which highlight the influence of ocean‐atmosphere carbon exchange on past atmospheric CO2 and climate, and point to emerging opportunities for resolving the global radiocarbon‐ and carbon budgets over the last glacial cycle.

A RESPONSE TO COMMUNITY QUESTIONS ON THE MARINE20 RADIOCARBON AGE CALIBRATION CURVE: MARINE RESERVOIR AGES AND THE CALIBRATION OF 14C SAMPLES FROM THE OCEANS

“…Changes in atmospheric 14 C levels typically take time to be transferred to the surface ocean and are also significantly damped (Druffel and Suess 1983;Grottoli and Eakin 2007;Komugabe-Dixson et al 2016;Bard and Heaton 2021). This is a consequence of mixing between the surface layer of the ocean and the extremely large carbon reservoir in deeper ocean layers (in total, during late-Holocene/pre-industrial times, the ocean contained ∼ 60 times more carbon than the atmosphere), and the slow rate of air-sea CO 2 equilibration (Grottoli and Eakin 2007;Skinner and Bard 2022). The resultant smoothing, and phase shift, of changes in atmospheric 14 C levels that is inherent to the surface ocean causes considerable high-frequency variation in the overall MRA at any site-whereby atmospheric 14 C levels have changed but the ocean is still in the process of responding/ equilibrating-see Figure 1 for a model-based illustration of the effect on overall MRA of this ocean smoothing.…”

Climate Induced Thermocline Aging and Ventilation in the Eastern Atlantic Over the Last 32,000 Years

“…The clear aging of thermocline water and amplified glacial age depth gradients compared to today (Skinner & Bard, 2022) may reflect a combination of numerous processes, which, however, influence all southern oceans. Such processes are increasing sea ice cover in south polar regions inhibiting gas exchange and thus ventilation, and/or overall reduced ventilation of the deep oceans (AMOC weakening), and/or increased storage of respired CO 2 in the deepest layers and upward mixing into the thermocline ocean (Skinner & Bard, 2022). In the deep ocean, glacial haline stratification is thought to decrease vertical mixing across water masses as compared to the thermohaline stratification today, leading to more stratification (Adkins, 2013).…”

“…However, previous radiocarbon reconstructions above 1,000 m depth in the North Atlantic are derived mostly from foraminifera and show large scatter, ranging from very well-ventilated waters to strongly depleted waters with B atm ages of up to 2,000 years (compilation by Skinner and Bard (2022)). As stated by Skinner and Bard (2022), one outstanding challenge for the interpretation of radiocarbon as ventilation age tracer is to narrow down the observed scatter in marine radiocarbon reservoir age estimates. 230 Th/U dated CWCs overcome some of the impediments associated with the use of foraminifera (Skinner & Bard, 2022), and provide ventilation ages with higher precision regarding the absolute age determination and reservoir age uncertainty.…”

“…As stated by Skinner and Bard (2022), one outstanding challenge for the interpretation of radiocarbon as ventilation age tracer is to narrow down the observed scatter in marine radiocarbon reservoir age estimates. 230 Th/U dated CWCs overcome some of the impediments associated with the use of foraminifera (Skinner & Bard, 2022), and provide ventilation ages with higher precision regarding the absolute age determination and reservoir age uncertainty. Here, we study this part of the Atlantic thermocline ocean using framework forming CWCs to retrieve the evolution of the east Atlantic north-south ventilation difference since the last glacial.…”

The significance of Atlantic Water routing in the Nordic Seas: The Holocene perspective

“…Radiocarbon ventilation ages, defined as the ocean-atmosphere radiocarbon isotope disequilibrium (e.g. Soulet et al, 2016) and therefore broadly indicative of the time since the water was last in contact with the atmosphere are used to infer changes in past ocean circulation and air-sea gas exchange (Skinner and Bard, 2022). Radiocarbon measurements have been used to reconstruct past changes in ocean circulation, mostly over the last deglaciation (e.g.…”