CL5.1.3

The vast majority of organic carbon (OC) produced by life is respired back to carbon dioxide (CO₂), but roughly 0.1% escapes and is preserved over geologic timescales. By sequestering reduced carbon from Earth’s surface, this “slow OC leak” contributes to CO₂ removal and promotes the accumulation of atmospheric oxygen and oxidized minerals. Countering this, OC contained within sedimentary rocks is oxidized during exhumation and erosion of mountain ranges. By respiring previously sequestered reduced carbon, erosion consumes atmospheric oxygen and produces CO₂. The balance between these two processes—preservation and respiration—regulates atmospheric composition, Earth-surface redox state, and global climate. Despite this importance, the governing mechanisms remain poorly constrained. To provide new insight, we developed a method that investigates OC composition using bond-strength distributions coupled with radiocarbon ages. Here I highlight a suite of recent results using this approach, and I show that biospheric OC interacts with particles and becomes physiochemically protected during aging, thus promoting preservation. I will discuss how this mechanistic framework can help elucidate why OC preservation—and thus atmospheric composition, Earth-surface redox state, and climate—has varied throughout Earth history.

How to cite: Hemingway, J., Rothman, D., Grant, K., Rosengard, S., Eglinton, T., Derry, L., and Galy, V.: Mineral protection regulates the long-term global preservation of natural organic carbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-627, https://doi.org/10.5194/egusphere-egu21-627, 2021.

Foraminifera isolated from deep-sea sediments are among the most common materials in AMS radiocarbon analysis. These results are used to determine accurate age models for sediment sequences as well as to detect changes in deep-sea ventilation. Often, only small numbers of (monospecific) foraminifera shells can be isolated, in particular when studying benthic species in sediments from the polar regions. Therefore, these samples are often analyzed as CO₂ gas using MICADAS instruments, and the method can typically be used for samples of up to around 40 ka in age. For reliable results, an accurate determination and minimization of processing blanks is required.

Processing blanks for foraminifera samples may in part derive from acid hydrolysis of the carbonates. It has, however, been shown that contamination of the carbonate fossils, mainly from atmospheric CO₂ adsorbed on the porous surfaces of foraminifera, is the largest source of blank found in foraminifera samples. The removal of such contamination has been attempted by various leaching methods, which come at the risk of introducing additional contaminations. Alternatively, blank correction of AMS results may be achieved using fossil foraminifera from ancient deposits much beyond the range of the radiocarbon method.

Here we report results of a systematic test comparing the F¹⁴C levels obtained for fossil (>130 ka) and sub-modern monospecific planktic and benthic foraminifera samples using different blank correction approaches. Specifically, we compare leaching with dilute hydrochloric acid, blank correction relative to a leached and an un-leached fossil foraminifera standard, and blank correction relative to the IAEA-C1 certified carbonate standard. 

How to cite: Mollenhauer, G., Grotheer, H., Bonk, E., and Gentz, T.: Determining and improving the analytical blank for radiocarbon analyses of small foraminifera samples using MICADAS and the gas interface system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-915, https://doi.org/10.5194/egusphere-egu21-915, 2021.

We have implemented ¹⁴C and further abiotic tracers (³⁹Ar, CFC-12, and SF₆) into the state-of-the-art ocean circulation model FESOM2. Different to other global ocean circulation models, FESOM2 employs unstructured meshes with variable horizontal resolution. This approach allows for improvements in areas which are commonly poorly resolved in global ocean modelling studies such as upwelling regions, while keeping the overall computational costs still sufficiently moderate. Here, we present results of a transient simulation running from 1850-2015 CE tracing the evolution of the bomb radiocarbon pulse with a focus on the evolution of marine radiocarbon ages. In addition we explore the potential of ³⁹Argon to complement ¹⁴C dating of marine waters.

How to cite: Butzin, M., Sidorenko, D., and Köhler, P.: A multi-resolution ocean simulation of the anthropogenic radiocarbon transient, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3118, https://doi.org/10.5194/egusphere-egu21-3118, 2021.

Previous studies attempting to explain Pleistocene atmospheric CO₂ variations have focused on mechanisms that transfer carbon (C) between the oceanic, atmospheric and terrestrial reservoirs, with the underlying assumption that the total C inventory in these three Earth’s surface reservoirs remained constant during glacial-interglacial cycles. Under this framework, ocean C inventory would have been marginally increased by 500-1000 GtC (1-2%) during the glacial period. Here, we show that past ocean C inventory can be revealed by reconstructed bulk ocean ¹⁴C/¹²C (denoted as ∆¹⁴C) and atmospheric ¹⁴C production rates with an Earth system model - cGENIE. First, we develop a bulk ocean ∆¹⁴C record that spans the last 40 ka from thousands of benthic foraminifera and deep sea coral ∆¹⁴C data with a fairly good coverage of the global seafloor. We then run cGENIE under constant pre-industrial boundary conditions, with the only forcing being atmospheric ¹⁴C production rates reconstructed by geomagnetic field intensity records and ice core record of ¹⁰Be fluxes. Under most of the ¹⁴C production scenarios, the simulated bulk ocean ∆¹⁴C are significantly lower than our composite during the Last Glacial Maximum as well as the early deglaciation. Bulk ocean ∆¹⁴C is a metric controlled by ¹⁴C production rates and ocean C inventory, with the state of ocean circulation playing a minor role.  Our finding suggests either glacial ¹⁴C production was much higher and/or glacial C inventory was much lower than previously thought. Implications of both possibilities are discussed. In particular, the second possibility highlight the exchange of C and ALK between Earth’s surface and geological reservoirs as a critical missing piece in searching for a complete theory of glacial-interglacial atmospheric CO₂ variability.

How to cite: Shao, J., Stott, L., Ridgwell, A., Zhao, N., Adolphi, F., and Yu, J.: A smaller glacial ocean carbon inventory?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6700, https://doi.org/10.5194/egusphere-egu21-6700, 2021.

Due to nuclear bomb tests during mid 1950s and 1960s, enormous amount of bomb radiocarbon was introduced into the atmosphere and subsequently to the ocean. Corals growing in shallow oceanic region record the radiocarbon variations in ocean surface waters. The bomb radiocarbon signature embedded in coral can be useful in providing information about natural processes affecting the surface waters of the region. In this regard, coral based radiocarbon records from the Lakshadweep Islands and the Andaman Islands from the northern Indian Ocean has been analysed. The analysed coral ∆¹⁴C values of recent period show comparable or even higher than the atmospheric ∆¹⁴C values, suggesting that major fraction of bomb radiocarbon have transferred in to the ocean. The northern Andaman region show ∆¹⁴C decline rate of about 3.1 ‰ yr^-1 between 1978 to 2014. Whereas, the southern Bay of Bengal and the Lakshadweep records show relatively lower decline rate of 2.5 ‰ yr^-1 for the same period. Based on the coral and atmospheric radiocarbon values, air-sea CO₂ exchange rate over the Lakshadweep and Andaman region has been estimated. The bomb radiocarbon based estimate of air-sea CO₂ exchange rate over Lakshadweep is 13.4 ± 2.1 mol m^-2 yr^-1 and over northern Andaman is 8.8 ± 1.3 mol m^-2 yr^-1. The Lakshadweep region show net regional CO₂ flux of 2.5 ± 0.4 Tg C yr^-1, while the northern Andaman region shows value of -0.3 ± 0.04 Tg C yr^-1. This study discusses the spatial and temporal radiocarbon changes in the northern Indian Ocean and has implications to constraining the carbon flux over the region.

How to cite: Raj, H. and Bhushan, R.: Surface radiocarbon record from the northern Indian Ocean: understanding surface ocean processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15892, https://doi.org/10.5194/egusphere-egu21-15892, 2021.

This study investigates the impact of circulation changes on the distribution of radiocarbon in the ocean interior using simulations of the NEMO model forced by JRA-55 atmospheric reanalysis data. We performed four simulations from 1850 to 2017 with transient or fixed boundary conditions for atmospheric ∆¹⁴C and CO₂, forced with inter-annually varying or fixed neutral year JRA-55 reanalysis data. The difference between variable and steady-state ocean dynamics revealed the areas in the world ocean where the radiocarbon distribution is most affected by variable ocean circulation: the midlatitude North Pacific and North Atlantic, the midlatitude South Pacific and South Indian, and the Weddell Sea. The difference between fixed and transient atmospheric boundary conditions over the past few decades shows the impact on natural vs bomb ¹⁴C. We investigate the potential drivers of variability from gas exchange, mixing in the thermocline, and interior transport. In general, the impact of circulation changes on bomb and natural ¹⁴C shows similar patterns but the effect is larger for bomb ¹⁴C. Compared to observed decadal changes in ∆¹⁴C between the 1990s and 2010s, the model underestimates the changes in ∆¹⁴C and potential density suggesting that the model response to circulation change is rapid.

How to cite: Pookkandy, B. and Graven, H.: Ocean radiocarbon response to circulation changes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9643, https://doi.org/10.5194/egusphere-egu21-9643, 2021.

Radiocarbon in atmospheric methane (Δ¹⁴CH₄) is a powerful tracer of fossil methane emissions and can be used to attribute methane emissions to fossil or biogenic sources. However, few Δ¹⁴CH₄ measurements are reported since 2000^1,2, due to challenges in sampling enough carbon for ¹⁴C measurements and in assessing the influence of ¹⁴C emissions from nuclear power plants on the ¹⁴C observations.

At Imperial College London we addressed the sampling limitation by developing a unique sampling system that separates carbon at the point of sampling and uses small traps of molecular sieves. Collection of a sample is made by three main steps: 1) removal of CO₂ and CO from air, 2) combustion of CH₄ into CO₂ and 3) adsorption of the combustion-derived CO₂ onto the molecular sieve trap. ¹⁴C analysis of our samples was carried out at the accelerator mass spectrometry facility at UCI. This novel system has been used for collection of samples in central London and has been made portable for collection of samples in different settings. 

Here we describe the system and report the evaluation of the measurement uncertainty and the processing blank. We achieved a measurement precision of 6 ‰, which is similar to or better than the reported precision of the most recent observations^1,3.

¹ Townsend‐Small et al JGR 117(D7) 2012

² Sparrow et al Sci. Adv 4(1) 2018

³ Espic et al Radiocarbon 61( 5) 2019

How to cite: Zazzeri, G., Xu, X., and Graven, H.: A novel sampling system for radiocarbon measurements of atmospheric methane, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15015, https://doi.org/10.5194/egusphere-egu21-15015, 2021.

Compound-specific radiocarbon analysis (CSRA) of leaf waxes has revealed significant lag times before compounds are deposited in marine and lacustrine sediments. No such data so far exist for a cold and arid high altitude lake system, where carbon turnover and biomarker fluxes to sediments are expected to be relatively low. To elucidate transport dynamics of terrestrial leaf waxes in such an environment (MAT: -4°C, MAP <100mm), we determined CSRA-ages of selected long-chain n-alkanes in surface soil samples (0-10 cm), collected from alpine meadows in the catchment of Lake Karakul (Pamirs, Tajikistan), and in two sections  of a well dated sediment core from the same lake. We aimed to answer the question if there is a potential bias in the interpretation of biomarker records, in case the leaf wax compounds are significantly older than the sediment age-model suggests.

nC₂₉- and nC₃₁-alkanes in the soil samples exhibited variable ages, ranging from 105±79 to 2260±155 cal. yrs BP. In the two sediment core samples, three of the four obtained ages for nC₂₉ and nC₃₁ felt on the very lower ends of the 1ϭ-uncertainty ranges of modelled ages (based on AMS ¹⁴C_TOC and OSL dating results).

The large span of CSRA-ages of soils gives evidence for heterogeneous decomposition and transport conditions in the lake catchment. We hypothesize that compounds with longest pre-aging contributed in lower proportions to the accumulated lake sediments and further suggest that sedimentary leaf waxes represent compounds with intermediate turnover time in soils, for example originating from alluvial plains close to the shores. Overall, the obtained results give evidence that sedimentary leaf wax compounds in this cold and arid high altitude setting are potentially older than the conventional age-model indicates. On the other hand, these findings need to be interpreted in context of the generally large uncertainty ranges of such age-models, which are further influenced by unknown factors for example changes of reservoir effects. 

How to cite: Aichner, B., Rethemeyer, J., Gierga, M., Stolz, A., Mętrak, M., Wilk, M., Suska-Malawska, M., Sachse, D., Rajabov, I., Rajabov, N., and Mischke, S.: Compound-specific radiocarbon ages of soil and sedimentary leaf wax biomarkers in an arid high-altitude environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3392, https://doi.org/10.5194/egusphere-egu21-3392, 2021.