Displays

Ocean-atmosphere ¹⁴C disequilibria of the surface and deep ocean reflect past changes in the efficiency of ocean-atmosphere CO₂ exchange and ocean mixing, while it may also be related to variations in global-ocean respired carbon content. A full assessment of the oceanic mechanisms controlling deglacial changes in atmospheric CO₂ is complicated by a lack of high-resolution ¹⁴C ventilation age estimates from the Southern Ocean and other key regions due to low foraminiferal abundances in marine sediments in those areas. Here we present high-resolution deglacial ¹⁴C ventilation age records from key sites in the Atlantic and Indian Sector of the Southern Ocean obtained by radiocarbon analyses of small benthic and planktic foraminiferal samples (<1 mg CaCO₃) with the UniBe Mini-Carbon Dating System (MICADAS). Our analyses specifically circumvent foraminiferal sample size requirements related to “conventional” accelerator mass spectrometer analyses involving sample graphitization (>1 mg CaCO₃ in most laboratories). Complementing multi-proxy analyses of sea surface temperature (SST) changes at these sites allow the construction of a radiocarbon-independent age model through a stratigraphic alignment of SST changes to Antarctic (ice core) temperature variations. We demonstrate the value of refining the age models of our study cores on the basis of high-resolution sedimentary U- and Th flux estimates, which allows an improved quantification of surface ocean reservoir age variations in the past. The resulting deep-ocean ventilation age changes are compared against qualitative and quantitative indicators of bottom water [O₂] variations, in order to assess the role of Southern Ocean overturning dynamics in respired carbon changes at our study sites. We discuss the implications of our new radiocarbon- and bottom water [O₂] data for the ocean’s role in atmospheric CO₂ changes throughout the last deglaciation, and evaluate down-stream effects of southern high-latitude surface ocean reservoir age anomalies.

How to cite: Gottschalk, J., Anderson, R. F., Hodell, D. A., Martinez-Garcia, A., Mazaud, A., Michel, E., Skinner, L. C., Studer, A., Szidat, S., Thöle, L. M., and Jaccard, S. L.: On new developments in accelerator mass spectrometry and how they promote our understanding of global carbon cycle dynamics during the last deglaciation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12768, https://doi.org/10.5194/egusphere-egu2020-12768, 2020.

Carbon isotopic measurements in oceanic dissolved inorganic carbon (DIC) contribute to many oceanographic fields. For instance, radiocarbon (¹⁴C) has been essential to elucidate aspects related to ocean circulation, air-sea exchange, carbon cycling and biogeochemistry. Despite its importance as a tracer in oceanography, oceanic ¹⁴C has been less well studied than other tracers (e.g. CFCs) as disentangling the natural from the artificial component is not trivial. Another major limitation was the large volume seawater samples required for the decay counting of ¹⁴C. Advances in Accelerator Mass Spectrometry (AMS) allowed the reduction of the sample volume to a couple of liters, permitting to obtain spatially better resolved distributions of oceanic ¹⁴C during repeated GO-SHIP sections. Yet, methods for sample preparation were borrowed from decay counting and not optimized for AMS. Here, we present a method that we recently developed in the Laboratory of Ion Beam Physics (ETHZ) that allows the rapid (<5 hours) measurement of DI¹⁴C in small seawater samples with unprecedented precision (<2‰) (Casacuberta et al., 2019). The setup consists of an automated sampler designed to extract DI¹⁴C from 50 - 60 ml samples, by sparging the acidified seawater with helium gas to extract CO₂. The fully automated method is controlled via a LabVIEW program that runs through all consecutive steps: catalyst preconditioning, CO₂ extraction, CO₂ trapping and thermal CO₂ release from the trap into the reactor for graphitization, which is performed simultaneously for 7 samples. The method is optimized by introducing a Cu-Ag furnace that improves and accelerates the graphitization to less than 2 hours. As a proof of principle, we will show two sections of ¹⁴C corresponding to two recent expeditions carried out in the North Atlantic (OVIDE section) and the Fram Strait in 2018. The high precision of the results allows for the characterization of different water masses in the subpolar North Atlantic Ocean, which reflect the export of anthropogenic carbon to the abyssal waters as a result of deep-water formation in the Iceland-Scotland Overflow Water and the Denmark Strait Overflow Water. Results will be also compared to previously published oceanic Δ¹⁴C data in those regions. These studies already demonstrate the potential to use Δ¹⁴C as a powerful and cost-efficient tool to resolve oceanic circulation patterns, especially with respect to ventilation of the water column.

Casacuberta, N., Castrillejo, M., Wefing, A.-M., Bollhalder, S., & Wacker, L. (2019). High Precision ¹⁴C Analysis in Small Seawater Samples. Radiocarbon, 00(00), 1–12. https://doi.org/10.1017/rdc.2019.87

How to cite: Casacuberta, N., Castrillejo, M., Wefing, A.-M., Bollhalder, S., Kündig, K., Synal, H.-A., and Wacker, L.: Rapid and high precision C-14 analysis in small DIC seawater samples and its future application as an ocean tracer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3581, https://doi.org/10.5194/egusphere-egu2020-3581, 2020.

To constrain the accurate age of a marine sediment record, the radiocarbon (¹⁴C) ages need to be corrected for short-term and small-scale changes in planktic ¹⁴C reservoir ages (R_plank). Nevertheless, accurate records of past changes in R_plank are scarce. Here we present a high-resolution record of deglacial ¹⁴C ages measured on Globigerina bulloides in sediment core M77/2-59-1 from the northern boundary (~4°S, 997 m) of the Peruvian upwelling zone. The fine structure of jumps and plateau boundaries in the ¹⁴C record were tuned to synchronous, thus global structures in the atmospheric ¹⁴C record of Lake Suigetsu (Bronk Ramsey et al., 2012) and used as tie points for an age model with semi-millennial resolution, moreover to reconstruct deglacial changes in R_plank from 17 to 11 cal. ka. In our record, R_plank drops from 1250 ¹⁴C yr prior to 14 cal. ka to ~600 – 450 ¹⁴C yr until the plateau named Top of Younger Dryas. The drop suggests a major decrease in coastal upwelling, possibly the result of a southward (poleward) expansion of the Intertropical Convergence Zone and related shift in the southeastern trade wind belt during the Bølling-Allerød. Subsequent to 14 cal. ka our R_plank values are roughly similar to values obtained for thermocline waters near the equator from the age difference between ¹⁴C ages of wood chunks and ¹⁴C of G. ruber (Zhao & Keigwin, 2018). Prior to 14 cal. ka our R_plank are ~800 ¹⁴C yr higher, which corroborates the presumed latitudinal shift of coastal upwelling. Our ¹⁴C ages measured on G. bulloides differ in part from paired ¹⁴C ages of Neogloboquadrina dutertrei, indicating their habitat in different water masses prior to 14 cal. ka, in support of the upwelling affinity of G. bulloides. In addition, we used our R_plank values to accurately derive past ventilation ages of intermediate waters near 1000 m depth based on the difference of paired benthic and planktic ¹⁴C ages, which is important to constrain centennial to millennial scale changes in circulation influencing the extent of the Peruvian oxygen minimum zone.

References:

Bronk Ramsey, C., et al., Science, 338, 370–374, 2012.

Zhao & Keigwin, Nature communications, 9, 3077, 2018.

How to cite: Glock, N., Sarnthein, M., Doering, K., Mollenhauer, G., and Salvatteci, R.: Deglacial 14C reservoir ages of surface waters at the northern boundary of Peruvian coastal upwelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4727, https://doi.org/10.5194/egusphere-egu2020-4727, 2020.

Radiocarbon (¹⁴C) is an ideal tracer to study the uptake of carbon dioxide by the seas and the ocean circulation during the past 50,000 years. However, there are various issues impeding a straightforward interpretation of marine ¹⁴C records. The spatial and temporal variability of marine ¹⁴C records is superimposed by a systematic isotopic depletion of the sea surface with respect to the atmosphere. This effect is frequently expressed as Marine Reservoir Age (MRA), ranging from ~400 years in subtropical oceans to more than 1000 years in polar seas during the late Holocene. Prior to the Holocene, MRAs are poorly constrained through reconstructions. Moreover, the entire database of marine ¹⁴C records gets increasingly patchy and sparse the further one steps backwards in time. Model simulations provide a valuable interpretation tool and can help to fill spatial and temporal gaps. However, ¹⁴C paleorecords typically originate from continental margins, marginal seas, or tropical lagoons. These regions are not properly resolved by default coarse-resolution ocean models, which may result in regional model and hence interpretation biases. The alternative are marine ¹⁴C simulations with high(er) resolution, but the conventional approach involving uniform meshes results in computational costs which are prohibitive in most cases. To overcome these issues, we have implemented ¹⁴C into the state-of-the-art ocean model FESOM2 which employs unstructured meshes with variable resolution. This approach permits zooming into certain regions of interest while keeping the model resolution in other areas sufficiently moderate. Here, we present first simulation results considering the Anthropocene, the late Holocene, and the Last Glacial Maximum, focusing on the evolution of Marine Reservoir Ages.

How to cite: Butzin, M., Sidorenko, D., and Köhler, P.: Marine radiocarbon simulations carried out with a global multi-resolution ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7682, https://doi.org/10.5194/egusphere-egu2020-7682, 2020.

The influence of solar variability on the Earth’s climate is a major subject of interest for understanding past and predicting future climate changes. While the observational record of solar activity (e.g. sunspots) covers only the last about 400 yr, cosmogenic nuclides stored in tree rings (¹⁴C) or ice cores (¹⁰Be, ³⁶Cl) are used as proxies for solar activity and allow solar reconstructions reaching much further back in time ^1-3. Major drawbacks of cosmogenic nuclide based solar reconstructions are the presence of weather-induced noise (e.g. ¹⁰Be in ice cores) or the low temporal resolution of long precisely dated records (¹⁴C in tree rings). Here, we present a continuous, annually resolved ¹⁴C record from precisely dated tree rings covering the past about 1’000 yr (969-1933 AD) comprising almost 1’300 highest-precision ¹⁴C measurements. The annually resolved ¹⁴C record adds significantly to the radiocarbon calibration curve⁴, which has hitherto been based mainly on decay counting measurements. A multi box carbon cycle model is used to extract annual ¹⁴C production changes from the tree ring data. The resulting high-resolution record of ¹⁴C production is then used to reconstruct the solar modulation parameter over the last millennium. The comparison of solar modulation with global temperature provides evidence that low solar activity could have caused the temperature reduction during the Little Ice Age. The ¹⁴C record further reveals for the first time the presence of the eleven-year solar cycle over the past 1’000 yr. The amplitude of this so called Schwabe cycle is found to correlate with the general level of the solar modulation with high amplitudes during periods of strong solar modulation and vice versa.

¹ Bard, E., Raisbeck, G., Yiou, F. & Jouzel, J. (2000) Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus Series B-Chemical and Physical Meteorology 52, 985-992.

² Muscheler, R. et al. (2007) Solar activity during the last 1000 yr inferred from radionuclide records. Quaternary Science Reviews 26, 82-97.

³ Usoskin, I.G. (2017) A history of solar activity over millennia, Living Rev. Sol. Phys. 14, 3.

⁴ Reimer, P. J. et al. (2013) Intcal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years Cal Bp. Radiocarbon 55, 1869-1887.

How to cite: Wacker, L., Brehm, N., Bayliss, A., Christl, M., Synal, H.-A., Adolphi, F., Beer, J., Kromer, B., Muscheler, R., Solanki, S. K., Usoskin, I., Bleicher, N., Bollhalder, S., and Tyres, C.: Radiocarbon in tree-rings reveals the solar 11-yr cycle over the last millennium, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11118, https://doi.org/10.5194/egusphere-egu2020-11118, 2020.

Radiocarbon measurements of heterotrophically respired C (∆¹⁴C-CO₂) in laboratory soil incubations provide information about the age and source of microbially-available soil organic matter. However, due to the influence of “bomb” radiocarbon (from nuclear weapons testing in the mid-20^th century), measurements of ¹⁴C at a single time point can yield multiple solutions when modeling soil C cycling rates. Measuring ∆¹⁴C-CO₂ on archived soils would provide additional time points to assess which solution is appropriate. We had two hypotheses regarding the effect of archiving on ∆¹⁴C-CO₂: 1) long-term storage does not affect ∆¹⁴C-CO₂, and 2) drying and rewetting effects on ∆¹⁴C-CO₂ are limited to CO₂ released immediately following rewetting, without significant effects on CO₂ released after respiration rates equilibrate.

To address the first hypothesis, sample splits of soils collected at nine grassland and 21 forest sites (n=30) between 2004 and 2011 (for which ∆¹⁴C-CO₂ had been previously measured) were incubated again in 2018 after undergoing air-drying and storage. The difference in ∆¹⁴C-CO₂ measured before and after archiving was significant (p < 0.05); however, in line with our hypothesis, the number of years archived was not a significant predictor of the difference in a regression analysis.

To test the second hypothesis we first collected and analyzed ∆¹⁴C-CO₂ following the “pre-incubation” period, i.e. the period immediately following rewetting, as well as after the equilibrium respiration period for the subset of samples (six grassland, six forest) for which we had data on the original pre-incubation period. In this subset we observed different responses in forest versus grassland soils in the equilibrium respiration period: ∆¹⁴C-CO₂ decreased from the original value by 12.7 (±4.5) per mille in forests (p = 0.08), but increased by 22.2 (±6.7) per mille in grasslands (p < 0.05) (errors are twice the standard error of the mean difference). In contrast to our second hypothesis the ∆¹⁴C of the CO₂ released immediately following rewetting was not significantly different from the ∆¹⁴C of the CO₂ respired under equilibrium respiration conditions, despite the much higher rate of respiration following rewetting. A final incubation experiment comparing freshly collected soils that were dried but not archived was conducted to distinguish conclusively between rewetting and storage effects, but we are still awaiting the data.

In conclusion, the drying/rewetting effect appears to drive the differences between ∆¹⁴C-CO₂ measured in incubations before and after archiving, rather than duration of storage. The radiocarbon incubation technique for archived samples is promising: the 12 to 22 per mille differences observed are not insignificant, but in many cases should be within the range of acceptable error in a modeling context. The wider implication of our results is that drying and rewetting soils appears to mobilize a different pool of soil organic matter than would otherwise be available to microbes, an effect that persists throughout an incubation and affects grassland and forest soils differently. This effect applies to radiocarbon incubations in general and warrants further investigation.

How to cite: Beem Miller, J., Schrumpf, M., Guggenberger, G., and Trumbore, S.: Radiocarbon incubations of archived soils: insights into drying/rewetting effects and constraining soil C models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10075, https://doi.org/10.5194/egusphere-egu2020-10075, 2020.

Radiocarbon (¹⁴C) is a powerful tracer of the global carbon cycle that is commonly used to assess carbon cycling rates in various Earth system reservoirs and as a benchmark to assess model performance. Therefore, it has been recommended that Earth System Models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 report predicted radiocarbon values for relevant carbon pools. However, a detailed representation of radiocarbon dynamics may be an impractical burden on model developers. Here, we present an alternative approach to compute radiocarbon values from the numerical output of an ESM that does not explicitly represent these dynamics. The approach requires computed ¹²C stocks and fluxes among all carbon pools for a particular simulation of the model. From this output, a time‐dependent linear compartmental system is computed with its respective state‐transition matrix. Using transient atmospheric ¹⁴C values as inputs, the state‐transition matrix is then applied to compute radiocarbon values for each pool, the average value for the entire system, and component fluxes. We demonstrate the approach with ELMv1‐ECA, the land component of an ESM model that explicitly represents ¹²C, and ¹⁴C in 7 soil pools and 10 vertical layers. Results from our proposed method are highly accurate (relative error <0.01%) compared with the ELMv1‐ECA ¹²C and ¹⁴C predictions, demonstrating the potential to use this approach in CMIP6 and other model simulations that do not explicitly represent ¹⁴C.

How to cite: Metzler, H., Zhu, Q., Riley, W., Hoyt, A., Müller, M., and Sierra, C.: Mathematical Reconstruction of Land Carbon Models From Their Numerical Output: Computing Soil Radiocarbon From 12C Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8876, https://doi.org/10.5194/egusphere-egu2020-8876, 2020.

Our Sun erratically expels large amounts of energetic particles into the interplanetary space and towards Earth, which can be observed as so-called solar proton events (SPE). A strong SPE might cause major damage to satellites and could even disrupt transformers at the ground¹. This rises the questions how often strong SPEs occur. Since direct observations of SPEs are limited to the last decades, cosmogenic radionuclides can be used to detect such events further back in time. The production rate of cosmogenic nuclides, such as radiocarbon, is primarily dependent on the incoming flux of highly energetic galactic cosmic rays (GCR). Under normal conditions, the Sun’s magnetic field carried by the (low energy) solar protons shields us from (high energy) GCRs, resulting in a lower production of cosmogenic radionuclides when the Sun is active. During a SPE, however, the sudden and drastic increase of high the energy solar protons themselves may lead to an elevated production of cosmogenic radionuclides on Earth. Only recently, such sharp increases in cosmogenic nuclide production occurring within less than one year have been detected in several radionuclide records (¹⁰Be, ³⁶Cl, ¹⁴C) from ice core and tree ring records, and have been attributed to SPEs^2,3.

Until now, only three SPE could confidently be detected in cosmogenic radionuclide records^1,4,5. The reason for this is a general lack of accurately dated and annually resolved radionuclide records and/or the strong dampening of the production signal e.g. by the carbon cycle. To find and identify such events we measured radiocarbon in tree ring records at annual resolution with accelerator mass spectrometry (AMS). In this new, accurately dated and annually resolved ¹⁴C record spanning the past about 1000 yr we found several new candidates for SPEs. Their timing and amplitude in terms of cosmogenic nuclide production was characterized by using a global carbon cycle box model. Once unambiguously identified such spiked production increases recorded in the absolutely dated tree ring record have a great potential to be used as a global tool to synchronize other not well dated (climate) records with cosmogenic radionuclides (e.g. ¹⁰Be, ³⁶Cl).

1              Schrijver, C. J. et al. (2012) Estimating the frequency of extremely energetic solar events, based on solar, stellar, lunar, and terrestrial records. Journal of Geophysical Research: Space Physics 117

2              Miyake, F., Masuda, K. & Nakamura, T. (2013) Another rapid event in the carbon-14 content of tree rings. Nature communications 4, 1748

3              Mekhaldi, F. et al. (2015) Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4. Nature Communications 6, 8611

4              Miyake, F., Nagaya, K., Masuda, K. & Nakamura, T. A (2012) signature of cosmic-ray increase in AD 774-775 from tree rings in Japan. Nature 486, 240-242

5              O'Hare, P. et al. (2019) Multiradionuclide evidence for an extreme solar proton event around 2,610 B.P. ( approximately 660 BC). Proc Natl Acad Sci U S A 116, 5961-5966

How to cite: Brehm, N., Christl, M., Synal, H.-A., Muscheler, R., Adolphi, F., Bayliss, A., Knowles, T., Casanova, E., Nicolussi, K., and Wacker, L.: Detection of solar proton events by using radiocarbon in tree-rings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17774, https://doi.org/10.5194/egusphere-egu2020-17774, 2020.

The decrease in required sample sizes for radiocarbon (¹⁴C) analysis by accelerator mass spectrometry (AMS), which now is on the order of ten micrograms carbon or less provides the opportunity for precise dating of single specific compounds. However, background contamination associated with sample purification presents a major limitation to precise ¹⁴C dating at these low sample sizes. Many key target compounds are amenable to isolation using preparative chromatographic methods. Using preparative GC, for example, column bleed has been reported as the main contamination source. Although this contamination may be at sub-microgram levels^[1], removal is favorable for accurate dating of ultra-small samples. In synthetic and analytical chemistry, sublimation is a well-established approach for purification of semi-volatile compounds, and here we test it as an approach for purification of selected compounds for microgram-level ¹⁴C analysis. As commercial sublimation equipment usually is not designed for such small sample sizes, a custom-built micro-sublimation apparatus has been developed and tested for the purification of organic compounds in the sub-milligram range. The design of the microsublimation apparatus, which has been optimized to enable a streamlined protocol that minimizes contamination risks, will be presented. Experiments were performed with a range of different compound types, including fatty alcohols, alkanes and vanillin. Reproducibility with yields of up to 90% have been achieved. Stability of isotopic measurements and contamination sources will be discussed along with possible other application areas in the future.

[1]      E. Casanova, T. D. J. Knowles, C. Williams, M. P. Crump, R. P. Evershed, Anal. Chem. 2017, 89, 7090–7098.

How to cite: Heusser, C., Welte, C., Hattendorf, B., Montluçon, D., Günther, D., and Eglinton, T. I.: Purification of Organic Compounds Using Microsublimation for 14C Analysis , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8784, https://doi.org/10.5194/egusphere-egu2020-8784, 2020.

While high-precision radiocarbon (¹⁴C) measurements of carbonaceous samples using Accelerator Mass Spectrometry (AMS) have become routine, achieving a continuous radiocarbon record for carbonate archives (e.g. speleothems, corals) still requires labor-intensive and time-consuming sample preparation. By feeding laser ablation (LA) generated CO₂/CO online into a gas source AMS, however, these archives can be sampled continuously and with minimal preparation efforts.

The LA-AMS setup installed in 2013 at ETH Zurich [1] has recently been improved in order to achieve higher signal intensities and consequently higher measurement precision as well as simpler instrumental maintenance. By redesigning the sample cell and reducing the optical path length of the laser, the fluence on the sample could be increased from previously 1-2 J cm^-1 to now 8-23 J cm^-1, leading to more efficient generation of gaseous carbon from CaCO₃. The laser spot size was reduced from 110 μm x 680 μm to 75 μm x 140 μm, improving the overall spatial resolution of the setup. The background level of the method has been determined to have a F¹⁴C of 0.009 ± 0.002 and reaches a precision of less than 1% for modern samples.

To fully exploit the advantages of this unique technique, a LA-AMS specific data analysis software to disentangle [2] the quasi-continuous data stream is being developed. Features implemented include correlation of data with sampling location and plotting of all relevant measurement parameters as a function of sampling location (F¹⁴C,

How to cite: Wertnik, M., Welte, C., Wacker, L., Yeman, C., Hattendorf, B., Koch, J., Christl, M., Fohlmeister, J., Synal, H.-A., and Eglinton, T. I.: Rapid, continuous radiocarbon analysis of carbonate archives using laser ablation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9495, https://doi.org/10.5194/egusphere-egu2020-9495, 2020.

The difference of radiocarbon (¹⁴C) concentration between terrestrial and aquatic samples is called the freshwater reservoir effect (FRE). The FRE is a potential issue for archaeologists dating fish bones, shells, human bones, or food crusts on pottery from sites near rivers or lakes. Studies on the FRE showed its variability in space and time, significant variations within one river or lake, different aquatic plants, and animals, or even single fish species of the lake [1, 2 and the references therein]. Therefore, dating the artifacts, it is very important to understand the nature of the FRE by studying processes that determine the redistribution of carbon isotopes in water ecosystems. It is important to obtain new knowledge on temporal variation of the FRE of a water system as due to climate change and anthropogenic activities it could be completely different at ancient times since such periods as Mesolithic, Neolithic and Early Bronze Age when aquatic resources were an important contribution to human nutrition are relatively poorly studied. The objective of the research was to examine how known anthropogenic factors affected carbon cycling in the lake systems, including how these changes are reflected in carbon isotope variations as well as the FRA of lake sediments and different species of fish.

Two completely different lake systems of eastern Lithuania were studied. Lake Tapeliai belongs to the huge drainage system and is permanently affected by hydrological changes. When Lake Drūkšiai served as a cooling pond for the Ignalina Nuclear Power Plant, its average temperature increased by 3-4 °C. Results revealed that over the last century the estimated radiocarbon freshwater reservoir age (FRA) in sediments of Lake Tapeliai varied from 1136±112 y to 5733±122 y. These changes were caused by old organic carbon import to the lake from a neighboring peat bog. The FRA in samples of different fish species differed by up to 500 y, whereas the variations in the FRA measured in samples of the same species reached up to 300 y. Radiocarbon activity measurements in the samples of fish caught in Lake Drūkšiai during the operation of the nuclear power plant were performed. During 1984-1999 years measurements showed that ¹⁴C activity in fish slightly exceeded (up to 5 pMC) atmospheric activity. However, during 2000-2009 it exceeded by 40 pMC. Unfortunately, no information about increased activity levels of aquatic effluents or different chemical agents used could be found in INPP reports. Data of the fish scales ¹⁴C activity measurements are in good agreement with the data of the humic acid fraction of lake bottom sediments.

This data clearly indicates that there was an event in the year 2000 when substances from NPP with elevated ¹⁴C content were introduced into the lake, although not exceeding the permissible levels.

This research was funded by a grant (No. S-MIP-19-16) from the Research Council of Lithuania

References

[1] Heritage Science (2013) 1(1), 1–622.

[2] Quaternary Science Reviews (2012) 48: 67–79.

How to cite: Šapolaitė, J., Ežerinskis, Ž., Barisevičiūtė, R., Rakauskas, V., Butkus, L., Garbaras, A., Virbickas, T., Maceika, E., Pabedinskas, A., and Remeikis, V.: Investigation of carbon isotope ratio variations caused by natural and anthropogenic processes in lacustrine ecosystems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8265, https://doi.org/10.5194/egusphere-egu2020-8265, 2020.

Abstract

Dissolve black carbon (DBC) has been recently recognized as an important fraction of dissolved organic carbon (DOC) in both rivers and ocean. It is estimated that about 10% of the riverine DOC transported by the world rivers could be DBC. The sources and fate of DBC in both rivers and ocean, however, is not well known. In this study, we present radiocarbon (¹⁴C) and stable carbon isotope (¹³C) measurements of DBC in several large rivers in China, and in coastal and open ocean waters. DBC was concentrated using solid phase extraction (SPE) method and quantified by chemothermal oxidation (CTO) method.

Concentrations of DBC varied in rivers depending on the drainage basin of the river and accounted for 3.7-7.6% of the riverine DOC pool. DBC was slightly lower, accounted for 2.9-5.9% of DOC in coastal and open oceans. Carbon isotope results indicate that DBC δ¹³C values were all slightly enriched (by 2-3‰) than the values of DOC in both rivers and ocean. The DBC Δ¹⁴C values varied largely in rivers and the values were significantly higher than DOC Δ¹⁴C values in rivers but similar to DOC Δ¹⁴C values in the ocean. Using a two-end member isotope mass balance model, we calculated that the most DBC (80%) with relatively young ¹⁴C ages in the rivers was derived from biomass burning. Laboratory incubation studies also found that DBC released from recent charcoal was able to be utilized by bacteria, supporting the speculation that river transported DOC could be decomposed during estuaries mixing. Our study suggests that DBC is cycled in the same time scales with the DOC pool in the ocean and no extremely older DBC was identified as reported in other studies previously.

How to cite: Wang, X.: Sources and cycling of dissolved black carbon (DBC) in rivers and ocean revealed by carbon isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2047, https://doi.org/10.5194/egusphere-egu2020-2047, 2020.

On the basis of ¹⁴C plateau tuning we established a robust centennial-scale age control for last glacial-to-deglacial sediment sections in two marine sediment cores MD07-3088 and PS97-137 from the upper Chilean continental margin to facilitate a precise stratigraphic correlation between short-term changes in South Pacific oceanography and global paleoclimate signals recorded in ice cores from Antarctica and elsewhere (Küssner et al., in prep.). Age tie points and reservoir ages were derived from tuning a suite of planktic ¹⁴C plateaus to a suite of pertinent atmospheric ¹⁴C plateaus defined at Lake Suigetsu (Sarnthein et al., 2015). Off central Chile four tephra layers in Core MD07-3088 provide independent proof both for the age assignment and for short-term changes in planktic reservoir age we deduced by means of ¹⁴C plateau tuning. Reservoir ages derived from ¹⁴C plateau tuning at 11–16.5 cal. ka closely match, one-by-one, four reservoir ages that have been deduced from the difference between the ¹⁴C ages of planktic foraminifera associated with the tephra layers in marine sediments and the atmospheric ¹⁴C ages of plants associated with paired tephras analyzed nearby on land (Siani et al. 2013). – In Core PS97-137, near to the southern tip of Chile, sediments of the Last Glacial Maximum show a section with distinct lamination of 5-7 layers / cm depth, associated with atmospheric ¹⁴C Plateau 6a by plateau tuning. A rough count of the layers in the 160 cm long sediment section of the planktic ¹⁴C Plateau 6a gives a number that comes extremely close to the ~900 year-long time span of the atmospheric ¹⁴C Plateau 6a, thus provides independent proof for both the accuracy of the time interval assigned to correlated atmospheric ‘Plateau 6a’ and the approach of plateau tuning in general.

Küssner et al., Paleoceanography, in prep.,

Sarnthein et al., Radiocarbon, 2015, 57 (1), 129–151.

Siani et al., 2013, Nature comm., 4, 2758.

How to cite: Küssner, K., Sarnthein, M., Lamy, F., Michel, E., Mollenhauer, G., Siani, G., and Tiedemann, R.: Independent verification of age tie points and planktic reservoir ages deduced for two marine sediment records at the Chilean continental margin by 14C plateau tuning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5683, https://doi.org/10.5194/egusphere-egu2020-5683, 2020.

Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids synthesized by archaea (isoprenoid GDGTs) and bacteria (branched GDGTs), form the basis of a suite of molecular proxies used in terrestrial as well as marine environments. Compound-specific radiocarbon analysis has provided valuable insights into the sources and yielded constraints on transport dynamics of different biomarkers in the context of carbon cycle processes. To complement the existing biomarker radiocarbon toolbox, and to shed new light on the sources and fate of GDGTs, we developed a new method to measure GDGT radiocarbon compositions in natural samples.

Isoprenoid and branched GDGTs are isolated using two UHPLC silica columns in series coupled to a fraction collector set to eluent recovery at different time intervals. The accuracy of the method was tested using a modern and a radiocarbon-dead reference material. Procedural blanks show that the separation procedure adds less than 3 µg carbon with a Fm of 0.64.

The method is first applied to determine the Δ¹⁴C composition of isoprenoid and branched GDGTs in two soil core profiles from a temperate and subalpine forest ecosystem in order to explore the range of typical values encountered in natural systems.  The cores, which reach a depth of 80 cm and 40 cm respectively, have previously been analyzed with respect to radiocarbon characteristics of long-chain n-alkanes and fatty acids as well as bulk particulate and dissolved organic carbon (OC) [1]. For each core, GDGTs were separated and analyzed from 3 different depth intervals. The Δ¹⁴C of both isoprenoid and branched GDGTs decreases, at a similar rate as the bulk, by -350‰ and -200‰ along the temperate and the subalpine core respectively, hence confirming their potential for constraining transport-dynamics of soil-derived matter in rivers.

The radiocarbon age of GDGTs in a suite of fluvial sediments is older than expected under the assumption that topsoil-derived organic matter is the main source of the compounds. Potentially, this offset could be caused by rapid degradation of the compounds during transport and therefore alter the proxy signal on the way to sedimentary archives.

[1] van der Voort, T. S., et al., 2017 - Geophysical Research Letters 44, 23

How to cite: Gies, H., Montluçon, D., Lupker, M., van der Voort, T., Hagedorn, F., Haghipour, N., and Eglinton, T.: Radiocarbon analysis of isoprenoid and branched Glycerol Dialkyl Glycerol Tetraethers in soils and fluvial sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8732, https://doi.org/10.5194/egusphere-egu2020-8732, 2020.

Rapid changes in ocean circulation and polar temperature variability have been observed in glacial and deglacial paleoclimate records from marine and ice core archives. However, an obstacle to progress in understanding the ice-ocean-bedrock-climate interactions on centennial-millennial timescales is due to the paucity of sediment records with precise chronologies. The sediment archive along the continental margin of Dronning Maud Land provides an excellent opportunity for high resolution ¹⁴C dating as it contains sufficient amounts of planktonic foraminifers. We dated a 7 m long sediment sequence from core PS111/13 by means of ¹⁴C plateau tuning (Sarnthein et al., 2015) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change from 7000 to 30,000 years that can be linked to ice core chronologies.

Sarnthein et al., Radiocarbon, 2015, 57 (1), 129–151.

How to cite: Tiedemann, R., Müller, J., Lembke-Jene, L., and Mollenhauer, G.: Precise dating of centennial-millennial scale climate variations in sediment archives from the Antarctic continental margin off Dronning Maud Land by 14C plateau tuning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6636, https://doi.org/10.5194/egusphere-egu2020-6636, 2020.

The selection and pre-treatment of a reliable organic fraction from which to acquire radiocarbon dates is fundamental to obtain accurate chronologies. Sampling from tropical lakes is particularly challenging given the adverse preservation conditions and diagenesis in these environments. Our research is the first to examine and quantify the differences between the radiocarbon date results from different carbon fractions and pretreatments from the same depths from a tropical lake sediment core (1.72 m long) located in north Australia to assess which one(s) are more reliable. Six different organic fractions (bulk organics, pollen concentrate, cellulose, stable polycyclic aromatic carbon (SPAC), charcoal >250 um and charcoal >63 um), for a total of 27 radiocarbon dates, were compared in six different depths along the core. Acid-base-acid (ABA), modified ABA (30 % hydrogen peroxide + ABA), 2chlorOx (a novel cellulose pre-treatment method) and hydrogen pyrolysis (hypy) were used to pre-treat the correspondent organic fractions. The oldest date is 31,295 calibrated years before present (cal yr BP) and the youngest is 2,048 cal yr BP, spanning 29,247 years. The smallest offset between the minimum and the maximum age in a given depth was found to be 975 years (between SPAC and charcoal >63 um) and the largest 16,527 years (between pollen concentrate and SPAC). The SPAC fractions pre-treated with hypy consistently yielded older ages compared to all other fraction in most cases, while bulk organics yielded consistently younger ones. The magnitude and consistency of the offsets and the physical and chemical properties of the tested organic fractions suggest that SPAC is the most reliable fraction to date in tropical lake sediments and that hypy successfully removes contamination sourced from exogenous carbon.

How to cite: Rivera-Araya, M., Bird, M., Rowe, C., Ulm, S., and Levchenko, V.: The reliability of radiocarbon dates of different carbon fractions in the Australian tropical savannas: A case study from Sanamere Lagoon, northeast Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11983, https://doi.org/10.5194/egusphere-egu2020-11983, 2020.

The Northeast Atlantic alone has received 1.2 PBq of ¹⁴C as liquid and gaseous releases from European nuclear fuel reprocessing plants (NRPs) between the 1950s and present. The input of reprocessing-¹⁴C has the potential to elevate the regional ¹⁴C content of seawater, sediments and marine biota above the ambient levels expected from the bomb-¹⁴C. Yet, a comprehensive assessment of the time evolution of F¹⁴C in seawater is still missing for the Northwestern European Seas. Moreover, the least-well studied period of time (1990’s onward) corresponds to the largest liquid ¹⁴C releases reported by the Sellafield and La Hague NRPs. In this study, we aim at better constraining the temporal changes of F¹⁴C between the late 1960s and 2019, and to delimit the area of influence of reprocessing discharges with regard to ¹⁴C. To this end, we combine Accelerator Mass Spectrometry techniques and a novel archive of bivalve shells that inhabited the Irish Sea, the North Sea, Norway and the Bay of Biscay throughout the main period of reprocessing-¹⁴C discharge. The shells are made of aragonite, and thus, they can be used as an analogue of the past seawater F¹⁴C. The shell-based F¹⁴C data can be accurately placed in the temporal context because the animals have a known capture date and short lifespan of two years. The reconstructed F¹⁴C values vary between ~1 and ~3 after the 1970s. This range of F¹⁴C values is even larger than the one displayed by the atmospheric bomb peak (1 - 1.9). To investigate if the excess ¹⁴C is related to the reprocessing releases, we use a simple box model that simulates the seawater F¹⁴C by mixing bomb and reprocessing-¹⁴C, as well as the naturally occurring ^12,14C. In shells from the southern North Sea, the F¹⁴C increases 0.1-0.4 above ambient levels after the mid-1990s in response to increased discharge rates of liquid ¹⁴C from the La Hague plant. Similarly, the shells collected in the Irish Sea show two consecutive peaks in the mid-1990s (F¹⁴C ~ 2.0) and 2000s (F¹⁴C ~ 2.2) that can be attributed to peak discharge rates of liquid ¹⁴C reported by Sellafield. The F¹⁴C in shells from the eastern coast of the UK and Norway are within the range of the ambient values, which indicates the expected rapid dilution of the reprocessing signal with open ocean waters. In previous studies, the bomb-¹⁴C marine curve has been used as a benchmark, among others, to estimate the age and growth rate of calcifying animals, to date marine sediments, and to investigate water mass mixing and circulation timescales. Given the biases from the marine bomb-¹⁴C curve unraveled by the shell data, we suggest that liquid releases from the NRPs should not be disregarded when applying ¹⁴C as a chronological or circulation tool to marine samples collected in the Irish Sea and parts of the North Sea over the last 5 decades.

How to cite: Castrillejo, M., Richardson, C. A., Witbaard, R., Dekker, R., Welte, C., Wacker, L., Yeman, C., Casacuberta, N., Synal, H.-A., and Christl, M.: Sea shells record large biases from the marine bomb-14C curve in NW European seawater between the late 1960s and 2019, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1495, https://doi.org/10.5194/egusphere-egu2020-1495, 2020.

A novel technique making use of laser ablation coupled online to accelerator mass spectrometry (LA-AMS) allows analyzing the radiocarbon concentration (F¹⁴C) in carbonate samples at a spatial resolution down to ~100 µm within very short analysis times [1]. This new technique can provide radiocarbon data close to the spatial resolution of stable carbon isotope measurements and, thus, can help to interpret δ¹³C signatures, which otherwise are difficult to understand [2]. Conventional analytical methods applied to stalagmite samples for ¹⁴C analysis, where a micro-sample is drilled or milled and carbon is liberated by the addition of phosphoric acid provide exclusively the isotope composition of the CaCO₃, but not of organic matter also captured in stalagmites. LA-AMS allows accessing the ¹⁴C concentration of both materials opening up new opportunities for gaining insights into vegetation and soil dynamics.

SPA-127 is a stalagmite from Spannagel cave (W Austrian Alps) that grew between 8500 and 2500 a BP at an average rate of 25 μm/a [3]. δ¹³C and ¹⁴C were analyzed with high resolution along the full range of the 15 cm long specimen. During LA-AMS ¹⁴C analysis, positive anomalies in ion currents were observed in the older stalagmite section. These comparably higher CO₂ conversion efficiencies are associated with organic materials compared to CaCO₃ during LA. Lower F¹⁴C were observed along with these anomalies. The signal structure could be reproduced both after removing ~0.5 mm of the carbonate surface layer and on the stalagmite’s archive slab making possible contamination unlikely. So far, we deduce that the observed anomalies are caused by several flushing events in the early Holocene, in which ¹⁴C dead organic components (acids) entered the cave and were incorporated into the stalagmite matrix. Due to the high elevation of the cave and cold conditions during the glacial, the ancient organic acids most likely stem from the Eemian and were stored in the host rock.

[1] C. Welte, et al., (2016). Anal. Chem., 88, 8570– 8576.

[2] F. McDermott, (2004). Quat. Sci. Rev., 23, 901-918.

[3] Fohlmeister J. et al., (2013). Holocene, 23, 749–754.

How to cite: Welte, C., Fohlmeister, J., Wacker, L., Wertnik, M., Spötl, C., Yeman, C., Hattendorf, B., Christl, M., Eglinton, T. I., and Synal, H.-A.: Laser Ablation radiocarbon analysis of a high alpine stalagmite - a hint to an old organic carbon pool?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7298, https://doi.org/10.5194/egusphere-egu2020-7298, 2020.

The CO₂ flux from soil is a large and significant flux in most ecosystems and can account for more than 2/3 of total ecosystem respiration. In many cases, CO₂ flux from soil is estimated by the eddy covariance technique or by classical chamber method with measures of bulk concentration and isotopic composition of carbon dioxide. Whereas most these studies estimated CO₂ flux from the soil surface, we analyzed its concentration and isotope composition directly in soil profiles down to 5m depth.

This experiment was conducted in Sumter National Forest by NSF Calhoun CZO research program. A 10cm diameter auger was used to core up to 5 m depth and capped PVC pipe segments of 750 cm³ volume serve as gas reservoirs, each with two gas impermeable tubes that connected the gas reservoirs. Soil gas reservoirs are installed at 5m, 3m, 1.5m, and 0.5m depths from the soil surface. On a three-week interval, soil gases were extracted with a pump and analyzed in the field for CO₂ and O₂ concentration with samples collected in Tedlar bags for analysis. The samples were collected in summer 2016 under 3 different land uses: hardwood stands that are taken to be never cultivated; old-field pine stands, which had been used for growing cotton in 19^th century and then abandoned; and cultivated sites which were used growing cotton, but for the last 50-60 years for growing corn, wheat, legume, sorghum, and sunflowers.

The radiocarbon analyses in the soil CO₂ profile were conducted for the first time. It was discovered that concentration of 14C increased with depth and Δ¹⁴C changed from 40-60%o in the top 0.5m to about 80-140 ‰ at 5m depth depending on land use.

How to cite: Cherkinsky, A., Prasad, R., Sheng, H., Brecheisen, Z., and Richter, D.: 14C estimation of soil C02 turnover rates in Ultisols with different land use , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11431, https://doi.org/10.5194/egusphere-egu2020-11431, 2020.

Changes in the geometry of ocean Meridional Overturning Circulation (MOC) are crucial in controlling changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes in the MOC of different ocean basins still present a major challenge. The suite of jumps and plateaus in the record of past atmospheric radiocarbon (¹⁴C) concentrations offers a unique opportunity of age control and global correlation. The upper and lower boundaries of atmospheric ¹⁴C plateaus in the ¹⁴C records of both tree rings and Lake Suigetsu (age calibrated on the basis of Hulu U/Th model ages)­ provide a detailed stratigraphic ’rung ladder’ of ~30 age tie points from 29 to 10 ka that can be used for dating of planktic ¹⁴C records and an age correlation, by now employed to ~20 sediment cores obtained from key locations of MOC all over the global ocean. The age difference between paired planktic and benthic ¹⁴C ages provides an estimate of the ventilation age of deep waters since their last contact with the atmosphere. ¹⁴C ventilation ages of Last Glacial Maximum (LGM) deep waters reveal coeval opposed geometries of Atlantic and Pacific MOC. Similar to today, LGM Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. Vice versa, low ¹⁴C ventilation ages of N.E. Pacific deep waters suggest a reversed, anti-estuarine MOC during early Heinrich Stadial 1 with a ~1500 year-long flushing of the deep North Pacific up to the South China Sea, when the North Atlantic was marked by an estuarine circulation geometry, gradually starting near 19 ka. Elevated ¹⁴C ventilation ages of LGM deep waters reflect a major drawdown of atmospheric carbon. Subsequent massive age drops accompanying changes in MOC reflect major events of carbon release to the atmosphere as recorded in Antarctic ice cores. These contemporaneous features of the MOC and the carbon cycle offer a great test case for comparison with model simulation.

How to cite: Sarnthein, M. and Grootes, P. M.: 14C Ventilation ages suggest a brief reversal of ocean Meridional Overturning Circulation during deglacial ‘Heinrich Stadial 1’, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5354, https://doi.org/10.5194/egusphere-egu2020-5354, 2020.

Since Ohkouchi et al. (2002) pioneering work, compound specific radiocarbon (¹⁴C) dating has been largely used to explore ¹⁴C age discrepancies between co-deposited sedimentary components in a wide range of depositional settings. Older ¹⁴C ages of bulk organic carbon (OC) and alkenones relative to co-deposited planktonic foraminifera have been mainly attributed to lateral sediment transport processes by means of organic matter (OM)-mineral associations.

Definitive evidence for this hypothesis requires in-depth investigations at the mineral grain-size level. Here, we examine the radiocarbon signatures of OC and two molecular biomarkers widely used as paleothermometers (i.e., alkenones and glycerol diakyl glycerol tetraether (GDGTs)) associated to discrete sediment grain-size fractions collected from a range of continental margin settings. Our results evidence the pervasive influence of hydrodynamically-driven sorting processes on the OM content and composition of continental margin sediments, manifested in the ¹⁴C age variability of OC, alkenones and GDGTs residing in bulk sediments corresponding grain-size fractions. We find that OC and both, alkenones and GDGTs, preferentially reside within the fine silt fraction, which accounts for a substantial fraction of the bulk sediment mass. Therefore, fine silt exerts a strong influence on the ¹⁴C ages of these three components in bulk sediments. Given the propensity to resuspension and advection of fine silt under strong currents, the extent of its impact on the paleotemperature signal recorded by alkenones and GDGTs is also assessed. 

Ohkouchi, N., Eglinton, T.I., Keigwin, L.D., Hayes, J.M., 2002. Spatial and Temporal Offsets Between Proxy Records in a Sediment Drift. Science 298, 1224-1227.

How to cite: Ausin, B., Bruni, E., Haghipour, N., Welte, C., Gies, H., Bernasconi, S. M., and Eglinton, T. I.: Pervasive influence of fine silt sediments on the radiocarbon age of bulk sedimentary OC, alkenones and GDGTs via hydrodynamic mineral sorting processes in continental margins., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5778, https://doi.org/10.5194/egusphere-egu2020-5778, 2020.

The Younger Dryas stadial (YD) was a return to glacial-like conditions in the North Atlantic region that interrupted deglacial warming around 12900 cal BP (before 1950 AD). Terrestrial and marine records suggest this event was initiated by the interruption of deep-water formation arising from North American freshwater runoff, but the causes of the millennia-long duration remain unclear. To investigate the solar activity, a possible YD driver, we exploit the cosmic production signals of tree-ring radiocarbon (¹⁴C) and ice-core beryllium-10 (¹⁰Be). Here we present the highest temporally resolved dataset of ¹⁴C measurements (n = 1558) derived from European tree rings that have been accurately extended back to 14226 cal BP (±8, 2-σ), allowing precise alignment of ice-core records across this period. We identify a substantial increase in ¹⁴C and ¹⁰Be production starting at 12780 cal BP is comparable in magnitude to the historic Little Ice Age, being a clear sign of grand solar minima. We hypothesize the timing of the grand solar minima provides a significant amplifying factor leading to the harsh sustained glacial-like conditions seen in the YD.

How to cite: Sookdeo, A., Kromer, B., Adolphi, F., Beer, J., Brehm, N., Büntgen, U., Christl, M., Eglinton, T., Friedrich, M., Guidobaldi, G., Helle, G., Muscheler, R., Nievergelt, D., Pauly, M., Reinig, F., Tegel, W., Treydte, K., Turney, C., Synal, H.-A., and Wacker, L.: Tree-ring radiocarbon reveals reduced solar activity during Younger Dryas cooling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11654, https://doi.org/10.5194/egusphere-egu2020-11654, 2020.