A novel post-1950 CE atmospheric 14C record for the tropics using absolutely dated tree rings in the equatorial Amazon

In this study, we present a comprehensive atmospheric radiocarbon (14C) record spanning from 1940 to 2016, derived from 77 single tree rings of Cedrela odorata located in the Eastern Amazon Basin (EAB). This record, comprising 175 high-precision 14C measurements obtained through accelerator mass spectrometry (AMS), offers a detailed chronology of post-1950 CE (Common Era) 14C fluctuations in the Tropical Low-Pressure Belt (TLPB). To ensure accuracy and reliability, we included 14C-AMS results from intra-annual successive cuts of the tree rings associated to the calendar years 1962 and 1963 and conducted interlaboratory comparisons. In addition, 14C concentrations in 1962 and 1963 single-year cuts also allowed to verify tissue growth seasonality. The strategic location of the tree, just above the Amazon River and estuary areas, prevented the influence of local fossil-CO2 emissions from mining and trade activities in the Central Amazon Basin on the 14C record. Our findings reveal a notable increase in 14C from land-respired CO2 starting in the 1970s, a decade earlier than previously predicted, followed by a slight decrease after 2000, signaling a transition towards the fossil fuel era. This shift is likely attributed to changes in reservoir sources or global atmospheric dynamics. The EAB 14C record, when compared with a shorter record from Muna Island, Indonesia, highlights regional differences and contributes to a more nuanced understanding of global 14C variations at low latitudes. This study not only fills critical spatial gaps in existing 14C compilations but also aids in refining the demarcation of 14C variations over South America. The extended tree-ring 14C record from the EAB is pivotal for reevaluating global patterns, particularly in the context of the current global carbon budget, and underscores the importance of tropical regions in understanding carbon-climate feedbacks.

Methods
Sample selection: For this atmospheric tree-ring based 14C dataset, a Cedrela odorata specimen was sourced from an established 231-year tree-ring chronology (Granato-Souza et al., 2019, 2020; https://www.ncei.noaa.gov/access/paleo-search/study/24110). A wood segment (P45) with clear tree ring boundaries was then used, and 77 tree rings spanning the period 1940 to 2016 were individually sampled from it for high-precision 14C accelerator mass spectrometry (AMS) and stable isotope (δ13C) analysis.

Cellulose extraction:  Tree-ring cellulose extraction to alpha-cellulose was performed at three laboratories, i.e., the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI), the Lamont-Doherty Earth Observatory (LDEO), and the Australian Nuclear Science and Technology Organization (ANSTO). Once KCCAMS/UCI completed the microscope inspection of all tree rings singled out, samples and/or subsamples were accordingly redistributed to other labs for alpha-cellulose extractions. The largest load of samples went to LDEO for batch-mode chemical extractions using a semiautomated bath device. At the KCCAMS/UCI facility the tree ring slices of 1962 and 1963, and replicates of the counterpart whole rings, were extracted in 13mm culture tubes. The ANSTO 14C-AMS facility was responsible for 10 duplicates of raw wood tree rings between 1999 and 2000 for cross-laboratory validation. Expanded descriptions of sample preparation processes and 14C analysis alongside images of the main devices/apparatus used by each laboratory have been added to the main text and in the supplementary material of Santos et al. 2024 https://doi.org/10.1016/j.scitotenv.2024.170686.

Graphite production for 14C analysis and measurements: Graphite production from cellulose extracts for 14C-AMS analysis were employed by two 14C-AMS facilities, KCCAMS/UCI and ANSTO. KCCAMS/UCI produced filamentous graphite from in-house alpha-cellulose fibers and LDEO cellulose extracts using the sealed-tube Zn reduction method (Santos & Xu 2017), while ANSTO used the H2/Fe method for graphite production (Hua et al. 2001). Both 14C-AMS facilities employ distinct spectrometers during their 14C analysis, i.e., the KCCAMS/UCI runs an in-house modified NEC 0.5MV 1.5SDH-1 system (Beverly et al 2010) and ANSTO operates a 1-MV NEC VEGA instrument (Fink et al., 2004). High accuracy and precision were obtained at both 14C-AMS facilities based on consistent results of reference materials and replicated cellulose data.

Stable carbon isotope (δ13C) determination: Remaining aliquots of chemically extracted alpha cellulose containing 0.6 to 0.7 mg each were weighed out into tin boats to take stable isotope ratio mass spectrometer (IRMS) measurements, using a Delta Plus CFIRMS interfaced with a Fisons NA 1500NC elemental analyzer (EA) at the KCCAMS/UCI facility. This instrument has a typical precision of 0.1‰ based on measurements of several standards, such as USGS-24, IAEA-600, and Atropine (C17H23NO3). For further details, refer to Santos et al. 2024 https://doi.org/10.1016/j.scitotenv.2024.170686.

Citation information in this document:

Beverly, R.K., Beaumont, W., Tauz, D., Ormsby, K.M., von Reden, K.F., Santos, G.M., Southon, J.R., 2010. The Keck Carbon Cycle AMS Laboratory, University of California, Irvine: status report. Radiocarbon 52 (2), 301–309. DOI: https://doi.org/10.1017/S0033822200045343

Fink, D., Hotchkis, M., Hua, Q., Jacobsen, G., Smith, A.M., Zoppi, U., Child, D., Mifsud, C., van der Gaast, H., Williams, A., Williams, M., 2004. The ANTARES AMS Facility at ANSTO. Nucl. Instrum. Methods Phys. Res. B 223–224, 109–115. https://doi.org/10.1016/j.nimb.2004.04.025

Granato-Souza, D., Stahle, D.W., Barbosa, A.C., Feng, S., Torbenson, M.C., de Assis Pereira, G., Schongart, J., Barbosa, J.P., Griffin, D., 2019. Tree rings and rainfall in the equatorial Amazon. Climate Dynam. 52 (3), 1857–1869. https://doi.org/10.1007/s00382-018-4227-y.

Granato-Souza, D., Stahle, D.W., Torbenson, M.C., Howard, I.M., Barbosa, A.C., Feng, S., Fernandes, K., Schongart, J., 2020. Multidecadal changes in wet season precipitation totals over the Eastern Amazon. Geophys. Res. Lett. 47 (8) (p.e2020GL087478).  https://doi.org/10.1029/2020GL087478

Hua, Q., Jacobsen, G.E., Zoppi, U., Lawson, E.M., Williams, A.A., Smith, A.M., McGann, M.J., 2001. Progress in radiocarbon target preparation at the ANTARES AMS Centre. Radiocarbon 43, 275–282. DOI: https://doi.org/10.1017/S003382220003811X

Santos, G.M., Xu, X., 2017. Bag of tricks: a set of techniques and other resources to help 14C laboratory setup, sample processing, and beyond. Radiocarbon 59 (3), 785–801. DOI: https://doi.org/10.1017/RDC.2016.43