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by admin | Apr 19, 2023 | mainpost, vol25

K.E. Grant, R.G. Hilton, V.V. Galy

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Global patterns of radiocarbon depletion in subsoil linked to rock-derived organic carbon

K.E. Grant1,2,

1Department of Geography, Durham University, South Road, Durham, DH1 3LE, UK
2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA

R.G. Hilton3,

3Department of Earth Sciences, University of Oxford, South Parks Rd, Oxford, OX1 3AN, UK

V.V. Galy4

4Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA

Affiliations | Corresponding Author | Cite as | Funding information

K.E. Grant
Email: grant39@llnl.gov

1Department of Geography, Durham University, South Road, Durham, DH1 3LE, UK
2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA
3Department of Earth Sciences, University of Oxford, South Parks Rd, Oxford, OX1 3AN, UK
4Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, MA 02543, USA

Grant, K.E., Hilton, R.G., Galy, V.V. (2023) Global patterns of radiocarbon depletion in subsoil linked to rock-derived organic carbon. Geochem. Persp. Let. 25, 36–40. https://doi.org/10.7185/geochemlet.2312

European Commission European Research Council (ERC) Starting Grant (678779, ROC-CO2) ERC Consolidator Grant (101002563, RIV-ESCAPE).

Geochemical Perspectives Letters v25 | https://doi.org/10.7185/geochemlet.2312
Received 23 June 2022 | Accepted 6 March 2023 | Published 19 April 2023

Copyright © 2023 The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY 4.0

Keywords: radiocarbon, soil organic carbon, rock-derived organic carbon

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Abstract

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information

Organic matter stored in sedimentary rocks is one of the largest stocks of carbon at Earth’s surface. The fate of this rock organic carbon (OCpetro) during weathering in soils influences the geological carbon cycle, and impacts soil radiocarbon content that is used to quantify soil carbon turnover. Here, we assess the potential contribution of OCpetro to soils, using a mixing model generated by a global dataset of soil radiocarbon measurements (14C). Soils developed on sedimentary rocks (rather than on igneous substrate) have a paired OC content and 14C values consistent with OCpetro input, giving rise to apparent increase in soil residence time. We call for renewed assessment of OCpetro input to soils, in terms of its impact on soil radiocarbon inventories, and its potential to release carbon dioxide.

Figures

Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt.

Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020) dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares).

Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally.

Figure 1 Figure 2 Figure 3

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Introduction

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


Sedimentary rocks cover 64 % of Earth’s surface (Hartmann and Moosdorf, 2012

Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370

). Within the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007

Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048

), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017

Jackson, R.B., Lajtha, K., Crow, S.E., Hugelius, G., Kramer, M.G., Piñeiro, G. (2017) The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls. Annual Review of Ecology, Evolution, and Systematics 48, 419–445. https://doi.org/10.1146/annurev-ecolsys-112414-054234

). When soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018

Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463

; Kalks et al., 2021

Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021

). While OCpetro could be widespread in soils developed on sedimentary rocks, observations come from only a handful of weathering profiles, including OCpetro-rich rocks, such as black shales which are not globally representative (Petsch et al., 2001

Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332

). In addition, estimates of global OCpetro surface exposure are based on upscaling from geological maps (Copard et al., 2007

Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048

), but this input has not been assessed in soil carbon studies. While there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015

Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400

; Clarke et al., 2017

Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615

), we lack constraint on the input of OCpetro to soils.

Input of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014

Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5

; Hilton and West, 2020

Hilton, R.G., West, A.J. (2020) Mountains, erosion and the carbon cycle. Nature Reviews Earth & Environment 1, 284–299. https://doi.org/10.1038/s43017-020-0058-6

). Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002

Agnelli, A., Trumbore, S.E., Corti, G., Ugolini, F.C. (2002) The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C. European Journal of Soil Science 53, 147–159. https://doi.org/10.1046/j.1365-2389.2002.00432.x

), used to quantify atmospheric CO2 exchange by (Trumbore, 2000

Trumbore, S. (2000) Age of Soil Organic Matter and Soil Respiration: Radiocarbon Constraints on Belowground C Dynamics. Ecological Applications 10, 399–411. https://doi.org/10.1890/1051-0761(2000)010[0399:AOSOMA]2.0.CO;2

; Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

). Soils slow the rapid degradation of “modern” plant-derived vegetation (OCbio) through mineral and physical interactions, helping store OCbio in soils over decades to millennia (Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

). In the topsoil, the OC pool is dominated by plant and microbial organic matter and is generally 14C-enriched (Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

). In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015

Mathieu, J.A., Hatté, C., Balesdent, J., Parent, É. (2015) Deep soil carbon dynamics are driven more by soil type than by climate: a worldwide meta-analysis of radiocarbon profiles. Global Change Biology 21, 4278–4292. https://doi.org/10.1111/gcb.13012

; He et al., 2016

He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273

; Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

) show that 14C-depleted (“old”) OC is common around the world. These studies attribute “old” 14C ages to an increase in OC-mineral interactions, as the clay content and mineral to organic ratio increase with depth and mineral surfaces can reduce the bioavailability and thus increase the persistence of OCbio (Schmidt et al., 2011

Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E. (2011) Persistence of soil organic matter as an ecosystem property. Nature 478, 49–56. https://doi.org/10.1038/nature10386

). Soil C age distributions can have very long tails, with a small amount of old or 14C-dead C skewing the mean age (Sierra et al., 2018

Sierra, C.A., Hoyt, A.M., He, Y., Trumbore, S.E. (2018) Soil Organic Matter Persistence as a Stochastic Process: Age and Transit Time Distributions of Carbon in Soils. Global Biogeochemical Cycles 32, 1574–1588. https://doi.org/10.1029/2018GB005950

). However, inputs of 14C-dead OCpetro are not considered in He et al. (2016)

He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273

and Shi et al. (2020)

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

, despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio.

Here, we quantify OCpetro in globally distributed soil profiles using the International Soil Radiocarbon Database (ISRaD database, see Supplementary Information) (Lawrence et al., 2020

Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020

). We separate soil profiles developed on igneous rocks (IG: negligible OCpetro) versus sedimentary rocks (SED: potential OCpetro inputs) using global geological maps and compare bulk 14C signatures and % OC distributions. Alongside the global data set, using a mixing model analysis, we compare measurements made on river and soil OC in settings where OCpetro inputs have been demonstrated and where OCpetro is absent. Together, we assess the contribution of OCpetro to deep soils around the world and the implications for soil radiocarbon inventories and the surface carbon cycle.

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Evidence of OCpetro in soils and rivers using mixing models

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019

Hemingway, J.D., Rothman, D.H., Grant, K.E., Rosengard, S.Z., Eglinton, T.I., Derry, L.A., Galy, V.V. (2019) Mineral protection regulates long-term global preservation of natural organic carbon. Nature 570, 228–231. https://doi.org/10.1038/s41586-019-1280-6

); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001

Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332

; Hemingway et al., 2018

Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463

). To explore these scenarios, we examine cases where OCpetro contributes to bulk OC in river sediment loads and soils, with OCpetro inputs identified from geochemical proxies other than bulk 14C (e.g., C/N, δ13C, biomarkers, Raman spectroscopy, Ramped Pyrolysis Oxidation-14C). These can be compared to examples where OCpetro inputs are absent and OCbio decay/ageing acts alone from river sediments and soils on igneous bedrock (Fig. 1a, c). We find a notable contrast in the relationship between 1/[OC] and Δ14C in sedimentary and igneous bedrock settings. Patterns expected for mixing between OCpetro and OCbio appear to dominate in sedimentary settings.


Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt.
Full size image


An Andean Mountain river draining shales (Clark et al., 2017

Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615

) has river sediment OC Δ14C values that ranged from −0.5 ‰ to −839 ‰. Clark et al. (2017)

Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615

concluded that, on average, 0.37 ± 0.03 % of the suspended sediment mass was OCpetro. There is a significant linear trend between Δ14C values and 1/[OC] (p < 0.05, R2 = 0.75, Fig. 1a) that was attributed to binary mixing of OCpetro and biospheric OC supplied by erosion processes. In Himalayan rivers, the river OC also has a linear relationship between these variables (p < 0.05, R2 = 0.37), albeit with a shallower slope due to lower OCpetro contents (Galy et al., 2015

Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400

). In soils, OCpetro has been recognised in only a few studies (Petsch, 2014

Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5

; Hemingway et al., 2018

Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463

; Hilton et al., 2021

Hilton, R.G., Turowski, J.M., Winnick, M., Dellinger, M., Schleppi, P., Williams, K.H., Lawrence, C.R., Maher, K., West, M., Hayton, A. (2021) Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering. Water Resources Research 57, e2021WR029844. https://doi.org/10.1029/2021WR029844

; Kalks et al., 2021

Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021

). In Taiwan, high erosion rates continuously supply fresh bedrock to the surface and keep surface soils young, and these samples have a linear relationship between Δ14C and 1/[OC]. Mixing models show that OCpetro input to sediments and soils result in a linear trend between 1/[OC] and Δ14C (Supplementary Information, Eq. S-8; Fig. 1b). These can explain the main patterns seen in the Andes and Himalayan rivers and Taiwanese soil data.

In contrast to the river sediments draining sedimentary rocks, rivers from a basaltic catchment, the Efri Haukadalsá River in Iceland (Torres et al., 2020

Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632

), have 14C value ranging from −60 ‰ to −395 ‰ and display no relationship between 1/[OC] and Δ14C (Fig. 1c). Torres et al. (2020)

Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632

attributed the 14C-depletion to erosion of millennial-aged OC from soils. In tropical soils, formed along a climate gradient underlain by a 450 ka lava flow from Hawaii, [OC] remains high while OC ages due to Fe-oxide mineral-carbon stabilisation, and these materials have an almost vertical array in 1/[OC] and Δ14C (Fig. 1c; Grant et al., 2022

Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629

). These patterns in the data are more consistent with the paired evolution of 1/[OC] and Δ14C expected for OC loss and ageing of OC, explored here using a simple organic matter degradation model (Supplementary Information sections 4.1 and 4.2).

Overall, the river and soil data here show how 1/[OC] and Δ14C can be used to examine the role of OCpetro input. A domain of OC and Δ14C values can most easily be reached by OCpetro addition to the organic matter pool (Fig. 1b), as seen in the grey “mixing zone” of sedimentary soils. In the igneous soils, this mixing domain is harder to populate. We note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011

Rumpel, C., Kögel-Knabner, I. (2011) Deep soil organic matter—a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143–158. https://doi.org/10.1007/s11104-010-0391-5

; Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

).

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Widespread OCpetro input to global soils

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


To assess a potential, wider input of OCpetro into soils, we use the ISRaD database (Lawrence et al., 2020

Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020

) (see Supplementary Information). All database samples used have reported Δ14C values and measured (not extrapolated or modelled) [OC] (Fig. 2a). We characterise the parent material for each geo-located profile from the global lithological map database (GLiM) (Hartmann and Moosdorf, 2012

Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370

) if no parent material is specified in the database (79.4 % of entries). The soil radiocarbon profiles are split into sedimentary (SED) (80.9 %) and igneous (IG) (19.4 %) parent material, with 557 soil profiles and 2978 radiocarbon measurements (Supplementary Information) (Fig. 2e). There are more SED profiles (397 profiles, 2260 measurements) than IG profiles (160 profiles, 718 measurements), broadly mirroring the fact that ∼60 % of Earth’s terrestrial surface is composed of sedimentary rock (Hartmann and Moosdorf, 2012

Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370

). An important caveat is that we do not capture all Quaternary deposits such as loess or ashfall, which are important parent materials for soil (Baisden and Parfitt, 2007

Baisden, W.T., Parfitt, R.L. (2007) Bomb 14C enrichment indicates decadal C pool in deep soil? Biogeochemistry 85, 59–68. https://doi.org/10.1007/s10533-007-9101-7

). Rather than filter the database further, we note some of the SED samples on loess or ashfall may have very low OCpetro inputs.


Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020)

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares).
Full size image


Despite the complexities inherent in any global soil assessment (Lawrence et al., 2020

Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020

), characterising samples from ISRaD as from either SED or IG sites reveals patterns which can be accounted for by OCpetro inputs. First, the median and lower quartile Δ14C values of SED samples are lower than IG sites (Fig. 2c, Table S-2). Second, the distribution of SED and IG soil Δ14C values are significantly different at the 0.05 level (Kolomogorov-Smirnov Test). Third, the calculated profile-averaged Δ14C values and modelled mean soil age are significantly different between SED and IG profiles (p > 0.05, Supplementary Information section 1.3). Finally, the SED samples that reach lower Δ14C values (Fig. 2c) define the grey zone suggested by a binary mixing model (Fig. 1b). IG profiles do not commonly reach this range of compositions, with the 14C depleted samples either having retained higher % OC, or remain fairly 14C-enriched (Δ14C > −200 ‰) as OC is lost, i.e. have a shallower trajectory between 1/[OC] and Δ14C (Fig. 2d).

Having established potential OCpetro inputs at SED sites, the radiocarbon data can be used to quantify a maximum permissible OCpetro input. To do so, we assume all 14C-depletion is driven by OCpetro addition (i.e. no OCbio ageing) (Supplementary Information section 5.1). We calculate an average maximum proportion of OCpetro = 0.38 or 38 % (Fig. 3b) for all SED soils. This corresponds to an average 0.85 ± 0.1 % OCpetro in soils (Fig. 3c). This maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011

Graz, Y., Di-Giovanni, C., Copard, Y., Elie, M., Faure, P., Laggoun Defarge, F., Lévèque, J., Michels, R., Olivier, J.E. (2011) Occurrence of fossil organic matter in modern environments: Optical, geochemical and isotopic evidence. Applied Geochemistry 26, 1302–1314. https://doi.org/10.1016/j.apgeochem.2011.05.004

; Partin et al., 2013

Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031

).


Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally.
Full size image


Taken together, our analysis places a maximum bound on the OCpetro content to deep and 14C depleted soils forming on sedimentary rocks. While this estimation is an upper bound because OCbio ageing will occur in deep soils (Grant et al., 2022

Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629

), and does not consider loss of OCpetro during weathering, it is a reasonable starting point to understand how incorporation of OCpetro can influence mean soil age.

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OCpetro in soils leads to mean age overestimation

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


Input of OCpetro in soils formed on sedimentary rocks is consistent with patterns in the dataset (Fig. 2c, d). If OCpetro contributes to deep soils, how much could it alter carbon residence times based on 14C measurements? To provide a first constraint, we reconsider the Shi et al. (2020)

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

global weighted Δ14C accounting for a fraction of OCpetro. The mean age in subsoils (30−100 cm depth) was Δ14C = −391 ± 56 ‰ (∼3970 14C years). Accounting for our calculated maximum fraction of 0.38 OCpetro could mean the OCbio age calculated in the Shi et al. (2020)

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

dataset is equivalent to −17.7 ‰, or 150 14C years, a difference of ∼3800 14C years. Using a more modest mass fraction of 0.10 for OCpetro would shift the biospheric radiocarbon age younger by 800 14C years (from 3900 to 3100 14C yr). While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016

He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273

; Shi et al., 2020

Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

). An “apparent” increase in OCbio age by failing to account for OCpetro addition would imply soil carbon exchange fluxes with the atmosphere are underestimated. Accounting for the fate of OCpetro in soils is important for reducing uncertainty on the present and future role of soils in the anthropogenic carbon budget.

The input of OCpetro to soils is central to the long-term carbon cycle. It is important to assess the OCpetro input to soils, its oxidation during soil formation that can release CO2, and the potential influence of OCpetro on soil OC stabilisation mechanisms. We can compare our maximum OCpetro contents (Fig. 3) from the SED soil profiles from ISRaD to global sedimentary rock OC values (Fig. S-3). These sedimentary rock samples have higher OC contents than sedimentary soils, suggesting the widespread loss of OCpetro in global soils (Partin et al., 2013

Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031

). Traditionally, OCpetro is considered inert or unreactive on anthropogenic timescales. However, in Taiwanese soils, Hemingway et al. (2018)

Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463

found that ∼67 % of OCpetro was lost during soil formation, even during rapid erosion which limits time for soil weathering. This extensive OCpetro oxidation was attributed to microbial assimilation of OCpetro (Hemingway et al., 2018

Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463

). To move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008

Galy, V., Beyssac, O., France-Lanord, C., Eglinton, T. (2008) Recycling of Graphite During Himalayan Erosion: A Geological Stabilization of Carbon in the Crust. Science 322, 943–945. https://doi.org/10.1126/science.1161408

; Petsch, 2014

Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5

) which may have a distribution of reactivities and residence times in soils. Radiocarbon measurements suggest that OCpetro contribution to soils is a global feature and so needs to be considered to properly constrain soil carbon storage and turnover. Quantifying the fate of OCpetro in soils is then crucial to understanding the evolution of the long-term carbon and oxygen cycles.

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Acknowledgements

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


This work was performed under the European Research Council (ERC) Starting Grant (678779, ROC-CO2) and partly under an ERC Consolidator Grant (101002563, RIV-ESCAPE) to RGH. Partial writing of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 (LLNL-JRNL-837045).

Editor: Liane G. Benning

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References

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information

Agnelli, A., Trumbore, S.E., Corti, G., Ugolini, F.C. (2002) The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13C. European Journal of Soil Science 53, 147–159. https://doi.org/10.1046/j.1365-2389.2002.00432.x
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Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
View in article


Baisden, W.T., Parfitt, R.L. (2007) Bomb 14C enrichment indicates decadal C pool in deep soil? Biogeochemistry 85, 59–68. https://doi.org/10.1007/s10533-007-9101-7
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An important caveat is that we do not capture all Quaternary deposits such as loess or ashfall, which are important parent materials for soil (Baisden and Parfitt, 2007).
View in article


Clark, K.E., Hilton, R.G., West, A.J., Robles Caceres, A., Gröcke, D.R., Marthews, T.R., Ferguson, R.I., Asner, G.P., New, M., Malhi, Y. (2017) Erosion of organic carbon from the Andes and its effects on ecosystem carbon dioxide balance. Journal of Geophysical Research: Biogeosciences 122, 449–469. https://doi.org/10.1002/2016JG003615
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While there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015; Clarke et al., 2017), we lack constraint on the input of OCpetro to soils.
View in article
An Andean Mountain river draining shales (Clark et al., 2017) has river sediment OC Δ14C values that ranged from −0.5 ‰ to −839 ‰.
View in article
Clark et al. (2017) concluded that, on average, 0.37 ± 0.03 % of the suspended sediment mass was OCpetro
View in article


Copard, Y., Amiotte-Suchet, P., Di-Giovanni, C. (2007) Storage and release of fossil organic carbon related to weathering of sedimentary rocks. Earth and Planetary Science Letters 258, 345–357. https://doi.org/10.1016/j.epsl.2007.03.048
Show in context

Within the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017).
View in article
In addition, estimates of global OCpetro surface exposure are based on upscaling from geological maps (Copard et al., 2007), but this input has not been assessed in soil carbon studies.
View in article


Galy, V., Beyssac, O., France-Lanord, C., Eglinton, T. (2008) Recycling of Graphite During Himalayan Erosion: A Geological Stabilization of Carbon in the Crust. Science 322, 943–945. https://doi.org/10.1126/science.1161408
Show in context

To move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008; Petsch, 2014) which may have a distribution of reactivities and residence times in soils.
View in article


Galy, V., Peucker-Ehrenbrink, B., Eglinton, T. (2015) Global carbon export from the terrestrial biosphere controlled by erosion. Nature 521, 204–207. https://doi.org/10.1038/nature14400
Show in context

While there is widespread evidence for OCpetro in sediments from rivers around the world (e.g., Galy et al., 2015; Clarke et al., 2017), we lack constraint on the input of OCpetro to soils.
View in article
In Himalayan rivers, the river OC also has a linear relationship between these variables (p < 0.05, R2 = 0.37), albeit with a shallower slope due to lower OCpetro contents (Galy et al., 2015).
View in article


Grant, K.E., Galy, V.V., Haghipour, N., Eglinton, T.I., Derry, L.A. (2022) Persistence of old soil carbon under changing climate: The role of mineral-organic matter interactions. Chemical Geology 587, 120629. https://doi.org/10.1016/j.chemgeo.2021.120629
Show in context

In tropical soils, formed along a climate gradient underlain by a 450 ka lava flow from Hawaii, [OC] remains high while OC ages due to Fe-oxide mineral-carbon stabilisation, and these materials have an almost vertical array in 1/[OC] and Δ14C (Fig. 1c; Grant et al., 2022).
View in article
While this estimation is an upper bound because OCbio ageing will occur in deep soils (Grant et al., 2022), and does not consider loss of OCpetro during weathering, it is a reasonable starting point to understand how incorporation of OCpetro can influence mean soil age.
View in article


Graz, Y., Di-Giovanni, C., Copard, Y., Elie, M., Faure, P., Laggoun Defarge, F., Lévèque, J., Michels, R., Olivier, J.E. (2011) Occurrence of fossil organic matter in modern environments: Optical, geochemical and isotopic evidence. Applied Geochemistry 26, 1302–1314. https://doi.org/10.1016/j.apgeochem.2011.05.004
Show in context

This maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011; Partin et al., 2013).
View in article


Hartmann, J., Moosdorf, N. (2012) The new global lithological map database GLiM: A representation of rock properties at the Earth surface. Geochemistry, Geophysics, Geosystems 13, Q12004. https://doi.org/10.1029/2012GC004370
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Sedimentary rocks cover 64 % of Earth’s surface (Hartmann and Moosdorf, 2012).
View in article
We characterise the parent material for each geo-located profile from the global lithological map database (GLiM) (Hartmann and Moosdorf, 2012) if no parent material is specified in the database (79.4 % of entries).
View in article
There are more SED profiles (397 profiles, 2260 measurements) than IG profiles (160 profiles, 718 measurements), broadly mirroring the fact that ∼60 % of Earth’s terrestrial surface is composed of sedimentary rock (Hartmann and Moosdorf, 2012).
View in article


He, Y., Trumbore, S.E., Torn, M.S., Harden, J.W., Vaughn, L.J.S., Allison, S.D., Randerson, J.T. (2016) Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century. Science 353, 1419–1424. https://doi.org/10.1126/science.aad4273
Show in context

In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
View in article
However, inputs of 14C-dead OCpetro are not considered in He et al. (2016) and Shi et al. (2020), despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio
View in article
While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016; Shi et al., 2020).
View in article


Hemingway, J.D., Hilton, R.G., Hovius, N., Eglinton, T.I., Haghipour, N., Wacker, L., Chen, M.-C., Galy, V.V. (2018) Microbial oxidation of lithospheric organic carbon in rapidly eroding tropical mountain soils. Science 360, 209–212. https://doi.org/10.1126/science.aao6463
Show in context

When soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018; Kalks et al., 2021).
View in article
The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article
However, in Taiwanese soils, Hemingway et al. (2018) found that ∼67 % of OCpetro was lost during soil formation, even during rapid erosion which limits time for soil weathering.
View in article
This extensive OCpetro oxidation was attributed to microbial assimilation of OCpetro (Hemingway et al., 2018).
View in article


Hemingway, J.D., Rothman, D.H., Grant, K.E., Rosengard, S.Z., Eglinton, T.I., Derry, L.A., Galy, V.V. (2019) Mineral protection regulates long-term global preservation of natural organic carbon. Nature 570, 228–231. https://doi.org/10.1038/s41586-019-1280-6
Show in context

The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article


Hilton, R.G., West, A.J. (2020) Mountains, erosion and the carbon cycle. Nature Reviews Earth & Environment 1, 284–299. https://doi.org/10.1038/s43017-020-0058-6
Show in context

Input of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014; Hilton and West, 2020).
View in article


Hilton, R.G., Turowski, J.M., Winnick, M., Dellinger, M., Schleppi, P., Williams, K.H., Lawrence, C.R., Maher, K., West, M., Hayton, A. (2021) Concentration-Discharge Relationships of Dissolved Rhenium in Alpine Catchments Reveal Its Use as a Tracer of Oxidative Weathering. Water Resources Research 57, e2021WR029844. https://doi.org/10.1029/2021WR029844
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In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article


Jackson, R.B., Lajtha, K., Crow, S.E., Hugelius, G., Kramer, M.G., Piñeiro, G. (2017) The Ecology of Soil Carbon: Pools, Vulnerabilities, and Biotic and Abiotic Controls. Annual Review of Ecology, Evolution, and Systematics 48, 419–445. https://doi.org/10.1146/annurev-ecolsys-112414-054234
Show in context

Within the upper 1 m, there are an estimated 1100 petagrams of carbon (PgC) present as radiocarbon (14C) “dead” rock organic carbon (OCpetro) (Copard et al., 2007), equivalent to the carbon stock in soils derived from the biosphere (Jackson et al., 2017).
View in article


Kalks, F., Noren, G., Mueller, C.W., Helfrich, M., Rethemeyer, J., Don, A. (2021) Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon. Soil 7, 347–362. https://doi.org/10.5194/soil-7-347-2021
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When soil forms on sedimentary rocks, OCpetro is a “bottom up” input of carbon that can contribute to the soil OC pool, especially in deep soils with low OC concentrations (Hemingway et al., 2018; Kalks et al., 2021).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article


Lawrence, C.R., Beem-Miller, J., Hoyt, A.M., Monroe, G., Sierra, C.A., et al. (2020) An open-source database for the synthesis of soil radiocarbon data: International Soil Radiocarbon Database (ISRaD) version 1.0. Earth System Science Data 12, 61–76. https://doi.org/10.5194/essd-12-61-2020
Show in context

Here, we quantify OCpetro in globally distributed soil profiles using the International Soil Radiocarbon Database (ISRaD database, see Supplementary Information) (Lawrence et al., 2020).
View in article
To assess a potential, wider input of OCpetro into soils, we use the ISRaD database (Lawrence et al., 2020) (see Supplementary Information).
View in article
Despite the complexities inherent in any global soil assessment (Lawrence et al., 2020), characterising samples from ISRaD as from either SED or IG sites reveals patterns which can be accounted for by OCpetro inputs.
View in article


Mathieu, J.A., Hatté, C., Balesdent, J., Parent, É. (2015) Deep soil carbon dynamics are driven more by soil type than by climate: a worldwide meta-analysis of radiocarbon profiles. Global Change Biology 21, 4278–4292. https://doi.org/10.1111/gcb.13012
Show in context

In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
View in article


Partin, C.A., Bekker, A., Planavsky, N.J., Scott, C.T., Gill, B.C., Li, C., Podkovyrov, V., Maslov, A., Konhauser, K.O., Lalonde, S.V., Love, G.D., Poulton, S.W., Lyons, T.W. (2013) Large-scale fluctuations in Precambrian atmospheric and oceanic oxygen levels from the record of U in shales. Earth and Planetary Science Letters 369–370, 284–293. https://doi.org/10.1016/j.epsl.2013.03.031
Show in context

This maximum value is reasonable given known OC contents of sedimentary rocks (Graz et al., 2011; Partin et al., 2013).
View in article
These sedimentary rock samples have higher OC contents than sedimentary soils, suggesting the widespread loss of OCpetro in global soils (Partin et al., 2013).
View in article


Petsch, S.T. (2014) 12.8 - Weathering of Organic Carbon. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry. Second Edition, Elsevier, Amsterdam, 217–238. https://doi.org/10.1016/B978-0-08-095975-7.01013-5
Show in context

Input of OCpetro to soil is important for two reasons. First, the exposure of rocks to weathering can drive OCpetro oxidation, releasing CO2 and consuming O2, with global emissions of CO2 from OCpetro oxidation similar to those from volcanism (Petsch, 2014; Hilton and West, 2020).
View in article
In soils, OCpetro has been recognised in only a few studies (Petsch, 2014; Hemingway et al., 2018; Hilton et al., 2021; Kalks et al., 2021).
View in article
To move forward, we must recognise that, like OCbio in soil, OCpetro is a continuum of compounds which differ in chemistry, linked to the history of past sedimentation, diagenesis, and metamorphism (Galy et al., 2008; Petsch, 2014) which may have a distribution of reactivities and residence times in soils.
View in article


Petsch, S.T., Eglinton, T.I., Edwards, K.J. (2001) 14C-Dead Living Biomass: Evidence for Microbial Assimilation of Ancient Organic Carbon During Shale Weathering. Science 292, 1127–1131. https://doi.org/10.1126/science.1058332
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While OCpetro could be widespread in soils developed on sedimentary rocks, observations come from only a handful of weathering profiles, including OCpetro-rich rocks, such as black shales which are not globally representative (Petsch et al., 2001).
View in article
The 14C-depletion of soil OC can result from both: (1) ageing of OCbio from plant and microbial derived material, stabilised by physical/chemical protection or intrinsic refractory properties (Hemingway et al., 2019); and (2) mixing of OCpetro from rocks (14C-free relative to instrumental background) with OCbio (Petsch et al., 2001; Hemingway et al., 2018).
View in article


Rumpel, C., Kögel-Knabner, I. (2011) Deep soil organic matter—a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143–158. https://doi.org/10.1007/s11104-010-0391-5
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We note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011; Shi et al., 2020).
View in article


Schmidt, M.W.I., Torn, M.S., Abiven, S., Dittmar, T., Guggenberger, G., Janssens, I.A., Kleber, M., Kögel-Knabner, I., Lehmann, J., Manning, D.A.C., Nannipieri, P., Rasse, D.P., Weiner, S., Trumbore, S.E. (2011) Persistence of soil organic matter as an ecosystem property. Nature 478, 49–56. https://doi.org/10.1038/nature10386
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These studies attribute “old” 14C ages to an increase in OC-mineral interactions, as the clay content and mineral to organic ratio increase with depth and mineral surfaces can reduce the bioavailability and thus increase the persistence of OCbio (Schmidt et al., 2011).
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Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z
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Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
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Soils slow the rapid degradation of “modern” plant-derived vegetation (OCbio) through mineral and physical interactions, helping store OCbio in soils over decades to millennia (Shi et al., 2020).
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In the topsoil, the OC pool is dominated by plant and microbial organic matter and is generally 14C-enriched (Shi et al., 2020).
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In deeper soils (∼>30 cm), recent studies (Mathieu et al., 2015; He et al., 2016; Shi et al., 2020) show that 14C-depleted (“old”) OC is common around the world.
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However, inputs of 14C-dead OCpetro are not considered in He et al. (2016) and Shi et al. (2020), despite the recognition that OCpetro inputs result in an “apparent” ageing of soil OCbio
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We note the importance of OCpetro is likely to be most relevant in soils with low OCbio inputs or relatively old 14C mean ages, which could be particularly important in deep soils (Rumpel and Kögel-Knabner, 2011; Shi et al., 2020).
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(b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020) dataset for the SED and IG profiles.
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If OCpetro contributes to deep soils, how much could it alter carbon residence times based on 14C measurements? To provide a first constraint, we reconsider the Shi et al. (2020) global weighted Δ14C accounting for a fraction of OCpetro
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Accounting for our calculated maximum fraction of 0.38 OCpetro could mean the OCbio age calculated in the Shi et al. (2020) dataset is equivalent to −17.7 ‰, or 150 14C years, a difference of ∼3800 14C years.
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While a 38 % OCpetro contribution is likely an overestimation, the analysis shows that even modest OCpetro inputs could shift assessments of OCbio residence time in soils, yet OCpetro inputs are not considered in global assessments (He et al., 2016; Shi et al., 2020).
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Sierra, C.A., Hoyt, A.M., He, Y., Trumbore, S.E. (2018) Soil Organic Matter Persistence as a Stochastic Process: Age and Transit Time Distributions of Carbon in Soils. Global Biogeochemical Cycles 32, 1574–1588. https://doi.org/10.1029/2018GB005950
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Soil C age distributions can have very long tails, with a small amount of old or 14C-dead C skewing the mean age (Sierra et al., 2018).
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Torres, M.A., Kemeny, P.C., Lamb, M.P., Cole, T.L., Fischer, W.W. (2020) Long-Term Storage and Age-Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland. Geochemistry, Geophysics, Geosystems 21, e2019GC008632. https://doi.org/10.1029/2019GC008632
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In contrast to the river sediments draining sedimentary rocks, rivers from a basaltic catchment, the Efri Haukadalsá River in Iceland (Torres et al., 2020), have 14C value ranging from −60 ‰ to −395 ‰ and display no relationship between 1/[OC] and Δ14C (Fig. 1c.
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Torres et al. (2020) attributed the 14C-depletion to erosion of millennial-aged OC from soils.
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Trumbore, S. (2000) Age of Soil Organic Matter and Soil Respiration: Radiocarbon Constraints on Belowground C Dynamics. Ecological Applications 10, 399–411. https://doi.org/10.1890/1051-0761(2000)010[0399:AOSOMA]2.0.CO;2
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Second, OCpetro input can increase the mean age of soil OC based on radiocarbon (Agnelli et al., 2002), used to quantify atmospheric CO2 exchange by (Trumbore, 2000; Shi et al., 2020).
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Supplementary Information

Abstract | Introduction | Evidence of OCpetro in soils and rivers using mixing models | Widespread OCpetro input to global soils | OCpetro in soils leads to mean age overestimation | Acknowledgements | References | Supplementary Information


The Supplementary Information includes:
  • 1. ISRaD Database Screening and Geological Maps
  • 2. Summary of Radiocarbon (Δ14C) Data from ISRaD Soil Profiles
  • 3. Measured Soil and River Samples with Known OCpetro Inputs
  • 4. Model Expectations
  • 5. Maximum Permissible OCpetro Input to Soils
  • 6. Caveats in the Global Approach
  • Supplementary Tables S-1 to S-4
  • Supplementary Figures S-1 to S-3
  • Supplementary Information References


  • Download the Supplementary Information (PDF)
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    Figures



    Figure 1 Rock organic carbon imprints on organic carbon concentration (OC) and radiocarbon activity (Δ14C). (a) Samples from Taiwanese soils, Andes and Himalayan rivers, and black shale weathering profiles where OCpetro inputs are known, with linear fits through data shown. (b) Binary mixing model (lines) between OCbio (Δ14C = +200 ‰) and OCpetro (varying from 1 % to 0.1 % of total OC, Δ14C = −1000 ‰). The grey shaded area is where >50 % of total OC is OCpetro. (c) Samples where OCpetro is absent: soil from Hawaiian Pololu lava flow and Icelandic river samples draining basalt.
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    Figure 2 (a) The distribution of radiocarbon measurements for the SED and IG profiles, with corresponding box plots depicting the median and quartiles of the distributions. (b) The depth integrated Δ14C (displayed in ‰ and 14C years) vs. modelled mean ages extracted from the Shi et al. (2020)

    Shi, Z., Allison, S.D., He, Y., Levine, P.A., Hoyt, A.M., Beem-Miller, J., Zhu, Q., Wieder, W.R., Trumbore, S., Randerson, J.T. (2020) The age distribution of global soil carbon inferred from radiocarbon measurements. Nature Geoscience 13, 555–559. https://doi.org/10.1038/s41561-020-0596-z

    dataset for the SED and IG profiles. (c) The ISRaD soil horizons from SED soils plotted in 1/[OC] and radiocarbon (Δ14C). (d) The ISRaD soil horizons from IG soils plotted in 1/[OC] and radiocarbon activity (Δ14C). The grey shaded region defines a zone where mixing of OCpetro can produce compositions which are difficult to obtain via OC decomposition alone (Fig. 1b). (e) Locations of samples where direct measurement of OCpetro inputs have been identified (diamond, triangle), and soil profiles from the ISRaD database on igneous (IG, circles) and sedimentary rocks (SED, squares).
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    Figure 3 (a) The maximum fraction of [OCpetro] is calculated via a binary mixing model between OCbio and OCpetro in the globally distributed soil profiles (ISRaD database). (b) A histogram of the maximum fraction of OCpetro values for each soil sample. The mean fpetro is 0.38 ± 0.01, the median value is 0.36. (c) A histogram of calculated maximum OCpetro (wt. %) in each SED soil horizon. The average is 0.85 ± 0.1 wt. % OC and the median is 0.19 % OCpetro in SED soils globally.
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