Enhanced petrogenic organic carbon oxidation during the Paleocene-Eocene thermal maximum

E.H. Hollingsworth¹,

¹School of Ocean and Earth Science, University of Southampton, Southampton, UK

R.B. Sparkes²,

²Department of Natural Sciences, Manchester Metropolitan University, Manchester, UK

J.M. Self-Trail³,

³U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, VA, USA

G.L. Foster¹,

¹School of Ocean and Earth Science, University of Southampton, Southampton, UK

G.N. Inglis¹

¹School of Ocean and Earth Science, University of Southampton, Southampton, UK

Affiliations | Corresponding Author | Cite as | Funding information

Geochemical Perspectives Letters v33 | https://doi.org/10.7185/geochemlet.2444
Received 21 June 2024 | Accepted 4 November 2024 | Published 25 November 2024

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

Keywords: PETM, carbon cycle, Raman spectroscopy



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Abstract

The Paleocene-Eocene thermal maximum (PETM; ∼56 Ma) is a hyperthermal event associated with the rapid input of carbon into the ocean-atmosphere system. The oxidation of petrogenic organic carbon (OC_petro) may have released additional carbon dioxide (CO₂), thereby prolonging the PETM. However, proxy-based estimates of OC_petro oxidation are unavailable due to the lack of suitable techniques. Raman spectroscopy is used to evaluate OC_petro oxidation in modern settings. For the first time, we explore whether Raman spectroscopy can evaluate OC_petro oxidation during the PETM. In the mid-Atlantic Coastal Plain, there is a shift from disordered to graphitised carbon. This is consistent with enhanced oxidation of disordered OC_petro and intensified physical erosion. In the Arctic Ocean, the distribution of graphitised carbon vs. disordered carbon does not change, suggesting limited variability in weathering intensity. Overall, this study provides the first evidence of increased OC_petro oxidation during the PETM, although it was likely not globally uniform. Our work also highlights the utility of Raman spectroscopy as a novel tool to reconstruct OC_petro oxidation in the past.

Figures

Figure 1 A simplified schematic illustrating the Raman spectroscopy approach to evaluating OC_petro oxidation. Marine sediment composition with (a) a dominance of graphitised carbon (dark brown) and (b) graphitised and disordered carbon (light brown), indicating a high and low OC_petro oxidation efficiency, respectively. Given that OC_petro oxidation is a source of CO₂, this correlates with a (a) high and (b) low CO₂ flux.	Figure 2 ‘Sparkes’ plots combining data from the pre-PETM interval (including the pre-onset excursion; POE) vs. Core PETM (including the onset, body, and Recovery interval; see Hollingsworth et al., 2024 and references therein), at (a-b) SDB and (c-d) ACEX. The spectra are categorised into either graphitised carbon (dark brown) or disordered carbon (light brown). Examples of Raman spectra (black) with fitted peaks (coloured) for (e) graphitised carbon, from a SDB sample at 200.28 m, and (f) disordered carbon, from an ACEX sample at 385.11 mcd. Spectra have had a linear background removed during the automated process, and the fitted peaks include: G (1580 cm⁻¹), D1 (1350 cm⁻¹), D2 (1620 cm⁻¹), D3 (1500 cm⁻¹), and D4 (1200 cm⁻¹) (Sparkes et al., 2013).	Figure 3 The SDB record of (a) bulk sediment δ¹³C of carbonates (δ¹³C_carbonates; Lyons et al., 2019), (b) percentage graphitised carbon (this study), and (c) C₃₁ homohopane 22S/(22S + 22R) ratio (Lyons et al., 2019). The blue symbols within panel (b) are the duplicate measurements. The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery Phase I, Recovery Phase II, and post-PETM, based on Hollingsworth et al. (2024) and references therein. The POE in the pre-PETM interval is isolated using the definition from Babila et al. (2022).	Figure 4 The ACEX record of (a) bulk sediment δ¹³C of total organic carbon (δ¹³C_TOC; Elling et al., 2019), (b) percentage graphitised carbon (this study), and (c) C₃₁ homohopane 22S/(22S + 22R) ratio (Hollingsworth et al., 2024). The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery, and post-PETM, based on Hollingsworth et al. (2024) and references therein. Note the core gap from ∼388–384.5 mcd (Sluijs et al., 2006).
Figure 1	Figure 2	Figure 3	Figure 4

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Introduction

At the Paleocene-Eocene boundary, an abrupt carbon cycle perturbation gave rise to a global warming event (∼4–6 °C; Tierney et al., 2022

Tierney, J.E., Zhu, J., Li, M., Ridgwell, A., Hakim, G.J., Poulsen, C.J., Whiteford, R.D.M., Rae, J.W.B., Kump, L.R. (2022) Spatial patterns of climate change across the Paleocene–Eocene Thermal Maximum. Proceedings of the National Academy of Sciences of the United States of America 119, e2205326119. https://doi.org/10.1073/pnas.2205326119

), known as the Paleocene-Eocene thermal maximum (PETM; ∼56 Ma). The initial flux of carbon occurred within ∼3–21 kyr (the ‘onset’ of the PETM; see Kirtland Turner, 2018

Kirtland Turner, S. (2018) Constraints on the onset duration of the Paleocene-Eocene Thermal Maximum. Philosophical Transactions of the Royal Society A 376, 1–16. https://doi.org/10.1098/rsta.2017.0082

and references therein), yet the PETM persisted for a further 170 ± 30 kyr (the ‘body’ of the PETM; Zeebe and Lourens, 2019

Zeebe, R.E., Lourens, L.J. (2019) Solar System chaos and the Paleocene–Eocene boundary age constrained by geology and astronomy. Science 365, 926–929. https://doi.org/10.1126/science.aax0612

). The long duration of the PETM can be reproduced in carbon cycle models but requires a continuous or additional source of carbon into the ocean-atmosphere system (e.g., Bowen, 2013

Bowen, G.J. (2013) Up in smoke: A role for organic carbon feedbacks in Paleogene hyperthermals. Global and Planetary Change 109, 18–29. https://doi.org/10.1016/j.gloplacha.2013.07.001

). Proposed mechanisms for the additional source include the slow dissociation of oceanic methane hydrates (e.g., Zeebe, 2013

Zeebe, R.E. (2013) What caused the long duration of the Paleocene-Eocene Thermal Maximum? Paleoceanography 28, 440–452. https://doi.org/10.1002/palo.20039

), pulsed emissions from hydrothermal vent complexes (see Jin et al., 2024

Jin, S., Kemp, D.B., Shen, J., Yin, R., Jolley, D.W., Vieira, M., Huang, C. (2024) Spatiotemporal distribution of global mercury enrichments through the Paleocene-Eocene Thermal Maximum and links to volcanism. Earth-Science Reviews 248, 104647. https://doi.org/10.1016/j.earscirev.2023.104647

and references therein), and the oxidation of soil organic carbon (e.g., Bowen, 2013

Bowen, G.J. (2013) Up in smoke: A role for organic carbon feedbacks in Paleogene hyperthermals. Global and Planetary Change 109, 18–29. https://doi.org/10.1016/j.gloplacha.2013.07.001

) or petrogenic organic carbon (OC_petro) (Lyons et al., 2019

Lyons, S.L., Baczynski, A.A., Babila, T.L., Bralower, T.J., Hajek, E.A., Kump, L.R., Polites, E.G., Self-Trail, J.M., Trampush, S.M., Vornlocher, J.R., Zachos, J.C., Freeman, K.H. (2019) Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation. Nature Geoscience 12, 54–60. https://doi.org/10.1038/s41561-018-0277-3

).

Biomarker thermal maturity ratios can fingerprint OC_petro that has been subject to relatively low burial temperature (<165 °C). Changes in biomarker thermal maturity ratios from a global compilation of shallow-marine sediments indicate greater delivery of OC_petro into the ocean during the PETM (Lyons et al., 2019

; Hollingsworth et al., 2024

Hollingsworth, E.H., Elling, F.J., Badger, M.P.S., Pancost, R.D., Dickson, A.J., Rees-Owen, R.L., Papadomanolaki, N.M., Pearson, A., Sluijs, A., Freeman, K.H., Baczynski, A.A., Foster, G.L., Whiteside, J.H., Inglis, G.N. (2024) Spatial and Temporal Patterns in Petrogenic Organic Carbon Mobilization During the Paleocene-Eocene Thermal Maximum. Paleoceanography and Paleoclimatology 39. https://doi.org/10.1029/2023PA004773

). Based on modern observations (e.g., Hilton and West, 2020

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

; Soulet et al., 2021

Soulet, G., Hilton, R.G., Garnett, M.H., Roylands, T., Klotz, S., Croissant, T., Dellinger, M., Le Bouteiller, C. (2021) Temperature control on CO₂ emissions from the weathering of sedimentary rocks. Nature Geoscience 14, 665–671. https://doi.org/10.1038/s41561-021-00805-1

), it is likely that the increased erosion rates and higher temperatures during the PETM enhanced OC_petro oxidation. However, constraining the fraction of OC_petro that was oxidised to CO₂ (i.e. the oxidation efficiency) remains challenging in the geologic record. In order to calculate the mass of OC_petro-derived CO₂ released during the body of the PETM, Lyons et al. (2019

) used the present day lower and upper bounds in oxidation efficiency (15–85 %; Bouchez et al., 2010

Bouchez, J., Beyssac, O., Galy, V., Gaillardet, J., France-lanord, C., Maurice, L., Moreira-turcq, P. (2010) Oxidation of petrogenic organic carbon in the Amazon floodplain as a source of atmospheric CO₂. Geology 38, 255–258. https://doi.org/10.1130/G30608.1

; Hilton et al., 2014

Hilton, R.G., Gaillardet, J., Calmels, D., Birck, J. (2014) Geological respiration of a mountain belt revealed by the trace element rhenium. Earth and Planetary Science Letters 403, 27–36. https://doi.org/10.1016/j.epsl.2014.06.021

). This resulted in a wide range of estimates that span two orders of magnitude (10²–10⁴ PgC; Lyons et al., 2019

). To reduce the uncertainty, new techniques are required to determine OC_petro oxidation efficiency in the past. This will help reveal whether OC_petro oxidation is an important positive feedback mechanism during hyperthermals, and thus its potential role in future climate change.

Here, we explore the utility of Raman spectroscopy as a novel tool to reconstruct OC_petro oxidation in the geologic record. Raman spectroscopy is a non-destructive technique that assesses nm-scale differences in the crystallinity of carbonaceous materials. This enables the distinction between highly crystalline (i.e. graphite) to amorphous (i.e. disordered) carbon, and can therefore detect OC_petro that formed at burial temperatures up to 650 °C (Beyssac et al., 2002

Beyssac, O., Goffé, B., Chopin, C., Rouzaud, J.N. (2002) Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology 20, 859–871. https://doi.org/10.1046/j.1525-1314.2002.00408.x

). As the porous structure of disordered carbon makes it more susceptible to oxidation, a shift towards a dominance of graphitised carbon downstream has been suggested to indicate high OC_petro oxidation efficiency (Fig. 1). This has been used to evaluate OC_petro oxidation in modern settings (e.g., 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

; Bouchez et al., 2010

). However, this approach has rarely been applied in a geological context. Here, we employ Raman spectroscopy to identify OC_petro in PETM-aged sediments, and evaluate changes in OC_petro oxidation during the PETM.

**Figure 1** A simplified schematic illustrating the Raman spectroscopy approach to evaluating OC_petro oxidation. Marine sediment composition with **(a)** a dominance of graphitised carbon (dark brown) and **(b)** graphitised and disordered carbon (light brown), indicating a high and low OC_petro oxidation efficiency, respectively. Given that OC_petro oxidation is a source of CO₂, this correlates with a **(a)** high and **(b)** low CO₂ flux.

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Material and Methods

We investigated two shallow-marine sites that exhibit higher OC_petro mass accummulation rates (MAR) during the PETM (Hollingsworth et al., 2024

). The South Dover Bridge (SDB) core was drilled in the Salisbury Embayment (mid-Atlantic Coastal Plain; Fig. S-1) and shows a drastic increase in OC_petro delivery during the PETM (Lyons et al., 2019

). International Ocean Drilling Program Expedition 302 Site M0004A (ACEX; Fig. S-1), is located at the Lomonosov Ridge (central Arctic Ocean) and indicates minimal change in organic carbon source(s) during the PETM (Hollingsworth et al., 2024

).

We follow the Raman spectroscopy methodology outlined in Sparkes et al. (2013)

Sparkes, R.B., Hovius, N., Galy, A., Kumar, R.V., Liu, J.T. (2013) Automated analysis of carbon in powdered geological and environmental samples by Raman spectroscopy. Applied Spectroscopy 67, 779–788. https://doi.org/10.1366/12-06826

, which was specifically developed to facilitate the analyses of sedimentary rocks. Overall, 36 samples from SDB and 12 samples from ACEX were processed and analysed with an InVia Raman spectrometer (Renishaw). Firstly, wet sediments were either freeze dried or placed in a 50 °C oven overnight (Table S-2). The samples were then ground to a fine powder with a Planetary Mill Pulverisette 5 (Fritsch), for 2 min at 300 rpm. Between samples, the agate mill and balls were cleaned with isopropanol. The homogenised samples were then compressed between two glass slides to create a 1 cm² area which can be rastered under a 50x magnification microscope. The slides were systematically scanned by traversing at regular spaced intervals. All carbonaceous particles were first determined by brief exposure to a 514 nm Ar-ion laser (2 s; measurement window 1050–1915 cm⁻¹). Those confirmed were further inspected and photographed prior to a final spectrum being measured using longer exposure (60 s; measurement window 800–2200 cm⁻¹). Laser power was kept low enough (estimated to be <6 mW) to cause no noticeable thermal alterations to the targets. To increase the chance that the data is representative of the population, 10 spectra were collected for each sample. The peaks in each spectrum were fitted using an updated script (https://github.com/robertsparkes/raman-fitting/releases/tag/v1.1.5) of the automated process described in Sparkes et al. (2013)

, and manually checked for inconsistencies.

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Results

In total, 360 spectra from SDB and 120 spectra from ACEX were collected. The automated process resulted in only 7 spectra from SDB and 1 spectrum from ACEX being excluded due to a noise-to-signal ratio greater than 1:3. In previous studies, spectra were characterised as either: (i) highly graphitised carbon, (ii) mildly graphitised carbon, (iii) intermediate carbon, or (iv) disordered carbon (Sparkes et al., 2013

). Spectra from highly graphitised carbon have a single sharp peak at 1580 cm⁻¹ (G peak; Fig. 2e). Disordered carbon has a wider ‘G band’ at approximately 1600 cm⁻¹, produced by a convolution of the G (1580 cm⁻¹) and D2 (1620 cm⁻¹) peaks, alongside other peaks signifying disorder (D1; 1350 cm⁻¹, D3; 1500 cm⁻¹, and D4; 1200 cm⁻¹; Fig. 2f). Our data suggests a bimodal distribution (Fig. 2e,f), and as such, spectra were categorised as either graphitised or disordered carbon (Fig. 2a-d, Tables S-1 and S-2). This separation was based on peak burial temperatures, which were calibrated using the R2 and RA2 peak area ratio, and not the sum of peak width (G + D1 + D2) (see Sparkes et al., 2013

**Figure 2** ‘Sparkes’ plots combining data from the pre-PETM interval (including the pre-onset excursion; POE) *vs.* Core PETM (including the onset, body, and Recovery interval; see Hollingsworth *et al.,* 2024
Hollingsworth, E.H., Elling, F.J., Badger, M.P.S., Pancost, R.D., Dickson, A.J., Rees-Owen, R.L., Papadomanolaki, N.M., Pearson, A., Sluijs, A., Freeman, K.H., Baczynski, A.A., Foster, G.L., Whiteside, J.H., Inglis, G.N. (2024) Spatial and Temporal Patterns in Petrogenic Organic Carbon Mobilization During the Paleocene-Eocene Thermal Maximum. *Paleoceanography and Paleoclimatology* 39. https://doi.org/10.1029/2023PA004773
and references therein), at **(a-b)** SDB and **(c-d)** ACEX. The spectra are categorised into either graphitised carbon (dark brown) or disordered carbon (light brown). Examples of Raman spectra (black) with fitted peaks (coloured) for **(e)** graphitised carbon, from a SDB sample at 200.28 m, and **(f)** disordered carbon, from an ACEX sample at 385.11 mcd. Spectra have had a linear background removed during the automated process, and the fitted peaks include: G (1580 cm⁻¹), D1 (1350 cm⁻¹), D2 (1620 cm⁻¹), D3 (1500 cm⁻¹), and D4 (1200 cm⁻¹) (Sparkes *et al.,* 2013
Sparkes, R.B., Hovius, N., Galy, A., Kumar, R.V., Liu, J.T. (2013) Automated analysis of carbon in powdered geological and environmental samples by Raman spectroscopy. *Applied Spectroscopy* 67, 779–788. https://doi.org/10.1366/12-06826
).

At SDB, there is a mixture of both graphitised and disordered carbon in the pre-PETM interval (including the pre-onset excursion; POE) and 'Core' PETM (including the onset, body, and Recovery interval) (Fig. 2a,b). Compared to the pre-PETM, the PETM exhibits a statistically highly significant (P < 0.001) increase in the mean percentage of graphitised carbon (33 %; Fig. S-2a). However, this shift does not occur until ∼1.5 m above the onset of the PETM (Fig. 3b). At ACEX, there is a dominance of disordered carbon in the pre-PETM interval and Core PETM (Fig. 2c,d), and no statistically significant (P ≥ 0.1) change throughout the record (Fig. 4b, Fig. S-2c). Both sites show a higher percentage of graphitised carbon in the Recovery and post-PETM intervals than in the pre-PETM interval (Fig. S-2a,c).

**Figure 3** The SDB record of **(a)** bulk sediment δ¹³C of carbonates (δ¹³C_carbonates; Lyons *et al.,* 2019
Lyons, S.L., Baczynski, A.A., Babila, T.L., Bralower, T.J., Hajek, E.A., Kump, L.R., Polites, E.G., Self-Trail, J.M., Trampush, S.M., Vornlocher, J.R., Zachos, J.C., Freeman, K.H. (2019) Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation. *Nature Geoscience* 12, 54–60. https://doi.org/10.1038/s41561-018-0277-3
), **(b)** percentage graphitised carbon (this study), and **(c)** C₃₁ homohopane 22S/(22S + 22R) ratio (Lyons *et al.,* 2019
Lyons, S.L., Baczynski, A.A., Babila, T.L., Bralower, T.J., Hajek, E.A., Kump, L.R., Polites, E.G., Self-Trail, J.M., Trampush, S.M., Vornlocher, J.R., Zachos, J.C., Freeman, K.H. (2019) Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation. *Nature Geoscience* 12, 54–60. https://doi.org/10.1038/s41561-018-0277-3
). The blue symbols within panel **(b)** are the duplicate measurements. The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery Phase I, Recovery Phase II, and post-PETM, based on Hollingsworth *et al.* (2024)
Hollingsworth, E.H., Elling, F.J., Badger, M.P.S., Pancost, R.D., Dickson, A.J., Rees-Owen, R.L., Papadomanolaki, N.M., Pearson, A., Sluijs, A., Freeman, K.H., Baczynski, A.A., Foster, G.L., Whiteside, J.H., Inglis, G.N. (2024) Spatial and Temporal Patterns in Petrogenic Organic Carbon Mobilization During the Paleocene-Eocene Thermal Maximum. *Paleoceanography and Paleoclimatology* 39. https://doi.org/10.1029/2023PA004773
and references therein. The POE in the pre-PETM interval is isolated using the definition from Babila *et al*. (2022)
Babila, T.L., Penman, D.E., Standish, C.D., Doubrawa, M., Bralower, T.J., Robinson, M.M., Self-Trail, J.M., Speijer, R.P., Stassen, P., Foster, G.L., Zachos, J.C. (2022) Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum. *Science Advances* 8, eabg1025. https://doi.org/10.1126/sciadv.abg1025
.

**Figure 4** The ACEX record of **(a)** bulk sediment δ¹³C of total organic carbon (δ¹³C_TOC; Elling *et al.,* 2019
Elling, F.J., Gottschalk, J., Doeana, K.D., Kusch, S., Hurley, S.J., Pearson, A. (2019) Archaeal lipid biomarker constraints on the Paleocene-Eocene carbon isotope excursion. *Nature Communications* 10, 1–10. https://doi.org/10.1038/s41467-019-12553-3
), **(b)** percentage graphitised carbon (this study), and **(c)** C₃₁ homohopane 22S/(22S + 22R) ratio (Hollingsworth *et al.,* 2024
Hollingsworth, E.H., Elling, F.J., Badger, M.P.S., Pancost, R.D., Dickson, A.J., Rees-Owen, R.L., Papadomanolaki, N.M., Pearson, A., Sluijs, A., Freeman, K.H., Baczynski, A.A., Foster, G.L., Whiteside, J.H., Inglis, G.N. (2024) Spatial and Temporal Patterns in Petrogenic Organic Carbon Mobilization During the Paleocene-Eocene Thermal Maximum. *Paleoceanography and Paleoclimatology* 39. https://doi.org/10.1029/2023PA004773
). The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery, and post-PETM, based on Hollingsworth *et al.* (2024)
Hollingsworth, E.H., Elling, F.J., Badger, M.P.S., Pancost, R.D., Dickson, A.J., Rees-Owen, R.L., Papadomanolaki, N.M., Pearson, A., Sluijs, A., Freeman, K.H., Baczynski, A.A., Foster, G.L., Whiteside, J.H., Inglis, G.N. (2024) Spatial and Temporal Patterns in Petrogenic Organic Carbon Mobilization During the Paleocene-Eocene Thermal Maximum. *Paleoceanography and Paleoclimatology* 39. https://doi.org/10.1029/2023PA004773
and references therein. Note the core gap from ∼388–384.5 mcd (Sluijs *et al*., 2006
Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Dickens, G.R., Huber, M., Reichart, G., Stein, R., Matthiessen, J., Lourens, L.J., Pedentchouk, N., Backman, J., Moran, K. (2006) Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum. *Nature* 441, 610–613. https://doi.org/10.1038/nature04668
).

To assess whether our semi-quantitative method (i.e. 10 spectra per sample) can accurately represent the population, four samples from SDB were analysed in duplicate (Fig. 3b, Table S-1). With the exception of 202.37 m (s.d. = 35 %), the measurements yielded similar values (average s.d. = 7 %). However, interpretations of small scale variability should be made with caution and we suggest that future studies increase the number of spectra per sample.

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Discussion

Fingerprinting OC_petro delivery during the PETM. Raman spectroscopy has most commonly been used to fingerprint OC_petro in modern river catchments and continental shelves (e.g., Galy et al., 2008

; Bouchez et al., 2010

; Sparkes et al., 2018

Sparkes, R.B., Maher, M., Blewett, J., Dog˘rul Selver, A., Gustafsson, Ö., Semiletov, I.P., van Dongen, B.E. (2018) Carbonaceous material export from Siberian permafrost tracked across the Arctic Shelf using Raman spectroscopy. Cryosphere 12, 3293–3309. https://doi.org/10.5194/tc-12-3293-2018

, 2020

Sparkes, R.B., Hovius, N., Galy, A., Kumar, R. V., Liu, J.T. (2020) Survival of graphitized petrogenic organic carbon through multiple erosional cycles. Earth and Planetary Science Letters 531, 115992. https://doi.org/10.1016/j.epsl.2019.115992

). Thus, we first compare our results to published biomarker thermal maturity ratios from the same PETM-aged shallow-marine cores (Lyons et al., 2019

; Hollingsworth et al., 2024

). The C₃₁ homohopane 22S/(22S + 22R) ratio is plotted alongside the percentage of graphitised carbon at SDB (Figs. 3c, S-2b) and ACEX (Figs. 4c, S-2d). Higher C₃₁ S/(S + R) values suggest greater delivery of OC_petro, with values closer to 0.6 indicating input of OC_petro formed during early stages of the oil window.

Overall, there are similarities between the percentage of graphitised carbon and the C₃₁ S/(S + R) ratio at both sites, whereby SDB exhibits large fluctuations (Fig. 3) and ACEX shows relatively low and stable values (Fig. 4). At SDB, the percentage of graphitised carbon and C₃₁ S/(S + R) ratio increases between the pre-PETM interval and both the POE and PETM intervals (Fig. S-2a,b). However, a lag within the PETM interval in the percentage of graphitised carbon (Fig. 3b), C₃₁ S/(S + R) ratio (Fig. 3c), and bulk carbon isotope of organic carbon (δ¹³C_org), suggests a delayed response of ∼10–20 kyr (Lyons et al., 2019

). This confirms that Raman spectroscopy can be applied to fingerprint OC_petro delivery in the past, and could be particularly powerful in permitting the inclusion of study sites with post-depositional diagenesis (Sparkes et al., 2020

). Our new data supports previous findings indicating enhanced OC_petro delivery at SDB (Lyons et al., 2019

) and no drastic changes in organic carbon source(s) at ACEX (Hollingsworth et al., 2024

) during the PETM. Assuming that the graphitised carbon and thermally mature biomarkers at SDB came from the same source, the increase in graphitised OC_petro MAR during the PETM is calculated to be 2 x 10^-2 gC cm^-2 kyr⁻¹ (based on Hollingsworth et al., 2024

).

Characterising and identifying the potential sources of OC_petro. Kopp et al. (2009)

Kopp, R.E., Schumann, D., Raub, T.D., Powars, D.S., Godfrey, L.V., Swanson-Hysell, N.L., Maloof, A.C., Vali, H. (2009) An Appalachian Amazon? Magnetofossil evidence for the development of a tropical river-like system in the mid-Atlantic United States during the Paleocene-Eocene thermal maximum. Paleoceanography 24, 1–17. https://doi.org/10.1029/2009PA001783

initially hypothesised that Cretaceous-aged upland deposits, such as the Potomac Group, could have been a source of sediment to the Salisbury Embayment during the PETM. Indeed, similarities were found with the biomarker thermal maturity ratios from the Raritan Formation of the upper Potomac Group and the PETM-aged SDB core (Lyons et al., 2019

). In addition, the δ¹³C_org values and source rock properties (e.g., T_max) are comparable (Lyons et al., 2019

). As there is a correlation between the percentage of graphitised carbon (Figs. 3b, S-2a) and the C₃₁ S/(S + R) ratios (Figs. 3c, S-2b), this suggests that the graphitised OC_petro may have also been sourced from the Raritan Formation. However, the presence of graphitised OC_petro implies burial temperatures (∼350–650 °C; Beyssac et al., 2002

) that should severly diminish or completely destroy biomarkers. Therefore, the graphitised OC_petro was likely reworked into the Raritan Formation and subsequently re-exhumed alongside the thermally mature biomarkers during the PETM. This is consistent with present day observations of graphite particles surviving transport over thousands of kilometres (e.g., Galy et al., 2008

) and persisting over multiple erosion cycles (e.g., Sparkes et al., 2020

). Decoupling between the percentage of graphitised carbon and the C₃₁ S/(S + R) ratios during the POE could thus be reflecting two distinct sources of OC_petro (Figs. 3b,c, S-2a,b).

At ACEX, the relative abundance of disordered carbon (Figs. 4b, S-2c) and low C₃₁ S/(S + R) values (Figs. 4c, S-2d) throughout the record indicates that the OC_petro is derived from a more thermally immature source rock (i.e. protolith). This is consistent with the excellent preservation of pollen and spores (Sluijs et al., 2008

Sluijs, A., Röhl, U., Schouten, S., Brumsack, H.-I., Sangiorgi, F., Sinninghe Damsté, J.S., Brinkhuis, H. (2008) Arctic late Paleocene–early Eocene paleoenvironments with special emphasis on the Paleocene-Eocene thermal maximum (Lomonosov Ridge, Integrated Ocean Drilling Program Expedition 302). Paleoceanography 23, 1–17. https://doi.org/10.1029/2007PA001495

), and the presence of biomarkers diagnostic of peats and/or lignite deposits (e.g., C₃₁ αβ hopanes; Hollingsworth et al., 2024

), in these sediments. The dominance of disordered carbon also implies limited availability of graphite-rich source rocks.

Evaluating OC_petro oxidation during the PETM. The mid-Atlantic Coastal Plain during the PETM has been referred to as the ‘Appalachian Amazon’ due to similarities with sediments from the modern Amazon shelf, including features indicative of hyperpycnal flow (Self-Trail et al., 2017

Self-Trail, J.M., Robinson, M.M., Bralower, T.J., Sessa, J.A., Hajek, E.A., Kump, L.R., Trampush, S.M., Willard, D.A., Edwards, L.E., Powars, D.S., Wandless, G.A. (2017) Shallow marine response to global climate change during the Paleocene-Eocene Thermal Maximum, Salisbury Embayment, USA. Paleoceanography 32, 710–728. https://doi.org/10.1002/2017PA003096

), and the presence of kaolinite (Gibson et al., 2000

Gibson, T.G., Bybell, L.M., Mason, D.B. (2000) Stratigraphic and climatic implications of clay mineral changes around the Paleocene/Eocene boundary of the northeastern US margin. Sedimentary Geology 134, 65–92. https://doi.org/10.1016/S0037-0738(00)00014-2

) and magnetofossils (see Kopp et al., 2009

and references therein). Present day observations of two Amazon tributaries show the preferential oxidation of the less recalcitrant disordered OC_petro and a consequential relative increase in graphite downstream (Bouchez et al., 2010

). The Amazon is mostly supplied by low grade metamorphic rocks from the Andes, which are subject to a long residence time within the extensive meandering rivers typical of large catchments. This results in the estimated loss of up to ∼90 % of OC_petro during transport (Bouchez et al., 2010

; see Dellinger et al., 2023

Dellinger, M., Hilton, R.G., Baronas, J.J., Torres, M.A., Burt, E.I., Clark, K.E., Galy, V., Ccahuana Quispe, A.J., West, A.J. (2023) High rates of rock organic carbon oxidation sustained as Andean sediment transits the Amazon foreland- floodplain. Proceedings of the National Academy of Sciences 120. https://doi.org/10.1073/pnas.2306343120

and references therein). The progressive shift towards more graphitised carbon along the land to sea transect is also seen in modern rivers that have a contribution of high grade metamorphic rocks, such as in the Himalayas (Beyssac et al., 2004

Beyssac, O., Bollinger, L., Avouac, J.P., Goffé, B. (2004) Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material. Earth and Planetary Science Letters 225, 233–241. https://doi.org/10.1016/j.epsl.2004.05.023

). However, the Himalayas have a more efficient sediment routing system that promotes burial of OC_petro in the Bengal Fan (Galy et al., 2007

Galy, V., France-Lanord, C., Beyssac, O., Faure, P., Kudrass, H., Palhol, F. (2007) Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system. Nature 450, 407–410. https://doi.org/10.1038/nature06273

).

As the Amazon shelf is the closest analogue for the mid-Atlantic Coastal Plain, we argue that the shift from a dominance of disordered to graphitised carbon at SDB represents enhanced oxidation of disordered OC_petro during the PETM. We note that the oxidation and/or deposition of OC_petro can also occur in floodplains (e.g., Scheingross et al., 2021

Scheingross, J.S., Repasch, M.N., Hovius, N., Sachse, D., Lupker, M., Fuchs, M., Halevy, I., Gröcke, D.R., Golombek, N.Y., Haghipour, N., Eglinton, T.I., Orfeo, O., Schleicher, A.M. (2021) The fate of fluvially-deposited organic carbon during transient floodplain storage. Earth and Planetary Science Letters 561, 116822. https://doi.org/10.1016/j.epsl.2021.116822

). However, this would lead to the loss of all types of OC_petro from the fluvial load, thus maintaining an equal distribution of graphitised carbon vs. disordered carbon between the pre-PETM and PETM intervals. An average ∼5 °C of warming during the PETM (Tierney et al., 2022

) may have also caused OC_petro oxidation to increase by one fold, based on observations made by Soulet et al. (2021)

in modern river catchments.

At SDB, there is a pronounced ∼20 fold increase in linear sedimentation rates, and higher OC_petro MAR, during the PETM (Lyons et al., 2019

). Given the widespread evidence for intense precipitation events (see Carmichael et al., 2017

Carmichael, M.J., Inglis, G.N., Badger, M.P.S., Naafs, B.D.A., Behrooz, L., Remmelzwaal, S., Monteiro, F.M., Rohrssen, M., Farnsworth, A., Buss, H.L., Dickson, A.J., Valdes, P.J., Lunt, D.J., Pancost, R.D. (2017) Hydrological and associated biogeochemical consequences of rapid global warming during the Paleocene-Eocene Thermal Maximum. Global and Planetary Change 157, 114–138. https://doi.org/10.1016/j.gloplacha.2017.07.014

and references therein) and exacerbated erosion rates (e.g., John et al., 2008

John, C.M., Bohaty, S.M., Zachos, J.C., Sluijs, A., Gibbs, S., Brinkhuis, H., Bralower, T.J. (2008) North American continental margin records of the Paleocene-Eocene thermal maximum: Implications for global carbon and hydrological cycling. Paleoceanography 23, 1–20. https://doi.org/10.1029/2007PA001465

), greater exhumation of graphitised OC_petro can also explain the shift from a dominance of disordered to graphitised carbon. For example, a more active palaeo-Potomac river could have sourced new graphite-rich rocks from regions in the Appalachian Mountains. The lower magnitude change in the relative abundance of graphitised carbon during the POE would therefore imply lower oxidation efficiency and/or abated physical erosion (Figs. 3b, S-2a). At ACEX, intense precipitation may have countered warming-induced OC_petro oxidation by decreasing residence time in the rivers and/or amplifying floodplain storage. Higher sedimentation rates also promotes the burial and preservation of organic matter (e.g., Galy et al., 2007

). However, the limited presence of graphite in this system could suppress signals that indicate changes in the oxidation of disordered OC_petro.

Overall, Raman spectroscopy reveals that OC_petro oxidation acted as a positive feedback mechanism in the mid-Atlantic Coastal Plain during the PETM. However, this change is not observed with smaller scale carbon cycle perturbations such as the POE (see Babila et al., 2022

Babila, T.L., Penman, D.E., Standish, C.D., Doubrawa, M., Bralower, T.J., Robinson, M.M., Self-Trail, J.M., Speijer, R.P., Stassen, P., Foster, G.L., Zachos, J.C. (2022) Surface ocean warming and acidification driven by rapid carbon release precedes Paleocene-Eocene Thermal Maximum. Science Advances 8, eabg1025. https://doi.org/10.1126/sciadv.abg1025

and references therein), whilst data from the Arctic Ocean suggests that enhanced OC_petro oxidation may not be globally uniform. Quantifying oxidation efficiency and subsequent CO₂ release via Raman spectroscopy requires ground truthing our approach in modern settings with different weathering intensities. It is also important to constrain the composition of the source rock(s), and understand the influence of other abiotic (e.g., rainfall type, temperature, O₂ availability, and mineral association) and biotic (e.g., microbial activity) processes on oxidation efficiency (e.g., see Hilton and West, 2020

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

and references therein; Soulet et al., 2021

). Crucially, this study exemplifies the utility of Raman spectroscopy to fingerprint OC_petro delivery and – when combined with biomarker thermal maturity ratios – expands the type of OC_petro detected and thus sites that can be investigated. It also highlights the potential for Raman spectroscopy to reconstruct OC_petro oxidation in the geological past.

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Acknowledgements

EHH acknowledges funding from NERC (Grant NE/S007210). GNI is supported by a GCRF Royal Society Dorothy Hodgkin Fellowship (DHFR1191178) with additional support via the Royal Society (RFERE231019, RFERE210068). This research used samples provided by the U.S. Geological Survey (USGS) and the International Ocean Drilling Program (IODP). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. We are grateful to Hayley Andrews from the Manchester Metropolitan University (MMU) for support with the Raman spectrometer. We thank Bart van Dongen for proposing this potential collaboration at the 2023 British Organic Geochemistry Meeting (BOGS). We also thank Bob Hilton and an anonymous reviewer for providing constructive feedback that greatly improved the manuscript.

Editor: Claudine Stirling

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Data Availability Statement

All the new data in this study are available in the Supporting Information.

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References

Show in context

The POE in the pre-PETM interval is isolated using the definition from Babila et al. (2022).
View in article
However, this change is not observed with smaller scale carbon cycle perturbations such as the POE (see Babila et al., 2022 and references therein), whilst data from the Arctic Ocean suggests that enhanced OC_petro oxidation may not be globally uniform.
View in article

Show in context

This enables the distinction between highly crystalline (i.e. graphite) to amorphous (i.e. disordered) carbon, and can therefore detect OC_petro that formed at burial temperatures up to 650 °C (Beyssac et al., 2002).
View in article
However, the presence of graphitised OC_petro implies burial temperatures (∼350–650 °C; Beyssac et al., 2002) that should severly diminish or completely destroy biomarkers.
View in article

Show in context

The progressive shift towards more graphitised carbon along the land to sea transect is also seen in modern rivers that have a contribution of high grade metamorphic rocks, such as in the Himalayas (Beyssac et al., 2004).
View in article

Show in context

In order to calculate the mass of OC_petro-derived CO₂ released during the body of the PETM, Lyons et al. (2019) used the present day lower and upper bounds in oxidation efficiency (15–85 %; Bouchez et al., 2010; Hilton et al., 2014).
View in article
This has been used to evaluate OC_petro oxidation in modern settings (e.g., Galy et al., 2008; Bouchez et al., 2010).
View in article
Raman spectroscopy has most commonly been used to fingerprint OC_petro in modern river catchments and continental shelves (e.g., Galy et al., 2008; Bouchez et al., 2010; Sparkes et al., 2018, 2020).
View in article
Present day observations of two Amazon tributaries show the preferential oxidation of the less recalcitrant disordered OC_petro and a consequential relative increase in graphite downstream (Bouchez et al., 2010).
View in article
This results in the estimated loss of up to ∼90 % of OC_petro during transport (Bouchez et al., 2010; see Dellinger et al., 2023 and references therein).
View in article

Bowen, G.J. (2013) Up in smoke: A role for organic carbon feedbacks in Paleogene hyperthermals. Global and Planetary Change 109, 18–29. https://doi.org/10.1016/j.gloplacha.2013.07.001

Show in context

The long duration of the PETM can be reproduced in carbon cycle models but requires a continuous or additional source of carbon into the ocean-atmosphere system (e.g., Bowen, 2013).
View in article
Proposed mechanisms for the additional source include the slow dissociation of oceanic methane hydrates (e.g., Zeebe, 2013), pulsed emissions from hydrothermal vent complexes (see Jin et al., 2024 and references therein), and the oxidation of soil organic carbon (e.g., Bowen, 2013) or petrogenic organic carbon (OC_petro) (Lyons et al., 2019).
View in article

Show in context

Given the widespread evidence for intense precipitation events (see Carmichael et al., 2017 and references therein) and exacerbated erosion rates (e.g., John et al., 2008), greater exhumation of graphitised OC_petro can also explain the shift from a dominance of disordered to graphitised carbon.
View in article

Show in context

This results in the estimated loss of up to ∼90 % of OC_petro during transport (Bouchez et al., 2010; see Dellinger et al., 2023 and references therein).
View in article

Elling, F.J., Gottschalk, J., Doeana, K.D., Kusch, S., Hurley, S.J., Pearson, A. (2019) Archaeal lipid biomarker constraints on the Paleocene-Eocene carbon isotope excursion. Nature Communications 10, 1–10. https://doi.org/10.1038/s41467-019-12553-3

Show in context

The ACEX record of (a) bulk sediment δ¹³C of total organic carbon (δ¹³C_TOC; Elling et al., 2019), (b) percentage graphitised carbon (this study), and (c) C₃₁ homohopane 22S/(22S + 22R) ratio (Hollingsworth et al., 2024).
View in article

Show in context

However, the Himalayas have a more efficient sediment routing system that promotes burial of OC_petro in the Bengal Fan (Galy et al., 2007).
View in article
Higher sedimentation rates also promotes the burial and preservation of organic matter (e.g., Galy et al., 2007).
View in article

Show in context

This has been used to evaluate OC_petro oxidation in modern settings (e.g., Galy et al., 2008; Bouchez et al., 2010).
View in article
Raman spectroscopy has most commonly been used to fingerprint OC_petro in modern river catchments and continental shelves (e.g., Galy et al., 2008; Bouchez et al., 2010; Sparkes et al., 2018, 2020).
View in article
This is consistent with present day observations of graphite particles surviving transport over thousands of kilometres (e.g., Galy et al., 2008) and persisting over multiple erosion cycles (e.g., Sparkes et al., 2020).
View in article

Show in context

The mid-Atlantic Coastal Plain during the PETM has been referred to as the ‘Appalachian Amazon’ due to similarities with sediments from the modern Amazon shelf, including features indicative of hyperpycnal flow (Self-Trail et al., 2017), and the presence of kaolinite (Gibson et al., 2000) and magnetofossils (see Kopp et al., 2009 and references therein).
View in article

Show in context

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

Show in context

Based on modern observations (e.g., Hilton and West, 2020; Soulet et al., 2021), it is likely that the increased erosion rates and higher temperatures during the PETM enhanced OC_petro oxidation.
View in article
It is also important to constrain the composition of the source rock(s), and understand the influence of other abiotic (e.g., rainfall type, temperature, O₂ availability, and mineral association) and biotic (e.g., microbial activity) processes on oxidation efficiency (e.g., see Hilton and West, 2020 and references therein; Soulet et al., 2021).
View in article

Show in context

Changes in biomarker thermal maturity ratios from a global compilation of shallow-marine sediments indicate greater delivery of OC_petro into the ocean during the PETM (Lyons et al., 2019; Hollingsworth et al., 2024).
View in article
We investigated two shallow-marine sites that exhibit higher OC_petro mass accummulation rates (MAR) during the PETM (Hollingsworth et al., 2024).
View in article
International Ocean Drilling Program Expedition 302 Site M0004A (ACEX; Fig. S-1), is located at the Lomonosov Ridge (central Arctic Ocean) and indicates minimal change in organic carbon source(s) during the PETM (Hollingsworth et al., 2024).
View in article
‘Sparkes’ plots combining data from the pre-PETM interval (including the pre-onset excursion; POE) vs. Core PETM (including the onset, body, and Recovery interval; see Hollingsworth et al., 2024 and references therein), at (a-b) SDB and (c-d) ACEX.
View in article
The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery Phase I, Recovery Phase II, and post-PETM, based on Hollingsworth et al. (2024) and references therein.
View in article
The time intervals are as follows: pre-PETM, PETM (onset/body), Recovery, and post-PETM, based on Hollingsworth et al. (2024) and references therein.
View in article
The ACEX record of (a) bulk sediment δ¹³C of total organic carbon (δ¹³C_TOC; Elling et al., 2019), (b) percentage graphitised carbon (this study), and (c) C₃₁ homohopane 22S/(22S + 22R) ratio (Hollingsworth et al., 2024).
View in article
Thus, we first compare our results to published biomarker thermal maturity ratios from the same PETM-aged shallow-marine cores (Lyons et al., 2019; Hollingsworth et al., 2024).
View in article
Our new data supports previous findings indicating enhanced OC_petro delivery at SDB (Lyons et al., 2019) and no drastic changes in organic carbon source(s) at ACEX (Hollingsworth et al., 2024) during the PETM.
View in article
Assuming that the graphitised carbon and thermally mature biomarkers at SDB came from the same source, the increase in graphitised OC_petro MAR during the PETM is calculated to be 2 x 10^-2 gC cm^-2 kyr⁻¹ (based on Hollingsworth et al., 2024).
View in article
This is consistent with the excellent preservation of pollen and spores (Sluijs et al., 2008), and the presence of biomarkers diagnostic of peats and/or lignite deposits (e.g., C₃₁ αβ hopanes; Hollingsworth et al., 2024), in these sediments.
View in article

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The initial flux of carbon occurred within ∼3–21 kyr (the ‘onset’ of the PETM; see Kirtland Turner, 2018 and references therein), yet the PETM persisted for a further 170 ± 30 kyr (the ‘body’ of the PETM; Zeebe and Lourens, 2019).
View in article

Show in context

Proposed mechanisms for the additional source include the slow dissociation of oceanic methane hydrates (e.g., Zeebe, 2013), pulsed emissions from hydrothermal vent complexes (see Jin et al., 2024 and references therein), and the oxidation of soil organic carbon (e.g., Bowen, 2013) or petrogenic organic carbon (OC_petro) (Lyons et al., 2019).
View in article
Changes in biomarker thermal maturity ratios from a global compilation of shallow-marine sediments indicate greater delivery of OC_petro into the ocean during the PETM (Lyons et al., 2019; Hollingsworth et al., 2024).
View in article
In order to calculate the mass of OC_petro-derived CO₂ released during the body of the PETM, Lyons et al. (2019) used the present day lower and upper bounds in oxidation efficiency (15–85 %; Bouchez et al., 2010; Hilton et al., 2014).
View in article
This resulted in a wide range of estimates that span two orders of magnitude (10²–10⁴ PgC; Lyons et al., 2019).
View in article
The South Dover Bridge (SDB) core was drilled in the Salisbury Embayment (mid-Atlantic Coastal Plain; Fig. S-1) and shows a drastic increase in OC_petro delivery during the PETM (Lyons et al., 2019).
View in article
The SDB record of (a) bulk sediment δ¹³C of carbonates (δ¹³C_carbonates; Lyons et al., 2019), (b) percentage graphitised carbon (this study), and (c) C₃₁ homohopane 22S/(22S + 22R) ratio (Lyons et al., 2019).
View in article
Thus, we first compare our results to published biomarker thermal maturity ratios from the same PETM-aged shallow-marine cores (Lyons et al., 2019; Hollingsworth et al., 2024).
View in article
However, a lag within the PETM interval in the percentage of graphitised carbon (Fig. 3b), C₃₁ S/(S + R) ratio (Fig. 3c), and bulk carbon isotope of organic carbon (δ¹³C_org), suggests a delayed response of ∼10–20 kyr (Lyons et al., 2019).
View in article
Our new data supports previous findings indicating enhanced OC_petro delivery at SDB (Lyons et al., 2019) and no drastic changes in organic carbon source(s) at ACEX (Hollingsworth et al., 2024) during the PETM.
View in article
Indeed, similarities were found with the biomarker thermal maturity ratios from the Raritan Formation of the upper Potomac Group and the PETM-aged SDB core (Lyons et al., 2019).
View in article
In addition, the δ¹³C_org values and source rock properties (e.g., T_max) are comparable (Lyons et al., 2019).
View in article
At SDB, there is a pronounced ∼20 fold increase in linear sedimentation rates, and higher OC_petro MAR, during the PETM (Lyons et al., 2019).
View in article

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We note that the oxidation and/or deposition of OC_petro can also occur in floodplains (e.g., Scheingross et al., 2021).
View in article

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Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Dickens, G.R., Huber, M., Reichart, G., Stein, R., Matthiessen, J., Lourens, L.J., Pedentchouk, N., Backman, J., Moran, K. (2006) Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum. Nature 441, 610–613. https://doi.org/10.1038/nature04668

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Note the core gap from ∼388–384.5 mcd (Sluijs et al., 2006).
View in article

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This is consistent with the excellent preservation of pollen and spores (Sluijs et al., 2008), and the presence of biomarkers diagnostic of peats and/or lignite deposits (e.g., C₃₁ αβ hopanes; Hollingsworth et al., 2024), in these sediments.
View in article

Show in context

Based on modern observations (e.g., Hilton and West, 2020; Soulet et al., 2021), it is likely that the increased erosion rates and higher temperatures during the PETM enhanced OC_petro oxidation.
View in article
An average ∼5 °C of warming during the PETM (Tierney et al., 2022) may have also caused OC_petro oxidation to increase by one fold, based on observations made by Soulet et al. (2021) in modern river catchments.
View in article
It is also important to constrain the composition of the source rock(s), and understand the influence of other abiotic (e.g., rainfall type, temperature, O₂ availability, and mineral association) and biotic (e.g., microbial activity) processes on oxidation efficiency (e.g., see Hilton and West, 2020 and references therein; Soulet et al., 2021).
View in article

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We follow the Raman spectroscopy methodology outlined in Sparkes et al. (2013), which was specifically developed to facilitate the analyses of sedimentary rocks.
View in article
The peaks in each spectrum were fitted using an updated script (https://github.com/robertsparkes/raman-fitting/releases/tag/v1.1.5) of the automated process described in Sparkes et al. (2013), and manually checked for inconsistencies.
View in article
In previous studies, spectra were characterised as either: (i) highly graphitised carbon, (ii) mildly graphitised carbon, (iii) intermediate carbon, or (iv) disordered carbon (Sparkes et al., 2013).
View in article
This separation was based on peak burial temperatures, which were calibrated using the R2 and RA2 peak area ratio, and not the sum of peak width (G + D1 + D2) (see Sparkes et al., 2013).
View in article
Spectra have had a linear background removed during the automated process, and the fitted peaks include: G (1580 cm⁻¹), D1 (1350 cm⁻¹), D2 (1620 cm⁻¹), D3 (1500 cm⁻¹), and D4 (1200 cm⁻¹) (Sparkes et al., 2013).
View in article

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Raman spectroscopy has most commonly been used to fingerprint OC_petro in modern river catchments and continental shelves (e.g., Galy et al., 2008; Bouchez et al., 2010; Sparkes et al., 2018, 2020).
View in article
This confirms that Raman spectroscopy can be applied to fingerprint OC_petro delivery in the past, and could be particularly powerful in permitting the inclusion of study sites with post-depositional diagenesis (Sparkes et al., 2020).
View in article
This is consistent with present day observations of graphite particles surviving transport over thousands of kilometres (e.g., Galy et al., 2008) and persisting over multiple erosion cycles (e.g., Sparkes et al., 2020).
View in article

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At the Paleocene-Eocene boundary, an abrupt carbon cycle perturbation gave rise to a global warming event (∼4–6 °C; Tierney et al., 2022), known as the Paleocene-Eocene thermal maximum (PETM; ∼56 Ma).
View in article
An average ∼5 °C of warming during the PETM (Tierney et al., 2022) may have also caused OC_petro oxidation to increase by one fold, based on observations made by Soulet et al. (2021) in modern river catchments.
View in article

Zeebe, R.E. (2013) What caused the long duration of the Paleocene-Eocene Thermal Maximum? Paleoceanography 28, 440–452. https://doi.org/10.1002/palo.20039

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Zeebe, R.E., Lourens, L.J. (2019) Solar System chaos and the Paleocene–Eocene boundary age constrained by geology and astronomy. Science 365, 926–929. https://doi.org/10.1126/science.aax0612

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