Widespread and intense wildfires at the Paleocene-Eocene boundary
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Abstract
Figures and Tables
Figure 1 Location map of Wilson Lake (WL), Randall’s Farm (RF) and other sites discussed in text; Millville (MV), Bass River (BR). Modified after Self-Trail (2017). | Figure 2 SEM images of charcoal fragments from WL and RF. | Figure 3 Charcoal abundance (# >63 μm/gram) and δ13Ccharcoal (this study) plotted with δ13Cbulk carbonate and spherules/gram discovered at WL (Schaller et al., 2016) and RF (this study). Total depth range at RF is ~2 times WL, allowing for direct comparison. Note difference in isotope scale between WL and RF. Ro measurements, Mr/Ms at WL (Kent et al., 2017), and FMR α at RF (Kopp et al., 2009) are shown. |
Figure 1 | Figure 2 | Figure 3 |
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Introduction
The P-E boundary is marked by a negative CIE in marine and terrestrial records and 5-8 °C global warming (Kennett and Stott, 1991
Kennett, J.P., Stott, L.D. (1991) Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353, 225-229.
; see McInerney and Wing, 2011McInerney, F.A., Wing S.L. (2011) Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth and Planetary Sciences 39, 489-516.
, for review). Although the source(s) and magnitude of low δ13C carbon released into the ocean-atmosphere system at the P-E boundary are debated, impact ejecta recently discovered at the CIE onset indicates that an extraterrestrial impact played a role. Glassy spherules are found in a discrete stratigraphic layer within the CIE onset on the U.S. Atlantic Margin at multiple sites spread over 1,000 km (Schaller et al., 2016Schaller, M.F., Fung, M.K., Wright, J.D., Katz, M.E., Kent, D.V. (2016) Evidence of an Extraterrestrial Impact at the Paleocene-Eocene boundary. Science 354, 225-229.
). The ejecta at Wilson Lake B (WL; Schaller et al., 2016Schaller, M.F., Fung, M.K., Wright, J.D., Katz, M.E., Kent, D.V. (2016) Evidence of an Extraterrestrial Impact at the Paleocene-Eocene boundary. Science 354, 225-229.
) and at Randall’s Farm (RF; this study, Fig. S-1) are formed as either solidified melt-droplets or vapour condensates. On the palaeo-continental shelf, the CIE onset coincides with the base of the thick Marlboro Clay that extends from Virginia to New Jersey (Fig. 1).In this study, we document unusually abundant microscopic (~100 μm) black charcoal shards near the base of the Marlboro Clay at WL and RF; these shards bear characteristics of well-preserved charred plant material and their distribution in the sediment column forms an abrupt peak immediately above the spherule level. SEM imaging – the most diagnostic method of studying charcoalified plant material (Scott and Collinson, 1978
Scott, A.C., Collinson, M.E. (1978) Organic sedimentary particles: results from SEM studies of fragmentary plant material. In: Whalley, W.B. (Eds) SEM in the Study of Sediments. Geoabstacts, Norwich, 137–167.
) – reveals three-dimensional plant features with excellent preservation, including homogeneous cell walls and honeycomb structures (Figs. 2, S-2). This paper explores two hypotheses for the origin of the charcoal following the impact ejecta within the CIE: 1) warming-induced, continental drying and ecosystem change; and 2) as a direct consequence of the impact.top
Evidence for Synchronous, Widespread Intense Wildfires
Charcoal is the most diagnostic artefact of wildfires (Belcher et al., 2003
Belcher, C.M., Collinson, M.E., Sweet, A.R., Hildebrand, A.R., Scott, A.C. (2003) Fireball passes and nothing burns - The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America. Geology 31, 1061-1064.
). Overlying the P-E impact ejecta, we observe a 24-fold increase in charcoal pieces (>63 µm)/gram at WL, and 21-fold increase at RF, a remarkable change above a low but consistent background (Figs. 3, S-3). The material is typically brittle, black, opaque in thin section, and preserves anatomical detail. Raman spectroscopy is widely used to characterise carbonaceous materials (Ulyanova et al., 2014Ulyanova, E.V., Molchanov, A.N., Prokhorov, I.Y., Grinyov, V.G. (2014) Fine structure of Raman spectra in coals of different rank. International Journal of Coal Geology 121, 37-43.
; Wang et al., 2014Wang, S., Cheng, H., Jiang, D., Huang, F., Su, S., Bai, H. (2014) Raman spectroscopy of coal component of Late Permian coals from Southern China. Spectroscopy 132, 767-770.
), and helps confirm the presence of charcoal (Fig. S-4). Wavelength dispersive X-ray spectroscopy (WDS) indicates that the black shards are 58-84 % carbon by weight. Charcoal exhibits high reflectance (Ro) compared to other maceral groups as the result of heating; higher reflectance indicates higher combustion temperatures (Scott and Jones, 1991Scott, A.C., Jones, T.P. (1991) Microscopic observations of Recent and fossil charcoal. Microscopy and Analysis 25, 13-15.
). Samples yield average Ro values between 0.38 % and 2.99 %, corresponding to temperatures between 340-550 °C (Scott and Jones, 1991Scott, A.C., Jones, T.P. (1991) Microscopic observations of Recent and fossil charcoal. Microscopy and Analysis 25, 13-15.
) (Table S-1). Lowest Ro values indicating vitrinite (uncharred plant material) are found below the boundary; highest Ro values occur within the charcoal peak at both RF (1.24 %) and WL (2.99 %), and decrease above; this distribution of Ro values supports a widespread and intense singular wildfire event.The ejecta found at the base of the Marlboro Clay, have all the characteristics of instantaneous air-fall deposit (Schaller et al., 2016
Schaller, M.F., Fung, M.K., Wright, J.D., Katz, M.E., Kent, D.V. (2016) Evidence of an Extraterrestrial Impact at the Paleocene-Eocene boundary. Science 354, 225-229.
), and have been dated by K-Ar to 54.9 ± 3 Ma (Schaller and Fung, 2018Schaller, M.F., Fung, M.K. (2018) The extraterrestrial impact evidence at the Paleocene-Eocene boundary and sequence of environmental change on the continental shelf. Philosophical Transactions of the Royal Society A, 376.
). Because the depositional and cooling ages of the spherules appear to be indistinguishable, the impact ejecta may provide a time-equivalent marker and baseline for the start of the CIE across the mid-Atlantic Salisbury Embayment. The charcoal peak at both localities occurs stratigraphically above both the spherule layer and the start of the CIE; based on this sequence, we infer that: 1) the charcoal was deposited after the impact; and 2) synchronous wildfires produced the charcoal.Several lines of evidence indicate that the charcoal-rich layers are neither artefacts of reworked older sediments nor simply due to sedimentation change. (1) The charcoal peak lies 12 and 104 cm above the ejecta layer at WL and RF, respectively, indicating that the charcoal is not older burned material reactivated by the impact event. (2) The charcoal increases substantially above the change from sands to clayey silts, and decreases to near-background levels well within the clay. (3) There is a single observed pulse of charcoal, not multiple peaks as would be expected from reworking of charcoal.
There is a stratigraphic and causal link between our charcoal observations and an unusual and widespread abundance of magnetic nanoparticles in the Marlboro Clay (Lanci et al., 2002
Lanci, L., Kent, D.V., Miller, K.G. (2002) Detection of Late Cretaceous and Cenozoic sequence boundaries on the Atlantic coastal plain using core log integration of magnetic susceptibility and natural gamma ray measurements at Ancora, New Jersey. Journal of Geophysical Research 107, 2216.
), which resulted from soil pyrogenesis (Kent et al., 2017Kent, D.V., Lanci, L., Wang, H., Wright, J.D. (2017) Enhanced magnetization of the Marlboro Clay as a product of soil pyrogenesis at the Paleocene-Eocene Boundary? Earth and Planetary Science Letters 473, 303-312.
). The stratigraphic level of the charcoal increase at RF and WL coincides with this unique change in magnetisation (Kopp et al., 2009Kopp, 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, PA4211.
and Kent et al., 2017Kent, D.V., Lanci, L., Wang, H., Wright, J.D. (2017) Enhanced magnetization of the Marlboro Clay as a product of soil pyrogenesis at the Paleocene-Eocene Boundary? Earth and Planetary Science Letters 473, 303-312.
, respectively; Fig. 3), indicating that they share a common origin and were deposited contemporaneously. The unique magnetisation is observed in ten sites spanning ~400 km in the Salisbury Embayment (Kopp et al., 2009Kopp, 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, PA4211.
), providing an independent line of evidence indicating that the wildfire activity was regionally extensive (See SI). Elevated fern spores and charred plant material within the CIE at the Mattawoman Creek-Billingsley Road site in the southern Salisbury Embayment (Self-Trail et al., 2017Self-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 and Paleoclimatology 32, 710-728.
) provides further evidence that the wildfires were widespread and contemporaneous (within the level of resolution possible). The dramatic increase and dominance of fern spores in the Marlboro Clay is consistent with the opportunistic nature of ferns immediately following wildfire (Ainsworth and Kauffman, 2009Ainsworth, A., Kauffman, J.B. (2009) Response of native Hawaiian woody species to wildfires in tropical forests and scrublands. Plant Ecology 201, 197-209.
).We use the nadir of the CIE onset as a tie point to correlate the sequence of events recorded at WL and RF (Fig. 3). Remarkably, the highest charcoal abundance occurs prior to the CIE nadir, highlighting the following sequence of events: 1) impact event (deposition of spherule layer); 2) widespread wildfires (significant increase in charcoal); and 3) CIE nadir. The significant increase in charcoal invites the obvious question: what triggered the intense, widespread wildfires that produced the charcoal spike? Here, we consider two scenarios to explain the elevated charcoal abundance at the P-E boundary: 1) continental drying and subsequent wildfires; and 2) impact-induced wildfires.
Scenario 1: Warming-induced, continental drying and subsequent wildfires
Large scale continental drying associated with Paleocene-Eocene Thermal Maximum (PETM) warming could put vegetation at risk for wildfires. These wildfires could be triggered by lightning strikes and be regionally widespread. The first of these many fires would burn extensively, probably contemporaneously, over a wide area, be very intense, and lead to deposition of the large charcoal peak we observe in the sedimentary record. This would likely be followed by subsequent smaller wildfire events producing repeated charcoal pulses, which we do not observe.
Under this scenario, continental interiors experienced aridification. Studies from the U.S. western interior indicate rainfall decreased by ~40 % near the PETM onset, but do not conclude if this aridification was more than regional (Wing et al., 2005
Wing, S.L., Harrington, G.J., Smith, F.A., Bloch, J.I., Boyer, D.M., Freeman, K.H. (2005) Transient floral change and rapid global warming at the Paleocene-Eocene boundary. Science 310, 993-996.
). Palaeosols in Wyoming also indicate decreased precipitation at the onset (Kraus and Riggins, 2007Kraus, M., Riggins, S. (2007) Transient drying during the Paleocene–Eocene Thermal Maximum (PETM): analysis of paleosols in the bighorn basin, Wyoming. Palaeogeography Palaeoclimatology Palaeoecology 245, 444-461.
). Conversely, modelling results from northern and mid-latitudes (including Wyoming) indicate a 20-25 % increase in soil and atmospheric moisture during the PETM (Bowen et al., 2004Bowen, G.J., Beerling, D.J., Koch, P.L., Zachos, J.C., Quattlebaum, T. (2004) A humid climate state during the Palaeocene/Eocene thermal maximum. Nature 432, 495-499.
), and 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, PA4211.
speculated on an “Appalachian Amazon” in Eastern North America. The latter is consistent with the apparent runoff increase responsible for the Apectodinium acme at the P-E onset in the Marlboro Clay (Sluijs et al., 2007Sluijs A., Brinkhuis, H., Schouten, S., Bohaty, S.M., John, C.M., Zachos, J.C., Reichart, G.-J., Sinninghe Damsté, J.S., Crouch, E.M., Dickens, G.R. (2007) Environmental precursors to rapid light carbon injection at the Paleocene/Eocene boundary. Nature 450, 1218-1221.
). Hydrologic studies on the PETM thus have not provided clear evidence for a pattern of dryer or wetter conditions, but observations suggest the mid-Atlantic margin was wet.Scenario 2: Impact-induced wildfires
The charcoal peaks at WL and RF occur stratigraphically above the ejecta layer; therefore, the wildfires could have been ignited by the P-E boundary impact. In this scenario, the massive thermal energy of an impact vapour plume and subsequent ejecta fallout (Melosh et al., 1990
Melosh, H.S., Schneider, N.M., Zahnlek, J., Latham, D. (1990) Ignition of global wildfires at the Cretaceous/Tertiary boundary. Nature 343, 251-254.
) ignited widespread wildfires, producing abundant charcoal that was transported to the shelf via the next major hydrologic event, resulting in the observed depositional sequence. Wildfires following the Cretaceous-Paleogene boundary impact event (see SI for review) provide a model for the P-E boundary impact and potential ignition mechanism, though we note that the P-E boundary impact was smaller.The δ13Ccharcoal at WL and RF provides insight into our two proposed wildfire ignition scenarios and sequence of events (Fig. 3). The δ13Ccharcoal from the charcoal peak exhibits pre-excursion values, and δ13Ccharcoal remains constant through the start of the CIE onset in bulk carbonate. Pre-excursion δ13Ccharcoal values indicate that: 1) the charcoal peak resulted from biomass that grew prior to the impact event and climatic perturbation; and 2) there was no vegetation growth within the time represented by the section of steady δ13Ccharcoal values through the CIE onset and charcoal peak. If the vegetation had grown during the CIE onset, as expected during a millennial-scale event, the δ13Ccharcoal would display excursion values because new plant growth would record the changing atmospheric δ13C. In contrast, our δ13Ccharcoal decreases above peak charcoal abundance and above the start of the CIE onset in bulk carbonate. At WL, two low charcoal abundance samples above the peak exhibit pre-excursion δ13Ccharcoal values, hence the pre-excursion δ13Ccharcoal values in the charcoal peak do not represent a mixture of pre-excursion and excursion carbon, diluted by an over-abundance of pre-excursion vegetation that burned along with it. Because there was apparently no time for new plant growth, we conclude that sediment deposition between the impact ejecta and the δ13Ccharcoal excursion was relatively fast (no more than centennial scale).
top`>Climate Change or Impact Event?
Under the drying scenario, the climate must have become hotter and drier as a result of the carbon cycle event. The environmental perturbation and warm temperatures associated with the PETM created conditions favourable for wildfires, yet peak charcoal occurs within the start of the onset and before the nadir of the excursion, which coincides with most observations of environmental perturbation. This suggests that the PETM climate perturbation did not cause the widespread wildfires and charcoal deposition; rather, wildfires were the consequence of an earlier event. The drying scenario is also inconsistent with the apparent massive increase in sedimentation rate at Salisbury Embayment shelf sections, which would have required an accelerated hydrologic cycle and increased precipitation and runoff, although we note that wildfires would result in significant soil loss. Interestingly, fern spikes of up to 90 % within the Marlboro Clay in the southern Salisbury Embayment suggest warm, wet environments (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 and Paleoclimatology 32, 710-728.
) because ferns need moist environments to complete their lifecycle (Mehltreter et al., 2010Mehltreter, K., Walker, L., Sharpe, J. (2010) Fern ecology. Cambridge University Press, Cambridge.
), making them particularly rare in arid climates (Aldasoro et al., 2004Aldasoro, J.J., Cabezas, F., Aedo, C. (2004) Diversity and distribution of ferns in sub-Saharan Africa, Madagascar and some islands of the South Atlantic. Journal of Biogeography 31, 1579-1604.
). If drying were the source of the P-E boundary wildfires, then there should be evidence of fires during subsequent Early Eocene Hyperthermal events, yet charcoal studies through the Early Eocene Climatic Optimum in Germany do not indicate any increase in fire activity (Robson et al., 2014Robson, B.E., Collinson, M.E., Riegel, W., Wilde, V., Scott, A.C., Pancost, R.D. (2014) A record of fire through the Early Eocene. Rendiconti Online Societa Geologica Italiana 31, 187-188.
).Although our findings are most consistent with impact-induced wildfires at the P-E boundary, either ignition scenario is feasible. Importantly, both continental drying and impact-induced wildfires are in agreement with a fast scenario (Wright and Schaller, 2013
Wright, J.D., Schaller, M.F. (2013) Evidence for a rapid release of carbon at the Paleocene-Eocene thermal maximum. Proceedings of the National Academy of Sciences of the USA 110, 15908-15913.
), supported by δ13Ccharcoal analyses. In contrast, a 4 kyr onset (Zeebe et al., 2016Zeebe, R.E., Ridgwell, A., Zachos, J.C. (2016) Anthropogenic carbon release rate unprecedented during the past 66 million years. Nature Geoscience 9, 325-329.
) requires the absence of plant growth or wildfires in the hinterlands for thousands of years; this is unlikely because fire has a revitalising effect on ecosystems and vegetation growth is quickly renewed following wildfires (Ahlegren, 1974Ahlegren, C.E. (1974) Introduction. In: Kozlowski, T.T., Ahlgren, C.E. (Eds) Fire and Ecosystems. Academic Press, New York, 1-5.
). A sustained period without vegetative growth or wildfires for millennia is highly improbable (Bond and Wilgen, 1996Bond, W.J., van Wilgen, B. (1996) Fire and plants. Chapman and Hall, London.
) when the lifetime of a typical large woody plant is less than a few centuries; therefore, we would expect resumption of at least background charcoal flux with post-CIE δ13C values during those millennia, which is not observed (Fig. 3). The charcoal record could consist of a mixture of charcoal that burned over a number of years; decades seem feasible, yet thousands of years is extremely unlikely, because we would expect to see multiple pulses of charcoal in the record (especially in proximal site RF), which we do not. The charcoal is so well preserved and displays such delicate features that we find it unlikely the material remained exposed on the landscape for thousands of years before being aggregated by floods.A more realistic explanation of the charcoal record is that the P-E boundary sediments were deposited relatively rapidly during the start of the CIE onset. This explanation is consistent with mechanics of charcoal production and transport (see SI). Extremely high sedimentation rates are common in response to widespread landscape clearance by forest fires, resulting in massive post-wildfire debris flows and deposition of large volumes of sediment (Wells, 1987
Wells, W.G. II. (1987) The effects of fire on the generation of debris flows in southern California. Reviews in Engineering Geology 7, 105-113.
). Modern fires increase soil erosion rates by ~30-fold compared to pre-fire levels (Swanson, 1981Swanson, F.J. (1981) Fire and geomorphic processes. In: Mooney, H.A., Bonniksen, T.H., Christensen, N.L., Lotan, J.E., Reiners, W.A. (Eds) Fire Regimes and Ecosystem Properties. USDA Forest Service General Technical Report WO-26.
). In addition, rapid mud accumulation rates are observed in the modern Amazon shelf (Kuehl et al., 1986Kuehl, S.A., DeMaster, D.J., Nittrouer, C.A. (1986) Nature of sediment accumulation on the Amazon continental shelf. Continental Shelf Research 6, 209-225.
), demonstrating that high sedimentation rates during the CIE onset are plausible, especially during an enhanced hydrologic cycle (Kopp et al., 2009Kopp, 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, PA4211.
) with ample sediment supply. Effects of wildfires in coastal California show a significant increase in sediment yield, and up to ~35 times greater than average during the subsequent wetter year (Warrick et al., 2012Warrick, J.A., Hatten, J.A., Pasternack, G.B., Gray, A.B., Goni, M.A., Wheatcroft, R.A. (2012) The effects of wildfire on the sediment yield of a coastal California watershed. GSA Bulletin 124, 1130-1146.
). These studies highlight how an increase in sedimentation and erosion rates occurs during the next major hydrologic event and is therefore an immediate (decadal scale) consequence of wildfires; this provides a framework for our fast sedimentation model. Regardless of the trigger mechanism, burnt terrestrial plant matter and eroded forest soils were transported to continental shelves in a sediment deluge during the next storm event, producing the base of the Marlboro Clay (see SI).To summarise, a synchronous, more than twenty-fold increase in charcoal abundance is identified at the P-E boundary in two locations, indicating that intense widespread wildfires occurred during a climatic warming event. We consider two hypotheses to explain the source of the wildfires: 1) warming-induced, continental drying during the PETM; and 2) thermal radiation and/or ejecta fallout from the P-E boundary impact event. Although we cannot definitively rule out either scenario, our data strongly support the impact-induced wildfire hypothesis. Regardless of the trigger mechanism, a critical finding of this study is that the carbon isotope composition of individual charcoal grains exhibit pre-excursion values and remain constant through the onset of the carbon isotope excursion, indicating a rapid onset of the CIE that challenges the rate paradigm for the very early stages of the event by thousands of years.
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Acknowledgements
We thank D. Kent, K. Miller, and W. Broecker for helpful discussions, and two anonymous reviewers for their thoughtful suggestions. We express our gratitude to M. Mastalerz from Indiana University for reflectance measurements. Funding was provided by the Comer Family Foundation, and NSF award #1737100.
Editor: Sophie Opfergelt
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References
Ahlegren, C.E. (1974) Introduction. In: Kozlowski, T.T., Ahlgren, C.E. (Eds) Fire and Ecosystems. Academic Press, New York, 1-5.
Show in context
In contrast, a 4 kyr onset (Zeebe et al., 2016) requires the absence of plant growth or wildfires in the hinterlands for thousands of years; this is unlikely because fire has a revitalising effect on ecosystems and vegetation growth is quickly renewed following wildfires (Ahlegren, 1974).
View in article
Ainsworth, A., Kauffman, J.B. (2009) Response of native Hawaiian woody species to wildfires in tropical forests and scrublands. Plant Ecology 201, 197-209.
Show in context
The dramatic increase and dominance of fern spores in the Marlboro Clay is consistent with the opportunistic nature of ferns immediately following wildfire (Ainsworth and Kauffman, 2009).
View in article
Aldasoro, J.J., Cabezas, F., Aedo, C. (2004) Diversity and distribution of ferns in sub-Saharan Africa, Madagascar and some islands of the South Atlantic. Journal of Biogeography 31, 1579-1604.
Show in context
Interestingly, fern spikes of up to 90 % within the Marlboro Clay in the southern Salisbury Embayment suggest warm, wet environments (Self-Trail et al., 2017) because ferns need moist environments to complete their lifecycle (Mehltreter et al., 2010), making them particularly rare in arid climates (Aldasoro et al., 2004).
View in article
Belcher, C.M., Collinson, M.E., Sweet, A.R., Hildebrand, A.R., Scott, A.C. (2003) Fireball passes and nothing burns - The role of thermal radiation in the Cretaceous-Tertiary event: Evidence from the charcoal record of North America. Geology 31, 1061-1064.
Show in context
Charcoal is the most diagnostic artefact of wildfires (Belcher et al., 2003).
View in article
Bond, W.J., van Wilgen, B. (1996) Fire and plants. Chapman and Hall, London.
Show in context
A sustained period without vegetative growth or wildfires for millennia is highly improbable (Bond and Wilgen, 1996) when the lifetime of a typical large woody plant is less than a few centuries; therefore, we would expect resumption of at least background charcoal flux with post-CIE δ13C values during those millennia, which is not observed (Fig. 3).
View in article
Bowen, G.J., Beerling, D.J., Koch, P.L., Zachos, J.C., Quattlebaum, T. (2004) A humid climate state during the Palaeocene/Eocene thermal maximum. Nature 432, 495-499.
Show in context
Conversely, modelling results from northern and mid-latitudes (including Wyoming) indicate a 20-25 % increase in soil and atmospheric moisture during the PETM (Bowen et al., 2004), and Kopp et al. (2009) speculated on an “Appalachian Amazon” in Eastern North America.
View in article
Kennett, J.P., Stott, L.D. (1991) Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature 353, 225-229.
Show in context
The P-E boundary is marked by a negative CIE in marine and terrestrial records and 5-8 °C global warming (Kennett and Stott, 1991; see McInerney and Wing, 2011, for review).
View in article
Kent, D.V., Lanci, L., Wang, H., Wright, J.D. (2017) Enhanced magnetization of the Marlboro Clay as a product of soil pyrogenesis at the Paleocene-Eocene Boundary? Earth and Planetary Science Letters 473, 303-312.
Show in context
Figure 3 [...] Ro measurements, Mr/Ms at WL (Kent et al., 2017), and FMR α at RF (Kopp et al., 2009) are shown.
View in article
There is a stratigraphic and causal link between our charcoal observations and an unusual and widespread abundance of magnetic nanoparticles in the Marlboro Clay (Lanci et al., 2002), which resulted from soil pyrogenesis (Kent et al., 2017).
View in article
The stratigraphic level of the charcoal increase at RF and WL coincides with this unique change in magnetisation (Kopp et al., 2009 and Kent et al., 2017, respectively; Fig. 3), indicating that they share a common origin and were deposited contemporaneously.
View in article
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, PA4211.
Show in context
Figure 3 [...] Ro measurements, Mr/Ms at WL (Kent et al., 2017), and FMR α at RF (Kopp et al., 2009) are shown.
View in article
The stratigraphic level of the charcoal increase at RF and WL coincides with this unique change in magnetisation (Kopp et al., 2009 and Kent et al., 2017, respectively; Fig. 3), indicating that they share a common origin and were deposited contemporaneously.
View in article
The unique magnetisation is observed in ten sites spanning ~400 km in the Salisbury Embayment (Kopp et al., 2009), providing an independent line of evidence indicating that the wildfire activity was regionally extensive (See SI).
View in article
Conversely, modelling results from northern and mid-latitudes (including Wyoming) indicate a 20-25 % increase in soil and atmospheric moisture during the PETM (Bowen et al., 2004), and Kopp et al. (2009) speculated on an “Appalachian Amazon” in Eastern North America.
View in article
In addition, rapid mud accumulation rates are observed in the modern Amazon shelf (Kuehl et al., 1986), demonstrating that high sedimentation rates during the CIE onset are plausible, especially during an enhanced hydrologic cycle (Kopp et al., 2009) with ample sediment supply.
View in article
Kraus, M., Riggins, S. (2007) Transient drying during the Paleocene–Eocene Thermal Maximum (PETM): analysis of paleosols in the bighorn basin, Wyoming. Palaeogeography Palaeoclimatology Palaeoecology 245, 444-461.
Show in context
Palaeosols in Wyoming also indicate decreased precipitation at the onset (Kraus and Riggins, 2007).
View in article
Kuehl, S.A., DeMaster, D.J., Nittrouer, C.A. (1986) Nature of sediment accumulation on the Amazon continental shelf. Continental Shelf Research 6, 209-225.
Show in context
In addition, rapid mud accumulation rates are observed in the modern Amazon shelf (Kuehl et al., 1986), demonstrating that high sedimentation rates during the CIE onset are plausible, especially during an enhanced hydrologic cycle (Kopp et al., 2009) with ample sediment supply.
View in article
Lanci, L., Kent, D.V., Miller, K.G. (2002) Detection of Late Cretaceous and Cenozoic sequence boundaries on the Atlantic coastal plain using core log integration of magnetic susceptibility and natural gamma ray measurements at Ancora, New Jersey. Journal of Geophysical Research 107, 2216.
Show in context
There is a stratigraphic and causal link between our charcoal observations and an unusual and widespread abundance of magnetic nanoparticles in the Marlboro Clay (Lanci et al., 2002), which resulted from soil pyrogenesis (Kent et al., 2017).
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McInerney, F.A., Wing S.L. (2011) Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth and Planetary Sciences 39, 489-516.
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The P-E boundary is marked by a negative CIE in marine and terrestrial records and 5-8 °C global warming (Kennett and Stott, 1991; see McInerney and Wing, 2011, for review).
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Mehltreter, K., Walker, L., Sharpe, J. (2010) Fern ecology. Cambridge University Press, Cambridge.
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Interestingly, fern spikes of up to 90 % within the Marlboro Clay in the southern Salisbury Embayment suggest warm, wet environments (Self-Trail et al., 2017) because ferns need moist environments to complete their lifecycle (Mehltreter et al., 2010), making them particularly rare in arid climates (Aldasoro et al., 2004).
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Melosh, H.S., Schneider, N.M., Zahnlek, J., Latham, D. (1990) Ignition of global wildfires at the Cretaceous/Tertiary boundary. Nature 343, 251-254.
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In this scenario, the massive thermal energy of an impact vapour plume and subsequent ejecta fallout (Melosh et al., 1990) ignited widespread wildfires, producing abundant charcoal that was transported to the shelf via the next major hydrologic event, resulting in the observed depositional sequence.
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Robson, B.E., Collinson, M.E., Riegel, W., Wilde, V., Scott, A.C., Pancost, R.D. (2014) A record of fire through the Early Eocene. Rendiconti Online Societa Geologica Italiana 31, 187-188.
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If drying were the source of the P-E boundary wildfires, then there should be evidence of fires during subsequent Early Eocene Hyperthermal events, yet charcoal studies through the Early Eocene Climatic Optimum in Germany do not indicate any increase in fire activity (Robson et al., 2014).
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Schaller, M.F., Fung, M.K., Wright, J.D., Katz, M.E., Kent, D.V. (2016) Evidence of an Extraterrestrial Impact at the Paleocene-Eocene boundary. Science 354, 225-229.
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Atlantic Margin at multiple sites spread over 1,000 km (Schaller et al., 2016).
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The ejecta at Wilson Lake B (WL; Schaller et al., 2016) and at Randall’s Farm (RF; this study, Fig. S-1) are formed as either solidified melt-droplets or vapour condensates.
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Figure 3 Charcoal abundance (# >63 μm/gram) and δ13Ccharcoal (this study) plotted with δ13Cbulk carbonate and spherules/gram discovered at WL (Schaller et al., 2016) and RF (this study).
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The ejecta found at the base of the Marlboro Clay, have all the characteristics of instantaneous air-fall deposit (Schaller et al., 2016), and have been dated by K-Ar to 54.9 ± 3 Ma (Schaller and Fung, 2018).
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Schaller, M.F., Fung, M.K. (2018) The extraterrestrial impact evidence at the Paleocene-Eocene boundary and sequence of environmental change on the continental shelf. Philosophical Transactions of the Royal Society A, 376.
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The ejecta found at the base of the Marlboro Clay, have all the characteristics of instantaneous air-fall deposit (Schaller et al., 2016), and have been dated by K-Ar to 54.9 ± 3 Ma (Schaller and Fung, 2018).
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Scott, A.C., Collinson, M.E. (1978) Organic sedimentary particles: results from SEM studies of fragmentary plant material. In: Whalley, W.B. (Eds) SEM in the Study of Sediments. Geoabstacts, Norwich, 137–167.
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SEM imaging – the most diagnostic method of studying charcoalified plant material (Scott and Collinson, 1978) – reveals three-dimensional plant features with excellent preservation, including homogeneous cell walls and honeycomb structures (Figs. 2, S-2).
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Scott, A.C., Jones, T.P. (1991) Microscopic observations of Recent and fossil charcoal. Microscopy and Analysis 25, 13-15.
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Charcoal exhibits high reflectance (Ro) compared to other maceral groups as the result of heating; higher reflectance indicates higher combustion temperatures (Scott and Jones, 1991).
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Samples yield average Ro values between 0.38 % and 2.99 %, corresponding to temperatures between 340-550 °C (Scott and Jones, 1991) (Table S-1).
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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 and Paleoclimatology 32, 710-728.
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Figure 1 [...] Modified after Self-Trail (2017).
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Elevated fern spores and charred plant material within the CIE at the Mattawoman Creek-Billingsley Road site in the southern Salisbury Embayment (Self-Trail et al., 2017) provides further evidence that the wildfires were widespread and contemporaneous (within the level of resolution possible).
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Interestingly, fern spikes of up to 90 % within the Marlboro Clay in the southern Salisbury Embayment suggest warm, wet environments (Self-Trail et al., 2017) because ferns need moist environments to complete their lifecycle (Mehltreter et al., 2010), making them particularly rare in arid climates (Aldasoro et al., 2004).
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Sluijs A., Brinkhuis, H., Schouten, S., Bohaty, S.M., John, C.M., Zachos, J.C., Reichart, G.-J., Sinninghe Damsté, J.S., Crouch, E.M., Dickens, G.R. (2007) Environmental precursors to rapid light carbon injection at the Paleocene/Eocene boundary. Nature 450, 1218-1221.
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The latter is consistent with the apparent runoff increase responsible for the Apectodinium acme at the P-E onset in the Marlboro Clay (Sluijs et al., 2007).
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Swanson, F.J. (1981) Fire and geomorphic processes. In: Mooney, H.A., Bonniksen, T.H., Christensen, N.L., Lotan, J.E., Reiners, W.A. (Eds) Fire Regimes and Ecosystem Properties. USDA Forest Service General Technical Report WO-26.
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Modern fires increase soil erosion rates by ~30-fold compared to pre-fire levels (Swanson, 1981).
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Ulyanova, E.V., Molchanov, A.N., Prokhorov, I.Y., Grinyov, V.G. (2014) Fine structure of Raman spectra in coals of different rank. International Journal of Coal Geology 121, 37-43.
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Raman spectroscopy is widely used to characterise carbonaceous materials (Ulyanova et al., 2014; Wang et al., 2014), and helps confirm the presence of charcoal (Fig. S-4).
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Wang, S., Cheng, H., Jiang, D., Huang, F., Su, S., Bai, H. (2014) Raman spectroscopy of coal component of Late Permian coals from Southern China. Spectroscopy 132, 767-770.
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Raman spectroscopy is widely used to characterise carbonaceous materials (Ulyanova et al., 2014; Wang et al., 2014), and helps confirm the presence of charcoal (Fig. S-4).
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Warrick, J.A., Hatten, J.A., Pasternack, G.B., Gray, A.B., Goni, M.A., Wheatcroft, R.A. (2012) The effects of wildfire on the sediment yield of a coastal California watershed. GSA Bulletin 124, 1130-1146.
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Effects of wildfires in coastal California show a significant increase in sediment yield, and up to ~35 times greater than average during the subsequent wetter year (Warrick et al., 2012).
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Wells, W.G. II. (1987) The effects of fire on the generation of debris flows in southern California. Reviews in Engineering Geology 7, 105-113.
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Extremely high sedimentation rates are common in response to widespread landscape clearance by forest fires, resulting in massive post-wildfire debris flows and deposition of large volumes of sediment (Wells, 1987).
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Wing, S.L., Harrington, G.J., Smith, F.A., Bloch, J.I., Boyer, D.M., Freeman, K.H. (2005) Transient floral change and rapid global warming at the Paleocene-Eocene boundary. Science 310, 993-996.
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Studies from the U.S. western interior indicate rainfall decreased by ~40 % near the PETM onset, but do not conclude if this aridification was more than regional (Wing et al., 2005).
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Wright, J.D., Schaller, M.F. (2013) Evidence for a rapid release of carbon at the Paleocene-Eocene thermal maximum. Proceedings of the National Academy of Sciences of the USA 110, 15908-15913.
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Importantly, both continental drying and impact-induced wildfires are in agreement with a fast scenario (Wright and Schaller, 2013), supported by δ13Ccharcoal analyses.
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Zeebe, R.E., Ridgwell, A., Zachos, J.C. (2016) Anthropogenic carbon release rate unprecedented during the past 66 million years. Nature Geoscience 9, 325-329.
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In contrast, a 4 kyr onset (Zeebe et al., 2016) requires the absence of plant growth or wildfires in the hinterlands for thousands of years; this is unlikely because fire has a revitalising effect on ecosystems and vegetation growth is quickly renewed following wildfires (Ahlegren, 1974).
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Supplementary Information
The Supplementary Information includes:
- Materials and Methods
- Supplementary Text
- Tables S-1 and S-2
- Figures S-1 to S-5
- Supplementary Information References
Download the Supplementary Information (PDF)