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Palaeocene–Eocene Thermal Maximum Prolonged by Fossil Carbon Oxidation

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Palaeocene–Eocene Thermal Maximum Prolonged by Fossil Carbon Oxidation ARTICLES https://doi.org/10.1038/s41561-018-0277-3 Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation Shelby L. Lyons 1*, Allison A. Baczynski1, Tali L. Babila2, Timothy J. Bralower1, Elizabeth A. Hajek 1, Lee R. Kump1, Ellen G. Polites1, Jean M. Self-Trail3, Sheila M. Trampush4, Jamie R. Vornlocher5, James C. Zachos2 and Katherine H. Freeman 1 A hallmark of the rapid and massive release of carbon during the Palaeocene–Eocene Thermal Maximum is the global negative carbon isotope excursion. The delayed recovery of the carbon isotope excursion, however, indicates that CO2 inputs continued well after the initial rapid onset, although there is no consensus about the source of this secondary carbon. Here we suggest this secondary input might have derived partly from the oxidation of remobilized sedimentary fossil carbon. We measured the bio- marker indicators of thermal maturation in shelf records from the US Mid-Atlantic coast, constructed biomarker mixing models to constrain the amount of fossil carbon in US Mid-Atlantic and Tanzania coastal records, estimated the fossil carbon accumula- tion rate in coastal sediments and determined the range of global CO2 release from fossil carbon reservoirs. This work provides evidence for an order of magnitude increase in fossil carbon delivery to the oceans that began ~10–20 kyr after the event onset 2 4 and demonstrates that the oxidation of remobilized fossil carbon released between 10 and 10 PgC as CO2 during the body of the Palaeocene–Eocene Thermal Maximum. The estimated mass is sufficient to have sustained the elevated atmospheric CO2 levels required by the prolonged global carbon isotope excursion. Even after considering uncertainties in the sedimentation rates, these results indicate that the enhanced erosion, mobilization and oxidation of ancient sedimentary carbon contributed to the delayed recovery of the climate system for many thousands of years. limate events of the Earth’s past reveal the broad array of fossil C in Bighorn Basin palaeosols17 and coastal sediments from Earth’s responses to global carbon cycle perturbations and Tanzania18 and Wilson Lake19, and transported terrestrial material thus provide valuable insights into the potential impacts of that included detrital magnetite20 and fern spores18 in Mid-Atlantic C 1 future anthropogenic climate change . The Palaeocene–Eocene coast records. Global evidence for remobilization of terrestrial and Thermal Maximum (PETM) serves as the best-known analogue fossil C during the PETM demonstrates the need to assess fossil for anthropogenic climate change due to the significant amount of C oxidation as an additional CO2 source after the PETM onset. carbon and geologically rapid rate of release2. During the PETM, Here we present the first constraints on fossil C accumulation 13 an injection of 4,500–10,000 PgC of C-depleted CO2 into Earth’s in marginal marine settings. Further, we use these to constrain the ocean–atmosphere system over < 20,000 years resulted in a global amount of CO2 released from fossil C oxidation. Our results indi- warming of 5–9 °C and a global carbon isotope excursion (CIE) of cate that fossil C accumulation in sediments increased by an order 2–7 3–5‰ recorded in terrestrial and marine sediments . of magnitude (between 15- and 50-fold), which resulted in CO2 3 Collectively, carbon isotope records of the PETM reveal a rapid release on the order of 10 PgC over the PETM body. 13 4–20 kyr onset, followed by 70–100 kyr of sustained negative δ C CO2 released from increased remobilization and oxidation of values, referred to as the CIE body. A subsequent ~50 kyr recov- fossil C is a sufficient and plausible mechanism to explain sustained 13 ery phase indicated by a return to pre-event δ C values suggests a elevated atmospheric CO2 levels and produce a prolonged global 8,9 period of CO2 drawdown . To reproduce the shape and magnitude CIE. This work demonstrates that changes in continental weath- of the CIE, models require an additional release of at least 1,480 PgC ering of carbon-rich sedimentary deposits during global warming 13 10 (if δ C = − 50‰) during the PETM body . The carbon input has events may act as a net source of thousands of petagrams of CO2 on been simulated as a pulsed carbon cycle feedback, continued release 10–100 kyr timescales. from methane hydrate reservoirs or sedimentary carbon oxidation 6,10–13 in response to the initial rapid CO2 release . Elevated accumulation of fossil C in coastal sediments Today, ancient sedimentary organic carbon (fossil C) reservoirs Shallow marine records from the Maryland Coastal Plain present emit CO2 when sedimentary deposits are exhumed, eroded and an opportunity to quantify landscape-scale fluxes of remobilized transported. During transport, 15–85% of the remobilized fossil organic matter from a major drainage basin to the marine system. C is oxidized to CO2, spanning the range observed in active margins to Upper Palaeocene and Lower Eocene sediments from a < 150 m passive margins with mobile mud belts14,15, which makes fossil C oxi- palaeowater depth21 were obtained from the Maryland Coastal dation a potential major source of atmospheric CO2. Globally, PETM Plain region of the Salisbury Embayment, specifically from inner records contain evidence of erosion and sediment transport across shelf sites South Dover Bridge (SDB) and Cambridge–Dorchester landscapes, including eroded kaolinite in marine records16, increased Airport (CamDor)22 (Fig. 1). SDB and CamDor are opportune 1Department of Geosciences, Pennsylvania State University, University Park, PA, USA. 2Earth & Planetary Sciences Department, University of California, Santa Cruz, CA, USA. 3Eastern Geology and Paleoclimate Science Center, US Geological Survey, Reston, VA, USA. 4Department of Geological Sciences, University of Delaware, Newark, DE, USA. 5School of Geosciences, University of Louisiana at Lafayette, Lafayette, LA, USA. *e-mail: [email protected] 54 NatURE GEOscIENCE | VOL 12 | JANUARY 2019 | 54–60 | www.nature.com/naturegeoscience NATURE GEOSCIENCE ARTICLES a 76° W 74° W PA NJ N 40° N MD South Jersey High Atlantic Ocean Fall line DE DC VA SDB Delaware y r nt y u e Bay a b B s m li y e CD a a k S b a e m p E a s e 0 25 50 miles h C MD 02550 km 38° N b –185 –185 –185 –190 –190 –190 y –195 –195 –195 Depth (m) –200 –200 –200 PETM Marlboro Cla –205 –205 –205 –210 –210 –210 –4 –2 02–30 –25 –20 00.5 1 cst vfs fs 13 13 δ Ccarbonate δ Corganic %TOC c 200 200 200 205 205 205 y 210 210 210 215 215 215 Depth (m) 220 220 220 PETM Marlboro Cla 225 225 225 cst vfs fs 230 230 230 –4 –2 02–30 –25 –20 012 13 13 δ Ccarbonate δ Corganic %TOC Fig. 1 | Percent organic carbon and carbonate and organic isotope records plotted against depth at SDB and CamDor inner shelf sites. a, Location of SDB and CamDor (CD) cores (modified from Self-Trail et al.48). The Upper Palaeocene Aquia formation is overlain by 13.8 m (CamDor) and 14.9 m (SDB) of Lower Eocene Marlboro Clay, above which is a contact with the Eocene Nanjemoy Formation. b,c, %TOC, carbonate and organic CIEs are presented for the inner-shelf Palaeo–Potomac transect records of SDB (b) and CamDor (c). The grey shaded areas represent the PETM Marlboro Clay, bounded by 13 red dotted lines. The start of the positive δ Corg enrichment is marked by a green dotted line. Lithology is designated as: c, clay; st, silt; vfs, very fine sand; fs, fine sand. records to assess sediment and organic matter provenance changes during the PETM19,22. Coupled with previously published records at the land–sea interface due to their increase in sedimentation rate from Tanzania18,20, we assessed changes in fossil C accumulation in from 1.1 to 16.4 cm kyr–1 and increase in terrestrial sediment flux coastal environments during the PETM. NatURE GEOscIENCE | VOL 12 | JANUARY 2019 | 54–60 | www.nature.com/naturegeoscience 55 ARTICLES NATURE GEOSCIENCE 185 190 y ) 195 Depth (m 200 TM Marlboro Cla PE 205 210 cstvfs –30 –25 –20 0 0.5 1 0 0.2 0.4 0.6 0.6 0.4 0.2 0 123 13C (‰ VPDB) δ organic Ts/(Ts + Tm) C31HH-S/(S + R) C29M/H NH/H 210 212 214 216 ) 218 220 Depth (m 222 TM Marlboro Clay PE 224 226 228 230 cstvfs –30 –25 –20 0 0.2 0.4 0.6 0 0.2 0.4 0.6 1 0.5 0 00.5 1 13C (‰ VPDB) δ organic C31HH-S/(S + R) C32HH-S/(S + R) C29M/H NH/H Fig. 2 | Biomarker thermal maturity ratios at SDB (top) and CamDor (bottom) demonstrate a source change during the PETM body. Thermal maturity ratios are plotted alongside the organic isotope record and shaded by timing during the PETM (light grey, pre-PETM; dark grey, PETM onset; black, 13 PETM body at δ Corg enrichment onset; white, post-PETM). Ts/(Ts + Tm), C31-homohopane (C31HH)-S/(S + R), C32-homohopane (C32HH)-S/(S + R) and norhopane/hopane (NH/H) increase with thermal maturity; C29-moretane/(moretane + hopane) (C29M/H) decreases with thermal maturity. The PETM interval is bounded by red dotted lines. VPDB, Vienna Pee Dee Belemnite. We confirmed the terrestrial carbon flux to the shelf at SDB by the consistent 2α-methylhopane index values ((2α -methylhopane/ 33 measuring n-alkane indicators of marine/terrestrial provenance, (2α -methylhopane + C30-hopane)) , which is typically, though not including the terrigenous/aquatic ratio ((n-C27 + n-C29 + n-C31)/ exclusively, used as an indicator of cyanobacteria-derived biomass 23–25 33,36 33 (n-C15 + n-C17 + n-C19)) (refs.
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