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Estimating Regional Flood Discharge During Palaeocene-Eocene Global

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Estimating Regional Flood Discharge During Palaeocene-Eocene Global www.nature.com/scientificreports OPEN Estimating regional food discharge during Palaeocene-Eocene global warming Received: 8 January 2018 Chen Chen1, Laure Guerit1,2, Brady Z. Foreman 3, Hima J. Hassenruck-Gudipati4, Accepted: 6 August 2018 Thierry Adatte5, Louis Honegger1, Marc Perret1, Appy Sluijs6 & Sébastien Castelltort 1 Published: xx xx xxxx Among the most urgent challenges in future climate change scenarios is accurately predicting the magnitude to which precipitation extremes will intensify. Analogous changes have been reported for an episode of millennial-scale 5 °C warming, termed the Palaeocene-Eocene Thermal Maximum (PETM; 56 Ma), providing independent constraints on hydrological response to global warming. However, quantifying hydrologic extremes during geologic global warming analogs has proven difcult. Here we show that water discharge increased by at least 1.35 and potentially up to 14 times during the early phase of the PETM in northern Spain. We base these estimates on analyses of channel dimensions, sediment grain size, and palaeochannel gradients across the early PETM, which is regionally marked by an abrupt transition from overbank palaeosol deposits to conglomeratic fuvial sequences. We infer that extreme foods and channel mobility quickly denuded surrounding soil-mantled landscapes, plausibly enhanced by regional vegetation decline, and exported enormous quantities of terrigenous material towards the ocean. These results support hypotheses that extreme rainfall events and associated risks of fooding increase with global warming at similar, but potentially at much higher, magnitudes than currently predicted. Alluvial deposits within the Tremp-Graus Basin of northern Spain (~35°N palaeolatitude) show a change from strata dominated by overbank palaeosols to an anomalously thick and widespread, conglomeratic fuvial unit that coincides with the early phase of the PETM1–3. Tis was interpreted to refect the development of a vast braid plain due to an abrupt and dramatic increase in seasonal rainfall1. Late Palaeocene foodplain deposits near the town of Aren (Esplugafreda Formation; Fig. 1) are intercalated with coarse sandstones and clast-supported conglomerates flling isolated single- and multi-storey ribbon fuvial channels deposits4. Levels of gypsum, ubiquitous microc- odium remains, abundant carbonate nodule horizons, and reddish palaeosols indicate deposition in generally semi-arid alluvial plains4,5. A member of the overlying Claret Formation that formed ~40 kyr prior to the PETM represents a 30 m thick incised valley fll (IVF) made of coarse- and fne-grained fuvial sediment, which displays an erosional base with maximum relief of ~30 m and maximum width of ~5 km (ref.2). Te IVF member is overlaid by an extensive sheet-like pebbly calcarenite and clast-supported conglomerate unit, the Claret Conglomerate (CC), which has typical thicknesses of 1 to 4 m and locally up to 8 m (ref.1). Studies on organic carbon have demonstrated that this unit occurs afer the onset of the carbon isotope excursion6–8 (CIE) and terminates prior to the peak of the CIE1,2,7 (Fig. 1), suggesting the Claret Conglomerate formed during the early phase of the PETM over a time span of ~10 kyrs1 (ref.1) or less. Te CC ends abruptly and is overlaid by ~20 m of fne-grained yellowish soil mainly made up of silty mudstones with abundant small carbonate nodules and gypsum layers, which span the majority of the carbon isotope excursion and its recovery1. Afer the PETM, an interval of red soils marks the return to Palaeocene-like conditions. A suite of carbon isotope records using bulk organic, pedogenic carbonate nodules, and compound-specifc proxies reinforce the correlation between the PETM and this unique sedimentologic 1Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205, Geneva, Switzerland. 2Géosciences Environnement Toulouse, 14 av. Edouard Belin, 31400, Toulouse, France. 3Department of Geology, Western Washington University, Bellingham, Washington, 98225, USA. 4Jackson School of Geosciences, The University of Texas at Austin, 2305 Speedway Stop, C1160, Austin, Texas, USA. 5ISTE, Geopolis, University of Lausanne, 1015, Lausanne, Switzerland. 6Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Heidelberglaan 2, 3584CS, Utrecht, Netherlands. Correspondence and requests for materials should be addressed to S.C. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:13391 | DOI:10.1038/s41598-018-31076-3 1 www.nature.com/scientificreports/ Figure 1. Study area in palaeogeographic context (modifed from ref.3) and simplifed stratigraphic column with main formations, ages and carbon isotopic profle showing the negative δ13C excursion in soil carbonates1 (blue profle) and in organic carbon8 (green profle). IVF: Incised Valley Fill. PETM: Palaeocene-Eocene Termal Maximum. ys and rs: yellowish and reddish soils. Arrows indicate main palaeofow directions in the Late Palaeocene. interval from stratigraphic sections located in both proximal and distal portions of the Tremp-Graus Basin1–3,6–9. However, there is some lingering disagreement in the precise timing of the sedimentologic response in relation to the onset, body, and recovery portions of the PETM3,6–8. Tis uncertainty is likely related to correlation impre- cision, the timescales of formation of diferent proxies, taphonomic preservation issues, and the inherent incom- pleteness of the terrestrial stratigraphic record on timescales shorter than 10 kyrs3,6–8,10. It should be noted that there is clear evidence for tectonic and eustatic influences on deposition in the Tremp-Graus Basin throughout the Late Cretaceous and early Paleogene. Compressional tectonics between the European and Iberian plates instigated active thrusting within the Pyrenees and formation of the Tremp-Graus foreland basin11–13. Structural relationships, subsidence analyses, and changing basin sedimentation rates indi- cate that the compressional tectonic regime produced discrete intervals of active thrusting and tectonic quies- cence14,15; However, these major episodes of thrusting are uncorrelated with the PETM and occurred during the late Santonian-late Maastrichtian (preceding the PETM) and the middle Illerdian-middle Lutetian intervals (post-dating the PETM)14. Te intervening period, which includes the PETM interval, experienced slow, uniform subsidence rates12,15,16. Te basin was also subjected to eustatic variability and the foreland basin inundated sev- eral times during the Late Cretaceous and early Paleogene3. Most pertinent is the sea level fall and subsequent rise documented by the IVF unit underlying the Claret Conglomerate, however, subsequent study has estab- lished this eustatic fuctuation preceded the PETM interval and was likely not the primary driver of the Claret Conglomerate3. Tus, in the absence of compelling independent evidence for a tectonic or eustatic forcing on the Claret Conglomerate and yellow palaeosol interval, and its tight correlation with several isotope records we proceed under the inference that the observed change in stratigraphy was driven by climatic shifs associated with high atmospheric carbon dioxide levels during the early phase of the PETM. Previous studies have inferred a quali- tative increase in seasonality, extreme events, and intra-annual humidity during the PETM based on the Claret Conglomerate1,2. Unfortunately, there is no detailed analysis of palaeosols in the Tremp-Graus Basin comparable to extensive studies of PETM paleosols in the Bighorn Basin of Wyoming, USA17–19. However, existing data sug- gest that the shif from red-to-yellow-to-red bed palaeosols implies these altered hydrologic conditions persisted throughout the PETM18. To quantify the magnitude of change in water and sediment discharge recorded by the fuvial systems in the basin, we frst reconstruct pre-PETM and PETM fuvial palaeoslopes and equilibrium fow velocities from feld estimates of grain size and channel depth data. We then extract average channel widths from a published cross-section of the Claret Conglomerate and combine with fow velocities to obtain frst-order estimates of vol- umetric discharge during channel forming events before and during the early PETM. Conspicuously, the Claret Conglomerate is temporally restricted to the early portion of the PETM (Fig. 1) and may only be representative of the climatic transition from baseline Paleocene conditions to the elevated pCO2 conditions of the PETM. Tus, our quantitative reconstructions of discharge and precipitation extremes must be conservatively restricted to the early phase of the PETM. However, there is also a possibility that the temporal brevity of the Claret Conglomerate is related to non-linear behavior of the geomorphic system to the PETM-forcing. Several modeling, experimental, and feld studies suggest the propagation of environmental signals will be modifed by alluvial systems20,21. Tis hypothesis has yet to be thoroughly vetted in the Tremp-Graus Basin and as such we restrict our inferences of hydrologic changes to the early PETM but note the possibility that they may be representative of the PETM as a whole. SCIENTIFIC REPORTS | (2018) 8:13391 | DOI:10.1038/s41598-018-31076-3 2 www.nature.com/scientificreports/ Figure 2. Outcrop panoramic view and line drawing with location of feld grain size measurement stations. PETM Claret Conglomerate is in pink above blue IVF interval (colors as on Fig. 1). Green line is the Mid- Palaeocene Unconformity separating Lower Palaeocene
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