Estimating regional flood discharge during Palaeocene-Eocene global warming

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 difficult. 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 fluvial sequences. We infer that extreme floods 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 flooding increase with global warming at similar, but potentially at much higher, magnitudes than currently predicted.

abruptly and is overlaid by ~20 m of fine-grained yellowish soil mainly made up of silty 48 mudstones with abundant small carbonate nodules and gypsum layers, which span the majority 49 of the carbon isotope excursion and its recovery 1 . After the PETM, an interval of red soils marks 50 the return to Palaeocene-like conditions. 51

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To quantify the magnitude of change in water and sediment discharge recorded by the 58 fluvial systems in the basin, we first reconstruct pre-PETM and PETM fluvial palaeoslopes from 59 grain size and channel depth data. We then use palaeoslope and channel width data to invert 60 equilibrium flow velocities and obtain first-order estimates of volumetric discharge during 61 channel forming events before and during the PETM. 62 We estimated channel depth from fining upward sequences and bar clinoforms 6  The average D 50 of the pre-PETM is 21.2±5 mm (1σ, N=26) and the average of the CC is 75 19.5±4 mm (N=22). Average channel depth is 1.1±0.6 m to 1.4±0.6 m, respectively for the 76 Palaeocene and PETM (Fig. 3a)    Claret Conglomerate is related to their larger width compared to the Esplugafreda Formation as 114 the total basin width likely did not change spanning the PETM. Moreover, during the PETM the 115 extreme (close to 100%) channel density prohibits assessments of whether more than one of 116 these braid-belts was active at any given time. In contrast, the very low channel density of ~5% 117 during the Esplugafreda Formation ( Fig. 2) suggests only one active channel at a given time.

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Combining average flow velocity, average channel height and average channel width yields a 119

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Channel-forming discharge in alluvial river systems is typically dictated by flood 129 recurrence on timescales of 1.5-3 years 8,10 , and slopes adjusted to sediment flux and grain size 130 distribution 11,12 . Therefore the parameters measured in this study unlikely relate to mean annual 131 precipitation conditions, but rather to (inter-) annual rainfall variability and/or extreme 132 precipitation events. These extreme events may be related to transport of the extension. In this case, the measured grains were randomly selected in a 1x1 m 2 area. Finally, the 209 grain-size distribution was also determined from pictures for outcrops with access issues 51 . 210 Pictures were taken with a Nikon Coolpix S2700 camera with 16Mpixels resolution from a 211 distance of ca. 1 meter, and a ruler was included on each picture for scale. The average resolution 212 of the pictures thus obtained is ~0.12 mm/pixel. Excluding the edges of the pictures, all visible 213 grains were measured using JMicrovision software 52 . This method corresponds to an areal-by-214 number sample that must be converted to an equivalent grid-by-number sample to be comparable 215 to other samples. A conversion factor of 2 was used in this study 51,53,54 216 217

Clausius-Clapeyron changes in precipitation 256
Precipitation extremes are expected to scale with temperature change at a rate given by the 257 Clausius-Clapeyron equation, which governs change in water-holding capacity of the atmosphere 258 at a rate of 7% per degree 38 . Cumulating this rate 5 times to account for a 5°C increase in 259 temperature during the PETM amounts to a ~40% increase in precipitation, i.e. 1.4 times the 260 initial pre-PETM value. So-called "super" Clausius-Clapeyron scaling involves a doubling (i.e. 261 14%) of the above rate for average temperatures above 12°C, which implies a 1.93-fold increase 262 in precipitation from initial value for a 5°C global warming.