Middle Eocene Climatic Optimum (MECO) and its imprint in the continental Escanilla Formation, Spain

. The Middle Eocene Climatic Optimum (MECO) is a global warming event mainly described in the marine domain but less in the terrestrial domain. This study presents a comprehensive geochemical record of the MECO from the Escanilla Formation, a fluvial sedimentary succession in the southern Pyrenees, Spain, based on 15 a suite of sampled paleosols, fluvial stromatolites and pedogenic nodules. Our multiproxy approach involves using carbon and oxygen stable isotope compositions to identify the regional preservation of the MECO, calculate chemical weathering intensity and identify the clay mineralogy of paleosols, as well as to estimate mean annual precipitation using temperature estimates based on clumped isotope compositions of carbonates. Results indicate that the Middle Eocene interval in the south central Pyrenees was characterized by low weathering rates under 20 warm and arid climatic conditions. This is further supported by the presence of smectite, palygorskite, illite, and chlorite, which suggest seasonal rainfall but under generally dry conditions resulting in weak chemical weathering. Importantly, an isotopic excursion indicates a regional, terrestrial impact of the MECO, highlighting that fluvial sedimentary successions even in active foreland basins can represent particularly interesting terrestrial archives of past changes in global climate.

Analyses were carried out on a suite of sampled paleosols (N = 45), stromatolites (n = 9), and pedogenic nodules (n = 9).Powders of bulk paleosol samples were prepared and analyzed for a suite of geochemical indicators, including total organic carbon (TOC) content, Rock-Eval parameters, organic carbon isotope compositions ( 13 Corg), carbonate carbon and oxygen isotopes ( 13 Ccarb and  18 Ocarb), major elements, and clay mineral 105 assemblages.Stromatolite and pedogenic nodule powders were analyzed for  13 Ccarb and  18 Ocarb and clumped isotope compositions (δ 47 and ∆47).We summarize the methods below, and details are available in the Supplementary Material (Supplementary Text S1).

Organic carbon isotope compositions
The carbon isotope compositions of organic matter in paleosol samples were analysed in the Stable Isotope 110 Laboratory of the Institute of Earth Surface Dynamics, University of Lausanne (IDYST-UNIL).Samples first underwent de-carbonatation with 10 % v/v HCl, then thoroughly washed with deionized water and dried at 40 °C for 48 h.The  13 Corg measurements were made using a Carlo Erba 1100 (Fisons Instruments, Milan, Italy) elemental analyser connected to a Thermo Fisher Scientific Delta V Plus isotope ratio mass spectrometer, both operated under continuous helium flow.Measured  13 C values were calibrated and normalized using international 115 reference materials and in-house standards [Spangenberg 2006[Spangenberg , 2016] ] and reported in per mil (‰) vs. Vienna Pee Dee Belemnite limestone standard (VPDB).The precision of the  13 Corg values were better than 0.1 ‰.

Carbonate isotope compositions
Carbon and oxygen isotope compositions ( 13 Ccarb,  18 Ocarb) were determined in the Stable Isotope Laboratory, IDYST-UNIL.Bulk paleosol samples containing >10 wt.% CaCO3, including carbonate samples, were analysed 120 using a Thermo Fisher Scientific Gas Bench II carbonate preparation device connected to a Delta V Plus isotope ratio mass spectrometer according to a method adapted after Spötl and Vennemann (2003).

Major elements
SiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5, Cr2O3, NiO, and loss on ignition (LOI), were measured in the powdered bulk paleosol samples by X-ray fluorescence (XRF; Phillips PANalytical PW2400 spectrometer) at the Institute of Earth Sciences of the University of Lausanne (ISTE-UNIL).The analyses were performed on 130 fused glass discs prepared with 1.2000 ± 0.0005 g ignited sample powder and 6.0000 ± 0.0005 g of lithium tetraborate (Li2B4O7).The concentrations of the major elements were expressed as wt.% oxides.The analytical precision (1 σ) assessed by replicate analyzed of international reference materials is 0.4 %.

Weathering indices
The chemical index of alteration (CIA, %) defined by Nesbitt and Young (1982) was used to quantify the degree 135 of weathering.CIA values were determined using the molar ratio of immobile Al2O3 and the mobile oxides CaO2, Na2O, and K2O in the silicate fraction (details in Text S1).

Mean annual precipitation
Mean annual precipitation (MAP, mm yr -1 ) was calculated using the equation proposed by Sheldon et al. (2002) (Text S1).This approach is based on an empirical relationship between the CIA and MAP of modern soils, and 140 while values are well-supported for soil types from different climatic settings, multiple proxies such as paleobotanical estimates must be used to confirm the results.Topography, chemical composition of parent material, diagenesis of older paleosols are some of the factors that may limit the application of this proxy [Sheldon et al., 2009].

Clay mineralogy
Clay mineralogical assemblages in paleosol samples were determined by X-ray diffractometry (XRD) at the ISTE-UNIL laboratories.Samples were prepared following the procedure described in Adatte et al. (1996).Analyses 160 were made using a Thermo Fisher Scientific ARL X-TRA diffractometer and the intensities of the XRD peaks characteristic for each mineral, were used to estimate the relative percentage of clay minerals in the < 2 µm and 2-16 µm size fractions.

δ 13 C of bulk paleosol organic matter
The  13 Corg values of the paleosols have a range between -26.0 and -20.4 ‰ with an average value of -23.2 ‰ (Fig. 3).A negative CIE is marked by a 3 ‰ shift from the base of the section (from 0 to 30 m), where the onset begins, followed by a plateau of low values (30-50 m) that gradually return to higher values 60 m upwards.Such a large magnitude of the excursion in  13 Corg was previously identified for the PETM within the intermontane 170 Piceance Creek Basin of western Colorado (USA), where a negative CIE of about 3 ‰ was reported [Foreman et al., 2012], while the  13 Corg record from the Middle Eocene Alano di Piave section deposited in the marginal Tethys Ocean showed a negative CIE of about 1 ‰ [Spofforth et al., 2010].Nevertheless,  13 Corg patterns are similar to the trends observed in the benthic foraminiferal carbon stable isotope record ( 13 Ccib) from ODP sites 738 (Fig. 3), indicating a general agreement in the change of  13 C values, even though absolute differences in the 175 magnitude of excursions exist.The presence of a synchronous negative CIE therefore suggests that the MECO can be recognized in the continental Escanilla Fm.  195 Vegetation plays a key role in influencing landscape response to terrestrial hydroclimates during global warming events such as the PETM [Foreman et al., 2012].For instance, de-vegetated fluvial banks enhance sediment erodibility, bank erosion, and lateral channel mobility, resulting in peak discharge and bedload sediment downstream flux even under a negligible increase in precipitation [Gran et al., 2001;Barefoot et al., 2021].Sparse vegetation in the fluvial Escanilla Fm reflects the response of the Escanilla rivers to peak discharge and sediment 200 flux events, which modified the stratigraphic architecture described as alternating sequences of high amalgamation (HA) and low amalgamation (LA) intervals [Sharma et al., 2023] The δ 13 Ccarb values in paleosol bulk carbonates have a range from -2.6 to -1.0 ‰ with an average of -1.6 ‰ (Fig. 4).A transient decrease in  13 Ccarb values is observed at 40 m where a negative CIE of about 1.0 ‰ magnitude is considered to represent the MECO negative CIE in the Escanilla Fm, followed by an increase in  13 Ccarb values to -1.5 ‰ towards the top of the section (Fig. 4).These values are synchronous to the negative CIE recorded in paleosol organic matter, although a difference in magnitude is apparent.Such differences in magnitude do exist 215 between CIE from different terrestrial archives [e.g., Gallagher et al., 2019].For instance, the magnitude of negative CIE during the PETM is on average greater in paleosol carbonates by 2 ‰ than in preserved paleosol organic matter [Cotton et al., 2015;Gallagher et al., 2019].However,  13 C records from the Escanilla Fm show a contrasting trend, likely indicating a difference in the soil carbon cycling between the MECO and the PETM, which requires further research.Additionally, the largest magnitude of excursion recorded by  13 Corg (Fig. 3) https://doi.org/10.5194/egusphere-2023-894Preprint.Discussion started: 24 May 2023 c Author(s) 2023.CC BY 4.0 License.perhaps also suggests that the organic carbon isotope record is a more sensitive proxy for the MECO, for reasons that are beyond the scope of this work.
13 Ccarb values in carbonates (stromatolites and pedogenic nodules) have a range from -4.9 ‰ to -1.1 ‰ with an average value of -2.7 ‰.A negative trend 20 m onwards culminates just above the OC at 55 m and likely represents the MECO negative CIE in stromatolites and pedogenic nodule samples (Fig. 4).Further,  13   The  18 Ocarb values in paleosol bulk carbonates have a range between -6.7 and -4.2 ‰ with an average of -5.8 ‰ (Fig. 5).A positive OIE of about 0.5 ‰ magnitude, suggestive of an increase in freshwater 18 O content and hence perhaps warming, occurs at the 40 to 60 m interval and likely represents peak MECO conditions in the Escanilla 240 Fm.Peak warming also corresponds to the OC where highest discharge and flux estimates have been predicted [Sharma et al., 2023].Following the positive OIE,  18 Ocarb values decline and return to relatively stable values of around -6.0 ‰, 60 m onwards until the top of the Olson section, and may represent the post-MECO cooling phase (Fig. 5).
The δ 18 Ocarb values in stromatolites and pedogenic nodules have a range from -8.1 to -4.7 ‰ with an average value 245 of -7.1 ‰; they crudely match the δ 18 O-trend in paleosol bulk carbonates.An increase in  18 Ocarb occurs above the OC at 60 m and most probably represent also the peak MECO warming (Fig. 4).A large spread in replicate measurements of some samples could be due to heterogeneities in carbonate composition within the same sample.In summary, stable isotope data from the Escanilla Fm at Olson is compatible with climate perturbations through excursions similar to the isotope excursions in the marine records, even though there are differences in the 255 magnitude of excursions.These excursions have also been identified downstream in the time-equivalent marine sediments in the Jaca Basin, Spain [Peris Cabré et al., 2023].This indicates the preservation of MECO climate perturbation signals in the source-to-sink Escanilla sediment routing system.Furthermore, irrespective of the presence of authigenic and detrital carbonates in paleosol samples, the CIE of organic matter suggests that the MECO can be regionally recognized in the Escanilla Fm. 260

Weathering conditions
CIA values vary between 9 and 43 % (average of 19 %) and remain almost constant throughout the section.A limited peak in CIA values may represents peak weathering conditions during the MECO (Fig. 6).However, such low values are broadly indicative of weak chemical weathering, and if true, suggest a prevalence of physical 265 weathering and erosion during the Middle Eocene.This contrasts with the assumption of intense weathering trends during global warming and recently published results for earlier hyperthermals [Tanaka et al., 2022].For instance, during the PETM, CIA values have been estimated to be in the 75-85 % range [Stokke et al., 2021].These relatively low CIA values could be related to the long-term trend of low silicate weathering in response to elevated pCO2 and warming levels during the Middle Eocene, as indicated by Os isotopes [van der Ploeg et al., 2018].

270
Current understanding of the Earth's carbon cycle suggests strengthening of the negative silicate weathering feedback in response to rising atmospheric pCO2 [Colbourn et al., 2015;Penman et al., 2020].However, the strength of the feedback depends primarily on several local/regional environmental variables such as temperature and precipitation, in addition to geomorphology and lithology [e.g., Richey et al., 2020;Deng et al., 2022].Our estimated weathering intensities could thus be indicative of a local dry and arid climate with a weakened 275 hydrologic system in northern continental Spain during the Middle Eocene.Such low weathering rates, if confirmed at a scale relevant to the global carbon cycle, could also explain the sustained elevated carbon levels for a longer duration than during the previous hyperthermals, highlighting the different dynamics of the MECO [Sluijs et al., 2013;van der Ploeg et al., 2018].

285
MAP values in the Olson section range from 270 to 570 mm yr -1 with an average of 330 ± 182 mm yr -1 .Values stay constant at 300 mm yr -1 until 40 m followed by a small increase in precipitation up to about 370 mm yr -1 that corresponds to the OC (peak MECO conditions), before returning to an average value of 340 mm yr -1 until the top of the section (Fig. 7).Overall, these values consistently predict arid (semi-arid) conditions in this area of the southern Pyrenees during the Middle Eocene and are coherent with the high  13 Corg values measured in our section 290 and linked with water-stressed environments.In other localities of the Ebro Basin, in northeastern Spain, climate interpretations based on palynological, pollen taxa and floral diversity studies [Cavagnetto and Anadón 1996;Haseldonckx, 1972] suggest warm climate and humid vegetation, with preservation for instance of mangrove swamp vegetation along the coast.The absence of evidence of humid climate in Olson could be due to its inland https://doi.org/10.5194/egusphere-2023-894Preprint.Discussion started: 24 May 2023 c Author(s) 2023.CC BY 4.0 License.location, away from the coastline in a better drained floodplain setting.Such regional differences in climate could 295 also be the result of a climate transition phase during the Middle Eocene, oscillating from a warm tropical Early Eocene to a cold and arid Early Oligocene, expressed differently in different regions and at possibly different sampled intervals.
Interestingly, arid conditions, based on palynological analysis, have also been identified in the Xining Basin in NW China at ~40 Ma [Bosboom et al., 2014].However, this has been linked to post-MECO cooling rather than 300 transient warming during the MECO.MAP estimates reported in this study are considerably lower when compared to Middle Eocene contemporary bio-climate analysis of south-eastern Australian flora, which predict MAP values between 1530 and 2370 mm yr -1 , consistent with modern rainforest climates [Greenwood et al., 2003], while MAP estimates from continental central Europe are in the range of 1100 to 1400 mm yr -1 [Mosbrugger et al., 2005].
Other proxy data from southern France indicate a MAP less than 500 mm yr -1 in the Bartonian [Kocsis et al., 305 2014], and is similar to our calculation from Spain.Our estimates are also comparable to modern-day MAP values of northern Spain, which range from 400 to 1000 mm yr -1 [Hijmans et al., 2005;Huyghe et al., 2018].Considering the high Ca content (> 20 wt.%) in sampled paleosols, our MAP values possibly underestimate true precipitation values due to an artificial reduction in the ratio of Al to other metals when calculating the chemical index of weathering (CIW, Text S1) [Stein et al., 2015].Nevertheless, this does not explain the regional disparities in 310 precipitation regimes during the Middle Eocene, and values reported here should be regarded as representative of a local regional signal, most likely influenced by the ongoing Pyrenean orogeny and the resulting orographic (rainshadow) effects [Vacherat et al., 2017;Huyghe et al., 2018], similar to the modern situation.For each sample, individual values are presented along with the respective mean value ± standard error (SE).We compare our terrestrial temperature estimates to sea surface temperatures (TEX86 and U K 37) having a 3-point moving average (Bijl et al., 2010).to 43 °C (Fig. 7).Each stromatolite was analyzed 1 to 3 times for replicate measurements.Standard error for replicate measurements range from 0.006 to 0.011 ‰.Δ 47 values of carbonate in pedogenic nodules have a range 325 from 0.629 to 0.704 ‰, translating into temperatures of 18 to 45 °C (Fig. 7).Each pedogenic nodule was analyzed 4 to 8 times for replicate measurements having standard error ranging from 0.006 to 0.009 ‰.It is, however, important to note that while measurement reproducibility was good, care must be taken while interpreting Δ 47 data as a number of potentially significant uncertainties are associated with it.For instance, Δ 47 temperatures may not necessarily reflect primary formation temperatures but could instead be the result of a combination of primary 330 formation temperatures and secondary effects such as potential diagenetic temperatures that bias primary https://doi.org/10.5194/egusphere-2023-894Preprint.Discussion started: 24 May 2023 c Author(s) 2023.CC BY 4.0 License.compositions, although secondary overprinting is unlikely to produce cooler temperatures [Hren and Sheldon 2019].A large(r) spread recorded in a few samples could be likely due to inhomogeneity in the analyzed sample powder.Furthermore, peak temperatures are found only within 25 m of the Olson section without any change in lithology.In addition, a similar temperature range in both stromatolites and pedogenic nodules further suggests 335 that analyzed samples most likely did not undergo significant diagenetic alteration after their formation.
Mean temperatures vary from 32.1 ± 1.8 °C to 38.6 ± 0.72 °C in the lower half of the section until 50 m followed by an increase just above the OC with a maximum mean temperature of 42 °C at 80 m. 90 m upwards until the top of the section, values return to an average value of 30 °C.Available magnetostratigraphic age constrains for the Escanilla Fm and its linear correlation to the GPTS, allow the identification of a potential lag between marine 340 and terrestrial MECO climate signals.Interestingly, recorded peak temperatures occur about 400 Kyr after peak MECO warming in the marine realm, suggesting, if our estimates are correct, a significant lag of peak temperature on the continents with respect to oceans.Soil carbonates typically form during warm and dry periods and consequently have a strong summer seasonal bias in T (Δ 47 ) values [Quade et al., 2013;Burgener et al., 2016;Huth et al., 2019].As a result, Δ 47 temperature from pedogenic nodules may be biased towards warm month mean 345 temperatures (WMMT) and on average exceed mean annual temperatures by 10 to 15 °C [Quade et al., 2013].
Δ 47 temperatures from the Escanilla Fm also suggest a land-sea temperature gradient of 5 to 10 °C when compared to different sea surface temperature (SST) records, that is from ODP site 1172 (Tasmania, Pacific;Bijl et al., 2010) and IODP sites U1408 and U1410 (northwest Atlantic Ocean; van der Ploeg et al., 2023) indicating an amplifying effect due to continentality and change in the season of carbonate growth.Similar continental 350 temperature sensitivity was previously also identified through clumped temperatures of pedogenic carbonates in the continental interiors of SW Montana, USA [Methner et al., 2016].
Δ 47 temperatures were further used to calculate the  18 O of fluids in equilibrium with carbonates using the temperature dependent fractionation factor of Epstein et al., (1953).Δ 47 temperatures and  18 Ocarb values give water δ 18 O values of -6.5 to -0.8 ‰ (average of -3.5 ‰) for T = 25 °C (see Text S1), which although consistent 355 with a range of fresh meteoric water at low latitude, are still on the higher end, most likely signifying 18 O-isotope enrichment due to excess evapotranspiration under arid climatic conditions, consistent with estimates from other proxies used in this study.Clay mineral assemblages in paleosols are also important paleoclimatic indicators and provide integrated records of the overall climate [Singer, 1984].Smectite, palygorskite, illite, and chlorite form up to 98 % of the identified mineral assemblages in the studied section (Fig. S1; supplementary material).Smectite, which typically forms under seasonal rainfall conditions with a pronounced dry season [e.g., Singer, 1984;Tabor et al., 2014], constitutes on average 17 % of the total identified clay mineral assemblage while individual values range up to 58 %.

365
Palygorskite often indicates an arid and semi-arid environment where evapotranspiration exceeds precipitation [e.g., Birkeland, 1984;Singer, 2002;Meunier, 2005], and constitutes up to 25 % (average of 10 %) of the total clay mineral assemblage.The presence of palygorskite together with smectite suggests a hot climate with some rain intervals [Xie et al., 2013;Luiz da Silva et al., 2018] and thus corroborates our low MAP and clumped isotope temperature values.Illite content is between 11 and 53 % (average of 36 %) while chlorite content is between 7 370 to 64 % (average of 35 %) in the analyzed paleosols.High amounts of illite and chlorite are typically found in sediments formed by physical weathering and are thus indicative of weak, incipient chemical weathering [Tabor et al., 2014], and support our calculated CIA values.

Figure 1
Figure 1 (A) Field image depicting the sampled Escanilla Fm. (B) Lithostratigraphic framework of the Escanilla Fm at Olson 85

Figure 3 .
Figure 3. Dispersed organic carbon isotope compositions ( 13 Corg) in paleosol samples (white circles) presented with a 7-point moving average for the change in benthic foraminiferal (genus Cibicidoides) carbon isotope record ( 13 CCib) from ODP site 180 . Such large flux in the Escanilla Fm at Olson during the MECO is also consistent with the clastic progradation event observed in the Jaca Basin [Peris Cabré et al., 2023], indicating the transmission and preservation of sedimentary flux signals in the sedimentary record from source-to-sink.Lastly, enhanced channel mobility during the MECO could also have lead to the export of large 205 quantities of fine-grained (floodplain) sediment to the marine domain and potentially affected the carbonate environments developing at that time on the southern Margin of the Jaca-Pamplona trough (e.g.Garcès et al., 2023).4.2 δ 13 C and δ 18 O of bulk paleosol carbonates 210 Ccarb 225 values from stromatolites and pedogenic nodules show a consistent 1 to 2 ‰ negative offset when compared to values from paleosol bulk carbonates which can be explained by the presence of detrital Mesozoic carbonates ( 13 Ccarb = 0 ‰; Zamarreno et al., 1997] in the bulk sediments from the source area.Since bulk soil composition consists of both pedogenic and detrital components, we estimate 60 % pedogenic origin of the carbonates in the Escanilla paleosols assuming normal paleosol values of -1.5 ‰ and that stromatolites and pedogenic nodules 230 represent endmembers having a  13 Ccarb value of -2.5 ‰ (by simple mass balance where the bulk soil composition = pedogenic proportion × pedogenic composition + detrital proportion × detrital composition).

Figure 4 .
Figure 4. Carbon isotope compositions ( 13 Ccarb) from paleosol bulk carbonates (white circles) with a 7-point moving average and from stromatolite and pedogenic nodules (grey circles) for the Olson section.Also marked is the MECO onset and peak 235

Figure 6 .
Figure 6.Chemical index of alteration (CIA) values from paleosols (white circles) with a 7-point moving average to quantify weathering in the Escanilla Fm at Olson.

Figure 7 .
Figure 7. Mean annual precipitation (MAP) values (white circles) from the Olson section are presented by a 7-point moving average, followed by clumped isotope (Δ 47 ) temperatures from stromatolites (grey circles) and pedogenic nodules (grey triangles).For each sample, individual values are presented along with the respective mean value ± standard error (SE).We of carbonate in stromatolites have a range from 0.634 to 0.704 ‰, translating into temperatures of 18 https://doi.org/10.5194/egusphere-2023-894Preprint.Discussion started: 24 May 2023 c Author(s) 2023.CC BY 4.0 License.
mineralogical analysis of paleosols, stromatolites, and pedogenic nodules, provide new insights into terrestrial records of the MECO in the Ainsa Basin of the southern Pyrenees, Spain.A negative CIE measured for organic matter indicates that the MECO is recognized in the fluvial Escanilla Fm, further supported by the presence of carbon and oxygen isotope excursions in whole rock carbonates, nodules and stromatolites, demonstrating that continental sedimentary successions can also serve as climate archives, and highlighting stable 380 isotope proxies as a powerful dating and correlation tool in notably difficult to date fluvial successions.Low CIA values in the Escanilla Formation suggest a poor silicate weathering feedback to elevated CO2 levels and a prevalence of physical erosion during the Middle Eocene.This is compatible with an arid climate, i.e., a locally diminished hydrological cycle, supported by MAP estimates and clay mineral assemblages in the fluvial sedimentary succession.Carbonate clumped isotope thermometry suggests high temperatures with a possible ~ 385 400 Kyr lag in peak temperatures with respect to the MECO timing in the oceanic domain, and an amplifying effect of 10 to 15 °C (for summer temperatures) on continents compared to temperature records from the deepsea.References https://doi.org/10.5194/egusphere-2023-894Preprint.Discussion started: 24 May 2023 c Author(s) 2023.CC BY 4.0 License.