In this manuscript, the authors analyze the lithium and neodymium isotopes of two sections from the southern Pyrenees, deposited during the PETM event (~ 56 Ma ago). They report a positive excursion of ~0.8‰ in the clay-size fraction lithium isotopic ratios during the PETM in both sections, while the neodymium isotope data indicate stable sediment provenance. They suggest the positive excursions in lithium isotopes reflect a decline in weathering intensity and congruency, potentially linked to the enhanced formation of secondary clays during continental weathering. To better understand how Earth’s climate regulates itself, it is essential to unravel the linkages among denudation, weathering and climate. I believe this paper improves our understanding of chemical weathering at hyperthermal events. The language of this manuscript is well-revised and clear enough, and the data broadly support the authors' interpretations. However, several sections of the manuscript raise important questions or require clarification, as outlined below. Overall, I recommend to accept this manuscript after minor revisions.

First of all, the authors propose increased denudation and weathering flux during the PETM, while also suggesting that weathering intensity (W/D) declined (e.g. Lines 525-533). I am wondering are there any direct geological or sedimentological evidences for increased erosion or tectonic uplift in the southern Pyrenees during the PETM? Because in the absence of uplift, one would expect weathering intensity to rise with increasing temperature and precipitation.

Second, the manuscript suggests that increased kaolinite abundance reflects enhanced secondary clay formation (Lines 547–549). However, kaolinite abundance alone may not reliably indicate the total formation flux of secondary clays, as it does not account for other clay mineral phases. I think it’s not easy to determine the total formation flux of secondary clays, but one can consider an extreme case: the observed kaolinite enrichment could also result from the dissolution of other clays.

Third, several studies (e.g., Pistiner & Henderson, 2003; Golla et al., 2021) suggest that lithium isotope fractionation is mineral-dependent. Since the authors already present clay mineralogy data, it would strengthen the paper to discuss how variations in mineral assemblages might influence the lithium isotopic fractionation factor (Δ⁷Li_water-clay) and, consequently, the observed δ⁷Li values.

Finally, the manuscript assumes that δ⁷Li variations in the clay-size fraction reflect continental weathering processes exclusively, with negligible contribution from marine authigenic alumino-silicate clays. However, an increased fraction of marine authigenic alumino-silicate clays during the PETM could also elevate the clay δ⁷Li values. Could the authors provide some additional constraints to rule out this possibility?

Below are some minor issues:

In Lines 111-112, the term “weathering efficiency” is ambiguous. I guess this refers to the CO₂ consumption flux as suggested by Bufe et al., 2024. It would be better if the authors can make this clearer.

In Lines 115-116, How can weathering rates increase with erosion under kinetically-limited regime? I suppose this is only the case for supply-limited regimes.

In Lines 493-495, the authors suggest enhanced clay formation in lowlands during the PETM. Is there evidence to support this? Could increased runoff instead dilute solute concentrations, thereby suppressing clay precipitation? Also, is it realistic to assume that enhanced clay formation in lowlands could compensate for reduced formation in uplands?

In Lines 517-522, I might be wrong, but higher temperatures during the PETM would likely reduce the lithium isotope fractionation (i.e. decrease Δ⁷Li_water-clay) during clay formation. The secondary clay formed at this warm period should have a closer δ⁷Li to the starting solution (or river water) (δ⁷Li_{secondary-clay} = δ⁷Li_water – Δ⁷Li_water-clay). Assuming a constant δ⁷Li value of river water (δ⁷Li_water) during the period of interest, the clay formed at syn-PETM should have higher δ⁷Li values, rather than lower values.

In Figure 1C, there are three different blue areas (from light to dark), but only two of them are explained in the legend. Additionally, the meaning of the white-striped area in panel B is unclear and should be clarified.

General comments:

This manuscript reports new geochemical (δ⁷Li, εNd, δ¹³C) and mineralogical data on two well-preserved Palaeocene-Eocene sedimentary floodplain sections in the Pyrenees. The objective is to reconstruct the regional changes in chemical weathering and physical erosion during the PETM warming event and the aftermath to characterize the response of the Earth system to warming events. The authors found a positive δ⁷Li excursion in the clay fraction of both sections during the main body of the PETM. They interpret these results as reflecting a decrease of the weathering intensity (higher chemical erosion and even higher physical erosion) during the PETM because of the intensification of the hydrological cycle and a more extensive clay formation in the floodplains.

Overall, this is a neat study and a valuable scientific contribution to the research field on the PETM. It is well written, the data are of high quality, and the interpretation and arguments are convincing. There are only a few aspects of the manuscript that should be improved, in particular the presentation of the interpretative framework and the implications, before final acceptance. My recommendation is publication after minor revisions. The main revisions I suggest are:

• Clarify the interpretative framework of Li isotopes in detrital sediments (section 1.2). In my opinion, some statements in this section are not in accordance with results from published studies (see more details below).
• Discuss in more details the comparison and implications with other Li isotopes PETM records. I suggest moving the paragraph lines 464-473 after discussing the results from the Pyrenees and modifying it to compare directly the results from various records and places. Why a positive excursion in the Pyrenees and a negative excursion in other settings? Is it because of the different geomorphic environment (deep regolith and transport-limited weathering in the pyrenees)? How does it compare with the other studied floodplain section, in the Bigorn basin (Ramos et al., 2022)?
• Discuss in more detail the reasons for the very low δ⁷Li values in the Esplugafreda section during the recovery period. This does not appear to be a simple return to initial conditions, but rather a further increase in weathering intensity (lower δ⁷Li) associated with regolith deepening and soil shielding relative to pre-PETM conditions. If this is the case, what are the implications for CO₂ removal through silicate weathering, given that such highly weathered settings are characterized by a weak weathering response and therefore act as inefficient sinks for atmospheric CO₂ (Penman et al., 2020)? The timescale of the PETM recovery and the role of silicate weathering remain debated, and this study could provide useful new constraints.
• Add a figure with the average values (δ⁷Li, εNd, δ¹³C, mineralogy) during each period (pre, POE, onset, body and recovery) for both sections (on the same figure). This would really help the reader to have a global overview of the main results and evolution through time.

In addition, the potential influence of marine clay formation (reverse weathering) on Li isotopes is not mentioned or discussed. Do you have any evidence to dismiss it for the Rin section? The study from Zhang et al., (2022) on the Ainsa paleo-delta shows that marine clay formation results in lowering of the δ⁷Li of sediments.

Specific comments:

- Line 111: define « weathering efficiency ». Note that this hypothesis of maximum weathering rate at moderate erosion rate (the “speed limit”) proposed by some authors (Dixon and von Blanckenburg, 2012; Gabet and Mudd, 2009) has been challenged by others (e.g. Larsen et al., 2014; West, 2012).

- Lines 139-145: This does not appear to be consistent with findings reported in previous studies. At high W/D both clay formation and clay dissolution processes are taking place, either in different part of the weathering profile (top regolith vs deep regolith; e.g. Chapela Lara et al., 2022; Clergue et al., 2015) or different part of the catchment (e.g. see figure 10 in Dellinger et al., 2015 ; figure 3 in Henchiri et al., 2016; Fries et al., 2019). Therefore, at high W/D, rivers do transport modern neoformed clays from weathering profiles, and hence there is NO evidence for “minimal clay neoformation” (on the contrary).

- Lines 150-154: what is the evidence for “fewer of the coarser-grained primary minerals carried in rivers are expected to be transported into offshore sites, the clay weathering signal may generally be recorded more clearly in marine sites”? I see several problems here:

First, the Dellinger et al., 2017 relationship has been observed for fine sediments (< 63 microns) not “coarse-grained primary minerals”.

Second, several studies on the Yangtze estuary (Yang et al., 2021, 2025) show the opposite sediment transport process to the one suggested is suggested by the authors, i.e. preferential transport of coarse-grained primary minerals offshore relative to clay minerals (“the offshore transport of SPM in the Changjiang Estuary may result in the preferential flocculation and deposition of clay minerals during the flooding season, while primary minerals or other fine-grained particles tend to be resuspended and carried seaward by currents” (Yang et al., 2021)).

Third, even if there are fewer primary minerals in the <2 μm fraction relative to the <63 μm fraction, primary clays and other minerals (feldspar, micas) are still present in the <2 μm fraction (e.g. Liu et al., 2025)

I suggest revision of the presented interpretation framework (Fig. 2) in accordance with existing publications.

- Section 4.1: is there any correlation between δ⁷Li and the mineralogy of clays? I think this should be discussed somewhere in the manuscript since different clays could have distinct fractionation factor during process of Li incorporation or adsorption into clays.

- Section 4.3: were the Li concentration measured on those samples? Any weathering-related changes in Li/Ti or Li/Al in both sections?

- Lines 494-495: see also all the studies that report increase dissolved Li isotope composition and clay formation in rivers when crossing large floodplains (e.g. Bagard et al., 2015; Dellinger et al., 2015; Maffre et al., 2020; Pogge von Strandmann et al., 2017; Pogge von Strandmann and Henderson, 2015). The proposed interpretation here, i.e. “the shift towards increased incongruent weathering, characterized by enhanced clay formation in the floodplain deposits” is consistent with all these studies on modern rivers with floodplains.

- Lines 497-499: see also the study from Xu et al., (2021)

- Lies 502-504: This is not clear to me, why faster runoff results in a positive δ⁷Li excursion in the clays?

- Figure 2 & 3: could be combined (panel A and Panel B)

- Figure 4: I suggest indicating the different periods (pre, POE, onset, body and recovery) on the right of the figure to help the reader not so familiar with the PETM timing and evolution.

References:

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Chapela Lara, M., Buss, H.L., Henehan, M.J., Schuessler, J.A., McDowell, W.H., 2022. Secondary Minerals Drive Extreme Lithium Isotope Fractionation During Tropical Weathering. Journal of Geophysical Research: Earth Surface 127, e2021JF006366. https://doi.org/10.1029/2021JF006366

Clergue, C., Dellinger, M., Buss, H.L., Gaillardet, J., Benedetti, M.F., Dessert, C., 2015. Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles). Chemical Geology 414, 28–41. https://doi.org/10.1016/j.chemgeo.2015.08.015

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Fries, D.M., James, R.H., Dessert, C., Bouchez, J., Beaumais, A., Pearce, C.R., 2019. The response of Li and Mg isotopes to rain events in a highly-weathered catchment. Chemical Geology 519, 68–82. https://doi.org/10.1016/j.chemgeo.2019.04.023

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