Synopsis

Using two new isotope and stratigraphic sections in the west-central Pyrenees, Cabré et al study the connection between the Middle Eocene Climatic Optimum (MECO) and sediment supply. Working in the Tremp-Jaca Basin, the authors collected geochemical and sedimentological data from two stratigraphic sections. These sections target two separate deltas, which were part of a common depositional system that deepened westward, created by subsidence in the Pyrenees foreland basin. The authors measured stable carbon and oxygen isotopes, trace and major elements, as well as organic matter composition and maturity. They synthesize these data into a new understanding of this basin during the MECO.

Their main findings are supported by two key observations: 

(1) The authors observe that the MECO coincides with two episodes of delta progradation in this basin. The authors assert that this is due to enhanced sediment supply due to changes in hydroclimate. They reject a suite of alternative hypotheses (enhanced uplifting, eustatic sea level, etc.)

(2) The authors observe that patterns in stable isotopes of oxygen from their stratigraphic sections parallel patterns in the global ocean and other basins, patterns of carbon isotopes do not. Based on this observation, they assert that because the deltas are prograding into a restricted ocean basin, the carbon isotope signature is dominated by local effects (provenance, local chemistry) rather than a global input of depleted carbon. It is not made clear why oxygen isotopes are not affected by these processes. The authors assert that diagenesis effects have not altered their samples substantially. 

Based on these main findings, the authors conclude that in the Iberian peninsula, climate-induced episodes of enhanced erosion are connected to transient warming and changes in hydroclimate. This bolsters similar findings from strata across the Pyrenees during the Eocene. The MECO is relatively long-lived (~ 500kyr) compared to its shorter counterpart hyperthermals during the EECO (e.g. PETM (~ 200kyr)). Given this, the authors suggest that the MECO may be a good test case for understanding feedbacks between the carbon cycle, hydrological cycle, and other Earth systems.

Overall Comments

I read this paper with interest, and a fair bit of initial skepticism. The authors have identified a good case study for an important problem. In many stratigraphic studies like this one, there are major issues of resolution and timing. This is particularly a problem with Eocene hyperthermals, where the hypothesized duration of a climate ``episode'' or ``event'' is brief enough (< 100kyr) that it can be distorted, disguised, or destroyed entirely by sediment transport processes in the sedimentary basin. On the other hand, if a climate episode is very long (> 1 Myr), then there are few terrestrial basins that can record the full length of the event without major issues of changing subsidence rates and basin filling, etc. This basin, and episode of interest, appears to lie in a kind of optimal middle ground, where the MECO is long-lived enough that one can be reasonably sure to capture the signal despite transport processes, and yet short enough given the basin size to ignore tectonic issues.

With the aid of this well-chosen field site, the authors have done a really nice job of constraining the problem. By leaning on a firm grounding in regional literature and collecting a comprehensive suite of data, the authors have persuaded me that their model is the most likely scenario. I found the observations to be appropriate, well-reasoned, and well-documented. The analysis of these observations is careful, and clearly articulated. 

Overall, I think this is a very well-executed study, it will have substantial appeal to readers of Climate of the Past across disciplines. I expect it may spark some increased interest in studying the MECO, and may serve a touchstone for motivating studies that join the best insights of sedimentary geology and paleoclimate. I have a few minor comments and questions for the manuscript, but after those have been addressed, I think it would make a good contribution to CP. 

For details, see the attached document that lists minor comments. The authors should feel free to reach out to me if they have any questions about my review.

Eric Barefoot

Title: Fluvio-deltaic record of increased sediment transport during Middle Eocene Climate Optimum (MECO), Southern Pyrenees, Spain

Authors: Cabré et al.

In this paper, the question of the MECO event is addressed by focusing on its preservation (potential diagenetic overimprint) in the South-Pyrenean Foreland Basin (SPFB). This peculiar environment offers a potential high-resolution climate record in a region characterized by its high sedimentation rate. Despite a few discrepancies concerning d¹³C_carbpeaks, major climate signals are preserved. By highlighting processes occurring in a foreland basin, this study provides a complementary approach for the MECO event to previous works only based on deep-sea records.

Scientific interest

In my opinion, the present manuscript is good. The paper is well written and considerable care has been taken to check that isotopic trends were not disturbed. Consequently this paper will be a useful resource for the community.

Recommendation: minor revisions

section 5. 2. The major issue concerns the lack of interpretations for d¹⁸O fluctuations (and to a lesser extent this comment can be also applied to the d¹³C_carb). Most of the time, this section only contains descriptions of isotopic trends without giving quantitative estimates about the MECO deduced from data presented in the section 4.

lines 467-469 and 497-499. To my understanding, one of the main findings of the MS seems to be the significant increase of erosion and sediment transport during the MECO. However the figure 11 clearly shows a sedimentation rate held constant through time (pre/syn/post-MECO). This point deserves more attention.

Lines 500-501. A such increase of the hydrological cycle during the MECO should, by its action on the weathering (and the alkalinity delivered), inhibit the ocean acidification process (or the authors assume that the acidification takes place in the early stage of the MECO before the enhancement of the hydrological cycle ?). Consequently the authors should reword some of their points mentioned as potential implications (section 5.5) to be consistent with their results.

Figure 3 (caption) Where are the grey lines mentioned in the caption ?

lines 366 and 368: please do not mix r and r²