Dear editor and author,

This paper is a solid work on the climate control on a Lower Jurassic hemipelagic succession in the Basque-Cantabrian Basin that contain interesting approaches to understand factors controlling its accumulation. Data, interpretations and discussion are very well organized (although some parts are not balanced: see comment 20; and the discussion is quite long and complex). Without a doubt, the paper deserves to be published. However, concerning descriptions (and related interpretations and discussions) four main aspects require to be deeply explained:

- hemipelagic character of the successions (see manly comments 1, 6);

- significance of color (see comments 10, 16, 18) and MS data (see comments 17, 18);

- criteria for definition of couplets (precession cycles) and bundles (eccentricity cycle) (see comment 15);

- characterization of the black shale package as a whole (see comments 7, 13, 26).

Other changes are suggested in order to state clear some concepts and descriptions.

Introduction

1. Pelagic rhythmites are presented as one of the key sedimentary successions recording orbital controlled climate changes (first paragraph). However, the studied succession is hemipelagic. It would be interesting to include: 1^st) a brief definition of the term hemipelagic in the context of the studied BCB; 2^nd) a brief explanation (and references) on the role of orbital-induce climate variations on this particular kind of sediments, compared to the pelagic ones.

Geological setting

1. Lines 83-84: “which connected the Boreal Sea with the southern Tethyan Ocean”. Better: “which connected the Boreal Sea with the northwestern Tethyan Ocean”.
2. Line 87: “source area was located in the semiarid belt”. What do you mean thin “source area”, emerged land?, shallow platform carbonate source area? Please, explain better.
3. Specify if the distribution of the humid and semi-arid zones was stable for the entire Early Jurassic.
4. Use in Fig. 1, Early/Lower Jurassic instead of Lias.
5. Line 104: “Pliensbachian (192.9–184.2 Ma) hemipelagic successions of the BCB..”. I suggest deleting the time duration: I suppose the studied succession has not been time-calibrated so accurately. The sedimentary environment of the successions requires a deep explanation. Notice the term “outer ramp” appears for the first time in the discussion (line 778). See also lines 677-679 “restricted paleogeographic setting”). Revise also lines 841-843 (“basins depleted in oxygen”: be careful, it sounds like a circular reasoning).
6. Line 106: use “packages of alternating black shales and limestones/marly limstones” instead of “black shale intervals”. it is important to state clear these black shales do not include only shales but also intercalated limestone/marly limestones. I think the word Interval has a time connotation.
7. Lines 130-132: “and 1 km north-west of a coeval section studied by others at the train station in the same locality…with which a bed by-bed correlation can be readily carried out.”. This sentence is more appropriate for the discussion (see also comment 26). In any case, it requires a deep explanation of how this correlation was made, without (I suppose) lateral continuity of outcrops.
8. Lines 132-137: Please state clearer the location and thickness of the studied succession. As far I understand the studied succession is 22.5 m thick and includes: the uppermost 2.5 m of the Puerto Pozazal Formation and the lowermost 20 m of the Camino Formation (including the first x-thick black shale package of this unit). However, in line 140 “30.40 m thick” is mentioned.

Materials and methods

1. The average color of samples is used for cyclostratigraphic (spectral) analysis. However, there is not any analysis to elucidate the sedimentary vs. diagenetic significance of this feature.
2. Thin sections are mentioned in results, but not included here.
3. Please, explain the lithology of the studied bundle and samples: line 167: fifty-seven samples, include also here the values of x samples/bed; line 177: central part of each bed, include here also the total number of samples.

Results

1. Lines 209-210: Concerning lithological terms, “limestones or marly limestones” and “marls or shales”. Do you have calcimetric analysis of the entire succession to differentiated these lithologies?. Concerning the term “shale”, please see previous comment 7. The black shale package has to be presented.
2. Description of lithologies and texture. In Fig. 2 (log), marly limestones of limestones with different texture are not drawn. I suggest to draw them. Also state clear the description of each lithology separately (also limestones and marly limestones; do they have bioturbation?) and then compare their main differences.
3. Lines 236-244 on couplets and bundles. This paragraph has to be separated in a subsection. The criteria for differentiating couplets are unclear: why the couples marl/shale to limestone/marly limestone (and not at the contrary)?; the “lithological contrast” for bundles is also very unclear (see also comment 13 on carbonate content of the entire succession). Do you see significant features at the boundaries of couplets or bundles or any trends withing couplets or bundles?.
4. Color trends: lines 244-258 “The variations in colour values are more significant in the central couplets of bundles than at bundle boundaries. This suggests that, as shown in previous studies… colour values are representative of the carbonate content of the samples.”. See previous comment 15 on “lithological contrast” for bundles (not well explained” and also comment 10 (significance of color). To use the similar trend in color and carbonate content in C35 to C44 as supporting criterion, it is necessary to discuss there was not a diagenetic imprint in both color and carbonate content.
5. Did you perform analysis of susceptibility-temperature (k-t) curves to know the type and abundance of magnetic minerals? The following sentence is not clear (as far I understand you interpret the presence of ferromagnetic minerals indirectly): Lines 264 “The MS of hemipelagic deposits is commonly determined by their paramagnetic components (mostly detrital clays; Kodama and Hinnov, 2015). However, in Santiurde this parameter does not show a great correlation with colour (r: 0.48, p<0.001, all section; Fig. S1) or calcium carbonate (r: 0.36, p<0.001, between C35 and C44; Fig. S1). Therefore, the Santiurde relationship suggests that the MS signal is more likely controlled by ferromagnetic minerals, such as magnetite (Fig. S2).” Revise also lines 750-755.
6. Spectral analysis of MS data (lines 283-285). MS data do not correlate with color and carbonate content; however, their spectral analysis corroborate the results of the spectra analysis of color. Please, explain this apparent contradiction.
7. Lines 310-311. “In general, %CaCO3 fluctuates in line with lithology, limestones and marly limestones (average: 66.36%) being richer than marls and shales (average: 34.86%).”. What do you mean? In fact, carbonate content is the criterion to differentiate these lithologies.
8. In 4.2. Detailed analysis of Bundle 9 (C35-C44 interval), pure descriptions are included in 4.2.1 to 4.2.4; however, 4.2.5 and 4.2.6 contain interpretation/discussion of the results, including the interpretation of oxic/anoxic conditions of the different lithologies (without any reference to the other results). This imbalance should be corrected.

Discussion

1. Line 459. “Origin of inorganic sedimentary fluctuations”. I suggest deleting “inorganic”. This term is obscure.
2. Lines 470-472 (secondary cements..), line 474 (bed geometry): these descriptions should be explained also in Results.
3. Lines 476-478. “Quite the opposite, the characteristics of the beds are continuous for more than 1 km between the Santiurde motorway and railway sections”. See comment 8.
4. Lines 485-487: “In general, the diagenetic characteristics observed in the Santiurde rhytmites are typical of processes related to organic matter decay during burial (Rosales et al., 2001). This sentence is not informative. Please explain in which way.
5. Lines 488-493 about periodicities. Do you have data on the time span of the studied succession to compare with your results? I would be interesting to know how many cycles are then represented in the entire succession and BS package.
6. The discussion lacks a proper explanation of the BS package as a whole (how many precession or eccentricity cycles includes, what short- and long-term factors controlled its accumulation.

Regards,

Beatriz Bádenas

Review Martinez-Braceras, CP, 2024

The manuscript on ‘Orbitally forced environmental changes during the accumulation of a Pliensbachian (Lower Jurassic) black shale in northern Iberia’ by Martinez-Braceras and co-authors investigates factors driving sedimentary rhythms in a Pliensbachian black shale interval. This is a timely approach, as sedimentary rhythms in Mesozoic successions are commonly used to construct astronomically tuned time scales, although the exact mechanisms driving lithological alternations (especially on the precessional scale) are often insufficiently understood. The authors present a multi-proxy study to discuss a suite of processes in detail, shedding light on the periodic nature of regional anoxia that resulted in the deposition of organic matter. This is a very thorough study and merits publication in Climate of the Past if some minor points can be addressed.

As the stratigraphic interval has been studied previously in the same region, it would be relevant to report any independent age information. This would include biostratigraphic, magnetostratigraphic and chemostratigraphic events, and if available (correlation to) radioisotopic ages. If no age information is available, the authors should make it clear that the interpretation of orbital forcing of the sedimentary rhythms is based solely on the cycle hierarchy.

The photograph of the section (Fig 2) shows very clear banding patterns, with individual lithological alternations varying in intensity and showing grouping in bundles. The time series analyses of the colour signal show periodicities at 6.6, 1.67, 1 and 0.37 m, where the ‘intermediate’ periodicities have some peculiarities. The periodicity at 1.6 m is not very strongly present in the time series analysis, whereas it is prominent in the log and view of the section. The periodicity at 1 m seems a bit different from what would be expected with a cycle hierarchy of eccentricity-modulated precession and obliquity (20:5:2:1). The longest periodicity that is strongly present in the spectral results likely reflects the influence of 405 kyr cycles, but its expression in the section is not clear. It would be good if the authors could comment on the reasons for the seeming discrepancies in the lithological patterns and the spectral analysis result.

A related point is that the periodicities detected through time series analyses are consistently shorter than those observed. The number of interpreted bundles (14) in a 31 m interval suggest that the imprint of short eccentricity actually resulted in a 2.2 m cycle rather than a 1.6 m one. The 6.6 m periodicity has a much stronger peak in the spectrum but based on the number of individual alternations (62), it is only present 3 times in the studied section, and has a length of approx. 10 m rather than 6.6 m. The individual alternations have an average thickness of 31/62=0.5 m rather than 0.37 m. I do not understand the origin of this discrepancy. It has been observed that the highest amplitude cycles may have higher sedimentation rates (in the absence of dissolution) and these thicker couplets may dominate the time series analyses results. But here, the time series results indicate shorter periodicities. I would recommend the authors to evaluate other power spectra methods, to see whether these give similar results, and to report on the imposed settings more elaborately. Also, it would be interesting to generate a power spectrum for CaCO3 in the studied high-resolution interval, to see whether the statistically identified periodicity driving the limestone marl alternations corresponds to the observed thickness of the alternations.

It would be good to indicate how couplets and eccentricity bundles are defined here precisely, along the lines of: ‘The term couplet, as used here, refers to a lithological alternation, consisting of a resistant limestone bed with a more weathered marl or shale bed, starting at the base of the marl or shale. These couplets vary in their amount of lithological contrast between the marl/shale and the limestone. The variations in lithological contrast result in a grouping into bundles of five (four to six) couplets, counting from the base of the lightest coloured marls, reflecting the least lithological contrast with their bounding limestones.’

The L/M ratio being close to 1 is taken as indication that carbonate productivity and dilution varied hand in hand. Besides this ratio, it would be interesting to plot the thickness of the couplets and the thickness of the individual beds, to see whether for example thicker couplets coincide with thicker limestones or not.

A range of geochemical methods is applied to carefully investigate the factors controlling the production and preservation of organic matter. Changes in P-EF seem to suggest elevated productivity in the dark levels, but this is contrasted by δ15Norg, δ13Corg and Ba-EF, which are explained to suggest lower productivity. I wonder if the authors can comment on whether, instead of increased productivity, enhanced preservation would be sufficient to explain the observed patterns.

The δ13C changes are addressed in many parts of the manuscript, and perhaps the readability would benefit from grouping all information about δ13C together, or a paragraph summarizing it.

Minor points

L 21: change ‘involved processes’ to ‘processes involved’

L 22: change ‘The study’ to ‘This study’

L 23: change ‘black shales’ to ‘black shale intervals’, change ‘revealed’ to ‘reveals’

L 25: the phrase ‘with the prevalence of precession, short eccentricity and long eccentricity cycles’ could be replaced by something a like: ‘and were likely driven by eccentricity-modulated precession’ to be more precise

L 32: the comma has to be deleted to understand what the active verbs are in the sentence

L 34: change waters to water

L 36: change maximum to maximal

L 37: typo in diminished

L 38: delete seawater

L 39: add and before contributed

L 40: change exportation to export

L 43: change seawaters to water

L 46: change orbital to orbitally

L 50: change few to a few

L 51: add ‘and temporal’ after latitudinal

L 56: move ‘erode the seabed’ and ‘or’ to before ‘interrupt’

L 57: delete ‘, sedimentation’

L 80: delete on

L 82: replace Armorican by ‘the Armorican Massif’

L 83: replace being part of by ‘within’

L 99: delete was

L 106: Here and in other occasion: I recommend avoiding the abbreviation BS which is in English is commonly used to refer to bullshit.

L 111: replace United Kindom with ‘the United Kingdom’

L 116: put in before inland

L 120: replace on by of

L 208: weather resistant and weather recessive does not sound correct. You could delete ‘weather’ or you could explain that the beds are either resistant or susceptible to weathering.

L 216: as L 208

L 218: add the before marls

L 220: add and before trace

L 234: delete weather (2x)

L 277: replace which peaks at by with a main periodicity of

L 288: to help the reader, please mention the width of the filters in the main text, expressed as periodicities. Consider explaining why the bandwidths of the two filters are very different (half of the centre frequency vs one fourth of the centre frequency).

L 293: I suggest to replace the word chronostratigraphy by cyclostratigraphic interpretation. Ideally, an integrated chronostratigraphy would include information from bioevents, magnetostratigraphy, chemostratigraphy, radioisotopic dating, etc.

L 299: add cycles after m, replace corresponds by correspond

L 311: add ‘in CaCO3’ after richer

L 313: delete counterpart

L 340: add with before maximum

L 375: replace ‘the amplitude of the oscillations’ by ‘amplitude of variability’

L 422: replace seawater by water, and ‘concentration was’ by ‘concentrations were’

L 434: replace whose by which, add of before their

L 557: add with before that

L 565: replace records by record

L 567: replace alternation by alternations

L 577: replace if by when

L 606: add than after higher

L 625: replace indurate by indurated

L 632: replace indurate by indurated

L 655: replace originate by lead to

L 660: add strength of before biological pump

L 664: replace distortions by alterations

L 672: replace come by coincide

L 676: replace ‘and greater OM with relatively higher’ by ‘and more OM with a relatively higher’

L 681: the PEf record actually does not always have its maxima in black shales. Perhaps mention the P concentrations themselves to strengthen the observation.

L 706: typo in would

L 721: replace sea bottom by either sea floor or bottom water

L 766: replace bottom by floor

L 769: replace waters by water

L 785: typo in oxygenation

L 835: replace are no evidences by is no evidence

L 844: replace depth by depths

L 862: typo in diagenetic

L 885: add the before OM

L 1076: replace supplies by supply

L 1090: delete sea

L 1096: typo in significant

Figure 4: replace ‘Relief in the outcrop’ by ‘weathering profile’. Consider reverting the colour axis back, so that the peaks coincide with protruding beds and are more easily compared with the log.

Figure 13: This is an excellent summary of your findings and the different orbital configurations are explained well. Instead of Ti/Al and Si/Al, I recommend using the enrichment factors that you use in the text and other figures. As the role of productivity is not so well constrained, I recommend using a question mark after the claim of increased productivity. Similarly, you could consider including only low/high OM preservation in the text within the figure (rather than including the transport)