Comments to Lyle and Olivarez Lyle

I have reviewed this manuscript and found very interesting! The new dataset is clearly novel and the discussion is clear and interesting. I have just few remarks and questions and thus ask for minor revisions.

Sincerely

Baptiste Suchéras-Marx



Introduction

l28: Why using Corg rather than POC?

l30-31: You never talk about bioturbation throughout the manuscript. Why so? Could you add argument to exclude this process as consumption of organic matter within the sediment.

Analytical Methods

l120: Could you please add a sentence about uncertainty (2 sigma) for wt%CaCO3 and wt%Corg in order to evaluate the reliability of variations in your record.

l128: Concerning site 884, the error is up to 0.4wt%. Those data are not shown but comparing to Fig.2, the variations are within this error. Could this bias the use of this site for your interpretations?

l149-151: Could you please explain how you estimate the proportion of terrigenous, authigenic oxide and authigenic clay-based Ba at site U1338 in order to evaluate the reliability of the 93% +/- 4% biogenic BaSO4 you have calculated?

Age Models and Mass Accumulation Rates (MAR)

l198: You say that “ages should still be good to +/- 0.2 Ma”. How can you tell? Please describe how you made this estimation.

l203-206: How the sampling density – which is often relatively low resolution for magnetostratigraphy – could influence the reliability of your age model?

Results

l220: Your data “averaged 0.043+/-0.014 wt%”. This is extremely low. Coming back on previous comment, what are the uncertainties of measurements of Corg?

Fig. 2: Panel B is in mg/cm2/kyr but in section 3, you said it would be in g/cm2/kyr. Please use the same unit throughout the manuscript. By the way, I would recommend to use g/m2/a, which is more coherent in term of unit (the former Bubnoff unit).

Still in Panel B, DSDP Site 574 show a peak at 12 Ma. Why so? Is it related to a major change in sedimentation rate and thus does the age model is reliable for this interval?

Discussion

l373: You site Honjo et al., 1982. With all the respect I have for pioneer work of Honjo, maybe more recent estimation and complex studies have been donw since then.

l438: You mentioned that “modeling and observations of plankton distribution point to a loss of Corg primarily within the surface ocean layers”. This statement about model is circular because models are designed this way, increase temperature = more organic oxidation. Please reevaluate the use of modelling studies in your argumentation.

Review of “Variations in the Biological Pump through the Miocene: Evidence from organic carbon burial in Pacific Ocean sediments”

By Lyle and Olivarez-Lyle

Dear Editor,

The manuscript cp-2024-34 by Lyle and Olivarez Lyle follows up their earlier work published in 2006 “Missing organic carbon in Eocene marine sediments: Is metabolism the biological feedback that maintains end-member climates?”. In this paper, following the Metabolic Theory of Ecology, the authors for the first time introduced the idea that temperature dependency of metabolic rates may act as a positive feedback to the ocean biological carbon pump on geological time scales. They postulated that under warm climate conditions, such those in the Eocene, enhanced heterotrophs metabolic rates would increase organic matter remineralization in the water column leading to low organic carbon sequestered in deep ocean sediments, explaining lower than expected organic carbon accumulation rates in the equatorial Pacific at this time. This was a consequential paper in paleoceanography, spurring a wealth of studies aimed at investigating the efficiency of the biological pump under different climate states. 

With a similar approach, in manuscript cp-2024-34, the authors focus on the trends of biogenic sedimentary components in the equatorial Pacific from about 21 Ma to present. They present new organic carbon and CaCO3 percentages and accumulation rates from 5 sites and measurements of total Barium for 3 of them. They convincingly show a major shift in the pattern of sedimentation from low biogenic components between 21-14 Ma to higher and variable values from 14 Ma to present. Because the studied sites were always in an approximately equatorial position over the studied interval, a long term decrease in productivity starting at 14 Ma is not a likely explanation. Instead, the authors suggest changes in the efficiency of the biological pump due to the progressive cooling of upper ocean temperatures from the middle Miocene to modern. 

I agree with their interpretation and think this is an interesting study which adds to the mounting evidences of a biological pump operating differently depending on background climate, and becoming progressively more efficient with the cooling trend of the last 15 Ma. 

I think though, that the message of the paper would be much strengthened by a better contextualization of their data with coeval climatic trends. In particular:

1) The strong link that the authors suggest between the warmth of the Miocene Climate Optimum (17-14 Ma) and low sedimentation of biogenic components is not apparent from the dataset. The dataset shows low sedimentation of biogenic component also for the older interval between 21-17 Ma. Instead, what is really apparent is the increase in biogenic sedimentation from the late middle Miocene on. I would hence recommend the authors not to put so much emphasis on the MCO per se, but rather on the generally warmer early to middle Miocene climate compared to today.

2) For the reason above, I also suggest the authors to show their data against climatic records covering the entire age range of their records. For instance, they could additionally show the sea surface temperature record from Auderset et al. 2022 /10.1038/s41586-022-05017-0 from 22 Ma on. This record shows an SST warming trend from about 22 Ma culminating in the MCO, which fits very well with the evidences shown here for low biogenic sedimentation over this whole time interval.  

3) Using the mid-to low latitude SST record from Auderset et al. 2022, would also help going around the issue of presenting records from an equatorial region against Northern Hemisphere mid latitude temperatures as it is now, which is not ideal and I do not particularly like given other, more suitable records are available. 

Minor

1) I find confusing that the complete records of data generated are shown only for 2 of the measured sites (U1338 and U1337) (Figs. 3-4). Can you please have the same figure also for the other 3 sites for completeness and to allow an overview on the studied region?

2) For the same reason I think the data from sites 884 and 1208 should also be shown in Fig. 2, although with the caveat that the sedimentary regime at these sites is less constrained.

3) A data availability statement is missing. 

Typos:

Line 37 the use of the word "eras" is a bit too colloquial.

Boscolo-Galazzo misspelled in lines 38 and 69.

Boscolo-Galazzo et al. 2018 missing in the reference list.

"Total" repeated twice in line 146.

Reference missing in line 383 for the sentence ending with "abyssal depth".

"Sediment" repeated twice in line 475.