|Manuscript Review BG-2021-245|
“The influence of near surface sediment hydrothermalism on the TEX86 tetraether lipid-based proxy and a new correction for ocean bottom lipid overprinting” by Bentley et al.
In this manuscript, Bentley et al. use data recently published elsewhere to assess the potential overprint of sedimentary tetraether production on the TEX86 proxy near a hydrothermal vent system. The authors detect substantial loss of tetraethers in some of the investigated sediment cores, but find no selective degradation of specific tetraethers. Nonetheless, TEX86 does not report the expected SST and instead seems to be influenced by benthic production. Finally, the authors suggest a model that corrects for the influence of benthic production and diagenesis/catagenesis.
In my opinion, this manuscript needs major revisions before it can be published. The text suffers from a lack of specificity when defining terms or laying out arguments, which often makes very difficult to assess any implications of arguments. Furthermore, although benthic production is identified as the major control on TEX86 overprints, there is a surprising lack of discussing ecological controls on TEX86 (i.e., community composition and growth conditions/substrate availability/habitat; these factors are indeed mentioned in the introduction). This aspect appears to be quite important given that the suggested correction – while successful at removing a certain degree of overprint – fails at reconstructing the actual measured SSTs, an aspect not given any further attention.
l. 34: The sentence should be rephrased to read “…of tetraether lipids produced by archaea and bacteria in soils and sediments…” or something similar. Thaumarchaeal GDGT proxies are not readily applied to soil samples (I guess the idea here is a comparison with bacterial brGDGTs?).
l. 38: please specify “environmental and/or geochemical variations”.
l. 39: do temperature changes have to be driven by climate change? Other mechanisms could affect seasonality or temperature depth gradients (expansion/compression of the thermocline).
l. 45: the phrase “…across TEX86 GDGT lipid classes” is confusing here since IPLs have not been introduced yet.
l. 48: delete “when it is”?
l. 50: TEXH86 has not yet been introduced
l. 59: please specify “geochemical and physical conditions”.
l. 63: how do you assess that TEX86 is “the most widely used” archaeal paleotemperature proxy?
l. 64: the reference to Tab. 1 should probably be deleted here?
l. 68: given that the entire introductory paragraph places an emphasis on marine systems (SSTs, ocean bottom sediments, marine planktonic archaea…), I would suggest to delete the reference to lake sediments.
l. 72: Blessing et al.?
l. 72 and 97: “TEX86-based lipids”? The index is based on lipids, not vice versa.
l. 77: is there a reference that would support this statement?
l. 82: change to “Mg/Ca or clumped …”?
l. 84: introduce alkenone paleothermometry for readers not familiar with Uk’37.
l. 92: Ho & Laepple (2016) do not discuss benthic production of GDGTs and there is substantial criticism of the 0-900m depth calibration suggested by these authors (see https://doi.org/10.1038/ngeo2997)
l. 99-101: highly controversial.
l. 107: the use of “deeper waters” may be confusing for some readers who understand the qualifier to indicate depths below the thermocline/below the ammonium maximum.
l. 116: this sentence should be more specific, Kim et al. do not calibrate against 0-900m depth.
l. 120: delete “by”
l. 122: abbreviation IPL not yet introduced in main text.
l. 130-134: sentence could be split into two more digestible sentences.
l. 139: “elevated sedimentation rates” in comparison to?
l. 153-154: This sentence suggests that additional measurements were conducted for this study rather than specifically stating that the data from Bentley et al. (2022) are used.
l. 213: The statement that TEXH86 is “for sediments outside the polar region” is somewhat misleading given the 15°C threshold.
l. 235: why would MilliQ water be used for the final 3 “washes”, if the mixture is not liquid-liquid extracted again?
l. 236: “gentle stream”?
l. 255-256: which mode was used? Targeted/untargeted? And what is the resolution of the Q-ToF?
l. 274: please express the reproducibility in %. Given that many GDGTs or IPLs have concentrations below 1µg/g sediment (Tab. S1 and S2), the absolute deviation is not informative (there are few IPLs that occur at >4µg/g).
l. 282: heterotrophic loss is not addressed in this section.
l. 285-287: brGDGTs not mentioned above and numbering (typically Roman numerals) may be confusing here.
l. 287-288: in Fig. 2, the concentrations in core 3 do not decrease systematically.
l. 290: please add information how the age of the sediments was determined, fallout radionuclides?
l. 299: specify “extreme vent fluid conditions”
l. 303: maybe rather “contain 0-4 cyclopentyl moieties…”
l. 306: please add regression/correlation coefficient (or other statistical means) to demonstrate the “tight control” of porewater temperature.
l. 310-315: why are 1G-GDGTs with abundances of ~8 μg/g DW in core 4 used for the study, but 2G-GDGTs with abundances of ~7 μg/g DW are not further considered? Maybe expressing the abundances in % would be more convincing here.
l. 317-320: BIT, CBT, MBT not introduced yet. Overall, these two sentences have very limited meaning, please be more specific. Which paleoclimate records (these proxies are used for different purposes), what is “environmentally scaled loading” (which parameters), what are “other ocean floor sediment systems” (this could mean anything from lateral influx to benthic production, I guess)?
l. 323: when referring to “parallel lines”, you actually mean to refer to similar slopes?
l. 332: there seems to be something wrong with the numbering of the equations throughout the text.
l. 339-341: it would be helpful to quickly summarize the results here/provide some statistical analysis, so the reader can follow the argument made.
l. 340: “similar slopes”
l. 342: RI and MI not introduced yet.
l. 346-347: this trend is primarily driven by the TEX86 values obtained for core 1 whereas core 2 displays TEX86 values mostly in agreement with the values determined for the habitable zone. It would be nice to see this aspect and possible explanations for this pattern addressed in the text. Do these data points correspond to the highest porewater temperatures?
l. 353-354: please be more specific, which isotope systems?
l. 356: cGDGTs not introduced yet.
l. 358: MI has been used in other than cold seep systems as well. Somewhere in this paragraph, it should be mentioned that contributions from Euryarchaeota are implied in such high-MI settings.
l. 366: please be more specific, sulfate reduction rates?
l. 367-369: I would not define 0.2-0.38 as “very low” values. How is any thermal control tested, please be more specific (include simple statistical analysis such as regression/correlation coefficients).
l. 369: what does “this” refer to, the MI values or the lack of thermal controls?
l. 378: please add reference when citing the upper thermal limit of life.
l. 396: McClymont et al. (2012) used POM from sediment traps for this purpose, not sediments.
l. 408: if they were determined by vertical fluxes alone.
l. 411: the “period of ~37.5 to 75 yrs” refers to which depth interval?
l. 412: the “shallow-surface sample” is typically described as core-top.
l. 415: please specify “larger ranges”.
l. 417: 10–21 cmbsf, i.e., the non-habitable zone mentioned above.
l. 419-420: this could also be triggered by other processes than temperature that influence the archaeal community composition.
l. 442-443: alternative ecological controls should be listed and discussed here (I assume data such as O2 or NH4 concentrations are not available to be compared to the TEX86 values?).
l. 451: abbreviation IPLTEX86 should be introduced.
l. 454-459: the 1G-GDGT-derived TEX86 values can again also be influenced by other ecological factors than temperature alone.
l. 458: the “living lipid pool” – it has been well established that IPLs are much more persistent than initially thought and that sedimentary IPL pools can contain substantial fossil components.
l. 461-480: I am missing a discussion about the other potential ecological controls on TEX86 values and, as is, the discussion only distinguishes ‘living’ and ‘dead’ archaea, but does not address the influence on TEX86 through changes in the contributions of Thaumarchaeota / Euryarchaeota (ANME?)/Bathyarchaeota etc. as well as potential ecotype contributions (even if the observed TEX86 trends are entirely determined by temperature, these archaea may not have the same response to adapt their membrane to temperature).
l. 479: if TEX86 “may entirely record ocean bottom sediment porewater temperatures”, why are the calculated SSTs not maxed out in all samples (e.g., at 0.9=40°C)?
l. 481: it would be very helpful if the symbols in Fig. 5C would allow the reader to differentiate the origin of the samples in the clusters. One would assume that cluster C is only made up of samples from the non-habitable zone, but this does not seem to be the case (includes 4 samples from core 3, which is ‘habitable’ in its entirety. Are there any trends, are these the lower samples or is there no pattern whatsoever?
l. 490: above, an argument is made that diagenesis is not selective. Why would this be included in the correction?
l. 498: Is d0-n an additional depth scaling parameter or a descriptor? Some initial info should be added here.
l. 505-509: what is the definition of the “deeper water column” here? This information is essential to understand the implication here. Does “deeper water column” refer to the bathypelagic or indeed to the thermocline (as is done in the introduction). In case of the latter, this is a major pitfall of the suggested correction since the majority of IPLs/GDGTs is synthesized in the thermocline/in the ammonium maximum. In case of a bathypelagic origin, the contribution of GDGTs may indeed be negligible, but then I wonder why this point is not made earlier in this section and the term bWCTEX86 is not simply omitted from equation 7.
l. 514: in line 314, the number is ~7 μg/g DW
l. 514-515: I understand the mass balance argument, but the fact that 2G-GDGTs only come with 1 or 2 cyclopentyl moieties could actually bias TEX86 much more profoundly (thus, is not a good argument).
l. 516-517: I find this sentence about ‘increasing accumulation’ confusing considering that there is significant loss of GDGTs in most cores.
l. 517-524: I am getting lost here, there is too much information missing. What is the definition of “allochthonous”? Why sum the GDGT concentrations for the entire depth interval (as implied by the upper bound n) if the goal is to correct the TEX86 values in a distinct depth interval? Shouldn’t the correction only scale the abundance of GDGTs in that particular interval against the original input rather than summing the concentration in the total sediment column? Why would the core-top not already have lost some of the IPLs? Sedimentation rates of 0.4–0.2 cm/yr imply that a 0-2cm core-top integrates 5-10 years of deposition.
l. 526-532: this section is at odds with the discussion regarding the kinetic model and the last two sentences directly contradict each other “…the selectivity of lipid classes being adsorbed to a protokerogen is undeterminable. More importantly, for this site it is insignificant…”.
l. 536-538: there is no discussion on IPL turnover rates in section 3.1 and only a sentence linking GDGT turnover rates to hydrothermalism (citing the findings of Bentley et al. 2022). How GDGT and IPL concentrations compare downcore would be a useful addition to Fig. 2 or at least it could be expressed here using statistical means (Kruskal-Wallis).
l. 557: write “partial least squares”
l. 564-565: what is the explanation for this 3°C offset relative to the actual SST? This seems to be an important discussion to be had given that the suggested TEX86-correction effectively accounts for a similar offset (some 3-4°C).
l. 566: how is a “high degree of influence” assessed? It will be worthwhile to compare the offset in light of the calibration residuals.
l. 568-569: please rephrase this sentence to be more meaningful. The fact that the “microbial community influences TEX86 measurements” is the reason it was proposed as a proxy. Be more specific.
l. 575: The caption should state that reconstructed SST are combined SS+WC SSTs at stated in the text.
l. 597-598: what is the meaning of “global changes by 2–4°C mean completed deglaciation” and, more importantly, which reference does this statement refer to?
l. 599: above, it is explicitly stated that the deeper water column is not considered in this study.
l. 601-604: the overprint in other sedimentary systems may likely be much lower, especially in core-top sediments (less vivid benthic communities in non-seep settings, substantially less diagenesis/catagenesis)
Overall, the quality of the Excel figures is not great. The data in Figures 3-6 will not be discernible for people with deuteranopia, I would recommend either using different colors or different symbols to differentiate cores. The grid lines in Figures 3-6 also obscure the data (I don’t think they are needed, but if the authors prefer them, they should recede into the background). References to background colors also do not correspond to the colors displayed (e.g., pink in Fig. 3 or blue in Fig. 4).
Some other remarks
The use of the phrase “lipid classes” throughout the text is incorrect when referring only to differences in the biphytane chains rather than headgroups. The phrase “loading” has a statistical connotation and may be confusing. There is an inconsistent delimiter use throughout the text when reporting numbers (and it often suggests much higher precision than can be expected).
The authors should give the text a final read to check spelling/grammar/punctuation and identify text residuals from previous manuscript versions.