Review:

Impact of the Late Miocene Cooling on the loss of coral reefs in the Central Indo-Pacific

by Petrick et al., 2024 CP

The authors present a TEX86-based reconstruction of sea surface temperatures (SSTs) for the Queensland Plateau (NE-Australia, ODP Site 811) for the period between 11 and 2 Ma. The dataset presented is a synthesis of new and published data by the authors (Petrick et al., 2023, Sci. Repts.). The described SSTs clearly document the transient cooling in the late Miocene between 7 and 5 Ma known as Late Miocene Cooling (LMC). However, the magnitude of the transient cooling documented is unusually large at ~4 °C. The authors discuss in detail a possible mechanistic relationship between LMC cooling and subsequent reef drowning known as “Pliocene Reef Gap”. All biochemical analytical procedures are described in detail and the quality of the data is discussed; technically speaking, the data set presented can be considered excellent.

Science

Chapter 1 (Introduction) provides an overview of global reef patterns in the Miocene and Pliocene on the basis of current literature knowledge. However, the structure of the chapter is not stringent and suffers from duplication and repetition. Also, in many sentences the content is difficult to follow or the relationships between sentences are illogical (e.g. “Therefore”, line 39). More problematic, however, is the fact that the corresponding author does not always summarize cited work adequately. For this reason, I checked some of the cited publications again. For example, it is not true that C4 grasses spread worldwide during the LMC (the C3/C4 transition was at ~40° latitude in both hemispheres, and the C4 grasses never reached western Europe and the Mediterranean region; Cerling et al., 1997, Nature). Another example is the interesting Brachert et al. (2020) paper, where I found no discussion of a link between biomineralization performance and sea level changes (line 74), nor modern reef corals to be “hypercalcified” (line 70) or the coral reef habitat to have changed (line 295). Furthermore, in the abstract (lines 9-10) the author claims that little is known about changes in “aquatic ecosystems” relative to terrestrial ecosystems – this statement simply denies the incredible wealth of knowledge that exists about ancient oceanic ecosystems.

The results and discussion chapters (chapters 2 and 3) lack any description of lithologies and depositional sequences encountered at site 811 for the period discussed. As the authors are using a new age model, the lithological information is not readily available from the literature. For an assessment of the TEX86 results, however, it should rather be an integral part of this publication. As is, chapter 3.2 of the results presents a SST reconstruction for the periods before, during and after the LMC. However, this text is rather confusing and the results remain irreproducible, as they lack clear definitions of the time intervals used and the number of measurements used for calculating average SSTs. I suggest the authors insert a table listing all of the information needed. Adding an error bar in the figures (2, 4) may also help the reader in evaluating the data. In the discussion chapter (chapter 4), the authors do not present any discussion of their findings related to lithological data from ODP site 811 nor any other site located nearby, e.g., ODP sites 824 and 825. Rather, the authors step directly into a more global discussion with a Mg/Ca dataset from the “nearby” northern Indian Ocean. I agree that the similarity of the two datasets is impressive (Fig. 4). However, given the fact that SSTs ~31 °C before and after the LMC were critically high for coral reef growth (and 3-4 °C warmer than modern SSTs; line 154) and SSTs of the LMC were within the classical reef window range (and modern SSTs), I had expected a discussion on the role of a hot ocean for reef health, both, in the geological past and recent future. This deficit leaves the reader to speculate, whether the Pliocene Reef Gap might be due to the global warming following the LMC?

Technical aspects

Many sentences are linguistically imprecise and the use of tenses is not always logical (historical facts and developments should be presented in past tense, for example). Although being a rather general bad habit in many scientific publications, I mention wordings like “… the end of the LMC at our site…” as inappropriate – it must read as “… the end of the LMC at ODP site 811…” or “… at the site studied…”. Spaces are used quite liberally in the text (especially when a citation is given at the end of a sentence). The formatting of the headings (line 213) or the bibliography is “free-style”. Overall, I think the manuscript has the character of a draft and needs to be strongly revised or re-written (and shortened).

Recommendation

The merit of this publication is the validation of the TEX86 proxy as a robust method for SST reconstructions of shallow-water carbonates. This new approach will allow for a better, direct understanding of the temperature regimes behind shallow-water carbonate deposits and contribute to the ongoing debate on ancient cool, warm, and hot carbonate systems. The publication must be improved technically, however, which requires a major revision.