We appreciate the attention that Gregg (2024) brings to our recent article in the Journal of Sedimentary Research. In this article, Ryan et al. (2023) use petrographic and Δ47 data from nine dolomite samples in the Paleocene–Eocene Umm er Radhuma Formation in Qatar to show that nonplanar dolomite textures can form below the theoretical dolomite critical roughening temperature (CRT) proposed by Gregg and Sibley (1984). From our reading, the key criticisms of Gregg (2024) can be summarized as follows: i) the dolomites presented in Ryan et al. (2023) do not exhibit nonplanar textures, ii) the Δ47-derived temperatures presented in Ryan et al. (2023) do not accurately reflect the conditions during dolomite crystallization, and iii) Ryan et al. (2023) unfairly criticize the kinetic theory of crystal growth of Jackson (1958, 2004) and the dolomite CRT proposed by Gregg and Sibley (1984).
Although we are confident that the criticisms of Gregg (2024) can be summarily dismissed with information and arguments presented in the original article by Ryan et al. (2023), we offer the following responses.
Dolomite Texture: The petrographic methods applied by Ryan et al. (2023) for defining dolomite texture faithfully follow those published in Gregg and Sibley (1984) and Sibley and Gregg (1987). Through objective point counting, Ryan et al. (2023) quantitatively determined the percentage of dolomite crystals with compromise boundaries that preserve crystal-face junctions (as defined in Gregg and Sibley 1984). According to Gregg and Sibley (1984), dolomites in which < 30% of their crystal boundaries preserve crystal-face junctions are defined as nonplanar. Table 1 in Ryan et al. (2023) shows that the analyzed UER dolomite samples fall below the 30% threshold, and should therefore be classified as nonplanar. Despite the quantitative data presented by Ryan et al. (2023), Gregg (2024) claims that the images provided in Ryan et al. (2023) are unconvincing. As evidence, Gregg (2024) offers only an opinion stating that, “My immediate impression, looking at these, was that they are planer-S [sic] dolomite.” No other data are provided by Gregg (2024). The qualitative method of Gregg (2024) is not surprising. It is rare, in fact, for the quantitative measurements to be presented. Instead—and in contrast to the point-count analysis provided in Table 1 of Ryan et al. (2023)—most claims of nonplanar dolomite texture in the literature are based on a subjective comparison between what is observed in a standard thin section and the 2-D illustrations published in Gregg and Sibley (1984) and Sibley and Gregg (1987). To further the critique, Gregg (2024) refers to additional petrographic criteria required to define nonplanar textures, which include using polished thin sections for dolomite crystals > 50 µm, the presence of undulose extinction, and using unstained thin sections. However, none of these criteria are part of the original definition outlined in the methods papers of Gregg and Sibley (1984, 1986). To summarize, Ryan et al. (2023) presented quantitative data based on the published petrographic criteria for determining dolomite texture. Using these criteria, it was determined that the dolomites of the UER exhibit nonplanar texture.
Crystallization Temperature: Δ47 clumped-isotope data presented in Ryan et al. (2023) were obtained using state-of-the-art analytical methods, and were collected using two different instruments, with multiple standards used to compare the instruments and thus represent the best possible measurements available. The temperatures for each sample are based on at least five replicate measurements, with some having six to eight. The average Δ47-derived temperatures from all of the nonplanar samples are remarkably consistent, and exhibit a narrow range between 38.8°C and 54.2°C. The average Δ47-derived temperature for the nonplanar UER dolomites is 43.6°C with calculated uncertainties suggesting temperatures could be as low as 29.1°C. These relatively low Δ47-derived temperatures align well with the shallow-burial history of the UER (He and Berkman 2003; Van Buchem et al. 2014; Rivers and Larson 2018) and fluid-inclusion thermometry—the one nonplanar-dolomite sample that was evaluated exhibited no two-phase liquid–vapor inclusions (J. Reynolds, personal communication), which is consistent with crystallization temperatures below 50°C (Goldstein 1993, 2012; Goldstein and Reynolds 1994). In our article, Ryan et al. (2023) presented objective, repeatable quantitative measurements showing that dolomite crystallization temperatures fall below the proposed CRT range of 50–100°C for nonplanar dolomites.
Dolomite CRT Theory: Most of Gregg (2024) focuses on the theoretical and mathematical foundation of crystal-growth theory of Jackson (1958). However, Ryan et al. (2023) made no effort to criticize this theory. Rather, we used our empirical observations to show that application of this theory to predict the formation temperatures of sedimentary dolomites, which form in complex natural environments, is unreliable, and should probably be revised or abandoned. In addition to our own observations, data from hundreds of published laboratory experiments (Bullen and Sibley 1984; Sibley 1990; Zempolich and Baker 1993; Sibley et al. 1994; Kaczmarek and Sibley 2007, 2011, 2014; Kaczmarek and Thornton 2017; Rose 2021; Zhang et al. 2024) and observations from the literature (Shukla 1986; Dawans and Swart 1988; Jones 2005; Huang et al. 2014) indicate that there are factors other than temperature and fluid saturation state that control dolomite crystal texture. This has also been shown to be true for other carbonate minerals as well (e.g., Hashim and Kaczmarek 2020).
In summary, the applicability of dolomite texture to infer formation temperatures will be adjudicated by our science, but at this point, we stand by the observations and interpretations of the dolomite textures and Δ47-derived crystallization temperatures provided in Ryan et al. (2023), and we reiterate the key finding that dolomite textures are an unreliable indicator of formation temperature.
