Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

The modern era of understanding carbonate rocks, effectively starting in the early 1960s, was created by the pioneering studies of many workers, and John Anthony Dawson (Tony) Dickson played a major part in this revolution. His legacy has led to a better appreciation that the crystallography, mineralogy and geochemistry of carbonates records a complex interplay of environmental, biotic and hydrogeological processes during geological time. As a result, carbonates are now routinely analysed for climatic, atmospheric, oceanographic and evolutionary signals, as well as delineating porosity and resource potential. Modern tropical carbonate sediments are generally aragonite and highMg calcite with high porosities (Enos & Swatsky, 1981), yet ancient carbonates (Miocene and older) are generally calcite or dolomite with far lower porosity. Hence the multitude of processes that comprise carbonate diagenesis are fundamental to the history of most carbonates. Deciphering primary signals, and distinguishing them from diagenetic ones, requires a forensic level of microscopic study, and these techniques have been developed and tirelessly promoted by Tony Dickson. Now, as illustrated by the papers in this issue, we can add modelling and the ever more sophisticated use of geochemical proxies to this armoury. This special issue of The Depositional Record focusses on a legacy of careful and inventive research on carbonate diagenesis in honour of Tony.


| INTRODUCTION
The modern era of understanding carbonate rocks, effectively starting in the early 1960s, was created by the pioneering studies of many workers, and John Anthony Dawson (Tony) Dickson played a major part in this revolution. His legacy has led to a better appreciation that the crystallography, mineralogy and geochemistry of carbonates records a complex interplay of environmental, biotic and hydrogeological processes during geological time. As a result, carbonates are now routinely analysed for climatic, atmospheric, oceanographic and evolutionary signals, as well as delineating porosity and resource potential.
Modern tropical carbonate sediments are generally aragonite and high-Mg calcite with high porosities (Enos & Swatsky, 1981), yet ancient carbonates (Miocene and older) are generally calcite or dolomite with far lower porosity. Hence the multitude of processes that comprise carbonate diagenesis are fundamental to the history of most carbonates. Deciphering primary signals, and distinguishing them from diagenetic ones, requires a forensic level of microscopic study, and these techniques have been developed and tirelessly promoted by Tony Dickson. Now, as illustrated by the papers in this issue, we can add modelling and the ever more sophisticated use of geochemical proxies to this armoury. This special issue of The Depositional Record focusses on a legacy of careful and inventive research on carbonate diagenesis in honour of Tony.

| GEOLOGICAL CAREER OF JOHN ANTHONY DAWSON (TONY) DICKSON
Tony grew up in north-west England and attended Queen Mary College at the University of London receiving BSc and PhD degrees in Geology. His PhD thesis was "The structure and sedimentology of the Carboniferous rocks of the Castletown Area, Isle of Man".
Tony's first major contribution to carbonate sedimentology involved the development of a technique for staining carbonate rocks. This technique allows the discrimination of calcite from dolomite in thin sections. The stain revolutionised carbonate petrography and rapidly became 'standard procedure' throughout the world. An important innovation in the staining technique was the display of iron-rich zones in both calcite and dolomite allowing the detailed history of carbonate cements to be unravelled simply from a stained thin section. His 1965 paper in Nature described the technique which was applied more extensively the following year in a paper published by the Journal of Sedimentary Petrology (Dickson, 1965(Dickson, , 1966.
Tony's papers from the 1970s reflect his keen observational skills, particularly in carbonate petrography. His detailed knowledge of mineralogy and petrography, coupled with details available from his staining technique and cathodoluminescence, brought Tony widespread recognition as one of the top international carbonate researchers. Tony taught at the Universities of Wales and Nottingham before moving to the University of Cambridge in 1981.
John Hudson stated in 1977 that that 'we must take our limestones to pieces' to reconstruct diagenetic histories (Hudson, 1977), and in a revolutionary study, Dickson and Coleman (1980) did just that on a succession of the early Carboniferous Ronaldsway Member located on the Isle of Man to the west of the UK mainland. Some aspects, however, of their findings remained unresolved due to issues which have dogged diagenetic studies for decades, specifically the limitations of interpreting δ 18 O values of phases because while the value of the calcite cement was known, the values of the precipitating fluid and the temperature of precipitation remained unknown. These two unknowns can now be investigated using newer techniques, and the clumped isotope geothermometer (Δ 47 and Δ 48 values) now allows temperature to be directly determined. Dickson et al. (2023) revisit those same Carboniferous carbonates using clumped isotopes to reveal the next chapter in this story.
In the 1980s, Tony's work was a highly sophisticated blend of petrography and geochemistry. His 1983 paper on modelling of crystal growth explained carbonate crystal growth patterns in insightful and novel ways (Dickson, 1983). He was asked to write a summary of the 'state-ofthe-art' of carbonate diagenesis in 1985 (Dickson, 1985). His other papers during the 1980s determined stable isotope and trace element composition of different generations of diagenetic carbonate, allowing interpretation of diagenetic histories and fluid movements during burial. Those sampling techniques and interpretations have since served as a model for many subsequent studies.
An important paper in Nature documented disequilibrium stable isotopic variations in calcites (Dickson, 1991), and his 1992 paper on aragonite neomorphism with Robert Maliva was a significant advance in our understanding of this process (Maliva & Dickson, 1992). His publications (with other co-authors) on limestones in Texas and New Mexico, including the Permian Capitan Reef, helped redefine depositional and diagenetic models that had been unchallenged for years (Kirkland et al., 1993;Wood et al., 1994Wood et al., , 1996. A major paper on the preservation of Palaeozoic Mg calcite (Dickson, 1995) provided further insight into how this unstable mineral can be preserved.
Tony continued to explore diverse aspects of carbonate diagenesis in the new millennium, including the aragonite to dolomite transformation, Ediacaran biomineralisation in Namibia, Floridian aquifer geochemistry, and breccia textures in the UK. Another area where his detailed work has had a significant international impact is on the changing Mg composition of sea water through time. His 2002 Science paper set the stage for this when he showed that Mg/Ca ratios in echinoderms changed systematically with geological time which has implications for constraining the evolving chemistry of Phanerozoic ocean water (Dickson, 2002). He documented further details in Dickson (2004).
Tony's work on the evolution of porosity during early and late diagenesis provided great insight into the development and preservation of subsurface reservoir quality in carbonate rocks throughout the world (UK, US, Angola, Kazakhstan; Dickson & Kenter, 2014;Saller et al., 1994Saller et al., , 1999aSaller et al., , 1999bSaller et al., , 2016. He collaborated on many US projects and published in many US journals which had some unanticipated results. His manuscripts in this volume required spelling corrections, and when he returned a revised version to the UK editor he despaired, "I am saddened to realise my English has unwittingly become Americanised." Tony has always had a sly sense of 'humor' (humour).
Tony continues to publish important work. His legacy of integrating careful field observations, impeccable laboratory techniques, meticulous petrography, and deeply insightful analysis and interpretation has enriched the study of carbonate diagenesis and continues to do so. Tony has always incorporated new techniques and instrumentation to delve deeply into the evolution of carbonate mineralogy and fluidrock interactions. For his major contributions to the science of carbonate sedimentology, he was awarded the SEPM Pettijohn Medal for Excellence in Sedimentology in 2013.
Tony's reputation for giving superb and beautifully illustrated presentations is unparalleled, and his sheer enjoyment in the beauty of a carbonate in thin section spans the continents. Perhaps most importantly, Tony has been an excellent teacher and mentor for more than 50 years as well as a highly collegial collaborator, and a leader of memorable field trips for numerous students and colleagues (Figures 1 and 2). His dedication to getting students into the field is an extremely important part of his life and career ( Figure 3). He remains an inspiration to geologists internationally.

| PAPERS IN THIS SPECIAL ISSUE
This 'Special Issue' contains insightful articles on many aspects of carbonate diagenesis, combining petrography and geochemistry with other techniques as Tony Dickson has done throughout his career. The papers in this issue can be divided into five groups, (1) Carbonate crystal growth, (2) Controls on Marine Diagenesis, (3) Meteoric and Burial Diagenesis, (4) Insights obtained from the study of Tufas and (5) Dolomitisation.

| Carbonate crystal growth
This section contains three papers (Diaz et al., 2023;Dickson, 2023;Immenhauser et al., 2023). The paper by Immenhauser et al. (2023) shows spectacular images that elucidate the formation of helictites that represent a bizarre form of carbonate precipitate. Modern to late Pleistocene structures composed of aragonite and calcite from three caves in western Germany differ significantly in their crystal fabrics and internal geometry, and have grown in different ways. The initial crystalline fabric influences the flow of fluid through the structures and exerts a critical control on their form and mineralogy. Indeed, the authors show how fabric and morphology are highly sensitive to even minor environmental and crystallographic changes. As a result, each cave and even different sites within a single cave may be characterised by a unique combination of parameters that change over seasonal, and longer, cycles. Diaz et al. (2023) use experiments with and without indigenous microbial flora to evaluate the precise role of microbial communities in precipitating carbonate and forming aggregate grains, carbonate components that F I G U R E 1 Tony Dickson on a field trip with colleagues at Stair Hole, Dorset. Photo credit: Art Saller.

F I G U R E 2 Tony with colleagues
Rosalind Rickaby and Rachel Wood on the Dorset coast. Photo credit: Art Saller. cover vast areas of the present-day Bahamas. Diaz and coworkers employ Scanning Electron Micrographs of samples with the attendant microbial biota to show significant alteration within 30-60 days, including binding of the grains by extracellular polymeric substances and the formation of micritic bridges, cements and encrusted aggregates, linked to heterotrophic and autotrophic microbial processes. In contrast, samples where microbes had been killed, showed almost no alteration. This study demonstrates the importance of microbes in mediating marine cementation, allowing cementation to occur at very rapid rates.
The final paper in this section (Dickson, 2023) extends the classic work on the morphology of limestone calcite cements done in the 1960s (Dickson, 1965(Dickson, , 1966. Cements can be initiated by either (1) nucleation, when new crystals start but are not attached to their substrate, or (2) seeding, when new crystals are seamlessly connected to and influenced by substrate crystals. After seeding, epitaxial cement growth starts with many sub-crystals that coalesce distally, followed by layered 'mantle growth'. Seeding on different sized crystals causes variations in epitaxial growth rate with faster growth on large crystals resulting in a disorganised cement fabric; the variation in epitaxial growth rate is perpetuated into mantle growth. Tony's stunning photomicrographs of cement zonation from staining, cathodoluminescence and charge contrast images along with coloured drawings illustrate the variable dynamics of calcite crystal growth as pores are filled.

| Controls on marine diagenesis
In this section the influence of marine diagenesis is examined in Precambrian and Neogene carbonates. Fairchild et al. (2023) assessed the cap carbonate from Svalbard which follows the second of two global glaciations of the Cryogenian Period, and which possess distinctive negative carbon isotope signatures that are woven into Snowball Earth theory. This posited a multi-million year hydrological shutdown leading to ultra-high atmospheric carbon dioxide levels, triggering deglaciation and rapid carbonate precipitation. An alternative is that carbonates were indicative of post-glacial terrigenous sediment starvation. An atmospheric 17 O signal provides independent proof of high pCO 2 that was trapped in sulphate in synglacial lacustrine carbonates of Svalbard, and in this new study this terrestrial signal is also found preserved in the succeeding marine cap dolomite. This requires dolomitisation by sea floor diagenesis, demonstrated by hardground formation, faithfully preserving the 17 O anomaly. The carbon isotope profiles are also reflective of marine water evolution, not globally, but representative of the depositional basin. A plausible exponentially declining sedimentation rate model for the Svalbard cap bridges the two extreme hypotheses of ultra-fast and slow carbonate deposition. Xiong et al. (2023) compile data to document the changing carbonate mineralogy and geochemistry of early marine cements through that interval, in order to track the evolution of sea water chemistry coincident with the Cambrian Explosion. Dickson (2002) pioneered such work in his seminal Science paper where he analysed exceptionally well-preserved echinoderm stereom to constrain Mg/Ca sea water values throughout the Phanerozoic. The Xiong et al. compilation shows that the cycles of aragonite/high-Mg calcite were more variable than previously recognised, with the first inferred transition from a high-Mg calcite to low-Mg calcite sea occurring during early Cambrian stage 3. In addition, Ce F I G U R E 3 Tony (lower group, 3rd from right) with Cambridge students on a field trip that he organised to West Texas. Photo credit: Art Saller. anomaly data also reveals two minor oxygenation events in the late Ediacaran and Cambrian Stage 3. Even though a dominant theme through Tony's research has been to advocate the careful use of early marine cements to track environmental changes, they still represent an underused and potentially high-resolution archive.
The diagenesis of more recent carbonates are examined in the study of Saller and Winterbottom (2023) who studied the drowned late Pleistocene reefs around Hawaii and Eocene to Lower Miocene carbonates at Enewetak Atoll. In both areas, diagenesis in samples from 300 m to 1505 m below sea level is related to carbonate saturation of sea water which decreases with depth. Dissolution of aragonite and Mg calcite with precipitation of calcite cements with 2-8 mol% MgCO 3 occur in sea water undersaturated with respect to aragonite and high-Mg calcite, but supersaturated with respect to low-Mg calcite. These observations support variations in carbonate saturation related to variations in pCO 2 (not changes in Mg/Ca ratios) being the main control on marine diagenesis and carbonate mineralogy in ancient oceans.

| Meteoric and burial diagenesis
During Tony's term as a junior lecturer at what is now Cardiff University, he developed an interest in the early Mississippian oolites in South Wales, which lead to him supervising two PhD research projects on these units after moving to Nottingham University. The results of one of these studies are re-evaluated by Raven (2023) who shows how the integration of palaeosol, meteoric cement and allochem isotopic data from a series of stacked oolitic sand bodies allows the recognition of global isotopic trends, regional climate effects and broad local hydrological changes. The Kinderhookian-Osagean Boundary Excursion, which had not been recognised when the work was undertaken, is evident in allochem (grain) δ 13 C values.
Chalk diagenesis, a longstanding interest of Tony's, is often viewed as a simple process related to the effects of increased burial and cementation related to pressure solution. The highly depleted oxygen isotopes of the Cretaceous Niobrara Formation chalks of north-east Colorado, USA, are globally anomalous and were investigated by Simon et al. (2023). Using clumped isotopes of calcite (Δ 47 ), pyrolysis data and strontium and neodymium isotopes, a clearer appreciation of the unique history of these chalks is elucidated. Crucially, clumped isotopic data and isotopic modelling of that data reveals that while regional heat pulses and local variability in geothermal gradients of ca 20°C/km at the tens of kilometre scale were present, it was the influxes of 18 O-depleted meteoric groundwater in association with continuous calcite recrystallisation during deep burial that resulted in the anomalous depleted oxygen isotope values.
In the final paper in this section Dickson et al. (2023) revisit the classic study of Dickson and Coleman (1980) in which diagenetic components of a section of the early Carboniferous Ronaldsway Member on the Isle of Man were 'taken apart'. The two unknowns of the value of the precipitating fluid and the temperature of precipitation are here investigated using the clumped isotope geothermometer (Δ 47 and Δ 48 values), allowing temperature to be directly determined. The clumped isotopes of both the diagenetic components, and even what were considered refractory grains (calcitic brachiopods and crinoid ossicles), are all shown to be reset by either recrystallisation at high temperatures or solid-state reordering.

| Insights from obtained from the study of Tufas
Tufas and other non-marine carbonates are repositories of critical information about past environmental change but understanding the processes leading to the capture of these records is critical. Broughton (2023) documents the earliest stages of carbonate formation in a Holocene tufa from south-eastern British Columbia, western Canada. Precipitation is interpreted as mediated by EPS (extracellular polymeric substances). Initially amorphous calcium carbonate (ACC) nucleates on EPS substrates with partial transformation to monohydrocalcite and subsequently to nanocrystalline calcite. The precipitation of ACC is proposed as a critical step in the earliest phase of tufa formation, potentially having been underappreciated in earlier studies because of recrystallisation into micrite and spar fabrics. Tufas also provide opportunities to reconstruct Quaternary climates and hydrology but often undergo extensive alteration due to weathering. De Wet et al. (2023) use the exceptionally preserved tufas of the arid Calama area of the Atacama Desert in Chile to identify a range of depositional settings, regional changes in groundwater composition and changed climate towards the late Pleistocene, as well as wetter time periods during the Quaternary within the drainage basin headwater area. The δ 18 O and δ 13 C values are used, respectively, to identify evaporation effects and the influence of volcanogenic carbon on biological signals in the carbonates.

| Dolomitisation
Two papers on dolomitisation consider different processes affecting steep margined carbonate platforms in rift-settings. Frazer et al. (2023), using reactive transport modelling, investigate the controls on faultassociated dolomitisation by geothermal sea water convection in a syn-rift carbonate platform. Their model also evaluates the contribution of incorporating stratigraphic growth and fault propagation. They compare these results with the earliest phase of dolomitisation found in the Derbyshire Platform of northern England, a Mississippian aged flat-topped, steep sided platform that underwent a polyphased dolomitisation, described by Breislin et al. (2023). This first phase of dolomite formed a template for later phases of dolomitisation on the same platform. The effects of this later phase, producing fault-controlled dolomite bodies, is documented further by Breislin et al. (2023). This paper, documents dolomitisation by hot, saline brines sourced from the adjacent hangingwall basin, related primarily to strikeslip crustal faults and reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded, demonstrating the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults connecting the carbonate platform to the adjacent basin.

ACKNO WLE DGE MENTS
We, the guest editors of this volume, are indebted to our reviewers who spent countless hours reviewing and improving the papers in this Special Issue. It is a thankless job, and we sincerely appreciate them. Those reviewers include: Dave Eby, Jay Gregg, Leslie Melim, Adrian Immenhauser, Malcolm Wallace, Giovanna de la Porta, John Dunham, David Budd, Cedric John, Bea Garcia-Fresca, Chaojin Lu, Dave Katz, Nicolas Blanco and Elizabeth Trower. We also thank all of the authors and the copy editor (Greta Mackenzie) for their hard work in making this a good Special Issue fitting of John Anthony Dawson (Tony) Dickson. We have no conflict of interest to declare.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analysed in this study.