A dolomitization event at the oceanic chemocline during the Permian-Triassic transition

The Permian-Triassic boundary (PTB) crisis caused major short-term perturbations in ocean chemistry, as recorded by the precipitation of anachronistic carbonates. Here, we document for the first time a global dolomitization event during the Permian-Triassic transition based on Mg/(Mg + Ca) data from 22 sections with a global distribution representing shallow- to deep-marine environments. Ten of these sections show high Mg/(Mg + Ca) ratios bracketing the PTB, recording a short-term spike in dolomite formation. The dolomite consists mainly of micron-scale anhedral to subhedral crystals that are associated with abundant fossilized bacterial bodies and extracellular polymeric substances, suggesting that dolomite precipitation was induced by microbial metabolic activity. Sections showing a dolomite spike at the PTB are widely distributed geographically, but mostly encountered in mid-shelf to upper-slope settings. Because the dolomitization event coincided with a rapid expansion of oceanic anoxia and high rates of sulfate reduction, we hypothesize that it was triggered by enhanced microbial sulfate reduction within the oceanic chemocline.


INTRODUCTION
The Permian-Triassic boundary (PTB) crisis, representing the largest mass extinction of the Phanerozoic (Erwin, 2007;Song et al., 2013), was linked to major perturbations of the ocean-atmosphere system, including increases in pCO 2 and large fluctuations in oceanic alkalinity and carbonate saturation levels (Payne and Kump, 2007;Retallack and Jahren, 2008). Changes in the seawater carbonate system are reflected in the widespread precipitation of "anachronistic" carbonates, e.g., microbialites, synsedimentary seafloor cements, ooids, and pisoids (e.g., Baud et al., 2005;Kershaw et al., 2012;Woods, 2014) and in controversial evidence for localized chemical erosion of marine carbonates (Payne and Kump, 2007). PTB oceans were also influenced by concurrent changes in seawater temperatures (Sun et al., 2012), pH values (Clarkson et al., 2015), and redox conditions (Wignall and Twitchett, 2002;Elrick et al., 2017;Zhang et al., 2018aZhang et al., , 2018b. One aspect of oceanic conditions during the PTB crisis that has received little attention to date is dolomite formation rates. An increase in dolomite in the uppermost Permian-lowermost Triassic was first noted by Wignall and Twitchett (2002) and has been attributed to an increased weathering flux of Mg-bearing clays to the ocean (Algeo et al., 2011). Here, we examine the distribution of dolomite in 22 PTB sections, documenting a global dolomitization event and identifying additional controls on its formation during the Permian-Triassic transition. Our data set shows that mid-depth sections experienced the largest increase in dolomite formation, suggesting the important influence of redox conditions, in addition to seawater alkalinity and Mg concentrations.

MATERIALS AND METHODS
We studied 22 widely distributed PTB sections, including 10 from the eastern Tethys (South China craton), four from the southern Tethys (northern Gondwana margin), five from the western Tethys, and three from Panthalassa ( Fig. 1A; Fig. DR1 in the GSA Data Repository 1 ). Based on sedimentary characteristics (see details in the Data Repository), these sections were grouped into three environmental categories ( Fig. 1B): (1) microbialite-dominated shallow-shelf sections at water depths <~30 m, (2) deep-shelf to slope sections at water depths of ~30-300 m, and (3) deep-basinal sections at water depths >~300 m.
A total of 2034 carbonate samples, including 544 from shallow-shelf sections, 1189 from deep-shelf to slope sections, and 271 from deep-basinal sections (Figs. DR2-DR4), were analyzed by X-ray fluorescence (XRF) or inductively coupled plasma-optical emission spectrometry (ICP-OES) to determine their Mg/(Mg + Ca) ratios. The analytical precision is better than 1% of reported values based on replicate analyses. Our composite Mg/ (Mg + Ca) record extends from the middle Permian to the Middle Triassic.
We measured 26 PTB dolostone samples from seven sections (Table  DR1) in the southern Tethys, eastern Tethys, and Panthalassa regions for carbon and oxygen isotopes using a Thermo Scientific GasBench II-MAT253 at the State Key Laboratory of Biogeology and Environmental Geology (China University of Geosciences, Wuhan). Results were corrected based on the NBS 19 limestone standard and reported in per mil (‰) relative to the Vienna Pee Dee belemnite (V-PDB) standard; replicate analyses yielded a <0.1‰ error.

Testing for Detrital Input and Diagenetic Influences
Siliciclastic material can influence measured Mg/(Mg + Ca) ratios, and so samples were screened to minimize this effect: (1) all clay-rich samples (Al >4%) were removed; and (2) the remaining samples were checked for detrital influence by cross-plotting Mg and Ca against Al, and if a significant correlation was detected, the detrital fraction of total Mg and Ca was calculated from the regression line and subtracted from total Mg and Ca to yield carbonate Mg and Ca (see Fig. DR5 for examples).
In addition to detrital influences, sediment Mg/(Mg + Ca) ratios may also have been affected by late burial dolomitization. Therefore, dolostone samples that exhibit evidence of late burial diagenesis were excluded in this analysis, such as from the Anisian interval of the Ursula Creek section, in which calcareous and phosphatic nodules are embedded in the dark-gray shales. Moreover, samples from evaporitic environments, such as from the Spathian interval of the Zal (peritidal setting) and Jesmond (lagoonal setting) sections, are not considered to reflect secular variations of dolomite in the open ocean, because a restricted evaporitic environment has an altered water chemistry that is different from that of the open ocean.
Cross-plots of Mn/Sr versus Mg/(Mg + Ca) were used to test whether elevated Mg/(Mg + Ca) values were a product of late burial alternations; the study sections reveal no correlation (n = 33, R 2 = 0.02; see Fig. DR6), providing no evidence of a link between dolomitization and late burial diagenesis. Dolostones from China show bright cathodoluminescence patterns indicative of Mn(II) incorporation without Fe quenching (Fig.  DR7), which is a feature of early suboxic diagenesis associated with organogenic dolomite (Mazzullo, 2000).

Spatio-Temporal Distribution of PTB Dolomite
Stratigraphically, PTB dolomite peaks are mainly restricted to the lower Griesbachian (Fig. 3C). Among the 10 sections that contain abundant dolomite, nine have dolomite peaks in the Hindeodus parvus conodont zone and/or the Otoceras ammonoid zone, and one section (Yangou [China]) has a slightly younger peak in the Isarcicella isarcica conodont zone. These PTB dolomite peaks followed closely on the Permian-Triassic mass extinction (Song et al., 2013), implying a potential link between these events.
Many of the PTB dolostones are found in sections deposited in deepshelf to slope settings (Fig. 2). Shallow-shelf settings (n = 16) are dominated by microbialites consisting of pure calcium carbonate, according to data from Collin et al. (2015) and this study (see Fig. 3B). In contrast,   nearly all (eight of nine) sections deposited in deep-shelf to slope settings display a Mg/(Mg + Ca) peak across the PTB (Fig. 3C). For the deepest, basinal sections, Mg/(Mg + Ca) ratios are only slightly higher in the Griesbachian than the Changhsingian (Fig. 3D).

Water-Depth Control on Dolomite Formation and Its Implications
The spatial distribution of facies at the PTB, i.e., microbialite-dominated shallow-shelf, dolostone-dominated deep-shelf, and shale-dominated deep-basin sediments, reveals the influence of water depth on dolomite formation. Dolostones in high-energy shelf settings (e.g., Selong [Tibet], Kamura) are only a few decimeters thick and thicken toward deep-shelf to slope settings (decimeters to meters). These dolostones consist mainly of bioclastic wackestone with a homogenous dolomitic matrix. Overall, dolomite formation was favored by quiet, low-energy shelf settings colonized by abundant sulfate-reducing microbes (Fig. 4A).
Associated with these fossilized microbes is dark-gray mineralized organic matter inferred to be extracellular polymeric substances (EPSs) (Fig. 4B).
Secular variations in dolomite abundance have been linked to changes in seawater chemistry, such as changing Mg/Ca ratios and sulfate concentrations (Burns et al., 2000). However, recent experiments have shown that Mg/Ca ratio, SO 4 2concentration, salinity, and pH are of secondary importance compared to microbial activity during dolomite formation (Wright and Wacey, 2005;Krause et al., 2012;Bontognali et al., 2014), suggesting that microbial activity (e.g., sulfate reduction) may have been a key factor in the PTB dolomitization event.
Sulfate-reducing bacteria and associated EPSs have been shown to be critical for the formation of dolomite in numerous experiments (Warthmann et al., 2000;Wright and Wacey, 2005;Bontognali et al., 2014). It is thought that microbial sulfate reduction can overcome the energy barrier to the dehydration of hydrated Mg 2+ ions and promote dolomite precipitation (Bontognali et al., 2014). The Early Triassic was characterized by    very hot surface ocean temperatures (Sun et al., 2012), which resulted in an upward shift of the chemocline to inner-to middle-shelf areas, leading to expansion of oceanic anoxia to shallow depths (Wignall and Twitchett, 2002;Grice et al., 2005). The peak of PTB dolomite formation at intermediate water depths was likely the result of microbial sulfate reduction below a relatively shallow oceanic chemocline, a hypothesis supported by U-isotope evidence of expansion of global-ocean anoxia during the earliest Triassic (Zhang et al., 2018a(Zhang et al., , 2018b.We therefore suggest that enhanced microbial sulfate reduction was an important factor in both sulfate drawdown (Luo et al., 2010;Song et al., 2014) and the widespread dolomite precipitation during the Early Triassic (Fig. 4).

CONCLUSIONS
A total of 2034 Mg/(Mg + Ca) data points from 22 sections provide evidence of a global dolomitization event during the Permian-Triassic transition. The dolomite is composed of micron-scale anhedral to subhedral crystals associated with fossilized bacterial bodies and extracellular polymeric substances. The PTB dolomite spike was concentrated in midshelf to upper-slope settings and coincided with the rapid expansion of oceanic anoxia, suggesting that widespread dolomite precipitation may have been induced by microbial sulfate reduction below the chemocline of a shallowly stratified ocean. Such dolomitization events may be manifested in the geological record at other times of widespread ocean anoxia and thus constitute an underexplored phenomenon.