Mercury Anomaly in Oligocene–Miocene Maykop Group Sediments (Caucasus Continental Collision Zone): Mercury Hosts, Distribution, and Sources

: The Oligocene–Miocene Maykop Group sediments, mainly composed of illite–smectite, store mercury in strongly variable concentrations from 10 to 920 µ g/kg. Extremely high Hg levels (98–920 µ g/kg) coupled with abnormal mercury-to-total organic carbon (TOC) ratios (Hg/TOC = 109 to 3000 µ g/kg/wt%; TOC = 0.2 wt% to 1.2 wt%) were measured in the Middle Maykop marine shales that were deposited in the deepwater Indol–Kuban Basin under anoxic conditions. The Middle Maykop shales contain up to 70% of total mercury in sulﬁde form. In heavy mineral fractions, abundant Hg-bearing pyrite (with up to 4810 µ g/kg Hg in hand-picked concentrates) is accompanied by sporadic cinnabar. Relative to the Middle Maykop sediments, the Upper Maykop shales have much lower Hg concentrations and Hg/TOC ratios: 10 to 63 µ g/kg (34 µ g/kg on average) and 7.7 to 137 µ g/kg/wt% (39 µ g/kg/wt% on average), respectively. Mercury sequestration is inferred to occur mostly by binding in sulﬁde hosts in the Middle Maykop anoxic deep-sea sediments and in organic matter, Fe 3+ -(oxy)hydroxides, and clay particles in the Upper Maykop shales which were deposited in a more oxygenated environment. Mercury inputs to the marine shales during Maykopian sedimentation were possibly associated with local Oligocene–Lower Miocene volcanic activity in the Caucasus Continental Collision Zone. At the same time, the mode of Hg binding in sediments was controlled by redox conditions which changed from anoxic to disoxic and suboxic at the Middle-to-Upper Maykop transition. at IGM (Novosibirsk). The accelerating voltage of 20 kV and nA beam current were used in low- (40–60 Pa) or high-vacuum modes at 20 s count time.

The Oligocene-Early Miocene Maykop Group shales, which are widespread in the Caucasus segment of the Alpine-Himalayan orogenic belt, store a record of significant paleogeographic changes. The Maykop monotonic sequence, which was deposited in restricted epicontinental basins of the Paratethys, is quite uniform lithologically but demonstrates stratigraphic and regional differences [11,12]. The Maykop Group sediments are key oil-generating rocks in the South Caspian and Black Sea Basins, and in the Caucasus region [13][14][15][16][17], though the Lower Oligocene, Upper Oligocene, and Lower Miocene sediments from particular sub-basins differ as to the petroleum potential [18,19]. At the same time, the Maykop shales feed numerous mud volcanoes (MVs) in the Indol-Kuban Trough and in the Kura intermontane basin, with the Middle and Lower Kura sub-basins [20][21][22][23][24][25].
All MVs of the Kerch Peninsula in the southern periphery of the Indol-Kuban Trough erupt and expulse homogeneous liquefied shale sediments belonging to the Maykop Group. According to Mg-Li geothermometry of aqueous fluids, small and large Kerch MVs root at depths of 1.0-1.5 km (Upper Maykop Formation) and 2.5-3.5 km (Middle Maykop Formation), respectively [23,24]. The erupted mud has extremely high B, Li, As, and Sb, and quite high Zn and Se content, but trace element loading varies markedly between different MVs [24]. Boron contents correlate with the sediment mobilization depth: the highest, up to 1500 ppm B, in illite-dominated mud expulsed by the largest MVs from as deep as 3.5 km, and within 66-250 ppm in smectite-dominated mud from shallower depths (1-1.5 km) at small MVs [24]. The liquefied Maykop Group sediments extruded by large and small MVs also show contrasting enrichments in Hg. Mud masses from large MVs exhibits an extreme enrichment of mercury (up to 920 µg/g) [26], whereas Hg concentrations in the mud masses extruded by small MVs commonly do not exceed typical Hg content in shales (<62.4 µg/g) [3]. The causes of this variability, however, have never been specifically investigated.
We studied mercury in samples of the Middle and Upper Maykop shales extruded by eight typical large and small MVs of the Kerch Peninsula. The data on measured total Hg, sulfidic mercury, total organic carbon, and total sulfur, along with mineralogy and signatures of Maykopian deposition conditions, were used to reveal main mercury hosts and to specify the causes of mercury anomalies in the Oligocene-Early Miocene Maykop Group sediments.

Kerch Peninsula and Maykop Group Sediments: Geological Background
The Oligocene-Lower Miocene  sediments of the Maykop Group, known as Maykop Shale, are widespread in the Crimea-Caucasus region [11,12,18,19]. They are mainly pelitic rocks with low to zero carbonate percentages, which were deposited in anoxic-disoxic conditions on the northern shelf of the Eastern Paratethys. They are main source and reservoirs rocks for hydrocarbons in the South Caspian and Black Sea Basins, and in the Caucasus region [13][14][15][16][17]. The Maykop shales feed numerous mud volcanoes, which count at least 400 in the Caucasus segment of the Alpine-Himalayan belt [15,20,21,[23][24][25]27,28].
The study area is located in the southern side of the Indol-Kuban Trough ( Figure 1), a large sedimentary basin extending from the Caucasus foothills in the east through the Taman Peninsula as far as the Kerch Peninsula in the west [21]. Details of local geology and history were reported in multiple earlier publications [15,20,[23][24][25]29]. The Late Mesozoic through Paleogene deposition in the Indol-Kuban basin produced voluminous sedimentary sequences with up to 3000 m thick Maykop Group deep-sea facies at the top. The Lower and Middle Maykop sediments occupy the greatest part of the Kerch Peninsula. The Middle Maykop strata are less than one hundred thousand meters thick in the southern part of the peninsula but reach 1000 to 2500 m in the north. Drilling in the Bulganak MV field stripped sediments of this age at a depth of 2000 m. The Upper Maykop Formation is especially widespread in the southwestern plains, and in synclines of the northeastern Kerch Peninsula [15]. Since the Middle Miocene, the peninsula area underwent orogenic movements and related clay diapirism with extrusion of plastic mud, which is found in cores of large anticlinal folds at sites of active and fossil mud volcanoes. Mud volcanism, which is an efficient mechanism for the transport of buried sediments back to the surface [27], culminated during the Middle-Late Miocene in the region and has decayed lately. The MV fields of the peninsula (19 fields altogether) are located within areas of thick mudrocks, and the solid ejecta of mud volcanoes mainly consist of the Maykop Shale material [15,23,24].
For this study, we selected eight key onshore mud volcanoes in the Kerch Peninsula ( Figure 1 and Table 1). In previous publications, they were characterized and systematized according to size, activity patterns, major-and trace-element compositions of fluid and mud components, and reconstructed fluid generation depths [23][24][25][26]29,30]. The origin depths of expulsed aqueous fluids estimated using Mg-Li geothermometry are 2.5 to 3.5 km in the large Bulganak MV field and Big Tarkhan MV (Middle Maykop mature illitic sediments) and 1.0-1.7 km beneath small MVs (Soldatsko-Slobodsky, Eny-Kale, Tobichek MVs). In the latter case, the fluids originate from the Upper Maykop sand-clay (smectite-illite) sediments of moderate maturity [23,24]. Generally, lower crust or mantle inputs into MV fluids are minor or negligible [27,31], which is also true for the special case of the Kerch-Taman MVs [20][21][22]25,[32][33][34]. Group of ten small bubbling seepages (15-50 cm in diameter) fringed by bitumen crusts The MV edifices are characterized and typified using the classification of Kopf [27].

Sampling
During the field trip of September 2017, we examined eight sites of onshore mud volcanoes of the Kerch Peninsula (Korolevsky, Soldatsko-Slobodsky, Big Tarkhan, Vladislavovsky, Nasyr, Eny-Kale, and Tobichek MVs, as well as the Bulganak MV field) and collected about eighty samples of erupted solid and liquefied mud masses composed of homogeneous Middle and Upper Maykop shales (Table 1). Heavy minerals were extracted from large (100 kg) samples of Middle and Upper Maykop shales. In the first step, the samples are disaggregated and washed with water to remove the clay fraction. This material (≤2 mm) was then subjected to several procedures of gravity and electromagnetic separation and several fractions were extracted: concentrates of sulfides, Fe-rich carbonates, Fe 3+ -(oxy)hydroxides, clastic ultrastable heavy minerals, and a light fraction with predominant quartz sand particles and fossil remnants composed of CaCO 3 . Cinnabar and pyrite were hand-picked under a binocular microscope from sulfide fractions. The samples for analyses of mineralogy, major-and trace-element chemistry, and particle size distribution were packed into sealed plastic bags while those for mercury analyses (total Hg and mercury species) were promptly transported to the laboratory and frozen to prevent loss of volatile Hg. All solid samples were shielded from sunlight to avoid transformations of mercury species [35].

Analytical Procedures
Analytical procedures were carried out at the Analytical Center for Multielemental and Isotope Research of the Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Siberian Branch (IGM, Novosibirsk, Russia), the South Ural Research Center of Mineralogy and Geoecology (SU FRC MG, Miass, Russia), and the Institute of Soil and Agrochemistry (ISA, Novosibirsk, Russia).
Total Hg content in mud samples. Bulk contents of Hg in the mud samples were measured by flameless atomic absorption spectrometry (AAS) on a Lumex RA-915M Hg analyzer with an RP-91C pyrolysis attachment, at IGM (Novosibirsk). Prior to analysis, the mud samples were freeze-dried; pyrite monomineral fractions were powdered in a mortar and homogenized. The technical specifications of the instrument allow for avoiding special preconditioning of solid samples. The national standard of soil (SDPS-3), certified for heavy metals and mercury, was used to calibrate the spectrometer and to check the quality of analyses. All samples were run in triplicate.
Mercury species in sediment samples. Thermal release analysis in combination with atomic absorption spectrometry was applied to study mercury speciation in powdered and homogenized solids (freeze-dried mud masses and pyrite monomineral fractions), at IGM (Novosibirsk). The method can resolve sulfide (HgS) and selenide mercury (HgSe), and the sum of HgCl 2 and HgSO 4 . The measurements were performed on a Lumex RA-915M Hg analyzer with an RP-91C pyrolysis attachment, at detection limits of 0.20 to 0.70 ng/g for different species. Analytical details were reported previously [36].
Organic carbon content. Total organic carbon (TOC) in sediments was measured by infrared spectroscopy on a Shimadzu Total Organic Carbon Analyzer, TOC-V CSH (Shimadzu Corporation, Kyoto, Japan), at ISA (Novosibirsk). The method implies IR detection of CO 2 produced by HCl digestion of bicarbonates and carbonates. TOC was determined by catalytical combustion of samples to CO 2 and re-measured by IR spectroscopy. The detection limit was 4 ppb and the analytical accuracy was better than 1.5%.
Sulfur content. Major elements (including total sulfur) in bulk rock samples were analyzed by the ICP-AES (inductively coupled plasma atomic emission spectroscopy) technique on a ThermoJarrell IRIS Advantage atomic emission spectrometer (ThermoJarrell Intertech Corporation, Atkinson, WI, USA), at IGM (Novosibirsk). The preconditioning procedure included fusion of powdered whole-rock samples with lithium borate [37]. Sulfate sulfur content was determined by the barium sulfate gravimetric method [38]. Total sulfur was determined on a separate sample, and sulfide sulfur was calculated by difference.
Particle size distribution. Particle sizes were measured at ISA (Novosibirsk), on a MicroTec Fritsch Analysette-22 diffraction pattern analyzer (Fritsch GmbH, Idar-Oberstein, Germany) with a 655 nm wavelength laser according to ISO 33320 [39]. Bulk mud samples were ultrasonified by a 600 W homogenizer at 22.5 kHz for 3 min in a 0.4% Na 4 P 2 O 7 solution. Replica analyses on each sample yielded 2-sigma error as high as 10%.
X-ray diffraction analysis (XRD) was applied to bulk samples of Maykop shales and separate mineral fractions. Mineral phases (≥1%) were identified by XRD in powdered samples. The relative percentages of clay minerals in ≤1 µm fraction and the patterns of illiteand smectite-dominated mixed-layer phases were determined in representative samples. All specimens were analyzed on a Shimadzu XRD-600 diffractometer (Shimadzu Corporation, Kyoto, Japan) (CuKα radiation with a graphite monochromator, λ = 1.54178 Å), at SU FRC MG (Miass). The scans were recorded from 6-60 • 2 θ at 0.05 • 2 θ increments with 5 s scanning time per step. SIROQUANT V.4 software was used to calculate the proportions of minerals. The procedure of layered silicates extraction and analytical details were summarized previously [24].
Scanning electron microscopy (SEM). The minerals were identified from energydispersive spectra (EDS), backscattered electron (BSE) images, and elemental maps (EDS system). The measurements were performed on a Tescan Mira 3MLU scanning electron microscope (Tescan Orsay Holding, Brno, Czech Republic) equipped with an Oxford AZtec Energy Xmax-50 microanalyses system (Oxford Instruments Nanoanalysis, Abingdon, UK), at IGM (Novosibirsk). The accelerating voltage of 20 kV and 1 nA beam current were used in low-(40-60 Pa) or high-vacuum modes at 20 s count time.

Particle Size Distribution
Silt-sized particles predominate in all sediments erupted from eight mud volcanoes in the Kerch Peninsula (48.9-87.7%), but the distribution of sand-and clay-size particles slightly differ (Table 2 and Figure 2). The samples from Big Tarkhan, Vladislavovsky, Nasyr, and Tobichek MVs have quite high percentages of the sand fraction (25.1-46.7%) while those from Bulganak, Soldatsko-Slobodsky, and Korolevsky MVs contain a significant amount of clay particles (4.7-26.2% sand and 7.3-18.2% clay). In terms of mineralogy, the silt and clay fractions are composed mainly of illite-or smectite-dominated mixed-layer illite-smectite. According to XRD of Maykop shales, the percentages of illite are the greatest (62 to 74%) in the clay fractions of the Bulganak samples, which contain 30% of illite and ≤10% of smectite in total (average, n = 13). Illite in the bulk mud samples from other MVs is notably less abundant: 20% (n = 8) for Soldatsko-Slobodsky, 16% (n = 4) for Nasyr and Tobichek, 13% (n = 3) for Korolevsky, 12% (n = 6) for Vladislavovsky, and 10% (n = 2) for Eny-Kale MVs. Mixed-layer illite-smectite in the samples from these edifices has a predominant smectite component (15 to 37%;~25% on average). The share of kaolin in these sediments does not exceed 25%, and other phases (mainly in the sand fraction) are~22 to 48 wt% quartz,~7 to 11 wt% high-silica plagioclase, and 2 to 10 wt% K-feldspar.

Organic Carbon and Sulfur
The contents of total organic carbon in the mud masses from the sampled MVs (Tables 3 and 4) are low in most of the samples, especially in Bulganak, Soldatsko-Slobodsky, Korolevsky, and Big Tarkhan MVs (0.6-0.8 wt% on average). The average TOC values are relatively high (1.2-1.3 wt%) in bulk samples from Vladislavovsky and Eny-Kale MVs and the highest (3.2-3.4 wt%) in those from the Nasyr and Tobichek MV sites known for oil seepages and abundant bitumen crusts. The high TOC contents in these cases are due to secondary oil impregnation.    The contents of total sulfur (TS) vary within a large range, 0.08 to 0.65 wt% S (0.22 wt% on average). The ranges of sulfidic sulfur and its ratio to total sulfur (S sulfidic /TS) are, respectively, 0.07 to 0.27 wt% and 0.2 to 1 (Tables 3 and 4). The variations of total sulfur and shares of its sulfidic and sulfate species correlate with the percentages of gypsum, barito-celestine, and sulfides.

Sulfidic Mercury and Sulfide Mineralization
Sulfidic mercury (HgS) was found in the Maykop sediments expulsed by Bulganak (100-230 µg/kg) and Korolevsky (140-250 µg/kg) MVs accounting for 22 to 44% and 15 to 71% of total Hg, respectively. The heavy mineral fractions of these sediments contain diverse authigenic sulfides: abundant pyrite, rarer sphalerite and marcasite, and sporadic cinnabar (Figures 3 and 4). Identification of polymorphic FeS 2 modifications was based on morphological difference between pyrite and marcasite crystals.  Pyrite mainly occurs as framboids or pseudomorphs after fossil organic remnants. Coarse (up to 200 µm) perfect octahedral, cubic-octahedral, or rarely cubic pyrite crystals most likely result from recrystallization of framboids and finer imperfect grains ( Figure 3A-F). Hand-picked pyrite concentrates contain 2600-4140 µg/kg THg for Korolevsky MV and 1300-4810 µg/kg THg for Bulganak MV. Hg contents are the highest, >2500 ng/g, in pyrite coexisting with cinnabar. Such high concentrations may be due to the incorporation of Hg as an impurity in the FeS 2 structure, and to the presence of HgS microinclusions. Cinnabar, with its average composition of Hg 0.97 S 1.03 , exists as intricately intergrown anhedral micrograins with trigonal etch pits, coarse (up to 400 µm) subhedral grains with sculptured faces, and sharp edges and corners, or less often as perfect crystals (up to 500 µm) with pinacoidal and rhombohedral faces (Figure 4). The morphology of cinnabar grains, with flat faces and well-defined edges and corners, looks similar in different shale samples. Note that in the Middle Maykop shales, cinnabar grains and crystals are untwinned, whereas twins are common to cinnabar from hydrothermal mineralization sites.
Sulfidic mercury in the Upper Maykop shales expulsed by Vladislavovsky, Soldatsko-Slobodsky, Nasyr, and Tobichek small MVs is as low as 13 µg/kg, and is absent from several samples (e.g., all mud samples from Eny-Kale MV lack detectable HgS). Mud masses from these volcanoes contains abundant Fe 3+ -(oxy)hydroxides in the heavy fraction, rare pyrite and marcasite, and no cinnabar. The hand-picked pyrite concentrates extracted from the samples of Upper Maykop shales are likewise poorer in mercury (e.g., 670-1090 µg/kg Hg in the concentrate from Soldatsko-Slobodsky MV).

Potential Mercury Sources in Maykop Group Sediments
Natural mercury comes to marine systems from terrestrial, atmospheric, and deep-sea (submarine exhalations from geothermal vents) sources [40][41][42][43][44]. Rivers carry terrestrial Hg derived from crustal and atmospheric sources to coastal sediments. Terrestrial Hg of atmospheric origin may potentially be transported to coastal sediments bound to organics or clay particles [45]. However, the contribution of terrestrial sources mostly depends on the presence of eroded Hg-rich rocks in the provenance areas. Such clastic inputs, as well as riverine transport of dissolved or absorbed Hg species, may be one of the key prerequisites of Hg enrichment in marine sediments. As it was previously inferred for the Kerch-Taman MV province [11,12,46], the clastic input to the deepwater Indol-Kuban basin during the Maykop deposition was mainly from mature kaolinite weathering profiles in the southern Russian Plate. Minerals of ultrastable assemblages were mainly derived from peraluminous high-grade metamorphic and granitic rocks of the Ukrainian shield, while the Caucasian Mountain system was the provenance for assemblages of scarce nonresistant minerals [23].
Mercury in the Maykop Group sediments may potentially come from numerous mercury deposits in the southern East European Craton (Dnieper-Donets basin and Donetsk orogenic system) associated with the Central-Donets deep fault. The mineralization of the largest Nikitovka Hg field situated within the arch of the Glavny anticline is controlled by faults in Middle and Upper Carboniferous coal-bearing sediments [47]. Mercury mineralization in the Nikitovka field was produced by low-temperature (≤250 • C) hydrothermal fluids and was classified as telethermal [48]. Its age remains a subject of controversy despite the known stratigraphy of the host rocks and the sources of the hydrothermal fluids. The available time estimates range from~360-240 Ma, the last phase of the Hercynian orogeny [49], to~210-110 Ma [50], and even~65 Ma corresponding to the Laramide orogeny [51]. The latter value is reconstructed from thermobarometry data for gas-liquid inclusions in hydrothermal minerals, using estimates of paleopressure, erosion rates, and cutout, which bracket the ore formation event between~60 Ma and the Neogene [52]. However, the Hg ore zone did not contribute to the clastic fill of the Indol-Kuban Trough as it was buried at a depth of~450 m in the time of Maykopian sedimentation.
Volcanism reasonably appears to be a potential agent in the mercury of the Maykop Group sediments deposited in the deepwater Indol-Kuban basin, which is located in the Caucasus segment of the Alpine-Himalayan orogenic belt. The Kerch-Taman MV province lies in a geodynamically unstable zone of continental collision, in the northern border of the Caucasian orogenic wedge (Figure 1). The Caucasus history included multiple episodes of arc and collisional magmatism [60], but volcanic eruptions abated in the Oligocene, early during the collisional event [61][62][63][64][65]. Since then, magmatism was restricted to syncollisional intrusions located in the southernmost Lesser Caucasus and along the Great Caucasus-Fore-Caucasus boundary and produced Oligocene-Lower Miocene (32-17 Ma) gabbro, monzonite, syenite, diorite, and granite intrusions [60]. However, a few volcanic bodies were found recently in the territory of present Armenia [66]: 23.7 Ma subvolcanic rhyodacites, 17.6 Ma andesite, and 16.6-17.7 Ma rhyolite lava sheets.

Mercury Loading of Maykop Group Sediments: Sulfides versus Organic Matter
The complex global cycle of mercury includes its deep-sea burial and ultimately ends in long-term Hg sequestration in marine sediments [42][43][44]67]. A large portion of mercury is scavenged by organic-rich particles, which are the major sink of Hg in the marine environment. Therefore, organic matter and Hg reasonably show strong correlation in different types of sediments [3,[68][69][70][71]. The Hg/TOC ratio in sediments is a good check of whether Hg anomalies are related to increased atmospheric Hg deposition or rather result from variation in sedimentation environment or changes the provenance areas [1,3,4,72,73]. In this respect, we studied the role of organic matter as a potential Hg carrier in the Maykop marine sediments with 0.3 to 1.5 wt% TOC (Tables 3 and 4).
Organic matter drawdown was recognized to be the dominant Hg sequestration mechanism over geologic time [3,[68][69][70][71], while increased deposition of sulfidic mercury, with corresponding Hg/TOC peaks, was related to large igneous province eruptions [1,3,6]. In the Hg-TOC diagram ( Figure 6A), the Middle Maykop shales fall within the field 'C' with low TOC (0.2 wt% < TOC < 2 wt%) and high Hg (>100 mg/kg) contents, which represent excess Hg drawdown in systems with high rates of Fe sulfide burial. Indeed, sulfidic Hg makes up to 70% of the total mercury budget in the Middle Maykop shales (Table 3), and the heavy mineral fractions contain abundant Hg-bearing pyrite (up to 4810 µg/kg THg in hand-picked concentrates) together with sporadic cinnabar (Figure 3). The Middle Maykop shale Hg data plot above the maximum normal drawdown line ( Figure 6B), i.e., Hg concentration in the environment during deposition and sediment maturation systematically exceeded the maximum sorption capacity of organic matter. Meanwhile, most of our data fall below the sulfide drawdown line ( Figure 6B) which delineates the maximum capacity of joint drawdown by TOC and sulfide (pyrite in the meaning of Grasby et al. [3]), while~15% of the Middle Maykop samples with the highest Hg contents fall above the sulfide drawdown line. It should be stressed that the same samples contain cinnabar (HgS) along with Hg-bearing pyrite. Therefore, both geochemical and direct mineralogical evidence allows us to infer that the abnormally high mercury level in the Middle Maykop sediments must be due to Hg binding in sulfide hosts.  Tables 3  and 4. (A) Fields A, B, and C correspond to different proposed modes of Hg sequestration according to [3,6]. (B) Hg and TOC data for Phanerozoic sediments (gray squares) according to the compilation of Grasby et al. [3]. Sulfide dragdown and maximum normal drawdown lines are according to [3].
The average Hg concentrations in the Upper Maykop shales delivered to the surface by Vladislavovsky (34 µg/kg), Soldatsko-Slobodsky (33 µg/kg), and Eny-Kale (42 µg/kg) MVs (Table 4) are slightly lower than the upper continental crust value (50 µg/kg) [75] and far below the average for sedimentary rocks, mainly shale (62.4 µg/kg) reported by Grasby et al. [3]. The Hg/TOC ratios, with the range from 7.7 to 137 (39 on average), are also lower than the respective ratios in the Middle Maykop shales (Tables 3 and 4). The Upper Maykop dataset plots in the field 'A' (Figure 6A), with low TOC and Hg reflecting OM-mediated Hg drawdown under normal marine conditions and background Hg emissions. However, this dataset falls below the maximum normal drawdown line ( Figure 6B), together with most of the available Hg and TOC data for the Phanerozoic sediments [3]. These inferences are consistent with mineralogical evidence that the Upper Maykop shales are poor in pyrite, which contains only 670-1090 µg/kg Hg and lacks Hg sulfides. Very low sulfidic mercury in bulk samples of these sediments (≤13 µg/kg) was also confirmed by direct measurements ( Table 4).
The Upper Maykop shales erupted by Nasyr and Tobichek MVs ( Figure 6; Table 4) have relatively high TOC values (2.7 to 4.1 wt%, X av = 3.3 wt%) coupled with low Hg (10 to 64 µg/kg, 26 µg/kg on average) and Hg/TOC values (5.8 to 12 µg/kg/wt%, X av = 7.7 µg/kg/wt%). These samples partly fall in the field 'B' ( Figure 6A) with high TOC (>~1.5 wt%) and variable Hg. A Hg-TOC relation such as this was attributed  to high primary productivity environments, where OM drawdown begins to limit marine Hg levels, but the high TOC values in our case appear to be due rather to later secondary oil impregnation.
The analyzed Maykop Group samples show obvious correlation between the contents of Hg and the total percentage of sulfides (mainly FeS 2 phases). Previously, a similar correlation was reported for sediments which mark the Early Jurassic Toarcian OAE [76] and for those deposited during the Late Ordovician Mass Extinction [77]. The Hg and S sulfidic concentrations in the Maykop shales correlate at R 2 = 0.67, but the correlation between Hg and TS is only R 2 = 0.35 (Figure 7, Tables 3 and 4). The latter poor correlation may result from the presence of sporadic sulfate mineralization (gypsum, barite, celestine) in addition to authigenic sulfides. Gypsum vein mineralization is common in the Upper Maykop strata [23]. The lack of obvious relationship between Hg and TS at low values of TS was also reported for the Late Permian through Middle Triassic shale-dominated sequence [3,5,6]. The Hg and TS correlation may be also biased by organic matter sulfurization, which leads to overestimation of sulfidic sulfur linked with organosulfur compounds [78,79]. Therefore, reliable estimates of Hg bound in sulfide carriers can be obtained using the contents of sulfidic mercury measured directly in bulk sediments and estimates of pyrite percentages and its Hg content [3]. Because sphalerite and HgS modifications may be extra Hg hosts in sediments, the mineralogy of sulfide concentrates should also be taken into account.  (Tables 3 and 4).
Authigenic pyrites from the Middle Maykop shales show 34 S depletion relative to seawater sulfate (about +20‰) [80]. Pyrite from the Middle Maykop shale extruded by Bulganak MVs has a large range of measured δ 34 S values: from +4 to +8.9‰ CDT (Canyon Diablo Troilite). Note that δ 34 S of cinnabar (+7.5‰ CDT) likewise fits into this range, which indicates that sulfidic sulfur comes from a single source, most likely, seawater sulfate [23,24]. The heavy S isotopic composition of sulfides and high total trace element loading of pyrite suggest depressed biogenic reduction of seawater sulfate during its formation. Prolific microbially mediated sulfate reduction leads to precipitation of relatively abundant pyrite with lighter 34 S compositions and lower trace element loading [23,81,82]. Under oxygendeficient conditions, mercury shows the strongest chloride complexation and highest hydrolysis constant, which should retard its reactivity with [HS − ] and allow prevalent incorporation of Hg into FeS 2 . On the other hand, Hg with its water exchange rate higher than Fe, is able to form its separate mineral prior to FeS 2 formation [83], as we revealed in the Middle Maykop sediments.
Thus, the accumulation of Hg in the Maykop Group sediments, which feed the Kerch MVs, was controlled by several factors. In anoxic deposition environments, mercury either forms its separate phases (namely HgS) prior to formation of authigenic Fe sulfides or incorporates into FeS 2 during early maturation of shales [23,83]. In both cases, large amounts of Hg become sequestered in sulfide carriers and accumulated in sediments together with sulfides of other chalcophile elements (Fe, Cu, Ni, Co, Zn, Cd, Ag, etc.). This mode of mercury loading is common to the Middle Maykop illitic shales making the mud reservoir for Bulganak, Korolevsky, and Big Tarkhan MVs, having the deepest roots in the area. Smectite-rich sediments erupted by small MVs of the Kerch Peninsula come from the shallower Upper Maykop strata that contain relatively abundant Fe 3+ -(oxy)hydroxides but less pyrite. In this case, organic matter, along with Fe 3+ -(oxy)hydroxides and clay particles, should be considered as the main sink for the more limited Hg resources.

Conclusions
The mercury variations and hosts in the Oligocene-Miocene Maykop Group deepwater shales erupted by mud volcanoes in the Kerch Peninsula provide some insights on Hg sources and sequestration mechanisms in the course of the Maykopian marine deposition in the Indol-Kuban basin.
(i) The Middle unit of the Maykop shale sequence is remarkable by its significant Hg anomalies and Hg/TOC spikes, which exceed the respective values in most marine shales worldwide and are comparable with those in the intervals of extinction and/or ocean anoxic events. However, both Hg and TOC values in the Upper Maykop shales are an order of magnitude lower and remain within the range for Phanerozoic sediments.
(ii) The Oligocene-Lower Miocene volcanism, which acted locally in the Caucasus Continental Collision Zone, appears to be the most probable mercury source for the Maykop Group sediments deposited in anoxic conditions of the deepwater Indol-Kuban basin. The Hg concentration in the environment during the deposition and maturation of the Middle Maykop marine shales systematically exceeded the maximum sorption capacity of OM. This estimate is supported by other features of these sediments: Hg and Hg/TOC spikes, two-to eight-fold Hg enrichments of pyrite over that from the Upper Maykop counterparts, and the presence of cinnabar. The Hg anomaly in the Middle Maykop shales may record high Hg loading caused by synsedimentary episodes of local volcanism in the Caucasus region.
(iii) The variability of Hg enrichment in the Maykop Group sediments may also result from local redox fluctuations in the deposition environment. Abundant Hg-bearing pyrite and sporadic cinnabar are main Hg hosts in the Middle Maykop deep-sea anoxic sediments. Direct measurements confirm that up to 70% of Hg in bulk sediments occurs as sulfidic mercury. Unlike these, the main Hg scavengers in the Upper Maykop shales, which were deposited in a more oxygenated environment, are organic matter, Fe 3+ -(oxy)hydroxides, and clay particles.

Data Availability Statement:
The data set is presented directly in the present study.