Ancient acid rains in Ediacaran – an alternative story for sulfate sedimentation

Vast deposits of anhydrite and magnesite widely distributed in Ediacaran strata of East Siberia near the Riphean unconformity. Anhydrite-rich rocks are not look like of evaporitic origin nd mostly nodules and the layers of chicken-wire structure otherwise disseminated as tiny sulfate forms amongst the terrigenous rocks. Here we propose an alternative point of view for anhydrite appearance – the enrichment of Sulphur because of the slashing increase the content of sulfur in the Ediacaran atmosphere due to high volcanic activity. It is suggested that the ancient Earth's atmosphere could have also been inuenced by powerful sulfuric acid rains that eroded the Precambrian dolomites causing their aggressive degradation. Chemical reactions with dolomite and sulfuric acid showed that in the early stages an unstable phase of bassanite occur which later stabilized as anhydrite after its heating as an analogue of aging. Aggressive acids have caused global process of dolomite karstication of the Siberian craton with appearance in Ediacaran strata in addition to the sulfate phases, including magnesite and sulphurous phases of pyrite and barite.


Introduction
Apparently, the Proterozoic epoch as a whole was characterized by an extensive accumulation of sulfate deposits. Extensive strata of anhydrite in the Precambrian are known in many regions of the world causing numerous disputes of its high content and unusual appearance with context of isotopic data 1,2,3,4,5,6,7 .
It is well known that the minerals of the sulfate group (anhydride and gypsum) are the product of precipitation from an oversaturated salt solutions in salted basins, or in other words they have an evaporate origin 8 . Three crystalline Calcium-sulfate phases are known in the Nature: hydrated phases of gypsum (CaSO 4 ·2H 2 O) and bassanite (CaSO 4 ·0.5H 2 O), and crystalline anhydrite (CaSO 4 ) as the nal phase and product of transformation for the rst two. However, practical experiments on the deposition of anhydrite in the laboratory showed that anhydrite has restricted conditions for primary sedimentation comparing with the hydrated gypsum and bassanite 9,10 . They also showed that the transition of hydrated sulfates to anhydrite requires long time and may not transform even after two years 10 .
The sulphate-bearing rocks of the Ediacaran of the Siberian platform have been studied. Rocks of mixed composition -multi-layer deposits of overlapping strata of a terrigenous-sulfate-carbonate sequence were drilled in deep wells at a depth of more than 2 kilometers. However, it is di cult to imagine how terrigenous rocks such as siltstones and sandstones could be deposited together with monomineralic anhydrite strata and actively mixed at the same time. One of these ways could be the ingress of clastic material into the carbonate basin -salted sabkha contaminated with clastic material. However, the sulfate layers and concretions in the form of separate nodules look isolated and were formed rather like high energy streams of solutions intruded in terrigenous sequences, subsequently acting as a cementing material for clastic rocks ( Fig.1-2). The thickness of the sulphatic interlayers are generally more than 20-30 centimeters, but they can reach up to 1-2 meters. Their numerous quantities among the terrigenous rocks indicates that they cannot form as classic evaporitic sediments.
At the same time, the surface of the Riphean of the Siberian Platform at the point of unconformity on the border of the Ediacaran and Riphean is pitted and transformed by the processes resembling the karst with the active development of voids and cavities of dissolution, both hollow and lled in new mineralsquartz, dolomite, magnesite, anhydrite. Karst or signs of karst formation are revealed in almost every deep well of basin, which indicates a fairly largescale manifestation of it on a large territory -almost the entire territory of the Siberian platform. In addition, karst zones often develop inside the Riphean strata at a depth of several meters from the surface of the unconformity. They are often lled with anhydrite, which clearly is not the product of sedimentation, but have secondary origins.
Another important feature of the lling of the Ediacaran basins of the Siberian Platform is the appearance carbonate (more often), and terrigenous layers lled with magnesite (MgCO 3 ), having a regional distribution and developed over a distance of many hundreds of kilometers in the Nepsko-Botuobinskaya anteclise 11 . Also, in the strata of the Siberian platform there are talc ndings with a stratiform occurrence and minor admixtures (1-2%) of pyrite, barite and celestine (only as traces). The general picture of deposited layers assumes the multiple strati cation and change of the terrigenous (marine) sedimentation environments of chemogenic origin, primarily carbonate, which in turn often changes to a halogen one of sulfate composition in the absence of a chloride component.
Chemical reaction in general form can be described as: Ca,Mg (CO 3 ) 2 + 2H 2 SO 4 = CaSO 4 + MgSO 4 + 2H 2 CO 3 Experimentally, the obtained solution (after full interaction) was decanted after keeping it overnight and then the dry precipitate was studied by XRD method (Fig. 3). The XRD data after the reaction showed that the dolomite was almost completely dissolved and the nal product of the reaction was bassanite (about 80%) and anhydrite (about 20%). Repeated XRD measurements after 6 months did not show any changes. However, the gradual heating of the sample in a mu e furnace showed the transformation of unstable bassanite into anhydrite (Table). We assume that bassanite, as it heats up, which can be equated with the ageing of the sample in geological time, completely turns into anhydrite as it evolves. At the same time, the content of silicon (quartz) in all samples were unchanged (Fig. 3).

Discussion
It is known that concentration of sulfate was not so high in Precambrian time causing its scarcity in seawater of the Paleoproterozoic and Neoproterozoic eras 12,13,14,6 . Fakhrae and his coauthors have explained the appearance of Sulphur in signi cant concentrations by an input sulfate into the ocean as result of weathering of sulphides on the land 14 . However, this way requires enormous elds of sul de deposits and long time for weathering and dissolution with the following transportation towards the sea basins.
Thus, it is assumed that the source of sulfur for the deposition of sulfates in the Precambrian time could be in the form of sulfuric acid rains. They actually dissolved the dolomite causing the chemical disintegration and separation of Ca and Mg. This effect, for example, was proved by experiments in which the MgCO 3 content in the dolomite composition changed when it interacted with sulfuric acid 15 . Ca and Mg subsequently formed their new phases -calcium bound with the sulfate-ion to form bassanite, and magnesium returned as carbonate in the form of magnesite or the high-magnesium dolomite, which is also, had high distribution in Precambrian 16 . It is widely known that magnesite is often adjacent to anhydrite 4,17 while the host rocks for both minerals are dolomite. The source of primary sulfur could be powerful volcanic events during the late Neoproterozoic and earlier in the Middle Proterozoic, which also contains multi-meter thicknesses of sulfate rocks around the world 18 .
As a consequence of industrialization and climate change, we are seeing frequent sulfuric acid rains in our modern era, causing the erosion of carbonates (marble statues, buildings, reefs etc.). However, the most extensive sulfuric acid rains as a geological process can be observed now on Venus, where this acid forms enormous clouds in the Venus atmosphere. McGouldrick with co-authors argued that the present atmosphere of Venus is saturated by clouds composed of a sulfuric acid/water solution creating acid rains/aerosols on the surface of planet. They also argued that the early Martian atmosphere was also sulfur-rich 19 . In addition to these, signs of sulfuric acid rains were observed on the surface of Mars in the form of the appearance of basanite on the planet`s surface. It has been proven in veins of Gale crater in Martian surface where bassanite was formed by dehydration of gypsum and where it is an important mineral phase 20,21 as well as the presence of jarosite (one more product of Sulphur activity and subsequent transformations) in Martian sediments 22 .
Volcanic activity provides the input of Sulphur in the atmosphere where it is mixed with atmospheric moisture and water and form acid clouds come in the form of rains or in the form of an aerosol. However, the difference comparing with ancient Earth is fact that neither the Venus nor the Mars have hydrosphere unlike the Earth where sea and lake water reservoirs could "extinguish" and sediment the products of the reaction of sulfuric acid rains turning and burying them in the form of precipitations as the sediments and rocks.
Simultaneously, this process caused a powerful aggressive karst formation on the dolomite surface in Precambrian lithosphere. These solutions eroded the Riphean dolomite rocks and formed the karstic processes causing the dissolution and distractions of carbonate rocks. The combination of aggressive sulfuric acid with alkaline carbonates led to their active dissolution of the Riphean dolomites and interaction with subsequent neutralization, with the formation of hydrated bassanite, magnesite, undissolved dolomite and water, as well as probably amorphous silica -SiO 2 .
All these compounds were actively washed off by water-acid currents owing down from the mountains and highland forms of Precambrian relief forming the deposits saturated with bassanite. Aggressive acid solutions fall into the relic sedimentary basins of shallow seas and lakes of the Ediacaran time and was buried with different types of other sediments -terrigenous (clayey and sandy-silty), carbonate and others, intensely mixed with them. Bassanite, as unstable mineral over time serves as a precursor phase for its further conversion to the gypsum and anhydrite. Thus, peculiar sulfate-terrigenous sequences were formed creating bizarre structural shapes of sulfate rocks. At the same time, magnesium sulfate, as an easily soluble compound in water, was actively carried by the waters also into sedimentary depressions, where it was also deposited together with terrigenous sediments and formed extensive magnesite mixtures, and later got buried together with terrigenous and carbonate rocks (Fig. 4). Thus, we suppose that the Neoproterozoic era was the epoch of extensive sulfuric acid rains that can cause catastrophic geological events of all ancient spheres of the Erath where anhydrite-rich strata could be the result of chemical neutralization after sulphuric acid rains. However, epochs of acid rain could be distributed in Mesoproterozoic time and PostCambrian on the boundaries of geological periods supported by other catastrophic events 27 . Moreover, such rains could play crucial role in vast distribution of other unusual ancient events explaining for example high silicon compounds that could have been involved in the same story and form huge talc elds 28 as well as also explain the abnormal appearance in Precambrian of sul des (the problem of "heavy pyrite") 3,29,30 and barite 31,32,33 . Method XRD-patterns were recorded using Rigaku Ultima IV X-ray diffractometer in Bragg-Brentano geometry, with a Cu anode, performed a qualitative characterization of the minerals. The X-ray tube voltage of 40 kV, the current of 30 mA, and a power of 1.2 kW. Whole rock samples were scanned from 5 to 60° 2θ at a scanning speed 1° per minute with a step of 0.02°. XRD-patterns were analyzed employing the program Siroquant, using the database ICDD-PDF2.