Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Silica recycling and replaced evaporites - 4. Proterozoic atmospheric transitions and saline microporous chert reservoirs

John Warren - Friday, October 07, 2016

Sulphate and oxygenation levels across the Proterozoic

Proterozoic (2.5 – 0.542 Ga) saline sediments encompass significant transitions in evaporite style and chemistry within an evolving atmospheric and oceanic framework. Lithospheric changes tie to a cooling and biologically-evolving earth as earth-scale plate tectonics move to a system set comparable with that operating today. The Proterozoic eon is divided into three eras: the Paleoproterozoic, Mesoproterozoic and Neoproterozoic. Thick sequences of halite are only found as the actual bedded salts in sediments of the Neoproterozoic (and the Phanerozoic), while calcium sulphate residues and beds occur in all three Proterozoic eras, especially in parts of the Paleoproterozoic and the Neoproterozoic.

Paleoproterozoic era sediments (2.5-1.6 Ga) contain isotopic evidence the first significant oxygenation event in the world's atmosphere, largely driven by the increasing dominance of cyanobacterial photosynthesis. Neoproterozoic sediments (1.0 – 0.542Ga) contain evidence of the second oxygenation event, which is associated with the evolution of widespread multicellular life and CaCO3/siliceous carapaces. By the end of the Neoproterozoic, the world oceans had chemistries, temperatures and salinities similar to those of the Phanerozoic (Blamey et al., 2016). The intervening Mesoproterozoic (1.6- 1.0 Ga) retains evaporitic residues with aspects of both the late Archean and the Phanerozoic.

The oxygenation of Earth’s atmosphere-ocean system occurred in two steps: 1) the Paleoproterozoic “Great Oxygenation Event” (GOE ≈ 2.3 Ga), which refers to the transition from a pervasively reducing Earth-surface system to one with an oxygenated atmosphere and oxygenated shallow seas, and 2) the “Neoproterozoic Oxygenation Event” (NOE), when the Earth’s atmosphere and ocean are understood to have become persistently oxygenated down to the deep ocean bottom (Turner and Bekker, 2016; Scott et al., 2014). The GOE is indicated by a Proterozoic carbon isotope anomaly known as the “Lomagundi event,” a positive carbon isotope excursion between ca. 2.22 and 2.06 Ga, interpreted to be the result of high organic carbon burial and attendant accumulation of atmospheric oxygen (Figure 1; Bekker and Holland, 2012)). A long interval spanning the remainder of the Paleoproterozoic and much of the Mesoproterozoic followed the Lomagundi event, typified by lower levels of atmospheric oxygen and little variation in carbon isotope values. This ended in the late Neoproterozoic with dramatic fluctuations, of escalating magnitude, in the biogeochemical carbon cycle and attendant fluctuations within an overall increasing oxygen content (Figure 2). By the end of the Neoproterozoic, not just shallow shelf waters but much of the deep-ocean water column was consistently oxygenated (see article 1 in this series)


 

Chert evolution

 We have already seen how low levels of oxygen and high levels of CO2 in the Archean favoured the precipitation of nahcolite, and its hydrothermal silica association, atop subsealevel isolated saline sumps in microcontinents and island arcs in a saline waterworld (article, 28 August 2016). The hydrothermally-dominated silica of the silica-rich Archean oceans is reflected in more negative 30Si isotope values in widespread marine cherts of that time, compared with most Proterozoic cherts (Chakrabarti et al., 2012; see also Figure 3b). However, shorter-term fluctuating levels of atmospheric oxygen in the Proterozoic also influenced drop-out salinities for gypsum in Mesoproterozoic marine brines. In brines derived from the modern, well-oxygenated world oceans, as in evaporite successions deposited throughout the Phanerozoic, the sulphate minerals gypsum and anhydrite precipitated from evaporating seawater after aragonite or calcite, but before halite (see 26 August, 2015, blog for more detail). At lower seawater sulphate levels across the Archean and much of the Proterozoic, gypsum and anhydrite precipitated after halite, even at Na and Cl concentrations similar to those of the modern ocean.

This is why some post-Lomagundi, Paleoproterozoic marine evaporite successions show clear evidence of halite precipitation before gypsum or anhydrite or even an absence of gypsum or anhydrite with halite (e.g., ≈ 1.88 Ga Stark Formation; Pope and Grotzinger, 2003). The post-halite precipitation of calcium sulphate is construed as evidence for a limited marine sulphate reservoir and little atmospheric oxygen (Scott et al., 2014). In contrast, Lomagundi- age sedimentary successions contain evidence for sulphate precipitation before halite (Melezhik et al., 2005; Bekker et al., 2006; Schröder et al., 2008). Sulphur isotope values of marine sulphates (in CaSO4, barite) and sulphides in marine pyrite also record expansion and contraction of anoxic oceanic settings. That is, a higher burial rate of pyrite in anoxic settings is indicated by a positive shift in the sulphur isotope values of sulphates, whereas ocean oxygenation creates a negative shift in values (e.g., Claypool et al., 1980; Strauss, 1997). Furthermore, expansion of the area of anoxic oceanic settings decreases the size of the seawater sulphate reservoir, resulting in more variable sulphur isotope values of sulphate evaporites, barites, and other carbonate-associated sulphates (Figure 2; Kah et al., 2004). This applies in particular in the Mesoproterozoic when only tshallow oceanic waters were consistently oxygenated.

When we look at silica mobility and chert styles across a Proterozoic milieu of evolving oxygen and sulphate levels we see some aspects similar to the Phanerozoic and others more akin to the high-silica oceans of the Archean. Maliva et al. (2005) and Perry and 2014 show that the latter part of the Paleoproterozoic era (post-Lomagundi) is marked by the end of widespread primary and early diagenetic silica precipitation in normal marine subtidal environments. However, silica precipitation continued apace in the deeper marine in waters that were still anoxic. The Paleoproterozoic is defined by the “rusting” of the shallower parts (shelves and upper slopes) of the world’s ocea,n as dissolved oxygen levels increased and the accumulation of widespread Banded Iron Formations (BIFs) occurred, including the huge deposits of NW Australia.

So where and when do we see nodular cauliflower chert after sulphate in the Proterozoic?

Some of the oldest silicified nodular sulphates with cauliflower chert textures and actual relict anhydrite are found in the Huronian Gordon Lake Formation (≈2.4 Ga; Chandler, 1988). The nodules are commonest in mud chip breccia at the base of sandstone, siltstone and mudstone upward-fining storm cycles. Anhydrite nodule relicts are composed of a mosaic and meshworks of blocky crystal laths. Earlier laths are preserved in silicified outer rims of many cauliflower chert nodules, with texture alignments similar to that of Recent displacive sabkha anhydrite nodules. Completely silicified nodules are composed of megaquartz, some calcite-cored, or of jasper, with replacement textures identical those documented the Phanerozoic by Milliken (1979). Similarly-textured cauliflower cherts are found in the Mallapunyah Formation (1650 Ma) in the Paleoproertozoic sediments of the McArthur Basin in the Northern Territory of Australia (Warren, 1999). Thin sections through 5-30cm diameter Mallapunyah nodules (in a redbed host) can still retain small relict highly-birefringent laths of anhydrite, but most of the former felted cores to the nodules are now composed of mimetic silica. There are also older sedimentary chert nodules in the McArthur Basin succession located in units below the level of the Mallapunyah, but they are smooth walled not rugose-surfaced features.

Unfortunately, the term cauliflower chert is loosely defined and is used to describe cemented features in Meosproterozoic and earlier sediments and metasediments, which are not true calcium sulphate evaporite replacements.Although termed cauliflower features, they do not have surface textures resembling the florets of a cauliflower (see article 2 in this series - July 31, 2016). For example, aggregates and clusters of growth-aligned barite crystals in the Archean of South Africa are described as cauliflowers when they should be described as bladed, palmate crystal aggregates (Reimers and Heinrich, 1997). Interstingly, Chowns and Elkins (1974) in a study of cauliflower cherts occurrences across the USA list no examples older than Cambrian. Using a tighter definition of cauliflower chert and recognising that this term should not be interchangeable with crocodile-skin chert it seems that Proterozoic occurrences of cauliflower chert nodules largely mirror times when oxygen levels were sufficiently high in the world's ocean to allow sulphate in solution. In the Paleoproterozoic and Mesoproterozoic only the upper parts of the ocean column, including waters covering the world's continental shelves (and derived evaporite basins) were sufficiently oxygenated to allow the formation of cauliflower chert after nodular anhydrite. However in some Neoproterozoic basins, especially if located in sumps in a highly-restricted brine layered seafloor, the levels of anoxia in the ponded bottom brines facilitated the accumulation of laminar microporous chert in association with evaporites or their early replacements


Primary laminated hypersaline silica chert in an evaporite basin at the Precambrian-Cambrian boundary

An organic-rich laminated porous chert known as the Athel or Al Shamou silicilyte consists of up to 90% microcrystalline quartz along with dolomite, magnesite, anhydrite and halite (Rajaibi et al., 2015). It occurs at the Precambrian-Cambrian boundary in the subsurface of the South Oman Salt Basin, Sultanate of Oman, where it acts as a light-oil reservoir  (Ramseyer et al., 2013; Amthor et al., 2005). Fully encased in variably halokinetic salt masses, it was first discovered during the 1990's hydrocarbon exploration activities of Petroleum Development Oman. This laminated microporous and variably fractured chert, has its source of silica and its mode of precipitation tied to an anoxic, sulphur-rich, stagnant and highly saline basin. Its homogeneous silica distribution and high Si isotope values (avg. d30Si = +0.83 ± 0.28), coupled with a low molar Ge/Si ratio (<0.25 x 10-6) in its microcrystalline quartz matrix imply dissolved silica in concentrated seawater as the Si source, and hydrothermal or biogenic (e.g. sponge-derived) silica are excluded.

Silica precipitation from a seawater-sourced brine was likely the result of a dramatic increase in salinity in response to halokinetic salt dissolution atop and adjacent to the edges of transtensional depressions on a deep basin floor in the South Oman Salt basin, thus markedly reducing the solubility of amorphous silica in these brine-filled seawater depressions. This saturation triggered the formation of silica gel. The gel accumulated at the base of a brine-layer covered basin floor, forming a soft silica-rich layer bound into bacterial mats, giving rise to its fine-scale lamination. The mean number of laminae in this laminated chert is ca. 32 per year suggesting that layering is non-annual and controlled by processes such as fluctuations in nutrient supply, lunar driven re-mixing or diagenetic segregation. The transformation of the silica-gel to microcrystalline quartz occurred below 45°C indicating a less than -4.5‰ d18O composition of the pore-water during microcrystalline quartz formation. The  microporous hydrocarbon filled nature of this ancient chert and the fact the hydrocarbon-filled micropores are still distinct after more than 500 million years after they filled (Figure 4d, e) is  why when artificially fractured the silicilyte can act as a hydrocarbon reservoir (See Rajaibi et al. 2015 and Warren, 2016; Chapter 10 for a summary of relevant literature).
 

 

What does this mean for other evaporite-associated laminar cherts in the Phanerozoic?
Beds of laminated chert (not replaced, but precipitated, primary silica accumulations) are unusual across the Phanerozoic, but cauliflower cherts as replacement of calcium sulphate evaporites are not (Chowns and Elkins, 1974). Spanning the Precambrian boundary, the Athel Silicilyte is the first Phanerozoic example of a laminar chert-hypersaline association. Then, there is the somewhat younger but world-famous Devonian to early Carboniferous Caballos Novaculite. This chert location was the site of a world-famous set of arguments as its origin, between two world-renown professors, Dr Earle McBribe and Dr Robert Folk, both on the faculty of the University of Texas at that time. Folk argued for a shallow-water peritidal hypersaline depositional setting, McBride for a deep a marine setting but still with possible hypersaline indicator textures (Folk, 1973; Folk and Mcbride, 1976; McBride and Folk 1977).

The Caballos Novaculite outcrops in the Marathon Uplift of Texas, while its lithologic and time equivalent, the Arkansas Novaculite, outcrops in the Ouachita Mountains of Arkansas and Oklahoma (Figure 5). Novaculite chert) in outcrop is very resistant to erosion so that layers of novaculite stand out as characteristic ridges and dip slopes in the Ouachita and Marathon mountains (Figure 5). This outcrop forms and its hard abrasive nature gives it the name novaculite, which in its Latin root novacula, means razor-stone. When some novaculite is fractured in the subsurface, there is sufficient connected porosity to form a fractured reservoir play, as in Arkansas and Texas. There, some 30 years ago, oil and gas fields such as Isom Springs in Oklahoma and McKay Creek, Pinion and Thistle fields in West Texas were discovered in the Caballos and Arkansas novaculite-chert. The chert reservoir is most productive when it is highly fractured, occurs within complex thrust faults and has had any enclosed carbonate material leached from its chert matrix, so creating microporosity (Figure 6; Godo et al., 2011).

Chert beds in the Caballos Novaculite are composed of equant grains of microcrystalline quartz, minor amounts of illite and radiolaria, and trace amounts of pyrite, carbonate and other minerals and organic matter. The chert beds are generally interpreted as having formed by the silicification and alteration of precursor sediment, sometimes massive, other times finely laminated. Some beds retain occasional evidence biogenic silica derived from radiolaria, while underlying levels retain can contain abundant siliceous sponge spicules. Fractures and crackle breccias developed in a grey chert following lithification; green siliceous sediment, whose lithification was impeded by clay, filled these pre-orogenic fractures.
Beds of red shale, chert pebble and cobble conglomerate, sandstone, limestone, dolomite, and lumpy manganiferous and jasperitic chert make up no more than 3% of the chert and shale members of the Caballos, but still are of controversial origin and environmental significance. The chert conglomerate beds, for example, are interpreted as tidal-channel deposits by Folk and as mass-flow deposits by McBride. Jasper beds texture are considered bizarre by many geologists who have worked on them: they are lumpy, uneven beds 0.2 to 2 m thick composed of cherry-red chert with local geopetal cavities, contorted laminae. manganiferous zones, cauliflowerlike quartz-filled nodular cavities sometimes with hollow centres and variably filled or partially filled with zebraic chalcedony, lutecite, quartzine, pseudocubic quartz crystals, and filamentous structures resembling algae. These beds are interpreted by Folk as the product of diagenetic alteration of sabkha evaporite nodules and siliceous ooze, during and following subaerial exposure with soil development, and by McBride as the product of diagenetic alteration of evaporite beds deposited in deep water and sandwiched between radiolarian ooze. Synthesis of evidence on the origin of both the novaculite and chert and shale members leads to contrasting interpretations of water depth during deposition. However, if the evaporite solution breccias, recognised as such by both authors and the cauliflower cherts as replaced diagenetic anhydrite clusters then the depositional setting is akin to that of the Athel Silicilyte (namely a deepwater holokinetic hypersaline evaporite setting not unlike a siliceous DHAL association (see Warren, 2016; Chapter 9, for discussion of the DHAL literature).
Economic implications of understanding what defines a silicilyte versus novaculite versus tripolite versus diatomaceous ooze
We have already seen that microporous cherts when fractured are possible reservoir rocks as illustrated by the saline-associated Athel Silicilyte and the Caballos Novaculite. The former retains its microporosity because of an early hydrocarbon charge into existing microporosity (Rajaibi et al. 2013; Amthor et al., 2005), while the reservoir quality of the latter was enhanced by diagenetic leaching of finely dispersed carbonate material, likely when it was caught up in the Ouachita Orogeny (Figure 6; Godo et al., 2011). Like the Athel, the Arkansas (Caballos) Novaculite is thought to be self-sourcing in terms of reservoir hydrocarbons (Zemmels et al., 1985).
The term silicilyte is defined by Rajaibi et al. (2013) as a "locally-used" term to describe porous organic-rich laminated chert, it is a succession of microcrystalline quartz that is preserved within salt-encased slabs, 300 to 400 m thick, at a depth of 4 to 5 km in the South Oman Salt Basin. As mentioned earlier, the term novaculite comes from its outcrop expression and its "razorstone" properties and does not necessarily have a direct connection to microporosity in the reservoir portions of the unit (Figure 5). Outcrops of weathered microporous chert zones in the upper part of the Arkansas (Caballos) Novaculite are called tripoli or rottenstone (Figure 6). When present in this finely powdered microporous form, it is quarried and crushed for use as a polishing abrasive in metalsmithing and woodworking. When present as a very hard dense rock, it can be cut and shaped for use as a whetstone or razorstone. Before European settlement, novaculite was a source for numerous arrow-tips, spear tips and knives. 
Etymological variations of the terms tripoli, novaculite and silicilyte as forms of chert, as currently used in the geological literature are interesting and geologically confused. This is particularly true if the writer did not 1) understand there are various origins to laminar sometimes microporous cherts, and  2) that there various possible silica sources and precipitation/replacement mechanisms can be halotolerant bacteria, or other silica sources that can be tied to marine sponges and yet others to radiolaria and diatomaceous oozes. Hence, there is the time-related aspect of biogenic chert evolution tying back to the source of the silica in some cherts and the presence or lack of salinity indicators in a laminar chert and chert nodules (e.g., cauliflower versus crocodile-skin cherts).
A diatomaceous ooze is a form of opaline silica made up of accumulations of siliceous frustules of diatoms in normal marine pelagic sediments and when it retains microporosity is sometimes called tripolite or tripolitic earth. Diatomaceous oozes, the precursor to this type of tripolitic earth is often laminated, but is harsh to the feel and scratches glass. To add to the confusion there are microporous mesohaline diatomaceous oozes called the Tripoli unit in many Messinian subbasins.
Tripoli or tripolite powder is also the term used to describe the form of microporous laminar chert used as an abrasive and collected from weathered zones of the Caballos and Arkansas Novaculites. The Palaeozoic age of the Caballos and Arkansas Novaculite means it cannot contain diatoms. Diatoms evolved in the Cretaceous and have been the dominant source of remobilised silica in marine chert nodules ever since. Palaeozoic silica remobilized into chert is often related to nearby sponge spicule horizons, or less often to radiolarian beds. The further back in time, and perhaps the more saline the marine bottom water, the greater the likelihood of a microbial association with chert precipitation and remobilisation.
Even in Tertiary strata some microporous diatomaceous earths can have a normal-marine, organic-enriched depositional association and can constitute a fractured microporous hydrocarbon reservoir. This is the case with the Miocene-age fractured microporous chert reservoirs that produce today in the Santa Barbara Basin of offshore California (Reid and McIntyre, 2001). Cores and coastal outcrops of the Monterey Formation show this type of marine-deepwater diatomaceous ooze is interlayered with microbial (bacterial-archeal) methanogenic organic-rich dolomites. Then, there are the deepwater diatomaceous saline oozes in the Miocene units that immediately underly Messinian evaporites in the Lorca and other sub-basins across the Mediterranean (Rouchy et al., 1998). These diatomaceous organic-rich oozes were deposited as pelagic sediments in saline waters on stratified bottoms that herald the creation of saline bottom water layers related to the onset of hypersaline conditions. Their depositional setting is in restricted basins with increasingly saline bottoms, driven by tectonic isolation and drawdown that soon after precipitated the salts of the Messinian Salinity Crisis. Finally, there are the lacustrine diatomaceous oozes accumulating at the base of density-stratified water columns in lakes of African Rift Valley. This type of laminar ooze occurred in the deeper parts of the lake floor and in Lake Magadi and Lake Natron define units that immediately predate significant lake drawdown episodes that are defined by the type 1 (sodium bicarbonate) evaporite layers. These laminar diatomaceous chert beds contain nodules with the characteristic surface shrinkage textures of crocodile-skin chert (see article 1 in this series of four).
So, in terms of silica with an evaporite association, "one size does not fit all.' There are multiple saline settings, both depositional and diagenetic, with silica sources evolving with life across the Proterozoic into the Phanerozoic. Interactions between biology and brine chemistry control the accumulation of silica in various forms in a range of evaporite settings, ranging across the marine to deep halokinetic seafloors to lacustrine basins. Once we understand how to recognise cauliflower chert and its possible association with laminar saline cherts of the Proterozoic and the Phanerozoic, then particular chert styles can help to define the evolution of atmospheric oxygen and the saline versus non-saline origins of some organic-rich laminar biogenic microporous cherts.

References

 

Al Rajaibi, I. M., C. Hollis, and J. H. Macquaker, 2015, Origin and variability of a terminal Proterozoic primary silica precipitate, Athel Silicilyte, South Oman Salt Basin, Sultanate of Oman: Sedimentology, v. 62, p. 793-825.

Amthor, J. E., K. Ramseyer, T. Faulkner, and P. Lucas, 2005, Stratigraphy and sedimentology of a chert reservoir at the Precambrian-Cambrian boundary: the Al Shomou Silicilyte, South Oman Salt Basin: Geoarabia, v. 10, p. 89-122.

Bekker, A., and H. D. Holland, 2012, Oxygen overshoot and recovery during the early Paleoproterozoic: Earth and Planetary Science Letters, v. 317-318, p. 295-304.

Bekker, A., J. A. Karhu, and A. J. Kaufman, 2006, Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America: Cambrian, v. 148, p. 145-189.

Blamey, N. J. F., U. Brand, J. Parnell, N. Spear, C. Lécuyer, K. Benison, F. Meng, and P. Ni, 2016, Paradigm shift in determining Neoproterozoic atmospheric oxygen: Geology.

Chakrabarti, R., A. H. Knoll, S. B. Jacobsen, and W. W. Fischer, 2012, Si isotope variability in Proterozoic cherts: Geochimica et Cosmochimica Acta, v. 91, p. 187-201.

Chandler, F. W., 1988, Diagenesis of sabkha-related, sulphate nodules in the early Proterozoic Gordon Lake formation, Ontario, Canada: Carbonates and Evaporites, v. 3, p. 75-94.

Chowns, T. M., and J. E. Elkins, 1974, The origin of quartz geodes and cauliflower cherts through the silicification of anhydrite nodules: Journal Sedimentary Petrology, v. 44, p. 885-903.

Claypool, G. E., W. T. Holser, I. R. Kaplan, H. Sakai, and I. Zak, 1980, The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation: Chemical Geology, v. 28, p. 199-260.

Folk, R. L., 1973, Evidence for Peritidal Deposition of Devonian Caballos Novaculite, Marathon Basin, Texas: Bulletin American Association Petroleum Geologists, v. 57, p. 702-725.

Folk, R. L., and E. F. McBride, 1976, The Caballos Novaculite revisited Part I: ”Origin of novaculite members": Journal of Sedimentary Petrology, v. 46, p. 659-669.

Godo, T. J., P. Li, and M. E. Ratchford, 2011, Exploration for the Arkansas Novaculite Reservoir, in the Southern Ouachita Mountains, Arkansas: AAPG Search and Discovery Article #90124 © 2011 AAPG Annual Convention and Exhibition, April 10-13, 2011, Houston, Texas.

Heinrichs, T. K., and T. O. Reimer, 1977, A sedimentary barite deposit from the Archean Fig Tree Group of the Barberton Mountain Land (South Africa): Economic Geology, v. 72, p. 1426-1441.

Kah, L. C., T. W. Lyons, and T. D. Frank, 2004, Low marine sulphate and protracted oxygenation of the Proterozoic biosphere: Nature, v. 431, p. 834-838.

McBride, E. F., and R. L. Folk, 1977, The Caballos Novaculite revisited; Part II, Chert and shale members and synthesis: Journal of Sedimentary Research, v. 47, p. 1261.

Melezhik, V. A., A. E. Fallick, D. V. Rychanchik, and A. B. Kuznetsov, 2005, Palaeoproterozoic evaporites in Fennoscandia: implications for seawater sulphate, the rise of atmospheric oxygen and local amplification of the delta C-13 excursion: Terra Nova, v. 17, p. 141-148.

Milliken, K. L., 1979, The silicified evaporite syndrome; two aspects of silicification history of former evaporite nodules from southern Kentucky and northern Tennessee: Journal Sedimentary Petrology, v. 49, p. 245-256.

Pope, M. C., and J. P. Grotzinger, 2003, Paleoproterozoic Stark Formation, Athapuscow Basin, Northwest Canada: Record of cratonic-scale salinity crisis: Journal of Sedimentary Research, v. 73, p. 280-295.

Ramseyer, K., J. E. Amthor, A. Matter, T. Pettke, M. Wille, and A. E. Fallick, 2013, Primary silica precipitate at the Precambrian/Cambrian boundary in the South Oman Salt Basin, Sultanate of Oman: Marine and Petroleum Geology, v. 39, p. 187-197.

Reid, S. A., and J. L. McIntyre, 2001, Monterey Formation porcellanite reservoirs of the Elk Hills field, Kern County, California: Bulletin American Association Petroleum Geologists, v. 85, p. 169-189.

Rouchy, J. M., C. Taberner, M. M. Blanc-Valleron, R. Sprovieri, M. Russell, C. Pierre, E. Di Stefano, J. J. Pueyo, A. Caruso, J. Dinares-Turell, E. Gomis-Coll, G. A. Wolff, G. Cespuglio, P. Ditchfield, S. Pestrea, N. Combourieu-Nebout, C. Santisteban, and J. O. Grimalt, 1998, Sedimentary and diagenetic markers of the restriction in a marine basin: the Lorca Basin (SE Spain) during the Messinian: Sedimentary Geology, v. 121, p. 23-55.

Schroder, S., A. Bekker, N. J. Beukes, H. Strauss, and H. S. van Niekerk, 2008, Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the 2.2-2.1 Gyr shallow-marine Lucknow Formation, South Africa: Terra Nova, v. 20, p. 108-117.

Scott, C., B. A. Wing, A. Bekker, N. J. Planavsky, P. Medvedev, S. M. Bates, M. Yun, and T. W. Lyons, 2014, Pyrite multiple-sulfur isotope evidence for rapid expansion and contraction of the early Paleoproterozoic seawater sulfate reservoir: Earth and Planetary Science Letters, v. 389, p. 95-104.

Strauss, H., 1997, The isotopic composition of sedimentary sulfur through time: Palaeogeography Palaeoclimatology Palaeoecology, v. 132, p. 97-118.

Turner, E. C., and A. Bekker, 2016, Thick sulfate evaporite accumulations marking a mid-Neoproterozoic oxygenation event (Ten Stone Formation, Northwest Territories, Canada): Geological Society of America Bulletin, v. 128, p. 203-222.

Warren, J. K., 1999, Evaporites: their evolution and economics: Oxford, UK, Blackwell Scientific, 438 p.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released Feb. 2016: Berlin, Springer, 1854 p.

Zemmels, I., P. L. Grizzle, C. C. Walters, and F. R. Haney, 1985, Devonian Novaculites as Source of Oil in Marathon-Ouachita Thrust System (Abstract): Bulletin American Association Petroleum Geologists, v. 69, p. 318-319.


 

Seawater chemistry (2 of 2): Precambrian evolution of brine proportions

John Warren - Wednesday, August 26, 2015

We saw in the previous Salty Matters article (part 1 of 2) that ionic proportions of major ions in seawater and oceanic salinity have changed through the Phanerozoic and so influenced the make-up of bittern precipitates once the lower salinity salts (carbonates, gypsum and halite) had precipitated. In the Phanerozoic, seawater was dominantly a Na-K-Mg-Ca-Cl (Ca-rich) brine that changed periodically to a Na-K-Mg-Cl-SO4 (SO4-rich) type, as in the modern ocean. This oscillation across 600 million years forces  number of questions, for example, do similar oscillations in ocean chemistry extend back across the Precambrian? How consistent is the chemistry of the world’s oceans since the early Archean? Does the evaporite evidence in Precambrian sediments support a notion of a primordial reducing atmosphere and/or higher levels of bicarbonate in an early Archean ocean?

Some authors postulate that there have been no significant changes in the major ion proportions in seawater and hence the evaporation mineral series for the past 4 Ga (Morse and Mackenzie, 1998). Others assert that the Archean was dominantly a time of little or no atmospheric oxygen and that ocean waters were reducing anoxic fluids and so sulphate levels were low and sulphide levels high in evaporative marine waters (Krupp et al., 1994). Yet others propose that the bicarbonate to calcium ratio was so high in Archean and Palaeoproterozoic seawater compared to today that all the calcium was used up in widespread abiotic marine aragonite and Mg-calcite precipitates (Sumner and Grotzinger, 2000). In this case trona or nahcolite are likely marine evaporites in the early Archean bitterns (see Figure 1 in part 1). Still others have theorised cyclic changes in oceanic chemistry occurred across much of the Precambrian were similar to those of the Phanerozoic. Such changes were perhaps related to changes in styles and rates of sea floor spreading-hydrothermal circulation in midoceanic ridges (Channer et al., 1997) and the development of tonalitic continents (Knauth, 1998). 

Given that the world's oldest known halites occur in the Bitter Springs Formation in the Amadeus Basin of Australia and that they were deposited some 840 Ma, we can only extend a halite chevron inclusion-based study of ocean chemistry back to that time. These brines were sulphate-depleted, while recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545 Ma) in Pakistan, contains solitary inclusions indicating SO4-rich brines (Kovalevych et al., 2006). This supports a similar late Neoproterozoic ocean chemistry to today, as do proportions derived from primary fluid inclusions from the Neoproterozoic Ara Formation of Oman (ca. 545 Ma). It seems that  SO4-rich seawater existed during latest Neoproterozoic time. In contrast while recrystallised halite from the somehat older Bitter Springs Formation contains brine inclusions that are entirely Ca-rich, implying ambient basin brines and the mother seawater were Ca-rich some 830-840 Mas. These combined data, supported by the timing of aragonite and calcite seas, as preserved in various marine carbonates, suggest that during the Neoproterozoic, significant oscillations of the chemical composition of marine brines, and seawater occurred over the last 250 million years of the NeoProterozoic, and that the end-members were similar to those of the Phanerozoic oceans. It seems that Ca-rich seawater dominated for a substantial period of Late Precambrian time (more than 200 Ma) from 850 Ma, until some 650 Ma, this was replaced by SO4-rich seawater, returning to Ca-rich seawater at 530 Ma. 

The detail for much of the remaineder of the Precambrian back to 4 Ga is far less precise than when modelling inclusion chemistries based on actual halites. The oldest documented chevron halite is 850Ma and the oldest bedded anhydrite is 1.2Ga, beyond that, only evaporite pseudomorphs are available to study. So, beyond the 850 Ma record established by halite inclusions in the Bitter Springs Fm., can other Precambrian evaporites especially the calcium sulphates with a record that extends back patchily to the Mesoproterozoic, give indirect clues as to a chemical scenario for the world’s paleo-oceans and brine?

 

Pseudomorphs, especially of halite hoppers, occur in marine rocks as old as Archean, but are far more common, as are the actual salts, in Proterozoic strata (Figure 1; Warren, 2016). Halite or its pseudomorphs characterise areas of widespread marine chemical sedimentation from the Archean to the present. CaSO4 pseudomorph distribution is more enigmatic. In the 1980s and 1990s, the oldest documented CaSO4 pseudomorphs were thought to cm-sized growth-aligned barytes and cherts in 3.45 Ga metasediments in the Pilbara/North Poleregion of Western Australia. They were interpreted as replacing primary bottom-nucleated gypsum (Figure 2; Barley et al., 1979; Lowe, 1983; Buick and Dunlop, 1990). These barytes and cherts occur in volcaniclastics in association with what are possibly the world’s oldest stromatolites (Hofmann et al., 1999; Allwood et al., 2007). Similar growth-aligned baryte crystals, which initially were also interpreted as likely primary gypsum pseudomorphs, occur in the Nondweni greenstones in South Africa, some 3.4 Ga (Wilson and Versfeld, 1994).

 

Sequences in both regions are now completely silicified or barytised. At the time they were first documented, the recognition of what were considered shallow-water Early Archean gypsum pseudomorphs at North Pole, Pilbara Craton, caused a re-evaluation of models of a totally reducing Archean atmosphere (Dimroth and Kimberley, 1975; Clemmey and Badham, 1982). The presence of free sulphate in surface brines of the Archean world was thought to imply an at least locally oxygenated hydrosphere. Gypsum precipitating in Archean ocean waters also meant calcium levels in the ocean waters were in excess of bicarbonate, as is in the modern oceans. The presence of free-standing gypsum on the seafloor is incompatible with any model of the Early Archean ocean as a “soda lake.”

However, in both the Pilbara and the South African sequences there are no actual calcium sulphate evaporites preserved, only growth-aligned crystal textures, now preserved as baryte or chert. Textures in baryte ore from Frasnian sediments in Chaudfontaine, Belgium, are near identical to those observed at North Pole, Australia. The Belgian barytes are primary shallow subsea-bottom precipitates with no precursor mineral phase (Figure 2 inset; Dejonghe, 1990). Some workers in the Pilbara feel that the growth-aligned Archean baryte in this region is also a primary seafloor precipitate, formed in the vicinity of hydrothermal vents (Vearncombe et al., 1995; Nijman et al., 1999; Runnegar et al., 2001). As such, it is not secondary after gypsum. A similar hydrothermal discharge model has been developed for aligned barytes in the Barberton Greenstone belt (de Ronde et al., 1994, 1996). 

Based on this more recent analysis, levels of Archean sulphate in the world ocean were probably less than a few percent of the current levels and probably remained so until the evolution of a widespread oxygen-producing biota into the Proterozoic (Figures 3, 4; Habicht and Canfield, 1996; Kah et al., 2004). Barium sulphate is highly insoluble in modern oxygenated seawater. To carry large volumes of barium or sulphur (as sulphide) in seawater solution to the precipitation site required anoxic conditions. If the aligned baryte crystals are primary, their formation still requires sulphate to be locally present on the seafloor, at least in the vicinity of the depositional site. A possible source for local sulphate production in the shallow waters that characterised the North Pole site was shortwave ultraviolet photoxidation of volcanic SO2, indicating an inorganic association (Runnegar et al., 2001). Within barytes in the same 3.47-Ga-old barytes there are microscopic sulphides. These sulphide inclusions show a d34S of 11.6‰, possibly indicating microbial sulphate reduction with H2 as electron donor in what was an anoxic seafloor (Canfield et al. 2004; Shen et al., 2009).

According to Nijman et al. (1999) the occurrence of the North Pole baryte in sedimentary mounds atop growth faults meant sulphate was locally derived via boiling of escaping hydrothermal vent waters enriched in Ba, Si and sulphide. As these hydrothermal waters vented beneath marine water columns perhaps 50 metres deep, they boiled or violently degassed. Consequent mixing with normally stratified seawater, caused instantaneous oxidization of sulphide into sulphate that then, on cooling, combined with the Ba to precipitate as growth-aligned baryte crystals on the seafloor. Conflicting notions (replaced gypsum versus primary baryte) mean that at this stage of our understanding, the bedded baryte evidence cannot be reliably used to support an evaporite paragenesis of gypsum and so infer an Archean ocean with ionic proportions similar to those of today.

Archean and Proterozoic distributions of gypsum have been further complicated by the misidentification of primary aragonite splays and pinolitic siderite marbles as gypsum replacements (Warren 2016; Chapter 15). When these misidentifications are removed from the record it is obvious that calcium sulphate precipitating directly from Archean seawater to form widespread beds did not occur, and that precipitation of aragonite as thick crusts on the sea floor was significantly more abundant than during any subsequent time in earth h istory. In contrast to gypsum, halite pseudomorphs are found throughout the Precambrian (Figure 1;e.g. Boulter and Glover, 1986). 

Grotzinger and Kasting (1993) argue that high levels of atmospheric CO2 meant HCO3/Ca ratios were much higher in the Archean and the Palaeoproterozoic oceans than today. All the calcium in seawater was deposited as marine cementstones and other alkaline earth precipitates well before bicarbonate was depleted and there was no Ca left over to precipitate as gypsum. The early Archean ocean was perhaps a Na–Cl–HCO3 sea, and not the Na–Cl ocean of today (Kempe and Degens, 1985; Maisonneuve, 1982). This early Archean hydrosphere had a chemistry similar to that found in modern soda lakes like Lake Magadi and Lake Natron (pathway I brines) and hence the term “soda-lake oceans” (see Figure 1 in part 1) This rather different marine brine chemistry would have precipitated halite and trona/nahcolite, not halite/gypsum. It probably meant that if gypsum did ever precipitate from Archean seawater it did so only in minor amounts well after the onset of halite precipitation. Excessive sodium in the ocean may help explain the ubiquity of stratiform albitites in much of the Archean. They would have formed throughout the marine realm as early diagenetic replacements of labile volcaniclastics/zeolites in volcanogenic/greenstone terranes).

A case for nahcolite (NaHCO3) as a primary evaporite, along with halite, in the 3.42 Ga rocks of the Barberton greenstone belt was documented by Lowe and Fisher-Worrell (1999). Sugitani et al. (2003) reported silicified nahcolite (the high CO2 form of sodium carbonate salts) in ≈3.2 Ga rocks in the northern part of the Eastern Pilbara block, Western Australia. Coarse, upward-radiating, silicified evaporite crystals in the ca. 3.47–3.46 Ga Strelley Pool Chert (Lowe, 1983) show the same habit, geometry, and environmental setting as nahcolite in the Barberton belt and also probably represent silicified NaHCO3 precipitates (Lowe and Tice, 2004).


Marine nahcolite in the 3.5-3.2 Ga sedimentary record is thought to be evidence of surface temperatures around 70±15°C (Figures 3b, c, 4; Lowe and Tice, 2004). Contemporary early Archean nahcolite (NaHCO3) as a primary evaporitic mineral in a very aggressive weathering regime, in the absence of land vegetation, is best explained by a mixed CH4 and CO2 atmospheric greenhouse. CH4/CO2 ratios were <<1 and pCO2 was at least 100-1000 times the present value, perhaps as high as several bars (Kaufman and Xiao, 2003). The formation of large areas of continental crust at 3.2-3.0 Ga, including the Kaapvaal and Pilbara cratons, resulted in the gradual depletion of atmospheric CO2 through weathering and a lack of marine nahcolite since the early Archean. By 2.9-2.7 Ga, declining pCO2 was associated with climatic cooling and siderite-free soils. 

Transitory CH4/CO2 ratios of ~1 may have resulted in the sporadic formation of organic haze from atmospheric CH4, and are reflected in one or more isotopic excursions involving global deposition of abnormally 13C-depleted organic carbon in sediments of this age. Surface temperatures of <60°C after 2.9 Ga may have allowed an increase in the distribution and productivity of oxygenic photosynthetic microbes (and a decrease in sulphur dependent thermophiles). Eventual lowering of newly formed continental blocks by erosion, reduced loss of atmospheric CO2 due to weathering, and continued long-term tectonic recycling of CO2 resulted in rising pCO2 and decreasing CH4/CO2 ratios in the later Archean and eventual re-establishment of a mainly CO2 greenhouse. Similar events may have been repeated in the latest Archean and earliest Proterozoic, but gradually rising production of O2 effectively kept CH4/CO2 ratios to <<1.

 

By 2.2-2.0 Ga and perhaps as early as 2.5 Ga, reliable examples of pseudomorphs after primary marine-sourced calcium sulphate first appear in the rock record, but aside from the Karelian beds associated with the Lomagundi Event (LE), widespread stratiform sulphate beds of anhydrite do not appear until 1.2 Ga (Figure 5a). Undeniable CaSO4 nodular and lenticular pseudomorphs are widespread in latest NeoArchean of South Africa and Palaeoproterozoic to Mesoproterozoic sediments of the McArthur Basin, Northern Territory, Australia, and in rocks of Great Slave Lake in northern Canada. For example, in the Malapunyah Formation (1.65 Ga) of the Northern Territory, Australia, the outer portions of numerous decimetre to metre-diameter silicified anhydrite nodules still retain outlines of felted anhydrite laths (pers. obs). The oldest reliable sulphate pseudomorphs after anhydrite and gypsum in Australia come from Palaeoproterozoic cherts in the 2.0-2.2 Ga Bartle Member of the Killara Formation, western Australia (Pirajno and Grey, 2002). These cherts locally retain small amounts of anhydrite (verified by XRD, as well as appearing as highly birefringent flecks in thin sections). Other widespread but younger sulphate pseudomorphs occur in the 1.2 Ga Amundsen Basin in the Canadian Arctic Archipelago. Actual CaSO4 beds outcrop in the 1.2 Ga Society Cliff Formation in Baffin and Bylot Islands of the Canadian Archipelago (Kah et al., 2001, 2004). Sulphate evaporite pseudomophs and nodules in all these Neoproterozoic basins are hosted in sedimentary layers up to tens of metres thick and with lateral extents measured in hundreds of square kilometres. All were laid down in shallow marine, coastal, and alluvial environments under an increasingly oxygenated Meso- to Neoproterozoic atmosphere (Jackson et al., 1987; Walker et al., 1977). After passing from the Archean, by the Mesoproterozoic the hydrosphere contained free sulphate and Ca/HCO3 ratios were lower, leading to a decrease in molar-tooth, herringbone and other carbonate textures indicative of widespread inorganic calcium carbonate saturation in shallow oceanic waters (Figure 6). However, oceanic mother brines for these now-widespread calcium-sulphate evaporites were largely H2S rich with only moderate levels of oxygen in the atmosphere until some 800 Ma (Figure 3a).

The work of Kah et al. (2004) shows that prior to 2.2 Ga, when oxygen began to accumulate in the Earth’s atmosphere, sulphate concentrations in the world’s oceans were low, <1 mM and possibly <200 μM (Figure 5). By 0.8 Ga, oxygen and thus sulphate levels had risen significantly. Sulphate levels were between 1.5 and 4.5 mM, or 5–15% of modern values, for more than a billion years after initial oxygenation of the Earth’s biosphere some 2.2-2.4 Ga and mid -ocean depth waters were anoxic for most of that time (Brocks et al., 2005). Marine sulphate concentrations probably remained low, no more than 35% of modern values, for nearly the entire Proterozoic. A significant rise in biospheric oxygen, and thus oceanic sulphate, may not have occurred until the latest Neoproterozoic (0.54 Ga), just before the Cambrian explosion, when sulphate levels may have reached 20.5 mM, or 75% of present day levels. This is a time when thick sulphate platforms first characterised the salt basins of Oman, prior to that most actual calcium sulphate is in the form of nodules or relatively thin beds.

In a refinement of the sulphate model, Bekker and Holland (2012) note that free sulphate bottom-nucleated sulphate evaporites and not just pseudomorphs were present during the Lomagundi Event (2.22 to 2.06 Ga), and then became relatively scarce once more until some 1.2 Ga. For example, there is a 200 m thick stratigraphic interval of sulphate evaporites of Lomagundi-age, preserved in a shallow-water open-marine siliciclastic and carbonate succession (Lower Jatuli informal group) of Karelia, Russia (Morozov et al., 2010). The Lomagundi Event defines the most extreme and longest lasting isotope excursion of carbon in the world’s marine carbonate record. Bedded gypsum pseudomorphs in the Malmani Group some 2.5 Ga (Gandin and Wright, 2007; Eriksson and Warren, 1983) implies that elevated oceanic sulphate levels that typify the Lomagundi Event may have extended a little further back in time, at least locally (Figure 5).

At the same time as the Lomagundi event, the average ferric iron to total iron (expressed as Fe2O3/Fe|Fe2O3|) ratio of shales increased dramatically. At the end of the Lomagundi Event (LE), the first economic sedimentary phosphorites were deposited, and the carbon isotope values of marine carbonates returned to ≈0.0‰VPDB (Figure 2.50). Thereafter marine sulphate evaporites and phosphorites again became scarce, while the average Fe2O3/Fe|Fe2O3| ratio of shales decreased to values intermediate between those of the Archean and Lomagundi-age shales.

In support of this notion of an “oxygen overshoot,” sulphur isotope work by Reuschel et al. (2012) on the 2.1 Ga dolomitic Tulomozero Fm, which entrains abundant CaSO4 pseudomorphs, concluded that there was a minimum level of 2.5 mM sulphate in the world ocean at that time (Figure 5).

Bekker and Holland (2012) argue the short appearance of sulphate evaporites in Logamundi and the other associated events can be regarded as a ca. 200 Ma “glitch” in the gradual oxidation of the atmosphere–ocean system. It was driven by a positive feedback between the rise in atmospheric O2, the oxidation of pyrite in rocks undergoing weathering, a decrease in the pH of soil and ground water, and an increase in the phosphate flux to the oceans. This sequence led to a major increase in the rate of organic matter burial, a rise in atmospheric oxygen, a large increase in the 13C value for marine carbonates, the deposition of marine evaporites containing gypsum and anhydrite, and the formation of the first commercially important phosphorites. The end of the LE was probably brought about by the weathering of sediments deposited during the LE.

In yet another proposal of hydrosphere-atmosphere evolution, Huston and Logan (2004) argue that the presence of relatively abundant bedded sulphate deposits before 3.2 Ga (as the contentious Archean barytes and chert mentioned earlier) and after 1.8 Ga (as CaSO4 salts), and the peak in banded iron formation abundance between 3.2 and 1.8 Ga, and the aqueous geochemistry of sulphur and iron, when taken together suggest that the redox state and the abundances of sulphur and iron in the hydrosphere varied widely during the Archean and Proterozoic. They propose a layered hydrosphere prior to 3.2 Ga in which sulphate was enriched in an upper oceanic layer, whereas the underlying layer was reduced and sulphur-poor. The sulphate was produced by atmospheric photolytic reactions with volcanic gases in a reducing atmosphere. Mixing of the upper and lower water masses allowed the banded barytes to form prior to 3.2 Ga and created an ocean chemistry where nahcolite was a marine evaporite. Between 3.2 and 2.4 Ga, decreasing volcanogenesis and sulphate reduction removed sulphate from the upper layer, producing broadly uniform, reduced, sulphur-poor and iron-rich oceans.

Whatever the origin of the early Archean baryte and chert, around 2.2 - 2.4 Ga, as a result of increasing atmospheric oxygenation, the flux of sulphate into the hydrosphere by oxidative weathering was greatly enhanced, producing layered oceans, with sulphate-enriched, iron-poor surface waters and reduced, sulphur-poor and iron-rich bottom waters. Gypsum evaporites were increasingly likely as marine precipitates. The rate at which this process proceeded varied between basins depending on the size and local environment of the basin. By 1.8 Ga, the hydrosphere was relatively sulphate-rich and iron-poor throughout. Gypsum was now a widespread marine evaporite. Variations in sulphur and iron abundances suggest that the redox state of the oceans was buffered by iron before 2.4 Ga and by sulphur after 1.6 to 1.8 Ga (Figure 1).

Gypsum in combination with halite was the marine evaporite association from then until now. Seawater was predominantly a Na-Cl±SO4 ocean. Neoproterozoic stratiform sulphates along with widespread halokinetic halite, occur in the Bitter Springs Formation of the Amadeus basin, central Australia (0.8 Ga), its equivalents in the Officer Basin, the Callana beds of the Flinders Ranges and the younger Infracambrian salt basins of the Arabian (Persian) Gulf (≈0.545 Ga; Wells, 1980; Cooper, 1991; Mattes and Conway-Morris, 1990; Edgell, 1991).


The transition to calcium sulphate textures in evaporite pseudomorphs mirrors a marked change in the style of marine carbonates that began around 2.2 to 2.3 Ga when herringbone calcite and precipitated carbonate beds become much less common and the precipitation mode shifted from the seafloor to the water column (Figure 6; Sumner and Grotzinger, 1996, 2000). The boundary also corresponds to the “rusting” of the oceans when oxygen levels became high enough to precipitate widespread banded iron deposits on the seafloor. Microdigitate stromatolites cross this boundary with little effect, suggesting the marked decrease in dissolved iron exerted little influence on them.

The relative scarcity of actual Pre-Phanerozoic salts, not pseudomorphs, especially in the Archean has been used by some to argue that conditions were less favourable for widespread evaporite deposition in the early Precambrian (Cloud, 1972). Others, myself included, feel that the relative scarcity of preserved evaporites in older sequences reflects the greater likelihood of fluid flushing, evaporite dissolution and metasomatism in progressively older rocks. It is likely that oceanic calcium-sulphate evaporites were less common in the Archean, and that sodium carbonates mixed with halite were dominant evaporite salts in the seawater-fed saline giants in appropriate tectonic seepage depressions of the Early Archean. But widespread evaporite deposition from sodium-dominated brines did occur throughout the Archean in large drawdown basins isolated from a surface connection with the ocean. A paucity of preserved bedded evaporite salts in the Precambrian reflects an increased probability of partial or complete evaporite dissolution, remobilization and metasomatism with increasing geological age (see meta-evaporite).

In what is an inclusion study of oldest actual halite, Spear et al., (2014) characterised marine brine chemistry using brine inclusions in the 830 Ma salt of the Browne Formation, Officer Basin, Australia (equiv. to Bitter Springs Fm.). It seems that concentrations of the major ions in these inclusions, except K+ and possibly SO42−, fall within the known range of Phanerozoic seawaters. This ananlysis suggests that mid-Neoproterozoic marine sulphate concentrations were lower (≈90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy from the Khewra Salt of Pakistan and the Ara salt in Oman indicate seawater sulphate levels had risen significantly, to 50%-80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen.

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