Organic geochemical signals of freshwater dynamics controlling salinity stratification in organic-rich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil)

Journal Pre-proofs Organic geochemical signals of controlling freshwater dynamics on salinity stratification in organic-rich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil) Laercio Lopes Martins, Hans-Martin Schulz, Hélio Jorge Portugal Severiano Ribeiro, Caroline Adolphsson do Nascimento, Eliane Soares de Souza, Georgiana Feitosa da Cruz PII: DOI: Reference:

S0146-6380(19)30195-0 https://doi.org/10.1016/j.orggeochem.2019.103958 OG 103958

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Organic Geochemistry

Received Date: Revised Date: Accepted Date:

22 May 2019 8 October 2019 14 November 2019

Please cite this article as: Martins, L.L., Schulz, H-M., Ribeiro, H.J.P., do Nascimento, C.A., de Souza, E.S., da Cruz, G.F., Organic geochemical signals of controlling freshwater dynamics on salinity stratification in organicrich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil), Organic Geochemistry (2019), doi: https://doi.org/10.1016/j.orggeochem.2019.103958

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Organic geochemical signals of controlling freshwater dynamics on salinity stratification in organic-rich shales in the Lower Permian Irati Formation (Paraná Basin, Brazil)

Laercio Lopes Martinsa,b,*, Hans-Martin Schulza, Hélio Jorge Portugal Severiano Ribeirob, Caroline Adolphsson do Nascimentob, Eliane Soares de Souzab, Georgiana Feitosa da Cruzb aGFZ

German Research Centre for Geosciences, Sec. 3.2 Organic Geochemistry,

Telegrafenberg, D-14473, Germany bLaboratory

of Petroleum Engineering and Exploration (LENEP), North Fluminense

State University (UENF), Macaé, Rio de Janeiro 27925-535, Brazil

*Corresponding author at: Laboratory of Petroleum Engineering and Exploration (LENEP), North Fluminense State University (UENF). Amaral Peixoto Road, Km 163, Brennand Avenue, Imboassica. Macaé, Rio de Janeiro 27925-535, Brazil. E-mail address: [email protected]

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Abstract: The Lower Permian Irati Formation in the northeastern and central eastern Paraná Basin (southern Brazil) was investigated in terms of bulk and molecular organic geochemistry in order to enlighten the complex depositional paleoenvironment during deposition of Irati black shales. The geochemical data reflect temporal and spatial variations of freshwater incursions promoting salinity stratification and high primary productivity in surface waters with the establishment of photic zone euxinia. Overall, highly concentrated C17 long-chain alkylnaphthalenes point to algal blooms as a result of freshwater inflows into the Irati Sea, mostly in the central eastern basin segment, and resemble concentration variations of pristane, phytane and nC17. The freshwater inflows were also retraced by increasing chroman ratios toward the top of the investigated units along with the general decrease of further palaeosalinity indicators, e.g., gammacerane, tetracyclic (C24), β-carotane, and the herein suggested γ-carotane and lexane indexes. The existence of a water column stratification promoting organic matter preservation is supported by the detection of tetrahydrophenanthrene at high concentrations, in addition to the known prominent occurrence of gammacerane. The detection of C10 to C31 aryl isoprenoids in addition to C16 and C18 pseudohomologue aryl isoprenoids points to the presence of photic zone euxinia. In general, samples from the central eastern basin were deposited in a deeper marine setting with lower salinity than samples from the northeastern basin, which displays signals of a shallower marine environment with reducing bottom water conditions, both deposited under freshwater influxes.

Keywords: Irati Formation; Paraná Basin; Gondwana; Permian; Black shale; Photic zone euxinia; Salinity stratification; Chromans; Carotenoids

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1. Introduction The Lower Permian Irati Formation in the Paraná Basin (southern Brazil) is one of the largest oil-shale deposits in the world (Tissot and Welte, 1984; Faure and Cole, 1999; Milani et al., 2007b), with black shale containing up to 26 wt.% total organic carbon (TOC; Alferes et al., 2011; Reis, 2017). These organic-rich sediments are natural archives not only for the depositional environment and biodiversity of southwestern Gondwana but also for the paleoclimatic change caused by the northward migration of western Gondwana (Goldberg, 2004). The large organic portion is considered a result of freshwater influxes that influenced primary organic matter production and preservation (Goldberg and Humayun, 2016). Despite a variety of organic and inorganic geochemical and micropaleontological data about the Irati Formation, there is still considerable debate about its depositional environment, mostly concerning the salinity of the water and the existence of stratification of the water column (Faure and Cole, 1999; Goldberg and Humayun, 2016). While the black shale intercalations were considered as lacustrine deposits under fresh/brackish or saline water conditions (Correa da Silva and Cornford, 1985; Faure and Cole, 1999; Summons et al., 2008), another hypothesis assumes an epicontinental hypersaline environment with a dysoxic-anoxic bottom water setting (Mello et al., 1993; Milani et al., 2007a). The deposition in a salinity-stratified water column has also been reasonably suggested assuming a positive water balance due to freshwater incursions (Goldberg and Humayun, 2016; Reis, 2017; Reis et al., 2018), and is herein considered as a working hypothesis. This prevailing uncertainty about the water chemistry during deposition of black shales intercalated in the Irati Formation is also a matter of debate regarding simultaneous processes in the Karoo Basin when the Whitehill Formation was deposited (extent of Paraná-Karoo Basins in Fig. 1a, modified from Faure and Cole, 1999). It is hypothesized that both the Irati and Whitehill Formations were formed from the deposition of mud rich in organic matter in a large and highly reducing basin, possibly under the direct influence of freshwater influx 3

(Du Toit, 1927; Milani and Wit, 2008; Goldberg and Humayun, 2016). The Whitehill Formation is made up of pyrite-rich black shale that was deposited under reducing bottom water conditions over wide areas that likely extended into the Paraná Basin when the Irati Formation formed (Milani and Wit, 2008). Organic geochemical data and mineralogical signals suggest that the preservation of organic carbon in the sediments of the Whitehill Formation was controlled by a stable water stratification, comprising anoxic marine bottom water overlain by lighter freshwater with high productivity (Schulz et al., 2016; 2018). Here we report about organic geochemical biomarkers used to assess the prevailing conditions during black shale deposition of the Irati Formation. Conceptually, we compare results about black shale samples from two different quarries in the Paraná Basin (Brazil) located in the São Mateus do Sul County (State of Paraná) and Amaral Machado Mining company area (State of São Paulo; Fig. 2). It is our aim to gain a deeper understanding of a complex depositional system that was directly influenced by freshwater incursions in a likely hypersaline epicontinental depositional setting.

2. Geological Setting 2.1. Paraná Basin The Paraná Basin (Fig. 2) is a large intracratonic sedimentary basin in South America extending over approximately 1.5 million square kilometers. The sedimentary filling of the Paraná Basin took place from Ordovician to Upper Cretaceous times within six large-scale units (or supersequences) that are separated by regional unconformities. The Gondwana I supersequence (Fig. 3) is the sedimentary result after the maximum glaciation, when intense erosional matter influx into the basin was due to the melting of the Gondwana ice cap. The sediments were deposited within a transgressive-regressive cycle controlled by marine incursions (and later retreats) of the Panthalassa Ocean into Gondwana. The lower sedimentary cycle can be directly linked to the frequent mass flows that occurred 4

during the melting of the polar ice cap (Milani and Ramos, 1998). Maximum flooding took place during the Artinskian age and continental depositional systems prevailed during the Triassic. Sedimentation during this Gondwana I supersequence was accompanied by a progressive closure of the Paraná Basin against marine incursions from the west and led to the gradual development of an intracratonic basin with an arid continental interior during the Mesozoic (Milani et al., 2007b). The lithological inventory of the Gondwana I unit (Fig. 3) is heterogeneous. It encompasses the Itararé Group with a maximum thickness of 1,500 m. This group consists of continental sandstones and five fining upward peri-glacial sequences made up of glaciomarine turbidites, diamictites, and rhythmites (Vesely and Assine, 2006; Vesely, 2007), which are correlated with the Dwyka Group in southern Africa (Milani and de Wit, 2008). This group is overlain by deltaic sandstones (Rio Bonito Formation), purely marine shales (Palermo Formation), shales and carbonates of the Irati Formation, and marine to continental siltstones (Serra Alta, Teresina, and Rio do Rastro Formations). The final member (Pirambóia Formation) is composed of fluvial and aeolian sandstones (Milani et al., 2007b; Reis et al., 2018).

2.2. Irati Formation Based on SHRIMP zircon age data and palynofossils, the Irati Formation was dated at 278.4 ± 2.2 Ma (Santos et al., 2006), which corresponds to the Artinskian/Kungurian boundary of the Cisuralian (Lower Permian; Rocha-Campos et al., 2011). The sediments have an average thickness of about 40 m, locally reaching a maximum of 70 m (Mendes et al., 1966), and are biostratigraphically correlated to the Whitehill Formation of South Africa (275-279 Ma; Stollhofen et al., 2008). The chronostratigraphy of both black shales indicates the existence of a giant anoxic inland sea that formed at the end of the Early Permian across southern Africa and southern South America. 5

According to Reis et al. (2018), the Irati Formation comprises eight chemostratigraphic units (A-H; Fig. 3). The classification is based on type and content of organic matter, which is characteristic for the specific physicochemical conditions during deposition. Two lithological units occur (Fig. 3): the Taquaral Member in the lower part that is mainly composed of marine shales, and the overlying Assistência Member that comprises limestones with intercalated shales and marlstones, including two bituminous shale beds (units E and H in the chemostratigraphic column, Fig. 3). The deposition of the Assistência Member is considered to be the result of changing salinity and redox conditions in a restricted marine environment (Rodrigues et al., 2010; Alferes et al., 2011). The organic matter of the Irati black shales is predominantly of algal origin resulting in an amorphous, liptinitic kerogen (Correa da Silva and Cornford, 1985; Milani et al., 2007a) with hydrogen index (HI) values averaging 600 mg HC/g TOC. The sediments thus have good hydrocarbon potential (Peters et al., 2005) based on a type I/II kerogen (Mendonça Filho, 1992; Mello et al., 1993; Afonso et al., 1994; Reis et al., 2018). The Irati Formation is mainly thermally immature but has reached the oil window in some areas because of heating from diabase intrusions (Sousa et al., 1997; Araújo et al., 2000; Araújo et al., 2005; Santos et al., 2009). It is also a source rock for the heavy oil stored in the tar sands of the Pirambóia Formation (Cerqueira and Santos Neto, 1986).

3. Experimental 3.1. Samples The black shale sample set consists of 17 outcrop samples of the Irati Formation, which were collected at two sites in the Paraná Basin (Fig. 2). Ten samples were from a quarry in São Mateus do Sul City (State of Paraná), and were collected from two different stratigraphic horizons of the Irati Formation (Fig. 3). The lower and upper horizons correspond to the chemostratigraphic units E and H, respectively, according to correlations with Reis (2017). 6

The other seven samples were from a quarry in the Amaral Machado Mining company area (Saltinho city, State of São Paulo) and correspond to the chemostratigraphic unit H, also according to Reis (2017). All samples were collected from the base to the top of the outcrops (Fig. 3).

3.2. Extraction and separation The Irati rock samples were crushed and pulverized prior to a 48-hour extraction with 300 mL of dichloromethane (chromatographic grade from Sigma-Aldrich) using a Soxhlet system. Asphaltenes were precipitated from approximately 50 mg of soluble organic matter according to Theuerkorn et al. (2008). The maltenes were concentrated and separated into saturate, aromatic, and NSO fractions by Medium Pressure Liquid Chromatography (MPLC) according to Radke et al. (1980).

3.3. Total organic carbon (TOC) content and Rock Eval analysis TOC determination and Rock Eval pyrolysis were performed by Applied Petroleum Technology (APT) AS (Oslo, Norway). The TOC content was obtained using a Leco SC632 instrument in which the finely crushed rock samples were diluted in HCl at 60 °C to remove inorganic carbon, and then introduced into the Leco combustion oven. The amount of carbon in the sample was measured as carbon dioxide by an IR-detector. Rock Eval analyses were performed using a Rock-Eval 6 instrument, with the following temperature program: initial temperature of 300 °C held for 3 min, then heat increased by 25 °C min-1 to the final temperature of 650 °C. Jet-Rock was run on every tenth sample as an external standard and checked against the acceptable range given in NIGOGA, 4th Edition (Weiss et al., 2000).

3.4. GC-FID 7

The aliphatic fractions of the Irati shale samples obtained from MPLC were diluted in nhexane and analyzed by gas chromatography with flame ionization detection (GC-FID) using an Agilent 6890 device equipped with an HP Ultra 1 capillary column (50 m x 0.2 mm x 0.33 µm). The injector temperature was set to 40 °C and heated at a rate of 11.6 °C s-1 to 300 °C and held for three minutes. The GC oven was programmed from 40 °C (2 min isothermal) to a final temperature of 300 °C (65 min isothermal) at a heating rate of 5 °C min-1 with a constant helium flow rate of 1 mL min-1. The FID was operated at 310 °C. Androstan (5 mg mL-1) was used as an internal standard (IS) for the quantification of nalkanes and isoprenoids.

3.5. GC-C-IRMS The stable carbon isotopic composition of aliphatic hydrocarbons was measured by GCC-IRMS (gas chromatography-combustion-isotope ratio mass spectrometry). The GC-CIRMS system consisted of a GC unit (7890N, Agilent Technology, USA) connected to a GC-C/TC III combustion device coupled via open split to a Delta V Plus mass spectrometer (ThermoFisher Scientific, Germany). The organic substances of the GC effluent stream were oxidized to CO2 in a combustion furnace held at 940 °C on a CuO/Ni/Pt catalyst. CO2 was transferred on line to the mass spectrometer to determine stable carbon isotope ratios. 2 µl of the aliphatic fractions were injected into the programmable temperature vaporization inlet (PTV, Agilent Technology, USA) with a septumless head working in split/splitless mode. The injector was held at a split ratio of 1:2 and an initial temperature of 230 °C. Upon injection, the injector was heated to 300 °C at a programmed rate of 700 °C min-1 and held at this temperature for the remainder of the analysis time. The aliphatic fractions were separated on a fused silica capillary column (HP Ultra 1, 50 m x 0.2 mm ID, 0.33 µm FT, Agilent Technology, Germany). The temperature program began at 80 °C (2 min isothermal) and was then increased at a rate of 5 °C min-1 to 320 °C, then finally increased at a rate of 1 8

°C min-1 to 325 °C, which was held for 15 min. Helium, set to a flow rate of 1.0 mL min-1, was used as a carrier gas. All saturated hydrocarbon fractions were measured in triplicate with a standard deviation of ≤ 0.5‰ for most of the compounds and samples. The quality of the carbon isotope measurements was checked regularly by measuring n-alkane standards (nC15, nC20, nC25) with known isotopic composition (provided by Campro Scientific, Germany).

3.6. GC-MS The aliphatic and aromatic fractions were analyzed by a DSQ Thermo Finnigan Quadrupole MS coupled to a gas chromatograph. The gas chromatograph was operated in the splitless mode with an initial temperature of 50 °C heated to 300 °C at 10 °C s-1, with the final temperature held for 10 min. The GC was equipped with a Thermo PTV injection system (Thermo Electron Corporation) and a BPX5 (SGE) capillary column (50 m x 0.22 mm x 0.25 µm). The GC oven was programmed from 50 °C (1 min isothermal) to a final temperature of 310 °C (30 min isothermal) at a heating rate of 3 °C min-1. Helium was used as a carrier gas with a constant flow rate of 1 mL min-1. Full scan mass spectra were recorded from m/z 50 to 600 Da in the aliphatic fraction, and from 50 to 330 or 50 to 600 Da in the aromatic fraction, at a scan rate of 1.5 scans s-1. Androstan (5 mg mL-1) and ethylpyren (1 mg mL-1) were used as an IS for the quantification of aliphatic and aromatic biomarkers, respectively.

4. Results and discussion 4.1. Bulk organic matter The TOC values of the analyzed Irati shale samples range from 4.14 to 22.10 wt.% (Fig. 4). Higher TOC values characterize samples from the São Mateus do Sul profile, and generally increase from the base to the top of each unit (except for sample SM 2.6 from the 9

top of lower São Mateus do Sul). The HI values are in the range of 348 to 631 mg HC/g TOC (Supplementary material Fig. S1a). As the oxygen index (OI) values are generally lower than 17 mg CO2/g TOC, except for samples AMa 1 and 5 from the base of Amaral Machado (29 and 20 mg CO2/g TOC, respectively), the kerogen can be classified as type I (to II; Supplementary material Fig. S1a). Samples AMa 1 and 5 are better classified solely as type II kerogen based on higher OI values as shown in Fig. S1a. All Irati shale samples are thermally immature according to Rock Eval analysis (Tmax < 435 °C according to Espitalié et al., 1977; except for sample AMa 24; Supplementary material Fig. S1b).

4.2. Reconstruction of the depositional environment Taking the paleoceanographic setting of the restricted Irati-Whitehill Sea into account (according to Faure and Cole, 1999; Araújo et al., 2001), samples from Amaral Machado are likely from the northeastern margin of the Irati-Whitehill Sea in a shallower water position (1 in Fig. 1b), while samples from the São Mateus do Sul site are from a deeper basin segment (2 in Fig. 1b). Here we present selected key biomarkers (I to X, chemical structure in the Appendix) that were thus far unreported in the Irati Formation to unravel the depositional environment during deposition of black shale (Fig. 5).

4.2.1. Freshwater incursions The aliphatic fractions of the organic-rich Irati shale samples are mainly composed of low-molecular-weight isoprenoids iC14, iC15 (farnesane), iC16, iC18 (norpristane), iC19 (pristane), and iC20 (phytane), together with low-weight n-alkanes, mainly nC15 and nC17 (Fig. 4 and 6a). The predominance of isoprenoids is an indication of high photosynthetic activity, while the high abundance of nC17 among the n-alkanes is caused by prominent algal contribution (Gibert et al., 1975; Correa da Silva and Cornford, 1985; Afonso et al., 1994; Alferes et al., 2011). This feature, in addition to elevated HI values between 348 and 631 mg 10

HC/g TOC (Supplementary material Fig. S1), suggests microbial bloom events (possibly Botryococcus) in surface waters due to freshwater incursions into the Irati Sea (Faure and Cole, 1999). These data are in accordance with previous findings that the Irati, Whitehill, and Huab Formations are stratigraphical equivalents, and that such microbial blooms covered an extensive area and are likely the most expansive microbial bloom events in Earth’s history (Faure and Cole, 1999). In addition, the C17 long-chain alkylnaphthalene (C17 2-aN, I) was identified in total ion current chromatograms (TICC) of the aromatic fraction (Fig. 6b) with higher abundance for samples from the lower section at São Mateus do Sul. Its spectrum is characterized by a molecular ion (M+.) at m/z 226, a relatively high-abundance ion at m/z 115, and base peaks at m/z 141 and 142 (Supplementary material Fig. S2, according to Lu et al., 2011). The high concentrations of this diaromatic compound in the Irati shale samples is a further indicator of the great algal contribution, such as could be derived from, e.g., Botryococcus braunii and Gloeocapsomorpha Prisca (Peters et al., 2005). The quantitative variability of this aromatic biomarker resembles the concentration pattern of the isoprenoids pristane and phytane (Fig. 4) and is evidence for a decreasing algal contribution from the bottom to the top in the lower section in São Mateus do Sul. Similar fossilized blooms of the unicellular microalga Botryococcus braunii are typically known from freshwater environments (Grice et al., 1998), and Reis et al. (2018) found evidence of fossilized Botryococcus algae in the base of unit E, suggesting deposition in a proximal environment under fluvial influence. The decrease in the C17 2-aN concentrations toward the top of the São Mateus do Sul lower unit is accompanied by an increase in the hopane/sterane ratio (Fig. 7), with its upper unit presenting the same trend for the latter ratio (except only the sample SM 3.7), suggesting a favorable paleoenvironment for bacterial growth. There are relatively high hopane proportions in the m/z 191 chromatograms (Fig. 6c) as indications of significant bacterial activity (Brassell et al., 1986; Alferes et al., 2011; Reis et al., 2018). On the other hand, 11

samples from Amaral Machado have generally lower C17 2-aN concentrations than samples from São Mateus do Sul, with a slight increase toward the top that could indicate higher algal contributions during deposition of samples of the upper AMa profile due to freshwater influx. Methyltrimethyltridecylchromans (MTTCs, II) have also been used as tracers of freshwater incursions, likely reflecting the conditions in the upper part of a stratified water column that is composed of lighter water (Sinninghe Damsté et al., 1993; Tulipani et al., 2015). The distribution of these compounds has been applied as a proxy for paleosalinity, e.g., the chroman ratio (CR; 5,7,8-triMe-MTTC/total MTTCs), which is presumably a reliable paleosalinity indicator according to empirical studies (Schwark and Püttmann, 1990; Sinninghe Damsté et al., 1993; Wang et al., 2011; Tulipani et al., 2013). These compounds are still of unclear specific origin and formation pathways. A potential formation is related to (i) biosynthesis by organisms in phytoplankton (Sinninghe Damsté et al., 1993), (ii) early diagenetic condensation reactions of phytol with alkylphenols (Fig. 5b; according to Li et al., 1995; Tulipani et al., 2015), or (iii) cyclization of alkylphenols (Barakat and Rullkötter, 1997). The 8-Me-MTTC, 5,8- and 7,8-diMe-MTTC and 5,7,8-triMe-MTTC chromans were identified in the aromatic fractions (summed m/z 121 + m/z 135 + m/z 149 chromatograms in Fig. 8), in which their spectra are characterized by a molecular peak at m/z 386, 400, and 414 and a relatively high abundance peak ion at m/z 121, 135, and 149, respectively (Supplementary material Fig. S3; according to Sinninghe Damsté et al., 1987). In particular, the concentration of 8-Me-MTTC varies significantly and is similar to the sum of MTTCs (Fig. 4). The chroman ratio (definition in Table S1) markedly increases from the base to the top of the units (Fig. 7), and thus indicates a progressively decreasing salinity of the surface water likely due to influx of freshwater (Goldberg and Humayun, 2016).

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Chroman ratios < 0.5 in basal samples from Amaral Machado (AMa 1, 5, and 7) and basal samples from the lower São Mateus do Sul section (SM 2.1, 2.2, and 2.3) can be attributed to a mesosaline to hypersaline environment. In contrast, samples from the top of both sections, as well as all samples from the upper São Mateus do Sul, were deposited under normal marine conditions (CR > 0.6, Fig. 9). Such a hypothesis is supported by plotting the CR against the Pr/Ph ratio (Fig. 9a; according to Schwark et al., 1998). Here, it is important to note that Pr/Ph ratios < 1 characterize the basal part at the Amaral Machado site (AMa 1 and 5) and may refer to a hypersaline environment (according to Ten Haven et al., 1987). Similar low Pr/Ph ratios of 0.53 and 0.93 in combination with higher values of the 27αααR/29αααR sterane ratio and C35 homohopane indices from the basal samples AMa 1 and AMa 5 (Fig. 7) were also considered as evidence of reducing and hypersaline conditions (Fu et al., 1986; Peters and Moldowan, 1991; Reis, 2017). The higher abundance of the 27αααR over 29αααR sterane is also an indication of a greater contribution of the primarily marine red algae in the OM to these basal samples, pointing to a marine depositional setting (Kodner et al., 2008; Sheath and Vis, 2015; Reis et al., 2018). In addition, phytane has a different stable carbon isotope composition than pristane at the base of the Amaral Machado section (samples AMa 1 to AMa 11; Fig. 4), which may refer to different precursors (Freeman et al., 1990) such as methanogens and/or halophilic archaea (Brassell et al., 1986). Since the concentration of phytane is higher than pristane in these samples, higher contributions of archaea are assumed (according to Brassell et al., 1986), corroborating harsh depositional conditions since these organisms can thrive in extreme environments such as in sulfide-rich seas. An increase of the chroman ratio toward the top of the units is accompanied by decreasing gammacerane and β-carotane indexes (Fig. 9b and c), and suggests a decreasing salinity of the surface water due to freshwater incursions, culminating in less saline surface water during deposition of samples at the top of the units. Other paleosalinity indexes mimic 13

a similar trend such as the tetracyclic index and the C31R/C30 ratio (Fig. 7; see also Table 1S; according to Mello et al., 1995, and Peters et al., 2005). For example, the C31R/C30 ratio is generally higher than 0.25 for marine source rocks (Peters et al., 2005), as for the herein analyzed Irati shale samples (Fig. 7). The distribution of chromans thus indicates a progressive freshwater influx into the Irati Sea. Dissolved nutrients stimulated primary productivity which caused an intensified organic flux to the anoxic sea floor (Goldberg and Humayun, 2016). This potential scenario is supported by the increasing TOC contents toward the top of the units (Fig. 4). The freshwater incursions, with low densities due to low contents of total dissolved solids, formed oxygenated surface water which was underlain by immiscible heavier marine water. Such density differences promoted stable water stratification during which anoxia of the bottom water developed, which is a key factor for the preservation of organic matter. Similar recent scenarios are known from the deep basins of the Baltic Sea (Emeis et al., 1998) and the Black Sea (Demaison and Moore, 1980). However, an alternative scenario can be glacier meltwater-driven nutrient upwelling (Cape et al., 2019).

4.2.2. Salinity stratification Gammacerane (III) can be considered an indicator for stratification of the water column (Harvey and MacManus, 1991; Sinninghe Damsté et al., 1995). Accordingly, high gammacerane indexes of the Irati shale samples (Fig. 7) indicate the presence of water column stratification (Fig. 5a and c). In addition, C18 1,1,7,8-tetramethyl-1,2,3,4tetrahydrophenanthrene (THP, IV) was identified in the aromatic fractions of all Irati shale samples (Fig. 4 and 6b). THP is a diaromatic hydrocarbon that possibly originates from tetrahymanol and was synthesized by anaerobic ciliates feeding predominantly on bacteria at the interface of stratified water layers (Sinninghe Damsté et al., 1999). Its spectrum is characterized by a molecular ion at m/z 238, a base peak at m/z 223, and fragment ions at 14

m/z 179, 193, and 208 (Supplementary material Fig. S4; according to Sinninghe Damsté et al., 1999). This aromatic compound is the most prominent signal in the TICC of aromatic fractions of most samples (Fig. 6b). This obvious abundance advocates the presence of anaerobic ciliates, and consequently salinity stratification in the Irati Sea. Another suggested origin of THP is aromatization of monoaromatic tricyclic terpanes (Azevedo et al., 1994). A set of C17, C18, and C19 tetrahydrophenanthrenes have also been detected in Permian Tasmanite Oil Shale (Peters et al., 2005). However, only the C18 THP was detected in the m/z 223 chromatogram in the aromatic fraction of the Irati shales. Its high concentration together with gammacerane refers to a similar origin of both and provides further support for a persistent salinity stratification during black shale sedimentation. The higher abundance of both biomarkers in TOC-rich samples from São Mateus do Sul could explain the better preservation conditions of organic matter. In addition to the detection of β-carotane (V), which is known from lacustrine saline and hypersaline environments, further aliphatic compounds derived from carotenoids were identified in the mass chromatogram for m/z 125 (according to Jiang and Fowler, 1986; Fig. 10). Such series of compounds have not been described for the black shales in the Irati Formation yet. The most abundant compounds are 1,1,3-trimethyl-2-alkylcyclohexanes (TAs) with 16, 18, and 19 carbons (VI), lexane (saturated lexene, VII), and γ-carotane (VIII; concentrations in Fig. 4). The mass chromatogram for m/z 125 also highlights the occurrence of other isomers of these alkylcyclohexanes and is evidence for the diversity of saturated carotenoid compounds. These aliphatic carotenoids were identified based on published mass spectra (Supplementary material Fig. S5; according to Anders and Robinson, 1971; Jiang and Fowler, 1986), and have similar concentration trends as β-carotane, which suggests the same biological source. Furthermore, salinity likely controls β-carotane concentrations (Jiang and Fowler, 1986; Mello et al., 1988; Chen et al., 1996). Here, the β-carotane index decreases toward the top 15

of all units (Fig. 7). Since the concentrations of others compounds with a carotenoid skeleton also decrease toward the top of the units in São Mateus do Sul similarly to β-carotane (Fig. 4), we suggest two new molecular proxies to assess paleosalinity: the γ-carotane index and the lexane index (Table S1; Fig. 7). It is important to note that β-carotane index values are distinctly lower in samples from Amaral Machado. A lack of C16 1,1,3-trimethyl-2alkylcyclohexanes, lexane, and γ-carotane in these samples points to precursors other than terrestrial OM (which is actually controversial to the findings of Murphy et al., 1967, and Ding et al., 2017). However, the likely source of the detected carotenoids is algae, and fits the occurrence of the algal biomarker C17 2-aN, along with pristane and nC17. β-Carotene, the presumable precursor of aliphatic carotenoids, is a highly labile and readily oxidized biomarker, as are the aromatic carotenoids in general (Murphy et al., 1967; Ding et al., 2017). Better preservation of their diagenetic products by reduction during diagenesis in the Irati shale samples is thus the result of anoxic and reducing conditions in the bottom water (according to Brocks et al., 2005). The sedimentary occurrence of βcarotane has also been attributed to a saline lacustrine environment of the Irati Formation (Summons et al., 2008). However, the higher concentrations of these aliphatic carotenoids in samples from the lower and upper sections in São Mateus do Sul point to a pronounced layer of oxygenated surface water with high algal productivity.

4.2.3. Photic Zone Euxinia (PZE) Aryl isoprenoids ranging from C13 to C31 (IX) were identified in all Irati shale samples (vertical variation of C16, C18, and C19 aryl isoprenoid concentrations in Fig. 4, and representative m/z 134 chromatograms of selected samples in Fig. 11). Their spectra are characterized by base peaks at m/z 133 and 134 and molecular ions at m/z 190, 218, 246, and 260, for instance, of the most abundant C14, C16, C18, and C19 aryl isoprenoids (Supplementary material Fig. S6; spectra interpretation according to Summons and Powell, 16

1987; Requejo et al., 1991; Sousa Júnior et al., 2013). Most mass spectra of the identified aryl isoprenoids are characterized by higher m/z 133 than m/z 134, as shown for C14, C16, C18, and C19 compounds, which suggest a 2-alkyl-1,3,4-trimethyl substitution pattern that is characteristic for diaromatic carotenoids such as isorenieratene, found in the Chlorobiaceae family of photosynthetic sulfur bacteria (Requejo et al., 1991). Aryl isoprenoid homologues with carbon numbers higher than 31 were not detected in any sample, and this is in agreement with the maximum chain length of the proposed precursors (Requejo et al., 1991). The distribution of aryl isoprenoids in the mass chromatogram for m/z 134 of the Irati shales resembles the distribution in petroleum samples from the Sergipe-Alagoas Basin (Northeastern Brazil; Sousa Júnior et al., 2013). Moreover, compounds with < 40 carbon atoms were detected together with higher abundances of C16, C18, and C19 aryl isoprenoids, but lack C12, C17, C23, and C28 members of the series, which is common because of the branching arrangement of irregular isoprenoids with a linked tail-to-tail (Sousa Júnior et al., 2013). However, other aromatic carotenoids were not detected in the m/z 134 chromatogram, including isorenieratene (cf. Sousa Júnior et al., 2013). The identification of these monoaromatic biomarkers in the Irati shales, which are diagenetic or catagenetic products of isorenieratene, points to the presence of photic zone euxinia during deposition (Koopmans et al., 1996; Schaeffer et al., 1997; Clifford et al., 1998, Saito et al., 2014). Hence, photosynthetic green sulfur Chlorobiaceae and purple sulfur Chromatiaceae bacteria as primary producers (Fig. 5c) significantly contributed to the preserved OM of the Irati Sea (Summons and Powell, 1987; Grice et al., 2005; Tulipani et al., 2015). The concentration gradients of the aliphatic carotenoids are in stark contrast to the aryl isoprenoid concentrations (see upper section in São Mateus do Sul; Fig. 4), corroborating distinct biological sources (Koopmans et al., 1997). Besides aryl isoprenoids, the co-occurring C16 and C18 pseudohomologue aryl isoprenoids (X-a and X-b) in the Irati shale samples (high abundance of the C16 one in the 17

TICC of aromatic fraction in Fig. 6b) could also be interpreted as biomarkers for Chromatiaceae sulfur bacteria (Brocks and Schaeffer, 2008) and thus photic zone euxinia. Their spectrum is characterized by a base peak at m/z 173 and molecular peak ions at m/z 216 and 244, respectively (Supplementary material Fig. S7; according to Brocks and Schaeffer, 2008). Series of these compounds likely originate from diagenetic cyclization products of okenone, a red-colored aromatic carotenoid pigment solely found in purple sulfur bacteria associated with the family Chromatiaceae (Brocks and Schaeffer, 2008). As the concentration profiles of these aromatic biomarkers resemble those of aryl isoprenoids (Fig. 4), a similar origin is assumed. These results are consistent with the co-occurrence of gammacerane and tetrahydrophenanthrene, likely biomarkers for bacterivorous ciliates. As a result, an oxicanoxic interface in a stratified water column can be retraced, with a favorable environment for microbial communities such as, e.g., Chlorobiaceae (Fig. 5c and d). Further evidence for underlying anoxic bottom water is the lack or rare occurrence of benthic invertebrates due to the hostile environment. Such benthic invertebrates occur in the overlying Serra Alta and Teresina Formations, the sediments of which deposited in dysoxic/oxic waters (Matos et al., 2017).

Conclusions The biomarker inventory of the TOC-rich shales in the Irati Formation points to temporal and geographic variations of the depositional conditions. Such changes relate to freshwater incursions that promoted microbial bloom events in the Irati Sea, in addition to salinity stratification with the establishment of photic zone euxinia. Samples from the northeastern basin area display signals of a shallower marine environment with higher salinity and reducing conditions, while samples from the central eastern basin area show signals of

18

thicker oxygenated surface water with higher algal productivity in a deeper marine setting with lower salinity. In particular, the detection of C17 long-chain alkylnaphthalene in higher abundance, together with similar concentration patterns of Pr, Ph, and nC17, allows the reconstruction of algal blooms during deposition of the basal samples in the lower section at São Mateus do Sul. The water stratification can be traced back by the detection of high tetrahydrophenanthrene (THP) concentrations, in addition to the known occurrence of gammacerane, which indicates the presence of bacterivorous ciliates feeding predominantly on bacteria that live in the anoxic interface of a stratified water column. The build-up of stratification was driven by freshwater incursion(s) and can be demonstrated by an increasing chroman ratio toward the top of the units along with the general decrease of further paleosalinity parameters (e.g., gammacerane, tetracyclic, and β-carotane indexes). In addition to β-carotane, other aliphatic carotenoid compounds such as γ-carotane and lexane were detected, resembling a similar salinity variability. The salinity stratification probably led to the development of photic zone anoxia and is reflected by aryl isoprenoids and the high concentrations of the C16 and C18 pseudohomologue aryl isoprenoids.

Acknowledgments This work was supported by GFZ German Research Centre for Geosciences (Potsdam, Germany) and by North Fluminense State University – UENF (Macaé, Brazil). The authors thank GFZ staff for all support, especially Anke Kaminsky, Cornelia Karger, Doreen Noack, and Kristin Günther. The authors are also grateful to BG E&P Brasil Ltda., a current subsidiary of Shell Brasil Petróleo Ltda., for funding the sampling field trip. Sampling was enabled on behalf of the "Commitment to Research and Development Investments" together with the ANP (National Agency of Petroleum, Natural Gas and Biofuels) based on an application of resources (clause of R&D contracts in the Exploration and Production of 19

Petroleum in Brazil). We also thank Suryendu Dutta and one anonymous reviewer for the relevant comments that improved the quality of this manuscript.

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Figure captions Fig. 1. (a) Map depicting the extent of the Paraná, Karoo, and Huab basins in southwestern Gondwana at time of Irati black shales deposition (278.4 ± 2.2 Ma; according to Santos et al., 2006), showing the approximated locality of samples studied herein: (1) Amaral Machado, and (2) São Mateus do Sul (modified from Faure and Cole, 1999); (b) Paleogeographic sketch of the restricted Irati-Whitehill Sea, showing areas with distinct depths of the sea due to low dip angles of the seafloor in the Paraná Basin in the NE-SW direction (modified from Araújo et al., 2001). Based on that, samples of Amaral Machado are from shallow restricted sea (1), while samples of São Mateus do Sul are from deep restricted sea (2). Fig. 2. Geographic localization of the Paraná Basin in South America (modified from Zalán et al., 1990) and outcrops of the Irati Formation (dashed red line; modified from Chaves et al., 1988). Samples used herein were collected in the Amaral Machado Quarry (State of São Paulo in the NE; green square) and in São Mateus do Sul City (State of Paraná in the E; orange/blue circle). Fig. 3. Geological time scale (according to Santos et al., 2006; Milani et al., 2007b), lithostratigraphy and correlation between the Irati shale samples used in this work and the chemostratigraphic units A-H defined by Reis et al. (2018). The unit E comprises samples SM 2.1 to 2.6, while unit H comprises samples SM 3.1 to 3.7 and AMa 1 to AMa 24, as depicted in the chemostratigraphic section. Fig. 4. Total organic carbon (TOC), δ13C of pristane (Pr) and phytane (Ph), and TOCnormalized biomarker concentration in the Irati shale samples. Fig. 5. (a) Schematic reconstruction of the paleoenvironment during deposition of the black shales in the Irati Formation at two sites in the Paraná Basin, indicating (b) the mechanism to generate chromans (according to Li et al., 1995; Tulipani et al., 2015), (c) the water interface with photic zone euxinia (PZE), and (d) selected key biomarkers used in this study 31

linked to their precursor microorganisms. Moreover, it is indicating the contribution of terrigenous organic matter suggested by the woody material previously identified in the Irati black shales (Correa da Silva and Cornford, 1985; Mendonça Filho et al., 1992). Fig. 6. (a) Representative chromatograms gained by GC-FID, (b) total ion current chromatograms (TICC) of aromatic fraction, (c) m/z 191, and (d) m/z 217 chromatograms of aliphatic fraction of Irati shale samples AMa1 and SM 2.1. IS: internal standard. Fig. 7. Parameters to assess organic matter input and the depositional environment of the Irati shale samples. Calculation procedures presented in Supplementary material Table S1. Fig. 8. Representative summed mass chromatograms (m/z 121 + 135 + 149) showing the variability of MTTCs (II) in the Irati shale samples. CR: Chroman ratio, based on the peak areas from the mass chromatograms (5,7,8-triMe-MTTC/total MTTCs). Fig. 9. Salinity considerations based on plotting the chroman ratio against Pr/Ph ratio (a), gammacerane index (b), and β-carotane index (c). Fig. 10. Representative m/z 125 chromatograms of aliphatic fractions of Irati shale samples SM 2.1 and SM 3.1 showing the distribution of aliphatic compounds derived from carotenoids (V, VI, VII, and VIII). Fig. 11. Representative m/z 134 chromatograms of aromatic fractions of Irati shale samples AMa 5, AMa 23, SM 2.1, and SM 3.7, showing the distribution of C10 to C31 aryl isoprenoids (IX).

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Highlights  Algal bloom events reconstructed by detection of C17 long-chain alkylnaphthalene;  Freshwater inflows in the Irati Sea retraced by variation in MTTCs distribution;  Water column stratification reflected by the occurrence of tetrahydrophenanthrene;  The detection of aliphatic carotenoids series allows paleosalinity reconstructions;  Development of Photic Zone Euxinia pointed out by detection of aryl isoprenoids.

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Declaration of interests

X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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