Nearshore euxinia in the photic zone of an ancient sea
Introduction
Zones of marine anoxia, such as those occurring in oxygen minimum zones (OMZs) or restricted basins, play an essential role in the global carbon (Cicerone and Oremland, 1988, Wishner et al., 1995, Morrison et al., 1999, Sarma, 2002, Brüchert et al., 2003, Lengger et al., 2012), sulphur (Canfield et al., 2010), and nitrogen cycles (e.g. Dugdale et al., 1977, Tyrrell, 1999, Naqvi et al., 2000, Codispoti et al., 2001, Schunck et al., 2013). In modern oceans photic zone euxinia (PZE) does occur in OMZs (Brüchert et al., 2003, Weeks et al., 2004) but is rarely persistent and does not yield a strong molecular signature in underlying sediments. Instead, PZE is observed almost exclusively in stratified basins (Arctic and Antarctic fjords, Black Sea) (Overmann et al., 1992, Repeta, 1993, Sinninghe Damsté et al., 1993, Smittenberg et al., 2004). Consequently, the Black Sea, in which density and chemical stratification are strongly coupled, has been used as an analogue for ancient euxinic basins (Arthur and Sageman, 1994). Crucially, such models mechanistically link observations of anoxia in marginal settings to basin-scale anoxia. Although geochemical records of some Mesozoic oceanic anoxic events (OAEs) suggest that ocean anoxia does extend into deep basins (Wagner et al., 2004, van Breugel et al., 2006), recent work suggests that this was not necessarily associated with basin-scale stratification (Kuypers et al., 2002, Kuypers et al., 2004a, Monteiro et al., 2012). Moreover, evidence for anoxia in many ancient basins is restricted to nearshore settings (e.g. Jenkyns, 1985, Jenkyns, 1988, Wignall and Newton, 2001). This raises the possibility that ancient sedimentary basins could have been oxygenated even when very strong evidence for PZE occurs in marginal marine settings. This can be tested directly in the Southern Permian Basin, because the access to over a dozen cores spanning the platform to central basin allows for nearly unprecedented (at least for the pre-Cretaceous) spatio-temporal profiling.
The Late Permian had a greenhouse climate, with mid-Northern latitude minimum summer land temperatures being more than 15 °C higher than the present day (Kiehl and Shields, 2005). This favoured the deposition of carbonates and evaporites in the east-west trending epicontinental Zechstein Sea, located in northern Pangea at palaeolatitudes of 10–20°N (present central and eastern Europe) (Fig. 1). The sea was relatively shallow with maximum depths estimated to be 300–350 m (Smith, 1980). The Northern Permian Basin (NPB) was fed with seawater from the Panthalassic Ocean entering through the Boreal Sea, a narrow strait between Greenland and Scandinavia. The Southern Permian Basin (SPB) was located in the subtropical belt under warm and arid conditions, facilitating formation of sabkha-type systems and carbonate platforms around the basin. Such climatic conditions and the restricted character of the SPB led to the formation of four major sedimentary cycles (the first three, PZ1–PZ3, are carbonate–evaporite cycles and the fourth, PZ4, is a terrigenous–evaporitic cycle) in the Polish sector of the SPB in Zechstein (Lopingian) time (Wagner and Peryt, 1997).
It has long been thought that the Zechstein water-column was salinity stratified with anoxic bottom waters (Brongersma-Sanders, 1971, Turner and Magaritz, 1986, Grotzinger and Knoll, 1995, Taylor, 1998). Although it is well established that the initial transgressive lowermost Zechstein mudrocks (Kupferschiefer, i.e., base of the PZ1, Fig. 2) were deposited under photic-zone euxinic conditions (Oszczepalski, 1989, Schwark and Püttmann, 1990, Gibbison et al., 1995, Grice et al., 1996a, Grice et al., 1996b, Grice et al., 1997, Pancost et al., 2002, Paul, 2006), the subsequent deposition of carbonates and evaporites of the first Zechstein cycle took place under varied oxic/suboxic to anoxic bottom-water conditions (Kluska et al., 2013, Peryt et al., 2014). This clearly shows that the epicontinental Zechstein Sea experienced euxinia in the PZ1, but it remains unclear how extensive this was spatially and what governed the apparently pronounced temporal variations in this and PZ2–PZ3 cycles. The subtropical environment, resembling the modern Abu Dhabi and Qatar carbonate–evaporite environments, resulted in the deposition of petroleum reservoir and source rocks in the second Zechstein carbonate cycle, called the Main Dolomite (ca. 254 Ma — Szurlies, 2013), which is particularly productive in the Polish sector of the SPB (Słowakiewicz and Gąsiewicz, 2013, Słowakiewicz et al., 2013, Kosakowski and Krajewski, 2014). However, although extensively studied, there is still much controversy and speculation with respect to organic matter (OM) productivity and the overall depositional environment of the Main Dolomite carbonate basin.
In this project we have used geochemical techniques to evaluate water-column redox conditions and changes in sources of OM during deposition of the Zechstein second carbonate cycle (Main Dolomite, Ca2, Fig. 2) in the Polish part of the SPB. A shelf to basin reconstruction of environmental conditions was achieved by analysing nearly 150 core samples from 14 wells. This allows us to evaluate for the first time significant spatial and temporal variations in the palaeowater-column redox state in the Main Dolomite sea and use that to interrogate models for anoxia. Specifically, we have quantified isorenieratene and chlorobactene derivatives (isorenieratane, chlorobactane and aryl isoprenoids) which are produced by brown and green strains of photosynthetic anaerobic green sulphur bacteria (Chlorobiaceae), respectively, in order to assess whether PZE was a pervasive characteristic of the PZ2 carbonate sea in Late Permian time. We also present a comparison of contemporaneous shelf, slope and basin PZE biomarkers. Our conclusions are integrated with other biomarker signatures for past redox conditions (homohopane ratios, bisnorhopane and gammacerane abundances) and changes in OM source (hopane and sterane distributions), as well as sedimentological, carbon and oxygen isotopic and trace element data. All parameters exhibit pronounced spatial and temporal variability and indicate that photic zone euxinia was restricted to the marginal part of the Ca2 sea.
Section snippets
Samples and lithology
One hundred and thirty six samples of stromatolite, thrombolite, biolaminated/laminated wackestone–mudstone, oolitic packstone/grainstone and calcareous mudstone were selected from the subsurface Main Dolomite sections of northern (Kamień Pomorski [KP] and Pomerania [PP] platforms), and central sectors of the Polish part of the SPB (Fig. 1). Representative samples (Table 2) were taken from basinal (wells Piła IG-1, Złotów-2, Lipka-1, Okonek-1), toe-of-slope and embayment (wells Błotno-3,
Microscopy and microanalysis
Thin-sections were examined for microfacies characterisation using traditional petrographic methods. Scanning electron microscope (SEM) studies of the nanofacies were carried out on polished thin-sections and freshly broken surfaces, using an FEI-Philips ESEM-FEG Quanta 200F, operating in a range of 5–20 kV with working distance between 6 and 15 mm. Samples were carbon- or gold-coated, respectively, depending on whether they were prepared for microanalysis or textural study with the SEM. The
Microfacies
The general model for the Main Dolomite facies in the eastern part of the SPB is well established as a shallow-water carbonate platform with interior peritidal flat – evaporitic sabkha, an extensive shallow-water lagoon and platform-margin carbonate sand shoal and microbialites, passing basinwards through slope, toe-of-slope, and basin-plain environments (e.g. Gąsiewicz, 2013, Słowakiewicz et al., 2013).
The thickness of the Main Dolomite carbonates is usually between 30 and 120 m on the
Conclusions
Biomarker, mineralogical and sedimentological data from the northern Zechstein Sea margin in Poland, where the Main Dolomite was deposited in a shallow-marine carbonate platform setting, suggest that euxinia periodically impinged on the toe-of-slope and slope. However, redox-sensitive biomarkers such as bisnorhopane, chlorobactane and isorenieratane (and its derivatives) are absent in samples from basinal and outer ooid shoal facies, and homohopane indices are below 0.1, all suggesting oxic or
Acknowledgements
We would like to thank Ian Boomer (Birmingham University) for performing carbon and oxygen analyses. We also wish to thank NERC for the partial funding of the mass spectrometry facilities at Bristol (contract no. R8/H10/63). We appreciate the constructive comments of David Bottjer and Paul Wignall to improve our manuscript. MS is supported by a Mobility Plus programme postdoctoral fellowship of the Ministry of Science and Higher Education of Poland and Shell Exploration & Production.
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2018, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :Like the situation of these inner shelf basins on the Yangtze Platform, Late Permian euxinia–anoxia had been reported in other inner shelf/restricted basins. In the Zechstein Sea in Europe, an evaporitic basin with very different water chemistry to South China, geochemical studies have also shown that euxinia developed and periodically impinged on shallow marine carbonate parts of the basin in the Late Permian (Słowakiewicz et al., 2015). However, these authors argue that euxinia did not occur across the whole basin, but was restricted to marginal settings based on the absence or low biomarker indices for euxinia in other facies.