Spectral gammaray logs and palaeoclimate change?

GEOLOGICAL JOURNAL Geol. J. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/gj.2552

Spectral gamma-ray logs and palaeoclimate change? Permian–Triassic, Persian Gulf EBRAHIM GHASEMI-NEJAD1, ALASTAIR RUFFELL2*, HOSSEIN RAHIMPOUR-BONAB1, MOHAMMED SHARIFI1, BEHZAD SOLTANI1 and EBRAHIM SFIDARI1 1

Department of Geology, University College of Science, University of Tehran, Tehran, Iran School of Geography, Archaeology and Palaeoecology, Queen’s University, Belfast, Northern Ireland, UK

2

Spectral gamma ray (SGR) logs are used as stratigraphic tools in correlation, sequence stratigraphy and most recently, in clastic successions as a proxy for changes in hinterland palaeoweathering. In this study we analyse the spectral gamma ray signal recorded in two boreholes that penetrated the carbonate and evaporate-dominated Permian–Triassic boundary (PTB) in the South Pars Gasfield (offshore Iran, Persian Gulf) in an attempt to analyse palaeoenvironmental changes from the upper Permian (Upper Dalan Formation) and lower Triassic (Lower Kangan Formation). The results are compared to lithological changes, total organic carbon (TOC) contents and published stable isotope (δ18O, δ13C) results. This work is the first to consider palaeoclimatic effects on SGR logs from a carbonate/evaporate succession. While Th/U ratios compare well to isotope data (and thus a change to less arid hinterland climates from the Late Permian to the Early Triassic), Th/K ratios do not, suggesting a control not related to hinterland weathering. Furthermore, elevated Th/U ratios in the Early Triassic could reflect a global drawdown in U, rather than a more humid episode in the sediment hinterlands, with coincident changes in TOC. Previous work that used spectral gamma ray data in siliciclastic successions as a palaeoclimate proxy may not apply in carbonate/evaporate sedimentary rocks. Copyright © 2014 John Wiley & Sons, Ltd. Received 21 September 2013; accepted 16 January 2014 KEY WORDS

Permian–Triassic; spectral gamma-ray; Persian Gulf

1. INTRODUCTION – THE ISSUE TO BE ADDRESSED Determining episodes of palaeoclimate change that are related to evolving palaeoweathering in sediment hinterlands is often based on time-consuming (if accurate) analysis of proxy data from clay mineralogy, stable isotopes and pollen (Schnyder et al., 2006). Hence, when Ruffell and Worden (2000) followed the work of Parkinson (1996) in suggesting that the rapid and non-destructive use of Th/K and Th/U ratios from spectral gamma-ray (SGR) measurements could serve as a proxy for hinterland palaeoclimate change, a series of papers were published that used this method (Hladil et al., 2003; Schnyder et al., 2006). Complications to this simple model were identified by the early workers (see above) and include diagenetic effects, the presence of U and Th-bearing heavy minerals; K-bearing glauconite and Th-hosting organic matter in mudrocks (Schnyder et al., 2006). These problems were overcome by the above authors *Correspondence to: A. Ruffell, School of Geography, Archaeology and Palaeoecology, Queen's University, Belfast, Northern Ireland, BT7 1NN, UK. E-mail: [email protected]

by focusing their studies on shallow-buried (diagenetically less-altered) mudstones or mudstone intervals with low glauconite, heavy mineral and organic matter contents. In addition, comparison of two or more independent, climaterelated variables (e.g. pollen compared to clay mineralogy) provides an internal test to the validity of interpretations. Two other potential problems not considered by these pioneering studies were where K, U and Th reside in limestone-dominated successions and the effects of global anoxia in controlling U contents in seawater and sediments: these issues are examined here.

2. PHANEROZOIC PALAEOCLIMATE PROXIES AND SPECTRAL GAMMA RAY LOGS A range of proxies are used in attempting to reconstruct Phanerozoic palaeoclimates, from the geographic observation of facies belts (Hallam, 1984), through cool–warm or arid–humid stratigraphic indicators (e.g. dropstones, fossil types (Price, 1999)), mineralogy (commonly clay minerals (Schnyder et al., 2006)), geochemistry (Si/Al ratios, Th/K Copyright © 2014 John Wiley & Sons, Ltd.

E. GHASEMI-NEJAD ET AL.

and Thu/U ratios (Ruffell and Worden, 2000)) and isotopes (usually δO and δC (Price, 1999; Veizer et al., 1999; Sharp, 2007)). Measurement of stable isotope ratios is the most widely used method for palaeoclimate reconstruction (Hoefs, 2009), although diagenetic factors must be considered for isotope measurements (as is the case for the above proxies). The use of SGR data from borehole logs in attempting palaeoenvironmental reconstructions is standard oil industry practice (Slatt et al., 1992), as is using outcrop-based SGR logs in palaeoclimate analysis (Osmond and Ivanovich, 1992; McRoberts et al., 1998; Ruffell and Worden, 2000; Schnyder et al., 2006). However, using borehole-based logs for palaeoclimate analysis (taking these borehole-based interpretations further to attempt reconstruction of changing palaeoclimates in the sediment sourcelands) is a new idea, with few tests of whether this is possible (Ghasemi-Nejad et al., 2010). Given the caveats we describe above, the method should work, following the positive results of Parkinson (1996) and subsequent workers (see works cited above). The natural radioactive content of sediments (mainly clastic, or chemical with a clastic component) measured by SGR logs can be used for palaeoclimatic determination, as long as a range of complicating factors (radiogenic isotope host minerals, redox changes, diagenesis) are taken into consideration, and the interpretations verified by some independent analyses such as lithostratigraphy, stable isotopes, climate-sensitive fossil organisms (Osmond and Ivanovich, 1992; McRoberts et al., 1998; Ruffell and Worden, 2000; Bond and Zaton, 2003; Hladil et al., 2003). More specifically, SGR data have been compared to established palaeoclimate proxies, such as palynological data (Ghasemi-Nejad et al., 2010), TOC and clay mineralogy (Schnyder et al., 2006) and carbon isotopes (Ruffell et al., 2002). Radioactive elements such as potassium (K), uranium (U) and thorium (Th) exist in many rocks. In sediment hinterlands, under chemical weathering, K and U dissolve more easily, while Th is considered less soluble (Osmond and Ivanovich, 1992), except in the presence of certain acids (e.g. humic acid). Generally, high Th/K and Th/U ratios in soils and weathered rocks indicate humid hinterland palaeoclimates (Osmond and Ivanovich, 1992; Ruffell and Worden, 2000), where few complicating factors such as unusual minerals or where significant shallow (e.g. redox changes) or deep (e.g. regional mineralising fluid flow) diagenesis occur. A problem in using SGR logs and isotopes in palaeoclimate reconstructions is that both these proxies are influenced by sea-level change (Zeebe, 2001), and can be affected by diagenesis. SGR data can be obtained by non-destructive, automated, rapid and inexpensive survey methods, either from outcrop, or from borehole measurements, as in this study. Here, we try to demonstrate the stratigraphic relationship between changes in SGR logs and other proxies (stable isotopes, lithology and TOC) in two boreholes Copyright © 2014 John Wiley & Sons, Ltd.

(informally termed SP.1 and SP.2) from in the South Pars Gas Field in the Persian Gulf, South of Iran (Fig. 1). We do this because basing interpretations on one data source can lead to significant error. Should another proxy data source compare well to the SGR data, then an interpretation gains strength. For palaeoenvironmental studies, where the problems of, say, diagenesis are being avoided, data from changing lithologies (e.g. tillites for cold climates, evaporites for arid), palynology and stable isotopes are commonly used. Where past changes in hinterland weathering (arid–humid cycles) occurred, the method appears to work (see references above). However, no work has been attempted to test the limits of the theory, either in diagenetically-altered successions, carbonate-dominated rocks, or where anoxia has dominated. This work avoids the diagenetic question by examining relatively unaltered rocks, but does tackle the question of whether hinterland palaeoclimate changes can be detected in a carbonate succession where changing anoxia is recorded. In the succession studied, no palynological data is available, so lithology and published data from isotopes are used as proxies for comparison to the borehole SGR data.

Figure 1. Location map of the South Pars Gas Field, Persian Gulf, south of Iran (Aali et al., 2006; Aali and Rahmani, 2011). ‘Border’ indicates the boundary between Iran in the north and Kuwait, Saudi Arabia, Bahrain, Qatar and the United Arab Emirates to the south.

Geol. J. (2014)

PERMIAN–TRIASSIC SPECTRAL GAMMA RAY LOGS, PERSIAN GULF

3. GEOLOGICAL SETTING The Qatar Arch is located in the inner part of the Arabian Platform, which is bounded by the Zagros Fold Belt to the north and northeast. The South Pars–North Field supergiant gas reserve was formed due to the buoyant nature of the Qatar Arch (Kashfi, 1992, 2000; Alshahran and Nairn, 1997; Sharland et al., 2001). Consequently, the anticlinal

structure of the South Pars field (part of the Qatar Arch) is subdued with dips in Phanerozoic strata, generally less than 10 (Aali et al., 2006). The generalized stratigraphy of the studied succession in the South Pars Field is shown in Figure 2. The Permian–Early Triassic deposits have been divided into the Faraghan (Early Permian), Dalan (Late Permian) and Kangan (Early Triassic) formations (Kashfi, 2000): it is the latter two formations we are concerned with here,

Figure 2. A: Generalized stratigraphy of the upper Permian–lower Triassic at the South Pars (from Rahimpour-Bonab et al., 2009, including their megasequences [for reference]), B: Detailed lithostratigraphic log of K3 and K2 units: conventional legend is used and explained in the text column. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

Copyright © 2014 John Wiley & Sons, Ltd.

Geol. J. (2014)


but describe the preceding succession to provide context. The Faraghan Formation (about 100 m thick) comprises alternations of sandstone, shale and limestone, rests disconformably on the Devonian Zakeen Formation sandstones and is overlain disconformably by the Dalan Formation (Ghavidel-Syooki, 2003), where the later Permian to Early Triassic story of the Persian Gulf really begins. The Dalan Formation is more than 680 m thick and mainly comprises limestone (Lower Dalan), anhydrite and subordinate dolomite (Nar Member), with subordinate limestones, dolomite and dolomitic limestones (Upper Dalan: see Kashfi, 2000). The upper Dalan has been informally divided into two submembers of K4 and K3 (op cit.). This lithostratigraphic nomenclature is not ideal, but is what is generally known and published from the area, which we adhere to. This formation is in turn overlain by the Early Triassic Kangan Formation, which is some 193 m thick and comprises limestone, dolomite, anhydritic dolomite with thin (centimetre to decimetre) shale layers from bottom to the top, and has been divided by Ghavidel-Syooki (2003) into K2 and K1 members respectively (Fig. 2). Although the PTB is continuous in some parts of Iran (Abadeh and Julfa areas in southwestern and northwestern Iran, respectively), and is known from a complete succession in Oman (Krystyn et al., 2003) the uppermost Permian (~254/252 to 250 Ma) and the lowermost Triassic (Scythian 252/250 to 246/248 Ma) deposits have not been recorded in the Persian Gulf (Baharlouei, 2012). Dates for the Late Permian–Early Triassic stages vary in the literature (with the PTB itself better constrained at 252 Ma), thus the time gap in the Persian Gulf successions (including those studied here) maybe around 6 million years. SGR logs and stable isotopes recorded in two wells drilled in the Persian Gulf are here used to consider whether the palaeoenvironmental (especially, any possible palaeoclimate changes) of the Late Permian and Early Triassic can be interpreted. The wells are named A and B, because of economic restrictions on data release.

4. PERMIAN–TRIASSIC BOUNDARY (PTB) EVENTS This work is not a discussion of global events that occurred in the Late Permian and Early Triassic, but rather a test of whether SGR logs can be used to characterize hinterland palaeoclimates in carbonate succession with changing depositional anoxia. The PTB succession is chosen as a major event in world history, across which we have hydrocarbon industry- and exploration- (e.g. DSDP/ODP) standard SGR logs. The PTB mass extinction reportedly killed between 75 and 95% of organisms world-wide and the recovery after this calamity took between 3 and 15 million years, depending on species and habitats (Hallam and Wignall, 1997; Benton and Twitchett, 2003; Bottjer, 2004; Fraiser Copyright © 2014 John Wiley & Sons, Ltd.

and Bottjer, 2005a,b, 2007; Pruss et al., 2005; Racki and Wignall, 2005; Erwin, 2006, 2007; Algeo et al., 2007; Knoll et al., 2007; Brayard et al., 2009; Marshall and Jacobs, 2009; Posenato, 2009). Extraterrestrial agents, tectonics/volcanism, anoxic events in oceans, eustatic sea-level changes and global variations in palaeoclimate have all been mentioned as controlling factors in this extinction event, which occurred as the Pangaean Supercontinent assembled (e.g. Schopf, 1974; Isozaki, 1997, 2009; Racki and Wignall, 2005; Erwin, 2006). A sea-level fall across the PTB could have caused the number of marine niches to reduce and many families of invertebrates to become extinct (Newell, 1963; Schopf, 1974; Simberloff, 1974). During the Permian, temperature increased gradually from the Southern Hemisphere glacial low, with a dramatic change in the Early Triassic, when the late Palaeozoic Icehouse Earth turned to Greenhouse conditions (Chumakov and Zharkov, 2000). One further cause of the end-Permian mass extinction has been suggested to be tectonically-induced climatic instability and marine regression (Erwin, 1990). All of the above-proposed mechanisms would have operated differently in various locations around the world. For instance, sea-level fall would have a large impact on broad, shallow shelf environments, compared to narrower, steeper shorelines. What is not in dispute are the great changes that occurred at the PTB: here we examine what is recorded in the SGR profiles of two oilfield borehole logs that penetrated Triassic and Permian successions 5. METHODS 5.1. SGR measurements In petroleum exploration, a range of tools are lowered down a borehole in order to understand the nature of the stratigraphy and lithologies. A standard tool comprises the gammaray logger, which may record total counts, or use photomultiplier arrays to separate the gamma-ray spectra (Slatt et al., 1992). The most commonly-occurring radioisotopes from rocks are K, U and Th, the ratios of which we use here (see above). Our measurements are thus provided directly from exploration operations, however, all gammaray detectors, and especially borehole-based devices are calibrated against standards and continuously checked. The methods used in interpreting SGR data must take into account a number of possible uses the information has (outlined below). SGR data are used in sequence stratigraphy (Van Wagoner et al., 1990), reservoir parameters (Davies and Elliot, 1996), differentiation of diagenesis and mineralogy (Hurst, 1990) and evaluation of source-rocks (Myers and Wignall, 1987). Using a spectral gamma ray device, one can determine the abundance of these radioactive isotopes. Major K-bearing minerals include mica, feldspars, clays and chloride salts. U and Th may be found in such materials as clays, Geol. J. (2014)


feldspars, phosphates, organic matter and heavy minerals. Potassium is soluble and can be leached from sediment source rocks during weathering. Uranium behaves similarly, but with lower intensity; Th is almost insoluble and its total percentage increases through time if chemical weathering is dominant (Ruffell and Worden, 2000; Schnyder et al., 2006). Therefore, the Th/K and Th/U ratio increases in sediments under a number of different controls: (i) sea-level changes influencing clay mineral content; (ii) early diagenesis (changes in redox) or late diagenesis (fluid movement/mineralisation) and (iii) in sediments whose original source locations enjoyed humid palaeoclimates (Schnyder et al., 2006). Clays and clay-bearing rocks are suitable minerals for SGR studies, as long as they have not undergone significant diagenetic alteration that could have mobilized any or some of K, U or Th. Variation in clay mineralogy (which is controlled by chemical and physical weathering of source rocks, transport and settling) affects the results gained from spectral gamma ray data. Different clay minerals have different U and Th contents, so there is likely to be some relationship between uranium and kaolinite (Ruffell and Worden, 2000). Kaolinite is found in hot and humid conditions and shows correspondence with high Th/K or Th/U ratios (Osmond and Ivanovich, 1992). Such a similar relation has also been recorded between Th/K + U ratio and illite (Chamberlain, 1984; Myers and Bristow, 1989; Slatt et al., 1992). Heavy minerals also present different amounts of U and Th (Hurst, 1990). In chemical rocks such as limestones, with low clay contents (~10%), K and U values may show a different pattern, which is discussed below. 5.2. TOC and isotope measurements The standard sampling interval in deep (typically petroleum exploration) borehole-based geochemical analysis is usually 18 m, although in higher resolution studies, a 9 m interval may be applied (Hunt, 1996). To account for the role of diagenesis, samples with minimal diagenesis (lack of veining, fractures, mineral growths) were chosen using a 10× magnification hand lens. Some possible microscopic diagenetic effects may be present nonetheless, which will likely appear as outliers in the data that do not compare to other proxies. In this study, 78 chips (ranging from 0.5 to 2 cm) were selected following the standard method mentioned by Hunt (1996) and analysed via a Rock-Eval6 machine. The experiment comprises heating (firstly at 300 °C then up to 850 °C with a temperature gradient of 25° per minute) using a small quantity of rock (100 mg). In this experiment, geochemical parameters of the rock, from which TOC is extracted were quantitatively estimated. Details of the sampling and pyrolysis methods can be found in Sfidari et al. (2012). Carbon and oxygen isotopic data of the two wells used here were available from earlier studies (Rahimpour-Bonab et al., 2009; Tavakoli and Rahimpour-Bonab, 2012). Copyright © 2014 John Wiley & Sons, Ltd.

6. RESULTS AND OBSERVATIONS Comparative changes in δ18O, δ13C, Th, K, and U values, TOC, and lithology (evaporate contents) were used in this study. K, U and Th values were obtained from conventional SGR well logs, δ18O and δ13C from published stable isotope analysis and TOC values from Rock-Eval pyrolysis. For each palaeoenvironmental proxy, we first summarize the main results, as details are included in the figures, followed by some observations on the data, given the context and interpretive methods (above). 6.1. Isotopes The oxygen isotope ratio in sedimentary rocks may be used for the evaluation of palaeotemperatures (Kasting et al., 2006). When evaporation occurs, light isotopes are removed and heavy ones remain in the ocean water, causing an increase in/ratios in precipitated carbonates. Isotopic data show an increasing trend of δ18O from about 8‰ in the early Phanerozoic to about 0‰ at present (Veizer et al., 1999). The isotopic composition of oxygen in carbonates becomes lighter with increasing geological age (Veizer and Hoefs, 1976; Veizer et al., 1999), which can be correlated with diagenesis and elevated seawater temperature (Sharp, 2007). Figure 3 shows the δ18O data throughout the upper Permian to lower Triassic of Kangan and upper Dalan strata (from Rahimpour-Bonab et al., 2009, and Tavakoli and Rahimpour-Bonab, 2012), with low values of δ18O (negative values) through the K2 unit. There is a distinctive difference in both phases of carbonates. For example, observations show dolomites are 2–3‰ heavier than limestones, indicating a possible mineralogical control on the isotopic composition of carbonates (Hoefs, 2009). The ratio of 13C to 12C, measured in sedimentary rocks, reflects the input of these forms of carbon into the sedimentary system, controlled initially by changes in the global carbon cycle, the flux of the two isotopes into the depositional basin (usually the oceans), and its capture in carbon-hosting materials such as organic matter and carbonates. Positive 13C excursions have been related in previous studies (see Veizer et al., 1999) to times when the carbon cycle was accelerated, such as in a Greenhouse Earth scenario. Many works (see above for a very brief review) use the carbon isotope across the PTB to both define its stratigraphic position, and also to demonstrate the major change in Earth systems (especially the change to a Triassic Greenhouse climate, see Joachimski et al., 2012). 6.2. Gamma-ray and SGR data The gamma ray curve shows (in general) a smooth decreasing trend from the base of the K3 towards the top of the K2 unit (Fig. 3), with some minor spikes in the Late Permian and Geol. J. (2014)


Figure 3. Spectral gamma ray logs and isotope data (published, after Rahimpour-Bonab et al., 2009) throughout the Permian–Triassic beds in borehole SP.B. Right hand side bars are 5 m increments. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

Triassic, but overall decreasing through the Triassic strata. Uranium is elevated in the Permian K3 unit, with a spike at the PTB. The K curve is variable through K3 (Permian), with elevated contents in the Early Triassic, then a decrease up section, with an overall different pattern from that of total counts and U, showing more consistency with changing Th trends: a similar pattern was shown by Raddadi et al. (2005) from western Alps carbonates, France. The trend presented by K contents is concordant with that of the Th pattern, representing an increase then decrease in both elements. The Th curve reflects overall more frequent changes through the K3 and K2 units but more similarity to the K curve. Fluctuations in Th might be ascribed to different factors (e.g. weathering activities, diagenetic processes and heavy mineral effects). Th/K is variable through the Permian K3, with some isolated spikes (unrelated to overall trends). Th/U is more informative, staying low through the Permian, rising across the PTB and into the Early Triassic, and then remaining low through the Triassic K2. The sharp change around the K3–K2 boundary is coincident with a previouslyrecorded sea-level fall (Rahimpour-Bonab et al., 2009).

6.3. TOC The amount of total organic carbon is affected by sea-level changes (coincident or not with changes in oxygenation) and oceanic and hinterland productivity. During sea-level rises, the continental shelf area increases and anoxic conditions may increase, or in areas/times of limited circulation, dominate sedimentation. When continents are free from ice, there is limited circulation in sea waters and therefore, anoxic conditions extend and source rocks with high amounts of TOC are developed. Despite the six degree increase in temperature at the PTB (Holser et al., 1989), smaller continents were gathered to form the supercontinent Pangaea at this time (Scotese and Golonka, 1992). Overall, a combination of these factors (increase in temperature and Copyright © 2014 John Wiley & Sons, Ltd.

formation of the huge continent) caused a eustatic sea-level fall at the PTB. As a consequence, anoxic conditions may have changed and a reduced amount of TOC could be expected. As shown on Figure 4 the results are in accord with other evidence of a global sea-level fall at this time, when TOC reached its lowest values at this boundary is shown by some researchers (e.g. Algeo et al., 2007; Maurer et al., 2009). A positive relationship has also been recorded between U and the TOC curves, including across the PTB (Ehrenberg et al., 2008) although many factors affect this relationship. 6.4. Lithological data Evaporitic layers are more abundant in the K3 unit, decreasing upwards and vanishing just below the PTB. The presence of microkarst recorded by Rahimpour-Bonab et al. (2009) in the lower Triassic rock unit implies meteoric weathering and atmospheric moisture in this interval. In general, lithological evidence has shown a great climate variation at the passage of the Permian–Triassic, from a semi-arid condition in the Late Permian to a semi-humid condition in the Early Triassic (Esrafili-Dizaji and Rahimpour-Bonab, 2009).

7. THEORIES The δ18O ratio shows some fluctuations that may be due to diagenetic processes, as shown by previous studies (Heydari et al., 2000; Rahimpour-Bonab et al., 2009; Tavakoli and Rahimpour-Bonab, 2012). In arid areas, where evaporation is high, meteoric waters can have a heavy oxygen isotopic composition, therefore meteoric diagenesis could change δ18O values to higher values in marine carbonates (Sharp, 2007). When the amount of buried organic carbon increases, 12 C will be preferentially removed from ocean water. During the oxidation of organic matter, 12C values in Geol. J. (2014)


Figure 4. Borehole formation data for selected Th/U and Th/K ratios from spectral gamma ray logs in the two boreholes (SP.A and SP.B) studied. This figure is available in colour online at wileyonlinelibrary.com/journal/gj

sediments decrease, since it is removed from organic matter more than 13C. Therefore, the ratio of 13C/12C increases in precipitated sediments (Fig. 4). Values of δ13C show an increasing trend from about 1 ± 1‰ through the Phanerozoic to +4 ± 2‰. However, a decrease of about 2‰ across the PTB and subsequent fluctuations of about +2‰ are recorded during the Mesozoic and Cenozoic (Veizer et al., 1999). Unlike oxygen isotopes, diagenetic processes usually have a lesser effect on carbon isotopes, so transportation of carbon from the parent rock or minerals to secondary minerals is not as critical, although diagenesis can still alter the C isotope content of a Copyright © 2014 John Wiley & Sons, Ltd.

rock and its minerals. δ13C values show a constantly-decreasing trend through the Permian strata preserved, which is in accord with previous research (e.g. Heydari et al., 2000; Heydari and Hassanzadeh, 2003; Rahimpour-Bonab et al., 2009). The sharp decrease across the later Permian to Early Triassic strata has been suggested to be the result of eustatic regression occurring in the latest Permian (Tavakoli and Rahimpour-Bonab, 2012), albeit here accentuated by a time-gap. A decreasing trend through the measured Permian succession could be ascribed to reoxidation of formerlystored 12C-enriched organic materials and lower rate of Geol. J. (2014)


burial caused by eustatic regression (Erwin, 2006, 2007; Heydari et al., 2000; Horacek et al., 2007; Fig. 4). High values of uranium throughout the K3 sector of the curve could indicate the dominance of an arid climate in the weathered hinterlands during sedimentation of this part of the succession. The major fall in U at the K3–K2 boundary might be considered as a result of the sea-level fall and an indicator of oxidizing conditions that might be the result of a change in palaeoclimate (Sun et al., 2012). Overall, low values of uranium occur in the Triassic strata (Fig. 4). The gradual increase in Th/U values in the upper part of the curve may reflect the dominance of a more humid climate after the major climatic turnover, or a rise in U contents of seawater. A major sea-level fall could also be concluded at the K3–K2 that affects the Th/U ratio (Fig. 4). The similarity presented by the K and Th curves could be explained as following. Based on the Lewis Theory, cations and anions are in general divided into two groups of hard and soft (Pearson, 1968). When soft cations bind with soft anions and/or hard cations with hard anions they form strong ionized bonds while, binding soft and hard cations or anions cause the formation of weakly-ionized bonds (House, 2008). A bond between a potassium ion, which is a hard cation, and a carbonate ion (e.g. CO3 2 ) which is a hard anion, generates a strongly-ionized bond, which is more stable and cannot be broken easily. This remains constant in the environment within carbonates while a reaction between a uranium ion (U+6) which is a soft cation with carbonate ion CO3 2 generates a weakly-ionized bond which is more soluble and removed from carbonates more easily. This is quite similar to those of Th, which is insoluble and remains constant in carbonates. Moreover, many uranium complexes are unstable and more soluble and have less chance of entering the calcium carbonate structure. On the other hand, the potassium radian is very close to that of calcium and could enter calcium carbonate as a rare element more easily than that of uranium and could enter the solid phase of calcium carbonate. The thorium radian is also close to that of calcium and is insoluble and could enter to the solid phase of calcium carbonates (Faure, 1986). These similarities between potassium and thorium (regarding carbonate ions) could explain the concordance between the potassium and thorium curves in the carbonate rock units studied here. This possibility requires further geochemical modelling and experimentation, which is beyond the remit of this work.

8. CONCLUSIONS In this study, SGR logs from two boreholes drilled in the Persian Gulf were examined for changes in K, U and Th contents that may reflect changing palaeoenvironmental conditions in the sediment source hinterlands. Specifically, we were interested in testing previously-proposed hypotheses Copyright © 2014 John Wiley & Sons, Ltd.

that changing Th/K and Th/U ratios may reflect the level of K and U dissolution and removal from soils under humid weathering conditions in the sediment sourcelands. Such K and U depleted sediment (especially clays from weathered precursor minerals) could be transported and deposited as sedimentary rocks, should diagenesis and settling be negligible. Any change in such ratios could reflect palaeoclimate variations, which across the Permian–Triassic boundary would be of significance. Changing lithologies, δ18O, δ13C and TOC contents were used as semi-independent proxies for palaeoenvironmental change. Together, these various data suggest a fall in sea-level across the Permian–Triassic boundary (some fluctuations in oxygen variation maybe related to diagenesis), coincident with a change in Th/U ratios across the PTB. This decrease in U could be due to a change from arid (Permian) to semi-arid (Early Triassic) palaeoclimates: this would be consistent with Retallack (1996); Ziegler (2001) and Heydari and Hassanzadeh (2003) who all suggest strengthening of a Greenhouse-type climate system (as opposed to the Icehouse of the Carboniferous-Permian) at this time, during the build-up of the Pangaean Mega-monsoonal system. The δ18O values of carbonates could be interpreted as the influence of a more humid (or less arid) climate in the Early Triassic, with meteoric diagenesis reducing δ18O values in marine carbonates. Moreover, δ18O values decrease less in dolomite than in calcite, possibly reflecting greater stability and low diagenetic potential in dolomite relative to calcite. Below the K3–K2 boundary, U values are higher, decreasing after the P/T boundary. Alternatively, Ehrenberg et al. (2008) have described a global drawdown in U (from global anoxia) at the PTB, an equally valid reason for the pattern seen on the SGR logs described here. Although the Th/K values are good proxies for palaeoclimatic interpretations in clastic rocks such as shales (Schnyder et al., 2006), they may not be so valid for palaeoclimatic interpretations in carbonate rock units, as they form strong bonds with carbonate ions which may not be broken easily and remain constant in the environment within carbonates. In summary, two conflicting reasons exist for the changing Th/U ratios observed here: palaeoclimate change (supported by the disappearance of evaporite lithologies) and global U drawdown due to anoxia and changing depositional redox conditions. Isotope data can be interpreted as supporting both reasons. Changing Th/K ratios are inconsistent with Th/U ratios: in times of changing hinterland weathering the two sets of ratios should occur together (K and U behaving similarly in these scenarios: see Ruffell and Worden (2000) and Schnyder et al. (2006)). As the occurrence of K in carbonates can be explained by the likely mineral host, the Th/K ratio appears to be unrelated to hinterland weathering. As the chemical composition of carbonates and evaporates is so much more removed from terrigenous source than clastic rocks, a marine origin for K and U contents is more logical for this succession, supporting the global drawdown of U theory. Spectral gamma ray logs in Geol. J. (2014)


pure or evaporite/carbonate-dominated successions are thus not considered as reliable proxies for hinterland palaeoeclimate change, and should be used with great caution, if at all for palaeoclimate reconstruction.

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Scientific editing by Zhong-qiang Chen


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