Brachiopods are abundant in the Oman Khuff Formation and similar brachiopod faunas are present at a few horizons in the same formation in Central Saudi Arabia. Following extensive systematic and biostratigraphic studies of these faunas, specimens from the base of the Midhnab Member of the Khuff Formation of Saudi Arabia (Buraydah Quadrangle), and from Member 3 of the Khuff Formation of the Huqf outcrop of Oman were assessed for isotope geochemistry (Sr, O and C). Dating using 87Sr/86Sr alone is not conclusive. Five pristine Oman brachiopods from biostratigraphically well-constrained lower Wordian horizons record a range of 87Sr/86Sr values that form a separate cluster offset from the current Sr isotope seawater curve, which defines the Early Permian and earliest Mid-Permian. The 87Sr/86Sr of the pristine Saudi Arabian brachiopod sits in an area which corresponds to a wide scatter of 87Sr/86Sr in the seawater curve data. However, the Saudi Arabian data does indicate that the Midhnab Member is likely younger than Member 3 of the Khuff Formation of the Huqf outcrop.
The well-preserved brachiopod carbonate allows deductions to be made about the palaeotemperature of the Oman Khuff Formation Member 3 seawater using its oxygen isotope composition (δ18O). Assuming δ18O of seawater < –0.5‰, then palaeotemperature derived from brachiopods in the Oman horizons would be +25°C, +22°C and +17°C respectively. This is consistent with the trend of shallowing within Member 3, suggested by facies.
The Khuff Formation (Steineke et al., 1958) comprises the Middle Permian to Lower Triassic carbonate rocks of the Arabian Peninsula and is a hydrocarbon-producing unit in Bahrain, Iran, Qatar, Saudi Arabia and the United Arab Emirates. Its deposition started in the Mid-Permian and accompanied the drifting of Cimmerian terranes from Gondwana (Konert et al., 2001). The Khuff Formation was mainly deposited as a post-rift carbonate succession (Maurer et al., 2009;,Koehrer et al., 2010) on a passive continental margin of the newly forming Neo-Tethys Ocean during a period of relative tectonic quiescence and steady subsidence (Muttoni et al., 2009).
In the Haushi Outcrop area in Oman, the Khuff Formation reaches a maximum thickness of 30 to 40 m and is divided into three members (Figures 1 and 2; Angiolini et al., 2003). Member 1 represents the onset of carbonate shelf sedimentation succeeding the organic-rich, marsh–alluvial to estuarine environments of the uppermost Unit B of the Gharif Formation (Berthelin et al., 2003,,2006; Broutin et al., 1995;,Angiolini et al., 2004;,Stephenson, 2011). The lower part of Member 1 (Unit C of Angiolini et al., 2003) consists of paralic to coastal sands interpreted as tidal sand-flat to barrier-beach deposits, variably reworked by waves and tidal currents in a lagoonal or bay environment (Angiolini et al., 2003). Carbonate sedimentation became progressively more common in Unit D (the upper part of Member 1) which consists of inner- to outer-shelf sediments deposited at water depths between fair weather wave-base and storm wave-base. Member 2 is characterised by limestones and marlstones with open-marine fauna, and records transition to outer-shelf conditions mostly below storm wave-base. Abundance of bioclastic storm layers in the lower part of Member 3 suggest deposition around storm wave-base, but sedimentary features in the topmost beds of Member 3 suggest shallower waters, well above storm wave-base (Angiolini et al., 2003).
In Central Saudi Arabia (Figures 1 and 3), the terrigenous sediments of the lowest Khuff Formation Ash Shiqqah Member are succeeded by the Huqayl Member which consists of calcarenite, gypsiferous claystone, dolomite, and solution breccias related to subsurface evaporites (Angiolini et al., 2006). The succeeding Duhaysan Member is the first true calcareous subtidal to littoral unit of the Khuff Formation (Le Nindre et al., 1990b), and it is succeeded by the Midhnab Member which displays a succession ranging from marine fossiliferous limestones at the base, to gypsiferous and dolomitic rocks deposited in restricted palaeoenvironments, in the upper part. Locally, in north Central Saudi Arabia, the topmost continental facies of the Midhnab Member include lacustrine limestone, sandstone channels and claystone in meandering river systems and swamps. These facies contain drifted wood and plant remains (Vaslet et al., 2005).
The Khuff Formation throughout the Arabian Peninsula contains a rich fauna including brachiopods, bivalves, gastropods, conularids, bryozoans, echinoderms, barnacles, ostracods, fishes, asterozoans, nautiloids, conodonts, fusulinids, echinoderms, small foraminifers and ammonoids (Angiolini et al., 1998, 2003, 2004; Angiolini and Bucher, 1999). However, only the lower part of the Khuff Formation in Oman has been dated with certainty using this fauna, as early Wordian (Angiolini et al., 2003, 2004).
The age of the lower part of the Khuff Formation in Central Saudi Arabia is in dispute. Vaslet et al. (2005) suggested that (1) the lowest part of the Ash Shiqqah Member is Capitanian in age; (2) the overlying Huqayl Member is Wuchiapingian in age; (3) the Duhaysan Member is Wuchiapingian to Changhsingian in age; and (4) the Midhnab Member is Changhsingian in age. However, the conodont ‘Jinogondollella’ cf. altaduensis from the base of the Midhnab Member suggests an older, late Capitanian age (Nicora et al., 2006) and the ostracod species recorded in the lower Midhnab Member (Hollinella herrickana Girty) has an Early to Mid-Permian range (Crasquin-Soleau et al., 2005). The Midhnab Member was also dated as Changhsingian by Vachard et al. (2005) based on the occurrence of the foraminifer Paradagmarita and of a new species of Glomospirella? similar to a species from the early Changhsingian of South China. However, the genus Paradagmarita has a Wuchiapingian to Changhsingian distribution (Zaninetti et al., 1981;,Pronina, 1999;,Groves and Altiner, 2005;,Maurer et al., 2009).
Brachiopods are abundant in the Oman Khuff Formation and similar brachiopod faunas are present at a few horizons in the same formation in Central Saudi Arabia. Following collection of these faunas, it was suggested that the brachiopods may be useful in elucidating palaeoenvironment and age through their geochemistry. Articulate brachiopod shells (Subphylum Rhynchonelliformea) are known to record the primary geochemical signal of ancient seawater, as the low-Mg-calcite of their shell is generally resistant to diagenetic change (see Compston, 1960;,Lowenstam, 1961;,Popp et al., 1986; Veizer et al., 1986,,1999; Korte et al., 2003, 2005; Angiolini et al., 2009). Articulate brachiopods secrete a two- or three-layered calcite shell below an outer proteinaceous periostracum. The primary layer is granular/acicular and usually low in δ13C and δ18O; the secondary layer is fibrous or laminar and is thought to precipitate in isotopic equilibrium with ambient seawater (Lowenstam, 1961;,Brand, 1989;,Grossman et al., 1991;,Brand et al., 2003;,Parkinson et al., 2005); and the tertiary layer, when present, is coarse prismatic and locally confined inside the shell and is also thought to be in equilibrium with ambient seawater. No ‘vital fractionation effects’ leading to departure from isotopic equilibrium greater than ± 1‰ have been recorded in the slow-growing secondary and tertiary layers of modern articulate brachiopods (Carpenter and Lohmann, 1995;,Curry and Fallick, 2002;,Brand et al., 2003;,Korte et al., 2005;,Parkinson et al., 2005), and therefore it is these layers that are generally sampled for their stable isotope composition.
In this study, we assess the degree of diagenetic alteration in the brachiopod shells by scanning electron microscopy (SEM), cathodoluminescence (CL), and geochemistry. We therefore ascertain which brachiopods are most likely to record primary environmental information in the isotope ratios of their shells. We use these data to (1) test the possibility that 87Sr/86Sr can be used to date the Khuff Formation in the Arabian Peninsula (since the 87Sr/86Sr ratio gradient is very steep in the Mid-Permian (Korte et al., 2006,,McArthur and Howarth, 2004); and (2) investigate the palaeotemperature of Khuff Formation seawater using δ18O.
MATERIALS AND METHODS
Twenty brachiopods belonging to the following six species were analysed: Omanilasma husseinii Angiolini and Zarbo in Angiolini et al. (2006), O. desertica Angiolini and Zarbo in Angiolini et al. (2006), Juresania omanensis Hudson and Sudbury (1959), Derbyia cf. D. diversa Reed (1944), Callispirina sp. ind., and Kotlaia cf. K. bistriata Reed (1944).
The brachiopod fauna from the Midhnab Member (Figure 1) was collected at outcrop by D. Vaslet and Y.-M. Le Nindre in 2002 (Angiolini et al., 2006). This fauna represents one of the few records of Permian brachiopods so far reported from Saudi Arabia (Vaslet et al., 1985; Le Nindre et al., 1990a, b; Al-Aswad, 1997). The fossiliferous locality is located in the Buraydah quadrangle (Vaslet et al., 1985) at 26°07’01”N and 44°02’26”E, 5 km to the north of the town of Midhnab. The brachiopods occur in the lowest part of the subunit 2 of the Midhnab Member (Figure 3; Angiolini et al., 2006), in bioclastic limestones with marine fauna, alternating with marly limestones, which correspond to the maximum flooding surface (MFS PKm) of Central Saudi Arabia (Le Nindre et al., 1990b; Sharland et al., 2001;,Vaslet et al., 2005). This very rare brachiopod fauna includes Kotlaia sp. ind. of the order Orthida and Omanilasma husseinii of the order Terebratulida. Kotlaia is a long-ranging genus, spanning the Mid-to Late Permian time interval. Omanilasma is known from the early Wordian of Oman and from the Wordian Ruteh Limestone of north Iran (Crippa and Angiolini, 2011). Seven specimens were analysed by CL, SEM and geochemistry, and for their isotope composition (Table 1).
The brachiopod fauna of the Khuff Formation of the Haushi Outcrop area of Interior Oman was collected by L. Angiolini from 1995 to 2001 (Angiolini and Bucher, 1999, Angiolini et al., 2003, 2004). It is diverse, comprising more than 2,000 specimens belonging to more than 30 species. Thirteen specimens were analysed by cathodoluminescence, SEM and geochemistry, and for their isotope composition (Table 1) from section K7 at 21°02′30″N and 57°42′00″E (Figure 2) in the Acritosia sp.-Globosobucina sp. Biozone and the Grandaurispina ghabaensis - Kozlowskia tescorum Biozone (Angiolini et al., 2003, 2004).
Ultrastructural SEM Analyses
Well-preserved non-luminescent secondary layer fibres were observed in five out of seven specimens of Omanilasma husseinii from Saudi sample level KH0211 (Plates 1 and 2). The secondary layer fibres have a keel and saddle profile with a thickness of 3–5 μm and are slightly deflected around very thin punctae (diameter 8–13 μm). Occasionally, secondary layer fibres show some dissolution and minor coalescence. The maximum measured thickness of the secondary layer is 202 μm.
Most of the thirteen specimens analysed from the Khuff Formation of Oman were well preserved in that all the secondary fibres were intact. In general the Oman specimens have a thicker secondary shell and are better preserved than those of Saudi Arabia. Diagenetic alteration appears not to be related to taxonomy or stratigraphic position as the three altered specimens belong to two species of different orders (K. cf. K. bistriata and J. omanensis), and occur in the same beds as other well-preserved specimens. However, specimens of J. omanensis, characterised by a secondary laminar shell, tend to be more easily altered externally around the hollow bases of spines (Plate 2) and appear luminescent.
A specimen of a different taxon, Callispirina sp. ind. (AO73-107, Table 1), was also analysed. It has very well-preserved secondary layer fibres, however these are deflected around coarse and dense punctae, which may have diagenetically altered fillings.
The well-preserved non-luminescent secondary shell of the Oman specimens of O. husseinii is much thicker than the conspecific Saudi Arabia ones, reaching thicknesses of more than 500 μm. The secondary layer fibres have a keel and saddle profile with a thickness of 3–5 μm and are deflected around larger punctae (diameter 30–45 μm) (Plates 2 and 3). The single specimen of O. desertica (AO210-153) has the same ultrastructure as O. husseinii. Punctae seem to have been filled soon after decomposition of the soft parts as the fibres surrounding them do not show any sign of dissolution.
Cathodoluminescence analysis (CL) was performed using a cold cathode luminoscope (Nuclide ELM2) operating at 10 kV voltage with a current beam of 5–7 mA. CL is a screening technique widely used to assess preservation of brachiopod shells (Popp et al., 1986), as they commonly show no luminescence in absence of significant geochemical alteration. To overcome one of the drawbacks of conventional optical CL (i.e. the fact that beam current conditions vary in the different studies), we analysed all the thin sections of the brachiopod shells with the same instrument operating under the same beam conditions.
Geochemistry and Isotopes
The brachiopods were sampled for trace element geochemistry and isotope analysis by drilling a portion of the longitudinal section of each shell, avoiding specialised areas such as the muscle attachment areas, articulation points and inter-areas. The powder was split into two parts, one for trace element geochemistry and 87Sr/86Sr, and the other for δ13C and δ18O isotopes.
Subsamples for trace element geochemistry and 87Sr/86Sr were dissolved in ultrapure acetic acid. The reserved acetic acid leachate was evaporated to dryness and the residue taken up in 1% nitric acid. Trace element geochemistry was obtained by ICP-AES on a Fison/ARL 3580 simultaneous/sequential spectrometer with Gilson auto sampler.
For strontium isotopes, a subsample of ca. 1–3 milligrams was weighed into a Savillex© FEP beaker and the carbonate dissolved in ultrapure 1M acetic acid (CH3COOH). After centrifuging, half of the CH3COOH solution was reserved for geochemical analysis and the other half evaporated to dryness. The residue was taken up in 2.5M HCl and strontium was separated by conventional cation exchange techniques using Biorad AG 50W-X8 ion exchange resin. Sr samples were loaded on rhenium (Re) filaments together with a tantalum oxide (TaO) activator following the method of Birck (1986) and isotope ratios were measured on a Finnegan MAT Triton operated in static mode. Analyses were made at two separate times and the relevant mean values obtained for the international isotope standard NBS 987 were 0.710234 ± 0.000008 (2σ, n = 11) and 0.710237 ± 0.000007 (2σ, n = 19). For consistency all the measured ratios reported in Table 1 are normalised relative to the accepted value of 0.710248 for NBS 987. Replicate determinations (n = 100) of the north Atlantic seawater standard yielded 0.709175 ± 0.000011 (2σ), relative to 0.710248 for NBS 987.
Carbon and oxygen isotope analysis was achieved using a GV IsoPrime mass spectrometer with multiprep device. Isotope values (δ13C, δ18O) are reported as per mil (‰) deviations of the isotopic ratios (13C/12C, 18O/16O) calculated to the VPDB scale using a within-run laboratory standard (KMC) calibrated against the international NBS standards (NBS18 and 19). Analytical reproducibility for these analyses was better than 0.1‰ for δ13C and δ18O.
The results of the SEM, CL, geochemical and isotope analyses are presented in Table 1. Saudi specimens of O. husseinii have Mn concentrations between 67.3–112 ppm, which are well below the cut-off of 200–250 ppm suggested by Bruckschen et al. (1999) and Korte et al. (2003) for pristine brachiopods, and in the range of values measured in extant brachiopods (Brand et al., 2003). Sr however is rather depleted being > 400 ppm only in specimen KH0211-2 and > 300 in specimen KH0211-7A. According to Popp et al. (1986) Mn concentrations are a better indication of chemical diagenesis in brachiopods than Sr. High Fe and low Sr concentrations are present in sample KH 0211-3, although similar values are found in KH0211-9A where the SEM analysis indicated that the calcite is more pristine. However KH 0211-9A has the lowest Sr concentration and very similar δ18O to the two samples deemed not well preserved after SEM analysis and is therefore excluded from the pristine data set.
The Oman specimens of O. husseinii and O. desertica from beds AO72, AO73 and AO210 (Table 1) have Mn concentrations between 105–431 ppm. Sr is less depleted than in the Saudi Arabian specimens of the same species, reaching 480–612 ppm in the most pristine samples and up to 1,282 ppm in less well preserved specimens. All samples with Mn above 200 ppm are excluded from the pristine data set. The specimen of Callispirina sp. appears to be unaltered but its coarse and dense punctae indicate that the isotope data should be viewed with caution. The poorly preserved specimen of Kotlaia cf. K. bistriata from bed AO80 further down the sequence within the Oman Khuff Formation, was found to have a high Mn content.
The δ13C and δ18O data from the nine pristine brachiopods from both sections show relatively stable δ13C, and a range in δ18O values. In the Saudi Arabian samples, δ18O ranges between −4.5 to −2.7‰ (mean −3.8, SD=0.8), while the Oman values are −3.1 to −0.7‰ (mean −2.0, SD = 1.0).
The 87Sr/86Sr of Saudi Arabian Khuff samples KH0211-3 and KH0211-10A (0.707198 and 0.707146 respectively) confirm that these samples have suffered some degree of diagenetic alteration. KH0211-9A also shows elevated 87Sr/86Sr (0.707069), which supports the decision to exclude this specimen from the pristine data set. The mean of 87Sr/86Sr yielded by the remaining samples is 0.707011 ± 0.000056 (2σ). However, the value obtained for the best-preserved specimen KH0211-2 (which met all the required conditions to be considered a pristine shell) is 0.706988 which is indistinguishable within error from the 87Sr/86Sr of 0.706988 for KH0211-11.
Samples from the five Oman pristine brachiopod range from 0.707217 to 0.707280 (Table 1 and Figure 4). Poorly preserved samples have increasing 87Sr/86Sr ratios and higher Mn concentrations. Sample AO73-44 has high Sr, Mn, and 87Sr/86Sr of 0.707731 suggesting that the source of the diagenetic fluids is significantly radiogenic and Sr and Mn rich. The specimen of Callispirina sp. yielded a higher 87Sr/86Sr ratio (0.707381 AO73-107) than the pristine O. husseinii samples suggesting that the coarse and dense punctae may have diagenetically altered fillings.
The 87Sr/86Sr data of the five pristine early Wordian brachiopods forms a separate cluster when plotted with the Sr seawater data of Korte et al. (2006, their appendix A; Figure 4). However, the seawater Sr data for the Early Permian and earliest Mid-Permian is sparse and scattered; the data also shows wide variation of 87Sr/86Sr from multiple brachiopods at single horizons or horizons that are close together. 87Sr/86Sr for the Oman brachiopod horizons in ascending stratigraphic order are: AO210 (0.707231 and 0.707217); AO73 (0.707280); and AO72 (0.707267 and 0.707243) (Figure 4). According to the general decreasing trend of Korte et al. (2006), the oldest (lowest) level, AO210, would most likely provide the highest 87Sr/86Sr ratio, while the converse should be true of AO72. However Korte et al. (2006) have no data within the Wordian, and therefore there could be an increasing trend (shown by the Oman data) within the overall general decreasing trend.
At present therefore, there is too much uncertainty in the distribution of 87Sr/86Sr over this period. Our firmly biostratigraphically-dated horizons suggest that the 87Sr/86Sr curve, at least in the early Wordian, requires refining. Despite this, a steep 87Sr/86Sr gradient seems to exist between the Asselian and the Capitanian (Figure 4; see also McArthur and Howarth, 2004), and if more data were available, brachiopod or other types of pristine carbonate would be a useful method for dating within this time period, independently of biostratigraphy. We recommend that further data be gathered which will allow a better-constrained curve to be constructed.
87Sr/86Sr for the most pristine Saudi Arabian specimens of O. husseinii would plot in a less steep part of the curve where there is a much greater spread of 87Sr/86Sr values in the Korte et al. (2006) data (Figure 4), making the curve harder to define. Thus the accuracy with which the Midhnab Member horizon can be dated is lower than that of Oman Khuff Formation Member 3. The range that could be suggested by the value 0.706988 from the best-preserved Saudi Arabian specimen (KH0211-2) is Capitanian to Wuchiapingian which means that it is not possible to resolve the different ages suggested by foraminifera, conodonts and ostracods (see earlier discussion), however it does indicate that the Midhnab Member is probably younger than Member 3 of the Khuff Formation of the Huqf Outcrop of Interior Oman.
The stable-isotope data allows inferences to be made about palaeoclimate. The δ13C data suggest an open-marine environment, consistent with Permian brachiopods from elsewhere (Korte et al., 2006), while δ18O allows deductions to be made about the palaeotemperature of the Khuff Formation sea (Figure 5). Using Leng and Marshall’s (2004) rearrangement of the expression of Kim and O’Neil (1997): T°C = 13.8 – 4.58(c – w) + 0.08 (c – w)2 and in the absence of ice caps where seawater δ18O sw (w) is assumed to be < –0.5 ‰ (Craig, 1965), the palaeotemperature for the Oman beds AO72, AO73 and AO210 would be +25°C, +22°C and +17°C, respectively (Figure 5). This is consistent with the trend envisaged for Member 3 by Angiolini et al. (2003), with a shallowing from the basal bed AO210, deposited at depth of some tens of metres, to the upper part of the member which was deposited in progressively shallower and thus warmer waters (AO72).
The Saudi Arabian brachiopod from bed KH02-11 gives a palaeotemperature of +29°C, using the δ18O mean value of –3.6‰. However, if evaporation rates were higher than in Oman as suggested by evaporitic facies in close proximity to bed KH02-11, the isotopic composition of seawater could have been higher than the δ18O value of seawater (−0.5‰) adopted here. Today, low latitude seawater has a more positive δ18O than that of high latitudes, where the seawater is 0.25‰ more negative than the global average of 0‰ (Zachos et al., 1994). If seawater δ18O was –0.25‰ or higher during the period when the Saudi Arabian brachiopod lived, then palaeotemperature would have been above 30°C in the KHO211 bed of the Midhnab Member.
The following are thanked for their technical support: Carol Arrowsmith, Mauro Brunetti and Curzio Malinverno for preparation of the samples and Agostino Rizzi for SEM technical assistance. MHS, MJL and DPFD publish with the permission of the Director, British Geological Survey. The authors wish to thank GeoArabia Designer Nestor “Niño” Buhay, for designing the paper for press.
ABOUT THE AUTHORS
Mike Stephenson is Head of Science (Energy) at the British Geological Survey (BGS). His education has included a BSc, MSc and PhD from the University of Sheffield and Imperial College, London as well as various postgraduate teaching qualifications. He began his career as a schoolteacher in rural Africa and stayed there for almost ten years, but returned to pursue research. Mike’s scientific work is mainly concerned with the geology of Arabia, and he has published over 60 papers on this and other regions as well as working extensively as a consultant for oil companies. Mike now runs the Energy Programme at BGS including carbon capture and storage, hydrocarbons, renewables and unconventional energy. He sits on the boards of several journals and is Editor-in-Chief of an Elsevier geological journal. He has a special professorship at the University of Nottingham and is the Director of the Nottingham Centre for Carbon Capture and Storage, a joint venture between the BGS and the University of Nottingham.
Lucia Angiolini is a Professor of Palaeontology at the Department of Earth Sciences, University of Milan, Italy. She received a PhD in Earth Sciences from the University of Milan in 1994, where she is now Associate Professor of Paleontology. Lucia has 15 years experience in Permian brachiopods from the Peri-Gondwana region and the Cimmerian blocks from Turkey to the Himalayas through Oman, Iran and Karakorum. Her research interests include, besides pure taxonomy, quantitative biostratigraphy, palaeobiogeography based on multivariate analyses, and Permian correlation between Gondwanan and Tethyan realms.
Melanie Leng is Professor of Isotope Geosciences at the University of Leicester and also heads the Stable Isotope Group at the NERC Isotope Geosciences Laboratory, British Geological Survey. Melanie’s research interests include the study of isotopes in biogenic silica (IBiS), the development of stable isotope techniques in relation to current environmental issues, current and past freshwater contributions into the Antarctic Ocean and effects on ocean circulation, climate influences on Northern Europe over the Holocene (last 10,000 years) and major climatic boundary thresholds through geological time.
D.P. Fiona Darbyshire has worked on projects in the UK, Europe, Africa, South America and South East Asia and published over 50 papers. Her scientific career has been largely devoted to using radiogenic isotope techniques, Rb-Sr and Sm-Nd, to study the age and petrogenesis of granites and Precambrian basement rocks. A particular interest in the temporal relationship between magmatism and mineralisation led to the application of these techniques to directly date ore minerals and fluid inclusions in gangue quartz. More recently Fiona has used high precision measurements of Sr isotopes to study natural groundwater systems and to provide age constraints on fossil shell calcite. She was Deputy Head of the NERC Isotope Geosciences Laboratory until her retirement in 2006 and is now an Honorary Research Associate at the British Geological Survey.