CARBON 13 AND OXYGEN 18 ISOTOPE RECORD OF THE EARLY EOCENE NAMMAL FORMATION SALT RANGE PAKISTAN.
The Nammal Formation is the lowermost unit of the Early Eocene succession in the Salt Range Pakistan. It is well exposed throughout the Salt Range. The Nammal Formation having 30 to 35 meters thickness is predominantly composed of nodular limestone interbedded with marl and shale. The present study was focussed on stable carbon 13 and oxygen 18 isotopic analysis based on data from two stratigraphically importantsections. The samples from the Nilawahan section provided with the d13C values varied between 1.34 to -1.56 (VPDB) and values of d18O fluctuated between -4.47 to -6.59 (VPDB). Likewise the sample analysis of BadshahPur section exhibited that the d13C values changes from1.09 to -1.65 (VPDB) and d18O values rangefrom -4.17 to -6.85 (VPDB). The isotopic records of carbon 13 and oxygen 18 indicated the shallow marine deposition of the Nammal Formation under tropical conditions.
It highlighted the palaeoclimatic and diagenetic conditions of the Nammal Formation at the time of deposition in the Salt Range region.
Key words: Early Eocene Nammal Formation Carbon13 and Oxygen18isotopes Salt Range.
Isotopic analysis of stable oxygen and carbon is of a particular importance in various geochemical techniques for the interpretation of sedimentary and diagenetic conditions and environments (Hudson 1977; Anderson and Arthur 1983; Marshall 1992 and Corfield 1995). Naturally abundant stable isotopes of oxygen (18O and 16O) and carbon (13C and 12C) are mostly used by means of their ratios between samples the variations of 18O/16O and 13C/12C ratios are measured by high-precision mass spectrometry (Fairchild et al. 1988). The abundance of 18O and 13C in a sample is conventionally reported as the per mil (=mg/g or ) difference in delta (d) notation (d18O and d13C) between isotope ratios in the sample and those in the international Pee Dee Belemnite (PDB) standard which has d18O and d13C values of 0 (Hudson 1977).
Increasingly negative or more depleted d values with respect to PDB imply a relative increase in the lighter isotopes (16O 12C) while more positive or enriched values indicate a relative increase in the heavier isotopes (18O 13C) (Nelson and Smith 1996).
The d 18O values of a carbonate precipitated from water depends mainly on the d 18O salinity and temperature of the water (Nagarajan et al. 2008;Strasseret al. 2012). More negative values of the d18O are indicative of decreasing salinity and increasing temperature (Hudson 1977). The d13C of a carbonate precipitated from water primarily reflects the source of biocarbonate dissolved in the waters (Hudson 1977; lrwin et al. 1977; Coleman 1993 and Mozley and Burns 1993). Thus in natural waters the carbonate precipitation at or near isotopic equilibrium tend to reflect characteristic range of d18O and d13C isotope values that shows reasonably closely their genetic environment (Nelson and Smith 1996).
Stable carbon 13 and oxygen 18 isotopic analysis of the Nammal Formation has been carried out for the very first time from Nilawahan and Badshahpur sections in the Salt Range (Fig. 1). The aim of this study is to interpret the palaeoclimate of depositional environment and diagenetic conditionsof the Nammal Formation in the Tethys Sea and impact of global Palaeocene-Eocene thermal maxima on deposition of Early Eocene in Eastern Tethys.
The Nammal Formation is predominantly composed of well-bedded nodular limestone shale and marl. The limestone and marl have light-grey to bluish colour while the shale is grey to olive-green in colour. The nodules of limestone are 10-12 cm in diameter and at places 16-20 cm. The lithological variations in the Nammal Formation divided it into four well defined unitsas follows; i) Alternate marl and limestone ii) Well-bedded limestone with chert nodules iii) Limestone interbedded with shale iv) Dolomitic limestone.
The Nammal Formation has its lower contact with the Patala Formation which is recorded in coal mines (Fig. 2a). It has sharp and wavy upper contact with the Sakesar Limestone (Fig. 2b).A detailed lithological log of the Nammal Formation exposed in the Nilawahan and BadshahPur sections along with its chemostratigraphy is presented in Figures 3 and 4.
Eighteen carbonate samples were collected from the Nilawahan and BadshahPur stratigraphic sections (Fig. 1)from Palaeocene-Eocene boundary (Nammal Formation lower contact with Patala Formation) to upper contact of the Nammal Formation with the Sakesar Limestone in the Salt Range for analysis of their 18O and 13C abundance.
The samples were processed at the Cornel University (New York USA) Isotope Laboratory at Thermo Delta V isotope ratio mass-spectrometer (IRMS) interfaced with a Temperature Conversion Element Analyzer (TC/EA). Isotope results for 13C are reported in delta (d) notation as per mil () with an accuracy of 0.31 deviations from the Vienna Pee Dee Belemnite (VPDB) reference standard and 18O delta values are corrected for primary reference Vienna Standard Mean Oceanic Water (VSMOW). Summary statistics of the isotope data for the analyzed samples from bottom to top in both sections are given in Table 1.
RESULTS AND DISCUSSION
At Nilawahan section in the lower part of the Nammal Formation (sample NN-1 to NN-3) the d 13C isotope ratios varies from -1.56 (VPDB) to -1.06 (VPDB) and d 18O isotope ratios fluctuates from -6.59 (VPDB) or 24.13 (SMOW) to -5.93 (VPDB) or 24.75 (SMOW) while from sample NN-4 the d 13C values suddenly changed from negative to positive values. Sample numbers NN-4 to NN-5 the d 13C showed variation in isotope ratios from 1.00 (VPDB) to higher positive values of 1.34 (VPDB) and d 18O isotope ratios decreased in negative values from -5.85 (VPDB) to -4.65 (VPDB) or 24.89 (SMOW) to 26.12 (SMOW). In the middle parts of the formation (from NN-6 to NN-8) the d 13C isotope ratios decreased in positive values from 1.02 (VPDB) to 0.99 (VPDB) and d 18O isotope ratios increases in negative values from -4.91 (VPDB) or 25.86 (SMOW) to -5.11 (VPDB) or 25.60 (SMOW).
Towards the top of the formation(from NN-9 to NN-10) the d 13C isotope ratios followed the same positive trend and decreased in positive values from 0.73 (VPDB) to 0.23 (VPDB) and d 13C isotope ratios also decreased from -5.36 (VPDB) or 25.39 (SMOW) to -4.47 (VPDB) or 26.31 (SMOW).
At BadshahPur section sample BN-1 to BN-2the d 13C isotope ratios variedfrom negative value of -1.46 (VPDB) to -1.65 (VPDB) and d 18O isotope ratios varied from -6.49 (VPDB) or 24.23 (SMOW) to -6.85 (VPDB) or 23.85 (SMOW) upward in sample BN-3 decreased in negative value of -1.32 (VPDB) ofd 13C isotope ratios and also in d 18O isotope decreased to -5.62 (VPDB) or 25.13 (SMOW). In the middle part of the formation like wise in Nilawahan section the d 13C isotope ratios (sample BN-4 and BN-5) followed the same trend of positivevalues ranges between1.09 (VPDB) to 0.87 (VPDB) and d 18O isotope ratios varied from -4.55 (VPDB) to -5.10 (VPDB) or 26.23 (SMOW) to 25.66 (SMOW). Towards the top from sample BN-6 to BN-7 d 13C isotope ratios decreased from 0.21 (VPDB) to 0.00 (VPDB) and d 18O isotope ratios fluctuated from -4.34 (VPDB) or 26.42 (SMOW) to -4.17 (VPDB) or 26.62 (SMOW).
After zero d 13C isotope ratios there is an increase in value to 0.43 (VPDB) and d 18O isotope ratios also increased in negative value of -5.01 (VPDB) or 25.75 (SMOW).
The negative or more depleted d values with respect to PDB at the base of the Nammal Formation indicated a relative increase in the lighter isotopes (16O 12C) (cf. Nelson and smith 1996). Increase of d 13C values upward in the Nammal Formation could be related to reduced recycling of 12C into the environment and accumulation in the Nammal marls with high organic content (Patterson and Walter 1994; Immenhauser et al. 2003; Colombie et al. 2010;Strasser et al. 2012) marking the transgression phase. High and gradually increasing organic activity assimilated the inorganic carbon decreasing the overall concentration of CO2 and hence raising the values of 13C from lower boundary to upper boundary of Nammal Formation that was confirmed with increased concentrations of foraminifers fauna upwards in the Nammal Formation.
The higher values showed high photosynthetic productivity that relatively depicted moderately warm conditions supporting life (Patterson and Walter 1994; Immenhauser et al. 2003; Colombieet al. 2010; Strasser et al. 2012). In warm temperatures the solubility of CO2 decreased resulting in higher values of 13C. Relatively lower d 13C and more negative d 18O values in the start of Early Eocene was due to the short term effects of the PETM (Palaeocene-Eocene Thermal Maximum) and consistently increasing values showed a gradual increase in temperature in a warm climate. However experiments have shown that there was no linear relationship between temperature and d 13C values (Rau et al. 1989) but18O values influenced the temperature. More the negative values of the 18O higher will be the temperature.
Although greenhouse conditions prevailed during the Eocene (Zachoset al. 2001;Gingerich 2006; Jahren 2007; Hollis et al. 2009) and substantial faunal and floral changes occurred which were caused by the almost continuous global cooling that followed the Early Eocene Climatic Optimum (Zachos et al.2001; Francis and Poole 2002; Dupont et al. 2007). Most information about Eocene climate dynamics is still derived from the marine realm (Zachos et al. 2001). The results of this study have been plotted in a d 13C versusd 18O (Fig. 5) cross plot diagram (Hudson 1977) in which plot of the Nammal Formation samples was in the fields of marine limestones.
Lighter values of d 18O with its most negative value of -6.85 PDB indicated a temperature of about 50oC inferred tropical environments at time of deposition of the Nammal Formation. Likewise in many tropical carbonate deposits in this study the range of d 18O values in all samples was supportive of cementationunder shallow to moderate burial conditionsof shallow marine environments (Nagarajanet al. 2008;Swei and Tucker 2012).
Table-1.Summary of statistics for 18O and 13C isotopes. Here sample ID denotes to the name of the sample where N-N is for Nilawahan-Nammal samples and same as B-N for BadshahPur- Nammal samples. d18O vs. VSMOW is the corrected isotope delta value for 18O measured against the primary reference Vienna Standard Mean Oceanic Water. d13C vs. VPDB is the corrected isotope delta value for 13C measured against the reference standard Vienna Pee Dee Belemnite (VPDB).
###Sections###Sample ID###13 C vs. VPDB###18 O VPDB###18 O VSMOW
Conclusions: The oxygen and carbon isotope analysis from measured Nilawahan and BadshahPur stratigraphic sections revealed the carbonate sedimentation of the Nammal Formation under tropical conditions of shallow marine environments.
Acknowledgements: The authors are highly thankful to the University of the Punjab for providing funds and research facilities. The authors also wish to express their gratitude to Aftab Ahmad Butt Muhammad Ashraf and Shafeeq Ahmed for their suggestions and guidance.
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|Publication:||Pakistan Journal of Science|
|Date:||Sep 30, 2014|
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