CARBON AND OXYGEN ISOTOPES: PRINCIPLES AND APPLICATION IN

GEOLOGICAL STUDIES

BY

OLADIMEJI AKINSILE UNIVERSITY OF ILORIN, ILORIN,

NIGERIA

PRESENTATION OUTLINE Introduction

Objectives of the review

Principles and instrumentation

Fractionation processes

Application of carbon and oxygen in geological studies

Selected case studies

Conclusions

What is isotope ? Basics of isotope Stable and unstable isotopes

TABLE 1: ATOMIC WEIGHT OF CARBON(C)) and OXYGEN(O) ISOTOPE ( Walker et al 1989)

ELEMENT ISOTOPE ATOMIC WEIGHT(amu)

ABUNDANCE (%)

CARBON ( 6) 12C 12.00000 98.90

13C 13.00335 1.1

OXYGEN (8) 16O 15.9994 99.76

17O 15.994915 0.04

18O 17.999160 0.20

OBJECTIVES OF THE REVIEW

To discuss the principles and instrumentation involved in the carbon and oxygen isotope.

To discuss the application of carbon and oxygen isotopes in geological studies.

PRINCIPLES

Element occur in the earth system in multiple isotopes , both stable and unstable

Proportions of stable isotopes in the earth are constant

The lighter isotopes (16O , 12C ) are more reactive than heavier isotopes (18O, 13C)

In higher temperature,lighter isotopes are preferentially lost.

MEASURING ISOTOPES

Prepare sampleIntroduce sample Convert to gasFlow through GC to seperate CO2Inject reference gas to IRMSInject sample to IRMS

Figure 1:Schematic diagram of Isotope-ratio mass spectrometer used to measure stable isotope ratios, with gas bench in foreground After Williams, 1998.

ISOTOPE STANDARD

VSMOW – Vienna standard mean ocean water (bunch of ocean water kept in austria) . The standard for the oxygen isotope is 0.002004

PDB- Pee Dee Belemnite – Fossil of a belemnite from the Pee Dee formation in canada . The standard carbon oxygen isotope is 0.011056

CDT- Canyon Diablo Troilite

Source: After J. Hoefs (1980) (Stable isotope Geochemistry)

FRACTIONATION PROCESSES

This is a reaction or process which selects for one of the stable isotope of a particular elements. If the process selects for the heavier isotope. The reaction product is heavy and vice versa

Types Equilibrum isotope fractionation kinetic isotope fractionation

CalculationsIt is given as per mil (‰) difference (δ) compared

to a standard.For Oxygen isotope this is calculated as

For carbon isotope it is calculated as

The result is multiplied by 1000 simply to make the resulting ratio

more meaningful

APPLICATIONS Geochronology it is assumed that the atmosphere is in equilibrum with respect to 14C poduced by

the cosmic ray flux which is equal to the rate of decay of 14C , so that the abundance in the atmosphere remains constant

N = N = Noe-lt

Where No is the number of atoms present at t = 0. This is the basic form of the radioactive decay equation.N = number of atoms left t = time when organism from which the sample was taken died

Fig 2 : diagram showing the 14C dating curve After Libby and Arnold (1949)

Paleoenvironment :To distinguish between the sedimentary rocks of marine origin and those deposited in terrestial environment.

Fig 3 :Distribution of δ18O and δ13C values in various types of marine carbonates . After milliam, . J. D., 1974)

Geothermometry – the utility of oxygen isotopes has to do with fractionation that occurs during evaporation and condensation of water and CaCO3

T(°C) = 16.9 – 4.38 (dc –dw) + 0.10(dc-dw)2 where dc = the equilibrum oxygen isotope of calcite dw= oxygen isotopic composition of the water from which the

calcite was precipitated Paleoceanography• Paleoclimate reconstruction

Fig 4: Diagram showing δ18O trends in the glacial-interglacial swings in the longterm record cooling of the deep ocean. K.G. Miller et al.,Paleoceanography 2, 1, 1987.

Fig 5: Schematic illustration of the relationship between continental glaciations and the 18O content of ocean water during the last 150,00 years .

*Note that 18O values become less negative (more heavy oxygen) during stages of continental glaciation and lowered sea level .After Williams et al., 1988

Hydrothermal ore deposit. This is the large interaction of cooling magma and large volume of meteoric water helps in understanding its origin and related alteration in the country rocks which has a deep seated origin.Low δ13C value of -4

to -12 suggesting deep seated origin. While most carbonates minerals formed early in hydrothermal veins have value of -6 to -9 but those formed in paragnetic sequence are rich in 13C and may have positive value.

Fossil fuel it can be used to detect fossil fuel contribution to the

atmosphere because the burning of fossil fuel has increased the CO2 content of the atmosphere and has caused immeasurably decrease in 13C value because of the enrichment in 12C.

To achieve maximum stratigraphic precision and age control

Fig 6: showing carbon isotope stratigraphy of bulk carbonates sediments from deep sea drilling project (After shackleton, N.J and M.A Hall, 1984)

Figure 7: Geological map of dahomey embayment Nigeria part

Carbon and oxygen isotopic compositions and diagenesis of the Ewekoro Formation in the eastern Dahomey Basin, southwestern Nigeria O.A Adekeye, S.O Akande. R.B. Bale and , B.D. Erdtmann 2005

Table.2 : Regional Stratigraphic Setting of Dahomey Basin (Modified from Idowu et al., 1993)

Table 3 . Carbon and Oxygen isotopic compositions of the selected Ewekoro Carbonates in Sagamu Quarry Sample No Locations Age Sample description %o PDB

δ13C%o PDBδ18O

Unaltered oyster shellsS-1aiS1aiiS3aS13aiS13aii

Sagamu quarry Paleocene Unaltered oyster

shell

0.860.910.870.590.62

-4.46-4.31-4.49-4.51-4.41

Unaltered sea urchins S1bS3bS13b

Sagamu QuarryPaleocene

Unaltered sea urchins

1.010.14-0.14

-3.41-4.12-4.83

Whole sampleS5aS10aS11aS12aS18aS18aiiS25aS28a

Sagamu QuarryPaleocene Bulk sample

-1.390.07-0.230.34- 0.83- 0.58- 0.24- 0.23

-4.77-4.39-4.65-5.37-5.37-5.13-5.06-5.02

Recrystallized calcite sampleS1cS3cS5bS10bS11bS12bS13cS20s

Sagamu QuarryPaleocene Recrystallized

calcite

0.07-1.22-0.80-1.18-1.020.12-2.44-2.26

-4.46-6.32-7.34-7.68-6.54-6.24-7.12-7.53

Fig 8: Plot showing carbon and oxygen composition in the Ewekoro Formation

RESULT AND INTERPRETATION From the table, it is concluded that the carbon and oxygen isotopic

compositions of the carbonate rocks vary from -3.41 ‰ to -7.68 ‰ δ18O PDB and ±1.01 ‰ to -2.44 ‰ δ13C PDB for tertiary sea water suggest that the heavier oxygen and carbon are recorded in the unaltered carbonates and the lighter fractions in the late stage recrystallized carbonates, due to the indicative of isotopic fractionation.

Estimated formational paleotemperature for the carbonates vary from 26.650c to 32.520c based on the calculations from and the unaltered carbonates δ18O values indicating only very limited burial and lack of severe hydrothermal alterations .

CASE STUDY TWO

• Depositional environment and carbon/oxygen isotopic composition of the Paleocene carbonates exposed around Kalambaina, Sokoto basin, Nigeria

  O.A Adekeye, S.O Akande, B.D and Erdtmann, B.D (2013)

Figure 9: Geological sketch map of northwestern Nigeria showing Kalambaina Formation type locality (modified from Obiosio et al., 1998).

Figure 10: Generalised stratigraphic setting of the Sokoto Basin (modified from Obiosio et al., 1998)

Table 4: showing carbon and oxygen isotopic composition of the selected kalabaina carbonates in kalambaina quarry

Carbon and oxygen isotopic composition vary from -5.29%o to -6.55%o δ 18O PDB and -1.24%o to -3.40%o PDB δ13C in the whole rock

RESULT AND INTERPRETATION The lighter oxygen and carbon fractions in the samples is a reflection of the

depletion of heavier isotope of oxygen and carbon as a result of late stage recrystallization, which suggests that carbonates of the Kalambaina Formation were deposited in shallow marine environment during the Paleocene marine transgression into the Sokoto basin. No evidence of hydrothermal alteration was observed in the carbonates, brings organic rich water with low 13C/12C ratio (δ13C) into the shallow marine water environments of the primary carbonate build up (Marshall, 1992).

Fig 11: Synthetic sedimentological log of Jibou Formation in Rona area and location of samples under study (R1-R46 )

Rona Limestone is a lens shaped body located in the NW of transylvania having a local development. The classic outcrop is located westwards from the neighbourhood of jibou and rona , the deposit has maximum thickness of about 300m,towards the NE, it gradually becomes thinner until it disappears in the area of caselor and Gardbrookes (Popescu, 1984) The age of Rona Limestone is LatePaleocene - Early Eocene (Gheerbrant et al., 1999).

GEOLOGICAL BACKGROUND

CARBON AND OXYGEN ISOTOPE RATIOS IN RONA LIMESTONE, ROMANIASTELA CUNA1, DANA POP2, ALEXANDRU HOSU2

METHODOLOGY AND RESULT Carbon and oxygen isotope measurements were done on 10 samples and

the carbon 13 range from δ13C (-3.12 to -6.78) and δ18O (-9.41 to -14.13)

Table 5 : Showing the carbon and oxygen isotopic data

• Table 5: carbon and oxygen isotope ratios in carbonate rocks according to

previous references

Fig. 12:Distribution of δ18O vs. δ13C (‰ PDB) values in various types of carbonate rocks, including this study (After Veizer, 1983)

RESULT AND INTERPRETATION The oxygen and carbon isotope data indicate a freshwater

depositional environment with the δ13C mean value (-4.96‰ PDB) is, generally, representative for fresh-water carbonates of the Tertiary period.

Conclusion Isotopic variations in stable and non stable isotopes are often produced by

exchange reactions, kinetic reactions in biological system such as evaporation or diffusion. The fractionation of stable isotope decreases with increase in temperature hence stable isotopes show greater fractionation in sedimentary systems than in magamatic rocks.

Carbon and oxygen isotope has a very wide application in geological studies including, the study of igneous, sedimentary and metamorphic rocks, application to temperature (paleothermometry), study of hydrothermal ore deposits, determination of fossil fuel to know the various carbon contents, and to distinguish marine from non marine carbonates.

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