Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies

Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies

Table of Contents

Hydrogen Isotopes

There are two naturally occurring stable isotopes of hydrogen, which occur in the following proportions:

  • ¹H: 99.9844%
  • ²H (Deuterium): 0.0156%

Hydrogen isotopes show the largest relative mass difference between two stable isotopes. This leads to significant variations in hydrogen isotope ratios in naturally occurring materials. These isotopes are widespread in nature, present in forms like H₂O, OH⁻, H₂, and hydrocarbons.

Measurement of Hydrogen Isotopes

Hydrogen isotopes are measured in parts per thousand relative to the SMOW standard and calculated similarly to oxygen isotopes, expressed as δD ‰. Precision is usually between 1 and 2‰. The δD values for the SLAP standard relative to SMOW are -428‰. O/H ratios are typically measured on H₂ gas, produced from the reduction of water at high temperatures.

Summary of δD Values

A summary of δD values for common rock types and waters is provided. Mantle values typically range from -40‰ to -80‰, as reported by Oeloule et al. (1991). Values as low as -125‰ have also been reported. The MORB reservoir is thought to have δD = -80 ± 5‰ (Kyser and O’Neil, 1984).

Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies
Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies

Calculating the Isotopic Composition of Water from Mineral Compositions

The isotopic composition of waters from different geological settings can be measured directly as ‘fossil’ water preserved in fluid inclusions (Ohmoto and Rye, 1974; Richardson et al., 1988). However, ‘fossil’ water is usually sampled indirectly, and its isotopic composition is determined from the isotopic composition of minerals that were in equilibrium with it.

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If isotopic equilibrium between a mineral and a hydrothermal solution can be assumed, laboratory calibrations of equilibria between rock-forming minerals and water can calculate the hydrothermal solution’s isotopic composition. Experimental calibrations exist for both oxygen and hydrogen isotopes, allowing precise specification of water’s isotopic composition. The calculation requires knowledge of the equilibration temperature, which may need to be estimated or measured independently using techniques like fluid inclusion thermometry.

An example is provided by Hall et al. in their study of the Climax molybdenum deposit, Colorado. Here, fluid inclusion thermometry provided the hydrothermal fluid’s temperature, and the δ¹⁸O and δD composition of the water was calculated from the isotopic composition of muscovite and sericite using muscovite-water experimental calibrations.

The Isotopic Composition of Natural Waters

The isotopic composition of natural waters can be obtained either by direct measurement or by calculation using the outlined method. Taylor (1974) describes six types of naturally occurring water, the compositions of which are summarized on a δD vs. δ¹⁸O diagram. The isotopic character of these waters can be used to trace the origin of hydrothermal solutions.

Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies
Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies

(a) Meteoric Water
Meteoric water shows the greatest variation of all natural waters. The δD-δ¹⁸O variations define a linear relationship, known as the meteoric water line, which can be represented by the expression:

\(\delta D\,(\text{‰}) = 8\delta^{18}O + 10\)

(Taylor, 1979). The δ¹⁸O and δD values for meteoric water vary according to latitude. Values are close to zero for meteoric waters on tropical oceanic islands, whereas at high latitudes in continental areas, δ¹⁸O values can be as low as -20‰ to -25‰, and δD values range between -150‰ to -250‰. Both the extreme variation and the linear relationship arise from the condensation of H₂O from the Earth’s atmosphere. The extreme variation reflects the progressive lowering of δ¹⁸O in an air mass as it leaves the ocean and moves over a continent. The linearity of the relationship indicates that fractionation is an equilibrium process and that the fractionation of D/H is proportional to ¹⁸O/¹⁶O.

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Ocean Water

Present-day ocean water is very uniform in composition, with values of δ¹⁸O = 0‰ and δD = 0‰. Exceptions occur in areas with high evaporation rates, such as the Red Sea, where elevated δ¹⁸O and δD values are observed, or in areas where seawater is diluted with freshwater. Muehlenbachs and Clayton (1976) suggested that ocean water’s oxygen isotopic composition is buffered by exchange with the ocean crust, a view supported by Gregory and Taylor (1981) in their study of the Semail ophiolite, Oman.

Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies
Hydrogen Isotopes: Measurement, Variations, and Applications in Geological Studies

The isotopic composition of ancient seawater is less certain. Evidence from the oxygen isotope composition of marine carbonates indicates global changes in ocean isotopic chemistry during the Tertiary. These changes are believed to result from the storage of isotopically light oxygen in polar ice and are known well enough to be used as a stratigraphic tool. Similar changes are reported in the Ordovician from the oxygen isotope chemistry of unaltered brachiopod shells (Marshall and Middleton, 1990). However, uncertainties in such models, especially before the Plio-Pleistocene, include the impact of diagenetic changes on calculated former ocean water compositions (Williams et al., 1988).

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Geothermal Water

Modern geothermal water is meteoric in origin but has its isotopic compositions transposed to higher δ¹⁸O values through isotopic exchange with country rocks. δD values remain the same as in the parent meteoric water or are slightly enriched. Similarly, ocean-floor geothermal systems have δ¹⁸O values between +0.37‰ and +2.37‰, close to unmodified seawater (Campbell et al., 1988).

Formation Water

Formation waters from sedimentary basins show a wide range of δ¹⁸O and δD values. Individual basins have water compositions that define a linear trend representing mixing between meteoric water and another source, such as trapped seawater, or between meteoric water and country rock.

Metamorphic Water

Attempts have been made to calculate the δD and δ¹⁸O values of water in equilibrium with metamorphic minerals across various metamorphic grades (Taylor, 1974; Rye et al., 1976; Sheppard, 1981). A combination of these values creates a metamorphic water ‘box’ with δ¹⁸O values between +3‰ and +25‰ and δD values between -20‰ and -65‰.

Magmatic Water

Calculating magmatic water composition is challenging due to interactions between magmas and groundwater. However, primary magmatic waters calculated by Taylor (1974) define a region on δD vs. δ¹⁸O diagrams between δD values of -40‰ and -80‰ and δ¹⁸O values of +5.5‰ and +9.0‰. Sheppard (1977) showed that magmatic waters associated with Permian granites of southwest England, produced by intracrustal melting, plot in a different field with δD values of -40‰ to -65‰ and δ¹⁸O values of +9.5‰ to +13‰.

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CSIR NET Exam: EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES

Exam Pattern: EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES 

 PART APART BPART CTOTAL
Total questions205080150
Max No. of Questions to attempt15352575
Marks for each correct answer224200
Marks for each incorrect answer (Negative marking for part A & B is @ 25%, and part C is @ 33%)0.50.51.32

The candidate is required to answer a maximum of 15, 35, and 25 questions from Part-A, Part-B, and Part-C, respectively. If more than the required number of questions are answered, only the first 15, 35, and 25 questions in Part A, Part B, and Part C, respectively, will be taken up for evaluation.

Below each question in Part A, Part B, and Part C, four alternatives or responses are given. Only one of these alternatives is the “correct” option to the question. The candidate has to find, for each question, the correct or the best answer.

Syllabus

EARTH, ATMOSPHERIC, OCEAN AND PLANETARY SCIENCES

PAPER I (PART B)

  1. The Earth and the Solar System

    • Milky Way and the solar system.
    • Modern theories on the origin of the Earth and planetary bodies.
    • Earth’s orbital parameters, Kepler’s laws of planetary motion.
    • Geological Time Scale; space and time scales of processes in the solid Earth, atmosphere, and oceans.
    • Radioactive isotopes and their applications.
    • Meteorites: chemical composition and primary differentiation of the Earth.
    • Basic principles of stratigraphy.
    • Theories about the origin of life and fossil records.
    • Earth’s gravity, magnetic fields, and thermal structure: Geoid and spheroid concepts; Isostasy.
  2. Earth Materials, Surface Features, and Processes

    • Gross composition and physical properties of important minerals and rocks.
    • Properties and processes responsible for mineral concentrations.
    • Distribution of rocks and minerals in Earth’s units and India.
    • Physiography of the Earth; weathering, erosion, and soil formation.
    • Energy balance of Earth’s surface processes.
    • Physiographic features and river basins in India.
  3. Interior of the Earth, Deformation, and Tectonics

    • Basic concepts of seismology and Earth’s internal structure.
    • Physico-chemical and seismic properties of Earth’s interior.
    • Stress and strain concepts; rock deformation.
    • Folds, joints, and faults; causes and measurement of earthquakes.
    • Interplate and intraplate seismicity; paleomagnetism.
    • Sea-floor spreading and plate tectonics.
  4. Oceans and Atmosphere

    • Hypsography of continents and ocean floors: continental shelves, slopes, abyssal plains.
    • Physical and chemical properties of seawater; residence times of elements.
    • Ocean currents, waves, tides, thermohaline circulation, and conveyor belts.
    • Major water masses, biological productivity, and fluid motion.
    • Atmospheric structure and heat budget; greenhouse gases and global warming.
    • General circulation, monsoon systems, ENSO, cyclones, and local systems in India.
    • Marine and atmospheric pollution, ozone depletion.
  5. Environmental Earth Sciences

    • Properties of water and the hydrological cycle.
    • Energy resources: uses, degradation, alternatives, and management.
    • Ecology, biodiversity, and natural resource conservation.
    • Natural hazards and remote sensing applications.

PAPER I (PART C)

I. Geology

  1. Mineralogy and Petrology

    • Point group, space group, and lattice concepts.
    • Crystal field theory, mineralogical spectroscopy, and bonding in mineral structures.
    • Genesis, properties, and crystallization of magmas.
    • Metamorphic structures, textures, and thermobarometry.
    • Petrogenesis of Indian rock suites: Deccan Traps, charnockites, ophiolites, and more.
  2. Structural Geology and Geotectonics

    • Stress and strain analysis; Mohr circles.
    • Geometry and mechanics of folds, faults, and ductile shear zones.
    • Plate boundaries, mantle plumes, and Himalayan orogeny.
  3. Paleontology and Applications

    • Life origin theories, evolution models, and mass extinctions.
    • Applications of fossils in age determination, paleoecology, and paleogeography.
    • Micropaleontology in hydrocarbon exploration.
  4. Sedimentology and Stratigraphy

    • Classification of sediments and sedimentary rocks.
    • Sedimentary environments and basin evolution.
    • Stratigraphic principles, correlation methods, and sequence stratigraphy.
    • Phanerozoic stratigraphy of India.
  5. Marine Geology and Paleoceanography

    • Ocean floor morphology, ocean circulation, and thermohaline processes.
    • Factors influencing oceanic sediments and paleoceanographic reconstruction.
  6. Geochemistry

    • Atomic properties, periodic table, thermodynamics of reactions, and isotopes in geochronology.
    • Applications of stable isotopes in Earth processes.
  7. Economic Geology

    • Ore formation processes, mineral deposit studies, and petroleum geology.
    • Coal and unconventional energy resources.
  8. Precambrian Geology and Crustal Evolution

    • Evolution of Earth systems and Precambrian characteristics of India.
    • Precambrian–Cambrian boundary.
  9. Quaternary Geology

    • Quaternary stratigraphy, climate variability, and human evolution.
    • Dating methods and tectonic geomorphology.
  10. Applied Geology

  • Remote sensing and GIS.
  • Engineering properties of rocks; construction investigations.
  • Methods of mineral exploration and groundwater studies.

II. Physical Geography

  1. Geomorphology: Landform processes, DEM analysis, extraterrestrial geomorphology.
  2. Climatology: Radiation balance, wind systems, ENSO, and climate classification.
  3. Biogeography: Plant and animal associations, Indian biogeography, and conservation.
  4. Environmental Geography: Man-land relationships, hazards, and ecological balance.
  5. Geography of India: Physical geography, climatology, agriculture, and population characteristics.

III. Geophysics

  1. Signal Processing: Fourier transforms, filters, and signal analysis.
  2. Field Theory: Newtonian potential, Green’s theorem, and seismic wave propagation.
  3. Numerical Analysis and Inversion: Least squares, optimization, and pattern recognition.
  4. Gravity and Magnetic Methods: Data interpretation and anomaly analysis.
  5. Seismic Methods: Ray theory, reflection/refraction techniques, seismic stratigraphy.
  6. Well Logging: Techniques for lithology, porosity, and fluid saturation interpretation.

(IV) METEOROLOGY

1) Climatology

  • Same as under Geography.

2) Physical Meteorology

  • Thermal Structure of the Atmosphere and Its Composition.
  • Radiation:
    • Basic laws – Rayleigh and Mie scattering, multiple scattering.
    • Radiation from the sun, solar constant, effect of clouds, surface and planetary albedo.
    • Emission and absorption of terrestrial radiation, radiation windows, radiative transfer, Greenhouse effect, net radiation budget.
  • Thermodynamics of Dry and Moist Air:
    • Specific gas constant, adiabatic and isentropic processes, entropy and enthalpy.
    • Moisture variables, virtual temperature, Clausius–Clapeyron equation.
    • Adiabatic processes of moist air, thermodynamic diagrams.
  • Hydrostatic Equilibrium:
    • Hydrostatic equation, variation of pressure with height, geopotential, standard atmosphere, altimetry.
  • Vertical Stability of the Atmosphere:
    • Dry and moist air parcel and slice methods, tropical convection.
  • Atmospheric Optics:
    • Visibility and optical phenomena – rainbows, haloes, corona, mirage, etc.

3) Atmospheric Electricity

  • Fair weather electric field in the atmosphere and potential gradients.
  • Ionization in the atmosphere, electrical fields in thunderstorms.
  • Theories of thunderstorm electrification, structure of lightning flash, mechanisms of earth-atmospheric charge balance, and the role of thunderstorms.

4) Cloud Physics

  • Cloud classification, condensation nuclei, growth of cloud drops and ice-crystals.
  • Precipitation mechanisms: Bergeron–Findeisen process, coalescence process.
  • Precipitation of warm and mixed clouds, artificial precipitation, hail suppression, fog and cloud dissipation.
  • Radar observation of clouds and precipitation:
    • Radar equation, rain drop spectra, radar echoes of hailstorms, tornadoes, hurricanes, and rainfall measurements.

5) Dynamic Meteorology

  • Basic Equations and Fundamental Forces:
    • Pressure, gravity, centripetal and Coriolis forces.
    • Continuity and momentum equations (Cartesian and spherical coordinates).
    • Scale analysis, inertial flow, geostrophic and gradient winds, thermal wind.
    • Divergence and vertical motion, Rossby, Richardson, Reynolds, and Froude numbers.
  • Atmospheric Turbulence:
    • Mixing length theory, planetary boundary layer equations, Ekman layer, eddy transport of heat, moisture, and momentum.
  • Linear Perturbation Theory:
    • Internal and external gravity waves, inertia waves, gravity waves, Rossby waves, wave motion in the tropics, barotropic and baroclinic instabilities.
  • Atmospheric Energetics:
    • Kinetic, potential, and internal energies; conversion into kinetic energy; available potential energy.

6) Numerical Weather Prediction (NWP)

  • Computational instability, filtering of sound and gravity waves.
  • Filtered forecast equations, barotropic and baroclinic models.
  • Objective analysis, data assimilation techniques, satellite applications in NWP.

7) General Circulation and Climate Modelling

  • Observed zonally symmetric circulations, meridional circulation models.
  • General circulation modelling principles: grid-point and spectral GCMs.
  • Climate variability phenomena: ENSO, QBO, MJO, etc.
  • Ocean-atmosphere coupled models.

8) Synoptic Meteorology

  • Weather observations and transmission, synoptic charts.
  • Synoptic weather forecasting, prediction of weather elements, and hazardous weather phenomena.
  • Tropical Meteorology:
    • ITCZ, monsoons, tropical cyclones, jet streams.
  • Extra-Tropical Features:
    • Jet streams, extratropical cyclones, anticyclones.
  • Air masses and fronts: sources, classification, frontogenesis, and associated weather.

9) Aviation Meteorology

  • Meteorological role in aviation, weather hazards during takeoff, cruising, and landing.
  • In-flight hazards: icing, turbulence, visibility issues, gusts, wind shear, thunderstorms.

10) Satellite Meteorology

  • Polar orbiting and geostationary satellites.
  • Applications in identifying synoptic systems, cyclones, temperature estimation, rainfall prediction, and temperature/humidity soundings.

(V) OCEAN SCIENCES

1) Physical Oceanography

  • T-S diagrams, mixing processes, characteristics of water masses.
  • Wind-generated waves, shallow and deep-water wave dynamics.
  • Coastal processes: wave reflection, refraction, diffraction, littoral currents, rip currents, tsunami, and more.
  • Ocean Circulation:
    • Global conveyor belt circulation, Ekman’s theory, upwelling processes.

2) Chemical Oceanography

  • Composition of seawater, chemical exchanges, and classification of elements.
  • Element chemistry under special conditions (estuaries, vents, etc.).
  • Carbonate chemistry, biological pumps, and sedimentary deposit factors.

3) Geological Oceanography

  • Topics as listed under “Marine Geology & Paleoceanography.”

4) Biological Oceanography

  • Classification of marine environments and organisms.
  • Primary and secondary production, factors affecting biodiversity.
  • Human impacts on marine communities and climate change effects.

 

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