GEOL 340 Sedimentology and Stratigraphy
Lecture 30
1. Chemostratigraphy
oxygen isotopes
carbon isotopes
Overview of the radiogenic dating schemes (handout)
The world's oceans have undergone longterm secular variations in their geochemical and isotopic compositions
Correlation of these changes with standard curves permits high-resolution relative and sometimes absolute age dating of stratigraphic sections
General Fractionation Concepts
a. atomic structure
electrons (- charge)
nucleus (+ charge)
protons (+ charge equal but opposite to electron) atomic number
Z
neutron (neutral but more mass than electrons) neutron number
N
nucleons = protons and neutrons = mass number A
notation = mass #/atomic # placed before element
b. isotope = atoms of the same chemical elements with the same
atomic number but a
different number of neutrons so a different mass number (Table
18.6, p. 656)
c. extent of fractionation a function of the vibrational frequency
of molecules
vibrational frequency proportional to 1/mass of atom
light isotopes have < bond energy and therefore > chemical
reactivity
Important Considerations
a. what is the relative abundance (concentration) of the element
in the system
b. what is the magnitude of the isotopic fractionation
c. diagenetic alteration of original isotopic compositions
Oxygen Isotopes - track both inorganic and organic mechanisms
Stable Fractionation Mechanisms
a. temperature
ToC = 16.9 - 4.38(dcalcite-dwater) + 0.10(dcalcite-dwater)2
d18O = x 1000
standards:
PDB (Pee Dee Belemnite)
SMOW (standard Mean Ocean Water)
colder temperatures = lower d18O
b. evaporation
c. biological mediation
d. mineralogy
SPECMAP (Fig. 18.13, p. 659)
Carbon Isotopes - track primarily organic mechanisms
Stable Fractionation Mechanisms
a. biological mediation
12C preferentially incorporated into organic matter (figure
on handout)
drawing of marine CO2 reservoir
b. mineralogy (minor)
Oceanic Anoxic Event correlations (figure on handout)