chapter 15 chronostratigraphy and geologic time

Chapter 15Chronostratigraphy and Geologic Time

Chronostratigraphy: the establishment of time relationship among rock units.

Stratotypes: the type representative of a named stratographic unit. It constitutes the standard for the definition and recognition of that boundary (i.e. outcrops at Chestnut Hill Mall represent the Roxbury conglomerate, but on a grander scale.)

Isochronous units: rock units formed during the same span of time and everywhere bounded by synchronous surfaces, which are surfaces on which every point has the same age.

Geochronologic Unit

Eon

Era

Period

Epoch

Age

Chron

Corresponding Geochronostratigraphic Unit

Eonothem

Erathem

System

Series

Stage

Chronozone

Geologic time scale

Radiochronology: An absolute-age dating method based on the existing ratio between radioactive parent elements (such as U-238) and their radiogenic daughter isotopes (such as Pb-206).

The equation for calculating radiometric age is:

t = (1/λ)ln(N-1(D – Do) + 1)

Where N is the number of parent atoms, ln is log base e, D is the total number of daughter atoms, Do is the number of original daughter atoms and λ is the decay constant.

λ = 0.693 / T½

where T ½ is the half-life of the radioactive element.

With some mathematical magic we develop the following:

N = Noe-λt

Where N = observed number disintegrations/hr/g, No = initial number of disintegrations/hr/g, λ = the decay constant and t = time elapsed.

Practice radioactive decay problem:

N = Noe-λt t = (1/λln(No/N))

The decay constant (λ) is C-14 is 1.2x10-4 years, (t½=5730)

No = 920 disintegrations/hr/gC

Prehistoric caves were discovered in the Lascaux cave in France. Charcoal from the site was analyzed and the level of radioactivity was found to be N = 141 disintegrations/hr/g.

Estimate the age of the paintings.

λ = 1.2x10-4, N = 141, No = 920, No/N = 6.5

t = (1/λln(No/N))

t = 8,333(ln6.5)

t = 15,629 years before present

Contemporaneity of sedimentary rocks to an associated, datable volcanic ash layer

Determining the ages of sedimentary rocks indirectly by

(A)Bracketing between two igneous bodies

(B) Bracketing between regionally metamorphosed sedimentary rocks and an intrusive igneous body.

U-Series Disequilibrium Methods of Dating

For a closed-system for a sufficiently long time, secular equilibrium will be achieved and the relative abundance of each isotope will be constant.

When the system enters disequilibrium due to separation of either parent or progeny, or subsequent decay, the reestablishment of equilibrium can be used as a dating method.

For example, when 234U decays to 230Th in sea water, the 230Th rapidly drops out of solution because, unlike uranium, thorium is very insoluble. In this case, the 230Th that accumulates in the sediments is said to be unsupported, as it is now separated from its parent isotope.

230Th Dating of Marine Sediments

As we saw in the last example, 230Th is unstable in the marine environment. In fact it has a mean residence time of about 300 years.

Given that the addition and removal of U (230Th’s parent) to the ocean is in balance, then 230Th is produced at a constant rate. This means that as long as there has been no disruption to the sediment layers on the sea floor, the uppermost layer will represent present-day 230Th deposition to the sediments.

230Th = 9.217 x 10-6 y-1.

t = 108,495 ln(230Thinitial / 230Thmeasured)

Example 6-4

The 230Th activity is measured for a marine sediment core. The top layer of the core has a 230Th activity of 62dpm. At a depth of 1m, the 230Th activity is 28dpm. Calculate the age of the sediment at a depth of 1m.

t = 108,495 ln(62/28) = 86,246 y

Rate = (sediment thickness / time) = 1m / 86,246 y = 1.16 cm /1000 y.

Correlation by Stable Isotope Events

Oxygen Isotopes and the water cycle during glacial/interglacial periods

Lighter 16O isotopes evaporate with seawater and are returned to the ocean through precipitation and runoff. When it is colder 16O is incorporated into continental ice sheets which causes the oceans to become enriched with heavier 18O that has not evaporated and precipitated onto ice sheets.

The (delta notation)

= [(Rsamp – Rstd) / Rstd] x 1000

= [(Rsamp / Rstd) -1] x 1000

same as

Again R is the ratio of the heavy to light isotope, and measured with a mass spectrometer.

Rstd is element specific…

Table 6-7. Stable isotope ratios for stan dards*

Element Standard Ratio

Hydrogen V-SMOW 2H/1H = 155.76 x 10-6

Carbon PDB 13C/12C = 1123.75 x 10-5

Oxygen V-SMOW 18O/16O = 2005.2 x 10-6

PDB 18O/16O = 2067.2 x 10-6

Nitrogen NBS-14 15N/14N = 367.6 x 10-5

Sulfur CDT 34S/32S = 449.94 x 10-4

*Data from Kyser (1987).

Units are per mil “‰”

Climate Change

Because the fractionation of H and O in water changes with temp, isotopicmeasurements of ice-cores are used to estimate paleoclimate.

1) Isotopic composition of snow reflects air temp.

2) Colder air = more negative D and 18O

3) Warmer air = less negative D and 18O

4) Works the same in both hemispheres

5) Once snow is packed into glacier, ice stratigraphy not disturbed, paleothermometer locked into place

Ice ages can confound this approach to some extent because by locking upa bunch of ocean water into glaciers, the overall D and 18O of all watergets less negative. This effect is small relative to the temp effect.

Arctic

Antarctic

When records from both hemispheres agree, it is a global climate change

When the disagree, it is a local climate change

(δ18O = the per mil deviation from the standard)

Carbon

Can we use 13C to detect Fossil fuel contributions to Atmospheric CO2?

-7‰

Factors that influence the δ13C of the ocean water:

Primary productivity (organisms preferentially incorporate light carbon 12C)

CO2 interchange with the atmosphere

Increased rates of erosion and runoff of organic rich

Increased rates of sediment burial in the ocean thereby removing sediments containing fine organic matter from interaction with seawater.