Chapter 15 Chronostratigraphy 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 – D o ) + 1) Where N is the number of parent atoms, ln is log base e, D is the total number of daughter atoms, D o is the number of original daughter atoms and λ is the decay constant. λ = / T ½ where T ½ is the half-life of the radioactive element. With some mathematical magic we develop the following: N = N o e -λt Where N = observed number disintegrations/hr/g, N o = initial number of disintegrations/hr/g, λ = the decay constant and t = time elapsed.

Practice radioactive decay problem: N = N o e -λt t = (1/λln(N o /N)) The decay constant (λ) is C-14 is 1.2 x years, (t ½ =5730) N o = 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 = 1.2 x 10 -4, N = 141, N o = 920, N o /N = 6.5 t = (1/λln(N o /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 234 U decays to 230 Th in sea water, the 230 Th rapidly drops out of solution because, unlike uranium, thorium is very insoluble. In this case, the 230 Th that accumulates in the sediments is said to be unsupported, as it is now separated from its parent isotope.

230 Th Dating of Marine Sediments As we saw in the last example, 230 Th 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 ( 230 Th’s parent) to the ocean is in balance, then 230 Th 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 230 Th deposition to the sediments. 230 Th = x y -1. t = 108,495 ln( 230 Th initial / 230 Th measured )

Example 6-4 The 230 Th activity is measured for a marine sediment core. The top layer of the core has a 230 Th activity of 62dpm. At a depth of 1m, the 230 Th 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 16 O isotopes evaporate with seawater and are returned to the ocean through precipitation and runoff. When it is colder 16 O is incorporated into continental ice sheets which causes the oceans to become enriched with heavier 18 O that has not evaporated and precipitated onto ice sheets.

The  (delta notation)  = [(R samp – R std ) / R std ] x 1000  = [(R samp / R std ) -1] x 1000 same as Again R is the ratio of the heavy to light isotope, and measured with a mass spectrometer. R std is element specific… Units are per mil “‰”

Climate Change Because the fractionation of H and O in water changes with temp, isotopic measurements of ice-cores are used to estimate paleoclimate. 1)Isotopic composition of snow reflects air temp. 2)Colder air = more negative  D and  18 O 3)Warmer air = less negative  D and  18 O 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 up a bunch of ocean water into glaciers, the overall  D and  18 O of all water gets 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

(δ 18 O = the per mil deviation from the standard)

Carbon Can we use  13 C to detect Fossil fuel contributions to Atmospheric CO 2 ? -7‰

Factors that influence the δ 13 C of the ocean water: Primary productivity (organisms preferentially incorporate light carbon 12 C) CO 2 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.