|
Home | Announcements
| Syllabus | Calendar
| Lecture Notes | Grades
| Geology 370: Glacial Geology
|
Home | Announcements
| Syllabus | Calendar
| Lecture Notes | Grades
| Traditional Ice Age Chronology
Wisconsin Wurm Weichsel Sangamon R-W Eem Illinoian Riss Saale Yarmouth M-R Holstein Kansan Mindel Elster Aftonian G-M Cromer Nebraskan Gunz Menap Revised chronology based on marine oxygen isotope stages
even numbers generally refer to glacials
odd numbers generally refer to interglacials
Wisconsin & Illinoian are only names still in use
Law of Superposition
younger beds overlie older beds
younger beds truncate older beds
Law of Ascendancy and Descendancy
exoposed beds at higher elevations are older than exposed beds at lower elevations
Limiting Age Estimates
maximum: event (unit) must be younger than the dated material
outwash must be younger than the stump
minimum: event (unit) must be older than the dated material
moraine must be older than the tree
C. Paleomagnetism
Movement of magnetic pole
chron: major and complete reversal
one reversal looks like another, so need other information to date units
Normal polarity: magnetic minerals point northwards; reverse polarity - southwards
Brunhes - normal
last reversal 780,000 YBP
Matuyama - reversed
Allows correlation of isolated stratigraphic sections over broad regions
Polarity time scale: Mankinen & Wentworth (2003) USGS Open File Report 03-187 http://pubs.usgs.gov/of/2003/of03-187/
Introduction
isotope: same number of protons, different number of neutrons
radioactive isotopes disinegrate & radiate particles at a fixed rate
half life: time it takes to disintegrate half of original amount
Radiocarbon Dating
3 carbon isotopes: 12C, 13C, and 14C
ratio of 14C/12C shifts as 14C disintegrates
half life (by international agreement) is 5568 +- 30 years; all ages are YPB; present is year 1950
age range: 100-70,000 years
limitations
Other Radiometric Dating Techniques
K/Ar Dating - age range: older than 500,000 years
Uranium Series Dating - age range: 1000-350,000 years
Thermoluminescence Dating
Fission Track Dating
3 Oxygen Isotopes: 16O and 18O most common
Fractionation
ratio at which these isotopes enter chemical compounds is temperature dependent
16O lighter so evaporates preferentially; 18O heavier so condenses preferentially
during interglacials 18O/16O ratio in oceans stays constant
measure how much 18O/16O ratio deviates from isotope proportions found in modern oceans
d18O %o is zero for standard marine ocean water
during glacials:
16O preferentially evaporated from oceans; 18O concentrated in oceans
ocean 18O/16O ratio is higher than in non-glacial seawater; d18O is positive
marine shells enriched in 18O during glacials
16O deposited on ice sheets; 16O concentrated in ice
ice sheet 18O/16O ratio is lower than in non-glacial seawater; d18O is negative
d18O is most widely used proxy for:
changes in global ice sheet volume
changes in global temperatures
Challenges
ice core interpretation
marine core interpretation
Methods of paleoenvironmental reconstruction:
stratigraphic sequencing
paleomagnetism
radiometric dating
oxygen isotopes
plus many other methods
Currently accepted Ice Age record:
Holocene (recent) - isotope stage 1
Wisconsin Glacial - isotope stages 2, 3, 4 (and 5 a, b, c, d?)
Sangamon Interglacial - isotope stage 5e (and 5 a, b, c, d?)
Illinoian Glacial - isotope stages 6 and 8
All earlier glacials/interglacials referred to by their isotope stage
