Embed Size (px)
Citation preview
Objectives
1) How did high-latitude continental climate respond to
Cenozoic climate change?
2) Can we infer Cenozoic paleotopography in southern
Alaska using volcanic glass dD values?
AcknowledgementsBenjamin Deans wishes to acknowledge the help of Cates Laboratories for use of their PLM and colleagues at UTA for their encouragement.
High-Latitude Continental Response to Cenozoic Climate Change in Southern Alaska Based on Volcanic Glass Hydrogen Isotope Composition
Benjamin Deans1, Majie Fan1, Jeff A. Benowitz2, Emily Finzel3
1Department of Earth & Environmental Sciences, University of Texas at Arlington, Arlington, TX, 76019, USA
2College of Natural Science & Mathematics, University of Alaska Fairbanks, Fairbanks, AK, 99775, USA
3Department of Earth & Environmental Sciences, University of Iowa, Iowa City, IA 52242, USA
Works CitedCailleteau, Celina et al. 2008. On the effect of glass composition in the dissolution of glasses by water. Journal of Non-Crystalline Solids. 354(2):117-123
Dallegge, TA, Layer, P. W. 2004. Revised 40Ar/39Ar chronostratigraphy of the Kenai Group using low-potassium bearing minerals, Cook Inlet, Alaska. Canadian Journal of Earth Science. 41(10): 1141-1158.
Fan, M et al. 2014. Middle Cenozoic uplift and concommitant drying in the Central Rocky Mountains and adjacent Great Plains: Geology. 42(6):547-550
Finzel, ES et al. 2016. Long-term fore-arc basin evolution in response to changing subduction styles in southern Alaska. Tectonics. 35(7):1735-1759
Friedman, J et al. 1993. Deuterium fractionation as water diffuses into silicic volcanic ash. Climate change in continental isotopic records. 78:321-323
Johnson, TM. 2014. A fluvial isoscape of southcentral Alaska. Alaska Pacific University, Ann Arbor
Sharp, Z. 2007. Stable isotope geochemistry. 1st ed. Upper Saddle River, NJ. Pearson Education, Inc.
Zachos, J. et al. 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science. 292(5517):686-693.
Results and Interpretation
Stratigraphy
Figure 3. Generalized stratigraphic column in the study area and field
picture of an ash layer in the Tordrillos Mountains.
Samples
1) 21 rhyolitic volcanic ash samples from fluvial Sterling
Formation on the west side of the Kenai Peninsula.
Elevations of sampling sites are ~200 m.
2) 3 volcanic ash samples from the West Foreland Formation
on the Talkeetna and Tordrillos Mountains. Elevations of
sampling sites are ~800 m.
Cook
Inle
tK
enai
Pen
insu
la
Talkeetna
Mts.
Tordrillos
Mts.
Anchorage, AK
-17000000.000000
-17000000.000000
-16800000.000000
-16800000.000000
-16600000.000000
-16600000.000000
8300000
.00
00
00
8300000
.00
00
00
8400000
.00
00
00
8400000
.00
00
00
8500000
.00
00
00
8500000
.00
00
00
8600000
.00
00
00
8600000
.00
00
00
8700000
.00
00
00
8700000
.00
00
00
8800000
.00
00
00
8800000
.00
00
00
8900000
.00
00
00
8900000
.00
00
00
Alaska
Canada
Russia
Bering Sea
Gulf of Alaska
Arctic Ocean
130°0'0"W
130°0'0"W
150°0'0"W
150°0'0"W
170°0'0"W
170°0'0"W
70°0'0"N 70°0'0"N
65°0'0"N 65°0'0"N
60°0'0"N 60°0'0"N
55°0'0"N 55°0'0"N
50°0'0"N 50°0'0"N
Figure 1. Sampling locations around the Cook Inlet. Red dots
represent samples >38 Ma. Blue dots represent samples <12 Ma.
Hydrated Volcanic Glass
Dense gel layer acts like a diffusion barrier and locks water
molecules within the hydrated glass.
Conclusions
• Cenozoic paleometeoric water δD values in south Alaska do not follow the trend of deep-sea temperature.
• The Kenai mountain belt in the Kenai Peninsula was at least 1 km above sea level during the last 10 Ma.
• The Talkeetna and Tordrillos Mountains were higher during the late Eocene than today.
Volcanic Glass in Alaska
Volcanic glass collected showed characteristic traits of:
• Optically isotropic
• Refractive Index of ~1.50
• Thin walled with or without circular to suboval vesicles
Figure 2. Left column: vesicular thin-walled volcanic glass. Right
column: tabular thin-walled volcanic glass. From top to bottom plane
polarized light, crossed polarized light with gypsum plate inserted, and
dispersion staining objective.
• 3 samples from the West Foreland Formation on the Talkeetna
and Tordrillos Mountains, dated to be 44-38 Ma, have δD
values ranging from -167‰ to -128‰ (VSMOW). The derived
paleometeoric water δD values range from -139‰ to -98‰
(VSMOW). These values are comparable to modern river water
δD values in the study area. Given that late Eocene was
warmer than today, these comparable δD values may suggest
that the study area was higher during the late Eocene than
today.
• 21 samples from the fluvial Sterling Formation on the Kenai
Peninsula, dated to be younger than 12 Ma, have δD values
ranging from -181‰ to -97‰ (VSMOW). The derived
paleometeoric water δD values range from -153‰ to -66‰
(VSMOW). These values have larger variation than modern river
water δD values at ~200 m on the Kenai Peninsula.
• The high paleometeoric water δD values most likely reflect
evaporation of surface water on the floodplain.
• The low paleometeoric water δD values, occurred at 8-6 Ma,
most likely reflect river water charged by high-elevation
precipitation.
• Mountains on the Kenai Peninsula were at least ~1 km above
sea level during the last 10 Ma.
Figure 4. 21 samples from the Kenai Peninsula.
Figure 5. River water δD values in southern Alaska (Johnson, 2014).
• Angular shape
• Tabular
Ash layer
Blue: water
Red: Si atoms
Orange: other elements
Gel layer
Hydrated glass
Pristine glass
Cailleteau et al.,
2008
Cailleteau et al.,
2008Fan et al., Geology, 2014
-190 -170 -150 -130 -110 -90 -70 -50
0
2
4
6
8
10
12
14
1.522.533.544.55
Glass δD (VSMOW)
Ag
e (
Ma)
Benthic foraminifera δ18O (VPDB) (Zachos et al., 2001)
Samples <12 Ma
δ18O (VPDB)
δD (VSMOW)
Modern river water δD range
(Johnson, 2014)
Figure 6. 3 samples from the Talkeetna and Tordrillos Mts.
-190 -170 -150 -130 -110 -90 -70 -50
37
38
39
40
41
42
43
44
45
00.511.522.53
Glass δD (VSMOW)
Ag
e (
Ma
)
Benthic foraminifera δ18O (VPDB) (Zachos et al., 2001)
Samples >38 Ma
δ18O (VPDB)
δD (VSMOW)
Modern δD range (Johnson, 2014)
Cooler Warmer
Cooler Warmer
