Jay Banner Department of Geological Sciences Environmental Science Institute April 19, 2005
Outline 1. Background: Isotope systematics – stable & radiogenic 2. Tracing atmospheric vs. hydrologic processes 3. Edwards aquifer examples 4. The temporal evolution of groundwater
Isotopes
• Different isotopes of the same element have the same number of protons and electrons, but different numbers of neutrons.
1 Stable Isotope Systems
• Many elements of low atomic weight have two or more stable isotopes, e.g., Hydrogen - 1H, 2H (= D) δD Carbon - 12C, 13C δ13C Oxygen - 16O, 18O δ18O
• The different masses cause isotopes to behave differently in physical and chemical processes.
18 16 • Consider two water molecules: H2 O and H2 O
Radiogenic Isotope Systems
Some elements undergo radioactive decay due to inherent instability of their nuclei, producing a daughter product of another element.
The daughter product produced is termed ‘radiogenic’. The relative amounts of radiogenic vs. stable isotopes of the same element in rocks, minerals, and waters provide information about age and sources of dissolved ions.
2 Isotope Fractionation
• Isotope fractionation: the development of differences in isotopic composition as a result of physical and chemical processes.
• The degree of fractionation depends on the relative weights of the isotopes. – Commonly fractionated: H, C, N, O, S (stable) – Negligible fractionation: Sr, Nd, U, Pb (radiogenic)
H and O isotopes in the hydrologic cycle
• Distillation and fractionation during cycle • Effects of – Evaporation – Rainfall amount – Topography – Temperature – Climate change
3 Map of North America showing contours of the average hydrogen isotope values (δD or 2H/1H values) of meteoric surface waters. An identical pattern is found for oxygen isotopes (δ18O or 18O/16O) in meteoric waters.
Hydrogen isotopes in meteoric water Taylor (1974)
Oxygen in hydrologic cycle cartoon
4 Barbados Monthly Rainfall
Jones et al. (2000)
H-O isotopes in rain and groundwater on Barbados
5 Barb Rain oxygen vs time
Rain Groundwaters Groundwater
Jones and Banner (2003)
6 What is strontium?
• An alkaline earth element with 4 naturally occurring isotopes (84Sr, 86Sr, 87Sr, 88Sr).
• The 87Sr/86Sr ratio (δ87Sr) varies in nature with age and composition of aquifer rock and soil.
• Groundwaters reacting with rock and soil may reflect these variations.
Radiogenic isotope variations in a karst aquifer
7 87Rb – 87Sr radioactive decay system
87 → 87 + β- 37Rb 38Sr
Half life = 48.8 b.y.
87 87 87 λt Sr = Sro + Rb (e –1)
87 86 87 86 87 86 λt Sr/ Sr = ( Sr/ Sr)o + Rb/ Sr (e –1)
y = b + m x
Sr isotope evolution of the Earth
0.720
0.716 Archean Crust
Proterozoic Crust 87Sr 0.712 86 Sr 0.708 Fig. 1
0.704 Mantle Initial Earth Mantle 0.700
0 1 2 3 4 5 Age (Ga)
Banner (2004)
8 Sr isotopes in the oceans through time
0.710
0.709 Sr
86 0.708 Sr/ 87
0.707
0.706 Q Tertiary Cretaceous Jurassic Triassic Permian Penn. Miss. Devonian Silurian Ordovician Cambrian PC 0 100 200 300 400 500 600 Age (Ma)
Burke et al. (1982)
Musgrove et al. 2001
4
1-
2- 3- 4- Honey Creek
9 www.edwardsaquifer.net/
10 Soils = High Sr isotope ratios Low Mg/Ca and Sr/Ca values
Limestone = Low Sr isotope ratios High Mg/Ca and Sr/Ca values
Methods
• Soils are leached using a NH4Ac solution to approximate the ion exchange by infiltrating rainwater as it moves through the soil.
• Cave waters and soil leachates are analyzed for elemental composition using ICP-mass spectrometry.
• Sr is extracted from waters using ion-exchange chemistry.
• Waters are analyzed for Sr isotope ratios using thermal- ionization mass spectrometry.
11 Edwards Aquifer: Regional groundwater compositions
Water-dolomite interaction Water-calcite interaction
phreatic vadose
Soil waters
Cretaceous limestones
Edwards Aquifer: Local groundwater compositions
Inner Space Natural Bridge dripwaters dripwaters
IS soil waters NB soil waters
12 Inner Space Cavern drip site ISST: Temporal Variations in 87Sr/86Sr and Effective Precipitation 0.70868 8
6 0.70866 87Sr/86Sr
4
0.70864
2
0.70862 0 Sr
86 0.7086 -2 Sr/ 87
-4 0.70858
-6
0.70856 monthly effective precipitation (inches) precipitation effective monthly effective precipitation -8
0.70854 -10
0.70852 -12 May 2002 June 2003 Aug. 2004 4/19/2001April 2001 11/5/2001 5/24/2002 12/10/2002 6/28/2003 1/14/2004 8/1/2004 2/17/2005
Edwards Limestone
13 Vadose Flow Routes in Karst
After Smart & Friederich (1987)
Vadose Groundwater Flow Model
After Smart and Friederich (1987)
14 Rainfall – Flow route model summary
High recharge = conduit flow = high 87Sr/86Sr
Low recharge = diffuse flow = low 87Sr/86Sr
Honey Creek Guadalupe River Study Area Honey Creek and Setting ● Preliminary data: Sr isotopes ●● ● for spring waters in the control ● watersheds (Bravo and Delta Springs, and the treatment watershed) Collaborators: treatment MaryLynn Musgrove Libby Stern
●Bravo Spring control ●Cotton Spring ●Delta Spring ●Echo Spring ● ● ●Other springs ● Rainfall simulation site 20’ contour
15 HCSNA Sr Isotope Variations
NH4Ac soil leachates
HCSNA spring waters
HCSNA Sr Isotope Variations
NH4Ac soil leachates control watershed treatment HCSNA spring waters watershed
16 Temporal Variations in Spring Water 87Sr/86Sr and Effective Precipitation
Effective preciptation: rainfall less evaporation
Impact of Urbanization on Austin-area streams
Lance Christian Geological Sciences MS thesis
17 Current Study: Street Indicators of Urbanization
Bull Creek
Shoal Creek West Bull Creek
Barton Creek Waller Creek
Williamson
Slaughter
Big and Little Bear Onion Creek
Source data: City of Austin Amy Hobbs, Dept. of Architecture, Univ. of TX
Strontium Isotope Versus Building Area Urbanization Indicator (for watersheds: Waller, Shoal Bull, West Bull, Williamson, and Slaughter)
0.70900
0.70880 R2 = 0.923
0.70860
0.70840
87Sr/86Sr
0.70820
0.70800
0.70780
0.70760 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 building area per watershed (%)
18 Temporal Evolution of Groundwater: An Isotopic Perspective
19 Speleothems
Speleology, n. [Greek speleon, cave; ology, study of] the exploration and study of caves
Speleothems, n. [Greek speleon, cave; them, them things] them things that grow in caves
20 Applicability of Speleothems to Studies of the Temporal Evolution of Groundwater
• Wide geographic coverage
• Dateable and continuous on a range of time scales
• Range of tracers: Stable and radiogenic isotopes Trace elements, inclusions
Pittman, 1999
The West Indies
21 Tectonic Setting of Barbados
Torrini et al. (1985)
Barbados, West Indies
After Scoffin, 1987
22 1 mm 1 mm
Southeast Barbados
23 Modern Barbados Reef
24 Horse Hill
Saharan dust transport patterns
The New York Times Oct 29, 1992
25 NASA image of Saharan Dust over Canary Islands
NASA 3/6/98
Barb speleo Sr v time eps
Banner et al. (1996)
26 Vadose Flow Routes in Karst
After Smart & Friederich (1987)
Barb Speleo Sr record
27 Holocene climate records, Mesoamerican tropics
• Lake levels • Lake chemistry: mineralogy stable isotopes
• Pollen, vegetation •Fires • Archeology
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