Groundwater-mediated Holocene hydrologic variability
in the Northern Rockies: the Ovando Valley
Mark Shapley
Emi Ito
Limnological Research Center
University of Minnesota
Joe Donovan
Dept of Geology and Geog.
West Virginia University
Research Questions
1) How do stable isotope and solute
dynamics in groundwaterdominated lakes govern climate
paleorecords ?
2) What modes of Holocene
variability are visible through the
‘groundwater filter’?
Why not avoid the paleorecord complexities
of groundwater influenced lakes?
1) There is important information to be
gained about hydrogeologic response to
climatic variation.
2) MOST lake paleorecords in continental
climates are influenced by groundwater
fluxes. Better to work with it.
3) Sedimentological signals are actually
simpler in lakes DOMINATED by
groundwater fluxes.
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Northern Great
Plains
Why Ovando?
Hydraulics generate wide geochemical variation
useful for comparative study
Hydrology and water management issues make
paleo-groundwater history interesting
(Neither is unique, both are well-met)
Daily mean discharge, ft3/sec
10000
Runoff
1000
2002 data
100
1-Jan
1-Mar
Groundwater-supported base flow
1-May
1-Jul
1-Sep
Period of record: 1898-2003
1-Nov
- average base flow ~ 750 cfs
- typical drought year base flow 400-500 cfs
- water rights actions 700 cfs
- survival of T&E fish species impacted - ?? cfs
31-Dec
Archive:
CaCO3 minerals precipitated from lake waters
Data sets:
Isotopic composition, elemental composition, mineralogy
Information carried:
(Something about) fluid balance
(Something about) solute balance
(Something about) groundwater recharge and fluxes
Calcite and aragonite: CaCO3 polymorphism
Elevated Mg:Ca (threshold disputed) suppresses calcite
precipitation, allowing aragonite formation to proceed
Mg:Ca is increased by CaCO3 formation (either polymorph)
If alkalinity initially exceeds Ca++, evaporative concentration
favors CaCO3 formation and resulting Ca depletion relative to Mg
(until Mg phases rreach saturation)
Bottom line: lacustrine inorganic aragonite is (usually) an
indicator of elevated lake concentration
18O in evaporative lakes
Inflow composition weighted mean of all sources
Lighter isotopes fractionate preferentially into vapor
phase; δ18O and δD in water rise systematically
Endogenic carbonate minerals incorporate oxygen from
water molecule (another fractionation); signal of water
composition locked in (barring diagenesis!)
Qpi, δ18Οpi
Qe, δ18Οe
Qgi, δ18Οgi
VL, δ18ΟL
Sediment
storage
Qgo, δ18Ο (~ δ18Ο L)
Oxygen isotope balance
components
gas evasion
Qpi, Cpi
Qeo, Cgo (~ 0)
CL(con)
Qgi, Cgi
Sediment
storage
Qgo, Cgo (~CL(cons))
Solute balance
components
Core imagery ‘bar code’ corrleates with endogenic mineral content
δ18O(carb)
δ13C(carb)
δ18O(carb)
Modeling 18O and 2H sensitivity in lake water
Model Structure: Transient fluid and isotope balance
Monthly climate and lake state forcing
Solute-constrained groundwater fluxes
Lake basin hypsometric model
Equilibrium and kinetic fractionation
Evaporation: climatically dumb, isotopically not-so-dumb
What are the messages?
1) Groundwater fluxes express lots of Holocene
variability
2) There is a (noisy) multidecadal periodic component
3) The implied hydroclimatic (recharge) variation is
not trivial
4) Only the latest Holocene was consistently as dilute
as the modern era
5) Amplitude of geochemical variability was enhanced
during the mid-Holocene
Collaborators: Emi Ito Joe Donovan Eric Grimm
Research support from the following sources is gratefully acknowledged:
-Graduate Assistance in Areas of National Need (GAANN), funded by the US
Department of Education.
-The GeoFluids Program of the Department of Geology and Geophysics, funded
by the National Science Foundation.
-The Paleorecords of Global Change RTG, a UM interdepartmental initiative
-The UM Quaternary Paleoecology Program.
-The Montana Department of Environmental Quality.
-Interested citizens of the Ovando Valley, Montana.
-Moral and intellectual support from numerous LRC friends and from the
PRAIRIe consortium.