Cold Evaporites
Peter
Webb (Emeritus, Department of Geological Sciences, Ohio State University,
Columbus, Ohio, USA)
4 May 1999
Talking
today about cold evaporites (vs. hot evaporites) and looking at changes in
water systems of dry valleys in Antarctica due to global climate change.
There
is a good correlation between levels of CO2 & methane and
climates. Now, CO2 and CH4 levels are rising,
possibly due to anthropogenic causes, possibly causing global warming. CO2
is from combustion. CH4 sources are more complicated - comes
from everything from sheep passing gas to release of CH4 from gas
hydrates. CO2 levels are now ~350-360 ppm; they were ~275 ppm
in the early-mid 1700s. Temperatures also have been rising. What is
the effect on high latitude warming?
We
can use evaporites (salts & brines) as a thermometer.
“Cold” Marine & Terrestrial - various processes produce brine in the following
environments: polar marine ice shelf, polar marine sea ice, polar marine open
ocean, polar marine polynie/polynya, polar terrestrial glacier, polar
terrestrial surficial, polar terrestrial lacustrine. The produced brines
are mostly CaCl2 (precipitates as hydrous calcium chloride - antarcticite,
CaCl2·6H2O) and NaCl (halite).
"Hot" Marine & Terrestrial - various processes produce brine in these
environments: marine shoreline-delta (sabkha), marine shorelines (salinas),
deep sea marine, terrestrial subaqueous, terrestrial subaerial. The
produced brines are mostly NaCl (halite), gypsum (CaSO4·2H2O),
and MgCl2 (chloromagnesite).
Major constituents of seawater - NaCl (78%), MgCl2 (9%), MgSO4 (7%), CaSO4
(4%), KCl (2%).
Major constituents of the Dead Sea - MgCl2 (52%), NaCl (30%), CaCl2
(12%).
Major constituents of Don Juan Pond/Lake Vander in Antarctica - CaCl2·6H2O (antarcticite)
(90%), NaCl (halite) (<10%), other salts.
The
Dead
Sea is very saline. Saline-fluvial formations/rocks occur on either
side of the sea adjacent to Masada. Old shorelines are readily apparent
on the land adjacent to the Dead Sea. Don’t get waves
in Dead Sea water, really - water is too thick and soupy. There is a
mix of halite (NaCl) and MgCl2 (chloromagnesite) crystals on the
shores of the Dead Sea - oily waves - tired, slow waves - very thick
water. The Dead Sea is a good example of a hot evaporite setting.
Now,
let’s look at Antarctic lakes/ponds and see the changes since the 1950s.
Forty years ago, things were colder. Now, get more floods and higher lake
levels. Now, we’re interested in water systems in Antarctica. As
long as there is snow cover (cold enough for it), there isn’t meltwater.
Sunlight gets reflected back, and little water is around during the
summer. Now, increased levels of water are present in valley lake
systems. Lake Vander
is an old Pliocene fjord, now drained and a terrestrial valley with a a lake
and smaller ponds in the valley. It was 4 miles long in 1958. Now,
it is 7-8 miles long. The enlarging of the lake is only due to 1 thing -
more meltwater from the glaciers. Lake levels have been monitored in
cases, such as Lake
Bonney. Real monitoring started in ~1970. Lake levels have been
rising in several lakes. There are salts in these lakes - are they
terrestrial or marine in origin? They are marine in origin, from being
ex-fjords. Salts are the result of weathering (probably from the Jurassic
Ferrar Dolerite). The salt brines in these systems are rather
acidic. Lake Vander has risen 8 meters since records have been kept.
As the lake grows and deepens, just freshwater is added to the top - no
turnover or convection occurs between the layers in the lake. In
pre-Holocene times, Lake Vander was probably a very saline lake, like the Dead
Sea. Temperature rises with depth in Lake Vander - up to 25˚ C near
the bottom. The traditional explanation is that the heat is from
geothermal sources. The modern explanation is that the heat is not
geothermal, but due to communication of sunlight downward by the vertically
oriented C axes of surface ice on the lake - heat can’t escape.
Polar evaporites - see
frost polygons in Antarctic valleys. CaCl2·6H2O (antarcticite) is often
around - a white powder, but not snow. The presence of CaCl2·6H2O
indicates temperatures reached -50˚ C at some point in the year (during
winter). When this stops precipitating, we know winter
temperatures have reached a new temperature threshhold. High winds in
area also drives evaporation of water from brines rising up cracks of frost
polygons. Polar evaporites are crystallizing at temperatures much colder
than Dead Sea evaporites.
A
southern, polar, dried up lake - a plug of evaporites, with different minerals
in different areas of plug. The wind blows off water as it upwells in
aquifers; highly saline water has very depressed freezing point - so water
upwelling occurs throughout winter.
There
are stromatolites in some of these lakes. Mats of antarcticite (CaCl2·6H2O)
coalesce in lakes, surrounded by halite crusts. The Don
Juan Pond system is 4 km long by 1 km wide.
Freight car hypothesis
- observation that trains over spongy land causes water table to rise suddently
and takes ~hour(s) to return to normal. The same effect occurs at Don
Juan Pond - thermal change in adjacent rock glacier causes movement of rock glacier
- presses down on aquifer and water gets pushed up as brine to surface into
lake.
High
winds causes Don Juan levels to fall suddenly - water is evaporated away.
Summer - CaCl2-dominant
brines circulate in aquifer and discharge to surface at -16˚ C.
Winter - Ice crystals
form at <-20˚ C; halite crystals form at -40˚ C; antarcticite
(CaCl2·6H2O) (hygroscopic) crystallizes subaerially at
-56˚ C.
Summer - Dissolution
of hygroscopic antarcticite and hydrohalite (NaCl·2H2O) in the
presence of warm air and meltwater with the return of the brine phase.
Prediction
- Don Juan Pond will increase in size and lake level. Don Juan Pond will
become Don Juan Lake eventually.
The
pH of these brines is low: 4-5.
~
-57˚ C is the coldest temperature a super briney water can be and still be
liquid.
