Cold Evaporites

Peter Webb (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.

 


 

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