Presentation on theme: "Licancabur: exploring the highest lake on Earth. Oral exam, Hock Topic 1, v.1.0 9 Sept. 2003."— Presentation transcript:
Licancabur: exploring the highest lake on Earth. Oral exam, Hock Topic 1, v Sept. 2003
GOAL: provide a quantitative physical explanation for a temperature anomaly observed at Licancabur Volcano crater lake. The site: Volcan Licancabur Motivation Observations—water temperature anomaly –H 2 O physics Hypotheses & tests Modeling lake mass, energy balance Proposed future work
Map: de Silva and Francis Volcan Licancabur 22 50’S, 67 53’W Crater lake: 5916 m ~90 x 70 x 4 m Twater~ 0-6 C pH ~ 8.5 TDS ~ 1.05 ppt
Motivation Terrestrial –Unexplored (e.g. Rudolph 1955; Leach 1986) –One of the highest (~5916 m) lakes on Earth –Volcanology/Limnology Unclassified wrt world’s volcano lakes Martian –Terrestrial analog to ancient paleolakes? intense UV flux (~85 W/m 2 ) and a cold (-13 °C), dry (< 200 mm/yr), oxygen-depraved (~48% pO 2 (0)) atmosphere –Harsh physical environment—Survival strategies of endemic organisms
Observations –No eruptions in recorded history. –Evidence of recent activity youthful lava flows, well-preserved summit crater, absence of glacial geomorphic features (de Silva and Francis 1991). –The region surrounding the volcano is geothermally active springs ranging from ~17-37 C and elevated heat flow (Hock et al. 2002). Despite sub-freezing air temperature and a 80 cm ice cover, summit lake has ~6 C bottom water (Leach 1986) –Summer surface water ~4.9 C, salinity ~1.05 ppt (Hock et al. 2002)
…H 2 O physics …bottom water temperature should equal the temperature of maximum density for water under these conditions. (S,T,p) freshwater has max~1.00 g/cc at ~4 °C T max(S,p) Licancabur (~4 m depth) waters have predicted T max~3.74 °C Licancabur: T max~3.74 Sea level freshwater: T max~4.00 Licancabur: Tobs~6.00
GOAL: provide a quantitative physical explanation for a temperature anomaly observed at Licancabur Volcano crater lake. The site: Volcan Licancabur Hypotheses & tests 1. Measurement error 2. Heliothermic 3. Volcanic Analysis Modeling lake mass, energy balance Proposed future work
1.Measurement error – there is no temperature anomaly. 2.Heliothermic – saline bottom waters are heated by solar insolation and sediment radiative cooling. 3.Volcanic – the lake hosts a diffuse hydrothermal system that supplies energy and fluid to the system. Hypotheses Measured bottom water temperature at Licancabur is ~2 °C warmer than predicted T ρmax for lake water
1. Measurement error? Leach 1986: –Difficult conditions Diver, early spring Overlying 80 cm ice cover –Instrument accuracy (d, T) unknown 2. Heliothermic? Saline bottom waters heated by sun –Thermal-density instability is prevented by an increased solute concentration (Wetzel 2001). –Only a very small increase in salinity is required to explain the observed temperature anomaly Example: Hot Lake, Washington. Even under ice cover, the bottom temperature of this ~4 m deep lake in a salt mine reaches 30 °C! (after Kirkland et al. 1983) ρ S=1.05 = ρ S=2.0 = ρ S=1.05 =
: Simplified model of a crater lake atop a passively degassing volcano. From the International Association of Volcanology and Chemistry of the Earth’s Interior Committee on Volcanic Lakes website: As a surface expression of terrestrial degassing and the interaction between the Earth’s mantle and hydrosphere, volcanic lakes host unique physical, chemical, and biological environments. “Neutral-dilute” problem: Volcanic lakes within dormant craters, --may be virtually indistinguishable from a typical freshwater reservoir (e.g. Crater Lake, OR) No fumaroles Diffuse, not discrete (seafloor-type) venting. Low T, neutral pH, low dissolved solids content Address with physical modeling 3. Volcanic
Analysis Hypothesis Water columnConductive heat flow Mass, energy balance model Measurement error Heliothermic Volcanic T(zmax): ~4 °C T(zmax): ~6 °C n/a S(z): salinity-based stratification S(z): well mixed. Low bottom water salinity Seasonally-dependent heat flow Heat flow sufficient to drive water column convection n/a Isothermal profile Seasonally-independent mixing Acidic bottom water Tw(z): increase w/o mixing Elevated heat flow Low heat flow w/o observed thermal fluid input Volcanic inputs as unknowns… Net outflow No determinable net flow, net inflow
GOAL: provide a quantitative physical explanation for a temperature anomaly observed at Licancabur Volcano crater lake. The site: Volcan Licancabur Hypotheses & tests Modeling lake mass, energy balance Terms, equations Results Proposed future work
Mass balance W met = W evap + W out Energy balance E sw + E lw = E rad + E evap + E cond + E met + E out Lake waters Precipitation Evaporation Seepage, outflowVolcanic input? Solar/atmospheric radiation Radiative cooling Evaporation, conduction Groundwater, snow “Drainage” loss Observations of stability on ~10 year timescale: assume hydrologic and energetic steady state.
Terms in the balance… Term DependenceAssumptions W met I, A c I~200 mm y -1 ; Pasternack and Varekamp 1997; Nunez et al W evap E evap Pasternack and Varekamp 1997 W out W out =0 E sw φ Linacre 1992 E lw T air, CC(φ,z); Linacre 1992 E rad TwTw T w ~5 ºC; Davies et al. 1971, Henderson-Sellers 1986 E evap (T w -T air ), W, (e s - e 2 ) T w ~5 ºC; W~6 m s -1 ; Ryan and Harleman 1973 E cond E evap Brown et al E met A c, I, (T w -T precip ) T precip =0 C; Pasternack and Varekamp 1997 E out W out, HW out =0
2002 Results [Hock et al. 2002, Hock et al. 2003] Model: –May support volcanic hypothesis—input on the order of ~10 6 W and a few m 3 H 2 O/day. Field data needed. Water chemistry –first measurements! –pH~8.5, TDS~1.05 ppt –Rock forming elements (Fe, Al, Mg, others) enriched wrt local geothermal, meteoric waters –Also enriched in SO 4, Cl, F—principal anions found in magmatic hydrothermal fluids
GOAL: provide a quantitative physical explanation for a temperature anomaly observed at Licancabur Volcano crater lake. The site: Volcan Licancabur Hypotheses & tests Modeling lake mass, energy balance Proposed future work Constrain model using field data 2003 field campaign, beyond
Constrain model using field data 1) Mass outflux by seepage and outflow = 0 2) Air temperature and cloud cover average functions of latitude and elevation (Linacre 1992) Readout temperature loggers 3) All meteoric input at 0 C Install meteorology station; measure precipitation and account for latent heat of melting in model 4) The lake remains unfrozen Readout surface water temperature logger 5) Vapor pressure approximation assumes year-round temperatures <0 C Readout temperature loggers 6) Average crater wind speed was estimated ~6.7 m/s Log wind speed in crater
2003 campaign Collect all of the deployed data loggers –Investigate mixing with time-dependent T(d) profiles CTD probe –Investigate heliothermic hypothesis with Deploy a simple meteorological station 1) quantify analogy between the Licancabur summit environment and paleoenvironments on Mars 2) validate data for wind speed (a critical term in evaporative flux estimates) and precipitation (critical to meteoric input estimates) Model the equilibrium chemistry of a pH 8.5 freshwater body in contact with andesitic sediments Analog to Mars –quantify the environmental parameters that underlie the analogy to ancient Mars and, in particular, martian paleolakes—compare with climate models? Scout additional sites; adaptations of biology; human physiology; education and public outreach…
Summary As one of the highest lakes on Earth and an end-member of the physical environments on Earth where lakes and liquid water are stable, the Licancabur crater lake is of considerable interest to terrestrial limnology, biology, and volcanology. My proposal represents the first thorough characterization of this environment and a quantitative physical explanation for the anomalous warmth of its waters.
Energy balance terms TermExpressionReference Incident shortwave radiation (solar) [W/m 2 ]: E sw φ-0.22φ φ 3 Linacre 1992 Incident longwave radiation from atmosphere [W/m 2 ]: E lw (208+6T air )( C 2 )Linacre 1992 Longwave radiative (blackbody) loss [W/m 2 ]: E rad ε w σT w 4 Davies et al. 1971; Henderson-Sellers 1986 Evaporation energy flux [W/m 2 ]: E evap [2.7(T lv -T av ) 1/3 +3.2W 2 ](e s -e 2 )Ryan and Harleman 1973 Conductive heat loss [W/m 2 ]: E cond 0.61[(T lake -T air )/(e s -e 2 )]E evap Brown et al Precipitation energy flux [W/m 2 ]: E meteoric aI(T lake -T precip )c p Pasternack and Varekamp 1997 Mass balance terms TermExpressionReference Precipitation mass flux [m 3 /day]: W meteoric IA c Pasternack and Varekamp 1997; Nunez et al Evaporative mass flux [m 3 /day]: W evap E evap /abPasternack and Varekamp 1997
If we assume that the source water for these features have similar composition, then enrichment in rock forming elements may be representative volcanic hydrothermal fluid input as fluid flowing up to the summit is allowed more time to react with local lithologies. Since solute enrichment is not uniform across the analytes in the summit lake waters, it is unlikely that this chemistry is a result of evaporative concentration alone.
Schematic “box model” of energy and mass balance in a volcanic crater lake; the terms represent those used for this model. The two volcanic input arrows at the bottom of the lake represent unknowns, and are solved for in the model. Wout and Wseep are set to zero as a conservative estimate. Physicochemical classification scheme for volcanic lakes (from Pasternack and Varekamp 1997). Dashed lines indicate physically-imposed thresholds; representative temperature (T) and total dissolved solids (TDS) values are given. Volcanic lake systematics Physical and chemical differences between lakes reflect the complex interaction between volcanic (e.g. the timescale and intensity of volcanic heat and fluid input) and nonvolcanic (e.g. atmospheric conditions, precipitation) phenomena Given a crater that can hold water, a volcanic lake in steady state requires an energetic and hydrologic balance between volcanic heat and mass input and output to the environment.
Thermopile temperature gradient probe deployment (buried probe top indicated by red arrow). Surface and underwater soil heat flux measurements were made using this lightweight, high-sensitivity probe at lower elevation lagunas and hot springs. Preliminary calculations show conductive heat flux values ranging from near global average (~0.06 W/m 2 ) to nearly two orders of magnitude greater near the hot spring.