HYDROTHERMAL ACTIVITY AND CARBON-DIOXIDE DISCHARGE AT SHRUB AND 

UPPER KLAWASI MUD VOLCANOES, 

WRANGELL MOUNTAINS, ALASKA

By Michael L. Sorey, Cindy Werner, Robert G. McGimsey, 

and William C. Evans 

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 00-4207 

Menlo Park, California 

2000 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

For additional information write to: 

William Evans 

U.S. Geological Survey 

345 Middlefield Road 

Menlo Park, California, 94025 

i
CONTENTS 

ABSTRACT 1

INTRODUCTION 1

1999 ACTIVITY 4

Shrub Mud Volcano 4

Upper Klawasi Mud Volcano 4

MEASUREMENT TECHNIQUES 5

Gas and Water Sampling 5

Vent Degassing 5

Diffuse Gas Flux from Soils 6

RESULTS AND DISCUSSION 6

Gas and Water Compositions 6

Vent Gas Discharge 10

Diffuse Gas Discharge 10

CONCLUSIONS 11

ACKNOWLEDGMENTS 12

REFERENCES 12

FIGURES 

1. Map of the Glennallen, Alaska region 2 

2. Map of Shrub mud volcano 3 

3. Photograph of Vent #42 in the Main Vent Area at Shrub mud 4 

4. Photograph of summit crater area on Upper Klawasi mud volcano 5 

5. Photograph of Vent #3, in the Main Vent Area at Shrub mud 5 

6. Photograph of equipment used to measure the rate of degassing from vent #6 6 

7. Map of diffuse carbon dioxide flow from soils near the summit of Shrub mud volcano 9 

TABLES 

1. Chemical analysis of gas samples collected in June 1999 from bubbling spring vents at Shrub and Upper Klawasi 

mud volcanoes 7 

2. Chemical analyses for water samples collected in June 1999 and July 1973 at Shrub and Upper Klawasi mud 

volcanoes 7

3. Chemical analyses for water samples collected in June 1999 and July 1973 at Shrub and Upper Klawasi mud 

volcanoes 8

 ii 
HYDROTHERMAL ACTIVITY AND CARBON-DIOXIDE DISCHARGE AT SHRUB 

AND UPPER KLAWASI MUD VOLCANOES, 

WRANGELL MOUNTAINS, ALASKA 

By Michael L. Sorey, Cindy Werner, Robert G. McGimsey, and William C. Evans 

ABSTRACT 

Shrub mud volcano, one of three mud volcanoes of

the Klawasi group in the Copper River Basin, Alaska, 

has been discharging warm mud and water and CO2

–rich 

gas since 1996. A field visit to Shrub in June 1999 found 

the general level of hot-spring discharge to be similar, 

but somewhat more widespread, than in the previoustwo

years. Evidence of recent animal and vegetation deaths 

from CO2 exposure were confined to localized areas 

around various gas and fluid vents. Maximum fluid 

temperatures in each of three main discharge areas, 

ranging from 48-54°C, were equal to or higher than 

those measured in the two previous years; such 

temperatures are significantly higher than those 

observed intermittently over the past 30 years. At Upper

Klawasi mud volcano, measured temperatures of 23-

26°C and estimated rates of gas and water discharge in 

the summit crater lake were also similar to those 

observed in the previous two years. Gas discharging at 

Shrub and Upper Klawasi is composed of over 98% CO2 

and minor amounts of meteoric gases (N2, O2, Ar) and 

gases partly of deeper origin (CH4 and He). The rate of 

CO2 discharge from spring vents and pools at Shrub is 

estimated to be ~10 metric tonnes per day. This 

discharge, together with measured concentrations of 

bicarbonate, suggest that a total CO2 upflow from depth 

of 20-40 metric tonnes per day at Shrub. 

Measurements were made of diffuse degassing 

rates from soil at one ~300 m2

 area near the summit of 

Shrub that included vegetation kill suggestive of high 

CO2 concentrations in the root zone. Most of measured 

gas flow rates in this area were significantly higher than 

background values, and a CO2 concentration of 26 

percent was measured at a depth of 10 cm where the gas 

flow rate was highest. Although additional 

measurements of diffuse gas flow were made elsewhere 

at Shrub, no other areas of vegetation kill related to 

diffuse degassing and high soil-gas CO2

 concentrations 

could be seen from the air. 

Chemical and isotopic compositions of the gas and

water discharging at Shrub and Upper Klawasi indicate 

derivation from a com-bination of mantle (magmatic) 

and crustal (marine sedimentary rock) sources and 

suggest a common fluid reservoir at depth. In particular, 

both the total dissolved carbon and values of 13C in CO2

are similar for fluids and gas sampled at each area, and 

do not appear to have changed with the onset of 

increased spring temperatures and fluid discharge at 

Shrub. This suggests that the underlying cause of the 

recent changes in discharge rate and temperature at 

Shrub is not an increase in the rate of input of magmatic 

heat and volatiles, but rather increases in the 

permeability of the upflow conduits that connect the gasrich reservoir to the surface. 

INTRODUCTION 

Shrub mud volcano is situated in the Copper 

River Basin some 27 km east of Glennallen, Alaska 

(figs. 1 and 2). The first known description of Shrub 

and its activity was presented by Nichols and Yehle 

(1961), who visited the volcano in 1955 and 1956. At 

that time, a vegetation-free basin containing numerous 

small, gassy spring-fed pools was located in the area 

now occupied by the active vents in the Main Vent 

Area (fig. 2). Gas and water samples were collected 

from a mineral spring near the summit of Shrub in 

July 1973 by Ivan Barnes (U.S. Geological Survey), 

whose unpublished field notes list a vent temperature 

of 18°C. A brief examination of Shrub in August 1991 

revealed no spring activity, nor any indication of 

recent activity (Richter and others, 1998a). In the 

spring of 1997, however, Shrub was observed to be 

vigorously venting potentially dangerous CO2–rich gas 

and warm saline mud. This thermal discharge may 

have begun on a smaller scale in 1996, according to 

reports from a helicopter pilot who observed active 

mud springs low on the north flank of the cone (fig. 2) 

and possible activity on the summit area (Richter and 

others, 1998a). 

Since June 1997, the U.S. Geological Survey 

(USGS) and the National Park Service have monitored 

the activity at Shrub and nearby Upper Klawasi mud 

volcano. Site investigations were conducted on June 21 

and 30, 1997 and August 31, 1998, and an aerial 

inspection was made on December 2, 1997. During the 

visit on June 30, 1997, gas samples were collected from 

a vent near the summit of Shrub (Richter and others, 

1998a,b). Video surveys of spring activity at Shrub and 

Upper Klawasi were made in June 1997 and July 1998. 

1
Shrub

Gulkana

Glennallen

Glen Hwy

Richardson

Copper

 Center

Upper

Klawasi

MUD

VOLCANOES

Lower

Klawasi

Mt. Drum

05 10 15 MILES

0 5 10 15 20 KILOMETERS

N

Copper

Tazlina River

AK

Glennallen

62°

145°

Highway

River

Base from U.S. Geological Survey Gulkans A-3 quadrangle, Alaska; mud deposits from U.S. Bureau of Land Management aerial 

photography taken August 14, 1988

Figure 1. Index map of the Glennallen, Alaska region showing locations of Shrub, Upper Klawasi, and Lower Klawasi mud 

volcanoes and Mt. Drum (from Richter and others, 1998a). 

One particularly striking feature was the appearance 

of narrow bands of alder and birch browned to heights of 

as much as 2 m above the ground surface, extending 

downhill toward the northwest from a vent area on the 

north flank of the Shrub (fig. 2). On June 21, 1997, this 

vent area consisted of a ~5 m-diameter pit filled with 

bubbling mud; Richter and others (1998a) speculated 

that the bands of browned vegetation may have been 

caused by significant amounts of CO2

–rich gas pouring 

out of the pit into rivers of gas. 

Video surveys were initially used to map the extent 

of mudflows that occurred over the 1996-1998 period 

(Richter and others, 1997a,b). Subsequent mapping (fig.

2) represents revisions based on aerial photo-graphs 

taken in August, 1998. Most of the 1996-1997 mud 

flows originated from violently discharging vents 

located just below the summit. During visits in the 

summer of 1998 and 1999, mud and gas were quietly 

discharging from bubbling mud pools at these same vent 

areas. Total mud production since 1996 is estimated at 

500,000 m3

 (Richter and others, 1998b). 

Shrub is one of three large mud volcanoes in the 

Copper River Basin that are collectively referred to as 

the Klawasi group. Each mud volcano is elevated some 

50-100 m above the surrounding terrain and constructed

entirely of erupted material derived from the underlying 

glacio-lacustrine deposits (fig. 2). The mud volcanoes 

occur approximately 20 km west of Mt. Drum, a large 

Pleistocene volcano that was last active about 240,000 

years ago, and about 60 km northwest of Mt. Wrangell, 

an active shield volcano (Richter and others, 1994). 

Shrub was virtually inactive prior to 1997. Minor 

activity was observed in the mid-1950’s; Nichols and 

Yehle (1961) report the occurrence of small, gassy pools 

in a basin some 30 feet below the summit in July 1956, 

and list a temperature of 12°C for these features. A water 

sample was obtained from a summit “mineral spring” in 

July 1973, but this feature was reported dry during a 

visit in August 1981 (Motyka and others, 1989). 

The other two mud volcanoes in this group, Upper 

and Lower Klawasi, have been active throughout at least 

the past 40 years and have yielded temperature and 

chemical data on the mud and gas discharges (Nichols 

and Yehle, 1961; Grantz and others, 1962; Reitsema, 

1979; and Motyka and others, 1989). These 

investigations showed that Upper and Lower Klawasi 

2
periodically erupt mud at temperatures of 20-25°C and 

mixed with Ca-poor, Na-HCO3 -rich saline water and 

CO2–rich gas. The chemistry and gas composition of 

water discharged at Shrub is similar to that at the other 

Klawasi vents, but since 1996 has occurred at higher 

temperatures (40-54°C). Motyka and others (1989) 

suggest that the warm saline water is connate in origin 

and that the CO2 derives from degassing of deep-seated 

magma and contact decarbonization of underlying 

Mesozoic limestone beds. At least one of the oil and gas 

exploration wells drilled near the town of Glennallen 

encountered fluid pressures in excess of hydrostatic at 

depths of 1,700 m during drilling through the Mesozoic 

sequence (Reitsema, 1979). Bottom-hole temperatures in

this and other deep exploration wells in the region range 

from 49-59°C (Motyka and others, 1989), similar to 

present-day vent temperatures at Shrub. 

3

Figure 2. Map of Shrub mud volcano showing vent locations and temperatures on June 23, 1999, extent of mud deposits in 

August (from Richter and others, 1998b), and inset sketches of vent areas (not to scale). Approximate location of the area of 

diffuse gas flow measurements shown in figure 7 is denoted by an open rectangle between the Main Vent and Fissure Vent 

area. 

1996 Mud

Dead bird or animal

1997 Mud

Old

Mud/Gas

pit

Probably includes

1996 mud

Bands of C -affected

vegetation in 1997

O2

Stream muddy to

Copper River,

approx. 15 miles

to west

+

2943

3

10 11

2

Lake

Fissure Vent

1999

0 500 1000 1500 FEET

0 100 200

Contour Interval 100 ft.

300 400 500 METERS

gassy

spring

Old

Mud

Dry

4-5 m wide

~ 5 m deep

30 m

54°C

Mud/Gas Pit

1999

Dead bird

in small pit

Dry

Large

upwelling

vent Minor

bubbles

+

25-30 m

~ 4 m

48.9°C

+

Undercut

1.5 x 1 m

8 x 10 m

1.8 x 2 m 1.2 x 7 m

2 m deep

1.2 x 0.8 m

20 cm

2-3 m deep

~ 40 m

41.9°C

15.7°C

44.1°C

48°C

14.2°C 9.5°C

10°C

41.3°C

4

1

2

3

7

5

6

Main Vent Area

1999

2800

2600

N

+

2700

Numerous small

mud vents
1999 ACTIVITY 

Shrub Mud Volcano 

Shrub mud volcano was visited on June 23 and 24, 

1999, at which time the general level of activity (mud, 

water, and gas discharge) was observed to be less 

intense, but more widespread than during visits the 

previous two years. Several new thermal and nonthermal

spring vents were noted near the summit. Measured 

vent temperatures around the summit and north flank 

were slightly higher than in previous years. Each of the 

discharging pools and streams appeared to carry 

significant mud loads, and although there was no active 

mud fountaining or erupting, mud from recent eruptions 

was splattered against trees around several vents (fig. 3). 

The map in fig. 2 is identical to that in Richter and 

others (1998b), except for the addition of inserts 

depicting details of each of the three main discharge 

areas on or near the summit - Main Vent Area (MVA) 

on the west side, Fissure Vent Area (FVA) on the east 

side, and Mud/Gas Pit Area (MGPA) on the north side 

below the summit region. Particularly noteworthy 

changes in activity at Shrub between 1999 and previous 

years include: 

1. The appearance ofseveral cold, degassing pools at the 

MVA (vents #5, #6, and #7). 

2. The disappearance of gas/mud/water discharge from 

the collapse pit at the MGPA. 

3. The appearance of a new gassy hot spring 30 m 

downhill from the collapse pit at MGPA with a 

temperature (54°C) that was 5-7°C hotter than in any 

of the MGPA and FVA vents visited in 1997 and 

1998. 

We found carcasses of dead birds and snowshoe 

hares on the inner slopes of several degassing pits (for 

example vents #4 and #5 at MVA) and a few small 

holes, or point sources, with cold CO2 gas venting 

through the soil. At one location between MVA and 

FVA, such a vent was surrounded by a ~300 m2 area of 

soil with anomalously high diffuse gas flow rates (up to 

5,600 g/d/m2

) and a spring seep discharging water at 

13°C. In 1997, this same area contained several small 

(1-2 cm diameter) holes audibly venting CO2

, and in 

1998 vegetation kill and mud seeps accompanied water 

and gas discharge from a small vent in the same area. 

A few diffuse gas flow measurements made at the 

MGPA were all anomalously high (~400 g/d/m2

), 

including those made at a site ~30 m southwest of MGPA 

where there was a stand of alders with dead or wilting 

leaves, most prevalent on the lower branches. These 

conditions appeared to extend over a small isolated area, 

unlike the elongated zones of browned vegetation 

observed in 1997 farther down the north flank. A possible 

explanation for the leaf kill observed in 1999 is that 

elevated CO2

 concentrations in the root zone affected 

oxygen and nutrient uptake by the tree roots. 

Upper Klawasi Mud Volcano 

During our visit on June 24, 1999, the general level 

of activity at Upper Klawasi mud volcano was similar to 

that during visits the previous two years. Measured 

temperatures in the summit pool at Upper Klawasi 

ranged from 23-26°C. Temperatures of 29-31°C in this 

pool had been measured prior to, and during visits in 

1998, and Motyka and others (1989) report temperatures 

of 13-19°C for pool samples collected in the 1980's. Gas 

discharge occurs primarily from vents that produce a 

general upwelling near the center of the 30-40 m 

Figure 3. Vent #42, a 44°C gassy pool in the Main Vent

Area at Shrub mud volcano (see Figure 2 for location). Photo

shows mud spattered on the branches of adjacent trees from

previous eruptions in this vent. 

4
diameter pool; upwelling was observed to occur every 

20-25 minutes during our visit on June 24, 1999. The 

size of the roil created by this upwelling grew with time 

during each cycle to cover approximately half of the 

total area of the pool (fig. 4). Surrounding the roil was a 

10-m-wide zone of dark, floating organic fluid. 

An old rim, ~10 m from the edge of the current 

pool, rises about 5 m above the pool level. A dozen orso

mature spruce trees were dead around the edge of the 

pool (inside this older rim) and in the outflow zone from 

the pool. Fresh deposits of gray mud filled much of 

the outflow channel. Measurements of diffuse degassing 

through the soil both inside and outside the old crater 

rim showed no evidence of anomalous discharge rates. 

MEASUREMENT TECHNIQUES 

Gas and Water Sampling 

Gas samples were collected from thermal pools at 

MVA (vent #3, fig. 5), FVA (the upwelling vent noted in 

fig. 2), and the 54°C spring in the MGPA. Each sample 

was collected using a funnel placed over a bubbling vent 

and connected by tygon tubing to an evacuated glass 

sample bottle. After purging air from the sampling line 

using either a hand pump or mouth suction, the outlet of 

the sample bottle was closed and the inlet opened. Gas 

composition and carbon isotopic values ( 13C) were 

determined at the USGS laboratories in Menlo Park, 

California, using methods described by Evans and others

(1981). 

Water samples were collected from the same vents

noted above, and from the cold degassing vent at MVA 

(vent # 6, fig. 2). Bulk water samples were filtered by 

hand in the field, but because of vigorous degassing in 

the spring vents, pH values were not determined in the 

field. Instead, values of pH, specific conductance, 

alkalinity, dissolved inorganic carbon (DIC), and CO2

concentration ([CO2

]) were determined in the laboratory, 

along with anion concentrations in each water sample. 

We then corrected the measured CO2 and DIC 

concentrations for the effects of gas loss by computing 

pH values expected for CO2 saturation at the measured 

vent temperatures and elevations (~3,000 ft above sea 

level). In previously published geochemicalstudies, only

laboratory pH values are reported. 

Vent Degassing 

Figure 4. Summit crater area on Upper Klawasi mud 

volcano, showing formation of a roil created by periodic 

upwelling of warm (23°C), CO2

-charged water. 

Figure 5. Vent #3, a 48°C gassy pool adjacent to a much 

larger pool (vent #2) in the Main Vent Area at Shrub mud 

volcano (see figure 2 for location). Photo shows the filling 

of the larger pool with smoke from a smoke bomb set off in 

vent #3 in order to visualize the overland flow of CO2. 

The rate of degassing from vent #6 at MVA was 

measured with equipment consisting of a large cylinder 

of flexible plastic fitted at the top with a smaller cylinder

into which a hot-wire anemometer was inserted for gasvelocity measurements (fig. 6). The degassing rate at 

this site required that the exit area of the top cylinder be 

reduced in order to increase gas exit velocity. This was 

5
Figure 6. Equipment used to measure the rate of degassing from 

cold-temperature vent #6 (see figure 2 for location). Gas flowed 

upward through the plastic cylinder and exited through a 

smaller-diameter neck at the top, where its velocity and 

temperature were measured. The mass flow rate per unit 

surface area was 0.46 t/d/m2. 

accomplished with duct tape. The gas discharge 

(volumetric) was then computed as the product of the 

velocity and exit area. Assuming that most of the gas is 

CO2

, the mass discharge was determined from the 

density of CO2

 at the vent temperature. Although this 

equipment could not be used directly at the thermal 

vents because the approach conditions were too 

dangerous, we were able to extrapolate the results from 

vent #6 to the other gassy pools using ratios of the pool 

areas to that of vent #6. 

Diffuse Gas Flux from Soils 

Measurements of diffuse gas flow rate from the 

soil were made with an accumulation chamber placed on 

the soil and connected by a gas-pumping system to an 

infrared CO2 analyzer. This equipment is portable and 

has been extensively used at other volcanic areas to map

the distribution and magnitude of diffuse gas flow (see, 

for example, Sorey and others, 1999). We made 

individual measurements of diffuse flow at some 10 sites 

around Shrub and a more detailed grid of 24 

measurements in the region between MVA and FVA 

(location of the grid given by the open square near the 

summit in fig. 2). Rates of diffuse gas flow are measured 

in units of grams per day per square meter (g/d/m2

), 

whereas total flux over a discharge area is specified in 

metric tonnes per day (t/d). 

At the grid-measurement area, we also 

measured the concentration of CO2 in soil gas at a 

depth of 10 cm. For this, we inserted a perforated steel 

probe and connected it with tygon tubing to an 

infrared meter with an internal pump. This same meter 

was used to check for dangerous gas concentrations in 

various depressions both before and after entering. No 

such concentrations where found. 

RESULTS AND DISCUSSION 

Gas and Water Compositions 

 Results of chemical and isotopic analyses of 

water and gas samples are listed in tables 1 and 2. Gas 

compositions are reported in volumetric percent of dry 

gas normalized to 100%. Samples from MVA, FVA, 

MGPA, and Upper Klawasi contain over 98% CO2 and 

minor amounts of N2

, O2 , Ar, and He. Methane 

contents were quite low (<0.02%) in all but the 

samples from Main Vent Area (0.47%) and the Fissure 

Vent Area (0.08%). The gas compositions are virtually 

the same as those determined for a vent sampled at the 

FVA in 1997 (Richter and others, 1998a). 

 Waters from the thermal vents at Shrub and the 

summit vent at Upper Klawasi are saline, containing 

high concentrations of chloride and bicarbonate and 

moderate concentrations of sulfate (table 2). This is also 

true for water issuing from the cold degassing pool at 

Shrub, although lower anion concentrations suggest 

dilution with fresher water (table 2). Cation 

concentrations were not measured on the 1999 samples. 

Analyses of previous samples from Upper Klawasi and 

Shrub show high ratios of Na/Ca relative to seawater 

(Motyka and others, 1989). These characteristics have 

been interpreted as resulting from addition of magmatic 

CO2 to connate waters in marine sedimentary 

formations, causing water-rock reactions with 

limestones and silicates and calcite pre-cipitation 

(Motyka and others, 1989). 

6
Table 1. Chemical analysis of gas collected in June 1999 from bubbling vents at Shrub and Upper Klawasi mud volcanoes 

[MGPA, Mud/Gas Pit area (54°C spring in figure 2); FVA, Fissure Vent area (large upwelling vent in figure 2); MVA, Main 

Vent area (feature #3 in figure 2); values in normalized volume percent] 

Gas Component MGPA FVA MVA Upper Klawasi1

He 0.0002 0.0007 0.0012 <0.0002 

H2 <0.0002 <0.0002 <0.0002 <0.0002 

Ar 0.0022 0.0040 0.0021 0.0253 

O2 0.0873 0.0896 0.0643 0.5020 

N2 0.1762 0.4561 0.5699 1.7792 

CH4 <0.0002 0.0804 0.4681 0.0020 

CO2 99.7341 99.3686 98.8938 97.6916 

C2H6 <0.0002 0.0005 0.0006 <0.0002 

H2S <0.0005 <0.0005 <0.0005 <0.0005 

CO <0.001 <0.001 <0.001 <0.001 

C3H8 <0.0005 <0.0005 <0.0005 <0.0005 

C4H10 <0.0005 <0.0005 <0.0005 <0.0005 

1Sample taken from bubbling vent near northern shoreline of a muddy, water-filled summit crater. 

Table 2. Laboratory values of chemical analyses for water samples collected in June 1999 and July 1973 at Shrub and Upper Klawasi mud 

volcanoes 

[Concentrations expressed in mg/L; MVA, Main Vent area (feature #3 in figure 2); FVA, Fissure Vent area (large upwelling vent 

in figure 2); MGPA, Mud/Gas Pit area (54C spring in figure 2); Shrub cold spring, feature #6 in figure 2, --, no data] 

Site Temp1 pH Alkalinity CO2 DIC3 Cl SO4 Br 

 (°C) 

Shrub MVA 20 7.73 7,442 220 1,520 11,100 340 27 

Shrub FVA 20 7.79 9,644 264 1,970 10,200 460 24 

Shrub MGPA 20 7.67 9,644 352 1,990 10,000 470 24 

Shrub cold spring 20 6.83 7,716 1,960 2,050 8,880 260 22 

Upper Klawasi2 20 7.52 8,857 480 1,872 11,300 500 28 

Shrub spring4 18 7.20 10,000 1,040 2,240 10,000 460 -- 

1All temperatures are lab measurements except for a field value listed for Shrub Spring. 

2Upper Klawasi sample taken from bubbling vent near northern shoreline of a muddy, water-filled summit crater. 

3Dissolved inorganic carbon, in mg/L = (alkalinity/61 + CO2/44) x 12. 

4Sample from mineral spring near summit collected July 9, 1973 by Ivan Barnes and analyzed by the U.S. Geological Survey, Menlo Park, California.

7
Table 3. Chemical analyses for water samples collected in June 1999 and July 1973 at Shrub and Upper Klawasi mud volcanoes, with 

parameters corrected for field conditions, assuming pH values for CO2

 saturation at 0.9 atmospheres (i.e. for an elevation of 3,000 feet 

above sea level) 

[Concentrations expressed in mg/L; MVA, Main Vent area (feature #3 in Figure 2); FVA, Fissure Vent area 

(large upwelling vent in figure 2); MGPA, Mud/Gas Pit area (54°C spring in figure 2); Shrub cold spring, feature 

#6 in figure 2] 

Site T pH Alkalinity CO2 DIC 

 (°C) (mg/L) (mg/L) (mg/L)

Shrub MVA 48 7.2 7,440 818 1,690 

Shrub FVA 49 7.22 9,640 818 2,120 

Shrub MGPA 54 7.23 9,640 800 2,110 

Shrub cold spring 6 6.82 7,720 2,490 2,200 

Upper Klawasi 23 7.01 8,860 1,430 2,140 

Shrub spring1 18 7.02 10,000 1,670 2,420 

1Sample collected from mineral spring near summit in July 1973 by Ivan Barnes (U.S. Geological Survey).

The computed pH values for field conditions are 

significantly lower than those measured in the laboratory

for all but the cold vent at MVA (table 3). For the 

thermal vents, these differences reflect the fact that pH 

increases as gas is lost during and after sampling. The 

lower pH and higher gas concentration under field 

conditions also results in higher (computed) values of 

total inorganic carbon, although the effect on DIC 

concentrations is less than on CO2 con-centrations 

because most of the DIC exists as bicarbonate. For the 

cold vent at MVA, the computed field pH is virtually the

same as the laboratory value, so that measured and 

computed CO2 and DIC concentrations are much closer. 

In this case, the effect of correcting for the difference in 

temperature between field (6°C) and laboratory 

conditions (20°C) is to offset the effect of gas loss 

during and after sampling, because raising the 

temperature also raises gas pressure and lowers the pH 

required for gas saturation. 

Motyka and others (1989) report a laboratory pH 

value of 7.2 and an alkalinity of 10,000 mg/L for a July 

1973 sample from the Shrub “mineral spring” (table 2). A 

vent temperature of 18°C was measured (unpublished field 

notes by Ivan Barnes), but was not reported by Motyka and 

others (1989). Comparison of the chemistry of this sample 

with that of present-day thermal waters shows that they are 

basically the same water. In terms of our computed 

parameters such as field pH and dissolved CO2 

concentration, the colder temperature “mineral spring” 

sampled in 1973 would have held more gas at a lower pH, 

and hence would have held a slightly greater amount of 

total dissolved carbon than our 1999 thermal spring 

samples. This suggests that higher vent temperatures at 

present result in more gas loss as the fluid approaches the 

land surface. 

Carbon isotopic values, expressed as 13C for CO2, 

range from –4.7 to –5.5‰ for gas sampled in 1999 at 

Shrub and Upper Klawasi. For comparison, Richter and 

others (1998a) obtained a value of –4.64 for gas sampled 

from a vent in the FVA in 1997, while Motyka and others 

(1989) list values of –4.3 to –4.8‰ for gas sampled at 

Upper Klawasi and Shrub (1973-1985). These data indicate 

a narrow range in 13C for gases from each mud volcano 

and show that values for our 1999 samples are similar to 

those determined in previous years. 

Comparison of the computed results for different 

vents shows that DIC values are remarkably similar for 

each feature sampled at Shrub and Upper Klawasi. The 

cooler vents (“Shrub cold spring” and “Shrub spring”), 

however, carry significantly more CO2 than the hotter 

vents because of the inverse relation between CO2

saturation and temperature. For vent #6 at MVA, the 

concentration of dissolved CO2, in mg/L, exceeds the corresponding concentration of DIC. This, of course, would 

not be the case if both quantities were expressed in units of 

moles/L. 

Motyka and others (1989) also list 13C values of –

6.5 to –6.7‰ for fumarolic gas sampled from the North 

Crater of the Mt. Wrangell summit caldera, some 60 km to 

the northeast of the Klawasi group mud volcanoes. Citing 

corresponding values of 3He/4He for this vent of 5.3 and 

8
1

2

3

4

5

6 7

8 9

10

11 12 13

14

15

16

17

18

19

20

21

22 23

24

S

S

S

S S

246 8 10 12 14

2

4

6

8

10

12

14

16

18

20

22

Meters

CO2 Flow Rate g/d/m2

M

eters

50

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

1 5,600

2 230

3 188

4 100

5 5,500

6 5,600

7 133

8 1,360

9 172

10 1,845

11 1,140

12 1,035

13 10

14 44

15 906

16 43

17 18

18 10

19 10

20 200

21 177

22 234

23 50

24 161

S 0

S 0

S 0

S 0

S 0

Co

Site Flow

# (g/d/m )

2

2

Figure 5. Results of measurements of diffuse carbon dioxide flow from soils near the summit of Shrub mud volcano made on 

June 24, 1999. See figure 2 for the approximate location of the measurement grid. A commercially available grid-based 

contouring program was used to contour the data and to compute a total gas flux of 0.23 t/d. Five synthetic values of gas flow 

(labeled S) were added in order to better constrain the contours to the southeast and southwest. 

9
6.0 Ra, these authors argue that the Mt. Wrangell gas 

represents the probable composition of mantle volatiles in 

this region. Lower helium isotopic ratios (2.1-4.1 Ra), 

including a value of 3.54 for Shrub (Richter and others, 

1998a) and heavier 13C values in the gases from the 

Klawasi group mud volcanoes (Motyka and others, 1989) 

suggest an additional component from crustal sources 

and/or carbon-isotope fractionation associated with the 

formation of bicarbonate and precipitation of calcite. 

Vent Gas Discharge 

The total CO2 discharged from spring waters at the 

summit of Shrub mud volcano is the sum of CO2 dissolved

in the spring-fed outflow streams and CO2 bubbles 

discharging directly from the vents. One way to compute 

the total CO2 discharge is to multiply an average value of 

CO2 concentrations dissolved in the vents (field 

conditions) by the total flow rate of streams draining these 

vents. From table 3 we would use concentrations of 820 

and 2500 mg/L, respectively, for gas content under field 

conditions at the hot and cold-water vents. Then, for visual 

estimates of a total hot-water flow of 50 L/s and a total 

cold-water flow of 10 L/s, we compute a gas discharge of 

~7 t/d. 

An alternative approach is to compute the rate of gas 

loss occurring directly at the pools and spring vents, using 

our measured degassing rate at the cold vent #6 at the 

MVA and measurements and estimates of the areas of 

bubbling pools. At vent #6, we measured a degassing rate 

of 0.46 t/d from an upwelling roil over a collector area of 

0.8 m2 (fig. 6), yielding a unit area rate of 0.58 t/d/m2. At 

the thermal pools, gas is lost both from upwelling roils and 

numerous point sources of bubbles. For this calculation we 

assume that (1) the average rate of degassing from the 

thermal pools is equal to that measured at the cold pool and 

(2) the total area of degassing pool surfaces is 10-20 m2. 

This yields a range of 6-12 t/d for the total rate of CO2 

degassing from vents, a result which is comparable to the 

~7 t/d derived from estimates of stream flow rates. 

We are also interested in estimating the discharge of 

total inorganic carbon (DIC), effectively represented by the 

sum of the discharges of CO2 and bicarbonate phases. 

Note that computations involving sums of concentrations 

of different carbon species are done using molar 

concentrations and then converted to units of mg/L using 

the appropriate molecular weight (12 g/mole for carbon, 44 

g/mole for CO2 and 61 g/mole for HCO3). At the land 

surface, we measure an average DIC value of 2,100 mg/L; 

multiplying this concentration by the total fluid upflow rate

noted in the previous paragraph (60 L/s) yields a discharge 

of 10 t/d of total carbon. 

Deeper into the fluid upflow system, the fractions of 

dissolved CO2 and bicarbonate vary with pH and hence 

with depth, while the total DIC remains constant, except 

for possible loss of dissolved carbon from calcite 

precipitation. For example, at the surface where pH = 7.2, 

CO2 makes up only 10 percent of the DIC, whereas at 

depths where the pH value is as low as perhaps 5.5, the 

total dissolved carbon is distributed as 90 percent CO2 and

only 10 percent HCO3. Thus, at depths where pH is below 

5.5, essentially all of the DIC is derived from dissolved 

CO2. Under such conditions, the total carbon flux of 10 t/d 

derived in the last paragraph would be associated with a 

CO2 flux of 40 t/d. Thus, all the DIC reaching the surface 

at Shrub could be supplied by this rate of magmatic 

degassing of CO2. 

If, instead, DIC is supplied by a combination of 

magmatic degassing and reactions between this gas and 

carbonate-rich reservoir rocks, it is possible that enough 

carbon could be liberated to double the DIC flux in fluids 

flowing upward from the reservoir. Under this condition, a 

magmatic CO2 degassing rate of 20 t/d is sufficient. 

Again, however, these computations of a range of 20-40 t/d 

for the magmaticCO2 flux rate do not take into account the 

likelihood that some carbon is lost to calcite precipitation 

and travertine formation at or near the land surface. 

To place gas flux of 20-40 t/d in some context, we 

can compare with values deter-mined for Mammoth 

Mountain, California, where diffuse CO2 discharge 

through soils has resulted in extensive tree kills over some 

0.4 km2 (100 acres) of pine forest. Measurementsat 

Mammoth show that diffuse discharge, although variable 

between the different tree-kill areas, averages about 50 t/d 

at each area and totals some 300 t/d. An additional CO2 

flux of ~50 t/d leaves the mountain dissolved in the 

ground-water system (Sorey and others, 1999). 

Diffuse Gas Discharge 

Measurements of diffuse degassing at ~10 isolated 

sites on Shrub mud volcano yielded values that were each 

less than 10 g/d/m2, except in the area of vegetation kill 

between the MVA and FVA and in the general vicinity of 

the MGPA. At MGPA, values of 400-430 g/d/m2 were 

found on the banks of both active and inactive vents and in 

a small growth of alders, located ~30 m southwest of the 

54°C hot spring, where there was evidence of recent leaf 

kill. 

At the vegetation-kill area near the summit, gas-flow 

measurements were made over a grid of 24 sites covering a 

14x22 m area, or ~300 m2 (fig. 7). This area included a 

13°C spring seep (next to grid point #1) with travertine 

deposits and mud lining the outflow channel, suggesting 

higher flow rates in the past. Diffuse gas flow rates near 

5,600 g/d/m2 were measured at points 1, 5, and 6; 

elsewhere within the grid, flow rates ranged from 10 to 

5,600 g/d/m2. A CO2 concentration of 26 percent at 10 cm

10
depth was found at point #6. As seen at Mammoth 

Mountain, such high gas concentrations can cause 

vegetation kill as a result of oxygen deprivation and 

nutrient uptake inhibition within the root zone (Sorey and 

others, 1998). 

A commercially available contouring program was 

used to analyze the measured gas flow data at this grid area

on Shrub. An initial run made using only the 24 measured 

points yielded a total flux of 0.29 t/d. To compensate for 

the sparseness of the data in the southeastern and 

southwestern corners of the area, five “synthetic” grid 

points with low flow rate values were added. This yielded 

the contour map shown in fig. 7, from which a total flux of 

0.23 t/d was determined. The area inside the 50 g/d/m2 

contour is 195 m2, and the average flow rate here is 1,200 

g/d/m2. Although this total flux is rather low compared to 

gas discharge from the springs, the measured rates of 

diffuse degassing in this area are sufficiently high to 

require more than just a shallow, biogenic source. A likely 

possibility is near-surface gas loss from the conduits 

channeling water toward the surface. A more complete 

flux survey on Shrub might delineate other areas of 

anomalous flow, but it seems that such areas will be found 

only in close proximity to the gassy springs. 

CONCLUSIONS 

Visual observations suggest that total rates of 

discharge of water, mud, and gas from the summit region 

and northern flank of Shrub mud volcano have not changed 

significantly over the 1997-1999 period. There does 

appear to have been a general expansion of each of the 

areas of discharge, as the surficial conduit system has more 

fully developed, and flow rates of some vents have 

declined. The chemical characteristics of hot water and gas

sampled in 1999 were similar to published results from 

previous years, as were the carbon-isotopic compositions. 

Continuous, high flow-rate discharge of hot water and gas 

discharge from the summit region over this 3-year period is 

unique compared to activity observed intermittently over 

the past 30 years. 

Several new features (both thermal and nonthermal) 

developed between July 1998 and June 1999, including a 

54°Cgassy hotspring on the north flank of the volcano and 

several small thermal and nonthermal features at the Main 

Vent Area. The hot spring in the Mud/Gas Pit Area occurs 

some 30 m downhill and to the east of the large collapse

pit from which gas and mud discharge were observed in 

previous years. At the Main Vent Area, the salinity and 

dissolved carbon content of water from the cold pool are 

similar, but slightly diluted, compared to those in water 

in the nearby hot pools. The origin of this cold, CO2–

charged water is uncertain. It could be derived from the 

same source as the hot saline fluid at Shrub, but reaches 

the surface along a longer, more tortuous path or is 

cooled by passage through near-surface permafrost 

layers. In the future, gas samples should be collected 

from any cold pools or springs for comparison with gas 

from the thermal vents to delineate whether different gas 

sources are involved for the thermal and nonthermal 

features. 

Carbon and helium isotopic compositions of the 

gas discharging at Shrub and Upper Klawasi mud 

volcano suggest derivation from a combination of 

magmatic and crustal sources. The high salinity and 

dissolved carbon content of these spring waters also 

suggest inputs of magmatic volatiles to fluids in 

sedimentary rock aquifers. The uniformity in fluid and 

gas compositions between vents at Shrub and Upper 

Klawasi, both now and in the past, along with similar 

vent temperatures at these two volcanoes in the decades 

prior to the 1990s, suggest similar sources at depth. 

The relatively high-temperature discharge 

conditions occurring at Shrub since 1996 seem to require 

either (1) increases in heat and volatile inputs at depth or 

(2) an increase in the rate of fluid upflow. In the first 

case, an increase in heat input in the form of magmatic 

steam should be accompanied by additional inputs of 

magmatic CO2 and He. However, observed values of 

DIC, 13C, and/or 3

He/4He for recent samples are within 

the ranges of values determined for previous gas and 

fluid samples from all three Klawasi Group mud 

volcanoes. In the second case, an increase in the rate of 

fluid upflow did accompany the onset of thermal activity 

at Shrub in 1996. Greater flow rates would tend to 

lessen the temperature drop between the source reservoir 

and the surface, for the same rate of conductive heat loss 

from the flow conduits. We note also that present-day 

vent temperatures at Shrub are similar to bottom-hole 

temperatures in ex-plorations wells completed in the 

Mesozoic basements rock beneath the Copper River 

Basin. Thus, the higher vent temperatures measured over

the past few years are most likely a result of more rapid 

rates larger volumes of fluid upflow from the deep 

source reservoir. Such increases in upflow would require 

corresponding increases in the permeability of faultcontrolled upflow conduits. 

We estimate the present rate of CO2

 discharge from 

springs and pools at Shrub to be ~10 t/d, and suggest 

that this discharge, along with the discharge of dissolved 

bicarbonate, requires inputs of roughly 20-40 t/d of 

magmatic CO2 at depth. These rates are comparable to 

CO2

 fluxes measured at Mammoth Mountain, California, 

both as diffuse flux at individual tree-kill areas and 

dissolved in the shallow ground-water system. 

Anomalous diffuse gas discharge at Shrub has been 

observed in areas immediately adjacent to some of the 

thermal features and from a more extensive area of 

11
normal-temperature soil located between the Main Vent

Area and the Fissure Vent Area. Total CO2 flux from the 

latter area is ~0.23 t/d, and high CO2 concentrations in 

the root zone there have caused some vegetation kill. 

Gas flow from dry vents that settles in localized 

depressions in the soil surface is probably responsible 

for the demise of small birds and animals. 

Although the potential for sudden, large-volume 

releases of CO2

 from vents at the summit and on the 

northern flank of Shrub clearly exists, no evidence was 

found of the occurrence of such events in 1998 or 1999, 

and no lethal atmospheric concentrations of CO2

 were 

found. Such events likely occurred prior to June 1997, 

when extensive browning of vegetation along narrow 

bands on the north flank were observed during field 

investigations. 

ACKNOWLEDGMENTS 

Logistical support for field investigations in 1999 

was provided by the U.S. Geological Survey’s Alaska 

Volcanic Observatory (AVO). During these 

investigations, helpful dis-cussions regarding the 

geology and hydrology of the Wrangell Mountains were 

provided by Don Richter and Chris Waythomas, USGS, 

AVO. The map in fig. 2, based on earlier maps in OFR 

98-128, was prepared by Kristi Wallace, USGS, AVO 

REFERENCES 

Evans, W.C., Banks, N.G., and White, L.D., 1981, Analyses of 

gas samples from the summit crater, in The 1980 

Eruptions of Mount St. Helens, Washington, edited by 

P.W. Lipman and D.R. Mullineaux: U.S. Geological Survey 

Professional Paper 1250, p. 227-232. 

Grantz, A., White, D.C., Whitehead, H.C., and Tagg, A.A., 1962, 

Saline Springs, Copper River Lowland, Alaska: American 

Association of Petroleum Geologists Bulletin, v. 46, p. 

1890-2002. 

Motyka, R.J., Poreda, R.J., and Jeffrey, W.A., 1989, 

Geochemistry, isotopic compo-sition, and origin of fluids 

emanating from mud volcanoes in the Copper River 

basin,Alaska: Geochemica et Cosmochimica Acta, v. 53, 

p. 29-41. 

Nichols, D.R., and Yehle, L.A., 1961, Mud volcanoes in the 

Copper River Basin, Alaska, in Geology of the Arctic,

edited by G.O. Raasch: International Symposium on 

Arctic Geology, 1st Calgary, 1960, Proceedings, v. 2, p. 

1063-1087. 

Reitsema, R.H., 1979, Gases of mud volcanoes in the Copper 

River Basin, Alaska: Geochemica et Cosmochimica Acta, 

v. 43, p. 183-187. 

Richter, D.H., Moll-Stalcup, E.J., Miller, T.P., Lanphere, M.A., 

Dalrymple, G.B., and Smith, R.L., 1994, Eruptive history 

and petrology of Mount Drum volcano, Wrangell 

Mountains, Alaska: Bulletin of Volcanology, v. 56, p. 29-

46. 

Richter, D.H., Symonds, R.B., Rosenkrans, D.S., McGimsey, 

R.G., Evans, W.C., and Poreda, R.J, 1998a, Report on the 

1997 activity of Shrub mud volcano, Wrangell-St. Elias 

National Park and Preserve, south-central Alaska: U.S. 

Geological Survey Open-file Report 98-128, 13 pp. 

Richter, D., McGimsey, R., and Rosenkrans, D., 1998b: 

Addendum to U.S. Geological Survey Open-file Report 

98-128, 3 pp. 

Sorey, M., Evans, B., Kennedy, M., Rogie, J., and Cook, A., 

1999, Magmatic gas emissions from Mammoth Mountain: 

California Geology, v.52, no.5, p. 4-16 

. 

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