Oxygen and Hydrogen Isotope Systematics of Lake Baikal, Siberia: Implications for Paleoclimate Studies

Oxygen and Hydrogen Isotope Systematics of Lake Baikal, Siberia: Implications forPaleoclimate StudiesAuthor(s): Robert R. Seal II and Wayne C. Shanks IIISource: Limnology and Oceanography, Vol. 43, No. 6 (Sep., 1998), pp. 1251-1261Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2838974 .

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Limnol. Oceanogr., 43(6), 1998, 1251-1261 C 1998, by the Amencan Society of Limnology and Oceanography, Inc.

Oxygen and hydrogen isotope systematics of Lake Baikal, Siberia: Implications for paleoclimate studies

Robert R. Seal II U.S. Geological Survey, 954 National Center, Reston, Virginia 20192

Wayne C. Shanks III U.S. Geological Survey, 973 Denver Federal Center, Denver, Colorado 80225

Abstract We interpret oxygen and hydrogen isotope data for water samples from Lake Baikal, Siberia, its tributaries and

other local rivers, and local precipitation in terms of the known water budget for the modem lake in order to gain insight into past limnological and climatic processes that influenced the lake. Lake Baikal is remarkably uniform in its isotopic composition (8180 = -15.8 ? 0.2%o; AD = -123 ? 2%o) and lies slightly to the right of the global meteoric water line, which suggests significant evaporation. Water is supplied to the lake by over 300 rivers and streams. The oxygen isotope values (8180) of the rivers in the Baikal catchment range from -13.4 to -21.2%o. The hydrogen isotope values (SD) for the same area range from -103 to -156%o. Both these ranges generally conform to the global meteoric water line. The weighted average isotopic composition of input to the lake (rivers plus precipitation) is - 15.2%o for 8180 and -1 16%o for AD, values higher than those of the modem lake. Therefore, the isotopic composition of the modem lake cannot be related to the modem input through simple evaporation. Instead, modeling of the isotopic mass balance of the lake suggests that inputs (precipitation and influx from rivers) and outputs (evaporation and outflow) are not at a steady-state equilibrium under current climate conditions. We found previous input to the lake had lower 8180 and AD values than modem input, which reflects cooler climates in the past compared with modem conditions. Under constant climate conditions, steady-state conditions are not expected to be reached by the lake for at least 700 yr because of its large size and the long residence time of water in the lake.

Lake Baikal (Fig. 1), in southeastern Siberia, is the world's largest (23,000 km3) and deepest (1,637 m) lake; it is also one of the oldest modem lakes (ca. 25 million years ago) and contains approximately 20% of the freshwater on the Earth's surface. The ecosystem of Lake Baikal is home to dozens of species of endemic flora and fauna. The lake has recently been the focus of attention because it presents unique opportunities to study ancient climates in continental settings because it contains a sedimentary record that extends from the present to 25 million years ago and because of its high-latitude, continental setting (Colman et al. 1995). Furthermore, international attention has also been directed toward the threat of industrialization to the lake. Despite the importance of Baikal, little effort has been focused on the oxygen and hydrogen isotope compositions of the lake waters and the surrounding environment. In the simplest terms, an understanding of the isotopic variations of the waters is fundamental to understanding the water budget of Lake Baikal.

A variety of geochemical and isotopic data for Lake Bai- kal have been published previously. Major and minor element chemistry has been presented by Falkner et al. (1991,

Acknowledgments We greatly appreciate discussions with and assistance by L. Gran-

ina, T Callender, P. Hearn, S. Colman, E. Karabanov, L. Martin, M. Grachev, and the captains and crews of the RNV Vereshchagin and R/V Papanin. J. Edmond graciously supplied samples from the Lena and Aldan Rivers. K. Falkner provided samples from the Bai- kal water column, surrounding hot springs, and rivers. L. Granina provided additional samples from rivers. Discussions with J. K. Bohike and T. Coplen are also appreciated. M. Grachev, J. K. Bohlke, and P. Hearn provided insightful reviews of the manuscript.

1996), Votintsev (1985), Leibovich-Granina (1987), and Cal- ender and Granina (1997). Vetshteyn et al. (1967) presented some preliminary oxygen and hydrogen isotopic data from the lake, which unfortunately were referenced to Moscow tap water and cannot be unambiguously converted to the Vienna Standard Mean Ocean Water scale. Shanks and Cal- lendar (1992) published oxygen and hydrogen isotopic data for pore waters from lake-bottom geothermal vent sites. Lo- monosov (1974) presented limited oxygen and hydrogen isotope data from geothermal areas throughout the Baikal rift. Grinenko et al. (1994) reported sulfur isotope values for dissolved sulfate in the lake.

This study includes over 100 oxygen and hydrogen isotopic analyses of water for the lake, surrounding rivers and streams, and terrestrial hot springs around the lake and interprets these data in terms of the known water budget of the modem lake. We do this to gain insight into past limnological processes that affected the lake. We use data from the Lena and Aldan Rivers, the rivers of the Baikal catchment, and local precipitation to define the range of variations in regional meteoric waters. We also use data from the rivers of the Baikal catchment and local precipitation to constrain the isotopic composition of water entering the lake. Data from terrestrial hot springs in the region provide insight into the regional hydrologic setting of the lake and the origins of thermal springs on the lake bottom (Shanks and Callender 1992).

Setting

Lake Baikal is situated within the Baikal depression in the Baikal rift zone. The rift is asymmetric, with the graben floor

1251



1252 Seal and Shanks

0y ;utsv 8 0 1520

,\S Sn=zhnaa . l510

12 1030 1040 1050 1060 1070 1080 1090 1100 1110 1120 1130

Fig. 1. Map of Lake Baikal region showing the location of rivers in the lake's catchment. Also shown are the Lower Angara River, the only effluent from the lake, and the Lena and Kirenga Rivers, which have their headwaters near the lake but do not flow into it. Dashed contours in the lake represent water depths at 400-rn intervals. Lake-water sample sites are shown as solid squares or circles. Hot spring samples are shown as open squares. Only the northern part of the Selenga drainage is shown.

sloping to the west (Hutchinson et al. 1992; Mats 1993). The rift zone separates the Siberian craton (Eurasian plate) from the Mongolian fold belts (Amur plate). Active rifting has occurred at least since the Oligocene era (Logatchev and Florensov 1978). The lake consists of three basins, the North, Central, and South Basins. Akademician Ridge, a horst, separates the North and Central Basins and the delta of the Selenga River separates the Central and South Basins.

The modem climate of the region is dominated by cold winters (average monthly temperatures, -12 to - 19?C) and warm summers (average monthly temperatures, 8 to 14?C) and a mean annual temperature of -1.7?C (Kozhov 1972). The area receives approximately 45.7 cm of precipitation per year (IAEA 1992).

The lake surface is at an elevation of 455 m above sea level and covers 31,000 km2. Over 300 rivers and streams supply water to the lake from a catchment area of over 588,000 km2 (Kozhov 1963). Approximately 72% of the riverine input is supplied by three rivers, the Barguzin, the Up- per Angara, and the Selenga. The Barguzin and the Upper Angara Rivers have relatively small catchments, but the Se- lenga River, the most significant in terms of volume, drains much of northern Mongolia, from as much as 5? latitude to the south. Outflow from the lake is through the Lower An-

gara River, which exits in a westerly direction from the South Basin, flows into the Yenisey River, and eventually empties into the Arctic Ocean. The headwaters of the Lena and Kirenga Rivers are within 10 km of the western shore of the North Basin, but neither river drains into the lake. The Kirenga flows into the Lena, which flows north to the Arctic Ocean.

Annual river input is 61.1 km3 and annual input from precipitation is 12.4 km3 (Gronskaya and Litova 1991). Thus, on the basis of modern surface-water input rates, the entire volume of Lake Baikal is replaced in 313 yr. Vertical exchange of waters due to thermal convection in the spring and autumn and wind-forced convection is limited to the upper 200 to 300 m (Shimaraev et al. 1993). Deep-water renewal is accomplished as jets of denser surface waters sink. The increased density of the jets is related to either thermobaric effects in surface waters (Shimaraev et al. 1993) or higher dissolved solids in riverine input compared with the ambient lake (Hohmann et al. 1997).

Analytical methods

Water samples were analyzed for their oxygen and hydrogen isotopic compositions using CO2 and H2 equilibration



Stable isotopes of Lake Baikal 1253

Table 1. Oxygen and hydrogen isotope data for rivers, hot springs, and precipitation feeding into Lake Baikal, Siberia.

8180 AD Sample sites Date (%o) (%0)

Lena and Aldan Rivers 101 -19.3 -146.3 102 -18.4 -138.3 103 -18.5 138.9 105 -21.1 -159.0 106 -17.9 -140.7 107 -20.3 -151.6 108 -20.3 -152.5 109 -19.7 -148.3 110 -16.9 -125.6 111 -21.6 -165.0 112 -21.0 -158.6 113 -21.1 -157.8 114 -20.8 -156.9 115 -19.0 -143.0 116 -20.9 -158.6 117 -21.8 -165.0 118 -19.3 -145.6 119 -17.7 -131.8 120 -21.2 -158.6 121 -21.5 -161.0 122 -18.9 -141.5 123 -20.1 -152.4 124 -17.6 -132.4 125 -19.6 -147.6 126 -20.3 -153.0 127 -20.7 -155.7 128 -19.4 -145.4 129 -16.3 -134.7 130 -20.1 -151.8 131 -19.7 -147.6 132 -20.0 -150.6 133 -20.0 -149.4 134 -17.4 -147.4

Baikal Drainage Upper Angara 29 July 92 -20.2 -152.2 Upper Angara 17 September 92 -19.3 -143.7 Kichera 31 July 91 -19.6 -148.0 Kichera 29 July 92 -20.5 -153.7 Kichera 17 September 92 -19.2 -143.9 Rel 26 July 91 -21.2 -156.0 Rel 29 July 92 -20.8 -153.7 Teya 29 July 92 -18.2 -138.4 Kabanya 28 July 92 -21.3 -156.1 Tompuda 29 July 92 -21.6 -160.3 Tompuda 18 September 92 -20.9 -152.8 Davsha 29 July 92 -21.7 -157.7 Barguzin 25 July 92 -15.5 -127.9 Barguzin 5 August 91 -16.6 -125.0 Barguzin 1 October 92 -17.9 -132.8 Turka 5 August 91 -19.6 -144/5 Turka 31 July 92 -20.0 -144.9 Turka 19 September 92 -20.0 -144.9 Buguldeika 8 September 92 -16.3 -126.7 Buguldeika 29 August 92 -16.5 -122.6 Buguldeika 5 August 91 -15.9 -119.0 Snezhnaya 1 September 92 -16.5 -117.8 Snezhnaya 2 September 92 -16.4 -119.0 Goloustnaya 30 August 92 -16.3 -122.2

Table 1. Continued.

6180 AD Sample sites Date (%o) (%o)

Goloustnaya 6 August 91 -15.1 -118.0 Utulik 2 September 92 -17.0 -127.5 Khara-Murin 3 September 92 -17.1 -122.3 Mysovaya 31 August 92 -21.1 -153.4 Mishika 31 August 92 -20.4 -149.6 Selenga-Sevevnaya 29 August 92 -13.9 -103.1 Selenga-Srednyaya 20 September 92 -14.4 -108.2 Selenga-Harauz 20 September 92 -14.5 -108.6 Selenga June 91 -13.4 -102.5 Selenga 31 July 92 -11.4 -95.8 Lower Angara 31 August 92 -15.8 -121.0 Lower Angara 4 August 92 -15.9 -123.5

Hot Springs Khakhusi (cold) June 91 -20.7 -157.0 Khakhusi (hot) June 91 -21.5 -158.0 Kotelnikousky June 91 -20.5 -156.0 Kotelnikousky 31 July 91 -20.5 -158.0 Smeyney June 91 -22.4 -164.0 Davsha June 91 -20.8 -158.5

Precipitation Akademician Ridge 26 July 92 -12.4 -104.0 Selenga 24 July 92 -12.6 -111.8 Listvyanka 3 August 92 -15.3 -114.9

techniques, respectively (Epstein and Mayeda 1953; Coplen et al. 1991) in the Stable Isotope Laboratory of the Water Resources Division, U.S. Geological Survey, Reston, Vir- ginia. We report data using the 8 notation, expressed per mill relative to the Vienna Standard Mean Ocean Water scale. The precision (1u) of the oxygen and hydrogen isotope data are ?0.1%o and ? 1%o, respectively, or better.

Results

Rivers-Thirty-three oxygen and hydrogen isotope analyses were obtained for samples from the Lena and Aldan Rivers, located about 900 km northeast of the northern end of Lake Baikal. The 8180 values range from -21.6 to - 16.3%o; the AD values range from - 165 to - 132%o (Table 1; Fig. 2). Thirty-five samples from 18 of the largest rivers in the Baikal catchment were collected in the summer months; their 8180 and AD values range from -21.7 to -11.4%o and from - 158 to -96%o, respectively (Table 1; Figs. 2, 3). In general, the data from the Lena and Aldan Rivers and the rivers of the Baikal catchment conform to the global meteoric water line (Craig 1961). Data for the Lower Angara River, the only effluent from the lake, are discussed below.

Precipitation-Three analyses of rain in the summer of 1992 (Table 1) were obtained, and the International Atomic Energy Agency (IAEA) also reported 14 analyses for monthly averages from the Irkutsk, Russia station (Fig. 3). The 17 analyses span a wider range than the river data; 8180 and AD values range from -25.1 to -9.3%o, and from -213 to -65%o, respectively (Table 2; IAEA 1992). As with the river




-50 . . . . . . . . . o Lena and Aldan Rivers

* Baikal Catchment Rivers xx

-100 + Baikal Precipitation +

X Irkutsk Precipitation (IAEA)

AD -1500

-200

-250 r -30 -25 -20 -15 -10 -5

6180

Fig. 2. Oxygen and hydrogen isotopic composition of meteoric waters from the Baikal catchment and the Lena/Aldan drainage 900 km to the northeast. GMWL indicates the global meteoric water line.

data, the precipitation data generally conform to the global meteoric water line (Fig. 2).

Lake waters and Lower Angara River effluent-Thirty- two samples of lake water and two samples of effluent from the Lower Angara River were analyzed for their 818O and AD values (Table 2; Fig. 3). With the exception of one surface sample from site 340 on Akademician Ridge, all the analyses were identical within analytical uncertainty. 818O

averaged -15.9 ? 0.2%o (2uf), and AD averaged -123 ? 2%o (2uf). The lack of variation with depth indicates that the lake is well mixed (Fig. 3), a conclusion reached by previous workers on the basis of a variety of geochemical indicators (Falkner et al. 1991; Weiss et al. 1991; Edgington et al. 1996; Peeters et al. 1997). The sample from site 340 was collected in July 1992 and yielded 818O values of - 14.4%o and AD values of -11 8%o. The more positive isotopic values, relative to the main mass of the lake, are consistent with evaporation from the surface of the lake.

200

400

600

a~1000

1200

1400

1600

180C -16 0 -15 5 -15.0 -14.5 -14 0 -126 -124 -122 -120 -118

8180 AD

Fig. 3. Profiles showing very constant oxygen and hydrogen isotope values of lake-water samples from the North, Central, and South Basins. Samples were collected by Falkner et al. (1997). The surface-water sample with the most positive isotopic values was collected near site 340 in July 1992 and undoubtedly reflects evaporation at the lake's surface. Analytical uncertainty (2o-) is illustrated by the brackets at the bottom of each data set.

Table 2. Oxygen and hydrogen isotope data for Lake Baikal, Si- beria.

Station Depth (m) Date 6180 (%o) 8D (%o)

336 0 July 92 -15.9 -123.7 336 1,600 July 92 -15.9 -125.5 337 0 July 92 -15.8 -121.6 337 5 July 92 -15.9 -123.0 337 100 July 92 -16.0 -123.1 337 385 July 92 -16.0 -122.5 337 400 July 92 -15.8 -122.5 337 400 July 92 -15.9 -123.1 338 0 July 92 -15.9 -122.2 338 375 July 92 -15.8 -122.6 338 375 July 92 -15.9 -123.3 340 0 July 92 -14.4 -118.1 LBS4 20 23 June 91 -15.8 -123.7 LBS4 80 23 June 91 -15.9 -124.4 LBS4 300 23 June 91 -15.8 -123.5 LBS4 500 23 June 91 -15.9 -121.6 LBS4 800 23 June 91 -15.8 -122.9 LBS4 1,200 23 June 91 -15.9 -123.4 LBS4 1,400 23 June 91 -15.9 -123.0 LBS4 1,620 23 June 91 -15.9 -122.7 LBS7 20 26 June 91 -15.8 -124.2 LBS7 160 26 June 91 -15.9 -123.8 LBS7 350 26 June 91 -15.8 -124.0 LBS7 550 26 June 91 -15.9 -123.2 LBS7 750 26 June 91 -15.9 -124.2 LBS7 817 26 June 91 -15.8 -123.4 LBS9 80 29 June 91 -15.8 -123.7 LBS9 200 29 June 91 -15.8 -122.1 LBS9 450 29 June 91 -15.7 -123.0 LBS9 800 29 June 91 -15.8 -121.3 LBS9 1,200 29 June 91 -15.8 -123.2 LBS9 1,400 29 June 91 -15.7 -123.4

Terrestrial hot springs-Four samples from terrestrial hot springs from around the North Basin of the lake were analyzed (Table 1; Fig. 4). The most notable feature of the hot springs data is the distribution along the meteoric water line. The meteoric signature indicates that the geothermal fluids

-110 . . . . . . . .

* Lake Waters O Vent Site Pore-Waters 00 * Terrestrial Hot Springs (p

-130

AD /

-150

-170 - -23.0 -22.0 -21.0 -20.0 -19.0 -18.0 -17.0 -16.0 -15.0

6180

Fig. 4. Isotopic variations among terrestrial hot springs relative to lake waters, pore waters, and lake-bottom pore waters near vent sites (Shanks and Callendar 1992).




Table 3. River flow and isotopic compositions used in mass-balance calculations.

Average Fraction flow of total 8180* 8D*

River (mi3 s-') flow (%o) (%o)

Selenga 946.0 0.616 -13.5 -103.6 Upper Angara 263.0 0.171 -19.8 -148.0 Barguzin 124.0 0.081 -17.3 -128.9 Snezhnaya 50.0 0.033 -16.5 -119.0 Turka 48.4 0.032 -19.9 -146.0 Khara-Murin 26.3 0.017 -17.1 -122.3 Tompuda 24.7 0.016 -21.3 -156.6 Utulik 17.4 0.011 -17.0 -127.5 Rel 11.7 0.008 -21.0 -155.0 Goloustnaya 7.9 0.005 -15.7 -120.1 Buguldeika 4.5 0.003 -16.2 -123.0 Kabanya 4.3 0.003 -21.3 -156.1 Teya 3.8 0.002 -18.2 -138.4 Mysovaya 3.2 0.002 -21.1 -153.4 Davsha 0.8 0.001 -21.7 -157.7 Total 1,536.0 1.000 Average -15.5 -117.4 Flow data from Afanasjev (1976) and Leibovich-Granina (1987). * For rivers for which there are multiple analyses (see Table 1), the isotopic value used is the average of all available data.

have not undergone significant isotopic exchange with the country rock through which they flowed. Furthermore, the proximity of the values for the hot springs to the meteoric water line also indicates that lake water is not an important component of these fluids. Thus, recharge for the hot springs is hydrologically upgradient from the lake.

Origin of vent fluids

The oxygen- and hydrogen-isotope and major-cation geochemistry of sedimentary pore waters from an underwater thermal vent site at Frolikha Bay in the North Basin were described by Shanks and Callender (1992). They concluded that the source of the water from the vent site was meteoric in origin, and they found that neither evaporation nor water- rock interaction can adequately explain the chemical and isotopic data. They speculated that the recharge region for the hydrothermal system responsible for the vent fluids lies northeast of the lake, in the Barguzin mountain range.

A comparison of isotopic data from the pore waters from the Frolikha Bay vent site with data from lake waters and terrestrial hot springs clearly suggests that the isotopic signature of the pore waters is the result of mixing between the lake waters and regional geothermal fluids (Fig. 4). The isotopic composition of the terrestrial hot springs is similar to the range of values found in the rivers of the North Basin on both sides of the lake and supports the hypothesis of Shanks and Callender (1992). If the average value for the terrestrial hot springs (8180 = -21.3%o; AD = -159%o) is a good representation of the isotopic composition of geothermal waters around the North Basin, then the pore-water sample with the lowest isotopic values consists of approximately 9% hydrothermal fluid and 91% lake water.

River Flow 25.0

0 _ 2 * Selenga

20.0 * I Upper Angara

A Barguzin Z 15.0 0 AllRi4vers

10.0

5.0

0.0 1 2 3 4 5 6 7 8 9 10 11 12

Month Fig. 5. Seasonal variations in percentage of annual flow of riv-

ers draining into Lake Baikal (data from Afanasjev 1976). Approx- imately 70 to 85% of flow is from May through September. Ap- proximately 72% of the total riverine influx comes from the Selenga, Upper Angara, and Barguzin Rivers.

Mass balance

Inputs-Numerous workers have addressed the mass balance of water in the Lake Baikal hydrologic system, and all have yielded comparable results for various time intervals of measurements (Afanasjev 1959, 1976; Vikulina and Kash- inova 1973; Gronskaya and Litova 1991; Khaustov et al. 1991). Inputs to the lake can be divided into three sources: rivers, atmospheric precipitation, and groundwater. Of these sources, the groundwater is the most difficult to quantify, but all studies have assigned a value of <4.5% of the total net flux into the lake. The present study does not directly include groundwater in the mass-balance considerations because of the low proportion of the total water budget that it constitutes and the difficulty in trying to define a bulk representative isotopic composition for groundwater influx. However, its isotopic composition should fall within the range of values found in river waters from the Baikal catchment. The last assumption is supported by the fact that the isotopic composition of terrestrial hot springs, described above, is indistinguishable from the composition of rivers that empty into the North Basin (Table 1).

The present study uses the net fluxes reported by Gron- skaya and Litova (1991) to model the isotopic geochemistry of the Baikal hydrologic system. Gronskaya and Litova (1991) reported annual riverine input of 61.1 km3 (83.2%) and annual precipitation into the lake of 12.4 km3 (16.3%). The riverine input is supplied by over 300 rivers and streams. Approximately 80% of this water is supplied by the 15 rivers listed in Table 3; published flow data is available for these rivers (Afanasjev 1976; Leibovich-Granina 1987). The most significant of these rivers are the Selenga (61.6%), the Upper Angara (17.1%), the Barguzin (8.1%), the Snezh- naya (3.3%), and the Turka (3.2%). Of the rivers analyzed in the present study, flow data are not available for the Kich- era River and the Mishikha River.

The water supplied to the lake by rivers varies drastically by season (Fig. 5). For the Selenga River and the rivers that empty into the North, Central, and South Basins, between 70 and 85% of the annual water input arrives in the lake




Irkutsk Precipitation

6180 = 0.361 T(?C) - 16.798 r2= 0.768

-10.0 - a 2.6

6180 n =14

-20.0-

-30.0 - -30.0 -20.0 -10.0 0.0 10.0 20.0

T?C Fig. 6. 8180 analyses of seasonal variations in precipitation col-

lected at the Irkutsk station (IAEA 1992). Corresponding AD values vary from -213 to -65%o.

during the period from May to September (Afanasjev 1976); therefore, isotopic analyses of samples collected in this period should provide a good approximation of the bulk riverine input to the lake. This proposal is also supported by several studies that have investigated seasonal variations in the isotopic composition of high-latitude rivers elsewhere in the world. Cameron et al. (1995) found that the isotopic composition of the Fraser River (British Columbia, Canada) varied less than 2%o for 8180 and less than 5%o for SD between July 1993 and November 1993. The mean annual flow of the Fraser River (3,630 m3 S-1) is over twice that of the entire Baikal catchment and four times that of the Selenga River (Table 3). Mook (1970) found seasonal variations of less than 1.5%o for 8180 for rivers in the Netherlands, the largest of which was the Rhine. Likewise, Rank and Papesch (1991) found seasonal 8180 variations of less than 1.5%o for Austrian rivers. In contrast, Fritz (1981) reported seasonal 8180 variations of approximately 10%o for the Red River in Manitoba, Canada. However, the flow of the Red River varies from a low of 10 m3 s-1 to a peak of approximately 100 m3 s-5 and thus may not be representative of larger rivers such as the Selenga, Upper Angara, and Barguzin (Table 3).

Between 1991 and 1992, isotopic data for the Baikal catchment are available for the months of June, July, August, and September (Table 1), which represent approximately 65% of the mean annual flow (Fig. 5). If May and October are included, then the percentage of the mean annual flow increases to approximately 89%. Thus, the mean annual isotopic composition of the Selenga is reasonably represented by data from June through September, although flow during the remaining months may have lower isotopic values and would lower the average value for the Selenga slightly. On the basis of mass-balance calculations and the published seasonal variations cited above, the average value for the Se- lenga should be no more than 0.4%o lower than the average value used in the calculations described below. However, the potentially lower average oxygen and hydrogen isotopic values for the Selenga may be offset in part by higher isotopic values in the base-flow period from October through May for rivers in the north, such as the Upper Angara and Bar- guzin, which have a greater input from spring and summer

Water Budget 35.0

*Rivers . 30.0 0 U Evaporation

25.0 - o Precipitation

20.0

1510- o 10.0

O5.0

~ .0

~)-5.0

-10.0 1 2 3 4 5 6 7 8 9 10 11 12

Month Fig. 7. Seasonal variations in river-water influx, precipitation,

and evaporation in Lake Baikal (from Afanasjev 1976). Eighty-three percent of the total water influx is from rivers and 16% is directly from precipitation. Nineteen percent of the water loss is from evaporation, and the balance is due to outflow through the Lower Angara River.

snow melt. Rank and Papesch (1991) documented seasonal variations in the 8180 value of Austrian rivers dominated by snow melt that resulted in low 8180 values in the summer and high values in the winter, that is, the opposite of what would be expected on the basis of seasonal variations of temperatures of precipitation.

For all rivers, between 44 and 52% of the water originated as groundwater (Afanasjev 1976). The amount of water that originated as snow rather than rain varies systematically with the latitude of the drainage basin. The maximum amount of snow-derived water is found in the rivers emptying into the North Basin (32%), and the minimum amount is found in the Selenga (15%), which drains much of northern Mongolia to the south. Conversely, the Selenga has the highest proportion of rain-derived water (37%), whereas the rivers of the North Basin have the lowest proportion (20%). The greater amount of snow melt in the rivers of the North Basin is reflected in their earlier and distinctly higher peak flow in June rather than July (Fig. 5). Average monthly precipitation values for the IAEA monitoring station at Irkutsk are presented in IAEA (1992) and plotted in Fig. 6. Total riverine inflow and precipitation peak in June and July, respectively (Fig. 7).

Average SD and 8180 values for rivers with multiple isotopic analyses were used in the mass-balance calculations (Tables 1, 3). The values for individual rivers were then weighted according to annual flow to yield an isotopic value for 8180 of-15.5%o and a value of -117%o for SD for the bulk annual riverine input to the lake. The isotopic composition of precipitation into the lake is approximated by the data from the IAEA monitoring station at Irkutsk (IAEA 1992). A weighted average of the monthly precipitation values yields an isotopic value for 8180 of -13.3%o and an isotopic value for 6D of -103%o. Thus, the modem annual total input to the lake from rivers and precipitation is -15.2%o for 8180 and -116%o for 6D (Fig. 8). It is important to note that the bulk modern input has both 8180 and 6D values that are higher than those in the lake. Therefore,




0 1 2 3 4 5 -12.0 - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

1 .. .. ........ -13.0 ..--- -- -- -- - -:-.-- ::-R:-- . . . . . ... : : : : : : : : : : : .. ...- --

-14.0 -

6180 -15.0

-16.0

-17.0

-17.0 O

_ :

~~~~~~~~~~~~~~~. :... ....... ..:......... . :. .... :... . ... - ........ ............. ...::_ .,--------'.,,..' ,' -, . -'.," ' ." . .... ... .... ...

-110 N

6D -120

-130

-140 0 1 2 3 4 5

Turnover Time (T)

Fig. 8. Evolution of oxygen and hydrogen isotope characteristics of lake with time. T = 0 represents the end of the Little Ice Age (A.D. 1890), the beginning of the current span of relatively stable global climatic conditions. The vertical zone at T = 0.33 represents the present day. The horizontal zone represents the ap- proximate analytical uncertainty for oxygen and hydrogen isotope determinations. Therefore, within analytical uncertainty, it will take over two turnover times for Lake Baikal to reach the analytical uncertainty of steady-state conditions with respect to its oxygen and hydrogen isotope systematics.

the modem river and precipitation inputs to the lake alone are not representative of the time-integrated input to the lake that resulted in the modem lake isotopic composition because evaporation produces an enrichment of heavier isotopes in the residual water.

Outputs-Loss of water from the lake mainly occurs as effluent through the Lower Angara River at the southwestern corner of the lake and through evaporation. Groundwater outflow is estimated to be <2% of the total output (Tseitlin 1959). Because of the small amount, the difficulty in trying to quantify it, and the lack of an isotopic fractionation effect, groundwater outflow will not be considered in the mass- balance considerations of the present study. Effluent through the Lower Angara constitutes 81% of the water loss from the lake and evaporation constitutes 19% (Gronskaya and Litova 1991). Other published estimates of evaporative loss from the lake range to a low of 13% (Afanasjev, 1959, 1976; Tseitlin 1959; Vikulina and Kashinova 1973; Khaustov and Lomonosov 1992). Average monthly values for evaporation are presented by Afanasjev (1976). Figure 7 shows that over 80% of the evaporation occurs in October, November, and December. The lake is typically covered by ice from early

January to mid-May (Peeters et al. 1997). On a long-term basis, the net inputs equal the net outputs, so there is no net change in the volume of the lake (Gronskaya and Litova 1991). Effluent through the Lower Angara does not result in an isotopic change in the composition of the lake; two analyses of the Lower Angara River are analytically identical to the those of lake (Tables 1, 2).

Isotopic effects associated with evaporation

Evaporation has a distinct effect on the isotopic composition of the lake. The physical processes responsible for isotopic fractionation during evaporation were first described by Craig and Gordon (1965). The isotopic effects associated with evaporation can be described by the combination of an equilibrium and a kinetic isotope effect. Subsequent workers have expanded the equations of Craig and Gordon (1965) and refined several of their parameters to more easily describe evaporation processes in lake settings for both steady- state and transient conditions (Gat 1981a,b; Gilath and Gon- fiantini 1983; Gonfiantini 1986). The present study uses the formulations of Gilath and Gonfiantini (1983) to describe evaporation in a lake of constant volume, one in which the riverine and precipitation inputs are equal to the riverine effluent and evaporation.

A necessary prerequisite for accurate modeling of the isotopic effects associated with evaporation is the determination of whether steady-state or transient conditions best describe Lake Baikal. Evaporation from a water body results in a shift in oxygen and hydrogen isotopic composition of the residual water to values that are more positive than the starting composition. Thus, the more positive values of the composition of the combined modem river and precipitation input to lake (8180 = - 15.2%o; SD = -1 15.6%o) compared to the modem lake (8180 = - 15.9%o; SD = - 123.2%o) indicate that the modem river and precipitation contributions alone cannot account for the modem lake composition through evaporation. Thus far, the potential compositional effect of groundwater contributions has not been directly addressed. The potential significance of groundwater contributions can be assessed by first assuming that the water balance of the lake has reached steady-state conditions. For steady-state conditions, the isotopic composition of the lake is described by the equation of Gilath and Gonfiantini (1983):

8L = 8 a(l - h + AE) + (E/I)(AAoh + E)

6L (Q/I)ot(l - h + AE) + (EII)

where 8 is the isotopic composition (either 8180 or SD); 6L

refers to the lake composition, 6A refers to the atmospheric composition, 8, is the starting composition of the input prior to evaporation, a is the temperature-dependent equilibrium liquid water-vapor isotopic fractionation factor (Majoube 1971), h is the relative humidity, AE is the kinetic enrichment factor, which is a function of the relative humidity, E = a - 1 + AE, E is the amount of evaporation, Q is the amount of effluent, and I is the total input into the lake (riverine and precipitation). The temperature of evaporation at the lake surface is also needed to calculate ot.

The 8L and 6, values are either measured directly or esti-




mated based on the mass balances of river flow and precipitation. Average monthly variations of temperature and humidity are reported in Panova (1976) and Galazii (1969). Expressions for AE are presented by Gonfiantini (1986). Val- ues for E, Q, and I (riverine input and precipitation) are from Afanasjev (1976). The source of the isotopic values is described above. Evaporation is modeled using the average November temperature (0?C) and relative humidity (65%) (Galazii 1969; Panova 1976), which are representative of peak evaporative conditions (Fig. 7; Afanasjev 1976). The isotopic composition of the atmospheric moisture is assumed to be represented by the composition of moisture generated by evaporation from the lake, calculated using equilibrium and kinetic fractionation factors discussed by Gilath and Gonfiantini (1983); the calculations yield a value for 3180A of -32.1%o and a value for 6DA of -215%o. With these estimates for temperature, humidity, and the composition of atmospheric moisture, we used Eq. 1 to calculate the isotopic composition of the lake prior to evaporation (8,). Assuming steady-state conditions, we found that 8180 = - 19.0%o and SD = - 140.6%o. These isotopic values are much lower than those calculated on the basis of modem inputs (8180 =

- 15.7%o; SD = -1 15.6%o), suggesting that the lake is not in steady-state equilibrium.

The potential importance of groundwater contributions can be addressed from mass-balance considerations using the equation

GWbGW + RP8RP = SS, (2)

where GW is the proportion of groundwater, RP is the proportion of river and precipitation input, SS is the calculated steady-state bulk input, and 8 is the isotopic composition. If the isotopic composition of the groundwater is hypotheti- cally assumed to be equal to the average value for the terrestrial hot springs around the North Basin (8180 =

-21.3%o; SD = - 159%o), then the unrealistic amount of approximately 61% groundwater input is needed to yield the calculated steady-state input value. In reality, this estimate for the composition of the average groundwater is undoubtedly too low because the composition of groundwater from the more southerly extent of the lake should be higher and would require an even greater contribution of groundwaters. Conversely, if the greatest estimate of groundwater input to the lake of 5% (Afanasjev 1959) is used, then the equally unrealistic groundwater composition values of 8180 =

-91.2%o and iD = -616%o can be calculated. In other words, under no reasonable circumstances can groundwater contributions to the lake account for the modem isotopic composition of the lake through evaporation assuming steady-state conditions. Therefore, it must be concluded that despite its impressive age and volume of water, Baikal is in a transient state with respect to the isotopic characteristics of its water budget.

The isotopic composition of the lake in a transient condition can be described using the equation of Gilath and Gonfiantini (1983):

1ot(l - h + AE) + (E/I)(6AA h+ E) (Q/I)o(1 - h + AE) + (EII)

{li- + - h + AE) T

+ 8 0exp -(QII) + (3l-) i) LeX[ otJc(l1-h +AE)JT] (3

where t is time, T is turnover (or residence) time, and 80 is the lake composition at time zero.

The meaningful application of Eq. 3 to the interpretation of the isotope systematics of Lake Baikal requires some knowledge of past climatic variations in the region. Histor- ical records of Chinese climatic variations can provide some constraints on historical climates in East Asia (Zhang and Crowley 1989). Chinese records indicate relatively constant climates back to the end of the Little Ice Age at about A.D. 1890, at which point temperatures dropped to approximately 1.5?C below current values. From A.D. 1890 to 1100, temperatures remained low, with several minor fluctuations, none of which reached modern temperatures.

Equation 3 is constructed in terms of the turnover time (I) for water in the lake. On the basis of modern input rates, the turnover time for Baikal is approximately 313 yr. There- fore, on the basis of the Chinese records, East Asian climates have been stable for the last 100 yr, or roughly one-third of a turnover period (t = 0.33) for Baikal. Equation 3 can be used to determine how close Baikal is to a steady state, assuming that the modern mass balance, in terms of both volume and isotopic composition, is representative of the past 100 yr.

Several features of these calculations are noteworthy (Fig. 8). First, the curves in Fig. 8 clearly indicate that Baikal is in a transient state relative to the end of the Little Ice Age. The lake will not reach a steady state within the current analytical uncertainty of isotopic methods until it has existed in its present state for an additional 2.5 turnover times (780 yr) or within absolute uncertainty until approximately 5 turnover times (1,565 yr). At a steady-state condition, the composition of the lake will be as follows: 8180 = - 13.1%o and SD = - 103.5%o. The composition of the lake in A.D. 1890 at the end of the Little Ice Age (t = 0) had the following values: 8180 = - 17.5%o and SD = -133.4%o (Fig. 9). Be- cause of the relatively stable climate for close to 800 yr prior to the end of the Little Ice Age, the input to the lake before 1890 can be assessed using Eq. 1. A 8180 value of -20.6%o and a SD value of - 150.5%o explain the input prior to evaporation.

For both the steady-state and transient model calculations, the greatest uncertainty lies in the estimation of the isotopic composition of the atmospheric moisture associated with evaporation (OA) and in selecting representative values for the air temperature at the lake surface and the relative humidity associated with evaporation.

The isotopic composition of atmospheric moisture is more problematic, but possible compositions can be constrained. The most likely sources of atmospheric moisture include the lake itself and the oceans. The isotopic composition of the water vapor over the ocean has a 8180 value of - 13.0%o and




-100

-110. 4 -C3g Modern Input

-120 LIA Inpu Modern Lake

8D -130 6D -130 ?~~~~~~~890 Lake -140 LIA Input

w/o Selenga Modern Input -150 LIA Calculated. w/o Selenga

SS Input -160 . .

-25.0 -20.0 -15.0 -10.0 6180

Fig. 9. Isotopic values of modem riverine plus precipitation input to the lake compared to modem lake waters and estimated input during the Little Ice Age (LIA) from A.D. 1100 to 1890. Values calculated without input from the Selenga River are for comparative purposes. The Little Ice Age-calculated steady-state (SS) input was calculated based on steady-state evaporation without constraints from modem riverine or precipitation isotopic data. The Little Ice Age inputs, with or without input from the Selenga, were calculated assuming a mean annual temperature that was 1.5?C lower than present-day values and using the temperature dependence of precipitation isotopic composition (defined in Fig. 6).

a AD value of -94%o. However, the isotopic composition of oceanic water vapor is subject to fractionation related to the removal of water by precipitation as the air mass moves across the continent (Dansgaard 1964). Through this process, the isotopic composition of the air mass will evolve into compositions more similar to those associated with evaporation from Lake Baikal. For comparison, the isotopic composition of water vapor in equilibrium with the weighted- mean annual isotopic composition of precipitation at Irkutsk (8180 = -21.9%o; AD = - 147%o) can be calculated using the mean annual temperature of precipitation of 9.8?C and the fractionation factors of Majoube (1971). Figure 10 com- pares the calculated inputs to the lake using transient-state Eq. 3 and the various atmospheric moisture compositions with the composition of the modem lake and modem riverine, precipitation, and combined inputs. All calculated inputs lie within the range for all individual modem rivers. The calculated input based on water vapor evaporated from the lake and the water vapor in equilibrium with mean annual precipitation in Irkutsk lie well within the range limited by the modem composition of the lake and the lowest values from modem rivers. Because of varying weather pattems, the isotopic composition of water vapor in equilibrium with precipitation at Irkutsk may not be representative of the water vapor associated with evaporation at the lake. Further- more, because Lake Baikal contains 20% of the freshwater on the Earth's surface, the isotopic composition of the lake must undoubtedly exert a very strong influence on the composition of atmospheric moisture in the region. Therefore, we propose that the calculated input to the lake based on the calculated isotopic composition of water vapor evaporated from Baikal is the best estimate of the composition of the time-integrated input to the lake before evaporation.

Compared with variations in the composition of atmo-

-100 0 Steady-State Lake

Modern Input -120

SD // Modern Lake

-140 No-Selenga Input Initial (Baikal) Initial (IAEA)

Initial (Ocean)

-160 -25 -20 -15 -10

6180

Fig. 10. Calculated initial lake waters for differing isotopic values of atmospheric humidity (8A, Eq. 3). The arrows indicate the evaporation trend from the "initial" lake at the end of the Little Ice Age to modem values, and then to future steady-state conditions, assuming long-term climatic stability.

spheric moisture, variations in the temperature and relative humidity have lesser but contrasting effects on the calculated input prior to evaporation. Variations in temperature are more prominent in the hydrogen isotopic composition of the calculated input, whereas variations in relative humidity produce a more pronounced effect in the oxygen isotope compositions.

Implications for past climates and future geochemical trends

The higher 8180 and AD values of the modem river and precipitation input compared with the lake values, and the unrealistic conditions required for groundwater contributions to account for a lower bulk input composition, suggest that the mass balance of the water budget for the lake is currently not at a steady state. Transient-state calculations indicate that the isotopic composition of the lake at the end of the Little Ice Age was significantly lower than that of the modem lake, which is consistent with cooler climates in the past. If steady-state conditions are assumed for the end of the Little Ice Age, then the calculated input prior to evaporation is even lower and near the lower limit observed for all rivers emptying into the lake. The values for the calculated steady- state input (8180 = -20.6%o; AD = -150.5%o) represent a lower limit on possible compositions. The cooler temperatures that prevailed during the Little Ice Age alone cannot account for the lower isotopic compositions of the lake and input because the effects of cooler temperatures on the isotopic composition of precipitation and resulting river waters are minimal. Linear-regression analysis of the isotopic and temperature data from the Irkutsk station (IAEA 1992) indicates a temperature dependence of 0.36%o TC-' for oxygen isotope composition of precipitation and 2.96%o 0C-I for hydrogen isotope composition of precipitation. In other words, for a 1.5?C decrease in temperature, the oxygen and hydrogen isotopic composition of meteoric waters would be 0.5%o and 4.4%o lower, respectively. The magnitude of the temperature effect is at least an order of magnitude too low to




account for the inferred ancient lake and input compositions alone. Thus, some other factor must be called upon to explain the lower ancient lake and input compositions.

The most obvious and simplest explanation is that the Se- lenga River, which supplies over 60% of the river flow to the lake, experienced substantially decreased flow in the past. Conversely, increased flow from the northern rivers in the past could produce the same effect, or a combination of increased flow in the north and decreased flow from the Se- lenga. Historical records of climates in northern China suggest drier periods in the past, which is in permissive support of at least lower flow from the Selenga River in the past (Zhang and Crowley 1989).

Conclusions

Although Lake Baikal seems permanent, as it is the oldest, deepest, and most voluminous lake in the world, it is really a dynamic hydrologic system. The mass-balance relations of the oxygen and hydrogen isotope geochemistry of the lake and the surrounding rivers and local precipitation indicate that a period of greater than 1,200 yr of stable water flux and constant climate is needed for the Baikal to reach a steady state. However, global paleoclimate studies record typical climatic variations on a much shorter time scale; 1,200 yr of stable climate will probably never be realized (Crowley and North 1991). Paleoclimate studies that assume that the modern lake's geochemical signature represents modern climates may be in error, particularly for paleoclimate indicators based on lake constituents with long residence times.

This statement applies only to paleoclimate indicators that rely on long-residence-time geochemical tracers. Falkner et al. (1991) concluded that most of the major elements in Bai- kal (alkalinity, SO42-, Ca, Mg, Na, and K) have long residence times, equivalent to that of water in the lake; Falkner et al. (1997) determined that many of the minor and trace elements have much shorter residence times that range from 310 yr (Cr) to as short as 5 yr (particle-reactive Al). Deep- water renewal, including dissolved gases, also occurs at a much faster rate (6.2 to 11.2 yr; Peeters et al. 1997). Paleo- climate indicators that rely on primary productivity, such as biogenic silica accumulation profiles (Colman et al. 1995), or elemental and isotopic partitioning, such as the oxygen isotope composition of diatoms or sponge spicules, are also independent of the residence time of water.

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Received: 26 November 1997 Accepted: 15 April 1998 Amended: 7 May 1998



Documents

Oxygen and Hydrogen Isotope Systematics of Lake Baikal, Siberia: Implications for Paleoclimate Studies