Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, U.S.A.

CHEMICAL GEOLOGY INCLUDING

ELSEVIER

ISOTOPE GEOSCIENCE

Chemical Geology 136 (1997) 135-152

Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, U.S.A. J.K. BiShlke a,*, G.E. Ericksen b,1, K. Revesz a a MS 431, National Center, U.S. Geological Survey, Reston, VA 20192, USA b MS 954, National Center, U.S. Geological Survey, Reston, VA 20192, USA

Received 14 November 1995; accepted 13 August 1996

Abstract Natural surficial accumulations of nitrate-rich salts in the Atacama Desert, northern Chile, and in the Death Valley region of the Mojave Desert. southern California, are well known, but despite many geologic and geochemical studies, the origins of the nitrates have remained controversial. N and O isotopes in nitrate, and S isotopes in coexisting soluble sulfate, were measured to determine if some proposed N sources could be supported or rejected, and to determine if the isotopic signature of these natural deposits could be used to distinguish them from various types of anthropogenic nitrate contamination that might be found in desert groundwaters. High-grade caliche-type nitrate deposits from both localities have 615N values that range from - 5 to + 5%o, but are mostly near 0%o. Values of 615N near 0%o are consistent with either bulk atmospheric N deposition or microbial N fixation as major sources of tile N in the deposits. 6 lSo values of those desert nitrates with 615N near 0%~ range from about + 31 to + 50%0 (V-SMOW), significantly higher than that of atmospheric 02 ( + 23.5%o). Such high values of 6180 are considered unlikely to result entirely from nitrification of reduced N, but rather resemble those of modem atmospheric nitrate in precipitation from some other localities. Assuming that limited modem atmospheric isotope data are applicable to the deposits, and allowing for nitrification of co-deposited ammonium, it is estimated that the fraction of the nitrate in the deposits that could be accounted for isotopically by atmospheric N deposition may be at least 20% and possibly as much as 100%. 634S values are less diagnostic but could also be consistent with atmospheric components in some of the soluble sulfates associated with the deposits. The stable isotope data support the hypothesis that some high-grade caliche-type nitrate-rich salt deposits in some of the Earth's hyperarid deserts represent long-term accumulations of atmospheric deposition (possibly in the order of l04 yr for the Death Valley region, 107 yr for the Atacama Desert) in the relative absence of soil leaching or biologic cycling. The combined N and O isotope signature of the nitrate in these deposits is significantly different from those of many other natural and anthropogenic sources of nitrate. Keywords: Nitrates; N-15/N-14; O-18/O-16; S-34/S-32; Atacama Desert; Mojave Desert

1. Introduction

* Corresponding author. Deceased.

Natural near-surface accumulations of nitrate-rich salts occur locally in some of the Earth's driest deserts, but the sources of the nitrogen and modes of

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J.K. BiThlke et al. / Chemical Geology 136 (1997) 135-152

accumulation are not well understood. The desert nitrates that have been studied most intensively are the unique surficial ("caliche"-type) deposits in the Atacama Desert of northern Chile, which were the major global sources of fixed nitrogen for nearly 100 years prior to the 1930s (Whitehead, 1920; Ericksen, 1981, 1983). Similar but smaller caliche-type nitrate deposits have been described from the clay-hills in the Death Valley region of the Mojave Desert, California (Noble et al., 1922; Noble, 1931; Ericksen et al., 1988). Some other examples of near-surface nitrate-rich salts have been reported from arid regions of Antarctica (Claridge and Campbell, 1968; Wada et al., 1981) and in playa lake sediments in the southern Great Basin, U.S.A. (Leatham et al., 1983). The global importance of desert nitrates as mineral resources has diminished; however, various types of natural occurrences are being recognized increasingly as potential sources of nitrate contamination in surface waters and groundwaters in arid regions (e.g., Strathouse et al., 1980; Rosenthal et al., 1987; Marrett et al., 1990; Barnes et al., 1992). Surficial nitrate deposits accumulated in dry climates may be leached into surface waters or groundwaters when conditions become wetter, either naturally or artificially, because of climate change, local flooding, waste disposal, or irrigation. The relative contributions of various natural sources of nitrogen in desert soils is not well known. Fixation of atmospheric nitrogen can result from electrochemical and photochemical reactions in the atmosphere and from organic processes at ground level. Among the organic processes, nitrogen fixation by bacteria (including cyanobacteria) in soils and lichens may account for a large portion of the nitrogen present in desert soils (Rychert et al., 1978). Nitrogen fixation by bacteria associated with higher plants and subsequent degradation of plant litter are additional sources of nitrogen in desert environments (Comanor and Staffeldt, 1978; Famsworth et al., 1978; Shearer et al., 1983). Atmospheric precipitation and deposition of aerosols, including nitrate formed by electrochemical and photochemical processes, also may contribute significant amounts of nitrogen to deserts (Claridge and Campbell, 1968; West, 1978). Despite their widespread occurrence, desert nitrates have not been widely studied isotopically.

Wada et al. (1981) report that desert soil nitrates from the Wright Valley, one of the "dry valleys" of Antarctica, have 615N = -23.4 to - 11.5%o. Those authors suggest that the "nitrate in Antarctic soils is mainly derived from atmospheric precipitation which carries NO x depleted in 15N" and that the ~$N depletion possibly was enhanced by fractionation 70

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Fig. 1. Index map showing the principal nitrate districts and sample localities in the Atacama Desert, northern Chile.

137

J.K. BUhlke et al. / Chemical Geology 136 (1997) 135-152

et al., 1957; Shearer et al., 1974) indicate that the nitrate nitrogen is slightly depleted in 15N with respect to atmospheric N 2, which is in contrast to most organically-derived nitrogen in soils, sediments, groundwaters, and surface waters that are more commonly relatively enriched in lSN (Hoering, 1955; L&olle, 1980; Hiibner, 1986; Owens, 1987)• In this paper we report new N isotope data for some of the most highly concentrated naturally occurring nitrate deposits in the Atacama Desert (Fig. 1) and the Death Valley region of the Mojave Desert (Fig. 2). For subsets of the samples we also report O isotope analyses of the nitrate and S isotope analyses of the soluble sulfate. The data are evaluated with respect to: (1) the origin of the desert nitrate deposits; and (2) the isotope "signatures" of natural desert nitrates that might distinguish them from ni-

during transport from tropical or temperate latitudes. Old groundwaters in the western Kalahari Desert, Namibia, which were recharged in Pleistocene and Holocene times, have nitrate concentrations as high as 3.1 m M and haw~ 8tSN = +5 to +8%o (Heaton, 1984). Heaton (1984) suggests that the groundwater nitrate was derived by natural processes from organic matter in recharge-area soils. Leatham et al. (1983) report that nitrate extracted from unsaturated playa sediments in the Basin and Range region of Nevada, east of the Death Valley region, has ~]SN = +5.5 to + 10.1%o. Those authors suggest that the high nitrate concentrations in the playa sediments are largely the result of nitrogen fixation by cyanobacteria and subsequent mineralization and nitrification of the organic matter. A few analyses of samples from the Chilean nitrate deposits (Hoering, 1955; Parwel

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Furnace Creek Formabon X 8401 Sample locality

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Fig. 2. Index map showing sample localities in the clay-hills and Lake Tecopa beds in the Death Valley region of the Mojave Desert, California, U.S.A.

N a (ppm)

C1/N b

125,000 170,000 110,000 92,000

53,000 25,000 50,000 126,000 160,000 170,000 89,000 64,000

100,000 28,000 160,000 100,000 100,000

43,000 94,000 110,000 50,000 100,000

0.039 2.263 0.002 0.001 0.017 0.000

0.017 0.000

0.399 1.966

0.047 2.094 0.002

0.012 0.033

0.001 0.000 0.026 0.000

0.003 0.013

0.744 0.011 0.014

0.947 0.557

0.727 0.001 0.312 1.114

S/N b

8401 - - salt 8401 - - bulk 8402 - - salt

21,000 7,700 13,000

6.94

0.93 +4.5

Mojave Desert, southern California, USA:

1221 193 E2 143 32A (1) 32A (2) 205 167

Taltal district:

87C-20 193A 375 8304 (1) 8304 (2)

Tocopilla district:

35A 461 458 91C-01 91C-02

S. Tarapac6 district:

439 443 444 447

N. Tarapac6 district:

Atacama Desert, northern Chile:

Sample ID

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-0.8 +0.9 -0.4 - 2.6 - 2.6

-2.4 - 1.0 +0.6 - 0.8 - 1.6

- 1.0 - 1.0 -4.9 - 2.2

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+2.7 + 2.9

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+ 1.4 +0.9

+21.3

+43.4 + 48.5

+38.9 +44.5

+43.1 +45.1

+35.6 +45.3 +42.6 + 42.0

+42.7

- 1.4

-0.6 +0.8 -0.4 - 2.4

+50.4 +40.5

+43.1 +42.6 +37.8 + 40.5

~ 18O d

-0.7 +0.6

- 1.3 -0.8 -4.8 - 2.0

615N d

- 19.4 -21.7

+0.4

+ 5.4 +4.5

+2.0 +4.8

-0.2

-2.8

634 S e

35°42'N 35°42'N 35°42'N

24°55'S 25°07'S 25°10'S 25°13'S 25°13'S 25°13'S 25°14'S 25o20' S

21°51'S 22°20'S 22°20'S 22o45' S 22°45'S

20o13'S 20°24'S 20°32'S 20°41' S 20°41'S

19°40'S 19°39'S 19°39'S 19°39' S

Lat.

Chemical and isotopic data for desert nitrates from northern Chile and southern California, U.S.A.

Table 1

116°12'W 116°12'W 116°12'W

70°10'W 69°55'W 69o52,W 69°49'W 69°49'W 69o49,W 70°08'W 69047' W

69°34'W 69°45'W 69°37'W 69047' W 69°47,W

69o52,W 69°42'W 69°46'W 69043' W 69o43'W

69°55'W 69°59'W 69°59'W 69059' W

Long.

Dolores; blocky NaNO 3 crystals San Antonio; blocky NaNO 3 crystals San Antonio; blue granular mixed salts San Antonio; acicular NaNO 3 crystals

Santa Fe; granular mixed salts Maria Elena; humberstonite-rich salt Maria Elena; hollow rounded pellets Pedro de Valdivia; granular mixed salts Pedro de Valdivia; granular mixed salts

Bully Hill, Upper Canyon area (22,300 ppm N) g Bully Hill, Upper Canyon area (3,700 ppm N) Bully Hill, Upper Canyon area

Pampa Yumbas; salt-cemented soil Of. Alemania; humberstonite-rich salt Of. Chile; pinkish granular mixed salts Of. Lautaro; blocky NaNO 3 crystals Of. Lautaro; acicular NaNO 3 crystals Of. Lautaro; acicular NaNO 3 crystals Of. Santa Luisa; mixed salts Of. Flor de Chile; granular mixed salts

Of. Of. Of. Of. Of.

Of. Humberstone; salt-cemented soil Of. San Pablo; acicular NaNO 3 crystals Pintados; granular mixed salts Of. Victoria; platy darapskite crystals Of. Victoria; granular mixed salts

Of. Of. Of. Of.

Source f; description

t.~ tj

I

t.n

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2,700 80 325 17 9

49,000 63,000 1,500 65,000 67,000 2,000 13,000 5,300 1.49

13.6

2.98 12.9 21.5

0.08

1.61

1.14 0.69 1.02

0.19 0.10

2.92 1.65

+7.2 +6.7 + 3.1 0.0 0.0

+0.7 +0.1 + 1.1 -0.5 0.0 +0.9 +0.9 + 1.3

+0.8 +0.2

+31.3

+31.5 +33.1

+5.4 +5.4 + 7.2 + 16.5

+9.7

+ 10.2 +9.9

35°50'N 35°50'N 35°50'N 35°50'N 35°50'N 35°50 'N 35°39'N 35°39'N 35°39'N 35°53'N 35°57 'N 35°02'N 35°02 'N 35 °02'N

116°36'W 116°36'W 116°36'W 116° 36'W 116° 36'W 116°36'W 116°28'W 116°28'W 116°28'W 116°16'W 116°16'W 117°02' W 117°02'W 117°02'W

Confidence Hills (49,500 ppm N) Confidence Hills (71,500 ppm N) Confidence Hills (23,000 ppm N) Confidence Hills Confidence Hills (73,400 ppm N) Confidence Hills (38,400 ppm N) Saratoga Hills (12,800 ppm N) Saratoga Hills (4,200 ppm N) Saratoga Hills (4,200 ppm N) Zabriskie area (1,900 ppm N) Zabriskie area Barstow Syncline; brownish soil Barstow Syncline; green clay-rich soil Barstow Syncline; green clay-rich soil

f Of. = Oficina (nitrate beneficiation plant). g ppm N in parentheses (Death Valley region) are from Ericksen et al. (1988); bulk isotope samples are from different pieces and some have different N concentrations as measured from combustion yields ( C I / N and S / N were calculated from the values given in Ericksen et al., 1988).

for 2-4 subsamples with an average deviation from the mean of 0.4%o. Sulfur isotope composition of soluble SO4z- relative to V-CDT in %o; duplicate analyses of 4 samples had an average deviation from the mean of 0.2%o.

average deviation from the mean of < 0.1%o. d Nitrogen and oxygen isotope compositions of NO~- relative to air and V-SMOW, respectively, in %o (after removal of SO~-, CO~-, PO43- followed by graphite combustion; N isotopes from noncondensible gases recombusted with Cu20 + CaO); ~ 15N values are for 1-2 subsamples with an average deviation from the mean of 0.1%o; ~ tSo values are

a

N (ppm by weight): semiquantitative values ( + 5-10%) calculated from the N isotope combustion yields (pure NaNO 3 has 165,000 ppm N). b Element ratios: in molar units, from nitrate, chloride, and sulfate analyses (Ericksen et al., 1988; G. Ericksen and J.K. B~hlke, unpublished data). Nitrogen isotope composition of NO~- relative to air in %~ (original samples combusted with Cu + CuO + CaO); each reported value is the average for 2 - 4 subsamples with an

8405B - - salt 8405C - - salt 8405 - - bulk 8413 - - salt 8414 - - salt 8414 - - bulk 8419 - - salt 8419 - - bulk (1) 8419 - - bulk (2) 8424 - - bulk 8425 - - bulk B 1B - - salt BIG - - salt B2G - - salt

I

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J.K. Brhlke et al. / Chemical Geology 136 (1997) 135-152

trates originating in animal wastes, synthetic fertilizers, or other anthropogenic sources.

2. Sample descriptions 2.1. A t a c a m a Desert, C h i l e

The Atacama Desert is one of the most arid regions on Earth. Measurable rainfall ( > 1 ram) may occur locally as infrequently as once in 5-20 years, and relatively heavy rains (several centimeters) as infrequently as only once or twice in a century. Nevertheless, the Atacama Desert has widespread and abundant groundwater that is recharged by streams and subsurface flow from the Andean Highlands (Ericksen, 1981; Fritz et al., 1981; Alpers and Whittemore, 1990; Magaritz et al., 1990). The nitrate deposits in the Atacama Desert are caliche-type accumulations of unusual highly oxidized saline constituents (including nitrate, iodate, perchlorate, and chromate) in an otherwise more normal saline desert environment characterized by abundant and widespread accumulations of chloride and sulfate minerals (Ericksen, 1981). The distribution of commercial-grade or potentially commercialgrade nitrate deposits is given in Fig. 1; the area covered by lower-grade nitrate-rich caliche is not well known but probably much larger. North of the latitude of Antofagasta (Fig. l), the nitrate deposits are mainly along the eastern side of the Coastal Range and, locally, in basins of interior drainage within the Coastal Range. South of Antofagasta, the deposits are more widely distributed from the Coastal Range eastward across the Central Valley into the Andean Front Ranges. The geologic characteristics of the deposits and their mineralogic and chemical compositions are roughly similar throughout the region (Ericksen, 1981). In the nearly rainless Atacama Desert, highly soluble salts including nitrate have accumulated in permeable regolith and fractured bedrock to depths ranging from about a meter to several meters over a relatively long period of time (possibly 15 Ma of hyper-arid climate; Ericksen, 1981). Ericksen (1986) summarizes the general characteristics of two major types of nitrate ore, one in which saline minerals cement unconsolidated regolith and another in which they form impregnations

and veins in bedrock. The mined nitrate ores in the region are estimated to have roughly sub-equal bulk molar concentrations of CI-, SO4z-, and NO 3 (Grossling and Ericksen, 1971; Ericksen, 1986). Ericksen et al. (1988) report compositions of average ores treated at beneficiation plants at Maria Elena (C1/N = 1.9, S / N = 1.2 in molar units) and at Pedro de Valdivia ( C 1 / N = 1.1, S / N = 0 . 7 in molar units) during the years 1932-1967. Nitrate-rich samples from the Atacama Desert were collected from open-pit and shallow underground mine workings at several localities between latitudes 19° and 26°S (Fig. 1; Table 1). The samples came from depths ranging from a few tens of centimeters to ~ 4 m. The analyzed samples include pure nitratine [NaNO3] crystals from veins, granular mixtures of nitrate-bearing salts from veins and regolith cements, and powdery saline masses and disseminated salts from porous soils. Samples 439, 443, 447, 461, and 32A (Table 1) are pure nitratine, sample 91C-01 is an aggregate of platy darapskite [Na3(SOnXNO3). HzO] crystals, and samples 193 and 193A consist of powdery humberstonite [K3NaTMg2(SO4)6(NO3) 2 • 6H20] with admixed bloedite [Na2Mg(SO4) 2 • 4H20], nitratine, and halite [NaC1]. The other Atacama Desert samples (except samples 35A, 1221, and 375) are granular mixtures of nitrate, chloride, and sulfate salts in which nitratine and halite are the most abundant minerals. Sample 8304 consists of nitrate-rich vein material with unusually high concentrations (in the order of 102103 ppm each) of iodate, perchlorate, and chromate. Samples 35A and 1221 consist of friable salt-cemented silt. Sample 375 is the commercial product of the Maria Elena beneficiation plant; it consists of 2-3-mm-diameter pellets formed from a spray of molten NaNO 3.

3. Death Valley region, California The nitrate deposits in the Death Valley region of the Mojave Desert, southern California, differ from those in the Atacama Desert in size, age, mineralogy, and host soil type, perhaps in part because of past and present climatic differences. In the Death Valley region, the nitrate deposits (Fig. 2) occur at shallower depths ( < 50 cm) in relatively impermeable

J.K. B6hlke et al. / Chemical Geology 136 (1997) 135-152

clay-rich soils (Noble et al., 1922; Noble, 1931; Ericksen et al., 1988). Average annual rainfall in the Death Valley region today ( ~ 5 cm) is sufficient to leach highly soluble salts from permeable soils. Furthermore, the present-day arid climate of the Mojave Desert in the vicinity of Death Valley apparently was preceded by a somewhat wetter climate and more extensive woodland plant communities in the Pleistocene (Benson et al., 1990; Spaulding, 1990), so the nitrate deposits now present in the clay-hills may have accumulated largely within the last 15 ka or less. At least two distinct types of nitrate deposits are preserved in the Death Valley region (Table 1; Fig. 2; Ericksen et al., 11988). The relatively high-grade clay-hills deposits in the Confidence Hills, Saratoga Hills, and Upper Canyon areas (Fig. 2) are in clayrich soils on intensely folded and faulted sequences of gypsum-bearing playa sediments of the Miocene and Pliocene(?) age Furnace Creek Formation. X-ray diffraction analyses of bulk samples of nitrate-bearing caliche from those localities indicate that the saline components are dominated by halite, nitratine, darapskite, glauberite [Na2Ca(SO4)2], anhydrite [CaSO4], and gypsum [CaSO4 • 2H20]. The clay-hills samples, which were collected from shallow pits and trenches, consist of nitrate-rich, hard, salt-cemented clay from layers 10-20 cm thick that are overlain by 20-30 cm of porous clayey soil having sparse disseminated salts and underlain by normal compact clays that make up the clay-hills. In contrast, samples from the Lake Tecopa beds (Fig. 2) are from a sequence of relatively undeformed Pleistocene lake sediments and contain halite as the principal saline mineral, with significant quantities of borax [NazB4Os(OH) 4 • 8HzO], tincalconite [NaEB4Os(OH)4. 3HzO], and trona [Na3(CO3)(HCO3). 2H20]. The nitrate mineral in that material is presumed to be nitratine but was not detected by X-ray diffraction in bulk samples. The Lake Tecopa samples are from thin, saltcemented, clay-rich soil on compact clay-rich lake beds. Three additional samples were collected from the folded sedimentary sequence exposed in the Barstow Syncline in the Mud Hills region of the Mojave Desert southwest of Death Valley (Noble, 1931). Those samples were taken from the top 10-15 cm of porous, partially salt-cemented, material on expo-

141

sures of upturned greenish and brownish fine-grained sediments.

4. Analytical methods Saline materials for isotopic analysis were ground gently in a glass mortar. Splits of samples 8304 and 32A from the Atacama Desert were prepared by grinding different specimens from the same localities. Several samples from the Death Valley region were prepared both as bulk samples and as dried salt leachates. To obtain the leachates, splits of the nitrate-bearing soils were disaggregated in de-ionized water. The water containing soluble salts was decanted and evaporated in an oven at 50-100°C (Ericksen et al., 1988). Nitrogen isotope analyses were performed on N 2 gas following sealed tube combustion procedures modified slightly from that of Kendall and Grim (1990) (BiShlke et al., 1993). Samples of nitrate-rich salts and salt-cemented soils were loaded with either Cu + CuO + CaO or Cu + Cu20 + CaO into Vycor or quartz glass tubes, which were evacuated, sealed, baked at 850°C for 2 hr, then cooled slowly. The combusted tubes were broken at the inlet of a Finnigan MAT 251 stable isotope ratio mass spectrometer and analyzed against a laboratory reference gas. Each sample was split, prepared, and analyzed 2-3 times. The data were calibrated against splits of a laboratory standard NaNO 3 salt that were prepared, combusted, and analyzed along with the samples. The lab standard (RSIL-N9) had 615N = +3.0%o, referenced to +0.4%0 for IAEA-N1 and +20.4%0 for IAEA-N2. The average deviation from the mean for individual analyses of standards and samples was +___0.1%o. Oxygen isotope analyses were performed on CO z produced by reaction of KNO 3 with graphite, as described by Revesz et al. (1996). Salt samples were dissolved in de-ionized water (0.1-1.0 g in 50 ml), from which carbonate and sulfate were removed by acidification and by addition of BaCI 2 to precipitate BaSO 4, which was filtered out. The nitrate-bearing filtrates were converted by ion exchange to KCIKNO 3 solutions, which were then freeze-dried. The dried salts were mixed with graphite plus catalysts and loaded into quartz tubes, which were evacuated,

142

J.K. Bi~hlke et al. / Chemical Geology 136 (1997) 135-152

sealed, baked at 520°C for 24 hr, then cooled slowly. CO 2 and N 2 were extracted from the tubes, purified, and analyzed separately. Noncondensible (N 2) gas splits were recombusted with Cu + Cu20 + CaO (BiShlke et al., 1993) to remove minor interference at masses 28 and 29 caused by trace amounts of CO. 6~5N values obtained by this procedure were in agreement with those obtained previously by direct combustion of salts with Cu (Table 1). 6180 values of the CO 2 were calibrated against CO 2 prepared by the same procedure from the international KNO 3 reference material IAEA-N3, for which Revesz et al. (1996) report a 6180[NO3] value of +22.7%0 with respect to V-SMOW, from analyses of gas and solid reaction products, using fractionation factors and normalization procedures from Coplen et al. (1983), with overall reproducibilities of ~ +0.4%0. The abundances of CO, NO, N20, and NO 2 were too small to cause significant isotope fractionation or to reduce the O and N yields measurably (Revesz et al., 1996). Analyses of laboratory standards containing NO;-, C1, and SO42- concentrations bracketing those of the samples were not significantly different from those of pure KNO 3. Sulfur isotopes were measured on SO 2 that was prepared by combusting BaSO 4 (filtered from salt solutions after addition of BaCI 2) with Cu20 + SiO 2 and purified by vacuum distillation, as described by Carmody et al. (1996). ~34S values were calibrated against SO 2 prepared from NZ-1 (-0.3%0) and NBS-127 (+20.9%o), and are reported relative to VCDT with reproducibilities of ~ + 0.2%0 (Table 1).

5. Results and discussion

5.1. Nitrogen isotopes in nitrate The Atacama Desert nitrates have 6~5N values ranging from - 4 . 9 to +4.1%o, but the majority of the values are less than 0%0 and the mode is at ~ -1%o (Table 1; Fig. 3). The 6tSN values are relatively uniform within each locality and do not vary systematically with the nitrate mineralogy (nitratine, darapskite, humberstonite) or the type of occurrence (veins or disseminated salts). The 615N values do not vary systematically with either latitude or alti-

10

t~

t-

0 -6

-4

-2

0

2

4

6

8

10

12

~15 N [NO3-]

Fig. 3. Summaryof nitrogen isotope variationsin nitrates from the Atacama (Chile) and Mojave (California, U.S.A.) deserts. Data are from Table 1 (column 5), with 4 additional values for the Atacarna Desert taken from Hoering (1955), Parwel et al. (1957), and Shearer et al. (1974).

tude; however, the highest 615N values ( > + 1.0%o) are all from the southernmost nitrate fields [Taltal district at 1700-2300-m above mean sea level (a.s.1.) elevation], and the lowest value (-4.9%0) is from the northernmost nitrate field (Tarapacfi district at ~ 1000-m a.s.1, elevation) (Fig. 1). Excluding the Taltal district, the mean ~ 15N value for the samples listed in Table 1 is - 1.3 ___ 1.4%o (n = 13). Including all localities listed in Table 1, the mean ~15N value is 0.0 + 2.2%0 (n = 20). The new data extend significantly the range of 6 ISN values reported previously by Hoering (1955; 615N = - 2.6 _+ 0.5%o for a single analysis of "nitratite" from Tarapac~), by Parwel et al. (1957; 615N = - 2 . 3 _+ 1%o and - 1.2 _+ 1%o, for two analyses of nitrate caliche from the Tocopilla district) and by Shearer et al. (1974; 615N = -- 1.4%o for a single sample of "Chilean nitrate"). The nitrate samples from the Death Valley region have 615N values ranging from - 0 . 5 to +7.2%0 (Table 1), overlapping but slightly higher on average than the Atacama Desert values (Fig. 3). Among the samples from the Death Valley region, the lowest 615N values ( < + 1.5%~) were obtained from some of the most highly concentrated deposits in the Confidence Hills and Saratoga Hills, including the socalled "blister caliche" in the Confidence Hills (Noble et al., 1922; Ericksen et al., 1988). Samples from upturned beds in the Barstow Syncline also have relatively low 615N values (0 to +3%0; Table 1).

J.K. BiJhlke et al. / Chemical Geology 136 (1997) 135-152

The salt-cemented soils from the Upper Canyon area (Fig. 2), which are superficially similar to those of the Saratoga Hills, have slightly higher 615N values (between + 4 and + 5%o), and the nitrate-rich soils on the Lake Tecopa beds in the Zabriskie area (Fig. 2) have still higher values ( ~ + 7%0). Previously suggested potential sources of the N in the Atacama Desert deposits include bird guano, leaching of volcanic rocks, organic-rich sea-spray, atmospheric deposition, microbial nitrogen fixation in the desert soils and playas, and ascending hydrothermal fluids (reviewed by Ericksen, 1981; Chong, 1994). Ericksen et al. (1988) propose a microbial origin for some of the Mojave Desert deposits. The N isotope data alone cannot distinguish with certainty between all of these potential sources because of variations in source characteristics and because of possible fractionations during transit or accumulation, but some sources may be considered less likely than others in comparison with the normal isotopic compositions of common materials. Precipitation and dry deposition from the atmosphere (including aerosols, rain, snow, fog condensate, and dew) may be significant sources of Nbearing salts in deserts (Claridge and Campbell, 1968; West, 1978). ~15N values for nitrogen compounds in rain and atmospheric particulate matter are variable but commonly average less than or approximately equal to 0%o (Hoering, 1957; Moore, 1977; Freyer, 1978; Heaton, 1986; Garten, 1992). Heaton (1987) estimates that the bulk average flux of fixed N from the atmosphere to the land surface in South Africa has ~ 15N = -- 3%0. Analyses reported by Paefl and Fogel (1994) indicate a bulk weighted average value between 0 artd - 1%o for wet deposition onto the coastal western Atlantic Ocean. If the isotopic composition of N (including oxidized and reduced species) in modem atmospheric deposition is not much different from that of pre-industrial N deposition, then the available N isotope data could be consistent with the hypothesis that atmospheric deposition has been a significant source for the N in the high-grade caliche-type desert nitrate deposits. Micro-organism:~ that fix atmospheric N 2 typically have bulk 815N values near - 1 + 2%0 (Hoering and Ford, 1960; Delwiche and Steyn, 1970; Hiibner, 1986). The dry soils of the Atacama Desert contain micro-organisms that are capable of nitrogen

143

fixation, but the population densities are several orders of magnitude smaller than those in soils of milder deserts (Ericksen, 1981). Nevertheless, N 2fixing micro-organisms could be abundant and active locally in wet surficial sediments and playa lakes, or periodically when the dry soils are moistened by condensate from heavy winter fogs in the Coastal Range or by infrequent rains. Furthermore, although rates of microbial activity and fixation of atmospheric nitrogen may be generally low in the region in modem times, they may have been greater during wetter climate periods in the past, as in the Pleistocene when there were ephemeral lakes in the Atacama Desert and numerous perennial lakes in the Andean highlands (Stoertz and Ericksen, 1974). Thus, because N2-fixing micro-organisms characteristically have a relatively restricted range of ~ XSN values that is similar to most of the values in the high-grade desert nitrate deposits, the N isotope data could be consistent with the hypothesis that fixation of atmospheric nitrogen by soil micro-organisms was a significant source of the nitrogen in the nitrate deposits (Ericksen et al., 1988). Ammonium and nitrate derived from in situ degradation and oxidation of animal wastes, including bird guano, almost invariably have 8 ~SN values significantly greater than 0%o, typically greater than + 10%o (Kreitler, 1975; Heaton, 1986; Mizutani et al., 1986). Thus, the N isotope data for the calichetype desert nitrates (6~5N < 4%0) indicate that they probably are not oxidized residues of animal wastes. Ammonia gas derived from such wastes could have much lower 6 lSN values (Kirschenbaum et al., 1947; Kreitler, 1975) and, if re-deposited in another location downwind, could be a source of isotopically light N in atmospheric deposition. Wind-transported sea-spray containing organicrich surface films has been considered as a possible source of iodine and nitrogen, both of which are uniquely abundant in the Chilean nitrate deposits (Ericksen, 1981). However, the majority of reported 615N values for marine nitrogen-beating substances such as phytoplankton, zooplankton, dissolved ammonium, nitrate, and particulate organic nitrogen (PON) are significantly greater than 0%0 (Cline and Kaplan, 1975; Saino and Hattori, 1980; Kaplan, 1983; Altabet and McCarthy, 1985; Minagawa and Wada, 1986; Owens, 1987). Some marine micro-organisms

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may fix atmospheric N 2 to form compounds with 615N<0%o, but even where those organisms are relatively abundant, they commonly account for only a minor fraction of the total near-surface PON, which still may have 615N > 0%0 (Wada, 1980; Minagawa and Wada, 1986). Suspended PON with bulk 615N values near 0%0 may be widespread at shallow depths in the open ocean for reasons other than N 2 fixation at some times of the year (Altabet and McCarthy, 1985), but it is not clear that those substances would be preferentially released from the ocean and transported to the continents. Thus, the hypothesis that the nitrate deposits resulted from accumulation of droplets or particles containing marine organic compounds does not appear to be strongly supported by the N isotope data. However, this does not preclude significant marine components of N in atmospheric deposition that may have been altered chemically and isotopically when released from the ocean or by subsequent oxidation-reduction reactions in the atmosphere. Potential volcanic sources of fixed nitrogen include ammonium salts in fumarole discharges, ammonium in rock-forming minerals, and water-soluble salts in erupted material. The widespread and abundant young ( < 20 Ma) volcanic rocks in the central Andes, particularly the highly permeable rhyolitic ash-flow tufts, have been considered to be important potential sources of chloride, sulfate, and borate in salars of the Andean Highlands as well as in the nitrate deposits and salars of the Atacama Desert (Ericksen, 1961, 1981). More than 1000 young eruptive centers in the central Andes volcanic complex could be sources of saline materials in thermal springs and fumaroles. However, there is no evidence that the volcanic rocks or associated thermal waters of that region contain unusually high concentrations of primary nitrogen. Moreover, the 615N values for a variety of arc volcanic rocks and granitic rocks and minerals from various parts of the world range from about + 2 to + 16%o (Zhang, 1988; Boyd et al., 1993), generally higher than the average values of the high-grade desert nitrates. A few samples of ammonium chloride deposits from fumarole vents in Mexico and Italy also have relatively high 6~5N values of + 11 to + 13%o (Hoering, 1955; Parwel et al., 1957), but comparable data are not available for the regions of this study. Nitrogen-bearing sedimen-

tary and metamorphic rocks could yield N / C I ratios as high as those in the nitrate ores, but those rock types are not exposed in northern Chile, and they also may generally have 615N values greater than + 1%o (Haendel et al., 1986; Bebout and Fogel, 1990).

6. Oxygen isotopes in nitrate The 6180 values in a subset of the Atacama Desert nitrates range from about + 36 to +50%0 (Table 1), all higher than that of atmospheric 02 ( + 23.5%0; Kroopnick and Craig, 1972). There is no obvious systematic variation of 6180 values with location or sample type, but there may be a weak positive correlation between 6180 and 615N (Fig. 4). Three of the Death Valley nitrates have 6180 values in a similar range ( + 31 to + 33%0); while a single sample has a relatively low 6180 value (+21%o). There are only a few other studies of O isotope compositions of nitrate with which to compare these results. Nitrate ions formed by microbial oxidation of reduced N (nitrification) have been shown to contain atoms of O from both the water and the 02 that were present in the oxidizing environment. For example, Anderson and Hooper (1983) report that NO~- produced by microbial (Nitrosomonas) oxidation of NH 3 derives 1 atom each from H 2 0 and 02; while Hollocher (1984) reports that NO 3 produced by microbial (Nitrobacter) oxidation of NO 2 derives its additional O atom from H20. In the absence of fractionation effects, those results would imply that the 6180 value of the NO 3 might be given to a first approximation by" 6180[NO3] = ~6180[H2 O] + 16180[02]

(1)

By analogy with the production of sulfate (Van Stempvoort and Krouse, 1994), it may be expected that significant deviations from this simple mass-balance equation could result from variations in the oxygen sources and from isotope fractionation effects during nitrification; nevertheless, the 6 x80 values of some groundwater nitrate thought to have formed by nitrification of reduced N appear to be roughly (though not exactly) consistent with those given by Eq. (1) (Amberger and Schmidt, 1987;

J.K. B6hlke et al. / Chemical Geology 136 (1997) 135-152

80

60

03

0

Z.

40

-

D_t."

-10

-5

m

0 %

20

0

5

10

q515N[NO3"] Fig. 4. Variation of nitrogen and oxygen isotopes in nitrate. Data for caliche-type desert nitrate deposits are from Table 1 (circles = Atacama Desert; squares= Mojave Desert). Data for modern precipitation are from Durka et al. (1994; Germany), and Kendall et al. (1995; Colorado, U.S.A.). Hypothetical 6180 values of nitrate formed by nitrification of reduced N were calculated using Eq. 1 and assuming 61sO[H20] = - 2 0 to 0%~ (precipitation, coastal fog, and groundwaters) or - 2 0 to + 20%~ (including highly evaporated surface waters), as reported for the Atacama Desert (Fritz et al., 1981; Aravena et al., 1989; Alpers and Whittemore, 1990); similar ranges could be obtained for nitrification values in the Mojave Desert. Schematic correlation lines are shown for denitrification (D) (relative fractionation of O and N isotopes from B/Sttcher et al., 1990) and for hypothetical mixtures ( M ) of atmospheric nitrate (615N= +5%0; 6180 = +70%~) and nitrified atmospheric ammonium (6 is N = - 5%0; 61 s O = + l(F/~c).

Schmidt and Voerkelius, 1989; B~Sttcher et al., 1990; Aravena et al., 19!)3; K. Revesz and J.K. BiShlke, unpublished data). The 6180 values of near-surface waters that might have been present in northern Chile and southern California could be highly variable, but presumably most would have been less than that of atmospheric 02 (+23.5%o). As the 8180 values of many of the caliche-type desert nitrates are significantly higher than that of atmospheric 02 , they provide evidence for a nitrate source other than microbial oxidation of reduced N. 6180[NO3] values higher than that of atmo-

145

spheric 02 have been reported for nitrate in atmospheric deposition (Fig. 4). Durka et al. (1994) report t ~ 1 8 0 [ N O 3 ] v a l u e s o f + 5 3 to +73%o for nitrate in precipitation in Germany (including results from Voerkelius, 1990). Similarly, Kendall et al. (1995) report 6~80[NO~] values ranging from about +40 to +70%o for rain from a relatively remote area of Colorado, and from about + 20 to + 60%0 for accumulated snowpack from localities in Colorado and the northeast U.S.A. The reasons for the high 6 laO[NO3] values in the atmosphere are not known, but presumably are related to fractionation during oxidation-reduction reactions involving a variety of N- and O-bearing species. Based on the available information, the 6180 data support the hypothesis that a significant fraction of the caliche-type nitrate deposits in both field areas may be the result of accumulation of N in the form of nitrate from atmospheric deposition. Because atmospheric deposition contains reduced forms of N along with nitrate (Eriksson, 1952; Duce et al., 1991; Cornell et al., 1995), it may be expected that the isotopic composition of nitrate representing the total of atmospherically deposited N would have 61sO[NO3] values between those of atmospheric nitrate and microbially oxidized N. In northern Chile at elevations up to ~4500 m a.s.l., reported 8180[H2 O] values range from about - 2 0 to -5%o for precipitation and shallow groundwaters, up to ~ -1%o for coastal fog, and as high as +20°/oo for highly evaporated surface waters in salars (Fritz et al., 1981; Aravena et al., 1989; Alpers and Whittemore, 1990). According to Eq. (1) (assuming no fractionation on nitrification), nitrate formed by microbial oxidation of ammonium from either atmospheric or other sources in contact with common waters in northern Chile could h a v e t~lSO[NO3] values between about - 6 and + 21%o (Fig. 4). If the few a v a i l a b l e t~lSO[NO3] data for precipitation in Europe and the U.S.A. ( + 40 to + 70%o) are applicable to long-term accumulations in northern Chile, if fractionation terms missing from Eq. (1) are not large, and if the recorded range of ~180 values of the local waters ( - 20 to + 20%o) is representative of those available during nitrification, then it may be inferred that the nitrate deposits in the Atacama Desert contain at least 20% and possibly 100% atmospherically-derived N. The lower limit for the

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J.K. Bi~hlke et aL / Chemical Geology 136 (1997) 135-152

atmospheric N contribution (20%) would correspond to a situation in which atmospheric deposition contained only nitrate (no reduced N) with relatively low t~180[NO3] (q-40~oo), and the majority of the nitrate in the deposits formed by nitrification of non-atmospheric reduced N (e.g., N fixed microbially at the Earth's surface) only in highly evaporated waters having 6180 = +20%o (6180[NO3] = +21%o). The upper limit for the atmospheric N contribution (100%) would correspond to most situations in which the atmospheric deposition rate of reduced N was approximately equal to or greater than that of nitrate. Similar results may be obtained for the clay-hills nitrate deposits near the south end of Death Valley that have 615N values near 0%o and 6180[NO3] values greater than + 30%e (6180 values of meteoric waters in that region may range from about - 2 5 to + l(F/oc; Benson and Klieforth, 1989). Isotopic fractionation or variations in the relative amounts of H 2 0 or 02 supplying O during nitrification could alter the estimated proportions of oxidized and reduced N contributing to the deposits, but fractionation would have to be extreme to remove the requirement for a significant source component with a high 6180 value, such as atmospheric nitrate. The simplest interpretation of the combined N and O isotope data would be consistent with either: (1) atmospheric deposition containing subequal amounts of nitrate and reduced N, or (2) nitrate-rich atmospheric deposition with some locally microbially fixed N; but the data would not seem to favor reduced N (originating in rocks, waters, or organic materials) as the sole source of the caliche-type nitrate deposits. Molar ratios of (N-org.+ NH~-)/NO 3 in aerosols and atmospheric deposition generally are significantly greater than 0, and commonly are > 1 in relatively uncontaminated regions of the world (Eriksson, 1952; Heaton, 1987; Duce et al., 1991; Savoie et al., 1993; Comell et al., 1995), so the isotope data could well reflect accumulation and oxidation of bulk N in atmospheric deposition alone.

7. Sulfur isotopes in soluble sulfate The average 634S value of soluble sulfate in the Atacama nitrate deposits ( + 2%o) is much lower than

that of marine sulfate ( + 21%o), but similar to values in some arc volcanic rocks and sulfide ores, and similar to those of atmospherically deposited sulfate in non-marine environments (Krouse and Grinenko, 1991). Thus, the S isotope data could be consistent with either atmospheric or local rock sources for the sulfate in the Atacama deposits. The 634S values from nitrate deposits in the Death Valley region indicate several different sources of S. In the caliche deposits with apparently atmospheric N sources, the S isotope values ( + 5 to + 10%o) may indicate a mixed source of sulfate or they could be consistent with an atmospheric source in a marginal marine setting where storm systems generally come from offshore. Similar values have been obtained from groundwaters and playa lake sediments in other parts of the Mojave Desert (Rosen, 1991; J.K. Btihlke and R.A. Schroeder, unpublished data). In the Lake Tecopa beds, where 615N values were relatively far from atmospheric, one ~345 value ( + 17%c) is close to that of marine sulfate and may indicate a local sedimentary sulfate source of S. In contrast, the negative ~345 values of the Bully Hill deposits ( - 2 2 to - 19%o) could indicate a local sedimentary sulfide source of S.

8. Desert nitrate deposits as long-term accumulations of atmospheric deposition By comparison with the available data for modem precipitation in other parts of the world, the 615 N[NO 3] and 818 O[NO 3 ] values of the high-grade caliche salt deposits in the Atacama Desert and the Death Valley region of the Mojave Desert are consistent with the hypothesis that a significant fraction (possibly 100%) of the nitrate was derived from N in atmospheric deposition. However, it is not known for certain whether that comparison is entirely valid because the N and O isotopic compositions of nitrate in remote or pre-anthropogenic atmospheric deposition are not well known. Because some of the reactions leading to the formation and occurrence of nitrate in the atmosphere presumably would be similar whether or not there were anthropogenic N emissions, it is possible that the distinctive isotopic characteristics of modem atmospheric nitrate (low 615N, high 3180) are not entirely due to man's activities.

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Conversely, if it were assumed that the nitrate-rich hyperarid desert salt deposits do contain long-term integrated accumulations of atmospheric deposition, then our data could be viewed as evidence that the isotopic composition of atmospheric nitrate has not been dramatically altered by anthropogenic contributions. More isotopic analyses of atmospheric deposition in remote areas and in the geologic record are needed to determine if that is the case. Modern precipitation studies would be difficult in areas like the Atacama Desert where precipitation is infrequent. Further work is also needed to determine if nitrate with low ~lSN and high ~180 may be formed by nitrification or other processes in soils, groundwaters, or surface waters. Despite being known for their unusual nitrate concentrations, the high-grade caliche-type deposits are not necessarily anomalously enriched in N with respect to the other major anions, when compared to atmospheric aerosols and precipitation. Recent precipitation in many areas of the world has roughly equal (within a factor of 2-3) average molar amounts of N, S, and C1 (F!riksson, 1952; Mayewski et al., 1986; Mayewski and Legrand, 1990; NADP, 1990), as do the bulk nitrate ores of the Atacama Desert (Grossling and Ericksen, 1971; Ericksen, 1986; Ericksen et al., 1988). Even far from the effects of recent anthropogenic emissions, the concentrations of nitrate plus reduced N in the atmosphere are high enough that nitrate could be a significant anion component in the oxidized equivalent of atmospheric deposition (Eriksson, 1952; Claridge and Campbell, 1968; Duce et al., 1991; Savoie et al., 1989, 1993). While regional enrichment of N in the atmosphere can occur for a variety of natural and anthropogenic reasons, and may be a factor in the vicinity of the high-productivity marine environment off the coast of Chile, local N sources such as biologic fixation of atmospheric N 2 in r~henear-surface environment may not necessarily be required to account for the bulk nitrate concentrations of the deposits. The chemical and isotopic data together could be consistent with the hypothesis (Claridge and Campbell, 1968) that unusually high nitrate concentrations in some hyperarid desert salts are due to an unusual lack of biologic acitivity or leaching rather than to specific enrichment. Whatever the source of the N, it is considered likely that remobilization by winds and

147

by numerous episodes of meteoric wetting and drying has caused heterogeneities in the compositions of the deposits themselves. Remobilization and(or) selective consumption of nitrate at various scales could be responsible in part for the regional distribution, vertical zonation, and complex crystallization patterns of the different salts in the deposits (Mueller, 1968; Ericksen, 1981; Searl and Rankin, 1993). Relatively dilute accumulations of nitrate in desert soils and regoliths may be much more widespread than the unusual high-grade caliche-type deposits that are the focus of this study (e.g., Hunter et al., 1982). Some of those have significantly higher 815N values and higher C1/N and(or) S / N ratios (Table 1; Leatham et al., 1983; J.K. BiShlke and J. Densmore, unpublished data) that may indicate various accumulation mechanisms and/or post-accumulation transformations involving isotopic fractionation. Even where N is deposited from the atmosphere or fixed microbially, large variations in nitrate 815N values could result from regional differences in the environments of accumulation (e.g., moisture) that could affect rates of organic recycling, volatilization, nitrification, and denitrification near the Earth's surface (Delwiche and Steyn, 1970; Kreitler, 1975; Klubeck et al., 1978; West and Skujins, 1978; Westerman and Tucker, 1978; Htibner, 1986). Positive correlations between 8180 and 8~5N values of nitrate could result from partial denitrification (B&tcher et al., 1990), or possibly from varying proportions of atmospheric nitrate and nitrified atmospheric ammonium in areas where ~ISN[NH+]
9. Implications for groundwater contamination studies Man's activities have greatly increased the concentrations of nitrate in many of the world's shallow groundwaters, and nitrogen isotope analyses have been widely used to distinguish between different anthropogenic sources of nitrate contamination. For example, numerous studies indicate that groundwater

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t~15N[NO3] values less than ~ +5%° are likely to result from contamination of aquifers by nitrate derived from synthetic fertilizers, whereas ~ 15N[NO3] values greater than ~ + 10%o are more likely to result from contamination by nitrate derived from animal waste products (e.g., Kreitler, 1975; Gormly and Spalding, 1979; Heaton, 1986). More limited data indicate that O isotopes may be useful in distinguishing nitrate from atmospheric deposition (with ~180> +30%o), synthetic fertilizer nitrate (with 180 relatively close to that of atmospheric O2), and nitrate formed by nitrification of reduced N compounds (with 8180 generally significantly less than that of atmospheric 02) (Amberger and Schmidt, 1987; BiSttcher et al., 1990; Durka et al., 1994; Kendall et al., 1995). Contamination of groundwater by nitrate from natural sources in desert soils having t515N n e a r 0%0 (Fig. 3) may not be distinguishable readily on the basis of N isotopes from that caused by synthetic nitrate fertilizers, but it may be distinguishable from that derived from most anthropogenic animal waste and sewage effluent sources. There are no known anthropogenic sources of nitrate with the combination of low 815N and high 8180 found in the natural caliche-type desert nitrates (Fig. 4). Both short- and long-term climate cycling might result in accumulation of nitrate salts near the Earth's surface during dry periods and incorporation of those salts in recharging groundwater during wet periods (Hendry et al., 1984). Surficial accumulations of nitrate salts also could be the sources of sudden and/or recent increases in groundwater nitrate as a result of flooding, irrigation, or wastewater disposal (Rosenthal et al., 1987). In the Mojave Desert near Victorville, California, nitrate in some groundwater wells (815N = + 2 to +4%0; J.K. BiShlke and R.A. Schroeder, unpublished data) is isotopically similar to some natural surficial accumulations of desert nitrate in the nearby Barstow area (815N = 0 to +3%0; Table 1), but distinct from nitrate derived from septic tank seepage pits (SlSN = + 7 to + 15%o; Schroeder et al., 1996). Those data indicate that the small amounts of nitrate in the local groundwater sampled by the wells is not likely to have come from the septic tank seepage pits, but could have been derived from natural surficial sources. O isotope data can increase the probability of success in such studies, provided

the various potential sources can be characterized locally.

10. Summary and conclusions With the exception of atmospheric deposition and microbial fixation of atmospheric N2, m o s t of the potential natural sources of saline components would yield generally higher 815N values and/or lower N/C1 ratios than those observed in the most highly concentrated caliche-type nitrate deposits in the Atacama Desert and Death Valley region. 8180[NO~ -] values higher than that of atmospheric 02 indicate that a significant component of the nitrate in the deposits may be the result of direct deposition of atmospheric nitrate. ~34S[soluble SO2- ] values indicate a variety of sources. If modern atmospheric deposition is not dramatically different isotopically from pre-industrial atmospheric deposition, then the stable isotope data indicate that some of the highgrade caliche-type nitrate deposits in two of the Earth's driest deserts largely may be the result of long-term accumulation of atmospheric deposition (possibly 10 4 yr for the Death Valley region, 10 7 yr for the Atacama Desert) in the relative absence of biologic N cycling or aqueous leaching. Conversely, if it were assumed that the caliche nitrates result from atmospheric deposition alone, then the data could be cited in support of the hypothesis that the isotopic compositions of N and of nitrate in the modern contaminated atmosphere are not dramatically different from pre-industrial values, despite recent increases in N deposition rates in many parts of the world. Those conclusions are based on a limited global dataset of O isotopes in N-bearing systems and may require modification if other sources of nitrate with low 8~5N and high B180 can be identified. Variations in the isotopic compositions of natural desert nitrates may be correlated in part with local or regional climate and soil characteristics that affect the degree of isotope fractionation occurring during oxidation-reduction reactions between N species during accumulation of the nitrate salts. Natural surficial accumulations of nitrate salts in deserts may become sources of high nitrate concentrations in groundwaters and surface waters when climate change results in greater rainfall, or as a

J.K. Brhlke et al. / Chemical Geology 136 (1997) 135-152

result of flooding, waste disposal, or irrigation. The available data indicate that nitrate salts accumulated naturally at the Earth's surface in desert regions can have a range of isotopic compositions that may be distinguishable locally from some anthropogenic nitrate sources. The high-grade caliche deposits described here are not necessarily representative of natural desert nitrates in general, and further studies are in progress to expand the data base.

Acknowledgements Analyses were performed in the Reston Stable Isotope Laboratory, U.S. Geological Survey (USGS), with essential support and services from T. Coplen. Use of brand names in this paper is for identification purposes only and does not constitute endorsement by the USGS. C. Gwinn and J. Hannon assisted with N and O isotope analyses; R. Carmody performed the S isotope analyses; J. Hopple, S. Anderson, G. Miller, and J. Jackson also assisted with sample preparation. R. Carmody, N. Clauer, P. Deines, and an anonymous reviewer provided helpful comments on the manuscript. George Ericksen died while this paper was in review, and no doubt would have had further suggestions for its improvement. J.K.B. would like to dedicate this paper to George's memory because, while it clearly is not definitive, it does reflect George's enthusiasm for new sources of information about a subject that occupied him (off and on) for several decades.

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Aravena, R., Suzuki, O. and Pollastri, A., 1989. Coastal fog and its relation to groundwater in the IV region of northern Chile. Chem. Geol. (Isot. Geosci. Sect.), 79: 83-91. Aravena, R., Evans, M.L. and Cherry, J.A., 1993. Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems. Ground Water, 31: 180-186. Barnes, C.J., Jacobson, G. and Smith, G.D., 1992. The origin of high-nitrate ground waters in the Australian arid zone. J. Hydrol., 137: 181-197. Bebout, G.E. and Fogel, M.L., 1990. Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history. Geochim. Cosmochim. Acta, 56: 2839-2849. Benson, L.V. and Klieforth, H., 1989. Stable isotopes in precipitation and ground water in the Yucca Mountain region, southern Nevada: paleoclimatic implications. In: D.H. Peterson (Editor), Aspects of Climate Variability in the Pacific and the western Americas. Am. Geophys. Union, Geophys. Monogr., 55: 4159. Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, C.G., Robinson, S.W., Smith, G.I. and Stine, S., 1990. Chronology of expansion and contraction of four Great Basin lake systems during the past 35000 years. Palaeogeogr., Palaeoclimatol., Palaeoecol., 78: 241-286. BShlke, J.K., Gwinn, C.J. and Coplen, T.B., 1993. New reference materials for nitrogen-isotope-ratio measurements. Geostand. Newsl., 17: 159-164. B&tcher, J., Strebel, O., Voerkelius, S. and Schmidt, H.-L., 1990. Using isotope fractionation of nitrate-nitrogen and nitrateoxygen for evaluation of microbial denitrification in a sandy aquifer. J. Hydrol., 114: 413-424. Boyd, S.R., Hall, A. and Pillinger, C.T., 1993. The measurement of 615N in crustal rocks by static vacuum mass spectrometry: Application to the origin of the ammonium in the Cornubian batholith, southwest England. Geochim. Cosmochim. Acta, 57: 1339-1347. Carmody, R., Busenberg, E., Plummer, L.N. and Coplen, T.B., 1996. Methods for the collection and preparation of sulfate in ground water for sulfur isotope analysis. (In preparation.) Chong, G., 1994. The nitrate deposits of Chile. In: K.-J. Reutter, E. Scheuber and P.J. Wigger (Editors), Tectonics of the Southern Central Andes. Springer, New York, N.Y., pp. 303-316. Claridge, G.G.C. and Campbell, I.B., 1968. Origin of nitrate deposits. Nature (London), 217: 428-430. Cline, J.D. and Kaplan, I.R., 1975. Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical north Pacific Ocean. Mar. Chem., 3: 271-299. Comanor, L. and Staffeldt, 1978. Decomposition of plant litter in two western North American deserts. In: N.E. West and J. Skujins (Editors), Nitrogen in Desert Ecosystems. Dowden, Hutchinson, and Ross, Stroudsburg, Pa., pp. 31-49. Coplen, T.B., Kendall, C. and Hopple, J., 1983. Comparison of stable isotope reference samples. Nature (London), 302: 236238. Cornell, S., Rendell, A. and Jickells, T., 1995. Atmospheric inputs of dissolved organic nitrogen to the oceans. Nature (London), 376: 243-246.

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