2001 Chemical Weathering in Pescadero Creek

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Chemical weathering in a region of active orogeny: Pescadero Creek Watershed, California by R.D. Phillips and S. Rojstaczer Division of Earth and Ocean Sciences and Center for Hydrologic Science Duke University
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  GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 15, NO. 2, PAGES 383-391, JUNE 2001 Chemical weathering in a region of active orogeny: Pescadero Creek Watershed, California R. D. Phillips and S. Rojstaczer Division of Earth and Ocean Sciences nd Center or Hydrologic Science, Duke University, Durham Abstract. Base flow chemical signals were used to determine he weathering eactions hat control he groundwater hemistry of a geologically heterogeneous, ountainous watershed. Major ion signatures esult rom cyclic salt and formation water inputs and from the weathering of easily oxidized or highly soluble minerals, such as pyrite, calcite, and dolomite. f montmorillonite s the dominant secondary mineral product, hen the bulk of silicate weathering probably nvolves volcanic rocks. Spatial rates of base flow chemical denudation ange from 0.0004 o 0.02 mm yr 1, and emporal ates ange rom 0.0006 o 0.6 mm yr 1. The mean chemical enudation ate or he watershed s 0.03 mm yr-1, which s comparable o some f the world's most rapidly weathering arge drainage basins. Because highly soluble or easily oxidized minerals contribute he bulk of the chemical signal o basin waters, spatial and temporal ates of chemical denudation re constrained argely by recharge and discharge ather than local variations n lithology. 1. Introduction Solute discharges f the world's major rivers indicate hat areas of active orogeny end to have elevated chemical-weathering ates relative to the rest of the Earth's surface [Summerfield and Hulton, 1994]. The implications of silicate weathering n these regimes on the global CO2 budget and climate are potentially very significant e.g., Volk, 1993; Moore and Worsley, 1994]. It is, however, difficult to quantify the influence of tectonic uplift upon weathering through the use of field measurements. Measurements of mass flux from tectonically active watersheds in conjunction with geochemical modeling may allow for such analysis. However, hydrogeochemical nvestigations n tectoni- cally active, mountainous watersheds re often complicated by extreme geologic heterogeneity and coastal settings. n these watersheds, groundwaters may acquire chemical signals from diverse ithologies, ormation waters, and atmospherically epos- ited cyclic salts. Consequently, extensive spatial sampling is required o characterize he processes y which the groundwaters obtain their chemical signatures. Measurements of chemical denudation generally have been confined o large-scale watersheds, nd there is little information on fine-scale variability in denudation ates. In this study, we use base flow chemical signals to determine the weathering reactions hat control groundwater chemistry n a coastal central California watershed Pescadero Creek) and use chemical and discharge ata to calculate spatial and temporal ates of chemical denudation. The Pescadero Creek watershed is located in a region of active tectonic uplift [Anderson and Menking, 1994], and the Mediterranean climate of the area aids in the spatial sampling of basin groundwaters. irtually no precipitation alls during the summer months, so streams are dominated by base flow conditions and are essentially n integrated surface expres- sion of the groundwater at any point in the system. Our measurements re used in conjunction with previous seasonal measurements f water quality [Baldwin, 1967a, 1967b; Steele, Copyright 2001 by the American Geophysical Union. Paper number 1999GB001187. 0886-6236/01 / 1999GB001187512.00 1968] in the watershed. In this region, silicate weathering apparently is only a minor contributor relative to chemical denudation aused by carbonate and possibly pyrite dissolution. Rates of denudation are comparable o those estimated or the major active continental mountain belts in the world. 2. Study Area 2.1. Geography The Pescadero Creek watershed is located in the Santa Cruz Mountains of California between latitudes 37ø11'30 N and 37ø19'N and between ongitudes 22ø7'30 W nd 122ø24'30 W (Fiõure ). The watershed ncompasses total rea f 211.6 km, and elevation ranges from sea level to 820 m. The topography of the eastern portion of the basin is dominated by the rugged, rregular relief of the Santa Cruz Mountains and yields westward to the flat marine terraces of the coastal regions [Baldwin, 1967b]. Owing to its relatively low human population, anthropogenic ffects on the watershed appear to be minimal. Annual precipitation anges from 50 cm at the coast to 140 cm at the ridges of the Santa Cruz Mountains [California Department of Water Resources, 1966]. Approximately 90% of the annual precipitation alls between the months of November and April [Steele, 1968] as frequent storms move inland from the northern Pacific. 2.2. Geologic Setting The geology of the Pescadero basin has been well docu- mented by Cummings et al. [1962], Nilsen and Brabb [1979], Clark [1981], Brabb and Pampeyan [1983], and Brabb [1989] among others. The lithology of the Pescadero Creek watershed consists mainly of a thick sequence of Tertiary marine clastic sedimentary rocks (Figure 2). Some carbonate and volcanic rocks are interspersed mong the marine sandstones, mudstones, and shales and assorted Quaternary deposits that overlie the Tertiary strata. The entire sequence s underlain by crystalline basement [Clark, 1981]. The sedimentary marine units com- monly contain formation water pockets that cause ion concen- 383  384 PHILLIPS AND ROJSTACZER: CHEMICAL WEATHERING IN ACTIVE OROGENY REGION San 37o30 ' 37o15 ' ..  Pacific Ocean 15 km 1122ø30 Francisco 'ã' B a y ã--'ã ã..ã ã.. -:. .. Figure 1. Location and geologic setting of the Pescadero reek watershed adapted rom Nilsen and Brabb [1979]). tration to increase with depth, especially n the lower Tertiary units such as the Butano Sandstone and the San Lorenzo Formation [dohnson, 1980]. Formation water persists in the system because t tends to stagnate n synclinal axes where it cannot be flushed by fresh groundwater Akers and rackson, 1977]. Because f the complex geology, t is difficult, f not impossible, to quantify mineralogy on a basin-wide scale. Common primary silicate minerals nclude quartz, micas, and potassium, odium, and calcium elspars. Dolomite, calcite, gypsum, anhydrite, halite, and pyrite are also common Clark, 1981 . Uplift rates of mafine terraces n the Santa Cruz Mountains are estimated o be 0.9-1.6 mm/yr [Anderson nd Menking, 1994]. The Pescadero asin s cut by a number of active aults, ncluding the north-south rending San Gregofio ault in the western part of the basin and he east-west rending Butano ault in the eastern art of the basin. 3. Methods 3.1. Field Methods Field samples and measurements ere taken in June and July of 1992 and 1993. Base flow samples were collected or analysis of major chemical constituents nd were filtered through 0.45 Iãm Gelman Supor-450 membrane ilters into polyethylene bot- tles. Filters were flushed with 300 mL of sample water before each sample was bottled. Samples o be analyzed for cations were acidified to a pH of 2 using concentrated itric acid, and samples o be analyzed or anions and silica were left unacidi- fled. At each sample location (Figure 3), measurements of temperature, pH, specific conductivity, and salinity were taken directly from stream waters. Discharge was also measured at each sample ocation, using a current meter and calibrated od. However, given the difficulty of measuring he low discharge f the rocky-channeled treams, he discharge data are assumed o be accurate within only 10%. 3.2. Laboratory Methods Water samples collected n the field, as well as stream and well samples collected n the summer of 1992, were subse- quently analyzed or major chemical constituents. ajor cations were determined by DCP spectrometry, nd major anions were determined by ion chromatography. Carbonate alkalinity was determined by titration and Gran evaluation as described by Gieskes and Rogers [1973]. Silica concentrations were deter- mined colofimetrically using a Gilford spectrophotometer. The 1992 base flow samples had a mean charge balance error of-0.9% with a standard deviation of 4.6%, and the 1993 base flow samples had a mean charge balance error of 1.5% with a standard deviation of 3.8% (Table 1). Analytic precision was better than 5% in the determination of all chemical constituents. 3.3. Cyclic Salt Corrections The importance of atmosphefic deposition o fiver-dissolved load has been documented n numerous studies, ncluding nves- tigations n the Pescadero asin by Baldwin [1967b] and Steele [1968]. Steele [1968] collected and analyzed bulk precipitation during he rainy season of 1966-1967 and estimated hat atmos- pheric wet deposition accounted or 25% of the C1- transported by Pescadero Creek at the United States Geological Survey (USGS) gauging station. However, this estimate ignored the significant contribution of dry particulates and fog during the summer months. Baldwin [1967a] indirectly calculated hat the combination of atmospheric wet and dry deposition made up 35% of the C1- transported y Pescadero reek at the gauging station. Two high-elevation ase low samples ollected n this study have C1- concentrations hich are 31 and 34%, respectively, of the base flow C1- concentration measured at the USGS gauging station. These samples, collected at the headwaters of Peters Creek and Lambert Creek (Figure 3), are probably epresentative of the recharge C1- composition ecause hey likely have shallow transport paths that have been well flushed of additional C1- sources. Because Baldwin based his calculation on data collected at the USGS gauging station, his figure may underestimate he atmos- pheric contribution of C1- to the coastal ributafies owing to the preferential ettling of heavier marine aerosols loser o the ocean. However, given the available data, the cyclic C1- component of base low was assumed o be 31% of observed ase low signals and was subtracted ccordingly. he remaining cyclic ions were subtracted based on their molar ratios to C1-. The molar ratios were taken rom Knops [ 1994], who measured wet and dry atmospheric deposition at the Hastings Natural History Reservation, a site  PHILLIPS AND ROJSTACZER: CHEMICAL WEATHERING IN ACTIVE OROGENY REGION 385 122ø22'30 122 5' . :-. ......... ' '.ã -. ã.ã .... -. ã ã :ã ..?ã -' - ;% -. ,;oã*ã hale nd Mudstone -'.] uaternaã Deposits ã*ã ãaqueros andstone ãuãisima ormation [ gindego Basalt ã Santa Cãz Mudstone c-----ã utano pper Sandstone ember '*':'** anta Maãgariã Sandstone :ã ãigeon oiãt ãormation Figure 2. Geologic map of the Pescadero reek watershed adapted rom Brabb and Pampeyan 1983] and Brabb [1989]). located 110 km southeast f the Pescadero asin but with very similar geographic haracteristics. 4. Results 4.1. Geochemistry of Basin Waters The base flow chemical data from 1992 and 1993 are shown in Figure 4. Despite the extreme ithologic variability of the basin, nearly all samples all in the calcium-bicarbonate ield. The lack of compositional ariation suggests hat the waters of the Pesca- dero basin receive heir respective ignals rom the same suite of weathering reactions. Of the possible mineral sources, calcite, dolomite, and pyrite are the most ubiquitous nd easily weathered [Clark, 1981]. However, saline formation waters are also found throughout he sedimentary nits of the basin [Johnson, 1980] and appear o mix with groundwaters nd contribute a significant portion of the chemical signal to discharge Baldwin, 1967a; Steele, 1968]. If weathering eactions nvolving carbonate nd sulfide miner- als and formation water inputs account or the observed hemical signals, then definite stoichiometric elationships between the ions should be present. f saline formation waters are indeed responsible or the observed a + and C1- signatures, 1:1 stoichiometric elationship hould e present etween Na + K +) and CI-, assuming + also has a sea-salt rigin Stallard nd Edmond, 1983]. Figure 5 shows that the data parallel the 1:1 trend but show a slight offset because of excess cations. The excess cations are probably due to the weathering of Na- and K- aluminosilicates revalent n the clastic sedimentary ocks n the basin [Stallard and Edmond, 1983; Clark, 1981]. Similarly, weathering reactions involving calcite, dolomite, and pyrite should produce a 1:1 stoichiometric alance between he equiv- alents f (Ca + + Mg +) and HCO3- + SO4 -) [Stallard nd Edmond, 1983]. Figure 6 shows that the data very closely approximate his 1:1 trend. Data points that fall below the trend line reflect the presence f excess nions, which are necessary o balance the excess cations derived from the weathering of the aforementioned Na- and K-aluminosilicates [Stallard and Edmond, 1983]. The importance of weathering eactions nvolving carbonates and sulfides is further exemplified by a ternary plot of the equivalent roportions f Ca + + Mg +, HCO3-, nd SO42- resent in the sample waters. Figure 7 shows hat within the bounds of analytic precision, ase low samples enerally lot within the field expected or a weathering egime dominated y carbonates nd sulfides. ontributions f Ca + and Mg + from silicate mineral weathering would also fall in this field, but it is not possible o distinguish rom stream chemistry alone the weathering eactions producing these ions [Stallard and Edmond, 1983]. Because contributions f Ca + and Mg + rom silicate eathering annot be theoretically differentiated rom those of carbonate nd sulfate weathering, only the silicate weathering eactions nvolving SiO2, Na +, and K + can be nferred rom he chemical ata. To determine the Na + derived rom silicate eathering, he adjusted 1- values were subtracted rom he adjusted a + values nder he assump- tion that these ions are contributed n equimolar amounts by formation waters. - 4.2. Rates of Chemical Weathering and Denudation Under steady state conditions, he total flux of solutes n the system, Qt, is given by the following equation: Qt = Qr + Qw + Qa + Qg + Qa + Qf - Qp Qb, where Q,, is the flux derived from bedrock weathering, Qw is the flux from atmospheric wet deposition, Qa is the flux from atmospheric ry deposition, g is the flux from atmospheric CO2 gas, Qa is the flux from anthropogenic ources, f is the contribution rom formation ater, Qp s the loss of solutes ue  386 PHILLIPS AND ROJSTACZER: CHEMICAL WEATHERING IN ACTIVE OROGENY REGION Table 1. Chemical and Discharge Data for the Pescadero Creek Watershed Sample Date Ca, Mg, Na, K, Fe, Mn, HCO3, SO4, C1, Number Collected mg L- ã mg L- ã mg L- ã mg L- ã mg L-ã mg L-ã mg L-ã mg L- ã mg L- 112 June 26, 1992 98.6 20.6 41.5 3.81 0.09 0.01 254.5 114.6 59.5 113 June 24, 1992 75.0 17.7 40.0 4.14 0.07 0.00 151.3 153.1 37.5 114 June 26, 1992 88.5 19.5 40.8 3.97 0.13 0.01 202.0 142.5 51.8 115 June 26, 1992 86.8 14.4 31.3 2.90 0.16 0.02 227.6 72.9 38.5 116 June 26, 1992 99.4 22.3 43.5 3.59 0.70 0.30 316.7 78.0 55.1 117 June 26, 1992 93.7 20.5 40.0 4.32 0.08 0.15 216.0 80.8 60.1 118 June 23, 1992 81.0 14.0 40.0 2.65 0.07 0.01 242.9 118.4 46.5 119 June 27, 1992 88.1 19.1 41.0 3.62 0.08 0.02 278.3 87.0 46.6 120 June 27, 1992 101.7 26.2 48.8 6.47 0.17 0.01 364.9 111.0 56.4 121 June 27, 1992 54.0 35.4 59.4 18.01 0.18 0.00 404.0 45.4 163.7 122 June 27, 1992 88.0 21.1 44.6 5.01 0.18 0.01 303.3 90.2 47.9 123 June 27, 1992 49.8 30.9 56.1 11.17 0.16 0.06 331.9 66.8 74.9 124 June 27, 1992 86.7 21.3 46.5 5.44 0.16 0.01 303.3 89.2 58.8 127 June 29, 1992 51.5 30.2 55.5 10.23 0.11 0.01 229.4 129.6 71.3 128 June 29, 1992 68.7 42.7 50.0 10.66 0.16 0.09 256.9 167.5 47.4 129 June 29, 1992 85.9 22.5 47.8 5.76 0.11 0.00 301.4 92.4 63.7 130 June 29, 1992 61.6 37.9 54.0 10.35 0.10 0.01 245.3 157.3 68.4 143 July 6, 1992 90.4 18.2 33.8 2.32 0.08 0.00 353.3 61.0 23.2 144 July 6, 1992 82.2 18.6 40.1 3.93 0.20 0.06 223.3 108.6 49.1 145 July 6, 1992 93.2 23.4 37.1 4.14 0.08 0.01 360.0 82.9 24.5 146 July 6, 1992 83.8 17.7 36.8 3.02 0.20 0.04 283.1 78.7 36.0 147 July 6, 1992 51.1 31.9 54.2 15.22 0.16 0.03 339.9 127.2 30.9 148 July 6, 1992 108.6 25.8 47.2 4.71 0.12 0.02 367.3 103.0 56.6 149 July 7, 1992 82.1 23.8 50.8 5.84 0.26 0.09 305.7 98.5 71.2 150 July 7, 1992 67.3 19.5 37.0 3.39 0.20 0.03 228.8 70.4 57.0 151 July 7, 1992 79.2 21.7 46.5 5.56 0.12 0.02 291.7 90.8 57.2 153 July 12, 1992 153.6 33.7 42.7 3.93 0.11 0.01 311.2 214.8 25.3 154 July 12, 1992 66.9 19.8 37.9 2.98 0.10 0.01 261.2 54.6 46.6 155 July 12, 1992 106.9 24.1 41.0 3.11 0.13 0.01 344.8 122.2 19.6 156 July 12, 1992 82.6 21.7 41.8 3.29 0.12 0.02 292.3 100.4 46.9 157 July 12, 1992 86.7 15.6 27.4 2.11 0.08 0.01 313.6 52.4 18.2 158 July 18, 1992 45.6 18.4 36.6 4.00 0.08 0.01 263.6 38.7 29.7 159 July 18, 1992 54.1 16.5 33.7 3.07 0.14 0.04 247.1 52.3 28.7 201 July 7, 1993 92.0 21.0 41.3 3.65 0.28 0.05 284.4 86.7 45.3 202 July 7, 1993 103.7 24.8 48.5 3.51 0.16 0.02 285.6 101.9 66.3 205 July 7, 1993 91.8 21.4 42.5 3.83 0.11 0.01 283.7 90.9 47.8 206 July 7, 1993 64.2 18.0 41.1 3.26 0.17 0.04 183.7 78.1 53.5 207 July 8, 1993 92.3 18.1 32.3 2.51 0.13 0.03 282.5 79.0 37.2 208 July 8, 1993 47.8 10.3 27.6 1.99 0.01 0.00 145.2 45.8 36.6 209 July 8, 1993 46.0 10.2 20.8 2.45 2.12 0.49 123.3 66.2 25.6 210 July 8, 1993 98.5 19.5 35.4 2.65 0.12 0.01 271.5 81.8 41.8 211 July 9, 1993 45.4 9.4 20.1 2.08 0.08 0.02 131.2 43.5 30.5 212 July 9, 1993 98.8 21.2 38.2 3.22 0.15 0.02 270.3 87.3 40.1 213 July 9, 1993 99.2 21.4 38.0 3.22 0.17 0.02 288.6 89.7 40.4 214 July 9, 1993 26.2 17.1 34.4 4.01 0.45 0.35 195.3 4.4 51.5 215 July 11, 1993 38.5 11.9 29.5 2.30 0.96 0.03 166.0 28.3 37.1 216 July 11, 1993 12.2 9.5 37.2 3.01 0.75 0.01 105.0 26.0 40.3 217 July 11, 1993 28.9 17.8 46.8 4.98 4.87 0.22 80.5 131.6 50.4 218 July 11, 1993 33.9 11.8 31.4 2.46 0.81 0.04 153.2 31.4 42.5 219 July 11, 1993 88.4 22.9 48.0 4.02 0.29 0.03 279.5 90.1 53.5 220 July 11, 1993 58.6 31.0 63.2 3.21 0.64 0.04 264.8 147.5 56.8 221 July 11, 1993 87.5 23.4 48.3 4.02 0.29 0.05 273.4 100.3 55.4 222 July 11, 1993 82.6 43.7 63.8 2.38 0.50 0.61 278.3 128.4 90.0 223 July 12, 1993 35.6 17.7 24.6 2.36 0.61 0.00 205.0 32.2 19.4 224 July 12, 1993 83.4 18.4 24.8 1.83 0.16 0.00 251.4 66.8 28.8 225 July 13, 1993 73.8 10.1 23.7 1.94 0.06 0.00 231.9 34.1 32.0 226 July 13, 1993 97.7 20.4 39.0 3.31 0.18 0.18 259.9 99.9 46.1 227 July 14, 1993 98.9 19.8 34.9 2.55 0.05 0.01 288.0 81.0 35.7 228 July 14, 1993 61.8 11.6 18.2 1.57 0.13 0.02 173.9 59.4 22.4 229 July 14, 1993 99.5 19.6 35.6 2.50 0.03 0.00 299.0 78.7 38.0 230 July 14, 1993 59.8 10.2 21.1 1.47 0.08 0.01 174.5 45.7 26.5 231 July 14, 1993 33.1 6.6 22.2 1.36 0.87 0.16 106.8 23.5 35.5 232 July 15, 1993 54.7 8.6 24.3 1.68 0.15 0.01 183.1 31.7 35.5 233 July 15, 1993 92.1 21.1 43.4 3.69 0.03 0.00 300.8 93.0 48.8 234 July 15, 1993 71.3 12.5 28.8 1.99 0.00 0.00 207.5 64.3 34.4 235 July 15, 1993 89.4 20.6 43.5 3.62 0.03 0.00 299.6 99.6 46.0 236 July 15, 1993 70.9 14.4 29.4 2.18 0.02 0.01 202.6 64.5 40.5 237 July 18, 1993 45.8 23.6 48.1 2.55 0.41 0.21 202.0 73.3 57.8 238 July 18, 1993 20.1 21.1 34.3 5.07 0.19 0.00 57.4 82.9 62.7 239 July 18, 1993 85.4 67.2 68.9 6.13 0.14 0.04 108.6 114.8 82.0 SiO2, Discharge, mg L ã L s ã 23.6 1.59 17.5 1.22 20.2 1.64 20.1 0.48 29.1 2.89 17.0 1.87 20.1 2.38 23.0 32.00 29.6 15.80 43.5 0.82 24.4 46.38 49.7 0.99 24.5 48.79 54.6 0.91 50.6 0.74 24.8 78.92 47.9 2.27 25.2 20.27 17.7 10.31 28.5 3.03 21.9 31.86 48.9 0.85 26.1 10.51 25.5 59.81 * 0.57 * 76.03 * 1.02 * 6.14 22.8 2.27 22.0 12.60 24.3 13.88 24.6 1.39 23.6 1.81 23.8 133.40 23.4 2.78 24.1 107.26 24.7 6.06 * 64.53 20.4 0.22 21.8 2.10 22.6 112.59 22.4 1.50 22.9 124.42 23.1 128.67 * 0.00 18.8 59.35 23.9 12.40 20.8 2.83 19.6 51.51 24.1 157.30 19.5 1.10 23.7 164.78 22.8 0.62 * 0.02 * 2.66 22.2 1.22 22.3 22.65 23.1 66.55 17.9 9.43 22.9 61.33 18.5 1.47 17.6 4.67 18.4 2.35 24.1 117.83 18.2 0.40 23.7 124.14 17.8 1.70 36.4 0.96 43.8 0.00 18.9 0.88
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