A modern vs. Permian black shale—the hydrography, primary productivity, and water-column chemistry of deposition

Chemical Geology 206 (2004) 177 – 197 www.elsevier.com/locate/chemgeo

A modern vs. Permian black shale—the hydrography, primary productivity, and water-column chemistry of deposition D.Z. Piper *, R.B. Perkins 1 U.S. Geological Survey, M/S 902, 345 Middlefield Road, Menlo Park, CA 94025, USA

Abstract The sediment currently accumulating in the Cariaco Basin, on the continental shelf of Venezuela, has an elevated organiccarbon content of approximately 5%; is accumulating under O2-depleted bottom-water conditions (SO24 reduction); is composed dominantly of foraminiferal calcite, diatomaceous silica, clay, and silt; and is dark greenish gray in color. Upon lithification, it will become a black shale. Recent studies have established the hydrography of the basin and the level of primary productivity and bottom-water redox conditions. These properties are used to model accumulation rates of Cd, Cr, Cu, Mo, Ni, V, and Zn on the seafloor. The model rates agree closely with measured rates for the uppermost surface sediment. The model is applied to the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, a phosphate deposit of Permian age in the northwest United States. It too has all of the requisite properties of a black shale. Although the deposit is a world-class phosphorite, it is composed mostly of phosphatic mudstone and siltstone, chert, limestone, and dolomite. It has organic-carbon concentrations of up to 15%, is strongly enriched in several trace elements above a terrigenous contribution and is black. The trace-element accumulation defines a mean primary productivity in the photic zone of the Phosphoria Basin as moderate, at 500 g m 2 year 1 organic carbon, comparable to primary productivity in the Cariaco Basin. The source of nutrient-enriched water that was imported into the Phosphoria Basin, upwelled into the photic zone, and supported primary productivity was an O2 minimum zone of the open ocean. The depth range over which the water was imported would have been between approximately 100 and 600 m. The mean residence time of bottom water in the basin was approximately 4 years vs. 100 years in the Cariaco Basin. The bottom water was O2 depleted, but it was denitrifying, or NO3 reducing, rather than SO24 reducing. Published by Elsevier B.V. Keywords: Phosphoria; Cariaco basin; Trace elements; Primary productivity; Redox

1. Introduction The geochemistry of several black shales resembles the geochemistry of sediments accumulating * Corresponding author. Tel.: +1-650-329-5187; fax: +1-650329-5373. E-mail address: [email protected] (D.Z. Piper). 1 Present address: Department of Geology, Portland State University, P.O. Box 751, Portland, OR, 97207-0751, USA. 0009-2541/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.chemgeo.2003.12.006

today in shallow-silled basins such as the Black Sea, Cariaco Basin, and several fjords at high latitude. Both are enriched in organic matter and commonly in trace metals (Vine and Tourtelot, 1970; Coveney et al., 1987; Calvert, 1990; Crusius et al., 1996). The modern basins are characterized by a strong depletion of O2 in the bottom water (sp. SO42 reduction), lending support to the interpretation that black shales were likewise deposited under conditions of O2 depletion in the water column (Wignall, 1994). Research

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of the modern basins (Richards, 1975; Jacobs et al., 1985, 1987; Izdar and Murray, 1991; Muller-Karger et al., 2001) has examined their bathymetry, hydrography, primary productivity, flux of organic matter and siliciclastic debris onto the seafloor, and water-column chemistry, all of which contribute to the sediment composition (Piper and Dean, 2002). A comparison of trace-element accumulation rates in these environments with accumulation rates in black shales may offer a unique opportunity to decipher the oceanography of deposition for the ancient deposits. Depletion of O2 in the bottom water of the modern basins is maintained in large part by a weak bottomwater circulation that is reflected in a bottom-water residence time of as much as several hundred years, e.g., the Black Sea (Izdar and Murray, 1991). The circulation likely contributes to the retention of sediment fractions that rain onto the seafloor. Open-ocean continental shelves such as the Peru shelf also exhibit O2 depletion in the water column, specifically denitrification. There, a high flux of organic matter settling through the water column (von Brockel, 1981) is more important in maintaining O2 depletion than it is in the silled basins. The high flux results from exceptionally high primary productivity (Codispoti, 1980; Chavez and Barber, 1987), driven by strong upwelling of nutrient-enriched seawater into the photic zone. An intense bottom current, an integral part of the circulation that drives the upwelling (Chavez and Toggweiler, 1995), imparts a relatively short residence time on the bottom water. It contributes to the masking of any simple relationship between the geochemistry of the sediment and the chemistry and biology of the water column (Pedersen et al., 1992; Arthur et al., 1998), owing to reworking of the seafloor sediment. The circulation maintains an O2 flux across the seafloor sufficient to support an abundant bottom fauna, even under the conditions of extreme O2 depletion (Arthur et al., 1998). The fine-grained and laminated texture of many black shales, texture of authigenic phases (Cook, 1968), and high trace-element concentrations suggest that the circulation in the shallow-silled basins is more representative of the circulation that characterized the black-shale depositional environments than is the seawater circulation on the Peru continental shelf. The bathymetry of the ancient basins may have differed significantly from the bathymetry of the silled basins. The black shales were deposited in epicratonic

seas, where the sill-to-basin depth relation may have been less of a factor in contributing to O2 depletion in the water column (Wignall, 1994). The oceanward margin of some of the ancient depositional environments could have been defined by an island arc, or uplifted highland, but possibly more important to the eventual composition of the sediment, many of these basins were several hundred kilometers wide. Examples include the Chainman and Deseret Basins of Mississippian age (Poole and Claypool, 1984; Sandberg and Gutschick, 1984) and the Phosphoria Basin of Permian age (McKelvey et al., 1959; Sheldon, 1989) in the western United States. Their expanse, with or without an outer shoaling margin, would have partially isolated them from a major ocean-boundary current and limited the accumulation rate per unit area of terrigenous debris. These aspects of deposition imposed a lower level of upwelling and primary productivity than on open-ocean continental shelves, a bottom-water residence time that was perhaps intermediate between the residence time of bottom water in shallow-silled basins and on continental shelves, and a more prominent marine signal of deposition, relative to the terrigenous signal, than in both modern environments. In summary, no single modern marine environment seems to be an analog to the environment(s) in which black shales accumulated. The different oceanographic properties for a suitable analog are present today, but only in several separate marine environments. The Cariaco Basin (Fig. 1A) may represent one such partial analog. The level of upwelling and primary productivity (Walsh et al., 1999; Varela et al., 1999), flux of organic matter settling through the water column (Muller-Karger et al., 2001), residence time of bottom water (Deuser, 1973), redox conditions and trace-element profiles in the water column (Jacobs et al., 1987; Emerson and Huested, 1991; DeBaar et al., 1988), and sediment accumulation rate, texture, color, and composition (Peterson et al., 1991; Hughen et al., 1996; Lin et al., 1997; Piper and Dean, 2002) have been determined. Our first aim is to establish the relationship between the trace-element geochemistry of the sediment in this basin and the hydrography and redox profile of the water column, and flux of organic matter and siliciclastic debris to the seafloor. The trace-element model is then applied to the Meade Peak Phosphatic Shale Member of the Phos-

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Fig. 1. Location maps for (A) the modern Cariaco Basin and (B) the Permian Phosphoria Basin, showing the generalized paleogeography, modern-day latitude and longitude coordinates, paleolatitude, and state boundaries.

phoria Formation, a world-class phosphate deposit in the northwest United States (Fig. 1B). The deposit has the requisite properties of a black shale (Wignall, 1994). It contains up to 15% organic carbon, was

deposited under conditions of O2 depletion in the water column (Piper, 2001), is enriched in trace elements (Piper et al., 2000), and is black. Ore-grade carbonate fluorapatite is concentrated in two units

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(McKelvey et al., 1959; Fig. 2), but the greater part of the deposit varies from phosphatic mudstone to phosphatic siltstone, interbedded with shale, carbonate, and chert units. The trace-element accumulation model provides absolute values for primary productivity in the Phosphoria Basin, source and residence time of bottom-water, and redox conditions of the water column. Earlier investigations have interpreted these properties only in very general terms that, nonetheless, differ significantly from the model values. Many assumptions must be made to relate the trace-element geochemistry of the Meade Peak to the dynamics of the Cariaco Basin. Although we cannot evaluate all of these assumptions as well as we would like, we have included best estimates of

their associated errors in calculating cumulative magnitudes of uncertainties for the model parameters.

2. Sources of data Sediment data for the Cariaco Basin (Piper and Dean, 2002) were obtained from the analysis of 194 samples collected from piston core PL07-39PC (Fig. 1A). Chemical data for the Meade Peak (Piper et al., 2000) were obtained from 75 rock samples collected in the mine adit at the Hot Springs Mine (Gulbrandsen and Krier, 1980) in southeast Idaho (Fig. 1B). The Hot Springs section (Fig. 2) is minimally weathered as evidenced by its dark color, induration, high total

Fig. 2. (A) Permian correlation chart for southeast Idaho and eastern Wyoming and (B) stratigraphic column of the Meade Peak Phosphatic Shale Member in southeast Idaho, modified from McKelvey et al. (1959) and Maughan (1994). In (A), series names, stage names, and ages (in m.y., in the stage column) are recommended by the International Commission on Stratigraphy. Ages to the far left of the correlation chart are from Harland et al. (1990). In (B), thicknesses (0 – 50) are given in meters. The carbonate below 0 m is the Grandeur Member of the Park City Formation; the chert above 51.75 m is the Rex Chert Member of the Phosphoria Formation. Chemical analyses show that dolomite is the dominant carbonate present in the Meade Peak in southeast Idaho and western Wyoming (Gulbrandsen and Krier, 1980).

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organic-carbon content (mean >6%), and abundance of sulfide minerals—pyrite and sphalerite—present in all thin sections examined (Perkins and Foster, 2004).

3. The Cariaco Basin The Cariaco Basin consists of two basins separated by a 900-m-deep saddle (Fig. 1). Bottom water is isolated from the deep water of the Caribbean Sea by a 146-m-deep sill. The sill limits exchange of water between the Cariaco Basin and Caribbean Sea, imposing a residence time on bottom water of approximately 100 years (Deuser, 1973). The slow renewal of bottom water has allowed oxidation of settling organic matter via bacterial respiration to exhaust the O2 and NO3 supply and establish SO42 -reducing conditions below approximately 300 m depth (Richards, 1975). Oxygen respiration occurs in the uppermost approximately 250 m of the water column and denitrifying conditions at the intermediate depth (Hashimoto et al., 1983). 3.1. Bottom-water source of trace elements The redox profile (Fig. 3) limits trace elements that are deposited from the bottom water by adsorption/ precipitation reactions (Jacobs et al., 1987), the hydrogenous fraction. Trace elements removed from seawater by inorganic reactions under oxic conditions, such as Mn, are reduced to a soluble ionic state under conditions of O2 depletion (Fig. 4). The REE are adsorbed onto particulate phases under oxic to denitrifying conditions and returned to solution in SO42 reducing bottom waters (DeBaar et al., 1988). Cr and V are reduced and accumulate under denitrifying to SO42 -reducing conditions (Piper, 2001), whereas Cd, Cu, Mo, and Zn precipitate from bottom water only under conditions of SO42 reduction (Jacobs et al., 1985, 1987; Landing and Lewis, 1991). Thus, the suite of trace elements that accumulates from bottom water is determined by the redox condition in the bottom water. The latter two groups are enriched in the Cariaco Basin sediment above terrigenous and biogenic inputs (Piper and Dean, 2002). The rate at which a suite of trace elements accumulates from bottom water is determined by the rate of their renewal via the renewal of bottom water, or

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advection, expressed as the bottom-water residence time. Advection further contributes to the bottomwater redox level itself, as the redox level represents a balance between the rate of supply of oxidants—O2, NO3 , and SO42 —via advection and the rate of their depletion via bacterial oxidation of labile organic matter settling through the water column. The latter is determined by the level of primary productivity (Muller-Karger et al., 2001), but ultimately by the rate of upwelling. These relations demonstrate the central role of the basin hydrography—advection of bottom water and upwelling into the photic zone—to the overall accumulation of trace elements from seawater. 3.2. Photic-zone source of trace elements Organic matter and its hosted trace elements, or trace nutrients, represent the biogenic fraction (Table 1). Its deposition is governed largely by primary productivity, which averages 1.5 g C m 2 day 1 (Varela et al., 1999). It is limited, as noted earlier, by the renewal of nutrients to the photic zone via upwelling. Primary productivity is highest during the spring and winter (Muller-Karger et al., 2001). During summer and autumn, trade winds weaken and upwelling decreases. On average, 5.8% of the organic matter fixed by algae in the photic zone (Fig. 3) is exported to 300 m depth (Muller-Karger et al., 2001), but only 1.8% settles onto the seafloor at 1400 m. The difference represents the organic matter oxidized at depth by SO42 -reducing bacteria, as the organic matter settles through the water column (Muller-Karger et al., 2001). The organic matter that settles through the water column during the winter/spring is deposited as light-colored laminae, owing to their high concentrations of biogenic CaCO3 and SiO2. Seawater depth profiles of trace elements in the Caribbean Sea and Atlantic Ocean, vs. PO43 , have been used to estimate the stoichiometry of the organic matter (Table 1). This procedure assumes the profiles reflect solely oxidation of the host organic matter at depth and advection. The assumption seems reasonable for Cd (Piper and Dean, 2002). Depth profiles of many of the trace nutrients within the oligotrophic environment of the open ocean, however, can be adjusted by several non-biogenic processes, e.g., preformed nutrient concentrations in water masses and scavenging at depth by Mn and Fe oxides (Boyle et

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Fig. 3. Schematic for the Cariaco Basin, showing the relation of ‘‘model’’ accumulation rates of the marine and terrigenous fractions of Mo (in parenthesis) and Cd to current conditions of primary productivity (55 mg C cm 2 year 1), water-column redox, bottom-water residence time (100 years), and terrigenous input. Depths are not to scale. The two curves in the left side of the figure show seawater depth profiles for Mo (  10 3) and Cd (Jacobs et al., 1987; Emerson and Huested, 1991). Model accumulation rates (Table 1) and sediment concentrations of Cd and Mo, given at the bottom of the figure, compare closely with measured values (Piper and Dean, 2002).

al., 1977; Measures et al., 1983; Sherrell and Boyle, 1992; Whitfield, 2001). Thus, direct measurements of trace-element contents in organic matter are preferable for establishing its stoichiometry.

For the North Pacific Ocean, trace-element concentrations in plankton and suspended organic matter have been reported in several recent studies. For the most important trace nutrients of this study (Cd, Cu,

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Fig. 4. Schematic representation of bacterial respiration vs. depth in the water column of marine basins. Changes in O2, NO3 , and H2S concentration with depth are representative of Saanich Inlet, British Columbia (Emerson et al., 1979). Half-cell reduction reactions are shown for the three major electron acceptors. Stabilities of trace-element compounds are shown at right, calculated from thermodynamic constants (see Piper, 2001). In the case of Cr, it precipitates possibly as Cr(OH)3, or is adsorbed onto settling particles, under conditions of mild denitrification. It is stable throughout the O2-depleted region of the water column. Under oxic conditions, Cr is in the oxidized and soluble CrO24 valence state (Murray et al., 1983). MnO2 responds oppositely; it is reduced to a soluble valence state (probably Mn2 +) under increasingly reducing conditions.

Ni, and Zn), the calculated values for the Pacific Ocean, based on their seawater-depth profiles, differ from measured plankton values by no more than a factor of 2 (Table 2). The measured values will be used below when considering the Meade Peak, even though they are representative of the modern ocean, which is separated from the Phosphoria Sea by approximately 260 m.y. (Fig. 2). The accumulation rates of the trace elements—Cd, Cr, Cu, Mo, Ni, V, and Zn—within the biogenic fraction are calculated from the flux of organic matter into the zone of sulfate reduction, not the lesser amount that settles onto the seafloor. The 5.8% is used owing to the way the hydrogenous fraction of these trace nutrients is calculated. Their accumulation

from bottom water (see below) is determined from their depletion in the bottom water, relative to their concentration in the water imported at depth. The calculation does not include the trace-element fraction added to the bottom water via the oxidation of settling organic matter. 3.3. Terrigenous source of trace elements The summer/autumn decrease in trade winds and upwelling reflects a shift of the intertropical convergence zone. It is accompanied by an increase of rainfall along the northern coast of South America (Peterson et al., 1991), resulting in an elevated flux into the basin of a third sediment fraction, the terrig-

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Table 1 Concentrations of trace elements in seawater and organic matter in the Cariaco Basin and their rates of deposition in the surface sediment Metal Trace-element concentration

Mo Cd Cu Zn V Ni Cr

Trace-element accumulation rate

Cariaco Basin source water (ppb)a

Cariaco Basin bottom water (ppb)a

Organic matter (ppm)b

Terrigenous contribution (Ag cm 2 year

10.7 0.012 0.069 0.124 1.48 0.15 0.22

9.1 0.001 0.028 0.043 0.484 0.15 0.088

2 5.5 5 30 3 18 2

0.015 0.002 0.243 0.736 1.251 0.478 0.883

Biogenic contribution 1 c ) (Ag cm 2 year

0.018 0.049 0.047 0.268 0.027 0.159 0.018

Hydrogenous contribution 1 c ) (Ag cm 2 year

0.963 0.006 0.029 0.049 0.596 0 0.053

Calculated rate (Ag cm 2 year

1

Measured rate ) (Ag cm 2 year

1

)

1 c

)

0.996 0.057 0.319 1.053 1.874 0.638 0.954

1.024 0.061 0.318 1.187 2.326 0.771 1.224

Calculated rates are based on trace-element concentrations in organic matter (the biogenic contribution) and terrigenous debris (the terrigenous contribution) and the flux of each through the water column, and on the bottom-water residence time and difference in elemental concentrations in source water and bottom water (the hydrogenous contribution). The measured rates represent accumulation rates based on the concentrations of trace elements in the surface sediment and the bulk sediment accumulation rate. a Concentrations of Mo, Cd, Cu, V, and Ni in source water are values of water in the Cariaco Basin between 100- and 250-m depth (Jacobs et al., 1987; Emerson and Huested, 1991). Concentrations of Zn and Cr in source water are of open-ocean seawater in the North Atlantic (Bruland and Franks, 1983) and North Pacific (Murray et al., 1983) Oceans at approximately 1500-m depth, respectively. Concentrations of metals in bottom water are from Jacobs et al. (1987), except for Zn and Cr. Their bottom-water concentrations assume 65 and 60% removal, respectively, from source water via precipitation/adsorption reactions. These estimates are based on concentration profiles of Zn in other basins that exhibit oxygen depletion in the bottom water (Fig. 6b) and Cr profiles in the water column in Saanich Inlet (Emerson et al., 1979), in which approximately 60% of the Cr(VI) is reduced at depth to Cr(III). b The Cd and Ni concentrations in organic matter are based on their seawater depth profiles and that of phosphate in the Caribbean Sea (Jacobs et al., 1987) and the phosphate concentration in organic matter (Redfield et al., 1963). The Cu and Zn concentrations are based on these values in the North Atlantic Ocean (Bruland and Franks, 1983). The Cr, Mo, and V values are given in Brumsack (1986). Actual analyses of plankton from the Atlantic Ocean or Caribbean Sea are too few to establish mean concentrations. c The current accumulation rate of terrigenous sediment is 7.36 mg cm 2 year 1; the flux of organic matter into the bottom water is 8.89 mg cm 2 year 1; the volume of bottom water above the core site is 60 l cm 2, or 600 m; and the residence time of bottom water is 100 years.

enous fraction. The seasonally elevated accumulation of this fraction is deposited as a dark lamina that, together with the lighter colored lamina of the winter/ spring period, forms an annual varve (Hughen et al., 1996). This fraction contains a complex inventory of trace elements (Table 1). Its composition is determined from plots of Al2O3 vs. the trace elements (Fig. 5), for which alumina is assumed to be partitioned totally into the terrigenous fraction. The procedure has been established by an examination of both modern and ancient sedimentary deposits representing a wide spectrum of environments (Leinen, 1977; Dymond, 1981; Piper et al., 2000). The regression curves for Al2O3 vs. Fe2O3, Ga, Th, and La (plus the other rare-earth elements) in the Cariaco Basin sediment extrapolate to the origin and to approximate world shale average values. The terrigenous contribution of these and several additional trace elements totally dominates their bulk sediment concentration. A

major drawback of the stoichiometric procedure is that it fails to identify a seawater contribution when the concentration of that contribution constitutes less than the precision of the technique. 3.4. Model accumulation rates The total accumulation rate of each trace element can be calculated as the sum of the three fractions, based on the trace-element stoichiometry of organic matter and terrigenous debris (Piper and Dean, 2002), and its level of removal from bottom water under the conditions of SO42 reduction, or its concentration in the water fluxing into the bottom of the basin vs. its current concentration in the bottom water (Jacobs et al., 1987; Emerson and Huested, 1991). As examples, consider the accumulation rates of Mo and Cd (Fig. 3) under the current oceanographic conditions. The immediate source of both upwelling water (Muller-

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Table 2 Selected major-oxide and trace-element concentrations in terrigenous debris, seawater, and plankton World shale average (terrigenous fractions) (%, Ag g 1)a Al2O3 Fe2O3 PO34 Cd Cr Cu La Li Mo Ni Th V Zn

15.5 (18.1, 16.6) 6.84 (6.6, 5.4) 0.24 (0.24, 0.25) 0.2 (0.2, 0.2) 83 (120, 140) 35 (33, 34) 41 (41, 41) 76 (82, 76) 2.0 (2, 2) 42 (65, 105) 12 (12, 14) 130 (170, 120) 100 (100, 100)

North Atlantic seawater (1500 m) (Ag kg 1)

North Pacific seawater (1500 m) (Ag kg 1)

North Pacific plankton measured (mg g 1)b

North Pacific plankton calculated (mg g 1)c

North Atlantic plankton calculated (mg g 1)c

123 0.035

285 0.12 0.22 0.13

32 0.012 0.002 0.011

27 0.013 0.007 0.009

27 0.0069

10.4 0.34

10.6 0.59

0.002 0.019

0.040

0.041

0.13

1.8 0.52

0.003 0.110

0.011 0.060

0.030

0.095

0.005

a

World shale average values are from Wedepohl (1969 – 1978). Concentrations in parentheses are the terrigenous fraction in the following (Cariaco Basin, Meade Peak). The oxides are in percent; the trace elements are in ppm. b Measured concentrations are reported in Collier and Edmond (1983), Elderfield and Greaves (1983), and Brumsack (1986). c Calculated values are based on seawater depth profiles of trace elements and PO43 in the North Pacific and North Atlantic Oceans (Bruland, 1983; Bruland and Franks, 1983; Murray et al., 1983; Collier, 1984, 1985) and a single PO43 concentration in plankton of 2.7% (Redfield et al., 1963).

Karger et al., 2001) and bottom water (Jacobs et al., 1987) is the water at a depth of about 100 –250 m. The 15% reduction of the Mo concentration in the bottom water represents 95% of the Mo accumulating on the seafloor; 80% of the accumulating Cd is associated with settling organic matter. Therefore, Mo defines the level of SO42 reduction and the rate of advection of bottom water; Cd largely defines the rate of organic matter settling into the bottom water, driven by primary productivity, but ultimately by the rate of upwelling into the photic zone. The fluxes of Al2O3, as well as Ga and Th, uniquely define the flux of the terrigenous fraction. The contribution of Cd and Mo by this fraction is minor, but it represents the largest contribution of Cu, Ni, V, and Zn (Table 1). Even so, the calculated accumulation rates for Cu, Ni, V, and Zn agree closely with their measured rates (Table 1), where the measured rate is obtained from the sediment accumulation rate (Lin et al., 1997) and bulk sediment composition. Molybdenum correlates with organic matter (Piper and Dean, 2002), even though organic matter represents a minor Mo source under bottom-water conditions of SO42 reduction (Fig. 3). The two commonly

correlate in black shales as well (Brumsack, 1986; Coveney et al., 1987). A possible explanation for this correlation is the dual role of organic matter: (1) of carrying Mo directly to the seafloor and (2) of contributing to the level of H2S in the bottom water, which determines the degree of Mo removal from the bottom water (Fig. 6a), all else being equal. This interpretation assumes that the current concentration of organic matter bears some positive relationship with the initial concentration of organic matter. The same argument may possibly account for correlations between organic matter and V and Cr. Cu and Zn may or may not correlate with organic matter (Piper and Dean, 2002), as their rate of accumulation from bottom water is largely a function of advection under virtually the full range of H2S concentrations (Fig. 6b). Cadmium accumulation from bottom water is independent of the H2S concentration (Fig. 6b), but its biogenic input to the sediment overwhelms the bottom-water hydrogenous input. The correlation between Ni and organic matter likely reflects the accumulation of Ni within the marine fraction solely as a trace nutrient. Indeed, the elevated concentration of Ni is strong support for the interpretation that it and the other trace elements accumulate as trace nutrients.

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Fig. 5. The relations between the concentrations of Al2O3 and the concentrations of Fe2O3 and selected trace elements. The solid curves give the relations for the world shale average, or WSA (Wedepohl, 1969 – 1978), mixed with diluting phases, for example SiO2 of biogenic origin, calcite – dolomite, and organic matter. In frame (c), the elevated concentration of La in the Meade Peak, above the curve for WSA, is interpreted to represent the seawater contribution. In frame (d), V shows a seawater contribution in both populations of samples. The y-axis in (d) has been selected to show clearly the enrichment of V in the Cariaco Basin. Expansion of the y-axis to 4000 ppm V shows that roughly 30% of the Meade Peak samples have a concentration above 500 ppm. The shaded area gives the range of V in the terrigenous fraction of the two deposits. Other elements that show an enrichment in both deposits, above the terrigenous contribution, include Cd, Cr, Cu, Mo, Ni, and Zn. Sb, Se, and U are also enriched in the Meade Peak (Piper et al., 2000; Piper, 2001). Based on the model, they should show an enrichment in the sediment from the Cariaco Basin as well, but they have not been measured.

3.5. Basin hydrography An upwelling rate in the Cariaco Basin can be calculated from the uptake rate of nutrients in the photic zone and their concentrations in the upwelling water. For Cd and PO43 , which are taken up essentially totally by phytoplankton in the photic zone (Bruland, 1983), the upwelling rate is approximately 760 m year 1, similar to the numerical model rate developed by Walsh et al. (1999). A mean rate of bottom-water advection of 11 m year 1 is calculated from the thickness of the O2-depleted

water column (1100 m) and the bottom-water residence time (100 years). This rate is supported by the agreement of measured and modeled rates of accumulation of Mo.

4. The Phosphoria Basin The bathymetry of modern basins is a major factor contributing to their hydrography. The depth of the Phosphoria Basin has been estimated at 200 – 1000 m (McKelvey et al., 1959). We can only assume a depth

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Fig. 6. The percentage of Cd, Cu, Ni, and Zn (a) and Mo and V (b) removed from SO42 -reducing bottom waters of modern marine basins (Kremling, 1983; Jacobs et al., 1985, 1987; Emerson and Huested, 1991; Landing and Lewis, 1991), relative to their mean concentrations in the sources of bottom water, vs. the mean concentration of H2S in the bottom waters. The curves represent regressions of best fit, of the form y = C1 + C2log [x], for which R = 0.75 (Cd), 0.31 (Cu), 0.00 (Ni), 0.70 (Zn). The equation for curve (1) for Mo vs. H2S has R = 0.96. It does not include the datum from Framvaren Fjord. Curve (2), R = 0.66, includes all Mo data. Curve (a) was used in our calculations. Depth profiles of Cd, Cu, and Zn in the Baltic Sea, Black Sea, and Framvaren Fjord exhibit strong depletions in the bottom water; from which all three may precipitate as sulfides (Jacobs et al., 1985). In the Cariaco Basin, only Cd and Cu have been measured, where they also are strongly depleted in the bottom water (Jacobs et al., 1987). Zn has not been reported, but its depletion in the other basins suggests that its depletion in the bottom water of the Cariaco Basin is similar to its depletion in these other basins. The concentration of Ni is essentially constant with depth in all of the basins for which it has been reported. The relation between DV and H2S clearly does not exhibit a simple regression.

in our calculations of 350 m. In an earlier paper, we (Piper, 2001) speculated why the basin might have been somewhat deeper, but was unlikely to have been shallower. Assuming a greater depth of 550 m would certainly change the absolute values of our calculations but would not change our overall interpretations. The basin was 300– 400 km wide and covered more than 300,000 km2 (Sheldon, 1989). Similar to the Cariaco Basin, the Phosphoria Basin was partially isolated from the open ocean by the remnant of the Antler Uplift to the west (Tisoncik, 1984; Maughan, 1994). It was further partially isolated from the highintensity eastern boundary current of the open ocean by its lateral expanse. For the Cariaco Basin, the trace-element profiles of the water column, which give the degree of removal of trace elements from the bottom water, and stoichiometry of plankton and settling organic matter (largely algal) were obtained from recent work. We can only assume values for the Phosphoria Sea based on the modern ocean, but even estimates of the stoichiometry of plankton in the modern ocean have a degree of uncertainty, as previously noted. Agreement of the hydrography based on each of the different

trace elements and agreement of the hydrography with the hydrography of upwelling environments in the ocean today should provide a check on our assumed concentrations of trace elements in the three different source fractions—basinal bottom water, organic matter, and terrigenous debris—and their rate of accumulation. Only the duration of deposition of 10 m.y. is available for the Phosphoria Formation (Hein, 2004) and approximately 5 m.y. for the Meade Peak (Piper, 2001). Both seem reasonable from the ages and uncertainty of ages that bracket the Meade Peak (Fig. 2), although a significantly briefer period for the Meade Peak is favored by Wardlaw (1999). The single date of 5 m.y. allows us to calculate merely a mean bulk sediment accumulation rate. At the Hot Springs Mine, the rate was 2.7 mg cm 2 year 1 or 22% of the current rate for the Cariaco Basin of 12.2 mg cm 2 year 1 (Lin et al., 1997). The high seawater-derived trace-element concentrations in the Meade Peak alone, between 10  (Ni) and 300  (Zn) higher than in the Cariaco Basin (except for Mo, see below), support a mean bulk sediment accumulation rate that was much less than the rate in the Cariaco

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Basin. A difference of 4.5-fold would seem to represent a minimum. Any loss of sediment through reworking would have led to an underestimate of the depositional rate and of the intensity of the circulation. Conversely, the sediment depositional rate may have been overestimated, owing to possible reworking of sediment in the margins of the basin (Piper and Link, 2002) and its redeposition in the more phosphogenic areas. The lack of abrasion of fossils (McKelvey et al., 1959) and absence of sedimentary features such as cross-bedding, rip-up clasts, and scour and fill features throughout the area of ore-grade phosphate (Gulbrandsen, personal communication) suggest that the latter was more likely than the former. That is, the rate at which the marine fractions rained onto the seafloor, interpreted to have been derived from the immediately overlying water column, may have been overestimated. Furthermore, upwelling and bottom-water advection during any time interval surely differed from the mean rate, as determined from the average sediment and trace-element accumulation rates. The lithology (Fig. 2) clearly suggests that the rates varied temporally. However, quantifying mean rates should advance our understanding of the processes that controlled the composition of the sediment and how changes in those processes interacted to shift that composition from the mean. 4.1. Terrigenous source of trace elements The terrigenous contribution to the bulk sediment was approximately 30%, giving its accumulation rate at 0.8 mg cm 2 year 1. The slow accumulation rate for this fraction confirms speculation of earlier studies that the Phosphoria Basin was a sediment-starved basin. The composition of this fraction is similar, although not identical, to the world shale average and even to the average composition of the terrigenous fraction in the Cariaco Basin (Table 2, Fig. 5a and b). The slightly elevated Th concentrations of several samples, which are distributed throughout the Meade Peak, tend to have the highest marine contributions of La and U (Piper et al., 2000), suggesting that Th has a marine contribution in these samples. Otherwise, the group of trace elements partitioned essentially totally

into this fraction is the same as was partitioned into this fraction in the Cariaco Basin. They include, but are not limited to, Co, Cs. Ga, Li, Rb, Ta, and the oxides Fe2O3 (Fig. 5a), K2O, and TiO2. The terrigenous contribution of those trace elements that had a seawater contribution (Table 2) is similarly determined from x/y-plots with Al2O3 (Fig. 5c and d). 4.2. Bottom-water redox Both deposits exhibit enrichments of Cd, Cr, Cu, Mo, Ni, V, and Zn above a terrigenous contribution (Piper et al., 2000; Piper and Dean, 2002), but only the Meade Peak is enriched in La (Fig. 5d) and the other REE. The enrichment of La in the one deposit, and not in the other, addresses differences in the redox conditions that existed in the water column of the two basins and, indirectly, the hydrography. The partitioning of REE in suspended phases of the open ocean (Sholkovitz et al., 1994) shows that the REE are scavenged from seawater under conditions of O2 respiration. They are returned to solution under SO 42 -reducing conditions in the Cariaco Basin (DeBaar et al., 1988), owing to the reduction of the likely dominant carrier phases of Mn and Fe oxides (German et al., 1991). Some disagreement surrounds the behavior of the REE under denitrifying conditions (German et al., 1991, 1992; Sholkovitz, 1992). A minimum in their seawater profiles at a depth corresponding to the O2-minimum zone in the eastern Pacific Ocean (DeBaar et al., 1985) and at the denitrifying/SO42 reduction interface of the water column in the Black Sea (Schijf et al., 1991) suggests that they are scavenged from the water column under conditions of both O2 respiration and denitrification, possibly even with enhanced removal from the water column under conditions of denitrification. Their enrichment in the Meade Peak then indicates bacterial conditions of O2 respiration to denitrification in the bottom water. The REE patterns of samples from the Meade Peak (Fig. 7) that are most strongly enriched in carbonate fluorapatite are similar to the seawater pattern (DeBaar et al., 1985), confirming a dominant seawater source. This pattern is characterized by a negative Ce anomaly and enrichment of the heavy REE relative to the light REE. Pelletal apatite accumulating currently

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Fig. 7. Rare-earth element (REE) concentrations for selected samples from the Meade Peak (curves only) and the Cariaco Basin (curves and data points for La, Ce, and Nd only), normalized to WSA on an element-by-element basis, vs. atomic number. The three groups of samples from the Meade Peak are representative of samples composed dominantly of carbonate fluorapatite, of siliciclastic debris, and of dolomite plus calcite. Each group has a distinct REE pattern and concentration. The four samples from the Cariaco Basin contain approximately 50% terrigenous debris, 10% organic matter, and 40% biogenic opal plus calcite.

on the Peru shelf (Piper et al., 1988) under denitrifying conditions (Codispoti, 1980) exhibits this same pattern. Samples from the Meade Peak, composed mostly of terrigenous debris, and all samples from the Cariaco Basin have a REE pattern similar to that of the world shale average, although slightly lower REE concentrations. The lower REE concentrations and weak negative Ce anomalies (Fig. 7) likely represent dilution by and a minor contribution from biogenic SiO2 and CaCO3. Both phases have low REE concentrations and a negative Ce anomaly (Elderfield and Greaves, 1983; Palmer, 1985). Their combined concentration in the Cariaco Basin sediment is between 35% and 65% (Piper and Dean, 2002), and in the Meade Peak between 5% and 95%. Bottom-water redox conditions of the Phosphoria Sea can be delineated further from the relationships between trace elements in the marine fraction of the deposit. Copper can be used as the master variable for the marine fraction (Fig. 8) because (1) its concentration is well above its detection limit, giving a precision to the measurement of its bulk concen-

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tration of roughly 5%, (2) it can be strongly enriched above a terrigenous contribution, giving high precision to our calculation of its marine fraction, and (3) it should be deposited from seawater solely as a biogenic fraction under bottom-water conditions of denitrification. Under bottom-water conditions of SO42 reduction, it is also deposited as a sulfide (Jacobs et al., 1985). Cd, Mo, and Zn are removed from bottom water via precipitation/adsorption reactions under SO42 -reducing conditions as well (Fig. 4), and they too are trace nutrients. If deposition of the Meade Peak occurred under bottom-water conditions of denitrification, these trace elements should have interelement ratios that equal the interelement ratios of plankton. The Cr/Cu and V/Cu ratios alone should be shifted to higher values, toward their ratios in seawater, as Cr and V are reduced to insoluble valence states in seawater under conditions of NO3 reduction and SO42 reduction. The trace elements are deposited as a terrigenous fraction under all conditions of bacterial respiration, requiring that interelement relationships in the marine sediment fraction must be established independently of their terrigenous contribution. The Meade Peak samples have Mo/Cu ratios in the marine fraction that fall near the plankton trend, whereas the Cariaco Basin samples have Mo/Cu ratios approximately 10 times higher and close to the seawater trend (Fig. 8a). This same division is seen in plots of Mo vs. Zn, Mo vs. Ni, and Mo vs. Cd. The samples represent two distinct populations that tend toward the values expected for a strictly biogenic source for the Meade Peak samples and a dominantly hydrogenous source for the Cariaco samples. The distribution of the Cariaco samples below the seawater ratio reflects differences in the removal of Mo under fluctuating levels of SO42 reduction (Fig. 6), of complexation and sorption reactions, and kinetics. Some degree of Mo diffusion from denitrifying bottom waters to SO42 -reducing sediment pore waters (Brumsack, 1986) may account for the Meade Peak samples lying slightly above the plankton line. Additionally, brief periods of SO42 reducing conditions in the Phosphoria Sea, driven by an increase in either primary productivity or bottomwater residence time that interrupted denitrifying conditions, may have contributed to the slight enrichment of Mo above a biogenic contribution in the

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Fig. 8. The relations between the marine fraction of Cu and the marine fraction of other selected trace elements in samples from the Cariaco Basin and the Meade Peak. Curves represent elemental ratios for seawater from the North Pacific and North Atlantic Oceans and measured plankton values from the North Pacific Ocean, extrapolated into the sediment field. The plankton curves represent a best estimate, based on concentrations reported by Brumsack (1986) and Collier and Edmond (1983). Seawater concentrations of trace elements are based on values reported by Boyle et al. (1976, 1977), Murray et al. (1983), Bruland (1983), Bruland and Franks (1983), and Collier (1984, 1985). The distributions of Sb, Se, and U in the Meade Peak (not shown) resemble the distribution of V (Se and U), or of Mo (Sb). They were not measured for the Cariaco Basin sediment.

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Meade Peak samples. The low concentration of NO3 in seawater of 40 Amol l 1, i.e., its low organicmatter-oxidation potential, makes this second alternative equally plausible. Whichever scenario is correct, the two groups of samples clearly reflect accumulation under different mean bottom-water conditions of bacterial respiration (Figs. 5c and 8). The Zn/Cu plot also shows a striking contrast between the Cariaco Basin and Meade Peak samples. Nearly all of the former lie on the seawater side of the plot and most of the latter lie on the plankton side of the plot (Fig. 8b), as expected for their accumulation under SO42 -reducing and denitrifying conditions, respectively. A comparison of the sediment data with the Zn/Cu ratio of plankton, as calculated from the concentration depth profiles in the North Atlantic Ocean seawater (Fig. 8b), strengthens this interpretation. The Cariaco Basin sediment samples scatter along the North Atlantic Ocean seawater curve, which is offset significantly from the plankton curve, whereas the Meade Peak samples fall along the North Pacific Ocean plankton curve. The Cd/Cu ratios for both deposits cover the limited range of plankton and seawater Cd/Cu ratios (Fig. 8c). The plot shows that Cd is of little use in discriminating between denitrifying and SO42 -reducing conditions of deposition. The Ni/Cu ratios of the two populations are similar (Fig. 8d), which supports the lack of Ni removal from bottom water under the full range of redox conditions (Jacobs et al., 1987). A shift in the ratios of the Cariaco samples, toward higher Cu values, might have been expected, owing to the precipitation of Cu under SO42 -reducing conditions (Table 1, Fig. 4). The lack of such a shift may reflect an underestimation of the Ni concentration in the terrigenous fraction (Piper and Dean, 2002), but more likely, the minor contribution of bottom water to the total Cu inventory of approximately 10% (Table 1). Elemental ratios of V/Cu and Cr/ Cu in both sets of samples (Figs. 8e and f) are shifted away from the plankton trend and toward the seawater trend. Their distributions reflect Cr and V having had a biogenic and hydrogenous input under both denitrifying and SO42 -reducing conditions, that is, of being reduced to insoluble valence states within the bottom water of both basins. The PO43 concentrations of the two deposits are even more strongly different than are their trace-

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element concentrations. Phosphate averages approximately 20% in the Meade Peak (Piper et al., 2000; Piper, 2001) and 0.3% in the Cariaco Basin sediment (Piper and Dean, 2002). The bulk sediment accumulation rate was certainly important in contributing to the accumulation of ore-grade PO43 in the Meade Peak, but by itself, seems inadequate to account for the virtual absence of carbonate fluorapatite in the sediment of the Cariaco Basin through dilution. The difference in redox properties of the two water columns, coupled with the difference in bulk sediment accumulation rates, may have largely account for this difference in sediment composition. One possible mechanism of retaining PO43 involves Fe cycling (Froelich et al., 1988), which might account for the frequent association of carbonate fluorapatite with pyrite. The PO43 released via the oxidation of organic matter in nearsurface sediment that does not precipitate directly as carbonate fluorapatite may be adsorbed by Fe(OH)3, followed by its precipitation when the Fe(OH)3 is reductively redissolved at depth in the sediment. The mechanism would further seem to have precluded SO42 -reducing conditions in the bottom water (Fig. 4) of the Phosphoria Sea. However, its contribution to phosphogenesis under denitrifying conditions on modern continental shelves has not been fully established (Schuffert et al., 1994). 4.3. Primary productivity The flux of organic matter to the seafloor of the Phosphoria Sea is calculated from the accumulation rates of biogenic elements and a stoichiometric equivalent of modern marine plankton (Table 3). The flux, based on Cd, Cu, Ni, and Zn, ranges from 0.14 to 0.33 g m 2 day 1, as organic carbon. The mean of these values is sufficiently close to the value obtained from the accumulation of PO43 , of 0.19 g C m 2 day 1, to suggest that all of these elements had a predominantly biogenic source. The values are close to values calculated for the Enoch Valley mine site (Piper, 2001). The value based on the marine Mo accumulation rate (0.75 g C m 2 day 1) is much larger, further suggesting Mo had a relatively minor hydrogenous input. The flux of organic matter settling into the bottom water of the Cariaco Basin at 275 m depth was

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Table 3 Calculation of primary productivity and hydrography for the Phosphoria Basin

PO34 Cd Cu Mo Ni Zn Averageg

Average concentration marine fraction (mg kg 1)a

Accumulation rate (mg m 2 day 1)b

Influx of organic matter to seafloor (g C m 2 day 1)c

Primary productivity (g C m 2 day 1)d

Upwelling rate (m year 1)e

Water column residence time (years)f

216,000 63 58 58 173 1390

16 0.0046 0.0042 0.0042 0.013 0.100

0.19 0.14 0.14 0.75 0.24 0.33 0.21

1.2 0.91 0.91 5.0 1.6 2.2 1.4

137 93 66 0.96 53 474 180

2.5 3.8 5.3 360 6.7 0.74 4.1

a Total mean concentration of 75 samples corrected for the terrigenous fraction contribution, based on the metal/Al2O3 ratio in the world shale average and the Al2O3 concentration in the samples. The terrigenous contribution represents < 1 – 14% of the average measured bulk concentrations. b Accumulation rates of phosphate and the trace elements in the marine fraction alone, for a 53-m-thick section with a measured density of 2.5 g cm 3 (Gulbrandsen and Krier, 1980). Duration of deposition was 5.0 m.y. c Influx of organic matter (as organic carbon) to the sea floor = accumulation rate/assumed concentration in plankton (‘‘North Pacific Plankton Measured,’’ Table 2)  358 g C kg 1 (in plankton). d Primary productivity = influx of organic matter to sea floor  6.67 (assuming 15% of primary productivity is deposited on sea floor). e Upwelling rate = rate of metal needed to maintain primary productivity, e.g., (accumulation rate  6.67) / assumed conc. in deep seawater (‘‘North Pacific Seawater’’, Table 2)  365 days year 1  0.001 m3 l 1. f Calculated residence time (years) = assumed height of water column (350 m)/upwelling rate (m year 1). g The average values exclude the PO43 and Mo data. The PO43 data are excluded to allow calculations based on its mean concentration to be compared to the calculations based on the mean trace-element concentrations. See text for explanation for excluding Mo.

reported by Muller-Karger et al. (2001) to be 5.8% of primary production. Applying this value to the Phosphoria Basin requires primary productivity to have been 3.6 g C m 2 day 1. This value is significantly higher than measurements for the ocean today. It may indicate an additional source of PO43 and trace elements. The rate of accumulation of PO43 in modern phosphogenic environments also seems to exceed that which is supplied by primary productivity (Froelich et al., 1988; van Cappellen and Berner, 1988). Most researchers invoke an input of PO43 as fish debris (Suess, 1981), in addition to the input from algal debris. Alternatively, the amount of primary productivity exported to depth, of 5.8%, may represent a minimum. Muller-Karger et al. (2001) did not include in their calculations an upwelling event that resulted in the complete clogging of their sediment traps, even to 1400 m depth. Such an event, however brief and few in number, would drive up the percentage of organic matter exported from the photic zone. Lastly, sediment-trap investigations on the Peru shelf environment have reported up to 19% of the organic matter settling out of the photic zone (von Brockel, 1981).

Sediment traps record the results of a single cycle of primary productivity. The greater width of the Phosphoria Basin than the Cariaco Basin and continental shelves that show pelletal carbonate fluorapatite accumulation (350 –450 km vs. less than 50 km) and a residence time for water in the Phosphoria Basin, in the range of 4 years (Piper, 2001), would have promoted the recycling of nutrients between the photic zone and bottom water of the basin, from the time of their import from the open ocean at depth to their export back into the open ocean in the surface Ekman layer. Such recycling of PO43 in the ocean, several tens of times, leads to the eventual removal of 100% of the riverine input, as the ocean is in steady state. Assuming 15% of the organic matter was exported to the seafloor, primary productivity would have been 1.4 g C m 2 day 1 (Table 3). The closeness of this value to primary productivity measured on the Venezuela shelf (1.5 g C m 2 day 1) seems plausible as both deposits accumulated in the belt of easterly trade winds and both environments had a surface seawater circulation that was isolated from the much more robust surface

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currents of the open ocean, such as the Peru/Chile current. Primary productivity in that environment currently exceeds 2 g C m 2 day 1 (Chavez and Barber, 1987) and may have been even higher during the Last Glacial Maximum (Hebbeln et al., 2002). 4.4. Water-column residence time The residence time of water in the basin can be calculated from the level of primary productivity, nutrient concentrations in the imported water, stoichiometry of plankton (Redfield et al., 1963), and depth of the basin. The calculation assumes that primary productivity was limited by the import of nutrients at intermediate depth from the open ocean and their upwelling into the photic zone (Piper and Link, 2002). We assume seawater concentrations of nutrients equaled those in the North Pacific Ocean between 500 and 1500 m depth (Boyle et al., 1976, 1977; Bruland, 1983) and their concentrations in phytoplankton equaled reported measured values (Table 2). For Cd, Cu, Ni, and Zn, we calculate a watercolumn residence time that ranged from 0.74 (Zn) to 6.7 years (Ni) with an average of 4.1 years, close to the value of 2.5 years obtained for PO43 alone (Table 3). At the Enoch Valley mine site (Piper, 2001), PO43 gave a residence time of about 4.5 years. The value for Mo of 360 years further suggests a hydrogenous input. The residence time of bottom water cannot be calculated from the accumulation rates of Cr, REE, and V, the trace elements that accumulated from the bottom water. Their level of removal from bottom water in modern basins, under conditions of denitrification, has not been established. Even their degree of removal from bottom water under conditions of SO42 reduction is problematic (Fig. 6a). Their degree of removal from the water column of the Phosphoria Sea can be estimated from the above estimate of the rate of advection. For Cr, 50% of that imported accumulated on the seafloor as a hydrogenous fraction and for V, 5%. For the Mo contribution from bottom water, less than 0.5% of that imported accumulated as a hydrogenous fraction. The values seem reasonable (none are greater than 100%), and their order reflects the increasingly negative redox condition under which the three are

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reduced to less soluble valence states (Fig. 4). The uncertainty of these calculations, and the above calculations, is surely large, but the agreement among the different trace elements and PO43 supports the overall model. The REE patterns (Fig. 7) suggested a rather straightforward explanation for their accumulation. Their mean accumulation rates, however, were roughly twice as great as their rates of import from the open ocean, assuming a seawater concentration for La of 50 pmol l 1 (DeBaar et al., 1985). Possible additional sources—hydrothermal, atmospheric, and riverine—could have provided an additional input of REE, but none of these sources would have had a seawater REE pattern (Elderfield and Greaves, 1983). Fish debris might have been a source of REE (and PO43 ) to the sediment, but it seems unlikely that that flux into the basin would have been so strongly one way. The apparent behavior of the REE during diagenesis of reducing near-shore sediments (Elderfield and Sholkovitz, 1987) seems only to add to the short fall. We are left with the explanation that seawater imported into the basin had a significantly higher concentration of REE than open-ocean seawater has today. 4.5. Source of bottom water Within the bottom water, denitrifying bacterial respiration, rate of oxidation of organic matter, and residence time combine to place a limit on the O2 concentration of the seawater imported from the open ocean. A range of dissolved O2 concentrations, from 7% to 22% of saturation, was earlier calculated (Piper and Link, 2002). Calculations for the Hot Springs section give a range of 0– 13% of saturation (Table 4). Calculations based on a section cored at a depth of 4000 m and 65 km to the northeast (Fig. 1A) give comparable values (Perkins et al., 2003). The low O2 concentration required that the water imported into the basin came from an O2-minimum layer of the open ocean. The O2-minimum in the northeastern Pacific Ocean today extends from approximately 125 to 600 m depth and has an O2 concentration as low as 5% saturation. The nutrientenriched water upwelling over the continental shelves of North and South America in the Pacific

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Table 4 Oxygen saturation (maximum and minimum) of open-ocean seawater imported into the Phosphoria Basin Parameter

Value

Influx of organic matter to seafloor

0.21 g C m 2 day 1 (640 Amol C cm 2 year

Scenario I: no NO3 utilized Rate of organic matter oxidized at depth

Source or equation )

580 Amol C cm 2 year 1 + 88 Amol N, or 176 Amol C equivalent cm 2 year 1

Rate of O2 consumed

756 Amol O2 cm

O2 content of imported seawater—a maximuma

45 Amol l

Scenario II: NO3 utilized Influx of NO3 Organic matter oxidized at depth by NO3

Table 3 1

2

year

1

1

; 13% saturation at 5 jC

Maximum rate (i.e., potential rate) of organic matter oxidized at depth by NO3

680 Amol cm 2 year 1 580 Amol C cm 2 year 1 + 88 Amol N, or 66 Amol C equivalent cm 2 year 1 680 Amol NO3 cm 2 year 1  1.25, or 850 Amol C equivalent cm 2 year 1

Rate of organic matter oxidized at depth by O2

0 Amol C cm

Rate of O2 consumed O2 content of influxed seawater—a minimuma

0 Amol cm 2 year 1 0 Amol l 1; 0% saturation at 5 jC

a b

2

year

1

C106H263O110N16P + 138O2 = 106CO2 + 16NO3 + HPO42 + 122H2O + 18H+, and assuming 90% of influx oxidized; 10% accumulated rate of O2 required to oxidize C and N via above reaction amount of oxygen consumed/upwelling rate (Table 3)

assumed concentrationb  upwelling rate C106H263O110N16P + 94.4NO3 + 92.4H+ = 106CO2 + 55.2N2 + HPO42 + 177.2H2O 763 Amol C cm 2 year 1 + 115 Amol N, or 87 Amol C equivalent cm 2 year 1 (partitioning based on electron transfers in above equation) organic matter oxidized < maximum rate of organic matter oxidized by influx of NO3 ; no organic matter is oxidized by O2

The maximum value precluded oxic conditions in the bottom water, whereas the minimum precluded SO42 -reducing conditions. Concentration of NO3 in North Pacific Ocean was 40 Amol l 1 (Boyle et al., 1976).

Ocean and North and South Africa in the Atlantic Ocean is derived from about 100 to 250 m (Codispoti, 1980), further delineating circulation in the Phosphoria Sea.

5. Estimate of uncertainty Errors associated with the calculations can be estimated (ignoring covariance) as the square root of the sum of the squares of percentage errors assigned to individual parameters. The largest errors introduced in our calculations are (1) those associated with the assumed values for the Meade Peak depositional period of 5 F 1 m.y., or 20% error, (2) trace-element concentrations in plankton during Permian time, which we estimate at 60%, based on the spread observed in calculated and measured values of Cd, Cu, Ni, and Zn in modern oceans,

(3) the seawater concentration of trace elements of F 100%, based on the variation observed between modern oceans, and (4) the percentage of organic matter produced in the photic zone, which ultimately accumulates at the sediment surface, of 15 F 10%, or 70%. Relatively minor errors are associated with rock density, measured section thickness and traceelement contents, and the percentage of organic carbon associated with plankton-derived organic matter. Despite all of the potential errors, the error associated with our calculation of primary productivity (1.4 g C m 2 day 1) is likely within 100%. The upper limit of its range is close to the maximum value of modern upwelling environments (Chavez and Barber, 1987). The residence time of water in the basin, of 4.1 years, is correct within a factor of 3, limiting it to an order of magnitude less than the 100 years for bottom water in the Cariaco Basin (Deuser, 1973).

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6. Summary and conclusions Measured trace-element accumulation rates in the Cariaco Basin closely match calculated accumulation rates. The calculated rates use a stoichiometric model for phytoplankton and terrigenous debris and the rate of primary productivity, bottom-water residence time, and trace-element removals from bottom water via precipitation/adsorption reactions. The success of modeling the trace-element composition of the sediment, based on the hydrography and geochemistry of the water column in this modern basin, supports a trace-element model for black shales that gives the hydrography and water-column chemistry of the ancient basins. Applying the model to the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, the accumulation rate of PO43 , Cd, Cu, Mo, Ni, and Zn identified a mean level of primary productivity at 1.4 g C m 2 day 1 that required an upwelling rate of approximately 170 m year 1. Primary productivity was significantly less than it is in the strongly upwelling environments of open-ocean continental shelves, but within the range observed in modern inland seas. The flux of biogenic debris settling out of the photic zone, combined with an average water-column residence time of 4.1 years, maintained denitrifying redox conditions in the bottom water. Denitrifying conditions allowed for the accumulation of Cr and V from bottom water as a hydrogenous fraction, but precluded the precipitation of Cd, Cu, Mo, and Zn as sulfides. The O2 concentration of imported nutrient-enriched seawater from the open ocean necessary to maintain the level of primary productivity and elemental accumulation rates in the Phosphoria Basin was between 0% and 15% of saturation. Its source was an O2-minimum zone, which, in the northeastern Pacific Ocean today, is present between approximately 100 and 600 m. Establishing the hydrography of the Phosphoria Basin, as similar to the hydrography of modern upwelling environments, supports a trace-element composition for seawater in the Permian Ocean that was within the range of compositions of the modern ocean. The REE are a possible exception. Their accumulation rate exceeded their import into the basin from the open ocean by a minimum of a factor of 2, assuming a modern seawater composition. The sea-

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water REE pattern of the deposit precludes a hydrothermal or riverine source and mass balance precludes a major contribution by fish debris.

Acknowledgements The research was supported by the US Geological Survey, Western Minerals Research Program, Surveys and Analysis Project. The manuscript was greatly improved from reviews by R. Rosenbauer, J. Rytuba, M. Arthur, and an anonymous reviewer. [LW]

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