Lithologic controls on osmium isotopes in the Rio Orinoco

tters 252 (2006) 138–151www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Lithologic controls on osmium isotopes in the Rio Orinoco

Cynthia Chen a,⁎, Mukul Sharma a, Benjamin C. Bostick b

a Radiogenic Isotope Geochemistry Laboratory, Department of Earth Sciences, Dartmouth College, Hanover NH 03755, United Statesb Soils and Aqueous Geochemistry Laboratory, Department of Earth Sciences, Dartmouth College, Hanover NH 03755, United States

Received 8 June 2006; received in revised form 6 September 2006; accepted 18 September 2006Available online 24 October 2006

Editor: R
.W. Carlson
Abstract

The seawater 187Os/188Os ratio has been increasing over the last 50Ma and at present is∼1.06 reflecting that about 80% of Os in theoceans is derived from the continents that provide Os with an average 187Os/188Os ratio of about 1.26. Whether the past variations inseawater Os isotopes record changes in the bulk Os flux with an average isotopic composition or the changes in the average 187Os/188Osratio of the continental inputs or a combination of these two pathways is not clear. The fundamental importance of lithology in controllingcontinental weathering rates and dissolved ion yields has been well established for the major cations. It is dramatically manifested in theSr isotopes of rivers that are affected by selective dissolution of carbonates. Accordingly, for Os isotopes it has been proposed thatdisproportionate contributions of Os with extremely high 187Os/188Os ratio come fromweathering of organic-rich black shales. Here weexamine the effect of the weathering of specific lithologies on osmium isotopic composition and concentration in water samples of theOrinoco River basin, a major river basin draining both black shales exposed in the Northern Andes and granites of the PrecambrianGuyana Shield. The Northern Andes was uplifted ~17 Ma, which corresponds to the time of a dramatic rise in 187Os/188Os ratio. Theosmium isotopic composition and concentrations vary considerably in the tributaries and the main stem of the Orinoco; the highest Osconcentrations are observed in the rivers draining the Andes. However, waters draining the Andes are not highly radiogenic(187Os/188Osb1.2). A sample of black shale from the area is also not radiogenic. A surprising number of samples from tributariesdraining the Shield are non-radiogenic. Os isotopic composition of Shield waters is often much lower than the bedload and underlyingbedrock lithology. We explain this discrepancy in terms of the limited input of Os from the bedrock. The data reported here for theOrinoco and its tributaries do not support the hypothesis that increased weathering of highly radiogenic black shales is the underlyingcause of the rising 187Os/188Os ratio of seawater during the last 17 million years.© 2006 Elsevier B.V. All rights reserved.

Keywords: osmium; black shale weathering; Orinoco; isotopes; ocean

1. Introduction

The intent of this study is to investigate if weatheringof black shales exposed in the Northern Andes controlsthe Os isotopic composition of the Orinoco River and toevaluate the role that weathering of black shales in recent-

⁎ Corresponding author. Tel.: +1 603 646 0285; fax: +1 603 646 3922.E-mail address: [email protected] (C. Chen).

0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.09.035

ly uplifted terrains may play in affecting the seawater Osisotopic composition. The similarities between the marineOs and Sr isotope records have long been recognized,especially with regards to their dramatic rise during theCenozoic [1–3]. Increasing 87Sr/86Sr is interpreted toindicate enhanced continental weathering [4–7] andconsequent removal of atmospheric CO2 that may alsoexplain the global cooling trend seen for the last 40 Ma[8–11]. However, the validity of seawater Sr as a proxyfor global silicate weathering has come into question

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http://dx.doi.org/10.1016/j.epsl.2006.09.035

139C. Chen et al. / Earth and Planetary Science Letters 252 (2006) 138–151

[12– 15]. Moreover, the long oceanic residence time of Srprovides limited temporal resolution that is inadequate toresolve more rapidly evolving geological processes suchas glacial–interglacial changes in ocean chemistry.Consequently, better proxies for silicate weatheringintensity are needed. The clear correlation of Os to Sr inthe marine record, its relatively short residence time(∼104 yr) [16–18], and its potential relationship to theorganic carbon cycle [19], have been a driving force forobtaining a better insight into the Os budget of the oceans.Interpretation of the Os isotope record, however, hasproven difficult primarily due to a lack of understandingof the values of critical parameters controlling the past andpresent Os isotopic composition of seawater (e.g., [20]).

The 187Os/188Os ratio of oceans reflects a balance bet-ween radiogenic continental (187Os/188Os∼1.26) [21,22]and unradiogenic (187Os/188Os∼0.13) hydrothermal andextraterrestrial sources [18,21,23,24]. The marine187Os/188Os ratio has increased from ∼0.7 at 17 Ma tothe modern value of 1.06. The observed increase is likelydue to an enhanced continental input [1,20], which couldresult from two scenarios with opposing effects on theatmospheric CO2 budget: (1) weathering of silicate por-tion of the continental crust consuming CO2 and (2)oxidative weathering of organic-rich black shales thatwould provide CO2. Moreover, an increase in the radio-genicOs flux from theweathering of silicate portion of thecontinents could be confined to terrains that were upliftedduring the past 17 Ma or could be global in extent. Also,there are two different hypotheses regarding how organicmatter weathering can increase Os isotopic composition:(1) accelerated weathering of organic-rich sediments overthe past 17 Ma [19] or (2) weathering of organic-richsediments at a constant rate but with increasing propor-tional flux of 187Os due to in-growth from the decay of187Re [25]. Any combination of these proposed mechan-isms is possible. For example, it is generally thought thatthe increasing seawater Os isotopic composition is pri-marily a result of the weathering of radiogenic blackshales exposed in uplifted terrains [1,26,27]. This hypo-thesis is supported by the fact that black shales have highconcentrations of potentially radiogenic Os and are easilyweathered [28–30]. Tectonically uplifted terrains showaccelerated weathering and progressively expose andremove new lithologies. They are a logical choice toprovide high amounts of extremely radiogenic Os to theoceans. However, rivers draining the Himalayan orogenappear not to control the seawater Os isotopic composi-tion [20,31], indicating that the uplift of the Himalayas,which is broadly coincident with the rise of 187Os/188Osduring Cenozoic, cannot be solely responsible for theobserved increase.

The Northern Andes were uplifted starting ∼17 Ma[32], a time period that closely corresponds to the observedincrease in seawater 187Os/188Os. Extensive outcrops ofblack shales are present in the region, which are weatheringand providing high amounts of Re [33] and Se [34] to thetributaries of the Orinoco draining the Northern Andes.One previous measurement of Os near the mouth of theOrinoco River also suggested the presence of a highlyradiogenic source, presumably black shales, in the Orinocowatershed [17]. Here we investigate the evolution of the Osisotopic composition and concentration in Orinoco river asit flows along the border of Precambrian Guyana Shield inthe south and east and recentAndean foredeep sediments ofthe Llanos to the north and west; the latter is drained byrivers from the Northern Andes, including those withextremely high Re and Se contents. Through theexamination of the major tributaries of the Orinoco,sediments and exposed rocks in the region, we criticallyevaluate the role of specific lithologies especially the blackshales, and their contribution to the oceanic osmiumbudget. This work is an extension of the paper presented atthe 2005 Fall AGU meeting [35].

2. Geologic setting

An excellent description of the geologic setting of theOrinoco river drainage basin is given in Edmond et al.[36]. Briefly, the Orinoco located in Venezuela and Co-lombia is the 3rd largest river by discharge (36,000 m3/s)in theworld [37], draining an area of 990,000 km2 (Fig. 1).The climate is seasonally tropical with the rainy seasonextending from approximately April to November. Thisdistinct wet–dry season, driven by the movement of theIntertropical Convergence Zone, causes discharges on theOrinoco to vary by a factor of 10 throughout the year.Precipitation varies from 1000 mm/yr in the north to4000 mm/yr in the southern shield area. Temperaturevariations are primarily due to elevation differences andare generally between 20–25 °C.

The Orinoco drainage basin can be divided into threedistinct geological regions, the Precambrian Shield, theAndes, and the Llanos (Fig. 1). The right bank tributariesdraining the southeastern third of the Orinoco basin flowthrough the Guyana Shield, which is dominated byintrusive Precambrian igneous rocks. The left banktributaries draining the northern and western portions ofthe basin are sourced in the northern sections of the Andesmountain range and Llanos. The Andes contains a diversemixture of evaporites, limestones, shales, sandstones,metamorphics, granites and calc-alkaline volcanics, butexposures of shales and limestones dominate [38].Carbonate and evaporite weathering of the Andes controls

Fig. 1. Map showing sampling locations. Numbers represent sampling locations. Samples were collected along the mainstem of the Orinoco and itstributaries draining the three distinct lithologic regions, Andes, Llanos, and Precambrian Shield. The Andes is the source of rapidly eroding Mesozoicblack shales. The Llanos is the floodplains at the foot of the Andes collecting eroded sediments and the Precambrian Shield is comprised primarily ofhighly weathered granites.

140 C. Chen et al. / Earth and Planetary Science Letters 252 (2006) 138–151

the major element chemistry of the Orinoco [36,39]. Theforeland basin collecting the alluvial sediments derivedfrom the Andes is the Llanos. The low relief inconjunction with monsoonal precipitation pattern in theLlanos means that this large region is seasonallyinundated. All rivers draining the eastern slopes of theNorthern Andes flow through the Llanos on their way tothe Orinoco; some rivers in the Llanos are sourcedcompletely within the Llanos itself. The Orinoco Riverhas evolved along the lithologic contact between theGuyana Shield bedrock (right bank) and the sediments ofthe Llanos (left bank); the main-channel of the river formost part drains on the bare rock.

The Orinoco river basin is the perfect setting forexamining the effects of lithology on Os isotopic com-position in river water because of the stark lithologiccontrast between regions within the watershed. The riverflows through a region of the Guyana Shield that consistsmainly of Precambrian granites. Assuming that thegranites are similar to those exposed in other shieldareas, they should have high 187Os/188Os ratios (≥2.5)

but low Os contents (∼10 pg g−1) [40–42]. In contrast,the principal lithologies in the Northern Andes areMesozoic carbonates and black shales; the latter includethe oil-bearing La Luna formation (Late Cretaceous). Ifthe Andean black shales have an age of about 85Ma theyshould have an initial 187Os/188Os of 0.65 [25]. If their187Re/188Os ratios show a range similar to that observedfor the Mesozoic organic-rich mudrocks (200–1600;[43]), we expect that their present-day 187Os/188Os to bebetween 1 and 3. The high Os concentrations in blackshales, 100 to 4000 pg g−1 for the Mesozoic mudrocks[43], in conjunction with the efficient mobilization of Osduring weathering suggests that waters draining blackshales should have elevated Os concentrations [29].Rivers draining the Andes show extensive black shaleweathering based on the observed high Re [33] and Se[34] concentrations, thus Os concentrations of the leftbank rivers should be much higher than the Shield. TheLlanos is composed of young sediments from the Andesand its bulk Os isotopic composition should thus besimilar. An understanding of the impacts of lithology on


Os in rivers and their mixing can be gleaned throughcomparisons of water and sediment samples from thetributaries and the Orinoco mainstream.

3. Methods and samples

Samples were collected from the Orinoco and itsvarious tributaries over two weeks in December 2004when the river was at an intermediate flow stage. Ateach sampling location (Fig. 1), pH, Eh and temperaturewere measured using a portable pH meter and water wascollected for both major and trace-metal analysis. Watersamples were drawn directly from the river using a pre-cleaned syringe. An acid cleaned 0.2 μm polypropylenefilter was then placed on the syringe and water filteredinto pre-cleaned, pre-acidified (ultrapure HNO3)250 mL HDPE Nalgene containers.

Major and trace element measurements were made onwater samples immediately after returning to thelaboratory. Acidified samples were analyzed directly byinductively coupled plasma-optical emission spectrosco-py (ICP-OES) on a Thermo Intrepid II for the major andminor cations. Quantificationwas based on comparison toa three-point standard curve for a standard containing allelements, which typically afforded 3–4 orders ofmagnitude linear range in quantification. For samplescontaining high levels of Na, Mg or Ca, dilutions wereperformed prior to analysis; however trace elementconcentrations were determined on undiluted samples.Concentrations were blank corrected by subtracting theacid blank from the measured concentrations. For mostelements, the detection limits were between 10 and 20ppb. Typical errors were b5% for most elements.

Anion concentrations were determined using ionchromatography on un-acidified samples. Concentra-tions of anions were measured by ion chromatographyand conductivity detection using a Dionex DX500 withchemical conductivity suppression. Ion exchange eluentis HCO3

−/CO32−, which precludes measurement of these

species. Quantification is conducted by comparison ofanalytes to 6-point linear calibration curves, whichafford linearity over 3–4 orders of magnitude.

Os was extracted from water samples using methodsdescribed in [20]. Approximately 50 mL of water wasfrozen in an ethanol and dry ice mixture in a pre-cleanedglass ampoule. Once the water was frozen, 190Os tracersolutionwas added. Once frozen, 200 μL of 8%m/v CrO3

in H2SO4, to help oxidize all Os to OsO4, was added andfrozen. The glass ampoule containing the frozen sample,tracer and CrO3 solutionwas then flamed sealed. Once thesample in the glass ampoule had thawed, it was placed inan oven at 180 °C and heated for 40+ h to ensure sample-

spike equilibration. The glass ampoule was then removedfrom the oven, chilled to 4 °C, scored open and itscontents emptied into a distillation apparatus. OsO4 wasdistilled for 3 h from the water sample and trapped in icecooled HBr. The resulting hexabromoosmate was thenfurther purified using micro-distillation [44]. The overallchemical separation procedure has excellent yields(∼90%) and low blanks (Table 1).

Each sediment sample was dried at 80 °C to a constantweight, and then homogenized in a Spex zirconia ballmill. Rock samples were crushed with a jaw crusher,powdered in a tungsten carbide shatterbox, and homog-enized using the zirconia ball mill. Approximately 1 g ofpowder was weighed into quartz glass Carius tubes. Ostracer solution and reverse aqua regia (3 mL of HCl and5 mL of HNO3) were then added to the Carius tubes. Allreagents were chilled to minimize any reactions thatwould cause the loss of Os to the atmosphere. Once theregents were added, the tubes were quickly sealed andplaced in an Anton Par High Pressure Asher. The samplewas then heated to 300 °C at a confining pressure of 128bar for 16 h. After cooling to room temperature, theCarius tubes were opened and OsO4 extracted usingliquid bromine and then trapped in HBr [44]. The samplewas then purified using the micro-distillation procedure.The procedure has yields of ∼70% and blanks b1%.

Purified Os samples dissolved in 0.5 μL of HBr wereloaded onto zone-refined Pt filaments (H. Cross) anddried at 0.8 A. Ba(OH)2 solution was then loaded as anemitter solution, and filament was gently heated to 1.2 Afor 6 s to dry. Os isotopes were measured on the Triton atDartmouth using a Secondary Electron Multiplier oper-ated in ion counting mode. Typically, 150 ratios for allisotopes were taken for each sample resulting in aninternal precision of better than 0.2% (2σRSD). Repeated(n=46) measurements of 1 pg laboratory standard (MPIOs-1 Standard) gave 187Os/188Os=0.1080±2% (2σRSD). The average is somewhat higher than the estab-lished 187Os/188Os ratio of 0.1069 for the standard andpossibly caused by 187Re interference. NoRe interferencecorrection was made, however, because Re correctionworsened the data quality probably due to non-Reinterferences on mass 233 (185Re16O3). Several duplicatemeasurements were made during the course of the study.Repeat measurements of water sample (VEN 19) did notgive reproducible results (compare VEN 19a and 19b;Table 1). This sample when analyzed again (VEN 19c)yielded [Os] and 187Os/188Os that were identical to VEN19b. The reason for the discrepancy is not clear. Duplicateanalyses of other samples (waters: VEN 8, VEN 10;sediment/rocks: VEN 27, Granite, Shale) yielded resultsthat are identical within error (Table 1).

Table 1Os concentration and isotopic composition of waters and sediments/rocks from the Orinoco drainage basin

SampleID

River DateSampled

Coordinates Type Water Sediment/Rock

SampleWt (g)

187Os188 Os

187Os188Oscorr

Conc(fg/g)

SampleWt (g)

187Os188Os

187Os188Oscorr

Conc(pg/g)

VEN 4 Portuguesa 12/8/04 N 7.95699 Andean 50.41 1.144 1.148a 62.1a 0.99819 1.105 1.105 74.3W 67.50390

VEN 4b Portuguesa 12/8/04 N 7.95699 Andean 52.51 1.120 1.123 84.3 – – – –W 67.50390

VEN 6 Apure 12/8/04 N 7.93558 Andean 49.97 0.877 0.889 14.6 1.01071 0.865 0.865 79.8W 67.54229

VEN 8 Apure 12/8/04 N 7.90256 Andean 52.59 1.176 1.186 21.8 – – – –W 67.51091

VEN 8b Apure 12/8/04 N 7.90256 Andean 50.03 1.184 1.195 22.0 – – – –W 67.51091

VEN 9 Capanaparo 12/9/04 N 7.02520 Llanos 50.11 0.830 0.873 3.5 1.06091 0.966 0.966 123.6W 67.56521

VEN 10 Cinaruco 12/9/04 N 6.55313 Llanos 51.17 0.395 0.411 3.1 1.14185 1.053 1.054 83.8W 67.50629

VEN 10b Cinaruco 12/9/04 N 6.55313 Llanos 50.88 0.431 0.447 3.6 – – – –W 67.50629

VEN 11 Meta 12/9/04 N 6.21630 Andean/Orinoco

51.59 1.318 1.351 8.2 1.02310 1.469 1.469 52.8W 67.43144

VEN 12 Orinoco 12/9/04 N 5.67691 Orinoco 49.88 0.479 0.490 6.4 1.02899 1.596 1.597 14.6W 67.63378

VEN 14 Atabapo 12/11/04 N 3.97992 Shield 49.87 0.331 0.340 4.1 – – – –W 67.69565

VEN 15 Guaviare 12/11/04 N 4.05424 Orinoco/Andean

50.43 0.612 0.624 7.9 – – – –W 67.71662

VEN 17 Orinoco 12/11/04 N 4.00761 Shield 51.42 1.076 1.144b 3.1b 0.99289 1.634 1.640 4.1W 67.53753

VEN 17b Orinoco 12/11/04 N 4.00761 Shield 50.10 0.675 0.695 5.7 – – – –W 67.53753

VEN 19 Orinoco 12/11/04 N 4.01696 Shield 51.28 0.382 0.386 11.4 – – – –W 67.65893

VEN 19b Orinoco 12/11/04 N 4.01696 Shield 49.24 0.276 0.277 19.4 – – – –W 67.65893

VEN 19c Orinoco 12/11/04 N 4.01696 Shield 50.40 0.269 0.270 21.9 – – – –W 67.65893

VEN 20 Orinoco 12/12/04 N 4.26637 Orinoco 49.96 0.658 0.681 5.0 – – – –W 67.78977

VEN 21 Orinoco 12/12/04 N 4.42977 Orinoco – – – – 1.02821 3.745 3.751 11.3W 67.76992

VEN 25 Orinoco 12/13/04 N 8.14722 Orinoco 50.12 0.968 0.999 6.2 – – – –W 63.54963

VEN 27 Caura 12/13/04 N 7.40053 Shield 53.85 0.971 1.012 4.2 1.05000 5.544 5.551 18.3W 65.19524

VEN 27c Caura 12/13/04 N 7.40053 Shield – – – – 1.01820 5.506 5.513 19.0W 65.19524

VEN 35 Orinoco 12/16/04 N 8.38607 Orinoco 50.96 0.676 0.688 10.0 1.10065 1.291 1.293 5.1W 62.67365

AF Angel Falls 12/15/04 N 5.96735 – 51.08 0.417 0.424 7.9 – – – –W 62.52994

Granite – 12/11/04 N 4.07747 – – – – – 1.01793 29.42 29.65b 9.3b

W 67.71072Granite b – 12/11/04 N 4.07747 – – – – – 1.01259 18.69 18.78 10.9

W 67.71072Granite c – 12/11/04 N 4.07747 – – – – – 1.09191 18.23 18.31 11.3

W 67.71072

(continued on next page)


Table 1 (continued)

SampleID

River DateSampled

Coordinates Type Water Sediment/Rock

SampleWt (g)

187Os188 Os

187Os188Oscorr

Conc(fg/g)

SampleWt (g)

187Os188Os

187Os188Oscorr

Conc(pg/g)

Shale – 12/7/04 N 9.72940 – – – – – 1.07131 1.115 1.116 42.5W 67.30738

Shale b – 12/7/04 N 9.72940 – – – – – 1.01682 1.124 1.125 44.7W 67.30738

Water187Os188Oscorr

has been corrected for procedural blank of [188Os]=0.039 fg with 187Os/188Os=0.156. Sediment/Rock187Os188Oscorr

has been corrected for

procedural blank of [188Os]=0.056 fg with 187Os/188Os=0.378.a Underspiked.b Low ion intensity and high Re interference.

Table 2Major and trace element concentrations from the Orinoco drainage basin

ID Ca K Mg Na Al Fe Li Mn P S Sr Zn

(mg/L) (mg/L) (mg/L) (mg/L) (μg/L) (μg/L) (μg/L) (μg/L) (μg/L) (mg/L) (μg/L) (μg/L)

VEN 4 40.39 2.81 6.68 7.24 14 12 7.3 1 45 10.20 215 10VEN 6 10.88 2.28 2.29 4.19 14 86 3.2 9 72 2.03 47 24VEN 8 18.31 2.33 3.45 4.73 42 92 4.0 4 37 4.09 90 4VEN 9 1.21 0.72 0.53 1.69 ND 151 1.3 5 ND 0.28 8 61VEN 10 0.10 0.10 0.06 0.79 26 19 1.2 5 2 0.03 1 20VEN 11 6.07 0.97 1.39 2.46 14 58 2.0 61 ND 3.64 22 53VEN 12 1.39 0.72 0.32 1.36 ND 112 0.8 27 23 0.30 9 127VEN 14 0.07 0.10 0.03 0.51 197 70 0.4 22 5 0.13 1 45VEN 15 3.12 0.75 0.64 1.42 45 106 1.4 3 9 0.55 20 28VEN 17 0.64 0.88 0.25 1.52 40 61 0.9 6 9 0.12 8 28VEN 19 0.53 0.96 0.21 1.71 12 65 0.8 1 6 0.12 7 33VEN 20 1.03 0.72 0.31 1.34 43 137 0.6 6 6 0.20 9 22VEN 25 3.84 0.84 1.15 2.35 34 98 1.6 2 18 2.24 20 29VEN 27 1.02 0.88 0.44 1.82 ND 89 1.1 2 0 0.58 10 35VEN 35 3.04 0.83 0.77 1.74 43 82 1.0 21 6 0.89 16 19AF 0.31 0.11 0.23 0.50 149 163 0.4 4 ND 0.04 1 23MDL 0.01 0.04 0.005 0.01 10 5 1 1 5 0.01 1 5

ID F− Cl− NO2− Br− NO3

− SO42− Si TZ+ TZ+⁎ Si/TZ+⁎

(mg/L) (mg/L) (μg/L) (mg/L) (μg/L) (mg/L) (mg/L) (molc/L) (molc/L) (mol/mol)

VEN 4 0.19 4.51 ND ND 266 10.92 5.56 2952 2143 0.09VEN 6 0.08 2.44 ND ND ND 1.78 4.31 972 792 0.19VEN 8 0.11 2.85 48 ND 276 3.97 4.64 1463 1134 0.15VEN 9 0.05 0.58 ND 0.12 ND 0.14 2.96 196 170 0.62VEN 10 ND 0.60 ND ND ND ND 4.25 47 30 5.09VEN 11 0.11 2.59 ND ND 153 3.73 3.13 549 243 0.46VEN 12 ND 0.27 ND ND 47 0.15 2.69 173 156 0.61VEN 14 ND 0.06 ND ND ND ND 0.79 31 30 0.95VEN 15 ND 0.36 ND ND ND 0.50 3.42 289 248 0.49VEN 17 ND 0.07 ND ND ND ND 3.67 141 139 0.94VEN 19 ND 0.18 ND ND 38 ND 3.83 142 137 0.99VEN 20 0.06 0.13 ND ND 31 0.19 2.96 154 139 0.76VEN 25 0.03 0.70 ND ND 36 0.82 3.19 410 339 0.34VEN 27 0.03 0.37 ND ND 107 ND 4.72 189 179 0.94VEN 35 0.05 0.81 ND ND ND 0.80 3.27 312 239 0.49AF ND 0.27 ND ND 30 0.10 1.32 59 45 1.04MDL 0.01 0.01 10 0.05 30 0.005 0.02

ND: Not detected; MDL: Minimum detection limit; TZ+ = 3[Al] + 2[Ca] + [K] + 2[Mg] + [Na]; TZ+⁎ = TZ+ − [Cl] − 2[SO4].



4. Results

Selected results are given in Tables 1 and 2 andplotted in Figs. 2–4. A complete dataset is available asan online supplement.

4.1. Major elements

Results of major element analysis are plotted in Fig. 2as a function of the tributary type. As expected, the ternaryplot of major cations (Fig. 2A) shows that the Orinoco is amixture of Andean and Shield endmembers. The Andeantributaries have much higher concentrations of cations,especially Ca that comes from carbonate and evaporiteweathering [36]. The Shield rivers have very low levels ofdissolved cations but are dominated by Na and K; the

Fig. 2. Major element composition of water samples as a function oftributary source type. The ternary plot of major cations (A) indicatesthat Shield rivers are highly enriched with Na and K. Andean rivershave much more Ca and the main channel of the Orinoco falls on themixing line between Andean and Shield endmembers. Using theclassification derived from Gibbs [45] (B) Andean samples are rock-weathering dominated while Shield samples are precipitation domi-nated. The Orinoco mainstem is a mixture of Andean and Shieldwaters, but is compositionally much closer to that of the Shield.

Fig. 3. Fig. A is a mixing plot for Os. The arrows indicate thedownstream flow of water in the Orinoco. The Andean samples seemto cluster at one end possibly delimiting an endmember. The rainwater(open diamond) plots towards the lower end of the points suggestinganother potential endmember. Levasseur’s data (filled diamond) ishigh suggesting a third endmember with low [Os] and high187Os/188Os. In Fig. B, the Andean samples have a nearly perfectone to one relationship between sediment 187Os/188Os and water187Os/188Os suggesting equilibrium between rock and waters.However, Shield and Orinoco samples do not fall on the one to oneline suggesting that rock weathering is not the dominant source of Osto the water. Instead, samples are shifted towards the left of the line,closer to the isotopic composition of precipitation (Angel Falls),indicating a larger contribution of precipitation/surface runoff in Shieldrivers. The Orinoco is a mixture between the Andean and Shieldsamples plotting between the two endmembers. VEN 20 and VEN 21,indicated on figure as (20, 21), sample the same lithology but areseparated by ∼10 km.

former comes from seasalt aerosol and the latter fromweathering of feldspar [36]. Fig. 2B separates out thetributary types using the river classification derived from[45]. Once again there are clear clusters defined bytributary types with Andean samples falling in the regionclassified as rock-weathering dominated and the Shieldsamples falling in the precipitation dominated regions.Although the Orinoco mainstem is a mixture of Andeanand Shield waters, it falls almost completely in theprecipitation-dominated region, similar to the Shield.

Fig. 4. Intensity of silicate weathering as it relates to Os. Osconcentration is related to the concentration of dissolved cations(TZ+

⁎) (A). Andean samples have [Os] positively correlated to TZ+

⁎,

while Shield rivers have [Os] equal to or higher than would bepredicted by their TZ+

⁎. The Os isotopic composition of Andean

samples is variable but near 1.2, while the Shield samples vary from0.3 to 1 due to differences in the source of Os in each region (B).


4.2. Osmium in water, rocks and sediments

The osmiumconcentrations inwaters range from3.1 to84 fg g−1 with 187Os/188Os ratios from 0.34 to 1.35(Table 1; Fig. 3). TheOrinocomainstem seems to bemoreenriched and less radiogenic than indicated by a singleanalysis from Levasseur et al. [17] (Fig. 3A). Concentra-tions are low for all samples except for the tributariesdraining the Andes, where they are ∼8 times higher thanthose found in the main channel of the Orinoco ortributaries draining the Shield (Fig. 3A). The average [Os]and 187Os/188Os of global rivers are 9 fg g−1 and 1.38,respectively [25]. In comparison, the average Andeanrivers have [Os]=41 fg g−1 and 187Os/188 Os=1.1. AnAndean black shale sample (Fig.1; Table 1) has187Os/188Os=1.12 and [Os]=43.6 pg g−1. While the187Os/188Os for this sample falls within the expectedrange, the [Os] value is lower than expected and likely aresult of low Corg (=0.20 wt.%). That there are moreenriched bedrocks exposed in the drainage region of theAndean tributaries is evident from the inspection of

sediment [Os] (=74 to 124 pg g−1; Table 1). TheAndean waters, sediments and shale samples havesimilar 187Os/188Os ratios (Fig. 3B). The average Osconcentration of the Shield rivers is about 6 fg g−1 andis identical to that of the tributaries of the McKenzieriver draining the Canadian Shield [41]. The average187Os/188Os ratio of the Shield rivers is 0.7, which ismuch lower than the Andean rivers. The Shield riversflow above a highly radiogenic bedrock as reflected in agranite sample (Fig. 1; Table 1) that has a 187Os/188Os∼18 and [Os]=11 pg g−1. They also carry a much moreradiogenic bedload as shown by sediment samples. It isthus evident that the Shield rivers, which display a rathernon-radiogenic Os isotopic composition are in isotopicdisequilibrium with the Shield rocks (Fig. 3B).

The presence/absence of isotopic equilibrium betweenwaters and bedrocks may reflect different weatheringregimes. The chemical composition of the Andean watersis governed by weathering of primary aluminosilicatesand carbonates. In contrast, the composition of the Shieldwaters is controlled by quartz and kaolinite weatheringproducing dissolved Si [46]. As a result, the ratio of Si tothe total dissolved cation concentration (Si/TZ+⁎=moleratio of Si and sum of Na, Mg, K, Ca corrected for seasaltcontribution) is markedly different for the two sourceareas (Table 2). The Os concentration of the left bankrivers is positively correlated with TZ+⁎ (R2 =0.90;Fig. 4A) indicating that weathering of primary mineralsprovides appreciable Os to the rivers. In Shield rivers, theTZ+⁎ is low because weathering primarily involvespreviously weathered and cation-depleted material (e.g.,laterites) rather than the underlying bedrock. Consequent-ly, the Os concentration in the Shield samples is low andnot related to TZ+⁎ (Fig. 4A) or dissolved Si. The differentweathering regimes result in distinct Os isotopic compo-sitions; the isotopic composition of Andean samples isgenerally higher but less variable than those of the Shield(Fig. 4B).

5. Discussion

5.1. Water and analyte mass balance

River discharge information is necessary to estimate themass balance of solutes in the river system and to estimatepossible temporal isotopic variations. Ideally, this can becalculated for the entire watershed using a digital elevationmodel that incorporates precipitation and evaporationrates. Unfortunately, the available information at this timeis not sufficient to calculate the discharge rates. Instead,weused a limited number of river gauge data that areavailable. The discharge datawere obtained froma number


of sources and are summarized in Table 3 [12,39,47–49].The right and left bank tributaries contribute roughlyequally to the total flowof theOrinoco.Discharge is highlyseasonal with low flow occurring between January andApril and high flow occurring between July and October(Fig. 5). Shield rivers such as theCaura peak just before theOrinoco does while Andean rivers such as the Apure peakjust after the Orinoco. The discharge and concentrationvalues were checked by performing a mass balance forseveral conservative elements (Na, Sr, and Li) throughoutthe watershed (Table 3). The mass balance is difficult toachieve because concentrations vary depending on theflow stage of the river. During high flow, concentrationsare reduced and during low flow, concentrations areelevated [39,48,50]. Fortunately, analyte concentrations inDecember for many elements are relatively near theirweight-averaged concentrations and can thus be regarded,to the first order, as representative of mean flow stage.However, the temporal variability for each element canvary considerably, as can the period of peak discharge.Additionally, there is significant variation in the dischargefrom year to year. Given the relatively short time series formost of our discharge data, the flow rates used in the massbalance may not be fully representative of the annualaverage during our sampling interval. Nonetheless, we

Table 3Mass and isotope balance table

Discharge(m3/s)

Na⁎a

(mg/L)Na⁎ massbalance(g/s)

Sr(μg/L)

Sr massbalance(g/s)

Left bank (Andean and Llanos)Guaviare 5040b 1.2 5977 20.2 102Meta 4560b 0.8 3586 22.2 101Apure 2421c 2.9 6966 90.4 219

Right bank (Shield)Caura 3297c 1.6 5219 9.6 31Orinoco+Ventuari

4800b 1.6 7633 8.4 41

MainstemTotalRiversd

20118 29381 494

[email protected]

20118e 1.5 30347 18.7 375

% Deviation −3% 24%

a Na⁎=Na–Cl (mol/L) and converted to mg/L.b Meade [49].c Discharge is annual average for 1982–1985 from Lewis [47].d Total Rivers=Guaviare+Meta+Apure+Caura+Orinoco+Ventuari.e Discharge scaled to match that of total rivers.

achieve a reasonably goodmass balance for Na, Sr, Li, andOs, and thus we believe that Os is behaving conservativelyin the Orinoco watershed.

The calculated flux of elements in the watershed ishigher than measured because of the overrepresentationof Andean rivers in the balance. Due to the availabilityof discharge values, the Andean rivers in the massbalance represent 60% of the total river flux. Since mostdissolved cations are found in the Andean tributaries,this causes an apparent overestimation of cation flux.Even with an overrepresentation of the Andean rivers,an isotope balance shows that the calculated 187Os/188Osis very close to what we measured at Ciudad Bolivar.Additionally we can examine the impact of black shaleweathering on the Orinoco by simply subtracting the Oscontribution of the Apure which drops the 187Os/188Os ofthe Orinoco from 0.99 to 0.91. This is a relatively smalleffect and points to the limited influence of black shaleweathering on the Os budget of the Orinoco River.

5.2. Os in the Orinoco

5.2.1. Andean and Llanos riversThe high Os concentrations in the Andean rivers are

consistent with prior observations of high levels of Se

Li(μg/L)

Li massbalance(g/s)

Os(fg/g)

Os massbalance(mg/s)

187Os/188Os Calculated187Os/188Os

1.4 7.1 7.9 40.3 0.622.0 9.1 8.2 37.3 1.354.0 9.7 22.0 53.4 1.20

1.0 3.5 4.2 13.9 1.010.8 3.8 7.2 34.6 0.77

33.2 179.5 0.99

1.3 26.2 6.2 126.3 1.00

21% 30%Total Rivers w/o Apure [email protected]/o Apure

0.94

Fig. 5. Average monthly discharges for the Apure, Caura and Orinoco rivers compiled from various literature sources [12,39,47–49]. The Apure andCaura are our Andean and Shield endmembers, respectively. Discharge is highly seasonal for all rivers with high flow occurring in late summer/earlyfall and low flow during spring. The Shield rivers peak before the Orinoco and the Andean rivers peak after the Orinoco. The differences in [Os] andisotopic composition between [17] and this study are possibly due to differences in flow conditions and contributions from a more radiogenicgroundwater component.


[34] and Re [33] attributed to weathering of black shales.However, despite elevated Os concentrations, there is noevidence of highly radiogenic 187Os/188Os values in theAndean river samples. The highest value we measured inwater samples is 1.35, somewhat higher than the averagecontinental material with an 187Os/188Os of 1.26 [21,22].An Andean black shale sample also has 187Os/188Os=1.12(Fig. 1; Table 1). However, a plot of 187Os/188Os insediments versus 187Os/188Os in water in paired samples(Fig. 3B) indicates that sediments and the associated watershave the same isotopic composition. This suggests that theAndean samples are in equilibrium with the sourcematerial. The dearth of radiogenic 187Os/188Os ratios inthe Apure, which receives waters from nearly the entirebreadth of the Northern Andes is surprising. As we couldnot sample the headwaters of lower order tributariessupplying water to the Apure, we cannot be certain if therelatively non-radiogenic character of the Apure waters is afeature inherited from the black shales or acquired duringpassage through the Llanos. Regardless it is clear that theweathering of black shales in the Andes does not supplyhighly radiogenic Os to the oceans.

5.2.2. Shield riversShield rivers have a significantly different Os isotopic

composition and concentration than Andean tributaries.The Orinoco and Shield river samples exhibit disequilib-rium between sediment and dissolved Os isotopiccomposition. These samples fall to the left of thesediment-water isotope equilibrium line in Fig. 3Bwhere the water 187Os/188Os is lower than the associatedsediment samples. Why do the Shield and thus Orinoco

water samples have such low 187Os/188Os values? Thediscrepancy between the bedrock/sediment and water187Os/188Os ratio indicates that there are different sourcesof non-radiogenic Os in waters. Given that the Shieldwater chemistry is controlled by the weathering ofsecondary minerals such as kaolinite [46,51], could thenon-radiogenic Os be derived from laterites?We have notmeasured any soil sample but note that lateritic soilsdeveloped on ancient granite could be much lessradiogenic than the bedrock [42]. Alternatively, the Shieldtributaries are precipitation dominated, thus it is possiblethat the low 187Os/188Os values reflect those in localrainwater. Although we have not measured rainwaterdirectly to test this possibility, we collected water fromAngel Falls, a ∼1 km waterfall from a sandstone tepui.The only source of water on the tepui is precipitationwhich has a residence time of weeks [51] and its chemicalcomposition indicates that very limited weathering hasoccurred (Table 2). Assuming that the Angel Falls water isa reasonable proxy for rainwater, we find that it has an187Os/188Os ratio of 0.42, which is much lower thanaverage continental crust. This could also explain why theShield rivers are less radiogenic. More research is neededto characterize these potential sources and to evaluate howeach of them influences dissolved Os concentrations andisotopic compositions.

5.2.3. Main channelTheweathering in theGuyana Shield is transport limited

and thus little dissolvedmaterial is found inwaters drainingthis terrain [46,52]. Although our data suggest that themainchannel of Orinoco itself is a mixture between Andean and


Shield tributaries, the former supply a disproportionateamount of the dissolved material to the Orinoco [39] (see[36] for detailed calculations). The Os concentration and187Os/188Os values of the Orinoco main channel are muchcloser to values found in the Shield than those of theAndean tributaries. This similarity reflects the differencesin relative discharge. Rio Portuguesa, directly draining theNorthern Andes, has Os concentration an order ofmagnitude higher (84 fg g−1) than the mainstem Orinoco(6 fg g−1). Rio Apure, a major left-bank tributary of theOrinoco, collects water from the Rio Portuguesa and otherNorthern Andean tributaries. Although its discharge is∼7% of total Orinoco discharge, it accounts for ∼25% ofthe Orinoco's total dissolved load [47], suggesting highweathering rates in the Andes. Its Os concentration is lower(22 fg g−1) than that of the Portuguesa but elevated overthat of the mainstem and Shield implying some black shaleweathering. However, the 187Os/188Os of the Portuguesaand the Apure are only ∼1.2. Other major Andeantributaries, Rio Meta and Rio Guaviare, constitute a muchlarger proportion of total Orinoco discharge, ∼14% each,but neither of these rivers has high 187Os/188Os ratios. TheGuaviare and Meta rivers also have low Os concentrations(∼8 fg g−1) and low dissolved Se relative to rivers drainingthe Northern Andes [34].

In a pilot study of Os isotopes in world-rivers,Levasseur et al. [17] measured one sample from theOrinoco, which came from Ciudad Bolivar (Fig. 1). Thissample gave [Os]=3.3 fg g−1 and 187Os/188Os=1.53.Our analysis of the Orinoco at Ciudad Bolivar yields Osconcentration of 6.2 fg g−1 with 187Os/188Os=1.00. Thedifference between these values indicates that concen-tration and isotopic composition of Os can varyconsiderably at this site. Ciudad Bolivar is upstream ofamajor Orinoco tributary, Rio Caroni (∼12% of Orinocodischarge). The Caroni was sampled close to its source atAngel Falls (Fig. 1) yielding [Os]=7.9 fg g−1 and187Os/188Os=0.42. A sample of the Orinoco mainstemafter the confluence of the Caroni at Ciudad Guyana(Table 1) yields [Os]=10 fg g−1 and 187Os/188Os=0.69,suggesting that the Caroni retains its non-radiogeniccharacter while flowing northwards to meet with theOrinoco. More significantly, the limited data presentedhere suggest that the Orinoco that flows into the AtlanticOcean is at times much less radiogenic than the globalaverage!

5.3. Limited groundwater contribution to the Orinocoriver?

The observed 187Os/188Os ratio of Orinoco at CiudadBolivar is variable and requires further explanation. We

note that no Andean sample, water or sediment that wemeasured has 187Os/188Os ratios close to 1.53, the ratiomeasured for the Orinoco at Ciudad Bolivar in 1998.Even if the isotopic composition of the left-banktributaries reached 3, the maximum reasonable ratio forMesozoic shales, it would be unlikely to increase theisotopic composition of the mainstem of the Orinoco,except during extreme high flow periods that accompanylow flow periods in the Shield. Thus the isotopiccomposition difference between our measured valueand that of Levasseur's is unlikely to be explained bychanging conditions in the Andes. Instead the variationsmay have an origin in the Shield. In the Shield, wemeasured a granite sample having 187Os/188Os ∼18.5.Although its Os concentration is low (11 pg g−1), itshighly radiogenic isotopic composition means that onlya small amount of granite would need to be weathered toinfluence the isotopic composition drastically.

One major difference between the two Ciudad Bolivarsamples is the time of year when sampled (Fig. 5).Levasseur's sample was collected in June when the riverwas approaching high stage. Our sample for the same sitewas collected in December, when the river was at anintermediate stage after the peak stage. Isotopic composi-tion and concentration can vary depending on river stage asdemonstrated by [12] for Sr. For Sr, concentrations are lowand isotopic composition is more radiogenic during highflow [12]. Similarly, during high flow, Os concentrationsare low and isotopic compositions are more radiogenic.Thus, we speculate that the differences in the isotopiccomposition and concentration for the two samples can beexplained quite simply by varying the amount of ground-water, which would presumably contain radiogenic Osisotopic composition due toweathering of primary bedrockin the Shield region. Future studies that involve time seriesanalysis and ground and rainwater sampling are needed tofurther address the issues raised by this study.

5.4. Black shale weathering in recently uplifted terrainsand the Os budget of the oceans

The mean residence time of Os in the oceans (τ̄Os) isof the order of 104 yr. It is short enough to assumeachievement of a steady state on a million year time-scale, and longer than the ocean mixing time(∼1500 yr), such that Os isotopic evolution of thedeep oceans may track changes induced by significantchanges in the contributing sources. The seawater187Os/188Os has increased in the last 66 Ma, exceptbetween 29 and 17 Ma when there was no change. Therate of increase of 187Os/188Os ratio in the last 17 Ma hasbeen quite rapid (0.019 Ma−1) and concomitantly there


has been a decrease in the δ13C of the oceans. It hasbeen recognized for over a decade that rapid oxidativeweathering of black shales exposed in a watershed canprovide disproportionate amounts of Re, V, U, Mo etc.to the rivers and CO2 to the atmosphere. Given the highRe/Os ratios of certain black shales producing poten-tially labile 187Os, Ravizza and Esser [19] surmisedaccelerated weathering of black shales could provide thedriver necessary to explain the observed rate change ofseawater 187Os/188Os and δ13C at 17 Ma. Alternatively,it could be caused by weathering at a constant rate oforganic-rich sediments that become increasingly radio-genic due to in-situ growth of 187Os [25]. While bothmechanisms can drive the seawater 187Os/188Os to moreradiogenic values, the first invokes an increase in totalOs flux and the second calls for a proportional increasein 187Os flux due to decay of 187Re.

The most suitable regions that expose black shales toweathering and erosion are recently uplifted terrains,specifically those that have seen an increase in upliftrates during the last 17 Ma. The Himalayan and Andeanorogenic belts contain large sections of black and grayshales that have been tectonically active throughout theCenozoic. A large block of the western Tibetan plateauand also the Northern Andes have seen increasing upliftrates over the last 17 Ma. Extensive work in theHimalayas has suggested that while black shales provideradiogenic Os, the total amount of Os provided from theweathering and erosion of the Himalayas is b10% oftotal input and cannot explain the marine Os isotopicevolution [20]. This study indicates that although theMesozoic black shales in the Northern Andes are notextremely radiogenic, their weathering does provide adisproportionate amount of Os to the Orinoco (e.g., theApure that supplies only 7% of the total flow provides∼27% of the total Os). However, the 187Os/188Os ratioof the Orinoco is not dominated by the black shalesignal. Indeed, the remaining 73% Os has average187Os/188Os∼1 (Table 3). The significance of this resultis that the Os dissolved in the Orinoco does not point tothe dominance of this singular lithology. These obser-vations indicate that weathering of black shales may notbe a common cause for the time-correlated changes inthe seawater 187Os/188Os and δ13C at 17 Ma. This ob-servation is in accordance with our previous conclusion[20,53] that bulk of Os derived from the continents isderived from weathering of average crust. However, therelationship between Os and cation release duringchemical weathering appears to be complex (Fig. 4).

What is then the cause of the rapid change in187Os/188Os ratio at 17 Ma? Could it be a consequenceof the increasing uplift rates and weathering in the Andes

and the western Tibetan plateau? We evaluate this ques-tion below using the Os mass balance model given in [20]assuming that the Os budget of the South American andHimalayan rivers is dominated by weathering and erosionalong the Andes and the Himalayan–Tibetan plateau. Ifthe time scale for changes of the input ismuch greater than104 yr and on a 106 yr time scale any phase lag isnegligible, the quasi-steady state Os isotopic ratio inseawater (SW) at time τ is given by [20]:

ð187Os=188OsÞτSW ¼ X COsðτÞð187Os=188OsÞτC

þ ð1−X COsðτÞÞð187Os=188OsÞH;

where XOsC (τ)=1/ (1+ROs(τ)) is the fraction of continent-

derived Os and ROs(τ)≡J188OsH /J188OsC represents ratio of

flows of 188Os from Hydrothermal plus Cosmic (H) andContinental (C) sources. As (187Os/188Os)H is constant, achange in (187Os/188Os)C

τ and/or ROs is needed to engen-der the change in the pace of increase of seawater187Os/188Os at 17 Ma.

Considering the implications of the isotopic shiftsinferred for the time interval between the present and17 Ma ago let us first assume that the isotopic shift weredue only to changes in the isotopic composition of thecontinental component. Using the nominal referencevalues: (187Os/188Os)C=1.54 [17], (

187Os/188Os)H=0.126,and (187Os/188Os)SW = 1.06, this would require(187Os/188Os)C to increase by 0.33 to 1.54 (today). If thisshift in the continental component were due to a shift of187Os/188Os in a suite of rivers contributing a fraction “f ” ofthe net riverine Os input, then this would require that therebe a shift Δ(187Os/188Os)/ f in the isotopic composition ofthese rivers. If f were 0.07 (=the fractional Os contributionof the Andean and Himalayan rivers), this would require ashift of Δ(187Os/188Os)=4.7 over the past 17 Ma in thissuite of rivers. The present day discharge averaged187Os/188Os of these rivers is only 1.54. Clearly, theserivers have not driven the observed increase in the seawater187Os/188Os. If on the other hand, we assume that theobserved shift is due only to changes in fraction of Oscoming from the continents, this would require XOs

C tochange from 0.51 (17 Ma ago) to 0.66 today. In otherwords, J188Os

C should be a factor of 1.8 less than the present-day value. The present day total riverine flux of is1533 mol/yr. If the riverine flux was 852 mol/yr (=1533/1.8) 17 Ma ago, we require that a combination of theAndean andHimalyan rivers are contributing 681mol/yr ofOs today.However, the present dayOs input ofAndean andHimalayan rivers is only 209 mol/yr. These observationsindicate that the input of high 187Os/188Os from sources thatdo not comprise a large or dominant input cannot be thebasis of the increase of 187Os/188Os in the oceans and that


large global changes in ROs(τ) and/or (187Os/188Os)C

τ arerequired to produce the observed effects [20].

6. Conclusions

The following are the principal conclusions of thisstudy:

1. Despite a significant contribution of Os throughblack shale weathering to the Apure, the Os is nothighly radiogenic and as a result the Orinoco is not aradiogenic source of Os to the ocean.

2. Although the Andes does not seem to be providinghighly radiogenic Os, the Precambrian Guyana Shieldcontains rocks that are very radiogenic. The Shieldrather than the Andes may be the source of radiogenicOs to the Orinoco.

3. The Orinoco is dominated by water of rainwatercomposition, which appears to be non-radiogenic;little is known regarding Os isotope chemistry inprecipitation which should be further investigated.

4. The highly seasonal nature of flow regimes has dramaticimpact on both Os concentrations and isotopiccompositions. Changing compositions in the Orinocomainstem can potentially be accounted for by varyingcontributions between left and right bank rivers duringdifferent flow regimes and/or differing groundwatercontribution in the Shield.

5. Our measurements of Os isotopic composition andconcentrations in water and sediment samples of theOrinoco River basin indicate that the uplift andsubsequent weathering of the Northern Andes cannotbe directly responsible for the increase of 187Os/188Osvalues in the seawater. The data require that weathe-ring of organic-rich sediments in recently upliftedterrains is not driving the marine Os isotopic compo-sition towards more radiogenic values.

Acknowledgments

This study was instigated by the late John Edmondwho after reading our conclusions from the Himalayanrivers pointed out that the place to investigate black shaleweathering was the Orinoco where this rock was present“in spades”. Funds were made available by NSF (OCE-0099231, EAR-0130631) to M.S. The field work inVenezuela had to be postponed several times due topolitical instability and as a result this study could not beconducted during the time period originally allotted fordoing it. We thank Rodey Batiza (NSF, OCE) for hiscontinuous support. We are also grateful to MartinPalmer for providing us with unpublished data and Josh

Landis for his help with major and trace elementanalyses. Comments from an anonymous reviewer, B.Peucker-Ehrenbrink and by the Editor R. W. Carlsonsignificantly improved the manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.epsl.2006.09.035.

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Lithologic controls on osmium isotopes in the Rio Orinoco