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Arsenic In The Rivers Of The Amazon Basin

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As is high in Amazonian waters coming from the Andes

As is high in Amazonian waters coming from the Andes

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  • 1. THEMATIC CONTRIBUTION Arsenic in the rivers of the Amazon Basin Wilson Scarpelli Mineral Exploration Consultant wiscar@attglobal.net ABSTRACT The rivers flowing from the Andes bring more than 5 tons of arsenic per year to the Atlantic Ocean as part of their load of transported sediments and also dissolved in the waters. The combined arsenic content, of about 1 mg/m3, is close to the 10 mg/m3 maximum acceptable for potable waters in most of the countries. Following their deposition in the Amazon Fan area, the most important carriers of arsenic, iron oxides and hydroxides, are diagenetically reduced to sulfides, carbonates and phosphates, liberating additional soluble arsenic which causes higher concentrations of arsenic along the shore. KEYWORDS Arsenic, arsenic in Amazon Basin waters, Amazon Basin, Andean rivers, iron oxyhydroxide reduction Introduction The Amazon Basin, defined as the area drained by the rivers that contribute to the Amazon River, covers about 6,110,000 km2, over large portions of the territories of Brazil, Bolivia, Peru, Ecuador, and Colombia. As shown in Fig. 1, the basin expands in two quite different geomorphological areas, and each one contributes in a different way to its development. In the west, for 3,200 kilometers the basin drains the eastern cordilleras of the Andes, from south of La Paz, in Bolivia, to near Bogota, in Colombia. This area has high elevations and is made up of very long ridges, with elevations greater than 4,000 meters, steep slopes, a thin cover of vegetation, and a very intense rate of erosion. The rivers leaving it contain a high bedload of sediments, which gives them a distinctive brown color. To the east of the Andes, surface elevations rarely surpass a few hundred meters, rainfall is intense, vegetation cover is thick, the rocks are deeply weathered and the rate of erosion is relatively low. The rivers draining this portion of the basin transport a small bedload of sediments, and the waters are relatively free of sediments and are occasionally dark due to accumulations of organic matter. TERRÆ 2(1-2):20-27, 2005 20
  • 2. TERRÆ 2(1-2):20-27, 2005 W. Scarpelli Figure 1 – The western portion of the Amazon Basin, shown Bogotá in light gray, covers the Boa Vista eastern cordilleras of the 3 Flow of 209,000 m 3/sec Andes, where the erosion Macapá 600,000 tpy solids Solimões River Solimões River is intensive and Quito 65% of the solids 65% of the solids responsible for 97% of the Óbidos Manaus sediment which is Belém Santarém transported seaward. The DEEP WEATHERING eastern portion, shown in variations of green, is characterized by deep Cruzeiro do Sul Madeira River 32% of the solids weathering and low levels Porto Velho of erosion Araca Cuiabá Intesive Brasília La Paz Erosion 2,000 km Negro River Manaus Solimões River Figure 3 – The encounter of the brown waters of the Amazon River with Madeira River the clean waters of the Tapajós river, at Santarém, Pará Figure 2 – The encounter of the brown waters of the Andean Solimões River with the dark clean waters of the Rio Negro in the north. The Andean Madeira River is shown in the lower right As result of the contrasting bedloads of the zon Basin. As mentioned later, the paper indicated rivers draining the two areas, a characteristic of the high values of arsenic in both the Amazon and the Amazon Basin are the “encounters of waters”, as Guamá Rivers at the mouth of the Amazon. those of Manaus and of Santarém (Figs. 2 and 3), High quality information regarding water flow when the brownish and sediment loaded waters volume and the transported materials is available coming from the Andes encounter and mix with from the sampling, measurements, and assay work the dark clean waters of the other rivers. done by ANEEL (the national agency of electrical energy) at its 60 monitoring stations located along the most important rivers of the Brazilian portion ANEEL sampling of the basin. The information is being compiled and com- An unpublished paper by S.F.P Pereira, of the . prehensively studied through the HiBAM Project Laquanam (the environmental and analytical chem- (Hidrology and Geochemistry of the Amazon istry laboratory of the University of Pará) triggered Basin), a joint program made up of an association the interest to search through the literature to find out about the distribution of arsenic in the Ama- of the CNPq (Research National Council, of 21
  • 3. W. Scarpelli TERRÆ 2(1-2):20-27, 2005 Brazil) with the IRD (institute of VENEZUELA S GUYANA research for the development, of E 5 SURINAME Atlantic D France). The activities of HiBAM Ocean Fr. G. COLOMBIA N are carried out with the partici- <0.1 nco Trom A <0.1 Bra pation of ANA (the national wa- beta <0.1 0 <0.1 Ne <0.1 0.3 gro ter agency), ANEEL, UnB (Uni- 0.7 s 0.1 0.5 1.0 <0.1 0.2 1.4 0.9 versity of Brasília), and the IRD. 0.1 0.7 s 0.8 õe 0.8 0.2 Among the many papers pres- 0.6 lim 0.7 1.5 0.4 -5 s jó So 0.4 pa ira s enting the information compiled Xingu uru Ta e PERU ad P M BRAZIL by the HiBAM Project, Seyler 0.6 aia and Boaventura (2001) pres- -10 Aragu 0.6 A 0.6 ented tables with good quantita- N 0.8 tive information obtained at the D E ANEEL sample stations. They S -15 show that from 1965 to 1990 the BOLIVIA flow of the Amazon River avera- Pacific 0.6 3.8 Ocean ged 209,000 m3 per second at the ARSENIC IN SOLUTION -20 3 (mg/m ) Andean Drainage Non-Andean Drainage Óbidos station, in the state of Pa- (and sampled points) (and sampled points) rá. This rate of flow corresponds 75 70 65 60 55 50 45 to 6,500,000,000 m3 annually. They also show that the river car- VENEZUELA S GUYANA E ried 600,000,000 tons of sedi- 5 SURINAME Atlantic D ments to the ocean each year, and Ocean Fr. G. COLOMBIA N <0.1 nco that 62% of this tonnage arrives Trom A <0.1 Bra from the Solimões River, 35% beta <0.1 0 <0.1 Ne 0.4 gro 0.1 <0.1 0.2 s from the Madeira River, and only 0.4 O.6 <0.1 0.2 1.2 0.2 0.1 3% from all of the other rivers. 3.8 s 0.6 0.4 õe <0.1 0.1 lim 0.1 0.7 2.4 These numbers explain quite -5 ós 1.2 So aj ra p rus Xingu ei Ta well the reasons for the brown- PERU ad Pu M BRAZIL ish color of the rivers with wa- 0.6 aia -10 ters coming from the Andes, like Aragu 4.9 4.2 the Beni, the Madeira, the So- A N 18.6 D limões, the Amazon and others. E S -15 They also explain the reason for BOLIVIA the contrast of color of the wa- ters of these rivers and the wa- Pacific 0.6 3.8 ARSENIC IN Ocean ters of rivers that originate in the -20 SUSPENSION Andean Drainage Non-Andean Drainage 3 (mg/m ) Brazilian Shield, like the Negro, (and sampled points) (and sampled points) 75 70 65 60 55 50 45 the Tapajós, and others. Figure 4 – Arsenic content of the rivers of the Amazon Basin as In their tables, Seyler and determined by sampling at ANEEL monitoring stations. Andean Boaventura (2001) also present rivers are shown in brown to represent their high load of figures of the amount of solids sediments. They show higher concentrations of arsenic in both being transported, and of the ar- soluble (above) and suspended (below) forms senic content in both the trans- ported solids and dissolved sol- HiBAM data on arsenic in the Amazon Basin ids in the waters. Their numbers are shown in the columns on the left hand side of Table 1. They The concentrations of arsenic in the rivers are made it possible to calculate, for each sampling sta- shown in Figures 4 and 5, on which the rivers with tion, the percentage of arsenic transported as sol- Andean waters are drawn in brown. The illustra- ids, the total concentration of arsenic in the rivers, tions show the contents of arsenic in soluble form, and the total mass of arsenic being transported in in suspended solids, the total concentration of tons per day. 22
  • 4. Calculated Values Reported Sampling and Assay Data As total mass Distance from Date of As in the As in solution As in the As total grade River flow Suspended As in solution 3 3 3 3 3 River (mg/m 3) sediments (mg/m 3) (mg/m 3) (m 3/sec) (mg/m 3) (ton/day) sea (km) sampling sediments (g/m33) sediments (g/ton) Solimões 2,500 26/oct/1995 20,115 166.5 14.5 1.53 1.53 2.41 3.94 6.85 Javari 27/oct/1995 1,565 127.6 5.8 0.36 0.36 0.74 1.10 0.15 Itaquaí 27/oct/1995 793 148.3 7.9 0.44 0.44 1.17 1.61 0.11 TERRÆ 2(1-2):20-27, 2005 Solimões 2,200 28/oct/1995 24,251 74.5 15.7 1.38 1.38 1.17 2.55 5.34 Içá 31/oct/1995 5,354 41.4 3.5 0.15 0.15 0.14 0.29 0.14 Solimões 29/oct/1995 32,539 46.0 9.7 0.49 0.49 0.45 0.94 2.63 Jutaí 3/nov/1995 1,143 13.5 3.4 0.15 0.15 0.05 0.20 0.02 Solimões 1,900 3/nov/1995 34,333 60.9 10.0 1.01 1.01 0.61 1.62 4.80 Juruá 4/nov/1995 1,045 56.3 10.1 0.85 0.85 0.57 1.42 0.13 Japurá 4/nov/1995 10,264 28.5 13.6 0.33 0.33 0.39 0.72 0.64 Solimões 1,380 7/nov/1995 46,847 63.7 3.0 0.88 0.88 0.19 1.07 4.34 Purus 9/nov/1995 2,534 38.6 3.1 0.55 0.55 0.12 0.67 0.15 23 Solimões 1,200 10/nov/1995 52,477 127.1 2.8 0.77 0.77 0.36 1.13 5.10 Branco 8/jul/1996 11,960 22.7 2.1 <0.05 <0.05 0.05 0.05 0.05 Negro 20/jun/1996 9,790 9.9 0.2 <0.05 <0.05 0.00 0.05 0.04 Rivers with Andean waters and sediments are shown in negrite Negro 23/jun/1996 15,840 11.6 0.2 <0.05 <0.05 0.00 0.05 0.07 Negro 27/jun/1996 23,000 10.3 0.2 <0.05 <0.05 0.00 0.05 0.10 Negro 9/jul/1996 52,640 17.0 1.7 <0.05 <0.05 0.03 0.05 0.23 Negro 1,250 12/jul/1996 64,680 8.9 7.9 0.05 0.05 0.07 0.12 0.67 Beni 2,000 1/apr/1994 2,856 937.0 19.9 0.83 0.83 18.65 19.48 4.81 Madre Diós 2,050 2/apr/1994 5,092 424.0 10.0 0.61 0.61 4.24 4.85 2.13 Mamoré 3/apr/1994 8,391 409.0 11.9 0.61 0.61 4.87 5.48 3.97 Madeira 1,950 12/apr/1998 29,000 302.0 (2) 0.59 0.59 ( 0.60 ) ( 1.19 ) ( 2.98 ) Madeira 1,200 15/nov/1995 5,132 21.3 2.7 0.69 0.69 0.06 0.75 0.33 Amazonas 1,000 15/nov/1995 75,017 46.1 81.9 0.73 0.73 3.78 4.51 29.20 Trombetas 16/nov/1995 1,258 14.8 17.1 0.12 0.12 0.25 0.37 0.04 Table 1 – Arsenic transported in the rivers of the Amazon Basin as observed with samples collected under the framework of the HiBAM project at the network of stations operated by ANEEL in the Brazilian Amazon. Tapajós 18/nov/1995 6,027 3.5 17.7 0.11 0.11 0.06 0.17 0.09 Amazonas 650 17/nov/1995 81,090 44.2 2.8 0.68 0.68 0.12 0.80 5.63 W. Scarpelli
  • 5. W. Scarpelli TERRÆ 2(1-2):20-27, 2005 arsenic, and the tonnage of the VENEZUELA S GUYANA element transported per day. E 5 SURINAME Atlantic It is quite clear that the D Ocean Fr. G. COLOMBIA N Andean waters have dramatically <0.1 nco Trom A <0.1 higher concentrations of arsenic, Bra beta <0.1 0 <0.1 Neg occasionally not too far from the 0.7 0.8 <0.1 ro s 0.2 0.9 1.6 0.1 0.3 2.6 0.1 maximum limit of potability, 1.1 4.5 s 1.4 1.1 õe which is 10 μg/liter, or 10 grams 0.2 lim 0.7 1.1 3.9 0.8 -5 s jó So 1.6 pa ira s per cubic meter, as defined in Xingu ru Ta e PERU ad Pu M BRAZIL year 2000 by the Brazilian Health 1.2 aia Ministry. When the concentra- -10 Aragu 5.5 4.8 tions of arsenic in solution and A N 19.5 D in solids are added, the limit of E S -15 potability is occasionally sur- BOLIVIA passed. Pacific The discrepancies observed 0.6 3.8 Ocean TOTAL GRADE OF -20 between the sites reflect differ- ARSENIC (mg/m3) Andean Drainage Non-Andean Drainage (and sampled points) (and sampled points) ences in the flow regimens and 75 70 65 60 55 50 45 should not be taken as represent- ing errors in sampling or assay- VENEZUELA S GUYANA ing. Along the Amazon River it- E 5 SURINAME Atlantic self, one of the reasons for the D Ocean Fr. G. COLOMBIA N differences is the continuous <0.1 nco Trom A <0.1 process of deposition and ero- Bra beta <0.1 0 0.1 Neg 0.6 sion of the transported solids on 0.2 5.6 ro s <0.1 2.6 4.8 0.7 0.1 5.3 the banks of the river (Martinelli 4.3 0.1 29.2 0.1 s 0.2 5.1 õe <0.1 et al. 1993). 0.3 0.2 lim 6.8 -5 ós 0.1 So aj ra p The largest difference oc- s Xingu ei ru Ta PERU ad Pu M BRAZIL curred with the samples taken at 3.0 aia the last two sampling stations in -10 Aragu 2.1 4.0 the Amazon River, near the A N 4.8 D mouths of the Trombetas and the E S -15 Tapajós rivers. The values ob- BOLIVIA served at these stations revealed Pacific that 29.2 and of 5.6 tons of ar- 0.6 3.8 ARSENIC Ocean -20 senic were being transported per TRANSPORT Andean Drainage Non-Andean Drainage (tons/day) (and sampled points) (and sampled points) day, respectively. While the true 75 70 65 60 55 50 45 value could be between these Figure 5 – The total grades of arsenic in Amazonian rivers are shown two figures, it is quite clear that above in soluble and suspended forms. Below, the tonnage of 5.6 or 29.2 tons per day of arsenic arsenic transported per day, considering the concentrations of represent a very large figure. arsenic and water flow volume The source seems to be the sulfide-related Source of the arsenic transported mineralizations of gold and base metals that occurs by the Amazon Basin rivers in the eastern Andean ridges, many related to north- south faults and occurring at elevations on the order The information presented clearly indicates of 4,000 meters. Most of these steep sloped ridges that the arsenic is coming from the Andes, which receive high rainfall due to moisture rich winds from the forest area to the east, and are eroded at a is the source of 97% of the solids being transported relatively fast rate (Figs. 6 and 7). by the Amazon River to the ocean. 24
  • 6. TERRÆ 2(1-2):20-27, 2005 W. Scarpelli ROSÁRIO CAMP Figures 6 and 7 – Rosario de Araca, east of La Paz, Bolivia, one of the source areas for sediments containing arsenic. The Rosario de Araca camp is located over an arsenic-bearing auriferous deposit, at mid-slope of a 4,700-meter high cordillera. The photo at left shows the steep gulleys cutting through the mineralized area, and the photo at right, taken from the camp, shows extensive sedimentary terraces at the base of the mountain, submitted to intense erosion Arsenic at the mouth of the Amazon River and from the foreland region of the Andes. Most of the grains are quartz, feldspars and micas, In the paper which drew attention to the issue followed by minor oxides and silicates. Quartz and of arsenic in Amazon waters, S.F.P Pereira presented . feldspars are round or angular, usually with iron- values of dissolved arsenic in waters of the mouth rich coatings. of the Amazon and the Guamá Rivers, that is, at McDaniel et al. (2002) detailed the relatively west, north, and south of the Marajó Island. As the unweathered conditions of the sediments sampled two rivers are connected by water channels west in the fan area and concluded, after comparative of the Marajó Island, Andean water from the studies of Pb and Nd isotopes, “that muds of the Amazon also flows through the Guamá River. Amazon Fan are derived dominantly from the Her paper (Fig. 8) revealed arsenic contents Andean highlands. Furthermore, during their similar to those observed in the upstream sections journey to the Atlantic Ocean, they were not of the Amazon River, but with an increase of values affected by the extreme weathering conditions such in direction of the sea. From concentrations of 3 as those that exist in the Amazon drainage basin to 5 mg/m3 in front of Macapá and Belém, the con- today”. This conclusion was similar to that made centration reached 10 to 14 mg/m3 in the delta. earlier by Gibbs (1967), who identified angular These high concentrations are greater than the grains of fresh feldspars in the sediment load of maximum presently accepted by most countries for the Amazon River. waters for human consumption, which is 10 mg/m3. Burns (1997) describes the presence of a laterally persistent horizon rich in secondary diagenetic iron sulfides, phosphates and carbonates High values of arsenic at the Amazon Fan in the fan area, at about one meter of depth. He points out that these minerals form after dissolution There are several reports describing the and re-crystallization of iron oxides and hydroxides. sediments of the Amazon Fan area, among them The reactions occur under the reducing anoxic those based on cores of the Ocean Drilling Program. conditions created by the continuous burial of the The observations support a proposed mechanism sediments. to explain the greater values of dissolved arsenic in Sullivan and Aller (1996), examining the shal- the coastal area of the state of Amapá. Nanayama (1997) describes the constituent low holes drilled along the lines ‘ost’ and ‘rtm’ shown minerals of the sediments deposited in the Amazon at Figure 8, verified that during the diagenetic for- Fan, identifying assemblies derived from the mation of iron carbonates, sulfides and phosphates, Precambrian Brazilian and Guyana Shields, from the arsenic contained in the iron oxides and the Tertiary Sediments of the Amazon River Basin, hydroxides was liberated to the pore water of the from arc-volcanic rocks of the Andean Cordillera sediments, with the maximum grade of arsenic in 25
  • 7. W. Scarpelli TERRÆ 2(1-2):20-27, 2005 pore water seen at depths of LAQUANAM SAMPLING AND SAMPLING HOLES OF THE 0.25 to 1.5 meters (Fig. 9). A MULTIDISCIPLINARY AMAZON SHELF SEDIMENTARY STUDY sample with 300 ppb (300 mg/ m3) was reported at the hori- ost 3 zon rich in iron carbonates, Sampling sites ost 2 where liberation of ost 1 sulfates and phosphates. They AS from sediments express that “maximum pore- was observed rtm 3 water arsenic concentrations rtm 2 are one order of magnitude rtm 1 greater than levels reported in most other coastal marine en- Sampling of 14 LAQUANAM vironments”, and “oxidized 9 current current 3 9 (mg/m AS) 8 arsenic is associated with iron direction direction 16 oxyhydroxides in surface sedi- 8 100 km 7 Macapá Macapá ments, then subsequently re- 6 duced and released upon burial Santana Santana 63 <1 in the extensive suboxic zone”. 3 To aggravate the matters, “up- ward diffusion and intense physical reworking of sedi- 5 ments presumably releases the Belém Belém 5 dissolved portion of arsenic 7 7 fraction to the water column”. 2 1 Summary and conclusions Figure 8 – Soluble arsenic determined by LAQUANAM in the Amazon River in the north, and the Guamá River in the south. There is an increase of soluble arsenic towards the ocean. The figure also shows the The high rate of erosion in location of the two lines of holes, “ost” and “rtm”, of the Amazon the Andean cordilleras is the Shelf Sedimentary Study, which revealed that the diagenesis of iron oxides and hydroxides liberates arsenic to the ocean waters, a cause of the great volume of possible reason for the increase of arsenic in the coastal areas. sediments transported by the Andean rivers, giving them their characteristic light brown color. These rivers to sulfides, carbonates and phosphates when are responsible for 97% of the sediments covered by about one meter of new sediment load, transported by the Amazon River to the Atlantic and during this reduction, their adsorbed arsenic Ocean. is liberated in soluble form to the pore water of the Together with their transported minerals, sediments. The top layers of sediments are mostly quartz, feldspars and micas, which are often disturbed during storms and the newly formed coated with iron oxides and hydroxides, the soluble arsenic is released into the sea water. Andean rivers also transport high concentrations As a consequence of this process, the con- of arsenic, in solution or adsorbed in the iron oxides centration of arsenic is higher in the Amazon Fan and hydroxides. The arsenic, which originates from than in the Amazon River, and this should be of the common occurrences of arsenic-bearing some concern since this area is a source of marine sulfides along the Andean cordilleras, appears with sea foods. concentrations which approach the maximum value of arsenic accepted for potable water. The References samples collected at the monitoring stations of ANEEL indicate that more than 5 tons of arsenic Brasil. Ministério da Saúde. 2000. Portaria nº 1469/ are transported annually to the Atlantic Ocean. GM. Brasília: Ministério da Saúde. 15p. After their deposition in the Amazon Fan area, (Regulation on water quality for human the iron oxides and hydroxides are slowly reduced consumption). 26
  • 8. TERRÆ 2(1-2):20-27, 2005 W. Scarpelli ‘OST’ HOLES (NORTHWEST) ‘RTM’ HOLES (SOUTHEAST) 250 250 As (ppb = mg/m3) As (ppb = mg/m3) 200 200 150 150 100 100 50 50 0 0 5 15 25 35 55 75 95 115 135 155 185 215 245 5 15 25 35 55 75 95 115 135 155 185 215 245 Depth (cm) Depth (cm) 45 45 40 40 35 35 30 30 Fe (ppm) Fe (ppm) 25 25 20 20 15 15 10 10 5 5 0 0 5 15 25 35 55 75 95 115 135 155 185 215 245 5 15 25 35 55 75 95 115 135 155 185 215 245 Depth (cm) Depth (cm) 25m 55m 75m Figure 9 – Arsenic and iron in pore water of shallow sediments in the Amazon Fan area, as observed in holes OST-1,2,3 and RTM-1,2,3. The holes were drilled under sea water columns of 25 (blue lines), 55 (green lines) and 75 meters (red lines) of depth and were sampled every 10 centimeters of penetration to a depth of 2.5 meters. The marked increase of arsenic coincides with the horizon of reduction of iron oxides and hydroxides to iron sulfides, carbonates and phosphates Burns S.J. 1997. Early diagenesis in Amazon Fan Piper, A. Klaus, L.C. Peterson, (eds.). Proceedings sediments: processes and rates of reaction. In: of the Ocean Drilling Program; Scientific Results, 155:169-176. R.D. Flood, D.J.W. Piper, A. Klaus, L.C. Peterson, (eds.). Proceedings of the Ocean Drilling Nanayama F. 1997. An electron microprobe study of Program, Scientific Results, 155:497-504. the Amazon Fan. In: R.D. Flood, D.J.W. Piper, A. Klaus, L.C. Peterson, (eds.). Proceedings of the Ocean Gibbs R.J. 1967. The geochemistry of the Amazon Drilling Program; Scientific Results, 155:147-168. River System: Part I. The factors that control the salinity and the composition and concentration Pereira, S.F.P. [without a date]. Avaliação da contami- of the suspended solids. Geol. Soc. Am. Bull., nação por metais pesados no delta do rio Amazonas. 78(10):1203-1232. Belém: Universidade do Pará, LAQUANAM. (unpublished report). Kasten S., Freudenthal T., Gingele F.X., Schulz H.D. 1998. Simultaneous formation of iron-rich layers Seyler P.T., Boaventura G.R. 2001. Trace elements at different redox boundaries in sediments of the in the mainstream Amazon River. In: M.E. Amazon deep-sea fan. Geochim. Cosmochim. Acta, McClain, R.L. Victoria, J.E. Richey. (eds.). 2001. 62(13):2253-2264. The Biogeochemistry of the Amazon Basin. Oxford: Oxford University Press. Martinelli L.A., Victoria R.L., Dematte J.L.I., Richey J.E., Devol A.H. 1993. Chemical and mineral- Sullivan K.A., Aller R.C. 1996. Diagenetic cycling ogical composition of Amazon River floodplain of arsenic in Amazon shelf sediments. Geochim. sediments, Brazil. Appl. Geochem., 8(4):391-402. Cosmochim. Acta, 60(9):1465-1477. McDaniel D.K., McLennan S.M., Hanson G.N. 1997. Provenance of Amazon Fan muds: constraints Submitted in October, 2004 from Nd and Pb isotopes. In: R.D. Flood, D.J.W. Accepted in May, 2005 27