Anthropogenic radionuclides from the Cold War era above-ground nuclear tests are routinely used to date sediments around the globe (Ketterer
There are six principal radio-isotopes of Plutonium; 238Pu (t½ = 87.74 years), 239Pu (t½ = 241,100 years), 240Pu (t½ = 6,561 years), 241Pu (t½ = 14.29 years), 242Pu (t½ = 373,000 years) and 244Pu (t½ = 8.3×107 years). The isotopes 238Pu, 239Pu, 240Pu, 242Pu and 244Pu decay through α emission, whereas 241Pu decays through β emissions, producing 241Am (t½ = 432.6 years). Sediments where 240Pu + 239Pu (240+239Pu) activity is first detected represents pre nuclear bomb deposited sediments and hence pre early 1950s deposition. Because plutonium is relatively immobile under both freshwater and saltwater conditions (Ketterer
Anthropogenic radionuclides from atmospheric fallout have been widely used as a tracer of sedimentation processes in the Northern Hemisphere (Abril, 2003; Alhajji
The timeframe of when anthropogenic plutonium was deposited globally closely matches periods of rapid and extensive land use change in the Amazon Basin (Barlow
Six sediment cores were collected from Amazon floodplain lakes adjacent to the Amazon and Tapajos Rivers (PA2 and PA9 adjacent the Tapajos River, and PA10 adjacent the Amazon River), and another three PP, RM and PZC, adjacent to the Madeira River near the city of Porto Velho, Brazil (Fig. 1). The sediment cores were collected using percussion and rotation to minimize sediment compression. The sediment cores ranged from 25 to 70 cm length. The sediment cores were sub sampled in 2 cm intervals. Dry bulk density (DBD, g cm-3) was determined as the dry sediment weight (g) divided by the initial volume (cm3).
The laboratory preparation for Pu analysis followed the method of Ketterer
The leachates were diluted to 100 mL, filtered to remove solids, and the aqueous solutions were processed with TEVA resin (ElChrom, Lisle, Il, USA) in order to chemically isolate 3.0 mL Pu fractions in aqueous ammonium oxalate solution suitable for measurements by sector ICPMS. Pu determinations were performed using a VG Axiom MC operating in the single collector (electron multiplier) mode. The system was used with an APEX HF system (ESI Scientific, Omaha, NE, USA) with an uptake rate of 0.4 mL/minute. Qualitative mass spectral scans (averages of 50 sweeps over the mass range 237.4–242.6) were collected for selected samples prior to the electrostatic sector quantitative scanning of 238U+, 239Pu+, 240Pu+, and 242Pu+. Detection limits were evaluated based upon the analysis of two blanks. A detection limit of 0.01 Bq/kg of 240+239Pu is applicable for samples of nominal 25 gram mass (Ketterer
For 210Pb dating the sediment cores were analyzed for radionuclide concentrations in an HPGe gamma well detector (Appleby and Oldfield, 1992). Freeze dried and ground sediments were packed and sealed in gamma tubes. The 210Pb and 226Ra activities were calculated by using a factor that includes the gamma detector efficiency, as previously determined from certified reference material IAEA-300 (Baltic Sea Sediment) and the gamma-ray intensity. The 210Pb and 226Ra activities were measured using the 46.5 KeV and 351.9 KeV gamma peaks, respectively (Sanders
The dry bulk densities (g cm-3) are shown in Table 1. The Pu atomic ratios in the six sediment cores indicate that the Pu is originating from stratospheric fallout (plutonium isotopic ratios 240Pu/239Pu) which ranged from 0.17 to 0.19 in the sediment cores near the Tapajos and Amazon Rivers and 0.16 to 0.20 in the sediment cores near the Madeira River (Table 2). There is no consistent activity pattern down the sediment profiles that resembles the expected atmospheric deposition history. Therefore, it is not possible to assess a 1963 peak deposition date from this profile. However the material below the depth where 240+239Pu first appears was deposited pre-bomb (that is, prior to the early 1950s) (Sanders
Depth profiles of dry bulk density (DBD) (g cm-3) and mass accumulation rates (MAR) (g cm-2year-1) of the six sediment cores (PA2, PA9, PA10, RM, PP, PZC) studied in this work.Depth PA2 PA9 PA10 RM PP PZC DBD MAR DBD MAR DBD MAR DBD MAR DBD MAR DBD MAR 0–2 0.54 0.18 0.48 0.53 1.01 0.34 0.65 0.37 0.89 0.28 0.74 0.33 2–4 0.55 0.19 0.53 0.58 0.89 0.30 0.63 0.36 0.81 0.26 0.56 0.25 4–6 0.58 0.20 0.60 0.66 1.03 0.35 0.77 0.44 0.83 0.26 0.62 0.28 6–8 0.59 0.20 0.56 0.61 1.07 0.36 0.69 0.39 0.84 0.27 0.72 0.32 8–10 0.62 0.21 0.58 0.63 1.04 0.35 0.71 0.40 0.77 0.25 0.72 0.32 10–12 0.61 0.21 0.59 0.65 1.05 0.36 0.75 0.43 0.78 0.25 0.76 0.34 12–14 0.57 0.19 0.63 0.70 1.14 0.39 0.70 0.40 0.94 0.30 0.72 0.32 14–16 0.62 0.21 0.66 0.72 1.10 0.37 0.68 0.39 1.02 0.33 0.91 0.41 16–18 0.58 0.20 0.63 0.69 0.99 0.34 0.66 0.37 1.05 0.33 0.71 0.32 18–20 0.57 0.19 0.62 0.68 1.07 0.36 0.64 0.36 1.00 0.32 0.77 0.34 20–22 0.59 0.20 0.63 0.70 0.96 0.33 0.74 0.42 0.90 0.29 0.78 0.35 22–24 0.59 0.20 0.62 0.68 1.00 0.34 0.70 0.40 1.06 0.34 0.72 0.32 24–26 0.58 0.20 0.61 0.68 0.97 0.33 0.67 0.38 1.00 0.32 0.81 0.37 26–28 0.58 0.20 0.66 0.73 1.05 0.36 0.77 0.44 0.94 0.30 0.96 0.43 28–30 0.59 0.20 0.72 0.79 0.95 0.32 0.71 0.40 0.96 0.31 0.81 0.37 30–32 0.62 0.21 0.71 0.78 0.95 0.32 0.81 0.46 0.96 0.31 0.78 0.35 32–34 0.55 0.19 0.68 0.75 1.30 0.44 0.81 0.46 1.09 0.35 0.81 0.37 34–36 0.57 0.19 0.63 0.69 1.57 0.53 0.77 0.44 1.05 0.34 0.92 0.41 36–38 0.56 0.19 0.70 0.77 1.36 0.46 0.74 0.42 1.00 0.32 0.72 0.33 38–40 0.53 0.18 0.69 0.76 1.36 0.46 0.70 0.40 0.99 0.32 0.77 0.35 40–42 0.54 0.18 0.70 0.77 1.45 0.49 0.72 0.41 0.98 0.31 0.82 0.37 42–44 0.55 0.19 0.69 0.76 1.55 0.53 0.72 0.41 0.98 0.31 0.90 0.40 44–46 0.69 0.23 0.70 0.77 1.75 0.60 0.84 0.48 1.06 0.34 0.95 0.43 46–48 0.68 0.23 0.72 0.79 1.47 0.50 0.77 0.44 0.44 0.32 0.86 0.39 48–50 0.62 0.21 0.68 0.75 1.36 0.30 0.74 0.42 0.42 0.32 0.72 0.39
Sediment accumulation rates (mm year-1) and inventories (mBq cm-2)based on the 210Pb and 239+240Pu sediment dating methods.Study 210Pb 210Pb 239+240Pu 239+240Pu Major PA2 2.7 6.5 2.4 4.9 Tapajos PA9 18.6 7.0 10.2 1.0 Tapajos PA10 8.2 53.6 2.3 0.5 Amazon PP 6.2 22.4 3.4 2.7 Madeiras PZC 5.8 3.1 4.2 2.0 Madeiras RM 6.1 40.2 6.2 3.7 Madeiras
Fig. 2 shows the 210Pb, 226Ra and subsequent 210Pb(ex) sediment core profiles. A general downcore decrease in 210Pb(ex) activity was found in five of the six sediment cores (Fig. 2), with only PA10 showing substantial sediment mixing at the surface intervals. For sediment core PA9, 210Pb analysis was conducted from the surface down to 40–42 cm depth interval. However for the Pu analyses in this same sediment core, samples were combined in 4 cm intervals from the 42–46 until the 68–72 cm depth intervals. No 240+239Pu was found in the 62–66 and 66–70 cm intervals. Therefore the 1950 date was established to be at the 58–62 cm depth interval representing the onset of nuclear bomb testing, as 240+239Pu was only introduced into the atmosphere after the early 1950s.
The 240+239Pu and 210Pb inventories and fluxes shown on Table 2 are based on the entire core. These inventories give an integrated view of the total supply of 240+239Pu and 210Pbex to the water column from multiple sources. The 240+239Pu and 210Pbex input comes from the atmosphere and runoff, which should be included when considering the sources to the sedimentation inventories. The 210Pbex inventories may also include an
The 240+239Pu results are consistent with the 240Pu/239Pu of 0.180 ± 0.014 discussed by Kelley
The mass accumulation rate results indicate that even though higher plutonium peak concentrations are observed in the PA10 sediment core, the MAR is greater in the PA9 sediment core (Table 1). The lower 240+239Pu mass accumulation rates in the other sediment cores may be due to dilution from a large sediment load or possibly a result of grain size differences, as the waters vary from fine grain sediment-laden Amazon River to the relatively high concentrations of sand in the Tapajós Rivers (Junk
There appears to be large sediment focusing, a process whereby water turbulence moves sediment from shallower to deeper zones of a lake (Blais and Kalff, 1995), in the floodplain lakes in this work. This is because the initial atmospheric fallout flux of 240+239Pu is fairly well known and much lower than was found in this study (Kelley
The 240+239Pu inventories in this work are substantially greater than the global fallout atmospheric fluxes and greater than most other lakes around the globe. For instance, Lake Bosten, China 0.46 mBq cm-2 yr-1 (Liao
The 240+239Pu and 210Pb methods are interpreted as having comparable sediment accretion rates in three sites (PA2, PZC, RM) and substantially different in the other three floodplain lakes (PA9, PA10, PP) (Table 2). The difference in sediment accretion rates between these two dating methods may be attributed to the different time spans these two models represent. For instance, accretion derived from 240+239Pu represent a net accumulation rate relative to a 60-year timeframe whereas net accumulation rates calculated using the CIC approach with 210Pb represents an over 100 year timescale. When comparing net accumulation rates from these two methodologies, we are assuming a consistent deposition regime across these timescales. However this may not be the case along the Amazon Basin as a result of changes in seasonal rain (El Nino) and land use activites (such as deforestion) which will influence the sediment loads that are delivered to the floodplain lakes.
The 210Pbex inventories, shown in Table 2, give an integrated view of the sedimentary dynamics as the 210Pbex inventories vary greatly from one floodplain lake to another. These differences in 210Pbex inventories indicate differing depositional fluxes (Smoak
The 240+239Pu and 210Pb sediment dates in this work indicate that the Amazon floodplain lakes accumulate sediment at rates between 2.4 and 18.6 mm year-1 during the previous century. However the derived 240+239Pu and 210Pb sedimentation models produced substantially different sediment accretion rates in three of the six sites. These differences are attributed to the different time spans these independent models represent and/or the solubility of Pb and Pu along the sediment column. From the two independent dating methods used in this work which are based on different time scales, we conclude sedimentation rates have remained relatively stable near the Madera River and the 240+239Pu and 210Pb inventories indicate that there is greater sediment focusing at the Madeiras River floodplain lakes, as compared to the Tapajos and Amazon River floodplain lakes, during the previous century. Furthermore, this work shows that the detection of a recognizable 240+239Pu in sediment profiles of the Amazon floodplain lakes is promising in terms of future uses as a geochronometric tool in the this region. The geochronologies derived from the 240+239Pu and 210Pb dating methods outlined in this work are of an ideal timeframe for studying the effects of anthropogenic influences along the Amazon Basin during the 20th century.