The Fluvial Dispersal System. Peru. Amazon River Basin

Transcription

1 R. Aalto,, A. Aufdenkampe,, L. Maurice-Bourgoin 1

2 The Fluvial Dispersal System Basin-scale Mass Fluxes 3.1 Gtonnes/y eroded from Andes 1.4 Gtonnes/y enter mainstem Amazon 1.2 Gtonnes/y exit mainstem Amazon S 1.0 Bolivia Peru Amazon River Basin 2

3 ~ 500 Mtonnes/y 3

4 4

5 Channel Migration, Transect Location 5

6 16 Sediment exchanges due to channel migration Cutbank Erosion Flux (230) Bar Deposition Flux (222) Migration Transfer (Bar) (-8.0) Flux (Mtonnes/y) Flux (Mtonnes y -1 ) S Foredeep Sedimentary Basin Forebulge Secondary Basin UTM Latitude (m) UTM Latitude (m) N 6

7 7

8 Documentation of Stratigraphy A B C X-radiograph negatives of sediment cores. Oriented top up, 25 cm long, cracks denoted with arrows. (A) Point-bar deposit, with crossbedding of sand reflecting the energetic depositional environment of the channel. (B) Floodplain 3 km from the channel. Fine horizontal lamina and massive silt deposits from a low energy environment. (C) Floodplain 50 m from the river, depicting massive silt banding and fine mm-scale horizontal laminae. 8

9 A new means to date river sediment: 210 Pb CIRCAUS Total Activity = Excess 210 Pb + Supported 210 Pb (must normalize total activity to % clay for each sample!) Meteoric fallout of 210 Pb onto the floodplain ( 210 Pb adsorbed by clay within 5-10 cm of the surface). XS 210 Pb carried by river sediment acquired its excess activity upstream (from local fallout onto soil). 210 Pb Natural, Uranium-series fallout radionuclide (Radon decay in atmosphere) with a half life of 22.3 years. Near-annual dating possible, up to a century record. Adsorbs strongly to clays, effectively immobile. Easy to sample and measure with high precision. New CIRCAUS method developed empirically from 150 cores and 3 rivers (Aalto and Nittrouer, in review). 9

10 Mamore Transect 12 dpm g -1 dpm g -1 Cores are very similar in recording the same sediment accumulation event and are undisturbed for almost six decades! Depth (cm) Depth (cm) Deposited in 1943 (+/- 3.9 y). Cap date m from channel. Fine laminations. Deposited in 1944 (+/- 2.0 y). Cap, m from channel. Fine laminations. 10

11 Beni Transect 60 dpm g -1 dpm g -1 Deposited in 1973 (+/- 3.1 y). Cap date is m from channel. X- ray shows laminae. Depth (cm) Deposited in 1974 (+/- 1.8 y) and 1951 (+/- 1.9 y) and 2900 m from channel at time of deposition. Meteoric caps are incomplete. Fine laminae on X-ray. Depth (cm) Both ~1973 and ~1950 events recorded over several km2 area. No deposition since. 11

12 Rate (cm y -1 ) Spatial Distribution of Sediment Accumulation Natural Levees Distal Floodplain Distance from Channel (m) Aalto et al., 2003 Accumulation rates averaged for all data. (minimum 210 Pb CIRCAUS rates) 12

13 Fluxes to floodplain Floodplain Deposition Deposition for 10 km UTML reach (Mt (Mtonnes/a) y -1 ) Calculated Beni River Floodplain Deposition Proximal FP Deposition (+25.0) m FP Deposition (16.8) 1-2 km FP Deposition (13.4) > 2 km FP Deposition (41.6) Total FP Deposition (96.7) Foredeep Sedimentary Basin Forebulge Secondary S UTM (m) UTM Latitude (m) N13

14 Net System Storage (Mt y -1 ) Net Floodplain Sediment Storage (Mtonnes/yr) S Net Floodplain Sediment Storage for the Beni Foreland Beni Foredeep Basin Forebulge Net Annual System Storage: 97 Mt y -1 Secondary Basin UTM (m) UTM Latitude (m) N Total Net Annual Flux to FP: 97.1 Mt/yr. 14

15 Accumulation Events Accumulation timing across 2,000 km of floodplain, Beni and Mamore Rivers. ( 210 Pb CIRCAUS geochronology) Cold phase ENSO following warm phase? '49 '55 '67 '70 '73 '77 '83 '88 '98 Beni Mamore Date (ENSO year) Mamore floodplain inactive after early 70 s Sea Temperature Aalto et al.,

16 Solid Flux (Mt y -1 ) ? '67'70 '73 '77 '83 '88 ' Water Flux (m 3 s -1 ) Maximum 2-day increase (BOLD), maximum, and average discharge. (all accumulation events (yellow) associated with discharge rise > 8000 and max > cms. Bars show total sediment conveyed by bank-full floods (> 6000 cms), which comprise > 60% of the average sediment efflux to the fluvial dispersal system. Exceptional floods far surpass the mean annual flux (250 Mtonnes) and account for much of the sediment exchanged due to channel migration. Beni River Annual Discharge Summary 16

17 Slow Rise Flood Minimal Hydraulic Gradient Rapid Rise Flood Steep Hydraulic Gradient 17

18 Secondary basin Forebulge Foredeep Basin Beni River 18

19 19

20 Extreme Events orchestrate particle movement Sediment supply, exchange, and floodplain deposition are strongly associated with ENSOorchestrated floods that occur every ~8 years. Such rapid-rise floods may be important globally. Extrapolating to other Amazonian Rivers, Gtonnes of sediment are flushed from Andes, a similar amount is exchanged by migration, and more than half of the sediment efflux is deposited. Observations in the Andes and other research suggest sediment is mainly supplied to channels by landslide clusters destabilized by large storms. 20

21 Organic Carbon largely rides on sediment Organic carbon in sediments is intimately associated with mineral surfaces in most environments Consistent ratios of %OC to surface area Sorption appears to decrease the availability of OM to microbial degradation %OC Amazon River SPLITT Other Rivers WA Coast Sediments Surface Area (m2/g) Adapted from Keil et al., 1994 &

22 Sorption Experiment Results Sorption followed a Langmuir Isotherm Aufdenkampe et al.,

23 Sorption Kinetics & Isotherm Abiotic sorption occurs quickly, but plateaus. Microbial sorption advances slowly, but does not appear to plateau. Biomass is an insignificant fraction of the total carbon in the live experiments. 23

24 Sorptive Enrichment of Amino Acids 24

25 Sediment Budget Carbon Sorption and Burial Hypothesis 1, Erosion: During large flood events, fresh mineral sediments with high carbon sorption potential enter the river corridor, in the Andes primarily from deep hill slope failure (landslides). Hypothesis 2, Mixing & Deposition: Within the river, low OC sediments mix with fresh organic matter and mineral surfaces acquire normal OC/SA ratios via sorption processes. Approximately half of the sediments are deposited along with non-mineral-associated particulate organic matter (POM) and leaf detritus. Hypothesis 3, Sequestration: Fresh organic matter that is sorbed or complexed with mineral surfaces is essentially protected from remineralization on time scales of decades to centuries. Fresh non-mineral associated POM flood sediments will be sequestered on decadal scales or longer if it is deposited in deep settings. Assuming likely OC concentrations: Gt of carbon buried per ENSO event in the Amazon, Gt lost globally?? 25

26 Table 1: Hypothesized sediment and organic carbon budget for the Rio Beni, Bolivia. Depositional fluxes of sediments are well constrained from previous studies (Aalto 2002). Values for associated organic carbon (OC) are educated guesses based on published and unpublished data (Devol & Hedges 2001; Aufdenkampe 2002). Labile OC is the fraction which would have otherwise degraded in <5 years had it remained in its source environment. Sequestered OC is the formerly labile OC that has been newly placed in a physical or biochemical environment (i.e. a deep deposit or sorbed to a mineral surface) that slows its return to atmospheric CO 2 to > 50 years. Percentages are the mass fraction of the preceeding column found in the following column. In rivers within the Amazon with fine suspended sediment (FSS) concentrations >75 mg/l, the mean %OC of FSS is 1.18% (n = 262, Aufdenkampe et al., unpublished data from CAMREX) whereas the global mean of high sediment rivers from Meybeck (1982) is approximately 1.5%. Objective 1 constrain these values Objective 2 constrain these values Objective 3 constrain these values Objective 1 constrain these values Objective 2 constrain these values Objective 3 constrain these values Formerly Sediment Organic Carbon Labile OC Sediment Organic Carbon Labile OC Sequestered OC Mt y -1 % Mt y -1 % Mt y -1 Mt y -1 % Mt y -1 % Mt y -1 % Mt y -1 Sediment Sources Sediment Sinks Andean Forelands Vegetation & Litter % % 1.99 Point Bar Dep., deep % % % 1.32 Surface Soils % % 0.04 Point Bar Dep., shallow % % % 0.03 Gully errosion % % Floodplain Dep., proximal % % % 0.12 Hillslope failure, shallow % % 0.02 Floodplain Dep., distal % % % 0.22 Hillslope failure, deep % % 0.01 Bed Deposition % % % 0.04 Andean Subtotal: Foreland Subtotal: Forelands Cutbank Erosion % % 0.46 Downriver % % % 0.25 Source Total Sink Total Extrapolating to the entire Amazon Basin (given 3.1 Gt y -1 sediment erode from Andes): If this happens primarily during wet years (historically 1 out of 8): 35 Mt y -1 labile carbon sequestered by these processes 278 Mt labile carbon sequestered in a wet year 26

27 Clusters of Bedrock landslides 27

28 Clusters of colluvial hollow failures 28

29 Sediment Transport and Reaction with OC Mid-Andes High Andes Sub-Andes 29

30 Sediment Deposition on Point Bars 30

31 Future Work: Testing the Hypotheses Sediment research (focus on documenting storm events): Radionuclides ( 210 Pb, 137 Cs, 7 Be) & Fingerprinting Documentation of event-based landsliding in Andes Remote sensing (migration, morphology, inundation) Event sediment sampling (automated water samplers at gauges, rising-stage samplers on floodplains, boats ) Organic Carbon research (focus on rapid processes): How much OC with sediments at erosion & deposition? Where did deposited OC originate ( 13 C, lignin phenols) and how much fresh OC was sorbed to surfaces? On what timescales will OC remineralize? ( 14 C) Global Significance: negative 1-2 Gt y -1 atmospheric carbon anomaly strongly associated with ENSO. 31

32 Clusters of huge, deep-seated landslides 32

33 Missing Sink for Carbon Global CO 2 balance for 1990s Where is all the fossil fuel carbon going? 33

34 Ne Japurá us Pur ira e d a M Ma d dre e Mamoré s o i D Ben i gu n i X jó s uá Ju r a zo nas aí Am Ju t n Uc ay ali ó Marañ Am Içá as n o az Ta pa Na po gr o

35 Isotopic Compositions of Organic Matter in Rivers of the Amazon δ 13 C ( ) DOM FPOM CPOM Average C3 Plant Lowland Tributaries 1000 m 3000 m 2000 m Ranked in Order of Increasing Mean Basin Elevation 35