Erosion rates & patterns inferred from cosmogenic 10Be in the Susquehanna River Basin Joanna M. Reuter Thesis Defense March 11, 2005 Paul Bierman, advisor Motivation: • Applied questions – Chesapeake Bay sedimentation • Basic science questions – Topographic change over time – Relationships between erosion rate and landscape characteristics Erosion rates Cosmogenic 10Be in fluvial sediment Sediment yield Landscape characteristics • Tectonics • Climate • Vegetation • Topography • Bedrock geology Geographic information systems (GIS) Cosmic ray bombardment produces 10Be in quartz. Cosmic Rays O Quartz 10Be 10Be is a proxy for erosion rate. Time scale: 104-105 years 0 Depth 1 (meters) 2 3 0% 50 % Production Rate 100% Virtual Tour of the Susquehanna River Basin EXPLANATION Forested Agricultural Developed Barren EXPLANATION Cities & towns EXPLANATION Coal fields EXPLANATION Major dams Basin Selection and Sampling Approach USGS Basins EXPLANATION Glaciated Part glaciated Non-glaciated Advantage: Sediment yield Disadvantage: Complex basins GIS-selected Basins EXPLANATION GIS-selected basins GIS-selected Basins GIS-selected Basins: Nested Basins mean slope: 3.4º mean slope: 8.0º Bedrock Samples Summary of Groups of Samples • USGS Basins –mostly large, complex • GIS-selected Basins –small, simple • Bedrock Samples 10Be Concentrations and Inferred Erosion Rates 10BeNorm________________ Normalized 10Be (105 atoms g-1 quartz) 0.001 Himalayas 0.01 Bolivia 0.1 Great Smoky Mountains Europe Namibia 1 10 Australia Susquehanna Data sources: Matmon et al., 2003; Bierman and Caffee, 2001; Schaller et al., 2001; Morel et al., 2003; Safran et al., in press; Vance et al., 2003; Duncan, unpublished data 100 0 1 2 3 4 Region REG_ID_NUMBER 5 6 7 Results for USGS Basins x x if assumptions have been met 70 all AP SS VR SS VR SH PD SC Appalachian Plateaus Valley and Ridge Piedmont Series1 Series9 10Be Be erosion rate 60 50 10 erosion 40 rate (meters/ 30 million years) 20 16 ± 10 14 ± 4 10 0 0.1 10 1000 Basin area (km2) 100000 Slope 70 60 R2 = 0.72 50 Erosion rate 40 (meters/ million 30 years) R2 = 0.51 20 R2 = 0.37 10 no correlation 0 0 5 10 15 Mean slope of basin (degrees) 20 25 10Be Erosion of the Appalachians 60 60 Susquehanna Great Smoky Mountains 50 50 R2 = 0.54 Erosion 40 40 rate (meters/ 30 million 30 years) 20 20 10 10 00 0 5 10 15 20 25 30 Mean slope of basin (degrees) Smokies data from Matmon et al., 2003 Bedrock Samples Bedrock samples from Namibia: 4.9 m/My 4.0 4.0 2.5 m/My Source: Bierman and Caffee, 2001 Summary of Results • Erosion rate correlates positively with basin slope • No discernible relationship exists between lithology and erosion rate • Results for non-glaciated USGS basins are robust • For basins impacted by glaciation, 10Be results cannot be directly interpreted as erosion rates • Bedrock outcrops are eroding slowly 10Be Erosion Rates Compared to Sediment Yield 10Be vs. • Sediment generation • Time scale: 104-105 years • Representative of full rock erosion Sediment Yield • Export of sediment from the basin • Period of record, 2 to 29 years • Suspended load only; does not include dissolved load or bedload For comparison, present both as erosion rates (m/My) Source of sediment yield data: Gellis et al. (2005), Williams and Reed (1972), and unpublished data from A. Gellis 1000 10Be Appalachian Plateaus 100 Valley and Ridge Erosion rate (m/My) Piedmont 10 1000 Sediment yield 1 0 20 40 60 Cleared land (% of basin area) 80 100 100 2 R = 0.32 Erosion rate 10 (m/My) 1 0 20 40 60 80 Cleared land (% of basin area) 100 Testing Geomorphic Models of Topographic Change • Davis’s Geographical Cycle • Hack’s Dynamic Equilibrium • Statistical Steady State • Perturbations Geographical Cycle (Davis) “Youth” “Maturity” “Old Age” Image sources: http://www.staff.amu.edu.pl/~sgp/gw/wmd/wmd.html http://epswww.unm.edu/facstaff/gmeyer/eps481/481tectclimateveg_files/frame.htm#slide0032.htm Dynamic Equilibrium (Hack) x Image source: http://epswww.unm.edu/facstaff/gmeyer/eps481/481tectclimateveg_files/frame.htm#slide0032.htm Dynamic Equilibrium (Hack) slope Image source: Hack (1960) slope erosion rate erosion rate “It is assumed that within a single erosional system all elements of the topography are mutually adjusted so that they are downwasting at the same rate.” lithologic resistance lithologic resistance Dynamic Equilibrium (Hack) Statistical Steady State x Statistical Steady State Less stable More stable Geographical Cycle Peneplain x Mountains Perturbation Dynamic Equilibrium Uniformly eroding topography Statistical Steady State Changing topography, constant relief 15 10 5 0 0 10 20 Mean slope of basin (degrees) Mean slope of basin (degrees) 20 Be erosion rate (m/My) 25 35 10 Be erosion rate (m/My) 10 30 Series1 VR sandstone VR shale Linear (Series1) lithologic resistance lithologic resistance slope 35 slope erosion rate erosion rate Hack’s Equilibrium? 30 25 20 15 10 5 0 3.50 VR VR 2.50 shale sandstone1.50 Basin lithology 25 20 15 10 5 0 1 shale sandstone Basin lithology Less stable More stable Geographical Cycle Peneplain Mountains Perturbation x x Dynamic Equilibrium Uniformly eroding topography Statistical Steady State Changing topography, constant relief Is Relief Changing? • Slow erosion of ridges: – Bedrock samples – High elevation, low slope sandstone basins • Capacity for rapid stream incision: – Holtwood Gorge 7 m/My Mechanism for reduction of relief: Slope retreat 9 m/My 13 m/My Less stable More stable Geographical Cycle Peneplain Mountains Perturbation x x Dynamic Equilibrium Uniformly eroding topography Statistical Steady State Changing topography, constant relief ? Perturbation? Background: 10Be data from the Sierra Nevada Riebe et al. (2001) No base level fall, no correlation with slope: Base level fall, correlation with slope: 60 10Be erosion rate (m/My) R2 = 0.72 average: 22 50 40 30 20 10 0 60 0 erosion rate (m/My) 10 15 20 25 average: 13 50 10Be 5 R2 = 0.37 40 30 20 10 0 Gradient in erosion rates across basin would have to be maintained by differential rock uplift for statistical steady state to apply 60 0 erosion rate (m/My) 10 15 20 25 20 25 average: 9 50 10Be 5 40 30 20 R2 = 0 10 0 0 5 10 15 Slope (degrees) Less stable More stable Geographical Cycle Peneplain Mountains Perturbation x x x Dynamic Equilibrium Uniformly eroding topography Statistical Steady State Changing topography, constant relief Evidence for a Miocene Perturbation • Offshore sedimentary record: increased sediment delivery (Poag and Sevon, 1989; Pazzaglia and Brandon, 1996) • Fission track: period of rapid exhumation beginning in the Miocene (Blackmer et al., 2001) • Detrital chert and detrital fission track data suggest stream capture and drainage reorganization in the central Appalachians (Naeser et al., 2004) Rates and patterns of erosion in the Susquehanna River Basin may reflect ongoing adjustment to Miocene stream capture and baselevel fall. 60 10Be erosion rate (m/My) R2 = 0.72 average: 22 50 40 30 20 10 0 60 0 erosion rate (m/My) 10 15 20 25 average: 13 50 10Be 5 R2 = 0.37 40 30 20 10 0 60 0 erosion rate (m/My) 10 15 20 25 20 25 average: 9 50 10Be 5 40 30 20 R2 = 0 10 0 0 5 10 15 Slope (degrees) Global Compilation of 10Be Data from Fluvial Sediment: A Brief Overview Regions with 10Be erosion rate data from sediment Published data from: Bierman and Caffee, 2001; Brown et al., 1995; Brown et al., 1998; Clapp et al., 2000; Clapp et al., 2002; Granger et al., 1996; Heimsath et al., 1997; Heimsath et al., 2001; Hewawasam et al., 2003; Kirchner et al., 2001; Matmon et al., 2003; Morel et al., 2003; Riebe et al., 2000; Riebe et al., 2003; Schaller et al., 2001; Vance et al., 2003 In press, in preparation, and unpublished data from: Bierman, Duncan, Johnsson, Nichols, Reuter, Safran Topography Tectonics Climate Vegetation Topography Tectonics Climate Vegetation Lithology per million years) erosion rate (meters eros_rep________________ 10Be Vegetation 10000 1000 100 Susquehanna 10 1 0 20 40 60 80 TRCVMEAN Mean tree cover in basin (percent) 100 erosion rate (meters per million years) 10Be Vegetation n = 454 Mean tree cover in basin (percent) erosion rate (meters per million years) 10Be Climate n = 454 Mean annual precipitation in basin (millimeters) erosion rate (meters per million years) 10Be Climate seasonal uniform n = 454 R2 = 0.37 p < 0.001 Precipitation of 3 driest months/total precipitation erosion rate (meters per million years) 10Be Topography n = 454 R2 = 0.39 p < 0.001 Mean slope of basin (degrees) erosion rate (meters per million years) 10Be Topography n = 454 R2 = 0.44 p < 0.001 Mean basin elevation (meters) erosion rate (meters per million years) 10Be Tectonics n = 454 R2 = 0.50 p < 0.001 Peak ground acceleration (m/s2) with 10% probability of exceedance in 50 years • Tectonics Conclusions – Susquehanna River Basin erosion rates are relatively low and similar to other passive margin and tectonically quiescent settings • Topography – Slope matters – Elevation and erosion rate are not correlated within the region • Climate – Relatively uniform intra-annual distribution of precipitation, and correspondingly low erosion rates – Glaciation disrupts isotopic steady state and precludes simple interpretation of erosion rates from 10Be Conclusions • Vegetation and land use – 10Be results are robust to land use impacts – Contemporary sediment yields for the Piedmont are high relative to background 10Be sediment generation rates • Lithology – No clear impact of lithology on erosion rate in the Susquehanna River Basin • History – Rates and patterns of erosion may reflect ongoing adjustment to Miocene stream capture and base-level fall Acknowledgments • • • • • Funding: NSF, USGS, UVM Paul Bierman Jen Larsen, Megan McGee, Bob Finkel Milan Pavich, Allen Gellis Donna Rizzo, Beverley Wemple, Cully Hession • Luke Reusser, Matt Jungers, and all the other Geo grads, faculty, & staff • Eric Butler For example: Erosion rates Mean denudation rate d (m/1,000 years) – Sediment yield Landscape characteristics • Topography – Relief Mean relief h (m) from Ahnert, 1970 For example: Erosion rates Landscape – 10Be in fluvial sediment characteristics • Topography – Relief from Vance et al., 2003 Results for non-glaciated USGS basins are robust within a factor of 2 70 60 50 Erosion 40 rate (m/My) 30 from 10Be 20 10 0 0.1 10 70 Basin area (km2) 180 1000 100000 260 60 50 Erosion 40 rate (m/My) 30 from 20 sediment yield 10 0 0 10 1000 2 Basin area (km ) 100000 70 all AP SS VR SS VR SH PD SC Appalachian Plateaus Valley and Ridge Piedmont 10Be Be erosion rate 60 50 10 erosion 40 rate (meters/ 30 million years) 20 10 0 0.1 10 1000 2 Basin area (km ) 100000 Factors of possible importance for understanding sediment dynamics and/or interpreting 10Be data • Multiple lithologies, varying quartz content • Glaciation • Human impact – – – – – Agriculture Logging Development Coal mining Dams Results for USGS Basins if assumptions have been met 70 all AP SS VR SS VR SH PD SC Appalachian Plateaus Valley and Ridge Piedmont 10Be Be erosion rate 60 50 10 erosion 40 rate (meters/ 30 million years) 20 10 0 0.1 10 1000 Basin area (km2) 100000 Summary Landscape characteristic Metric Relates to 10Be erosion rate? Lithology Rock Type (Susquehanna) Erodibility metric (Rio Puerco) No Vegetation Tree cover No Mean annual precipitation No Seasonality of precipitation Yes Climate Yes Slope Topography Relief Elevation Yes Yes Yes Tectonics Yes Seismic hazard assessment 10BeNorm________________ Normalized 10Be (105 atoms g-1 quartz) 0.001 Himalayas 0.01 Bolivia 0.1 Great Smoky Mountains 1 Europe Namibia 10 Australia Susquehanna Data sources: Matmon et al., 2003; Bierman and Caffee, 2001; Schaller et al., 2001; Morel et al., 2003; Safran et al., in press; Vance et al., 2003; Duncan, unpublished data 100 0 1 2 3 4 Region REG_ID_NUMBER 5 6 7 Summary: 10Be and Sediment Yield • Sediment yield is out of equilibrium with 10Be in the Piedmont