UNRAVELING THE HISTORY OF A LANDSCAPE: Using geomorphology, tephrochronology, and stratigraphy Photo by: Josh Roering
Photo: Eric Bilderback
Photo by: Josh Roering Goal 1. Reconstruct the paleo-landscape 2. Confirm with the offshore sedimentary record
Why Waipaoa? Transient landscape Hillslope are currently adjusting to rapid channel incision post- Last Glacial Maximum (LGM) Evidence of widespread earthflows Understand how landscape responds to perturbations on short and long timescales (Roering Lab s bread and butter) Crosby & Whipple (2006)
Motivation
Waipaoa River Sedimentary System End of the Last Glacial Maximum (LGM) in the Waipaoa -approx. 18 ka -major shift in climate, vegetation, and fluvial processes Major pulse of rapid incision Recent (historical) changes to the system Photo: David Schmidt
Study Area Mangataikapua Catchment
Study Area Mangataikapua Catchment Mangataikapua Stream 15 km 2 sub-catchment of the Waipaoa River Periglacial environment Podocarp forest post- LGM Mostly converted to pastureland in early 20 th century Weak mélange of marine sedimentary rocks in sheared smectitic mudstone matrix Uplift rates: ~1mm/yr Incision rates: ~4mm/yr
Approximating Sediment Eroded Previous Studies ignored hillslopes (Corina s $$ maker hopefully) Corina looks at erosion from landslides Method: 1) Date landforms 2) LGM landscape Reconstruction 3) Approximate volume change between LGM and now 4) Compare to sediments (for Becky) Marden et al. (2008)
Tephra Record Photo/Figure: Eric Bilderback
Sample Collection Collect basal tephra Analyze glass composition Compared to known tephras
Tephrochronology No Tephra Unknown Taupo (1.7 ka) Waimihia (3.4 ka) Whakatane (5.5 ka Rotoma (9.5 ka) Opepe (10.1 ka) Rerewhakaaitu (17.6 ka) Omataroa (32.5 ka) A1 EMPA Analyzed Sample Sample Collected No Sample Collected N 500 m <5% of this catchment is relict (LGM) terrain!
Reconstruction
Landscape Roughness
Landscape Roughness Correlate age to roughness Gives estimated age for every point in landscape
Reconstruction Connect the dots of like age Use ridgelines and paleochannel elevations Reconstruct past hillslope configuration through time Use to calculate sediment flux
Erosion Rates from Reconstructions
Schematic of Hillslope Response Initially gravel filled valley buttresses slopes LGM (pre-rerewhakaaitu)
Schematic of Hillslope Response Initially gravel filled valley buttresses slopes Post-LGM river removes gravel and begins incising into bedrock Rerewhakaaitu (17.6 ka)-rotoma (9.5 ka)
Schematic of Hillslope Response Initially gravel filled valley buttresses slopes Post-LGM river removes gravel and begins incising into bedrock Slopes destabilized and fail as large block/rotational landslides Rotoma (9.5 ka)-waimihia/whakatane (4 ka)
Schematic of Hillslope Response Initially gravel filled valley buttresses slopes Post-LGM river removes gravel and begins incising into bedrock Slopes destabilized and fail as large block/rotational landslides Deposits mobilized as earthflows Waimihia/Whakatane (4 ka)-deforestation (0.1 ka)
Schematic of Hillslope Response Initially gravel filled valley buttresses slopes Post-LGM river removes gravel and begins incising into bedrock Slopes destabilized and fail as large block/rotational landslides Deposits mobilized as earthflows Earthflows re-initiate following deforestation Deforestation (0.1 ka)-present
Verifying My Story Use other proxies 1. Offshore sediment records 2. Offshore carbon-14 records Assumptions 1. Closed basin 2. Limited storage in floodplains 3. Same sediment Questions: What signature do landslides leave in the depositional record? What would we expect to see preserved offshore?
Offshore Story Brown, L., 1995. Holocene shoreline depositional processes at Poverty Bay, a tectonically active area, northeastern North Island, New Zealand. Quat. Int. 26, 21 33.
Offshore Story Grain size shifts Marden et al. (2008)
More grain size Leithold et al., 2013. Signals of watershed change preserved in organic carbon buried on the continental margin seaward of the Waipaoa River, New Zealand. Mar. Geol. 346, 355 365.
Offshore Story Where is older carbon? What is source of older, recycled, carbon? Carbon signature Leithold et al., 2013. Signals of watershed change preserved in organic carbon buried on the continental margin seaward of the Waipaoa River, New Zealand. Mar. Geol. 346, 355 365.
Comparison of Offshore and Onshore Hillslopes responded to channel incision ~10ka (lagged behind) See gravels offshore General decrease landsliding until ~3 ka See decrease in grain size Large, deep landslides occasionally See older, recycled wood
Take-Home Points 1. Accurately estimated paleo-landscape Geomorphologic story (quantified with tephra) 2. Completed source-to-sink story Assuming a closed system with limited storage 3. Used offshore stratigraphy to verify onshore geomorphology Offshore sediment records (grain size variation & gravel deposits) and carbon records (wood ages/sources)