Geologic Controls on Arctic Coastal Erosion Tom Ravens, Sasha Peterson
ABSTRACT Arctic coastal erosion rates are high and accelerating due to climate warming. However, our understanding of the mechanism responsible for ther erosion is not well-developed. In this paper, we identify two erosion mechanisms that are affecting the high bluffs on the north coast of Alaska: (1) niche erosion/block collapse and (2) bluff face thaw/slump. Further, we identify a geologic parameter - coarse sediment real density (CSAD) - that appears to control which of these erosion mechanisms is dominant. In this project we analyzed over 100 aerial photos of the coastal zone and attributed each of those photos to either niche erosion/block collapse (NEBC) or bluff face thaw/slump (BFTS). Also, we analyzed sediment data from 22 soil cores proximal to the erosion sites (11 from NEBC sites and 11 BFTS sites). On average, the sites subject to NEBC had a much lower coarse sediment density (45.89 g/cm 2) relative to the density at the BFTS sites (175.27 g/cm2 ). Thus, it appears that the type and amount of sediments in the coastal bluffs has a degree of control over the erosion mechanism. In addition, we examined the average eoersion rate (1950-2000 period) for sites subject to the two mechanisms finding that NEBC sites 2 rate at the BFTS sites was had an average erosion rate of 1.696 m/yr while the erosion only 0.588 m/yr. Coarse sediment areal denisty not only enables a prediction of the dominant erosion mechanism, but also the erosion rate. There is an interest in the forecasting of erosion rates in the Arctic. Based on this study, we can colnclude that accounting for the sediment charachter will enable a more reliable coastal erosion forecast.
INTRO/BACKGROUND Coastal erosion is the removal of beach and coastal sediments through the action of wave and tidal forces. In the Arctic this process is unique due to the presence of permafrost, sea ice, and a seasonal open water season. As the ice melts, coastlines are more vulnerable to erosion, and thus the Arctic expierences abnormally high erosion rates. This study recognizes two erosion mechanisms unique in the Arctic: niche erosion/block collapse and bluff face thaw/slump. NICHE EROSION/BLOCK COLLAPSE
LOCATION AREA
METHODS - Analyzed aerial photos to find sites suffering from either niche erosion/block collapse or bluff face thaw/slump. - Niche erosion/block collapse was recognized by the presence of collapse blocks. - Bluff face thaw/slump was recognized by the presence of a large beach, tall bluff height, and the prescence of sloughing material - Mapped soil cores and erosion sites to determine which erosion sites could be matched with soil core data. Erosion sites were primarily matched with soil cores based on proximity. - Used soil core data by means of bluff height, sand content and density to calculate coarse sediment areal density. - Mapped erosion sites on surficial geologic map. - Analyzed photos and maps for prescence of barrier islands at erosion site. - Graphed erosion rates versus coarse sediment areal denisty in sites with and without the prescence of barrier islands. - Graphed frequency of erosion mechanisms versus coarse areal sediment density.
COASTAL EROSION MECHANISMS ON NORTH SLOPE, AK 155 W
CONCLUSIONS
145 W
150 W
140 W
70 N
EXPLANATION Landmark Niche Erosion/Block Collapse Bluff Face/Thaw Slump
Fig 5. Map of North Slope Alaska showing the study area. Niche erosion/block collapse sites are marked in orange and dominate towards the West. Bluff face thaw/slump sites are marked in blue and dominate towards the East.
RESULTS
BLUFF FACE THAW/SLUMP
SURFICIAL GEOLOGY OF NORTH SLOPE, AK FREQUENCY OF EROSION MECHANISM 145 W
150 W
EXPLANATION
EXPLANATION
Landmark Niche Erosion/Block Collapse Old Marine & Alluvium Floodplain
Barrow
Coastal Delta
Lake
Barter Island 70 N
Photo courtesy of USGS
Landmark
Drew Point
N
N
AK-CAN Border Barter Island
CANADA
Photo courtesy of USGS
Niche Erosion/ Block Collapse Bluff Face Thaw/ Slump
Coastal Delta
Glaciomarine Drew Point
Glaciomarine Alluvial Fan
Old Marine & Alluvium Floodplain
Bluff Face/Thaw Slump
Frequency
155 W
Coarse Sediment Areal Density (g/cm2)
3D MODEL ACTIVE LAYER
ACTIVE LAYER ICE & FROZEN SEDIMENTS
COLLAPSED BLOCK
ICE & FROZEN SEDIMENTS
CONVECTIVE & RADIATIVE HEAT TRANSFER
NICHE GROWTH
Fig 6. Map showing erosion sites in relation to the surficial geology of the North Slope, Alaska. Niche erosion/block collapse sites are demonstrated in orange and dominantly are located on glaciomarine sediment. Bluff face thaw/slump sites are demonstrated in blue and dominantly located on old marine and alluvium sediment
add caption & finish editing in GIS
TABLE OF EROSION RATES AND COARSE SEDIMENT AREAL DENSITY
THERMAL EROSION & WAVE UNDERCUTTING
Fig 1. Niche erosion/block collapse charachterized by collapsed blocks, modified from Ravens 2012 and Fritz 2015
Fig 2. Bluff face thaw/slump charachterized by high elevation, presence of large beach, and sloughing material, modified from Ravens 2012 and Fritz 2015
Niche Erosion/ Block Collapse
Bluff Face Thaw/Slump
45.89 g/cm2
175.27 g/cm2
1.696 m/yr
0.588 m/yr
Average Coarse Sediment Areal Density (g/cm 2) Lorem ipsum
Average Erosion Rate (1950-2000)
Fig 7. Graph demonstrating the coarse sediment areal density differences between nich erosion/block collapse and bluff face thaw/slump. Niche erosion and block collapse has a lower coarse sediment areal denisty in comparison to a higher density in bluff face thaw/slump
COARSE SEDIMENT AREAL DENSITY VS EROSION RATE
Erosion Rate (1980-2000)
3D MODEL
Coarse Sediment Areal Density (g/cm2)
ipsum
Lorem ipsum Fig 8. Table demonstrating niche erosion/block collapse sites on average has a lower coarse sediment areal density (45.89 g/cm2) and a higher average erosion rate (1.696 m/yr). Bluff face thaw/slump on the other hand has a higher average coarse sediment areal density (175.27 g/cm2) and a lower average erosion rate (0.588 m/yr). Lorem ipsum
Fig 3. Map of north coast Alaska demonstrating the varying coastal erosion rates (Gibbs and Richmond 2015)
Fig 9. Graph depicting the Erosion rate versus coarse sediment areal density. The average coarse sedment areal denisty for niche erosion/block collapse sites, where a barrier island is present, is 41.98 g/cm2 , in comparison to an average coarse sediment areal density of 42.52 g/cm2 , for an absent barrier island. The average coarse sediment areal density for bluff face thaw/slump sites where a barrier island was present or absent, was found at 149.84 g/cm2 and 203.81 g/cm2 respectively
- Two erosion mechanisms are recognized through aerial photography. 1. Niche erosion/block collapse is charachterized by collapsed blocks. 2. Bluff face thaw/slump is charachterized by the presence of a large beach, tall bluff height, and the prescence of sloughing material. - Coarse sediment areal density appears to control which erosion mechanism is dominant and can further predict erosion rates. 1. Niche erosion/block collapse is found to have a lower coarse sediment areal density (45.89 g/cm2) and a higher average erosion rate (1.696 m/yr). 2. Bluff face thaw/slump is found to have a higher average coarse sediment areal density (175.27 g/cm2) and a lower average erosion rate (0.588 m/yr). - Niche erosion/block collapse sites are predominantly located towards the West side of the North Slope and East of Barrow. They are also predominantly located on glaciomarine sediment. - Bluff face thaw/slump sites are predominantly located towards the East side of the North Slope and they are predominantly located on old marine and alluvium sediment. - Accounting for sediment charachter may be beneficial for erosion forecasting in the Arctic.
REFERENCES - Fritz, M., Opel, T., Tanski, G., Herzschuh, U., Meyer, H., Eulenburg, A., & Lantuit, H. (2015). Dissolved organic carbon (DOC) in arctic ground ice. The Cryosphere (Online), 9(2), 737-752. - Gibbs, A.E., and Richmond, B.M. (2009). Oblique aerial photography of the Arctic coast of Alaska, Nulavikto Demarcation Point, August 7 -10, 2006, U.S. Geological Survey Data Series 436 , 6 p., 4 databases [http://pubs.usgs.gov/ds/436/] - Gibbs, A.E., Erikson, L.H., Jones, B. M., and Richmond, B.M. 2010. Characterizing Morphology and Erosional Trends of Permafrost Bluffs, Barter Island, Alaska. 2010 AGU Fall Meeting. EP23A-0772. - Gibbs, A.E., and Richmond, B.M., 2015, National assessment of shoreline change—Historical shoreline change along the north coast of Alaska, U.S.–Canadian border to Icy Cape: U.S. Geological Survey Open-File Report 2015–1048, 96 p., https://dx.doi.org/10.3 133/ofr20151048. - Jones, B. M., Arp, C. D., Jorgenson, M. T., Hinkel, K. M., Schmutz, J. A., and Flint, P. L. (2009a). Increase in the rate and uniformity of coastline erosion in Arctic Alaska.Geophys. Res. Lett., (36), L03503. - Ping, C.-L., G. J. Michaelson, L. Guo, M. T. Jorgenson, M. Kanevskiy, Y. Shur, F. Dou, and J. Liang (2011), Soil carbon and material fluxes across the eroding Alaska Beaufort Sea coastline, J. Geophys. Res., 116, G02004, doi:10.1029/2010JG001588. - Ravens, T. Kartezhnikova, M, Ulmgren, M., Yager, G., Jones, B., Erikson, L., Gibbs, A. Richmond, B., Zhang, J., Tweedie, C., and Aguirre, A. 2011. Arctic coastal erosion modeling. Presented at the AGU Fall Meeting, San Francisco, CA, Dec. 2011. - Ravens, T. M. , Jones B. M., Zhang, J., Arp, C. D., and J. A. Schmutz. 2012. Process-Based Coastal Erosion Modeling for Drew Point, North Slope, Alaska . J. of Waterway, Port, Coastal, and Ocean Engineering 138(2): 122-130.
ACKNOWLEDGMENTS - Dr. Tom Ravens for the research oppurtunity - USGS for the aerial photos - Dr. Chien Lu Ping et al for the soil core data from the North Slope - Dr. Jeniffer Aschoff for the poster template