Controls on turbidite deposition offshore the Chilean convergent margin

• Applicant: Professorin Dr. Anne Bernhardt
• Subject Area: Palaeontology
• Term: from 2012 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 216660054

• Final Report Year: 2017

Final Report Abstract

Turbidity currents are underwater, sediment-laden currents, and they transport large amounts of sediment from the continent to the ocean. The largest sediment accumulations on Earth, deep-marine sediment fans, are mainly formed by these types of currents. Turbidity currents often evolve from submarine landslides. These currents do not only transport terrigenous sediment but also organic carbon, nutrients, and pollutants to the ocean. Moreover, submarine landslides and turbidity currents can pose hazards to submarine infrastructure, such as submarine data cables, drilling platforms, wind parks, etc. Hence it is of utmost importance to understand when and at what frequencies these currents occur. Strong earthquakes pose an immense risk to societies. One eminent problem when predicting earthquakes is our insufficient knowledge about the long­term recurrence rates of such large seismic events, or the seismic cycle. In order to analyze seismic cycles, archives of past earthquakes of the past millennia are urgently needed, yet hard to find. Under certain circumstances sedimentary deposits of turbidity currents in marine sediment cores may provide such archives. The assumption is that seismic shaking during an earthquake triggers landslides along the rupture zone that evolve into turbidity current deposits. However, this approach is quite problematic as other triggers can launch turbidity currents and climatic boundary conditions can prevent or favor turbidity current activity. In this project, we aimed to test the controlling factors that favor or prevent sediment transport to the deep ocean by turbidity currents along the coast of Chile. Furthermore, we investigated whether the Chilean continental margin forms a suitable place for the compilation of paleoseismic archives. The most surprising result was that almost no turbidity currents were detected along north-central Chile during the past 12,000 years, whereas turbidite deposition is very frequent offshore from south-central Chile during that same time period. We suggest that it is the pronounced present-day rainfall gradient, which leads to dry conditions in north-central Chile and to very humid conditions in south-central Chile that is responsible for this sedimentation pattern. Hence, along the coast of north-central Chile, it is impossible to detect past earthquakes in the marine sediment over the past 12,000 years. Despite strong recurrent earthquakes, landslides and resulting turbidity currents hardly occur because the sediment input from the continent is too low and there is simply not enough loose sediment to remobilize during seismic shaking. The opposite is true offshore from south­central Chile, where plenty of sediments enter the ocean due to the large sediment input by onshore rivers. However, our age­dating techniques fail when too much terrestrial sediment is provided to the ocean, which prevents us from establishing earthquake recurrence rates. Therefore, this region is also unsuitable for the compilation of a paleoseismic record. Hence, when selecting a suitable study site for paleoseismology based on turbidity current deposits, researchers should take into account the delicate balance between sufficient sediment input and the applicability of radiocarbon dating techniques. Lastly, Chilean climate became much drier since the Last Glacial Maximum around 20,000-18,000 years ago. We compared the onshore aridification to the decline of turbidity current activity in the marine sediment cores of that same time period, and we can show that turbidity current activity declined contemporaneously to the onshore drying trend. This implies that turbidity current activity responds very rapidly to onshore climate change, even along regions with very distinct climate and geomorphic characteristics. This latter finding has interesting implications for predicting turbidite occurrence under changing climate conditions.

Publications

• (2017) Immediate propagation of deglacial environmental change to deep-marine turbidite systems along the Chile convergent margin. Earth and Planetary Science Letters 473 190–204
  Bernhardt, Anne; Schwanghart, Wolfgang; Hebbeln, Dierk; Stuut, Jan-Berend W.; Strecker, Manfred R.
  
• 2015, Controls on submarine canyon activity during sea-­level highstands: The Biobío canyon system offshore Chile, Geosphere
  Bernhardt, A., Melnick, D., Jara-Muñoz, J., Argandoña, B., González, J., Strecker, M.R.
  
• 2015, Turbidite paleoseismology along the active continental margin of Chile – feasible or not?, Quaternary Science Reviews, v. 120. p. 71-92
  Bernhardt, A., Melnick, D., Hebbeln, D., Lueckge, A., Strecker, M.
  
• 2016, Shelfal sediment transport by an undercurrent forces turbidity - current activity during high sea level along the Chile continental margin, Geology 44, 295–298
  Bernhardt, A., Hebbeln, D., Regenberg, M., Lueckge, A., Strecker, M.