UC San Diego
Geophysical constraints on sediment dispersal systems
- Author(s): Johnstone, Elizabeth Anne Carruthers
- Johnstone, Elizabeth Anne Carruthers
- et al.
Geophysical and geological approaches were employed to understand sediment dispersal systems and their response to various forcing functions (i.e., sea level fluctuations, tectonic deformation, sediment supply, and climate change). Two end member marine environments were studied; one with high precipitation and sediment discharge (Gulf of Papua, Papua New Guinea) and the other with low precipitation and sediment discharge (Oceanside Littoral Cell). The high-sedimentation rate in the Gulf of Papua (GoP) yields high-fidelity records of Earth history. As part of the NSF Margins Source-to-Sink (S2S) program, we acquired CHIRP and core data across the GoP continental shelf that complemented onshore and offshore research in the region. CHIRP seismic data imaged three Holocene sedimentary lobes. The older Central lobe is downlapped by two younger lobes to the north and south. Sediment analysis showed that the older Central lobe has an elemental signature similar to the younger Northern lobe with both sourced from the Purari River watershed and lobe migration appears to be climatically controlled. The Southern lobe has elemental signatures more consistent with the Fly River watershed. Our results suggest the northern rivers began depositing sediments on the shelf during the Holocene sea level rise in the central region of the GoP and migrated abruptly north at ~2 kybp. Conversely, during the early Holocene transgression, sediments in the Fly drainage system were sequestered onshore infilling accommodation created in the large low- relief coastal plain during the sea level rise. Upon infilling the onshore accommodation, the Fly River delivered sediment to the ocean and formed the Southern lobe. Such differences in onshore storage capacity may introduce a lag between low-gradient rivers (Type I) with a large coastal plain versus high-gradient river systems (Type II) with small coastal plains. The second study site is in the sediment-starved Oceanside Littoral Cell (OCL) of Southern California. Terrestrial Laser scanning was performed seasonally using Light Detection and Ranging (LiDAR) technology to perform time series modeling of sea cliff erosion. We established a repeatable and reliable protocol for efficiently conducting coastal sea cliff mapping. Changes in geomorphology were quantified to determine the forcing mechanisms controlling erosion. Results from our investigations provide insight into "hot spots" of erosion along the coast and the controlling processes. Subaerial and marine erosion are predominantly controlled by precipitation and wave energy, respectively. Beach elevation, a seasonally dependent variable, is the most important physical factor controlling whether a sea cliff is vulnerable to marine-based or subaerial erosion. Due to the heavy coastal development and important state revenue from beach tourism, it is critical to continue this type of time series research to understand fully these relationships. This study provides a baseline from which future change due to the rapid sea level rise (>3 mm /yr) and climate change can be assessed