Using physical and chemical sediment characteristics to distinguish fluvial and oceanic event deposits within mid-shelf depocenters
Abstract
Continental shelf stratigraphy can preserve a high-resolution record of episodic events driven by terrestrial, oceanographic and/or climate processes. Along river dominated margins, episodic events often result from fluvial floods, which produce distinct deposits on the shelf. However, for the small mountainous river systems along the US Pacific coast, storms also drive energetic ocean conditions that can increase sediment remobilization from waves and currents. Presumably, deposits from fluvial floods and wave supported remobilization events would impact seabed processes differently, and these differences may inform our understanding of the role event deposits play in biogeochemical processes and stratigraphy.
In this study we demonstrate a means by which these different types of episodic event deposits may be distinguished based on physical and chemical sediment characteristics. We collected and analyzed cores from the Monterey Bay shelf along the central California coast, focusing on areas proximal to the Salinas and Pajaro Rivers. Sediment analyses included bulk density, sedimentary fabric (CT imagery), grain size, X-ray diffraction (XRD), and X-ray fluorescence (XRF). Age control was provided through 210Pb/137Cs and 14C geochronology. We identified two types of event deposits: Type 1 was defined primarily by sedimentary fabric (sharp contacts and internal laminations), relatively low bulk density, increases in fine silt and clay, but no distinguishable changes in elemental components. Type 2, identified in upper sections of the cores, is characterized by uniform 210Pb activity with depth, increases in coarse silt and fine sand, and decreases in terrestrial elements such as Ti, Fe, and Zr. The Type 1 deposits we interpret to represent rapid settling out of terrestrially derived plumes from fluvial floods such as those in 1861-1862. Type 2 deposits are enriched in remobilized inner shelf and coastal sediment from large wave events that occurred during strong El Niño winters in the 1980s and 1990s. This study demonstrates the possibility of distinguishing between fluvial flood and wave remobilized deposits along the mid shelf. This distinction may provide insight into event frequency, as well as a basis to understand how these different types of episodic events may impact biogeochemical cycles.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B53I2175H
- Keywords:
-
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0428 Carbon cycling;
- BIOGEOSCIENCESDE: 1635 Oceans;
- GLOBAL CHANGEDE: 3022 Marine sediments: processes and transport;
- MARINE GEOLOGY AND GEOPHYSICS