Turbidity currents—downhill flows of sediment-laden water that occur in lakes and oceans—are one of the primary processes responsible for transporting sediment from coastal regions to the deep sea. This process results in the incremental development of sedimentary deposits, which become potential hydrocarbon reservoirs. The currents can reach velocities of up to 20 meters per second and travel hundreds of kilometers, which means they can also damage or destroy underwater pipelines, cables, and other equipment.
Exactly how turbidity currents achieve such high speeds has long been a topic of debate. For decades, scientists presumed that a turbidity current can self-accelerate because of a feedback loop: The entrainment of sediment makes the flow denser and hence faster, which in turn increases sediment entrainment. But measuring this self-reinforcing mechanism—which would strengthen the currents as they glide downhill—has proven very difficult.
Now Sequeiros et al. report the results of a series of experiments in which they document self-acceleration in laboratory-generated turbidity currents. Using video cameras and ultrasonic velocity profilers, the researchers measured this phenomenon in both the body and the front of three of the nine turbidity currents they generated in a 15-meter-long, 1.4-meter-deep flume with a 5% slope.
The team discovered that self-accelerating turbidity currents form when the discharge of water flowing into the flume exceeds a threshold while all other parameters, such as the concentration of suspended sediment, are held constant. The results suggest the increasing discharge raises the velocity of the flow high enough to initiate the feedback mechanism responsible for starting the process of self-acceleration. However, they also found that a very high discharge or too much sediment in suspension might hinder acceleration.
In a new study, scientists documented how currents that carry large amounts of sediment to the deep sea can accelerate as they glide downhill. The top image shows the change over time of instantaneous velocity profiles. Below is a photo of the turbidity current after the front crossed the velocity measuring point. Credit: Octavio Sequeiros
The team also observed that the material the self-accelerating currents deposited was more uniform in terms of its thickness and grain size than sequences that settled from decelerating currents. The authors suggest this characteristic could be used in the field to reveal more about the conditions under which turbidite sequences in the geologic record were deposited.
By carefully documenting the processes that control whether turbidity currents become self-accelerating, this study helps constrain the mechanisms by which even relatively small currents can disperse sediment to the deep oceans. Unraveling how these flows work is key to a better understanding of our oceans. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2018JC014061, 2018)
—Terri Cook, Freelance Writer
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