Quantifying the journey of a turbidity current: How water and sediment
discharges vary with distance in Monterey Canyon
Abstract
Turbidity currents transport vast quantities of sediment across the
seafloor and form the largest sediment accumulations on Earth. These
flows pose a hazard to strategically important seafloor infrastructure
and are important agents for the transport of organic carbon and
nutrients that support deep-sea ecosystems. Therefore, it is important
to quantify the scale of these flows, the amount of sediment they
transport, and the evolution of their discharge over time and space
along their flow path. Two modes of flow evolution have been proposed
based on experimental and numerical models. The first is termed
ignition, where flows entrain seafloor sediment, becoming more
voluminous and powerful and increasing their discharge. In the second
mode of evolution, called dissipation, sediment falls out of suspension,
so flows decelerate and lose discharge. Thus far, field-scale turbidity
currents have only been measured at a handful of sites worldwide, and
never in detail at multiple locations along their full course.
Therefore, it has not yet been possible to determine when, where, and
why flows diverge into these two modes in the deep sea, or how flow
discharge varies. The ambitious multi-institution Coordinated Canyon
Experiment measured turbidity currents at seven instrumented moorings
along the Monterey Canyon, offshore California. Fifteen flows were
recorded, including the fastest events yet measured at high resolution
(>8 m/s). This remarkable dataset provides the first
opportunity to quantify down-channel sediment and flow discharge
evolution of turbidity currents in the deep sea. To understand whether
flows ignite or dissipate, we derive total and sediment discharges for
each of the flows at all seven mooring locations down the canyon.
Discharges are calculated from measured velocities, and sediment
concentrations are derived using a novel inversion method. We observe
two distinct flow modes, as most flows rapidly dissipated in the upper
reaches of the canyon, while three ran out for the full 50 km array
length. We then explore why only these three flows ignited and discuss
the implications for canyon and channel capacity and evolution.