Abstract

Field measurements of oceanic turbidity currents, especially diluted currents, are extremely rare. We present a dilute turbidity current recorded by instrumented moorings 14.5 km apart at 1300 and 1860 m water depth. The sediment concentration within the flow was 0.017%, accounting for 18 cm/s gravity current speed due to density excess. Tidal currents of ∼30 cm/s during the event provided a “tailwind” that assisted the down-canyon movement of the turbidity current and its sediment plume. High-resolution velocity measurements suggested that the turbidity current was likely the result of a local canyon wall slumping near the 1300 m mooring. Frequent occurrences, in both space and time, of such weak sediment transport events could be an important mechanism to cascade sediment and other particles, and to help sustain the vibrant ecosystems in deep-sea canyons.

INTRODUCTION

Turbidity currents are sediment-laden, gravity-driven underflows in which the sediment grains are suspended by fluid turbulence (Kuenen and Migliorini, 1950). By definition, turbidity currents are driven downslope by the density excess (Δρ = ρ – ρa, where ρ is bulk density, and ρa is density of ambient water) arising from the presence of suspended sediment in the flow; thus the speed of turbidity currents can be very low or very high, depending on the sediment concentration. Most oceanic turbidity currents were accidental findings, i.e., they were discovered a posteriori in response to broken submarine cables (Heezen and Ewing, 1952; Hsu et al., 2008) or damaged instruments (Inman et al., 1976; Paull et al., 2010). Weaker turbidity currents in the ocean can only be recorded from field experiments specifically designed for measuring live events, experiments that have proven very difficult (Hay, 1987; Xu et al., 2004). Because field instruments often do not survive in strong flows, detailed measurements of oceanic turbidity currents are extremely rare. In this paper we present field data from acoustic Doppler current profiler (ADCP) and turbidity sensors that recorded a dilute turbidity current in one of the world’s largest canyons, the Monterey Submarine Canyon, offshore California, United States. We identify the signature of the event, estimate its flow and sediment properties, and discuss its contribution to sediment transport. Our hypothesis of canyon-wall slumping as a cause of the dilute turbidity currents can be applied not only to large canyons like the Monterey, but also the small ones that are ubiquitous along the world’s coastlines.

DATA

Two moorings were deployed in the Monterey Submarine Canyon axis at 1300 m (mooring B1) and 1860 m (mooring B2) (Fig. 1). They were outfitted with similar instrumentation (Table 1), although the sampling settings were slightly different. Mooring B2 was deployed for 13 months (March 2010 through April 2011), and B1 was deployed for 4 months (December 2010 through April 2011). The detailed multibeam bathymetric map of the canyon floor (Paull et al., 2011) showed an increase of bed slope from 1.6° at B1 to 3.7° through the narrow gap called Navy Slump, and to 2.6° between water depths of 1470 and 1750 m. In the study area, semidiurnal constituents dominate the tidal currents, and the spring-neap two-week cycle is also pronounced (Xu and Noble, 2009).

The echo intensities, averaged over the four beams of each ADCP, are used here as a qualitative proxy for sediment concentration; higher echoes represent greater concentration. The background echo is removed by subtracting the minimum value of each ADCP bin from the raw data to produce the net echo. The turbidity events were identified based on the anomalies in the net echo intensity. Quantitative estimates of sediment concentration were obtained from transmissometers and turbidity sensors.

Two turbidity currents were recorded in the year-long deployments. A stronger event (TC1) was recorded at mooring B2 on 24 May 2010, before B1 was deployed (Fig. 2). Its peak down-canyon velocity 10 m above bed (MAB) reached 260 cm/s (Fig. 2B). The transmissometer at 13 MAB was totally occluded, with the maximum attenuation coefficient approaching 80 m−1, equivalent to 4.0 g/L (0.15%), for a mixture of half fines and half sands (based on calibrations by Xu et al., 2002). A much weaker event (TC2) was recorded on 25–26 December 2010 by both moorings. This paper focuses on TC2, because the high-resolution, high-frequency current and turbidity data from both moorings allowed us to (1) examine the TC2 evolution, (2) study the role of tidal currents in modulating the event, and (3) discuss the origin and cause of the event.

DILUTE TURBIDITY CURRENT

TC2 started at 23:16 (Greenwich Mean Time) on 25 December 2010, identified by a rapid rise in turbidity and chlorophyll [with peak values of 264 nephelometric turbidity units (NTU) and 0.8 ug/L, respectively] and the echo intensities at B1 (Fig. 2). The front of TC2 arrived at B2 at 07:40 on 26 December (the spike at 03:30, a single data point, is a spurious data anomaly; see following discussion), and the ADCP echo stayed high for more than 24 h (Fig. 2G). The two transmissometers at 13 and 30 MAB at B2 also signaled the arrival of the turbid plume, but the variation in attenuation coefficient (Fig. 3D) was much weaker than either the ADCP echo intensity (Fig. 2G) or the turbidity meters on mooring B1 (Fig. 3C).

During the event, the depth-averaged velocity at B1 had a maximum of 52 cm/s (Figs. 4A and 4B). This velocity consisted of two parts: part one (forumla) was the depth-averaged tidal current that happened to also flow down canyon during the event (Figs. 3A and 4A); part two (forumla) was the gravity current generated by Δρ, the velocity profiles of which had the well-defined shape of a gravity flow.

Given the distance (length, L = 14.5 km) and time differential (Δt = 504 min, 23:16–07:40) between the two sites, the average down-canyon speed of the turbid plume <U> = Lt = ∼48 cm/s. Both the turbidity sensor at B1 and the ADCP at B2 were sampled at a 10 min interval (Table 1), so the possible error in estimating the arrival times at both sites was ±5 min; therefore, the uncertainty was ∼2%. Even though the arrival time at B2 was clearly identified in the ADCP echo (Figs. 2G and 3D), the absence of characteristic velocity profile suggested that TC2 had dissipated before arriving at B2. The <U> of TC2 could be somewhat different than 48 cm/s.

DISCUSSION

Tidal Modulation of Turbidity Current

When the speed of a turbidity current is similar to that of tidal currents, understanding the magnitude and phase relations between the two is of great importance. The TC2 peak turbidity at 10 MAB at B1 was 264 NTU (Fig. 3C), equivalent to a sediment concentration of ∼450 mg/L (0.017%), according to the correlation relationship shown by Downing (2005). Applying this concentration to a Chezy-type equation,  

graphic
(Bowen et al., 1984), with typical values of channel slope (β = 1.6° at B1), drag coefficient (f = 3 × 10−3), thickness (height, h = 20 m), density of sediment (ρs = 2650 g/L), and potential density of seawater at 1300 m water depth (ρ0 = ρB1 = 1027.4 g/L), it predicts that a concentration of C = 0.017% can sustain a gravity current of forumla = 18 cm/s. The depth-averaged tidal current (forumla), which can be estimated from a typical spring tide because TC2 occurred during spring tide, ranged from 28 to 36 cm/s. As expected, the sum of forumla and forumla closely resembles the TC2 depth-averaged speed, forumla (52 and 44 cm/s; profiles 3 and 4 in Fig. 4A). Because forumla is almost twice as much as forumla, it provided a “tailwind” to enhance the down-canyon movement of TC2. Moreover, the fact that forumla ≈ <U> reinforces our conclusion that the sediment plume associated with TC2 arrived at B2 at 07:40, not at 03:30, on 26 December. The value of <U> would have to be 95 cm/s if the arrival time were 03:30. Presumably, a turbidity current traveling at 95 cm/s would not suddenly stop, even with an opposing tidal current of ∼30 cm/s (Fig. 3E).

Note that <U> is an average value over the canyon length between the two moorings, whereas forumla is a snapshot at a particular time. The latter velocity includes the gravity current, forumla, that is strongly slope (β) dependent. With all other parameters in the Chezy equation unchanged, the greater bed slopes of 3.7° and 2.6° would have increased forumla to 28 cm/s and 24 cm/s, respectively.

Fate of the Dilute Turbidity Current

The elevated chlorophyll and sediment concentrations indicated the presence of fresh sediment that contained either (1) particles originated from the canyon head, or (2) particles draped on the canyon walls after settling through water column; both tend to have higher chlorophyll content. The coincidence of the highest sediment concentration and a burst of strong cross-canyon flow (toward the west; Figs. 3A–3C, and profiles 1 and 2 in Fig. 4C) suggest the latter. The water column at B1 was clearly stratified at the beginning of TC2 (Figs. 3A and 3B; see the GSA Data Repository1), when flows within the bottom ∼10 m were cross canyon, whereas the water above flowed along canyon. It is unlikely that TC2 first started at the canyon head, e.g., by wave-induced liquefaction, and then passed by B1, because (1) surface waves and turbidity events were not correlated (Figs. 2 and 3); and (2) a transit current from upstream should not flow across canyon at B1. Instead, the near-bed cross-canyon current burst, the only occurrence of such flow separation in the entire deployment at B1, strongly suggested that TC2 was started by a local slumping on the northeastern canyon wall in the vicinity of B1, an event analogous to the gravity flows induced by commercial trawling (Palanques et al., 2006). How such a slump was triggered is unclear, and the exact location of the slump cannot be identified without repeated high-resolution multibeam surveys. Numerous slumping marks near B1 (Fig. 1) suggest that such failures have occurred here in the recent past.

All turbidity currents dissipate and eventually diminish due to friction and reduced Δρ resulting from mixing and sediment settling out. Weaker turbidity currents like TC2 are expected to collapse sooner, providing that they have similar compositions. The lack of a gravity current profile at B2 suggests that the ground-hugging gravity flow had dissipated prior to arriving at B2. The inverted echo profiles at B2 (Fig. 3D) were believed to result from sediment plumes that had lofted due to buoyance reversal, ρ < ρa (Sparks et al., 1993). The initial bulk density of TC2 was 1027.9 g/L [ρ – ρB1(1 – C) + ρsC)], and the density of bottom water at B2 was ρB2 = 1027.7 g/L (from measured temperature of 2.4 °C, salinity of 34.6 PSU, at 62 MAB). However, there were not adequate data to determine when and where TC2 became lighter than the ambient water. Other factors such as the slope, size, and curvatures of the channel also affect the actual movement of TC2. The nearly 90° bend of the canyon between B1 and B2, as well as the very sharp turns of the thalweg near the Navy Slump (Fig. 1), inevitably contributed to the flow complexity.

Cascade of Down-Canyon Sediment Transport

There are enough measurements at B1 to obtain an order-of-magnitude estimate of sediment transport by TC2. The duration (t) of the event was ∼2 h (Figs. 3A and 3B). The mean sediment concentration (C) over the 2 h period was 0.2 g/L. The mean speed (velocity, V) was 47 cm/s (Fig. 4A). The canyon cross-section area (A) at site B1 was 1.2 × 104 m2 (600 m width × 20 m flow thickness). The total down-canyon sediment transport by TC2 (Q = C × A × V × t) was ∼8.5 × 106 kg, or ∼4900 m3 assuming a nominal porosity of 0.65. With the assumption that the majority of this sediment was deposited in a 10 km section of the canyon (from 1470 m, where β decreases from 3.7° to 2.6° [Paull et al., 2011], to site B2) where the thalweg is 500 m wide, the thickness of the deposit is ∼1 mm. Because of the slow speed of TC2, this thin deposit is likely fine grained (silts and clays). Similarly, the sediment transport by the much larger TC1 passing through site B2 can also be estimated: t = 2 h, A = 3 × 104 m2 (1000 m width × 30 m flow thickness), C = 2.0 g/L, and V = 100 cm/s, producing a total down-canyon sediment transport of ∼4.3 × 108 kg (2.5 × 105 m3). This is more than 50 times greater than the transport by TC2.

The transport by these two events probably represented the lower and upper bounds among the instrumented records of turbidity currents in the Monterey Submarine Canyon to date, but both are rather small compared to published data related to canyons and turbidity currents, locally and globally. For example, the annual inputs to the Monterey Submarine Canyon head from the Salinas River and the littoral drift are 2 × 106 m3 and 0.4 × 106 m3, respectively (Eittreim et al., 2002). Sediment failures at the head of the Scripps Canyon (Marshall, 1978) and Var Canyon (Malinverno et al., 1988) are 10 × 106 m3 and 8 × 106 m3, respectively. None of the modern-day turbidity currents in the Monterey Submarine Canyon was able to move sediments passing 3000 m water depth; the last event that did so was ∼100 yr ago (Paull et al., 2009). However, because events like TC1 and TC2 occur much more frequently both in space and time (Xu et al., 2004), they may play an important role in cascading sediments and other particles down canyons to be entrained by later larger flows. Furthermore, the movement of fresh organic matter in these frequent cascading sediments probably helps support the vibrant ecosystems in submarine canyons.

CONCLUSIONS

The evolution of a dilute turbidity current is documented with high-resolution flow and sediment data collected at 1300 and 1860 m water depth. The turbidity current is inferred to have started by canyon-wall slumping. The sediment concentration within the turbidity current was very small (0.017%), so the speed of gravity flow due to density excess was only 18 cm/s, lower than the depth-averaged speed of tidal currents. Because the tidal currents also flowed down canyon during the event, the turbidity current had a “tailwind” while traveling toward the mooring at 1860 m, where only the remnant of the plume was recorded.

The sediment transport capacity of this dilute turbidity current was minimal. However, the event represented a significant process that has not previously been well documented in submarine canyons, i.e., turbidity currents triggered by localized natural wall slumping. Frequent occurrences of such events at a variety of magnitudes taking place at different water depths are likely to play an important role in cascading sediments and other particles down submarine canyons.

We thank Kurt Rosenberger, Joanne Feirrera, and Patrick Whaling for preparing the instruments and processing the data. The crews of the R/V Western Flyer are acknowledged for ably deploying and recovering the moorings. The David and Lucile Packard Foundation provided support. Comments from Marlene Noble, Russell Wynn, and two anonymous reviewers are appreciated.

1GSA Data Repository item 2013035, two figures showing the stratified flows at the beginning of the turbidity current, and the velocity profiles during a typical spring tide, is available online at www.geosociety.org/pubs/ft2013.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.