Caught in the Act...again! December 17-19, 2002
Two turbidity events flowing down Monterey Canyon were recorded in December 2002 by a benthic mooring located in 1300 m depth in the axis of the canyon. These pre-Christmas events occurred almost exactly one year after another turbidity flow in the same area (Paull et al. 2002), and, together with 2-3 other events observed in the canyon over the past year, indicate that such events are much more frequent than expected.
Two benthic moorings, one located in the axis of Monterey Canyon (Map - Mooring 1) another on Smooth Ridge (Map - Mooring 2), were deployed in 1300 m depth during November 2002, and scheduled for recovery in April, 2003. ROV observations of Mooring 1 during related benthic studies on March 25, 2003 revealed that the sediment trap on the mooring was inundated with sandy sediment (see trap photo). The mooring was recovered immediately, with all gear fouled with sediment, including the float package.
Analysis of the current meter data indicated that 2 events occurred in the canyon over 2 days (17 & 19 December 2002). These events are evident from both the current record and the depth record from the ADCP (Figure 1, top 2 panels). The 4-month record of current flow (Figure 2, top panel) shows the typical tidal and fortnightly variability in current flow in the canyon (flow to the SSE is down-canyon), as well as 2 short events in December 2002, when southward and eastward current speeds were considerably higher than average. Closer inspection of these events (Figure 3, top panel), showed that current speeds were at least 150 cm/s, and may have reached 500+ cm/s for brief (i.e. ~15-30 minutes) periods.
Speeds of 500 – 600 cm/s were reported during the peak flow at the beginning of the second (19 Dec) event, but these data are unreliable. Unreliable data were recorded for 2 data records (data are recorded every 15 minutes), indicated that extremely high currents lasted for 16 – 44 minutes. Current speeds higher than normal persisted for roughly 2-4 hours. A time-series profile of current speed above the bottom is shown in Figure 3.
Lateral forces on the mooring during these high currents were strong enough to deflect the mooring greatly. The total mooring height is normally ~35 m above the bottom, with the ADCP positions 30 m above bottom, looking down on the seafloor. The depth of the ADCP increased by 13 and 25 m during event 1 & 2, respectively (Figure 1, panel 2), during these flow events, effectively laying the mooring down during peak flows (Figure 4).
Surprisingly, the ADCP, normally oriented downward, was inverted (upward -looking) during the ~1/2 h peak flow during event 2. Tilt and pitch of the ADCP also indicate significant deflection of the mooring during these events. Modeling of the drag on the mooring components indicated that maximum speeds of ~3.6 m/s were required to defect the mooring toward the seafloor by 25 m (Figure 5).
There is some evidence that the mooring was moved during these events. The long term depth record from the ADCP (Figure 1, second panel) shows that the mooring depth increased after the second event, apparently due both to the added weight of the entrained sediment in the sediment trap, as well as movement of up to 200 m of the entire mooring (based on deployment and recovery navigation). The mooring slowly rose in depth a week or so after event 2, suggesting that some of the sediment load fouling the mooring was flushed during high tidal currents during spring tides. However, the rapid increase in the depth of the mooring observed during mid-March 2003 is not associated with other anomalous data and is presently inexplicable.
Although rapid current speeds persisted for at most a few hours, suspended sediment was above normal for a day or more. Optical backscatter (detected by a HOBI Labs HS2) showed that suspended sediment was above normal for >24 h (Figure 2). Backscatter strength reported by the ADCP (not shown) also showed that these events were detectable for several days. Fluorescence also increased during these events, but not to the degree detected during other periods, suggesting that the chlorophyll signal was simply part of the suspended material entrained in the turbidity flow.
Temperature increased immediately during these events, with peak temperatures of 5.3 and 3.8 degrees C at the beginning of events 1 and 2, respectively. High temperatures are caused by the shallower, warmer water entrained in the turbidity flow and transported down-canyon. Using these temperature data, we can calculate a conservative estimate of the origin of the flow can be made from its temperature. Event 1, with a temperature of 5.3 oC, must have originated in ~630 m depth or shallower (based on a typical temperature profile from central Monterey Bay), since mixing and heat loss occurs during the turbidity flow. Event 2 may have originated nearer our mooring (~900 m), since its temperature anomaly is lower. Alternatively, the smaller T-anomaly of event 2 may indicate a more distant source, which underwent greater mixing and dilution before reaching our mooring. Moreover, it is possible that each event originated near our mooring on the walls of the canyon, and entrained shallower, warmer water as the turbid flow sank to the axis and flowed down-canyon. A location in the axis of Monterey Canyon at 630 m depth is ~20 km “up-canyon” from the benthic mooring, suggesting that these events traveled a considerable distance.
Loss of RINs and BINs from the Canyon Dynamics site (yellow squares on map) during these events indicated that the same event(s) that impacted the Canyon Dynamics site continued through the canyon to our mooring, a distance of at least 25-30 km.
Was the event confined to the canyon? Apparently so. Although we have no other current speed data from outside the canyon, another sediment trap was deployed on Smooth Ridge in 1300 m depth (~20 km from the canyon mooring). This sediment trap showed low sediment flux and no anomalous sedimentation events during the same period.
What initiated this turbidity flow? For the 2001 event (Paull et al), no clear signal was related to the turbidity flow. For this event, however, a huge storm with 6 m waves lasted for several days prior to, and during, these events (Figure 1,2). This is the same storm that detached the MOOS mooring only days earlier.
How frequent are turbidity events in Monterey Canyon? In addition to the Christmas events that occurred during the past 2 years, the RINs & BINs of the Canyon Dynamics Project were moved by turbidity flows during summer 2002, and apparently again in March, 2003. This is considerably more frequent that expected, with large volumes of sediment transported in these events. The MOOS Science Experiments, planned for 2004/2005 will attempt to measure the flow of materials and carbon in the lower canyon near Shepard Meander near 3500 m. It will be interesting to see if the upper canyon events penetrate to near the abyssal plain. Oceanographic El Nino events were named originally for warm water that would arrive around Christmastime off the coast of Peru. We’ll be ready for this year’s "El Nino Event" in the canyon, but our mooring will have an even larger anchor weight.