Nature ref. number: 2006-02-01981B
Supplementary Information
Collection and processing details of the various data sets and trap particle samples are
provided below.
CTD data. Seven data channels (pressure, temperature, conductivity, elapsed time, light
transmission, fluorescence and altimetry) were used at an acquisition rate of 24 Hz. The
altimeter allowed to profile down to a few tens of centimetres above the seabed and to
precisely control the altitude of water sampling. A low-pass filter was used to
compensate for the different time responses of the sensors and to remove salinity spikes.
A ship-roll and minimum probe velocity filter (< 0.05 m s-1) was applied to each cast to
disallow pressure slowdowns and reversals. After filtering, the downcast portion of each
cast was pressure-averaged and sequenced into 1 decibar pressure intervals. The one
decibar pressure, temperature and conductivity data were used to derive hydrographic
parameters (depth, potential temperature, salinity and potential density anomalies) from
the equation of state of seawater EOS80 (Fofonoff and Millard, 1983). The beam
attenuation coefficient due to absorption and diffusion by water and suspended particles
was derived from the light transmittance using the Beer-Lambert law. A least-square
linear relation was derived between suspended sediment concentration in water samples
and beam attenuation coefficient.
Current meter data. Unfiltered raw data (temperature, pressure, speed, direction, and
optional conductivity and optical backscattering values) were checked for spurious data
points and drifts. Temperature and conductivity data were fine tuned using
contemporary CTD casts collected next to the moorings. Salinity and potential density
anomalies were computed using the equation of state of seawater EOS80 (Fofonoff and
Millard, 1983). Optical backscattering data were converted into suspended sediment
concentration following the calibration curve given in Guillen et al. (2000).
Sediment trap samples. A detailed description of the PPS3 sediment trap and the
sample processing applied during this experiment can be found in Heussner et al.
(1990), and the main steps are therefore only summarized here. The trap collecting cups
were filled before deployment with a buffered 5% (v/v) formaldehyde solution in 0.45
m filtered seawater. This poisoning solution prevents degradation of trapped particles
and limits the mechanical disruption of swimming organisms (“swimmers”) that
occasionally enter the traps during sampling. After recovery, the closed cups were
stored in the dark at 2-4°C until they could be processed in the laboratory, within a
maximum delay of a few weeks. After decantation of the supernatant, particles were
wet-sieved through a 1 mm nylon mesh to retain the largest swimmers. Smaller ones
were removed under a dissecting microscope using fine tweezers. The original samples
were then precisely divided into subsamples for subsequent analyses using a rotary
splitting method. Replicate subsamples were filtered onto Whatman GF/F filters, rinsed
with distilled water, and oven-dried at 40°C. Following Nieuwenhuize et al. (1994),
organic carbon content was analyzed in 25% HCl acidified samples on a Thermo NA
2100 elemental analyzer. Nitrogen content was analyzed on a FISONS NA2000
elemental analyzer. For total mass flux calculations, four subsamples were filtered
through pre-weighed, rinsed (with distilled water) and dried (overnight at 40ºC)
Millipore cellulose acetate filters (0.45 m pore size, 47 mm diameter). Total mass
fluxes were calculated from the sample dry weight, the trap collecting area and the
sampling interval. Grain size analysis was conducted using a Sedigraph 5100 for the
fine fraction, and a settling tube for the coarse fraction.
Side scan sonar data. High-resolution sonographs were acquired during a collaborative
cruise between the University of Barcelona, the Moscow State University and the
Training Through Research initiative of UNESCO. Data acquisition includes computer
data logging and control, real time true altitude detection, record annotation, input from
subsea acoustics and DGPS, real time slant range correction, computer generated TVG
curve for optimum display and record/playback capability. Onboard post processing of
raw data using proprietary software included water column removing, correction of raw
navigation data, fitting of the side scan sonar images in geographical coordinates,
formation of the seabed section image in a given scale, side scan sonar slant range
correction and final transformation of scale and reduction of the seabed image to
geographical coordinates. Sonographs were plotted using a high resolution greyscale
thermographic recorder.
Supplementary References
Copin-Montegut, C & Copin-Montegut, G. Stoichiometry of carbon, nitrogen and
phosphorus in marine particulate matter. Deep-Sea Res. I, 30, 31–46 (1983).
Durrieu de Madron, X., Zervakis, V., Theocharis, A. & Georgopoulos, D. Comments to
"Cascades of dense water around the world ocean”. Progr. Oceanogr. 64 (1), 83-90
(2005).
Fofonoff, N.P. & Millard, R.C. Algorithms for computation of fundamental properties
of seawater. UNESCO report 44, 15-24 (1983).
Guillen, J., Palanques, A., Puig, P., Durrieu de Madron, X. & Nyffeler, F. Field
calibration of optical sensors for measuring suspended sediment concentration in the
Western Mediterranean. Sc. Mar. 64 (4), 427-435 (2000).
Heussner, S., Ratti, C. & Carbonne, J. The PPS 3 time series sediment traps and the trap
sample processing techniques used during the ECOMARGE experiment. Cont. Shelf
Res. 10, 943-958 (1990).
Nieuwenhuize, J., Maas, Y.E.M. & Middelburg, J.J. Rapid analysis of organic carbon
and nitrogen in particulate materials. Mar. Chem. 45, 217– 224 (1994).
Redfield, A. C., Ketchum, B. M. & Richards, F. A. in The Sea (ed Hill, M.N.) 2, 26-77
(Wiley, New York, 1963).
Supplementary Figure Legends
Supplementary Figure 1  Log term records from the axis of the Lacaze-Duthiers
Canyon since October 1993. a, Temperature at mid-depth (470 m, in blue) and nearbottom (1000 m, in red) (extended and modified from Durrieu de Madron et al., 2005).
Winter cooling characterizes DSWC events. See location of mooring in Fig. 1. b,
Sediment trap measured total mass flux of settling particles at 1000 m depth. The
signatures of the 1999 and 2005 DSWC events in terms of downward particle transport
are well visible, with a x5 increase of fluxes over background values, despite the partial
data gap at the onset of the 2005 event.
Supplementary Figure 2  Time integrated along-canyon cumulative suspended
sediment transport for seven submarine canyons in the Gulf of Lion. Data are
integrated over the cascading season from January to early May 2004. Instruments were
located 5 m above bottom in canyon heads at 300 m water depth. The plot shows that
the dominant downcanyon suspended sediment transport in the Cap de Creus Canyon
was two orders of magnitude higher than in all the other canyons but the LacazeDuthiers Canyon, for which the difference was one order of magnitude. Canyons in the
legend are ordered from east (Planier Canyon) to west (Cap de Creus Canyon) from top
to bottom in figure legend. Locations are shown as orange dots in Fig. 1. Note the
change in scale at 500 kg m-2.
Supplementary Figure 3  Near-bottom time series of potential temperature,
potential density anomaly, current speed and suspended sediment concentration.
Values are from three different depths (200, 500 and 750 m) along the axis of the Cap
de Creus Canyon and embrace the 2005 cascading event. Notice the strong betweendepth similarity of time series, except for SSC at 200 m. Note also the lack of SSC
response towards the end of the DSWC episode for current speeds equal to those at the
beginning of the event, which indicates the progressive winnowing and exhaustion of
easily resuspendable sediment from the shelf and upper canyon regions. See location of
moorings in Fig. 1.
Supplementary Figure 4  Time series before cascading (a) and during cascading
(b) in winter 2004-05. The parameters illustrated are current speed, suspended
sediment concentration (SSC), sand fraction (>63 µm) and fluxes from the currentmeter and trap deployed on the 500 m mooring (purple dot in Fig. 1) within the Cap de
Creus Canyon. Note the difference in sand content between sediment particles trapped
at 30 m above the seafloor before and during the 2005 cascading event.
Supplementary Figure 5  C/N regression fit for trap particles collected in the Cap
de Creus and Lacaze-Duthiers canyons during autumn stratified conditions and
DSWC events. Note the different slopes and intersections on the C axis for both
datasets. The value of the C/N ratio (6.5) and the positive intercept on the C axis (0.4
mmol g-1) for the particles transferred during DSWC events point to the fresh and rich
in extracellular carbohydrates nature of the organic matter. Such C/N ratio is very close
to the Redfield ratio for phytoplankton (Redfield et al., 1963) and the ratios for surface
particulate matter (Copin-Montegut and Copin-Montegut, 1983) in the North-western
Mediterranean. Data are from the 300 m depth moorings (orange dots in Fig. 1).
Supplementary Figure 6  Coastal regions in the world where DSWC has been
observed (redrawn from the list compiled by Ivanov et al., 2004, and Durrieu de
Madron et al., 2005).