Maximum downwelling speeds of small-scale plumes are in the range of 2 - 3 cm/s. After 48 hours most of the initial sediments is deposited. Thus convective mixing driven by sedimentation will come to an end some hours later.
5. SUMMARY AND C0NCLUSIONS
The numerical studies suggest that sediment-driven gravity currents contribute to a ventilation of intermediate and deep waters of the ocean. The results are a part of a sequence of case studies (see, Kampf & Fohrmann, Deep water ventilation triggered by turbidity currents: Numerical investigations, submitted to JGR, 1997). Studies with a stronger shelf sediment source of 1.2 cm suggest that sediment plumes reach the abyssal plane. Due to a reduction of plume dynamics along the flat abyssal plane, high accumulation areas of sediments form close to the foot of the continental slope. The lateral spreading of the plume, however, is limited since plume properties are mixed away very quickly by upward directed convection. This limitation in lateral spreading is in agreement with field observations in the Sulu Sea (Quadfasel et al., 1990). The efficiency of sediment plumes depends (a) on the volume of a shelf region; (b) on the amount of accumulated bottom sediments prior to a storm event; and (c) on the density stratification of the ambient ocean.
In shelf areas where storm activity is high or where tidal currents are strong, shelf waters are renewed on short time scales. Thus there is unlikely that deep reaching sediment plumes can be initiated.
Sediment plumes are initiated in a time scale of years but last for only some days. Thus it seems to be impossible to observe acvtive plume events. The effects of sediment plumes, however, can be concluded from the formation of high-accumulation areas of sediments at the foot of continental slopes and from convective stirring of the water column.
Acknowledgements. This work was funded by DFG (SFB313, TP-A2);and by BMBF (ARKTIEF, 03F0178C).
REFERENCES:
Backhaus, J.0., H. Fohrmann J. Kimpf and A Rubino (1997): Formation and export of water masses produced in Arctic shelf polyas - process studies of oceanic convection, ICES J. Mar, Sci., 54: 366-382.
Fohrmann, H. (1996): Sedimente in bodengefuhrten Dichtestromungen - Numerische Fallsludien, ph.D. thesis published as report of the Sonderforschungsbereich 313 at the University of Kiel, 66: 106.
Kampf, J., and J,0. Backhaus (1998): Shallow, brine-driven free convection in polar oceans: Nonhydrostatic process studies, JGR (to appear).
Kerr,R.C. (1991): Erosion of stab1e density gradients by sedimentation-driven convection, Nature, 353, 423-425.
Quadfasel D., H. Kudrass, and A Frische (1990); Deep-water renewal by turbidity currents in the Sulu Sea, Nature, 343, 320-322.
