REG UIAR ISSUE
G e o l o g i c a l p e r t u r b a t i o n s a n d c o n s e q u e n c e s o f e x t r e m e a n d
u n e x p e c t e d p h e n o m e n a i n t h e o c e a n
James P.M. Syvitski University of Colorado • Boulder, Colorado USA
r e a v e
y o u ever w o n d e r e d h o w the ocean and seafloor react to rare but high energy extents, such as tsunamis, volcanic eruptions, extreme river floods, massive iceberg calving events, s u b m a r i n e slides and debris flows, or m a y b e the perfect storm that could tear apart the ocean bottom? During the 1999 TOS Science Meeting, held in Reno, N e v a d a(Oceanography
V.12, No. 2 and supplement) a group of experts were invited to reflect on geological perturbations and conse- quences of such extreme events.Jasim I m r a n and James Syvitski p r e s e n t e d an overview on h o w rivers during extreme floods, m a y carry so m u c h sediment that their fluid density m a y exceed the density of the ambient ocean water. When that threshold is passed, the flood waters can flow across the seafloor as a b o t t o m b o u n d a r y current. These erosive currents m a y be responsible for, or at least con- tribute to submarine canyon formation, and the tur- bidite fans located off of m a n y canyons. The physical risk to offshore installations (cables, oil rigs) can be e n o r m o u s w h e n taken on a global scale.
Jodie Bourgeois reviewed the deadly hazards related to tsunamis waves and h o w the ocean c o m m u n i t y is responding by deploying earl}, warning detection sys- tems for public notice and evacuation. N e w u n d e r w a t e r detection svstems coupled with i m p r o v e d numerical models, hold out the hope for better predictions on tsunami m a g n i t u d e and wave amplification in coastal waters. Thousands of lives are lost each decade, includ- ing the 1990s, even though very large events are not always involved. Even w h e n lives are saved through evacuation, coastal destruction remains immense.
Patricia Wiberg capitalized on the p o p u l a r novel
The Pelfect Storm
(Junger, 1997) to reflect on h o w ocean storms create sufficient seafloor bottom stresses that w h e n coupled with storm-generated currents can liter-ally rip apart seafloor sediment. Fines are r e m o v e d and only the coarsest sediment lags behind. Very large storms m a y scour the sea floor at water depths in excess of 100m. The science challenge is to couple the d y n a m - ics of b o t t o m b o u n d a r y layer transport, based on remotely sensed observations and numerical models, to the formation and preservation of event beds. Taken together this approach m a y be used to interpret the geo- logic record from continental shelves and recover the frequency and magnitude of large ocean storms.
Veronique Robigou carried the audience into the remote and hostile world of u n d e r w a t e r volcanoes where chemosynthetic life controls the ecosystem. Such extreme environments boggle the mind in terms of eco- logical adaptation (temperature, darkness, pressure, toxins). Although submarine volcanoes on spreading ridges o c c u p y only a small portion of the global seafloor, they s u p p o r t a significant biomass within the ocean. Significant is the episodic behavior of seafloor vents, where vigorous bursts of h y d r o t h e r m a l fluids s u d d e n l y escape and give rise to spectacular microbial blooms.
Anders Elverhoi discussed the greatest threat to offshore structures (platforms, cables, pipelines), that of submarine slides and the debris flows they spawn. A remarkable feature of u n d e r w a t e r debris flows is h o w fast the}, flow w h e n c o m p a r e d to their subaerial counterparts. This comparative difference, based on laboratory experiments, is counter-intuitive because the friction of an object moving in air is so m u c h less that the friction of a similar object m o v i n g in water.
Submarine debris flows can hydroplane: essentially they ride across the seafloor on a cushion of water, at least if the flow velocity has surpassed some critical threshold. This explains the long r u n o u t distances that some submarine debris flows apparently obtain.
Oceanography • VoL 13 • No. 3/2000 83
John Andrews discussed one of the hot topics in paleoceanography, the extreme variability in the flux of icebergs into the North Atlantic. The discovery of Heinrich (ice-rafted debris) layers in spatially distribut- ed sediment cores collect across the N. Atlantic have led to the postulation of massive calving events. Yet both the evidence and explanations are often contradictory.
Andrews described h o w basal instabilities within an ice sheet, such as the Laurentide, can lead to the destabi- lization of other ice sheets, through feedbacks in sea level and ocean-climate interactions. Sorting out these leads and lags between icesheet behavior and calving
dynamics has been difficult and yet the solution m a y help our understanding of deep water formation and thermohaline circulation.
Four of these six papers are presented here for the members and readers of
Oceanography,
as a special collection to help us understand some of the hottest topics in geo-oceanography.REFERENCES
Junger, S., 1997:
The Perfect Storm.
W.W. Norton & Co., 227 pp.84 Oceanography • VoL 13 • No. 3/2000
