Abstract Three case studies are used to exemplify the wide variety of controlling factors that combine to influence the development of modern turbidite systems, and how these vary with location and time. For example, Cascadia Basin in the Pacific Ocean off western North America, which is underlain by the Cascadia Subduction Zone, exhibits the dominant tectonic control of earthquake triggering for turbidity currents, the increased sediment-supply effects of the Mt. Mazama catastrophic volcanic eruption in 7626 yr B.P., the glacial climatic and sea-level lowstand control on rapid turbidite–system growth rates, and the recent anthropogenic control that reduces sediment supply rates. Lake Baikal in Russia shows how the rift-basin tectonic setting controls the number and type of sediment input points, the amount of sediment supply, and the consequent types of turbidite systems developed along different margins of the Baikal basin. Pleistocene glacial climatic changes, without changes in lake base level, causes increased sediment input and the rapid growth rate of Baikal turbidite systems that is three to five times greater than that during the Holocene interglacial climate. The Ebro turbidite systems in the northwest Mediterranean Sea exhibit control of system types by the Messinian salinity-crisis lowstand, of channel locations by oceanographic current patterns, and of sediment-supply increase by glacial climatic changes as well as recent decrease by anthropogenic changes. Both active-margin and passive-margin settings have some common controls such as climatic and sea-level changes, and develop similar types of turbidite systems such as base-of-slope aprons, submarine fans, and deep-sea or axial channels. Each margin also has specific local controlling factors, for example the volcanic events in Cascadia Basin, glacial climatic without erosional base-level control in Lake Baikal, and the Messinian extreme lowstand in the Mediterranean Sea. Comparison of modern turbidite systems points out new insights on external controls such as importance of: (1) earthquakes for triggering turbidity currents on active tectonic margins, (2) equal or greater Pleistocene climatic control compared to lowered base level for sediment supply, (3) direct glacial sediment input that results in doubled proximal channel size, (4) greatly reduced deposition rates in drained compared to ponded turbidite basins, (5) importance of ocean currents on location of turbidite systems and channel development, and (6) anthropogenic effects from river damming during the last century that sometimes reduces present sediment supply to turbidite systems by orders of magnitude. External Controls on Deep-Water Depositional Systems SEPM Special Publication No. 92 (CD version), Copyright © 2009 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-200-8, p. 57–76.

Abstract Four seeps and mud extrusion features at the lake floor were discovered in August 1999 in the gas hydrate area in Lake Baikal's South Basin. This paper describes these features in detail using side-scan sonar, detailed bathymetry, measurements of near-bottom water properties, selected seismic profiles and heat flow data calculated from the depth of the hydrate layer as well as obtained from in situ thermoprobe measurements. The interpretation of these data is integrated with published geochemical data from shallow cores. The seeps are identified as methane seeps and appear as mud cones (maximum 24 m high, 800 m in diameter) or low-relief craters (maximum 8 m deep, 500 m in diameter) at the lake floor. Mud cones (estimated to be approximately 50–100 ka old) appear to be older than the craters and have a different structural setting. Mud cones occur at the crest of rollover structures, in the footwall of a secondary normal fault, while the craters occur at fault splays. The seeps are found in an area of high heat flow where the base of the gas hydrate layer shallows rapidly towards the vent sites from about 400 m to about 160 m below the lake floor. At the site of the seep, a vertical seismic chimney disrupts the sedimentary stratification from the base of the hydrate layer to the lake floor. Integration of these results leads to the interpretation that focused destabilization of gas hydrate caused massive methane release and forced mud extrusion at the lake floor and that the gas seeps and mud diapirs in Lake Baikal do not have a deep origin. This is the first time that methane seeps and/or mud volcanoes associated with gas hydrate decomposition have been observed in a sub-lacustrine setting. The finding suggests that gas hydrate destabilization can create large pore fluid overpressures in the shallow subsurface (<500 m subsurface) and cause mud extrusion at the sediment surface.