Giant Pit Craters On The Seafloor Above Magma-induced Hydrothermal Vent Complexes

Massive injection of 13C depleted carbon to the ocean and atmosphere coincided with major environmental upheaval multiple times in the geological record. For several events, the source of carbon has been attributed to explosive venting of gas produced when magmatic sills intruded organic-rich sediment.
The concept mostly derives from studies of a few ancient sedimentary basins with numerous hydrothermal vent complexes (HTVCs) where craters appear to have formed across large areas of the seafloor at the same time, but good examples remain rare in strata younger than the Early Eocene.
We present geophysical data documenting at least 150 large (km-scale) craters on the modern seafloor across ∼148,000 km2 of Scan Basin in the southern Scotia Sea, a remote region offshore Antarctica.

Overview of the study area. (A) Map showing plate boundaries, tectonic structures, bathymetry and key features surrounding Scan Basin (SB). BB: Bruce Bank, DB: Discovery Bank, DoB: Dove basin, SFZ: Shackleton Fault Zone, SSR: South Scotia Ridge, SGI: South Georgia Islands. Yellow dots show the location of ODP and IODP sites. Blue box shows the study area of lower panel. (B) Map of Scan Basin located between Bruce Bank and Discovery Bank and north of South Scotia Ridge that highlights significant subsurface structures. Red: areas with mapped subsurface sills based on the isochrone map to the top of the sill intrusions. Black lines: inferred transport magma flow directions. Yellow letters (A, B and C): sill systems. Magenta colour marks the volcanic cones along the Discovery Bank. Black bold lines mark the localization of the seismic lines show in Figs. 3B and 4A and E. White box show the location of the field of giant craters covered with high-resolution bathymetry shown in Fig. 2. V1 and V2 are volcanic cones shown in Figs. 2 and 8A associated with the field A of pit craters. — Science Reports via PubMed
Seismic and bathymetric information reveals the craters relate to vertical fluid pipes extending above dome-shaped forced folds and saucer-shaped igneous sills. Presumably, magmatic intrusions deform overlying sediment and produce thermogenic gas, where buoyant hydrothermal fluids migrate upwards from sill flanks through V-shaped gas chimneys to the seafloor.
Fluid expulsion, driven by excess pore pressure, enhances vertical conduits and creates collapse structures on the seafloor. Age estimates for sill emplacement and crater formation come from correlations of seismic reflectors with bore hole data collected on IODP Expedition 382.
Sills intruded into sediment at least two times, first about 12–13 Ma (Middle Miocene), which occurred with deep intrusions of stacked composite sills, and once about 0.9 Ma and associated with volcanism along Discovery Bank, which may have reactivated previous fluid venting. Crater reactivation has occurred since 0.9 Ma, although probably episodically.
Importantly, at present-day, numerous craters related to sills and fluid pipes populate the seafloor above a young sedimentary basin, and the ocean and atmosphere are receiving massive quantities of 13C depleted carbon. The two phenomena are unrelated but, with changes in global climate and sedimentation, the craters could be filled simultaneously and give an impression in the rock record of rapid and coeval formation coincident with carbon emission.
Interpretations of ancient HTVCs and their significance to global carbon cycling needs revision with consideration of modern seafloor regions with HTVCs, notably Scan Basin.

High-resolution bathymetric images of the seafloor craters and pockmarks. (A) and (B) show 3D images from the South Field A and (C) and (D) show 3D images from the North Field B. a-f insets show the profiles and dimensions (width and depth) along the craters. (E)-Overview of the study area. Arrows show orientation of the 3D views (A–D). Dotted white line indicates location of 2-D seismic line shown in Fig. 3. V1 and V2 are volcanic cones associated with the field of pit craters. Information of the vulcanism in the Supplementary Figure S3. Note that colour scale of the bathymetry is different for individual panels. — Science Reports via PubMed
Giant pit craters on the modern seafloor above magma-induced hydrothermal vent complexes of Scotia Sea, offshore Antarctica, Science Reports via PubMed
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