PALAEOZOIC/Devonian 199 slowly rise to produce paroxysms of volcanic eruptions over huge geographic areas when they intersect the Earth’s surface Several flood-basalt episodes are known to have occurred at other times of biotic crisis, such as the eruption of the Siberian flood basalts during the Permo-Triassic mass extinction and the Deccan flood basalts during the Cretaceous–Tertiary mass extinction Vast regions of the bottoms of the Earth’s oceans were depleted in oxygen during the Late Devonian It has been argued that the great geographic extent of these anoxic water masses were produced in part by extensive submarine volcanism, and hence might be evidence for catastrophic volcanic episodes during the Late Devonian In addition, a major continental riftsystem is now known to have been active in the Ukraine region of Europe during the Late Devonian (the Pripyat–Dnieper–Donet rift), and many of the Earth’s flood-basalt fissure eruptions are associated with rifting and spreading of the Earth’s tectonic plates However, the volume of volcanic material erupted in the Pripyat–Dnieper–Donet rift appears to be relatively small (less than 10 000 cubic kilometres), and thus the intensity of the volcanism associated with the rift was not near the magnitude associated with the Siberian or Deccan flood basalt fissure eruptions (both of which produced well over a million cubic kilometres of volcanic material) Likewise, the extensive spread of anoxic bottom waters in the Late Devonian seas, and the characteristic black-shale deposits within these seas, has been argued to have been due more likely to greatly enhanced organic productivity in the phytoplankton The greatly increased influx of nutrients into the oceans, produced by the effects of extensive land plant cover on the land (Figure 1), is believed to have produced widespread eutrophication in Devonian shallow seas Bacterial degradation of phytoplankton rain and burial of the massive amounts of organic carbon fixed by eutrophication is a more probable cause of the Late Devonian black-shale deposits than submarine volcanism Another hypothetical way to produce a rapid drop in global temperature would be to impact the Earth with a large asteroid or comet, in essence to produce an ‘impact winter’ instead of a ‘volcanic winter’ As in the case of the hypothesised Late Frasnian glaciations, extensive searches for geologic evidence of major impact events in the Late Frasnian have produced very few hard data In contrast, considerable evidence exists for several impact events in the Early Frasnian, but about three million years before the biodiversity crisis in the Late Frasnian In particular, three impact events are currently dated to have occurred between 369 to 367 million years ago: the Alamo impactor, which produced ejecta debris that covers over 19 000 square kilometres of southern Nevada (USA) and which is estimated to have produced a crater 70 kilometres in diameter, the Siljan impactor, which produced a 52-kilometre-diameter crater in Sweden, and the Flynn Creek impactor, which produced a 3.5-kilometre-diameter crater in Tennessee (USA) These impacts all occurred on the Earth’s continents; however, an unknown number of additional impacts may also have occurred in the Earth’s oceans, for which we have no geologic record The ‘lag-time multiple impacts hypothesis’ proposes that these Early Frasnian impacts produced an abrupt increase in global temperature rather than a decrease The Alamo impactor in particular struck carbonate target rock, and would have produced a massive injection of carbon dioxide into the atmosphere The climatic effect of these known Early Frasnian impactors, and of other impacts as yet poorly dated but known to have occurred during the span of the Late Devonian, is argued to have produced an anomalous greenhouse interval that interrupted the Givetian to Carboniferous gradual decline in global temperatures (Figure 5) The collapse of this anomalous greenhouse interval, and the rapid drop in global temperatures produced by the resumption of the preexisting global cooling trend, is proposed to have produced the observed rapidity of temperature fall seen during the Late Devonian biodiversity crisis (Figure 5) This hypothesis would also account for Figure A proposed temperature curve for the Late Devonian, based upon the lag time multiple impacts hypothesis Several known impacts that occurred in the Early Frasnian are proposed to have produced an anomalous greenhouse interval, interrupt ing the gradual cooling trend from the Givetian to the Carbonifer ous Positions of the Late Devonian (F/F) and end Devonian (D/C) biodiversity crises are marked with arrows