GROWTH AND DEMISE OF PERMIAN BIOGENIC CHERT ALONG NORTHWEST PANGEA: EVIDENCE FOR END-PERMIAN COLLAPSE OF THERMOHALINE CIRCULATION

The Permian Chert Event (PCE) was a 30 Ma-long episode of unusual chert accumulation along the northwest margin of Pangea. The onset of the PCE occurred at about the Sakmarian-Artinskian boundary, where it coincides with a shift from warm- to cool-water carbonate sedimentation in shallow-water proximal areas. A landward and southward expansion of silica factories occurred during the Middle and Late Permian at which time warm-water carbonate producers disappeared completely from the northwest margin of Pangea. By Late Permian time, the deep- to shallow-water sedimentary spectrum was occupied by siliceous sponge spicules. Silica factories collapsed late during Permian time, an event associated with much warmer oceanic and continental climatic conditions. Chert deposition resumed in the distal oceanic areas early during the Middle Triassic (Anisian) after an 8 to 10 Ma interruption (Early Triassic Chert Gap). The conditions necessary for the onset, expansion and zenith of the PCE were provided by the thermohaline circulation of nutrient-rich cold waters along the northwest and west margin of Pangea. These conditions provided an efficient transportation mechanism that replenished the supply of silica, created a nutrient- and oxygen-rich environment favouring siliceous biogenic productivity, established cold sea-floor conditions hindering silica dissolution while increasing calcium carbonate solubility, and provided conditions adverse to organic and inorganic carbonate production. The northwest margin of Pangea was, for nearly 30 Ma, bathed by cold waters presumably derived from the seasonal melting of northern sea ice, the assumed engine for thermohaline circulation. The demise of silica factories was caused by the breakdown of these conditions and the establishment of a warmer marine environment accompanied by sluggish circulation and perhaps a reduced input of dissolved silica to the ocean. Complete thawing of northern sea ice ended thermohaline circulation and led to oceanic conditions inimical to the production, accumulation and preservation of biogenic silica.