PRECURSORS: ENVIRONMENTAL DISTURBANCE IN THE PANTHALASSIC REALM PRIOR TO THE TJB

The end of the Triassic period has been viewed in several ways, as either a canonical “Big Five” mass extinction, or as part of a prolonged depletion in biodiversity encompassing several distinct episodes. Here we examine several sections deposited in Panthalassic Ocean, focusing on geochemical evidence for deteriorating environmental conditions in the millions of years leading up to the Triassic-Jurassic boundary. We present a distinct mercury enrichment as evidence for volcanic influence starting as early as the Norian-Rhaetian boundary, coinciding with disturbances to the marine nitrogen cycle that may indicate the onset of more intense water column stratification. The Kurusu section, in the Mino-Tamba terrane of central Japan, is comprised of what are likely Milankovitch controlled chert-shale couplets, and an age model can be established using cyclostratigraphic principles. Applying this age model shows evidence for increased sulfur content, likely reflecting water column euxinia, over the ~800 kyr interval preceding the biostratigraphic TJB. This is similar to the 400-800 kyr duration of environmental disturbance estimated from the Kennecott Point section in Haida Gwaii, British Columbia (part of the Insular terrane). At Kennecott Point, this interval is taken to represent the development of euxinic conditions in the bottom water at depths of 200-500 m, prior to shoaling of the chemocline to photic-zone water depths. Both the Kennecott Point section and Mino-Tamba sections show evidence for increased organic productivity after the extinction horizon, which may represent the introduction of the deep-water reduced nitrogen pool into the photic zone; we explore the implications of using this horizon as an alternative to biostratigraphy for correlation purposes. Published shelf sections from western North and South America show environmental disturbances preceding the TJB on shorter timescales. This may reflect the gradual and intermittent development of water column euxinia and shoaling of the over the course of the Rhaetian, with sections deposited in deeper water feeling the first effects of the disturbance.