Onset of environmental disturbances in the Panthalassic Ocean over one million years prior to the Triassic-Jurassic boundary mass extinction

Abstract

While the end-Triassic mass extinction has been linked to emplacement of the Central Atlantic Magmatic Province (CAMP), evidence for environmental stresses appears hundreds of thousands of years prior to the extinction in some sections from the Panthalassic Ocean. In this study, we measured carbon, sulfur, and mercury concentrations in the Kurusu section, near Inuyama, Japan. These bedded radiolarian cherts are part of the Mino Terrane, an accretionary complex of late Paleozoic and Mesozoic sediments deposited at abyssal water depths in the open ocean, providing a unique window into the Triassic-Jurassic transition in pelagic settings. The rhythmically bedded nature of the sediments allowed construction of a floating astronomical age model tied to the radiolarian-defined Triassic-Jurassic boundary. Average linear sedimentation rates (LSR) of 0.07–0.48 cm kyr−1 and total organic carbon (TOC) concentrations of 0.07–0.22% yielded estimates of primary productivity rates (PPR) based on published transfer functions ranging from 2400 to 63,000 mg C cm−2 kyr−1, which are generally comparable to PPRs in the modern equatorial and subtropical Pacific. While mercury (Hg) concentrations are strongly correlated with sedimentary sulfide content throughout the section, a distinct increase in the ratio of Hg to sulfide near the Triassic-Jurassic boundary may record Hg input from CAMP volcanism. Below this level, a series of discrete spikes in sulfide content appear during the ~ 1.2 Myr before the extinction, recording a precursor interval of environmental stress that also correlates with changes in the composition of the planktonic community. We infer that these changes reflect the development of stratification in the water column, with more reducing conditions characterizing the thermocline below the surface mixed layer. Based on the evidence from Kurusu and comparisons to other Panthalassic sections, we propose a model in which water-column stratification began to develop in the open Panthalassic Ocean over one million years before the Triassic-Jurassic boundary. Evidence from sections deposited at slope depths suggests that this rising chemocline may have begun to impinge on the slopes of island arcs and the South American continental margin by ~ 400 kyr before the boundary. The end-Triassic extinction coincided with both the main phase of CAMP eruptions and the irruption of acidic, reducing deep waters into photic zone and shelf environments.