Globally enhanced Hg concentration and Hg and C isotopes in Permian-Triassic boundary successions: Possible linkage to volcanism

This study presents C and Hg (Hg/TOC) chemostratigraphy data on identified Permian-Triassic successions across the globe with an aim to develop a better understanding on the "mass extinction" event during this time interval. Besides, it would also contribute to the global database on C and Hg stratigraphy and reinforce the application of the latter as a reliable proxy in global correlation, especially in cases of sedimentary sequences deposited coevally with LIP volcanism. In this endeavor, the delta C-13 chemostratigraphic curves from thousands of kilometers apart from nine classical marine sedimentary sections from various paleolatitudinal zones as well as Hg/TOC stratigraphic pathways of eight of these sections, were examined in a transect across the Pangea supercontinent (similar to 252 Ma). A strong negative delta C-13(carb) shift at the Late Permian Mass Extinction (LPME), in Meishan D, Guryul Ravine and several other (Permian Triassic Boundary) PTB sections worldwide, is probably due to voluminous CO2 degassing during volcanism in the Siberian Traps Large Igneous Province (STLIP). The C-isotope shift at the LPME is much stronger than in the PTB or Early Triassic Mass Extinction (ETME). The Hg/TOC peaks are observed at LPME and ETME horizons in the Meishan D section, whereas the peaks were recorded at LPME and PTB horizons in Hovea-3, Ursula Creek, Idrijca and Rizvanusa sections. The Rizvanusa section displays a single peak at the ETME horizon, Zal and Abadeh successions at the LPME and ETME horizons, while the Misci section shows Hg/TOC enrichment at the LPME horizon. The largest Hg/TOC peaks at the LPME, PTB and ETME can be associated with the beginning of Stage 2 (extrusive hiatus) of STLIP. The origin of a Hg/TOC spike in the LPME-PTB interval in seven of these sections is unclear as well as the Hg/TOC spike below the LPME at the Misci and Zal sections. The latter may, possibly be, linked to the STLIP (Stage 1) or some local volcanism. In the delta Hg-202 (MDF) vs Delta Hg-201 (MIF) cross plot, the majority of samples from the extinction interval plot within the "volcanic-emission box" and a few samples, in the "sediment/soil/peat box." Hg isotope signatures underline mixing of volcanic and terrestrial Hg, generating four trends with negligible variation in Delta Hg-201. Lesser terrigenous-Hg influx was noticed in sections closer to the STLIP (Rizvanusa, Idrijca and Seres/Misci, all in Europe), recording close to zero Delta Hg-201 (MIF) values. Marked influence is observed in sections far away from the STLIP (Meishan, Ursula Creek and Hovea-3) that exhibit negative a Delta Hg-201 (MIF). The Zal and Abadeh sections, at intermediate distance from the STLIP, exhibit the highest, positive Delta Hg-201 values (Abadeh) and the lowest, negative Delta Hg-201 values (Zal). The Delta Hg-199 (MIF) vs Hg (n ng(-1)) cross plot testifies that volcanic Hg has been contaminated by terrigenously sourced-Hg influx, inducing the two major curved trends. The negative C-isotope excursions and Hg/TOC enrichments in these sections are nearly coeval, substantiating a synchronism between the P-T transition biotic crises (LPME and ETME) and the start of the stage 2 of the Siberian Traps (sill-intrusions).