Geochemical studies of marine sediments indicate that most Oceanic Anoxic Events (OAEs) appear coincident with Large Igneous Province (LIP) volcanism. OAE 1d records peculiar paleoceanographic changes and global carbon cycle perturbations, however, its association with volcanism has not yet been supported by robust geochemical evidence. To examine the potential role of volcanism we investigated the mercury (Hg) concentration and isotopic record of OAE 1d interval at the Youxia section, southern Tibet. The interval prior to OAE 1d is marked by a combined positive Δ199Hg and Hg content shift, which suggests a volcanic Hg source. These findings are consistence with a prominent increase in sea surface temperatures and atmospheric CO2 before OAE 1d. We suggest that eruption of the central portion of the Kerguelen LIP may have been the main source of the Hg anomaly and resulted in global environment perturbations that drove the onset of the anoxia event.

No abstract

No abstract

Paleoclimate during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2, 94.5–93.9 Ma) was characterized by elevated atmospheric CO2 concentrations and peak global temperatures. In this study, we employ δ66Zn measured on samples spanning OAE 2 in an expanded hemipelagic section in Tibet to trace changes in the major fluxes that influence biogeochemical cycles. The prominent feature of the δ66Zn record in the studied section is a continuous decrease from ~1‰ at the onset of OAE 2 to a minimum of ~0.2‰ within the Plenus Cold Event (ca. 94.3 Ma), followed by a stepwise recovery through the upper part of OAE 2. The negative shift in δ66Zn corresponds with higher terrigenous inputs, as revealed by previously published detrital index and TOC/TN (total organic carbon to total nitrogen) ratio records, and covaries with a notable decreasing trend recorded in compiled pCO2 data of different basins. We propose that influx of isotopically light Zn from weathered volcanic rocks associated with submarine large igneous provinces and/or (sub)tropic Indian continental volcanics is likely responsible for the δ66Zn decrease. We infer that the recovery of δ66Zn was caused by continued high primary production and an inevitable decline in the flux of light Zn as volcanic terrains were progressively weathered. The ultimate cessation of OAE 2 may have been a consequence of the same effect, with the nutrient supply from weathering reaching a minimum threshold to maintain productivity-anoxia feedback.

No abstract