Effect of volcanic activity on organic matter accumulation: Insights from the lower Aptian shales in the eastern Tethys

Abstract

Volcanic activity is generally accompanied by the widespread deposition of organic matter (OM)-rich sedimentary rocks throughout Earth’s history. However, the inherent connection between OM accumulation in deposits and volcanism remains controversial. In this study, we presented high-resolution organic and inorganic geochemical data of the lower Aptian shales in southern Tibet of eastern Tethys to reveal how volcanic activity affected OM accumulation. Enrichment factors of Mo and U as well as Th/U ratio indicated that the studied shales were deposited under oxic-to-suboxic bottom water conditions. Bioproductivity-related Ba/Al reflected a high biotic productivity during deposition. Multiple geochemical proxies revealed that volcanic material occurred within the lower Aptian deposits and volcanism was prevalent in this time. A significant positive correlation between OM content and bioproductivity proxy, combined with the weak or moderate negative correlations among OM content and redox, sedimentation rate and detrital input indicators, demonstrated that OM accumulation was primarily governed by the surficial bioproductivity level. Significantly, the good coupling of volcanic proxies with OM content and biotic productivity suggested that volcanic activity released abundant nutrient elements (e.g., N and P) into the ocean to stimulate the improvement of bioproductivity in surficial water column and then controlled OM accumulation during sedimentary period. This study underscores that volcanic activity plays a constructive function in OM accumulation by enhancing the bioproductivity during the lower Aptian shale deposition.