Effect of Intense Weathering and Postdepositional Degradation of Organic Matter on Hg/TOC Proxy in Organic-rich Sediments and its Implicationsfor Deep-Time Investigations

Mercury (Hg) enrichments in sediments are increasingly used as tracer for distal volcanism in deep-time investigations. The impact of changes in organic-matter deposition and preservation on sedimentary Hg sequestration is, however, poorly understood. In this study, we evaluate the potential role of intense weathering and postdepositional organic-matter degradation on the Hg/TOC proxy in sediments. For this, we investigate weathering profiles in organic-rich sediments from lowermost Toarcian sediments (T-OAE; Lafarge cement quarry, France) and organic-rich deposits from the uppermost Cenomanian-lowermost Turonian Bonarelli level (OAE2; Furlo and Monte Velo, Italy; Manilva and El Chorro, Spain). The comparison of Hg data along weathering profiles in lowermost Toarcian sediments indicates that recent intense oxidation of the originally organic-rich deposits has removed up to 89% of the Hg signal. The organic-rich sediments of the Furlo and Manilva sections are characterized by lower Hg/total organic carbon (TOC) ratios, which suggest important Hg scavenging by organic matter (OM) deposition. At the opposite, in equivalent successions, three significant positive Hg/TOC excursions persist at El Chorro and Monte Velo. These samples exhibit low Hydrogen Index (HI) values, which plot in the field of type-III OM. This resulted from postdepositional degradation of marine OM type II to type III, which largely modified the amount and the quality of OM. Consequently, the recorded Hg/TOC ratios do not reflect original Hg drawdown but postdepositional oxidation, suggesting that extreme care is needed in the evaluation of the history of OM preservation when using Hg as a proxy for volcanic activity.