Degradation of Hg signals on incipient weathering: Core versus outcrop geochemistry of Upper Permian shales, East Greenland and Mid-Norwegian Shelf

Abstract

Mercury (Hg) enrichment and elevated ratios of Hg to total organic carbon (Hg/TOC) in sedimentary rocks have often been linked to volcanism from large igneous provinces (LIPs). Primary Hg and TOC contents of sediments can be altered by secondary processes like extreme weathering. These effects must be evaluated before tying Hg anomalies in weathered rocks directly to LIP events. However, the effects of incipient weathering on Hg contents and Hg/TOC ratios are not known. In this study, we elucidate the behavior of Hg during incipient weathering by investigating visually pristine black shales from outcrops of the Ravnefjeld Formation in East Greenland (GRL) and comparing them to drill core equivalent intervals acquired from the same outcrop area and correlative shales from the mid-Norwegian shelf (MNS). By using geochemical investigations and principal component analysis, we characterize the main host phases of Hg and relate the different Hg contents of pristine samples from GRL and MNS to different Hg inputs during shale deposition. Compared with pristine drill core samples, incipiently weathered outcrop shales have up to 77% lower Hg contents and up to 64% lower Hg/TOC ratios. Incipient weathering causes the early degradation of Hg signals, which masks the primary Hg and Hg/TOC signals in sedimentary rocks. Therefore, we suggest that the presence and effects of weathering in sedimentary rock should be evaluated before discussing Hg signals.