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

Abstract

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.

Mercury deposition in Western Tethys during the Carnian Pluvial Episode (Late Triassic)

“…Different Hg and Hg/TOC signals could arise due to local differential diagenesis, as intense weathering and post-depositional oxidation of organic matter can change Hg concentrations in sediments and Hg/TOC values 30 , 39 . Enrichments in Hg and Hg/TOC may be controlled by early diagenetic degradation of organic matter, potentially amplifying the Hg/TOC ratios, especially in samples with very low TOC 40 .…”

Mercury Evidence of Intense Volcanism Preceded Oceanic Anoxic Event 1d

“…These results indicate that fluctuations in TOC are not controlling the Hg levels in our section. While recent studies highlight the impacts of intense weathering and post depositional organic‐matter degradation on the Hg/TOC proxy in marine sediments (Charbonnier et al., 2020) our samples were collected from the fresh part of the outcrop and weathering surface has been removed before chemical analyses. Alteration can also be tested by the Sr/Ca ratio and δ 18 O values of ancient marine sediments that may be altered significantly in cases of major change in carbonate precipitation rate (Lorens, 1981; Watkins et al., 2014), or during extensive carbonate diagenesis (Swart, 2015).…”

Cenomanian-Turonian sea-level transgression and OAE2 deposition in the Western Narmada Basin, India

“…Mercury anomalies in sediments worldwide have been identified as potential proxy for Large Igneous Province (LIP) volcanism associated with all five major mass extinctions in Earth's history (e.g., Grasby et al, 2016;Gong et al, 2017;Jones et al, 2017;Percival et al, 2017;Thibodeau and Bergquist, 2017;Bond and Grasby, 2020; review in Grasby et al, 2019;Keller et al, 2020). Similarly, three submarine LIPs have been identified as active during the Cretaceous Oceanic Anoxic Events (OAEs) (e.g., Aptian OAE1a, Aptian-Albian OAE1b, Cenomanian-Turonian OAE2, Valanginian, Toarcian) (e.g., Charbonnier and Föllmi, 2017a, b;Scaife et al, 2017;Percival et al, 2018;Them et al, 2019;Charbonnier et al, 2020) (Fig. 4).…”