Mercury enrichment indicates volcanic triggering of Valanginian environmental change

The Valanginian stage (Early Cretaceous) includes an episode of significant environmental changes, which are well defined by a positive δ13C excursion. This globally recorded excursion indicates important perturbations in the carbon cycle, which has tentatively been associated with a pulse in volcanic activity and the formation of the Paraná-Etendeka large igneous province (LIP). Uncertainties in existing age models preclude, however, its positive identification as a trigger of Valanginian environmental changes. Here we report that in Valanginian sediments recovered from a drill core in Wąwał (Polish Basin, Poland), and from outcrops in the Breggia Gorge (Lombardian Basin, southern Switzerland), and Orpierre and Angles (Vocontian Basin, SE France), intervals at or near the onset of the positive δ13C excursion are significantly enriched in mercury (Hg). The persistence of the Hg anomaly in Hg/TOC, Hg/phyllosilicate, and Hg/Fe ratios shows that organic-matter scavenging and/or adsorbtion onto clay minerals or hydrous iron oxides only played a limited role. Volcanic outgassing was most probably the primary source of the Hg enrichments, which demonstrate that an important magmatic pulse triggered the Valanginian environmental perturbations.

elemental mercury (Hg 0 ), which is globally distributed due to a rather long atmospheric residence time (0.5-1 year) 28 . After oxidation to reactive Hg 2+ , Hg is deposited in continental and marine environments by precipitation 29 (Fig. S1 in supplementary material). In sedimentary deposits, Hg is preferentially adsorbed onto organic matter, hydrous iron (Fe) oxides, and/or clay minerals 25,[29][30][31][32] (Fig. S1 in supplementary material). The Hg, total organic carbon (TOC) and/or phyllosilicate and iron contents are therefore generally correlated in modern and ancient sediments [29][30][31][32] . Consequently, the origin of sedimentary Hg anomalies is evaluated by discrmining between enrichments related to enhanced TOC, iron, and/or clay mineral contents and enrichments related to volcanic activity 29 . In fact, since the pioneering work of Sanei et al. 25 , anomalous enrichments in Hg contents observed in the geological records have been related to volcanic eruptions, such as the formation of the Siberian Traps leading to the end-Permian extinction event 31,33 , central Atlantic magmatic activity inducing end-Triassic mass extinction 34 , Karoo Ferrar LIP volcanism implied in the early Toarcian OAE 29 , and finally the build up of the Deccan Traps related to the K-T boundary extinction event 32,[35][36][37] .
Here we investigate the distribution of Hg contents in four Valanginian reference sections located in pelagic and hemipelagic environments in the Central Tethyan Realm (Lombardian Basin, Breggia section), the northern Tethyan margin (Vocontian Basin, Orpierre and Angles sections), and the narrow seaway connecting the Tethyan and Boreal Oceans (Polish Basin, Wąwał core) (Fig. 1). The sedimentary succession of Breggia consists of a monotonous pelagic limestone succession. The Orpierre and Angles sections are composed of hemipelagic marl-limestone alternations, whereas the Wąwał core is comprised of monotonous sandy to silty clays. The reported Hg concentrations are normalized against TOC, iron (Fe) and phyllosilicate contents in order to discriminate variations due to local adsorptive processes from those due to increased volcanic activity 29 (Figs 2  and 3). For the samples with less than < 0.2 wt. % TOC, Hg/TOC ratios are not considered to reflect a realistic values 31 and are consequently not shown here.

Results
The selected sections have not been the subject of significant diagenesis, benefit from a robust temporal frameworks, and the evolution of the Valanginian Weissert episode is well defined by δ 13 C stratigraphy 6

Discussion
The organic carbon record in the Vocontian Basin, expressed as TOC in weight percent, shows low values for the entire Valanginian interval, not exceeding 0.6 wt. % 11,16 . With regards to the Wąwał section, the TOC values are somewhat more elevated near the base and the top of the section to reach maxima of 1.24 and 1.44 w%, respectively 40 . However, the highest Hg contents are recorded during the Weissert episode where TOC values are below 0.7 wt% (Fig. 3). For all studied sections, the Hg/TOC ratios are rather well correlated with the overall Hg contents, and intervals of maxima in Hg/TOC ratios correspond to samples with relatively high Hg contents (Figs 2  and 3). At Breggia in the Lombardian Basin, at Breggia the TOC values remain constantly very low (< 0.05 wt. %), and unlikely influenced Hg sequestration in the studied samples. Furthermore, normalizing Hg concentrations against Fe contents does not remove the trend observed in overall Hg contents in all sections, which suggest that hydrous iron oxides did not influence Hg sequestration 26 . We have also calculated Hg/phyllosilicate ratios for each section (Figs 2 and 3), and it appears that correlation coefficients between Hg and phyllosilicate are very low and that the observed Hg variations can not be explained by the presence of clay minerals alone (R 2 = 0.01, 0.03, 0.15 and 0.13, at Orpierre, Angles, Breggia and Wąwał, respectively; Fig. S3 in the supplementary material). This implies that the Hg anomalies recorded in the studied sedimentary successions are not primarily controlled by organic-matter, hydrous iron oxides, and/or clay-mineral contents.
In analogy to previous studies, in which anomalous enrichments in Hg were interpreted as indicative of an increase in volcanic activity 25,29,[32][33][34]37 , we suggest that the Hg enrichments recorded in the European Valanginian sections are related to a rapid increase in atmospheric Hg concentrations associated with a massive release of volcanic Hg 0 into the atmosphere just before the early/late Valanginian transition. A distal volcanic ash deposit occurs in the sediments from the Vocontian Basin during the Campylotoxus Zone, thus witnessing volcanic eruptions during the late early Valanginian 41 . The source of the volcanic ash deposit has been attributed to volcanic activity associated with the Tethyan subduction zone (ref. 41, Fig. 1), and implies that subduction-related volcanism was active near the onset of Weissert episode. Such process could have been a source of Hg input into the atmosphere. However, an alternative and more likely candidate for the main pulse in Hg contents is Paraná-Etendeka LIP activity, represented by the large intraplate magmatic province situated in southern South America and in southwest Africa (Fig. 1). A recent revision of the 40 Ar/ 39 Ar dates 23 and U-Pb isotopic data on zircons 24,42 points indeed to the short-lived character of this volcanic episode (not exceeding 1.2 Myr) with a main pulse at ~135 Ma 42 .
The Hg anomalies documented herein are situated near the onset of the Weissert episode (Figs 2 and 3). They mark the starting point of major turnovers and crises recorded in marine and terrestrial ecosystems 3,[11][12][13][14][15][16]43,44 . They allow us to directly establish a connection between an increase in volcanic outgassing and the Valanginian environmental and ecological changes. The initiation of volcanic activity and the hypothesized associated released of CO 2 in the ocean-atmosphere system has been important enough to affect both the hydrological cycle and ocean chemistry. On one hand, it has accelerated the hydrological cycle, intensified silicate weathering, and fertilized the coastal environment, thereby affecting shallow-water ecosystems 3,14 . On the other hand, the excess amount of CO 2 in surface waters have most probably been accompanied with lowered pH that could have hampered the development of several pelagic carbonate producers 45 . The demise of shallow-water carbonate platforms The biostratigraphic framework is based on ammonites and calcareous nannofossils 6,16 . and the biocalcification crises in pelagic-environments coupled with a global increased in organic-matter burial and preservation, especially on continents, contributed to the positive shift in the δ 13 C records 46 . Volcanic activity was sufficiently important to impact the Valanginian biosphere and environment in a way that marine life severely suffered, whereas terrestrial life may have benefited from the prevailing warm and humid conditions, which lead to the development of widespread vegetation covers, favoring the evolution of herbivore life and eventually also providing a setting favorable to angiosperm evolution 8 .

Methods
A total of 429 Hg analyses were achieved using a Zeeman R-915F (Lumex, St. Petersburg, Russia) high-frequency atomic absorption spectrometer at the University of Lausanne. Analyses are based on the direct thermal evaporation of Hg from solid samples. Measurements were systematically conducted on two aliquots. The accuracy was confirmed by the analysis of certified reference materials (GSD-11 standard, Chinese alluvium: 72.0 ppb) 47 with a correlation coefficient of 0.99 and a standard residual deviation of 0.44. In complement, total organic carbon (TOC) at Orpierre and Breggia has been obtained by Rock-Eval TM6 analysis 48 at the University of Lausanne. Approximately 50 to 70 mg of powdered sample material has been subjected to pyrolysis followed by complete oxidation of the residual sample 49,50 . Samples were calibrated using the IFP160000 standard with an instrumental precision of < 2%.