Mercury vs total organic carbon

Phanerozoic mass extinctions have been linked to the emplacement of LIPs38,39,40 and the associated release of SO 2 and CO 2 6,41,42. Because mercury is the major pre-anthropogenic product of volcanism in Earth’s surface environments43, its accumulation in sediments is commonly used as a proxy for ancient volcanic events18,22,25. Changes in the content, early diagenetic degradation, or evaluated environmental loading of organic matter may influence both Hg and Hg/TOC enrichments30. In the case of the Zbrza section, in both the Sandbian and Rhuddanian stages, Hg was correlated with TOC content (R2 = 0.2 and 0.56, respectively), which does not indicate environmental Hg loading, especially in the case of Rhuddanian organic-rich shales. The opposite was true in intervals connected with LOME. No correlation was found between Hg and TOC content for either the late Katian (R2 = 0.01) or the Hirnantian (R2 = 0.07) (Fig. 2), suggesting that this pattern of Hg concentration was not a result of changing TOC content30. Moreover Jones et al.30 emphasised that the Hg/TOC ratio may be diagenetically inflated by loss of TOC during burial. In the Zbrza section all samples exhibit a rather high TOC value (specifically, ≥0.2 wt%), with the exception of sample ZB 28.45 (at the Katian-Hirnantian boundary), which has a low TOC content (0.1 wt%). However, due to the generally low level of thermal maturity of the Zbrza samples33, the loss of carbon during diagenesis is unlikely (Fig. 2, Table 1).

Moreover, we used Ni/TOC (ppm/wt%) to support the use of mercury as a volcanic indicator. Recently, Rampino et al.44 suggested a nickel anomaly as a link between the Siberian Traps eruptions and the latest Permian mass extinctions. For the Zbrza section, the Ni/TOC ratio corresponded to Hg/TOC data, with anomalies in the late Katian (Ni/TOC~967 ppm/wt%; Ni~175 ppm, while the background is approximately 65 ppm), on the Katian-Hirnantian boundary (Ni/TOC~404 ppm/wt%), and in the late Hirnantian immediately below the Ordovician-Silurian boundary (Figs 2 and 3; Table 1).

Mercury’s independence of redox-sensitive indicators

The strongest evidence for the occurrence of euxinic (persistently sulfidic lower water column) conditions in the photic zone of the water column is the identification of biomarkers from green sulphur bacteria (GSB)45,46. The most powerful and frequently used indicators of euxinia are isorenieratane and its diagenetic degradation products47,48,49,50,51 and Me,i-Bu maleimides, which are degradation products of bacteriochlorophyll pigments related to autotrophic sulphur bacteria (Chlorobiaceae)52,53,54. Framboidal pyrite populations provide another redox-sensitive indicator35. Based on Wignall & Newton35, we interpreted euxinic conditions for the samples with high number of very low-diameter pyrites (mean 3–5 μm; narrow size range), anoxic conditions for the samples with high number of low-diameter pyrites (mean 4–6 μm; some larger framboids), samples where framboids were moderately common with broad range of sizes were interpreted as dysoxic conditions and oxic conditions for the samples lacking framboids. It is noteworthy that intervals with strong positive Hg/TOC anomalies (late Katian, Katian-Hirnantian boundary, late Hirnantian) are characterised by the absence of ‘anoxic indicators’ such as biomarkers (aryl isoprenoids, maleimides) or framboidal pyrite (Figs 2 and 3; Table 1). Thus, these Hg spikes appear to be unconnected with organic matter accumulation in anoxic/ euxinic conditions. Moreover, the Zbrza section displays the high Hg/Mo ratio. Mo, as a redox-sensitive element, is associated with pyrite55 and increases in euxinic conditions. The three highest Hg/Mo spikes occur in the lower late Katian, at the Katian-Hirnantian boundary, and in the late Hirnantian, showing Hg’s independence of Mo and other redox-sensitive elements. Gong et al.31 in their study of Ordovician-Silurian strata from South China, presented Hg and Mo concentrations. As was true of the Zbrza section, their Hg/Mo spikes were related to Hg/TOC anomalies, which they interpreted as Hg flux to the ocean basin.

To sum up, the lack of strong Hg/TOC correlation, in combination with high levels of TOC concentration, the presence of Ni enrichments, and the absence of ‘indicators of anoxia/euxinia’ (no specific biomarkers, no framboids, very low Mo concentration), supports the interpretation that the positive Hg/TOC anomalies in the lower late Katian, on the Katian-Hirnantian boundary, and in the late Hirnantian (Figs 2 and 3) record enhanced environmental Hg loading.

Pre-LOME and LOME volcanic event

The Katian and Hirnantian strata in the Zbrza section display both Hg and Hg/TOC anomalies with a rather high concentration of TOC (≥0.2 wt%) at the same stratigraphic interval (Table 1, Fig. 2). For the lower late Katian, Hg reached approximately 3 times the background level (Hg/TOC ~2500 ppb/wt%) and can be roughly correlated with the mid-Boda change from cool to warm climate conditions4. The spike detected precisely on the Katian-Hirnantian boundary shows high values of Hg/TOC (~681 ppb/wt%), as well as a positive anomaly of Ni/TOC (~404 ppm/wt%). The late-Hirnantian anomaly is approximately double the Hg background level (Hg/TOC~470 ppb/wt%) and appears to be coeval with the termination of the Hirnantian glaciation (Table 1, Figs 2 and 4). Similar positive Hg/TOC anomalies were also reported in Wangjiawan (South China) and the Monitor Range (West Laurentia) by Jones et al.30, who distinguished the ornatus (=complexus) anomaly in the late Katian, coincident roughly with the lower late Katian anomaly from Zbrza, but characterised by a lower concentration of Hg and moderate TOC content (0.10–0.19%). The Katian-Hirnantian anomaly from the Zbrza section can be compared to the first pulse of the pacificus anomaly30 with extreme Hg enrichment (500 times background levels). Jones et al.30 postulate that Hg enrichments are products of enhanced environmental loading driven by LIP emplacement. Thus, the Hg enrichment in the Katian geochemical record (the ornatus anomaly) is interpreted as a volcanic event, that triggered severe cooling30,56,57. It has been suggested that the upper pacificus anomaly is connected with a volcanic eruption which triggered an albedo catastrophe and the rapid expansion of ice sheets30. The presence of numerous K-bentonites in the Upper Ordovician of the Zbrza section suggests that the Hg spikes may be related to increased delivery of volcanic ash. Trela et al.32 postulate that pyroclastic material was transported to the HCM by westerlies from the Avalonian volcanoes. Alternatively, Shen et al.58 claimed that expanded eukaryotic algal production contributed to a higher level of export efficiency during the late Katian, resulting in increased organic carbon burial and drawing down of CO 2 which ultimately triggered the Hirnantian glaciation. Enhanced algal production may have been connected with the delivery of nutrients to the ocean from volcanic sources. In the Zbrza section, the late-Hirnantian Hg anomaly was probably connected with the second phase of LOME, which can be linked with the global environmental effects of LIP-associated CO 2 (marine transgression, warming, anoxia)4,42. Jones et al.30 argued that the late-Hirnantian warming was caused by CO 2 release during the later phase of LIP emplacement and reduction of SO 2 emissions59 as aerosol albedo forcing waned.