Volcanism and basalt weathering drove Ordovician climatic cooling

Domain: earth_science
Published: 2025-12-12
Editorial outcome: major_revision

Abstract
Causal connections among major Ordovician environmental and biological events (i.e., long-term climatic cooling, Hirnantian Glaciation, Great Ordovician Biodiversification Event, and Late Ordovician Mass Extinction) remain in debate, and the hypothesis of volcanism-driven long-term cooling is untested. Here, we present both newly generated and compiled literature data for global volcanic activity (Hg geochemistry), sea-surface temperatures (conodont oxygen isotopes), and chemical weathering intensity (conodont strontium isotopes). This dataset documents a ~25-Myr-long interval of climatic cooling (~470-445 Ma), commencing around the onset of the Middle Ordovician, intensifying during the Late Ordovician, and ultimately culminating in the Hirnantian Glaciation. Cooling was associated with long-term intensified volcanic rock (basalt) weathering and atmospheric pCO2 drawdown, as well as episodic marine photic-zone euxinia, during and after major volcanic episodes. These relationships favor volcanism (i.e., continental large igneous provinces) as the primary driver of contemporaneous environmental and climatic changes, thus revealing complex modulation of life-environment co-evolution during the Ordovician Period.

Introduction
The Ordovician Period (485.4–443.8 Ma) was bookended by major biotic events in the marine realm, i.e., the early stages of the Great Ordovician Biodiversification Event (GOBE) at its onset and the Late Ordovician Mass Extinction (LOME), the first of the “Big Five” biocrises, near its termination1. During the GOBE, which spanned the early Tremadocian (Early Ordovician) to middle Katian (Late Ordovician) with an early Darriwilian peak2,3, the numbers of marine invertebrate species and genera increased by factors of ~6× and 3×, respectively. A key aspect of this diversification event was the proliferation of marine plankton (e.g., primary producers such as photosynthetic cyanobacteria), representing the “Ordovician Plankton Revolution“4. The LOME, which had a ~70–80% species-level extinction rate, occurred in two phases corresponding to the onset and termination of the Hirnantian Glaciation: LOME-1 in the early Hirnantian, which eliminated many tropical taxa, and LOME-2 during the mid-Hirnantian, which decimated the cool-water Hirnantia Fauna5. A precursor biocrisis (LOME-0) has recently been identified in the mid-Katian2,6. The driving mechanisms of the Ordovician marine biological radiations and extinctions have long been speculated upon without arriving at a consensus7,8.

Despite their complex internal structure and chemical composition9, the oxygen isotopic composition of conodonts (δ18Oconodont) is a valuable tool for study of sea-surface temperatures (SSTs) in the Paleozoic Era10. Conodont δ18O data have established that the GOBE coincided with a long-term global cooling trend with a total magnitude of ~10°C10–12, a pattern confirmed by clumped isotope analysis of carbonates13. Climatic cooling is postulated to have been a direct driver of evolution among marine invertebrates10, as have rising oxygen levels in the atmospheric-oceanic system7, although the pattern and trigger of the Ordovician climatic cooling remain contentious. For example, it is debated whether this cooling trend was a continuous long-term event or an episodic, multistage process10–12. Atmospheric CO2 levels, a key determinant of the long-term climate evolution of the Earth, are thought to have fallen through enhanced silicate weathering14 linked to the spread of the earliest land plants15, or possibly to elevated marine primary productivity16, which are not necessarily mutually exclusive mechanisms. Moreover, the patterns of change in climate and biodiversity in South China (i.e., the most intensively studied craton, and one that may have been the cradle of diversification) versus at a global scale are significantly different2, complicating an understanding of the causal relationship between Ordovician climatic cooling and contemporaneous bioevolutionary developments17.

Long-term (>50 Myr) greenhouse-icehouse climatic oscillations were commonly driven by fluctuations in dominance between continental and island-arc volcanism during geological history18. The protracted interval of Ordovician climatic cooling (leading to the Hirnantian Glaciation) is speculated to have been induced by volcanic activity19, offsetting transient warming associated with greenhouse gas emissions from volcanic eruptions16. However, integrated studies of volcanic and paleotemperature proxies are lacking to test this hypothesis, leaving the role of volcanism in the contemporaneous long-term cooling uncertain14.

Volcanism is the principal natural source of mercury (Hg) in the Earth-surface system20. Volcanic eruptions can emit substantial quantities of elemental mercury having a residence time of a few years (~1-2 yr) in the atmosphere, which facilitates its long-range transport and eventual deposition in both marine and terrestrial sediments21,22. Massive inputs of Hg during major volcanic events may exceed the absorption capacity of organic matter at the sediment-water interface, resulting in positive anomalies of the ratio of total mercury to total organic carbon (TOC) (Hg/TOC) in sedimentary successions that are signals of volcanic Hg inputs20. The isotopic composition of Hg serves as an additional powerful tool to decipher sources and mechanisms of Hg enrichment (e.g., Zhao et al.22). Hg isotopes can reveal both mass-dependent fractionation (MDF; δ202Hg) and mass-independent fractionation (MIF; Δ199Hg, Δ200Hg, Δ201Hg). Critically, MIF arises predominantly through aqueous or atmospheric photochemical interactions antecedent to Hg sedimentation, and its isotopic signals are resilient against post-depositional diageneticalteration. Mercury emissions from subaerial volcanoes typically exhibit near-zero MIF values with minimal variance20,23.

Here, we studied three biostratigraphically well-constrained Ordovician marine successions at Huanghuachang (n.b., the Global Stratotype Section and Point (GSSP) of the Lower/Middle Ordovician boundary), Chenjiahe (n.b., the global auxiliary stratotype section of the same boundary), and Wangjiawan (n.b., the GSSP for the base of the Hirnantian Stage), which are closely spaced (<15 km apart) near Yichang City (Fig. 1 and Supplementary Figs. 1-3). We generated paired high-resolution profiles of oxygen isotopes for conodont bioapatite (δ18Oconodont) as paleotemperature proxies, Hg-system chemistry (Hg/TOC, Δ199Hg and δ202Hg) as volcanic proxies, and strontium isotopes for conodont bioapatite (87Sr/86Srconodont) as a continental weathering proxy, with compilation of global published datasets, and additional constrains of Ordovician time framework based on published biostratigraphic and newly generated carbonate carbon isotope (δ13Ccarb) chemostratigraphic data. These data allowed us to evaluate the patterns and test volcanism as the main cause of Ordovician climatic cooling, providing key insights regarding life-environment co-evolution during this period.

Results
Sample analysis and global data compilation
Carbonate carbon isotopes (δ13Ccarb) exhibit major positive excursions from ~–2‰ to ~0‰ in the middle to upper Tremadocian, ~–2‰ to ~+1‰ in middle Floian to lower Dapingian, ~0‰ to ~+1‰ in the Darriwilian, and ~0‰ to ~+2‰ in the middle Sandbian to lower Katian (Fig. 2). These major excursions of δ13Ccarb are well preserved and comparable to representative global δ13Ccarb profiles (Supplementary Figs. 4-5). Together with intercalibrated biostratigraphic data, they provide a high-resolution temporal framework for the present study (Supplementary Note S1).

Our conodont in-situ oxygen isotope record (δ18Oconodont) shows a long-term secular increase from ~+15-17‰ in the Lower Ordovician Nanjinguan Formation (Fm) to ~+16-19‰ in the Middle Ordovician Dawan to Guniutan Fms, and to ~+18-20‰ in the Upper Ordovician Wufeng Fm (Supplementary Fig. 6). Diagenetic and taxon-related effects on δ18Oconodont, latitudinal changes of South China, and their impacts on SST reconstruction are discussed in Supplementary Note S2. Overall, these δ18Oconodont data document three major cooling episodes within the Ordovician in South China (Fig. 2): (1) the late Floian cooling event (LFCE), during the Prioniodus honghuayuanensis to Baltoniodus navis zones of the late Floian to middle Dapingian stages (upper Honghuayuan to lower Dawan Fms, ~200-215 m), (2) the late Darriwilian cooling event (LDCE), during the Yangtzeplacognathus protoramosus to Amorphognathus ordovicicus zones of the late Darriwilian to early Katian stages (upper Guniutan to lower Baota Fms, ~270-280 m), and (3) the Hirnantian Glaciation, during the Normalograptus extraordinarius-N. ojsuensis to N. persculptus zones of the late Katian to Hirnantian stages (upper Lingxiang to middle Guanyinqiao Fms, ~305–315 m).

To establish a comprehensive global SST record, we integrated the 170 newly generated δ18Oconodont values with 293 conodont δ18O data points from the literature representing multiple other locales (archived in Figshare, Fig. 2). In this compiled dataset, average δ18O values for the latest Cambrian (+16.2 ± 0.3‰), and the Lower, Middle and Upper Ordovician (+16.2 ± 0.8‰, +17.8 ± 0.7‰ and +19.4 ± 0.9‰, respectively) were used as baseline values for evaluation of cooling trends (Fig. 3). Relative to these baseline values, intervals of rapidly rising δ18Oconodont, representing stepwise cooling events, occurred during the late Floian (LFCE) and the late Darriwilian to early Katian (LDCE) (Fig. 2). Although the Hirnantian Glaciation interval was roughly documented in the newly generated data, it was not clearly evident in the globally compiled LOWESS curve (Supplementary Note S2). To evaluate the global representativeness of the reconstructed SST curves and further explore the duration of the Hirnantian Glaciation, we compared them to oxygen-isotope temperature records derived from conodonts, brachiopods, and bulk carbonates (Supplementary Fig. 7). The comparison reveals similar patterns of secular variation in SSTs, particularly during the LFCE and LDCE, as well as the onset of the Hirnantian glaciation around the mid-Katian and its peak near the end of the Hirnantian. These consistent trends support our inference of three stepwise cooling events during the Ordovician.

Conodont in-situ strontium isotopes (87Sr/86Srconodont) exhibit a slow decrease from ~0.7090 in the lower Tremadocian to ~0.7085 in the middle Darriwilian, followed by a more rapid decrease to ~0.7080 in the upper Katian (Supplementary Fig. 6). Our 87Sr/86Srconodont data exhibit trends similar to a Laurentian profile24 as well as a globally compiled dataset25 (Fig. 2).

Mercury content ([Hg]) is mostly below Phanerozoic average values (i.e., 34 ppb and 62 ppb for carbonate- and shale-dominant successions, respectively20, which were adopted as baseline values), interspersed with [Hg] plateaus (as high as ~200–400 ppb) above baseline values in the Sandbian and Hirnantian to basal Silurian (Supplementary Fig. 6). Carbonate-free Hg content ([Hg]cf), which is widely used to compensate for dilution of Hg caused by increasing carbonate deposition26, shows plateaus above the baseline values mostly in the middle Tremadocian to upper Floian, Sandbian, and Hirnantian to basal Silurian (Fig. 2). Moreover, the ratio of Hg to total organic carbon (Hg/TOC) exhibits plateaus above Phanerozoic average values (i.e., 144 ppb/wt% for all sedimentary rocks and 72 ppb/wt% for sedimentary rocks of TOC > 0.2%, which were adopted as baseline values20) (Fig. 2). Enrichment factors of Hg (HgEF) show high values above a threshold27 of 3× (Supplementary Fig. 8) in the lower Tremadocian, upper Floian, Sandbian, and Hirnantian to basal Silurian.

Evaluation of diagenesis suggests good preservation of Hg and validity of the Hg/TOC proxy in the study sections (Supplementary Note S3). Joint analysis using multiple Hg proxies (Supplementary Note S4) shows [Hg], [Hg]cf, Hg/TOC and HgEF mostly below baseline/threshold values in the Tremadocian, indicating a lack of Hg enrichment in that stage. Pronounced Hg enrichment is manifested in the upper Floian, Sandbian, and Hirnantian to basal Silurian in the study sections.

Ratios of mercury to aluminum (Hg/Al), iron (Hg/Fe) and manganese (Hg/Mn) show roughly parallel variations (Supplementary Fig. 9), marked by general decrease trends (from ~50 to <0.1 ppb/wt%, ~50 to 0.1 ppb/wt% and 0.1 to 0.001 ppb/ppm, respectively) in the Nanjinguan to Dawan Fms, followed by increases to maxima (~100 ppb/wt%, ~300 ppb/wt% and ~3 ppb/ppm, respectively) in the upper Guniutan to Miaopo Fms. After decreases to ~0.1 ppb/wt%, ~0.3 ppb/wt% and ~0.001 ppb/ppm, respectively, in the Baota and Lingxiang Fms, these Hg ratios rebound to pre-excursion values in the Wufeng and Longmaxi Fms.

Δ199Hg exhibits overall positive values with a gradually decreasing trend from ~+0.3‰ to near-zero values (~–0.05 to +0.05 ‰) in the Nanjinguan to Dawan Fms (Supplementary Fig. 6), with a few positive values (~+0.1 to +0.3‰) in the Honghuayuan and Dawan Fms. Δ199Hg mostly exhibits negative values in the Guniutan and Miaopo Fms (~–0.2 to ~+0.05‰), and variation from ~–0.05‰ to +0.2‰ in the Baota and Lingxiang Fms. δ202Hg mostly shows negative values (~–3 to 0 ‰) throughout the study sections, except for weakly positive values (~+0.2 to +0.7‰) in the Guniutan Fm (~467-462 Ma) and the Miaopo Fm (~459-454 Ma) (Supplementary Fig. 6), and it is negatively correlated with Δ199Hg (Supplementary Fig. 10). Diagenesis evaluations suggest good preservation of both Δ199Hg and δ202Hg signals in the study sections (Supplementary Note S3).

To accurately identify Hg anomalies within the study sections and delineate globally representative enrichment intervals, we integrated our newly generated dataset with published Hg geochemical data from 22 globally distributed Ordovician sections/cores in South China, Tarim, Baltica, Laurentia and Gondwana (Fig. 1), including 1433 Hg content ([Hg]), 1361 Hg/TOC ratios, 397 Δ199Hg values, and 391 δ199Hg values (archived in Figshare, Supplementary Fig. 8). These data derive from clastic rocks (mainly shales, 75%), limestones (24%), and volcanic ash layers (1%), of mainly Late Ordovician age. The newly generated and published Hg content and isotope data are in good agreement throughout the Ordovician (Supplementary Note S5). Multiple baseline parameters of [Hg], [Hg]cf, Hg/TOC and HgEF were further adopted to accurately identify Hg anomalies in the compiled dataset (Supplementary Note S4). Using a unified timescale28, the compiled datasets confirmed Hg enrichment intervals as revealed by our newly generated data, jointly depicting plateau values of [Hg]cf, Hg/TOC and HgEF and, thus, abnormal Hg enrichments during the late Floian, Sandbian, middle Katian, and late Katian to early Silurian (Fig. 2).

LOWESS-smoothed Δ199Hg and δ202Hg trends based on our study sections alone closely mirror that of the global composite trend based on both new and published data (Supplementary Note S5) (Fig. 2). Therefore, the identified Δ199Hg and δ202Hg signals throughout the Ordovician in the study sections represent a global pattern. Accordingly, we subdivided the compiled Ordovician Δ199Hg profile into four intervals, including (1) a decreasing trend of Δ199Hg values during the early Tremadocian to middle Floian (~485-473 Ma), (2) a dominance of near-zero Δ199Hg values during the late Floian, followed by a decreasing trend from positive to near-zero values during the earliest to early Dapingian (i.e., the LFCE) (~473-468 Ma), (3) a gradual decreasing trend to slightly negative values (~–0.05 to –0.1‰) through the Darriwilian (~468-459 Ma) until more negative Δ199Hg values (~–0.3 to –0.2‰) by the middle Sandbian, followed by a rebound to near-zero values in the late Sandbian (i.e., the LDCE) (~459-453 Ma), and (4) mostly near-zero Δ199Hg values followed by both near-zero and slightly positive values (~+0.05 to +0.1‰) during the early Katian to Hirnantian (~453-443 Ma).

Volcanism throughout the Ordovician Period
Volcanism, along with other factors, e.g., submarine hydrothermal activity, seawater redox conditions, terrestrial fluxes, and sediment accumulation rates, may collectively influence temporal fluctuations in sedimentary [Hg]21,22. In seawater, Hg complexes with organic matter to form methylmercury, and it reacts with sulfide to form mercuric sulfide (HgS), yielding compounds that commonly serving as major repositories of Hg20. However, mercury can also adsorb onto the surfaces of phosphate, clay minerals, and Fe-Mn oxides20. The concentration of phosphorus ([P]) can be utilized as an indicator of the relative abundance of phosphate minerals, and the concentration of aluminum ([Al]) can reflect the relative content of clay minerals such as kaolinite and illite, whereas the concentrations of iron ([Fe]) and manganese ([Mn]) indicate the presence of Fe-Mn oxides. In the study units, [Hg] exhibits pronounced positive covariation with TOC in the Chenjiahe (r=+0.77, n=50, p<0.001) and Wangjiawan sections (r=+0.49, n=52, p<0.001), but no significant covariation with [Al], [Mn], [Fe], or TS (Supplementary Fig. 11), implying that organic matter serves as the principal host of mercury. In comparison, [Hg] shows weak positive covariation with TOC, [Al] and [Fe] (r=+0.35, n=266, p<0.001; r=+0.41, n=82, p<0.05; r=+0.35, n=83, p<0.05, respectively), whereas no significant covariation with [Mn] and TS in the Huanghuachang section, suggesting the presence of Hg in multiple host phases (organic matter, clay minerals and Fe-oxides).

Sources of sedimentary Hg other than volcanic activity are possible. Hydrothermal activity generally results in lower 87Sr/86Sr signals for seawater and sedimentary rock (i.e., closer to the mantle endmember, ~0.7035)25, and, therefore, conodont 87Sr/86Sr ratio may proxy for hydrothermal activity. Previous studies have suggested that a majority of Sr in the studied conodont specimens (avg. ~13,000 ppm) was sourced from diagenetic fluids instead of bulk carbonate (avg. ~300 ppm)29, which means that large quantities of hydrothermally sourced Sr in contemporaneous seawater should be detectable by analysis of conodonts. In the study sections, the range of secular variation in 87Sr/86Srconodont (~0.7080 to 0.7090) is consistent with a primary marine Sr isotope signal (Fig. 2)24,25, providing no evidence of hydrothermal influence.

Seawater redox conditions commonly play a pivotal role in sedimentary Hg accumulation, mostly through modulation of microbial activity and the interplay of the C-S biogeochemical cycles, thus determining the speciation and concentration of mercury within the aquatic environment30,31. Therefore, redox oscillations can have a profound effect on Hg diffusion at the sediment-water interface, directly influencing its enrichment in sediment. To evaluate marine redox variation through the Ordovician, we used molybdenum and uranium-enrichment factors (MoEF and UEF), Corg/P ratios, and the cerium anomaly of the carbonate fraction (Ce/Ce*carb), for either clastic or carbonate successions (Supplementary Fig. 9; Supplementary Note S6). Overall, [Hg] shows no significant correlation to MoEF, UEF, Corg/P and Ce/Ce*carb, and the stratigraphic distribution of [Hg] peaks are not always within intervals of more reducing water mass conditions (i.e., lower Ce/Ce*carb, and higher MoEF, UEF and Corg/P) (Supplementary Figs. 9 and 11). However, a few high-[Hg] samples (e.g., limestone lenses in Miaopo Fm shale) correspond to more reducing conditions (i.e., lower Ce/Ce*carb), implying that reducing conditions promoted Hg accumulation in limited intervals of the study sections, probably through elevated net burial rates of organic matter and sulfide20.

Terrestrial fluxes serve as a major source of Hg to the ocean. The overall decrease in calcium carbonate content ([CaCO3]) and increase in [Al] observed in the study sections suggest local increases in the content of non-carbonate components (e.g., terrestrial materials) during the Ordovician (Supplementary Fig. 9). However, [Hg] exhibits no correlation to [CaCO3] or [Al] in the study sections (Supplementary Fig. 11), suggesting that the overall increasing trend of [Hg] through the Ordovician cannot be ascribed to Hg absorption by specific lithologic components.

Sedimentation rate can exert a significant influence on Hg uptake in sedimentary rocks by modulating their physical, chemical, and biogeochemical attributes. A low sedimentation rate facilitates the uptake of hydrogenous species (such as Hg complexes) by the sediment. In the present study, however, average linear sedimentation rate (LSR) shows no correlation with [Hg] at the substage level of resolution (Supplementary Fig. 12). Although condensed intervals with low sedimentation rates generally favor Hg accumulation, similar average [Hg] values (~2-3 ppb) are observed in the rapidly deposited Tremadocian (~24 m/Myr), the intermediate-rate Dapingian (~12 m/Myr), and the slowly deposited Floian (~4 m/Myr) stages at Huanghuachang (Supplementary Figs. 9, 12), suggesting that variation in sedimentation rates was not the dominant cause of observed [Hg] peaks.

In sedimentary successions, the Hg/TOC ratio is usually relatively stable around a background value20. Volcanically sourced Hg may be released directly during eruptions or emitted through magmatic heating of organic-rich sediment (e.g., coal beds)32, or directly to seawater through ocean-crustal hydrothermal activity. The erupted Hg is predominantly in a gaseous form and is subject to global atmospheric transport over a brief timeframe (~0.5 to 2 yr), during which it partakes in the global ocean’s biogeochemical cycles, thus leading to anomalously high Hg/TOC ratios (i.e., relative to the baseline value). Sedimentary rock Hg/TOC records have been widely used to evaluate secular variation in volcanic activity at a regional or global scale20. While Hg/TOC normalization is suitable for tracing volcanic Hg anomalies in the Chenjiahe and Wangjiawan sections, it is inappropriate for the Huanghuachang section, in which organic matter, clay minerals and Fe-oxides all serve as Hg hosts, necessitating the use of Hg/Al and Hg/Fe as complementary proxies32. In both study sections as well as others successions of the same age globally, plateau values of [Hg]cf, Hg/TOC, and HgEF are above baseline levels (Fig. 2 and Supplementary Fig. 12), and the corresponding plateau/peak values in both Hg/Al and Hg/Fe ratios (Supplementary Fig. 9) indicate major volcanic Hg inputs during the late Floian, Sandbian, middle Katian, and late Katian to early Silurian (Supplementary Note S5).

Apart from Hg anomalies, volcanic tuff layers constitute the most overt manifestation of regional volcanism. The multiple tuff layers present in the Middle and Upper Ordovician of the study sections, along with widespread (~0.5 M km²) tuffs of trachyandesitic to rhyodacitic composition across the Yangtze Platform (Supplementary Figs. 1-2), especially in Hirnantian successions, were likely derived from a continental volcanic arc associated with arc magmatism along the margin of the South China Craton during the convergence of the Yangtze and Cathaysia blocks33 (Supplementary Note S7). A global compilation of the spatiotemporal distribution of volcanic tuff layers (Supplementary Note S1) shows sparse occurrences in the Lower Ordovician, with increasing frequency in the Middle and Upper Ordovician of the USA, southern China, Europe, Argentina and Tarim Basin, China (Supplementary Fig. 13)34. These global occurrences have been attributed to continental arc volcanism linked to closure of the Iapetus Ocean35. Furthermore, these volcanic tuff layers are typically associated with elevated [Hg] and Hg/TOC ratios exceeding baseline values36, near-zero Δ199Hg values37, and host formations that display overall Hg anomalies (Supplementary Fig. 8). Although continental arc volcanism (persistently active during oceanic crust subduction) contributed Hg, the spatiotemporal distribution of volcanic tuff layers remains inconsistent with globally documented multiphase Hg enrichment intervals in Ordovician sedimentary successions. Therefore, continental arc volcanism was not the primary driver of the multiphase Hg enrichment intervals in the Ordovician.

Only a few continental large igneous provinces (LIP) of Ordovician age are currently known. The recently identified Alborz LIP in northern Iran38 has an estimated basalt volume of ∼0.5 M km3. This LIP originated from the initial rifting of Cimmerian terranes from the northern Gondwanan margin to form the Paleotethys Ocean. Accompanied by equatorward drift, its paleolatitude shifted from ~30°S in the Early Ordovician to ~10°S by the Late Ordovician. The Alborz LIP has been broadly dated to the Middle to Late Ordovician (~469 to 451 Ma), but the existing radiometric ages are insufficient to narrow its timing or to define a pattern of episodic activity38 (Fig. 2). Additionally, recent studies have identified a major tectono-magmatic event in the Proto-Qiangtang Ocean within peri-Gondwanan terranes (adjacent to South China) that extended from the late Cambrian to the late Middle Ordovician (~510–460 Ma) based on modest age constraints from zircon U-Pb dating39. This event (Pinghe) is regarded as a potential continental LIP with a volume of ∼2.5 M km3, although its classification remains insecure given that its composition of dominant S-type and subordinate A-type granites is also consistent with subduction-zone magmatism. Other as yet-unidentified Ordovician LIPs may have existed. Moreover, high rates of plate motion and oceanic lithosphere formation during the Ordovician, as evidenced by abundant ophiolites40 and extensive basaltic lavas (e.g., in the British Isles, northern Iran and West Junggar) (e.g., Parnell et al.41), exceed those of other periods of the Paleozoic Era42 (Supplementary Fig. 14). These factors likely contributed to contemporaneous global peaks in the production rates of volcanogenic massive sulfide deposits, sulfidic shales and ironstones39. Volcanism related to continental LIPs as well as subduction of oceanic lithosphere may have led to globally elevated basalt weathering fluxes, especially in (sub)tropical regions14, resulting in a prolonged negative shift of 87Sr/86Sr (by 0.0008) during the Darriwilian to Sandbian, followed by stabilization in the Katian43 (Fig. 2).

LIP volcanism is commonly the primary driver of anomalous Hg enrichment in marine sediments during major geological events such as mass extinctions20,27. The current clues regarding Ordovician LIPs suggest that such volcanism may have contributed to the global sedimentary Hg enrichments, although temporal links between LIP volcanism and the three stepwise cooling events remain to be further verified. In any case, chemical weathering of basalt could sequester atmospheric CO2 during the Ordovician56, predominantly through secular silicate minerals reacting with CO2 to produce carbonates.

Globally intensified continental LIP volcanism and weathering of volcanic rocks during the Ordovician is likely to have been the trigger for climatic cooling. This relationship is implied by concurrent positive excursions of δ18Oconodont, and plateau [Hg]cf, Hg/TOC and HgEF above baseline/threshold during the LFCE, LDCE and Hirnantian Glacian Glacian Glacian Glaciation, as well as plateau δ18Oconodont and sporadic Hg enrichments between the three major cooling events (Fig. 2 and Supplementary Fig. 8). Furthermore, the negative shift in Δ199Hg to near-zero values and absence of Hg enrichment in sedimentary rocks indicate weakened oceanic lithospheric subduction and continental LIP quiescence during the Early Ordovician, concurrent with a relatively warmer climate. These relationships suggest that the Ordovician climatic cooling (e.g., LFCE and LDCE) was primarily a volcanically driven, stepwise cooling event spread out over a ~40-Myr-long interval that culminated in the Hirnantian Glaciation. The inferred temporal concurrence of volcanism and the Ordovician climatic cooling have previously been ascribed to weathering of volcanic rocks14,46,55.

As a pivotal adjunct, the volcanic cooling effect was further amplified by enhanced marine primary productivity and organic carbon burial (e.g., peak TOC values in the LDCE interval, Supplementary Fig. 9), driven by elevated nutrient fluxes (e.g., phosphorus) from the weathering of terrestrial volcanic rocks16,43. This is supported by the long-term Ordovician climatic cooling and concurrent of positive excursions of δ13Ccarb (Fig. 2). These considerations establish connections between the GOBE, the Ordovician climatic cooling, and contemporaneous volcanism (e.g., Albanesi and Barnes58; Miller et al.59).

As important supplements to volcanism and basalt weathering, several other processes may have collectively accelerated continental volcanic rock weathering and pCO2 drawdown, hastening the development of the Late Ordovician icehouse. The Ordovician Period was an active phase of continental drift and rift formation. The entire ensemble of continents moved ~1500–2000 km northwards at a moderate rate (~5.0 cm per year), along with counterclockwise rotation (e.g., by 30° for the Laurentia and 45° for the Baltica), apart from Gondwana, which moved the slowest (at a speed of ~2.5 cm per year) and remained situated over the South Pole60. Meanwhile, continental rifting persisted throughout the Ordovician until the onset of collision in the Late Ordovician, causing reduced volume of ocean basins, progressively rose in sea levels, and a maximum sea level during the Late Ordovician (excluded a precipitous sea level fall during Hirnantian Glaciation) which is comparable to that in the mid-Cretaceous61. Active plate movement promotes changes in the land surface topography, leading to the formation of rifted margins (as well as island arcs, zones of plate collision, etc.), thus generations of more weatherable volcanic rocks. The abundance of passive continental margin reflects the scale of continental breakup and rifted margins, which are consistently high during the Cambrian greenhouse and Ordovician icehouse periods (and plateau levels of the Early Paleozoic)62, indicating that the changes of rifted margins probably not the cause of the Ordovician cooling.

In addition, our present study shows major global volcanism and the onset of Ordovician climatic cooling during the middle Floian (~475 Ma), which is ~5 Myr earlier than the earliest embryophytes in the Dapingian (~470 Ma) (Supplementary Note S9). Therefore, the delayed evolution of land plants, as well as small biomass of early land plants, and/or incomplete fossil record do not support terrestrialization as a trigger of the Ordovician cooling event. With the expansion of land plants during the Middle to Late Ordovician63, they would have played an increasingly important role in the climatic cooling process15. However, the exact causal relationship between land plant evolution and Ordovician cooling remains unclear due to a lack of definitive fossil evidence, which may be addressed in the future through more terrestrial plant fossil records and refined age constraints.

Overall, secular climatic cooling from the LFCE to the Hirnantian Glaciation was probably driven by continental LIP volcanism and basalt weathering, and may be further amplified by elevation of marine productivity (thus PZE). Concurrent with the three-phase climatic cooling, Hg enrichments in sedimentary rock were mainly driven by episodically intensified volcanism related to continental LIP under background of prolonged continental arc volcanism. Hg fractionation was largely driven by continental weathering (especially the basalt) and the following oceanic PZE in the ~485–473 Ma and ~468–448 Ma interval, and by volcanism in the LFCE and Hirnantian Glaciation.

Methods
Geological background and sample preparation
The Huanghuachang, Chenjiahe and Wangjiawan sections are situated in the Yiling District, Yichang City, South China (Supplementary Fig. 1). The paleogeography and lithostratigraphy of the study sections have been fully documented in prior studies29. Additional bio- and δ13Ccarb chemo-stratigraphy (Supplementary Figs. 3 and 5), and their intercalibration for the Ordovician time framework are discussed detailly in Supplementary Note S1. Here, we provide a concise summary focused on key stratigraphic features essential to this analysis. Paleogeographically, the study area was located on the north-central Yangtze Platform, which accumulated shallow-marine carbonate sediment that graded into argillaceous sands in inner-shelf settings to the northwest and slope facies to the southeast64. The study area exposure the Wuduhe Fm of the Upper Cambrian, and overlying 11 formations/beds (from base to top, the Nanjinguan, Fenxiang, Honghuayuan, Dawan, Guniutan, Miaopo, Baota, Lingxiang and Wufeng formations, Guanyinqiao Bed, and Longmaxi Fm) ranging continuously in age from the earliest to latest Ordovician and Ordovician-Silurian transition64 (Supplementary Figs. 2-3). Limestone, bioclastic limestone and muddy limestone are dominant in the study successions, except for thin (few meters) black shale intervals with limestone lenses in the Miaopo Fm, and black shales in the Wufeng and Longmaxi Fms. Lithological compositions of the successions were fully described in Zhang et al.29. Besides, multiple volcanic tuff depositions yielded, including a ~4 cm thick tuff layer in the upper Dawan Fm and a ~10 cm thick tuff layer in the middle Miaopo Fm at Huanghuachang, and at least five tuff layers of ~1 to 4 cm thick in the upper Wufeng Fm at Wangjiawan (Supplementary Fig. 2). Overall, the study successions record a gradual shift from carbonate platform facies in the Lower Ordovician to neritic facies in the Middle Ordovician and deep basinal facies in the Upper Ordovician, reflecting a long-term sea-level rise before the Hirnantian glacio-eustatic fall at the end of the Ordovician29.

The bulk carbonate rocks and few shales were collected from the study sections. Weathered surfaces and diagenetic veins in bulk rocks were removed, and the remaining sample was cleaned, air-dried and crushed. An aliquot of each sample was powdered using a rock mill to <200 mesh for bulk-rock geochemical analyses. To extract conodont specimens, lightly crushed samples were dissolved in 10% acetic acid for days, after which conodont elements (from a few to dozens in absolute frequency) were recovered from the insoluble residue using a binocular microscope. The color alteration index (CAI) of the study specimens ranges from 1 to 3, indicating probable preservation of primary environmental signals in the conodont apatite (Supplementary Note S2). Each conodont specimen was identified, including Ansella sp., Drepanodus sp., Paroistodus sp., Periodon sp., Oneotodus sp., Scolopodus sp., Drepanoistodus sp., Baltoniodus sp., Tripodus sp., Serratognathus sp., and Triangulodus sp. The samples used in this study represent the same suite that was used in Zhang et al.9,29.

Conodont samples were embedded in resin, followed by grinding and polishing to produce a flat surface suitable for in-situ oxygen isotope analysis. The densest albid crown was targeted for in-situ O isotope analyses in this study to minimize tissue-related effect, reduce biases in reconstructed temperature curve, and facilitate cross-case comparisons (Supplementary Note S2). Other batch of conodonts were affixed to double-sided adhesive tape on a silica glass for in-situ Sr-isotopic analysis. Albid crown is also thought to yield the most faithful record of (near-)primary seawater Sr isotope signals24, and it was targeted for the in-situ Sr isotope analyses in this study.

In-situ conodont oxygen isotopes and paleotemperature
In-situ oxygen isotope measurements were made using a Cameca IMS 1280 secondary ion mass spectrometry (SIMS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Oxygen isotopes were measured using the multi-collection mode on two off-axis Faraday cups. The intensity of 16O was typically 1×109 cps. The nuclear magnetic resonance probe was used to control stability of the magnetic field. A single analysis took ~5 min consisting of pre-sputtering (~120 s), automatic beam centering (~60 s) and integration of oxygen isotopes (20 cycles×4 s, total 80 s). SIMS measurements were performed using identical laser parameters (20 μm spot diameter, 80 s ablation duration) to minimize systematic biases.

The instrumental mass fractionation factor (IMF) was corrected using the Durango apatite standard. Measured 18O/16O ratios were normalized to Vienna Standard Mean Ocean Water compositions (V-SMOW, 18O/16O = 0.0020052), and then corrected for the instrumental mass fractionation factor (IMF) as follows:

δ18OM = (18O/16O)M / 0.0020052 - 1 × 1000 (‰) (1)

IMF = δ18OM(standard) - (δ18O)VSMOW (2)

(δ18O)sample = (δ18O)M - IMF (3)

Replicate analyses of the Durango apatite standard (which was analyzed after every five sample measurements) yielded an average value of +9.40 ± 0.16‰ (2SD; n=78), which is indistinguishable within analytical uncertainty from the reported value of +9.4 ± 0.3‰ (2SD)10. An average of ~5 measurements of δ18O were obtained for 1 to 3 specimen(s) from each stratum, yielding average 2SD variance of 0.8‰. All oxygen-isotope measurements were made during a single analytical session, during which the measured values of the Durango standard did not show any significant drift.

To estimate temperatures of conodont apatite precipitation, we made use of the equations of Pucéat et al.65 updated by Zhang et al.9 (Eq. 4):

T = 118.7 - 4.22 × (δ18Ophos + 0.9 - δ18Owater) (4)

where T is the temperature of precipitation in °C, δ18Ophos is the measured oxygen-isotope composition of conodont apatite, and δ18Owater is the oxygen-isotope composition of the primary or diagenetic fluid with which oxygen in conodont apatite equilibrated. Fluid δ18Owater of the Ordovician was roughly set at ‒4.0‰ SMOW, based on constrains from carbonate clumped isotopes13. According to the Eq. 4, a 1‰ increase in δ18Oconodont indicates a decrease in sea-surface temperature by ~4°C.

In-situ conodont strontium isotopes
In-situ 87Sr/86Sr ratios of conodont albid crown (87Sr/86Srconodont) were measured by laser-ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analysis at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) at the China University of Geosciences (Wuhan)66. A 193-nm ArF-excimer laser was used during the measurement, with a laser beam diameter of 60 μm. Three 87Sr/86Sr measurements were obtained for each specimen, yielding average 2SD of ~0.0006. Two natural apatite standards, Slyudyanka and MAD (Madagascar apatite), were used to monitor the accuracy of LA-MC-ICP-MS measurements, yielding average values 0.70776 ± 0.00017 (2SD) and 0.71176 ± 0.00016 (2SD), respectively, which are consistent with the reference 87Sr/86Sr values for Slyudyanka (0.70769 ± 0.00015 (2SD)) and MAD (0.71180 ± 0.00011 (2SD)).

Mercury content and isotopes
Mercury concentration was measured using a LECO AMA 254 mercury analyzer in the GPMR, and its isotopes were determined using a MC-ICP-MS with high sensitivity X skimmer cone at the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China21. For mercury concentration analyses, all samples were freeze-dried to prevent the decomposition of Hg. About 100 mg for mudstone or shales and 150–200 mg for limestone were analyzed. Data reliability was ensured by analysis of international standard coal samples 502–685 (40.5 ppb) after every 12 unknowns, then followed by a repeat, yielding reproducibility of sample concentrations being within 10%. Long-term monitoring of 502–685 (n=90) yield analytical uncertainty 2.5 ppb (1SD) for the analysis interval.

Pyrolysis method was used to extract Hg for its isotopic analyses. An international standard NIST SRM 997 Tl was used for simultaneous instrumental mass bias correction of Hg and 4 ng/mL SnCl2 solution was used to generate elemental Hg0 before being introduced into the plasma. International standard NIST SRM 3133 was measured after every 3 unknowns to monitor the stability of the instrument. We also analyzed NIST SRM 3177 after every 10 unknowns to examine the instrument accuracy. Hg concentrations of ∼2 ng/mL or 1 ng/mL of NIST SRM 3133 and NIST SRM 3177 solutions were prepared for matching measured sample solutions to reduce the matrix dependent mass bias. Hg isotopic composition is reported in δ202Hg notation in units of permille (‰) relative to the NIST SRM 3133 Hg standard:

δ202Hg(m) = [(202Hg/198Hg)sample / (202Hg/198Hg)standard - 1] × 1000 (5)

Mass independent fractionation (MIF) of Hg isotopes is expressed in Δ notation (ΔxxxHg), which describes the difference between the measured δxxxHg and the theoretically predicted δxxxHg value, using the following equations:

Δ199Hg = δ199Hg - (δ202Hg × 0.2520) (6)

Δ200Hg = δ200Hg - (δ202Hg × 0.5024) (7)

Δ201Hg = δ201Hg - (δ202Hg × 0.7520) (8)

Replicate analyses of the NIST 3177 Hg isotope reference standard (n=4) yielded the following: δ202Hg = −0.42 ± 0.06‰ (2SD); Δ199Hg = −0.04 ± 0.02‰ (2SD); Δ200Hg = +0.01 ± 0.04‰ (2SD); Δ201Hg = +0.01 ± 0.04‰ (2SD).

Carbonate carbon and oxygen isotopes
The analyses of δ13Ccarb and δ18Ocarb using a Thermo Fisher 253 Plus mass spectrometer at the GPMR. The analytical precision was better than 0.04‰ for δ13Ccarb and δ18Ocarb based on duplicate analyses of the national reference standards GBW-04416 (δ13Ccarb = +1.61‰, δ18Ocarb = −11.59‰) and GBW-04417 (δ13Ccarb = −6.06‰, δ18Ocarb = −24.12‰).

Elemental and total organic carbon contents
Major- and trace-element concentrations (e.g., Al, Fe and Mn, and Mo, U and Sr) of bulk-rock samples were measured using wavelength-dispersive XRF and inductively coupled plasma mass spectrometry (ICP-MS), respectively in the GPMR. Average analytical uncertainty is better than 5% (RSD—relative standard deviation) for major elements based on repeated analysis of national standards GBW07132, GBW07133, and GBW07407, and better than 2% (RSD) for trace elements based on international standards AGV-2, BHVO-2, BCR-2, and GSR-1.

In the same laboratory, total organic carbon (TOC) was measured using an Elementar Vario Micro Cube analyzer with a detection limit of 0.03% for carbon content. Powdered samples were digested with an excess of 2-N HCl at 50°C for 12 h, with periodic stirring to ensure completed digestion and inorganic carbon removal. Initial weight (m0) and residue weight (m1) were recorded post-digestion. The residue was dried, ground, and analyzed for carbon content (C%) via elemental analyzer. Total organic carbon (TOC) was calculated as: TOC = C% × (m1/m0). A standard sample and a repeat were analyzed after every 12 unknowns. Data quality was assessed through multiple analyses of standard sample GSS-8 (C% = 1.97%). Long-term monitoring of GSS-8 (n=51) yield analytical uncertainty 0.05% (1SD) for the analysis interval. Since all analyses were performed consecutively during the same period, analytical uncertainty is consistent across samples. Thus, while absolute TOC values may carry systematic offsets (in Hg/TOC normalization), temporal trends remain robust.

Total sulfur content (TS) was measured using a Thermo Fisher FlashSmart at Chengdu University of Technology. About 3-4 mg of bulk-rock powder was pyrolyzed, and the generated SO2 was purified through a chromatographic column before entering the elemental analyzer. Multiple analyses of standard sample GSS-1a (S% = 0.0726 wt.%) yielded a standard deviation of ~0.02%.

Rare earth and trace element concentrations of the carbonate fraction (e.g., La_carb, Ce_carb, Nd_carb, Th_carb) were conducted in the Wuhan Sample Solution Analytical Technology Co., Ltd., using ICP-MS66. About 100 mg sample powder was digested with 5% acetic acid at 60°C (24 h, sealed bath). After centrifugation, ~5 mL supernatant was evaporated to dryness (~105°C). Then, 0.5 mL concentrated HNO3 was added and evaporated, followed by 0.3 mL HNO3 and 5 mL ultrapure H2O (2 h heating). The solution was diluted to 100 g for ICP-MS. Duplicates analyzed after every 10 samples yielded relative errors of <5%.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "See letter to the editor and reviewed document attached.", "reviewer_2": "Review of the manuscript entitled \"Volcanism and basalt weathering drove Ordovician climatic cooling\" submitted to NC by\nZhao et al. (MS: NCOMMS-25-31073)\nA series of remarkable geological events occurred during the Ordovician, including the Great Ordovician Biodiversification\nEvent (GOBE), the Late Ordovician mass extinction (LOME),and the Hirnantian glaciation. The coupling relationships\namong climate, marine environment, and biological evolution have long been a central focus of Ordovician research. A\ngeneral cooling trend dominated the Ordovician climate, albeit with short-term fluctuations, and was accompanied by a\ncontinuous decline in seawater 87Sr/86Sr ratios. Debates on the linkage between Ordovician volcanism and climate change\nhave persisted for years; however, progress over the past two decades has been limited due to the poor preservation of\nvolcanic eruption records. In recent years, from the perspective of sedimentary geochemistry, especially the interpretation of\nHg chemical strata, the eruption records, time and scale of volcanic events have gradually been identified, but these studies\nhave mainly focused on the area near the Ordovician–Silurian (O/S) boundary. In this manuscript, the authors, for the first\ntime, present Hg-Sr-O isotope chemostratigraphy spanning the entire Ordovician from South China. They argue for a\ncorrelation between volcanic activity and global temperature changes, as well as potential driving mechanisms. The data\nappear reasonable and are well interpreted within the context of existing studies. Overall, I consider this work to be original\nand believe it enhances our understanding of Ordovician climate change. However, I have some concerns regarding the\ndata processing and interpretation in the manuscript. The main issues are as follows:\n(1) Biostratigraphy\nMy first major concern, and perhaps the most critical one, pertains to the stratigraphic framework of this study.\nThe authors selected three distinct sections from the Yichang area and ultimately assembled them into a composite Ordovician\nstratigraphic column. A robust biostratigraphic framework is essential for this study, particularly as the authors aim to\ncorrelate their geochemical data with global Ordovician events and to calculate chronostratigraphic ages and sedimentation\nrates. Although Figure 1 provides a rough correlation of the biostratigraphy, it is clearly oversimplified. As the authors\nmentioned, these sections are well-studied. To my knowledge, detailed biostratigraphic data are available for each\nindividual section, and the authors should independently present the conodont/ graptolite biozones for each profile. Such\ninformation is a necessary prerequisite for accurate interpretation of geochemical proxies. Moreover, given that the authors\nthemselves have conducted extensive conodont analysis, establishing a detailed biostratigraphic framework should be\nreadily achievable.\n(2) Do the Hg chemostratigraphic signals reflect global or local events?\nHg concentration profiles in chemostratigraphy can be influenced not only by global large-scale volcanic eruptions but also\nby local volcanic activity, redox conditions, sedimentation rates, intensity of chemical weathering, and lithofacies variations.\nThe studied sections show substantial lithofacies heterogeneity. As observed: (1) Hg concentrations differ by 1–2 orders of\nmagnitude between black shales and carbonates; and (2) high Hg content tends to occur in intervals with low sedimentation\nrates (Fig. S7). Thus, using a uniform background value for Hg concentrations and Hg/TOC ratios to identify volcanic events\nwarrants further scrutiny. Although the authors employ Hg isotopes to distinguish sources, it should be noted that many\nHg/TOC anomalies occur within black shale intervals near the Sandbian–Katian (S/K) and Ordovician–Silurian (O/S)\nboundaries. The influence of anoxic deposition on Hg isotope signatures is uncertain, potentially yielding either negative or\npositive Δ¹⁹⁹Hg values—especially when volcanic eruptions coincide with black shale deposition. In fact, two major volcanic\nash layers (bentonites) have been recorded in the Ordovician stratigraphy, corresponding to the S/K and O/S boundaries\n(Huff et al., 1996; Su et al., 2009), seemingly consistent with the intervals of Hg/TOC anomalies reported by the authors. I\nrecommend that the authors further test the geochemical response of their sections against these known volcanic events.\nRegarding the background levels of Hg, I suggest establishing lithofacies-specific baselines or calculating Hg burial fluxes\nas a more robust alternative.\n(3) Redox proxies\nThe authors use proxies such as MoEF, UEF, and C/P ratios to infer local redox conditions. These indicators may be valid\nfor shales (TOC >1%), but likely become unreliable in carbonate-dominated intervals. In particular, the hydrodynamic\nconditions of the basin can significantly affect the reliability of redox-sensitive trace metal proxies. I recommend\nsupplementing the dataset with Ce anomaly curves for the carbonate samples, which would help better constrain the redox\nconditions. Moreover, as the authors state, there is no correlation between redox proxies and Hg concentrations. However,\nthe manuscript also reports a positive correlation between Hg and TOC contents. This is contradictory, as elevated TOC\ncontent is commonly associated with anoxic conditions. This section of the manuscript requires major revision to clarify and\nresolve these inconsistencies.\n(4) Interpretation of basalt weathering based on Hg-Sr isotopes\nDuring basalt weathering, lighter Hg isotopes are preferentially released (Gao et al., 2023), theoretically causing seawater\nδ²⁰²Hg values to decrease and Δ¹⁹⁹Hg values to increase. While this pattern may be briefly reflected in the Late Darriwilian\nCooling Event (LDCE), it is important to note that the long-term decline in seawater 87Sr/86Sr is a more gradual and\nsustained trend. Furthermore, volcanic ash layers near the S/K and O/S boundaries typically exhibit arc-type geochemical\nsignatures, with source magmas likely evolving from mantle- to crust-derived compositions (Huff, 2008; Yang et al., 2022).\nMore importantly, the long-term evolution of chemical weathering intensity during the Ordovician remains poorly constrained.\nTherefore, it is not advisable for the authors to attribute the sustained decrease in seawater Sr isotopic ratios solely to\nenhanced basalt weathering. While Hg isotopes may offer partial insights, they are insufficient to fully resolve this complex\nissue.\n(5) TOC data and normalization\nThe authors do not provide sufficient details on the pretreatment procedures for TOC analysis in the methods section.\nFurthermore, the majority of the limestone samples have TOC contents below 0.1%. For such low-TOC samples, what are\nthe detection limits and analytical uncertainties of the instrumentation used? These directly affect the reliability of Hg/TOC\nnormalization. Although the authors claim that normalization does not artificially amplify natural anomalies, a systematic\nassessment of the reliability of this approach is still necessary. In many elemental analyzers used for C-S measurements, an\nerror margin of ±0.1% is common, and this should be explicitly addressed. In previous studies, strict criteria have been\napplied to eliminate false positives in Hg anomalies; for example, see Racki et al. (2019, Geology).\nOther comments\nFigure 1\n(1) In South China, the boundary between the Wufeng and Linxiang formations is regionally very consistent and lies at the\nbase of the Dicellograptus complexus biozone (Zhang et al., 2019), with a GTS 2020 age of approximately 449 Ma\n(Goldman et al., 2020).\n(2) In recent years, several studies worldwide have reported Middle–Late Ordovician Hg chemostratigraphic records, all\nunderpinned by robust biostratigraphic frameworks. By converting the sample collection height or depth into deposition age,\nthese data could be integrated into Figure 1. Such an approach would be highly appropriate for distinguishing Hg anomalies\ndriven by local versus global volcanic events.\nFigure 2\nAfter adding the published Hg data to Figure 1, panel 2B could be removed and replaced with a schematic of Hg isotope\nfractionation mechanisms. This substitution would aid readers in quickly grasping the key processes discussed in the text.\nLines 95-99: To improve readability and logical flow, this paragraph should be moved to follow line 81.\nLines 255-228: These two sentences are unclear and lack a coherent logical connection to the preceding text.\nLines 234-237: These redox proxies may not be valid for carbonate lithologies.\nLines 238-242: These conclusions contradict the previously noted correlation between Hg and TOC. Clearly, anoxic black\nshales are enriched in Hg, so the claim that redox conditions have no effect on Hg concentrations is not supported.\nLines 247-253: Sedimentation rate and trace element concentration are not typically linearly related; however, condensed\nintervals with low sedimentation rates do favor element accumulation.\nLines 261-264: Lithofacies exert a strong control on Hg concentrations. Using a single threshold for anomaly identification is\nnot appropriate. I recommend adopting lithofacies-specific thresholds and a rigorous discrimination protocol to identify\nvolcanic Hg events (see Racki et al., 2018).\nLines 265-275: The volcanic ash deposition record (photographs, thicknesses, frequencies) should be described in detail\nwithin the geological background and lithostratigraphic columns. These data are critical for your arguments, please include\nthem in the Supplementary Materials.\nLine 296: δ202Hg\nLines 325-336: Basalt weathering produces significant Hg isotope fractionation; please describe its specific impact on\nsedimentary Hg isotope signatures (see Gao et al., 2023).\nLines 344-355: As the authors note, bottom-water anoxia and photic-zone euxinia expanded during the Mid–Late Ordovician\n(Zhang et al., 2022; Liu et al., 2025). Although anoxic deep-basin sediments typically record negative Δ¹⁹⁹Hg and positive\nδ²⁰²Hg signatures, the studied sections lie on a shallow-water platform where redox proxies point to oxic deposition. From a\nmass-balance standpoint, expanding deep-water anoxia should drive opposite Hg-isotope trends in shallow settings. The\nauthors should therefore discuss this apparent discrepancy.\nLines 362-378: Since the preceding discussion is based entirely on Hg/TOC anomalies, I suggest relocating this paragraph\nto precede the main Discussion section.\nRef.\nGao et al., 2023. Tracing the source and transport of Hg during pedogenesis in strongly weathered tropical soil using Hg\nisotopes. Geochimica et Cosmochimica Acta, 361, 101–112.\nGoldman et al., 2020. The Ordovician Period. In: Gradstein, F.M., Ogg, J.G, Schmitz, M.D., Ogg, G.M. (Eds.), Geologic Time\nScale 2020. Amsterdam, Netherlands, Elsevier, pp. 631–694.\nHuff et al., 1996. Large-magnitude Middle Ordovician volcanic ashfalls in North America and Europe dimensions, emplace-\nment and post-emplacement characteristics. Journal of Volcanology and Geothermal Research, 73, 285–301.\nHuff, 2008. Ordovician K-bentonites: Issues in interpreting and correlating ancient tephras. Quaternary International, 178,\n276–287.\nRacki et al., 2018. Mercury enrichments and the Frasnian-Famennian biotic crisis: A volcanic trigger proved? Geology, 46,\n543–546.\nShen et al., 2019. Mercury in marine Ordovician/Silurian boundary sections of South China is sulfide-hosted and non-\nvolcanic in origin. Earth and Planetary Science Letters, 511, 130–140.\nSu et al., 2009. K-bentonite, black-shale and flysch successions at the Ordovician–Silurian transition, South China: Possible\nsedimentary responses to the accretion of Cathaysia to the Yangtze Block and its implications for the evolution of\nGondwana. Gondwana Research, 15, 111–130.\nZhang et al., 2019. Ordovician integrative stratigraphy and timescale of China. Science China Earth Sciences, 62, 61–88.\nZhang et al., 2022. Progressive expansion of seafloor anoxia in the Middle to Late Ordovician Yangtze Sea: Implications for\nconcurrent decline of invertebrate diversity. Earth and Planetary Science Letters, 598, 117858.", "reviewer_3": "This manuscript presents coupled δ18Oconodont, δ87Sr/δ87Srconodont and Hg system chemistry data (Hg/TOC, Δ199Hg\nand δ202Hg) from the South China GSSP sections. These data are highly significant for reconstructing volcanism and\nclimate change during the Ordovician period. While the scientific hypothesis is reasonable, the storyline and data\ninterpretation require significant improvement to meet the standards of Nature Communications. I therefore recommend\nmajor revisions to enhance the global significance of the environmental data in this study and strengthen the interpretation of\nthe data.\nDetailed comments are provided below.\n1. Attention should be paid to the limitations and multiple solutions of the Hg system. Volcanism identified during the\nOrdovician period lacks matched geological records, such as bentonite or volcanic ash (Huff, 2008). Furthermore, Hg\nisotope data from other continents is lacking for comparison purposes. Therefore, it is unclear whether the identified\nvolcanism represents a global pattern, a point which the authors should emphasize carefully. The gradual decrease in\nΔ199Hg correlates negatively with long-term global cooling (485-455 Ma), but this correlation disappears during the Late\nOrdovician. The authors should provide a clear and concise explanation of the relationship between Hg isotopes and\nclimate.\n2. With regard to weathering, I would advise the authors to incorporate published δ87Sr/δ87Sr data from Laurentia into their\ndiscussion of changes in oceanic δ87Sr/δ87Sr and the mechanisms behind them during the Ordovician period. Very limited\ndata is available from South China during the key transition period of the rapid decrease in δ87Sr/δ87Sr. Except for basalt\nand volcanic rock, the tropical arc would contribute to global weatherability. I therefore recommend that the authors integrate\nthese published data to demonstrate changes in global weatherability and its relationship with climate change (Macdonald\net al., 2019, Science; Conwell et al., 2022, Geology).\n3. I have a few further comments regarding the temperature reconstruction. To improve the reliability of the data, the authors\nshould provide a detailed analytical method for the δ18Oconodont, given that δ18O values within the same conodont can\nvary considerably. Furthermore, the location of South China gradually shifted from ~60°S to the Equator during the\nOrdovician period (Jin JS et al., 2020, Geology). Temperature data from South China may be affected by latitude and\ntemperature gradients, which could cause deviations from the global average temperature. Therefore, I suggest that the\nauthors integrate well-studied published temperature or δ18O data for comparison (Goldberg et al., 2021, PNAS; Scotese et\nal., 2021, ESR; Edwards et al., 2022, GSAB). This would establish a global pattern of climate change during the Ordovician\nperiod and make an important contribution to climate reconstruction during the Ordovician.\n4. While the weathering of basalt and the expansion of early land plants could explain the changes in Hg and Sr isotopes to\nsome extent, the evidence is insufficient to support these mechanisms. I therefore recommend that the authors focus on the\nthree global cooling events that occurred during the Ordovician period, strengthen the evidence of global volcanism, and\nestablish the long-term relationship between volcanism and climate change.\nAdditional minor comments:\n1) The relationship between Hg and TOC is weak (r = 0.35) in the Huanghuachang section, which covers the most of the\nOrdovician period in this study. Therefore, organic matter may not be the primary host for Hg in this section. Furthermore, the\ngradual increase in Hg content upwards may be due to an increase in water depth and mudstone content, as evidenced by\nthe lithological succession. Changes in Hg content may indicate that more non-carbonate material is absorbing Hg, rather\nthan reflecting changes in regional or global Hg flux. I therefore suggest adding carbonate content data to the discussion of\nthe mechanism of Hg changes.\n2) Carbon isotopes are an important tool in chemostratigraphy. However, this study did not report carbon isotope data for the\nGSSP sections, instead using the GTS 2020 curve. To enable more precise comparisons with the GTS 2020 curve, as well as\nto refine the stratigraphy of the sections in this study, I would recommend that the authors include carbonate and oxygen\nisotope data for these sections.\n3) Hg data from a single section can be interpreted in various ways and has limited global significance. If possible, I would\nadvise the authors to conduct Hg isotope analyses from other continents (e.g., Laurentia) to demonstrate that Hg isotope\nchanges in South China are a global phenomenon. This would greatly enhance the reliability and quality of the manuscript.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.", "reviewer_1_attached": "C1-1: I have reviewed the paper by He Zhao et al. entitled “Volcanism and basalt weathering drove Ordovician\nclimatic cooling”. This study is a classic chemostratigraphic work on isotopy (O, Hg, Sr) under\nbiostratigraphic control of conodonts that integrates paleontological data on eutracheophyte precursors, and\nTotal Organic Carbon analysis in the Ordovician of South China. Its purpose is understanding the cause of the\ncooling of the Ordovician Period and the subsequent diversification of life. The manuscript is well done and\nsuitable for this journal.\nThe article is well written, almost free of typographical errors (indicated in post scriptum). The\nmanuscript consists of an abstract, an introductory part, results on O, Sr and Hg isotopy, discussion on volcanic\nactivity and long-term global cooling, including early evolution of land plants and TOC implications. In\naddition, a section on methods is developed. References, 3 composite figures and supplementary material are\nnecessary complements to the text, in good shape and quality for publication.\nC1-2: The study is correct to my knowledge, recording a series of isotopic curves in 3 sections of S China\nunder previously established conodont biostratigraphic control, whose references should be added. The\nauthors argue that the Ordovician global cooling could have been caused by a complex interplay of CO2\nuptakes by early plant propagation and weathering of basalts after intense volcanism. In the specialized part\nof the article that concerns my knowledge, I urge the author to correct the names of several genera of conodonts,\nas indicated in the post scriptum and in the attached reviewed paper. Also, it is suggested to add a number of\nsignificant references throughout the manuscript.\nC1-3: I consider that the manuscript would be ready for publication after a moderate revision, on\ntypographical errors and figure formats, and especially revising the nomenclature of conodont genera. In\naddition, take care of the references to be added as suggested, following the indications of the post scriptum\nand the attached reviewed document. This contribution to Nature Communications is an original and valuable\nwork that could be considered for publication after present review and decision of editorial evaluation.\nSincerely,\nGuillermo L. Albanesi\nPS: Suggestions and corrections:\nC1-4: - Line 81: See also: Barnes, C.R., 2004, Ordovician oceans and climate, in Webby, B.D., Paris, F.,\nDroser, M.L., and Percival, I.G., eds., The Great Ordovician Biodiversification Event: New York, Columbia\nUniversity Press, p. 72–76.\nC1-5:- Lines 102-103: add the references of the papers related to the conodont biostratigraphy of the 3\nstratigraphic sections analyzed.\nC1-6: - Line 183: Floian.\nC1-7: - Line 191: Fm,\nC1-8: - Line 275: Also well reported in the Argentine Precordillera: Huff, W.D., Bergström, S.M., Kolata,\nD.R., Cingolani, C.A. and Astini, R.A. 1998. Orrlovician K-bentonites in the Argentine Precorrlillera: relations\nto Gondwana margin evolution. In: Pankhurst, R.I. and Rapela, C.W. (eds.) The ProtoAndean Margin of\nGondwana. Geological Society, London, Special Publications, 142, 107-]26.\nC1-9: - Line 278: See also: Barnes, C.R., 2004, Was there an Ordovician superplume event?, in Webby, B.D.,\nParis, F., Droser, M.L., and Percival, I.G., eds., The Great Ordovician Biodiversification Event: New York,\nColumbia University Press, p. 77-80.\nC1-10: - Line 311: Add a reference for the Ordovician System divisions and chrono-biostratigraphy, e.g.\nGoldman, D., Leslie, S. A., Liang, Y. and Bergström, S.M., 2022. Ordovician biostratigraphy: index fossils,\nbiozones and correlation. In: Harper, D. A. T., Lefebvre, B., Percival and I. G., Servais, T. (eds.), A Global\nSynthesis of the Ordovician System: Part 1. Geological Society, London, Special Publications, 532, 31–62.\nAlso, clarify that, aside on the left margin of the panel A, the formations are named.\nC1-11: - Line 429: See also: Ramos, V.A., Escayola, M., Mutti, D. and Vujovich, G.I., 2000. Proterozoicearly\nPaleozoic ophiolites of the Andean basement of southern South America. In: Dilek, Y., Moores, E.M., Elthon,\nD. and Nicolas, A. eds. Ophiolites and Oceanic Crust: New insights from field studies and the Ocean Drilling\nProgram: Boulder, Colorado, Geological Society of America, Special Paper 349: 331-349.\nC1-12: - Line 448: See also: Albanesi, G.L. and Barnes, C.R., 2000, Subspeciation within a punctuated\nequilibrium evolutionary event: Phylogenetic history of the Lower-Middle Ordovician Paroistodus originalis–\nP. horridus complex (Conodonta). Journal of Paleontology, v. 74, p. 492–502", "reviewer_3360": "SWAPEE>2.0.CO;2.\nC1-13: - Line 491: delete “is”.\nC1-14: - Line 530: The amount of conodont elements (absolute frequency) per sample should be indicated.\nAlso indicate how many pelagic conodont elements per sample were used for geochemical analyses.\nC1-15: - Lines 531-533: Caution: several genera of conodonts are misspelled and/or their identification are\ndoubtfuls: Belodella (probably Ansella), Pasoistodus (probably Paroistodus), Pesiodus (?Periodon),\nSessagtognathdus (probably Serratognathus), Trianglodus (probably Triangulodus).\nIn supplementary material you should correct: Conurodus (= Cornuodus), Protopandesodus\n(=Protopanderodus), Nususguathus (= Nasusgnathus), Belodena (= Belodella), Juanogn (=Juanognathus).\nC1-16: Other comments/suggestions in attached PDF file."}, "author_response": "R1-1: We sincerely thank the Reviewer for the time and positive feedback. We\nare pleased that our revisions have strengthened the manuscript and appreciate the\nendorsement of our conclusions on volcanism, weathering, and anoxia in Ordovician\ncooling. The Reviewer's insightful comments were invaluable for making these\nimprovements. We are grateful for the recommendation to publish in Nature\nCommunications.\nR2-1: We sincerely thank the Reviewer for the positive feedback and for\nrecommending our manuscript for acceptance. We are delighted that the Reviewer finds\nour revised manuscript significantly improved and recognizes the novelty of identifying\nEarly-Middle Ordovician volcanic events and their climate linkages. All further\nrevisions suggested have been carefully addressed in the point-by-point responses\nbelow and incorporated into the manuscript.\nR2-2: We thank the reviewer for this suggestion. Accordingly, we have marked the\npositions of both the GOBE and LOME events next to the biodiversity curve in Figure\n2.\nThe suggested age of “~499 Ma” appears to have been a typographical error, as\nthis value falls outside the Ordovician Period (~486.85–443.07 Ma) according to the\nGeologic Time Scale 2020 (Goldman et al., 2020). The reviewer’s query prompted us\nto re-examine the age constraint for the base of the Wufeng Formation. Based on\nprevious studies (Zhang et al., 2019), the base of the Wufeng Formation in South China\ncorrelates with the Dicellograptus complexus graptolite Zone, which has a basal age of\n~449 Ma (Goldman et al., 2020, Fig. 20.2) and is globally correlatable. We recognize\nthat the earlier label was indeed a typographical error (449 vs. 499). Accordingly, in the\nrevised Figure 2, we have placed the base of the Wufeng Formation at ~449 Ma, which\nis consistent with the age model in our data table and maintains alignment with the\ndiscussions and conclusions of the manuscript.\nR2-3: We have removed this sentence from the main text.\nR3-1: We sincerely thank the Reviewer for their positive evaluation of our revised\nmanuscript and for acknowledging the robustness of the new datasets and\ninterpretations. We are pleased that the additional evidence, including carbonate-free\nHg content (Hg ) and global compilation of Hg and bentonite records, along with Sr–\ncf\nC–Nd isotopes and paleolatitude considerations, have strengthened the case for\nvolcanic-derived Hg and its use as a global volcanism proxy during the Ordovician. We\nare grateful for the Reviewer's support and have carefully addressed all further\ncomments as detailed in the point-by-point responses below.\nR3-2: We thank the reviewer for this comment. In response to the observation that\nFigure 4 (showing geological responses to the Ordovician LIPs) received limited\ndiscussion in the main text, we have moved it to the Supplementary Information as the\nnew Supplementary Figure 13. We believe this placement allows the figure to still serve\nits purpose of helping readers quickly grasp the evidence for LIP activity, while keeping\nthe main text focused. We hope this adjustment is acceptable.\nR3-3: We thank the reviewer for this suggestion. First, we have provided an\nenhanced perspective in Supplementary Figure 2E to better display the characteristics\nof the volcanic ash layer. Furthermore, following the reviewer's recommendation, we\nhave added photomicrographs (both plane- and cross-polarized light image) of thin\nsections from the volcanic ash layers in both the Dawan and Wufeng Formations as new\nSupplementary Figures 2I-2L. These new images show typical minerals, such as quartz,\nfeldspar and sericite, and clay mineral, rock fragment as well as vesicle, thus providing\nfurther petrographic support for the identification of these layers as volcanic ash\ndeposits.\nWe also thank the reviewer for drawing our attention to this reference, which\nprovides supporting evidence for the presence of Middle Ordovician volcanic ash\ndeposits in the study area. The suggested publication has now been cited in\nSupplementary Note S5.\nR4-1: We thank the reviewer for their time and contribution to the peer-review\nprocess of our manuscript. We appreciate their participation in Nature Communications'\ninitiative to support early-career researchers in peer review, and we value their input on\nour work."}]

Decision letter