Was the Dan-C2 event a global hyperthermal event?
Our study provides a new terrestrial δ13C record of the LMWE and Dan-C2 event, expanding their global distribution, especially the Dan-C2 event. However, there are still inconsistencies in marine records of the Dan-C2 event (supporting information, Text S1, Fig. S1), which are summarized in the following aspects: 1) the negative excursion of δ13C is mainly recorded by bulk samples and planktonic foraminifera, while benthic foraminifera are rarely recorded; 2) the distribution is restricted to parts of the Atlantic Ocean and the Tethys Ocean; and 3) evidence of bottom-seawater warming is generally lacking.
To explore the possible reasons for the inconsistencies in the Dan-C2 event, we compared it to the Palaeocene-Eocene Thermal Maximum (PETM), which is the most studied hyperthermal event. The CIE of the PETM can reach 7‰ in terrestrial records and only 3‰ in marine records24,25,32. Two mechanisms have been proposed: 1) marine CIE records are commonly truncated by carbonate dissolution, so the most extreme values are not represented, leading to an incomplete CIE32,33; and 2) the terrestrial CIE is amplified by environmental changes and fractionation effects caused by photosynthesis in plants34. A similar phenomenon is also shown with the Dan-C2 event. The CIEs of δ13Corganic and δ13Ccarb in Boltysh crater (Fig. S1l) and the Nanxiong Basin (Fig. 1c) can reach ~ 3.0‰, whereas the marine CIEs are less than 1.5‰ (Fig. S1). Differences in the carbon isotope records during the PETM are not only manifested in marine and terrestrial records but also among different types of carbonate records (bulk samples, planktonic foraminifera, or benthic foraminifera) and even within the same type of carbonate but in different marine regions33 (supporting information Fig. S3 and Fig. 4). Ocean acidification during the PETM led to carbonate dissolution and shoaling of the carbonate compensation depth (CCD), causing the sediments to be clay-rich. The thickness of the clay layer increases with increasing palaeodepth32,33, indicating enhanced carbonate dissolution. The CIEs of δ13Cbulk and δ13Cbenthic have significant negative correlations with palaeodepth (Figs. 4a and 4b), suggesting that the dissolution of carbonate suppressed the amplitude of the CIE. In addition to carbonate dissolution, the sedimentation rate also affects the CIE. The CIEs of δ13Cbulk and δ13Cbenthic have positive correlations with sediment thickness (Figs. 4c and 4d). The greater the sediment thickness is, the greater the sedimentation rate, the more complete the carbonate isotope record, and the greater the CIE. Similarly, for the Dan-C2 event, the CIEs of δ13Cbulk show a significant negative correlation with palaeodepth (Fig. 4e) but a positive correlation with sediment thickness (Fig. 4f) and carbonate concentration (Fig. 4g), indicating that the CIEs of the Dan-C2 event are also influenced by the water depth, sedimentation rate, and carbonate dissolution, leading to some differences in the CIE records from different regions.
However, compared with the PETM, the Dan-C2 event was a short-lived and muted warming event that occurred under a specific environmental background, for instance, biotic turnover and drastic ecosystem changes caused by mass extinction. Under normal oceanic conditions, phytoplankton preferentially convert 12CO2 into organic matter through photosynthesis (primary productivity), and then, 12C-rich organic carbon is transported to the bottom water through a biological pump (export productivity), thus promoting carbon exchange between the surface and the deep ocean35. After the K-Pg boundary, a collapse occurred in the surface-bottom δ13C gradient36, which was initially thought to reflect either a collapse in primary productivity (“Strangelove Ocean”36) or export productivity (“Living Ocean”37) after the mass extinction. However, benthic faunal records show a lack of significant extinction of phytoplankton-dependent benthic foraminifera and an increased food flux to the seafloor in the southeastern Atlantic Ocean and the Pacific Ocean31. In addition, biogenic barium records indicate geographic heterogeneity in export productivity, an increase in the central Pacific Ocean, no changes in upwelling or shelf Atlantic sites, and decreases in the northeast and southwest Atlantic Ocean, Southern Ocean, and Indian Ocean38. Thus, the “Resilient Ocean” and “Heterogeneous Ocean” have been proposed to explain these phenomena39.
The spatial heterogeneity of primary/export productivity could cause heterogeneous carbon cycling processes between the surface and deep ocean, leading to inconsistent marine δ13C records from the Dan-C2 event. However, this heterogeneity was proposed to be due to the limited number of sites, which is insufficient to reveal a robust pattern, and the mechanism responsible for this heterogeneity is still unclear38. The geographic location40, circulation, nutrient runoff from land, and stratification38 are potential drivers of the spatially heterogeneous ocean and inconsistent records of the Dan-C2 event. For example, ODP site 1049C is located at a productive coastal upwelling site41 in the western North Atlantic, where a great deal of terrestrial materials are transported by surface ocean currents42; in addition, the export productivity is stable43, leading to strong negative excursions from all δ13Cbulk, δ13Cplanktic and δ13Cbenthic (Fig. S1a), especially negative excursions from δ13Cbenthic. Although increased export productivity was recorded at ODP sites 1209 and 1210 in the Central Pacific Ocean24,38, there are no records of Dan-C2 event, probably due to pelagic and oligotrophic environments, as well as the lack of nutrients from terrestrial sources. Evidently, the mass extinction and the dramatic changes it brought are the indispensable causes of the spatial heterogeneity of the Dan-C2 records, but proposing a reasonable model to explain this mechanism requires additional records in the future.
The roles of Deccan volcanism and orbital forcing in the carbon cycle
The large amount of CO2 released by large igneous provinces (LIPs) can cause perturbations to the global carbon cycle. In addition to LIPs, other carbon pools, such as peat and methane hydrates, which are modulated by eccentricity forcing, could also contribute to the carbon cycle during hyperthermals. For instance, during eccentricity minima, seasonally uniform annual precipitation is more suitable for carbon burial, whereas during eccentricity maxima, short wet seasons and prolonged dry seasons caused by “monsoon-like” precipitation could promote carbon release2. In addition, methane hydrates buried in the marine shelf become unstable and decompose in response to orbital-driven warming, leading to large quantities of light carbon being emitted to the atmosphere-ocean system, further perturbing the global carbon cycle44. Previous work has shown that the total mercury (Hg) concentration in the Nanxiong Basin has been anomalous from 66.4 to 65.6 Ma (Fig. 2b); combined with Hg isotope data, Ma et al.4 attributed these anomalies to volcanism in the central Deccan Traps. Both the LMWE and the Dan-C2 event temporally overlapped with the central Deccan volcanism (Fig. 2). Moreover, the LMWE and Dan-C2 event occurred during the last 405-kyr long eccentricity of the Maastrichtian and the first 405-kyr long eccentricity of the Danian, respectively (Figs. 2c, 2d, and 2e), and their CIE maxima were all within the maxima of the 405-kyr eccentricity cycle12. Moreover, 100-kyr short eccentricity cycles were significant in both terrestrial and marine records (except for the LMWE of ODP 1262; Fig. 3). These findings imply that both Deccan volcanism and orbital forcing contributed to the LMWE and Dan-C2 event.
However, there are noticeable differences between the LMWE and Dan-C2 event: 1) the magnitude of the CIE during the LMWE (~ 1.5‰ in the Nanxiong Basin,<0.5‰ in ODP 1262) is more muted than in the Dan-C2 event (2–3‰ in the Nanxiong Basin, 0.6‰ in ODP 1262); 2) the LMWE was characterized by both surface and deep sea warming1,18, while the Dan-C2 event was characterized by surface ocean warming, accompanied by little appreciable deep sea warming; 3) each of the double CIEs of the Dan-C2 event corresponds to a maxima of the 100-kyr eccentricity cycle, while the total duration of the onset, peak and recovery of the LMWE (200 ~ 300 kyr) was significantly longer than each CIE of the Dan-C2 event (Fig. 2); 4) the short eccentricity cycles are significant during the Dan-C2 event, whereas they are insignificant and even disappear during the LMWE in the marine record (Fig. 3); and 5) although the CIE maxima of both the Dan-C2 and the LMWE were all within the maxima of the 405-kyr eccentricity cycle, the onset of the LMWE occurred at the minima of the 405-kyr eccentricity cycle12,18 (Figs. 2c and 2e). These apparent differences suggest that Deccan volcanism and orbital forcing played different roles in driving the LMWE and Dan-C2 event, as well as in the global carbon cycle.
High-precision chronologies indicate that both the eruption rate and volume were low during the early stage of the central Deccan Traps26,27 (Fig. 2a). However, CO2 release has the potential to decouple from rates of surface volcanism because large amounts of CO2 can be released through passive degassing6,27, especially from intrusive magmas20. The reconstructed atmospheric CO2 concentration based on the pedogenic carbonate nodules showed higher pCO2 values during the LMWE than during the Dan-C2 event5,45 (Fig. S4), which is consistent with direct measurements of melt-inclusion CO2 concentrations, suggesting that early Deccan magmas were enriched with more CO220. Although the onset of the LMWE occurred at the minima of the 405-kyr eccentricity18 (Figs. 2c and 2e), several thousand Gt of carbon that degassed from the early Deccan magmas was sufficient to trigger the LMWE6,20. The δ13C composition of volcanic CO2 (~-5‰46) is much more positive than that of other sources, such as peat (δ13C≈-25‰47) and methane hydrates (δ13C ≈-60‰48); therefore, the massive amount of volcanic CO2 emitted through passive degassing of early Deccan magmas could have led to muted and prolonged δ13C negative excursions during the LMWE (Fig. 2), as well as disruption of the short-eccentricity cycle in the oceanic record (Fig. 3c, right). Although passive degassing triggered the LMWE, whether the amount of released CO2 was sufficient to cause ~ 4°C of warming is still debated19,20. Notably, 405-kyr long eccentricity cycles were significant during the LMWE according to both terrestrial and marine records (Fig. 3a), and even 100-kyr short eccentricity cycles were significant according to the terrestrial record (Fig. 3b, right), suggesting that Deccan CO2 outgassing likely enhanced the climate sensitivity to orbital forcing, leading to a global warming of ~ 4°C12,20. After the K-Pg boundary, with a decrease in CO2 released from the Deccan magma20,27, the carbon cycle was mainly controlled by orbitally driven carbon pools with more negative δ13C values, leading to larger CIEs (Fig. 2) and significant short-eccentricity cycles in both terrestrial and marine records (Figs. 3b left and 3c left). Due to the decrease in CO2 emissions, pCO2 and its warming effects were muted during the Dan-C2 event. The above speculated release scenario of Deccan volcanic CO2 was further substantiated by long-term ocean-atmosphere-sediment carbon cycle reservoir (LOSCAR)49 model simulations, which showed that prior to the K-Pg boundary, more CO2 was released through passive degassing20 (intrusive:extrusive = 5:1) or that half of the CO2 was released (50:50 outgassing scenario) but with a higher emission rate6. The greater volume and higher rate of CO2 release before compared to after the K-Pg boundary confirm that Deccan volcanism likely contributed to both the LMWE and Dan-C2 event but contributed more to the LMWE.
In conclusion, we provide a new terrestrial δ13C record of the LMWE and Dan-C2 event in low-latitude East Asia, which can be compared with marine records, further expanding the global distribution of these events. The inconsistency of marine records for the Dan-C2 event is related to the drastic ecosystem changes caused by mass extinction, especially in the heterogeneous ocean, while the specific mechanism remains to be revealed by additional studies. We hypothesize that Deccan volcanism and orbital forcing played different roles in the carbon cycles during the LMWE and Dan-C2 event. Deccan volcanic CO2 triggered the LMWE through passive degassing, disturbed the carbon cycle and amplified the sensitivity of the climate to orbital forcing, whereas the Dan-C2 event was mainly controlled by orbital forcing, with weakening of the volcanic perturbation.