Introduction

The Middle Devonian Kačák Episode1 is known as a minor global extinction event associated with a eustatic sea level rise2. The event interval, first recognized as significant lithological change from the Choteč Limestone to the Kačák Shale within the Prague Synform (Czech Republic), is characterized by global occurrence of bottom-water dys-/anoxia and expanded deposition of black shales1,3,4,5,6,7,8,9. The Kačák Episode was divided into two event intervals, named late Eifelian 1 Event (LEE1) and late Eifelian 2 Event (LEE2), reflecting its polyphased nature1,10. Although the Kačák Episode was initiated by black shale deposition during the late Eifelian about 388 million years ago (uppermost part of the kockelianus conodont Biozone), it did not terminate at the Eifelian–Givetian boundary but reached into the earliest Givetian (hemiansatus conodont Biozone)10,11. At the Eifelian/Givetian GSSP (Global Stratotype Section and Point) at Mech Irdane (Morocco), the Kačák Episode is temporally constrained to a relatively short interval of about 370 kyr12,13.

Biodiversity analysis has shown that mainly ammonoids5,14,15,16, conodonts10,11,17,18 and trilobites19,20 suffered from a certain taxonomic loss. However, it appears that extinction was not the main response of faunal communities to environmental changes but rather habitat tracking and speciation events. Several groups even produced new taxa, e.g., planktonic dacryoconarid tentaculitids like Nowakia otomari occurred with the onset of LEE121. An increased intraspecific variability is observed among some polygnathid and icriodontid conodont taxa10,11,18,22. Additionally, the bathymetric changes coherent with the late Eifelian eustatic sea level rise effected the paleogeographic distribution of benthic marine organisms23,24. Some of Rhenish-type brachiopod and trilobite taxa migrated from southern Euramerica (Ardenne area) to the northern shelf of Gondwana (Prague Synform) during the latest Eifelian24. Similar migration patterns are seen among rugose corals along the western Euramerican shallow shelf, where cosmopolitan taxa of the Old World Realm invaded and replaced the endemic coral community of the East Americas Realm23. Probably, not only because previously existing topographic barriers were removed and shelf areals broadened, but also because the inferred global warming allowed/forced extension of the latitudinal distribution of temperature-dependent taxa.

New insights about the late Eifelian global warming period known as the Kačák Episode were generated by analysis of latitudinally dependent paleotemperature patterns reconstructed from conodont apatite. So far, paleotemperatures were published from southern hemisphere low latitudes between 10° and 36° S for localities in France (Pic de Bissous)25 and Germany (Blankenheim, Hillesheimer Mulde, Schönecken-Dingdorf and Blauer Bruch)25,26. Additional data from Nevada, USA (northern Antelope mountains) documented a negative shift in the δ18Oapatite during the onset of the LEE1 (upper part of the kockelianus Biozone)27, and more recently, paleotemperature reconstructions from Belarus were provided for the latest Eifelian interval (ensensis Biozone)28,29.

Here we discuss late Eifelian to earliest Givetian paleotemperature estimates based on new δ18Oapatite data from a shallow to deeper water transect within the Carnic Alps (Austria, Italy) and an offshore section from the Prague Synform (Czech Republic). Together with palaeotemperature estimates from other localities within the Rheic Ocean and the North American shelf, mean sea surface temperatures for each region and biozone are calculated and compared (Fig. 1). Because paleotemperature estimates from shallow marine localities appear to be biased by varying salinities, the temperature record across the Kačák Episode is limited to more open marine deposits of the Carnic Alps.

Figure 1
figure 1

Paleolatitudinal position of published and newly studied localities during the Middle Devonian (late Eifelian to earliest Givetian). Localities reflecting more open marine conditions are marked in bold. Eifelian paleogeographic reconstruction modified after Scotese33.

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Keeping in mind that Middle Devonian coral-stromatoporoid reefs showed a remarkable wide latitudinal dispersal of up to 50° on either hemisphere with an acme in diversity just after the Kačák Episode during the early and middle Givetian (~ 387–382 million years ago)30,31,32, only a detailed study of the event interval can show how the marine biosphere dealt with environmental changes at that time. Estimation of paleotemperatures across the event interval contributes to uncover critical temperatures and to identify ecological limits of climate sensitive low latitude communities.

Geological settings

The Carnic Alps, part of the Proto-Alps34, were situated on the northern peri-Gondwana shelf located around 34° S during the Devonian. Today the area forms a more than 2000 m high mountain chain along the Austro-Italian border with Middle Devonian rocks well-exposed between the Biegengebirge in the west and Mount Osternig in the east (Fig. 2A,B)35. Coeval sections from different bathymetric settings were investigated (Fig. 2B). The section studied in the Val di Collina quarry (N46° 35′ 49.07″; E12° 55′ 29.22″) consists of back reef and reef environments belonging to the Spinotti and Kellergrat formations, respectively. The Zuc di Malaseit Basso (ZMB) section (N46° 33′ 19.06″; E13° 11′ 10.6″) was deposited within a distal slope setting of the Hoher Trieb Formation, while the Wolayer “Glacier” section (N46° 36′ 46.56″; E12° 52′ 33.66″) is represented by condensed pelagic deposits of the Valentin Formation. These sections were correlated using high-resolution conodont biostratigraphy11.

Figure 2
figure 2

(A) Location of the Carnic Alps at the Austro-Italian border and the Prague Synform in Czech Republic. (B) Simplified map of the central Carnic Alps with areas of occurrence of the formations analysed in this paper. Investigated sections are the Wolayer “Glacier” section (Valentin Formation), the Val di Collina quarry (Spinotti and Kellergrat formations) and the Zuc di Malaseit Basso section (Hoher Trieb Formation). (C) Detailed map of the Koněprusy area with location of the Jirásek quarry section I and II. The photo left of the map shows the Upper Dark Interval (UDI) of the uppermost part of the Acanthopyge Limestone at Jirásek quarry section I. Maps in (A–C) designed in CorelDRAW® X8 based on google earth images (maps data attribution incorporated within illustrations).

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The Middle Devonian Srbsko Formation in the Prague Synform (part of the Bohemian Massif) is generally developed in siliciclastic facies36 and was located around 28° S. The studied Jirásek quarry section I (N49° 54′ 50.2″; E14° 04′ 34.2″) represents a unique section where the stratigraphic equivalent of the Kačák Member of the Srbsko Formation is developed in carbonate facies. The so-called Upper Dark Interval (UDI), exposes a dark gray thin-bedded to nodular limestone interval representing the uppermost portion of the Acanthopyge Limestone (Choteč Formation, Fig. 2A,C)37. Upper Eifelian deposits are unconformably overlain by a limestone breccia, wherefore the middle/upper? part of the ensensis Biozone within the UDI was probably reworked18.

Results

In the Carnic Alps, the δ18Oapatite record of the ZMB section includes 11 measurements covering an interval between the lower part of the kockelianus Biozone (late Eifelian) and the entry of the timorensis Biozone (early Givetian). δ18Oapatite values are between 18.2 to 19.1‰ (VSMOW). Oxygen isotopes show a weak positive shift of 0.6‰ in the kockelianus Biozone, followed by a negative shift of 0.9‰ in the upper part of this biozone and gradually increasing to values of 18.8‰ at the hemiansatus/timorensis Biozone boundary (Fig. 3; Supplementary Fig. 1). Only few δ18Oapatite data were generated from shallow marine deposits due to the generally lower abundance of conodont elements. Samples from the Val di Collina quarry record only little variation during a short interval within the ensensis Biozone with values of 19.5 and 19.6‰ (Fig. 3; Supplementary Fig. 1). The condensed pelagic sequence of the Wolayer “Glacier” section yielded abundant conodonts, which were extracted from cm-thick limestone levels separated along slightly uneven stylolite layers. δ18Oapatite values measured across the Eifelian–Givetian boundary vary between 17.7 and 18.0‰ (Fig. 3; Supplementary Fig. 1).

Figure 3
figure 3

Comparison of δ18Oapatite isotope records during the late Eifelian and earliest Givetian (North–South sorted). Data from Belarus, Germany, North America and France compiled from Elrick et al.27, Joachimski et al.25, Königshof et al.26 and Narkiewicz et al.28.

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In the Prague Synform, the δ18Oapatite record at the Jirásek quarry section I includes 28 measurements covering the upper part of the kockelianus Biozone and most of the ensensis Biozone. Values range between 17.6 to 18.7‰ (VSMOW). The highest value of 18.7‰ is observed during the late kockelianus Biozone while the lowest value is observed before the entry of the ensensis Biozone (Fig. 3; Supplementary Fig. 1). Variations of about 1‰ are recognized within the sampled interval.

Discussion

For a better understanding of the paleotemperature record across the E/G boundary, results from this study are compared with published δ18Oapatite values25,26,27,28 (Fig. 3). In total, 67 measurements are documented from the late Eifelian (kockelianus and ensensis biozones) and 21 values from the earliest Givetian (hemiansatus Biozone). The highest resolution is documented for the late Eifelian with 28 δ18Oapatite values reported from the Prague Synform (28° S). All data included in this study derive from localities allocated in southern hemisphere latitudes between ~ 10–36° S. In addition to the common approach used for paleotemperature calculation from δ18Oapatite, we applied a latitude-dependent correction for δ18Oseawater following the isotope-enabled ocean–atmosphere general circulation model of the National Aeronautics and Space Administration Goddard Institute for Space Studies (NASA GISS) ModelE-R38. The model provides better constraints on variations in δ18Oseawater as consequence of latitude-dependent changes in the evaporation vs. precipitation ratio. A higher evaporation vs. precipitation ratio in the subtropics results in higher salinities and thus a higher δ18Oseawater whereas equatorial and higher latitudes are characterized by a lower δ18Oseawater. We applied the correction published for the early Paleocene, a relatively warm time interval, likely comparable to the Devonian.

Paleotemperature estimates

For paleotemperature calculation, the phosphate-water isotope fractionation equation of Lécuyer et al.39 is used. Because no or at least no permanent glacial ice sheet was established around 388 million years ago40,41 and flooded continental area reached a maximum within the entire Phanerozoic record during the Middle Devonian42, a δ18O value of − 1‰ VSMOW for Middle Devonian seawater is assumed. As result, δ18Oapatite data from eastern and southern Euramerica indicate lower temperatures compared with localities from more southern latitudes on the western shelf of Euramerica and northern Gondwana. Depending on data availability, calculated mean δ18Oapatite values of each area from north to south show highest temperatures of ~ 33 °C on the western shelf of Euramerica around 25° S during the pre-/onset of the LEE1 (kockelianus Biozone), ~ 32 °C on the northern Gondwana shelf at 28° S during the LEE1-LEE2 interval (ensensis Biozone) and ~ 31 °C within offshore deposits at 34° S during the late/post-LEE2 (hemiansatus Biozone) (Table 1). Mean paleotemperatures during the pre-/onset of the LEE1 are ~ 7 °C higher in western Euramerica at 25° S than in southern Euramerica at 24° S. The mean temperature difference between localities on the southern shelf of Euramerica (24° S) and northern Gondwana (28° S) during the LEE1–LEE2 interval is 5 °C. In the earliest Givetian (late/post-LEE2), the temperature offset between 24° S and 34° S decreases to ~ 4 °C. However, a gradual latitudinal decrease in mean sea surface temperature is observed between 25 and 34° S during the late Eifelian and earliest Givetian Kačák Episode (Table 1).

Table 1 Calculated regional paleotemperature ranges and mean values for each paleolatitude/country and biozone.
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Latitude correction for δ18Oseawater

Since the δ18O of seawater varies in part due to the evaporation to precipitation ratio, we calculated latitude dependent δ18Oseawater values using the GISS ModelE-R provided by Roberts et al.38 (Table 2). A comparison of both approaches (Tables 1 vs. 2) shows ~ 2 °C higher temperatures when applying a latitude correction for δ18Oseawater (Fig. 4A–C; Supplementary Fig. 2). Hottest mean temperatures of ~ 36 °C at 25° S during the pre-/onset of the LEE1 (kockelianus Biozone), ~ 34 °C at 28° S during the LEE1-LEE2 interval (ensensis Biozone) and ~ 33 °C at 34° S during the late/post-LEE2 (hemiansatus Biozone) were calculated using the data available for each biozone. Since SSTs from epeiric seas and continental margins are generally higher than average temperatures in the same latitude band43, we conclude that mean SSTs did not reach beyond 36 °C within southern hemisphere low latitudes during the polyphased Kačák Episode (Table 2).

Table 2 Paleolatitude corrected regional paleotemperature including the latitude correction values for δ18Oseawater and mean values calculated for each paleolatitude/country and biozone.
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Figure 4
figure 4

Comparison of paleotemperatures calculated from δ18Oapatite assuming δ18Oseawater = − 1‰ VSMOW vs. applying a latitude correction for δ18Oseawater during the (A,D) pre-/onset LEE1 (kockelianus Biozone), (B,E) LEE1–LEE2 (ensensis Biozone) and (C,F) late/post-LEE2 (hemiansatus Biozone). (A–C) Entire dataset. (D–F) Shallow marine localities excluded. x-axis: southern hemisphere latitude; y-axis: temperature in °C.

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Uncertainties in the latitudinal paleotemperature record

High SSTs of Devonian low latitudes

A recently published Devonian climate simulation suggests a global annual mean surface air temperature of 21 °C and tropical SST of 28 °C for the Middle Devonian44. The model44 suggests colder low latitude mean SSTs compared to SSTs calculated from δ18Oapatite of Devonian conodonts. Combining the new δ18Oapatite data from the Prague Synform (28° S) and the Carnic Alps (34° S) with other published data (Supplementary Fig. 2), mean tropical-subtropical SSTs reach 33 °C during late Eifelian/earliest Givetian time (Table 2: mean). However, Judd et al.43 call for caution when using paleotemperatures from shallow epeiric seas and continental margins as these are systematically higher compared to open-ocean temperatures from the same latitude band and thus result in an overestimation of global mean temperatures. Though, it seems a paradox, that especially conodonts from epeiric Belarus (10–14° S), the Eifel area (24° S), shallow marine platform deposits of the Carnic Alps (34° S) and from Pic de Bissous (36° S) indicate significantly lower paleotemperatures compared to those from slope and offshore deposits. A possible explanation could be an interval of arid climate conditions in tropical-subtropical realms linked with the Kačák Episode which led to increased evaporation in shallow shelf areas and thus higher δ18Oapatite values due to regionally increased salinity.

Zonal heterogeneity of low latitude SSTs

δ18Oapatite from the epeiric sea of eastern Euramerica (10–14° S) and shallow marine deposits of southern Euramerica (24° S) exhibit higher values which compare better with values of conodonts from northern Gondwana at 34–36° S rather than with those from more open marine settings between 25 and 34° S at least during the late Eifelian (Fig. 3, Supplementary Fig. 2). This could be explained by regional salinity variations as suggested for localities between 10 and 14° S28,29 or upwelling of colder intermediate waters in the Rheic Ocean45,46,47,48.

SSTs modelled for the Middle Devonian44 give significantly lower SSTs along the southern hemisphere Euramerican shelf compared to temperature estimates for open ocean settings of the same latitude. According to the Holocene SST record, a highest temperature of 32 °C (central equatorial Indian Ocean) and a lowest temperature of 17 °C (eastern equatorial Atlantic) between 0 and 5° S are documented49. As indicated by modern sea surface temperatures50, SST of low latitude waters does not always show a gradational temperature decrease towards higher latitudes. Hence, temperature variations within the low latitude belt like those documented in δ18Oapatite from Euramerican shallow shelf localities between 10 and 24° S likely reflect, similar to present day SSTs, regional factors as for example evaporation, runoff via large river delta systems, submarine groundwater discharge, karst water flow in coastal areas, ocean circulation, or upwelling zones along continental shelfs.

Mean SST record across the Eifelian/Givetian boundary

The interpretation of the paleotemperature trend across the Eifelian/Givetian boundary is difficult, because of the low number of isotope values available. Data of only two localities, at 34° S (ZMB section, Italy) and 36° S (Pic de Bissous sections, France), span the entire interval from the kockelianus until the hemiansatus Biozone. Additionally, δ18Oapatite values from the epeiric Belarusian Basin (10–14° S)28,29, as well as those derived from conodonts of shallow marine deposits at 24° S25,26, 34°S (this study, Supplementary Fig. 2) and 36° S25 are higher compared to values from conodonts of coeval more open marine habitats, contradicting expected values in relation to the latitudinal allocation. δ18Oapatite values of the three localities from different depositional settings within the Carnic Alps at 34° S, for example, differ significantly. Local conditions could have been responsible for such high zonal heterogeneity of SST. While the shallow marine carbonate platform setting (Val di Collina section) might have been affected by higher evaporation translating into higher δ18Oseawater and thus higher δ18Oapatite values, conodonts from pelagic deposits (Wolayer “Glacier” section) show lowest δ18Oapatite values likely reflecting normal-marine more open ocean salinity and temperature. This documents that δ18Oapatite values obtained from conodonts of tropical-subtropical shallow marine settings may not only represent seawater temperature but as well local salinity variations, as noted by several studies on δ18Oapatite before25,27,28,29,43,51,52. Thus, exclusively δ18Oapatite data from more open marine settings are used for calculation of mean paleotemperature. This results in higher mean SST as observed especially for mean SST calculated for the LEE1-LEE2 interval (ensensis Biozone), with a significant increase in temperatures from ~ 31 °C to ~ 34 °C (Table 2: mean vs. mean*). In summary, our data argue for at least 6 °C higher mean SST across the Kačák Episode compared to published model estimates suggesting mean values of 28 °C for the tropical-subtropical climate belt44.

Additionally, more open marine sections at 25–34° S show slightly decreasing mean SST from north to south during each biozone-interval (Fig. 4A–C vs. D–F, Table 2). A mean latitudinal temperature gradient of ~ 5 °C between 25 and 34° S is observed for the pre-/onset LEE1 interval (kockelianus Biozone) which is somewhat higher compared to the present day mean temperature gradient of ~ 4.5 °C for the same latitudinal range43. Calculating a mean latitudinal temperature gradient between 28 and 34° S, a decrease from ~ 3 °C during the pre-/onset LEE1 (kockelianus Biozone) to ~ 1 °C during the LEE1–LEE2 (ensensis Biozone) is observed (compare Fig. 4D–E). The present-day mean latitudinal temperature gradient between 28 and 34° S is ~ 3.3 °C43. Thus, we argue that global warming resulted in a minor latitudinal SST gradient in the subtropics and probably slightly higher latitudes that may have had an important effect for marine biota. Specifically, climate sensitive organisms like corals and stromatoporoids could spread more easily towards higher latitudes along broad shelfs. Hence, we suggest that the Kačák Episode could have triggered the early-middle Givetian acme of coral distribution and diversity.

Conclusion

The main results of this study can be summarized as follows:

  • New conodont δ18Oapatite data around the Kačák Episode (late Eifelian to earliest Givetian) from the Carnic Alps (34° S) and the Prague Synform (28° S) indicate mean SSTs between 31 and 34 °C.

  • SST of late Eifelian southern hemisphere low latitudes was at least 6 °C higher than previously assumed.

  • δ18Oapatite based paleotemperatures from conodonts of tropical-subtropical shallow water should only be used with great caution, confirming previous studies suggesting that salinity variations may affect the oxygen isotope values.

  • Global warming caused a flattening of the latitudinal temperature gradient in the subtropics and probably as well higher latitudes having an important effect for marine biota.

  • Specifically, climate sensitive organisms could spread more easily towards higher latitudes along broad shelfs. Hence, we suggest that the Kačák Episode could have triggered the early-middle Givetian acme of coral and stromatoporoid distribution and diversity.

Materials and methods

Conodont extraction

Conodont elements were extracted from limestones using 5% formic acid (Carnic Alps 2013, 2015; Prague Synform 2013) and 6% acetic acid (Prague Synform 2020) diluted in 5 L buckets filled with tap water. In order to prevent dissolution of conodont elements, the acid was exchanged twice a day until the carbonates were completely dissolved (generally after 7–10 days). In case of large amounts of insoluble residues, heavy liquid separation was applied at room temperature (sodium polytungstate; density: 2.79 g/cm3). The colour alteration index (CAI) of the conodont elements from the Carnic Alps is 4–5 and 5 (Supplementary Fig. 3) which agrees well with earlier observations of Brime et al.53, who published a comprehensive conodont CAI study of the Carnic Alps. Conodonts from the Jirásek quarry (Prague Synform) have a CAI of 3 (Supplementary Fig. 3).

Repository

Conodont elements from the Carnic Alps are stored at the Geological Survey of Austria in Vienna, Austria (Wolayer “Glacier” section: GBA-2016/013/0001-0018; Val di Collina quarry: GBA-2016/015/0001-0015) and at the Museo Friulano di Storia Naturale in Udine, Italy (ZMB section: MFSNgp 48161-48222). Conodont elements from the Prague Synform are stored at the Czech Geological Survey (Jirásek quarry section I and II: SV1-SV121). Permission was obtained from repository holding Institutions for the use of conodont materials included within this study.

Oxygen isotope analyses

Two analytical methodologies, thermal conversion-elemental analyser-isotope ratio mass spectrometry (TC-EA IRMS) and secondary ionization mass spectrometry (SIMS) are commonly used for measuring oxygen isotopes on biogenic apatite. While the first technique measures the oxygen isotopic composition of the PO4 group of several conodont specimens25, SIMS enables a high-resolution spot analysis, but measures δ18O of total oxygen (PO43−, CO32− and OH−) of single conodont elements54. Oxygen isotopes of bioapatite (conodont crown tissue without basal tissue preserved) within this study were analysed by TC-EA IRMS using a ThermoFinnigan Delta V Plus mass spectrometer at the Geozentrum Nordbayern (FAU Erlangen-Nürnberg, Germany). Depending on the amount of trisilverphosphate precipitated after dissolving conodont apatite in HNO3, triplicate measurements were conducted whenever possible. Values are reported in ‰ relative to VSMOW. The standard deviation of replicate sample analyses was ± 0.02 to ± 0.34 (1σ; see Supplementary Fig. 1). NBS 120c was measured as 21.7‰. Literature based SIMS values and TC-EA IRMS values that are calibrated with standards other than NBS 120c (21.7‰) were corrected for direct comparison (see Supplementary Fig. 2: − 0.7‰25; − 0.9‰27; − 0.6‰28). For paleotemperature calculation, the phosphate-water isotope fractionation equation of Lécuyer et al.39 is used: T(°C) = 117.4(± 9.5) − 4.50(± 0.43) (δ18Oapatite − δ18Oseawater) assuming δ18O of Devonian seawater as − 1‰ VSMOW.

Stratigraphic correlation of δ18Oapatite records

Biostratigraphically constrained δ18Oapatite values from the Carnic Alps11, the Prague Synform18 and the Blankenheim section of the Eifel area26 are correlated within biozonal boundary ranges via high-resolution magnetic susceptibility and geochemical data. Published data of other sections from Germany and from France25 are plotted according to the absolute age25 and correlated via conodont biozone boundaries55,56,57,58. Data from North America27 are correlated with the ZMB section of the Carnic Alps11 via the australis/kockelianus biozone boundary and the upper limit of the T-R cycle Id (calibrated to 393.37 Ma)11. Data published from Belarus28 are biostratigraphically constrained by conodonts to the ensensis Biozone and were already combined within the standard-corrected data of Joachimski et al.25.

Calculation of latitude dependent δ18O of seawater

Seawater δ18O is dependent on the ratio evaporation vs. precipitation which is higher in subtropical latitudes relative to equatorial and higher latitudes. The calculation of spatial gradients in δ18Oseawater follows Roberts et al.38, which prerequisites exact paleolatitude calculation for each locality. Since the climate conception of an ice-free Middle Devonian40,42 compares better with early Paleogene climate conditions rather than with preindustrial climates, we adopted latitude-dependent seawater δ18O estimates for the early Paleogene38.

Paleolatitude calculation

The calculation of paleolatitudinal positions for the various areas follows Scotese and Wright59 (Supplementary Fig. 2). Evaluation of the tectonic setting around Val di Collina quarry53 results in a slightly more southern paleolatitudinal position than suggested by Scotese and Wright59. In general, the paleo-position is calculated to a latitudinal precision of 1°, based on the equation used.