The evolution of the marine and terrestrial biosphere was affected by several critical periods in Earth’s history which are known as global mass extinctions. Global mass extinction events are defined by the loss of a considerable amount of fossil taxa at the global scale within a relatively short geological time period (a few thousand to hundred thousand years) as a result of a strongly increased extinction rate.
A loss of about 80% of all species of the marine realm is recorded for the Late Triassic (RAUP & SEPKOSKI 1988, JABLONSKI 1994). Most geoscientists suggest that this diversity decline represents a global mass extinction event at the end of the Triassic (ETME) about 201 million years ago which was caused by extensive volcanism and a massive increase of atmospheric CO2 (e.g. Ward et al. 2001, Hesselbo et al. 2002, Ruhl et al. 2011, Richoz et al. 2012, Blackburn et al. 2013). Some scientists, however, suggest a stepwise extinction process during the Late Triassic might have been responsible instead (Lucas & Tanner 2008, Zaffani et al. 2018). Reconstructions of palaeobiodiversity data suggest that the Late Triassic was a period of relatively high extinction rates, but that the diversity loss was primarily due to a low species origination rate (Bambach et al. 2004).
Carbon isotope (δ13C) analyses are among the most important tools for the detection of the global environmental crisis because global changes in δ13C reflect perturbations of the global carbon cycle. The end-Triassic interval is characterized by a significant and global negative shift in δ13C, which is recorded both in bulk rocks and organic matter at several Triassic/Jurassic boundary sections around the world (e.g. Hesselbo et al. 2002, Kürschner et al. 2007, Ruhl et al. 2009; Korte et al. 2019). But the record of carbon isotopes prior to this end-Triassic event is still poorly known because there are few continuous and high-resolution Rhaetian δ13C records published so far.
In recent years, our team has carried out several high-resolution carbon isotope studies in the Late Triassic of the Northern Calcareous Alps (NCA), a 500 km long east-west extending mountain chain in the northern part of Austria. The NCA is built of Late Permian to early Cenozoic sedimentary rocks and is one of the most suitable areas for studying Late Triassic marine sediments. The Rhaetian of the NCA includes shallow marine deposits of an extensive carbonate platform with intraplatform basins (Kössen Formation), reefal limestones of the Dachstein mountain complex, and oceanic sediments of the Hallstatt deeper shelf and basin (Zlambach Formation) at the western Tethys margin (Fig. 1 a,b). The carbon isotope data of the Kössen Formation have demonstrated that significant negative and positive δ13C shifts occur in the middle and late Rhaetian, prior to the ETME. The most prominent shift occurs in the Late Rhaetian (Mette et al. 2012, Korte et al. 2017, Rizzi et al. in review).
1. (a) Palaeogeographic reconstruction for the Western Tethyan margin during the Late Triassic (modified after Hornung et al., 2008). (b) Map showing the location of the Rossmoosgraben section and geological setting in Northern Calcareous Alps (modified after Kürschner et al., 2007). (c) Geological map (simplified after Schäfer, 1982). (d) Locality map of the studied sections.
Recently, the deeper shelf-to-basin successions of the western Tethys (Zlambach Formation) have also been analyzed at a high resolution with respect to isotope stratigraphy, sedimentology, and micropalaeontology (Austrian Science Fund (FWF) project No. 25782, Mette et al. 2019). The study was aimed to identify significant environmental deteriorations in the Rhaetian. For this purpose, a section of the Zlambach Formation has been analyzed for carbon isotopes, carbonate microfacies, benthic microfossils, and nannofossils (Fig. 1 c,d, Figs. 2, 3, 6, Table 1).
2. Biostratigraphy, lithostratigraphy, and carbon isotope record of the Zlambach Formation at the Rossmoosgraben sections (S=shales, M= marls, ML= marly limestones, L= limestones); ostracod total abundance (number of specimens) and species diversity (Shannon−Wiener Index)- foraminifera total abundance (N=number of specimens) and diversity (G=number of genera). The uppermost part of Section 1 and the lower part of Section 2 show a litho-and isotope stratigraphic congruence. A composite section of the Zlambach Formation at Rossmoosgraben has been constructed based on a significant correlation between the δ13Ccarb curve of section 1 and 2 (R2 = 0.85). Ammonoid occurrences (this study): 1=Choristoceras cf. marshi, 2=Choristoceras sp., 3=Cycloceltites arduini, 4=Gigantarcestes ? sp. (det. L. Krystyn). The range of the V. stuerzenbaumi zone and the C. ammonitiforme subzone is based on data of Weiland (1991).
3. Lithostratigraphy, sedimentary structures (lamination, bioturbation) and carbonate microfacies types (LF 1-4) of the Zlambach Formation at the Rossmoosgraben composite section (LF 1: A-D, LF 2: E-L, LF 3: M-N, LF 4: O-P); (S=shales, M= marls, ML= marly limestones, L= limestones).
Table 1. Carbonate lithofacies types of the Zlambach Formation at Rossmoosgraben section and their depositional environment.
Carbon isotope ratios of carbonate sediments can be altered due to carbonate redeposition and diagenesis and thus may not represent the primary carbon isotopic ratios of the sea water. Therefore, we carefully removed potentially altered δ13C data from the δ13C record by means of statistical analysis (Fig. 4).
4. (a) Cross-plot of δ13Ccarb versus δ18Ocarb (section 1); (b) Cross-plot of δ13Ccarb versus δ18Ocarb (section 2); (c) Cross-plot of % CaCO3 versus δ18Ocarb; (d) Cross-plot of % CaCO3 versus δ13Ccarb.
One of the most important features of the isotope studies is a prominent negative carbon isotope shift in the late Rhaetian which is primary in nature (Figs. 2, 5, 6). Based on the integrated sedimentological, geochemical, and micropalaeontological analysis, we could show that this carbon isotope negative excursion was most probably linked with the development of the Dachstein reef and the Hallstatt basin. This suggestion is based on carbonate microfacies results studied on basin deposits (Zlambach Formation).
5. Carbon isotope record and stratigraphic distribution of the main detrital proxies obtained from the Zlambach Formation at Rossmoosgraben composite section (S=shales, M= marls, ML= marly limestones, L= limestones); intervals with high Ti/Al, Si/Al and Zr/Al values are indicated by grey shading; intervals with high K/Al ratios are white.
6. Comparison of lithofacies types (LF 1-4) and sedimentary structures (lamination/bioturbation) of the Zlambach Formation at the Rossmoosgraben composite section with δ13Ccarb, δ18Ocarb, and %CaCO3.
These results provided important information about the development of adjacent carbonate platforms and reefs which largely depend on sea level changes. During sea-level highstand, carbonate platforms grow and progradate into the basin (regressive phase), and increasing amounts of detrital carbonate are shed as calciturbidites from the platform margins into the basin. During rapid sea level rise, carbonate platforms retrogradate (transgressive phase), and the calciturbidite input in the basin decreased.
Our microfacies and isotope results further show that the lower part of the Zlambach Formation includes several calciturbidites, and only minor δ13C variations occur. The prominent δ13C negative shift in the upper part of the Zlambach Formation, however, is accompanied by a rapid decrease of calciturbidite deposits (Fig. 5).
As mentioned above, the cessation of calciturbidites deposition may be either caused by a rapid retrogradation of the carbonate platform margin due to a strong sea level rise or by a cessation of carbonate platform margin growth due to pronounced environmental changes. At the same stratigraphical position, we also found a rapid abundance and diversity decline of both benthic microfossils (Fig. 2) and nannofossils (unpublished data). The stratigraphical coincidence of significant geochemical, sedimentological, and ecological changes thus suggests that the Dachstein carbonate platform growth was stopped in the Late Rhaetian due to environmental deterioration in the western Tethys realm.
Besides carbonate microfacies studies, our sedimentological research included also trace element analysis. Various trace elements (e.g. Ti, Si, Zr) are indicative of fluvial input of terrigenous clastics into marine basins which is largely controlled by sea-level changes and climate. We analyzed trace element ratios (e.g. Ti/Al, Si/Al, Zr/Al, Ca/Al) and found 16 short-term cyclic changes of detrital input from the platform (Fig. 5).
These cyclic changing trace element ratios of fluvial supply reflect rapid sea level changes and/or rapid climatic variations that influenced sediment supply and loading on the slope. The maxima of trace elements ratios of terrigenous origin and calciturbidite input can help to identify long-term sea level changes which are expressed by transgressive/regressive sedimentary sequences of different orders. According to our trace element data, the Zlambach Formation includes a third-order regressive-transgressive unit and two alternating fourth-order regressive and transgressive units (Fig. 7). The variations of calciturbidite input indicate two phases of platform progradation and retrogradation, which correspond to the fourth-order transgressive and regressive units identified by trace element data.
7. Lithostratigraphy, calciturbidites, T/R cycles, the occurrence of deep-water foraminifera (Oberhausellidae) at the Rossmoosgraben composite section and inferred platform/basin development (S=shale, M=marl, ML=marly limestone, L=limestone).
Finally, our geoscientific analysis of Late Triassic marine sediments suggests that there was at least one significant environmental crisis in the Tethys ocean prior to the end-Triassic event. The causes of this environmental crisis and its impact on marine biodiversity have to be detected by further integrated geochemical, sedimentological and palaeontological research in the Rhaetian oceanic deposits of the western Tethys.