HigH-resolution calcareous nannofossil biostratigrapHy across tHe toarcian oceanic anoxic event in nortHern italy: clues from tHe sogno and gajum cores (lombardy basin, soutHern alps) Stefano ViSentin1 & eliSabetta erba1* 1Dipartimento di Scienze della terra “ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, 20133 Milano, italy. *Corresponding author. e-mail: elisabetta.erba@unimi.it. to cite this article: Visentin S. & erba e. (2021) - High-resolution calcareous nannofossil biostratigraphy across the toarcian oceanic anoxic event in northern italy: clues from the Sogno and Gajum Cores (lombardy basin, Southern alps). Riv. It. Paleontol. Strat., 127(3): 539-556. Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy) vol. 127(3): 539-556. November 2021 Abstract: Calcareous nannofossil biostratigraphy was conducted across the toarcian oceanic anoxic event (t- oae) interval cored at Colle di Sogno and Gajum in the lombardy basin (Southern alps, northern italy). Drilling at both sites resulted in 100% recovery of unweathered material. the Sogno and Gajum Cores consist of pelagic marly limestones, marlstone, marly claystone, and a relatively expanded black shale interval named fish level considered the lithostratigraphic record of the t-oae at regional scale. Semiquantitative analyses of calcareous nannofloras allowed to achieve a high-resolution biostratigraphy of the latest Pliensbachian-early toarcian time interval. Several nannofossil biohorizons were detected, including zonal/ subzonal markers and additional events related to changes in abundance. the nannofossil biostratigraphic correlation of the Sogno and Gajum Cores indicates that, according to their paleogeographic settings, the succession recovered in the Sogno Core deposited on a pelagic plateau is continuous while a hiatus of ~600 kyrs was detected in the lower- most toarcian in the Gajum Core located on a slope of a structural high. the nJt 5 and nJt 6 Zones of the standard nannofossil zonation for the Mediterranean Province were identified in both the Sogno and Gajum Cores. Our findings allow an implementation of the reference biozonation with the separation of the nJt 6a and nJt 6b Subzones, and age revision of some secondary events. the zonation established for the Lusitanian Basin (Portugal) is only partially reproducible in the Lombardy Basin, confirming nan- noplankton paleoprovincialism during the early Jurassic requiring different zonal schemes in various areas. neverthe- less, we underline that the t-oae is unambiguously constrained by the fo of C. superbus crassus and the lo of M. jansae at supra-regional scale. Received: January 14, 2021; accepted: June 25, 2021 Keywords: Calcareous nannofossils; biostratigraphy; toarcian oceanic anoxic event; lombardy basin. IntroductIon the early toarcian oceanic anoxic event (t- oae) is recognized as one of the most severe and geographically widespread events of oceanic anoxia and organic-carbon burial in the Mesozoic (Jenkyns 1985, 1988, 2003, 2010). Coeval paleoenvironmental perturbations have been linked to volcanism of the Karoo-ferrar large igneous Province (liP) and its release of volcanogenic Co 2 , and/or thermogenic methane (CH 4 ) from sill intrusion into Gondwanan coals, and/or biogenic methane from dissociation of sub-seafloor clathrates (Hesselbo et al. 2000; Kemp et al. 2005; Mcelwain et al. 2005; Svensen et al. 2007; Jenkyns 2010; reolid et al. 2020). this dramatic episode of ecosystem adjustments, glob- al warming and altered ocean chemistry occurred during a crucial time for calcareous nannoplankton diversification as a major speciation episode took place in the late Pliensbachian-early toarcian time interval (bown 1987; Mattioli & erba 1999; bown Visentin S. & Erba E.540 et al. 2004; erba 2004, 2006; fraguas & Young 2011; Menini et al. 2019). new genera and species appeared and quickly evolved allowing a high-reso- lution biostratigraphy also across the t-oae (Mat- tioli & erba 1999; Mattioli et al. 2004; Casellato & erba 2015; ferreira et al. 2019). Moreover, major changes in abundance of some taxa were proved coeval in the late Pliensbachian-early toarcian time interval and probably related to large scale paleoen- vironmental stress preceding and across the t-oae (erba 2004; Mattioli et al. 2004, 2008; tremolada et al. 2005; fraguas et al. 2012; Casellato & erba 2015; Clémence et al. 2015; Menini et al. 2019; Visentin et al. 2021b). Calcareous nannofossils were proved to be extremely useful for high-resolution biostrati- graphic dating and correlations at low, medium, and high latitudes. two standard biozonations are avail- able for the early Jurassic: the one of bown (1987) revised by bown & Cooper (1998) established for the boreal realm (United Kingdom, Germany, the netherlands, central and northern france) and the zonal scheme of Mattioli & Erba (1999) specific for the tethyan area (italy, Southern and eastern Spain, South france, Hungary, Greece). More recently, two zonal schemes were published for the Canta- brian range in northern Spain (fraguas et al. 2015, 2018) and the lusitanian basin in Portugal (ferreira et al. 2019). Calcareous nannofossil biostratigraphy across the t-oae has been documented for several out- crop sections (see ferreira et al. 2019 for a review) and a few cores (Van de Schootbrugge et al. 2019; Visentin et al. 2021b). Core successions are, in many cases, preferable since unweathered lithologies usu- ally display a much better preservation and sampling can be conducted in much higher resolution result- ing in a more resolved taxonomy and biostratigra- phy. the Sogno and Gajum drilling project was, in fact, aimed at recovering high-resolution data from continuous and well-preserved sequences since out- cropping sedimentary rocks, and particularly black shales, are commonly badly degraded (erba et al. 2019b). Continuous coring is, thus, crucial to recov- er high-quality fresh material with potentially good preservation. in this work we present a detailed calcareous nannofossil biostratigraphy of two continuously cored successions representing significantly differ- ent geological settings within the lombardy basin (Southern alps) (erba et al. 2019b). the Sogno Core was drilled on the albenza Plateau (a pelagic structural high) while the Gajum Core was pene- trated in an inner basin along the slope of the Mt. Corni di Canzo structural high (fig. 1). both cores recovered a pelagic succession through the upper- most portion of the Domaro limestone (lmst.) and the lower part of Sogno formation (fm.) in- cluding the fish level, which represents the litho- logical expression of the t-oae in the lombardy basin (tintori 1977; Gaetani & Poliani 1978; erba & Casellato 2010; erba et al. 2019a, b). the calcareous nannofossil biostratigraphic investigation of the Sogno and Gajum Cores is aimed at building a robust framework to constrain the t-oae paleoenvironmental changes at local, regional, and supra-regional scale. in particular, nannofossil events will be used to date the fish level black shale interval pointing out synchrone- ity or diachroneity within the lombardy basin, and also for comparison with other basins at different latitudes. analogies and differences will be used to derive reproducibility and lateral extension of cal- careous nannofossil events. GeoloGIcal settInG and lIthostratIGraphy During the Jurassic, the lombardy basin was a relatively deep area bordered by the Cana- vese Zone to the West and the trento Plateau to the east (Gaetani 1975; 2010; bernoulli & Jenkyns 2009). During latest triassic to earliest Jurassic times, a rifting phase broke the southern margin of the Western tethys into depressions and pa- leohighs that are, from West to east: Monte nudo trough, lugano High, Generoso trough, Corni di Canzo High, albenza Plateau, Monte Cavallo High, Sebino trough, botticino High (Gaetani 1975, 2010) (fig. 1). in the troughs, lower Juras- sic partially resedimented sequences may reach a non-decompacted thickness of 3000 m (e.g., in the Generoso trough), while condensation and/ or hiatuses distinguish the paleohigh successions, often represented by reddish nodular facies. on the escarpments connecting the highs to the deep- er parts, sedimentation was affected by slumps, resedimented intervals and locally submarine brec- cias, typically interrupting condensed and seldom incomplete successions (Gaetani & erba 1990; Toarcian Oceanic Anoxic Event in Northern Italy 541 Gaetani 2010) (fig. 1). the lower Jurassic succes- sions of the Lombardy Basin were influenced by local-regional tectonics, but also shaped by global paleoenvironmental perturbations including the t- oae (erba et al. 2019a). lower toarcian sections contain a distinctive black shale interval that is the sedimentary record of oxygen-depleted seafloor of the tethys ocean. in the lombardy basin the low- er toarcian black shale interval was named livello a Pesci/fish level (tintori 1977): it generally has a thickness between 0.5 and 5 m, but can reach a few tens of meters in the most expanded sections. black shales are ubiquitous in the deeper settings of the lombardy basin, but are commonly absent on paleo-highs (tops and upper slopes), usually resulting from stratigraphic gaps and/or extreme condensation. the Colle di Sogno and Gajum sites were se- lected as pelagic records (Gaetani & erba 1990; Ca- sellato & erba 2015) for continuous coring of the lower toarcian fish level. Within the lombardy basin, these sites represent different depositional settings on a pelagic structural high (albenza Pla- teau) and in an inner basin along the slope of a structural high (Mt. Corni di Canzo). at both loca- tions, the fish level is present in the lower part of the Sogno formation (Gaetani & Poliani 1978; Gaetani & erba 1990; Casellato & erba 2015) providing the opportunity to date, investigate and model the anoxic interval. at Colle di Sogno, the type-section of the Sogno fm. was formalized along the road SP 179 on the northern slope of Mt. brughetto (Gaetani & Poliani 1978). the pelagic calcilutites of the Do- maro limestone (lmst.) fm. are suddenly followed by the Sogno fm. consisting of marlstones, calcare- ous marlstones, marly limestones, and black shales. the upper Pliensbachian-lower bajocian Colle di Sogno section was characterized stratigraphically through litho-, bio-, chemo-, magneto-, and cyclo- stratigraphy (Gaetani & Poliani 1978; Jenkyns & Clayton 1986; Gaetani & erba 1990; Hinnov et al. 2000; Channell et al. 2010; Casellato & erba 2015). the Gajum outcrop is located in a small lateral cut of the ravella Valley (close to the Canzo village), where the carbonate-rich lithologies of the Domaro lmst. are followed by dark grey, clay-rich lithologies of the Sogno fm. overlain by reddish nodular limestones of the rosso ammonitico lombardo. erba et al. (2019b) described in detail the lithostratigraphy of the Sogno and Gajum Cores. Lithologic units were defined taking into account lithological features and sedimentary structures. for each core, at least four dip measurements were taken during lab preparation to calculate the stratigraphic thickness of the drilled sections. the composite So- gno Core section recovered a complete upper Pliens- bachian–lower toarcian interval (total stratigraphic thickness = 25.33 m) represented by the uppermost part of the Domaro lmst. and the lower part of the No rth Am eri ca S. America Africa TethysAdria 20°N 10°N 10°S 0° Paleohigh Basin Fault Sogno core Gajum core 0 5 km Lecco Como Monte Cavallo Albenza Corni di Canzo Co mo L ak e A d d a r iv er sea level Albenza Plateau Monte Nudo Trough Lugano High Monte Generoso Trough Corni di Canzo High Monte Cavallo High W. Sebino Trough Botticino High E. Sebino Trough Trento Platform 2 km 50 km LOMBARDY BASIN (Jurassic) Garda Escarpment Jurassic DRILLING AREAA B Triassic Paleozoic fig. 1 - location of the Sogno and Gajum drilling sites relative to a) paleo- and b) current geography (after erba et al. 2019b). Visentin S. & Erba E.542 S 4.82 10.52 9.54 14.55 15.93 21.35 19.10 22.92 24.35 25.47 26.83 11.87 1 2 3 5 6 1.5 7 8 9 10 11 13 14 15 24.99 16.86 20 21 22 23 24 25 26 27 10 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 0 1 2 Lithostratigraphic Unit SOGNO Core 4 BRECCIA 19.45 BRECCIA 12 fo ld S S S S S S SS Fracturedf f f f f f f grey 5Y 5/1 black 5Y 2.5/1 lig ht b rownish grey 2.5Y 6/2 very dark grey 5Y 3/1 dark grey 5Y 4/1 dark grey 7.5YR 4/0 greyish b rown 2.5Y 5/2 dark greyish b rown 2.5Y 4/2 grey 2.5Y 6/0 very dark greyish b rown 10 YR 3/2 grey 5Y 6/1 faultf pyrit e s st yloli t e emerald lamina bi valve biotu rb ation faint bioturb ation laminations Legend dus ky red 2.5YR 3/2 dark reddish brown 5 YR 3/3 oli ve 5Y 5/3 and 5Y 4/3 grey 5YR 5/1 oli ve grey 5Y 5/2 and 4/2 lig ht oli ve grey 5Y 6/2 reddish grey 10YR 5/1 dark reddish brown 2.5 YR 3/3 dark brown 7.5 YR 3/2 N JT 6 N JT 5 Calcareous nannofossil EVENTS and ZONES Schizosphaerella recovery (11.08 m) Schizosphaerella and M. jansae crises (16.78 m) Schizosphaerella decline (25.69 m) Watznaueria sp. 1 (9.06 m) M. jansae (11.25 m) C. superbus crassus (17.01 m) D. constans (17.16 m) L. sigillatus (26.69 m) C. poulnabronei (24.27 m) L. crucicentralis (23.65 m) L. velatus (21.46 m) D. ignotus (20.21 m) presence of L. hauffii (26.83 m) Samples b S O G N O F or m at io n E ar ly T O A R C IA N D O M A R O Li m es to ne la te st P LI E N S . A ge s U ni ts hiatus a b a F is h Le ve l F is h Le ve l fig. 2 - lithostratigraphy and biostratigraphy of the Sogno Core. Calcareous nannofossil events in red are the primary events used as zonal and subzonal boundaries. the Schizosphaerella decline, crisis, recovery, and M. jansae crisis are reported in blue. Toarcian Oceanic Anoxic Event in Northern Italy 543 Sogno fm. (fig. 2). the Gajum Core recovered the Domaro lmst. immediately overlain by the fish level black shales, followed by marlstones of the Sogno fm. and the rosso ammonitico lombardo (total stratigraphic thickness = 28.18 m) (fig. 3). MaterIals and Methods a total of 154 samples were selected for calcareous nanno- fossil biostratigraphy of the Sogno Core. the average sampling rate adopted is 16 cm through the core succession: the lowermost 9 m (core bottom to lower unit 8) and the uppermost 9 m (upper part of unit 4 to core top) were sampled at a slightly lower resolution of 20 cm whereas the fish level and the interval immediately preceding and following (upper part of unit 8, units 7, 6, 5 and lower part of unit 4) were sampled every 10 cm (fig. 2). biostratigraphic analyses were per- formed on smear slides prepared following the settling boxes method of Geisen et al. (1999) that was applied to conduct future quantitative investigations. according to this technique, a basic pH suspension of 0.1 g of dried rock powder and ammoniac water was homogenized and let settled for 24 h on a cover slide in a settling device. the cover slide was recovered, dried, and attached to a slide with the norland optical adhesive. a total of 156 samples were collected from the Gajum Core. the average sampling rate adopted is 20 cm with the exception of the Domaro lmst./Sogno fm. lithostratigraphic boundary correspon- ding to the inception of the black shales of the fish level (lowermost unit 6) sampled every 0.5-1 cm (fig. 3). Samples were prepared from limestones and marly limestones of the Domaro lmst., limy marlsto- nes, marlstones, marly claystones and black shales of the Sogno fm. and nodular limestones of the rosso ammonitico lombardo fm. Simple smear slides were prepared following the method of Monechi & thierstein (1985): a small amount of rock material was powdered adding few drops of bi-distillate water, without centrifuging, ultra- sonic cleaning or settling the sediment in order to retain the original composition. the obtained suspension was mounted onto a slide, covered with a cover slide fixed with the Norland Optical Adhesive. Smear slides of both cores were investigated using a light po- larizing microscope, at 1250X magnification. Calcareous nannofossil abundance and preservation were evaluated by examining at least 6 longitudinal traverses (750 fields of view) in each smear slide. Pre- servation of calcareous nannofossils was characterized adopting the codes described by roth (1983) for etching and overgrowth (see ap- pendices 2 and 3). for each sample, semi-quantitative abundances of individual taxa and total nannofossil assemblages were obtained as number of specimens in a microscope field of view as detailed in appendices 2 and 3. the biozonation scheme adopted is that of Mattioli & erba (1999) established for the tethyan realm. Calcareous nannofossil taxa recognized in this study are listed in appendix 1. the range charts of calcareous nannofossil taxa from the Sogno and Gajum Cores are re- ported in appendices 2 and 3. Plates 1 and 2 contain the photographs of the most common taxa. taxonoMIc notes in the present work we followed the recent taxonomic revi- sion for the genus Carinolithus conducted by Visentin et al. (2021a). Specifically, morphometric analyses allowed the separation of C. super- bus crassus from C. superbus superbus based on the stem width (SW). the former taxon has a SW > 1 µm whereas the latter is characterized by a SW ≤ 1 µm. As a consequence, Visentin et al. (2021a) concluded that C. superbus, whose fo is used as marker for the base of the nJ 6 and nJt 6 Zones in the reference biozonation schemes (bown & Cooper 1998; Mattioli & erba 1999; ferreira et al. 2019), is indeed C. superbus crassus. Visentin et al. (2021a) further demonstrated that the species C. cantaluppii is a diagenetic artefact of Carinolithus specimens due to intensive overgrowth (highly calcified C. poulnabronei and C. superbus) and, accordingly, we disregard this taxon. the taxonomic subdivisions of Casellato & erba (2015) were applied as follows: a) C. crassus is subdivided into C. crassus (length > 5 µm) and “small” C. crassus (length < 5 µm) ; b) M. jansae is divided into M. jansae and “thin” M. jansae; c) S. punctulata is divided into S. punctulata, “small” S. punctulata and “encrusted” S. punctulata. addi- tionally, similarly to Casellato & erba (2015), we consider the taxon Watznaueria sp. 1 corresponding to W. fossacincta of Mattioli & erba (1999) and ferreira et al. (2019). results calcareous nannofossil preservation and abundance Calcareous nannofossil preservation and abundance vary through the studied interval, show- ing comparable and consistent variations in the two cores. in the Sogno Core the preservation is gener- ally moderate to moderate/good varying from mod- erate/poor to moderate in the Domaro lmst. and from moderate/poor to good in the Sogno fm., with the exception of the fish level where a moderate/ poor to moderate/good preservation was observed. the degree of etching varies from e1 to e2, with stronger dissolution within the fish level, whereas the degree of overgrowth fluctuates between O0 and o2 and is relatively higher in the Domaro lmst. Sim- ilar results were obtained for the Gajum Core: a gen- erally moderate preservation shows variations from poor to moderate/good in the Domaro lmst. and within the fish level, moderate/poor to good in the uppermost part of the Sogno fm. and from moder- ate/poor to moderate within the rosso ammonitico lombardo fm. the degree of etching varies from e1 to e3 and, as for the Sogno Core, stronger dis- solution is observed within the fish level while the degree of overgrowth fluctuates between O0 and O3 with stronger evidence of overgrowth in the Doma- ro lmst. and occasionally in the rosso ammonitico lombardo fm. the calcareous nannofossil abundance is somehow lithology-dependent: nannofossils are rare to rare/frequent and rare/frequent to frequent in the Domaro lmst. of the Sogno Core and Gajum Core, Visentin S. & Erba E.544 respectively. Within the Sogno fm., nannofossils are generally frequent, fluctuating between rare to fre- quent/common in both cores. a drastic decrease in total abundance is observed in the fish level, char- acterized by extremely rare nannofossils with a few sporadic samples containing rare/frequent or fre- quent specimens. the rosso ammonitico lombar- do fm. is characterized by rare/frequent to frequent nannofossils. biostratigraphy Sogno Core a total of nine calcareous nannofossil events were recognized in the Sogno Core (fig. 2) and the nJt 5 (divided into the nJt 5a and nJt 5b Sub- zones) and NJT 6 Zones were identified. according to the zonation of Mattioli & erba (1999), the lowermost studied sample (26.83 m) is at- tributed to the nJt 5a Subzone for the presence of Lotharingius hauffii and the absence of Lotharingius sigil- latus. The first occurrence (FO) of L. sigillatus at 26.69 m (sample S3-C-473) marks the boundary between the nJt 5a and the nJt 5b Subzones. Within the nJt 5b Subzone additional biohorizons were rec- ognized, namely: the fos of Carinolithus poulnabronei (24.27 m), Lotharingius crucicentralis (23. 65 m), Lotha- ringius velatus (21.46 m), Discorhabdus ignotus (20.21 m) and Diductius constans (17.16 m). the fo of Carinoli- thus superbus crassus (17.01 m) defines the base of the nJt 6 Zone. the last occurrence (lo) of Mitrolithus jansae (11.25 m) and the fo of Watznaueria sp.1 (9.06 m) were detected within the nJ 6 Zone. following ferreira et al. (2019), we use here the lo of M. jansae to separate the nJt 6a and nJt 6b Subzones (fig. 2). the nJt 6/nJt 7 zonal boundary was not iden- tified because Discorhabdus striatus was not observed in the studied interval. Variations in semiquantitative abundance of a few taxa were recorded (see range chart in appendix 2) and previously defined acme-paracme levels were identified. Casellato & Erba (2015) identified the Schizosphaerella decline, crisis and recovery as well as the M. jansae crisis on the basis of absolute abundanc- es obtained from ultrathin section analyses. Howev- er, semiquantitative analyses (Casellato & erba 2015; Visentin et al. 2021b; this work) also allow the identi- fication of the above mentioned biohorizons. In the Sogno Core, the Schizosphaerella decline corresponds to a change from common to frequent abundance at the Domaro lmst./Sogno fm. boundary (25.69 m); the Schizosphaerella and M. jansae crises are marked by a change from frequent and continuous records to rare and discontinuous occurrences and corre- late with the base of the fish level (16.78 m). the Schizosphaerella recovery has been detected at the level where this taxon returns to be continuous and rare/ frequent above the fish level (11.08 m) (fig. 2). the calcareous nannofossil assemblages of the nJt 5b Subzone are dominated by Schizosphae- rella punctulata and M. jansae. among schizosphaer- ellids, S. punctulata is the most abundant taxon (rare to common) whereas “small” S. punctulata (extremely rare to frequent/common) and “encrusted” S. pun- ctulata (extremely rare to rare/frequent) are subordi- nated. Within genus Mitrolithus, M. jansae is slightly more abundant (extremely rare to frequent/com- mon) than “thin” M. jansae (extremely rare to rare/ frequent). Within genus Lotharingius a slight increase in abundance is observed for L. hauffii, L. frodoi and L. sigillatus in the upper part of the nJ 5b Subzone. nannofossil assemblages in the lower part of the nJt 6a Subzone, largely corresponding to the fish level, show a severe reduction in abundance of both Schizosphaerella and Mitrolithus (Schizosphaerella and M. jansae crises). in particular, both S. punctulata and “small” S. punctulata become extremely rare, and “encrusted” S. punctulata is recorded only sporadi- cally. as far as M. jansae is concerned, abundances become rare to extremely rare and discontinuous in the fish level, with a bigger drop in abundance of the normal morphotype. in the fish level Lotharingius becomes the dominant genus with L. hauffii, L. frodoi and L. sigil- latus as most abundant species. in this interval, an increase in abundance for genera Calyculus and Ca- rinolithus was also recorded (see range chart in ap- pendix 2). Calcareous nannofossil assemblages of the uppermost part of the nJt 6a Subzone are charac- terized by a relative recovery of schizosphaerellids (Schizosphaerella recovery) above the fish level, al- though with limited abundance: S. punctulata is ex- tremely rare to frequent and “small” S.punctulata is extremely rare to rare/frequent whereas “encrusted” S. punctulata remains sporadic. through the Sogno Core, genera Biscutum, Ca- lyculus, Carinolithus and Crepidolithus are characterized by a general rare abundance, whereas Bussonius, Di- ductius, Similiscutum, Parhabdolithus and Tubirhabdus are extremely rare. Toarcian Oceanic Anoxic Event in Northern Italy 545 Plate 1 Scale bars represent 2 µm. 1-2 - B. dubium, 1) cross-polarized light, 2) quartz lamina, Gajum Core, sample G C25 995 (26.94 m) 3-4 - B. finchii, 3) cross-polarized light, 4) quartz lamina, Sogno Core, sample S3 C12 330 (17.16 m) 5-6 - B. grande, 5) cross-polarized light, 6) quartz lamina, Gajum Core, sample G C25 994 (26.89 m) 7-8 - B. intermedium, 7) cross-polarized light, 8) quartz lamina, Sogno Core, sample S3 C12 330 (17.16 m) 9-10 - Calyculus spp. side view (SV), 9) cross-polarized light, 10) quartz lamina, Sogno Core, sample S3 C29 456 (25.5 m) 11-12 - Calyculus spp. distal view (DV), 11) cross-polarized light, 12) quartz lamina, Sogno Core, sample S3 C21 416 (22.95 m) 13-14 - C. poulnabronei, 13) cross-polarized light, 14) quartz lamina, Sogno Core, sample S3 C22 423 (23.42 m) 15-16 - C. superbus crassus, 15) cross-polarized light, 16) quartz lamina, Sogno Core, sample S3 C5 313 (13.42 m) 17-18 - C. cavus, 17) cross-polarized light, 18) quartz lamina, Gajum Core, sample G C16 623 (17.15 m) 19-20 - C. crassus, 19) cross-polarized light, 20) quartz lamina, Gajum Core, sample G C22 841 (23.09) 21-22 - C. crucifer, 21) cross-polarized light, 22) quartz lamina, Sogno Core, sample S1 C32 192 (11.86 m) 23-24 - C. granulatus, 23) cross-polarized light, 24) quartz lamina, Gajum Core, sample G C25 992 (26.85 m) 25-26 - D. ignotus, 25) cross-polarized light, 26) quartz lamina, Sogno Core, sample S3 C14 343 (17.9 m) 27-28 - L. barozii, 27) cross-polarized light, 28) quartz lamina, Gajum Core, sample G C7 275 (9. 15 m) 29-30 - L. crucicentralis, 29) cross-polarized light, 30) quartz lamina, Sogno Core sample S1 C7 35 (4.24 m) Visentin S. & Erba E.546 Gajum Core the calcareous nannofossil biostrati- graphic investigation of the Gajum Core resulted in the identification of seven events and of the NJT 5 and nJt 6 Zones (fig. 3). the lowermost inves- tigated sample (31.17 m) is assigned to the upper- most Pliensbachian nJt 5 Zone (nJt 5b Subzone) based on the presence of L. hauffii and L. sigillatus. the nJt 5/nJt 6 zonal boundary is correlatable with the base of the fish level, although we pre- cise that the fo of C. superbus crassus was observed 0.8 cm above the base of the lowermost black shale (26.932 m) (fig. 3). the fo of D. ignotus is placed at 26.938 m, in the topmost nJ 5b Subzone. Within the basal part of the nJt 6 Zone the fos of rare and discontinuous C. poulnabronei, L. crucicentralis, L. velatus and D. constans were detected within a 4.8 cm thick interval. These findings sug- gest a hiatus at the base of the black shale interval of the fish level eliding part of the latest Pliensba- chian-early toarcian time interval. in the nJt 6 Zone, the lo of M. jansae (13.74 m) was detected and used to identify the base of the nJt 6b Subzone of ferreira et al. (2019) (fig. 3). the uppermost investigated sample (3.08 m) is still included in the early toarcian nJt 6 Zone due to the absence of D. striatus, which is the zonal marker of the base of the nJt 7 Zone (Mattioli & erba 1999). the Schizosphaerella decline was not observed in the Gajum Core, further indicating the absence of the Domaro lmst./Sogno fm. boundary inter- val. the Schizosphaerella and M. jansae crises were detected at the base of the fish level (26.94 m). However, as explained above, the lithostratigraphic boundary between the Domaro lmst. and the Sog- no fm. is marked by a hiatus and, consequently, the Schizosphaerella and M. jansae crises are presumably apparent and not real events in the Gajum Core. the Schizosphaerella recovery was detected at 12.89 m in the upper part of the fish level where this genus become continuous and rare to frequent. the calcareous nannofossil assemblages of the nJt 5b Subzone are dominated by S. punctulata and M. jansae. among schizosphaerellids, S. punctu- lata and “small” S. punctulata are the most abundant morphotypes (rare/frequent to common/abundant and rare/frequent to common, respectively) where- as “encrusted” S. punctulata is subordinated (rare to frequent/common). Within the genus Mitrolithus, M. jansae is relatively more abundant (rare to fre- quent) than “thin” M. jansae (extremely rare to rare/ frequent) (see range chart in appendix 3). nannofossil assemblages of the nJt 6a Sub- zone corresponding to the fish level show a drastic reduction in abundance of both Schizosphaerella and Mitrolithus (Schizosphaerella and M. jansae crises). in particular, both S. punctulata and “small” S. punctu- lata become rare whereas “encrusted” S. punctu- lata is extremely sparse, although in some samples schizosphaerellids displays a frequent/common abundance. Similarly to the Sogno Core, within the fish level M. jansae is equally represented by both normal and thin morphotypes becoming discon- tinuous and generally rare. in the fish level Lotha- ringius is the dominant genus with L. hauffii as most abundant species. Plate 2 Scale bars represent 2 µm. 1-2 - L. frodoi, 1) cross-polarized light, 2) quartz lamina, Gajum Core, sample G C22 854 (23.39 m) 3-4 - L. hauffii, 3) cross-polarized light, 4) quartz lamina, Gajum Core, sample G C6 221 (8.14 m) 5-6 - L. sigillatus, 5) cross-polarized light, 6) quartz lamina, Gajum Core, sample G C20 810 (22.19 m) 7-8 - L. umbriensis, 7) cross-polarized light, 8) quartz lamina, Gajum Core, sample G C7 275 (9. 15 m) 9-10 - L. velatus, 9) cross-polarized light, 10) quartz lamina, Gajum Core, sample G C5 160 (6.84 m) 11-12 - M. elegans, 11) cross-polarized light, 12) quartz lamina, Sogno Core, sample S3 C30 466 (26.2 m) 13-14 - M. jansae, 13) cross-polarized light, 14) quartz lamina, Gajum Core, sample G C25 995 (26.94 m) 15-16 - “thin” M. jansae, 15) cross-polarized light, 16) quartz lamina, Sogno Core, sample S3 C24 441 (24.38 m) 17-18 - “thin” M. jansae, 17) cross-polarized light, 18) quartz lamina, Gajum Core, sample G C25 993 (26.87 m) 19-20 - M. lenticularis, 19) cross-polarized light, 20) quartz lamina, Sogno Core, sample S3 C12 330 (17.16 m) 21-22 - S. cruciulus, 21) cross-polarized light, 22) quartz lamina, Gajum Core, sample G C25 995 (26.94 m) 23-24 - S. punctulata, 23) cross-polarized light, 24) quartz lamina, So- gno Core, sample S3 C20 409 (22.5 m) 25-26 - S. punctulata, 25) cross-polarized light, 26) quartz lamina, Gajum Core, sample G C15 599 (16.55 m) 27-28 - “small” S. punctulata, 27) cross-polarized light, 28) quartz lamina, Sogno Core, sample sample S3 C24 439 (24.22 m) 29-30 - “encrusted” S. punctulata, 29) cross-polarized light, 30) quartz lamina, Sogno Core, sample S3 C20 401 (22 m) 31-32 - T. patulus, 31) cross-polarized light, 32) quartz lamina, Sogno Core, sample S3 C13 335 (17.45 m) 33-34 - Watznaueria sp.1, 33) cross-polarized light, 34) quartz lamina, Sogno Core, sample S1 C24 164 (10.47 m) Toarcian Oceanic Anoxic Event in Northern Italy 547 Plate 2 Visentin S. & Erba E.548 f f f 4.35 7.11 21.39 25.42 26.94 1 2 3 5 6 7 31.18 3.0 20 21 22 23 24 25 26 27 28 29 30 31 10 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 GAJUM Core Lithostratigraphic Unit F is h Le ve l 10.60 4 S O G N O F or m at io n E ar ly T O A R C IA N D O M A R O L m st . la te st P LI E N S B A C H IA N A ge s U ni ts R O S S O A M M O N IT IC O LO M B A R D O N JT 6 Calcareous nannofossil EVENTS and ZONES Schizosphaerella recovery (12.89 m) Schizosphaerella and M. jansae crises (26.938 m) M. jansae (13.74 m) C. sup. crassus (26.932 m) D. constans (26.884 m) C. poulnabronei (26.932m) L. crucicentralis (26.92 m) L. velatus (26.92 m) D. ignotus (26.938 m) presence of L. hauffii, L. sigillatus (31.17 m) N JT 5b Samples b a fig. 3 - lithostratigraphy and biostratigraphy of the Gajum Core. Calcareous nannofossil events in red are the primary events used as zonal and subzonal boundaries. the Schizosphaerella and M. jansae crises, and the Schizosphaerella recovery are reported in blue. legend as in figure 2. Toarcian Oceanic Anoxic Event in Northern Italy 549 in the uppermost part of the fish level, cal- careous nannofossil assemblages are characterized by a general recovery in abundance of schizosphae- rellids (Schizosphaerella recovery). However, only S. punctulata and “small” S. punctulata were observed (extremely rare to frequent/common) whereas “en- crusted” S. punctulata disappears. in the uppermost studied interval represented by the rosso am- monitico lombardo fm., the family Calyculaceae shows an increase in abundance of both Calyculus and Carinolithus, passing from extremely rare to fre- quent. through the Gajum Core, genera Biscutum and Crepidolithus are characterized by rare abun- dance, whereas Bussonius, Diductius, Similiscutum, Par- habdolithus and Tubirhabdus are extremely rare. dIscussIon correlation of the sogno core to the colle di sogno section. before coring, the section outcropping at Colle di Sogno was studied for calcareous nan- nofossil biostratigraphy and paleoceanography by erba (2004), Channell et al. (2010) and Casellato & erba (2015). the stratigraphic interval studied in detail by Casellato & erba (2015) spans from the upper Pliensbachian (Domaro lmst.) to the lower toarcian (Sogno fm.) and is comparable, but not identical, to the Sogno Core succession. in fact, the Sogno Core terminated at a stratigraphic level younger than the Colle di Sogno outcrop sec- tion and, therefore, some nannofossil events were not detected in the present work (fos of Calyculus spp., Biscutum grande, Bussonius prinsii, Lotharingius fro- doi and Bussonius leufuensis). Conversely, the upper 5 meters of the Sogno Core are younger than the top of the Colle di Sogno section (Casellato & erba 2015). as expected, the stratigraphic levels of fos and los found in the Sogno Core are fully consistent with data documented for the Colle di Sogno section by Casellato & erba (2015). this is the case for the fos of C. poulnabronei, L. crucicen- tralis, Watznaueria sp. 1, the lo of M. jansae, the Schizosphaerella decline, crisis, recovery, and the M. jansae crisis. only a few events, such as the fos of L. sigillatus, D. constans and C. superbus crassus, were detected at slightly lower stratigraphic levels in the Sogno Core relative to the Colle di Sogno outcrop. relatively higher incongruities regarding the fos of L. velatus and D. ignotus were detected below the fish level in the Sogno Core but within the fish level in the Colle di Sogno section. Such strati- graphic differences are, presumably, attributable to a better preservation and higher sampling rates of the Sogno Core, concurring to improved detection of rare taxa. in fact, observed discrepancies most- ly depend on the discontinuous range of taxa in their initial ranges (see range chart in appendix 2). Moreover, the generally better preservation state of the cored material increases the possibility of finding the most delicate taxa. Due to their scantiness, the los of S. cruci- ulus and M. lenticularis were only tentatively placed in the Sogno Core in the upper part of the fish level and above the fo of Watznaueria sp. 1, re- spectively, at stratigraphic levels comparable to those documented in the Colle di Sogno section (Casellato & erba 2015). as far as the lo of the standard morphotype of M. jansae is concerned, it was placed at the base of the fish level in the Colle di Sogno outcrop, based on absolute abundances obtained in thin sec- tions (Casellato & erba 2015: fig. 8). this datum was named last Common occurrence (lCo) of M. jansae and was correlated to similar events at su- pra-regional scale (Casellato & erba 2015). Howev- er, sparse specimens of M. jansae were document- ed within the fish level outcropping at Colle di Sogno (Casellato & erba 2015), consistently with data obtained for the Sogno Core (this study). the two morphotypes of M. jansae co-occur in the fish level, although rare and discontinuous, and the lo of this species was located above the top of the black shale interval both in the outcrop and in the Sogno Core. correlation of the sogno and gajum cores Calcareous nannofossil preservation, abun- dance, and biostratigraphy display quite consistent results between the two investigated core succes- sions. the stratigraphic interval spans from the up- per Pliensbachian (Domaro lmst.) to the lower to- arcian (Sogno fm./rosso ammonitico lombardo fm.), including the lithological expression of the t-oae (fish level) in both cores (fig. 4). the bio- stratigraphic results indicate that the Sogno Core Visentin S. & Erba E.550 recovered a slightly longer interval as documented by the fos of L. sigillatus in the lowermost part and Watznaueria sp.1 in the upper part of the Sogno Core. according to their geological settings, the successions recovered in the Sogno and Gajum Cores are characterized by a continuous and in- complete record, respectively (fig. 4). in fact, the fos of C. poulnabronei, L. crucicentralis, L. velatus and D. ignotus were detected at different stratigraphic levels in the Sogno Core, but in a few cm thick in- terval in the lowermost part of the fish level of the Gajum Core, revealing the presence of a hia- tus. this is further evidenced by the absence of the Schizosphaerella decline and the occurrence of the Schizosphaerella and M. jansae crises in correspon- dence of the base of the black shale interval. fig- ure 4 illustrates the biostratigraphic correlations of the two cored sequences, providing the estimate of the interval missing in the Gajum Core. Calcare- ous nannofossil events, however, suggest that the onset of the fish level is most probably preserved. Conversely, the lowermost toarcian (and possibly the topmost Pliensbachian) is not recorded in the Gajum Core due to the first co-occurrence of D. ig- notus, C. poulnabronei, L. crucicentralis, L. velatus and in the basal part of the fish level. indeed, the Doma- ro lmst. is immediately overlain by black shales of the fish level, while in pelagic complete sections, as for instance the Sogno Core (erba et al. 2019b) and the Colle di Sogno section (Gaetani & Poliani 1978; Casellato & erba 2015), the fish level is preceded by the lowermost part of the Sogno fm. We estimate that ~ 600kys are missing adopting the zonal scheme of Mattioli & erba (1999) and the time Scale of Gradstein et al. (2012) (fig. 5). the detection of a hiatus in the lower toarcian before the t-oae black shales is a rather common feature in Western tethyan areas (Wignall 1991; Morard et al. 2003; röhl & Schmid-röhl 2005; léonide et al. 2012; Mattioli et al. 2013; Pittet et al. 2014; Menini et al. 2019; Visentin et al. 2021b). the fo of C. superbus crassus shows relatively consistent results between the two cores although this biohorizon was detected at a slightly higher stratigraphic level in the Gajum Core (within the basal part of the fish level). this minor difference is possibly imputed to the scarcity of this taxon in its initial range or might suggest an earlier onset of anoxic sedimentation at Gajum. the Schizosphaerella and M. jansae crises were detected at the base of the fish level in both the Sogno and Gajum Cores, although in the latter core this event is presumably apparent (see discussion above). Schizosphaerellid abundances are different in the two studied successions: reflecting the rela- tively higher carbonate content in a few levels of the Gajum fish level, abundances of schizosphae- rellids vary from absent to common/abundant. the Schizosphaerella recovery and the lo of M. jansae (both standard and thin morphotypes) display very consistent results in the two cored successions. comparison with the tethyan nannofossil zonation the zonal scheme of Mattioli & erba (1999) proposed as reference for tethyan and lower lati- tude sections and, more specifically, for the Mediter- ranean province, is used to discuss the nannofossil biohorizons detected in this work (fig. 5). after 20 years of nannofossil biostratigraphic investigations of lower Jurassic sections the events and zones proposed by Mattioli & erba (1999) have been test- ed and partly revised (see ferreira et al. 2019 for a synthesis). it must be emphasized that the funda- mental structure – consisting in marker events and derived biozones - remains, however, confirmed. the nannofossil zonal biohorizons recogni- zed in the Sogno and Gajum Cores are fully consi- stent with the scheme of Mattioli & erba (1999) as discussed below, from bottom to top: * the fo of L. sigillatus defining the base of the nJt 5b Subzone was detected in the uppermost Pliensbachian Domaro lmst. in both cores. as dis- cussed by Casellato & erba (2015), the original use of this event to define the Pliensbachian/Toarcian boundary has been revised to a slightly older age within the latest Pliensbachian (see also discussion in ferreira et al. 2019); * the fo of C. superbus crassus used to deter- mine the base of the nJt 6 Zone was found at the onset of the t-oae black shale interval in both cores. We specify here that, according to Visentin et al. (2021a), C. superbus crassus corresponds to C. superbus of Mattioli & erba (1999); * the lo of M. jansae was detected above and in the uppermost part of the fish level black shales in the Sogno and Gajum Cores, respectively. al- though originally this biohorizon was proposed by Mattioli & erba (1999) as a secondary event to be Toarcian Oceanic Anoxic Event in Northern Italy 551 fig. 4 - Calcareous nannofossil biostratigraphic correlation of the Sogno and Gajum Cores showing that the lowermost toarcian So- gno Fm. is missing at Gajum. In the lower part of the figure, the Sogno and Gajum Cores are contestualised on the Albenza Plateau and on the flank of the Corni di Canzo High (after Erba et al. 2019b). 1 2 3 5 6 7 8 9 10 11 13 14 15 Lithostratigraphic Unit SOGNO Core 4 12 C. sup. crassus L. sigillatus L. velatus D. ignotus 20 21 22 23 24 25 26 27 10 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 0 1 2 S O G N O F or m at io n E ar ly T O A R C IA N D O M A R O Li m es to ne la te st P LI E N S . A ge U ni t N JT 6 N JT 5 b a ** * *** 1 2 3 5 6 7 GAJUM Core Lithostratigraphic Unit F is h Le ve l 4 A ge U ni t 20 21 22 23 24 25 26 27 28 29 30 31 10 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 S O G N O F or m at io n E ar ly T O A R C IA N D O M A R O Li m es to ne la te st P LI E N S B A C H IA N R O S S O A M M O N IT IC O LO M B A R D O N JT 6 C. poulnabronei L. crucicentralis L. velatus D. ignotus L. sigillatus N JT 5b *** ** D. constans Watznaueria sp. 1 C. sup. crassus b a b a D. constans M.jansae C. poulnabronei L. crucicentralis F is h Le ve l 0 5 km 100 m 0 1 km 250 m Gajum Core Sogno Core M.jansae Visentin S. & Erba E.552 confirmed, its reproducibility makes this datum ful- ly reliable at low latitudes (Casellato & erba 2015) and we use it to separate the nJt 6 Zone into 2 subzones as suggested by ferreira et al. (2019). the zonal scheme recently proposed by fer- reira et al. (2019) for the lusitanian basin is also con- sidered here (fig. 5). events, zones, and subzones are based on detailed investigation of several lusita- nian successions, including two GSSP sections, lo- cated during the Jurassic in the Western tethys and, thus, acting as a north-south migration pathway for several organisms, including calcareous nannoplank- ton and ammonites. ferreira et al. (2019) calibrated nannofossil biohorizons against ammonite Zones (aZs) as well as against carbon and oxygen isotope excursions where possible. Moreover, the work by ferreira et al. (2019) includes an updated and revised taxonomy and a comprehensive synthesis of bio- stratigraphies from the Western tethys. as far as the marker events are concerned, the fo of C. superbus crassus (= C. superbus in ferreira et al. 2019) and lo of M. jansae are comparable with our findings and the zonation of Mattioli & Erba (1999) (fig. 5). Conversely, ferreira et al. (2019) placed the fo of L. sigillatus within the nJt 4 Zone (base of their nJt 4e Subzone) of late Pliensbachi- an age (Margaritatus AZ), thus, at a definitively older stratigraphic level relative to Mattioli & erba (1999) and our data from the Sogno and Gajum Cores. fer- reira et al. (2019) stated that the earliest specimens of L. sigillatus recorded in the upper Pliensbachian of the Rabaçal section are significantly smaller (av- erage distal shield diameter of 4.34 µm; ferreira et al. 2017) than the size range provided in the original description by Stradner (1961) (6-8 µm). neverthe- less, since these early specimens show identical diag- nostic features and their sizes are not very different from those documented in the emendation by Goy (1981), ferreira et al. (2019) considered them within L. sigillatus. We notice that ferreira et al. (2019) re- port the first common occurrence (FCO) of L. sigil- latus at the stratigraphic level of the fo of L. sigil- latus of Mattioli & erba (1999) and as found in the Sogno and Gajum Cores (fig. 5). the fo of L. crucicentralis is also older in the zonal scheme of ferreira et al. (2019), who placed this event within the lowermost emaciatum aZ at the base of the nJt 5b Subzone (late Pliensbachi- an). These authors specified that several specimens of L. crucicentralis, although with diagnostic features matching those described by Medd (1971) and emended by Grün & Zweili (1980), are significantly smaller in size. in this study, we adopted taxonomic features of L. sigillatus and L. crucicentralis consistent 184 183 181 182 D. ignotus L. velatus L. crucicentralis L. sigillatus C. poulnabronei La te P LI E N S . D om ar o Lm st . Gajum Core (this study) D. striatus W. fossacincta W. colacicchii D. ignotus L. velatus L. crucicentralis C. superbus L. sigillatus C. poulnabronei Calyculus spp. L. barozii L. hauffii M. jansae E ar ly T O A R C IA N La te P LI E N S . sp in at um te nu ic os ta tu m se rp en tin us A ge A m . Z on e Standard Tethys Biozonation (Mattioli & Erba, 1999) N an no fo ss il Z on e D. constans D. ignotus L. velatus L. crucicentralis L. sigillatus C. poulnabronei E ar ly T O A R C IA N La te P LI E N S . A ge D om ar o Lm st . S og no F or m at io n Li th os tr at ig ra ph y Watznaueria sp.1 Sogno Core (this study) L. hauffii Schizosphaerella and M. jansae CRISIS Schizosphaerella DECLINE D. constans F is h D. striatus C. superbus M. jansae Portugal Biozonation (Ferreira et al., 2019) L. crucicentralis E ar ly T O A R C IA N La te P LI E N S . em ac ia tu m po ly m or ph um le vi so ni A ge A m . Z on e m ar ga rit at us L. hauffii Z. erectus W. fossacincta D. ignotus D. ignotus FCO L. velatus L. sigillatus FCO D. constans N JT 5 N JT 6 6a 6b 5c 5b C. sup. crassus E ar ly T O A R C IA N S og no F m . R A L Schizosphaerella RECOVERY A ge Li th os tr at ig ra ph y N JT 6 5b 5a N JT 5 N JT 4 4b N JT 6 5b 5a N JT 5 N JT 4 4b 5a C. poulnabronei m ar ga rit at us C. sup. crassus N an no fo ss il Z on e N an no fo ss il Z on e 6b 6a M. jansaeM. jansae L e ve l * ** *** * ** *** ** *** fig. 5 - Comparison of the results obtained for the Sogno and Gajum Cores with the zonations of Mattioli & erba (1999) and ferreira et al. (2019). Zonal and subzonal events are reported in red. Toarcian Oceanic Anoxic Event in Northern Italy 553 with those of ferreira et al. (2019), without using differentiation based on coccolith size. therefore, we conclude that the fos of L. sigillatus and L. cru- cicentralis are diachronous, being older in the lusita- nian basin relative to the Mediterranean province in the tethys ocean. among the secondary events, the fo of C. poulnabronei detetcted in the Sogno Core is consistent exclusively with that of Mattioli & erba (1999). fer- reira et al. (2019), indeed, placed this event within the uppermost Polymorphum aZ, just after the fo of C. superbus crassus. it is possible, however, that this discrepancy is mainly the result of the discon- tinuous occurrence of this taxon in its initial range rather than diachroneity. the fo of C. poulnabronei, indeed, was reported by ferreira et al. (2019) only in the Peniche section and looking at the range charts provided in the supplementary material C. poulnabro- nei occurs sporadically, thus not excluding a poten- tially older first occurrence. Conversely, the FO of D. ignotus detected in our study shows consistency with the biozonation of ferreira et al. (2019), while Mattioli & erba (1999) assigned a younger age to this event, between the fos of C. superbus crassus and the lo of M. jansae. the reason of this discrepancy is also probably attributable to scantiness of this taxon. in fact, according to Mattioli et al. (2013), D. ignotus has a Lazarus behaviour, first occurring in the earliest toarcian, being absent from the sediments corresponding to the t-oae and subsequently oc- curring consistently from the end of the t-oae upwards. this distribution was observed not only at Peniche, but also in sections of the Mediterranean province, namely, Valdorbia in central italy (Mattioli et al. 2013), amellago in Marocco (bodin et al. 2010) and la almunia in South-eastern Spain (Menini et al. 2019). Although our findings confirm that the fo of D. ignotus precedes the onset of the t-oae black shales within the nJt 5b Subzone of Mattioli & erba (1999), it is possibly coeval with the datum of ferreira et al. (2019) within their nJt 5c Sub- zone. the stratigraphic level of the fo of D. ignotus of the Mattioli & erba (1999) zonation appears to be equivalent to the fCo of this taxon in the zonal scheme of ferreira et al. (2019) (fig. 5). as far as the fo of L. velatus is concerned, our finding is inconsistent with the zonal schemes of ferreira et al. (2019) and Mattioli & erba (1999) who proposed an older (shortly before the fo of Z. erectus) and younger (between the fos of C. superbus crassus and D. ignotus) age, respectively. the fo of D. constans was not considered in Mattioli & erba (1999), and our results fit well with the datum pro- posed by ferreira et al. (2019). in our study, the fo of Watznaueria sp. 1 cor- responds to the fo of W. fossacincta in both the zo- nations by Mattioli & erba (1999) and ferreira et al. (2019). the taxonomic characterization of Watznau- eria sp. 1 described by Cobianchi (1992) and Casel- lato & erba (2015) are applied here. Differently from the biozonation of Mat- tioli & erba (1999), the fo of W. colacicchii, re- ported between the lo of M. jansae and the fo of Watznaueria sp. 1 (= W. fossacincta), was not found in the Sogno and Gajum Cores as this taxon is absent in the investigated interval. also Casellato & erba (2015) did not observe W. colacicchii in the Colle di Sogno section. ferreira et al. (2019) place the fo of W. colacicchii in the lower part of the nJt 7 Zone at the base of the bifrons aZ, thus at a stratigraphic level younger than the zonation of Mattioli & erba (1999) and the interval here investigated. nannofossil events reported by ferreira et al. (2019), but not by Mattioli & erba (1999), and not identified in the Sogno and Gajum Cores are, from the oldest to youngest, the fos of Axopodorhabdus atavus, Zeugrhabdotus erectus, Ethmorhabdus spp. and lo of Mazaganella protensa. this major difference is presumably linked to nannoplankton provincialism during the early Jurassic which further supports the necessity of two biozonation schemes applicable in the lusitanian basin and the Mediterranean Prov- ince, respectively. suMMary and conclusIons High-resolution calcareous nannofossil bio- stratigraphy carried out in two cores drilled in the Lombardy Basin allowed the identification of nine (Sogno Core) and seven (Gajum Core) events in the uppermost Pliensbachian Domaro lmst. and the lower toarcian Sogno fm. and rosso ammonitico lombardo fm. as expected, relative to the Colle di Sogno outcrop calcareous nannofossils are better preserved in the Sogno Core and the high-resolution sampling resulted in improved detection of events. the correlation of the Sogno and Gajum nanno- fossil biostratigraphy shows that, according to their geological settings, the succession recovered in the Visentin S. & Erba E.554 Sogno Core (located on the albenza Plateau) is con- tinuous while a hiatus of ~600 kyrs was detected at the base of the fish level in the Gajum Core. How- ever, nannofossil data suggest that the black shale interval is complete whereas the lowermost toarcian basal portion of the Sogno fm. (and possibly the topmost Pliensbachian Domaro lmst.) is missing in the Gajum Core. following the standard nannofossil zonation for the Mediterranean Province in the tethys ocean (Mattioli & erba 1999), the nJt 5 and nJt 6 Zones were identified in both the Sogno and Gajum Cores. furthermore, as proposed by ferreira et al. (2019), we adopt the lo of M. jansae to separate the nJt 6a and nJt 6b Subzones. in the Sogno and Ga- jum Cores, the fish level, regionally considered the lithological expression of the t-oae, results to be largely constrained between the fo of C. superbus crassus and the lo of M. jansae (nJt 6a Subzone), although black shales continue for a short inter- val above the lo of M. jansae in the Gajum Core. therefore, nannofossil biostratigraphy suggest that anoxic conditions did not cease simultaneously in nearby sites within the lombardy basin. other nannofossil events useful for the bio- stratigraphic characterization of the t-oae are the fo of D. constans and the Schizosphaerella - M. jansae crises at the base of the fish level and the Schizo- sphaerella recovery following the lo of M. jansae. the comparison between the results of our investigation and the zonal schemes established for the tethys ocean (Mattioli & erba 1999; ferreira et al. 2019) revealed consistencies and differences. Confirming the Mediterranean affinity of the stu- died sites, calcareous nannofossil biohorizons identi- fied in the Sogno and Gajum Cores are mostly cohe- rent with the biozonation of Mattioli & erba (1999), that is, however, here improved separating the nJt 6 Zone into two subzones. the zonation of ferreira et al. (2019) establi- shed for the Western tethys lusitanian basin is only partially reproducible, in practice only for the nJt 6 zonal markers, thus confirming the supra-regional value of the fo of C. superbus crassus and the lo of M. jansae. another event traceable from the Medi- terranean area to the lusitanian basin is the fo of Watznaueria sp. 1 in the nJt 6b Subzone. Some discrepancies might be explained by the abundance/rarity of taxa in different areas. in fact, ferreira et al. (2019) report the fCo of L. sigillatus at the stratigraphic level of the fo of L. sigillatus of Mattioli & erba (1999) and as found in the Sogno and Gajum Cores. Similarly, we detected the fo of D. ignotus within the nJt5b Subzone of Mattioli & erba (1999) at a stratigraphic level comparable to that of Ferreira et al. (2019), but definitively before the datum of the Mattioli & erba (1999) whose fo of D. ignotus appears to be equivalent to the fCo of this taxon in the zonal scheme of ferreira et al. (2019). Contrarily to Mattioli & erba (1999), we did not observe W. colacicchii in the ntJ 6b Subzone as in the zonation of ferreira et al. (2019), who report the fo of W. colacicchii in the lower part of the nJt 7 Zone at the base of the bifrons aZ, thus at a stra- tigraphic level younger than the investigated interval. Our finding of the FO of L. velatus is inconsi- stent with the zonal scheme of ferreira et al. (2019) while Mattioli & erba (1999) proposed a younger age between the fos of C. superbus crassus and D. ignotus. this disparity along with the absence of se- veral marker species used in the zonation of ferreira et al. 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