Multiphase response of palynomorphs to the Toarcian Oceanic Anoxic Event (Early Jurassic) in the Réka Valley section, Hungary

Abstract

Major palaeoenvironmental and palaeoceanographical changes occurred during the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE), due to a perturbation of the global carbon cycle and a crisis in marine ecosystems. The sequence of environmental change and regional differences during the T-OAE are not yet fully understood and organic-walled phytoplankton and other palynomorphs are well-suited, but under-utilised, in research into this event. Based on quantitative palynological analyses from a black shale-bearing succession at Réka Valley in the Mecsek Mountains of southwest Hungary, five sequential palynomorph assemblages are distinguished. These reveal major shifts in organic-walled phytoplankton communities, driven by palaeoenvironmental changes. In addition, palynofacies analysis helped to document changes in the composition of sedimentary organic matter, and to quantify the terrestrial input. Assemblage 1 is characterised by a moderately diverse phytoplankton community and high levels of terrestrial palynomorphs. Assemblage 2 records a significant peak of the euryhaline dinoflagellate cyst Nannoceratopsis. Assemblage 3 is distinguished by dominance of highly opportunistic prasinophytes and the temporary disappearance of all dinoflagellate cyst taxa. Assemblages 4 and 5 represent distinctive phases of a prolonged recovery phase with low diversity phytoplankton assemblages and intermittently high levels of terrestrially-derived palynomorphs. The successive disappearance of phytoplankton taxa and the gradual takeover by opportunistic euryhaline species at the onset of the T-OAE were related to several phenomena. These include reduced salinity in the surface waters, establishment of a stable pycnocline and deterioration of nutrient recycling, followed by oxygen deficiency throughout much of the water column. The high amount of terrestrially-derived palynodebris indicates intense runoff and freshwater input, driven by the early Toarcian warming and the enhanced hydrological cycle. Comparison with coeval European successions proves that the palaeoenvironmental changes during the T-OAE were not entirely synchronous, and local factors played a crucial role in influencing phytoplankton communities. In the Mecsek Basin, regional freshening of the surface waters and increased terrestrial input due to the proximity of the hinterland had a greater influence on phytoplankton communities compared to the open oceanic setting of the Tethys to the south.

The Early Jurassic palynostratigraphy of the Lusitanian Basin, western Portugal

“…These relative low diversity dinoflagellate cyst assemblages are broadly typical of upper Pliensbachian to Toarcian successions throughout Europe and adjacent areas (Riding, 1984a;Riding et al, 1991;Koppelhus and Nielsen, 1994;Bucefalo Palliani and Riding, 1997a,b;1999a;2000;Baranyi et al, 2016). Furthermore, the dinoflagellate cyst association reported herein is consistent with previous research on the Lower Jurassic palynology of the Lusitanian Basin (Davies, 1985;Oliveira et al, 2007b;Barrón et al, 2013).…”

“…Luehndea cirilliae is present in the upper Pliensbachian and lower Toarcian of Hungary (Baldanza et al, 1995;Baranyi et al, 2016). This species was also reported in the Pliensbachian of Brenha by Bucefalo Palliani and , but this taxon was not found in this study, or by Correia et al (2017a,b).…”

New multiproxy record of the Jenkyns Event (also known as the Toarcian Oceanic Anoxic Event) from the Mecsek Mountains (Hungary): Differences, duration and drivers

“…Major environmental change affected the plankton communities before and after the Jenkyns Event, but a largely invariable marine algal source for the OM during the CIE is further supported by recent results of Baranyi et al . (); this study of palynomorphs in the Réka Valley section documents a significant change in the dinoflagellate assemblage to an impoverished Nannoceratopsis ‐dominated community, broadly coeval with the initial two steps of the CIE preceding the onset of black shale deposition. This stage was followed by the temporary disappearance of all dinoflagellates, and high abundance of sphaeromorphs, regarded to belong to the opportunistic group of prasinophyte algae, together with a lack of terrestrially derived phytoclasts.…”

“…Although the carbonate fraction was affected by diagenetic overprint, the organic matter yielded negative δ 13 C org values between −28·6‰ and −30·9‰ (Varga et al ., ), raising the possibility of correlation with the minima of other European sections. A clay mineralogical study confirmed elevated weathering under hot and humid conditions (Raucsik & Varga, ), whereas changes in palynomorph assemblages were used to infer the sequence of environmental and biotic change (Baranyi et al ., ). Despite these advances, detailed comparison of this section with other coeval sedimentary records of the Jenkyns Event has been hindered by insufficient biostratigraphy [only preliminary ammonoid and calcareous nannoplankton studies (Baldanza et al ., ) exist] and the low resolution of previous carbon isotope data (Varga et al ., ).…”

Redox conditions and ecological resilience during Oceanic Anoxic Event 2 in the Western Interior Seaway

“…10) that have been considered 'disaster species' by Tappan (1980) due to their abundance in sediments deposited in low-oxygen conditions associated with environmental perturbations and biotic turnovers (e.g. van de Schootbrugge et al, 2005;Baranyi et al, 2016). Indeed, within this study high C 28 sterane values correspond with less oxygenated conditions (Fig.…”