Do spores herald the Toarcian Oceanic Anoxic Event?

The Cenomanian/Turonian boundary in light of new developments in terrestrial palynology

“…This peculiar characteristic, though, could be the result of a taphonomic artefact rather than environmental stress; therefore, further studies are needed to validate this external trait. The few occurrences of spore dyads and tetrads were less than 1% on average and, therefore, below the baseline frequency of 3-5% above which environmental influence on spore and pollen morphology can be assumed 1,8,12,63 . They reflect the natural variation of spore and pollen morphology (e.g., 64,65 ).…”

“…and Classopollis classoides were found as welded grains or as grains close/attached to one another. Classopollis classoides in the Cassis section had somewhat comparable sizes to dispersed Classopollis, which ruled out polyploidy 11,12,59 . Furthermore, this genus is known to frequently occur in tetrads 60 in low-energy shelf environments or oceanic sediments that were deposited far from land 61,62 .…”

“…These changes include for instance, a shift in plant-community dominance structure from pollen-dominated to a spore-dominated ecosystems (i.e. spore "spike") (e.g., [78][79][80][81][82][83][84] ), and the occurrence of malformed pollen and spores beyond background values (e.g., 2,5,[7][8][9][10][11][12] ). In both cases, the harmful effects of LIPs, such as halocarbons, SO 2 , and phytotoxicity, were ascribed as the potential causes 7,8,85 .…”

“…In palaeopalynology, teratomorphic spores and pollen have been successfully employed as a proxy for increased environmental disturbance. The occurrence of increased malformation in sporomorphs represents one of the more exotic proofs of stress response in biotas during the Devonian/Carboniferous boundary (DCB, 2 ), the Permian/Triassic boundary (PTB, [3][4][5] ), the Smithian/Spathian boundary (SSB, 6 ), the Triassic/Jurassic boundary (TJB, [7][8][9][10] ) and the Toarcian Oceanic Anoxic Event (T-OAE, 11,12 ). Although much progress has been made in studying the vegetation response to environmental perturbations throughout the Mesozoic, the Cretaceous, particularly the Cenomanian/Turonian boundary, is still poorly studied and understood.…”

“…The boundary interval is marked by an extinction event and turnover in planktonic foraminifera and radiolarians [16][17][18][19][20][21] , by an increase in sea level [22][23][24][25][26] , and by widespread dysoxic to anoxic conditions known as the Oceanic Anoxic Event 2 (OAE 2). The widespread anoxia-euxinia resulted in the depletion of isotopically light 12 C in seawater due to increased organic matter burial, globally expressed by a positive carbonate excursion (CIE) in both marine and terrestrial organic and inorganic carbon (e.g., [27][28][29][30][31] ). Along with the widespread anoxia, the onset of OAE 2 was accompanied by a rise in sea surface temperature (SST), known as the "Cretaceous thermal maximum", which most likely occurred as abrupt warming associated with an increase in atmospheric pCO 2 (e.g., 24,[32][33][34][35][36][37] ).…”