Masahiro Oba

Effects of soil erosion and anoxic-euxinic ocean in the Permian-Triassic marine crisis.

The largest mass extinction of biota in the Earth’s history occurred during the Permian–Triassic transition and included two extinctions, one each at the latest Permian (first phase) and earliest Triassic (second phase). High seawater temperature in the surface water accompanied by euxinic deep-intermediate water, intrusion of the euxinic water to the surface water, a decrease in pH, and hypercapnia have been proposed as direct causes of the marine crisis. For the first-phase extinction, we here add a causal mechanism beginning from massive soil and rock erosion and leading to algal blooms, release of toxic components, asphyxiation, and oxygen-depleted nearshore bottom water that created environmental stress for nearshore marine animals. For the second-phase extinction, we show that a soil and rock erosion/algal bloom event did not occur, but culmination of anoxia–euxinia in intermediate waters did occur, spanning the second-phase extinction. We investigated sedimentary organic molecules, and the results indicated a peak of a massive soil erosion proxy followed by peaks of marine productivity proxy. Anoxic proxies of surface sediments and water occurred in the shallow nearshore sea at the eastern and western margins of the Paleotethys at the first-phase extinction horizon, but not at the second-phase extinction horizon. Our reconstruction of ocean redox structure at low latitudes indicates that a gradual increase in temperature spanning the two extinctions could have induced a gradual change from a well-mixed oxic to a stratified euxinic ocean beginning immediately prior to the first-phase extinction, followed by culmination of anoxia in nearshore surface waters and of anoxia and euxinia in the shallow-intermediate waters at the second-phase extinction over a period of approximately one million years or more. Enhanced global warming, ocean acidification, and hypercapnia could have caused the second-phase extinction approximately 60 kyr after the first-phase extinction. The causes of the first-phase extinction were not only those environmental stresses but also environmental stresses caused by the soil and rock erosion/algal bloom event.

Short-term euxinia coinciding with rotaliporid extinctions during the Cenomanian-Turonian transition in the middle-neritic eastern North Atlantic inferred from organic compounds

Oceanic anoxic event 2 (OAE2), which occurred during the Cenomanian-Turonian (C-T) transition and lasted 1 m.y., is characterized by a positive global carbon isotopic excursion and stepwise extinctions in marine biota. To examine temporal variations in the dissolved oxygen content of the water column, shallow-marine C-T sediments from northern Spain were analyzed for concentrations of dibenzothiophenes, which are indicators of euxinic depositional environments, and 2,3,6-trimethylarylisoprenoids, which probably indicate photic-zone euxinia. The positive excursion in δ 13 C values of carbonates is accompanied by short-term (10 3 –10 4 yr) and long-term (10 5 yr) increases in dibenzothiophene and 2,3,6-trimethylarylisoprenoid concentrations, suggesting that the bottom water and photic zone of the eastern marginal sea of the North Atlantic Ocean were euxinic. Two of the short-term increases in organic compound concentrations took place just after the last occurrence of the planktonic foraminifers Rotalipora greenhornensis and R . cushmani . These transient maxima indicate that the extinction of both planktonic foraminifers was due to short-term OAEs lasting 10 3 –10 4 yr.

Oldest Fossil Records of Marine Protists and the Geologic History Toward the Establishment of the Modern-Type Marine Protist World

The oldest marine protist fossil goes back 1.8 Ga (Statherian, Paleoproterozoic), and the oldest dinosterane biomarkers 1.6 Ga (Calymmian, Mesoproterozoic). The probable heterotrophic agglutinated microfossil appeared when marine metazoans appeared in the Ediacaran. Multichambered foraminifers appeared around the start of biomineralization in Small Shelly Fossils in the early Cambrian. The first fossilizable radiolarian polycystine is likely to have appeared in the period of the Cambrian Explosion. After the initial appearance period, the emergence of fossilizable skeleton formative ability was concentrated in five short geological time intervals: (1) the Middle to Late Devonian for calcareous benthic foraminifers; (2) the Carnian to the Rhaetian (Triassic) for the “switching on” of fossilizable dinoflagellate cysts, nannoliths, coccoliths and calcareous cysts, and probably the molecular appearance of diatoms; (3) the Toarcian–Aalenian Ages for diversified dinoflagellates and coccolithophores, the establishment of symbiosis in radiolarian Acantharia and the appearance of planktic lifestyle in foraminifers; (4) the Albian–Maastrichtian Ages for the rapid accumulation of coccolithophores, the start of skeletogenesis both in silicoflagellates and marine centric diatoms, molecular appearance of both araphid and raphid diatoms, and the appearance of fossilizable araphid diatoms; and (5) the middle to late Eocene for the appearance of fossilizable raphid diatoms and radiolarian colonial collodarians and the continuous occurrences of ebridians. The establishment of the modern-type marine protist world was concluded in the late Eocene by the appearance of collodarians, the continuous occurrences of ebridians, and the substituted silicon precipitation marine protists as diatoms.