Australia: The Land Where Time Began
Triassic mass extinctions, poisoning of shallow seas after the end of the end-Triassic extinctions by hydrogen sulphide
According to the authors1 there were at least 5 mass extinction events that interrupted the evolution of life on Earth in the course of the past 600 My. Of these 1 occurred at the close of the Triassic. Massive volcanism and the burning of biomass led to a period in which the levels of CO2 in the atmosphere were high, the athors1 suggesting that most mass extinction scenarios indicate that it was the resulting perturbations of the environment that accounted for the loss of biodiversity in the oceans and on the land. In this paper the authors1 have based their reconstruction of changes in the shallow marine ecosystems of the Tethys Ocean, and redox chemistry in the oceans from earliest Jurassic black shales from the Hettangian age, that have been found in Germany and Luxemburg. Isorenieratane, a biomarker, is a fossilised pigment that is produced by green sulphur bacteria, was present at increased concentration in the shales. It is suggested by the abundance of green sulphur bacteria that there were prolonged periods in which the concentrations of hydrogen sulphide were high in the photic zone. Green algae, an indicator of anoxia, also proliferated in this period. It has been suggested by the authors1 that shallow epicontinental seas, that were hotspots of biodiversity in the oceans of the Mesozoic, were repeatedly affected by poisoning events caused by hydrogen sulphide that possibly slowed the marine ecosystems' recovery in the Early Jurassic.
Many benthic organisms were affected by the mass extinction event that occurred 201.6 Ma at the Triassic/Jurassic boundary (T/J boundary), such as scleractinian corals, calcareous algae and megalodont bivalves, as well as nektonic predators; ammonites undergoing a high diversity loss at the genus level, the survivors suggested by the authors1 to be the energy-saving passive floaters. The radiolaria, a dominant planktonic group of the Triassic, suffered a dramatic setback at the T/J boundary. The authors suggest it is uncertain whether the higher taxa of the oceans were affected by the extrinsic perturbations that included ocean acidification and anoxia that resulted from the increased partial pressure of CO2 (pCO2) or from intrinsic changes in the ecosystem that affected the marine food web base leading to the extinction of phytoplankton and zooplankton that greatly reduced or altered the primary production.
Palynological investigations of sections of the T/J boundary from across Europe have shown that there was a major turnover among marine phytoplankton at that time. There was a high extinction rate primarily among lineages of dinoflagellates and coccolithophorids that contained red plastids at the T/J boundary after they initially radiated rapidly during the Late Triassic. 3 species from 2 genera survived the T/J boundary extinction out of 10 genera of dinoflagellates that produced cysts, which, according to the authors 1 suggests an extinction rate of more than 80 % at the genus level. The record is even worse for the calcareous nannoplankton, with only a sparse population of the enigmatic Schizosphaerella punctulata in the oceans of the Early Jurassic. Nearly 10 My later, in the Pliensbachian, dinoflagellates and coccolithophorids began to recover rapidly. The authors1 suggest the T/J boundary was a major evolutionary bottleneck for red eukaryotic phytoplanktonic groups, these groups eventually becoming the dominant groups of primary production in the oceans of the present, though their rise was delayed by the conditions that prevailed at the T/J boundary.
At the T/J boundary the prevailing environmental conditions in the oceans appear to have favoured taxa of phytoplankton that contained green plastids, an example being prasinophyte green algae. It has been found that the rapid, widespread change from red to green alga taxa at the T/J boundary occurred in the Hettangian, evidence for which has been found in England, Denmark, Germany, Austria and Hungary. There was a resurgence of ancient acritarchs in the Early Jurassic as the marine food webs were reassembled, as had happened following the end-Permian mass extinction event. It has been suggested that the dominance of phytoplankton with green plastids was symptomatic of an increase of the pCO2 levels in the Early Jurassic, though the authors1suggest it could possibly have been driven by the bio- availability of trace elements that were redox-sensitive. According to the authors1 they used the abundance of biomarkers, and records of nitrogen isotopes and trace elements, and the palynological recprds in pristine core material from Germany and Luxemburg to study the link between changes that occurred in primary production, and redox condition changes across the T/J boundary.
The Mariental-1 core that came from north Germany and the Rosswinkel FR 204-201 core from Luxemburg, which contained a lithological record of black shale deposition that occurred repeatedly during the Late Triassic and Early Jurassic, provided the core material for their study. Age-diagnostic palynomorphs, especially pollen and the cysts of dinoflagellates, allowed the recognition of the T/J boundary in both cores. In the Mariental-1 core there is an expanded succession of the latest Rhaetian clay, silt and sandstone containing evidence of terrestrial ecosystem perturbations that occurred in the form a fern-spike interval (Triletes Beds; ref. 16; Fig 2), situated above the Cortorta Beds of Middle Rhaetian age. An erosional hiatus at the T/J boundary, in the Rosswinkel core, is marked by the absence of Argiles de Levallois, equivalent to the Triletes Beds in Luxemburg. Diagnostic palynomorphs from the latest Rhaetian are present beneath a transgressive lag deposit at the T/J boundary. A long-term sea level rise of second order leading to flooding on a continent-wide scale of epicontinental basins during the Early Jurassic is marked by this transgressive surface that is also present in the Mariental-1 core material.
Green sulphur bacteria - repeated photic-zone euxinia
The extent of euxenic conditions (zero O2 + free H2S) across the T/J boundary was evaluated by the authors1 using the temporal distribution and concentration of Isorenieratane as a biomarker. Isorenieratane, a fossil derivative of isorenieratene, a carotenoid, a specific pigment that is produced uniquely by brown strains of the Chlorobiaceae (green sulphur bacteria). Being obligatory anaerobic photolithoautotrophs, green sulphur bacteria require light and H2S simultaneously for growth. The shallowing of euxenic conditions into the photic zone (photic zone euxinia) is indicated by the presence of isorenieratane. In the Mariental-1 core there are no detectable amounts of isorenieratane and no diagenetic products such as aryl isoprenoids, in spite of the very high total organic carbon (TOC) with values up to 20 % being present in shales with a high content of organic matter below the T/J boundary, indicating that burial of organic carbon during the latest Rhaetian under conditions of high rates of carbon accumulation, and the resultant low levels of oxygen, though with no shallowing of the chemocline.
In the Mariental-1 and Rosswinkel cores the black shales, of Hettangian age, deposited directly after the T/J boundary, are fundamentally different. Isorenieratane first appears in the Mariental-1 core at 100.23 m, the onset of the PZE being suggested by this to possibly have been episodic, at least initially, a macrobenthic assemblage, that was moderately diverse, being present in this interval, brief repopulation apparently being allowed. Concentrations of isorenieratane increase abruptly from 98.40 m reaching a maximum of 29 μg g- TOC. By the time this concentration of isorenieratane is reached there is a complete absence of benthic fauna and bioturbation has stopped, between 98.2 m and 79.0 m; in ref.201. At 72.2 m isorenieratane was detected in small amounts, though the lack of benthic invertebrates and trace fossils suggest conditions continued to be unfavorable. There are at least 4 intervals in the Hettangian and Lower Sinemurian in which the concentrations of isorenieratane are enriched, as indicated by the Rosswinkel core the presence of only rare macrofossils is consistent with the 4 intervals in which isorenieratane is present. The first appearance of isorenieratane in the Rosswinkel core is at 65.4 m in grey laminated shales, and at the top of this black shale interval there is a second peak.
In the sandstone composing then Luxemburg Sandstone Formation any isorenieratane present is at concentrations below the detection limit, though large amounts of isorenieratane (>150 μg g- ) is again present in intercalations of grey and black marl from the Upper Hettangian and Lower Sinemurian. Directly following the T/J boundary many basins across the European epicontinental seaway are characterised by repeated phases of PZE. According to the authors1 their results are compatible with those obtained from the Frick section in Switzerland in which there are 4 levels in the insect marl from the Middle Hettangian and the arietes limestone from the Lower Sinemurian that contain high levels of isorenieratane and aryl isoprenoids. Biomarkers for green sulphur bacteria have not yet been found in England in the black shales of the Watchett Beds from the Early Hettangian of the preplanorbis beds that are basal layers in the Blue Lias Formation, though euxinia is indicated by paper-shale appearance, high levels of pyrite framboids of small size, together with the lack of benthic fauna, all point to euxinia in the water column at these locations. Samples of the black shale, of earliest Hettangian age, at the base of the Fernie Formation from 3 sections of British Columbia, Black Bear Ridge, Pardonet Creek and Ne-Parle-Pas Point, that were part of the margin of the Panthalassa Ocean were analysed by the authors1. These sections that had been tectonically deformed contained no isorenieratane, though they did contain large amounts of unresolved complex mixtures, with no C20+ compounds, the results indicating a high thermal overprint. Samples from the Kennecott Point section on the Queen Charlotte Islands, produced similar results that were attributed to exposed outcrop samples that had been biodegraded.
The authors1 suggest it is not likely the isorenieratane present in their samples was derived from green sulphur bacteria in microbial mats on the sediment-water interface and in the photic zone in water of limited depth, in spite of the nature of the shallow marine sediments that were analysed. Sedimentary facies from both localities cover a range from successions in which deltaic sediments alternate with siliciclastic sediments of shallow marine origin, and a deepening trend that was progressive into the Lower Jurassic, and no evidence of microbial deposits at either locality. According to the authors1 strong evidence exists for complete restructuring of redox conditions in the water column spanning the J/T boundary, as indicated by changes of the nitrogen-isotope and trace element records, and assemblages of phytoplankton.
The water column - redox state changes
In the Mariental-1 black shales, of Rhaetian age, nitrogen isotope values are higher (δ15Norg) than those of the Hettangian black shales. In the Rhaetian of southern Germany and England there are also enhanced values of 15N. Oxygen concentrations in the water column strongly regulate the nitrogen isotope signature variations in the oceans of the present. Denitifcation is favoured under suboxic conditions as a result of the deposition of organic matter in which the δ15Norg is enriched as the lighter 14NO3- molecules are strongly selected for by the denitrification process. If it is assumed complete nitrate utilisation occurs these δ15Norg are translated to sedimentary records of organic matter. Sedimentary organic matter with lower δ15Norg values are generated by processes with lower fraction factors, such as nitrogen fixation, nitrification and assimilation, such processes are allowed to operate when oxygen levels are either high or in anoxic conditions. Suboxic conditions in the Rhaetian resulting from most, though not all, of the oxygen being consumed by high levels of primary production are implied by the higher nitrogen isotope values. The nitrogen isotope values decrease for the first time at the Triletes Beds base, which is concomitant with a primary production decrease and the onset of extinction. In the upper part of the Triletes Beds values again increase then remain stable at about 3 ‰ for most of the Hettangian. The authors1 suggest this may indicate that productivity increased slightly compared with the uppermost Rhaetian. During the lowermost Hettangian there was a small negative excursion of the δ15Norg that coincided with peaks of isorenieratane, nitrogen fixation being the dominant process during these intervals.
According to the authors1 the trace element record indicates there was a progressive change from suboxic conditions in the Triassic to euxinic conditions in the Jurassic. Ni/(Ni+V) and V/Cr values at Mariental are well correlated with the trend in nitrogen isotopes (Supplementary Fig. 1). Increasingly more of the redox-sensitive vanadyl porphyrins, that are indicated by both proxy records, were preserved relative to the nickel porphyrins that are less redox sensitive, independent from from weathering flux. Sections of the marginal basins of the Panthalassa Ocean have similar trends that have been documented, an example being the Toarcian Oceanic Anoxic Event. Fe and Zn, both required as trace elements in the photosystem in the green-plastid superfamily, resulted from strong redox gradients in the water column in the Early Jurassic. The results from the Hettangian black shales of southern Germany are consistent with these results.
Green oceans, black shales in the Early Jurassic
When assemblages were compared the authors1 found that the contrasting modes of deposition of black shales before and following the T/J boundary are even greater. It is known that a rich suite of dinoflagellate cysts are preserved in sediments from the Upper Rhaetian, and these sediments are enriched in TOC. It has been found by palynological analysis of samples from Mariental and Rosswinkel that Rhaetogonyaulax rhaetica and Dapocodinium priscum both dinoflagellate cysts, are numerically dominant among the phytoplankton taxa that are preserved in the latest Rhaetian. It is suggested by the diverse communities of cyst-producing dinoflagellates that during the Rhaetian the epicontinental seas were largely oxygenated, as if the water was anoxic at the sea floor-water boundary the resting stage of the dinoflagellates, that are benthic, would not have been able to hatch. Phytoplankton communities underwent important changes in the sulphidic conditions that began immediately following the T/J boundary. There are not many dinoflagellate cysts in the Mariental and Rosswinkel deposits from the Hettangian, those that are present being dominated by a single species, Beaumontella langii. Prasinophyte green algae, such as Tasmanites, Cymatiosphaera, and Pleurozonaria, also including ancient acritarchs such as Micrhystridium, Veryhachium and Baltisphaeridium. According to the authors1 this major turnover of phytoplankton is recognised in Denmark, southern Germany, England, Hungary and Austria, as well as in northern Germany and Luxemburg.
They suggest there are a number of mechanisms that could possibly have driven the success of the chlorophytes in the Early Jurassic. The loss of nitrite and ammonium by anammox process, that result in surface waters being enriched in ammonium, may have been limited by the presence of sulphidic waters in some parts of the photic zone. As prasinophytes have a high nutrient-uptake quota for both ammonium and nitrate ions these conditions would have been beneficial to prasinophytes compared with dinoflagellates. The life cycle of prasinophytes is entirely restricted to the water column, their vegetative cysts (phycomata) remain in the water column until the motile cells are released, only then sinking to the ocean floor, unlike the dinoflagellates in which their cysts stay on the sea floor until they hatch.
Pulses of rising sea levels have been linked to black shale deposition before and after the T/J boundary, though during the Rhaetian sea level rise didn't result in PZE, indicating that there must have been other factors acting in concert for the redox state changes in the water column to be driven during the Early Jurassic. In the Hettangian black shales the high concentrations of isorenieratane are on a scale that is comparable to those of other black shale events such as occurred in the Cenomanian-Turonian Oceanic Oxygen Event. The authors1 suggest it may be even more significant that such high concentration values have also been found in the deposits from Permian/Triassic boundary about 252 Ma. They suggest the massive release of carbon dioxide that was released to the atmosphere during the volcanic activity of the Siberian Traps flood basalt province. At the T/J boundary a very similar scenario is suggested to have applied during the mass extinction event that temporarily overlaps the volcanic activity of the Central Atlantic Magmatic Province, a large igneous province that was emplaced when rifting began in Pangaea. The authors1 suggest that during the Early Jurassic the development of the PZE is compatible with an estimated doubling to quadrupling of pCO2 in the atmosphere as a result of the emplacement of the Central Atlantic Magmatic Province. A hydrological cycle that was accelerated and strong stratification of salinity in the epicontinental basins were products of the global warming that resulted. Recovery from the mass extinction of the end-Permian occurred rapidly on land, the conifer forests returning and the theropod dinosaurs radiated during the earliest Hettangian. Repeated poisoning of shallow seas by hydrogen sulphide, the very places that were hotspots of biodiversity in the Mesozoic resulted in the recovery of benthic and planktonic communities being delayed for a considerable length of time. It wasn't until about 10 My later in the Pliensbachian of the Early Jurassic that red-phytoplankton dinoflagellates and coccolithophorids recovered.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|