Stephen P. Hesselbo

Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13 C (refs 1,2,5,6). Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that—as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago—the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.

Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography

Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios (87Sr/86Sr) and carbon-isotope ratios (δ13Ccarb and δ13Corg) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (δ18O) in carbonate, sulphur-isotope ratios (δ34S) in carbonate-hosted sulphate, nitrogen-isotope ratios (δ15Norg) in organic matter, osmium-isotope ratios (187Os/188Os) in black shales and neodymium-isotope ratios (143Nd/144Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ13C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium-isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon-isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: A link to initiation of massive volcanism?

Mass extinction at the Triassic-Jurassic (Tr-J) boundary occurred about the same time (200 Ma) as one of the largest volcanic eruptive events known, that which characterized the Central Atlantic magmatic province. Organic carbon isotope data from the UK and Greenland demonstrate that changes in flora and fauna from terrestrial and marine environments occurred synchronously with a light carbon isotope excursion, and that this happened earlier than the Tr-J boundary marked by ammonites in the UK. The results also point toward synchronicity between extinctions and eruption of the first Central Atlantic magmatic province lavas, suggesting a causal link between loss of taxa and the very earliest eruptive phases. The initial isotopic excursion potentially provides a widely correlatable marker for the base of the Jurassic. A temporary return to heavier values followed, but relatively light carbon dominated the shallow oceanic and atmospheric reservoirs for at least 600 k.y.

Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals

The marine sedimentary record exhibits evidence for episodes of enhanced organic carbon burial known as ‘oceanic anoxic events’ (OAEs). They are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Causal mechanisms for the enhancement of organic carbon burial during OAEs are still debated, but it is thought that such events should draw down significant quantities of atmospheric carbon dioxide. In the case of the Toarcian OAE (∼183 million years ago), a short-lived negative carbon-isotope excursion in oceanic and terrestrial reservoirs has been interpreted to indicate raised atmospheric carbon dioxide caused by oxidation of methane catastrophically released from either marine gas hydrates or magma-intruded organic-rich rocks. Here we test these two leading hypotheses for a negative carbon isotopic excursion marking the initiation of the Toarcian OAE using a high-resolution atmospheric carbon dioxide record obtained from fossil leaf stomatal frequency. We find that coincident with the negative carbon-isotope excursion carbon dioxide is first drawn down by 350 ± 100 p.p.m.v. and then abruptly elevated by 1,200 ± 400 p.p.m.v, and infer a global cooling and greenhouse warming of 2.5 ± 0.1 °C and 6.5 ± 1 °C, respectively. The pattern and magnitude of carbon dioxide change are difficult to reconcile with catastrophic input of isotopically light methane from hydrates as the cause of the negative isotopic signal. Our carbon dioxide record better supports a magma-intrusion hypothesis, and suggests that injection of isotopically light carbon from the release of thermogenic methane occurred owing to the intrusion of Gondwana coals by Toarcian-aged Karoo-Ferrar dolerites.

Carbon-isotope composition of Lower Cretaceous fossil wood: Ocean-atmosphere chemistry and relation to sea-level change

The carbon-isotope composition of fossil wood fragments, collected through a biostratigraphically well-constructed Aptian (Lower Cretaceous) shallow-marine siliciclastic succession on the Isle of Wight, southern Britain, shows distinct variations with time. The results indicate that the stratigraphic signature of {delta}{sup 13}C{sub wood} through the Aptian was influenced primarily by fluctuations in the isotopic composition of CO{sub 2} in the global ocean-atmosphere system, as registered in marine carbonates elsewhere, and was not governed by local paleoenvironmental and/or paleoecological factors. Negative and positive excursions in {delta}{sup 13}C{sub wood} through the lower Aptian occur in phase with inferred transgressions and regressions, respectively -- a pattern that contrasts with that observed in many previous studies for different time intervals. The relationship between {delta}{sup 13}C variations and relative sea-level change is tentatively interpreted as a response to various climatic and eustatic factors, relating to rapid sea-floor spreading, thermal uplift of ocean floor, emplacement of plateaus, volcanic CO{sub 2} emissions, weathering, and sedimentary rate.

Nitrogen isotope evidence for water mass denitrification during the early Toarcian (Jurassic) oceanic anoxic event.

Bulk sedimentary nitrogen isotope (δ15Ntot) data have been generated from Lower Jurassic black, carbon-rich shales in the British Isles and northern Italy deposited during the early Toarcian oceanic anoxic event. A pronounced positive δ15Ntot excursion through the exaratum Subzone of the falciferum Zone (defined by characteristic ammonites in the British Isles) broadly correlates with a relative maximum in weight percent total organic carbon and, in some sections, with a negative δ13Corg excursion. Upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of δ15Ntot, and parallels may be drawn with Quaternary sediments of the Arabian Sea, Gulf of California, and northwest Mexican margin. The development of Early Toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity. Approximately coincident phenomena include the following: a relative climatic optimum, realignment of major oceanic current systems, and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event.

Strontium isotopes in Early Jurassic seawater

Abstract The analysis of well-preserved, well-dated belemnites and oysters from the Jurassic of Great Britain has resulted in a well-constrained, detailed seawater strontium isotope curve for the Early Jurassic. The preservation of fossil low-Mg calcite was monitored using Mn, Fe, δ 13 C , and δ 18 O . Iron was the most useful indicator, with about 75% of the samples containing more than 150 ppm Fe showing 87 Sr 86 Sr ratios elevated relative to adjacent points on the curve. High Mn concentrations less often correlated with elevated 87 Sr 86 Sr ratios; however, low Mn concentrations ( 87 Sr 86 Sr . δ 13 C and δ 18 O proved to be insensitive to diagenesis as it affects 87 Sr 86 Sr . The principal features of the strontium isotope curve include a rise to about 0.70772 in the latest Triassic and earliest Jurassic (Hettangian). From the Hettangian, the curve begins a roughly linear descent through the Sinemurian and Pliensbachian. Following a small levelling off and increase in the late Pliensbachian, the curve falls rapidly to its Early Jurassic minimum of 0.70706. It then gently increases through the Toarcian until the falciferum zone, where it shows an apparently abrupt increase to 0.70719 before continuing its slow increase to 0.70728 in the Aalenian-Bajocian (Middle Jurassic). This reversal of the downwards trend established in the Sinemurian and Pliensbachian had not been previously identified. The Sinemurian and Pliensbachian section of the curve potentially allows correlation and dating to within 1 or 2 ammonite subzones (±0.5 to 1 Ma).

Sea-level change and facies development across potential Triassic-Jurassic boundary horizons, SW Britain

The Late Triassic to Early Jurassic aged succession of SW Britain (the Penarth and lower Lias Groups) comprises mudstone, sandstone and limestone strata deposited in a variety of marine to non-marine environments. Faunal and floral characteristics of these successions have led to the proposal that one location in SW England, St Audries Bay, should serve as the Global Stratotype Section and Point (GSSP) for the base of the Hettangian Stage and, thus, for the Triassic–Jurassic (Tr–J) boundary. The sections of SW Britain have also been used previously to infer sea-level change history and relate this to potential kill mechanisms associated with the Tr–J boundary mass extinction. Chemostratigraphic, biofacies and lithofacies data are used here to suggest alternative models of sea-level change in relation to possible Tr–J boundary horizons in the sections of SW Britain. A sea-level lowstand surface of erosion is inferred to occur within the Cotham Member of the Lilstock Formation, a unit deposited in an environment that was often subaerially exposed. In contrast to previous interpretations, the top surface of the overlying Langport Member (here inferred to be deposited on a carbonate ramp of depositional or tectonic origin) represents a drowning event of at least regional extent. All horizons regarded as plausible levels at which to place the Tr–J boundary based on fossil distributions lie within strata deposited during relative sea-level rise. However, it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and, additionally, there is no positive evidence that sea-level fall had any relation to the genesis of proposed Tr–J marker horizons. It is unlikely that sea-level fall played a significant role in the Tr–J boundary extinctions in either a local or a global context.

Macroecological responses of terrestrial vegetation to climatic and atmospheric change across the Triassic/Jurassic boundary in East Greenland

Abstract The magnitude and pace of terrestrial plant extinction and macroecological change associated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified using paleoecological data. However, tracking the diversity and ecology of primary producers provides an ideal surrogate with which to explore patterns of ecosystem stability, collapse, and recovery and to explicitly test for gradual versus catastrophic causal mechanisms of extinction. We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland, spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossil leaf specimens, in an attempt to better constrain our understanding of the causes and consequences of the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnover of ecological dominants between Triassic and Jurassic plant communities, (2) marked structural changes in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in generic-level richness and evenness and change in ecological composition prior to the Tr/J boundary; all of these findings argue against a single catastrophic causal mechanism, such as a meteorite impact for Tr/J extinctions. We identify various key ecological and biological traits that increased extinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plant ecophysiological models. These include ecological rarity, complex reproductive biology, and large leaf size. Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however, species-level turnover in earliest Jurassic plant communities remained an order of magnitude higher than observed for the Triassic. We hypothesize, on the basis of evidence for geographically extensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the nature and magnitude of paleoecological changes recorded in this study reflect wider vegetation change across the whole region. How exactly these changes in dominance patterns of plant primary production affected the entire ecosystem remains an important avenue of future research.

First record of the Early Toarcian Oceanic Anoxic Event from the Southern Hemisphere, Neuquén Basin, Argentina

Abstract: The first record of the Early Toarcian Oceanic Anoxic Event (c. 183 Ma) from the Southern Hemisphere is described from the Neuquén Basin, Argentina, identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ13Corg) in bulk rock and fossil wood. The negative excursion of −6‰ in bulk organic carbon (falling to −31.3‰) crosses the boundary of the tenuicostatum–hoelderi Andean ammonite Zones, equivalent to the tenuicostatum–falciferum/serpentinum zones of Europe. These data indicate that the Early Toarcian Oceanic Anoxic Event was a global phenomenon. Supplementary material: A detailed stratigraphic log, chemostratigraphic data and nannofossil data are available at http://www.geolsoc.org.uk/SUP18411.