Stuart A. Robinson

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.

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.

Carbon-cycle perturbation in the Middle Jurassic and Accompanying Changes in the Terrestrial Paleoenvironment

Carbon‐isotope analyses of fossil wood from the Middle Jurassic Ravenscar Group, Yorkshire, NE England, reveal a significant excursion toward light isotopic values (δ13C change of −3 to −4‰) at about the Aalenian‐Bajocian boundary (∼174 Ma). A positive carbon isotopic excursion is also shown for the middle Bajocian (∼170 Ma) but is less clearly defined. These isotopic patterns are very similar to the few published marine carbonate records available for this time, in particular one based on belemnites from the Hebrides basin, NW Scotland, and others from pelagic limestones in Italy. The similarity of the terrestrial and marine isotope curves is an indication that the observed isotopic signal is a global phenomenon. Through parts of the Ravenscar Group (the Scarborough Formation), supplementary data from bulk organic carbon and palynofacies analysis confirm that isotopic curves based on bulk analyses may be strongly influenced by the balance of terrestrial versus marine organic matter present in the samples. The negative isotope excursion at the Aalenian‐Bajocian boundary marks a change from charcoal to coal as the dominant preservational mode of the macroscopic wood fossils, which is interpreted here as a shift to a more continuously humid climate in the Early Bajocian. Upsection, charcoal once again becomes common, reflecting a return to more fire‐prone (presumably seasonally arid) environments in the middle Bajocian. Paradoxically, floral assemblages associated with the lithological unit in which the negative excursion occurs display characteristics that would normally be interpreted as adaptations to water stress brought about by relative aridity or salinity. Preliminary analyses of leaf stomatal densities show some evidence of raised pCO2 relative to background values at about the level of the negative excursion.

Mid-Cretaceous pCO2 based on stomata of the extinct conifer Pseudofrenelopsis (Cheirolepidiaceae)

Stomatal characteristics of an extinct Cretaceous conifer, Pseudofrenelopsis parceramosa (Fontaine) Watson, are used to reconstruct atmospheric carbon dioxide ( p CO 2 ) over a time previously inferred to exhibit major fluctuations in this greenhouse gas. Samples are from nonmarine to marine strata of the Wealden and Lower Greensand Groups of England and the Potomac Group of the eastern United States, of Hauterivian to Albian age (136–100 Ma). Atmospheric p CO 2 is estimated from the ratios between stomatal indices of fossil cuticles and those from four modern analogs (nearest living equivalent plants). Using this approach, and two calibration methods to explore ranges, results show relatively low and only slightly varying p CO 2 over the Hauterivian–Albian interval: a low of ∼560–960 ppm in the early Barremian and a high of ∼620–1200 ppm in the Albian. Data from the Barremian Wealden Group yield p CO 2 values indistinguishable from a soil-carbonate–based estimate from the same beds. The new p CO 2 estimates are compatible with sedimentological and oxygen-isotope evidence for relatively cool mid-Cretaceous climates.

Evidence for global cooling in the Late Cretaceous.

The Late Cretaceous ‘greenhouse’ world witnessed a transition from one of the warmest climates of the past 140 million years to cooler conditions, yet still without significant continental ice. Low-latitude sea surface temperature (SST) records are a vital piece of evidence required to unravel the cause of Late Cretaceous cooling, but high-quality data remain illusive. Here, using an organic geochemical palaeothermometer (TEX86), we present a record of SSTs for the Campanian–Maastrichtian interval (~83–66 Ma) from hemipelagic sediments deposited on the western North Atlantic shelf. Our record reveals that the North Atlantic at 35 °N was relatively warm in the earliest Campanian, with maximum SSTs of ~35 °C, but experienced significant cooling (~7 °C) after this to <~28 °C during the Maastrichtian. The overall stratigraphic trend is remarkably similar to records of high-latitude SSTs and bottom-water temperatures, suggesting that the cooling pattern was global rather than regional and, therefore, driven predominantly by declining atmospheric pCO2 levels.

Fossil-wood carbon-isotope stratigraphy of the non-marine Wealden Group (Lower Cretaceous, southern England)

Fossil-wood carbon-isotope data are presented for the Wessex Formation, a non-marine unit within the Lower Cretaceous (Valanginian–Barremian) Wealden Group of the Isle of Wight and Dorset, southern England. The carbon-isotope values have a range (δ13C c. −26.6 to −19.8‰) that is consistent with that expected for Mesozoic C3 plants. Consideration of the Isle of Wight fossil-wood carbon-isotope data together with data from Dorset allows construction of a composite fossil-wood carbon-isotope curve for almost the entire Wealden Group. The new carbon-isotope curve is in smooth continuity with data previously published from the overlying lagoonal Vectis Formation and marine Lower Greensand Group of Aptian age. A tentative correlation with a Tethyan reference carbon-isotope curve allows the provisional application of stage-level chronostratigraphy to the Wealden Group. Correlations suggest that the Wealden Group sediments are dominantly of Hauterivian and Barremian age and that the Valanginian is either condensed or partially missing. The carbon-isotope data presented indicate that even during times of relative carbon-cycle quiescence atmosphere CO2 faithfully tracks the carbon-isotopic composition of the oceanic reservoir.

Atmospheric pCO2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England)

Abstract: Nodular soil carbonates (calcretes) are present in overbank facies of Lower Cretaceous, non-marine Wealden Beds (Wessex Formation) of southern England. Field evidence suggests that these calcretes formed mostly under semi-arid Mediterranean-type climatic conditions. Typical calcrete fabrics, identified petrographically, include floating detrital grains, corroded grain margins and circumgranular cracks defining peds. Localized alteration of primary micrites is mainly associated with large cracks where early non-ferroan diagenetic cementation and neomorphism was focused. Diagenetic ferroan calcites occur as void fills and yield relatively light carbon-isotope and oxygen-isotope compositions (δ13C= −15.0‰; δ18O= –6.3‰) compared to well-preserved micrite (δ13C= –10.2‰; δ18O= –4.0‰). Precise definition of δ13C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO2 (pCO2) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO2 was low during the mid-Early Cretaceous before rising to a previously defined mid-Cretaceous high. Data from calcretes in the Weald Clay highlight the need for selection of appropriate material and careful evaluation before pCO2 calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO2 into the soil-zone was either reduced or prevented.

Formation of “Southern Component Water” in the Late Cretaceous: Evidence from Nd-isotopes

Constraining deep-ocean circulation during past greenhouse climatic periods, such as the Cretaceous, is important for understanding meridional heat transfer processes, controls on ocean anoxia, and the relative roles of climate and tectonics in determining paleocirculation patterns. Ocean circulation models for the Late Cretaceous and early Paleogene suggest that significant deep-water production occurred in the Southern Ocean, but cannot constrain when this process commenced or what the temporal relationship was between opening tectonic gateways and Late Cretaceous climatic cooling. Nd-isotope data obtained from biogenic apatite (fish teeth and bones) are presented from lower bathyal and abyssal sites in the South Atlantic and Indian Oceans. During the mid-Cretaceous, relatively radiogenic Nd-isotope values suggest that deep-water circulation in these basins was sluggish with inputs likely dominated by seawater-particle exchange processes and, possibly, easily weathered volcanic terranes. In the Campanian–Maastrichtian the Nd-isotopic composition of proto-Indian and South Atlantic deep waters became less radiogenic, suggesting the onset of deep-water formation in the Southern Ocean (Southern Component Water, SCW), consistent with Paleogene reconstructions and ocean circulation models. A combination of Southern Hemisphere cooling and the opening of tectonic gateways during the Campanian likely drove the onset of SCW.

Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes : Dakota Formation, Western Interior Basin, USA.

Analysis of bulk sedimentary organic matter and charcoal from an Albian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of ∼3‰ in late Albian strata. Overlying Cenomanian strata have δ13C values of −24‰ to −23‰ that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Albian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive δ13C event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D2), as well as a major regressive phase throughout the globe at the Albian-Cenomanian boundary. Data from RCP confirm suggestions that some positive carbon-isotope excursions in the geologic record are coincident with regressive sea-level phases. We estimate using isotopic correlation that the D2 sequence boundary at RCP was on the order of 0.5 m.y. in duration. Therefore, interpretations of isotopic events and associated environmental phenomena, such as oceanic anoxic events, in the shallow-marine and terrestrial record may be influenced by stratigraphic incompleteness. Further investigation of terrestrial δ13C records may be useful in recognizing and constraining sea-level changes in the geologic record.

Widespread and synchronous change in deep‐ocean circulation in the North and South Atlantic during the Late Cretaceous

Modern thermohaline circulation plays a role in latitudinal heat transport and in deep-ocean ventilation, yet ocean circulation may have functioned differently during past periods of extreme warmth, such as the Cretaceous. The Late Cretaceous (100–65 Ma) was an important period in the evolution of the North Atlantic Ocean, characterized by opening ocean gateways, long-term climatic cooling and the cessation of intermittent periods of anoxia (oceanic anoxic events, OAEs). However, how these phenomena relate to deep-water circulation is unclear. We use a proxy for deep-water mass composition (neodymium isotopes; eNd) to show that, at North Atlantic ODP Site 1276, deep waters shifted in the early Campanian (∼78–83 Ma) from eNd values of ∼−7 to values of ∼−9, consistent with a change in the style of deep-ocean circulation but >10 Myr after a change in bottom water oxygenation conditions. A similar, but more poorly dated, trend exists in eNd data from DSDP Site 386. The Campanian eNd transition observed in the North Atlantic records is also seen in the South Atlantic and proto-Indian Ocean, implying a widespread and synchronous change in deep-ocean circulation. Although a unique explanation does not exist for the change at present, we favor an interpretation that invokes Late Cretaceous climatic cooling as a driver for the formation of Southern Component Water, which flowed northward from the Southern Ocean and into the North Atlantic and proto-Indian Oceans.