Seymour Island, in the James Ross Basin, Antarctica, contains a continuous succession of latest C... more Seymour Island, in the James Ross Basin, Antarctica, contains a continuous succession of latest Cretaceous sediments deposited in a shallow marine environment at high latitude, making it an ideal place to study environmental changes prior to the K–Pg mass extinction. We measured major and trace elements and conducted petrographic analysis of two sections from the Maastrichtian–Danian López de Bertodano Formation of Seymour Island. Several lines of evidence point to intermittently anoxic to euxinic conditions during deposition, including the presence of pyrite framboids with a size distribution suggesting syngenetic formation in the water column, and enrichments in several trace elements, including molybdenum, arsenic, copper, zinc, and chromium. Molyb-denum enrichments are clearly associated with enrichments in manganese and authigenic iron, suggesting " shuttling " of redox sensitive trace elements across a chemocline that fluctuated across the sediment-water interface. Comparisons with modern systems suggest relatively high-frequency redox variability, possibly over approximately annual timescales, which may be related to the annual cycle of polar sunlight and associated seasonal changes in primary productivity. Glauconitic horizons are associated with more reducing conditions, including at the K–Pg boundary, though this does not appear to be a uniquely euxinic interval; similar degrees of trace element enrichment are seen in other highly glauconitic intervals. While euxinia may have contributed to low diversity in the lowermost 'Rotularia Units', redox conditions do not seem to have been the primary control on the transition to a mollusc dominated fauna in the latest Maastrichtian. Redox conditions show little to no response to the eruption of the Deccan Traps or Maastrichtian climatic changes. Instead, intermittent euxinia appears to have been a characteristic feature of this high-latitude environment during the Cretaceous–Paleogene transition.
Conodont, C isotope and fossil and facies data are presented for the Capitanian (Middle Permian) ... more Conodont, C isotope and fossil and facies data are presented for the Capitanian (Middle Permian) mass extinction record seen in platform carbonates (Maokou and Wuchiaping formations) of South China, where limestones interdigitate with the volcanic succession of the Emeishan large igneous province. The Maokou Formation provides an extinction record marked by the loss of keriothecal-walled fusulinaceans and a turnover in calcareous algae. In sections within the Emeishan province this crisis occurs at the base of the oldest record of volcanism from the Jinogondolella altudaensis conodont zone (of mid-Capitanian age). Around the periphery of the Emeishan province this extinction level lies within the upper part of the Maokou Formation at a level where platform carbonate deposition was frequently interrupted by thick volcanic ash depositional events. The assemblages of the uppermost Maokou Formation are characterised by typical "Late Permian" taxa although these levels still lie within the Middle Permian (Guadalupian Series). "Disaster" taxa, such as Earlandia and Diplosphaerina are locally prolifically abundant in the aftermath of the mass extinction. The crisis is particularly noteworthy amongst photosynthetic taxa such as calcareous algae and fusulinaceans that probably harboured photosymbionts. Therefore, a kill mechanism involving cooling from explosive volcanism and potentially acid rain from sulphate aerosols appears appropriate. A composite δ 13 C carb curve, calibrated against a high-resolution conodont biostratigraphy reveals a major intra-Capitanian negative excursion (of N 5‰) superimposed on typically heavy (4-5‰) Middle Permian values. This curve can also be recognised in Panthalassan seamount carbonates of Japan although this requires reassignment of apparently Upper Permian limestones to the Middle Permian. In both China and Japan the Capitanian mass extinction occurs during the early stage of this major, intra-Capitanian negative excursion. Assuming typical platform carbonate accumulation rates, the records of the Maokou Formation suggest δ 13 C carb values fell at ∼0.01‰/kyr suggesting a catastrophic origin (such as gas hydrate destabilisation) is unlikely, although a volcanic source is possible.
Nitrogen isotope signatures (δ 15 N) provide powerful measures of the trophic positions of indivi... more Nitrogen isotope signatures (δ 15 N) provide powerful measures of the trophic positions of individuals, populations and communities. Obtaining reliable consumer δ 15 N values depends upon controlling for spatial variation in plant δ 15 N values, which form the trophic 'baseline'. However, recent studies make differing assumptions about the scale over which plant δ 15 N values vary, and approaches to baseline control differ markedly. We examined spatial variation in the δ 15 N values of plants and ants sampled from eight 150-m transects in both unlogged and logged rainforests. We then investigated whether ant δ 15 N values were related to variation in plant δ 15 N values following baseline correction of ant values at two spatial scales: (1) using 'local' means of plants collected from the same transect and using 'global' means of plants collected from all transects within each forest type. Plant δ 15 N baselines varied by the equivalent of one trophic level within each forest type. Correcting ant δ 15 N values using global plant means resulted in consumer values that were strongly positively related to the transect baseline, whereas local corrections yielded reliable estimates of consumer trophic positions that were largely independent of transect baselines. These results were consistent at the community level and when three trophically distinct ant subfamilies and eight abundant ant species were considered separately. Our results suggest that assuming baselines do not vary can produce misleading estimates of consumer trophic positions. We therefore emphasise the importance of clearly defining and applying baseline corrections at a scale that accounts for spatial variation in plant δ 15 N values.
Stable isotope ratios of sulfur (d 34 S) and oxygen (d 18 O) in marine sulfate respond to changes... more Stable isotope ratios of sulfur (d 34 S) and oxygen (d 18 O) in marine sulfate respond to changes in fluxes and processes in the global sulfur cycle. The two isotope systems respond on different time scales to different factors affecting the global sulfur cycle. Sulfur isotopes respond primarily to the relative fluxes of weathering-derived sulfate to the oceans versus the net flux of sulfur exported from the oceans as pyrite in marine sediments. At the present day, this response is relatively slow (~20 Ma) because of the large size of the marine sulfate reservoir; ancient oceans had lower sulfate concentrations and thus sulfate residence times may have been shorter. The d 34 S of marine sulfate is also sensitive to the development of a significant reservoir of H 2 S in ancient stratified oceans. Sulfate-oxygen isotopes respond primarily to changes in the cycling of sulfur in the oceans and marine sediments. Sulfur disproportionation, oxic and anaerobic sulfide oxidation all impart different d 18 O signatures to marine sulfate; the relative importance of these different processes is in turn controlled by the distribution of redox conditions and microbial ecology of sea-floor sediments. The mutual interpretation of changes in marine sulfate d 34 S and d 18 O can thus give rather precise information on the nature of changes in global sulfur cycling. Recently, records of isotope compositions of ancient marine sulfate have become available through the analysis of carbonate-associated sulfate (CAS) in limestones and marine biogenic barite, both of which provide far more continuous records than previous data based primarily on marine evaporite sulfate minerals.
Debate continues about the nature of the Cretaceous–Paleogene (K–Pg) mass extinction event. An ab... more Debate continues about the nature of the Cretaceous–Paleogene (K–Pg) mass extinction event. An abrupt crisis triggered by a bolide impact contrasts with ideas of a more gradual extinction involving flood volcanism or climatic changes. Evidence from high latitudes has also been used to suggest that the severity of the extinction decreased from low latitudes towards the poles. Here we present a record of the K–Pg extinction based on extensive assemblages of marine macrofossils (primarily new data from benthic molluscs) from a highly expanded Cretaceous–Paleogene succession: the López de Bertodano Formation of Seymour Island, Antarctica. We show that the extinction was rapid and severe in Antarctica, with no significant biotic decline during the latest Cretaceous, contrary to previous studies. These data are consistent with a catastrophic driver for the extinction, such as bolide impact, rather than a significant contribution from Deccan Traps volcanism during the late Maastrichtian.
... evolved that has a smaller associated carbon isotopic fractionation and thus produces biomass... more ... evolved that has a smaller associated carbon isotopic fractionation and thus produces biomass that is ... of these two realms has rather hindered the progress of ideas on the cause and effect ... Anysignificant release of light carbon to the atmosphere or the oceans should be readily ...
Abstract Chemical signatures of enrichment of highly reactive iron, and framboid size distributio... more Abstract Chemical signatures of enrichment of highly reactive iron, and framboid size distributions, are reported in turbidite sediments that host soft tissue pyritization (Beecher's Trilobite Bed, Upper Ordovician, and the Hunsrück Slate, Lower Devonian). These ...
Here we present two records of marine SO4-O isotopes derived from carbonate associated sulfate (C... more Here we present two records of marine SO4-O isotopes derived from carbonate associated sulfate (CAS) across the Toarcian anoxic event, one from a Tethyan open ocean margin (Tibet) and the other from within the European Epicontinental Sea (EES, Yorkshire, UK). Limestones from Tibet record average SO4-δ18O of+ 13.1±1.3‰ across the interval with no systematic change. Belemnites from Yorkshire however record considerably heavier values which change in a systematic way. These start at~+ 15‰ in the uppermost Pliensbachian ...
Carbonate-associated sulphate (CAS) extracted from a Permo^Triassic succession at Siusi in northe... more Carbonate-associated sulphate (CAS) extracted from a Permo^Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing N 34 S and N 18 O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H 2 S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-N 18 O directly above the P^T boundary, not accompanied by a large change in CAS-N 34 S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-N 34 S, but is preserved by declining CAS-N 18 O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical 'fossil' until the beginning of the Middle Triassic. ß
The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedde... more The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.
Two new records of seawater sulfate-sulfur isotopes derived from carbonate associated sulfate sug... more Two new records of seawater sulfate-sulfur isotopes derived from carbonate associated sulfate suggest that other isotopic records found in sediments from the European epicontinental sea (EES) during the Toarcian may represent principally local scale rather than global changes. The first of these, from belemnites from the Yorkshire coast in the UK (within the EES), describe a rapid change from +16.5& to +22.2& d 34 S VCDT between the semicelatum and exaratum ammonite subzones. Radiometric dating shows that the entire Toarcian represents around 5 myr and individual subzones (if equal in length) can be calculated to be around 400,000 years in duration. Hence, it is conservatively estimated that this 6& isotopic change occurred in 1.5 myr or less. This is considerably shorter than the modern residence time of sulfate in the oceans (20 myr) and requires either; that the concentration of oceanic sulfate in the Toarcian was much lower than its modern value, or; that the European epicontinental sea became isolated from the worlds oceans at this time. A second record from southern Tibet from sediments deposited on the margin of Tethys, whilst having less exact dating and a degree of scatter, shows no such consistent change and records an average value of around +18& d 34 S VCDT . The Tibetan data therefore favours the hypothesis that the EES developed a chemistry significantly different from that of the worlds oceans during the Toarcian. If correct, this suggests that carbon (and other) isotope data from Toarcian sediments from the EES only records regional variation and calls into question previous models of extensive synchronous global carbon burial, as well as the need for large scale methane release at this time.
Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgi... more Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgian (Late Jurassic) basin margin sections of the Boulonnais (N. France). Pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic-rich shale formation, and Th/U ratios and degree of pyritisation of Fe suggest severe oxygenrestriction, whilst palaeoecological and sedimentological evidence suggests that sea¯oor oxygenation events were frequent. Interbedded, hummocky cross-strati®ed sandstones indicate deposition above storm wave base. Correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen-poor waters that contracted in lateral extent during parasequence formation. The Boulonnais black shales are similar to other varieties of transgressive black shale, in that they are developed upon surfaces of marine starvation, but they differ in that they pass down-dip into better oxygenated facies. They are therefore classi®ed as transgressive, nearshore (TN) black shales that do not record the`feather edge' of basinal conditions. Deposition is interpreted to have occurred beneath a small volume of sub-thermocline bottom waters that were readily de-oxygenated, whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub-thermocline water. Why such conditions should pertain only during initial stages of base-level rise remains obscure. q
Triassic-Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spe... more Triassic-Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spectrum of depositional environments from the western Canadian continental margin. A primarily deep-water, anoxic record of deposition, indicated by V/V + Ni, V/Cr, Th/U indices and pyrite framboid and facies data, is seen throughout the Late Triassic-Early Jurassic (Norian-Hettangian) interval in a section at Black Bear Ridge. Abundant Monotis bivalve populations in Norian strata are interpreted to have lived on the seabed during transient oxygenation events that punctuated the background anoxic conditions. Monotis went extinct around the Norian/Rhaetian boundary and the deep-water assemblages are subsequently dominated by thin-shelled pectinids. An intra-Rhaetian sea-level fall generated a major hiatus in the proximal Black Bear Ridge section whilst distinctive lowstand strata were developed in the more distal and more complete Ne Parle Pas Point and Pardonet Creek sections. These consist of shallow-water, phosphatic ooid sand facies that contain diverse bivalves. The majority of the infaunal bivalves (but not the epifaunal ones) disappeared at the acme of the regression. This is the end-Triassic mass extinction event and it occurred shortly before a terminal Triassic flooding event. This transgressive event is associated with a rapid negative shift of δ 13 C org values interpreted to be the Initial Isotope Excursion of Hesselbo et al. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30, 251-254]. The Rhaetian record of Williston Lake thus records an extinction of shallow-water infaunal bivalves during a lowstand prior to the development of a transgressive record that is associated with a major, negative C isotope.
A record of the Early Jurassic mass extinction event is reported from eastern Tethyan (Tibetan) l... more A record of the Early Jurassic mass extinction event is reported from eastern Tethyan (Tibetan) locations for the first time. In the Mount Everest region a thick Lower Jurassic carbonate formation, here named the Yungjia Formation, is developed within the predominantly clastic Triassic–Jurassic succession. Within the formation a sharp transition from peloidal packstones/grainstones to thin-bedded, pyritic micrite-shales interbeds records a sharp pulse of deepening and development of dysoxic bottom waters. Both the lithiotid bivalves and the lituolid foraminifera are important constituents of the lower Yungjia Formation but they disappear at this flooding surface or a short distance below it. This extinction event is comparable to that seen at the base of the Pliensbachian/Toarcian boundary in western Tethyan platform carbonates but the Tibetan events occurred late in the Toarcian Stage as indicated by nannofossil biostratigraphy and C isotope chemostratigraphy. The Early Jurassic extinction event (and the associated spread of oxygen-poor waters) was therefore not synchronous throughout the Tethyan region.
Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the dee... more Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the deep confined zone of a dual porosity carbonate aquifer. Some of the fissure-waters show good evidence for bacterial sulphate reduction, with low concentrations of sulphide present which is strongly to moderately depleted in 34S relative to sulphate. The sulphur geochemistry is best interpreted as mixing between: (i) a reduced water with sulphide ∼60‰ depleted in 34S relative to sulphate; and (ii) a sulphate-rich water from up-dip in the aquifer. In addition, sulphide oxidation occurs where sufficiently oxidizing water is drawn in by abstractions. The large isotope fractionation factor associated with the sulphidic waters is probably the result of redox cycling of sulphur with little net reduction, rather than a true kinetic fractionation factor. By contrast, pore-waters in the “sulphate reducing zone” show little or no evidence for the effects of sulphate reduction, despite the fact that the pore-waters represent a significant reservoir of sulphate for reduction. Some pore-waters have been modified recognizably by diffusional exchange with the fissure-waters, but the aquifer matrix has not been colonized by sulphate reducing bacteria, probably because porethroats in the limestone are too small for bacteria to pass. Physical exclusion of bacteria from the aquifer matrix and limited diffusional exchange are likely to exert fundamental controls on bacterial redox processes in dual porosity aquifer systems and other systems with low permeability due to small pore interconnections.
Palaeogeography Palaeoclimatology Palaeoecology, Jan 1, 2005
The Khunamuh Formation of the Guryul Ravine section in Kashmir provides one of the most detailed ... more The Khunamuh Formation of the Guryul Ravine section in Kashmir provides one of the most detailed deep-water records of the end-Permian mass extinction in Perigondwanan palaeolatitudes. Examination of pyrite framboid size distributions and sediment microfabric reveals a distinct change in the uppermost Permian strata from nonlaminated, pyrite-free silty mudstones to finely laminated, silty shales rich in framboidal pyrite around 1 m above the base of the formation. The size frequency distribution of the framboids is identical to populations from severely oxygen-restricted modern depositional environments. This change to dysoxic benthic conditions coincides with the rapid demise of diverse Permian brachiopod and bryozoan assemblages. Thus, the link between extinction and anoxia is clearly seen at Guryul Ravine and it provides an interesting contrast to other Perigondwanan sections developed in shallower water where both the onset of anoxia and the extinction occurred somewhat later in the earliest Triassic.
Seymour Island, in the James Ross Basin, Antarctica, contains a continuous succession of latest C... more Seymour Island, in the James Ross Basin, Antarctica, contains a continuous succession of latest Cretaceous sediments deposited in a shallow marine environment at high latitude, making it an ideal place to study environmental changes prior to the K–Pg mass extinction. We measured major and trace elements and conducted petrographic analysis of two sections from the Maastrichtian–Danian López de Bertodano Formation of Seymour Island. Several lines of evidence point to intermittently anoxic to euxinic conditions during deposition, including the presence of pyrite framboids with a size distribution suggesting syngenetic formation in the water column, and enrichments in several trace elements, including molybdenum, arsenic, copper, zinc, and chromium. Molyb-denum enrichments are clearly associated with enrichments in manganese and authigenic iron, suggesting " shuttling " of redox sensitive trace elements across a chemocline that fluctuated across the sediment-water interface. Comparisons with modern systems suggest relatively high-frequency redox variability, possibly over approximately annual timescales, which may be related to the annual cycle of polar sunlight and associated seasonal changes in primary productivity. Glauconitic horizons are associated with more reducing conditions, including at the K–Pg boundary, though this does not appear to be a uniquely euxinic interval; similar degrees of trace element enrichment are seen in other highly glauconitic intervals. While euxinia may have contributed to low diversity in the lowermost 'Rotularia Units', redox conditions do not seem to have been the primary control on the transition to a mollusc dominated fauna in the latest Maastrichtian. Redox conditions show little to no response to the eruption of the Deccan Traps or Maastrichtian climatic changes. Instead, intermittent euxinia appears to have been a characteristic feature of this high-latitude environment during the Cretaceous–Paleogene transition.
Conodont, C isotope and fossil and facies data are presented for the Capitanian (Middle Permian) ... more Conodont, C isotope and fossil and facies data are presented for the Capitanian (Middle Permian) mass extinction record seen in platform carbonates (Maokou and Wuchiaping formations) of South China, where limestones interdigitate with the volcanic succession of the Emeishan large igneous province. The Maokou Formation provides an extinction record marked by the loss of keriothecal-walled fusulinaceans and a turnover in calcareous algae. In sections within the Emeishan province this crisis occurs at the base of the oldest record of volcanism from the Jinogondolella altudaensis conodont zone (of mid-Capitanian age). Around the periphery of the Emeishan province this extinction level lies within the upper part of the Maokou Formation at a level where platform carbonate deposition was frequently interrupted by thick volcanic ash depositional events. The assemblages of the uppermost Maokou Formation are characterised by typical "Late Permian" taxa although these levels still lie within the Middle Permian (Guadalupian Series). "Disaster" taxa, such as Earlandia and Diplosphaerina are locally prolifically abundant in the aftermath of the mass extinction. The crisis is particularly noteworthy amongst photosynthetic taxa such as calcareous algae and fusulinaceans that probably harboured photosymbionts. Therefore, a kill mechanism involving cooling from explosive volcanism and potentially acid rain from sulphate aerosols appears appropriate. A composite δ 13 C carb curve, calibrated against a high-resolution conodont biostratigraphy reveals a major intra-Capitanian negative excursion (of N 5‰) superimposed on typically heavy (4-5‰) Middle Permian values. This curve can also be recognised in Panthalassan seamount carbonates of Japan although this requires reassignment of apparently Upper Permian limestones to the Middle Permian. In both China and Japan the Capitanian mass extinction occurs during the early stage of this major, intra-Capitanian negative excursion. Assuming typical platform carbonate accumulation rates, the records of the Maokou Formation suggest δ 13 C carb values fell at ∼0.01‰/kyr suggesting a catastrophic origin (such as gas hydrate destabilisation) is unlikely, although a volcanic source is possible.
Nitrogen isotope signatures (δ 15 N) provide powerful measures of the trophic positions of indivi... more Nitrogen isotope signatures (δ 15 N) provide powerful measures of the trophic positions of individuals, populations and communities. Obtaining reliable consumer δ 15 N values depends upon controlling for spatial variation in plant δ 15 N values, which form the trophic 'baseline'. However, recent studies make differing assumptions about the scale over which plant δ 15 N values vary, and approaches to baseline control differ markedly. We examined spatial variation in the δ 15 N values of plants and ants sampled from eight 150-m transects in both unlogged and logged rainforests. We then investigated whether ant δ 15 N values were related to variation in plant δ 15 N values following baseline correction of ant values at two spatial scales: (1) using 'local' means of plants collected from the same transect and using 'global' means of plants collected from all transects within each forest type. Plant δ 15 N baselines varied by the equivalent of one trophic level within each forest type. Correcting ant δ 15 N values using global plant means resulted in consumer values that were strongly positively related to the transect baseline, whereas local corrections yielded reliable estimates of consumer trophic positions that were largely independent of transect baselines. These results were consistent at the community level and when three trophically distinct ant subfamilies and eight abundant ant species were considered separately. Our results suggest that assuming baselines do not vary can produce misleading estimates of consumer trophic positions. We therefore emphasise the importance of clearly defining and applying baseline corrections at a scale that accounts for spatial variation in plant δ 15 N values.
Stable isotope ratios of sulfur (d 34 S) and oxygen (d 18 O) in marine sulfate respond to changes... more Stable isotope ratios of sulfur (d 34 S) and oxygen (d 18 O) in marine sulfate respond to changes in fluxes and processes in the global sulfur cycle. The two isotope systems respond on different time scales to different factors affecting the global sulfur cycle. Sulfur isotopes respond primarily to the relative fluxes of weathering-derived sulfate to the oceans versus the net flux of sulfur exported from the oceans as pyrite in marine sediments. At the present day, this response is relatively slow (~20 Ma) because of the large size of the marine sulfate reservoir; ancient oceans had lower sulfate concentrations and thus sulfate residence times may have been shorter. The d 34 S of marine sulfate is also sensitive to the development of a significant reservoir of H 2 S in ancient stratified oceans. Sulfate-oxygen isotopes respond primarily to changes in the cycling of sulfur in the oceans and marine sediments. Sulfur disproportionation, oxic and anaerobic sulfide oxidation all impart different d 18 O signatures to marine sulfate; the relative importance of these different processes is in turn controlled by the distribution of redox conditions and microbial ecology of sea-floor sediments. The mutual interpretation of changes in marine sulfate d 34 S and d 18 O can thus give rather precise information on the nature of changes in global sulfur cycling. Recently, records of isotope compositions of ancient marine sulfate have become available through the analysis of carbonate-associated sulfate (CAS) in limestones and marine biogenic barite, both of which provide far more continuous records than previous data based primarily on marine evaporite sulfate minerals.
Debate continues about the nature of the Cretaceous–Paleogene (K–Pg) mass extinction event. An ab... more Debate continues about the nature of the Cretaceous–Paleogene (K–Pg) mass extinction event. An abrupt crisis triggered by a bolide impact contrasts with ideas of a more gradual extinction involving flood volcanism or climatic changes. Evidence from high latitudes has also been used to suggest that the severity of the extinction decreased from low latitudes towards the poles. Here we present a record of the K–Pg extinction based on extensive assemblages of marine macrofossils (primarily new data from benthic molluscs) from a highly expanded Cretaceous–Paleogene succession: the López de Bertodano Formation of Seymour Island, Antarctica. We show that the extinction was rapid and severe in Antarctica, with no significant biotic decline during the latest Cretaceous, contrary to previous studies. These data are consistent with a catastrophic driver for the extinction, such as bolide impact, rather than a significant contribution from Deccan Traps volcanism during the late Maastrichtian.
... evolved that has a smaller associated carbon isotopic fractionation and thus produces biomass... more ... evolved that has a smaller associated carbon isotopic fractionation and thus produces biomass that is ... of these two realms has rather hindered the progress of ideas on the cause and effect ... Anysignificant release of light carbon to the atmosphere or the oceans should be readily ...
Abstract Chemical signatures of enrichment of highly reactive iron, and framboid size distributio... more Abstract Chemical signatures of enrichment of highly reactive iron, and framboid size distributions, are reported in turbidite sediments that host soft tissue pyritization (Beecher's Trilobite Bed, Upper Ordovician, and the Hunsrück Slate, Lower Devonian). These ...
Here we present two records of marine SO4-O isotopes derived from carbonate associated sulfate (C... more Here we present two records of marine SO4-O isotopes derived from carbonate associated sulfate (CAS) across the Toarcian anoxic event, one from a Tethyan open ocean margin (Tibet) and the other from within the European Epicontinental Sea (EES, Yorkshire, UK). Limestones from Tibet record average SO4-δ18O of+ 13.1±1.3‰ across the interval with no systematic change. Belemnites from Yorkshire however record considerably heavier values which change in a systematic way. These start at~+ 15‰ in the uppermost Pliensbachian ...
Carbonate-associated sulphate (CAS) extracted from a Permo^Triassic succession at Siusi in northe... more Carbonate-associated sulphate (CAS) extracted from a Permo^Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing N 34 S and N 18 O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H 2 S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-N 18 O directly above the P^T boundary, not accompanied by a large change in CAS-N 34 S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-N 34 S, but is preserved by declining CAS-N 18 O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical 'fossil' until the beginning of the Middle Triassic. ß
The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedde... more The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.
Two new records of seawater sulfate-sulfur isotopes derived from carbonate associated sulfate sug... more Two new records of seawater sulfate-sulfur isotopes derived from carbonate associated sulfate suggest that other isotopic records found in sediments from the European epicontinental sea (EES) during the Toarcian may represent principally local scale rather than global changes. The first of these, from belemnites from the Yorkshire coast in the UK (within the EES), describe a rapid change from +16.5& to +22.2& d 34 S VCDT between the semicelatum and exaratum ammonite subzones. Radiometric dating shows that the entire Toarcian represents around 5 myr and individual subzones (if equal in length) can be calculated to be around 400,000 years in duration. Hence, it is conservatively estimated that this 6& isotopic change occurred in 1.5 myr or less. This is considerably shorter than the modern residence time of sulfate in the oceans (20 myr) and requires either; that the concentration of oceanic sulfate in the Toarcian was much lower than its modern value, or; that the European epicontinental sea became isolated from the worlds oceans at this time. A second record from southern Tibet from sediments deposited on the margin of Tethys, whilst having less exact dating and a degree of scatter, shows no such consistent change and records an average value of around +18& d 34 S VCDT . The Tibetan data therefore favours the hypothesis that the EES developed a chemistry significantly different from that of the worlds oceans during the Toarcian. If correct, this suggests that carbon (and other) isotope data from Toarcian sediments from the EES only records regional variation and calls into question previous models of extensive synchronous global carbon burial, as well as the need for large scale methane release at this time.
Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgi... more Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgian (Late Jurassic) basin margin sections of the Boulonnais (N. France). Pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic-rich shale formation, and Th/U ratios and degree of pyritisation of Fe suggest severe oxygenrestriction, whilst palaeoecological and sedimentological evidence suggests that sea¯oor oxygenation events were frequent. Interbedded, hummocky cross-strati®ed sandstones indicate deposition above storm wave base. Correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen-poor waters that contracted in lateral extent during parasequence formation. The Boulonnais black shales are similar to other varieties of transgressive black shale, in that they are developed upon surfaces of marine starvation, but they differ in that they pass down-dip into better oxygenated facies. They are therefore classi®ed as transgressive, nearshore (TN) black shales that do not record the`feather edge' of basinal conditions. Deposition is interpreted to have occurred beneath a small volume of sub-thermocline bottom waters that were readily de-oxygenated, whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub-thermocline water. Why such conditions should pertain only during initial stages of base-level rise remains obscure. q
Triassic-Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spe... more Triassic-Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spectrum of depositional environments from the western Canadian continental margin. A primarily deep-water, anoxic record of deposition, indicated by V/V + Ni, V/Cr, Th/U indices and pyrite framboid and facies data, is seen throughout the Late Triassic-Early Jurassic (Norian-Hettangian) interval in a section at Black Bear Ridge. Abundant Monotis bivalve populations in Norian strata are interpreted to have lived on the seabed during transient oxygenation events that punctuated the background anoxic conditions. Monotis went extinct around the Norian/Rhaetian boundary and the deep-water assemblages are subsequently dominated by thin-shelled pectinids. An intra-Rhaetian sea-level fall generated a major hiatus in the proximal Black Bear Ridge section whilst distinctive lowstand strata were developed in the more distal and more complete Ne Parle Pas Point and Pardonet Creek sections. These consist of shallow-water, phosphatic ooid sand facies that contain diverse bivalves. The majority of the infaunal bivalves (but not the epifaunal ones) disappeared at the acme of the regression. This is the end-Triassic mass extinction event and it occurred shortly before a terminal Triassic flooding event. This transgressive event is associated with a rapid negative shift of δ 13 C org values interpreted to be the Initial Isotope Excursion of Hesselbo et al. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30, 251-254]. The Rhaetian record of Williston Lake thus records an extinction of shallow-water infaunal bivalves during a lowstand prior to the development of a transgressive record that is associated with a major, negative C isotope.
A record of the Early Jurassic mass extinction event is reported from eastern Tethyan (Tibetan) l... more A record of the Early Jurassic mass extinction event is reported from eastern Tethyan (Tibetan) locations for the first time. In the Mount Everest region a thick Lower Jurassic carbonate formation, here named the Yungjia Formation, is developed within the predominantly clastic Triassic–Jurassic succession. Within the formation a sharp transition from peloidal packstones/grainstones to thin-bedded, pyritic micrite-shales interbeds records a sharp pulse of deepening and development of dysoxic bottom waters. Both the lithiotid bivalves and the lituolid foraminifera are important constituents of the lower Yungjia Formation but they disappear at this flooding surface or a short distance below it. This extinction event is comparable to that seen at the base of the Pliensbachian/Toarcian boundary in western Tethyan platform carbonates but the Tibetan events occurred late in the Toarcian Stage as indicated by nannofossil biostratigraphy and C isotope chemostratigraphy. The Early Jurassic extinction event (and the associated spread of oxygen-poor waters) was therefore not synchronous throughout the Tethyan region.
Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the dee... more Chemical and sulphur isotopic analyses are presented of fissure-waters and pore-waters in the deep confined zone of a dual porosity carbonate aquifer. Some of the fissure-waters show good evidence for bacterial sulphate reduction, with low concentrations of sulphide present which is strongly to moderately depleted in 34S relative to sulphate. The sulphur geochemistry is best interpreted as mixing between: (i) a reduced water with sulphide ∼60‰ depleted in 34S relative to sulphate; and (ii) a sulphate-rich water from up-dip in the aquifer. In addition, sulphide oxidation occurs where sufficiently oxidizing water is drawn in by abstractions. The large isotope fractionation factor associated with the sulphidic waters is probably the result of redox cycling of sulphur with little net reduction, rather than a true kinetic fractionation factor. By contrast, pore-waters in the “sulphate reducing zone” show little or no evidence for the effects of sulphate reduction, despite the fact that the pore-waters represent a significant reservoir of sulphate for reduction. Some pore-waters have been modified recognizably by diffusional exchange with the fissure-waters, but the aquifer matrix has not been colonized by sulphate reducing bacteria, probably because porethroats in the limestone are too small for bacteria to pass. Physical exclusion of bacteria from the aquifer matrix and limited diffusional exchange are likely to exert fundamental controls on bacterial redox processes in dual porosity aquifer systems and other systems with low permeability due to small pore interconnections.
Palaeogeography Palaeoclimatology Palaeoecology, Jan 1, 2005
The Khunamuh Formation of the Guryul Ravine section in Kashmir provides one of the most detailed ... more The Khunamuh Formation of the Guryul Ravine section in Kashmir provides one of the most detailed deep-water records of the end-Permian mass extinction in Perigondwanan palaeolatitudes. Examination of pyrite framboid size distributions and sediment microfabric reveals a distinct change in the uppermost Permian strata from nonlaminated, pyrite-free silty mudstones to finely laminated, silty shales rich in framboidal pyrite around 1 m above the base of the formation. The size frequency distribution of the framboids is identical to populations from severely oxygen-restricted modern depositional environments. This change to dysoxic benthic conditions coincides with the rapid demise of diverse Permian brachiopod and bryozoan assemblages. Thus, the link between extinction and anoxia is clearly seen at Guryul Ravine and it provides an interesting contrast to other Perigondwanan sections developed in shallower water where both the onset of anoxia and the extinction occurred somewhat later in the earliest Triassic.
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