Graham Shields-Zhou | University College London (original) (raw)

Books by Graham Shields-Zhou

Research paper thumbnail of Radiogenic isotope geochemistry in sedimentary and aquatic systems

Papers by Graham Shields-Zhou

Research paper thumbnail of Biogeochemical cycling through the Neoproterozoic-Cambrian transition in China: an integrated study of redox-sensitive elements

I investigated changes in the biogeochemical cycling the Precambrian-Cambrian transition on the Y... more I investigated changes in the biogeochemical cycling the Precambrian-Cambrian transition on the Yangtze Platform in South China by analyzing about 350 predominantly black shale samples from several sedimentary successions deposited during the interval from the Late Cryogenian to the Lower Cambrian. I focused on redox-sensitive trace-metal concentrations in these sediments deposited under anoxic conditions whereby special attention is paid to molybdenum, vanadium and uranium to try to pinpoint the onset of increasing atmospheric oxygen levels and the transition from possibly widespread euxinia to a pervasively oxygenated deep ocean. The measurements have been carried out using X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS).

Research paper thumbnail of Chapter 11 The record of Neoproterozoic glaciation in the Taoudeni Basin, NW Africa

Geological Society, London, Memoirs, 2011

ABSTRACT The Taoudéni Basin covers over 1 000 000 km2 of the West African Craton, bounded by Pan-... more ABSTRACT The Taoudéni Basin covers over 1 000 000 km2 of the West African Craton, bounded by Pan-African orogenic belts. Four supergroups separated by craton-scale unconformities are recognized, with Neoproterozoic glaciogenic deposits occurring at the base of Supergroup 2. The Jbéliat Group occurs along a continuous, 1300-km-long, narrow belt from the Adrar region of Mauritania to the eastern limit of the Hank in Algeria and comprises thin glacial drift capped widely by periglacial polygonal structures, with more complex glacial sequences preserved in palaeo-depressions. A thicker, variously marine and continental glaciogenic succession can be found in southern parts, while fully marine, glacially influenced successions are only known from the extreme SW of the basin. The ‘triad’ sequence of diamictites overlain by barite-bearing ‘cap’ dolostones and then by green shales and/or bedded cherts (silexites) is ubiquitous and has long been used to correlate the Supergroup 1/2 boundary across the basin and into the surrounding orogenic belts. The bedded cherts commonly show a volcanic influence and are cemented by early marine calcite at their base at Adrar, Mauritania. Although fossil-based age constraints are scarce and ambiguous, regional tectonic events indicate that ‘triad’ deposition occurred between the Bassaride (665–655 Ma) and Dahomeyide (610–580 Ma) orogens. Recent U–Pb zircon studies of ignimbrite tuffs provide a minimum age for the glaciation of c. 600 Ma. Correlation of supergroup 2 glacial deposits with the c. 635 Ma end-Cryogenian (‘Marinoan’) glaciation is likely and is supported by limited carbon and strontium isotope data. Barite is commonly found within the cap carbonate and may relate to methane seepage and/or unusual oceanographic conditions after deglaciation. Several studies have attributed sequence complexity within the post-glacial succession to isostatic reequilibration. The Taoudéni Basin represents a rare Neoproterozoic example of terrestrial tillites and associated periglacial facies.

Research paper thumbnail of Isotopic composition of seawater past and present (Sr, Nd, Pb, Os, Ce)

Lecture Notes in Earth Sciences, 1997

Research paper thumbnail of Trace and rare earth element geochemistry of black shale and kerogen in the early Cambrian Niutitang Formation in Guizhou province, South China: Constraints for redox environments and origin of metal enrichments

Precambrian Research, 2013

Abstract Early Cambrian black shales of South China not only host important sponge, arthropod and... more Abstract Early Cambrian black shales of South China not only host important sponge, arthropod and other soft-bodied fossils that have helped to trace early metazoan diversification, but also show extreme enrichments of a number of trace metals in particular Ni, Mo and V. In this study, we use a new approach by analyzing rare earth elements in kerogen extracted from the black shales, together with a number of redox-sensitive trace element compositions and total organic carbon (TOC) concentrations in an early ...

Research paper thumbnail of Strontium isotope stratigraphy

A Geologic Time Scale 2004, 2005

The 87 Sr/ 86 Sr value of Sr dissolved in the world's oceans varied though time in a known way, f... more The 87 Sr/ 86 Sr value of Sr dissolved in the world's oceans varied though time in a known way, facts that allow 87 Sr/ 86 Sr to be used to date and to correlate marine sedimentary rocks worldwide. In this work, the variation in 87 Sr/ 86 Sr is displayed graphically and the theory and practice of the methodology is discussed. Chapter Outline 7.

Research paper thumbnail of Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation

Nature Geoscience, 2014

Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Camb... more Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Cambrian boundary 1-3 , initiating a transition 4,5 between the stratified Precambrian 6 and more well-mixed Phanerozoic 7 sedimentary records, against the backdrop of a variable 8,9 global oxygen reservoir probably smaller in size than present 10,11 . Phosphorus is the longterm 12 limiting nutrient for oxygen production via burial of organic carbon 13 , and its retention (relative to carbon) within organic matter in marine sediments is enhanced by bioturbation 14-18 . Here we explore the biogeochemical implications of a bioturbation-induced organic phosphorus sink in a simple model. We show that increased bioturbation robustly triggers a net decrease in the size of the global oxygen reservoir-the magnitude of which is contingent upon the prescribed di erence in carbon to phosphorus ratios between bioturbated and laminated sediments. Bioturbation also reduces steady-state marine phosphate levels, but this e ect is o set by the decline in iron-adsorbed phosphate burial that results from a decrease in oxygen concentrations.

Research paper thumbnail of T. Goldberg, A. Mazumdar

Research paper thumbnail of OCEAN REDOX CONTROL ON NUTRIENT CYCLING IN THE EARLY NEOPROTEROZOIC

Research paper thumbnail of Isotope geochemistry of river water

Research paper thumbnail of Weathering

Research paper thumbnail of Integrated global correlation of the Late Neoproterozoic: exploring the glacial roots of the metazoa

The user has requested enhancement of the downloaded file.

Research paper thumbnail of A bioturbation-induced decrease in atmospheric oxygen across the Precambrian-Cambrian boundary

Research paper thumbnail of The Cryogenian Period

ABSTRACT The Cryogenian Period is characterized by evidence for at least two prolonged episodes o... more ABSTRACT The Cryogenian Period is characterized by evidence for at least two prolonged episodes of widespread, even global, glaciation and the contemporaneous reappearance of sedimentary iron formations. The pre-glacial portion marks the disappearance of early marine microsparite crack fill (molar tooth structure), a decline in diversity of stromatolite forms, proliferation then decline in organic-walled microfossil (acritarch) biodiversity, appearance of vase-shaped microfossils (possible testate amoebae), and the beginning at c. 800 Ma of a series of negative d13C excursions. The later Cryogenian is typified by very high d13Ccarb and d34Spyr values in its nonglacial part, and the onset of glaciation which culminates in the worldwide deposition of cap dolostones that marks the start of the Ediacaran Period. The chronometrically defined base of the Cryogenian Period (850Maec. 635Ma) is set to be replaced in coming years by a Global Stratotype Section and Point (GSSP).

Research paper thumbnail of Carbon Cycle

Encyclopedia of Geology, 2005

Research paper thumbnail of Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China

Precambrian Research, 2013

Abstract This study reports a high-resolution carbon and strontium isotope profile for the Ediaca... more Abstract This study reports a high-resolution carbon and strontium isotope profile for the Ediacaran–Cambrian Xiaotan section, situated in northeastern Yunnan, South China. Xiaotan section represents a more distal setting than the more condensed Meishucun section in eastern Yunnan, and is, biostratigraphically and chemostratigraphically, the best constrained section on the Yangtze platform covering the Ediacaran–Cambrian interval. The carbonate carbon isotopic ratios of the late Ediacaran, upper Dengying Formation to the ...

Research paper thumbnail of The DOUNCE event at the top of the Ediacaran Doushantuo Formation, South China: Broad stratigraphic occurrence and non-diagenetic origin

Precambrian Research, 2013

Research paper thumbnail of Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China

Precambrian …, Jan 1, 2011

The Precambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs ... more The Precambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet, when changes in chemical and physical conditions, together with key biological innovations, helped to trigger a biodiversity "explosion". The Yangtze Platform (South China), mainly characterized by continuous and unaltered successions of this age, is ideally suited to high-resolution, palaeomarine investigations of this event. In this study, ␦ 15 N and ␦ 13 Corg records from Xiaotan Section (Yunnan, SW China) were investigated in order to provide insight into variations in primary productivity, ecological developments and marine environment. The Ediacaran-early Cambrian Xiaotan section is characterized by relatively high sedimentation rates and variable lithologies (carbonates, cherts, phosphorites, sandstones and siltstones) that alternate through nearly 600 meters thickness. Organic carbon isotope values vary between −36‰ and −21‰, tightly following and helping to complete published ␦ 13 Ccarb trends. The base of the Cambrian is characterized in this section by a significant drop from −25‰ to −35‰, mirroring the ␦ 13 Ccarb fall observed at this and other sections worldwide. Higher in the section in the Dahai Member, values increase to −20‰, again demonstrating communication between DOC and DIC pools during the early Cambrian on 10 5 -10 6 year time scales. From the base of the overlying Shiyantou Formation, ␦ 13 Corg values remain around −30‰ until the end of the section, testifying to an interval of more modest change in DIC ␦ 13 C that would be consistent with biostratigraphic correlations. Nitrogen isotope values vary independently from carbon isotope trends, exhibiting several major ␦ 15 N cycles, with values fluctuating between +9‰ and −1‰. Robust trends in ␦ 15 N within the Dahai Member testify to changes in the ratio between nitrogen fixation and denitrification, possibly reflecting fluctuations in the water column chemocline as ␦ 13 C values reached their global acme. Up section, nitrogen isotope values show dampened cyclicity with values remaining between 0‰ and +5‰, indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles. The overall variability in ␦ 15 N closely resembles that of recent marine sediments, while the observed cyclicity is defined by several samples in all cases. These observations argue for good preservation of original isotopic signatures which bodes well for future N isotope studies of the Precambrian-Cambrian boundary.

Research paper thumbnail of Redox Changes in Early Cambrian black shales at Xiaotan section, Yunnan Province, South China

Precambrian …, Jan 1, 2011

The Xiaotan section, situated on the southwestern Yangtze Platform, South China, contains an unus... more The Xiaotan section, situated on the southwestern Yangtze Platform, South China, contains an unusually expanded sedimentary record, spanning the upper Neoproterozoic to the Cambrian and representing more than 40 My of marine sedimentation. Three distinct black shale units of the Shiyantou and Yuanshan formations were analysed for redox-sensitive trace elements and iron speciation, in order to constrain the evolution of the marine environment during the Early Cambrian metazoan biodiversification on the southwestern Yangtze Platform. For comparison we also include black shale samples of the Shiyantou Fm. from the Meishucun section, samples from a previous study covering the Shiyantou and Yuanshan Fm. at Dapotuo near Chengjiang, and samples from the strongly condensed Early Cambrian Niutitang Fm. from the Zhongnan section situated on the platform margin. Th/U and Fe HR /Fe T ratios reveal that bottom waters on the southwestern Yangtze Platform were commonly anoxic and ferruginous into the Cambrian, but became oxic after deposition of the black shales of the lower Yuanshan Formation, which elsewhere contains the Chengjiang Biota. Average Mo/TOC ratios, used as an indicator of the size of the Mo reservoir, increase up-section, and together with generally increasing S/C ratios suggest a trend towards normal (Phanerozoic) marine conditions. A distinctive Ni peak within the upper Shiyantou Fm. at Xiaotan and maximum Mo concentrations at the base of the Yuanshan Fm. can be correlated to the Ni-Mo sulphide layer within the lower Niutitang Fm. and thus indicate two separate pulses of sedimentary-exhalative nickel enrichment and subsequent, redox controlled molybdenum accumulation. A strong decoupling of organic carbon and pyrite sulphur within both sections further suggests upwelling of H 2 S-rich waters onto the platform. We argue here for an opening of the depositional environment on the northwestern Yangtze Platform which led to the upwelling of sulphidic waters, ventilation of the seafloor and ultimately its colonisation by newly emergent animal groups during the 'Cambrian Explosion'.

Research paper thumbnail of Cerium anomaly variations in Ediacaran–earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater

Precambrian Research, 2013

The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been s... more The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean. However, apart from the emergence of animal fossils, little supporting evidence has been found for the rise of oxygen in shallow seawater. Here we present Ce anomaly data, a redox proxy with a higher reduction potential than many other redox proxies, for carbonates from well-preserved marine successions (the Doushantuo Formation, Dengying Formation and lower Yanjiahe Formation of the Ediacaran Period, and the upper Yanjiahe Formation of early Cambrian) on the Neoproterozoic Yangtze Platform in the Yangtze Gorges area, in order to constrain the redox evolution of the shallow marine environment. Calculated Ce anomalies were screened, using the following criteria: Al < 0.35% and Fe < 0.45% in bulk rock, Th < 0.5 ppm, Sc < 2 ppm, REE < 12 ppm and Y/Ho > 36 in acetic-acid-leached carbonate, to target samples that preserve primary seawater REE features and Ce anomalies. The samples satisfying this screening show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Doushantuo Formation has Ce/Ce* values between 0.92 and 0.71, the Dengying Formation between 0.90 and 0.40, and the Yanjiahe Formation between 0.52 and 0.72. The Ce/Ce* values of the Doushantuo Formation suggest that shallow waters during the Doushantuo stage (635-551 Ma) were anoxic to suboxic. From the bottom to the top of the Dengying Formation, Ce/Ce* values decrease systematically, which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the Dengying stages (from 551 Ma to Ediacaran-Cambrian boundary). These results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the Ediacaran Period. (G.A. Shields-Zhou), myzhu@nigpas.ac.cn (M. Zhu). that period . However, details of this Neoproterozoic oxidation event remain unclear.

Research paper thumbnail of Radiogenic isotope geochemistry in sedimentary and aquatic systems

Research paper thumbnail of Biogeochemical cycling through the Neoproterozoic-Cambrian transition in China: an integrated study of redox-sensitive elements

I investigated changes in the biogeochemical cycling the Precambrian-Cambrian transition on the Y... more I investigated changes in the biogeochemical cycling the Precambrian-Cambrian transition on the Yangtze Platform in South China by analyzing about 350 predominantly black shale samples from several sedimentary successions deposited during the interval from the Late Cryogenian to the Lower Cambrian. I focused on redox-sensitive trace-metal concentrations in these sediments deposited under anoxic conditions whereby special attention is paid to molybdenum, vanadium and uranium to try to pinpoint the onset of increasing atmospheric oxygen levels and the transition from possibly widespread euxinia to a pervasively oxygenated deep ocean. The measurements have been carried out using X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS).

Research paper thumbnail of Chapter 11 The record of Neoproterozoic glaciation in the Taoudeni Basin, NW Africa

Geological Society, London, Memoirs, 2011

ABSTRACT The Taoudéni Basin covers over 1 000 000 km2 of the West African Craton, bounded by Pan-... more ABSTRACT The Taoudéni Basin covers over 1 000 000 km2 of the West African Craton, bounded by Pan-African orogenic belts. Four supergroups separated by craton-scale unconformities are recognized, with Neoproterozoic glaciogenic deposits occurring at the base of Supergroup 2. The Jbéliat Group occurs along a continuous, 1300-km-long, narrow belt from the Adrar region of Mauritania to the eastern limit of the Hank in Algeria and comprises thin glacial drift capped widely by periglacial polygonal structures, with more complex glacial sequences preserved in palaeo-depressions. A thicker, variously marine and continental glaciogenic succession can be found in southern parts, while fully marine, glacially influenced successions are only known from the extreme SW of the basin. The ‘triad’ sequence of diamictites overlain by barite-bearing ‘cap’ dolostones and then by green shales and/or bedded cherts (silexites) is ubiquitous and has long been used to correlate the Supergroup 1/2 boundary across the basin and into the surrounding orogenic belts. The bedded cherts commonly show a volcanic influence and are cemented by early marine calcite at their base at Adrar, Mauritania. Although fossil-based age constraints are scarce and ambiguous, regional tectonic events indicate that ‘triad’ deposition occurred between the Bassaride (665–655 Ma) and Dahomeyide (610–580 Ma) orogens. Recent U–Pb zircon studies of ignimbrite tuffs provide a minimum age for the glaciation of c. 600 Ma. Correlation of supergroup 2 glacial deposits with the c. 635 Ma end-Cryogenian (‘Marinoan’) glaciation is likely and is supported by limited carbon and strontium isotope data. Barite is commonly found within the cap carbonate and may relate to methane seepage and/or unusual oceanographic conditions after deglaciation. Several studies have attributed sequence complexity within the post-glacial succession to isostatic reequilibration. The Taoudéni Basin represents a rare Neoproterozoic example of terrestrial tillites and associated periglacial facies.

Research paper thumbnail of Isotopic composition of seawater past and present (Sr, Nd, Pb, Os, Ce)

Lecture Notes in Earth Sciences, 1997

Research paper thumbnail of Trace and rare earth element geochemistry of black shale and kerogen in the early Cambrian Niutitang Formation in Guizhou province, South China: Constraints for redox environments and origin of metal enrichments

Precambrian Research, 2013

Abstract Early Cambrian black shales of South China not only host important sponge, arthropod and... more Abstract Early Cambrian black shales of South China not only host important sponge, arthropod and other soft-bodied fossils that have helped to trace early metazoan diversification, but also show extreme enrichments of a number of trace metals in particular Ni, Mo and V. In this study, we use a new approach by analyzing rare earth elements in kerogen extracted from the black shales, together with a number of redox-sensitive trace element compositions and total organic carbon (TOC) concentrations in an early ...

Research paper thumbnail of Strontium isotope stratigraphy

A Geologic Time Scale 2004, 2005

The 87 Sr/ 86 Sr value of Sr dissolved in the world's oceans varied though time in a known way, f... more The 87 Sr/ 86 Sr value of Sr dissolved in the world's oceans varied though time in a known way, facts that allow 87 Sr/ 86 Sr to be used to date and to correlate marine sedimentary rocks worldwide. In this work, the variation in 87 Sr/ 86 Sr is displayed graphically and the theory and practice of the methodology is discussed. Chapter Outline 7.

Research paper thumbnail of Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation

Nature Geoscience, 2014

Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Camb... more Animal burrowing and sediment-mixing (bioturbation) began during the run up to the Ediacaran/Cambrian boundary 1-3 , initiating a transition 4,5 between the stratified Precambrian 6 and more well-mixed Phanerozoic 7 sedimentary records, against the backdrop of a variable 8,9 global oxygen reservoir probably smaller in size than present 10,11 . Phosphorus is the longterm 12 limiting nutrient for oxygen production via burial of organic carbon 13 , and its retention (relative to carbon) within organic matter in marine sediments is enhanced by bioturbation 14-18 . Here we explore the biogeochemical implications of a bioturbation-induced organic phosphorus sink in a simple model. We show that increased bioturbation robustly triggers a net decrease in the size of the global oxygen reservoir-the magnitude of which is contingent upon the prescribed di erence in carbon to phosphorus ratios between bioturbated and laminated sediments. Bioturbation also reduces steady-state marine phosphate levels, but this e ect is o set by the decline in iron-adsorbed phosphate burial that results from a decrease in oxygen concentrations.

Research paper thumbnail of T. Goldberg, A. Mazumdar

Research paper thumbnail of OCEAN REDOX CONTROL ON NUTRIENT CYCLING IN THE EARLY NEOPROTEROZOIC

Research paper thumbnail of Isotope geochemistry of river water

Research paper thumbnail of Weathering

Research paper thumbnail of Integrated global correlation of the Late Neoproterozoic: exploring the glacial roots of the metazoa

The user has requested enhancement of the downloaded file.

Research paper thumbnail of A bioturbation-induced decrease in atmospheric oxygen across the Precambrian-Cambrian boundary

Research paper thumbnail of The Cryogenian Period

ABSTRACT The Cryogenian Period is characterized by evidence for at least two prolonged episodes o... more ABSTRACT The Cryogenian Period is characterized by evidence for at least two prolonged episodes of widespread, even global, glaciation and the contemporaneous reappearance of sedimentary iron formations. The pre-glacial portion marks the disappearance of early marine microsparite crack fill (molar tooth structure), a decline in diversity of stromatolite forms, proliferation then decline in organic-walled microfossil (acritarch) biodiversity, appearance of vase-shaped microfossils (possible testate amoebae), and the beginning at c. 800 Ma of a series of negative d13C excursions. The later Cryogenian is typified by very high d13Ccarb and d34Spyr values in its nonglacial part, and the onset of glaciation which culminates in the worldwide deposition of cap dolostones that marks the start of the Ediacaran Period. The chronometrically defined base of the Cryogenian Period (850Maec. 635Ma) is set to be replaced in coming years by a Global Stratotype Section and Point (GSSP).

Research paper thumbnail of Carbon Cycle

Encyclopedia of Geology, 2005

Research paper thumbnail of Carbon and strontium isotope evolution of seawater across the Ediacaran–Cambrian transition: Evidence from the Xiaotan section, NE Yunnan, South China

Precambrian Research, 2013

Abstract This study reports a high-resolution carbon and strontium isotope profile for the Ediaca... more Abstract This study reports a high-resolution carbon and strontium isotope profile for the Ediacaran–Cambrian Xiaotan section, situated in northeastern Yunnan, South China. Xiaotan section represents a more distal setting than the more condensed Meishucun section in eastern Yunnan, and is, biostratigraphically and chemostratigraphically, the best constrained section on the Yangtze platform covering the Ediacaran–Cambrian interval. The carbonate carbon isotopic ratios of the late Ediacaran, upper Dengying Formation to the ...

Research paper thumbnail of The DOUNCE event at the top of the Ediacaran Doushantuo Formation, South China: Broad stratigraphic occurrence and non-diagenetic origin

Precambrian Research, 2013

Research paper thumbnail of Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China

Precambrian …, Jan 1, 2011

The Precambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs ... more The Precambrian-Cambrian boundary is claimed as one of the most crucial biological breakthroughs on our planet, when changes in chemical and physical conditions, together with key biological innovations, helped to trigger a biodiversity "explosion". The Yangtze Platform (South China), mainly characterized by continuous and unaltered successions of this age, is ideally suited to high-resolution, palaeomarine investigations of this event. In this study, ␦ 15 N and ␦ 13 Corg records from Xiaotan Section (Yunnan, SW China) were investigated in order to provide insight into variations in primary productivity, ecological developments and marine environment. The Ediacaran-early Cambrian Xiaotan section is characterized by relatively high sedimentation rates and variable lithologies (carbonates, cherts, phosphorites, sandstones and siltstones) that alternate through nearly 600 meters thickness. Organic carbon isotope values vary between −36‰ and −21‰, tightly following and helping to complete published ␦ 13 Ccarb trends. The base of the Cambrian is characterized in this section by a significant drop from −25‰ to −35‰, mirroring the ␦ 13 Ccarb fall observed at this and other sections worldwide. Higher in the section in the Dahai Member, values increase to −20‰, again demonstrating communication between DOC and DIC pools during the early Cambrian on 10 5 -10 6 year time scales. From the base of the overlying Shiyantou Formation, ␦ 13 Corg values remain around −30‰ until the end of the section, testifying to an interval of more modest change in DIC ␦ 13 C that would be consistent with biostratigraphic correlations. Nitrogen isotope values vary independently from carbon isotope trends, exhibiting several major ␦ 15 N cycles, with values fluctuating between +9‰ and −1‰. Robust trends in ␦ 15 N within the Dahai Member testify to changes in the ratio between nitrogen fixation and denitrification, possibly reflecting fluctuations in the water column chemocline as ␦ 13 C values reached their global acme. Up section, nitrogen isotope values show dampened cyclicity with values remaining between 0‰ and +5‰, indicating establishment of an equilibrium state in marine biogeochemistry and nutrient cycles. The overall variability in ␦ 15 N closely resembles that of recent marine sediments, while the observed cyclicity is defined by several samples in all cases. These observations argue for good preservation of original isotopic signatures which bodes well for future N isotope studies of the Precambrian-Cambrian boundary.

Research paper thumbnail of Redox Changes in Early Cambrian black shales at Xiaotan section, Yunnan Province, South China

Precambrian …, Jan 1, 2011

The Xiaotan section, situated on the southwestern Yangtze Platform, South China, contains an unus... more The Xiaotan section, situated on the southwestern Yangtze Platform, South China, contains an unusually expanded sedimentary record, spanning the upper Neoproterozoic to the Cambrian and representing more than 40 My of marine sedimentation. Three distinct black shale units of the Shiyantou and Yuanshan formations were analysed for redox-sensitive trace elements and iron speciation, in order to constrain the evolution of the marine environment during the Early Cambrian metazoan biodiversification on the southwestern Yangtze Platform. For comparison we also include black shale samples of the Shiyantou Fm. from the Meishucun section, samples from a previous study covering the Shiyantou and Yuanshan Fm. at Dapotuo near Chengjiang, and samples from the strongly condensed Early Cambrian Niutitang Fm. from the Zhongnan section situated on the platform margin. Th/U and Fe HR /Fe T ratios reveal that bottom waters on the southwestern Yangtze Platform were commonly anoxic and ferruginous into the Cambrian, but became oxic after deposition of the black shales of the lower Yuanshan Formation, which elsewhere contains the Chengjiang Biota. Average Mo/TOC ratios, used as an indicator of the size of the Mo reservoir, increase up-section, and together with generally increasing S/C ratios suggest a trend towards normal (Phanerozoic) marine conditions. A distinctive Ni peak within the upper Shiyantou Fm. at Xiaotan and maximum Mo concentrations at the base of the Yuanshan Fm. can be correlated to the Ni-Mo sulphide layer within the lower Niutitang Fm. and thus indicate two separate pulses of sedimentary-exhalative nickel enrichment and subsequent, redox controlled molybdenum accumulation. A strong decoupling of organic carbon and pyrite sulphur within both sections further suggests upwelling of H 2 S-rich waters onto the platform. We argue here for an opening of the depositional environment on the northwestern Yangtze Platform which led to the upwelling of sulphidic waters, ventilation of the seafloor and ultimately its colonisation by newly emergent animal groups during the 'Cambrian Explosion'.

Research paper thumbnail of Cerium anomaly variations in Ediacaran–earliest Cambrian carbonates from the Yangtze Gorges area, South China: Implications for oxygenation of coeval shallow seawater

Precambrian Research, 2013

The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been s... more The late Neoproterozoic ocean witnessed the naissance of animals which is believed to have been stimulated by an increase in oxygen levels in the ocean. However, apart from the emergence of animal fossils, little supporting evidence has been found for the rise of oxygen in shallow seawater. Here we present Ce anomaly data, a redox proxy with a higher reduction potential than many other redox proxies, for carbonates from well-preserved marine successions (the Doushantuo Formation, Dengying Formation and lower Yanjiahe Formation of the Ediacaran Period, and the upper Yanjiahe Formation of early Cambrian) on the Neoproterozoic Yangtze Platform in the Yangtze Gorges area, in order to constrain the redox evolution of the shallow marine environment. Calculated Ce anomalies were screened, using the following criteria: Al < 0.35% and Fe < 0.45% in bulk rock, Th < 0.5 ppm, Sc < 2 ppm, REE < 12 ppm and Y/Ho > 36 in acetic-acid-leached carbonate, to target samples that preserve primary seawater REE features and Ce anomalies. The samples satisfying this screening show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Doushantuo Formation has Ce/Ce* values between 0.92 and 0.71, the Dengying Formation between 0.90 and 0.40, and the Yanjiahe Formation between 0.52 and 0.72. The Ce/Ce* values of the Doushantuo Formation suggest that shallow waters during the Doushantuo stage (635-551 Ma) were anoxic to suboxic. From the bottom to the top of the Dengying Formation, Ce/Ce* values decrease systematically, which cannot be explained by a change in depositional depth of the carbonate but suggests that the shallow waters became more oxygenated during the Dengying stages (from 551 Ma to Ediacaran-Cambrian boundary). These results may provide direct evidence for an increase in oxygen levels in the shallow marine environment during the Ediacaran Period. (G.A. Shields-Zhou), myzhu@nigpas.ac.cn (M. Zhu). that period . However, details of this Neoproterozoic oxidation event remain unclear.

Research paper thumbnail of Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran–Cambrian transition in South China

Palaeogeography, Palaeoclimatology, Palaeoecology, 2014

Please cite this article as: Cremonese, Lorenzo, Shields-Zhou, Graham A., Struck, Ulrich, Ling, H... more Please cite this article as: Cremonese, Lorenzo, Shields-Zhou, Graham A., Struck, Ulrich, Ling, Hong-Fei, Och, Lawrence M., Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran-Abstract N and C org isotope results are presented from six sections along a West-East transect in the South China Basin (SCB) covering both shallow and deeper domains, in order to investigate biogeochemical cycling, stratigraphic correlation and isotope systematics over the crucial Ediacaran-Cambrian transition.  15 N bulk values range between -3‰ and +7‰, while  13 C org values range between -21‰ and -39‰. Similar isotopic trends have been identified for both these proxies across the basin, although hiatuses, differences in depositional setting and syn-depositional bacterial fermentation may have caused some inconsistencies. A trend towards negative N isotope values can be recognized above the PC-C boundary in both shallow and deeper basin realms across the Yangtze platform. This negative  15 N excursion is probably a response to photic zone anoxia and intense nitrogen fixation/assimilation by diazotrophic cyanobacteria and Green/Purple Sulfur Bacteria (GSB and PSB). The Xiaotan section and a composite section from the Yangtze Gorges area show meaningful similarities in their nitrogen isotope trends, interpreted as chemocline fluctuations in the water column that testify to rapid mixing of water overlying the shallow platform. Using carbon isotope stratigraphy, we correlate boundary strata across the platform to test the wider significance of nitrogen isotopic variations. Increased bioturbation and food-chain complexity across the Ediacaran-Cambrian transition probably led to more frequent variations in nitrate isotope composition and related pool dimension during the early Cambrian, reflecting the beginning of a new more biologically controlled era.