Alexander J. Dickson

• Professor (Full) at Royal Holloway University of London

The Paleocene-Eocene Thermal Maximum (PETM) ~56 million years ago, provides one of the best geological analogues for investigating how marine oxygen levels respond to rapid global warming and massive perturbations to the global carbon cycle. Various studies on PETM shelf sections have documented the deposition of an extensive organic-rich sapropel...

Lower Jurassic marine basins across the northwest European epicontinental shelf were commonly marked by deposition of organic-rich black shales. Organic-carbon burial was particularly widespread during the Toarcian Oceanic Anoxic Event (T-OAE: also known as the Jenkyns Event) with its accompanying negative carbon-isotope excursion (nCIE). Lower Toa...

The Paleocene–Eocene Thermal Maximum (PETM) is associated with climatic change and biological turnover. It shares features with the Oceanic Anoxic Events (OAEs) of the Mesozoic, such as transient global warming and biogeochemical perturbations. However, the PETM experienced a more muted expansion of marine anoxia compared to the Mesozoic OAEs (espe...

The isotopic composition of rhenium (Re) has potential for use as a proxy to infer changes in seafloor redox and/ or global oxidative weathering intensity. Despite an emerging dataset on this nascent isotope system in Earth's surficial environments, very little is known about processes that control Re isotope fractionation, nor the isotopic composi...

Oxidative weathering of organic carbon in sedimentary rocks is a major source of CO2 to the atmosphere over geological timescales, but the size of this emission pathway in Earth's past has not been directly quantified due to a lack of available proxy approaches. We have measured the rhenium isotope composition of organic‐rich rocks sampled from unw...

Understanding the behavior of mercury (Hg) in organic‐rich sediments as they undergo thermal maturation is important, for example, because enrichment of Hg in sedimentary deposits has become a widely used proxy for volcanism from Large Igneous Provinces (LIPs). In this study, we evaluate the effects of such processes on sedimentary Hg concentration...

The Paleocene–Eocene Thermal Maximum (PETM) was a global hyperthermal event ∼56 Ma characterized by massive input of carbon into the ocean–atmosphere system and global warming. A leading hypothesis for its trigger is the emplacement of the North Atlantic Igneous Province (NAIP), with extensive extrusion/intrusion of igneous material into nearby sed...

The Paleocene‐Eocene Thermal Maximum (PETM) was a transient global warming event and is recognized in the geologic record by a prolonged negative carbon isotope excursion (CIE). The onset of the CIE was due to a rapid influx of ¹³C‐depleted carbon into the ocean‐atmosphere system. However, the mechanisms required to sustain the negative CIE remains...

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic period (~201.4 million years ago). The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions. However, the global-scale pattern of anoxic conditions across...

Drilling for the International Continental Scientific Drilling Program (ICDP) Early Jurassic Earth System and Timescale project (JET) was undertaken between October 2020 and January 2021. The drill site is situated in a small-scale synformal basin of the latest Triassic to Early Jurassic age that formed above the major Permian-Triassic half-graben...

The early Paleozoic was marked by several carbon-cycle perturbations and associated carbon-isotope excursions (CIEs). Whether these CIEs are connected to significant (external) triggers, as is commonly considered to be the case for CIEs in the Mesozoic and Cenozoic, or result from small carbon-cycle imbalances that became amplified through lack of...

In ancient sedimentary systems, high molybdenum (Mo) enrichments are associated with strong euxinic conditions. Molybdenum isotopes (δ98Mo) have been used as a paleo-redox proxy for more than two decades. Within sulfidic waters, MoO42− can undergo thiolation: Sulfur (S) replaces molecular oxygen (O) to form thiomolybdate species (MoOxS(4−x)2−). Thi...

The U-isotope system is a well-established palaeo-redox proxy that potentially constrains the global extent of marine anoxia during average as well as extreme redox events throughout Earth's history. A typical archive that forms underneath a reducing water column and acts as an intense U sink is organic-rich black shale. However, the degree to whic...

The Sogno Core represents one of the deepest pelagic records of the Toarcian Oceanic Anoxic Event (T-OAE) in the Alpine-Mediterranean Tethys. New sedimentological, elemental, Rock-Eval, and biomarker data are presented here, with the aim of reconstructing the depositional conditions that characterized the sedimentation of this succession during the...

Nickel and zinc are both bio‐essential micronutrients with a nutrient‐like distribution in the modern ocean, but show key differences in their biological functions and geochemical behavior. Eukaryotic phytoplankton, and especially diatoms, have high Zn quotas, whereas cyanobacteria generally require relatively more Ni. Secular changes in the relati...

The pulsed extinction of marine organisms during the latest Triassic (the end-Triassic mass extinction, ETME) represents one of the largest mass extinction events in geological history. The ETME is thought to have been driven by large igneous province volcanism which caused perturbations to ocean redox chemistry and surface temperatures. However, t...

The controls on Zn burial fluxes into marine sediments are not well constrained by existing datasets. To address this problem, new Zn-isotope data have been generated from a globally distributed array of late-Holocene age sediments that accumulated in open ocean settings underneath a diverse range of depositional conditions (Namibian margin, West A...

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic Period. The end-Triassic extinction event was initiated by large igneous province volcanism and has tentatively been linked to oceanic redox change. However, the global-scale pattern of oceanic redox across the end-Triassic event is not...

The past ~550 million years of Earth’s history have been marked by numerous periods of global extinction, one of the most severe of which took place at the end of the Triassic period (ETME). The ETME has been strongly assoicated with marine redox change. However, the global scale pattern of oceanic redox across the ETME is not well constrained....

The de-oxygenation of marine environments is thought to have played a significant role in many of Earth’s major mass extinction events. This includes the end-Triassic mass extinction event (ETME), which witnessed the disappearance of conodonts, the near extinction of ammonoids and the most significant reef crisis across the entirety of the Phaneroz...

The de‑oxygenation of marine environments is thought to have played a significant role in many of Earth's major mass extinction events. This includes the end-Triassic mass extinction event (ETME), which witnessed the disappearance of conodonts, the near extinction of ammonoids and the most significant reef crisis across the entirety of the Phaneroz...

The early Toarcian Oceanic Anoxic Event (T-OAE) was associated with major climatic changes involving profound effects on the global carbon cycle. In this study, we present new carbon- and oxygen- isotope, CaCO3 and total organic carbon (TOC) records from two cores (Sogno and Gajum Cores) that recovered pelagic successions from north-western Tethys....

The isotope ratios of redox-sensitive metals in organic-rich rocks are critical tools for quantifying the timing and severity of deoxygenation and nutrient cycling in Earth’s past. The resilience of isotopic data to thermal alteration of the host sediments over millions of years of burial is, however, largely unknown. We present molybdenum, uranium...

Molybdenum (Mo) is a widely used trace metal for investigating redox conditions. However, unanswered questions remain that concentration and bulk isotopic analysis cannot specially answer. Improvements can be made by combining new geochemical techniques to traditional methods of Mo analysis. In this Element, we propose a refinement of Mo geochemist...

The metals strontium (Sr), lithium (Li), osmium (Os) and calcium (Ca), together with their isotopes, are important tracers of weathering and volcanism – primary processes which shape the long-term cycling of carbon and other biogeochemically important elements at the Earth's surface. Traditionally, because of their long residence times in the ocean...

Continental margin sediments have been identified as the dominant sink in the marine budget of cadmium (Cd). The isotopic composition of this important output flux is, however, unknown. Here we present, with measurements on the Argentine continental margin, the first observational constraints on the isotopic composition of Cd in modern marine oxic...

Despite increasing numbers of studies interpreting the end-Triassic mass extinction event (ETME) as being caused through global marine anoxia, there are currently few studies which present redox data from across the critical interval, nor high resolution redox studies, or studies which correlate redox and biodiversity data from the same site. H...

Intervals of extreme warmth are predicted to drive a decrease in the oxygen content of the oceans. This prediction has been tested for the acme of short (<1 million years) episodes of significant marine anoxia in the Phanerozoic geological record known as Oceanic Anoxic Events (OAEs). However, there is a paucity of data spanning prolonged multimill...

A multi-million-year decrease in global temperatures during the Eocene was accompanied by large reorganisations to ocean circulation, ocean chemistry and biological productivity. These changes culminated in the rapid growth of grounded ice on Antarctica during the Eocene–Oligocene climate transition (EOT), ∼34 million years ago. However, while it i...

The Paleocene Eocene Thermal Maximum (PETM) represents a major carbon cycle and climate perturbation that was associated with ocean de-oxygenation, in a qualitatively similar manner to the more extensive Mesozoic Oceanic Anoxic Events. Although indicators of ocean de-oxygenation are common for the PETM, and linked to biotic turnover, the global ext...

The emplacement of Large Igneous Provinces (LIPs) throughout the Phanerozoic Eon introduced vast quantities of mafic rocks to the Earth's surface, which were subsequently weathered into the oceans. Osmium isotope data can be used to track these LIP‐related weathering fluxes, providing a global fingerprint of the timing and magnitude of LIP emplacem...

This study uses organic-rich sediments from the Tarfaya Basin, Morocco, to assess the Cd- and Zn-isotope response to dramatic global palaeoenvironmental change during the Cenomanian–Turonian interval (Late Cretaceous). These organic-rich continental-margin deposits include an expression of Oceanic Anoxic Event 2 (OAE 2, ∼94 Ma), an interval associa...

The metals strontium (Sr), lithium (Li), osmium (Os) and calcium (Ca) and their isotopes are important tracers in the study of changes in weathering rates and volcanism, two main processes which shape the long-term cycling of carbon and other biogeochemically important elements at the Earth's surface. Traditionally, isotopic shifts of these four el...

The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and δ⁹⁸Mo from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) ‘iron-rich’ sediments containin...

Cores recovered from the Jurassic (Toarcian) Posidonienschiefer (Posidonia Shale) in the Lower Saxony Basin, Germany, contain calcite‐filled fractures (veins) at a low angle to bedding. The veins preferentially form where the shale is both organically rich and thermally mature, supporting previous interpretations that the veins formed as hydraulic...

The decline in dissolved oxygen in global oceans (ocean deoxygenation) is a potential consequence of global warming which may have important impacts on ocean biogeochemistry and marine ecosystems. Current climate models do not agree on the trajectory of future deoxygenation on different timescales, in part due to uncertainties in the complex, linke...

The concentrations and isotopic compositions of rhenium are presented from seawater samples obtained from the primary station for the Bermuda Atlantic Time Series Study in the North Atlantic Ocean and from the 40oS UK GEOTRACES expedition in the South Atlantic Ocean. Salinity-normalized Re concentrations in both locations range between ∼6.8–7.7 ppt...

The isotopic composition of Cd buried in marine sediments may preserve valuable palaeoenvironmental information on past ocean redox conditions or biological cycling. It is unclear, however, how the Cd-isotope composition of the sedimentary record reflects these processes. In this study, new Cd-isotope data are presented, along with δ¹³C, and Cd, Mo...

Earth’s history has been dominated by the large-scale disappearances of life in catastrophic events known as ‘Mass extinctions’. Mass extinction events have been closely linked to the large-scale release of isotopically light carbon, e.g. from large volcanic provinces, which is believed to have resulted in atmospheric, climatic and environmental ch...

The concentrations and isotopic compositions of molybdenum (Mo), zinc (Zn) and cadmium (Cd) in organic-rich marine mudrocks may be used to characterize ocean chemistry in the geological past. These approaches rely on the rarely tested assumption that the geochemical signatures of these metals are not affected by the thermal maturation of the organi...

Profound changes in upper ocean oxygenation have taken place in recent decades and are expected to continue in the future, but the complexity of the processes driving these changes has yet to be fully unraveled. Planktic foraminiferal I/Ca is a promising tool to reconstruct the extent of past upper ocean oxygenation, but a thorough assessment is ne...

Oxygen is a prerequisite for all large and motile animals. It is a puzzling paradox that fossils of benthic animals are often found in black shales with geochemical evidence for deposition in marine environments with anoxic and sulfidic bottom waters. It is debated whether the geochemical proxies are unreliable, affected by diagenesis, or whether t...

Mesozoic oceanic anoxic events (OAEs) were major perturbations of the Earth system, associated with high CO2 concentrations in the oceans and atmosphere, high temperatures, and widespread organic-carbon burial. Models for explaining OAEs and other similar phenomena in Earth history make specific predictions about the role and pattern of temperature...

Mercury (Hg) is increasingly being used as a sedimentary tracer of Large Igneous Province (LIP) volcanism, and supports hypotheses of a coincidence between the formation of several LIPs and episodes of mass extinction and major environmental perturbation. However, numerous important questions remain to be answered before Hg can be claimed as an une...

A newly located exposure of the Niveau Thomel, an organic-rich level at the Cenomanian-Tur-onian boundary, provides a highly expanded record of Oceanic Anoxic Event (OAE) 2, except for the lower relatively condensed glauconite-rich part of the section. The new locality, close to Barreme in the Vocontian Basin, SE France, is developed in deep-water...

The degree to which ocean deoxygenation will alter the function of marine communities remains unclear but may be best constrained by detailed study of intervals of rapid warming in the geologic past. The Paleocene-Eocene Thermal Maximum (PETM) was an interval of rapid warming that was the result of increasing contents of greenhouse gases in the atm...

The extent and persistence of anoxia in the South Atlantic Ocean during its early opening phase in the Early Cretaceous is not well constrained, hindering a holistic understanding of the processes and mechanisms that drive past changes in water column redox conditions, as well as the impacts of such changes on marine ecosystems. Here we provide hig...

The development of large ice-sheets across the Northern Hemisphere during the late Pliocene and the emergence of the glacial-interglacial cycles that punctuate the Quaternary mark a significant threshold in Earth's climate history. Although a number of different mechanisms have been proposed to initiate this cooling and the onset of major Northern...

The development of large ice-sheets across the Northern Hemisphere during the late Pliocene and the emergence of the glacial-interglacial cycles that punctuate the Quaternary mark a significant threshold in Earth's climate history. Although a number of different mechanisms have been proposed to initiate this cooling and the onset of major Northern...

Widespread deposition of organic-rich shales during the Late Cretaceous Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) occurred during a period of significant global paleo-environmental and geochemical change. It has been proposed that an increase in nutrient input to the ocean during OAE 2 was the key mechanism that generated and sustained high rates o...

Significance Past “Oceanic Anoxic Events” (OAEs) represent important carbon cycle perturbations that offer the opportunity to study Earth’s response to extreme climate warming. A fundamental limitation for understanding OAEs is quantifying the timing and total extent of ocean anoxia. We present a quantitative account of global redox conditions for...

The emergence of animal ecosystems is largely believed to have occurred in increasingly oxygenated oceans after the termination of the Sturtian and Marinoan glaciations. This transition has led to several hypotheses for the mechanism driving ocean oxygenation and animal evolution. One hypothesis is that enhanced weathering increased oceanic nutrien...

Oceanic Anoxic Event 2 (OAE 2), during the Cenomanian–Turonian transition (∼94 Ma), was the largest perturbation of the global carbon cycle in the mid-Cretaceous and can be recognized by a positive carbon-isotope excursion in sedimentary strata. Although OAE 2 has been linked to large-scale volcanism, several large igneous provinces (LIPs) were act...

The expansion and contraction of sulfidic depositional conditions in the oceans can be tracked with the isotopic composition of molybdenum in marine sediments. However, molybdenum-isotope data are often subject to multiple conflicting interpretations. Here I present a compilation of molybdenum-isotope data from three time intervals: the Toarcian Oc...

The Paleocene-Eocene Thermal Maximum (PETM) hyperthermal, ~ 56 million years ago (Ma), is the most dramatic example of abrupt Cenozoic global warming. During the PETM surface temperatures increased between 5 and 9 °C and the onset likely took < 20 kyr. The PETM provides a case study of the impacts of rapid global warming on the Earth system, includ...

Opening-mode veins in cores drilled from the mudrocks overlying and underlying the major Silurian salt décollement in the Appalachian plateau (Tioga and Lawrence Counties, Pennsylvania) have mineralogic and isotopic compositions generally matching those of their host mudrocks, suggesting opening and filling amid little cross-stratal fluid motion. C...

Molybdenum (Mo)-isotope chemostratigraphy of organic-rich mudrocks has been a valuable tool for testing the hypothesis that the Toarcian Oceanic Anoxic Event (T-OAE, Early Jurassic, ~183 Ma) was characterized by the spread of marine euxinia (and organic-matter burial) at a global scale. However, the interpretation of existing Mo-isotope data for th...

The key drivers controlling the redox state of seawater and sediment pore waters in low energy environments can be inferred from redox-sensitive trace elements (RSTE), molecular biomarkers and trace metal isotopes. Here, we apply a combination of these tools to the Upper Permian Kupferschiefer (T1) from the Thuringian Basin, deposited in the southe...

The Posidonienschiefer (Early Jurassic, ∼183 Ma) is the expression of the Toarcian Oceanic Anoxic Event (T- OAE) in Germany and Northern Switzerland and comparable to its correlative units in the UK (Jet Rock) and France (Schistes Carton) is composed of organic-rich, laminated clayey and calcareous mudstone. The carbon iso- tope record is marked by...

Large igneous provinces (LIPs) are proposed to have caused a number of episodes of abrupt environmental change by increasing atmospheric CO2 levels, which were subsequently alleviated by drawdown of CO2 via enhanced continental weathering and burial of organic matter. Here the sedimentary records of two such episodes of environmental change, the To...

Trace metal enrichments in sedimentary deposits are of prime interest because they are governed by processes that also control the production and preservation of organic matter. Consequently, trace metals have been used in reconstructions of the (palaeo)depositional environment of organic-rich deposits, but most of these studies have primarily focu...

During the Palaeocene–Eocene Thermal Maximum (PETM) about 56 million years ago, thousands of petagrams of carbon were released into the atmosphere and ocean in just a few thousand years, followed by gradual sequestration over approximately 200,000 years. If silicate weathering is one of the key negative feedbacks that removed this carbon, a period...

Oceanic Anoxic Event 2 (Cenomanian–Turonian: ca 94 Ma) represents a major palaeoceanographic phenomenon that took place during an interval of extreme global warmth when large amounts of organic matter entered the marine burial record, probably triggered by increased availability of nutrients for planktonic biota. Three sections (Eastbourne, Sussex,...

Despite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian-Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Ke...

The Mesozoic Era marked a time of greenhouse conditions on Earth, punctuated by a number of abrupt perturbations to the carbon cycle, such as Ocean Anoxic Events (OAEs). OAEs are typically marked in the stratigraphic record by the appearance of organic-rich shales, and excursions in carbon-isotope ratios registered in carbonates and organic matter....

It is well established that the burial of organic carbon in marine sediments increased dramatically at a global scale at the Cenomanian–Turonian boundary (Oceanic Anoxic Event 2: OAE-2, ∼94 Myr ago, Late Cretaceous). Many localities containing chemostratigraphic expressions of this event are not, however, enriched in organic carbon, and point to a...

During the Paleocene-Eocene Thermal Maximum (PETM) about 56 million years ago, thousands of petagrams of carbon were released into the atmosphere and ocean in just a few thousand years, followed by a gradual sequestration over approximately 200,000 years. If silicate weathering is one of the key negative feedbacks that removed this carbon, a period...