Tais Dahl | University of Copenhagen (original) (raw)

Papers by Tais Dahl

The Molybdenum (Mo) stable isotope composition of seawater provides a characteristic measure of t... more The Molybdenum (Mo) stable isotope composition of seawater provides a characteristic measure of the global redox state of the ocean. A method to infer seawater δMo values from the sedimentological record would offer a measure of ocean oxygenation through time, Arnold et al. [1]. Mo is readily scavenged in sulfidic settings and the δMo values in modern marine euxinic sediments fall close to the seawater value. Hence, euxinic sediments are good candidates to record seawater δMo values. We studied the Mo removal pathway and its isotopic consequences from sulfidic waters to the sediments in Lake Cadagno, Switzerland, by examining both water column and sedimentary samples. These allow us to assess conditions at which euxinic sediments reflect δMo in overlying waters. Our results show that Mo scavenging occurs through particle adsorption in the sulfidic water column, and support a model where molybdate (MoO4) reacts with H2S to form particle reactive oxy-thiomolybdates (MoO4-xSx) as propo...

GSA Today

Large datasets increasingly provide critical insights into crustal and surface processes on Earth... more Large datasets increasingly provide critical insights into crustal and surface processes on Earth. These data come in the form of published and contributed observations, which often include associated metadata. Even in the best-case scenario of a carefully curated dataset, it may be nontrivial to extract meaningful analyses from such compilations, and choices made with respect to filtering, resampling, and averaging can affect the resulting trends and any interpretation(s) thereof. As a result, a thorough understanding of how to digest, process, and analyze large data compilations is required. Here, we present a generalizable workflow developed using the Sedimentary Geochemistry and Paleoenvironments Project database. We demonstrate the effects of filtering and weighted resampling on Al 2 O 3 and U contents, two representative geochemical components of interest in sedimentary geochemistry (one major and one trace element, respectively). Through our analyses, we highlight several methodological challenges in a "bigger data" approach to Earth science. We suggest that, with slight modifications to our workflow, researchers can confidently use large collections of observations to gain new insights into processes that have shaped Earth's crustal and surface environments. 1 Supplemental Material: table of valid lithologies; map depicting sample locations; crossplot illustrating analytical uncertainty; flowchart of the proposed workflow; histograms showing the effects of progressive filtering, the distribution of spatial and age scales, and proximity and probability values; and results of sensitivity tests.

Earth and Planetary Science Letters

We report evidence for Milankovitch cycles in two drill cores from the Cambro-Ordovician Alum Sha... more We report evidence for Milankovitch cycles in two drill cores from the Cambro-Ordovician Alum Shale Formation of Scandinavia. The signal is preserved in elemental abundances recorded at high stratigraphic resolution by core scanning XRF analysis (0.2 mm resolution). The new data enable us to establish a floating timeline calibrated to the stable 405 kyr eccentricity cycle for a ~8.7 Myr interval across the Miaolingian-Furongian boundary. This interval spans the Steptoean Positive Carbon Isotope Excursion (SPICE), which is recorded in the d 13 Corg in the studied drill cores. We calculate the durations of the Olenus Superzone to 3.4 ± 0.2 Myr, the Parabolina Superzone to 1.9 ± 0.3 Myr, the Leptoplastus Superzone to 0.33 ± 0.18 Myr, the Protopeltura Superzone to 0.51 ± 0.20 Myr, and the SPICE event straddling the Paibian and lower main part of the Jiangshanian Stage to 3.0 ± 0.2 Myr. The sedimentation rate shows similar trends at both drilling locations and is inversely correlated to eustatic sea level changes in certain time intervals, opening tantalizing new prospects of using cyclostratigraphic analyses of shales to track eustatic sea level variations. The identification of obliquity cycles enables us to calculate the Cambrian Earth-Moon distance as well as the day length at ~493 Ma to 368.9 ± 2.3 •10 6 m and 21.78 ± 0.29 hours, respectively.

&lt;p&gt;Profound environmental and biological changes took place during the Camb... more &lt;p&gt;Profound environmental and biological changes took place during the Cambrian, yet, compared to other Phanerozoic intervals, the Cambrian time framework remains poorly constrained, which severely hinders a detailed understanding of the timing and progression of these major geological events. In this study, we report a radiometrically anchored astrochronologic framework across the late Cambrian interval, using high-resolution aluminum (Al) series (1 mm resolution) through the Alum Shale Formation in Scania, southernmost Sweden, based on the fully cored Albj&amp;#228;ra-1 well. Significant cycles with periods of 405 kyr (long eccentricity), 108 kyr (short eccentricity), 30.4 kyr (obliquity) and 18.8 kyr (precession), associated with long-term amplitude modulation of obliquity and precession, confirmed the orbital imprint on late Cambrian climate. Using the U-Pb dating at 486.78&amp;#177;0.53Ma for the Cambro-Ordovician boundary as anchor point, our timescale spans from ~483.9 to ~500.0 Ma, covering 7 trilobite superzones and 3 graptolite zones. The calibration indicates ages of 491.2&amp;#177;0.54 Ma, 493.9&amp;#177;0.67 Ma, 497.3&amp;#177;0.67 Ma and 500.4&amp;#177;0.67 Ma for the lower boundaries of provisional Stage10, Jiangshanian, Paibian and Guzhangian stages, respectively. This radiometrically anchored astrochronology also provides precise age constrains on regional superzones or even biozones within Scandinavia, and hopefully pave the way for better understanding the late Cambrian major geological events globally.&lt;/p&gt;

Geobiology, 2021

Geobiology explores how Earth's system has changed over the course of geologic history and ho... more Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses.

Geochimica et Cosmochimica Acta

Abstract The end-Permian mass extinction (EPME; ca. 251.94 Ma) is the most severe mass extinction... more Abstract The end-Permian mass extinction (EPME; ca. 251.94 Ma) is the most severe mass extinction in the geological record. Detailed paleobiological investigations show a very rapid EPME event, and recently published δ238U data show a large negative excursion and thus a massive shift to globally expanded anoxia at the main extinction phase in the latest Permian. The negative shift in δ238U is in correlation with a globally characterized negative δ13C excursion near the Permian-Triassic boundary (PTB). In some highly expanded PTB carbonate sections, however, there are two distinct negative δ13C excursions whereas uranium isotopes (δ238U) from such sections have not yet been examined, leaving a gap in the understanding of the global perturbations of marine redox conditions immediately following the EPME. Here, we present a new δ238U study of syn-depositional dolostones from a well-characterized and highly expanded drill core, which recorded two pronounced negative δ13C excursions across the PTB, from the Carnic Alps, Austria. This drill core extends 331-meters across the PTB and provides a unique opportunity to explore the detailed timing, duration, and extent of marine redox chemistry changes before, during, and immediately after the EPME. Our new δ238U record shows two negative shifts, which are correlated with the two negative δ13C excursions. The first negative δ238U excursion preceding the EPME confirms the recently published δ238U records from across the EPME and support that syndepositional marine dolostones can record δ238U trends of seawater similar to that of limestones. Modeling of uranium isotope cycling in the latest Permian and earliest Triassic oceans suggests two distinct stages of expanded marine anoxia separated by a brief interval (∼100 kyr) of reoxygenation across the PTB. The first anoxic episode lasted for ∼ 60 kyr while anoxic seafloor area expanded to cover >18% of the entire seafloor, coeval with the main EPME horizon, agreeing with marine anoxia as a proximate kill mechanism for the EPME. The second anoxic event was less intense compared to the first anoxic pulse but sustained for a longer duration. A global modeling of coupled C, P, and U cycles show that two pulses of volcanic carbon injection that drives global warming and increased phosphorus weathering rate can reasonably reproduce our data to match two phases of anoxia. The model also demonstrates that the loss of terrestrial vegetation in the EPME is crucial to generating an intervening interval of oxygenated ocean. Our new study adds to a growing body of evidence that the global marine redox conditions underwent rapid oscillations during the EPME event and continued afterward, which may have played a central role in delaying the marine ecosystem recovery in the Early Triassic.

Earth and Planetary Science Letters

Scientific Reports

oxygen is a prerequisite for all large and motile animals. it is a puzzling paradox that fossils ... more 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 the fossils are transported from afar or perhaps were not benthic. Here, we improved the stratigraphic resolution of marine anoxia records 100-1000 fold using core-scanning X-Ray Fluorescence and established a centennial resolution record of oxygen availability at the seafloor in an epicontinental sea that existed ~501-494 million years ago. The study reveals that anoxic bottom-water conditions, often with toxic hydrogen sulfide present, were interrupted by brief oxygenation events of 600-3000 years duration, corresponding to 1-5 mm stratigraphic thickness. fossil shells occur in some of these oxygenated intervals suggesting that animals invaded when conditions permitted an aerobic life style at the seafloor. Although the fauna evidently comprised opportunistic species adapted to low oxygen environments, these findings reconcile a long-standing debate between paleontologists and geochemists, and shows the potential of ultra-high resolution analyses for reconstructing redox conditions in past oceans. Deposition of black shales was widespread in the Cambrian oceans (541-488 Ma) 1. In the past decade, various geochemical redox proxies have been used to associate black shale facies to deposition under anoxic waters with hydrogen sulfide present in the water column. Basinal transects across continental shelves in various parts of the world outline a heterogeneous O 2 landscape in the Cambrian ocean with oxic surface waters overlying anoxic mid-depth waters, akin to modern-day oxygen minimum zone settings, but sometimes with free hydrogen sulfide (H 2 S) present at mid-water depth and anoxic and ferruginous deep waters 2-4. Areas with low O 2 availability in the ocean are called 'dead zones' because all animals require O 2 for respiration 5. The expansion of dead zones was arguably an efficient kill mechanism during several of the most devastating animal crises in Earth history, including during the Cambrian 6,7. When anoxia impinges on the seafloor, it changes the behavior of redox sensitive elements and leads to trace metal enrichments in the sediments. There is now evidence that the extent of seafloor anoxia waxed and waned over million year time scales 8-10. Yet, the stability of marine redox conditions on shorter time scales has not been investigated in ancient shales due to low temporal resolution of previous paleoredox studies. This problem has led to controversies between paleontologists and geochemists regarding strata where fossils of benthic animals are found in intervals containing geochemical evidence for anoxic bottom waters with poisonous H 2 S; e.g. the Alum Shale Formation of Scandinavia 8,10. As a solution to this paradox, it has been suggested

Geological Society of America Abstracts with Programs

Precambrian Research

The emergence of animal ecosystems is largely believed to have occurred in increasingly oxygenate... more 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 nutrient levels, primary productivity and organic carbon burial, and ultimately oxygenated the atmosphere and oceans. Another hypothesis suggests that an animal-driven reorganization of the marine biogeochemical cycles might have oxygenated the oceans. Through molybdenum (Mo), carbon (C), sulfur (S) isotopes and iron (Fe) speciation results from the Arena Fm, East Greenland, this study constrains ocean redox conditions during the Cryogenian, after the Sturtian deglaciation and before the major radiation of animals. Carbon and sulfur isotope stratigraphy is used to correlate the Arena Fm with other formations worldwide between the Sturtian and Marinoan glaciations (~720-635 Ma). The lower part of the Arena Fm (~25 m) consists of black shales deposited under locally euxinic conditions as evidenced by high proportions of highly reactive iron (Fe HR /Fe T > 0.38) and pyrite (Fe PY /Fe HR > 0.7). These black shales display small Mo enrichments (< 3 ppm) and low Mo/TOC compared to overlying shales and Phanerozoic euxinic sediments. The maximum δ 98 Mo value is observed in the basal Arena Fm (1.5‰). Many samples display lower δ 98 Mo than typical oceanic input fluxes, which can be explained by Mo isotope fractionation from a marine Mo pool with δ 98 Mo ~ 1.3‰, similar to that inferred from other Cryogenic euxinic basins. The combination of low [Mo] and δ 98 Mo suggests that widespread anoxia prevailed in the oceans at this time. Our data are consistent with most other studies from this time suggesting that ocean oxygenation was not linked to Snowball Earth deglaciation, but was delayed until animals effectively entered the scene.

Geochemical Perspectives Letters

The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans,... more The Phanerozoic radiation of bilaterian animals has been linked to oxygenation of Earth's oceans, due to the oxygen demand of the evolving animal ecosystems. However, how early animals may have regulated Earth's surface oxygen budget via self-stabilising feedbacks is poorly understood. Here, we report parallel positive uranium, carbon, and sulphur isotope excursions from carbonate successions in Siberia that document a brief global oxygenation episode 521-520 Myr ago, at the onset of diversification of larger arthropods known from the fossil record. Our data and model indicate that an abrupt increase in the sinking rate of marine organic matter expanded the oxygenated zone in the oceans and that reducing conditions returned 1.3 ± 0.8 Myr after the onset of this transient oxygenation episode, necessitating a strong negative feedback to the increasing levels of oxygen. We speculate that larger zooplankton could have sourced both oxygen and food to the seafloor, fueling bioturbation over wider areas and, thereby, stabilising O 2-rich habitats in the oceans. Thus, this reorganisation exemplifies how animal ecosystems might have influenced oxygen availability in Earth's surface environment soon after their establishment.

Reviews in Mineralogy and Geochemistry

$Research paper thumbnail of Molybdenum isotope fractionation and speciation in a euxinic lake—Testing ways to discern isotope fractionation processes in a sulfidic setting$

Chemical Geology

The molybdenum (Mo) isotope composition in euxinic shales has been used as a proxy for the global... more The molybdenum (Mo) isotope composition in euxinic shales has been used as a proxy for the global distribution of anoxic conditions in ancient oceans, and since more recently also as a proxy for sulfide concentrations in depositional environments. However, there is currently no way to distinguish isotope fractionation at low bottom water sulfide concentrations in 'local' basins from 'global' secular isotope variations associated with changing seawater composition. This uncertainty is challenging the use of Mo isotopes for paleoceanographic reconstructions. To explore this further, we present new data from sediments deposited over the past~9800 years in one of the best studied euxinic localities in the world: Lake Cadagno in Switzerland. The sample set allows us to test ways to discern isotope fractionation processes at play in a highly restricted euxinic basin. Most of our drill core samples (n = 18) show high δ 98 Mo values similar to previously studied shallow sediments, indicative of quantitative Mo removal from the water column (Dahl et al. 2010a). However, a few samples (n = 3) deposited between about 1200 and 3400 years ago carry low δ 98 Mo values and have been isotopically fractionated in the lake. Sedimentological and geochemical characterizations show that these δ 98 Mofractionated sediments formed during times of frequent injection of O 2-and sediment-rich river water into the deep sulfidic water column. A positive correlation between δ 98 Mo and sedimentary Mo contents suggests that isotope fractionation occurred during times of non-quantitative Mo removal, although Mn-oxide cycling at the chemocline might also contribute a subordinate proportion of (98 Mo-depleted) molybdenum into the sulfidic zone. Sedimentary Mo/U enrichments relative to oxic lake water further supports the hypothesis that a particulate Mo shuttle was most efficient during times of quantitative Mo removal. Therefore, periods with inefficient Mo capture are ascribed to incomplete conversion of molybdate to particle reactive Mo species when bottom water H 2 S levels were low or less stable than today. Using XAFS spectroscopy, we found that the two distinct Mo compounds predominating in the sediments (Mo IV-S and Mo VI-OS) are not diagnostic for isotope fractionation that has occurred in Lake Cadagno. Instead, we infer that δ 98 Mo-fractionated products (forming via a low-sulfide Mo pathway) can be subsequently altered with little or no isotopic imprint during remobilization and re-precipitation (e.g., at higher sulfide levels in the sediments) as well as during post-depositional oxidation. Future work could investigate local δ 98 Mo-fractionation processes expressed in other euxinic settings and explore other sedimentary metrics to constrain the steps involved in the euxinic burial pathway(s). One tantalizing prospect of this is to distinguish between local bottomwater sulfide levels and variations in the fraction of global seafloor anoxia from the Mo isotope composition in ancient euxinic mudrocks.

Proceedings of the National Academy of Sciences, 2016

The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but t... more The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435–392 Ma, and the appearance of fossil charcoal indicates O2 >15–17% by 420–400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial—the net long-term source of O2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today’s global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100),...

Proceedings of the National Academy of Sciences, 2016

Planavsky et al. (1) argue that variability in the V/Al of soils compromises our ability to detec... more Planavsky et al. (1) argue that variability in the V/Al of soils compromises our ability to detect V depletions and thus oxygenated bottom waters in the Xiamaling Formation. Indeed, because of such variability, we explored trace metal chemistry through several units of the Xiamaling Formation to establish V/Al background values and trace metal behavior. Unit 4 lacks trace metal enrichments, with V/Al values distributed around the crustal average (CA) (Fig. 1A), which we take to represent unaltered particles entering the basin. In contrast, unit 3 was enriched in Mo and U, with V/Al either depleted or similar to CA (Fig. 1A). These trace metal patterns are, in the modern ocean, uniquely found in organic-rich sediments depositing in oxygenated water (2). In contrast, unit 2 was enriched in V, Mo, and U, a pattern found under anoxic depositional conditions (2). If, following ref. 1, unit 3 bottom waters were similarly anoxic, then deposition somehow switched to a source so low in V/Al that the sediment V/Al remained ≤ CA despite enriching in V as expected under anoxic deposition. There is no modern precedent for such trace metal behavior under anoxic deposition (2), and we view this scenario as unlikely. We believe that unit 3 bottom water oxygenation is the most parsimonious with basin particle chemistry and modern analogs. Planavsky et al. (1) argue that our 2,3,6-trimethyl aryl isoprenoid (TMAI) biomarkers are a product of contamination. They cite previous work (3) finding no evidence of these biomarkers in six black shale outcrop samples. We share concerns about contamination and sample integrity, and therefore analyzed fresh core material collected with fresh water as a

GSA Today

Earth and Planetary Science Letters

Geobiology, 2021

Geochimica et Cosmochimica Acta

Earth and Planetary Science Letters

Scientific Reports

Geological Society of America Abstracts with Programs

Precambrian Research

Geochemical Perspectives Letters

Reviews in Mineralogy and Geochemistry

$Research paper thumbnail of Molybdenum isotope fractionation and speciation in a euxinic lake—Testing ways to discern isotope fractionation processes in a sulfidic setting$

Chemical Geology

Proceedings of the National Academy of Sciences, 2016