Chris Holmden

Environmental changes in the Late Ordovician–early Silurian: Review and new insights from black shales and nitrogen isotopes

The Late Ordovician (Katian-Hirnantian) through earliest Silurian (Rhuddanian) interval was a time of varying climate and sea level, marked by a peak glacial episode in the early-mid-Hirnantian. Synthesis of recently published data permits global correlation of at least two cycles of glacial advance and retreat with a distinct interglacial period that is recognizable in sequence-stratigraphic and chemostratigraphic records in many parts of the world. A period of warming and sea-level rise during the late Katian is marked by the widespread occurrences of oceanic anoxia in paleotropical and subtropical localities, mostly confined to regions of inferred upwelling and semirestricted marine basins. Nitrogen isotope data show that the regions of oceanic anoxia were marked by intense water-column denitrification in which cyanobacteria were the principal source of fixed N. In the overlying peak glacial interval of the Hirnantian, sedimentary successions from localities representing a wide range of water depths and paleolatitudes indicate that anoxia was restricted during the early-mid-Hirnantian. The shift to more positive N isotope values also suggests less intense water-column denitrification. In the overlying late Hirnantian and early Rhuddanian, the distribution of black shales reaches its greatest extent in the studied interval. Localities showing evidence of anoxia are globally spread over all paleolatitudes and water depths for which data are available, indicating a Rhuddanian ocean anoxic event comparable to examples from the Mesozoic. It is accompanied by a return to intensely denitrifying conditions within the water column, as indicated by the shift to negative N isotope values. The two phases of Hirnantian mass extinction coincide with rapid, climate-driven changes in oceanic anoxia. The first extinction occurred at the onset of glaciation and with the loss of anoxic conditions at the end of the Katian. The second extinction occurred at the demise of glaciation and coincided with the return of anoxic conditions during the late Hirnantian–early Rhuddanian. Integration of our N isotope data with graptolite biodiversity records suggests that the extinctions were profoundly influenced by changes occurring at the base of the marine food web, i.e., redox-driven changes in nutrient cycling and primary producer communities.

Two millennia of North Atlantic seasonality and implications for Norse colonies

δ18O values of mollusks recovered from near-shore marine cores in northwest Iceland quantify significant variation in seasonal temperature over the period from ∼360 B.C. to ∼A.D. 1660. Twenty-six aragonitic bivalve specimens were selected to represent intervals of climatic interest by using core sedimentological characteristics. Carbonate powder was sequentially micromilled from shell surfaces concordant with growth banding and analyzed for stable oxygen (δ18O) and carbon (δ13C) isotope values. Because δ18O values record subseasonal temperature variation over the lifetime of the bivalves, these data provide the first 2,000-year secular record of North Atlantic seasonality from ca. 360 cal yr B.C. to cal yr A.D. 1660. Notable cold periods (360 B.C. to 240 B.C.; A.D. 410; and A.D. 1380 to 1420) and warm periods (230 B.C. to A.D. 140 and A.D. 640 to 760) are resolved in terms of contrast between summer and winter temperatures and seasonal temperature variability. Literature from the Viking Age (ca. 790 to 1070) during the establishment of Norse colonies (and later) in Iceland and Greenland permits comparisons between the δ18O temperature record and historical records, thereby demonstrating the impact of seasonal climatic extremes on the establishment, development, and, in some cases, collapse of societies in the North Atlantic.

Carbon isotope chemostratigraphy of the Llandovery in Arctic Canada: Implications for global correlation and sea-level change

Abstract Stratigraphic and δ13C data from the Rhuddanian to lower Telychian succession on Cornwallis Island, Arctic Canada show evidence of a significant positive δ13C excursion in the upper Aeronian and weak positive shifts in the mid-Rhuddanian and uppermost Rhuddanian—lower Aeronian. The lower and upper Aeronian levels coincide with times of continental glaciation in Gondwana and these can be correlated with events recorded in the δ13C records from Dobs Linn (Scotland), Anticosti Island (Quebec), and Estonia. In most instances in the C-isotope records appear to coincide closely with local sea-level changes. The available data suggest that the global environmental impact of the late Aeronian glaciation was greater than that of the early Aeronian event. The data support the hypothesis that changes in degree of carbonate platform exposure and weathering resulting from of a combination of local and global (glacioeustatic) sea-level changes (particularly sea-level fall) were an important controlling factor in the generation of positive δ13C excursions in this time interval. These changes shifted the isotope value of the C-weathering flux entering shelf seas, which in turn resulted in positive δ13C excursions of varying magnitudes in shelf and deep basinal settings. These varied regionally in magnitude in close correspondence with differences in local sea-level histories.

143Nd/144Nd and Sm/Nd stratigraphy of Upper Ordovician epeiric sea carbonates

Abstract Stratigraphic changes in the eNd of epeiric sea carbonates from central North America track the submergence history of the interior craton during the Late Ordovician. Fluctuations in sea level changed the Nd isotope balance of the epeiric sea by modifying the flux of Nd weathered from the highlands of the Taconic Orogen (eNd = −6 to −9) and from the low relief Precambrian basement (eNd = −22 to −15) of the Transcontinental Arch and Canadian Shield. Transgressions over the Arch and Shield, which diminished the weathering flux of Nd from the Precambrian basement, are recorded as positive shifts in the eNd profiles of carbonates. Negative eNd shifts reflect regression and reexposure of the Precambrian basement to erosion. Correlation of Upper Ordovician carbonates by use of the eNd profiles demonstrates the potential for Nd isotope stratigraphy. Comparison of stratigraphic variations in carbonate Sm/Nd ratios with sea level curves, conodont paleoecology, and the eNd profiles suggest that our observed variations in Sm/Nd ratios are related to changes in depth. Increasing Sm/Nd ratios correlate with increasing depth, whereas decreasing Sm/Nd ratios correlate with decreasing depth. This relationship between Sm/Nd ratios and depth suggests Sm/Nd profiles have potentially wide applications in understanding the paleoceanography of ancient epeiric seas.

Influence of landscape on the apportionment of Ca nutrition in a Boreal Shield forest of Saskatchewan(Canada) using 87Sr/86Sr as a tracer

A 87Sr/86Sr tracer technique was used to apportion the supply of calcium (Ca) between atmospheric deposition and soil mineral weathering in a pristine Boreal Shield forest of northern Saskatchewan. To assess the impact of landscape variability on soil mineral weathering 87Sr/86Sr ratios, the watershed was divided into six study plots at low, middle, and high elevations along two toposequences - one consisting of mixed woods and the other consisting of black spruce. Apportionment analysis shows that none of the trees in the study plots depend entirely on soil mineral weathering as a source of Ca. Calcium pools in trees are shifted towards the atmospheric end-member (31-98%), probably because of the low soil mineral weathering fluxes combined with relatively high atmospheric deposition fluxes. These results need to be considered as the consequence of buildup and recycling of atmospherically derived Ca in the soil-vegetation system rather than the direct and large use of current atmospheric Ca inputs by the ...

Isotope Geochemistry and Plankton Response to the Ireviken (Earliest Wenlock) and Cyrtograptus lundgreni Extinction Events, Cape Phillips Formation, Arctic Canada

Several Canadian Arctic Silurian stratigraphic sections from the basinal facies of Cape Phillips Formation have been sampled across the Llandovery–Wenlock and early–late Homerian (late Wenlock) boundary intervals for integration of biotal (graptolite, radiolarian, palynomorph) and geochemical (13C) data for two well-known extinction events, the Ireviken and lundgreni Extinction (LEE) events. Graptolites, abundant and well preserved, provide a refined biostratigraphic base for other paleontologic and geochemical data. They were globally affected by both extinction events: about 64% reduction for the Ireviken and 90–95% for the LEE. Recovery from the LEE event was slow and diversity low through the late Homerian. Radiolarians—diverse (28 species), abundant, and beautifully preserved through the early Homerian―are sharply reduced slightly below the LEE boundary. Data for the late Homerian are more scattered, but it appears that diversity was low; few early Homerian taxa crossed the extinction boundary and new taxa appeared. Palynological studies around the LEE interval are at a preliminary level, but it appears that chitinozoans and microflora (acanthomorph acritarchs, prasinophytes, sphaeromorphs) were impacted by the extinction event. Chitinozoans, though seldom abundant, appear to disappear briefly across the LEE boundary, as do palynomorphs. Amorphous organic matter is abundant in the upper part of the lundgreni Zone; it is much less common in the early and middle–late Homerian and common in the latest part. Stable isotope geochemistry shows well-marked, positive excursions in the δ13Corganic fraction associated with the Ireviken event and LEE. The Ireviken excursion (C1) curve has a sharp base, reaches a peak in the early Wenlock, and then tapers more slowly. The LEE excursion (C4) peaks at, or slightly below, the early–late Homerian boundary. Both are positive excursions. Considering the limits of biostratigraphic placement of the boundaries, they were close to or coincident with regressions, particularly across the LEE interval. The δ13Corganic excursions are greater for inshore sections compared with the offshore section. The most parsimonious explanation for increased carbon content is accelerated weathering of carbonates exposed during a lowstand.

Reconnaissance hydrogeochemistry of economic deposits of sodium sulfate (mirabilite) in saline lakes, Saskatchewan, Canada

This paper briefly reviews the economic geology of sodium sulfate deposits hosted by post-glacial, internally-drained lakes in southern Saskatchewan, Canada, and reports on a reconnaissance survey of potential chemical and isotopic tracers that may be useful in identifying specific aquifer inputs to, and groundwater seepage from, the lakes. This is an initial step in a five-year project to advance the understanding of sodium sulfate deposits as modern ore forming systems. Hydrogen and oxygen isotopes suggest that Pleistocene glacial melt water is present as a component in some groundwaters discharging at the deposits. Major-ion chemistry suggests that mixing of water from two or more aquifers may be involved. Strontium isotopes show that different inter-till aquifers have quite distinct 87Sr/86Sr ratios. The data suggest that strontium isotopes may be useful in fingerprinting specific aquifer inputs in a context of potential 87Sr/86Sr evolution along groundwater flow paths, due to rock (till)–water interaction.

Geologically controlled bi-directional exchange of groundwater with a hypersaline lake in the Canadian prairies

Hypersaline lakes occur in hydrologically closed basins due to evaporitic enrichment of dissolved salts transported to the lakes by surface water and groundwater. At the hypersaline Lydden Lake in Saskatchewan, Canada, groundwater/lake-water interaction is strongly influenced by the geological heterogeneity of glacial deposits, whereby a highly permeable glaciofluvial sand/gravel deposit is underlain by glaciolacustrine deposits consisting of dense clay interspersed with silt/sand lenses. Pressure head distribution in a near shore area indicates a bi-directional flow system. It consists of topographically driven flow of fresh groundwater towards the lake in the sand/gravel aquifer and density-driven, landward flow of saline groundwater in the underlying glaciolacustrine deposits. Electrical resistivity tomography, and chemical and isotopic composition of groundwater clearly show the landward intrusion of saline water in the heterogeneous unit. The feasibility of bi-directional flow and transport is supported by numerical simulations of density-coupled groundwater flow and transport. The results suggest that the geologically controlled groundwater exchange processes have substantial influences on both inputs and outputs of dissolved minerals in hypersaline lakes in closed basins.RésuméLes lacs hypersalins se produisent dans des bassins hydrologiques fermés à cause de l’enrichissement par évaporation des sels dissous transportés dans les lacs par les eaux de surface et les eaux souterraines. Concernant le lac hypersalin de Lydden dans le Saskatchewan, Canada, l’interaction entre les eaux souterraines et les eaux de surface est fortement influencée par l’hétérogénéité géologique des dépôts glaciaires, caractérisée par un dépôt fluvioglaciaire hautement perméable de sables/graviers qui repose sur des dépôts glacio-lacustres constitués d’argiles denses entrecoupées par des limons et des lentilles sableuses. La distribution de la charge hydraulique dans une zone proche du rivage indique un système d’écoulement bi-directionnel. Il se compose de flux d’eaux souterraines douces influencés par la topographie vers le lac au sein de l’aquifère de sables/graviers et d’un contre flux d’eaux souterraines salées, influencé par la densité, au sein des dépôts glacio-lacustres sous-jacents. La tomographie de résistivité électrique et la composition chimique et isotopique de l’eau souterraine montrent clairement l’intrusion vers la terre de l’eau salée dans l’unité hétérogène. La vraisemblance d’un écoulement bi-directionnel et d’un transport est corroborée par des simulations numériques du flux d’eau souterraine, couplé à la densité et du transport. Les résultats suggèrent que les processus d’échange d’eaux souterraines contrôlés par la géologie ont des influences importantes aussi bien sur les apports que sur les exports des minéraux dissous dans les lacs hypersalins des bassins fermés.ResumenLos lagos hipersalinos se generan en cuencas hidrológicamente cerradas debido a un enriquecimiento evaporítico de sales disueltas transportadas hacia los lagos por el agua superficial y subterránea. En el lago hipersalino Lydden en Saskatchewan, Canadá, la interacción del agua subterránea/agua del lago está fuertemente influenciada por la heterogeneidad geológica de los depósitos glaciales, debido a que un depósito de arena/grava glaciofluvial altamente permeable subyace a depósitos glaciolacustres compuestos por una arcilla densa intercalada con lentes de limos/arenas. La distribución de la carga de presión en una zona cercana a la costa indica un sistema de flujo bidireccional. Consta de un flujo de agua dulce subterránea impulsado topográficamente hacia el lago en el acuífero de arena/grava y un flujo de agua subterránea salina hacia la tierra impulsado por la densidad, en los depósitos glaciolacustres subyacentes. La tomografía eléctrica de resistividad, la química y la composición isotópica de las aguas subterráneas muestran claramente la intrusión hacia tierra de agua salina en la unidad heterogénea. La viabilidad de flujo y transporte bi-direccional está soportada por simulaciones numéricas de flujo y por el acoplamiento del transporte de densidad del agua subterránea. Los resultados sugieren que los procesos de intercambio geológicamente controlados de aguas subterráneas tienen influencias importantes en la entrada y salida de minerales disueltos en lagos hipersalinos en cuencas cerradas.摘要由于输送到湖中的地表水和地下水中的溶解盐份的蒸发富集,超盐度的湖泊就会出现在水文上封闭的盆地中。在加拿大萨斯喀彻温省的Lydden超盐度湖,地下水/湖泊水相互作用受到冰川沉积地质上非均质性的强烈影响,而高度透水性的冰川水形成的砂/砾石沉积下伏着由含有粉砂/砂透镜体的致密粘土组成的冰湖沉积。近岸附近的压力水头分布表明,有双向的水流系统。包括砂/砾石含水层中地下淡水流地形上受驱动流向湖泊,以及下伏冰湖沉积中地下咸水流受密度驱使流向陆地。电阻系数断层摄影及地下水化学和同位素组分清楚地显示出异质单元中的咸水向陆地入侵。双向水流和传输的可行性得到了密度耦合的地下水流和传输的数值模拟支持。结果显示,受地质控制的地下水交换过程对封闭盆地超盐度湖泊中的溶解矿物的输入和输出有重要影响。ResumoLagos hipersalinos ocorrem em bacias hidrologicamente fechadas devido ao enriquecimento evaporítico dos sais dissolvidos transportados até o lago por águas superficiais e águas subterrâneas. No hipersalino Lago Lydden, em Saskatchewan, Canadá, a interação lago/aquífero é fortemente influenciada pela heterogeneidade geológica dos depósitos glaciais, na qual um depósito glaciofluvial de areia/cascalho altamente permeável é sustentado por depósitos glaciolacustres constituídos de argilas densas intercaladas com lentes de silte/areia. Distribuições de carga hidráulica em uma área próxima à costa indicam um sistema de fluxo bidirecional. Isso consiste em um fluxo de águas subterrâneas doces topograficamente orientado sob o lago no aquífero areia/cascalho, e um fluxo de águas subterrâneas salinas em direção a terra, orientado por densidade, nos depósitos glaciolacustres subjacentes. Tomografia de resistividade elétrica e a composição química e isotópica das águas subterrâneas claramente mostram uma intrusão de água salina em direção a terra na unidade heterogênea. A exequibilidade do fluxo bidirecional e transporte é respaldada por simulações numéricas de fluxo e transporte das águas subterrâneas com densidade acoplada. Os resultados sugerem que o processo de troca geologicamente controlado de águas subterrâneas tem uma influência substancial em ambas entradas e saídas de minerais dissolvidos em lagos hipersalinos em bacias fechadas.