Carol B. de Wet

Isotopic Imprint of Climate and Hydrogeochemistry on Terrestrial Strata of the Triassic-Jurassic Hartford and Fundy Rift Basins

ABSTRACT Late Triassic to Early Jurassic terrestrial sequences in the Hartford and Fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite. The isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine, playa, and fluvial environments. Dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the Hartford basin have variable isotopic compositions (13C = -5.8 to + 1.8 PDB; 18O = -7.2 to +0.7 PDB). Within any single symmetrical cycle of playa red mudstone--lacustrine gray, black, gray mudstone--playa red mudstone, there is a systematic change to relatively enriched 13C compositions in dolomite in the grayish black and black mudstones in the center of the cycle. These carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where 13C-depleted carbon was concentrated in organic matter that was then buried. Calcites from lacustrine, micritic, and biomicritic limestones of the Scots Bay Formation of the Fundy basin have stable isotopic compositions (13C = -4.6 to -2.2 PDB; 18O = -6.1 to -3.0 PDB) that become more enriched in 18O and 13C upward in shallowing depositional sequences. These isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels, followed by progressively lower inflow, resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface wat r. The lake waters were well oxygenated at all times. In the Hartford basin, caliche calcites in fluvial mudstones and sandstones have isotopic compositions (13C = -7.3 to -3.8 PDB; 18O = -8.0 to - 5.6 PDB) that reflect paleosol processes during climatic conditions that varied from warm and dry in Late Triassic time to relatively cooler and probably wetter in the Early Jurassic. Isotopic compositions of caliche calcites in redbeds in the Fundy basin indicate a parallel climate change from Late Triassic to Early Jurassic time, but also that the climate was relatively hotter and probably drier over the entir interval, as compared to the Hartford basin.

Disrupted coal and carbonate facies within two Pennsylvanian cyclothems, southern Illinois basin, United States

Two cyclothems contain an unusual suite of disrupted fabrics. This study9s interdisciplinary integration of coal maceral and sedimentological information offers significant new interpretations of coal-limestone interchange, both depositionally and diagenetically. Disrupted fabrics within the Herrin Coal and Providence Limestone are interpreted as representing previously undocumented interactive processes between coals and limestones. The Herrin Coal contains brecciated beds that are the result of marine transgression over a coastal-plain mire, representing high-energy conditions at the swamp-beach interface. This interface has rarely been documented in detail. Peat is inherently unstable prior to its conversion to coal, and we document this by interpreting conglomeratic beds in overlying strata. Differential compaction of Herrin peats generated subaqueous debris flows in the overlying Providence Limestone. This mechanism for syndiagenetic and early diagenetic sediment deformation has not been previously recognized. In addition, near-surface and surface processes produced other disrupted fabrics in the Providence Limestone, such as in situ mosaic breccias, nodular fabrics, and calcrete horizons. This study also documents that stratigraphically adjacent, alkaline carbonate sediments were further altered by undersaturated acidic pore waters, derived from compacting peats during early diagenesis. Our interdisciplinary approach to distinguishing and interpreting all of these disrupted fabrics led to a new appraisal of the importance of depositional and postdepositional interactions between coal and limestone. The presence of mire deposits can have a significant influence on the thickness, distribution, and fabrics in overlying strata. In cyclothems, climatic and eustatic changes may control overall depositional patterns, but peat compaction may determine certain facies.

Biological Clocks and Tenure Timetables: Restructuring the Academic Timeline

Despite decades of progressive social change by an active women’s movement, federal and state legislation, and adoption of academic affirmative action policies, women geoscientists have not reached a critical mass in higher education. Women comprise only 12.5% of geoscience faculty in U.S. colleges and universities and only 10% at Ph.D. granting institutions. Senior women faculty tend to be marginalized from the academic power structure. A combination of biological factors, lifestyle choices, dual career pressures, double standards for social and professional interactions, and gender-based discrimination creates an effective filter, reducing women in geoscience departments to a surprisingly low level. There are two rungs on the ladder where women proportionally leave the discipline at a higher rate than men. One is continuing on to obtain a Ph.D.; the other is prior to, or at tenure. The present time frame for achieving tenure and promotion was established by men, for men, decades ago. Such a time frame is incompatible with women’s biologic reproductive constraints, and as such, puts an unequal level of pressure and stress on women relative to their male professional counterparts. Only a significant change in the culture of science, and its traditional pathways, will create a geoscience community that has a sound base of gender equity. Strong leadership from innovative and far-sighted administrators and colleagues is required to introduce and foster institutional change that will reduce the conditions that leave women disadvantaged. Biological Factors—Childbearing Years and Tenure Trajectory Due to the inevitable tick of the biological clock, there is an unavoidable collision between a woman’s optimum childbearing years and her career trajectory. Although career and family issues affect both men and women, there are issues unique to women who are starting a family. The most fundamental gender specific issue is childbearing. Women face a difficult choice: wait to have children until their professional life is secure, but risk serious health consequences for their children (or selves), or bear their children earlier, and risk their professional success. Maternal mortality rates are four times higher among women 35 to 39 years old than those of ages 20 to 24, and babies of older women have higher mortality rates as well. In women age 35 or older, there is a four-fold increase in serious fetal complications, including stillbirth. This kind of emotional dilemma is what may lead some women leave the discipline. Those who stay in the profession experience tension that may seriously impact their quality of life, their career (research productivity, field and lab work), and their ability to successfully compete for jobs and grants. The overlap in biological and professional imperatives lasts for only a minor portion of a women’s life, perhaps only 6 years out of a 35-year career. The risk of giving birth to a child with Down syndrome at age 35 is ~ 1:270. By age 40 this risk is 1:106. Lethal chromosomal abnormalities are also more common with advanced maternal age, and risk of miscarriage is a significant concern. Only ~10% of woman under the age of 20 experience spontaneous miscarriage. By age 40 this risk has more than doubled, to almost 34%. Aging eggs, not simply overall good health, also affect a woman’s fertility, and the natural incidence of chronic illnesses that complicate pregnancy increases with maternal age. Conclusions Attrition from the geosciences is higher for women than men at two critical points: after the M.Sc. degree, and between assistant and associate professor. Due to the inevitable tick of the biological clock, there is an unavoidable collision between a woman’s optimum childbearing years and her career trajectory. Assuming that gender equity in science makes sense in terms of resources, diversified types of study, and balance, then causes for the rate of women’s attrition must be sought. Biological realities should to be acknowledged if we are to attain a critical mass of women in the geosciences. Stopping the tenure clock, allowing part time work for given time periods, and encouraging split positions are policies that already exist in some institutions. More responsive, flexible schemes for integrating work and family are essential to ensure women’s full participation in higher education. At one time, it was considered impossible for female students to go on field excursions because of a lack of facilities. This “impossible situation” has been overcome, and there is no reason to doubt that the issues we have described can also be overcome. Only by the retention and advancement of women can critical mass be achieved, after which women can begin to fill positions of power and influence. Women then can serve as role models for the next generation of scientists, encouraging more of them to enter and stay in science. This pool will then form the teaching basis for both academia and industry, and will potentially lead to a generation of managers and department heads who will be part of the ongoing transformation of the sciences. Only strong leadership today, by both administrators and faculty, can change the academic culture of priorities, workloads, reward structure and values to more closely reflect all of its constituencies and begin such a transformation. Read the full text at www.geosociety.org/pubs/gsatoday/. 24 NOVEMBER 2002, GSA TODAY Comentary

Origin of Meter-Scale Submarine Cavities and Herringbone Calcite Cement in a Cambrian Microbial Reef, Ledger Formation (U.S.A.)

Abstract Meter-scale submarine cavities in Middle Cambrian shelf-margin microbial reef strata indicate large-scale dewatering processes, in conjunction with substrate instability related to interreef channeling and shelf-edge downslope creep and slip. Syndepositional cement precipitation within the cavities preserved delicate microbial fabrics and stabilized the reef system. Radiaxial fibrous calcite and herringbone calcite cements line the cavity interiors isopachously. The two phases cannot be discriminated on the basis of Fe, Mn, or Sr contents, but do have different isotopic signatures. Slightly more negative δ13C values in herringbone calcite suggest that abrupt transitions between radiaxial fibrous and herringbone calcite cement are the result of rapid and repeated changes in pore-fluid oxygen levels. Storm-driven pore-water circulation renewed oxygenated seawater flow into the cavities, resulting in precipitation of radiaxial fibrous calcite. A threshold level of oxygen reduction resulted in the change to herringbone calcite precipitation. The pore fluids associated with herringbone calcite did not have elevated Mn or Fe concentrations, as suggested in previous studies. Herringbone calcite appears to be more susceptible to diagenetic alteration than radiaxial fibrous cement however, as indicated by greater resetting of oxygen isotope values.

Subtle signatures of seeps: Record of groundwater in a Dryland, DK, Olduvai Gorge, Tanzania

Few proxies exist to identify aridity in the depositional record, although drylands cover ca 30% of the modern continental surface. New exposures in a siliciclastic and carbonate sequence in an arid to hyperarid basin at Olduvai Gorge, Tanzania provide a unique multi‐proxy record of a 1·85 Ma landscape that was exploited by early humans. The 2 m thick sequence of clastics and carbonates that are exposed along a 450 m outcrop records climate change over a single precession (dry‐wet‐dry) cycle. Siliciclastic data (sedimentary structures, grain size, mineralogy) and biological data are combined with data for a 10 to 35 cm thick limestone (stable isotopes, elemental geochemistry, petrography) to generate a depositional facies model for a site DK (Douglass Korongo) on this dry rift basin landscape. This site was situated on a low gradient, distal portion of a volcaniclastic alluvial fan. The clastics are intercalated distal alluvial fan sandy silts and lake clays that accumulated in a low energy environment. Groundwater discharge and the alkaline springs and seeps during wet‐to‐dry change in climate made a freshwater carbonate‐rich environment. Bedded lithofacies (a lime mudstone with fossils) were deposited in shallow standing (spring‐fed) pools, while nodular lithofacies with calcite spherulites indicate permanently saturated ground (seeps). Both environments experienced similar diagenesis, that is, the precipitation of authigenic barite from supersaturated groundwater, desiccation and pedogenesis, and late‐stage calcite precipitation. Compositional and isotopic data suggest that a fresh groundwater‐fed system was available to early humans even during dry intervals of the precession cycle.

High-energy Shelf-margin Carbonate Facies: Microbial Sheet Reefs, Endolites, and Intraclast Grainstone—Ledger Formation (Middle Cambrian), Pennsylvania

Cambrian–Ordovician shelf-margin deposits of the great American carbonate bank (eastern North America) experienced significant regional dolomitization and/or metamorphism, but the Middle Cambrian Ledger Formation in south-central Pennsylvania contains a shelf-margin facies complex that includes exceptionally well-preserved microbialite sheet reefs riddled with centimeter- to meter-scale submarine cavities. The reefs and associated sands, composed of reef-related allochems, interfinger with ooid shoals, forming a high-energy shelf-margin facies association located near the seaward margin of the Middle Cambrian Laurentian platform. The Ledger Formations ooid shoal complex, exposed in the Magnesita Refractories quarry in York County, Pennsylvania, is pervasively dolomitized. Forthcoming research documents multiple stages of dolomitization and dedolomitization in the ooid dolostone; therefore, the ooid dolostone is not discussed here. In contrast to the ooid dolostone, most of the Ledger reef facies remains limestone. This has facilitated detailed interpretation of the reef depositional and diagenetic history, including new information presented here. Previous publications describe the Ledger reef geologic setting, mechanisms for generating the cavities, and petrographic and geochemical analyses of radiaxial fibrous and herringbone calcite fibrous submarine cements within the cavities. This chapter provides new information on the microbial reef sheet facies, describes a previously undocumented type of cryptic microbial morphology (endolite), and interprets a 1-m (3.3-ft)-thick intraclastic grainstone bed. Modern reefs in high-energy settings adapt by building robust coral frameworks that can withstand normal current activity and wave action. In the Middle Cambrian, coral framebuilders were absent, so to exploit high-energy ecological niches, organosedimentary constructers, primarily cyanobacteria (algae and bacteria), had to develop a similarly robust morphology. We propose that low-growing, thick, cohesive microbial sheets, such as documented here from the Ledger Formation, provided minimal wave resistance and, therefore, outcompeted stromatolites and thrombolites to form subtidal wave-resistant structures in such high-energy settings. Similar to modern reefs, these microbial sheets contain cavities across a range of scales from millimeter-size fenestrae to meter-size stromatactis-type voids capable of sheltering and supporting delicate shrubs of Epiphyton-like dendrites and cryptic endolites, as detailed later in this chapter. Microbial processes dominated all ecological niches, forming the substrate, colonizing cryptic spaces, and coating and encrusting other microbes. The reef microbialite consists of weakly bedded sheets composed of shrubs and stubby strands of calcified Epiphyton- and Angulocellularia-like elements. Centimeter-scale domal stromatolites, thrombolites, oncolites, dendrolites, and oval multiple-layered organosedimentary cryptic structures, termed “endolites,” form lenses and distinctive structures. Petrographically, the microbialite is expressed as clots, stringers, arborescent garlands, and dendritic shrubs. Stromatactislike and fenestral cavities within the microbialite formed primarily through processes of gas and water escape, although syndepositional slumping and channel undercutting produced other types of cavities and void spaces. Grainstone, composed of microbial clasts and fragments, accumulated as cross-bedded intrareef channel sands. Large stromatactislike cavities were stabilized with multiple generations of microdolomite-bearing calcite radiaxial fibrous and herringbone calcite cements and intercalated internal sediment. Cement morphology, internal sediment associations, stable isotopes, and trace element geochemistry suggest that the cements precipitated from marine fluids as magnesium calcite and subsequently stabilized to calcite during diagenesis. The Ledger microbial assemblage closely resembles living cryptic, mat, and domal cyanobacterial forms reported from the Tikehau Atoll, French Polynesia. Detailed descriptions of the cyanobacteria involved in creating the modern structures provide useful analogies for enigmatic Middle Cambrian fossil morphologies.