A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate?

ArticleinGeology 37(10):951-954 · October 2009with 70 Reads
Abstract
A large drop in seawater 87Sr/86Sr during the Middle Ordovician was among the most rapid in the entire Phanerozoic. New 87Sr/86Sr measurements from Nevada indicate that the rapid shift began in the Pygodus serra conodont zone of the upper Darriwilian Stage. We use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic Sr to the oceans. A close balance between volcanic outgassing and CO2 consumption from weathering produced steady pCO2 levels and climate through the middle Katian, consistent with recent Ordovician paleotemperature estimates. In the late Katian, outgassing was reduced while volcanic weathering continued, and resulted in a cooling episode leading into the well-known end-Ordovician glaciation.
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    The Middle-Late Ordovician transition (Darriwilian to Sandbian Age) witnessed a major pulse of the Great Ordovician Biodiversification Event (GOBE) and distinctive oceanic geochemical fluctuations, such as coeval negative C and Sr isotope excursions. In this study, investigations into geochemical variations, notably the Hg abundance (or Hg/TOC), have been carried upon the organic-rich black shale of the Mid-Upper Ordovician Saergan Formation to unravel the causes of this pulse. Based on these data, three phases were identified. Phase 1 (0 to 3 m) is characterized by rising Hg/TOC (up to 138 ppb/wt%) and Ti/Al values as well as high CIAcorr (corrected chemical index of alteration) values (68.9–72.3) with negligible enrichment of redox sensitive elements (RSE) and nutrient elements (e.g. U ≤ 5.2 ppm, V ≤ 153 ppm, Mo ≤ 1.8 ppm, P2O5 ≤ 0.2%), suggesting intensified volcanism, which could have emitted significant amount of greenhouse gases, thereby leading to climate warming. In contrast, Phase 2 (3 to 11 m) is characterized by decreasing Hg/TOC and Ti/Al, relatively low though slightly fluctuating CIAcorr values, generally depleted in RSE (except moderately enriched U up to 14.6 ppm) and increased P/Al and Ba/Al, implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling. Phase 3 (Sandbian Age: 11–13 m) witnessed continuous decrease in Hg/TOC ratio, an increase in Ti/Al and CIAcorr values, fairly low values of RSE enrichment and P/Al and Ba/Al ratios, indicating recurrent climate warming, and the potential for slowed oceanic circulation and attenuated upwelling of nutrient-rich deep waters onto the shallow shelf. These changes could have diminished bioproductivity and organic output onto the seafloor. This study offers insights into volcanic-climatic-oceanic interactions around the Mid-Late Ordovician transition while black shales were extensively deposited and during a major pulse of the GOBE.
  • Thesis
    Full-text available
    The last few decades have seen rapid expansion of large-scale paleontological databases that have allowed a much more methodologically rigorous approach to macroevolutionary patterns over geologic timescales. Studies have included insights into extinction, speciation, geographic range, ecological diversification, and the effects of climate change among others. However, these databases are heavily biased towards benthic shelly invertebrates, no doubt a reflection of their proportional contribution to the fossil record as a whole. Because of this it has not been clear how general observed patterns are to other groups, particularly planktic organisms which may face very different evolutionary pressures. Here, I examined some common correlates of extinction risk identified previously, apply them to a group of planktic organisms, graptolites, and compare the significance and effect size to results from benthic taxa. Of the properties associated with extinction risk geographic range is the most widely used and consistent. This property can be measured in a variety of ways that may impact results but there is relatively little published literature comparing different methods of measuring geographic range despite its widespread use. I explore how six measures of geographic range respond to changes in sample size as well their utility as correlates of extinction risk in the context of three disparate datasets. Finally, graptolites are of practical use in biostratigraphy and form the bulk of the global geologic timescale for the Ordovician and Silurian periods. Automated techniques have been developed to incorporate ever larger biostratigraphic datasets but the uncertainty in results has been difficult to characterize. I use a graptolite occurrence dataset assembled from the literature to build a novel ordinal composite for the Middle to Late Ordovician with the recently developed Horizon Annealing (HA) technique. From this composite I explore the limits and advantages of HA, with a particular focus on characterizing the uncertainty within and across the solution space of the ordinal composite. Analyses of 1114 graptolite species with general linear models found that all factors commonly associated with extinction risk in benthic taxa were also significant in this planktic clade. However, the magnitude of the effect of geographic range was much lower than typically reported. This is most likely because the relationship between geographic range and extinction risk is non-linear with the greatest gains in extinction risk between taxa with small geographic range values and those with moderate ranges. As graptolites generally have large geographic ranges, even substantial increases in geographic range only provide a modest decrease in extinction risk. Different measures of geographic range also varied substantially in their explanatory power, and unidimensional measures were particularly poor. When the covariance among factors was accounted for by the use of partial least squares regression the strongest correlate of extinction risk was overall commonness, which reflects the interdependent effects of sampling and geographic range. Among analyses overall commonness explained 12-30% of the total variance in extinction risk. Because these two properties reinforce one another they are essentially impossible to disentangle in fossil datasets where the number of occurrences per taxon is typically low. After taking account of the correlation geographic range and sampling, the individual contributions of either variate alone were either very low (<5%) or not significant at all. The second strongest correlates of extinction risk were clade and age cohort, which are strongly correlated in graptolite evolutionary history. These two variates individually explained as much as an additional 18% of the variance. While all individual measures of geographic range were significant correlates of extinction risk in graptolites their individual magnitudes varied substantially. To explore the cause of these differences I examined how six measures of geographic range (minimum spanning tree distance, convex hull area, maximum pairwise great circle distance, latitudinal range, longitudinal range, and equal area cell count) responded to sample size using three datasets: a simulated dataset with two differently shaped distributions of equal area, a fossil dataset of 381 brachiopod genera from the Paleobiology Database, and a set of 1152 modern bird species from the eBird database. Results showed that measures could be classified into two groups based on how their accuracy and precision responded to sampling. Group 1 measures (maximum pairwise distance, latitudinal range, and longitudinal range) rapidly became accurate and precise as sample size increased while Group 2 measures (minimum spanning tree, convex hull, and cell count) became accurate at a much slower rate, with one notable exception. In one of the simulated distributions the convex hull, as expected, became more precise as sample size increased, but unexpectedly, became less accurate at the same time. Group 2 measures also often showed a humped pattern where the lowest precision occurs at low, but not the lowest, sample sizes. This feature reflects the fact that variance at very small sample sizes is limited by either the method (cell count) or clustering of occurrences (minimum spanning tree and convex hull). Whereas the rank order of the value taxon geographic ranges was robust to variation in sample size across all measures, Group 1 measures had lower values at the smallest sample sizes. I further examined the six measures as correlates of extinction risk in the PBDB dataset using both logistic regression and general linear models. Logistic regression of the duration of generic survival beyond their stage of first occurrence showed that all measures of geographic range were significant and relatively robust to differences in how large versus small ranges were classified. General linear models of extinction risk also found that all measures of geographic range were significant but that the magnitude of effects varied significantly. Group 1 measures were relatively poor correlates of extinction risk in comparison to Group 2 measures. The relatively poor performance of Group 1 measures probably reflects that fact that these measure are not able to capture the complex nature of real distributions and because of methodological biases in these measures that make them vulnerable to outlier points and make some values more likely than others. The convex hull also suffers from vulnerability to outlier points and consistently overestimates geographic range. The minimum spanning tree method performs well as a correlate of extinction risk but is computationally expensive while equal area cell count performs even better and is not computationally expensive. For these reasons I advise the use of equal area cell count as a measure of geographic range in most cases, with minimum spanning tree as a supplement if the number of point occurrences is not too great. I also advise against the use of Group 1 measures and CH as measures of geographic range for extinction risk analyses. Finally, although previous automated biostratigraphic methods of ordination have proven powerful, the amount of uncertainty in solutions remains unclear, partially because such characterization is time-consuming. This is especially true for the Horizon Annealing (HA) approach, which uses more data than other methods. I examined how HA performed using a large dataset of 109 stratigraphic sections containing 136 graptolites species, one event bed, and three K-bentonites across 1549 horizons. The resulting composite generally agrees with published biozonations and independently developed composites affirming HA scales up effectively. To test for a previously hypothesized methodological bias (greater influence of first appearances than last appearances on determining the ordination of events in the composite), I ran the solution in reverse but did not find any evidence of this bias in the system. To reduce the initial burn-in time of searching in HA, I tested the use of a quasi-Bayesian scaffolding approach that starts the solution closer to traditional biozonations and found that it did significantly improve the penalty score of the solution, without any apparent bias. I further examined how effective the different ways of mutating the composite solution were at improving the solution during the search procedure. Mutations that allow changes in the spacing of collection levels within sections relative to the composite were much more effective than those that did not. I characterized uncertainty in the solution with three methods (vice, jackknife, and an island search). Vice was the fastest and most conservative measure, and it indicates an uncertainty range of 40 horizons on average, which corresponds to a temporal resolution of ~373 Kya. Finally, I characterized the uncertainty based on differences between three independent runs and found that although the global first and last occurrences of taxa were robust, the position of individual horizons varied more than was implied by any of the within-run uncertainty metrics. This result suggests that using HA composites to test dynamics based on patterns of occurrence within the temporal range of individual taxa should be done with caution. Although the accelerating pace of quantitative approaches to macroevolution have yielded more robust results than was previously possible there are substantial gaps in taxonomic coverage and methodological issues which have often been downplayed. The studies presented here attempt to address some of those issues. First, a large proportion of macroevolutionary studies have been based on patterns in shelly benthic invertebrates and the results then treated as if they apply to all groups of organisms. I demonstrate that while the same factors are often significant between these shelly benthic invertebrates and planktic graptolites the magnitude of relationships vary substantially. These contrasts in magnitude are reflective of the groups’ ecological strategies and can provide further insight into macroevolutionary processes that uniquely effect each one. Therefore, studies should be careful to interpret their results with regard to the ecology of the particular organisms being examined and to not be overly broad in claims about how generalizable a pattern is. Second, the treatment of geographic range in macroevolutionary studies has been inconsistent and the field of macroevolution would be well-served by some standardization. Here I found that unidimensional measures of geographic range, particularly the commonly used maximum pairwise distance, should not be used due to serious biases and limitations. More complex methods, minimum spanning tree distance and equal area cell count, were found to have the most desirable qualities as measures of geographic range and should be used in most macroevolutionary analyses. Finally, the use of ordinal composite built from biostratigraphic data for analyses of turnover, extinction, colonization, or other patterns requires a knowledge of the uncertainty in the system that was lacking. I demonstrated three methods of quantifying uncertainty in an ordinal composite as well as refining search methods that should allow for this powerful approach to be more widely utilized and understood.
  • Article
    Full-text available
    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 infl uenced by changes occurring at the base of the marine food web, i.e., redoxdriven changes in nutrient cycling and primary producer communities.
  • Article
    The extreme warmth associated with the mass extinction at the Permian-Triassic boundary was likely produced by a rapid build-up of carbon dioxide in the atmosphere from the eruption and emplacement of the Siberian Traps. In comparison to another hyperthermal event, the Palaeocene-Eocene Thermal Maximum, the Permian-Triassic event, while leaving a similar carbon isotope record, likely had larger amounts of CO2 emitted and did not follow the expected time scale of climate recovery. The quantities and rates of CO2 emission likely exhausted the capacity of the long-term climate regulator associated with silicate weathering. Failure was enhanced by slow rock uplift and high continentality associated with the supercontinental phase of global tectonics at the time of the Siberian Traps eruption.This article is part of a discussion meeting issue 'Hyperthermals: rapid and extreme global warming in our geological past'.
  • Article
    There are numerous K‐bentonites interlayered with black shales from the Late Ordovician‐Early Silurian that are widely distributed in South China. In this paper, we carried out mineralogical and geochemical investigations on K‐bentonite samples collected from 6 sections in South China. The petrological features of thin sections, X‐ray diffraction data, and major element results show that in addition to clay minerals, which are dominated by illite/smectite mixed layer and illite, the K‐bentonites also contain quartz, microcline, albite, pyrite, and zircon. Zircon U–Pb dating for two K‐bentonite beds (WXP‐BT2 and LBP‐BT1) by the LA‐ICP‐MS method yielded two weighted mean ²⁰⁶Pb/²³⁸U ages of 443.5 ± 1.9 and 440.4 ± 5.6 Ma, respectively. The concentrations of rare earth elements (REEs) range from 141.28 to 854.44 ppm, chondrite‐normalized REE patterns display a negative Eu anomaly and an enhanced enrichment in light REE. Samples plot in the fields of trachyte, trachyandesite, rhyodacite (dacite), and andesite in a plot of Nb/Y against Zr/TiO2 suggest that the K‐bentonites are most probably derived from felsic magmas with subalkaline to alkaline affinities. The discrimination diagrams (Y‐Nb, Y + Ta‐Rb, Y + Nb‐Rb) show that the tectonic setting of the source volcanoes ranges from a volcanic arc to within‐plate setting. The widely distributed bentonites suggest intensified volcanic ash eruptions in the early Late Ordovician, which released huge amounts of volcanic ash. The SO2 emission and the weathering of the volcanic ashes probably resulted in global climate cooling and indirectly caused the Hirnantian glaciation and the mass extinction at the end of the Ordovician Period.
  • Article
    Full-text available
    The Ordovician 87 Sr/ 86 Sr isotope sea-water curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the strati-graphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North Ameri-can Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian re-gion, and Ohio Valley. Our results indicate that conodont apatite may provide an accu-rate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in 87 Sr/ 86 Sr is estimated at ~5.0–10.0 × 10 –5 per m.y. Variable preservation of conodont elements limits the precision for individual stratigraphic horizons. Replicate conodont analyses from the same sample differ by an average of ~4.0 × 10 –5 (the 2σ standard de-viation is 6.2 × 10 –5), which in the best case scenario allows for subdivision of Ordovician time intervals characterized by the highest rates of fall in 87 Sr/ 86 Sr at a maximum reso-lution of ~0.5–1.0 m.y. Links between the increased rate of fall in 87 Sr/ 86 Sr beginning in the mid-late Darriwilian (Phragmodus po-lonicus to Pygodus serra conodont zones) and geologic events continue to be investigated, but the coincidence with a long-term rise in sea level (Sauk-Tippecanoe megasequence boundary) and tectonic events (Taconic orog-eny) in North America provides a plausible explanation for the changing magnitude and 87 Sr/ 86 Sr of the riverine Sr fl ux to the oceans.
  • Article
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    The Ordovician Period (485–443 Ma) is characterized by abundant evidence for continental-sized ice sheets. Modeling studies published so far require a sharp CO2 drawdown to initiate this glaciation. They mostly used non-dynamic slab mixed-layer ocean models. Here, we use a general circulation model with coupled components for ocean, atmosphere, and sea ice to examine the response of Ordovician climate to changes in CO2 and paleogeography. We conduct experiments for a wide range of CO2 (from 16 to 2 times the preindustrial atmospheric CO2 level (PAL)) and for two continental configurations (at 470 and at 450 Ma) mimicking the Middle and the Late Ordovician conditions. We find that the temperature-CO2 relationship is highly non-linear when ocean dynamics are taken into account. Two climatic modes are simulated as radiative forcing decreases. For high CO2 concentrations (≥ 12 PAL at 470 Ma and ≥ 8 PAL at 450 Ma), a relative hot climate with no sea ice characterizes the warm mode. When CO2 is decreased to 8 PAL and 6 PAL at 470 and 450 Ma, a tipping point is crossed and climate abruptly enters a runaway icehouse leading to a cold mode marked by the extension of the sea ice cover down to the mid-latitudes. At 450 Ma, the transition from the warm to the cold mode is reached for a decrease in atmospheric CO2 from 8 to 6 PAL and induces a ∼ 9 ◦C global cooling. We show that the tipping point is due to the existence of a 95 % oceanic Northern Hemisphere, which in turn induces a minimum in oceanic heat transport located around 40◦ N. The latter allows sea ice to stabilize at these latitudes, explaining the potential existence of the warm and of the cold climatic modes. This major climatic instability potentially brings a new explanation to the sudden Late Ordovician Hirnantian glacial pulse that does not require any large CO2 drawdown.
  • Article
    Precise biostratigraphic correlation of the base of the Paibian Stage, Cambrian, within and beyond the North China craton has been difficult, mostly because of the strong endemism of the trilobite fauna on the shallow platform facies. The use of the Steptoean Positive Carbon Isotope Excursion (SPICE) in defining the Guzhangian-Paibian stage boundary has become a standard practice. Thus, a chemostrati- graphic study of the Huangyangshan section from North China has been carried out. This section in Shandong Province clearly records the SPICE, with the most detailed fossil and carbonate carbon isotope data to date. Compared to other sections in the world, all the published SPICE in North China display both reduced peak and reduced difference (D13C) values. These may imply unique conditions on the North China craton during the upper Guzhangian to Paibian stages. This study shows that the SPICE is recorded in relatively condensed sections in North China; thus, 􏰀 0.5 m sampling density is suggested here in order to evaluate the presence of the SPICE in North China and condensed sections.
  • Article
    Full-text available
    The present study on bulk carbonate 87Sr/86Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87Sr/86Sr curve for the Tremadocian-KatianStages (485-445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87Sr/86Sr results from bulk carbonate from the Tremadocian-Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87Sr/86Sr and its utility for 87Sr/86Sr chemostratigraphy. A high-resolution 87Sr/86Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87Sr/86Sr (87Sr/86Srseawater). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87Sr/86Sr in order to determine the conditions under which 87Sr/86Srseawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87Sr/86Srseawater trend, but that its 87Sr/86Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10-20 × 10-5, compared to 5-10 × 10-5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87Sr/86Sr (Δ87Sr/86Sr) was determined by taking the difference between 87Sr/86Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87Sr/86Sr values. Cross plots comparing Δ87Sr/86Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87Sr/86Srseawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 oC. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87Sr/86Srseawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87Sr/86Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low (<10-100) or if pore fluid 87Sr/86Sr is similar to the seawater 87Sr/86Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87Sr/86Srseawater, with a sample variation on par with fossil materials (<10 × 10-5), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.
  • Article
    The Lower Palaeozoic includes not only one of the most devastating extinction events in earth history but also several minor episodes of global climatic change affecting all kinds of biota. In the last decade knowledge on palaeo-environmental changes during Ordovician and Silurian times such as anoxic-, or cooling events increased steadily. It has been stated for long time, that except for the Sahara glaciation, Ordovician and Silurian times were conform to greenhouse climates, but due to the increased documentation of cooling periods during this time span it seems worthwhile reviewing the relation of changed conditions in climate history and its significance for marine biota. Here we present an overview on all major events known from Ordovician and Silurian times and highlight the relations between coral diversity and palaeoclimate perturbations.
  • Article
    There are at least a dozen Phanerozoic continental flood basalts and oceanic plateaus (large igneous provinces) that roughly obey a "rule of two times one million" (volume of extruded lava of one million cubic kilometers and duration of volcanic activity ∼1 m.y.). The correlation between large igneous province ages and mass extinctions (and oceanic anoxia events) is excellent, but quantitative scenarios are still wanting. We hypothesize that the temporal sequences of extrusions determine the severity of extinction: Volcanic pulses separated by thousands of years allow the ocean-atmosphere system time to recover, whereas large volcanic pulses occurring in a shorter sequence may result in a runaway effect and cause a mass extinction. Detailed flow-by-flow magnetic stratigraphies of thick sections have identified directional groups (sequences of superimposed lava flows with the same paleomagnetic direction that cooled in a time too short to record secular variation). With help of this simple tool, many single eruptive events with a volume larger than 1000 km3, some in excess of 10,000 km3, emplaced in possibly less than a decade, have been identified. We review this evidence, mainly for the following flood basalt provinces: Columbia, Brito-Arctic, Deccan, Karoo-Ferrar, Central Atlantic magmatic province, and Siberian Traps. Large igneous province volcanism occurs in a highly discontinuous way, on embedded time scales, on the order of 10 m.y., 1 m.y., 100 k.y., and 10 yr. This provides constraints for models of plume-lithosphere interaction and magma production. A next step is to model the consequences of massive injection of gases that can be derived from these time and duration estimates. Early attempts are reviewed in a companion paper in this volume.
  • Chapter
    Rapid and sustained biotic diversification (“Ordovician radiation”) to reach highest diversity levels for Paleozoic; prolonged “hot-house” climate punctuated by “ice-house” intervals and oceanic turnover; strong fluctuations in eustatic level, global glaciation, and mass extinction at end of period; appearance and evolution of pandemic planktonic graptolites and conodonts important for correlation; moderate to strong benthic faunal provincialism; re-organization and rapid migration of tectonic plates surrounding the Iapetus Ocean; migration of South Pole from North Africa to central Africa, all characterize the Ordovician period. HISTORY AND SUBDIVISIONS Named after the Ordovices, a northern Welsh tribe, the Ordovician was proposed as a new system by Lapworth in 1879. It was a compromise solution to the controversy over strata in North Wales that had been included by Adam Sedgwick in his Cambrian System but which were also included by Murchison as constituting the lower part of his Silurian System. Although it was initially slow to be accepted in Britain, where it was instead generally called Lower Silurian well into the twentieth century, the Ordovician was soon recognized and used elsewhere, such as in the Baltic region and Australia.
  • Article
    The Ordovician and Silurian periods were times of major geological activity as regards palaeogeography, volcanism and climate change, the last of these evidenced by a series of cooling episodes and glaciations that climaxed in the Hirnantian (Late Ordovician). The presence of cryptospores in the Darriwilian (Middle Ordovician) marked the advent of higher plants on land. A critical survey of direct (mega- and microfossils) and some indirect evidence in succeeding rocks indicates the presence of algae, Bacteria, Cyanobacteria, Fungi, probable lichens, cryptophytes and basal tracheophytes. Similar associations of photosynthesizers and decomposers occur today in cryptogamic covers (CCs), for example biological crusts, except that bryophytes replace cryptophytes (basal embryophytes) and tracheophytes are absent. Thus, extant CCs, which make significant contributions today to global carbon and nitrogen fixation and prevention of erosion, provide an excellent analogue for the impacts of early land vegetation on both lithosphere and atmosphere. As a prerequisite to assessing impacts in Ordovician–Silurian times, with particular consideration of parameters used by climate modellers, the effects of a number of abiotic factors on the growth and survival of extant cryptogamic ground covers and their environmental impacts are reviewed. Factors include photosynthetically active radiation, ultraviolet radiation, temperature, water, oxygen, carbon dioxide, nitrogen, phosphorus, iron, surface roughness and albedo. A survey of the nature and extent of weathering facilitated by such vegetation concludes that it was limited based on depth of weathering when compared with that from rooted tracheophytes today, with minor effects on carbon dioxide drawdown. As global net productivity from Ordovician–Silurian CCs was very probably lower than today, and while the small fraction of intractable material in their organic carbon would have resulted in a more rapid turnover of terrestrial biomass, we conclude that there was decreased possibility of long-term organic carbon burial. Hence, there would have been very limited increase in atmospheric oxygen and decrease in carbon dioxide resulting from carbon burial.
  • Article
    Full-text available
    A pronounced positive δ¹³C excursion in the Hirnantian Age has been documented globally, reflecting large perturbations of carbon cycling in the Late Ordovician oceans. Increased organic-carbon burial or enhanced carbonate weathering during glacioeustatic sea-level regression has been proposed to account for this anomalous C-isotope excursion. To test the two competing hypotheses, we measured ⁸⁷Sr/⁸⁶Sr and δ¹³C of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative ⁸⁷Sr/⁸⁶Sr shifts that coincide with two prominent positive δ¹³C excursions and glacial advances. Numerical model simulations suggest that enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level fall is required to produce the observed drops of ⁸⁷Sr/⁸⁶Sr and the coeval large positive δ¹³C excursions, possibly with or without increased organic carbon burial.
  • Article
    Full-text available
    The greatest relative changes in marine biodiversity accumulation occurred during the Early Paleozoic. The precision of temporal constraints on these changes is crude, hampering our understand- ing of their timing, duration, and links to causal mechanisms. We match fossil occurrence data to their lithostratigraphical ranges in the Paleobiology Database and correlate this inferred taxon range to a constructed set of biostratigraphically defined high-resolution time slices. In addition, we apply capture–recapture modeling ap- proaches to calculate a biodiversity curve that also considers ta- phonomy and sampling biases with four times better resolution of previous estimates. Our method reveals a stepwise biodiversity increase with distinct Cambrian and Ordovician radiation events that are clearly separated by a 50-million-year-long period of slow biodiversity accumulation. The Ordovician Radiation is confined to a 15-million-year phase after which the Late Ordovician extinc- tions lowered generic richness and further delayed a biodiversity rebound by at least 35 million years. Based on a first-differences approach on potential abiotic drivers controlling richness, we find an overall correlation with oxygen levels, with temperature also exhibiting a coordinated trend once equatorial sea surface tem- peratures fell to present-day levels during the Middle Ordovician Darriwilian Age. Contrary to the traditional view of the Late Ordovi- cian extinctions, our study suggests a protracted crisis interval linked to intense volcanism during the middle Late Ordovician Katian Age. As richness levels did not return to prior levels during the Silurian—a time of continental amalgamation—we further argue that plate tectonics exerted an overarching control on biodiversity accumulation.
  • Article
    Although extinction risk has been found to have a consistent negative relationship with geographic range across wide temporal and taxonomic scales, the effect has been difficult to disentangle from factors such as sampling, ecological niche, or clade. In addition, studies of extinction risk have focused on benthic invertebrates with less work on planktic taxa. We employed a global set of 1114 planktic graptolite species from the Ordovician to lower Devonian to analyze the predictive power of species’ traits and abiotic factors on extinction risk, combining general linear models (GLMs), partial least-squares regression (PLSR), and permutation tests. Factors included measures of geographic range, sampling, and graptolite-specific factors such as clade, biofacies affiliation, shallow water tolerance, and age cohorts split at the base of the Katian and Rhuddanian stages. The percent variance in durations explained varied substantially between taxon subsets from 12% to 45%. Overall commonness, the correlated effects of geographic range and sampling, was the strongest, most consistent factor (12–30% variance explained), with clade and age cohort adding up to 18% and other factors <10%. Surprisingly, geographic range alone contributed little explanatory power (<5%). It is likely that this is a consequence of a nonlinear relationship between geographic range and extinction risk, wherein the largest reductions in extinction risk are gained from moderate expansion of small geographic ranges. Thus, even large differences in range size between graptolite species did not lead to a proportionate difference in extinction risk because of the large average ranges of these species. Finally, we emphasize that the common practice of determining the geographic range of taxa from the union of all occurrences over their duration poses a substantial risk of overestimating the geographic scope of the realized ecological niche and, thus, of further conflating sampling effects on observed duration with the biological effects of range size on extinction risk.
  • Article
    Full-text available
    Paired measurements of bulk carbonate (δ13Ccarb), organic matter (δ13Corg), and their difference (Δ13C) can be used to estimate changes in isotopic fractionation through time as a function of O2/CO2 in the atmosphere. However, because local scale processes can also affect Δ13C, it is essential to compare sections from widely separated water masses. Here we present new δ13Corg data from Ordovician carbonate rocks from the Great Basin, Oklahoma, and Appalachian Basin and compare with published δ13Ccarb records from these sections and paired δ13C values from other carbonate successions around North America. These new data complement previous studies that focused on Upper Ordovician δ13Ccarb excursions and now provide a composite Ordovician δ13Corg record. New Lower Ordovician (Tremadocian Stage) δ13Corg data range from ca. − 26 to − 28‰, decreasing throughout the Lower–Middle Ordovician (Floian–Dapingian Stages) to ca. − 29 to − 31‰. δ13Corg values remain at their lowest throughout the Sandbian and are similar to other published Upper Ordovician (Sandbian–Katian) δ13Corg data from North America. Δ13C values from well-preserved intervals generally vary between + 26 to + 28‰ throughout the Lower to Middle Ordovician (Tremadocian to early Darriwilian), but increase to + 31‰ during the mid–late Darriwilian and mid Sandbian, similar to published data from younger Late Ordovician positive δ13C excursions known as the Guttenberg (GICE) and Hirnantian (HICE) events. The overall Δ13C trend shows a ~ 3‰ increase throughout the Early–Middle Ordovician and coincides with a previously interpreted period of ocean cooling and some of the earliest pulses of global biodiversity of marine invertebrates and planktonic organisms. Modeling studies predict that pCO2 decreased during this time, suggesting that the effect of pCO2 on Δ13C may have been overwhelmed by other controls, such as an in increase in pO2 or a higher O2/CO2 ratio during this biodiversification event.
  • Thesis
    Full-text available
    The Tyrone Igneous Complex of Northern Ireland forms an integral part of the Grampian-Taconic orogen, linking the well documented sectors of Scotland, western Ireland and Newfoundland. The orogen records the accretion of a series of peri-Laurentian affinity arcs, ophiolites and microcontinental blocks to the Laurentian margin between the Late Cambrian and Middle Ordovician. The Tyrone Igneous Complex is broadly divisible into two distinct units: the c. 484-480 Ma ophiolitic Tyrone Plutonic Group and the structurally overlying c. 475-469 Ma arc-related Tyrone Volcanic Group. Both were intruded by a synvolcanogenic and syncollisional, to postcollisional high-level ensialic intrusive suite between c. 470 and 464 Ma associated with their coeval obduction to an outboard peri-Laurentian microcontinental block, the Tyrone Central Inlier, at c. 470 Ma. The Tyrone Plutonic Group is principally composed of amphibolite-facies layered and isotropic gabbros, sheeted dolerite dykes and rare pillow lavas. Tholeiitic suprasubduction affinity geochemical characteristics, Nd- isotope constraints, zircon inheritance, and the presence of late Fe-Ti enriched post-obduction dykes suggest the Tyrone Plutonic Group formed above a N-dipping subduction zone by the propagation of a spreading centre into a microcontinental block. The Tyrone Volcanic Group is characterized by mafic to intermediate lavas, tuffs, rhyolite, banded chert, ironstone and argillaceous sedimentary rocks cut by numerous high-level synvolcanogenic intrusive rocks. Geochemical signatures are consistent with formation in an evolving peri-Laurentian island-arc which underwent several episodes of rifting. High LILE and LREE enrichment, calc- alkaline geochemical signatures and strongly negative εNd t values suggest the Tyrone arc was at least partially founded on a fragment of microcontinental crust, which may have rifted off the Tyrone Central Inlier during the formation of the Tyrone Plutonic Group. Stong temporal, stratigraphic, and geochemical correlations with elements within the Annieopsquotch Accretionary Tract of Newfoundland suggest the Tyrone Igneous Complex represents a third phase of arc-ophiolite obuction in the Irish Caledonides during the Grampian-Taconic orogeny and may potentially host significant VMS mineralization. Through a combination of field mapping and petrochemistry several stratigraphic horizons have been identified in the Tyrone Igneous Complex, favourable for the formation and preservation of VMS deposits. Each is closely associated with hydrothermal alteration, synvolcanogenic faults and high-level synvolcanogenic intrusions of dolerite, gabbro, diorite and tonalite. Episodic rifting is recorded by the eruption of: abundant non-arc type Fe-Ti enriched eMORB (‘icelandite’), island-arc tholeiite, OIB-like alkali basalt, high-temperature tholeiitic rhyolites with flat to U-shaped REE profiles, and high-Zr rhyolites, within the calc-alkaline dominated sequence. Rift related mafic lavas occur in the hangingwall to VMS-style mineralization and are closely associated with ironstones (often Au-bearing) and/or argillaceous sedimentary rocks representing volcanic quiescence. Extensive hydrothermal alteration, characterized by Na-depletion, high Ba/Sr, Bi, Sb, Ni, CCPI, AI and variable MgO and CaO, allows specific target areas to be identified. In the lower bimodal-mafic Tyrone arc and backarc, hydrothermal alteration is associated with Zn-Cu mineralized float. Pb-Zn-Cu-Au mineralization occurs in silicified auriferous rhyolite domes/flows and/or volcaniclastic rocks of the syncollisional bimodal-felsic upper Tyrone arc. Ophiolite hosted Cu mineralization is characterized by chalcopyrite stringers hosted in sheeted dyke sequences.
  • Article
    Volcanism is a natural climate force that causes variations in temperatures. The Aptian Oceanic Anoxic Event 1a (OAE-1a) was preceded by a prominent negative C-isotope excursion attributed to major volcanism of the Ontong Java plateau (OJP), which presumably resulted in a pCO2 increase and a climatic change. However, the OJP alone may not adequately explain some important isotopic signatures such as the negative strontium-isotope excursion from 125 Ma to 113 Ma that is recorded in the corresponding marine deposits. We present an independent and hitherto undocumented case, the giant Aptian volcanism in the Songliao Basin and northeast Asia (SB-V) on the Cretaceous active continental margin between the Eurasian and the Pacific plates, which covered an area of ca. 2.3 × 106 km2, nearly matching the simultaneous case of the OJP. Intensive strong, explosive volcanic eruptions of the SB-V occurred at 121-109 Ma and introduced a large volume of fine-grained volcanic ash and degassing volatiles into the atmosphere. The Aptian isotopic ratios (87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) of marine carbonates from the Mid-Pacific shift in values between their Barremian pre-excursion high values and the negative magmatic values of the SB-V. The transient global cooling at the onset of the OAE-1a coincided with the beginning of the violent acidic eruption of the SB-V (119.9-120.2 Ma). We therefore infer that the SB-V must have played a role in the Aptian global climatic changes and OAE-1a through the heavy fall of volcanic dust and the outgassing of aerosol and greenhouse gases.
  • Article
    Precise measurements of 786 marine carbonate, evaporite, and phosphate samples of known age provide a curve of seawater 87Sr/86Sr versus geologic time through the Phanerozoic. Many episodes of increasing and decreasing values of 87Sr/86Sr of seawater have occurred through the Phanerozoic. The Late Cambrian Early Ordovician seawater ratios are approximately equal to the modern ratio of 0.70907. The lowest ratios, ˜0.7068, occurred during the Jurassic and Late Permian. The configuration of the curve appears to be strongly influenced by the history of both plate interactions and seafloor spreading throughout the Phanerozoic. The curve provides a basis for dating many marine carbonate, evaporite, and phosphate samples. Furthermore, diagenetic modifications of original marine 87Sr/86Sr values are often interpretable. Analysis of 87Sr/86Sr data, therefore, may provide useful information on regional diagenetic patterns and processes. All of the Cenozoic samples and some of the Cretaceous samples are from Deep Sea Drilling Project (DSDP) cores. With the exception of the DSDP samples, the curve was constructed only from samples containing at least 200 ppm Sr and not more than 10% dilute acid insoluble material. All measurements are made by comparison with standard SrCO3 (NBS SRM 987) for which a 87Sr/86Sr of 0.71014 is assumed. Precision is estimated to be ± 0.00005 at the 95% confidence level. Measured ratios of 42 modern marine samples average 0.70907, with a standard deviation of 0.00004. *Present addresses: (Denison) Suite 616, One Energy Square. 4925 Greenville Avenue, Dallas, Texas 75206; (Nelson) 2516 West Five Mile Parkway, Dallas, Texas 75233
  • Article
    Ranges of conodonts in stratigraphic sections at five localities in the Monitor and Antelope ranges of central Nevada are used graphically to assemble a standard reference section for the lower Middle Ordovician Whiterockian Series. The base of the series is officially 0.3 m above the base of the Antelope Valley Limestone in the stratotype in Whiterock Canyon (Monitor Range). The top is the level at which Baltoniodus gerdae makes a brief appearance in an exposure of the Copenhagen Formation on the flanks of Hill 8308 in the western Antelope Range. Graphic compilation of the sections considered in this report also indicates that a level correlative with the base of the Whiterockian Series in the stratotype section is 66 m above the base of the Antelope Valley Limestone in its de facto type section on Martin Ridge in the eastern part of the Monitor Range. Ranges, diversity, and the composition of the conodont faunas differ markedly in lithofacies adjacent to the basal boundary of the series; hence we are unable to identify a single conodont species, in a credible developmental sequence, to serve as biological marker of that boundary.
  • Chapter
    The Ordovician stratigraphic succession of eastern North America contains at least 60 altered volcanic ash beds, K-bentonites, one or more of which are distributed over an area of 1.5 million km2. The beds range in age from Ibexian to Cincinnatian with the greatest concentration in mid-Mohawkian strata. Most K-bentonites are not widely distributed, but a few can be correlated for hundreds, or even thousands, of kilometers by chemical fingerprinting techniques, tracing on wireline logs, and matching of detailed outcrop descriptions. The thickest and most widespread beds include the mid-Mohawkian Hockett (new), Ocoonita (new), Deicke, Millbrig, and Dickeyville K-bentonites, in ascendin order. These beds, comprising the Hagan K-bentonite Complex, can be traced confidently in outcrop and on wireline logs from the Sevier Basin of western Virginia and eastern Tennessee westward and northward into the Illinois and Michigan Basins. The Deicke, Millbrig, and Dickeyville extend even farther northwestward into the Mississippi Valley, a total distance of approximately 1,300 km. Evidence suggests that the Deicke and Millbrig may extend into the Arkoma Basin of eastern Oklahoma. Overall, the Ordovician K-bentonites are thicker, coarser, and more numerous toward the central and southern Appalachians, suggesting that the source volcanoes were situated in the region between what are now Alabama and Pennsylvania. Majorand trace-element analyses of whole rock K-bentonite samples indicate that the parental magmas consisted of a calc-alkaline suite ranging through andesite, rhyodacite, trachyandesite and rhyolite. Furthermore, the chemical compositions indicate a tectonomagmatic setting characterized by destructive plate-margin volcanics. Large volumes of volcanic ash ejected from plinian and co-ignimbrite eruptions along the active margin were carried by the prevailing southeast tradewinds for hundreds of kilometers northwestward and were deposited in shallow cratonic seas. Copyright © 1996, The Geological Society of America, Inc. (GSA). All rights reserved.
  • Article
    A high-resolution Sr isotope study of Middle to Upper Cambrian platform carbonates of the southern Great Basin significantly refines the structure of the existing seawater Sr isotope curve. Samples were selected using rigorous stratigraphie, petrographic, and geochemical criteria in order to minimize the effects of diagenetic alteration and contamination from noncarbonate components. Highest seawater 87Sr/86Sr values over the past 2 b.y. are constrained to <0.7093 and occurred during the latest Middle Cambrian to earliest Late Cambrian. Integrated with published Cambrian seawater 87Sr/86Sr values, these new data record the culmination of an increasing trend in seawater 87Sr/86Sr beginning in Early Cambrian time. This trend is interpreted to reflect increasing flux and/or isotopic ratio of riverine Sr delivered to Cambrian oceans in response to Pan-African orogenesis and attendant enhanced chemical weathering. High-resolution changes in seawater 87Sr/86Sr values are superimposed on the longer-term trend. Correlation between 87Sr/86Sr values and sea level leads us to propose a model that links changes in weathering rates and riverine Sr flux with variations in exposed continental surface area related to short-term (1-5 m.y.) sea-level events during greenhouse times.
  • Article
    The extensive work carried out during more than a decade by the International Subcommission on Ordovician Stratigraphy has resulted in a new global classification of the Ordovician System into three series and seven stages. Formal Global Boundary Stratotype Section and Points (GSSPs) for all stages have been selected and these and the new stage names have been ratified by the International Commission on Stratigraphy. Based on a variety of biostratigraphic data, these new units are correlated with chronostratigraphic series and stages in the standard regional classifications used in the UK, North America, Baltoscandia, Australia, China, Siberia and the Mediterranean-North Gondwana region. Furthermore, based mainly on graptolite and conodont zones, the Ordovician is subdivided into 20 stage slices (SS) that have potential for precise correlations in both carbonate and shale facies. The new chronostratigraphic scheme is also tied to a new composite δ13C curve through the entire Ordovician.
  • Article
    Full-text available
    One hundred and fifty-six 87Sr/86Sr ratios were measured from Ordovician and Silurian brachiopod shells, marine calcite cements, and conodonts in order to establish the secular strontium isotope curve for the coeval seawater. Preservation of the brachiopod shell material has been evaluated by petrographic and geochemical criteria and only the well preserved internal secondary layer of the shells has been utilized for strontium isotope measurements. The results document a gradual decrease in 87Sr/86Sr, from 0.7091 to 0.7087, from Tremadoc to Llandeilo, a sharp decline to 0.7078 during the late Llandeilo-early Caradoc; little change during Caradoc and the Ashgil; and a steady rise to 0.7087 through the Silurian. These long-term (107 yr) variations, with magnitudes in the range of 10−3, are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering.The gradual decrease in 87Sr/86Sr ratio during the Early Ordovician may record the reduction in uplift and weathering rates due to waning of the Pan-African orogenies. The rapid decline near the Llandeilo/Caradoc boundary suggests a strong hydrothermal flux likely due to increased sea-floor spreading and a possible superplume event. The latter may have caused the prominent transgressive phase, the largest in the Phanerozoic, which would have muted continental flux input. Although the Caradoc was the main interval for the Taconic Orogeny, its impact on the Sr continental flux may have been delayed until the early Llandovery. This effect, complemented by reworking of glacial deposits near the Ordovician-Silurian boundary and enhanced by phases of the Silurian Salinic Orogeny, may have combined to give the progressive increase in the strontium isotope ratio through the Silurian. The scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes.
  • Article
    Full-text available
    North American Ordovician strata record a large shift in their neodymium isotopic composition ( ) at around Nd period before orogenic sources became dominant. We conclude from this that, superimposed on a general westward regional shift in sediment sources with time, there were also complex local effects involving multiple (unmixed) sediment sources that persisted long after the initial pulse of orogenic material arrived. The combined "simultaneous" nature of the isotopic shift, an Ordovician sea-level high stand, and the emergence of the Appalachian-Taconian- Caledonian orogenic belt as a primary sediment source, leads us to conclude that by 450 Ma, seafloor south of North America was being supplied by well-mixed, isotopically homogeneous sediment delivered from uplifted fold-thrust belts and foreland basins of the Appalachian Taconian highlands. U-Pb detrital zircon ages from bracketing sandstone units reinforce the Nd evidence for a complete changeover in provenance between 465 Ma (abundant Archean-age zircons) and 440 Ma (no Archean-age zircons) in the Ouachita region.
  • Graptolite-rich shale in the Valley and Ridge province of Alabama, Georgia, Tennessee, and Virginia records subsidence and migration of the Sevier foreland basin, which formed in response to arc-continent collision during the Middle Ordocician Blountian phase of the Taconic orogeny. Rapid, tectonically induced subsidence of what had been a shallow carbonate platform led to pelagic deposition of graptolite shale in a deep basinal setting. The contact of the shale with the underlying carbonate rocks is a distinct record of initiation of subsidence. Graptolites were collected extensively and systematically from the shale at 50 localities in the southern Appalachians. Biostratigraphic datas from 22 localities were amenable to analysis by graphic correlation. Recorrelation of each of the 22 measured sections with the composite standard reference section allowed the stratigraphic base of the shale in each section to be expressed in terms of composite standard units, and such correlations permitted the diachronism of the basal shale contact to be measured with much greater precision than with zonal biostratigraphy. In addition, age differences of the basal shale contact between sections can be expressed in terms of time. With the temporal information provided by the composite standard reference section and original distances between sections determined from a palinspastic reconstruction, the rate of migration of the foreland basin as recorded in the diachronous basal shale contact was determined. 70 refs., 15 figs., 1 tab.
  • Article
    Full-text available
    The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated delta13C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive delta13C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian delta13C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric pCO2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level---which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of pCO2 by suppressing shelf-carbonate production---signals the transition to a Late Ordovician icehouse climate ˜10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.
  • The Middle Ordovician St. George Unconformity is the Sauk/Tippercanoe sequence boundary in western Newfoundland. It is a karst unconformity to disconformity to paraconformity. It formed in the initial stages of the Taconic Orogeny. Faulting active throughout late Ibexian and Whiterockian time is consistent with the passage of the forebulge through western Newfoundland. This and changing rates of subsidence influenced not only the shape of the unconformity, but also pre- and post-unconformity sedimentation, that is, facies, thickness, and cyclic sedimentation; the ordered timing of events upon the shelf and in coeval slope sediments; and a diachronous event stratigraphy from Newfoundland to Quebec. -from Authors
  • The unconformity influenced the distribution of postunconformity carbonates, including Middle Ordovician build-ups. It also influenced later Zn mineralization and possible localization of petroleum reservoirs in the basin.-from Authors
  • Article
    Integrated sequence stratigraphic, biostratigraphic, and chemostratigraphic analyses of three stratigraphic sections in central Nevada indicate that Late Ordovician glaciation-induced sea-level fall produced diachronous, stepwise faunal turnover in graptolites, conodonts, chitinozoans, and radiolarians, and also triggered a strong, but transient, positive δ13C excursion. This pattern is very different from that described for most mass extinction events.
  • Article
    Late Ordovician (Caradocian) age Viola limestones from the Arbuckle Mountains, Oklahoma, have Sr concentrations from 216 ppm to 3720 ppm (average 1600 ppm), δ18O values from -5.1‰ to -3.1‰ (PDB), and 87Sr/86Sr ratios from 0.70775 to 0.70793. Although textural and chemical evidence indicates that these limestones suffered diagenetic alteration, the elemental and isotopic compositions still reveal important information about the primary marine mineralogy and seawater composition during Late Ordovician time. First, the high Sr concentrations suggest that original Viola sediments were composed mainly of aragonite. Second, the highest δ18O value (-3.1‰) of the lime-stones, which is interpreted to be the least altered, constrains the δ18O values of seawater to >-1‰ (SMOW), assuming that ocean temperatures were 27° ± 5°C. Third, the 87Sr/86Sr ratios of Viola limestones probably better represent the 87Sr/86Sr ratios of coeval seawater because the ratios from these Sr-rich limestones show less scatter than those of similar-aged samples previously published.
  • Article
    The carbon cycle model GEOCARBSULF is extended by dividing the weathering of silicates into volcanic and non-volcanic rocks. The proportion of volcanic weathering is calculated as a function of time from the oceanic record of 87Sr/86Sr. The volcanic proportion is then used to modify the equations for calculating atmospheric CO2 by the addition of a new non-dimensional volcanic weathering factor. The effect of uplift and physical erosion on weathering is also modified by using only the distribution over time of the abundance of sandstones and shales, and not Sr isotopic data that had been used previously. Results indicate moderate change from GEOCARBSULF in the distribution of atmospheric CO2 over Phanerozoic time. This includes an increased minimum in CO2 during the Late Ordovician, in agreement with the presence of a continental glaciation at that time, and a shift of maximum Mesozoic values from the Jurassic to the Early Cretaceous.
  • Article
    Full-text available
    Seafloor hydrothermal systems clearly have played an important role in controlling the composition of seawater over geologic time. However, controversy exists concerning their role as drivers of secular variability in major element composition of seawater during the Phanerozoic. The history of Phanerozoic changes in calcium (Ca 2+) and magnesium (Mg 2+) concentration has been reconstructed through analysis of fluid inclusions in evaporites, and this history is inconsistent with global geochemical cycling models that link both volcanic CO 2 release and Mg 2+ uptake / Ca 2+ release to inferred rates of seafloor production. These models generate little variability in Mg 2+ concentrations because of compensating effects. Some models that recreate the observed trends either do not conserve alkalinity or do not link seafloor spreading, inferred from past variations in continental flooding presumed to reflect changing mid-ocean-ridge volumes, to volcanic CO 2 production. The most comprehensive models reproduce the variations in seawater Mg 2+ /Ca 2+ through time quite well, but not because this ratio is tied to variations in seafloor spreading rate, and despite considerable mismatches to the Mg 2+ record. This cacophony arises in part from our lack of quantitative understanding of fundamental relationships between heat flow, seafloor production rates, sea level, hydrothermal circulation rates, volcanic CO 2 release rates, and ocean chemical changes. Nevertheless, the current fluid inclusion data seem to indicate that ocean composition has an "attractor" that drives Ca 2+ and Mg 2+ toward equilibration with seafloor hydrothermal mineral assemblages [20 mm (millimolal) Ca 2+ and 0 mm Mg 2+ ], thwarted by other processes (weathering and riverine inputs, biogenic mineral precipitation) that drive the system away from this attractor.
  • Article
    Full-text available
    1] To evaluate recent Ordovician paleogeographic reconstructions, e Nd values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates. Isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography. The isotopic patterns show a consistent picture of the changes in the Ordovician oceans. The e Nd values for water masses associated with Laurentia are strongly negative (À28 to À18) in the Early Ordovician, evolving over time to higher values in the range À13 to À5. The Early Ordovician signature of Laurentia is in marked contrast to other cratons and microplates, which have a range of values from À10 to À5. Samples from South China show interesting signals that reflect a greater similarity to Laurentia than other peri-Gondwana terranes. The isotopic variations are a function of both regional geology and global tectonic processes, the most obvious being the Taconic Orogeny and onset of the closure of the Iapetus Ocean. Regional and global models of paleogeography are considered in light of these proxy signals. This study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses. The use of the Nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models. By integrating such information, a more multidisciplinary approach is possible. Components: 9581 words, 5 figures, 2 tables, 1 data set. isotopic composition of Ordovician conodonts as a seawater proxy: Testing paleogeography, Geochem. Geophys. Geosyst., 3(2), 10.1029/2001GC000195, 2002.
  • Article
    Full-text available
    1] Geothermal activity is common at the foot of the Higher Himalaya near the Main Central Thrust (MCT), Nepal Himalaya. We have sampled hot springs along a 150 km stretch of the Himalayan front and find that they carry large fluxes of CO 2 derived from metamorphic reactions. Hot spring fluids are saturated with CO 2 , have [DIC] from 1.3 to >100 mmol kg À1 and have d 13 C DIC values from À13% to +13% (PDB) . Analysis of CO 2 released by decrepitation of fluid inclusions from syn-and postdeformational quartz veins indicate that crustal fluids had d 13 C from À15% to +2% (PDB) , consistent with production of CO 2 from both thermal decomposition of organic matter and decarbonation at depth. Modeling of the hot spring fluid compositions indicates that they are strongly degassed. Combining our degassing calculations with estimates of the fraction of hydrothermal alkalinity in local rivers shows that the metamorphic degassing flux of CO 2 in the 32,000 km 2 Narayani basin of the central Himalaya is >1.3 Â 10 10 mol a À1 , exceeding the calculated consumption of CO 2 by chemical weathering for the Narayani River basin by a factor of four. Our study implies that the net impact of Himalayan orogenesis on the carbonate-silicate geochemical cycle is not large-scale drawdown of CO 2 because the weathering sink is substantially offset or even exceeded by the metamorphic source. Components: 10,247 words, 7 figures, 4 tables.
  • Article
    New paired carbonate and organic-carbon isotope analyses from Nevada, USA, together with a consideration of the effects of mountain-building and ice-sheet coverage of the continents on atmospheric pCO2, lead to a new hypothesis for the cause of the Late Ordovician glaciation. We suggest that the Taconic orogeny, which commenced in the late-middle Ordovician, caused a long-term decline in atmospheric pCO2 through increased weatherability of silicate rocks. Ice-sheet growth was triggered when pCO2 decreased to a threshold of ∼10× present atmospheric level and proceeded by positive ice-albedo feedback. In the midst of glaciation, atmospheric pCO2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets. At first, this enhanced greenhouse effect was overcompensated for by ice-albedo effects. Ultimately, however, atmospheric pCO2 reached a level which overwhelmed the cooling effects of ice albedo, and the glaciation ended. The isotope results can be interpreted to indicate that atmospheric pCO2 rose during the glaciation, consistent with other proxy information, although alternative interpretations are possible. The large, positive carbonate isotope excursion observed in Late Ordovician rocks around the world is explained as the expected response to increased carbonate-platform weathering during glacioeustatic sea-level lowstand, rather than as a response to increased organic-carbon burial.
  • Article
    Full-text available
    This study attempts to characterise the chemical weathering of basalts and to quantify the flux of carbon transferred from the atmosphere to the ocean during this major process at the surface of the Earth. To this aim, we have compiled different published chemical compositions of small rivers draining basalts. Basaltic river waters are characterised by relatively high Na-normalized molar ratios (Ca/Na: 0.2–3.9; HCO3/Na: 1–10; Mg/Na: 0.15–6) in comparison with those usually observed for river draining silicates. The data also show the climatic influence on basalt weathering and associated CO2 consumption. Runoff and temperature are the main parameters controlling the chemical weathering rate and derived CO2 consumption during basaltic weathering. From these relationships and digital maps, we are able to define the contribution of basalts to the global silicate flux. Taking account of this result, we estimate that the CO2 flux consumed by chemical weathering of basalts is about 4.08×1012 mol/year. The fluxes from the islands of Indonesia and regions of central America represent around 40% of this flux. The global flux of CO2 consumed by chemical weathering of basalts represents between 30% and 35% of the flux derived from continental silicate determined by Gaillardet et al. [Chem. Geol. 159 (1999) 3]. Finally, it appears that volcanic activity not only acts as a major atmospheric CO2 source, but also creates strong CO2 sinks that cannot be neglected to better understand the geochemical and climatic evolution of the Earth.
  • Article
    Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater 87Sr/86Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle–Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ18O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ18O in some samples, particularly those from the Lower Ordovician, maximum δ18O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ18O records the effect of decreasing tropical seawater temperatures across the Middle–Late Ordovician transition superimposed on seawater δ18O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ13C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ13C excursion up to +7‰. Nevertheless, an underlying trend in mean δ13C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater 87Sr/86Sr and δ18O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle–Late Ordovician event.
  • Article
    Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation.
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    This paper is included in the Special Publication entitled 'The proto- Andean margin of Gondwana', edited by R.J. Pankhurst and C.W. Rapela. Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig I, victoriae lunatus graptolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. elegans (P. suecicus) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.
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    A new high-resolution Paleozoic δ13Ccarb curve from the Great Basin shows an amount of variation that appears transitional between the highly unsettled Neoproterozoic and the increasingly stable Mesozoic to Cenozoic periods. Large positive excursions were common during cool periods (e.g., Late Ordovician-Silurian and Late Devonian-Early Mississippian), but rare during greenhouse climates. Some periods of stability in δ 13Ccarb lasted for > 107 yr and are interpreted to reflect negative feedbacks on productivity in a nitrogen-limited (low oceanic N/P) ocean in which anoxia led to increased denitrification. Suppression of N fixation, likely due to low levels of essential trace elements, is a requirement of N limitation. In contrast, cool periods that ventilated the oceans switched the ultimate limiting nutrient to P and allowed for δ13C excursions, which signal episodic organic carbon burial that could be sustained by positive feedbacks between productivity and anoxia.
  • Article
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    The Ordovician Period, long considered a supergreenhouse state, saw one of the greatest radiations of life in Earth's history. Previous temperature estimates of up to approximately 70 degrees C have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time. We present a very different global climate record determined by ion microprobe oxygen isotope analyses of Early Ordovician-Silurian conodonts. This record shows a steady cooling trend through the Early Ordovician reaching modern equatorial temperatures that were sustained throughout the Middle and Late Ordovician. This favorable climate regime implies not only that the oxygen isotopic composition of Ordovician seawater was similar to that of today, but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period.