Despite frequent use of conodonts as an archive for seawater Sr-87/Sr-86, available diagenetic screening tools are inconsistent predictors of primary seawater Sr-87/Sr-86 preservation. Here, we attempt to isolate variables affecting the preservation of seawater Sr-87/Sr-86 in conodonts. We present new Middle-Late Ordovician (similar to 470-450 Ma) conodont Sr-87/Sr-86 from mid-paleolatitude (30-45 degrees S) carbonate successions from central Sweden (Fj & auml;cka, K & aring;rg & auml;rde) and Estonia (Uuga Cliff, Viki borehole) which have low Conodont Alteration Index (CAI) values of 1-1.5. Coeval Sr-87/Sr-86 measurements from K & aring;rg & auml;rde and Estonian sections are offset from global seawater by similar to +2 x 10(-4) but preserve the overall structure of the seawater curve, suggesting minor diagenetic Sr exchange. Sr-87/Sr-86 measurements from the more argillaceous Fj & auml;cka section show extensive alteration with highly radiogenic values as much as similar to 6 x 10(-4) greater than global seawater and a stratigraphic trend that diverges from the seawater curve. This may be explained by highly radiogenic Sr contribution from the Kinnekulle K-bentonite and Fj & auml;cka shale units that occur at Fj & auml;cka section. We compiled 1164 conodont Sr-87/Sr-86 measurements spanning similar to 485-250 Ma and categorized measurements by host rock, sample preparation, paleolatitude, and CAI. Boxplots and Mann-Whitney U testing of residual values relative to the seawater curve (Sr-87/Sr-86(conodont) - Sr-87/Sr-86(seawater)) reveal that paleolatitude and associated differences in sedimentation rate affect preservation of seawater Sr-87/Sr-86 values in conodonts. The relative abundance of clay minerals did not significantly correlate with residual values for conodonts from mid- or low-latitude deposits. We recommend that future investigations of seawater Sr-87/Sr-86 records using conodont apatite target low-latitude settings with high sedimentation rate and CAI <= 2.

An increase in accumulation of phosphate-enriched sediment occurred in several areas of Earth's oceans during the later Ordovician, suggesting that some fundamental change (s) in seawater and/or pore water chemistry were taking place, either syndepositionally or during early diagenesis. One hypothesis is that widespread cooling of the water column led to this increase, but another is that the increase could have been forced primarily by changes in seawater chemistry that accompanied an influx of siliciclastic sediments associated with tectonically driven subsidence. Here, we report on findings from study of the upper similar to 30 m of the >200 m of Middle and Late Ordovician strata exposed near Tidwell Hollow, Blount County, Alabama, and what the results suggest about the competing hypotheses. This Ordovician sequence, deposited along the southeastern margin of Laurentia during the initial (Blountian) stage of the Taconic Orogeny, records the transition from a restricted peritidal carbonate shelf environment (the "Black River lithofacies") into a more normal marine carbonate environment (the "Trenton lithofacies"). In particular, the younger carbonate strata are notable for their measurably higher amounts of secondary phosphate minerals. This stratigraphic interval is also well-constrained chronostratigraphically by the presence of the Deicke and Millbrig K-bentonite beds (altered volcanic tephra layers), thus it is ideal for a focused geochemical, petrographic, and stratigraphic investigation of the increased phosphate content of a specific Upper Ordovician sequence. Analyses of bulk rock samples and of extracted collophane grains (via x-ray fluorescence, XRF, and in-situ laser ablation inductively coupled plasma mass spectrometer, ICPMS) suggest that phosphogenesis in these strata was episodic rather than slow and steady, with a depositional pattern of abrupt increases followed by abrupt declines. Observed trace element changes in the 12 m to 18 m sample interval include the occurrence of elevated Th/U ratios (maximum of 5.7) accompanied by Y/Ho ratios that range from 27 (base) to 51 (top). We attribute these changes to increasing silicate influx into the basin and an accompanying increase in available Fe, which would preferentially scavenge MREEs and then release them to the pore waters leading to MREE enrichment, which we also observe in this interval. In a rapidly subsiding basin, MREE enrichment could have resulted from initial restriction of bottom water circulation followed by gradually more open marine conditions. Some allochems and textures that are more characteristic of the older Black River lithofacies than the younger Trenton lithofacies (e.g. framework grains of calcareous green algae and Type 1 and Type 2 oncoids, fenestral lime mudstones, and associated small framestones of Tetradium sp. buildups) persist in relative abundance upsection into the similar to 10 m interval of strata above the Millbrig K-bentonite Bed, and this persistence supports the hypothesis that the Trenton transgression was not initially accompanied by widespread cooling of the Laurentian epicontinental sea. Instead, we propose that variable weathering rates, turbidity, and nutrient supply accompanied by relatively rapid subsidence could have governed the observed variability in phosphate deposition and elemental abundances in these sediments, and that this subsidence of the Laurentian margin could have been the driver of an uptick in phosphate deposition during the later Ordovician here, and perhaps at other locations as well where subsidence was occurring, given that increased phosphate deposition is a globally recognized and globally significant change in many Upper Ordovician sequences beyond eastern Laurentia.

Abstract Two remarkable events in the history of life on the Earth occur during the Ordovician Period (486.9–443.1 Ma). The first is an exceptionally rapid and sustained radiation of marine life known as the ‘Great Ordovician Biodiversification Event’ (GOBE), and the second is a catastrophic Late Ordovician mass extinction (LOME). Understanding the duration, rate and magnitude of these events requires an increasingly precise global correlation framework. In this chapter we review the major subdivisions of the Ordovician System, their Global Stratotype Section and Points, and the chronostratigraphic levels that define their bases. We also present a detailed set of correlation charts that illustrate the relationships between most of the regional graptolite, conodont and chitinozoan successions across the world.

The Williston Basin retained a high‐resolution record of global δ13C fluctuations during the ~13 Ma duration Upper Ordovician–lower Silurian, and is a possible missing link in correlations between Laurentian margins and its interior. This study integrates δ13C and conodont biostratigraphy to yield a high‐resolution, continuous composite spanning from the Upper Ordovician Aphelognathus grandis Zone into the lower Silurian Distomodus staurognathoides Zone. From oldest to youngest, six global δ13C excursions are identified: (1) Whitewater (=Moe) and (2) Elkhorn (=Paroveja) in the Amorphognathus divergens Zone; (3) LHICE (KaH?) in the Aphelognathus shatzeri Zone; (4) HICE in the Ozarkodina hassi Zone; (5) Early Aeronian within the Distomodus kentuckyensis–Pranognathaus tenuis zones and (6) late Aeronian within the Psuedolonchodina expansa–D. staurognathoides zones. This δ13C record helps to alleviate some of the uncertainties in determining the precise order, duration and timing of events influencing the biotic, climatic, oceanic and sea‐level evolution of the Upper Ordovician–lower Silurian successions across Laurentia.

The Ordovician Period (486.9–443.1 Ma) encompasses two extraordinary biological events in the history of life on the Earth. The first, the “Great Ordovician Biodiversification Event,” is a great evolutionary radiation of marine life and the second is a catastrophic Late Ordovician extinction. Understanding the duration, rate, and magnitude of these events requires an increasingly precise time scale. The Ordovician time scale is based on the subdivision of a Lower Paleozoic CONOP9 composite graptolite range chart derived from 837 stratigraphic sections and 2651 graptolite taxa with interpolated radioisotopic dates. Thirty-seven new radioisotope dates are used in the scaling of the new Ordovician time scale. The base of the Ordovician Period is defined at the level of the first appearance of the conodont Iapetognathus fluctivagus at the Green Point Newfoundland section. Its top, the base of the Silurian Period, is set as the level of the first appearance of the graptolite Akidograptus ascensus at Dob’s Linn, Scotland. For the first time an independently time-scaled CONOP9 composite conodont range chart is presented to facilitate the application of the time scale to carbonate facies sections.