The Geological of America Geologic Scale 1888 2013


1,† 2 3 4 J.D. Walker , J.W. Geissman , S.A. Bowring , and L.E. Babcock Invited Review 1Department of , University of , Lawrence, Kansas 66045, USA 2Department of Geosciences, ROC 21, University of at Dallas, Richardson, Texas 75080, USA, and Department of and Planetary , MSC 03 2040, 1 University of New Mexico, Albuquerque, New Mexico 87131, USA 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of , Cambridge, Massachusetts 02139, USA 4Department of Geology, Lund University, SE-223 62 Lund, , and School of Earth Sciences, Ohio University, Columbus, Ohio 43210, USA

ABSTRACT INTRODUCTION over the few in establishing both numerical and relative ages, describe selected The Geological Society of America has One of the most important aspects of proxies for time in the record, and note and sponsored versions of the in the geosciences is connecting what we examine comment on some challenges. This paper since 1983. Over the past 30 , the Geo- in the rock record with ages of events and mea- is intended to provide a general overview of logical Society of America Geologic Time sured rates and durations of geologic processes. geologic time scales. The most comprehensive Scale has undergone substantial modifi ca- In doing so, geoscientists are able to place esti- treatment of the geologic time scale is contained tions, commensurate with major advances mates on the rates of and evolutionary in the recent publication of Gradstein et al. in our understanding of chronostratig- changes, use astronomically forced depositional (2012), the most current defi nitive work on the raphy, , astrochronology, processes to tell time within sedimentary basins, geologic time scale from a global perspective. chemostratigraphy, and the geomagnetic examine the ways in which tectonic processes This book is the most recent in the of ma- polarity time scale. Today, many parts of change crustal and structure and infl uence jor publications by The Geological Society of the time scale can be calibrated with preci- and global climate patterns, London (Harland et al., 1964) and subsequently sions approaching less than 0.05%. Some and assess the temporal relations among - Cambridge University Press (Harland et al., notable time intervals for which collabora- tism, fl uid-rock interaction, and base/precious 1982, 1990; Gradstein et al., 2004; Ogg et al., tive, multifaceted efforts have led to dra- mineralization in many settings. A key re- 2008) and Elsevier (Gradstein et al., 2012). The matic improvements in our understanding quirement is establishing accurate ages of rocks current Geological Society of America Geologic of the character and temporal resolution that are directly associated with or bracket a geo- Time Scale (Fig. 1) incorporates of key evolutionary events include the Tri- logic or process. With efforts to estimate presented in the International Commission on assic-, -, and Neo- the of geologic events beginning ’s International Chronostratigraphic - boundaries (or over two ago, this was commonly ac- Chart (Cohen et al., 2012) and in Gradstein transitions). In developing the current Geo- complished by establishing a relative geologic et al. (2012). Numerical dates used for bound- logical Society of America Time Scale, we time scale using the ranges of and strati- ary positions are from Gradstein et al. (2012). have strived to maintain a consistency with graphic relationships. Beginning in the early The Geological Society of America (GSA) does efforts by the International Commission on twentieth , the relative geologic time scale not directly “maintain” an international geo- Stratigraphy to develop an international was calibrated using numerical information. logic time scale. Rather, the society provides a geologic time scale. A geologic time scale is the ordered com- geologic time scale in a concise, logically orga- Although current geologic time scales are pilation of numerical ages and relative de- nized and readable format that is largely based vastly improved over the fi rst geologic time terminations based on stratigraphic and other on the work of the International Commission on scale, published by in 1913, principles. Numerical ages, formerly called Stratigraphy (ICS) and related groups and pub- we note that Holmes, using eight numeri- “absolute ages” (Holmes, 1962), form a chrono- lications. GSA follows the work and recommen- cal ages to calibrate the Phanerozoic time scale typically expressed in thou- dations of these groups in promoting a better scale, estimated the beginning of the Cam- (ka) or millions (Ma) of years; relative understanding and use of geologic time through brian to within a few percent of the ages form a chronostratigraphic time scale. A its time scale. Many of these organizations in- currently accepted value. Over the past 100 geologic time scale is an invaluable tool for geo- clude geoscientists who are GSA members or years, the confl uence of process-based geo- investigating virtually any of members of the associated of GSA. logical thought with observed and approxi- Earth’s development, anywhere on the , mated geologic rates has led to coherent and and at almost any time in Earth’s . History of Chronometric- quantitatively robust estimates of geologic This paper describes the history of the devel- Chronostratigraphic Geologic Time Scales time scales, reducing many uncertainties to opment of the Geological Society of America the 0.1% level. Geologic Time Scale and provides a brief his- “For it was evident to me that the between tory of geologic time scales and their compo- the ranges, which lie above the of †E-mail: [email protected] nents. We also discuss important advances made Memphis, once was a gulf of the sea, like the

GSA Bulletin; March/April 2013; v. 125; no. 3/4; p. 259–272; doi:10.1130/B30712.1; 1 fi gure; 1 .

For permission to copy, contact [email protected] 259 © 2013 Geological Society of America Walker et al. 541 635 850 1000 1200 1400 1600 1800 2050 2300 2500 2800 3200 3600 4000 (Ma) BDY. AGES PERIOD MESO-




EOARCHEAN PROTEROZOIC ARCHEAN ka) for the ka) for rom 0.13 to 0.01 Ma. rom but only two are shown but only two are ositions follow Gradstein EON 750 (Ma) AGE 1000 4000 1250 1500 1750 2250 2750 3000 3250 3500 3750 2000 2500 315 265 269 272 279 290 296 299 304 307 323 331 347 359 372 383 388 393 408 411 423 426 427 430 433 439 441 444 445 453 458 467 470 478 485 490 494 497 501 505 514 521 541 419 509 529 260 254 252 (Ma) PICKS AGE 5 AGE 4 AGE 3 AGE 2 AGE 10 VISEAN MOSCOVIAN lian gian LATE LATE LATE LATE GIAN VERY PRIDOLI EARLY EARLY EARLY EARLY


LLANDO- Cisura- Guada-




MISSIS- PENNSYL- PERMIAN PERIOD 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 (Ma) AGE 237 66.0 72.1 83.6 86.3 89.8 93.9 100 113 126 131 134 139 145 152 157 164 166 168 170 174 183 191 199 201 209 228 241 247 250 252 (Ma) PICKS CENOMANIAN . The , , and Paleozoic are the of the Phanerozoic Eon. Names of the Eras of Phanerozoic The Cenozoic, Mesozoic, and Paleozoic are .





C32 MESOZOIC C30 C31 C33 C34 ANOM. 34 32 30 31 33


M3 M25 M0r M1 M10 M12 M14 M16 M18 M29 M20 M22 HIST POLARITY MAGNETIC 70 80 90 250 240 (Ma) 100 110 120 130 140 150 160 170 180 190 210 220 230 AGE GEOLOGIC TIME SCALE 200 2.6 1.8 3.6 5.3 7.2 0.01 11.6 13.8 16.0 20.4 23.0 28.1 33.9 37.8 41.2 47.8 56.0 59.2 61.6 66.0 (Ma) PICKS


MIOCENE Geological Society of America Geologic Time Scale America Geologic Time Geological Society of





et al. (2012) but are rounded to the nearest whole number (1 Ma) for the pre-Cenomanian, and rounded to one decimal place (100 and rounded the pre-Cenomanian, (1 Ma) for whole number to the nearest rounded et al. (2012) but are ages, The is divided into four provisional. series and stages of the Cambrian are to Pleistocene interval. Numbered chrono stratigraphic units follow the usage of the Gradstein et al. (2012) and Cohen et al. (2012). Age estimates of boundary p stratigraphic units follow the usage of Gradstein et al. (2012) and Cohen (2012). chrono Figure 1. Figure here. What is shown as Calabrian is actually three ages: Calabrian from 1.8 to 0.78 Ma, Middle from 0.78 to 0.13 Ma, and Late f 1.8 to 0.78 Ma, Middle from ages: Calabrian from What is shown as Calabrian actually three here. CHRON.

C3 C1 C7 C8 C4 C5 C9 C2 C6

C21 C23 C26 C11 C12 C13 C17 C30 C20 C27 C24 C25 C5A C5B C5C C15 C16 C18 C28 C29 C4A C5E C5D C7A C10 C6B C22 C2A C3A C19 C6A C6C ANOM. 9 1 7 2 8 6

3 4 5

30 29 27 23 22 24 28 10 11 16 18 21 25 20 3A 26 17 2A 7A 12 15 5D 6B 4A 5A 5B 5E 19 5C 6A 6C 13 HIST. POLARITY MAGNETIC 5 10 15 20 25 30 35 40 45 50 55 60 65 (Ma) AGE

260 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale about…Ephesos and the Plain of the Maiander, if it The discovery of radioactivity, and, more In his fi rst edition of On the Origin of the Spe- be permitted to compare small things with great. And specifi cally, the relation between radioactive cies by Means of , Charles small these are in comparison, for of the , which parent elements and their intermediate and ulti- Darwin provided a crude estimate of the age heaped up the in those regions none is worthy to be compared in volume with a single one of the mouths mate daughter products through a fundamental of Earth of several hundred million years of the , which has fi ve mouths.” half- of , was the seminal based on both geology and his assumption of —, likely the ’s fi rst , event that led to establishing the numerical phyletic gradualism. Interestingly, estimates fi fth century B.C., in his , 2.10.0-2. ages of geologic materials and ultimately de- of the of the Phanerozoic incorporat- veloping the fi rst chronometric time scale. The ing radiometric ages in the work of Holmes The quest to understand geologic time has fi rst attempt was by Arthur Holmes (1913) in (1913) at ~550 Ma (inferred from Holmes’ fi g. been integral to the geosciences for over 200 his book The Age of the Earth. Holmes (1913, 17) and Barrell (1917, p. 892) at ~552 Ma are years. fi rst formally presented the Chapter X) extensively reviewed the early well within a few percent of the currently ac- scientifi c hypothesis that Earth is ancient in a methods of geochronology using U as the cepted value. reading at the Royal Society of Edinburgh on parent element. Work at that time showed that The time scale was greatly refined by 4 April 1785. He concluded his revolutionary the decay of U produced He as a by-product Holmes in the edition of his book text, of the Earth, which was based on and probably had as its ultimate daughter the (Holmes, 1937). At that time he used almost his lectures to the Royal Society of Edinburgh element Pb. The half-life of U and thus pro- 30 U-He, U-Pb, and Th-Pb age determina- and published in 1788, with the now-famous duction rates of these elements were roughly tions. Because of the recognition of multiple and often-quoted sentence concerning the natu- established by the time of Holmes’ (1913) , geochronology relied not just on de- ral history of the Earth: “The result, therefore, publication. At the time of Holmes’ work, termining the parent-daughter ratio, but in the of our enquiry is, that we fi nd no ves- it was simply the ratio of U to He or Pb that cases of the U-Pb and Th-Pb decay systems, tige of a beginning,—no prospect of an end” was measured—the discovery that elements the actual atomic mass and, to some extent, (p. 304). His work in part inspired the great had multiple isotopes was reported separately the ratios of the daughter products advances in the geosciences over the following in the same (Soddy, 1913; Thomson, (Holmes, 1937, Chapter V). Holmes continued century, over many parts of the world, and, after 1913). Holmes (1913) reviewed all available to refi ne the time scale over the next 25 years. the discovery of radioactivity, prompted the ini- age determinations for U-Pb and U-He that Concurrently, analytical methods increased in tial attempts to quantify the age of the planet had bearing on tying numerical age estimates their precision while the number of elements Earth and geologic time. to meaningful geologic ages. In this effort, and thus used for isotopic age deter- The fi rst attempts by to quantify he noted that U-He age estimates were typi- minations increased. For example, Kulp (1961) the chronostratigraphic time scale and to es- cally too young, represented minimum ages, primarily incorporated K-Ar dates as numerical tablish some bounds on the fall and were most useful for younger (Cenozoic) estimates for his compilation of the time scale. under the category of “” methods. The rocks. Holmes established the pre-Cenozoic A subsequent symposium in honor of Holmes two most important of these involved consid- time scale using a total of fi ve U-Pb determina- resulted in a major effort toward developing a erations of the thickness of sedimentary tions. Of these, only three were from rocks of geologic time scale by a broader community and the salinity of the . Both of these ap- Phanerozoic age: an age of 340 Ma for the end of geoscientists (Harland et al., 1964). In the proaches relied on using estimated rates of geo- of the Carboniferous, 370 Ma for the end of the symposium volume, the authors reviewed all logic processes to establish a more quantitative Devonian, and 430 Ma for the end of the Ordo- of the signifi cant aspects of and developments time scale. For the fi rst hourglass, the rates of vician. Ultimately, Holmes used fi ve U-He and with the geologic time scale from numerical accumulation were compared to thick- fi ve U-Pb dates to calibrate his geologic time dates, to hourglass methods, and stratigraphic nesses of sedimentary strata of known chrono- scale. The ages of other parts of the chrono- constraints. In all, over 300 dates were used to strati graphic age to compute the duration of strati graphic time scale not covered by avail- construct a Phanero zoic time scale. of the rock unit. The second method able age estimates were approximated by using Over the next 20 years, the standardization relied on understanding the input from compilations of sediment thicknesses. Holmes of isotopic decay constants by Steiger and Jäger to the oceans to compute an overall age for the concluded this chapter by stating (p. 165): (1977) and major improvements to the chrono- oceans. In both cases, these were very rough strati graphic time scale, fostered by the Inter- approximations of the duration of processes, “Most of the available drawn from radio- national Union of Geological Sciences (IUGS) because the estimates of rates were inexact. active minerals has now been passed in review. As yet through the International Commission on Stra- it is a meager record, but, nevertheless, a record brim- Importantly, these estimates indicated that the ful of promise. Radioactive minerals, for the geologist, tigraphy (ICS) and its subcommissions, greatly Earth was much older than was accepted at are wound up at the time of their origin. After advanced the geologic time scale. In particular, the time. This was also true of estimates by a few years’ preliminary work, we are now confi dent the efforts of the ICS established worldwide Thomson (see below). Remarkably, during the that the means of reading these time-keepers is in our standard defi nitions of the relative geologic possession. Not only can we read them, but if they second half of the nineteenth century, prior to have been tampered with and are recording time in- time scale. ICS has and continues to establish the discovery of radioactivity, a number of correctly, we can, in most cases, detect the error and global chronostratigraphic standards known as workers recognized astronomically forced sedi- so safeguard ourselves against false conclusions.” global boundary section and points mentary deposits and used them as a means to (GSSPs) (see following discussion). The work calibrate geologic time (reviewed in Hilgen, Besides establishing a rough time scale, this of IUGS and ICS has culminated in more re- 2010). The fi rst attempts were built on the astro- work was radical in that it expanded the age of fi ned geologic time scales for the Phanero- nomical for ice ages, and they used Earth beyond any previous estimate. Up until zoic published by Harland et al. (1982), Odin eccen tricity maximums to tune deposits of the that time, the particularly infl uential work of (1982), and numerous other authors. The initial last glaciation but were also tuning and Lord had placed an upper limit of ca. GSA time scale (Palmer, 1983) relied heavily Cretaceous sedimentary rocks (Hilgen, 2010). 40 Ma on the age of Earth (Thomson, 1865). on the contributions by these authors. A major

Geological Society of America Bulletin, March/April 2013 261 Walker et al. treatment, including the statistical assessment scale that was North American–centered but RECENT ADVANCES AND THEIR of uncertainties of the estimated boundary ages, with more far-reaching consequences. With IMPACT ON THE GEOLOGIC was presented by Harland et al. (1989). Con- of the DNAG project and considerable TIME SCALE siderable progress was made over the next 15 changes in chronostratigraphic nomenclature years, with the next major and comprehensive taking place internationally, refi nements of the Here, we review some of the key advances revision to the geologic time scale published time scale were inevitable, and updated versions that have led to changes and refi nements of the by Gradstein et al. (2004). This publication were published as GSA’s 1999 Geologic Time geologic time scale. These include, of course, fully integrated the advances made in global Scale (Palmer and Geissman, 1999) and 2009 changes in stratigraphic and geochronologic stratigraphy through the work of the ICS, in- Geologic Time Scale (Walker and Geissman, approaches that are at the heart of a combined cluding astrochronology, and took advantage 2009). What the original DNAG time scale chronometric/chronostratigraphic time scale, of considerably more precise radioisotope apart from earlier versions of time scales pub- but also astrochronology, which is - geo chronol ogy including 40Ar/39Ar step heat- lished in textbooks and summary papers was izing the time scale effort, chemostratigraphy ing and single- laser methods as well as the overt application of integrated stratigraphic and related rock magnetic stratigraphy, and the U-Pb dating methods. and magnetostratigraphic data sources (includ- geomagnetic polarity time scale (see Table 1 for ing both relative and numerical age dating tech- brief descriptions of these methods). History of the Geological Society of America niques used to assemble the time scale) and the Geologic Time Scale of presentation. Advances in Stratigraphy—The The rationale behind and history of the work International Commission on Stratigraphy Besides being the 125th anniversary of the to develop the fi rst Geological Society of Amer- Geological Society of America, 2013 marks ica Geologic Time Scale itself were described Leading the way in the advancement of the 30th anniversary of the Geological Soci- by Palmer (1983) and Walker and Geissman chronostratigraphic information is the Inter- ety of America Geologic Time Scale, the 50th (2009). Allison (“Pete”) Palmer was the Cen- national Commission on Stratigraphy, including anniversary of the publication of the Vine- tennial Program Coordinator for GSA’s the many subcommissions formed by the ICS. Matthews-Morley-Larochelle hypothesis that DNAG project, and was charged with compil- These groups actively promote the acquisi- marine patterns adjacent to ing the results of the efforts of the Time Scale tion and dissemination of information vital to mid- ridges were a record of the polarity Advisory Committee, consisting of Z.E. Peter- making informed decisions about stratigraphic reversal history of the geomagnetic fi eld and man, J.E. Harrison, R.L. Armstrong, and W.A. boundary positions and numerical calibrations thus that the ridges were sites of ocean-fl oor Berggren. Pete Palmer had a clear passion for of those positions. A major goal of the ICS is spreading (Vine and Matthews, 1963), the 100th the work and devoted considerable to the to develop an unambiguous, globally applica- anniversary of the fi rst geologic time scale pre- project. An innovative contribution was the at- ble nomenclature for geologic time units and sented by Holmes (1913), and the 225th anni- tempt to organize the time scale onto a single their chronostratigraphic equivalents, a com- versary of Hutton’s (1788). 8.5 by 11 inch sheet of paper using the “tools mon language in which concepts are identical The GSA time scale grew out of the of the trade” in those days—Mylar, zipatone for all localities across the globe. This effort is of Geology (DNAG) project letters, a Leroy lettering machine, and a lot of part of a larger project to develop unambiguous (Palmer, 1983), which had as its goal a syn- patience. His efforts resulted in a unique lay- chronostratigraphic units for the entire geologic thesis of the geology then known of the North out for the time scale, in which each era of the time scale (Gradstein et al., 2004, 2012; Ogg American . Before that time, geologic Phanerozoic, and all of the Precambrian, was et al., 2008; Cohen et al., 2012). The main driv- information on North America had been scat- given identical column length. Scaling of the ing forces for the refi nements and restructuring tered through the . With a few excep- magnetic polarity time scale to fi t this format to the chronostratigraphic side of the geologic tions (e.g., Eardley, 1951; King, 1959), little required numerous trials. When Jim Clark, the time scale are changes in philosophy about in the way of comprehensive summaries of the director of publications for GSA, suggested that how stratigraphic units are defi ned, as well as existed. A necessary the geologic time scale should also be published high-resolution studies utilizing chronostrati- prerequisite for discussing the geology of the as a pocket wallet card, the effort became even graphic proxies, including , continent was a common chronostratigraphic more challenging. A key concern by both the ad- chemostratigraphy, , rock vocabulary and internally consistent sense of visory committee and Palmer was whether the magnetic stratigraphy, and astrochronology/ both the relative and numerical ages of geo- time scale should be North American centric or orbital tuning. logic and evolutionary events. This provided global. The advisory committee and Palmer rec- Over the past several decades, different strati- ages of specifi c stratigraphic units addressed in ognized the likely pitfalls and hurdles associated graphic have been applied to defi - the multivolume compilation involving scores with trying to develop a global time scale and nitions of chronostratigraphic units. Defi nitions of authors that resulted from the DNAG proj- set as a goal trying to develop a “common vo- in many places around the world were com- ect. The DNAG project was the fi rst detailed cabulary” for North America, still a suffi ciently monly based on the unit-stratotype concept, in synthesis of North American geology follow- great challenge. In discussions with Palmer which a type section serves as the standard of ing widespread acceptance of plate-tectonic (2012, personal commun.), he emphasized the reference for the defi nition and characterization theory and provided a sense of the evolution diffi culties with defi ning the base of the Ordovi- of a unit (Salvador, 1994). The lower and upper of the continent in the context of global events. cian (which has more recently been solved with boundaries of a unit are normally specifi ed by Before the DNAG compilation could proceed the defi nition of an Ordovician GSSP). The fi - reference to a type section. Beginning in the very far, a uniform geologic time scale had to nal product (Palmer, 1983) served the intended , the concept of a boundary-stratotype was be adopted, and it was the fi rst major product purpose well. “There were no naysayers, and no promoted. Under this concept, only the base of a of the DNAG initiative. This inevitably led to major disagreements,” stated Palmer. “The basic chronostratigraphic unit is formally defi ned, and advancement of a version of the geologic time subdivisions were all accepted.” it is marked by a point in strata (Salvador, 1994),

262 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale

TABLE 1. TIME DETERMINATION METHODS Methods Definition and application Astrochronology Study of cyclical variation in properties present in sequences interpreted to represent a response to Earth orbital parameters deriving from interaction with the , , and other . Variations can be expressed in physical sedimentary patterns (e.g., thicknesses) or chemical characteristics. Can be used for establishing durations in the Cenozoic and in older time intervals if calibrated by geochronologic results and for direct dating in rocks younger than ca. 35 Ma. Chemostratigraphy Study of chemical and isotopic variations in sedimentary sequences interpreted to reflect changes in seawater or catastrophic events (e.g., Ir anomaly at the end of the Cretaceous). Detailed variations in chemical signals may reflect astronomical forcing and can be used as a dating tool when signal changes at a high rate (Cenozoic Os and Sr). Can also be used for relative time by correlation of major unique signatures. Geochronology Study of radioisotopes and their decay products. Time measured by the ratio of radioisotopes to decay products using a decay constant. This is a numerical dating method. Magnetic polarity Study of the polarity record of Earth’s magnetic field as reflected in the paleomagnetic signatures of stratified rocks. This gives relative stratigraphy ages and allows for correlation of sequences. It can approximate a numerical method if one or, ideally, many parts of a sequence can be tied to a numerical age. Rock magnetic Study of the variations in the magnetic (concentration and phases) assumed to be originally deposited with the stratigraphy sedimentary sequence. At a fine-scale examination (e.g., bed or even lamina), this method can be a component of astrochronology. Stratigraphy Study of stratigraphic successions by using the physical arrangement of layers and sequences () as well as the content (biostratigraphy). This is a and correlation method. which is known as a GSSP, or global boundary Also by defi nition, the chronostratigraphic normally the evolutionary fi rst appearance of a stratotype section and point, more popularly re- unit subjacent to each GSSP includes all strata recognizable guide fossil. For at least one chrono- ferred to as a “golden spike.” known and unknown up to that , so there , this approach is impractical, Originally, the boundaries of most geologic can be no gap between adjacent chronostrati- in part because some available time proxies time units were drawn at positions in the strati- graphic units. exceed biostratigraphy in their ability to pro- graphic record where some sizable biotic change, High-resolution stratigraphic studies today, vide high-resolution stratigraphic information. widespread geologic event such as a glacial often enhanced with continuous drill cores, The base of the Holocene Series/Epoch, for advance or retreat, or an orogenic episode was are the norm, so more detailed information is example, has been defi ned at a point refl ecting recognized. Some of the formal nomenclature available now than when early versions of the a distinct switchover in deuterium content of of the geologic time scale refl ects this practice: geologic time scale were assembled. The result- glacial ice in . That point essentially “Phanerozoic” means “visible life,” Paleozoic, ing details are not always clear, and unambigu- coincides with the onset of warming at the end Mesozoic, and Cenozoic mean “ancient life,” ous sets of data in some cases have shown that of the and a change in thickness “middle life,” and “recent life,” respectively. In- geoscientists were sometimes mistaken in their of glacial ice layers, refl ecting increased dust formal terms applied to geologic time units, such assumptions of isochronous events. The fi rst content associated with the warming interval. as the “age of bryozoans” for the Ordovician, appearance of was once used through Chemostratigraphy, particularly using δ13C, pro- the “age of swamps” for the Carboniferous, much of the world as the marker for the base vides high-resolution stratigraphic information the “age of ” for the Mesozoic, and the of the Phanerozoic. Today, we recognize that for parts of the Paleozoic that have been chal- “age of ” for the Cenozoic, reinforce the appearance of trilobites on separate paleo- lenging to defi ne on the basis of biostratigraphic this approach to characterizing the progression was not synchronous, making such evidence alone. To , stable isotope data have of geologic time. This broad-brush view may an inferred singular “event” unsuitable on not been used as a primary tool for defi ning a have some merits, but to a large extent, the ap- its own as a defi nitive correlation tool. High- chronostratigraphic unit, but biostratigraphi- parent distinctions between strata contained resolution chronostratigraphic studies using cally calibrated isotopic excursions, particularly in superjacent chronostratigraphic units have multiple methods in combination now give us in δ13C, have some advantages in successions been enhanced by evolutionary overturns (ex- the ability to pinpoint an exact datum in strata where the biostratigraphic data are inconclusive tinctions, followed by recoveries and changing where a boundary can be drawn. For example, at a detailed scale (e.g., Maloof et al., 2010; patterns of ecologic dominance) and unconfor- the base of the Cenozoic coincides with the base Korte and Kozur, 2010). As we achieve a more mities of varying scale. Boundaries drawn at of an -bearing layer, as fi rst demon- highly calibrated record, assuming global syn- such are almost certain to be in strati- strated at , (Alvarez et al., 1980). chroneity of the Carbon record, for example, graphic gaps. In the past, chronostratigraphic On a global scale, numerous other stratigraphic may not remain valid. Since the completeness units based on the unit-stratotype concept were tools (among them, the biostratigraphic ranges of sections is variable, it may be that no single often drawn at previously unrecognized gaps in of nannoplankton and foraminiferans, proxy will resolve all issues making the use and the stratigraphic record. To be useful, a globally and , ratios of stable isotopes of carbon integration of multiple proxies required. synchronous boundary must be drawn at a hori- and , and magnetic polarity stratigraphy) The application of the GSSP approach has zon in a continuous of strata to en- provide information that adds to the anomalous helped stabilize the geologic time scale by sure that there are no gaps in the succession. A concentration of iridium at that single datum to adoption of chronostratigraphic nomenclature boundary-stratotype–based chronostratigraphic help defi ne its singular position. Geochronology ratifi ed by specialists the world over. Changes to unit is defi ned only at the base, so the “top” is also can be used to identify the base of the Ceno- the time scale resulting from these advances are automatically defi ned by the base of the overly- zoic with great precision (ca. 66.0 Ma), adding numerous and some time periods have been dra- ing unit, and there can be no gap in between. to the broad range of techniques that can be used matically restructured through formal decisions GSSPs are defi ned according to the boundary- to identify the boundary horizon globally. on boundary positions. The Cambrian / stratotype concept. By defi nition, every GSSP Most GSSPs in the Phanerozoic have been Period, for example, now has a ratifi ed base marks the base of a chronostratigraphic unit. defi ned to coincide with an evolutionary event, that is stratigraphically well below its position

Geological Society of America Bulletin, March/April 2013 263 Walker et al. in 1983, and the ratifi ed base of the Ordovician isotope anomalies that refl ect major changes in ferent laboratories when EARTHTIME began System, which marks the top of the Cambrian, seawater and . This included both 208Pb and 205Pb that were mixed is above its position in 1983. The 1983 position mainly results from dramatic increases in the with 235U or 233U + 235U or 233U + 236U. Two iso- of the Cambrian base, following Harland et al. precision of radioisotopic dates, especially topes of U allow mass-dependent fractionation (1982) was at the base of the Tommotian using 40Ar/39Ar () and U-Pb (zircon) to be evaluated during the run, greatly reduc- as used in and, at that time. Today, that methods. These increases have been stimulated ing one of the biggest sources of uncertainty position is within provisional Stage 2 of the by new laboratory methods, new generations of in U isotopic analyses. In addition, there are a Cambrian System and estimated to be about mass spectrometers, and scientifi c projects that few tracers containing 233U-235U-202Pb-205Pb that 525 Ma (570 Ma by 1983 standards). The hori- demand the highest possible precision. The U-Pb allow correction for fractionation of the Pb iso- zon is well within what was in 1983 considered method can be applied to rocks from as young topes as well. It was decided that the community to be Proterozoic strata. The base of the Ordovi- as 600 ka, to as old as the oldest known min- needed a single mixed tracer for all laboratories cian, after formal defi nition, is at a horizon near eral grains on the planet (ca. 4.4 Ga zircon), to interested in the EARTHTIME goal (producing 485 Ma (504 by 1983 standards). . The 40Ar/39Ar method is mainly used the highest-precision geochronology to estab- Formal decisions on chronostratigraphic for rocks that range in age from younger than lish a calibrated time line for Earth history) and boundaries have in some instances led to 600 ka through the Paleozoic, and even in some one was mixed, calibrated, and sent to several changes in the way we subdivide systems/ cases Archean rocks and meteorites. However, laboratories (Condon et al., 2007). periods . Largely as a result of the addition of a 40Ar/39Ar systematics are typically disturbed by At about the same time, U-Pb zircon geo- thick stratigraphic section below its traditional in rocks exposed to greenschist chronol ogy was forever changed by the develop- base, the Cambrian is now subdivided into four and higher grades of metamorphism. For rocks ment of a method that eliminates open-system series/epochs rather than the traditional three, younger than 600 ka, 40Ar/39Ar and U-series behavior (usually Pb loss) from zircon. The and the resulting names for the epochs are de- geochronology are best applied. The time in- method was developed by James Mattinson rived from localities, not relative stratigraphic terval refl ecting recent landscape evolution and (Mattinson, 2005) and involves high-tempera- position. The Silurian Period is also subdivided is best evaluated using several different ture annealing of grains followed by partial dis- into four epochs, the names of which are de- terrestrial cosmogenic . solution. Mattinson was able to show that the rived from localities. The Permian Period is As precisions improved (e.g., <0.1% for high-U parts of the grains that are most suscep- subdivided into three epochs, and their names U-Pb zircon methods), it was apparent that there tible to Pb loss can be preferentially removed, are likewise derived from localities rather than were both intertechnique and interlaboratory leaving relatively low-U domains with no evi- relative stratigraphic position. The Cenozoic systematic errors for the U-Pb and 40Ar/39Ar dence of Pb loss. Era has been considerably reorganized. It now methods. Thus the EARTHTIME project (http:// EARTHTIME workers did a series of blind consists of three periods, , Neogene, www.earth-time.org/ and European sister orga- tests where unknown and synthetic U-Pb and . Strata traditionally assigned to nization http://earthtime-eu.eu/) was initiated solutions were sent to a number of laboratories the uppermost Pliocene Series/Epoch (Neogene to develop a community-driven approach to with the EARTHTIME tracer. The fi rst blind test System/Period) are now assigned to the Gela- the calibration of Earth history using 40Ar/39Ar was done without the tracer, and dispersion was sian Stage/Age, which belongs in the Pleisto- and U-Pb methods. The project organized all close to 1%. The second test was done using the cene Series/Epoch (Quaternary System/Period). the major geochronology laboratories in the EARTHTIME tracer, and the synthetic solutions As a result, the Quaternary is 43% longer to- world in a collaborative effort to eliminate both indeed greatly decreased interlaboratory vari- than it was prior to addition of the Gela- inter laboratory and intertechnique and ability, and at least six laboratories now agree sian Stage/Age. Importantly, the base of the to critically assess both accuracy and precision at the 1‰ level. Finally, the community seeks to Holo cene Series/Epoch has been defi ned, and associated with numerical ages. The ultimate have common open-source data acquisition and its beginning is dated at 11,700 yr b2k (before goal is to calibrate Earth history, and the effort reduction protocols that can evolve as new ideas A.D. 2000). Terms such as , Recent, has already produced remarkable cooperation are developed. The same approach can be and is and have been dropped from the among geochronologists and the rest of the earth being used for all techniques from 14C to terres- geologic and chronostratigraphic vocabulary as science community, especially paleontologists, trial cosmogenic nuclides (http://www.physics. they no longer have any internationally sanc- astrochronologists, and stratigraphers. This has purdue.edu/primelab/CronusProject/cronus/). tioned standing. also spawned a new generation of students who The 40Ar/39Ar community is working in par- range from conversant to full practitioners of allel on developing fl uence monitors, Advances in Geochronology and the radioisotope geochronology and , including independent assessment of their age, EARTHTIME Effort stratigraphy, and astrochronology. A critical ele- other age standards, and data acquisition of fl u- ment of this globally collaborative approach is ence monitors. This community also has run two The past decade has seen dramatic advances the need for transparency for all laboratory tech- blind tests. The fi rst test, involving fi ve samples, in high-precision geochronology. These ad- niques, from sample preparation to data acqui- indicated considerable dispersion among labora- vances have been used to develop time lines for sition and data reduction to data reporting and tories but no systematic . In the second test, Earth history and deconvolve complex tectonic archiving. all fi ve samples were sent for one irradiation and and magmatic events. Importantly, the applica- The fi rst step for the U-Pb community was to then distributed, eliminating differences in reac- tion of high-precision geochronology to sedi- eliminate a large source of interlaboratory bias tors and irradiation protocols. The results were mentary sequences has allowed researchers to caused by the that each laboratory used a similar to the fi rst with ~2% dispersion among determine the age, duration, and possible syn- different isotopic tracer. Enriched tracers are laboratories, and this is much greater than the chroneity of global events such as , added to U-bearing accessory minerals prior reported analytical precisions (mostly 0.1%– rates of biological evolution and changes in to dissolution to allow the U/Pb ratio of the 0.5%). There are several potential explanations, , and the age and duration of stable mineral to be determined. Tracers used by dif- including differences in the data acquisition and

264 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale

reduction protocols in each laboratory (currently particular we’ve defi ned a new event—the Jaramillo further enforced the hypothesis that Earth’s fi eld there is not a common ), nonlinearity of event.’ I realized immediately that with that new time had reversed its polarity in the past. the mass spectrometer source/detector systems, scale, the Juan de Fuca Ridge could be interpreted These emerging data served to enforce the in terms of a constant spreading rate. And that was and different systems for cleaning sample gas fantastic, because we realized that the record was need to settle the controversy concerning the pos- using getters. A new approach to solving this more clearly written than we had anticipated. Now sibility that the fi eld was capable of reversing problem is under way. A pipette system fi lled we had evidence of constant spreading; that was very polarity, as workers recognized the potential of with gases of a known isotopic composition will important. Earth’s polarity history for establishing an inde- “I realized at once, having poured over the prob- travel to participating laboratories to eliminate is- lem for so long and so recently, that with this revised pendent scale of geologic time. In support of the sues associated with sample heterogeneity. There time scale it would be possible to interpret the Juan de fi eld reversal hypothesis, there was a growing will be three gasses in the pipette ; Fuca anomaly sequence with an essentially constant body of data showing positive “baked contact” one will be air, and the other two will be aliquots rate of spreading. To me, at that instant, it was all over, tests, where the magnetization in a host rock im- of a large natural sample of biotite irradiated dif- bar the shouting.” mediately adjacent to an intrusive , —Frederick J. Vine, recalling his meeting of 40 39 ferent such that the ratio of Ar*/ Ar is Brent Dalrymple at the 1965 GSA Annual or below a fl ow, was similar if not identical ~23.6 between the two . An impor- Meeting, , Kansas City, in Glen to that in the igneous rock, and thus of the same tant aspect of the pipette experiment is that each (1982) and Oreskes (2001), respectively. polarity. In his classic early text on paleomag- canister will have three calibrated pipette vol- netism, Irving (1964) described the baked umes of 0.1, 0.2, and 0.4 cm3, allowing for vary- It has been 50 years since the publication contact test (his fi g. 7.22, p. 172) that, once and ing the amount of gas delivered as 0.1, 0.2, 0.3 of the seminal contribution by Fred Vine and for all, would convincingly demonstrate that (0.1+0.2), 0.4, 0.5 (0.1+0.4), 0.6 (0.2+0.4), and Drummond Matthews (Vine and Matthews, reverse polarity magnetizations in rocks were 0.7 cc. Furthermore, it will be possible to mix 1963) in which marine magnetic anomaly pat- truly a geomagnetic fi eld phenomena, rather these gases with the same volume increments were interpreted in the context of seafl oor than some form of “self-reversal” characteristic to produce intermediate values. This approach spreading and mid-ocean ridges (the Vine- of a complicated magnetic mineralogy that is not should better elucidate the sources of interlabora- Matthews-Morley-Larochelle hypothesis). It common to most rocks. As Opdyke and Chan- tory dispersion. We are highly optimistic that was the recognition that Earth’s magnetic fi eld nel (1996) noted, the geomagnetic fi eld reversal within a few years, the 40Ar/39Ar community will was capable of reversing its polarity, and in hypothesis was gaining considerable support by have eliminated interlaboratory bias and have in- fact had done so numerous times, that allowed this time. First, all rocks of and ternal errors of ~0.3%, if not lower. their hypothesis to be proffered. This was at a Holocene age were of normal polarity. Second, As EARTHTIME community members have point in the early stages in the history of the reverse polarity magnetizations were common attempted to improve precisions of radioiso- development of the geomagnetic polarity time to rocks of age. Third, mag- topic dates, there has been a much deeper ap- scale and magnetic polarity stratigraphy, as the netizations with directions that were “transi- preciation of the ways in which to quantify most “ordering of sedimentary or igneous rock strata tional” between normal and reverse polarity sources of uncertainty and to fully propagate into intervals characterized by the direction of were identifi ed in both sedimentary and igne- associated errors. This in turn has led to new magnetization of the rocks, being either in the ous rocks; their presence would be consistent protocols for data acquisition, reduction, and direction of the present Earth’s fi eld (normal with changes in polarity states occasionally be- archiving through open-source software (Bow- polarity) or 180° from the present fi eld (reverse ing recorded. Fourth, self-reversal mechanisms ring et al., 2011; McLean et al., 2011), result- polarity)” (Opdyke and Channell, 1996). It was were identifi ed in laboratory experiments, but ing in more accurate uncertainties in calculated also at a point soon after the general acceptance in only a few types of rocks; the overwhelming U-Pb dates. Archiving of data will be achieved of the fact that the geomagnetic fi eld was capa- majority of experiments demonstrated that most through a collaboration with EarthChem (at ble of reversing its polarity—a concept that for rocks acquired a thermoremanent magnetization http://www.geochron.org), where data may be decades met with considerable argument and that replicated the ambient fi eld. Finally, many uploaded directly from the software and will al- debate in the scientifi c community. The fi rst baked contact tests, including some involving low future recalculation of dates as “constants” report of a magnetization in a rock (the natural the nearly ideal fi eld conditions postulated by such as the 238U/235U ratio in zircons (e.g., Hiess remanent magnetization, NRM) with a direc- Irving (1964), provided strong support for dual et al., 2012), U and K decay constants, and tracer tion reverse to that of the present geomagnetic polarities of the geomagnetic fi eld. calibration changes due to new fi eld of Earth was that of Brunhes (1906), who As described in The to Jaramillo (Glen, in any laboratory while preserving the original examined Pliocene lava fl ows from central 1982), tremendous and excitement uploaded data. This will allow any time scale . Several other studies over the ensuing raged between teams of geochronologists/paleo- to be updated using published and recalculated decades reported magnetizations reverse to that magnetists from laboratories at the U.S. Geo- dates. The 40Ar/39Ar community is also working of the present fi eld, the most notable of which logical Survey at Menlo Park and University toward a common platform for data reduction is that by Matuyama (1929), who suggested the of at Berkeley and their Australian and is linked to EarthChem for archiving of data possibility that the polarity of the geomagnetic counterparts at the Australian National Uni- through current data reduction software. fi eld was age dependent. With the development versity. The important geochronologic method of with suffi cient sensitivity to of / age dating was being per- Advances in the Geomagnetic Polarity measure the NRM of sedimentary rocks, mag- fected, and the “race” was on to establish a more Time Scale—Developments and Integration netic polarity stratigraphy began to blossom in refi ned polarity time scale, based on isotopic with the Geologic Time Scale the late and early . The earliest such age determinations of the very rocks examined “The critical thing at that meeting was that I met study in North America was that by Torreson for polarity information. By 1963, the combined Brent Dalrymple for the fi rst time. He told me in pri- et al. (1949), which demonstrated that magneti- geochronologic and magnetic polarity data were vate discussion between sessions, ‘We think that we’ve zations of reverse polarity were retained in rocks suffi ciently compelling in their internal consis- sharpened up the polarity reversal scale a bit, but in as old as the early Mesozoic. Numerous studies tency to allow Vine and Matthews to formulate

Geological Society of America Bulletin, March/April 2013 265 Walker et al. and have accepted for publication their now fa- spreading rates were not constant among ocean high-resolution, astrochronologically tuned - mous explanation of mid-ocean ridges and their plates and that they have not been larity record for the ca. 25+ Ma. time interval importance. In the context of both the geomag- constant since the early Mesozoic. obtained, which has been incorporated into the netic polarity time scale and global , Since 1983, the geomagnetic polarity time GSA geologic time scale. This advance permit- the Vine and Matthews (and Morley/Larochelle) scale has been extended to the base of the Tri- ted far more robust correlations of magnetic hypothesis of seafl oor spreading was certainly assic and in fact the very latest Permian using polarity data obtained from marine sections that not immediately embraced by the broad commu- magnetic polarity stratigraphy records from preserved more complete records of evolution- nity. Skeptics noted the fi ne structure of marine continental and marine sedimentary rocks, in ary change during the latest Triassic to earli- magnetic anomaly patterns adjacent to crests of combination with high-resolution geochrono- est Jurassic transition. This time interval (late mid-ocean ridges and pointed out inconsisten- logic data, biostratigraphy, and cyclostratig- Rhaetian to earliest Hettangian) is dominantly cies between anomaly patterns and the current, raphy, where possible. It may be possible to of normal polarity. Importantly, as summa- yet quickly evolving, polarity time scale. Much extend an accurate, chronologically defi ned rized by Lucas et al. (2011), individual sections of that skepticism disappeared following the polarity time scale into the latest , across the boundary have revealed between two Doell and Dalrymple (1966) paper in Science, prior to the Permian-Carboniferous Reverse and four reverse polarity zones inferred to be of in which they demonstrated that the last ~1 m.y. Superchron. This superchron is perhaps the short duration, and thus microzones. If possible, of Earth history was not of entirely normal singular most unusual of the geomag- the precise correlations of these microzones, in polarity, but rather consisted of some 700 ka netic fi eld during the entire Phanerozoic, as we marine and continental sections, across the Tri- of normal polarity, followed by an interval of have little evidence to contradict the likelihood assic-Jurassic boundary may ultimately prove ~100 ka of reverse polarity, an ~150 ka interval that the fi eld was exclusively of reverse polar- invaluable in understanding the timing of end- back to normal polarity, and a several 100 ka ity for some 55 Ma. The termination of the re- Triassic extinctions and the evolution interval of reverse polarity, which character- verse superchron, originally named the Kiaman of Jurassic recovery . ized what was then the early Pleistocene. They Magnetic Interval by Irving and Parry (1963), is Several recently obtained magnetic polarity named the short time interval of normal polar- close to the Wordian-Capitanian boundary (ca. stratigraphy records from both marine and ter- ity the Jaramillo “event,” after Jaramillo Creek 265 Ma). The base of the superchron, however, restrial strata across the Permian-Triassic bound- in the Valle Grande of the Jemez , is less well known. Opdyke et al. (2000) argued ary interval show that both the end-Permian north-central New Mexico, and also described that it might lie in lowermost ecological crisis as well as the - the longer intervals of constant polarity as mag- strata. For most of the remainder of the Paleo- calibrated biostratigraphic Permian-Triassic netic epochs (e.g., Brunhes normal, Matuyama zoic, the magnetic polarity record is relatively boundary both followed a key polarity reversal reverse, Gauss normal, and Gilbert reverse). As ill defi ned, as noted by several workers and sum- from a relatively short interval (subchron) of re- noted previously herein, Fred Vine fi rst learned marized by Opdyke and Channell (1996). The verse polarity to a considerably longer interval of this new observation in November 1965. By exception to this statement is an interval (super- (chron) of normal polarity (Li and Wang, 1989; 1967, the hypothesis of seafl oor spread- chron) of reverse polarity in the Early to Middle Scholger et al., 2000; Szurlies et al., 2003). An ing, and its many implications, had risen in Ordovician that may be some 30 Ma in duration. excellent example of the importance of inte- status to the theory of . Pavlov and Gallet (2005) proposed the name of grating biostratigraphic, cyclostratigraphic, and At the time of publication of the 1983 Geo- “Moyero” for the superchron, based on the lo- magnetic polarity stratigraphic records is the logic Time Scale (Palmer, 1983), our under- cation in Siberia where a fi rst complete record principally continental Permian ( standing of the geomagnetic polarity time scale of the interval was identifi ed. They also noted and Groups) and immediately over- had evolved by incremental refi nements from that the Middle Cambrian was a period of high lying epicontinental Triassic ( a chronology of the last few million years of polarity reversal frequency. Subsequent work by ) strata from Western and Central , Earth history to an increasingly robust defi - Pavlov et al. (2012) has further documented the which have yielded high-quality magnetic po- nition of Earth’s polarity history back to the chronostratigraphic framework for the Moyero larity stratigraphic records. In combination with Middle Juras sic. How this happened in consid- superchron. cyclo strati graphic records, Szurlies et al. (2003, erably less than 20 years involved additional Two important time intervals in Earth history 2012) estimated that the normal polarity chron high-quality radioisotopic dates obtained from for which considerable effort has been made to containing both the end-Permian crisis and the igneous rock sequences with well-defi ned po- understand and defi ne, at very high resolution, biostratigraphic Permian-Triassic boundary was larity records, improved magnetic polarity stra- the magnetic polarity time scale are the Late ~0.7 Ma in duration. tigraphy records, including those from deep-sea Triassic to earliest Jurassic and the late Middle , determination of the approximate Permian to earliest Triassic. Both of these time Chemostratigraphy ages of boundaries between some specifi c po- intervals include major events and larity epochs, and a global integration of marine the development of large igneous provinces, the Chemostratigraphy is the study of variations magnetic anomaly data into an interpretable rec- Central magmatic province, and Sibe- in the primary chemical and isotopic composi- ord of polarity changes, eventually back to the rian fl ood , respectively. tions of sedimentary rocks with time. A vari- . These efforts were summarized The magnetic polarity record of the Triassic, of chemostratigraphy is rock magnetic by Berggren et al. (1985a, 1985b) and and and particularly the , has evolved stratigraphy, where variations in the concen- Gradstein (1986). Having several key numeri- considerably since early work by Keith Run- tration and/or mineralogy of magnetic phases cal age estimates of polarity boundaries allowed corn and colleagues in the 1950s. The seminal are determined in sedimentary sequences (e.g., for further refi nements to the magnetic polar- work by Paul Olsen and Dennis Kent and col- Evans and Heller, 2003; Kodama et al., 2010). ity time scale based on the marine magnetic leagues on the continuously cored Upper Trias- Ideally, in chemostratigraphic and rock mag- anomaly record (Cande and Kent, 1992, 1995) sic sequence of the Newark Basin (Kent et al., netic stratigraphic approaches, it is assumed and better recognition of the fact that seafl oor 1995; Olsen et al., 1996a, 1996b) provided a that the chemical/rock magnetic signals pre-

266 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale served within sedimentary rocks provide a periods of time, punctuated by large, mostly been appreciated that the oxygen isotope record proxy for global seawater chemistry and/or en- short-lived and globally synchronous perturba- shows cyclic variation, and Emiliani (1955) was vironmental conditions at the time of, or shortly tions, aid in correlation. Also used is the heavier the fi rst to argue that these variations over the after, depo si tion. However, postdepositional system of 87Sr/86Sr, although data acquisition past 55 Ma have been the result of oscillations processes can overprint primary signals, and is more time consuming and expensive. Rock between glacial and stages. Thus, much care must be taken to avoid interpretation magnetic stratigraphy may utilize several rock the oxygen isotopic composition of minerals of signals that have been modifi ed by postdepo- magnetic parameters, including but not limited precipitated from seawater can be used to infer sitional processes as primary. If one assumes to bulk , intensity of an- ice volumes and ocean temperatures. that the variation in seawater chemistry is a hysteretic remanent magnetization (ARM), in- Variations in the oxygen isotopic composi- global and homogeneous signal, then one of the tensity of isothermal remanent magnetization tion of have played an important main uses of chemostratigraphy is to correlate (IRM), including saturation IRM (SIRM), and role in the calibration of the astronomical time stratigraphic variability in chemistry between S-ratios (IRM acquired by fi rst saturating the scale, related to the periods of orbital eccen- packages of sedimentary rocks across a basin sample in a high fi eld and then applying a back- tricity, obliquity, and (see detailed and/or around the globe. The relative tempo- fi eld IRM in a fi eld of –300 mT; this is divided discussion in the following section). Tilt and ral framework provided by chemostratigraphic by the SIRM). precession control the seasonal variations in correlation is enhanced using biostratigraphy, solar . This has led to dramatic varia- magnetostratigraphy, floating astrochronol- Isotopes of Carbon tions in the temperature of the Northern Hemi- ogy (see following section), and calibration of It is generally assumed that the carbon iso- sphere over the last 25 ka. Using the variations chemical signals with numerical time using topic composition of rocks (expressed in oxygen isotopes of in foraminiferan δ13 either well-established boundaries (e.g., extinc- as Ccarb) is set during equilibrium precipita- shells, it is possible to reconstruct past ice vol- tions, isotopic anomalies) or accurately dated tion from seawater and refl ects the isotopic umes and temperatures. Ice volume depends volcanic rocks. Most recently defi ned GSSPs composition of the contemporaneous dissolved largely on the amount of summer radiation at are at horizons close to some well-established inorganic carbon reservoir, which is in turn set high in the . Most chemostratigraphic shift that can be correlated by the relative fraction of inorganic to organic of the variation in δ18O can be related to orbital between sections (Gradstein et al., 2012). In carbon burial (e.g., Kump and Arthur, 1999). forcing of summer radiation. During glaciations, δ13 strata that are devoid of recognizable guide Excursions from Ccarb = 0 (e.g., Shields and there is less seasonal variability, the Earth-Sun fossils, chemostratigraphic correlation has Veizer, 2002; Maloof et al., 2010; Zachos et al., distance is high, and Earth’s orbit is character- been essential to unraveling the chronology of 2001) can be used for global correlation, espe- ized by high eccentricity and low tilt. During Earth history. Study of the has cially when calibrated with high-precision geo- interglacial periods, there is strong seasonality, benefi ted greatly from applying stable isotope chronology. For the Neoproterozoic, the large the Earth-Sun distance is lower, and Earth’s orbit δ13 chemostratigraphy to chronostratigraphic corre- amplitude of well-documented global Ccarb is characterized by low eccentricity and high tilt. lation (e.g., Knoll and Walter, 1992; Halverson excursions has led to controversy over the These orbital variations have led to the recogni- et al., 2005). canonical interpretation of these records. Like tion of ~50 different episodes of growth of con- A major to detailed chemostrati- the Phanerozoic, positive excursions are thought tinental ice or ice ages over the past 3 Ma and a graphic correlations across large distances is to refl ect enhanced organic carbon burial. The way to calibrate the astronomical time scale. that it is unlikely that multiple depositional cause of the negative excursions is debated, and Oxygen isotopic studies of older rocks are areas capture the same complete and continu- hypotheses include reconstitution of organic also of value and have been used with great ous record of sediment accumulation. In addi- carbon (e.g., Rothman et al., 2003; Fike et al., success in the Cretaceous. Unusually fresh tion, accumulation rates between locations often 2006) and catastrophic release (e.g., foraminifera from the Demerara Rise in the vary signifi cantly. In detail, these factors result Kennedy et al., 2001). western tropical Atlantic record the warmest in different structures of chemical variability, re- (~36 °C) sea-surface temperatures (SSTs) re- quiring care in comparing records. Nonetheless, Isotopes of Oxygen ported for Cretaceous–Cenozoic time (Wilson average global curves for some elements have Secular variations in marine 16O/18O, denoted et al., 2002). This in turn supports the idea of a been compiled and appear to work quite well as δ18O, have been a very important chemo- Cretaceous greenhouse. Other studies that have (e.g., Halverson et al., 2005; Saltzman, 2005; stratigraphic tool, especially in paleoclimate reconstructed Cretaceous paleotemperatures us- Zhu et al., 2006). Some stratigraphic positions, studies over the last 55 Ma (e.g., Zachos et al., ing oxygen isotopes include Norris and Wilson such as the base of the Cambrian (Corsetti and 2001). The basis for the method is that 16O is (1998), Bice et al. (2003), and Huber et al. Hagadorn, 2000; Amthor et al., 2003; Bowring preferentially enriched in vapor leaving (2002). The further back in time one goes using et al., 2007; Babcock et al., 2011), and the base the oceans, resulting in seawater that is rela- oxygen isotopes, the more carefully diagenetic of the Oligocene (Zachos et al., 2001), as well as tively enriched in 18O. that precipi- effects must be evaluated. some geologic events such as the end-Permian tate shells as well as CaCO3 refl ect the oxygen extinction (e.g., Shen et al., 2011; Cao et al., isotope composition of seawater. When the con- 2009) are marked by sharp, short-lived pertur- tinents have ice sheets, the ice is enriched in 16O Over much of Earth history, the 87Sr/86Sr bations in seawater chemistry that can be used from precipitation, and the oceans are depleted ratio of seawater has varied (e.g., Veizer, 1989; to correlate disparate sections. in 16O, so that the δ18O is increased. During Halverson et al., 2007), and it can be used to Light stable isotopes and, in particular, varia- times of no ice, precipitation is mixed back into evaluate the relative contributions of the major tions in 13C/12C and 18O/16O are the most widely the oceans, and there is little change in δ18O. sources of dissolved Sr, namely, riverine input applied chemostratigraphic tools, largely be- Temperature also controls the δ18O of precipita- from chemical of (1) continental cause of their relative low acquisition costs. The tion, with higher fractionations correlated with rocks that range from old radiogenic Sr from smooth trends in their global values over long lower mean annual temperatures. It has long areas and active orogens to (2) relatively

Geological Society of America Bulletin, March/April 2013 267 Walker et al. unradiogenic Sr contained in weathered lime- isotope records exist for the Cenozoic (e.g., cally deposited sediments, which can be tuned stones, and (3) the hydrothermal Sr fl ux, which Paytan et al., 1993) and Mesozoic (McArthur to the calculated together is similar to the MORB source. Samples amena- et al., 2001), but the resolution of the record with stratigraphic correlation tools, principally ble to 87Sr/86Sr include calcite, , decreases further back in Earth history (Veizer biostratigraphy and magnetostratigraphy. shells, and authigenic or biogenic , et al., 1999; Shields and Veizer, 2002; Halverson This has developed into the use of “target especially . and contami- et al., 2007). curves,” which provide numerical age calibra- nation by Sr residing in clay mineral lattices can tion, with no reliance on radioisotopic geo- perturb the primary seawater signal of 87Sr/86Sr. Chemostratigraphy in the Precambrian chronology, by matching local cyclostratigraphic Calcite samples must be carefully screened for The most recent addition of a new geologic records with a highly constrained model of the high Sr concentrations, 100 ppm to >2000 ppm, period occurred in 2004 (Knoll et al., 2004) astronomical parameters or the climatic expres- and in some cases, trace elements (e.g., Brand when the Ediacaran Period was defi ned with a sion of those parameters such as variations in and Veizer, 1980; Jacobsen and Kaufman, 1999). GSSP in southern . This period brack- solar insolation (the target curve). In the past Veizer (1989) provided a detailed review of ets the period of time from one of the last major decade , dramatic improvements in understand- the origins of strontium in seawater and the ice ages (Marinoan) to the base of the Cambrian. ing dynamics have resulted in the processes that affected the preservation of Because of the dearth of fossils with well-known extension of sophisticated models that predict the signal, including the effects of diagenesis. ranges in this period and the global of the the climatic and sedimentologic response of He constructed the fi rst Phanerozoic Sr sea water bounding events, carbon isotope chemostratig- Earth to the astronomical parameters at least as curve using CaCO3 from rocks and shells of dif- raphy (Ediacaran-Cambrian transition) and the far back as 40–50 Ma (Laskar et al., 2011). ferent ages. He argued that the rate of mixing signal of deglaciation (Cryogenian-Ediacaran A similar approach for older rocks that can- of ocean and the long-residence time of boundary) were used to defi ne the newest period not be directly referenced to the late Cenozoic strontium in seawater could potentially yield a (Knoll et al., 2004, 2006). These global isotope allows the use of “fl oating” segments of cycli- reliable signal. He assumed that by using the excursions, interpreted to be isochronous, and cally deposited sedimentary rocks to estimate least radiogenic (i.e., lowest 87Sr/86Sr) ratios for events such as the global glaciations that char- the amount of time between two stratigraphic a given time and using large amounts of data, acterize the Neoproterozoic offer much hope for horizons. There are many examples of spec- one could minimize the effects of diagenesis further subdivision of Earth history. tacular cycles preserved in both marine and and produce a robust estimate of the isotopic In summary, several chemostratigraphic continental sedimentary rocks that are defi ned composition of seawater with time. McArthur methods provide very powerful tools for global by, for example, color, , rock mag- (1994, 1998) and McArthur and Howarth correlation between radioisotopic tie points. netic properties, , and (2004) have taken the in the development Moreover, in addition to providing a framework organic carbon content. The signal is measured and appli ca tion of the approach to age determi- for relative time, chemostratigraphic variations and assigned relative ages using magnetic po- nation and standardization of the method, espe- are ultimately driven by evolving seawater com- larity stratigraphy and biostratigraphy, and cially when there are large and unidirectional position, and thus, when integrated with the fos- then, if possible, calibrated in numerical time changes, and its integration into the process of sil and tectonic records, great may be with high-precision geochronologic informa- refi ning the time scale. gained in understanding the history of oceanic tion on beds. These sections can The 87Sr/86Sr ratio of marine can circulation and timing of major changes in be used to correlate with other sections that be used for global correlations as well as inter- ocean chemistry. may not have cyclostratigraphy but include the pretations concerning the role of tectonics and same biostratigraphy or magnetic polarity stra- climate in determining the isotopic composi- Astrochronology tigraphy. When combined with geochronology tion of seawater. For example, the dramatic rise of volcanic horizons, one can use the cycles in 87Sr/86Sr after ca. 40 Ma (Burke et al., 1982; There are significant gravitational inter- to interpolate time with higher precision than Palmer and Elderfi eld, 1985; Edmond, 1992) actions between Earth and the Sun, Moon, and with geochronology alone or linear interpola- is attributed to the (radiogenic other planets. Earth’s orientation relative to the tion assuming constant sediment accumulation and high concentrations) derived from the ris- Sun changes in a quasi-periodic fashion as a re- rates. This has been used for sequences in the ing and rapidly eroding and carried sult of interactions between Earth’s axial preces- Cretaceous (e.g., Meyers et al., 2012) and in to the oceans by the Ganges and Brahmaputra- sion and the variable shape of its orbit induced much older strata, such as those deposited near Tsangpo systems. Raymo and Ruddiman by of other planets (Berger and Loutre, the Triassic-Jurassic transition (e.g., Whiteside (1992) suggested the rapid rise was related to 1990; Laskar et al., 2004; Hinnov and Hilgen, et al., 2007) and in the Carboniferous (Davydov an enhanced rate of continental weathering and 2012). The interactions result in cyclic oscilla- et al., 2010). thus climate. The Himalayas contain a broad tions in the eccentricity of Earth’s orbit and in The target curve and fl oating curve calibration range of rocks with radiogenic Sr, especially the tilt and precession of Earth’s axis. Orbital approaches have been important for developing metamorphic rocks, which were likely the domi- eccentricity variations have periods of ~100,000 an astronomical time scale in the construction of nant control on the Sr isotopic composition of years and 405,000 years, whereas the tilt varia- the global geologic time scale back to 250 Ma seawater at that time (Harris, 1995). It remains tion has a period of 41,000 years, and the “cli- (Gradstein et al., 2012). The astronomical time unclear whether this trend can be used in deep matic precession” has a period of ~21,000 years, scale alone has a resolution of 0.02–0.40 m.y. time to fi ngerprint the rise of ancient mountain and apart from the 405,000-year cycles, there for cyclostratigraphy in Neogene and Quater- ranges or to make inferences about climate. are multiple periods for each of these modes. nary sections, and at the low end, it is com- However, for rocks with good radioisotopic These in turn cause variations in solar radia- parable to radioisotopic dating but completely and biostratigraphic control, such a rise can be tion reaching Earth’s surface, which result in independent. For older rocks, the astronomical used as a chronometer with precisions of ~1 Ma climatic variations, called Milankovitch cycles. time scale can provide age models between or better. Very detailed and precise strontium The climatic variations are recorded as cycli- radio iso topi cally dated tie points allowing fi ne-

268 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale

scale resolution in thick sequences lacking other points for the Paleogene and all three Meso- models was recently published by Tsukui and geochronologic proxies. zoic periods. The geologic time scale includes Clyde (2012). In this paper, a highly resolved, Parallel improvements in both 40Ar/39Ar and absolute astronomical calibrations for the entire previously obtained geomagnetic polarity time U-Pb geochronology have made it possible to Cenozoic Era and Cretaceous Period; fl oating scale for the early-middle Eocene was combined pursue the intercalibration of radioisotopic dat- astronomical calibrations make up 85% of the with 40Ar/39Ar sanidine age determinations on ing and astrochronology (e.g., Kuiper et al., Jurassic and 75% of the Triassic time scales. 23 discrete ash-fall tuffs and other chronostrati- 2008; Meyers et al., 2012). When successful, The eccentricity of Earth’s orbit has a major graphic information, including polarity data, to this integration allows for unprecedented ac- variation with a period of 405,000 years. support a revised paleomagnetic time scale for curacy and precision in the of Shorter astronomical cycles with precession and the early to middle Eocene. The age calibration geologic time. The basic approach is to use obliquity periods are used to calibrate Neogene model suggests an approximate 2 Ma duration high-precision geochronology of volcanic ash and younger rocks. However, because the solar for the early Eocene climatic optimum, which beds with both U-Pb and 40Ar/39Ar techniques system exhibits chaotic diffusion, we can expect in turn coincides with the inferred age of the to yield an estimate of the duration between that their periods will change with time beyond -Bridgerian faunal transition. Com- each dated deposit. Despite the complexities 50 m.y., and the short cycles cannot be used to pared with a previous astrochronological model, involving systematic differences between U-Pb calibrate the older (>50–60 Ma) rock record. The the revised time scale is consistent with an age and 40Ar/39Ar dates, the age difference between 405,000-year eccentricity cycle remains more for the Paleocene-Eocene thermal maximum of each deposit should be the same but likely with stable through geologic time because it results 56.33 Ma. Ultimately, terrestrial sequences with different uncertainties. Then, the fl oating astro- from interactions of and . Workers relatively high accumulation rates and high- nomical model for the time interval can be are now using fl oating cyclostratigraphic rec- precision geochronology can better resolve the used for a much higher-resolution age model. ords that contain what they interpret to be the age and duration of polarity chrons, and can Ideally , bedded sedimentary rock sequences 405,000-year cycle to calibrate the geologic time be compared with astrochronologic data from with orbitally infl uenced cycles occur nearby or scale dating back to at least the end of the Paleo- more continuous marine sedimentary deposits. between the dated ash beds, so that one can be zoic era (ca. 252.2 Ma) and beyond. In summary, the integration of high-preci- sure the fl oating time scale can be applied to a There is potential to use the fl oating curve sion, high-accuracy geochronology with refi ned particular interval. This is a rare occurrence but approach to greatly increase the precision of astrochronological models and stable isotope, allows an opportunity to directly intercalibrate the geologic time scale. For example, Davydov chemostratigraphic, and magnetostratigraphic two independent radioisotopic chronometers et al. (2010) used U-Pb geochronology in the observations will allow us to better understand against an astrochronologic age model (Meyers Donets Basin to confi rm that individual high- the role of climate and ocean- inter- et al., 2012). frequency Carboniferous cyclothems and bun- actions on biological evolution. Another important application was reported dles of cyclothems into fourth- sequences by Kuiper et al. (2008), who used 40Ar/39Ar are the eustatic response to orbital eccentricity OUTSTANDING ISSUES geochronology to determine the age of the (100 and 405 ka) forcing. They produced a con- sanidine by astronomically calibrating tinuous age model for Pennsylvanian strata of Stratigraphic Challenges high-precision dates on ash beds and arriving the basin during this key interval of Earth his- at a value of 28.201 ± 0.046 Ma. Meyers et al. tory characterized by a major . Detailed The search for satisfactory horizons to use for (2012) directly tested this by comparing U-Pb biostratigraphy will allow the export of this age marking GSSPs in the Phanerozoic has been a and 40Ar/39Ar dates from the same bentonites model to sections throughout Euramerica. long protracted process, and many refi nements and concluded that the pairs of dates, using This approach has also been used in the re- in strategy and technical methods have been an assumed age for Fish Canyon sanidine of markable sedimentary sequence of dominantly introduced since the time that the fi rst GSSP, 28.201 Ma, were the same within uncertainty. lacustrine of the Newark Basin, where in- which marked the base of the Devonian, was They integrated the 40Ar/39Ar dates from three vestigation of the continuous sequence obtained ratifi ed in 1972. With the appearance of new bentonites and the orbital time scale with un- by a series of drill cores has allowed unprece- information (particularly detailed chemostrati- certainties of thousands of years to confi rm that dented resolution of a fl oating astronomical time graphic data) and reevaluation of sections, some 28.201 Ma is the best age estimate for Fish Can- scale for over 5800 m of composite stratigraphy early decisions as to specifi c time boundaries yon sanidine. Renne et al. (2010, 2011) arrived integrated with magnetic polarity stratig raphy now seem less well justifi ed, and may merit at a slightly older age estimate (28.291 Ma ± that includes 59 magnetozones and a sampling reexamination. One example is the basal Cam- 0.036) by comparing pairs of U-Pb zircon and density of about every 20 ka (Kent and Olsen, brian GSSP. Because of problems associated 40Ar/39Ar dates from rocks from a wide range of 1999; Olsen et al., 2011). Vertebrate biostratig- with global correlation of the Cambrian/Paleo- ages with the Fish Canyon sanidine age proposed raphy and geochronology of the section are zoic-Phanerozoic base using the biostratigraphy by Kuiper et al. (2008). In future intercalibration sparse, although several Central Atlantic mag- of trace fossils, chemostratigraphy has emerged studies, the sources of uncertainty associated matic province intrusions and lava fl ows at ca. as the de facto means of characterizing and cor- with the radioisotopic ages (analytical, tracer so- 201.4 Ma, near the end-Triassic extinction, are relating that horizon globally (Bowring et al., lution, or age of neutron fl uence standard, and used as an upper tie point. There is a pronounced 2007; Babcock et al., 2011). The Ediacaran- decay constants), as well as uncertainties in the 405 ka cyclicity, and it has been used for specu- Cambrian boundary in Newfoundland is based astro chronol ogy, must be considered. lations on the tempo and mode of dinosaurian on assemblages and has no body A Geologic Time Scale 2012 (Gradstein et al., origin, diversifi cation, and rise to ecological fossils and has no rocks suitable for geochronol- 2012) uses “absolute” astronomical calibrations dominance (Olsen et al., 2011). ogy or chemostratigraphy. It remains uncertain based on solar system dynamics for the Neo- A good example of combining magnetic whether the negative δ13C excursion used to ap- gene, and “fl oating” astronomical calibrations polarity information and geochronology and proximate the boundary does in fact coincide tied to radioisotopically dated stratigraphic testing the results against astrochronological with the Cambrian GSSP.

Geological Society of America Bulletin, March/April 2013 269 Walker et al.

Up to the present time, boundary positions in to 28.29 Ma (Renne et al., 2010, 2011). Many REFERENCES CITED the Archean and Proterozoic have mostly used workers in the community are using 28.201 Ma Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., purely chronometric boundaries called global as the preferred age, as determined by astro- 1980, Extraterrestrial cause for the Cretaceous-Tertiary standard stratigraphic ages (GSSAs), based on nomical calibration (Kuiper et al., 2008), extinction: Science, v. 208, p. 1095–1108, doi:10.1126 geochronologic data, rather than points in sec- as in many cases it appears to result in good /science.208.4448.1095. Amthor, J.E., Grotzinger, J.P., Schröder, S., Bowring, S.A., tions or GSSPs. The one exception is the ter- agreement between 40Ar/39Ar and U-Pb deter- Ramezani, J., Martin, M.W., and , A., 2003, Ex- minal system/period of the Proterozoic, the minations. The community is making great tinction of Cloudina and at the Precam- brian-Cambrian boundary in : Geology, v. 31, Ediacaran. Ultimately, it will be desirable, if pos- progress, but more work remains to be done. p. 431–434, doi:10.1130/0091-7613(2003)031<0431: sible, to replace GSSAs with GSSPs that have Our goal was to provide a synopsis of the EOCANA>2.0.CO;2. well-calibrated numerical ages. history of the Geological Society of America Babcock, L.E., Robison, R.A., and Peng, S.C., 2011, Cam- brian stage and series nomenclature of and The recent defi nition of the base of the Holo- Geologic Time Scale and the many different the developing global chronostratigraphic scale: Mu- cene Series using signals of climatic change facets of investigations that have led seum of Northern Bulletin, v. 67, p. 12–26. (Walker et al., 2009), rather than biostrati- to refi nements in geologic time scales. This Barrell, J., 1917, Rhythms and the measurement of geologic time: Geological Society of America Bulletin, v. 28, graphic markers, represents an important turn- overview highlights the continued importance p. 748–749. ing point in philosophy about GSSP defi nition. of integrated efforts to better understand the Berger, A., and Loutre, M.F., 1990, Origine des fréquences des eléments astronomiques intervenant dans le calcul Some nonbiostratigraphic proxies are superior Earth-life record of deep time on a global scale. de l’insolation: Bulletin de la Classe des Sciences, to biostratigraphic tools in certain parts of the A crucial aspect of progress in geochronology Académie Royale Belgique, Ser. 6 1(1[3]), p. 45–106. geologic column, and it is likely that nonbio- is the use of highly characterized mineral stan- Berggren, W.A., Kent, D.V., Flynn, J.J., and Van Couver- ing, J.A., 1985a, Cenozoic geochronology: Geologi- stratigraphic correlation techniques will an dards that span a range of ages. EARTHTIME cal Society of America Bulletin, v. 96, p. 1407–1418, increased role in the defi nition of GSSPs in the is actively seeking out standard samples that doi:10.1130/0016-7606(1985)96<1407:CG>2.0.CO;2. future, particularly those that will be defi ned in range in age from less than 1 Ma to well over Berggren, W.A., Kent, D.V., and Van Couvering, J.A., 1985b, The Neogene: Part 2. Geochronology and chrono- the pre-Phanerozoic. 500 Ma that can be analyzed for both 40Ar/39Ar stratigraphy, in Snelling, N.J., ed., The Chronology of and U-Pb (Schoene et al., 2005; Renne et al., the Geological Record: Geological Society of London Memoir 10, p. 211–260. Geochronology and Resolving 2010). Mineral separates will be distributed to Bice, K.L., Huber, B.T., and Norris, R.D., 2003, Extreme Discrepancies between U-Pb and participating laboratories, and, after analyses polar warmth during the Cretaceous greenhouse? Para- 40Ar/39Ar Methods are completed, the results will be published. dox of the late Turonian δ18O record at Deep Sea Drill- ing Project Site 511: , v. 18, 1031, Ideally, when a U-Pb laboratory publishes a doi:10.1029/2002PA000848. Ultimately, the geoscience community will study, they will also publish data for one or Bowring, J.F., McLean, N.M., and Bowring, S.A., 2011, succeed in building a time scale that can seam- more standards run during the same interval as cyber infrastructure for U-Pb geochro- nology: Tripoli and U-Pb_Redux: Geochemistry Geo- lessly use both high-precision 40Ar/39Ar data the study, as is currently done in many 40Ar/39Ar Geosystems, v. 12, Q0AA19, doi:10.1029 and U-Pb geochronologic data. The concern laboratories. /2010GC003479. 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The 1983 Geological Society of 87Sr/86Sr throughout Phanerozoic : Geology, v. 10, p. 516–519, doi:10.1130/0091-7613(1982)10 2012), and the possibility that in some vol- America Geologic Time Scale was constructed <516:VOSSTP>2.0.CO;2. canic rocks the U-Pb system in zircon does not with an emphasis on North American geology. Cande, S.C., and Kent, D.V., 1992, A new geomagnetic record the time of emplacement but rather a The Geological Society of America will con- polarity time scale for the and Ceno zoic: Journal of Geophysical Research, v. 97, short period of pre-eruptive residence. Because tinue to follow the ICS and other efforts such as p. 13,917-13,951. all 40Ar/39Ar approaches are relative dating Gradstein et al. 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(2010, 2011) inverted its membership, the larger scientifi c community, euxinic oceans and ecological presaging data sets of U-Pb zircon and 40Ar/39Ar dates to and the public to promote the scientifi c and edu- the end-Permian mass : Earth and Plan- arrive at a more accurate estimate of the age cational goals of the society. etary Science Letters, v. 281, p. 188–201, doi:10.1016 /j.epsl.2009.02.012. of Fish Canyon sanidine. This is a promising Cohen, K.M., Finney, S., and Gibbard, P.L., 2012, Inter- approach as we acquire many additional high- ACKNOWLEDGMENTS national Chronostratigraphic Chart: International 40 39 Commission on Stratigraphy, www.stratigraphy.org precision Ar/ Ar and U-Pb data sets on the (last accessed May 2012). (Chart reproduced for the same samples from several different localities We thank A.R. (Pete) Palmer for extensive dis- 34th International Geological Congress, Brisbane, cussion and encouragement. James Ogg shared pre- representing more and more of geologic time. Australia, 5–10 August 2012.) prints of parts of the Gradstein et al. (2012) book. Condon, D., Schoene, B., Bowring, S.A., Parrish, R., The estimate of the assumed age of the Fish J.C. Creveling , Linda Hinnov, Brad Cramer, and Paul McLean, N., Noble, S., and Crowley, Q., 2007, Canyon sanidine has varied from ca. 27.7 Ma Olsen are thanked for comments. EARTHTIME: Isotopic tracers and optimized solutions

270 Geological Society of America Bulletin, March/April 2013 The Geological Society of America Geologic Time Scale

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