ArticlePDF Available

The Geological Society of America Geologic Time Scale

March 2013
Geological Society of America Bulletin 125(3-4):259-272

DOI:10.1130/B30712.1

Authors:

J. Douglas Walker

University of Kansas

John Geissman

The University of Texas at Dallas

Babcock Loren

The Ohio State University

The Geological Society of America has sponsored versions of the geologic time scale since 1983. Over the past 30 years, the Geological Society of America Geologic Time Scale has undergone substantial modifications, commensurate with major advances in our understanding of chronostratigraphy, geochronology, astrochronology, chemostratigraphy, and the geomagnetic polarity time scale. Today, many parts of the time scale can be calibrated with precisions approaching less than 0.05%. Some notable time intervals for which collaborative, multifaceted efforts have led to dramatic improvements in our understanding of the character and temporal resolution of key evolutionary events include the Triassic-Jurassic, Permian-Triassic, and Neoproterozoic-Phanerozoic boundaries (or transitions). In developing the current Geological Society of America Time Scale, we have strived to maintain a consistency with efforts by the International Commission on Stratigraphy to develop an international geologic time scale. Although current geologic time scales are vastly improved over the first geologic time scale, published by Arthur Holmes in 1913, we note that Holmes, using eight numerical ages to calibrate the Phanerozoic time scale, estimated the beginning of the Cambrian Period to within a few percent of the currently accepted value. Over the past 100 years, the confluence of process-based geological thought with observed and approximated geologic rates has led to coherent and quantitatively robust estimates of geologic time scales, reducing many uncertainties to the 0.1% level.

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The Geological Society of America Geologic Time Scale

J.D. Walker1,†, J.W. Geissman2, S.A. Bowring3, and L.E. Babcock4

1Department of Geology, University of Kansas, Lawrence, Kansas 66045, USA

2Department of Geosciences, ROC 21, University of Texas at Dallas, Richardson, Texas 75080, USA, and

Department of Earth and Planetary Sciences, MSC 03 2040, 1 University of New Mexico, Albuquerque, New Mexico 87131, USA

3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge,

Massachusetts 02139, USA

4Department of Geology, Lund University, SE-223 62 Lund, Sweden, and School of Earth Sciences, Ohio State University,

Columbus, Ohio 43210, USA

ABSTRACT

The Geological Society of America has

sponsored versions of the geologic time scale

since 1983. Over the past 30 years, the Geo-

logical Society of America Geologic Time

Scale has undergone substantial modiﬁ ca-

tions, commensurate with major advances

in our understanding of chronostratig-

raphy, geochronology, astrochronology,

chemostratigraphy, and the geomagnetic

polarity time scale. Today, many parts of

the time scale can be calibrated with preci-

sions approaching less than 0.05%. Some

notable time intervals for which collabora-

tive, multifaceted efforts have led to dra-

matic improvements in our understanding

of the character and temporal resolution

of key evolutionary events include the Tri-

assic-Jurassic, Permian-Triassic, and Neo-

proterozoic-Phanerozoic boundaries (or

transitions). In developing the current Geo-

logical Society of America Time Scale, we

have strived to maintain a consistency with

efforts by the International Commission on

Stratigraphy to develop an international

geologic time scale.

Although current geologic time scales are

vastly improved over the ﬁ rst geologic time

scale, published by Arthur Holmes in 1913,

we note that Holmes, using eight numeri-

cal ages to calibrate the Phanerozoic time

scale, estimated the beginning of the Cam-

brian Period to within a few percent of the

currently accepted value. Over the past 100

years, the conﬂ uence of process-based geo-

logical thought with observed and approxi-

mated geologic rates has led to coherent and

quantitatively robust estimates of geologic

time scales, reducing many uncertainties to

the 0.1% level.

INTRODUCTION

One of the most important aspects of research

in the geosciences is connecting what we examine

in the rock record with ages of events and mea-

sured rates and durations of geologic processes .

In doing so, geoscientists are able to place esti-

mates on the rates of climate and evolutionary

changes, use astronomically forced depositional

processes to tell time within sedimentary basins,

examine the ways in which tectonic processes

change crustal and mantle structure and inﬂ uence

landscape evolution and global climate patterns,

and assess the temporal relations among magma-

tism, ﬂ uid-rock interaction, and base/precious

metal mineralization in many settings. A key re-

quirement is establishing accurate ages of rocks

that are directly associated with or bracket a geo-

logic event or process. With efforts to estimate

the chronology of geologic events beginning well

over two centuries ago, this was commonly ac-

complished by establishing a relative geologic

time scale using the ranges of fossils and strati-

graphic relationships. Beginning in the early

twentieth century, the relative geologic time scale

was calibrated using numerical information.

A geologic time scale is the ordered com-

pilation of numerical ages and relative age de-

terminations based on stratigraphic and other

principles. Numerical ages, formerly called

“absolute ages” (Holmes, 1962), form a chrono-

metric time scale typically expressed in thou-

sands (ka) or millions (Ma) of years; relative

ages form a chronostratigraphic time scale. A

geologic time scale is an invaluable tool for geo-

scientists investigating virtually any aspect of

Earth’s development, anywhere on the planet,

and at almost any time in Earth’s history.

This paper describes the history of the devel-

opment of the Geological Society of America

Geologic Time Scale and provides a brief his-

tory of geologic time scales and their compo-

nents. We also discuss important advances made

over the past few decades in establishing both

numerical and relative ages, describe selected

proxies for time in the rock record, and note and

comment on some future challenges. This paper

is intended to provide a general overview of

geologic time scales. The most comprehensive

treatment of the geologic time scale is contained

in the recent publication of Gradstein et al.

(2012), the most current deﬁ nitive work on the

geologic time scale from a global perspective.

This book is the most recent in the series of ma-

jor publications by The Geological Society of

London (Harland et al., 1964) and subsequently

Cambridge University Press (Harland et al.,

1982, 1990; Gradstein et al., 2004; Ogg et al.,

2008) and Elsevier (Gradstein et al., 2012). The

current Geological Society of America Geologic

Time Scale (Fig. 1) incorporates information

presented in the International Commission on

Stratigraphy’s International Chronostratigraphic

Chart (Cohen et al., 2012) and in Gradstein

et al. (2012). Numerical dates used for bound-

ary positions are from Gradstein et al. (2012).

The Geological Society of America (GSA) does

not directly “maintain” an international geo-

logic time scale. Rather, the society provides a

geologic time scale in a concise, logically orga-

nized and readable format that is largely based

on the work of the International Commission on

Stratigraphy (ICS) and related groups and pub-

lications. GSA follows the work and recommen-

dations of these groups in promoting a better

understanding and use of geologic time through

its time scale. Many of these organizations in-

clude geoscientists who are GSA members or

members of the associated societies of GSA.

History of Chronometric-

Chronostratigraphic Geologic Time Scales

“For it was evident to me that the space between

the mountain ranges, which lie above the City of

Memphis, once was a gulf of the sea, like the regions

For permission to copy, contact editing@geosociety.org

259

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

†E-mail: jdwalker@ku.edu

Invited Review

CELEBRATING ADVANCES IN GEOSCI ENCE

1888 2013

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260 Geological Society of America Bulletin, March/April 2013

HIST

ANOM.

CHRON.

C31

C32

C33

M0r

M10

M12

M14

M16

M18

M20

M22

M25

M29

RAPID POLARITY CHANGES

30 C30

C34

1C1

C2A

C3A

C4A

C6A

C6B

C6C

C10

C11

C12

C13

C15

C16

C17

C18

C19

C20

C21

C22

C23

C24

C25

C26

C27

C28

C29

C7A

C5A

C5B

C5C

C5D

C5E

30 C30

237

100

110

120

130

140

150

160

170

180

190

210

200

220

230

240

250

750

1000

1250

1500

1750

2000

2250

2500

2750

3000

3250

3500

3750

260

280

300

320

340

380

360

400

420

440

460

480

500

520

540

4000

GEOLOGIC TIME SCALE

PALEOZOIC

PERMIAN

DEVONIAN

ORDOVICIAN

SILURIAN

MISSIS-

SIPPIAN

PENNSYL-

VANIAN

CAMBRIAN CARBONIFEROUS

AGE

(Ma) EPOCH AGE PICKS

(Ma)

PERIOD

252

260

254

265

269

272

279

290

296

304

307

299

323

331

347

359

372

383

388

393

408

411

419

423

426

433

430

439

441

427

444

445

453

458

470

467

478

485

494

497

501

505

490

509

514

521

529

541

GZHELIAN

KASIMOVIAN

MOSCOVIAN

BASHKIRIAN

SERPUKHOVIAN

VISEAN

TOURNAISIAN

FAMENNIAN

FRASNIAN

GIVETIAN

EIFELIAN

EMSIAN

PRAGIAN

LOCHKOVIAN

315

PRECAMBRIAN

PROTEROZOICARCHEAN

AGE

(Ma) EON ERA

BDY.

AGES

(Ma)

1000

1200

1800

2050

2300

1400

1600

2500

2800

3200

3600

4000

Lopin-

gian

MIDDLE

Guada-

lupian

Cisura-

lian

LLANDO-

VERY

EARLY

FURON-

GIAN

Epoch 3

Epoch 2

TERRE-

NEUVIAN

LATE

LUDLOW

LATE

MIDDLE

WENLOCK

541

635

850

MESOZOIC

TRIASSIC JURASSIC CRETACEOUS

AGE

(Ma) EPOCH AGE PICKS

(Ma)

MAGNETIC

POLARITY PERIOD

PERIOD

LATE

EARLY

LATE

EARLY

MIDDLE

LATE

EARLY

MIDDLE

MAASTRICHTIAN

66.0

72.1

83.6

86.3

89.8

93.9

100

113

126

131

134

139

145

152

157

166

164

CAMPANIAN

SANTONIAN

CONIACIAN

TURONIAN

CENOMANIAN

ALBIAN

APTIAN

BARREMIAN

HAUTERIVIAN

VALANGINIAN

BERRIASIAN

TITHONIAN

KIMMERIDGIAN

OXFORDIAN

CALLOVIAN

BATHONIAN

BAJOCIAN

AALENIAN

TOARCIAN

PLIENSBACHIAN

SINEMURIAN

HETTANGIAN

NORIAN

RHAETIAN

CARNIAN

LADINIAN

ANISIAN

OLENEKIAN

INDUAN

168

170

174

199

191

201

209

241

228

252

250

247

RAPID POLARITY CHANGES

CENOZOIC

AGE

(Ma) EPOCH AGE PICKS

(Ma)

MAGNETIC

POLARITY PERIOD

HIST.

ANOM.

CHRON.

QUATER-

NARY PLEISTOCENE*

MIOCENEOLIGOCENEEOCENEPALEOCENE

PLIOCENE PIACENZIAN

0.01

1.8

3.6

5.3

7.2

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

ZANCLEAN

MESSINIAN

TORTONIAN

SERRAVALLIAN

LANGHIAN

BURDIGALIAN

AQUITANIAN

CHATTIAN

RUPELIAN

PRIABONIAN

BARTONIAN

LUTETIAN

YPRESIAN

DANIAN

THANETIAN

SELANDIAN

CALABRIAN

HOLOCENE

PALEOGENE NEOGENE

GELASIAN 2.6

183

CHANGHSINGIAN

WORDIAN

ROADIAN

WUCHIAPINGIAN

CAPITANIAN

KUNGURIAN

ASSELIAN

SAKMARIAN

ARTINSKIAN

PRIDOLI

LUDFORDIAN

GORSTIAN

HOMERIAN

RHUDDANIAN

TELYCHIAN

AERONIAN

SHEINWOODIAN

HIRNANTIAN

SANDBIAN

KATIAN

DARRIWILIAN

DAPINGIAN

AGE 10

JIANGSHANIAN

PAIBIAN

GUZHANGIAN

DRUMIAN

AGE 5

AGE 4

AGE 3

AGE 2

FORTUNIAN

FLOIAN

TREMADOCIAN

EDIACARAN

CRYOGENIAN

TONIAN

STENIAN

ECTASIAN

CALYMMIAN

STATHERIAN

OROSIRIAN

RHYACIAN

SIDERIAN

NEOPRO-

TEROZOIC

MESOPRO-

TEROZOIC

PALEOPRO-

TEROZOIC

NEOARCHEAN

MESO-

ARCHEAN

PALEO-

ARCHEAN

EOARCHEAN

HADEAN

EARLY

MIDDLE

LATE

Figure 1. Geological Society of America Geologic Time Scale. The Cenozoic, Mesozoic, and Paleozoic are the Eras of the Phanerozoic Eon. Names of

chrono stratigraphic units follow the usage of the Gradstein et al. (2012) and Cohen et al. (2012). Age estimates of boundary positions follow Gradstein

et al. (2012) but are rounded to the nearest whole number (1 Ma) for the pre-Cenomanian, and rounded to one decimal place (100 ka) for the Cenomanian

to Pleistocene interval. Numbered series and stages of the Cambrian are provisional. The Pleistocene is divided into four ages, but only two are shown

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 from 0.13 to 0.01 Ma.

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The Geological Society of America Geologic Time Scale

Geological Society of America Bulletin, March/April 2013 261

about…Ephesos and the Plain of the Maiander, if it

be permitted to compare small things with great. And

small these are in comparison, for of the rivers, which

heaped up the soil in those regions none is worthy to

be compared in volume with a single one of the mouths

of the Nile, which has ﬁ ve mouths.”

—Herodotus, likely the world’s ﬁ rst geologist,

ﬁ fth century B.C., in his Histories, 2.10.0-2.

The quest to understand geologic time has

been integral to the geosciences for over 200

years. James Hutton ﬁ rst formally presented the

scientiﬁ c hypothesis that Earth is ancient in a

reading at the Royal Society of Edinburgh on

4 April 1785. He concluded his revolutionary

text, Theory of the Earth, which was based on

his lectures to the Royal Society of Edinburgh

and published in 1788, with the now-famous

and often-quoted sentence concerning the natu-

ral history of the Earth: “The result, therefore,

of our present enquiry is, that we ﬁ nd no ves-

tige of a beginning,—no prospect of an end”

(p. 304). His work in part inspired the great

advances in the geosciences over the following

century, over many parts of the world, and, after

the discovery of radioactivity, prompted the ini-

tial attempts to quantify the age of the planet

Earth and geologic time.

The ﬁ rst attempts by geologists to quantify

the chronostratigraphic time scale and to es-

tablish some bounds on the age of Earth fall

under the category of “hourglass” methods. The

two most important of these involved consid-

erations of the thickness of sedimentary strata

and the salinity of the oceans. Both of these ap-

proaches relied on using estimated rates of geo-

logic processes to establish a more quantitative

time scale. For the ﬁ rst hourglass, the rates of

sediment accumulation were compared to thick-

nesses of sedimentary strata of known chrono-

strati graphic age to compute the duration of

deposition of the rock unit. The second method

relied on understanding the input from streams

to the oceans to compute an overall age for the

oceans. In both cases, these were very rough

approximations of the duration of processes,

because the estimates of rates were inexact.

Importantly, these estimates indicated that the

Earth was much older than was accepted at

the time. This was also true of estimates by

Thomson (see below). Remarkably, during the

second half of the nineteenth century, prior to

the discovery of radioactivity, a number of

workers recognized astronomically forced sedi-

mentary deposits and used them as a means to

calibrate geologic time (reviewed in Hilgen,

2010). The ﬁ rst attempts were built on the astro-

nomical theories for ice ages, and they used

eccen tricity maximums to tune deposits of the

last glaciation but were also tuning Miocene and

Cretaceous sedimentary rocks (Hilgen, 2010).

The discovery of radioactivity, and, more

speciﬁ cally, the relation between radioactive

parent elements and their intermediate and ulti-

mate daughter products through a fundamental

half-life of radioactive decay, was the seminal

event that led to establishing the numerical

ages of geologic materials and ultimately de-

veloping the ﬁ rst chronometric time scale. The

ﬁ rst attempt was by Arthur Holmes (1913) in

his book The Age of the Earth. Holmes (1913,

Chapter X) extensively reviewed the early

methods of geochronology using U as the

parent element. Work at that time showed that

the decay of U produced He as a by-product

and probably had as its ultimate daughter the

element Pb. The half-life of U and thus pro-

duction rates of these elements were roughly

established by the time of Holmes’ (1913)

publication. At the time of Holmes’ work,

it was simply the ratio of U to He or Pb that

was measured—the discovery that elements

had multiple isotopes was reported separately

in the same year (Soddy, 1913; Thomson,

1913). Holmes (1913) reviewed all available

age determinations for U-Pb and U-He that

had bearing on tying numerical age estimates

to meaningful geologic ages. In this effort,

he noted that U-He age estimates were typi-

cally too young, represented minimum ages,

and were most useful for younger (Cenozoic)

rocks. Holmes established the pre-Cenozoic

time scale using a total of ﬁ ve U-Pb determina-

tions. Of these, only three were from rocks of

Phanerozoic age: an age of 340 Ma for the end

of the Carboniferous, 370 Ma for the end of the

Devonian, and 430 Ma for the end of the Ordo-

vician. Ultimately, Holmes used ﬁ ve U-He and

ﬁ ve U-Pb dates to calibrate his geologic time

scale. The ages of other parts of the chrono-

strati graphic time scale not covered by avail-

able age estimates were approximated by using

compilations of sediment thicknesses. Holmes

concluded this chapter by stating (p. 165):

“Most of the available evidence drawn from radio-

active minerals has now been passed in review. As yet

it is a meager record, but, nevertheless, a record brim-

ful of promise. Radioactive minerals, for the geologist,

are clocks wound up at the time of their origin. After

a few years’ preliminary work, we are now conﬁ dent

that the means of reading these time-keepers is in our

possession. Not only can we read them, but if they

have been tampered with and are recording time in-

correctly, we can, in most cases, detect the error and

so safeguard ourselves against false conclusions.”

Besides establishing a rough time scale, this

work was radical in that it expanded the age of

Earth beyond any previous estimate. Up until

that time, the particularly inﬂ uential work of

Lord Kelvin had placed an upper limit of ca.

40 Ma on the age of Earth (Thomson, 1865).

In his ﬁ rst edition of On the Origin of the Spe-

cies by Means of Natural Selection, Charles

Darwin provided a crude estimate of the age

of Earth of several hundred million years

based on both geology and his assumption of

phyletic gradualism. Interestingly, estimates

of the duration of the Phanerozoic incorporat-

ing radiometric ages in the work of Holmes

(1913) at ~550 Ma (inferred from Holmes’ ﬁ g.

17) and Barrell (1917, p. 892) at ~552 Ma are

well within a few percent of the currently ac-

cepted value.

The time scale was greatly refined by

Holmes in the second edition of his book

(Holmes, 1937). At that time he used almost

30 U-He, U-Pb, and Th-Pb age determina-

tions. Because of the recognition of multiple

isotopes, geochronology relied not just on de-

termining the parent-daughter ratio, but in the

cases of the U-Pb and Th-Pb decay systems,

the actual atomic mass and, to some extent,

the isotope ratios of the daughter products

(Holmes, 1937, Chapter V). Holmes continued

to reﬁ ne the time scale over the next 25 years.

Concurrently, analytical methods increased in

their precision while the number of elements

and thus minerals used for isotopic age deter-

minations increased. For example, Kulp (1961)

primarily incorporated K-Ar dates as numerical

estimates for his compilation of the time scale.

A subsequent symposium in honor of Holmes

resulted in a major effort toward developing a

geologic time scale by a broader community

of geoscientists (Harland et al., 1964). In the

symposium volume, the authors reviewed all

of the signiﬁ cant aspects of and developments

with the geologic time scale from numerical

dates, to hourglass methods, and stratigraphic

constraints. In all, over 300 dates were used to

construct a Phanero zoic time scale.

Over the next 20 years, the standardization

of isotopic decay constants by Steiger and Jäger

(1977) and major improvements to the chrono-

strati graphic time scale, fostered by the Inter-

national Union of Geological Sciences (IUGS)

through the International Commission on Stra-

tigraphy (ICS) and its subcommissions, greatly

advanced the geologic time scale. In particular,

the efforts of the ICS established worldwide

standard deﬁ nitions of the relative geologic

time scale. ICS has and continues to establish

global chronostratigraphic standards known as

global boundary stratotype section and points

(GSSPs) (see following discussion). The work

of IUGS and ICS has culminated in more re-

ﬁ ned geologic time scales for the Phanero-

zoic published by Harland et al. (1982), Odin

(1982), and numerous other authors. The initial

GSA time scale (Palmer, 1983) relied heavily

on the contributions by these authors. A major

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262 Geological Society of America Bulletin, March/April 2013

treatment, including the statistical assessment

of uncertainties of the estimated boundary ages,

was presented by Harland et al. (1989). Con-

siderable progress was made over the next 15

years, with the next major and comprehensive

revision to the geologic time scale published

by Gradstein et al. (2004). This publication

fully integrated the advances made in global

stratigraphy through the work of the ICS, in-

cluding astrochronology, and took advantage

of considerably more precise radioisotope

geo chronol ogy including 40Ar/39Ar step heat-

ing and single-crystal laser methods as well as

U-Pb zircon dating methods.

History of the Geological Society of America

Geologic Time Scale

Besides being the 125th anniversary of the

Geological Society of America, 2013 marks

the 30th anniversary of the Geological Soci-

ety of America Geologic Time Scale, the 50th

anniversary of the publication of the Vine-

Matthews-Morley-Larochelle hypothesis that

marine magnetic anomaly patterns adjacent to

mid-ocean ridges were a record of the polarity

reversal history of the geomagnetic ﬁ eld and

thus that the ridges were sites of ocean-ﬂ oor

spreading (Vine and Matthews, 1963), the 100th

anniversary of the ﬁ rst geologic time scale pre-

sented by Holmes (1913), and the 225th anni-

versary of Hutton’s Theory of the Earth (1788).

The GSA time scale grew out of the Decade

of North America Geology (DNAG) project

(Palmer, 1983), which had as its goal a syn-

thesis of the geology then known of the North

American continent. Before that time, geologic

information on North America had been scat-

tered through the literature. With a few excep-

tions (e.g., Eardley, 1951; King, 1959), little

in the way of comprehensive summaries of the

geology of North America existed. A necessary

prerequisite for discussing the geology of the

continent was a common chronostratigraphic

vocabulary and internally consistent sense of

both the relative and numerical ages of geo-

logic and evolutionary events. This provided

ages of speciﬁ c stratigraphic units addressed in

the multivolume compilation involving scores

of authors that resulted from the DNAG proj-

ect. The DNAG project was the ﬁ rst detailed

synthesis of North American geology follow-

ing widespread acceptance of plate-tectonic

theory and provided a sense of the evolution

of the continent in the context of global events.

Before the DNAG compilation could proceed

very far, a uniform geologic time scale had to

be adopted, and it was the ﬁ rst major product

of the DNAG initiative. This inevitably led to

advancement of a version of the geologic time

scale that was North American–centered but

with more far-reaching consequences. With

progress of the DNAG project and considerable

changes in chronostratigraphic nomenclature

taking place internationally, reﬁ nements of the

time scale were inevitable, and updated versions

were published as GSA’s 1999 Geologic Time

Scale (Palmer and Geissman, 1999) and 2009

Geologic Time Scale (Walker and Geissman,

2009). What set the original DNAG time scale

apart from earlier versions of time scales pub-

lished in textbooks and summary papers was

the overt application of integrated stratigraphic

and magnetostratigraphic data sources (includ-

ing both relative and numerical age dating tech-

niques used to assemble the time scale) and the

style of presentation.

The rationale behind and history of the work

to develop the ﬁ rst Geological Society of Amer-

ica Geologic Time Scale itself were described

by Palmer (1983) and Walker and Geissman

(2009). Allison (“Pete”) Palmer was the Cen-

tennial Science Program Coordinator for GSA’s

DNAG project, and was charged with compil-

ing the results of the efforts of the Time Scale

Advisory Committee, consisting of Z.E. Peter-

man, J.E. Harrison, R.L. Armstrong, and W.A.

Berggren. Pete Palmer had a clear passion for

the work and devoted considerable energy to the

project. An innovative contribution was the at-

tempt to organize the time scale onto a single

8.5 by 11 inch sheet of paper using the “tools

of the trade” in those days—Mylar, zipatone

letters, a Leroy lettering machine, and a lot of

patience. His efforts resulted in a unique lay-

out for the time scale, in which each era of the

Phanerozoic, and all of the Precambrian, was

given identical column length. Scaling of the

magnetic polarity time scale to ﬁ t this format

required numerous trials. When Jim Clark, the

director of publications for GSA, suggested that

the geologic time scale should also be published

as a pocket wallet card, the effort became even

more challenging. A key concern by both the ad-

visory committee and Palmer was whether the

time scale should be North American centric or

global. The advisory committee and Palmer rec-

ognized the likely pitfalls and hurdles associated

with trying to develop a global time scale and

set as a goal trying to develop a “common vo-

cabulary” for North America, still a sufﬁ ciently

great challenge. In discussions with Palmer

(2012, personal commun.), he emphasized the

difﬁ culties with deﬁ ning the base of the Ordovi-

cian (which has more recently been solved with

the deﬁ nition of an Ordovician GSSP). The ﬁ -

nal product (Palmer, 1983) served the intended

purpose well. “There were no naysayers, and no

major disagreements,” stated Palmer. “The basic

subdivisions were all accepted.”

RECENT ADVANCES AND THEIR

IMPACT ON THE GEOLOGIC

TIME SCALE

Here, we review some of the key advances

that have led to changes and reﬁ nements of the

geologic time scale. These include, of course,

changes in stratigraphic and geochronologic

approaches that are at the heart of a combined

chronometric/chronostratigraphic time scale,

but also astrochronology, which is revolution-

izing the time scale effort, chemostratigraphy

and related rock magnetic stratigraphy, and the

geomagnetic polarity time scale (see Table 1 for

brief descriptions of these methods).

Advances in Stratigraphy—The

International Commission on Stratigraphy

Leading the way in the advancement of

chronostratigraphic information is the Inter-

national Commission on Stratigraphy, including

the many subcommissions formed by the ICS.

These groups actively promote the acquisi-

tion and dissemination of information vital to

making informed decisions about stratigraphic

boundary positions and numerical calibrations

of those positions. A major goal of the ICS is

to develop an unambiguous, globally applica-

ble nomenclature for geologic time units and

their chronostratigraphic equivalents, a com-

mon language in which concepts are identical

for all localities across the globe. This effort is

part of a larger project to develop unambiguous

chronostratigraphic units for the entire geologic

time scale (Gradstein et al., 2004, 2012; Ogg

et al., 2008; Cohen et al., 2012). The main driv-

ing forces for the reﬁ nements and restructuring

to the chronostratigraphic side of the geologic

time scale are changes in philosophy about

how stratigraphic units are deﬁ ned, as well as

high-resolution studies utilizing chronostrati-

graphic proxies, including biostratigraphy,

chemostratigraphy, magnetostratigraphy, rock

magnetic stratigraphy, and astrochronology/

orbital tuning.

Over the past several decades, different strati-

graphic philosophies have been applied to deﬁ -

nitions of chronostratigraphic units. Deﬁ nitions

in many places around the world were com-

monly based on the unit-stratotype concept, in

which a type section serves as the standard of

reference for the deﬁ nition and characterization

of a unit (Salvador, 1994). The lower and upper

boundaries of a unit are normally speciﬁ ed by

reference to a type section. Beginning in the

1970s, the concept of a boundary-stratotype was

promoted. Under this concept, only the base of a

chronostratigraphic unit is formally deﬁ ned, and

it is marked by a point in strata (Salvador, 1994),

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which is known as a GSSP, or global boundary

stratotype section and point, more popularly re-

ferred to as a “golden spike.”

Originally, the boundaries of most geologic

time units were drawn at positions in the strati-

graphic record where some sizable biotic change,

widespread geologic event such as a glacial

advance or retreat, or an orogenic episode was

recognized. Some of the formal nomen clature

of the geologic time scale reﬂ ects this practice:

“Phanerozoic” means “visible life,” Paleozoic,

Mesozoic, and Cenozoic mean “ancient life,”

“middle life,” and “recent life,” respectively. In-

formal terms applied to geologic time units, such

as the “age of bryozoans” for the Ordovician,

the “age of coal swamps” for the Carboniferous,

the “age of dinosaurs” for the Mesozoic, and the

“age of mammals” for the Cenozoic, reinforce

this approach to characterizing the progression

of geologic time. This broad-brush view may

have some merits, but to a large extent, the ap-

parent distinctions between strata contained

in superjacent chronostratigraphic units have

been enhanced by evolutionary overturns (ex-

tinctions, followed by recoveries and changing

patterns of ecologic dominance) and unconfor-

mities of varying scale. Boundaries drawn at

such horizons are almost certain to be in strati-

graphic gaps. In the past, chronostratigraphic

units based on the unit-stratotype concept were

often drawn at previously unrecognized gaps in

the stratigraphic record. To be useful, a globally

synchronous boundary must be drawn at a hori-

zon in a continuous succession of strata to en-

sure that there are no gaps in the succession. A

boundary-stratotype–based chronostratigraphic

unit is deﬁ ned only at the base, so the “top” is

automatically deﬁ ned by the base of the overly-

ing unit, and there can be no gap in between.

GSSPs are deﬁ ned according to the boundary-

stratotype concept. By deﬁ nition, every GSSP

marks the base of a chronostratigraphic unit.

Also by deﬁ nition, the chronostratigraphic

unit subjacent to each GSSP includes all strata

known and unknown up to that horizon, so there

can be no gap between adjacent chronostrati-

graphic units.

High-resolution stratigraphic studies today,

often enhanced with continuous drill cores,

are the norm, so more detailed information is

available now than when early versions of the

geologic time scale were assembled. The result-

ing details are not always clear, and unambigu-

ous sets of data in some cases have shown that

geoscientists were sometimes mistaken in their

assumptions of isochronous events. The ﬁ rst

appearance of trilobites was once used through

much of the world as the marker for the base

of the Phanerozoic. Today, we recognize that

the appearance of trilobites on separate paleo-

conti nents was not synchronous, making such

an inferred singular “event” unsuitable on

its own as a deﬁ nitive correlation tool. High-

resolution chronostratigraphic studies using

multiple methods in combination now give us

the ability to pinpoint an exact datum in strata

where a boundary can be drawn. For example,

the base of the Cenozoic coincides with the base

of an iridium-bearing clay layer, as ﬁ rst demon-

strated at Gubbio, Italy (Alvarez et al., 1980).

On a global scale, numerous other stratigraphic

tools (among them, the biostratigraphic ranges

of nannoplankton and foraminiferans, spores

and pollen, ratios of stable isotopes of carbon

and oxygen, and magnetic polarity stratigraphy)

provide information that adds to the anomalous

concentration of iridium at that single datum to

help deﬁ ne its singular position. Geochronology

also can be used to identify the base of the Ceno-

zoic with great precision (ca. 66.0 Ma), adding

to the broad range of techniques that can be used

to identify the boundary horizon globally.

Most GSSPs in the Phanerozoic have been

deﬁ ned to coincide with an evolutionary event,

normally the evolutionary ﬁ rst appearance of a

recognizable guide fossil. For at least one chrono-

stratigraphic unit, this approach is impractical,

in part because some available time proxies

exceed biostratigraphy in their ability to pro-

vide high-resolution stratigraphic information.

The base of the Holocene Series/Epoch, for

example, has been deﬁ ned at a point reﬂ ecting

a distinct switchover in deuterium content of

glacial ice in Greenland. That point essentially

coincides with the onset of warming at the end

of the Younger Dryas and a change in thickness

of glacial ice layers, reﬂ ecting increased dust

content associated with the warming interval.

Chemostratigraphy, particularly using δ13C, pro-

vides high-resolution stratigraphic information

for parts of the Paleozoic that have been chal-

lenging to deﬁ ne on the basis of biostratigraphic

evidence alone. To date, stable isotope data have

not been used as a primary tool for deﬁ ning a

chronostratigraphic unit, but biostratigraphi-

cally calibrated isotopic excursions, particularly

in δ13C, have some advantages in successions

where the biostratigraphic data are inconclusive

at a detailed scale (e.g., Maloof et al., 2010;

Korte and Kozur, 2010). As we achieve a more

highly calibrated record, assuming global syn-

chroneity of the Carbon record, for example,

may not remain valid. Since the completeness

of sections is variable, it may be that no single

proxy will resolve all issues making the use and

integration of multiple proxies required.

The application of the GSSP approach has

helped stabilize the geologic time scale by

adoption of chronostratigraphic nomenclature

ratiﬁ ed by specialists the world over. Changes to

the time scale resulting from these advances are

numerous and some time periods have been dra-

matically restructured through formal decisions

on boundary positions. The Cambrian System/

Period, for example, now has a ratiﬁ ed base

that is stratigraphically well below its position

TABLE 1. TIME DETERMINATION METHODS

Methods Deﬁnition and application

Astrochronology Study of cyclical variation in properties present in sedimentary rock sequences interpreted to represent a response to Earth orbital

parameters deriving from interaction with the Sun, Moon, and other planets. Variations can be expressed in physical sedimentary

patterns (e.g., bed 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 chemistry 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

stratigraphy Study of the polarity record of Earth’s magnetic ﬁeld as reﬂected in the paleomagnetic signatures of stratiﬁed rocks. This gives relative

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

stratigraphy Study of the variations in the magnetic mineralogy (concentration and mineral phases) assumed to be originally deposited with the

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 (lithostratigraphy) as well as the fossil

content (biostratigraphy). This is a relative dating and correlation method.

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in 1983, and the ratiﬁ ed base of the Ordovician

System, which marks the top of the Cambrian,

is above its position in 1983. The 1983 position

of the Cambrian base, following Harland et al.

(1982) was at the base of the Tommotian Stage

as used in Siberia and, at that time. Today, that

position is within provisional Stage 2 of the

Cambrian System and estimated to be about

525 Ma (570 Ma by 1983 standards). The hori-

zon is well within what was in 1983 considered

to be Proterozoic strata. The base of the Ordovi-

cian, after formal deﬁ nition, is at a horizon near

485 Ma (504 by 1983 standards).

Formal decisions on chronostratigraphic

boundaries have in some instances led to

changes in the way we subdivide systems/

periods . Largely as a result of the addition of a

thick stratigraphic section below its traditional

base, the Cambrian is now subdivided into four

series/epochs rather than the traditional three,

and the resulting names for the epochs are de-

rived from localities, not relative stratigraphic

position. The Silurian Period is also subdivided

into four epochs, the names of which are de-

rived from localities. The Permian Period is

subdivided into three epochs, and their names

are likewise derived from localities rather than

relative stratigraphic position. The Cenozoic

Era has been considerably reorganized. It now

consists of three periods, Paleogene, Neogene,

and Quaternary. Strata traditionally assigned to

the uppermost Pliocene Series/Epoch (Neogene

System/Period) are now assigned to the Gela-

sian Stage/Age, which belongs in the Pleisto-

cene Series/Epoch (Quaternary System/Period).

As a result, the Quaternary is 43% longer to-

day than it was prior to addition of the Gela-

sian Stage/Age. Importantly, the base of the

Holo cene Series/Epoch has been deﬁ ned, and

its beginning is dated at 11,700 yr b2k (before

A.D. 2000). Terms such as Tertiary, Recent,

and Anthropocene have been dropped from the

geologic and chronostratigraphic vocabulary as

they no longer have any internationally sanc-

tioned standing.

Advances in Geochronology and the

EARTHTIME Effort

The past decade has seen dramatic advances

in high-precision geochronology. These ad-

vances have been used to develop time lines for

Earth history and deconvolve complex tectonic

and magmatic events. Importantly, the applica-

tion of high-precision geochronology to sedi-

mentary sequences has allowed researchers to

determine the age, duration, and possible syn-

chroneity of global events such as extinctions,

rates of biological evolution and changes in

biodiversity, and the age and duration of stable

isotope anomalies that reﬂ ect major changes in

seawater and atmospheric chemistry. This

mainly results from dramatic increases in the

precision of radioisotopic dates, especially

using 40Ar/39Ar (feldspars) and U-Pb (zircon)

methods. These increases have been stimulated

by new laboratory methods, new generations of

mass spectrometers, and scientiﬁ c projects that

demand the highest possible precision. The U-Pb

method can be applied to rocks from as young

as 600 ka, to as old as the oldest known min-

eral grains on the planet (ca. 4.4 Ga zircon), to

meteorites. The 40Ar/39Ar method is mainly used

for rocks that range in age from younger than

600 ka through the Paleozoic, and even in some

cases Archean rocks and meteorites. However,

40Ar/39Ar systematics are typically disturbed by

metamorphism in rocks exposed to greenschist

and higher grades of metamorphism. For rocks

younger than 600 ka, 40Ar/39Ar and U-series

geochronology are best applied. The time in-

terval reﬂ ecting recent landscape evolution and

erosion is best evaluated using several different

terrestrial cosmogenic nuclides.

As precisions improved (e.g., <0.1% for

U-Pb zircon methods), it was apparent that there

were both intertechnique and interlaboratory

systematic errors for the U-Pb and 40Ar/39Ar

methods. Thus the EARTHTIME project (http://

www.earth-time.org/ and European sister orga-

nization http://earthtime-eu.eu/) was initiated

to develop a community-driven approach to

the calibration of Earth history using 40Ar/39Ar

and U-Pb methods. The project organized all

the major geochronology laboratories in the

world in a collaborative effort to eliminate both

inter laboratory and intertechnique biases and

to critically assess both accuracy and precision

associated with numerical ages. The ultimate

goal is to calibrate Earth history, and the effort

has already produced remarkable cooperation

among geochronologists and the rest of the earth

science community, especially paleontologists,

astrochronologists, and stratigraphers. This has

also spawned a new generation of students who

range from conversant to full practitioners of

radioisotope geochronology and paleontology,

stratigraphy, and astrochronology. A critical ele-

ment of this globally collaborative approach is

the need for transparency for all laboratory tech-

niques, from sample preparation to data acqui-

sition and data reduction to data reporting and

archiving.

The ﬁ rst step for the U-Pb community was to

eliminate a large source of interlaboratory bias

caused by the fact that each laboratory used a

different isotopic tracer. Enriched tracers are

added to U-bearing accessory minerals prior

to dissolution to allow the U/Pb ratio of the

mineral to be determined. Tracers used by dif-

ferent laboratories when EARTHTIME began

included both 208Pb and 205Pb that were mixed

with 235U or 233U + 235U or 233U + 236U. Two iso-

topes of U allow mass-dependent fractionation

to be evaluated during the run, greatly reduc-

ing one of the biggest sources of uncertainty

in U isotopic analyses. In addition, there are a

few tracers containing 233U-235U-202Pb-205Pb that

allow correction for fractionation of the Pb iso-

topes as well. It was decided that the community

needed a single mixed tracer for all laboratories

interested in the EARTHTIME goal (producing

the highest-precision geochronology to estab-

lish a calibrated time line for Earth history) and

one was mixed, calibrated, and sent to several

laboratories (Condon et al., 2007).

At about the same time, U-Pb zircon geo-

chronol ogy was forever changed by the develop-

ment of a method that eliminates open-system

behavior (usually Pb loss) from zircon. The

method was developed by James Mattinson

(Mattinson, 2005) and involves high-tempera-

ture annealing of grains followed by partial dis-

solution. Mattinson was able to show that the

high-U parts of the grains that are most suscep-

tible to Pb loss can be preferentially removed,

leaving relatively low-U domains with no evi-

dence of Pb loss.

EARTHTIME workers did a series of blind

tests where unknown zircons and synthetic U-Pb

solutions were sent to a number of laboratories

with the EARTHTIME tracer. The ﬁ rst blind test

was done without the tracer, and dispersion was

close to 1%. The second test was done using the

EARTHTIME tracer, and the synthetic solutions

indeed greatly decreased interlaboratory vari-

ability, and at least six laboratories now agree

at the 1‰ level. Finally, the community seeks to

have common open-source data acquisition and

reduction protocols that can evolve as new ideas

are developed. The same approach can be and is

being used for all techniques from 14C to terres-

trial cosmogenic nuclides (http://www.physics.

purdue.edu/primelab/CronusProject/cronus/).

The 40Ar/39Ar community is working in par-

allel on developing neutron ﬂ uence monitors,

including independent assessment of their age,

other age standards, and data acquisition of ﬂ u-

ence monitors. This community also has run two

blind tests. The ﬁ rst test, involving ﬁ ve samples,

indicated considerable dispersion among labora-

tories but no systematic bias. In the second test,

all ﬁ ve samples were sent for one irradiation and

then distributed, eliminating differences in reac-

tors and irradiation protocols. The results were

similar to the ﬁ rst with ~2% dispersion among

laboratories, and this is much greater than the

reported analytical precisions (mostly 0.1%–

0.5%). There are several potential explanations,

including differences in the data acquisition and

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reduction protocols in each laboratory (currently

there is not a common platform), nonlinearity of

the mass spectrometer source/detector systems,

and different systems for cleaning sample gas

using getters. A new approach to solving this

problem is under way. A pipette system ﬁ lled

with gases of a known isotopic composition will

travel to participating laboratories to eliminate is-

sues associated with sample heterogeneity. There

will be three gasses in the pipette experiment;

one will be air, and the other two will be aliquots

of a large natural sample of biotite irradiated dif-

ferent times such that the ratio of 40Ar*/39Ar is

~23.6 between the two experiments. An impor-

tant aspect of the pipette experiment is that each

canister will have three calibrated pipette vol-

umes of 0.1, 0.2, and 0.4 cm3, allowing for vary-

ing the amount of gas delivered as 0.1, 0.2, 0.3

(0.1+0.2), 0.4, 0.5 (0.1+0.4), 0.6 (0.2+0.4), and

0.7 cc. Furthermore, it will be possible to mix

these gases with the same volume increments

to produce intermediate values. This approach

should better elucidate the sources of interlabora-

tory dispersion. We are highly optimistic that

within a few years, the 40Ar/39Ar community will

have eliminated interlaboratory bias and have in-

ternal errors of ~0.3%, if not lower.

As EARTHTIME community members have

attempted to improve precisions of radioiso-

topic dates, there has been a much deeper ap-

preciation of the ways in which to quantify most

sources of uncertainty and to fully propagate

associated errors. This in turn has led to new

protocols for data acquisition, reduction, and

archiving through open-source software (Bow-

ring et al., 2011; McLean et al., 2011), result-

ing in more accurate uncertainties in calculated

U-Pb dates. Archiving of data will be achieved

through a collaboration with EarthChem (at

http://www.geochron.org), where data may be

uploaded directly from the software and will al-

low future recalculation of dates as “constants”

such as the 238U/235U ratio in zircons (e.g., Hiess

et al., 2012), U and K decay constants, and tracer

calibration changes due to new measurements

in any laboratory while preserving the original

uploaded data. This will allow any time scale

to be updated using published and recalculated

dates. The 40Ar/39Ar community is also working

toward a common platform for data reduction

and is linked to EarthChem for archiving of data

through current data reduction software.

Advances in the Geomagnetic Polarity

Time Scale—Developments and Integration

with the Geologic Time Scale

“The critical thing at that meeting was that I met

Brent Dalrymple for the ﬁ rst time. He told me in pri-

vate discussion between sessions, ‘We think that we’ve

sharpened up the polarity reversal scale a bit, but in

particular we’ve deﬁ ned a new event—the Jaramillo

event.’ I realized immediately that with that new time

scale, the Juan de Fuca Ridge could be interpreted

in terms of a constant spreading rate. And that was

fantastic, because we realized that the record was

more clearly written than we had anticipated. Now

we had evidence of constant spreading; that was very

important.

“I realized at once, having poured over the prob-

lem for so long and so recently, that with this revised

time scale it would be possible to interpret the Juan de

Fuca anomaly sequence with an essentially constant

rate of spreading. To me, at that instant, it was all over,

bar the shouting.”

—Frederick J. Vine, recalling his meeting of

Brent Dalrymple at the 1965 GSA Annual

Meeting, November, Kansas City, in Glen

(1982) and Oreskes (2001), respectively.

It has been 50 years since the publication

of the seminal contribution by Fred Vine and

Drummond Matthews (Vine and Matthews,

1963) in which marine magnetic anomaly pat-

terns were interpreted in the context of seaﬂ oor

spreading and mid-ocean ridges (the Vine-

Matthews-Morley-Larochelle hypothesis). It

was the recognition that Earth’s magnetic ﬁ eld

was capable of reversing its polarity, and in

fact had done so numerous times, that allowed

their hypothesis to be proffered. This was at a

point in the early stages in the history of the

development of the geomagnetic polarity time

scale and magnetic polarity stratigraphy, as the

“order ing of sedimentary or igneous rock strata

into intervals characterized by the direction of

magnetization of the rocks, being either in the

direction of the present Earth’s ﬁ eld (normal

polarity) or 180° from the present ﬁ eld (reverse

polarity)” (Opdyke and Channell, 1996). It was

also at a point soon after the general acceptance

of the fact that the geomagnetic ﬁ eld was capa-

ble of reversing its polarity—a concept that for

decades met with considerable argument and

debate in the scientiﬁ c community. The ﬁ rst

report of a magnetization in a rock (the natural

remanent magnetization, NRM) with a direc-

tion reverse to that of the present geomagnetic

ﬁ eld of Earth was that of Brunhes (1906), who

examined Pliocene lava ﬂ ows from central

France. Several other studies over the ensuing

decades reported magnetizations reverse to that

of the present ﬁ eld, the most notable of which

is that by Matuyama (1929), who suggested the

possibility that the polarity of the geomagnetic

ﬁ eld was age dependent. With the development

of magnetometers with sufﬁ cient sensitivity to

measure the NRM of sedimentary rocks, mag-

netic polarity stratigraphy began to blossom in

the late 1940s and early 1950s. The earliest such

study in North America was that by Torreson

et al. (1949), which demonstrated that magneti-

zations of reverse polarity were retained in rocks

as old as the early Mesozoic. Numerous studies

further enforced the hypothesis that Earth’s ﬁ eld

had reversed its polarity in the past.

These emerging data served to enforce the

need to settle the controversy concerning the pos-

sibility that the ﬁ eld was capable of reversing

polarity, as workers recognized the potential of

Earth’s polarity history for establishing an inde-

pendent scale of geologic time. In support of the

ﬁ eld reversal hypothesis, there was a growing

body of data showing positive “baked contact”

tests, where the magnetization in a host rock im-

mediately adjacent to an intrusive igneous rock,

or below a lava ﬂ ow, was similar if not identical

to that in the igneous rock, and thus of the same

polarity. In his classic early text on paleomag-

netism, Irving (1964) described the ideal baked

contact test (his ﬁ g. 7.22, p. 172) that, once and

for all, would convincingly demonstrate that

reverse polarity magnetizations in rocks were

truly a geomagnetic ﬁ eld phenomena, rather

than some form of “self-reversal” characteristic

of a complicated magnetic mineralogy that is not

common to most rocks. As Opdyke and Chan-

nel (1996) noted, the geomagnetic ﬁ eld reversal

hypothesis was gaining considerable support by

this time. First, all rocks of late Pleistocene and

Holocene age were of normal polarity. Second,

reverse polarity magnetizations were common

to rocks of early Pleistocene age. Third, mag-

netizations with directions that were “transi-

tional” between normal and reverse polarity

were identiﬁ ed in both sedimentary and igne-

ous rocks; their presence would be consistent

with changes in polarity states occasionally be-

ing recorded. Fourth, self-reversal mechanisms

were identiﬁ ed in laboratory experiments, but

in only a few types of rocks; the overwhelming

majority of experiments demonstrated that most

rocks acquired a thermoremanent magnetization

that replicated the ambient ﬁ eld. Finally, many

baked contact tests, including some involving

the nearly ideal ﬁ eld conditions postulated by

Irving (1964), provided strong support for dual

polarities of the geomagnetic ﬁ eld.

As described in The Road to Jaramillo (Glen,

1982), tremendous competition and excitement

raged between teams of geochronologists/paleo-

magnetists from laboratories at the U.S. Geo-

logical Survey at Menlo Park and University

of California at Berkeley and their Australian

counterparts at the Australian National Uni-

versity. The important geochronologic method

of potassium/argon age dating was being per-

fected, and the “race” was on to establish a more

reﬁ ned polarity time scale, based on isotopic

age determinations of the very rocks examined

for polarity information. By 1963, the combined

geochronologic and magnetic polarity data were

sufﬁ ciently compelling in their internal consis-

tency to allow Vine and Matthews to formulate

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266 Geological Society of America Bulletin, March/April 2013

and have accepted for publication their now fa-

mous explanation of mid-ocean ridges and their

importance. In the context of both the geomag-

netic polarity time scale and global tectonics,

the Vine and Matthews (and Morley/Larochelle)

hypothesis of seaﬂ oor spreading was certainly

not immediately embraced by the broad commu-

nity. Skeptics noted the ﬁ ne structure of marine

magnetic anomaly patterns adjacent to crests of

mid-ocean ridges and pointed out inconsisten-

cies between anomaly patterns and the current,

yet quickly evolving, polarity time scale. Much

of that skepticism disappeared following the

Doell and Dalrymple (1966) paper in Science,

in which they demonstrated that the last ~1 m.y.

of Earth history was not of entirely normal

polarity, but rather consisted of some 700 ka

of normal polarity, followed by an interval of

~100 ka of reverse polarity, an ~150 ka interval

back to normal polarity, and a several 100 ka

interval of reverse polarity, which character-

ized what was then the early Pleistocene. They

named the short time interval of normal polar-

ity the Jaramillo “event,” after Jaramillo Creek

in the Valle Grande of the Jemez Mountains,

north-central New Mexico, and also described

the longer intervals of constant polarity as mag-

netic epochs (e.g., Brunhes normal, Matuyama

reverse, Gauss normal, and Gilbert reverse). As

noted previously herein, Fred Vine ﬁ rst learned

of this new observation in November 1965. By

spring 1967, the hypothesis of seaﬂ oor spread-

ing, and its many implications, had risen in

status to the theory of plate tectonics.

At the time of publication of the 1983 Geo-

logic Time Scale (Palmer, 1983), our under-

standing of the geomagnetic polarity time scale

had evolved by incremental reﬁ nements from

a chronology of the last few million years of

Earth history to an increasingly robust deﬁ -

nition of Earth’s polarity history back to the

Middle Juras sic. How this happened in consid-

erably less than 20 years involved additional

high-quality radioisotopic dates obtained from

igneous rock sequences with well-deﬁ ned po-

larity records, improved magnetic polarity stra-

tigraphy records, including those from deep-sea

sediments, determination of the approximate

ages of boundaries between some speciﬁ c po-

larity epochs, and a global integration of marine

magnetic anomaly data into an interpretable rec-

ord of polarity changes, eventually back to the

Middle Jurassic. These efforts were summarized

by Berggren et al. (1985a, 1985b) and Kent and

Gradstein (1986). Having several key numeri-

cal age estimates of polarity boundaries allowed

for further reﬁ nements to the magnetic polar-

ity time scale based on the marine magnetic

anomaly record (Cande and Kent, 1992, 1995)

and better recognition of the fact that seaﬂ oor

spreading rates were not constant among ocean

lithosphere plates and that they have not been

constant since the early Mesozoic.

Since 1983, the geomagnetic polarity time

scale has been extended to the base of the Tri-

assic and in fact the very latest Permian using

magnetic polarity stratigraphy records from

continental and marine sedimentary rocks, in

combination with high-resolution geochrono-

logic data, biostratigraphy, and cyclostratig-

raphy, where possible. It may be possible to

extend an accurate, chronologically deﬁ ned

polarity time scale into the latest Mississippian,

prior to the Permian-Carboniferous Reverse

Superchron. This superchron is perhaps the

singular most unusual feature of the geomag-

netic ﬁ eld during the entire Phanerozoic, as we

have little evidence to contradict the likelihood

that the ﬁ eld was exclusively of reverse polar-

ity for some 55 Ma. The termination of the re-

verse superchron, originally named the Kiaman

Magnetic Interval by Irving and Parry (1963), is

close to the Wordian-Capitanian boundary (ca.

265 Ma). The base of the superchron, however,

is less well known. Opdyke et al. (2000) argued

that it might lie in lowermost Pennsylvanian

strata. For most of the remainder of the Paleo-

zoic, the magnetic polarity record is relatively

ill deﬁ ned, as noted by several workers and sum-

marized by Opdyke and Channell (1996). The

exception to this statement is an interval (super-

chron) of reverse polarity in the Early to Middle

Ordovician that may be some 30 Ma in duration.

Pavlov and Gallet (2005) proposed the name of

“Moyero” for the superchron, based on the lo-

cation in Siberia where a ﬁ rst complete record

of the interval was identiﬁ ed. They also noted

that the Middle Cambrian was a period of high

polarity reversal frequency. Subsequent work by

Pavlov et al. (2012) has further documented the

chronostratigraphic framework for the Moyero

superchron.

Two important time intervals in Earth history

for which considerable effort has been made to

understand and deﬁ ne, at very high resolution,

the magnetic polarity time scale are the Late

Triassic to earliest Jurassic and the late Middle

Permian to earliest Triassic. Both of these time

intervals include major extinction events and

the development of large igneous provinces, the

Central Atlantic magmatic province, and Sibe-

rian ﬂ ood basalts, respectively.

The magnetic polarity record of the Triassic,

and particularly the Late Triassic, has evolved

considerably since early work by Keith Run-

corn and colleagues in the 1950s. The seminal

work by Paul Olsen and Dennis Kent and col-

leagues on the continuously cored Upper Trias-

sic sequence of the Newark Basin (Kent et al.,

1995; Olsen et al., 1996a, 1996b) provided a

high-resolution, astrochronologically tuned po-

larity record for the ca. 25+ Ma. time interval

obtained, which has been incorporated into the

GSA geologic time scale. This advance permit-

ted far more robust correlations of magnetic

polarity data obtained from marine sections that

preserved more complete records of evolution-

ary change during the latest Triassic to earli-

est Jurassic transition. This time interval (late

Rhaetian to earliest Hettangian) is dominantly

of normal polarity. Importantly, as summa-

rized by Lucas et al. (2011), individual sections

across the boundary have revealed between two

and four reverse polarity zones inferred to be of

short duration, and thus microzones. If possible,

the precise correlations of these microzones, in

marine and continental sections, across the Tri-

assic-Jurassic boundary may ultimately prove

invaluable in understanding the timing of end-

Triassic vertebrate extinctions and the evolution

of Jurassic recovery faunas.

Several recently obtained magnetic polarity

stratigraphy records from both marine and ter-

restrial strata across the Permian-Triassic bound-

ary interval show that both the end-Permian

ecological crisis as well as the conodont-

calibrated biostratigraphic Permian-Triassic

boundary both followed a key polarity reversal

from a relatively short interval (subchron) of re-

verse polarity to a considerably longer interval

(chron) of normal polarity (Li and Wang, 1989;

Scholger et al., 2000; Szurlies et al., 2003). An

excellent example of the importance of inte-

grating biostratigraphic, cyclostratigraphic, and

magnetic polarity stratigraphic records is the

principally continental Permian (Rotliegend

and Zechstein Groups) and immediately over-

lying epicontinental Triassic (Buntsandstein

Group) strata from Western and Central Europe,

which have yielded high-quality magnetic po-

larity stratigraphic records. In combination with

cyclo strati graphic records, Szurlies et al. (2003,

2012) estimated that the normal polarity chron

containing both the end-Permian crisis and the

biostratigraphic Permian-Triassic boundary was

~0.7 Ma in duration.

Chemostratigraphy

Chemostratigraphy is the study of variations

in the primary chemical and isotopic composi-

tions of sedimentary rocks with time. A vari-

ant of chemostratigraphy is rock magnetic

stratigraphy, where variations in the concen-

tration and/or mineralogy of magnetic phases

are determined in sedimentary sequences (e.g.,

Evans and Heller, 2003; Kodama et al., 2010).

Ideally , in chemostratigraphic and rock mag-

netic stratigraphic approaches, it is assumed

that the chemical/rock magnetic signals pre-

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served within sedimentary rocks provide a

proxy for global seawater chemistry and/or en-

vironmental conditions at the time of, or shortly

after, depo si tion. However, postdepositional

processes can overprint primary signals, and

much care must be taken to avoid interpretation

of signals that have been modiﬁ ed by postdepo-

sitional processes as primary. If one assumes

that the variation in seawater chemistry is a

global and homogeneous signal, then one of the

main uses of chemostratigraphy is to correlate

stratigraphic variability in chemistry between

packages of sedimentary rocks across a basin

and/or around the globe. The relative tempo-

ral framework provided by chemostratigraphic

correlation is enhanced using biostratigraphy,

magnetostratigraphy, floating astrochronol-

ogy (see following section), and calibration of

chemical signals with numerical time using

either well-established boundaries (e.g., extinc-

tions, isotopic anomalies) or accurately dated

volcanic rocks. Most recently deﬁ ned GSSPs

are at horizons close to some well-established

chemostratigraphic shift that can be correlated

between sections (Gradstein et al., 2012). In

strata that are devoid of recognizable guide

fossils, chemostratigraphic correlation has

been essential to unraveling the chronology of

Earth history. Study of the Neoproterozoic has

beneﬁ ted greatly from applying stable isotope

chemostratigraphy to chronostratigraphic corre-

lation (e.g., Knoll and Walter, 1992; Halverson

et al., 2005).

A major complication to detailed chemostrati-

graphic correlations across large distances is

that it is unlikely that multiple depositional

areas capture the same complete and continu-

ous record of sediment accumulation. In addi-

tion, accumulation rates between locations often

vary signiﬁ cantly. In detail, these factors result

in different structures of chemical variability, re-

quiring care in comparing records. Nonetheless,

average global curves for some elements have

been compiled and appear to work quite well

(e.g., Halverson et al., 2005; Saltzman, 2005;

Zhu et al., 2006). Some stratigraphic positions,

such as the base of the Cambrian (Corsetti and

Hagadorn, 2000; Amthor et al., 2003; Bowring

et al., 2007; Babcock et al., 2011), and the base

of the Oligocene (Zachos et al., 2001), as well as

some geologic events such as the end-Permian

extinction (e.g., Shen et al., 2011; Cao et al.,

2009) are marked by sharp, short-lived pertur-

bations in seawater chemistry that can be used

to correlate disparate sections.

Light stable isotopes and, in particular, varia-

tions in 13C/12C and 18O/16O are the most widely

applied chemostratigraphic tools, largely be-

cause of their relative low acquisition costs. The

smooth trends in their global values over long

periods of time, punctuated by large, mostly

short-lived and globally synchronous perturba-

tions, aid in correlation. Also used is the heavier

system of 87Sr/86Sr, although data acquisition

is more time consuming and expensive. Rock

magnetic stratigraphy may utilize several rock

magnetic parameters, including but not limited

to bulk magnetic susceptibility, intensity of an-

hysteretic remanent magnetization (ARM), in-

tensity of isothermal remanent magnetization

(IRM), including saturation IRM (SIRM), and

S-ratios (IRM acquired by ﬁ rst saturating the

sample in a high ﬁ eld and then applying a back-

ﬁ eld IRM in a ﬁ eld of –300 mT; this is divided

by the SIRM).

Isotopes of Carbon

It is generally assumed that the carbon iso-

topic composition of carbonate rocks (expressed

as δ13Ccarb) is set during equilibrium precipita-

tion from seawater and reﬂ ects the isotopic

composition of the contemporaneous dissolved

inorganic carbon reservoir, which is in turn set

by the relative fraction of inorganic to organic

carbon burial (e.g., Kump and Arthur, 1999).

Excursions from δ13Ccarb = 0 (e.g., Shields and

Veizer, 2002; Maloof et al., 2010; Zachos et al.,

2001) can be used for global correlation, espe-

cially when calibrated with high-precision geo-

chronology. For the Neoproterozoic, the large

amplitude of well-documented global δ13Ccarb

excursions has led to controversy over the

canon ical interpretation of these records. Like

the Phanerozoic, positive excursions are thought

to reﬂ ect enhanced organic carbon burial. The

cause of the negative excursions is debated, and

hypotheses include reconstitution of organic

carbon (e.g., Rothman et al., 2003; Fike et al.,

2006) and catastrophic methane release (e.g.,

Kennedy et al., 2001).

Isotopes of Oxygen

Secular variations in marine 16O/18O, denoted

as δ18O, have been a very important chemo-

stratigraphic tool, especially in paleoclimate

studies over the last 55 Ma (e.g., Zachos et al.,

2001). The basis for the method is that 16O is

preferentially enriched in water vapor leaving

the oceans, resulting in seawater that is rela-

tively enriched in 18O. Organisms that precipi-

tate shells as well as CaCO3 reﬂ ect the oxygen

isotope composition of seawater. When the con-

tinents have ice sheets, the ice is enriched in 16O

from precipitation, and the oceans are depleted

in 16O, so that the δ18O is increased. During

times of no ice, precipitation is mixed back into

the oceans, and there is little change in δ18O.

Temperature also controls the δ18O of precipita-

tion, with higher fractionations correlated with

lower mean annual temperatures. It has long

been appreciated that the oxygen isotope record

shows cyclic variation, and Emiliani (1955) was

the ﬁ rst to argue that these variations over the

past 55 Ma have been the result of oscillations

between glacial and interglacial stages. Thus,

the oxygen isotopic composition of minerals

precipitated from seawater can be used to infer

ice volumes and ocean temperatures.

Variations in the oxygen isotopic composi-

tion of foraminifera have played an important

role in the calibration of the astronomical time

scale, related to the periods of orbital eccen-

tricity, obliquity, and precession (see detailed

discussion in the following section). Tilt and

precession control the seasonal variations in

solar radiation. This has led to dramatic varia-

tions in the temperature of the Northern Hemi-

sphere over the last 25 ka. Using the variations

in oxygen isotopes of calcite in foraminiferan

shells, it is possible to reconstruct past ice vol-

umes and temperatures. Ice volume depends

largely on the amount of summer radiation at

high latitudes in the Northern Hemisphere. Most

of the variation in δ18O can be related to orbital

forcing of summer radiation. During glaciations,

there is less seasonal variability, the Earth-Sun

distance is high, and Earth’s orbit is character-

ized by high eccentricity and low tilt. During

interglacial periods , there is strong seasonality,

the Earth-Sun distance is lower, and Earth’s orbit

is characterized by low eccentricity and high tilt.

These orbital variations have led to the recogni-

tion of ~50 different episodes of growth of con-

tinental ice or ice ages over the past 3 Ma and a

way to calibrate the astronomical time scale.

Oxygen isotopic studies of older rocks are

also of value and have been used with great

success in the Cretaceous. Unusually fresh

forami nifera from the Demerara Rise in the

western tropical Atlantic record the warmest

(~36 °C) sea-surface temperatures (SSTs) re-

ported for Cretaceous–Cenozoic time (Wilson

et al., 2002). This in turn supports the idea of a

Cretaceous greenhouse. Other studies that have

reconstructed Cretaceous paleotemperatures us-

ing oxygen isotopes include Norris and Wilson

(1998), Bice et al. (2003), and Huber et al.

(2002). The further back in time one goes using

oxygen isotopes, the more carefully diagenetic

effects must be evaluated.

Isotopes of Strontium

Over much of Earth history, the 87Sr/86Sr

ratio of seawater has varied (e.g., Veizer, 1989;

Halver son et al., 2007), and it can be used to

evaluate the relative contributions of the major

sources of dissolved Sr, namely, riverine input

from chemical weathering of (1) continental

rocks that range from old radiogenic Sr from

shield areas and active orogens to (2) relatively

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268 Geological Society of America Bulletin, March/April 2013

unradiogenic Sr contained in weathered lime-

stones, and (3) the hydrothermal Sr ﬂ ux, which

is similar to the MORB source. Samples amena-

ble to 87Sr/86Sr analysis include calcite, dolomite,

shells, and authigenic or biogenic phosphates,

especially conodonts. Diagenesis and contami-

nation by Sr residing in clay mineral lattices can

perturb the primary seawater signal of 87Sr/86Sr.

Calcite samples must be carefully screened for

high Sr concentrations, 100 ppm to >2000 ppm,

and in some cases, trace ele ments (e.g., Brand

and Veizer, 1980; Jacobsen and Kaufman, 1999).

Veizer (1989) provided a detailed review of

the origins of strontium in seawater and the

processes that affected the preservation of

the signal, including the effects of diagenesis.

He constructed the ﬁ rst Phanerozoic Sr sea water

curve using CaCO3 from rocks and shells of dif-

ferent ages. He argued that the rate of mixing

of ocean waters and the long-residence time of

strontium in seawater could potentially yield a

reliable signal. He assumed that by using the

least radiogenic (i.e., lowest 87Sr/86Sr) ratios for

a given time and using large amounts of data,

one could minimize the effects of diagenesis

and produce a robust estimate of the isotopic

composition of seawater with time. McArthur

(1994, 1998) and McArthur and Howarth

(2004) have taken the lead in the development

and appli ca tion of the approach to age determi-

nation and standardization of the method, espe-

cially when there are large and unidirectional

changes, and its integration into the process of

reﬁ ning the time scale.

The 87Sr/86Sr ratio of marine limestones can

be used for global correlations as well as inter-

pretations concerning the role of tectonics and

climate in determining the isotopic composi-

tion of seawater. For example, the dramatic rise

in 87Sr/86Sr after ca. 40 Ma (Burke et al., 1982;

Palmer and Elderﬁ eld, 1985; Edmond, 1992)

is attributed to the dissolved load (radiogenic

and high concentrations) derived from the ris-

ing and rapidly eroding Himalayas and carried

to the oceans by the Ganges and Brahmaputra-

Tsangpo River systems. Raymo and Ruddiman

(1992) suggested the rapid rise was related to

an enhanced rate of continental weathering and

thus climate. The Himalayas contain a broad

range of rocks with radiogenic Sr, especially

metamorphic rocks, which were likely the domi-

nant control on the Sr isotopic composition of

seawater at that time (Harris, 1995). It remains

unclear whether this trend can be used in deep

time to ﬁ ngerprint the rise of ancient mountain

ranges or to make inferences about climate.

However, for rocks with good radioisotopic

and biostratigraphic control, such a rise can be

used as a chronometer with precisions of ~1 Ma

or better. Very detailed and precise strontium

isotope records exist for the Cenozoic (e.g.,

Paytan et al., 1993) and Mesozoic (McArthur

et al., 2001), but the resolution of the record

decreases further back in Earth history (Veizer

et al., 1999; Shields and Veizer, 2002; Halverson

et al., 2007).

Chemostratigraphy in the Precambrian

The most recent addition of a new geologic

period occurred in 2004 (Knoll et al., 2004)

when the Ediacaran Period was deﬁ ned with a

GSSP in southern Australia. This period brack-

ets the period of time from one of the last major

ice ages (Marinoan) to the base of the Cambrian.

Because of the dearth of fossils with well-known

ranges in this period and the global nature of the

bounding events, carbon isotope chemostratig-

raphy (Ediacaran-Cambrian transition) and the

signal of deglaciation (Cryogenian-Ediacaran

boundary) were used to deﬁ ne the newest period

(Knoll et al., 2004, 2006). These global isotope

excursions, interpreted to be isochronous, and

events such as the global glaciations that char-

acterize the Neoproterozoic offer much hope for

further subdivision of Earth history.

In summary, several chemostratigraphic

methods provide very powerful tools for global

correlation between radioisotopic tie points.

Moreover, in addition to providing a framework

for relative time, chemostratigraphic variations

are ultimately driven by evolving seawater com-

position, and thus, when integrated with the fos-

sil and tectonic records, great insight may be

gained in understanding the history of oceanic

circulation and timing of major changes in

ocean chemistry.

Astrochronology

There are significant gravitational inter-

actions between Earth and the Sun, Moon, and

other planets. Earth’s orientation relative to the

Sun changes in a quasi-periodic fashion as a re-

sult of interactions between Earth’s axial preces-

sion and the variable shape of its orbit induced

by motions of other planets (Berger and Loutre,

1990; Laskar et al., 2004; Hinnov and Hilgen,

2012). The interactions result in cyclic oscilla-

tions in the eccentricity of Earth’s orbit and in

the tilt and precession of Earth’s axis. Orbital

eccentricity variations have periods of ~100,000

years and 405,000 years, whereas the tilt varia-

tion has a period of 41,000 years, and the “cli-

matic precession” has a period of ~21,000 years,

and apart from the 405,000-year cycles, there

are multiple periods for each of these modes.

These in turn cause variations in solar radia-

tion reaching Earth’s surface, which result in

climatic variations, called Milankovitch cycles.

The climatic variations are recorded as cycli-

cally deposited sediments, which can be tuned

to the calculated Milankovitch cycles together

with stratigraphic correlation tools, principally

biostratigraphy and magnetostratigraphy.

This has developed into the use of “target

curves,” which provide numerical age calibra-

tion, with no reliance on radioisotopic geo-

chronol ogy, by matching local cyclostratigraphic

records with a highly constrained model of the

astronomical parameters or the climatic expres-

sion of those parameters such as variations in

solar insolation (the target curve). In the past

decade , dramatic improvements in understand-

ing solar system dynamics have resulted in the

extension of sophisticated models that predict

the climatic and sedimentologic response of

Earth to the astronomical parameters at least as

far back as 40–50 Ma (Laskar et al., 2011).

A similar approach for older rocks that can-

not be directly referenced to the late Cenozoic

allows the use of “ﬂ oating” segments of cycli-

cally deposited sedimentary rocks to estimate

the amount of time between two stratigraphic

horizons. There are many examples of spec-

tacular cycles preserved in both marine and

continental sedimentary rocks that are deﬁ ned

by, for example, color, geochemistry, rock mag-

netic properties, sedimentary structures, and

organic carbon content. The signal is measured

and assigned relative ages using magnetic po-

larity stratigraphy and biostratigraphy, and

then, if possible, calibrated in numerical time

with high-precision geochronologic informa-

tion on volcanic ash beds. These sections can

be used to correlate with other sections that

may not have cyclostratigraphy but include the

same biostratigraphy or magnetic polarity stra-

tigraphy. When combined with geochronology

of volcanic horizons, one can use the cycles

to interpolate time with higher precision than

with geochronology alone or linear interpola-

tion assuming constant sediment accumulation

rates. This has been used for sequences in the

Cretaceous (e.g., Meyers et al., 2012) and in

much older strata, such as those deposited near

the Triassic-Jurassic transition (e.g., Whiteside

et al., 2007) and in the Carboniferous (Davydov

et al., 2010).

The target curve and ﬂ oating curve calibration

approaches have been important for developing

an astronomical time scale in the construction of

the global geologic time scale back to 250 Ma

(Gradstein et al., 2012). The astronomical time

scale alone has a resolution of 0.02–0.40 m.y.

for cyclostratigraphy in Neogene and Quater-

nary sections, and at the low end, it is com-

para ble to radioisotopic dating but completely

independent. For older rocks, the astronomical

time scale can provide age models between

radio iso topi cally dated tie points allowing ﬁ ne-

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scale resolution in thick sequences lacking other

geochronologic proxies.

Parallel improvements in both 40Ar/39Ar and

U-Pb geochronology have made it possible to

pursue the intercalibration of radioisotopic dat-

ing and astrochronology (e.g., Kuiper et al.,

2008; Meyers et al., 2012). When successful,

this integration allows for unprecedented ac-

curacy and precision in the measurement of

geologic time. The basic approach is to use

high-precision geochronology of volcanic ash

beds with both U-Pb and 40Ar/39Ar techniques

to yield an estimate of the duration between

each dated deposit. Despite the complexities

involving systematic differences between U-Pb

and 40Ar/39Ar dates, the age difference between

each deposit should be the same but likely with

different uncertainties. Then, the ﬂ oating astro-

nomical model for the time interval can be

used for a much higher-resolution age model.

Ideally , bedded sedimentary rock sequences

with orbitally inﬂ uenced cycles occur nearby or

between the dated ash beds, so that one can be

sure the ﬂ oating time scale can be applied to a

particular interval. This is a rare occurrence but

allows an opportunity to directly intercalibrate

two independent radioisotopic chronometers

against an astrochronologic age model (Meyers

et al., 2012).

Another important application was reported

by Kuiper et al. (2008), who used 40Ar/39Ar

geochronology to determine the age of the Fish

Canyon sanidine by astronomically calibrating

high-precision dates on ash beds and arriving

at a value of 28.201 ± 0.046 Ma. Meyers et al.

(2012) directly tested this by comparing U-Pb

and 40Ar/39Ar dates from the same bentonites

and concluded that the pairs of dates, using

an assumed age for Fish Canyon sanidine of

28.201 Ma, were the same within uncertainty.

They inte grated the 40Ar/39Ar dates from three

bentonites and the orbital time scale with un-

certainties of thousands of years to conﬁ rm that

28.201 Ma is the best age estimate for Fish Can-

yon sanidine. Renne et al. (2010, 2011) arrived

at a slightly older age estimate (28.291 Ma ±

0.036) by comparing pairs of U-Pb zircon and

40Ar/39Ar dates from rocks from a wide range of

ages with the Fish Canyon sanidine age proposed

by Kuiper et al. (2008). In future intercalibration

studies, the sources of uncertainty associated

with the radioisotopic ages (analytical, tracer so-

lution, or age of neutron ﬂ uence standard, and

decay constants), as well as uncertainties in the

astro chronol ogy, must be considered.

A Geologic Time Scale 2012 (Gradstein et al.,

2012) uses “absolute” astronomical calibrations

based on solar system dynamics for the Neo-

gene, and “ﬂ oating” astronomical calibrations

tied to radioisotopically dated stratigraphic

points for the Paleogene and all three Meso-

zoic periods. The geologic time scale includes

absolute astronomical calibrations for the entire

Cenozoic Era and Cretaceous Period; ﬂ oating

astronomical calibrations make up 85% of the

Jurassic and 75% of the Triassic time scales.

The eccentricity of Earth’s orbit has a major

variation with a period of 405,000 years.

Shorter astronomical cycles with precession and

obliquity periods are used to calibrate Neogene

and younger rocks. However, because the solar

system exhibits chaotic diffusion, we can expect

that their periods will change with time beyond

50 m.y., and the short cycles cannot be used to

calibrate the older (>50–60 Ma) rock record. The

405,000-year eccentricity cycle remains more

stable through geologic time because it results

from interactions of Jupiter and Venus. Workers

are now using ﬂ oating cyclostratigraphic rec-

ords that contain what they interpret to be the

405,000-year cycle to calibrate the geologic time

scale dating back to at least the end of the Paleo-

zoic era (ca. 252.2 Ma) and beyond.

There is potential to use the ﬂ oating curve

approach to greatly increase the precision of

the geologic time scale. For example, Davydov

et al. (2010) used U-Pb geochronology in the

Donets Basin to conﬁ rm that individual high-

frequency Carboniferous cyclothems and bun-

dles of cyclothems into fourth-order sequences

are the eustatic response to orbital eccentricity

(100 and 405 ka) forcing. They produced a con-

tinuous age model for Pennsylvanian strata of

the basin during this key interval of Earth his-

tory characterized by a major ice age. Detailed

biostratigraphy will allow the export of this age

model to sections throughout Euramerica.

This approach has also been used in the re-

markable sedimentary sequence of dominantly

lacustrine facies of the Newark Basin, where in-

vestigation of the continuous sequence obtained

by a series of drill cores has allowed unprece-

dented resolution of a ﬂ oating astronomical time

scale for over 5800 m of composite stratigraphy

integrated with magnetic polarity stratig raphy

that includes 59 magnetozones and a sampling

density of about every 20 ka (Kent and Olsen,

1999; Olsen et al., 2011). Vertebrate biostratig-

raphy and geochronology of the section are

sparse, although several Central Atlantic mag-

matic province intrusions and lava ﬂ ows at ca.

201.4 Ma, near the end-Triassic extinction, are

used as an upper tie point. There is a pronounced

405 ka cyclicity, and it has been used for specu-

lations on the tempo and mode of dinosaurian

origin, diversiﬁ cation, and rise to ecological

dominance (Olsen et al., 2011).

A good example of combining magnetic

polarity information and geochronology and

testing the results against astrochronological

models was recently published by Tsukui and

Clyde (2012). In this paper, a highly resolved,

previously obtained geomagnetic polarity time

scale for the early-middle Eocene was combined

with 40Ar/39Ar sanidine age determinations on

23 discrete ash-fall tuffs and other chronostrati-

graphic information, including polarity data, to

support a revised paleomagnetic time scale for

the early to middle Eocene. The age calibration

model suggests an approximate 2 Ma duration

for the early Eocene climatic optimum, which

in turn coincides with the inferred age of the

Wasatchian-Bridgerian faunal transition. Com-

pared with a previous astrochronological model,

the revised time scale is consistent with an age

for the Paleocene-Eocene thermal maximum of

56.33 Ma. Ultimately, terrestrial sequences with

relatively high accumulation rates and high-

precision geochronology can better resolve the

age and duration of polarity chrons, and can

be compared with astrochronologic data from

more continuous marine sedimentary deposits.

In summary, the integration of high-preci-

sion, high-accuracy geochronology with reﬁ ned

astrochronological models and stable isotope,

chemostratigraphic, and magnetostratigraphic

observations will allow us to better understand

the role of climate and ocean-atmosphere inter-

actions on biological evolution.

OUTSTANDING ISSUES

Stratigraphic Challenges

The search for satisfactory horizons to use for

marking GSSPs in the Phanerozoic has been a

long protracted process, and many reﬁ nements

in strategy and technical methods have been

introduced since the time that the ﬁ rst GSSP,

which marked the base of the Devonian, was

ratiﬁ ed in 1972. With the appearance of new

information (particularly detailed chemostrati-

graphic data) and reevaluation of sections, some

early decisions as to speciﬁ c time boundaries

now seem less well justiﬁ ed, and may merit

reexamination. One example is the basal Cam-

brian GSSP. Because of problems associated

with global correlation of the Cambrian/Paleo-

zoic-Phanerozoic base using the biostratigraphy

of trace fossils, chemostratigraphy has emerged

as the de facto means of characterizing and cor-

relating that horizon globally (Bowring et al.,

2007; Babcock et al., 2011). The Ediacaran-

Cambrian boundary in Newfoundland is based

on trace fossil assemblages and has no body

fossils and has no rocks suitable for geochronol-

ogy or chemostratigraphy. It remains uncertain

whether the negative δ13C excursion used to ap-

proximate the boundary does in fact coincide

with the Cambrian GSSP.

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Walker et al.

270 Geological Society of America Bulletin, March/April 2013

Up to the present time, boundary positions in

the Archean and Proterozoic have mostly used

purely chronometric boundaries called global

standard stratigraphic ages (GSSAs), based on

geochronologic data, rather than points in sec-

tions or GSSPs. The one exception is the ter-

minal system/period of the Proterozoic, the

Ediacaran. Ultimately, it will be desirable, if pos-

sible, to replace GSSAs with GSSPs that have

well-calibrated numerical ages.

The recent deﬁ nition of the base of the Holo-

cene Series using signals of climatic change

(Walker et al., 2009), rather than biostrati-

graphic markers, represents an important turn-

ing point in philosophy about GSSP deﬁ nition.

Some nonbiostratigraphic proxies are superior

to biostratigraphic tools in certain parts of the

geologic column, and it is likely that nonbio-

stratigraphic correlation techniques will play an

increased role in the deﬁ nition of GSSPs in the

future, particularly those that will be deﬁ ned in

the pre-Phanerozoic.

Geochronology and Resolving

Discrepancies between U-Pb and

40Ar/39Ar Methods

Ultimately, the geoscience community will

succeed in building a time scale that can seam-

lessly use both high-precision 40Ar/39Ar data

and U-Pb geochronologic data. The concern

that we presently face, however, is one that has

been recognized for some time, and that is the

systematic bias between U-Pb and 40Ar/39Ar

dates, which in some cases approaches ~1%,

with U-Pb dates being consistently older. The

causes of this systematic bias are not clear

and likely include inaccuracies in the decay

constants for both systems, inaccuracies in

the age of ﬂ uence monitors used in 40Ar/39Ar

geo chronol ogy, an inaccurate assumed value

for the 238U/235U ratio in zircons (Hiess et al.,

2012), and the possibility that in some vol-

canic rocks the U-Pb system in zircon does not

record the time of emplacement but rather a

short period of pre-eruptive residence. Because

all 40Ar/39Ar approaches are relative dating

methods, the age of the ﬂ uence monitor must

be independently determined, and sanidine

from the Fish Canyon tuff is the most com-

monly used. Renne et al. (2010, 2011) inverted

data sets of U-Pb zircon and 40Ar/39Ar dates to

arrive at a more accurate estimate of the age

of Fish Canyon sanidine. This is a promising

approach as we acquire many additional high-

precision 40Ar/39Ar and U-Pb data sets on the

same samples from several different localities

representing more and more of geologic time.

The estimate of the assumed age of the Fish

Canyon sanidine has varied from ca. 27.7 Ma

to 28.29 Ma (Renne et al., 2010, 2011). Many

workers in the community are using 28.201 Ma

as the preferred age, as determined by astro-

nomical calibration (Kuiper et al., 2008),

as in many cases it appears to result in good

agreement between 40Ar/39Ar and U-Pb deter-

minations. The community is making great

progress, but more work remains to be done.

Our goal was to provide a synopsis of the

history of the Geological Society of America

Geologic Time Scale and the many different

facets of deep time investigations that have led

to reﬁ nements in geologic time scales. This

overview highlights the continued importance

of integrated efforts to better understand the

Earth-life record of deep time on a global scale.

A crucial aspect of progress in geochronology

is the use of highly characterized mineral stan-

dards that span a range of ages. EARTHTIME

is actively seeking out standard samples that

range in age from less than 1 Ma to well over

500 Ma that can be analyzed for both 40Ar/39Ar

and U-Pb (Schoene et al., 2005; Renne et al.,

2010). Mineral separates will be distributed to

participating laboratories, and, after analyses

are completed, the results will be published.

Ideally, when a U-Pb laboratory publishes a

study, they will also publish data for one or

more standards run during the same interval as

the study, as is currently done in many 40Ar/39Ar

laboratories.

The Geological Society of America Geologic

Time Scale and Its Future

Palmer’s seminal work on the Geological So-

ciety of America Geologic Time Scale (Palmer,

1983) was important for the development of time

scales in general. This model has been adopted

by the ICS in the presentation of their geologic

time scale (International Commission on Stra-

tigraphy, 2012). The 1983 Geological Society of

America Geologic Time Scale was constructed

with an emphasis on North American geology.

The Geological Society of America will con-

tinue to follow the ICS and other efforts such as

Gradstein et al. (2012) in updating the chrono-

strati graphic scale and integrating it with the

chronometric one, and the GSA will maintain

its presentation of the time scale as a service to

its membership, the larger scientiﬁ c community,

and the public to promote the scientiﬁ c and edu-

cational goals of the society.

ACKNOWLEDGMENTS

We thank A.R. (Pete) Palmer for extensive dis-

cussion and encouragement. James Ogg shared pre-

prints of parts of the Gradstein et al. (2012) book.

J.C. Creveling , Linda Hinnov, Brad Cramer, and Paul

Olsen are thanked for comments.

REFERENCES CITED

Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V.,

1980, Extraterrestrial cause for the Cretaceous-Tertiary

extinction: Science, v. 208, p. 1095–1108, doi:10.1126

/science.208.4448.1095.

Amthor, J.E., Grotzinger, J.P., Schröder, S., Bowring, S.A.,

Ramezani, J., Martin, M.W., and Matter, A., 2003, Ex-

tinction of Cloudina and Namacalathus at the Precam-

brian-Cambrian boundary in Oman: Geology, v. 31,

p. 431–434, doi:10.1130/0091-7613(2003)031<0431:

EOCANA>2.0.CO;2.

Babcock, L.E., Robison, R.A., and Peng, S.C., 2011, Cam-

brian stage and series nomenclature of Laurentia and

the developing global chronostratigraphic scale: Mu-

seum of Northern Arizona Bulletin, v. 67, p. 12–26.

Barrell, J., 1917, Rhythms and the measurement of geologic

time: Geological Society of America Bulletin, v. 28,

p. 748–749.

Berger, A., and Loutre, M.F., 1990, Origine des fréquences

des eléments astronomiques intervenant dans le calcul

de l’insolation: Bulletin de la Classe des Sciences,

Académie Royale Belgique, Ser. 6 1(1[3]), p. 45–106.

Berggren, W.A., Kent, D.V., Flynn, J.J., and Van Couver-

ing, J.A., 1985a, Cenozoic geochronology: Geologi-

cal Society of America Bulletin, v. 96, p. 1407–1418,

doi:10.1130/0016-7606(1985)96<1407:CG>2.0.CO;2.

Berggren, W.A., Kent, D.V., and Van Couvering, J.A., 1985b,

The Neogene: Part 2. Geochronology and chrono-

stratigraphy, in Snelling, N.J., ed., The Chronology of

the Geological Record: Geological Society of London

Memoir 10, p. 211–260.

Bice, K.L., Huber, B.T., and Norris, R.D., 2003, Extreme

polar warmth during the Cretaceous greenhouse? Para-

dox of the late Turonian δ18O record at Deep Sea Drill-

ing Project Site 511: Paleoceanography, v. 18, 1031,

doi:10.1029/2002PA000848.

Bowring, J.F., McLean, N.M., and Bowring, S.A., 2011,

Engineering cyber infrastructure for U-Pb geochro-

nology: Tripoli and U-Pb_Redux: Geochemistry Geo-

physics Geosystems, v. 12, Q0AA19, doi:10.1029

/2010GC003479.

Bowring, S.A., Grotzinger, J.P., Condon, D.J., Ramezani, J.,

Newall, M.J., and Allan, P.A., 2007, Geochronologic

constraints on the chronostratigraphic framework of the

Neoproterozoic HUQF Supergroup, Sultanate of Oman:

American Journal of Science, v. 307, p. 1097–1145

http://dx.DOI.org/10.2475/10.2007.01.

Brand, U., and Veizer, J., 1980, Chemical diagenesis of a

multicomponent carbonate system: 1. Trace elements:

Journal of Sedimentary Petrology, v. 50, p. 1219–1236.

Brunhes, B., 1906, Recherches sur le direction d’aimenta-

tion des roches volcaniques: Journal of Physics, v. 5,

p. 705–724.

Burke, W., Denison, R., Hetherington, E., Koepnik, R.,

Nelson, M., and Omo, J., 1982, Variations of sea water

87Sr/86Sr throughout Phanerozoic shales: Geology,

v. 10, p. 516–519, doi:10.1130/0091-7613(1982)10

<516:VOSSTP>2.0.CO;2.

Cande, S.C., and Kent, D.V., 1992, A new geomagnetic

polarity time scale for the Late Cretaceous and

Ceno zoic: Journal of Geophysical Research, v. 97,

p. 13,917-13,951.

Cande, S.C., and Kent, D.V., 1995, Revised calibration of

the geomagnetic polarity timescale for the Late Creta-

ceous and Cenozoic: Journal of Geophysical Research,

v. 100, p. 6093–6095, doi:10.1029/94JB03098.

Cao, C. Love, G.D., Hays, L.E., Wanga, W., Shen, S., and

Summons, R.E., 2009, Biogeochemical evidence for

euxinic oceans and ecological disturbance presaging

the end-Permian mass extinction event: Earth and Plan-

etary Science Letters, v. 281, p. 188–201, doi:10.1016

/j.epsl.2009.02.012.

Cohen, K.M., Finney, S., and Gibbard, P.L., 2012, Inter-

national Chronostratigraphic Chart: International

Commission on Stratigraphy, www.stratigraphy.org

(last accessed May 2012). (Chart reproduced for the

34th International Geological Congress, Brisbane,

Australia, 5–10 August 2012.)

Condon, D., Schoene, B., Bowring, S.A., Parrish, R.,

McLean, N., Noble, S., and Crowley, Q., 2007,

EARTHTIME: Isotopic tracers and optimized solutions

on August 30, 2015gsabulletin.gsapubs.orgDownloaded from

The Geological Society of America Geologic Time Scale

Geological Society of America Bulletin, March/April 2013 271

for high-precision U-Pb ID-TIMS geochronology: Eos

(Transactions, American Geophysical Union), v. 88,

no. 52, abs. V41E–06.

Corsetti, F.A., and Hagadorn, J.W., 2000, Precambrian-

Cambrian transition: Death Valley, United States:

Geology v. 28, p. 299–302, doi: 10.1130/0091-7613

(2000)28<299:PTDVUS>2.0.CO;2

Davydov, V.I., Crowley, J.L., Schmitz, M.D., and Poletaev,

V.I., 2010, High-precision U-Pb zircon age calibration

of the global Carboniferous time scale and Milanko-

vitch band cyclicity in the Donets Basin, eastern

Ukraine: Geochemistry Geophysics Geosystems, v. 11,

Q0AA04, doi:10.1029/2009GC002736.

Doell, R.R., and Dalrymple, G.B., 1966, Geomagnetic

polarity epochs: A new polarity event and the age of

the Brunhes Matuyama boundary: Science, v. 152,

p. 1060–1061, doi:10.1126/science.152.3725.1060.

Eardley, A.J., 1951, Structural Geology of North America:

New York, Harper and Brothers, 624 p.

Edmond, J.M., 1992, Himalayan tectonics, weathering

processes and the strontium isotope record in marine

limestones: Science, v. 258, p. 1594–1597, doi:10.1126

/science.258.5088.1594.

Emiliani, C., 1955, Pleistocene temperatures: The Journal of

Geology, v. 63, p. 538–578, doi:10.1086/626295.

Evans, M.E., and Heller, F., 2003, Environmental Magne-

tism: Principles and Applications of Enviromag-

netics: San Diego, California, Academic Press, 299 p.,

doi:10.1002/jqs.858.

Fike, D.A., Grotzinger, J.P., Pratt, L.M., and Summons, R.E.,

2006, Oxidation of the Ediacaran ocean: Nature, v. 444,

p. 744–747, doi:10.1038/nature05345.

Glen, W., 1982, The Road to Jaramillo: Stanford, California,

Stanford University Press, Stanford, 459 p.

Gradstein, F.M., and 38 others, 2004, A Geologic Time

Scale 2004: Cambridge, UK, Cambridge University

Press, 589 p.

Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M.,

editors, 2012, The Geologic Time Scale 2012, vol. 1:

Boston, Elsevier, 1144 p., http://dx.doi.org/10.1016

/B978-0-444-59425-9.01001-5.

Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A.C.,

and Rice, A.H., 2005, Towards a Neoproterozoic com-

posite carbon-isotope record: Geological Society of

America Bulletin, v. 117, p. 1181–1207, doi:10.1130

/B25630.1.

Halverson, G.P., Dudas, F.O., Maloof, A.C., and Bowring,

S.A., 2007, Evolution of the 87Sr/86Sr composition of

Neoproterozoic seawater: Palaeogeography, Palaeo-

climatology, Palaeoecology, v. 256, p. 103–129, doi:

10.1016/j.palaeo.2007.02.028.

Harland, W.B., Smith, A.G., and Wilcock, B., 1964, The

Phanerozoic Time-Scale—A Symposium Dedicated

to Professor Arthur Holmes: Quarterly Journal of the

Geological Society of London, v. 120S, 458 p.

Harland, W.B., Cox, A.V., Llewellyn, P.G., Picton, C.A.G.,

Smith, A.G., and Walters, R.W., 1982, A Geologic

Time Scale: Cambridge, UK, Cambridge University

Press, 131 p.

Harland, W.B., Cox, A.V., Llewellyn, P.G., Smith, A.G., and

Walters, R., 1990, A Geologic Time Scale 1989: Cam-

bridge, UK, Cambridge University Press.

Harris, N., 1995, Signiﬁ cance of weathering Himalayan

metasedimentary rocks and leucogranites for the Sr

isotope evolution of seawater during the early Mio-

cene: Geology, v. 23, p. 795–798, doi:10.1130/0091

-7613(1995)023<0795:SOWHMR>2.3.CO;2.

Hiess, J., Condon, D.J., McLean, N., and Noble, S.R., 2012,

238U/235U systematics in terrestrial uranium-bearing

minerals: Science, v. 335, p. 1610–1614, doi:10.1126

/science.1215507.

Hilgen, F.J., 2010, Astronomical tuning in the 19th century:

Earth-Science Reviews, v. 98, p. 65–80

Hinnov, L.A., and Hilgen F.J., 2012, Cyclostratigraphy

and astrochronology, in Gradstein, F.M., Ogg, J.G.,

Schmitz, M.D., and Ogg, G.M., eds., The Geologic

Time Scale 2012: vol. 1: Boston, Elsevier, p. 63–83,

doi:10.1016/B978-0-444-59425-9.00004-4.

Holmes, A., 1913, The Age of the Earth: London and New

York, Harper, 196 p.

Holmes, A., 1937, The Age of the Earth (3rd ed.): London,

Nelson, 263 p.

Holmes, A., 1962, ‘Absolute age’ a meaningless term:

Nature , v. 196, p. 1238, doi:10.1038/1961238b0.

Huber, B.T., Norris, R.D., and MacLeod, K.G., 2002, Deep

sea paleotemperature record of extreme warmth during

the Cretaceous: Geology, v. 30, p. 123–126, doi:10.1130

/0091-7613(2002)030<0123:DSPROE>2.0.CO;2.

Hutton, J., 1788, Theory of the Earth; or an investigation of

the laws observable in the composition, dissolution, and

restoration of land upon the Globe: Transactions of the

Royal Society of Edinburgh, v. 1, Part 2, p. 209–304.

International Commission on Stratigraphy, 2012, Inter-

national Chronostratigraphic Chart: www.stratigraphy

.org (last accessed May 2012).

Irving, E., 1964, Paleomagnetism and its Applications to

Geological and Geophysical Problems: New York,

John Wiley and Sons, 399 p.

Irving, E., and Parry, L.G., 1963, The magnetism of some

Permian rocks from New South Wales: Geophysical

Journal of the Royal Astronomical Society, v. 7, p. 395–

411, doi:10.1111/j.1365-246X.1963.tb07084.x.

Jacobsen, S.B., and Kaufman, A.J., 1999, The Sr, C, and O

isotopic evolution of Neoproterozoic seawater: Chemi-

cal Geology, v. 161, p. 37–57, doi:10.1016/S0009-2541

(99)00080-7.

Kennedy, M.J., Christie-Blick, N., and Sohl, L.E., 2001,

Are Proterozoic cap carbonates and isotopic excur-

sions a record of gas hydrate destabilization following

Earth’s coldest intervals?: Geology, v. 29, p. 443–446,

doi:10.1130/0091-7613(2001)029<0443:APCCAI

>2.0.CO;2.

Kent, D.V., and Gradstein, F.M., 1986, A Jurassic to Recent

chronology, in Vogt, P.R., and Tucholke, B.E., eds.,

The Western North Atlantic Region: Boulder, Colo-

rado, Geological Society of America, The Geology of

North America, v. M, p. 45–50.

Kent, D.V., and Olsen, P.E., 1999, Astronomically tuned geo-

magnetic polarity time scale for the Late Triassic: Jour-

nal of Geophysical Research, v. 104, p. 12,831–12,841.

Kent, D.V., Olsen, P.E., and Witte, W.K., 1995, Late Trias-

sic–Early Jurassic geomagnetic polarity and paleolati-

tudes from drill cores in the Newark rift basin (eastern

North America): Journal of Geophysical Research,

v. 100, p. 14,965–14,998, doi:10.1029/95JB01054.

King, P.B., 1959, The Evolution of North America: Prince-

ton, New Jersey, Princeton University Press, 189 p.

Knoll, A.H., and Walter, M.R., 1992, Latest Proterozoic stra-

tigraphy and Earth history: Nature, v. 356, p. 673–678,

doi:10.1038/356673a0.

Knoll, A.H., Walter, M.R., Narbonne, G.M., and Christie-

Blick, N., 2004, A new period for the geologic time

scale: Science, v. 305, p. 621–622, doi:10.1126/science

.1098803.

Knoll, A.H., Walter, M.R., Narbonne, G.M., and Christie-

Blick, N., 2006, The Ediacaran Period: A new addition

to the geologic time scale: Lethaia, v. 39, p. 13–30,

doi:10.1080/00241160500409223.

Kodama, K.P., Anastasio, D.J., Newton, M.L., Pares, J.M.,

and Hinnov, L.A., 2010, High-resolution rock magnetic

cyclostratigraphy in an Eocene ﬂ ysch, Spanish Pyre-

nees: Geochemistry Geophysics Geosystems, v. 11,

QOAA07, doi:10.1029/2011GC003069.

Korte, C., and Kozur, H., 2010, Carbon-isotope stratigraphy

across the Permian-Triassic boundary: A review: Journal

of Asian Earth Sciences, v. 39, p. 215–235, doi:10.1016

/j.jseaes.2010.01.005.

Kuiper, K.F., Deino, A., Hilgen, F.J., Krijgsman, W., Renne,

P.R., and Wijbrans, J.R., 2008, Synchronizing rock

clocks of Earth history: Science, v. 320, p. 500–504,

doi:10.1126/science.1154339.

Kulp, J.L., 1961, Geologic time scale: Science, v. 133,

p. 1105–1114, doi:10.1126/science.133.3459.1105.

Kump, L.R., and Arthur, M.A., 1999, Interpreting carbon-

isotope excursions: Carbonates and organic matter:

Chemical Geology, v. 161, p. 181–198, doi:10.1016

/S0009-2541(99)00086-8.

Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia,

A.C.M., and Levrard, B., 2004, A long-term numeri-

cal solution for the insolation quantities of the Earth:

Astronomy & Astrophysics, v. 428, p. 261–285, doi:

10.1051/0004-6361:20041335.

Laskar, J., Fienga, A., Gastineau, M., and Manche, H., 2011,

La2010: A new orbital solution for the long term mo-

tion of the Earth: Astronomy & Astrophysics, v. 532,

doi:10.1051/0004-6361/201116836.

Li, H.M., and Wang, J.D., 1989, Magnetostratigraphy

of Permo-Triassic boundary section of Meishan of

Changxing: Zhejiang, Scientia Sinica, Series B, v. 32,

p. 1401–1408.

Lucas, S.G., Tanner, L.H., Donohoo-Hurley, L., Geissman,

J.W., Kozur, H.W., Heckert, A.B., and Weems, R.E.,

2011, Position of the Triassic-Jurassic boundary and

timing of the end-Triassic extinctions on land: Data

from the Moenave Formation on the southern Colo-

rado Plateau, USA: Palaeogeography, Palaeoclimatol-

ogy, Palaeoecology, v. 302, p. 194–205, doi:10.1016

/j.palaeo.2011.01.009.

Maloof, A.C., Porter, S.M., Moore, J.L., Dudás, F.Ö.,

Bowring , S.A., Higgins, J.A., Fike, D.A., and Eddy,

M.P. 2010, The earliest Cambrian record of animals

and ocean geochemical change: Geological Society of

America Bulletin, v. 122, p. 1731–1774, doi:10.1130

/B30346.1.

Mattinson, J.M., 2005, Zircon U-Pb chemical abrasion

(“CA-TIMS”) method: Combined annealing and multi-

step partial dissolution analysis for improved preci-

sion and accuracy of zircon ages: Chemical Geology,

v. 220, p. 47–66, doi:10.1016/j.chemgeo.2005.03.011.

Matuyama, M., 1929, On the direction of magnetization of

basalts in Japan, Tyosen, and Manchuria: Proceedings

of the Imperial Academy (Tokyo), v. 5, p. 203–205.

McArthur, J.M., 1994, Recent trends in strontium isotope

stratigraphy: Terra Nova, v. 6, p. 331–358, doi:10.1111

/j.1365-3121.1994.tb00507.x.

McArthur, J.M., 1998, Strontium isotope stratigraphy, in

Doyle, P., and Bennett, M.R., eds., Unlocking the

Stratigraphical Record: Chichester, UK, John Wiley

and Sons, p. 221–241.

McArthur, J.M., and Howarth, R.J., 2004, Strontium isotope

stratigraphy, in Gradstein, F., Ogg, J., and Smith, A.,

eds., A Geological Time Scale 2004: Cambridge, UK,

Cambridge University Press, p. 96–105.

McArthur, J.M., Howarth, R.J., and Bailey, T.R., 2001,

Strontium isotope stratigraphy: LOWESS Version 3:

Best ﬁ t to the marine Sr-isotope curve for 0–509 Ma

and accompanying look-up table for deriving numeri-

cal age: The Journal of Geology, v. 109, p. 155–170,

doi:10.1086/319243.

McLean, N.M., Bowring, J.F., and Bowring, S.A., 2011,

An algorithm for U-Pb isotope dilution data reduc-

tion and uncertainty propagation: Geochemistry Geo-

physics Geosystems, v. 12, Q0AA18, doi:10.1029

/2010GC003478.

Meyers, S., Siewert, S.E., Singer, B.S., Sageman, B.B.,

Condon, D.J., Obradovich, J.D., Jicha, B.R., and

Sawyer, D.A., 2012, Intercalibration of radioisotopic

and astrochronologic time scales for the Cenomanian-

Turonian boundary interval, Western Interior Basin,

USA: Geology, v. 40, p. 7–10, doi:10.1130/G32261.1.

Norris, R.D., and Wilson, P.A., 1998, Low-latitude sea-

surface temperatures for the mid-Cretaceous and the

evolution of planktonic foraminifera: Geology, v. 26,

p. 823–826, doi:10.1130/0091-7613(1998)026<0823:

LLSSTF>2.3.CO;2.

Odin, G.S., 1982, Numerical Dating in Stratigraphy:

Chichester , UK, John Wiley, 1094 p.

Ogg, J.G., Ogg, G., and Gradstein, F.M., 2008, The Con-

cise Geologic Time Scale: Cambridge, UK, Cambridge

University Press, 177 p.

Olsen, P.E., Kent, D.V., Cornet, B., Witte, W.K., and Schlische,

R.W., 1996a, High-resolution stratigraphy of the Newark

Rift Basin (early Mesozoic, eastern North America):

Geological Society of America Bulletin, v. 108,

p. 40–77, doi:10.1130/0016-7606(1996)108<0040:

HRSOTN>2.3.CO;2.

Olsen, P.E., Schlische, R.W., and Fedosh, M.S., 1996b,

580 ky duration of the Early Jurassic ﬂ ood basalt event

in eastern North America estimated using Milankovitch

cyclostratigraphy, in Morales, M., ed., The Continen-

tal Jurassic: Museum of Northern Arizona Bulletin 60,

p. 11–20.

Olsen, P.E., Kent, D.V., and Whiteside, J.H., 2011, Implica-

tions of the Newark Supergroup-based astrochronology

and geomagnetic polarity timescale (Newark-APTS)

for the tempo and mode of the early diversiﬁ cation

on August 30, 2015gsabulletin.gsapubs.orgDownloaded from

Walker et al.

272 Geological Society of America Bulletin, March/April 2013

of the Dinosauria: Earth and Environmental Science

Transactions of the Royal Society of Edinburg, v. 101,

p. 1–33, doi:10.1017/S1755691011020032.

Opdyke, N., and Channell, J.E.T., 1996, Magnetic Stratigra-

phy: San Diego, Academic Press, 346 p.

Opdyke, N., Roberts, J., Claoue-Long, J., Irving, E., and

Jones, P., 2000, Base of the Kiaman: Its deﬁ nition and

global stratigraphic signiﬁ cance: Geological Society of

America Bulletin, v. 112, p. 1315–1341, doi:10.1130

/0016-7606(2000)112<1315:BOTKID>2.0.CO;2.

Oreskes, N., ed., 2001, Plate Tectonics: An Insider’s History

of the Modern Theory of the Earth: Boulder, Colorado,

Westview Press, 424 p.

Palmer, A.R., 1983, The Decade of North American Geol-

ogy 1983 Geologic Time Scale: Geology, v. 11, p. 503–

504, doi:10.1130/0091-7613(1983)11<503:TDONAG

>2.0.CO;2.

Palmer, A.R., and Geissman, J.W., 1999, 1999 Geologic

Time Scale: Boulder, Colorado, Geological Society of

America, 1 p.

Palmer, M.R., and Elderﬁ eld, H., 1985, Strontium isotope

composition of sea water over the past 75 Myr: Nature,

v. 314, p. 526–528, doi:10.1038/314526a0.

Pavlov, V.E., and Gallet, Y., 2005, A third superchron during

the Early Paleozoic: Episodes, v. 28, no. 2, p. 1–7.

Pavlov,V.E., Veselovskiy, R.V., Shatsillo, A.V., and Gallet ,

Y., 2012, Magnetostratigraphy of the Ordovician

Angara/Rozhkova River Section: Further Evidence for

the Moyero Reversed Superchron: Izvestiya, Physics

of the Solid Earth, v. 48, p. 297–305.

Paytan, A., Kastner, M., Martin, E.E., Macdougall, J.D., and

Herbert, T., 1993, Marine barite as a monitor of sea-

water strontium isotope composition: Nature, v. 366,

p. 445–449, doi:10.1038/366445a0.

Raymo, M.E., and Ruddiman, W.F., 1992, Tectonic forcing

of late Cenozoic climate: Nature, v. 359, p. 117–122,

doi:10.1038/359117a0.

Renne, P.R., Mundil, R., Balco, G., Min, K., and Ludwig,

K.R., 2010, Joint determination of 40K decay constants

and 40Ar*/40K for the Fish Canyon sanidine standard,

and improved accuracy for 40Ar/39Ar geochronology:

Geochimica et Cosmochimica Acta, v. 74, no. 18,

p. 5349–5367, doi:10.1016/j.gca.2010.06.017.

Renne, P.R., Balco, G., Ludwig, K., Mundil, R., and Min,

K., 2011, Response to the comment by W.H. Schwarz

et al. on “Joint determination of 40K decay constants

and40Ar*/40K for Fish Canyon sanidine standard, and

improved accuracy for 40Ar/39Ar geochronology”

by P.R. Renne et al. (2010): Geochimica et Cosmo-

chimica Acta, v. 75, p. 5097–5100, doi:10.1016/j.gca

.2011.06.021.

Rothman, D.H., Hayes, J.M., and Summons, R.E., 2003,

Dynamics of the Neoproterozoic carbon cycle: Pro-

ceedings of the National Academy of Sciences of

the United States of America, v. 100, p. 124–129,

doi:10.1073/pnas.0832439100.

Saltzman, M.R., 2005, Phosphorus, nitrogen, and the redox

evolution of the Paleozoic oceans: Geology, v. 33,

p. 573–576, doi:10.1130/G21535.1.

Salvador, A., ed., 1994, International Stratigraphic Guide

(2nd ed.): Trondheim, Norway, and Boulder, Colorado,

USA, International Union of Geological Sciences and

the Geological Society of America, 214 p.

Schoene, B., Crowley, J.L., Condon, J.D., Schmitz, M.D.,

and Bowring, S.A., 2005, Reassessing the uranium

decay constants for geochronology using ID-TIMS

U-Pb data: Geochimica et Cosmochimica Acta, v. 70,

p. 426–445, doi:10.1016/j.gca.2005.09.007.

Scholger, R., Mauritsch, H.J., and Brandner, R., 2000, Permian-

Triassic boundary magnetostratigraphy from the southern

Alps (Italy): Earth and Planetary Science Letters, v. 176,

p. 495–508, doi:10.1016/S0012-821X(00)00026-1.

Shen, S., and 21 others, 2011, Calibrating the end-Permian

extinction: Science, v. 334, p. 1367–1372, doi:10.1126

/science.1213454.

Shields, G., and Veizer, J., 2002, Precambrian marine carbon-

ate isotope database: Version 1.1: Geochemistry Geo-

physics Geosystems, v. 3, doi:10.1029/2001GC000266.

Soddy, F., 1913, Radioactivity: Chemical Society Annual

Report on the Progress of Chemistry, v. 10, p. 262–288,

doi:10.1039/ar9131000262.

Steiger, R.H., and Jäger, E., 1977, Subcommission on geo-

chronology: Convention on the use of decay constants

in geo- and cosmochronology: Earth and Planetary Sci-

ence Letters, v. 1, p. 369–371.

Szurlies, M., 2013, Late Permian (Zechstein) magneto-

stratigraphy in Western and Central Europe, in Gas-

iewicz, A., Roscher, M., and Slowakiewicz, M., eds.,

Late Palaeozoic Climate Cycles: Their Evolutionary,

Sedimentological and Economic Impact: Geological

Society of London Special Publication (in press).

Szurlies, M., Bachmann, G.H., Menning, M., Nowaczyk,

N.R., and Kading, K.C., 2003, Magnetostratigraphy

and high-resolution lithostratigraphy of the Permian-

Triassic boundary interval in central Germany: Earth

and Planetary Science Letters, v. 212, p. 263–278,

doi:10.1016/S0012-821X(03)00288-7.

Szurlies, M., Geluk, M.C., Krijgsman, W., and Kurschner,

W.M., 2012, The continental Permian-Triassic bound-

ary in the Netherlands: Implications for the geomagnetic

time scale: Earth and Planetary Science Letters, v. 317–

318, p. 165–176, doi:10.1016/j.epsl.2011.11.043.

Thomson, J.J., 1913, Rays of positive electricity: Proceed-

ings of the Royal Society of London, ser. A, v. 89,

p. 1–20, doi:10.1098/rspa.1913.0057.

Thomson, W., 1865, The doctrine of uniformity in geology

brieﬂ y refuted: Proceedings of the Royal Society of

Edinburgh, v. 5, p. 512–513.

Torreson, O.W., Murphy, T., and Graham, J.W., 1949, Mag-

netic polarization of sedimentary rocks and the Earth’s

magnetic history: Journal of Geophysical Research,

v. 54, p. 111–129, doi:10.1029/JZ054i002p00111.

Tsukui, K., and Clyde, W.C., 2012, Fine-tuning the calibra-

tion of the early to middle Eocene geomagnetic polar-

ity time scale: Paleomagnetism of radioisotopically

dated tuffs from Laramide foreland basins: Geologi-

cal Society of America Bulletin, v. 124, p. 870–885,

doi:10.1130/B30545.1.

Veizer, J., 1989, Strontium isotopes in seawater through

time: Annual Review of Earth and Planetary Sci-

ence Letters, v. 17, p. 141–167, doi:10.1146/annurev

.ea.17.050189.001041.

Veizer, J., Ala, D., Azmy, K., Bruckschen, P., Bruhn, F.,

Buhl, D., Carden, G., Diener, A., Ebneth, S., Goddris,

Y., Jasper , T., Korte, C., Pawellek, F., Podlaha, O.G.,

Strauss, H., 1999, 87Sr/86Sr, δ13C and δ18O evolution

of Phanerozoic seawater: Chemical Geology, v. 161,

p. 59–88, doi:10.1016/S0009-2541(99)00081-9.

Vine, F.J., and Matthews, D.H., 1963, Magnetic anomalies

over oceanic ridges: Nature, v. 199, p. 947–949, doi:

10.1038/199947a0.

Walker, J.D., and Geissman, J.W, 2009, 2009 GSA Geologic

Time Scale: GSA Today, v. 19, no. 4–5, p. 60–61.

Walker, M., and 17 others, 2009, Formal deﬁ nition and dat-

ing of the GSSP (global stratotype section and point)

for the base of the Holocene using the Greenland

NGRIP ice core, and selected auxiliary records: Jour-

nal of Quaternary Science, v. 24, p. 3–17, doi:10.1002

/jqs.1227.

Whiteside, J.H., Olsen, P.E., Kent, D.V., Fowell, S.J., and

Et-Touhami , M., 2007, Synchrony between the CAMP

and the Triassic–Jurassic mass-extinction event?: Palaeo-

geography, Palaeoclimatology, Palaeoecology, v. 244,

p. 345–367, doi:10.1016/j.palaeo.2006.06.035.

Wilson, P.A., Norris, R.D., and Cooper, M.J., 2002, Testing the

Cretaceous greenhouse hypothesis using glassy forami-

niferal calcite from the core of the Turonian tropics on

Demerara Rise: Geology, v. 30, p. 607–610, doi:10.1130

/0091-7613(2002)030<0607:TTCGHU>2.0.CO;2.

Zachos, J., Pagani, M., Sloan, L., Thomas, E., and Billups,

K., 2001, Trends, rhythms, and aberrations in global

climate 65 Ma to Present: Science, v. 292, p. 686, doi:

10.1126/science.1059412.

Zhu, M.Y., Babcock, L.E., and Peng, S.C., 2006, Advances

in Cambrian stratigraphy and paleontology: Integrat-

ing correlation techniques, paleobiology, taphonomy

and paleoenvironmental reconstruction: Palaeoworld,

v. 15, p. 217–222, doi: 10.1016/j.palwor.2006.10.016.

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