Quick viewing(Text Mode)

The Geologic Time Scale

The Geologic Time Scale

The geologic scale

Felix M. Gradsteina,* and James G. Oggb being convenient multiples to organize our daily life and aMuseum of Natural , University of Oslo, N-0318 Oslo, productivity. 7 e day carries the record of light and dark, Norway; bDepartment of Earth and Atmospheric Sciences, Purdue the month the regularly returning shapes of the , University, West Lafayette, IN 47907, USA and the the cycle of the seasons and the apparent *To whom correspondence should be addressed (felix.gradstein@ path of the sun. All is clear, and we have grown up with nhm.uio.no) the notion that time is a vector, pointing from the pre- sent to the future. Events along its path mark the arrow Abstract of time, and the arrow is graded either in relative “nat- ural” units, or in units of duration—the standard second Construction and assembly of the and its multiples, like hours and , and millions of involves: (a) constructing a relative (chronostratigraphic) years. standard scale for key periods in the Earth’s rock record; (b) identifying high-resolution linear dates to calibrate this relative scale in linear time; (c) astronomically tuning Geologic time and the sediment record intervals with cyclic sediments or stable isotope sequences A majority of geologists consider time as a vector point- which have suffi cient or geomagnetic ties to be merged ing from the distant past to the present. Instead of “dis- in the standard scale, and increase its resolution; (d) inter- tant past,” the term “” has been coined in the polating for those relative time intervals where direct linear vernacular. What is exactly the concept of geologic time, age information is insuffi cient; and (e) estimating error bars what are its natural units, how are they deA ned, and how on the age of boundaries and on unit durations. do we use these units properly? A good understanding of geologic time is vital for every scientist who deals with Time is an indispensable tool for all of us. 7 e time kept events in the Earth sediment and rock record, or with by innumerable watches and a great variety of clocks the genetic record of evolution in living organisms, espe- regulates our everyday life, while the familiar cially those who strive to understand past processes and governs our weekly, monthly, and yearly doings. 7 ese determine rates of change. 7 is understanding takes eventually condense into the historical record of the place in a framework called Earth Geological History, a events over centuries. 7 e standard unit of modern time super calendar of local and global events. 7 e challenge keeping is the second, deA ned by a precise number of to this understanding is reading, organizing, and sort- vibrations of the cesium atomic clock. 7 e atomic second ing the Earth’s stone calendar pages. In the process, we is deA ned as the duration of 9.192.631.770 periods of the oJ en have to reconstruct the content of missing pages. radiation corresponding to the transition between two Correlation of the rock record between regions is a vital hyperA ne levels of the ground state of the Cesium 133 part of the reconstruction process. atom. 7 is value was established to agree as closely as One of the earliest reconstructions is by Nicolas Steno possible with the ephemeris second based on the Earth’s (1631–1687) who made careful and original stratigraphic motion. 7 e advantage of having the atomic second as observations. Based on these observations, Steno con- the unit of time in the International of Units is the cluded that the Earth’s strata contain the superimposed relative ease, in theory, for anyone to build and calibrate records of a chronological sequence of events that can be an atomic clock with a precision of 1 part per 1011 (or bet- correlated worldwide. Geological correlation formally is ter). In practice, clocks are calibrated against broadcast expressed in terms of A ve consecutive operations (each is time signals, with frequency oscillations in Hertz being followed by one or more examples): the “pendulum” of the atomic time-keeping device. 7 e tick of the second paces the quick heart beat, and (a) Rock units, like formations or well log intervals = traditionally was the 60th part of the 60th part of the lithostratigraphic correlation 24th part of the 24-h day, with the minute and the hour Kimmeridge Clay Formation of England

F. M. Gradstein and J. G. Ogg. 7 e geologic time scale. Pp. 26–34 in e Timetree of Life, S. B. Hedges and S. Kumar, Eds. (Oxford University Press, 2009).

HHedges.indbedges.indb 2626 11/28/2009/28/2009 11:24:44:24:44 PPMM The Geologic Time Scale 27

(b) Fossil units, like zones = biostratigraphic underlying and older than the typical rocks of the correlation next higher stage. 7 is is the concept of deA ning stage Turrilina alsatica benthic foraminifer zone units with type sections, commonly referred to as strato- (c) Relative time units = geochronologic (“Earth time”) type sections. 7 e principles and building blocks of this correlations were slowly established during cen- Period, , Age, turies of study in many discontinuous and incomplete polarity chron C29r outcrop sections. Inevitably, lateral changes in (d) Rocks deposited during these time units = between regions and lack of agreement on criteria, par- chronostratigraphic (time–rock) correlation ticularly in which were characteristics of a rela- Jurassic System, Eocene , Oxfordian Stage, tive unit of rock, have always resulted in a considerable polarity zone C29r amount of confusion and disagreement on stage nomen- (e) Linear time units or ages = geochronologic clature and stage use. Almost invariably classical stage correlation stratotypes turned out to only represent part of stages. 150 million years ago (Ma), 10,000 years ago (ka) Hence, a suite of global subdivisions with precise correl- ation horizons was required. Without correlation to a global reference scale, succes- sions of strata or events in time derived in one area are unique and contribute nothing to the understanding of Global section and point Earth history elsewhere. 7 e rules of hierarchy in geo- Now, relatively rapid progress is being made with deA n- logical correlation, from rocks and fossils to relative and ition of Global Stratotype Sections and Points (GSSPs) linear time, are carefully laid down in the International to A x the lower boundary of all geologic stages, using Stratigraphic Guide. An abbreviated copy of this “rule discrete fossil and physical events that correlate well in book” with further references may be found on the Web the rock record. For the ladder of chronostratigraphy, site of the International Commission on Stratigraphy this GSSP concept switches the emphasis from marking (ICS) under www.stratigraphy.org. the spaces between steps (stage stratotypes) to A xing the Before we deal with linear geological time, a few words rungs (boundaries of stages). are necessary about the common geological calendar Each progressive pair of GSSPs in the rock record also built from relative age units. 7 is chronostratigraphic precisely deA nes the associated subdivision of geologic scheme is not unlike a historical calendar in which soci- time. It is now 25 years ago that a “golden spike” struck etal periods, for example, the Minoan Period, the reign the A rst GSSP. 7 is event of historic proportions for of Louis XIV, the American Civil War, are used as build- the geologic time scale involved the boundary between ing blocks, devoid of a linear scale. Archeological relics the and Periods, or rather the lower deposited during these intervals (e.g., the Palace of Minos limit of the Devonian, at a locality called Klonk in on Crete, Versailles or spent cannon balls at Gettysburg, Czechoslovakia. respectively) comprise the associated physical and 7 e problem of the Silurian–Devonian boundary and chronostratigraphic record. A chronostratigraphic scale its consensus settlement in the Klonk section hinged on a is assembled from rock sequences stacked and seg- century-old debate known as the “Hercynian Question” mented in relative units based on their unique fossil and that touched many outstanding geoscientists of the nine- physical content. When unique local fossil and physical teenth century. 7 e issue came to the forefront aJ er 1877, records are matched with those of other rock sequences when Kaiser stated that the youngest stages (étages) of across the globe—in a process known as stratigraphic Barrande’s “Silurian System” in Bohemia correspond to correlation—a relative scale can be assembled that, when the Devonian System in the Harz Mountains of Germany calibrated to stage type sections, becomes a chronostrati- and other regions. Kaiser’s A ndings contrasted with the graphic scale. 7 e standard chronostratigraphic scale, in conventional nineteenth century wisdom that graptolite downloadable graphics format, is available from the ICS fossils became extinct at the end of the Silurian. Even- Web site. 7 is time scale is made of successive stages in tually, it became clear that so-called Silurian graptolites the rock record, like , , , in some sections occur together with so-called Devonian and so on, within the system. fossils in other sections, leading to the modern consen- Originally, each stage unit was a well-deA ned body sus that graptolites are not limited to Silurian strata. of rocks at a speciA c location of an assigned and agreed A bronze plaque in the Klonk outcrop shows the exact upon relative age span, younger than typical rocks of the position of the modern Silurian–Devonian Boundary,

HHedges.indbedges.indb 2727 11/28/2009/28/2009 11:24:47:24:47 PPMM 28 THE TIMETREE OF LIFE

which is taken at the base of the Stage, the At present over 55 GSSPs have been deA ned (Fig. 1; lowest stage in the Devonian. 7 e base of the Lochkovian see www.stratigraphy.org for details), but there are more Stage is deA ned by the A rst occurrence of the Devonian stages in the Eon in need of base deA nition. graptolite Monograptus uniformis in bed #20 of the Fortunately, a majority of those now have target deA ni- Klonk Section, northeast of the village of Suchomasty. tions, and are awaiting consensus on the best outcrop or 7 e lower Lochkovian index trilobites with representa- borehole section to place a “golden spike.” 7 us, with the tives of the Warburgella rugulosa group occur in the next deA nitions in place, we can proceed to scale the “deep younger limestone bed #21 of that section. time” stage units linearly. 7 e concept of the GSSP has gained acceptance among 7 is brings us to , referring to the geo- those stratigraphers who consider it a pragmatic and chronologic calendar of Earth events called the Geologic practical solution to the common problem that conven- Time Scale. While the chronostratigraphic scale is a tional stage type sections inevitably leave gaps, or lead convention to be agreed upon rather than discovered, to overlap between successive stages. 7 e boundary stra- calibration of the scale in seconds and (mega-) years totype very much relies on the notion that it is possible is a matter for discovery and estimation rather than to arrive at accuracy in correlation through the use of agreement. Like human time, linear geological time is events, like a , a global change in a expressed in units of standard duration—the second and stable isotope value, or the evolutionary appearance of hence (thousands or millions of) years. one or more prominent and widespread fossil taxa. 7 us, the limits of a stage can now be deA ned with multiple event criteria that to the best of our current knowledge Building a geological time scale are synchronous over the world. Delimiting successive 7 e ideal time scale is built from accurate radiometric stages in a clear and practical manner enhances their ages, taken precisely at stage boundaries throughout the value as standard units in chronostratigraphy and ulti- stratigraphic column in the Phanerozoic Eon. For more mately in geochronology. Without standardized units detailed resolution, the exact number of orbitally tuned neither the (relative) stratigraphic scale nor the (linear) sedimentary cycles is counted within each stage, such time scale can exist. that calibrations and correlation may be achieved within

seafloor orbital tuning direct dating detailed direct proportional scaled compositecubic spline fitting & Ma spreading dating subzone scaling standard max. likelihood 0 error estimation

90 Cretaceous

180 Jurassic 270

Carboniferous 360 Devonian Silurian 450

Cambrian 540 Methods used to construct Geologic Time Scale 2004 (GTS 2004) Fig. 1 Methods used to construct Geologic Time Scale 2004 (GTS2004) (1).

HHedges.indbedges.indb 2828 11/28/2009/28/2009 11:24:47:24:47 PPMM The Geologic Time Scale 29

20 thousand years for the last 540 million years or so . . . . pelagic sediments. 7 e present trend for the pre- If this sounds too good to be true, let it rest. Back to is to incorporate radiometric dates that have very small reality. analytical and stratigraphic uncertainties, and pass the Geologic reality is schematically illustrated in Fig. 1, most stringent tests. providing a quick overview of the actual method- 7 e fourth step, interpolating the stratigraphic and ology applied to construct Geologic Time Scale 2004 radiometric information, has much evolved. An early (GTS2004) (1), the most recent standard time scale. method already constructed the basic two-way graph, Before the , the A rst period of the Phanerozoic used until now. It plotted the cumulative sum of max- , the geologic time scale is less sophisticated, and imum global thickness of strata per based only on sparse radiometric dates. 7 e steps along the vertical axis and selected radiometric dates involved in Phanerozoic time scale construction may be from volcanic tuB s and other suitable layers along the summarized as follows: horizontal linear axis. 7 is best A t line method interpo- lated ages to the stages, but is a far cry from methods used (a) Construct a relative (chronostratigraphic) standard today that scale stages along the vertical axis with com- scale for the key periods in the Earth’s rock posite standards of fossil zones. In the mid-1990s, Frits record Agterberg and Felix Gradstein started to apply math- (b) Identify high-resolution linear age dates to calibrate ematical/statistical error analysis to the time scale ages, this relative scale in linear time which, for the A rst time, allowed them to assign fairly (c) Astronomically tune (see later) intervals with cyclic realistic error bars to ages of stage boundaries, sediments or stable isotope sequences which have a trend that persists today for the whole of Phanerozoic su1 cient fossil or geomagnetic correlation ties to below the Neogene. be merged in the standard scale, and increase its 7 e following is a simpliA ed introduction to the mod- resolution ern building tools depicted in Fig. 1. (d) Interpolate for those relative time intervals where direct linear age information is insu1 cient (e) Estimate error bars on the age of boundaries and on Music of the spheres unit durations. Let us start with a brief outline of the principle of the 7 e A rst step, integrating multiple types of stratigraphic sedimentary cycles approach to time scale building, as is information to construct the standard chronostrati- now standard for the last 23 Ma (Neogene), and provides graphic scale, is the most time consuming; it summarizes superior resolution and precision. Gravitational interac- and synthesizes centuries of detailed geological research tions of the Earth with the Sun, Moon, and other plan- and tries to understand all relative correlations and cali- ets cause systematic changes in the Earth’s orbital and bration to the standard. rotational system. 7 ese interactions give rise to cyclic 7 e second and third steps, identifying which radio- oscillations in the eccentricity of the Earth’s , and metric and cycle-stratigraphic studies to use as the pri- in the tilt and precession of the Earth’s axis, with mean mary constraints for assigning linear ages, are the ones dominant periods of 100,000, 41,000, and 21,000 years, that have much evolved. Historically, Phanerozoic time respectively. 7 e associated cyclic variations in annual scale building went from an exercise with very few and and seasonal solar radiation onto diB erent latitudes alter relatively inaccurate radiometric dates, as available to long-term climate in colder vs. warmer and wetter vs. the pioneer of the geologic time scale Arthur Holmes, dryer periods that lead to easily recognizable sediment- to one with many dates with greatly varying analytical ary cycles, such as regular interbeds of limy and shaly precision, as in the mid-1980s. Next, time scale studies facies. Massive outcrops of hundreds or thousands of started to appear of selected intervals, like , such cycles are observed in numerous geological basins, , or Ordovician, that selected a small for example around the Mediterranean, and in sediment suite of radiometric dates with high analytical precision cores from ocean-drilling sites. and relatively precise stratigraphic position. Counting of this centimeter to meter thick cycles in At the same time, a high-resolution Neogene time great detail over land outcrops and in ocean-drilling scale started to take shape, using orbital tuning of long wells, combined with the additional correlation aids sequences of sedimentary and/or isotope cycles provided by , oxygen isotope stra- in the Mediterranean region and in and PaciA c tigraphy, and , produced a very detailed

HHedges.indbedges.indb 2929 11/28/2009/28/2009 11:24:48:24:48 PPMM 30 THE TIMETREE OF LIFE

Neogene cycle pattern. 7 e critical step is the direct action. Because of uncertainty in the relation of the linkage of each cycle to the theoretical computed astro- intrusion to the host sediment, such dates may be of nomical scale of the 21,000, 41,000, and 100,000-year limited stratigraphic use. paleoclimatic cycles. 7 is astronomical tuning of the (b) Dating of volcanic P ows and tuB s as part of the geological cycle record from the Mediterranean and stratiA ed sedimentary succession. Atlantic by earth scientists at Utrecht and Cambridge (c) Dating of authigenic sedimentary minerals, mainly Universities such as Luc Lourens, Frits Hilgen, and Nick involving glauconite, found widespread in many Shackleton led to unprecedented accuracy and resolution marine sediments. Mild heating or overburden for the last 23 million years (2). In New Zealand, Tim pressure aJ er burial may lead to loss of argon, the Naish and colleagues have calibrated the upper Neogene daughter product measured in the 40K/40Ar clock record to the standard Neogene time scale. Using the in glauconite. Another problem is that glauconite high-resolution land-based cycle, isotope and magnetic also contains an abundance of tiny P akes that allow record in the Wanganui Basin, these authors thereby diB usion of Ar at low temperatures. 7 e result is transferred precise absolute ages to local shallow marine that glauconite dates may be too young. Because sediments and demonstrated the link between sequence of such problems which may be di1 cult to detect, and cycle stratigraphy. modern geologic time scales avoid dates based on EB orts are underway to extend the continuous astro- glauconite. chronologic scale back into and Eocene by Calibration of the decay constants or measurement applying a combination of cycle stratigraphy, improved standards can be enhanced by intercalibration to other astronomical projections, oxygen isotope stratigraphy, radiometric methods, or by dating rocks of a known and magnetostratigraphy to the deep sea record. age, for example a volcanic ash within an astronomically A special application of orbitally tuned cyclic sediment tuned succession. Astrochronologic and interlaboratory sequences is to “rubber-band” stratigraphically P oating recalibration of the 40Ar/39Ar monitor standard indi- units, like parts of , , and parts of Lower cates that many of the 40Ar/39Ar ages used in previous Jurassic, skilfully executed by specialists like Ursula Phanerozoic time scales are too young by about 0.5% to Rohl, Tim Herbert, and Graham Weedon. A quantitative 1.0%. For example, the 65.0 Ma age, assigned 10 years estimation of the duration of all cycles within a strati- ago to the top Cretaceous, is now 65.5 Ma. graphic unit allows estimates of their duration. techniques with less than 1% analytical error are providing suites of high-precision Decay of atoms U/Pb and Ar/Ar dates for the and Mesozoic. Surprisingly, perhaps, there are only seven direct age For rocks older than Neogene, the derivation of a numer- dates on period or stage boundaries (Fig. 1), with a major- ical time scale depends on the availability of suitable ity of the 200+ radiometric age dates used for GTS2004 radiometric ages. Radiometric dating generally involves “P oating” at some level within a stage. measuring the ratio of the original element in a mineral, 7 e integration of this level of chronometric precision like sanidine feldspar or zircon, to its isotopic daughter with high-resolution biostratigraphy, magnetostratig- products. 7 e age of a mineral may then be calculated raphy, or cyclic scales is a major challenge to time scale by means of the isotopic decay constant. Depending on studies. Even the most detailed biostratigraphic scheme the half-life of the element, several radiometric clocks probably has no biozonal units of less than 0.5–1.0 million are available; 40Ar/39Ar and the family of U/Pb isotopes year (my) duration, not to speak of the actual precision are the most common suites nowadays applied to the in dating a particular “stratigraphic piercing” point, for Phanerozoic, because of analytical precision and utility which an U/Pb age estimate would be available with an with tuB aceous beds in marine or non-marine sequences. analytical uncertainty of 0.1 to 0.5 my. Similarly, com- Radiometric dating of sedimentary rocks follows several bination of analytically less precise K/Ar dates with geological strategies: much more precise Ar/Ar or U/Pb dates in statistical (a) Dating of igneous intrusions within sediments interpolations creates a strong bias toward the latter, des- records the time of primary cooling, when the pite the fact that both may have equal litho-, bio-, and igneous rocks were emplaced and had cooled chronostratigraphic precision. su1 ciently (to a few hundreds of degrees Nevertheless, the combination of precise strati- centigrade) to set the radiometric decay clock in graphic deAnitions through GSSPs and accurate

HHedges.indbedges.indb 3030 11/28/2009/28/2009 11:24:48:24:48 PPMM The Geologic Time Scale 31

radiometric dates near these levels is paving the way A second geological method involves building a for a substantial increase in the precision and accur- zonal composite to scale stages. Several outstanding acy of the Geologic Time Scale. 7 e bases of Paleozoic, examples are documented in GTS2004 built by a large Mesozoic, and Cenozoic are bracketed by analyt- international team of scientists under the direction of ically precise ages at their GSSP or primary correlation Felix Gradstein and James Ogg. For this scale, Roger markers—542.0 ± 1.0 Ma, 251.0 ± 0.4 Ma, and 65.5 ± 0.3 Cooper and colleagues have built a very detailed com- Ma respectively—and there are direct age dates for the posite standard of graptolite zones from 200+ sections base , base Permian, base Jurassic, base in oceanic and slope environment basins for the upper- , base Cenomanian, and the base Oligocene. Most most Cambrian, Ordovician, and Silurian intervals. other period or stage boundaries lack direct age control. With zone thickness taken as directly proportional to 7 erefore, the third step, linear interpolation, also plays zone duration, the detailed composite sequence was a key role for the time scale. scaled using selected, high-precision age dates. For the Carboniferous through Permian, a composite stand- ard of , fusulinid, and ammonoid events Interpolation and statistics from many classical sections can now be calibrated to a Despite the progress in standardization and dating, parts combination of U/Pb and 40Ar/39Ar dates. A composite of the Mesozoic and Paleozoic Eras have sparse radio- standard of conodont zones was used for . metric records (see Fig. 2). Ideally, each of the 90+ stage 7 is procedure directly scales all stage boundaries and boundaries that comprise the Paleozoic, Mesozoic, and biostratigraphic horizons. Cenozoic Eras of the Phanerozoic Eon should coincide 7 e two-way graph of linear age vs. scaled stages requires with an accurate radiometric date from volcanic ash. a best A tting method, and that is where statistics comes However, this coincidence is rare in the geological record. into play, with cubic spline A tting and maximum likeli- 7 e combined number of fossil events and magnetic hood interpolation most suitable. On the time scale chart reversals far exceeds the total number of radiometric- (late 2008 edition; Fig. 3), a majority of Phanerozoic stage ally datable horizons in the Phanerozoic. 7 erefore, a boundaries for the A rst time show error bars; an exception framework of bio-, magneto-, and chronostratigraphy is the Neogene Period where analytical errors are negli- provides the principal fabric for stretching of the relative gible. 7 e error bars reP ect both radiometric and strati- time scale between dated tiepoints on the loom of linear graphic uncertainty; in addition, error bars were calculated time. For such stretching, interpolation methods that are on stage duration. Uncertainty in the duration of the age employed are both geological and statistical in . units is less than the error in age of their boundaries. Earlier, we mentioned the outdated method of plot- ting the cumulative global thickness of periods against selected linear age dates. Among the modern geological TS-Creator© scaling methods, an assumption of relative constancy of Now, Adam Lugowski, Ogg, and Gradstein are producing seaP oor spreading over limited periods of time is a com- an electronic version of the Geologic Time Scale with the mon tool for interpolating the Latest Cretaceous through international standard bio-magneto-sequence time scale Paleogene relative scale. Magnetic polarity chrons, the charts. 7 ere are charts for Paleozoic, Mesozoic, and units of magnetochronology, can be recognized both Cenozoic Eras and for each period. 7 is JAVA language on the ocean P oor as magnetic anomalies measured in package, called TS-Creator©, can be freely downloaded kilometers from the mid-ocean spreading center, and from the ICS website (www.stratigraphy.org). in marine sediments as polarity zones that contain bio- It contains tables of Cambrian through stratigraphic events and can be linked to linear time. stratigraphic events calibrated to GTS2004 ages. 7 ere Knowing the linear age of a few ocean crust magnetic are nearly 15,000 biostratigraphic, sea-level, and mag- anomalies (earth magnetic reversals or magnetochrons) netic zones and levels, plus a suite of geochemical allows interpolation of the ages of the intervening mag- curves. Documentation of zonal deA nitions, relative age netic pattern, which in turn can be correlated to the assignments, and how these events were recalibrated fossil record and geological stage boundaries. 7 e sub- to GTS2004 was also compiled. 7 is included updat- duction of pre- oceanic crust precludes such ing cross correlations and enhancing detail for selected an interpolation approach for older Mesozoic and the stratigraphic methods using trilobites, , grapto- Paleozoic strata. lites, ammonoids, fusulinids, chitinozoans, megaspores,

HHedges.indbedges.indb 3131 11/28/2009/28/2009 11:24:48:24:48 PPMM r A Stage/Age - Era Period Epoch K boundaries r Quatern. , 0 e L / r HolocenePliocene E b n S

e L A - P

g / - b

c r o M i e E U A R N o L z e Oligocene E n o L e

n g M

o

e Eocene e

l E C

a L M

P Paleocene E

Campanian Late

s Coniacian

u Turonian

o Cenomanian

100 e

c Albian a t e

r Early Aptian C Barremian

c i

o

z Late Oxfordian o c

i s

s Middle s e a

r M u

J Early 200

Norian Late c i

s s a i r Middle T Early a / i m

r e

P

300 n - a

i Late

n s n n

a Moscovian

e Middle u v l P y

o

s Early r e n Late f a i i p n p i o Middle Visean s b s i r s a s i Early C M

Famennian

n Late c a i i n o

o z

v Middle e o

D

400 e l Early

a Lochkovian Pridoli P n Ludlow / a

i Wenlock / r

u l i Llandovery

S /

n

a Late i c i

v Middle o

d r Early

On Stage 10 Stage 9 Legend 500 n Stage 7 GSSP

a i Series 3 Stage 5 r Direct radiometric constraint b Series 2 Stage 4 Stage 3 on GSSP rocks m a Stage 2 Other radiometric date

C Series 1 Stage 1

0 10 20 # radiometric dates

Fig. 2 Geologic Time Scale 2004 showing which stage and period boundaries have a Global Boundary Stratotype Section and Point (GSSP), which ones are dated directly and which other age dates were used. Intervals with sparse or no dates required interpolation.

HHedges.indbedges.indb 3232 11/28/2009/28/2009 11:24:48:24:48 PPMM HHedges.indb 33 e d g e s . i n d b

3 3 INTERNATIONAL STRATIGRAPHIC CHART ICS International Commission on Stratigraphy Ma Ma Ma Ma Era Era Era Era Age Age Age Age Eon Age Eon Age Eon Eon Age Stage Stage Stage GSSP GSSP GSSP GSSA GSSP Series Epoch Series Epoch Series Epoch Period Period Period Period System System System System Erathem Erathem Erathem Eonothem Eonothem Eonothem 145.5 ±4.0 359.2 ±2.5 542 Holocene Tithonian 0.0117 150.8 ±4.0 Upper 374.5 ±2.6 Neo- ~635 Upper Upper Kimmeridgian Frasnian 0.126 ~ 155.6 385.3 ±2.6 850 Pleistocene “Ionian” Oxfordian Givetian 0.781 161.2 ±4.0 Middle 391.8 ±2.7 1000 Callovian Eifelian * 1.806 164.7 ±4.0 397.5 ±2.7 Meso- 1200 Gelasian Bathonian Emsian proterozoic 2.588 Middle 167.7 ±3.5 Devonian 407.0 ±2.8 1400 Piacenzian Bajocian Lower Pragian 3.600 171.6 ±3.0 411.2 ±2.8 1600 Zanclean Aalenian Lochkovian 5.332 Jurassic 175.6 ±2.0 416.0 ±2.8 1800 Messinian Toarcian Pridoli Paleo- 7.246 183.0 ±1.5 418.7 ±2.7 2050 Tortonian Pliensbachian Ludfordian proterozoic 11.608 Lower 189.6 ±1.5 Ludlow 421.3 ±2.6 2300 Serravallian Sinemurian Gorstian Miocene 13.82 196.5 ±1.0 422.9 ±2.5 2500 Langhian Hettangian Homerian 15.97 199.6 ±0.6 Wenlock 426.2 ±2.4 Burdigalian Rhaetian Sheinwoodian 20.43 203.6 ±1.5 428.2 ±2.3 2800 Aquitanian Upper Silurian Telychian 23.03 216.5 ±2.0 436.0 ±1.9 P r e c a m b r i a n P C e n o z o i c Chattian Carnian Llandovery Aeronian Oligocene 28.4 ±0.1 ~ 228.7 439.0 ±1.8 3200 Rupelian Ladinian Rhuddanian 33.9 ±0.1 Middle 237.0 ±2.0 443.7 ±1.5 Priabonian Anisian Proterozoic Triassic Hirnantian 37.2 ±0.1 ~ 245.9 445.6 ±1.5 3600 Bartonian Olenekian Upper Katian Eocene 40.4 ±0.2 Lower ~ 249.5 455.8 ±1.6 Lutetian Induan Sandbian 48.6 ±0.2 251.0 ±0.4 460.9 ±1.6 4000

Ypresian Changhsingian a l e o z o i c P Darriwilian 55.8 ±0.2 468.1 ±1.6 P h a n e r o z o i c P P h a n e r o z o i c P Lopingian 253.8 ±0.7 h a n e r o z o i c P Middle (informal) PaleogeneThanetian Neogene Wuchiapingian 58.7 ±0.2 260.4 ±0.7 471.8 ±1.6 ~4600 Paleocene Selandian Capitanian Ordovician Floian ~ 61.1 265.8 ±0.7 Lower 478.6 ±1.7 Subdivisions of the global are Danian Guadalupian Wordian Tremadocian 268.0 ±0.7 formally defined by their lower boundary. Each unit 65.5 ±0.3 488.3 ±1.7 of the Phanerozoic (~542 Ma to Present) and the Maastrichtian Roadian Stage 10 70.6 ±0.6 270.6 ±0.7 ~ 492 * base of Ediacaran are defined by a basal Global Kungurian Furongian Stage 9 Standard Section and Point (GSSP ), whereas 83.5 ±0.7 Permian 275.6 ±0.7 ~ 496 * units are formally subdivided by Santonian Artinskian Paibian absolute age (Global Standard Stratigraphic Age, Upper 85.8 ±0.7 Cisuralian 284.4 ±0.7 ~ 499 Coniacian Sakmarian GSSA). Details of each GSSP are posted on the ~ 88.6 294.6 ±0.8 ~ 503 ICS website (www.stratigraphy.org). Turonian Asselian Series 3 Drumian Numerical ages of the unit boundaries in the 93.6 ±0.8 299.0 ±0.8 ~ 506.5 Phanerozoic are subject to revision. Some stages Cenomanian Gzhelian Stage 5 within the Cambrian will be formally named upon 99.6 ±0.9 Upper 303.4 ±0.9 ~ 510 * international agreement on their GSSP limits. Most P a l e o z o i cP M e s o z o i c Albian Kasimovian 112.0 ±1.0 307.2 ±1.0 Series 2 ~ 515 * sub-Series boundaries (e.g., Middle and Upper

Aptian Penn- Middle Moscovian Stage 3 Aptian) are not formally defined. Cretaceous 125.0 ±1.0 sylvanian 311.7 ±1.1 ~ 521 *

M e s o z o i c Colors are according to the Commission for the Barremian Lower Bashkirian Stage 2 Geological Map of the World (www.cgmw.org). Lower 130.0 ±1.5 318.1 ±1.3 ~ 528 * Hauterivian Upper Serpukhovian The listed numerical ages are from 'A Geologic ~ 133.9 328.3 ±1.6 542.0 ±1.0 Time Scale 2004', by F.M. Gradstein, J.G. Ogg, Valanginian Middle Visean This chart was drafted by Gabi Ogg. Intra Cambrian unit ages A.G. Smith, et al. (2004; Cambridge University Press) 140.2 ±3.0 Carboniferous 345.3 ±2.1 Missis- sippian with * are informal, and awaiting ratified definitions. and “The Concise Geologic Time Scale” by J.G. Ogg, Berriasian Lower Tournaisian 145.5 ±4.0 359.2 ±2.5 Copyright © 2008 International Commission on Stratigraphy G. Ogg and F.M. Gradstein (2008). * Definition of the Quaternary and revision of the Pleistocene are under discussion. Base of the Pleistocene is at 1.81 Ma December 2008 (base of Calabrian), but may be extended to 2.59 Ma (base of Gelasian). The historic “Tertiary” comprises the Paleogene and Neogene, and has no official rank.

Fig. 3 The International Stratigraphic Chart summarizes the set of chronostratigraphic units (geologic stages, periods) and their computed ages, which are the main framework for Geologic Time Scale 2004. Uncertainties on ages are expressed as two-sigma (95% confi dence). This version incorporates changes made by the International Commission up to 11/28/2009 1:24:49 PM / 2

8 December, 2008. / 2 0 0 9

1 : 2 4 : 4 9

P M 34 THE TIMETREE OF LIFE

nannofossils, foraminifers, dinoP agellates, radiolarians, 2. L. Lourens, F. Hilgen, N. J. Shackleton, L. Laskar, diatoms, strontium isotope, and C-org curves. D. Wilson, in Geologic Time Scale 2004, F. M. Gradstein, Numerical ages are calculated within the database J. G. Ogg, A. G. Smith, Eds. (Cambridge University Press, using the calibrations; therefore, all ages can be auto- New York, 2004), pp. 409–440. matically recomputed when control ages are improved in 3. S. C. Cande, D. V. Kent, J. Geophys. Res. Solid Earth 100, 6093 (1995). future time scales. Regional scales of selected areas (e.g., 4. S. A. Bowring, D. H. Erwin, Y. Isozaki, Proc. Natl. Acad. Russia, China, North America, and New Zealand) are Sci. U.S.A. 96, 8827 (1998). also included. 5. R. M. Carter, T. R. Naish, Eds., The High-Resolution TS-Creator© automatically takes the reference data- Chronostratigraphic and Sequence Stratigraphic Record base, gets instructions from the user on the stratigraphic of the Plio-Pleistocene Wanganui Basin (Institute interval and stratigraphic information to be displayed, of Geological and Nuclear Sciences, New Zealand, and then generates both on-screen and scalable-vector 1999). graphic (SVG) renditions that directly input into draJ - 6. F. M. Gradstein, et al., SEPM Spec. Publ. 54, 95 (1995). ing programs. Next, the user can click on a value, zone, 7. W. B. Harland et al., A Geologic Time Scale 1989 or boundary in the charts on the computer screen, and (Cambridge University Press, Cambridge, 1990). a window opens with an explanation of the calibration, 8. F. J. Hilgen, W. Krijgsman, C. G. Langereis, L. J. Lourens, deA nition, and interpolated age. 7 is “hot-linked” chart EOS Trans. Am. Geophys. Union 78, 285 (1997). 9. A. Holmes, Trans. Geol. Soc. Glasgow 21, 117 (1947). suite is currently a back-looking reference to information 10. A. Holmes, Trans. Edinburgh Geol. Soc. 17, 183 (1960). in the source tables, but in the future will also provide 11. S. L. Kamo et al., Earth and Planetary Science Letters 214, links to other tables and text from the GTS2004 book, 75 (2003). images of stage-boundary outcrops and fossil taxa, and 12. A. Martinsson, e Siluro-Devonian Boundary. Inter- the additional enhancementsanticipated during the national Union of Geological Sciences Series A, 5 major update for “Geologic Time Scale 2010.” (Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, 1977). 13. P. R. Renne et al., Geology 22, 783 (1994). Additional information on geologic time 14. N. J. Shackleton, S. J. Crowhurst, G. P. Weedon, J. Laskar, 7 e goal of this brief synopsis was to introduce the basic Phil. Trans. Roy. Soc. Lond. A 357, 1907 (1999). 15. G. P. Weedon, H. C. Jenkyns, A. L. Coe, S. P. Hesselbo, concepts involved in the construction of the geologic time Phil. Trans. Roy. Soc. Lond. A 357, 1787 (1999). scale. Further details can be found elsewhere (1–18). 16. T. D. Herbert, S. L. D’Hondt, I. Premoli-Silva, E. Erba, A. G. Fischer, SEPM Spec. Vol. 54, 81 (1995). 17. J. D. Obradovich, Geol. Assoc. Canada Spec. Pap. 39, 379 References (1993). 1. F. M. Gradstein, J. G. Ogg, A. G. Smith, A Geologic Time 18. U. O. Röhl, J.G., T. L. Geib, G. Wefer, Geol. Soc., Spec. Scale 2004 (Cambridge University Press, New York, 2004). Publ. 183, 163 (2001).

HHedges.indbedges.indb 3434 11/28/2009/28/2009 11:24:49:24:49 PPMM

Top View