Sea Level Variations

SEA LEVEL VARIATIONS OVER GEOLOGIC TIME

M. A. Kominz, Western Michigan University,Kalamazoo, MI, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0255

Introduction

Sea level changes have occurred throughout Earthhistory. The magnitudes and timing of sea levelchanges are extremely variable. They provide con-siderable insight into the tectonic and climatic his-tory of the Earth, but remain difRcult to determinewith accuracy.

Sea level, where the world oceans intersect thecontinents, is hardly Rxed, as anyone who has stoodon the shore for 6 hours or more can attest. But theever-changing tidal Sows are small compared withlonger-term Suctuations that have occurred in Earthhistory. How much has sea level changed? Howlong did it take? How do we know? What does ittell us about the history of the Earth?

In order to answer these questions, we need toconsider a basic question: what causes sea level tochange? Locally, sea level may change if tectonicforces cause the land to move up or down. How-ever, this article will focus on global changes in sealevel. Thus, the variations in sea level must be dueto one of two possibilities: (1) changes in the vol-ume of water in the oceans or (2) changes in thevolume of the ocean basins.

Sea Level Change due to Volume ofWater in the Ocean Basin

The two main reservoirs of water on Earth are theoceans (currently about 97% of all water) and gla-ciers (currently about 2.7%). Not surprisingly, forat least the last three billion years, the main variablecontrolling the volume of water Rlling the oceanbasins has been the amount of water present inglaciers on the continents. For example, about20 000 years ago, great ice sheets covered northernNorth America and Europe. The volume of ice inthese glaciers removed enough water from theoceans to expose most continental shelves. Sincethen there has been a sea level rise (actually fromabout 20 000 to about 11 000 years ago) of about120 m (Figure 1A).

A number of methods have been used to establishthe magnitude and timing of this sea level change.

Dredging on the continental shelves reveals humanactivity near the present shelf-slope boundary. Thesedata suggest that sea level was much lower a rela-tively short time ago. Study of ancient corals showsthat coral species which today live only in veryshallow water are now over 100 m deep. The car-bonate skeletons of the coral, which once thrived inthe shallow waters of the tropics, yield a detailedpicture of the timing of sea level rise, and, thus, themelting of the glaciers. Carbon-14, a radioactiveisotope formed by carbon-12 interacting with high-energy solar radiation in Earth’s atmosphere (seeCosmogenic Isotopes) allows us to determine theage of Earth materials, which are about 30 thou-sand years old.

This is just the most recent of many, largechanges in sea level caused by glaciers, (Figure 1B).These variations in climate and subsequent sea levelchanges have been tied to quasi-periodic variationsin the Earth’s orbit and the tilt of the Earth’s spinaxis. The record of sea level change can be esti-mated by observing the stable isotope, oxygen-18 inthe tests (shells) of dead organisms (see CenozoicClimate + Oxygen Isotope Evidence). When marinemicroorganisms build their tests from the calcium,carbon, and oxygen present in sea water they incor-porate both the abundant oxygen-16 and the rareoxygen-18 isotopes. Water in the atmosphere gener-ally has a lower oxygen-18 to oxygen-16 ratio be-cause evaporation of the lighter isotope requires lessenergy. As a result, the snow that eventually formsthe glaciers is depleted in oxygen-18, leaving theocean proportionately oxygen-18-enriched. Whenthe microorganisms die, their tests sink to the sea-Soor to become part of the deep marine sedimentaryrecord. The oxygen-18 to oxygen-16 ratio present inthe fossil tests has been calibrated to the sea levelchange, which occurred from 20 000 to 11 000 yearsago, allowing the magnitude of sea level changefrom older times to be estimated. This techniquedoes have uncertainties. Unfortunately, the amountof oxygen-18 which organisms incorporate in theirtests is affected not only by the amount of oxygen-18 present but also by the temperature and salinityof the water. For example, the organisms take upless oxygen-18 in warmer waters. Thus, duringglacial times, the tests are even more enriched inoxygen-18, and any oxygen isotope record revealsa combined record of changing local temperatureand salinity in addition to the record of globalglaciation.

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Figure 1 (A) Estimates of sea level change over the last 20 000 years. Amplitude is about 120 m. (B) Northern Hemisphereglaciers over the last million years or so generated major sea level fluctuations, with amplitudes as high as 125 m. (C) The long-termoxygen isotope record reveals rapid growth of the Antarctic and Greenland ice sheets (indicated by gray bars) as Earth’s climatecooled. (D) Long-term sea level change as indicated from variations in deep-ocean volume. Dominant effects include spreadingrates and lengths of mid-ocean ridges, emplacement of large igneous provinces (the largest, marine LIPs are indicated by graybars), breakup of supercontinents, and subduction of the Indian continent. The Berggren et al. (1995) chronostratigraphic timescalewas used in (C) and (D).

Moving back in time through the Cenozoic (zeroto 65 Ma), paleoceanographic data remain excellentdue to relatively continuous sedimentation on theocean Soor (as compared with shallow marineand terrestrial sedimentation). Oxygen-18 in thefossil shells suggests a general cooling for aboutthe last 50 million years. Two rapid increases in theoxygen-18 to oxygen-16 ratio about 12.5 Ma andabout 28 Ma are observed (Figure 1C). The forma-tion of the Greenland Ice Sheet and the Antarctic IceSheet are assumed to be the cause of these rapidisotope shifts. Where oxygen-18 data have been col-lected with a resolution Rner than 20 000 years,high-frequency variations are seen which are pre-sumed to correspond to a combination of temper-ature change and glacial growth and decay. Wehypothesize that the magnitudes of these high-frequency sea level changes were considerably less inthe earlier part of the Cenozoic than those observedover the last million years. This is because consider-ably less ice was involved.

Although large continental glaciers are not com-mon in Earth history they are known to have beenpresent during a number of extended periods (‘icehouse’ climate, in contrast to ‘greenhouse’ or warmclimate conditions). Ample evidence of glaciation is

found in the continental sedimentary record. In par-ticular, there is evidence of glaciation from about2.7 to 2.1 billion years ago. Additionally, a longperiod of glaciation occurred shortly before the Rrstfossils of multicellular organisms, from about 1 bil-lion to 540 million years ago. Some scientists nowbelieve that during this glaciation, the entire Earthfroze over, generating a ‘snowball earth’. Such con-ditions would have caused a large sea level fall.Evidence of large continental glaciers are also seenin Ordovician to Silurian rocks (&420 to 450 Ma),in Devonian rocks (&380 to 390 Ma), and in Car-boniferous to Permian rocks (&350 to 270 Ma).

If these glaciations were caused by similar mecha-nisms to those envisioned for the Plio-Pleistocene(Figure 1B), then predictable, high-frequency, peri-odic growth and retreat of the glaciers should beobserved in strata which form the geologic record.This is certainly the case for the Carboniferousthrough Permian glaciation. In the central UnitedStates, UK, and Europe, the sedimentary rocks havea distinctly cyclic character. They cycle in repetitivevertical successions of marine deposits, near-shoredeposits, often including coals, into Suvial sedimen-tary rocks. The deposition of marine rocks overlarge areas, which had only recently been

2606 SEA LEVEL VARIATIONS OVER GEOLOGIC TIME

nonmarine, suggests very large-scale sea level cha-nges. When the duration of the entire record istaken into account, periodicities of about 100 and400 thousand years are suggested for these large sealevel changes. This is consistent with an origin dueto a response to changes in the eccentricity of theEarth’s orbit. Higher-frequency cyclicity associatedwith the tilt of the spin axis and precession of theequinox is more difRcult to prove, but has beensuggested by detailed observations.

It is fair to say that large-scale (10 to '100 m),relatively high-frequency (20 000}400000 years;often termed ‘Milankovitch scale’) variations in sealevel occurred during intervals of time when conti-nental glaciers were present on Earth (ice houseclimate). This indicates that the variations ofEarth’s orbit and the tilt of its spin axis playeda major role in controlling the climate. During therest of Earth history, when glaciation was nota dominant climatic force (greenhouse climate), sealevel changes corresponding to Earth’s orbit did oc-cur. In this case, the mechanism for changing thevolume of water in the ocean basins is much lessclear.

There is no geological record of continental icesheets in many portions of Earth history. These timeperiods are generally called ‘greenhouse’ climates.However, there is ample evidence of Milankovitchscale variations during these periods. In shallowmarine sediments, evidence of orbitally driven sealevel changes has been observed in Cambrian andCretaceous age sediments. The magnitudes of sealevel change required (perhaps 5}20 m) are far lessthan have been observed during glacial climates.A possible source for these variations could be vari-ations in average ocean-water temperature. Waterexpands as it is heated. If ocean bottom-water sour-ces were equatorial rather than polar, as they aretoday, bottom-water temperatures of about 23C to-day might have been about 163C in the past. Thiswould generate a sea level change of about 11 m.Other causes of sea level change during greenhouseperiods have been postulated to be a result of vari-ations in the magnitude of water trapped in inlandlakes and seas, and variations in volumes of alpineglaciers. Deep marine sediments of Cretaceous agealso show Suctuations between oxygenated andanoxic conditions. It is possible that these variationswere generated when global sea level change re-stricted Sow from the rest of the world’s ocean toa young ocean basin. In a more recent example,tectonics caused a restriction at the Straits of Gibral-tar. In that case, evaporation generated extreme sealevel changes and restricted their entrance into theMediterranean region.

Sea Level Change due to ChangingVolume of the Ocean Basin

Tectonics is thought to be the main driving force oflong-term (550 million years) sea level change.Plate tectonics changes the shape and/or the arealextent of the ocean basins.

Plate tectonics is constantly reshaping surface fea-tures of the Earth while the amount of water presenthas been stable for about the last four billion years.The reshaping changes the total area taken up byoceans over time. When a supercontinent forms,subduction of one continent beneath another de-creases Earth’s ratio of continental to oceanic area,generating a sea level fall. In a current example, thecontinental plate including India is diving underAsia to generate the Tibetan Plateau and theHimalayan Mountains. This has probably generateda sea level fall of about 70 m over the last 50 millionyears. The process of continental breakup has theopposite effect. The continents are stretched,generating passive margins and increasing the ratioof continental to oceanic area on a global scale(Figure 2A). This results in a sea level rise. In-crements of sea level rise resulting from continentalbreakup over the last 200 million years amount toabout 100 m of sea level rise.

Some bathymetric features within the oceans arelarge enough to generate signiRcant changes in sealevel as they change size and shape. The largestphysiographic feature on Earth is the mid-oceanridge system, with a length of about 60 000 km anda width of 500}2000km. New ocean crust andlithosphere are generated along rifts in the center ofthese ridges. The ocean crust is increasingly old,cold, and dense away from the rift. It is the heat ofocean lithosphere formation that actually generatesthis feature. Thus, rifting of continents forms newridges, increasing the proportionate area of young,shallow, ocean Soor to older, deeper ocean Soor(Figure 2B). Additionally, the width of the ridge isa function of the rates at which the plates aremoving apart. Fast spreading ridges (e.g. the EastPaciRc Rise) are very broad while slow spreadingridges (e.g. the North Atlantic Ridge) are quite nar-row. If the average spreading rates for all ridgesdecreases, the average volume taken up by oceanridges would decrease. In this case, the volume ofthe ocean basin available for water would increaseand a sea level fall would occur. Finally, entireridges may be removed in the process of subduction,generating fairly rapid sea level fall.

Scientists have made quantitative estimates of sealevel change due to changing ocean ridge volumes.Since ridge volume is dependent on the age of the


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Figure 2 Diagrams showing a few of the factors which affect the ocean volume. (A) Early breakup of a large continent increasesthe area of continental crust by generating passive margins, causing sea level to rise. (B) Shortly after breakup a new ocean isformed with very young ocean crust. This young crust must be replacing relatively old crust via subduction, generating additionalsea level rise. (C) The average age of the ocean between the continents becomes older so that young, shallow ocean crust isreplaced with older, deeper crust so that sea level falls. (D) Fast spreading rates are associated with relatively high sea level.(E) Relatively slow spreading ridges (solid lines in ocean) take up less volume in the oceans than high spreading rate ridges(dashed lines in ocean), resulting in relatively low sea level. (F) Emplacement of large igneous provinces generates oceanicplateaus, displaces ocean water, and causes a sea level rise.


ocean Soor, where the age of the ocean Soor isknown, ridge volumes can be estimated. SeaSoormagnetic anomalies are used to estimate the age ofthe ocean Soor, and thus, spreading histories of theoceans (see Magnetics). The oldest ocean crust isabout 200 million years old. Older oceanic crust hasbeen subducted. Thus, it is not surprising thatquantitative estimates of sea level change due toridge volumes are increasingly uncertain and cannotbe calculated before about 90 million years. Sealevel is estimated to have fallen about 230 m($120 m) due to ridge volume changes in the last80 million years.

Large igneous provinces (LIPs) are occasionallyintruded into the oceans, forming large oceanicplateaus (see Igneous Provinces). The volcanism as-sociated with LIPs tends to occur over a relativelyshort period of time, causing a rapid sea level rise.However, these features subside slowly as the litho-sphere cools, generating a slow increase in oceanvolume, and a long-term sea level fall. The largestmarine LIP, the Ontong Java Plateau, was emplacedin the PaciRc Ocean between about 120 and 115 Ma(Figure 1D). Over that interval it may have gener-ated a sea level rise of around 50 m.

In summary, over the last 200 million years,long-term sea level change (Figure 1D) can belargely attributed to tectonics. Continental crustexpanded by extension as the supercontinents Gon-dwana and Laurasia split to form the continents wesee today. This process began about 200 Ma whenNorth America separated from Africa and continueswith the East African Rift system and the formationof the Red Sea. The generation of large oceansoccurred early in this period and there was an over-all rise in sea level from about 200 to about 90million years. New continental crust, new mid-ocean ridges, and very fast spreading rates wereresponsible for the long-term rise (Figure 1D). Sub-sequently, a signiRcant decrease in spreading rates,a reduction in the total length of mid-ocean ridges,and continent}continent collision coupled with anincrease in glacial ice (Figure 1C) have resulted ina large-scale sea level fall (Figure 1D). Late Creta-ceous volcanism associated with the Ontong JavaPlateau, a large igneous province (see Igneous Prov-inces), generated a signiRcant sea level rise, whilesubsequent cooling has enhanced the 90 million yearsea level fall. Estimates of sea level change fromchanging ocean shape remain quite uncertain. Mag-nitudes and timing of stretching associated withcontinental breakup, estimates of shortening duringcontinental assembly, volumes of large igneousprovinces, and volumes of mid-ocean ridges improveas data are gathered. However, the exact conRgura-

tion of past continents and oceans can only bea mystery due to the recycling character of platetectonics.

Sea Level Change Estimated fromObservations on the Continents

Long-term Sea Level Change

Estimates of sea level change are also made fromsedimentary strata deposited on the continents. Thisis actually an excellent place to obtain observationsof sea level change not only because past sea levelhas been much higher than it is now, but alsobecause in many places the continents have sub-sequently uplifted. That is, in the past they werebelow sea level, but now they are well above it. Forexample, studies of 500}400 million year old sedi-mentary rocks which are now uplifted in the RockyMountains and the Appalachian Mountains indicatethat there was a rise and fall of sea level with anestimated magnitude of 200}400m. This examplealso exempliRes the main problem with using thecontinental sedimentary record to estimate sea levelchange. The continents are not Rxed and move ver-tically in response to tectonic driving forces. Thus,any indicator of sea level change on the continents isan indicator of relative sea level change. Obtaininga global signature from these observations remainsextremely problematic. Additionally, the continentalsedimentary record contains long periods of non-deposition, which results in a spotty record of Earthhistory. Nonetheless a great deal of informationabout sea level change has been obtained and issummarized here.

The most straightforward source of informationabout past sea level change is the location of thestrand line (the beach) on a stable continentalcraton (a part of the continent, which was notinvolved in local tectonics). Ideally, its presentheight is that of sea level at the time of deposition.There are two problems encountered with this ap-proach. Unfortunately, the nature of land}ocean in-teraction at their point of contact is such that thosesediments are rarely preserved. Where they can beobserved, there is considerable controversy overwhich elements have moved, the continents or sealevel. However, data from the past 100 millionyears tend to be consistent with calculations derivedfrom estimates of ocean volume change. This is notsaying a lot since uncertainties are very large (seeabove).

Continental hypsography (cumulative area versusheight) coupled with the areal extent of preservedmarine sediments has been used to estimate past sea


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Figure 4 Example of the sequence stratigraphic approach to estimates of sea level change. (A) Multichannel seismic data (gray)from the Baltimore Canyon Trough, offshore New Jersey, USA (Miller et al., 1998). Black lines are interpretations traced on theseismic data. Thick dark lines indicate third-order Miocene-aged (5}23 Ma) sequence boundaries. They are identified by truncationof the finer black lines. Upside-down deltas indicate a significant break in slope associated with each identified sequence boundary.Labeled vertical lines (1071}1073) show the locations of Ocean Drilling Project wells (Deep-sea Drilling Methodology), used tohelp date the sequences. The rectangle in the center is analyzed in greater detail. (B) Detailed interpretation of a single third-ordersequence from (A). Upside-down deltas indicate a significant break in slope associated with each of the detailed sedimentpackages. Stippled fill indicates the low stand systems tract (LST) associated with this sequence. The gray packages are thetransgressive systems tract (TST), and the overlying sediments are the high stand systems tract (HST). (C) Relationships betweendetailed sediment packages (in B) are used to establish a chronostratigraphy (time framework). Youngest sediment is at the top.Each observed seismic reflection is interpreted as a time horizon, and each is assigned equal duration. Horizontal distance is thesame as in (A) and (B). A change in sediment type is indicated at the break in slope from coarser near-shore sediments (stippledpattern) to finer, offshore sediments (parallel, sloping lines). Sedimentation may be present offshore but at very low rates. LST, TST,and HST as in (B). (D) Relative sea level change is obtained by assuming a consistent depth relation at the change in slopeindicated in (B). Age control is from the chronostratigraphy indicated in (C). Time gets younger to the right. The vertical scale is intwo-way travel time, and would require conversion to depth for a final estimate of the magnitude of sea level change. LST, TST, andHST as in (B). Note that in (B), (C), and (D), higher frequency cycles (probably fourth-order) are present within this (third-order)sequence. Tracing and interpretations are from the author’s graduate level quantitative stratigraphy class project (1998, WesternMichigan University).

level. In this case only an average result can beobtained, because marine sediments spanning a timeinterval (generally 5}10 million years) have beenused. Again, uncertainties are large, but results areconsistent with calculations derived from estimatesof ocean volume change.

Backstripping is an analytical tool, which hasbeen used to estimate sea level change. In this tech-nique, the vertical succession of sedimentary layersis progressively decompacted and unloaded (Figure3A, B). The resulting hole is a combination of thesubsidence generated by tectonics and by sea levelchange (Figure 3B, C). If the tectonic portion can beestablished then an estimate of sea level change can

be determined (Figure 3C, D). This method is gener-ally used in basins generated by the cooling ofa thermal anomaly (e.g. passive margins). In thesebasins, the tectonic signature is predictable (ex-ponential decay) and can be calibrated to the well-known subsidence of the mid-ocean ridge.

The backstripping method has been applied tosedimentary strata drilled from passive continentalmargins of both the east coast of North Americaand the west coast of Africa. Again, estimates of sealevel suggest a rise of about 100}300m from about200}110Ma followed by a fall to the present level(Figure 1D). Young interior basins, such as the ParisBasin, yield similar results. Older, thermally driven


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Figure 5 Million year scale sea level fluctuations. Estimates from sequence stratigraphy (Haq et al., 1987; solid curve) have beenshifted down by 100 m to allow comparison with estimates of sea level from backstripping (Kominz et al., 1998; dashed curve).Where sediments are present, the backstripping results, with uncertainty ranges, are indicated by gray fill. Between backstripobservations, lack of preserved sediment is presumed to have been a result of sea level fall. The Berggren et al. (1995)biostratigraphic timescale was used.

basins have also been analyzed. This was the methodused to determine the (approximately 200 m) sea levelrise and fall associated with the breakup of a Pre-Cambrian supercontinent in earliest Phanerozoictime.

Million Year Scale Sea Level Change

In addition to long-term changes in sea level thereis evidence of Suctuations that are considerablyshorter than the 50}100 million year variationsdiscussed above, but longer than those caused byorbital variations (40.4 million years). These vari-ations appear to be dominated by durations whichlast either tens of millions of years or a half to threemillion years. These sea level variations are some-times termed second- and third-order sea levelchange, respectively. There is considerable debateconcerning the source of these sea level Suctuations.They have been attributed to tectonics and changingocean basin volumes, to the growth and decay ofglaciers, or to continental uplift and subsidence,which is independent of global sea level change. Asnoted above, the tectonic record of subsidence anduplift is intertwined with the stratigraphic record ofglobal sea level change on the continents. Syn-chronicity of observations of sea level change on

a global scale would lead most geoscientists to sug-gest that these signals were caused by global sealevel change. However, at present, it is nearly im-possible to globally determine the age equivalencyof events which occur during intervals as short asa half to two million years. These data limitationsare the main reason for the heated controversy overthird-order sea level.

Quantitative estimates of second-order sea levelvariations are equally difRcult to obtain. Althoughthe debate is not as heated, these somewhat longer-term variations are not much larger than the third-order variations so that the interference of the twosignals makes deRnition of the beginning, endingand/or magnitude of second-order sea level changeequally problematic. Recognizing that our under-standing of second- and third-order (million yearscale) sea level Suctuations is limited, a brief reviewof that limited knowledge follows.

Sequence stratigraphy is an analytical method ofinterpreting sedimentary strata that has been used toinvestigate second- and third-order relative sea levelchanges. This paradigm requires a vertical suc-cession of sedimentary strata which is analyzed in atleast a two-dimensional, basinal setting. Packages ofsedimentary strata, separated by unconformities, areobserved and interpreted mainly in terms of their


internal geometries (e.g. Figure 4). The unconformi-ties are assumed to have been generated by relativesea level fall, and thus, reSect either global sea levelor local or regional tectonics. This method of strati-graphic analysis has been instrumental in hydrocar-bon exploration since its introduction in the late1970s. One of the bulwarks of this approach is the‘global sea level curve’ most recently published byHaq et al. (1987). This curve is a compilation ofrelative sea level curves generated from sequencestratigraphic analysis in basins around the world.While sequence stratigraphy is capable of estimatingrelative heights of relative sea level, it does notestimate absolute magnitudes. Absolute dating re-quires isotope data or correlation via fossil data intothe chronostratigraphic timescale (see GeomagneticPolarity Timescale). However, the two-dimensionalnature of the data allows for good to excellentrelative age control.

Backstripping has been used, on a considerablymore limited basis, in an attempt to determinemillion year scale sea level change. This approach israrely applied because it requires very detailed,quantitative, estimates of sediment ages, paleo-environments and compaction in a thermal tectonicsetting. A promising area of research is the applica-tion of this method to coastal plain boreholes fromthe mid-Atlantic coast of North America. Here anintensive Ocean Drilling Project survey is underwaywhich is providing sufRciently detailed data for thistype of analysis (see Deep-sea Drilling Methodo-logy). Initial results suggest that magnitudes of mil-lion year scale sea level change are roughly one-halfto one-third that reported by Haq et al. However, inglacial times, the timing of the cycles was quiteconsistent with those of this ‘global sea level curve’derived by application of sequence stratigraphy (Fig-ure 5). Thus, it seems reasonable to conclude that,at least during glacial times, global, third-order sealevel changes did occur.

Summary

Sea level changes are either a response to changingocean volume or to changes in the volume of watercontained in the ocean. The timing of sea levelchange ranges from tens of thousands of years toover 100 million years. Magnitudes also vary signiR-cantly but may have been as great as 200 m ormore. Estimates of sea level change currently sufferfrom signiRcant ranges of uncertainty, both inmagnitude and in timing. However, scientists areconverging on consistent estimates of sea levelchanges by using very different data and analyticalapproaches.

See also

Cosmogenic Isotopes. Deep-sea Drilling Methodo-logy. Geomagnetic Polarity Timescale. IgneousProvinces. Magnetics.

Further ReadingAllen PA and Allen JR (1990) Basin Analysis: Principles

& Applications. Oxford: Blackwell ScientiRc Publications.Berggren WA, Kent DV, Swisher CC, Aubry MP (1995)

A revised Cenozoic geochronology and chronostratig-raphy. In: Berggren WA, Kent DV and HardenbolJ (eds) Geochronology, Time Scales and Global Strati-graphic Correlations: A UniTed Temporal Frameworkfor an Historical Geology. SEPM Special PublicationNo. 54, pp. 131}212.

Bond GC (1979) Evidence of some uplifts of large magni-tude in continental platforms. Tectonophysics 61:285}305.

CofRn MF and Eldholm O (1994) Large igneousprovinces: crustal structure, dimensions, and externalconsequences. Reviews of Geophysics 32: 1}36.

Crowley TJ and North GR (1991) Paleoclimatology. Ox-ford Monographs on Geology and Geophysics, no. 18.

Fairbanks RG (1989) A 17,000-year glacio-eustatic sealevel record: inSuence of glacial melting rates on theYounger Dryas event and deep-ocean circulation.Nature 6250: 637}642.

Hallam A (1992) Phanerozoic Sea-Level Changes. NY:Columbia University Press.

Haq BU, Hardenbol J and Vail PR (1987) Chronology ofSuctuating sea levels since the Triassic (250 millionyears ago to present). Science 235: 1156}1167.

Harrison CGA (1990) Long term eustasy and epeirogenyin continents. In: Sea-Level Change, pp. 141}158.Washington, DC: US National Research CouncilGeophysics Study Committee.

Hauffman PF and Schrag DP (2000) Snowball Earth.ScientiTc American 282: 68}75.

Kominz MA, Miller KG and Browning JV (1998) Long-term and short term global Cenozoic sea-levelestimates. Geology 26: 311}314.

Miall AD (1997) The Geology of Stratigraphic Sequences.Berlin: Springer-Verlag.

Miller KG, Fairbanks RG, Mountain GS (1987) Tertiaryoxygen isotope synthesis, sea level history, and conti-nental margin erosion. Paleoceanography 2: 1}19.

Miller KG, Mountain GS, Browning J et al. (1998) Ceno-zoic global sea level, sequences, and the New Jerseytransect; results from coastal plain and continentalslope drilling. Reviews of Geophysics 36: 569}601.

Sahagian DL (1988) Ocean temperature-induced changein lithospheric thermal structure: a mechanism forlong-term eustatic sea level change. Journal of Geology96: 254}261.

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Sea Level Variations

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