Sea Level, Ice, and Climatic Change (Proceedings of the Canberra Symposium, December 1979). IAHS Publ. no. 131.

Relative and average sea level changes, and endo-, epi-, and exogenic processes on the Earth

JOHN CHAPPELL Biogeography and Geomorphology, R. S. Pac. S., Australian National University, Box 4, Canberra 2601, Australia

ABSTRACT Past sea levels at any given epoch, identified geologically from positions of sea level-related indicators, differ between sites around the world due to vertical displacements of the Earth's surface. These are caused by endogenic (e.g. tectonic) processes and by epigenic (e.g. isostatic) responses. Geological records of sea level changes thus are relative only to local datum, and reduction to average changes requires estimation of these displacement factors. Available palaeo sea level data which are best as regards accuracy of age and position measurements come from the present interglacial episode (the last 6000 years), the last interglacial (120 OOO to 135 000 years BP), and a few points in between. Analysis of these data shows (a) that endogenic movements on a 10 year scale can be establish­ed, and that there is a suggestion that movements may have varied in rate within this time frame, in tectonic areas, and (b) that post-glacial isostatic changes across the globe are substantial and do not appear to be in accordance with contemporary isostatic models, in certain particulars. The positions of shorelines of 17 000 and 30 000 years BP need to be better established before global epigenic movement models can satisfactorily be tested. Paucity of good sea level data from late Pleistocene glacial periods has led to acceptance of <5180 records in deep sea cores as good surrogates of actual sea level/ice volume histories. Comparison with recent data on late glacial sea levels and <5 80 values, from coral reefs, shows that the deep sea core records do not accurately resolve changes at the 101* year level. Finally, effects of exogenic factors, especially the Milankovitch effect of orbital perturbations on climate and thence on sea level, are reviewed.

INTRODUCTION

The relationship between sea level and any landmass can change because the land moves vertically relative to the mean ocean floor, or because the quantity of water in the oceans changes, or because the dimensions of the ocean basins are altered. Separa­tion of these factors has been discussed since Darwin (1842) indicated, through his global map of coral reefs, that some regions are submerging while others are relatively stationary or

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412 John Chappell

are even emerging from the sea. Geological evidence for changes of the local or regional land/sea juxtaposition is manifest in such features as raised or submerged beaches and reefs, strati-graphic sequences bearing the imprint of transgressions and regressions of the sea, and in certain base-level-related features in lowland river systems. In all such records, dissection of the three factors which affect the land/sea level relationship is necessary if such studies are to realize their potential contribution to our knowledge of the physical behaviour of the Earth.

In what follows, average sea level indicates the global mean height of the sea surface on an hypothetical gauge attached to the centre of the Earth. All actual indications of sea level are related to a shoreline, either on a large landmass or on an island attached to the ocean floor, and will be referred to as relative sea levels. Average sea level ultimately is set, on a constant radius Earth, by the quantity of land (Fig. 1(a)) which results from crustal sial/sima differentiation (Fig. 1(b)), modified by the extent to which isostatic disequilibrium is induced by the upper mantle "heat engine" (Fig. 1(c)). Average sea level also can be altered by transferring liquid water from the oceans into ice sheets on land, although net change is reduced by isostatic compensation (Fig. 1(d)).

Relative sea level changes, which, if measured globally, can lead to estimates of average sea level changes (in principle), are caused by different sorts of forces. Endogenic forces originate within the Earth, occasioned by the internal heat engine, and cause vertical movements which are manifest directly

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Fig. 1 With a constant volume solid Earth and a constant volume of ocean water, V 0 , the average sea level on a gauge related to the centre of the Earth is at H with no land above sea surface, (a) Sea level is H' when land emerges. Quantity of land is fundamentally set by V0 and by sial-sima differentiation (b), modified (c) by emergence through tectonic uplift U, shifting the average sea level down to H t on the gauge. Ice cap formation lowers sea to H 0 (shown in (d)), but isostatic compensation depresses the land to Zj and raises the sea bed to S\, thus giving the final sea level Hj.

Relative and average sea level changes 413

as surface displacements associated with seismic events. These accumulate over time, producing average vertical rates as high as several metres per 1000 years in orogenic areas, leading to net displacements of any initial surface of up to tens of kilometres over 10 to lO8 years. Vertical expression of these movements is modified by epigenic forces principally caused by action of the hydrological cycle on the land surface, in turn conditioned by atmospheric circulation and meteorology. These forces redistribute surface loads, both by transfer of eroded sediments and by formation of ice sheets, and may interact with endogenic forces through upper mantle mass transfer (Fig. 1(d)) and by altering crustal stress fields.

The shape of the global water surface, constrained by gravi­tation potential and the effects of the Earth's rotation, does not bear a constant relation to the figure of the solid Earth when the latter is in isostatic disequilibrium due to the vast viscosity difference (about 20 orders of magnitude) between water and upper mantle material. This means that an average sea level change, due to ice sheet melting, say, will be perceived as relative change which varies around the globe. This compli­cates the task of separating average and relative changes; a task which must be performed when endogenic and epigenic factors are to be distinguished.

In addition to variations of endo- and epigenic processes in the geological past, and the matter of coupling between them, an issue of continuing interest concerns the ways in which both are affected by exogenic factors (from outside the Earth). Epigenic processes are affected directly by effects of radiation on climate; endogenic processes are directly affected by tidal forces, they may be indirectly affected by climate-induced effects on the Earth's rotation, and, on time scales greater than lO years, may be influenced by passage of the solar system through its galactic orbit. Certain of these are discussed in this review of sea level changes.

IDENTIFICATION AND CORRELATION OF RELATIVE SEA LEVEL CHANGES

Contemporary changes of relative sea level, directly measured from tide gauge records, have been used to test solid Earth models of isostatic adjustment for regions previously covered by late Pleistocene ice sheets (Chappell, 1974a), and can provide data on displacements associated with seismic events. In general, however, relative and average changes, and crustal movements of the broad categories introduced above, are identified geologically. The basic technique is to correlate, in an accurate chronological sense, deposits or erosion features which can be related to sea level, which occur in two quite different contexts - surficial, and stratigraphie.

Surficial indicators of sea level include ancient littoral and near-shore deposits, coral reefs, and wave cut platforms which have a physiographic expression today; few remnants of greater than late Cenozoic age are preserved. Stratigraphie indicators include the same sorts of features, as well as epineritic marine

414 John Chappell

deposits which can be related to an ancient sea level through palaeo-ecologic reconstruction, which are preserved within stratigraphie sequences. Cyclothemic sequences (Weller, 1958; Wanless, 1972) illustrate the latter. Changes of average sea level, which as noted above require global recognition, naturally are more difficult to interpret from stratigraphie than from surficial indicators, firstly because formations in which they are preserved may be tilted or folded, and secondly because correlation is difficult at the necessary level of chronological accuracy. For example, sea level changes associated with major Quaternary glacial cycles occur within 10 years, which is substantially less than the time error for correlations in the deeper geological past. This is not to say that long term changes of relative vertical movements of different landmasses, or of average sea level, cannot be identified by stratigraphie means, but indicates that the problem of separating the different factors of sea level change can best be addressed by examining the record of surficial indicators of Quaternary age.

Correlation between sea level indicators from different places can be done palaeontologically, through radiometric dating, or from the relationships between indicator deposits and traces of singular events such as widespread volcanic tepra falls or geomagnetic reversals. Each has limitations. Temporal accuracy of palaeontological correlation depends not only on speed of dispersal of a newly emergent species (which may be rapid for a pelagic micro-organism) but also on the relation between time of its evolutionary arrival as a distinct new form and time of deposition of the deposits in question (this long-standing issue, discussed by Arkell in 1923, applies in many contexts). The problem is greatly reduced when continuous sequences, which have a fixed correlation horizon determined by other .means, bear signatures in their fossil assemblages which can be attributed to global events such as major climatic changes. This strategy has been particularly successful with Quaternary deep sea cores, where the core top provides one horizon and the Brunhes-Matuyama magnetic reversal provides another. Limitations of radiometric dating broadly stem from the standard error of determination (ranging from about 0.2 to 10% of measured age, varying with method), and temporal association of the dated material and the sea level indicator. The latter constraint also applies to traces of singular events. Clearly, whatever the means of correlation, the materials on which it is based must be deposited at the time the sea level indicator is formed. These matters are not discussed further, but in what follows the uncertainties of correlations are indicated where appropriate.

THE QUESTION OF QUATERNARY HIGH SEA LEVELS

Maclaren in 1842 theorized that growth and decay or northern continental ice sheets, now known to have occurred repeatedly throughout Quaternary times, would cause global glacio-eustatic sea level fluctuations. In the following hundred years or so, it became almost dogma that flights of wave cut terraces,

Relative and average sea level changes 415

veneered by littoral deposits, occurring around coastlines in many parts of the world were representative of interglacial sea levels, and that these could be correlated on a rank-order basis, counting upwards from present sea level (Zeuner, 1959). A "standard" series of Pleistocene interglacial sea level heights was developed (average sea levels in the sense defined above) and Fairbridge (1962) gave these a chronology by correlating the sequence with interglacials recognized in deep sea cores by Emiliani (1955) and others. Flaws in the argument were perceivd by Cotton (1963) and Russell (1964), amongst others, who recognized that the terrace flights occurred on coasts subject at least to endogenic movements. It became increasingly clear that the practice, of correlating surficial indicators of sea level on a rank order basis, is specious, as is well shown by the related problem of correlating major Pleistocene glaciations across the Atlantic on a similar basis (Cooke, 1973). Thus, as matters stand at present, the only widely distributed sea level indicators which can be correlated with any degree of satis­faction come from the post-glacial (Holocene) period, the last interglacial episode of about 120 000 to 135 000 years ago, and a few interstadial points in between. As a further constraint, age determination of these is possible only for Holocene formations on a global basis using "*C as the dating method, and the earlier indicators lie essentially in the tropical coral reef areas, as corals are the only materials which so far have yielded sufficiently reliable age data by the Th/ U method, which is the only dating method of wide usage in this context. Hence, the task of separating average sea level changes, caused glacio-eustatically, and endo- and epigenic movements, from the pattern of relative sea level movements, will be addressed through these data alone. Problems raised by comparison of these results with other less direct indicators of global ice/ocean volume changes, such as O/ 0 ratio variations in deep sea cores will be discussed, before proceeding finally to examine aspects of the question of coupling between exo-, epi-, and endogenic processes.

ENDOGENIC AND EPIGENIC VERTICAL MOVEMENTS

First order endogenic uplift, in tectonic belts, and first order epigenic movements, at the centres of glacio-isostatically rising areas, differ in magnitude in both rate and duration. As far as these movements are known from the sorts of dating studies of indicators which were outlined above, tectonic uplift rates may range to 5 m per 1000 years but generally are less, and persist for 106 to 107 years (Chappell, 1974b; Chappell & Veeh, 1978a), while glacio-isostatic rates can exceed 50 m per lOOO years (e.g. Donner, 1968) but have relaxation times around 103

years and reverse in direction as ice caps wax and wane (i.e. roughly every 3 x lo"* years for the northern Quaternary ice sheets). Hence, epigenic movements are best examined with Holocene sea level indicators, while the slower tectonic rates are better estimated from displacements of the last interglacial (120 OOO-

416 John Chappell

year) indicators.

Endogenic movements Separation of endo- and epigenic components of Vertical displace­ment seems best approached, at present, by successively-reducing approximations. Hence, the last interglacial is the best interval to examine for endogenic movements on the grounds that it was of roughly similar duration-as the present interglacial and the ice/ocean distribution was similar to the present, i.e. epigenic differences from the present Earth's figure are most likely to have been small. Thus, the elevation of last inter-glacial shorelines, relative to the present, is the best basis we have at present for estimating tectonic vertical rates.

Age determination of most last interglacial sea level indicators is based on the 230Th/23l+U method (Thurber et al., 1965). Principally these are raised coral reefs, in which the reef crest facies reliably indicates the low tide level palaeo datum, although there are a few cases where non-coralline shorelines have been reproducibly dated by the same means, using molluscs (usually significantly less reliable than corals for the purpose; Blanchard et al., 1967). Elevations relative to present fall into two geo-tectonic groups - one comprised on locations remote from plate boundaries, and the other containing sites near plate convergence lines. Elevations within the first group fall in a rather narrow range in comparison with those in the second group. Summarizing the review in Chappell & Veeh (1978a), the first group contains Hawaii (4-7 m above present sea level), Tuamotos (4 m ) , Cook Island (2 m ) , Mauritius (1.5 m) , Western Australia (4 m ) , Seychelles (up to 9 m ) , and Florida (up to 10 m). In most cases, the exact elevation of the reef crest (palaeo datum) facies relative to modern datum is not given and it is possible that this range may contract with further work. Details of this last interglacial episode will emerge in a later section: at this stage we take the palaeo datum for the interval 120 OOO to 135 000 years as 6 ± 4 m above present, where the variance may be due to second order tectonic or other factors, as is the 6 m mean difference from present datum.

Raised reefs in the second group are higher by up to 2 orders of magnitude, making uncertainty in relative uplift rate arising from the ±4 m error in the first group small in comparison. Localities include Barbados (up to 50 m; Broecker et al., 1968), Ryukyu Group (up to 200 m; Konishi et al., 1974), northeast New Guinea (up to 400 m; Chappell, 1974b; Bloom et al., 1974), Timor (up to 60 m; Chappell & Veeh, 1978a). An interesting feature in all these sets of results is that the elevation of the 120 000-135 OOO year BP reef changes along the coast or island chain, indicating progressive increases of rate towards a maximum uplift zone. Similar deformation patterns are known from coastal terrace flights around other tectonic coasts which have not been as well dated as the raised coral reefs (e.g. California (Wahrhaftig & Birman, 1965); New Zealand (Wellman, 1971a, 1971b; Singh, 1971; Lewis, 1971)). Figure 2 illustrates uplift deformation at Huon Peninsula, New Guinea and the sort of raised coral reef evidence under discussion (we return to Fig. 2 when changes of average sea

Relative and average sea level changes 417

Deformation parallel to coast shown by terrace crests (reefs Il-SUb)

Fig. 2 Evidence for tectonic movements and relative sea level changes at Huon Peninsula, New Guinea (based on Chappell, 1974b). Top: Huon Peninsula coast showing localities X, Y, Z referred to in text, and margins of largest terraces. Roman numerals follow terrace identifications in Chappell (1974b). Middle: section from Holocene reef I to last interglacial terrace Vl la /Vl lb . Growth of barriers represents major sea level rise relative to the land; growth of fringing reef represents minor sea level rise or stationary relative sea level. Bottom: Longshore variation of terrace heights. Reefs II to V l lb . Note: asterisk in base of reef Ilia indicates site of samples dated by 2 3 0Th/2 3 4 U at 50 000 years.

level are examined). Finally, although these patterns of vertical deformation have been related to tectonic processes (e.g. Dubois et al., 1974), more complete integration with numerical models of convergence and subduction tectonics (such as developed by Sleep (1975) and Smith & Toksoz (1972)) remains to be done.

418 John Chappell

Epigenic movements The global consequence of global ice/ocean mass redistribution, following the last déglaciation, was demonstrated by O'Connell (1971) showing that the historical non-tidal acceleration of the Earth's rotation is due to continuing slow compensation of the equatorial bulge to the redistributed surface load. Resulting changes in the figure of the Earth as a whole are analysed successively by Walcott (1972), and by Clark et al. (1978). The latter proceed from a calculation based on a visco-elastic Earth to estimate the pattern of relative sea level changes which should be observable from Holocene sea level indicators around the globe. Chappell (1974a) used a layered visco-elastic model to examine the deformation which should occur near continental margins over the last 6000 years as a consequence of marine transgressions of the continental shelves.

The important calculations by Clark et al. (1978) show a fair degree of general agreement with Holocene sea level indicators although, as they note, certain divergences (particularly for eastern US data) suggest that lithosphère thickness and elasti­city should be included as factors in the Earth model. The 'flat Earth' model of Chappell (1974a), which includes this factor, gives a better prediction of the 6000-year shoreline data in this region, although it is otherwise limited in not being applicable on a global scale, as it stands. The global predictions for the Holocene by Clark et al., are further examined by Newman et al. (1981) and are not discussed further. It is clear, however, that such sea level data provide a power­ful means for evaluating the response of the Earth to epigenic load redistribution, and hence the rheology of the solid globe. It is not the place in this review to offer recalculations of rhéologie behaviour, but discrepancies between predictions and observed sea level indicators will be mentioned as we proceed.

Separation of endogenic and epigenic movements An issue of significance for theories of endogenic (tectonic) processes concerns the variation of vertical rates through time. Although variations over time scales of 107 to 108 years are well known from the geological record, it is unclear whether the rates vary over 10 to 10 years and, if so, whether this is due to modulation of the endogenic process by the epigenic effects of Quaternary glaciation. The best evidence available lies with the Holocene and last interglacial sea level indicators.

Differences between uplift rates estimated from elevations of the last interglacial and Holocene indicators clearly are best made for places where the rates are highest, to minimize uncertainties stemming from correction for the last interglacial palaeo datum (discussed above), and from the error which arises when relating a particular type of indicator to modern sea level datum. Uplift rate u = (h - p - d ± e)/t where h is elevation above modern datum, t is its age, p is estimated height of palaeo datum where this is suspected to have been different from present (i.e. 6 ± 4 m for the last interglacial), d is the epigenic correction (as estimated by Clark et al. (1978), or by similar means), and e is the error estimate associated with all of these.

Relative and average sea level changes 419

The Huon Peninsula, New Guinea, with highest uplift rates so far determined, provides an example. Substituting values given by Chappell & Polach (1976) from the section marked at position X in Fig. 2, and estimating d =1 ± 1 m, gives u (last interglacial) = 1.85 ± 0.15 m per 1000 years, and u (6000 year reef crest) = 1.2 ± 0.2 m per lOOO years. The palaeo datum estimated for 6000 years BP is 0 m, on the grounds that déglaciation was complete and that average sea level should have been as at present. The rate discrepancy implies either that the 6000 BP palaeo datum estimate is wrong, or that the estimated epigenic correction, d, is wrong, or that a post-6000 seismic event depressed the coast by about 3 m. As the first two factors should be constant along the Huon coast, and the last may be constant or may vary irregularly across faults (cf. Pig. 2), their effects can be assessed by repeating the calculation at points with very differ­ent uplift rates. Results for point Y on Fig. 2 (highest uplift rate) give a rate discrepancy of 1.0 m per 1000 years, and for point Z (lower uplift rate) give a discrepancy of 0.3 m per lOOO years. Hence, it appears that rate discrepancy increases with mean uplift rate, which may be interpreted as indicating rate variation on a 10 to 10 -year time scale. Further work of this type is needed before such variations are well established, and before the guestion of whether epigenic forces affect the endogenic processes can be answered.

AVERAGE SEA LEVEL CHANGES AND WATER VOLUME CHANGES

Following the introductory definitions, average sea level changes are changes of global mean distance of the ocean surface above the centre of the Earth. This concept is difficult to investi­gate, even allowing that epigenic readjustment to changing ice/ ocean distributions can be estimated, and we turn first to the matter of changes of oceanic water volume associated with Pleistocene glaciation.

Submerged shorelines dated at around 17 000-18.OOO years old, i.e. coeval with the last glacial maximum, have been recognized near the outer margins of continental shelves in many parts of the world. The depths of these should provide the basis for estimating the total oceanic water volume changes due to glaciation, particularly if the data are corrected for estimated isostatic displacements. The result should agree with estimates of northern continental ice volumes at the glacial maximum with a correction for any Antarctic ice volume change. Unfortunately, uncertainties at present are about as large as the estimated changes. Estimates of ice volume (reviewed briefly in Chappell, 1974a) range from 130 m of sea level equivalent (Flint, 1971) through 100-115 m (Paterson, 1972) down to 75 m (Clark et al., 1978). The gap between estimates based directly on ice sheet reconstructions, such as Flint's and Paterson's, can be narrowed by including lesser contributions to ice volume, such as extension of the Fennoscandian sheet into the Barents Sea (Schytte et al., 1968) and possible increase in Antarctica where the margins extended at the time of glacial low sea level,

420 John Chappell

leading to an estimate of 115-125 m of sea level equivalent (cf. Chappell, 1974a) . Greater problems remain with data from 17 OOO-year old submerged shorelines. Amongst the better dated cases are examples from northern Australia, where coral and intertidal algal rock samples, occurring on or closely below the edges of submerged terraces, have been dated between 14 000 and 18 700 years BP by lkC and 230Th/231*U methods (Veeh & Veevers, 1970; Jongsma, 1970). These lie 150-160 m below present sea level. According to the calculations of Clark et al. (1978), these should occur at the same depth as similar-aged shorelines on the southeastern US shelf; actual observations from the latter show depths of -75 m (reviewed by Clark et al.) to -90 m (Curray, 1960). Hence, it appears that present estimates of global iso-static (eplgenic) readjustment, following the last glaciation, are in error. Further careful exploration of 17 000-year shore­lines clearly is needed.

The problem of reconstructing oceanic water volume changes becomes more acute before the last glacial maximum. Ice volume reconstructions are very much less certain,as glacial margins before the maximum are imperfectly known. Indicators of relative sea level changes become controversial as to age and inter­pretation. The question of whether or not an interstadial high sea level occurred around 30 000-40 OOO years ago illustrates the point. Since the advent of radiocarbon dating very many authors have documented apparent indicators of relative sea level occur­ring in this time range, according to "*C determinations, and lying at anything between 10 m above and 60 m below present sea level. By the late 1960s it was widely accepted that sea level was within 15 m of present around 30 OOO years ago (e.g. Emery & Milliman, 1970; Faure & Elouard, 1967) , despite that Broecker (1965) had cautioned against acceptance of "*C determinations from marine carbonates in this age range, and that such a sea level was difficult to reconcile with evidence on land for considerable ice volumes. Reviewing the problem, Thorn (1973) examined 188 such studies and found that none could be rated as unequivocally superior, in terms of dating reliability or other­wise in terms of undisputable evidence for a valid sea level indicator. In fact, the only extensive evidence which appears to pass on both counts comes from the flights of raised coral reefs dated by 23,,Th/2 **U (and also 1>tC in the case of New Guinea) which, as discussed above, occur in regions of rapid endogenic uplift. Thus it is, that there has been a growing tendency to estimate oceanic water volume changes by indirect means, most notably from variations of ô O/ô O ratio in foraminifera in deep sea cores. There are difficulties in reconciling these results with such details of late Pleistocene sea levels as we have, however.

DEEP SEA CORE o180/<5160 RECORDS AND .WATER VOLUME CHANGES

Since the pioneering work of Emiliani (1955, 1966) it has been clear that oxygen isotope ratio variations in cores from continuously accumulating deep sea sediment constitute one of the

Relative and average sea level changes 421

most important records of Quaternary glacial history. The ratio, recorded in foraminiferal CaCOn, increases as ocean temperatures decrease and also as ice volume increases. The latter effect is due to preferential precipitation, in snow, of ô 0 relative to ô180 - a depletion which can be over 40°/oo relative to the value in mean ocean water. Relative magnitudes of the two effects on the isotope ratio was debated until Shackleton showed that variations of about 1.2°/0o occur throughout cores from equator­ial Pacific areas, which on the basis of foraminiferal palaeontology appear to have experienced only small temperature changes in Quaternary times (Shackleton & Opdyke, 1973, 1975). On the grounds that the last glaciation withdrew about 120 m of sea level equivalent from the oceans (see above), a general calibration has been adopted that oceanic <5180 increases by about 0.1°/oo for 10 m of glacio-eustatic lowering of sea level. Deep sea core records now have their sedimentary time scales calibra­ted to the 700 000-year Brunhes-Matuyama geomagnetic reversal (Shackleton & Opdyke, 1973, 1975), and glacial-interglacial cycles show satisfactory agreement with the same broad 10 -year cycle seen in the relative sea level curves recognized in flights of raised coral reefs (Chappell, 1974c).

Glacial water volume changes indicated by deep sea core 6 80 records appear to conflict with apparent sea level changes, however, when comparisons are made within a lo"*-year time frame. Figure 3 {top) shows relative sea level changes at Huon Peninsula over the last 40 000 years as interpreted from the raised reef sequence in Fig. 2. Subtraction of endogenic uplift from this figure gives a relative sea level curve for the northern Australian region, which should differ from the glacio-eustatic water volume curve only to the extent of epigenic movements. Mean uplift rate for the section in Fig. 2 is 1.6 ± O.l m per 1000 years, based on the last interglacial reefs, and may be as low as 1.2 m per 1000 years on the basis of the Holocene reef. Figure 3 (middle) shows the relative sea level curve with uplift subtracted, as a shaded band to cover the range of uplift rate estimates. The low sea level shown between 15 OOO and 20 OOO years is based on the deeply submerged terrace in Fig. 2, which was mapped by echo-sounder and dredge (Chappell, 1974b) but is not dated; it agrees well in position with the dated submerged terraces in northern Australia, mentioned earlier.

Superimposed on the reduced sea level curve in Fig. 3 are ô180 records from three important deep sea cores, V28-238 (Shackleton & Opdyke, 1973) from the equatorial Pacific, Caribbean core P6304-9 (Emiliani, 1966), and Panama basin core V19-28 (Ninkovitch & Shackleton, 1975). The 6180 range in the latter two cores exceeds that in V28-238 owing to glacial-interglacial temperature changes (cf. Shackleton & Opdyke, 1973). Also shown are ô 0 data from Tridacna clams from the Huon reefs (P. Aharon, personal communication, and in prep.), which are known to be in isotopic equilibrium with the surrounding sea water (Aharon et al., 1980). There are clear discrepancies of two types. Firstly, the deep sea core 6180 curves do not show the interstadial high sea levels at 30 000 and 40 000 years. Secondly, the ô O difference between the modern reef at Huon

422 John Chappell

0 Years BPxIOOO

-140m

10 0 Years BPxIOOO

P6304 -5.0

<

Fig. 3 Top: detail of reefs I to Ilia at Sialum (cf. Fig. 2). Middle: relative sea level changes for section at top, deduced from internal structures of the reefs (cf. Chappell, 1974; Bloom et al., 1974). Bottom: glacio-eustatic sea level changes relative to New Guinea-northern Australia, deduced by subtracting endogenic uplift from curve at centre, compared with S I 80 curves from deep sea cores (P6304-9, Caribbean, Emiliani, 1966; V28-238, equatorial Pacific, Shackleton & Opdyke, 1973; V19-26, Panama basin, Ninkovitch & Shackleton, 1975), and 5180 data from Huon Peninsula terraces, reef Ilia crest, reef II crest, and modern reef (solid vertical bars).

Peninsula and the 30 OOO BP reef is 0.8°/oo, similar to that in

core V28-238 but significantly less than in most other cores

across the same time interval. Alternative interpretations are

as follows:

(a) The deep sea core 6180 correctly indicates ocean water

volume changes and either (i) the dated evidence for sea level at

-150 m relative to the northern Australian region is wrong, and

the 17 OOO year relative sea level stood above -lOO m, or (ii)

Relative and average sea level changes 423

the -150 m low sea level is correct and the relative sea level at 30 OOO years was about -120 to -130 m, implying that uplift rates at Huon Peninsula were higher than the mean rate for the last 120 000 years by a factor of 3, and furthermore must have reduced to the mean rate before the Holocene, or (iii) that the -150 m low sea level is correct and uplift rate varied only within the limits set by the shaded curve in Fig. 3, if at all, and that epigenic movements caused the region to rise by at least 70 m between 30 000 and 17 OOO years BP and then to subside by the same amount before about 8000 years ago.

Possibility (i) should be eliminated, as the evidence of Veeh & Veevers (1970) and Jongsma (1970) for the -150 m relative sea level is at least as good as any of this type. Case (ii) , entailing radical variations of uplift rate, is eliminated because the assumption of locally uniform uplift since the last interglacial has yielded estimates of earlier interstadial sea levels (viz. at 62 OOO, 84 000, and 105 OOO years) which are in good agreement both within regions (Barbados, New Guinea, Ryukyus, and Timor) and between these regions (Bloom et al., 1974; Chappell & Veeh, 1978b). Case (iii) is discussed below.

(b) The deep sea core record at the lo'*-year level is smoothed

by bioturbation or other factors, during sedimentation, to such

an extent that shorter interstadial oscillations are lost.

Supporting this is the fact that the 6 8 0 shift, between 30 OOO

BP and present, is smaller at Huon Peninsula than in the deep

sea cores. This is contrary to expectation, because other

evidence shows that temperatures in New Guinea, around the last

glacial maximum, were at least 6°C lower than present (Bowler

et a l . , 1976; Webster & Streten, 1978). Hence, if the New

Guinea 30 000-year reef did not represent a substantial inter­

stadial event it would show a ô 8 0 shift exceeding that of core

V28-238, at least, and probably exceeding the shifts seen in the

other two cores. It is concluded that ice/water volume and/or

climatic oscillations shorter than 10 years in duration are

much supressed, or even lost from view, in deep sea cores with

normal sedimentation rates (i.e. rates of 2 to 4 cm per 1000

years).

(c) The 6* 0 differences can be reconciled with minimum

distortion by admitting partial smoothing in the deep sea core

records, allowing no temperature effects on ô 8 0 at Huon £ 1 8

Peninsula, and explaining the remaining discrepancy between o 0 and relative sea level curves in terms of global epigenic movements. This sort of juggling leads to very different patterns of epigenic movement from those calculated by Clark et al. (1978), who predict that the northern Australian region should register sea level changes rather closely accordant with actual oceanic water volume changes. In an interesting discussion, Morner (1976) conjectures that the figure of the geoid may have changed more radically than predicted by Walcott (1972) or by the subsequent results of Clark et al., due to quite hypothetical effects of changes of rotation rate on the Earth. Morner presents no calculations, however, and it seems likely that some of his notions, involving changes of geoidal shape originating at the core-mantle boundary, can be ruled out

424 John Chappell

on the grounds that lower mantle relaxation times exceed 10 years (cf. McKenzie, 1967). Nonetheless, the possibilities should be explored by careful assemblage of data on relative positions around the globe, of the 17 OOO and 30 000 years BP sea level indicators. The 30 000-year shorelines probably are the more accessible, and certainly many of the 188 examples reviewed by Thorn (1973) should be investigated further.

To conclude this section, the author's estimations are as follows. The analysis of glacial-interglacial global eplgenic movements by Clark et al. (1978) is not the last word on the matter. Addition of an elastic lithosphère to their model probably will not resolve the problem, although the gap between northern Australian and southeast US data on the c. 17 OOO year shorelines may be narrowed, particularly if it is allowed that the glacial-interglacial water volume change is around 120-130 m of sea level equivalent. Numerical calculations should be made of some of the effects discussed by Morner (1976) . Turning to the ô180 results, the discrepancy between the cores and Huon Peninsula at 30 000 cannot be resolved by juggling with epigenic movements. As stated, the magnitude of the discrepancy increases if ocean temperatures at Huon Peninsula were colder than present, 30 OOO years ago. In addition to the terrestrial evidence for lower temperatures (cited by Webster & Streten, 1978), 6180 measurements from other terraces in the flight shown in Fig. 2 indicate strong temperature effects (Aharon et al., 1980; Aharon, in prep. ) . I conclude that the deep sea core records are seriously smoothed, for events shorter than 101* years and that relative sea level curves derived from well dated flights of raised coral reefs constitute the best available records of ice/ ocean water volume changes, in the 101* to 10 year time frame, while the cores provide the best records in the lo to 10 year frame.

EXOGENIC EFFECTS ON OCEANIC WATER VOLUMES

In addition to effects of gravitation on endogenic processes, exogenic factors can be coupled to endo- and epigenic forces through effects on the climate. Anderson (1974) and Lambeck & Cazenave (1976) have shown that there is probably a causal chain whereby the zonal circulation affects rotation of the Earth, which in turn affects seismicity through acceleration stresses,-this seems substantiated by historical data. Whether such a chain effect is operative at the level of the much slower and larger Quaternary climatic fluctuations is unknown, although the question raises the interesting matter of lagged feedbacks between endogenic vulcanism and climate (Chappell, 1973). Similarly, epigenic forces induced by ice sheet growth and decay may influence endogenic vulcanism (Matthews, 1969), although present data from Quaternary records do not prove a relationship. These questions are laid aside, and to conclude this review the question of exogenic influence on climate is addressed on the Quaternary time scale, with relative sea levels and deep sea core results as the primary data.

Relative and average sea level changes 425

The principal argument relating external factors to climate, on a Quaternary time scale, has come to be known as the Milankovitch theory after the Czech mathematician who first made comprehensive calculations of the way in which the solar radiation pattern, received by the Earth, is slowly modulated by three perturbation factors in the Earth's orbit - precession, obliquity changes, and eccentricity changes. The theory was adopted by Zeuner (1959) to provide a chronology for Pleistocene glaciations, but was frequently discredited by meteorologists. Interest was revived when Broecker et al. (1968) showed that the 30Th/23l*U ages of Barbados coral terraces I, II, III corres­

ponded with Milankovitch peaks for maximum contrast of warm northern summer-cool northern winter differences. Similar correlations from New Guinea reef terraces were shown by Veeh & Chappell (1970), and Broecker & van Donk (1970) showed a close correspondence between glacial terminations registered by 6~80 patterns in deep sea cores and the cyclic variation of orbital eccentricity (with a period of about 100 000 years). Subsequent recomputation of the Milankovitch curves to successively higher levels of accuracy (Vernekar, 1972; Berger, 1978) and refinement of the deep sea core time scale, through use of the Brunhes-Matuyama geomagnetic reversal, has led to widespread acceptance of the theory, at the 10 -year scale (Hays, 1978; Imbrie,unpub.). It also is accepted that the precessional component, with a period of about 20 000 years, has strongly modulated late Quaternary climates and glaciation, through the evidence for high relative sea levels (representing interstadials) which are now known to occur simultaneously in Barbados, New Guinea, and Timor at the times when northern summer-winter radiation differences are maximal (Chappell, 1976). Evidence supporting the Milankovitch theory for the last 700 OOO years is summarized in Fig. 4. Relative sea level changes for the three principal sets of coral reef terraces are reduced to a single curve by removing endo- and epigenic factors, as far as this is possible. Major peaks correlate well with Shackleton & Opdyke'1 s (1973) deep sea core ô 0 curve, as shown. The record of climatic change on land, interpreted by Kukla (1970) from Czechoslovakian loess sequences, also is shown. The time scale applied here is based on C dates and the position of the Brunhes-Matuyama reversal in the sequence (Kukla, 1970) and on the position of the 108 OOO years BP geomagnetic 'Blake event' (Kukla & Koci, 1972). Between the two geomagnetic markers the loess time scale is interpolated, and hence the correlations shown may be fictitious. However, the pattern of correlations between the three geologic records is very good, especially for the last 2oo 000 years where the dating is most secure. Also shown in Fig. 4 are the times when northern summer-winter radiation contrast was maximum (potential termination events) and when the contrast was minimum (glacial initiation or growth conditions). Again, correspondence with the geological records is striking, although it must be pointed out that the geological age-errors increase with age value, and beyond about 150 OOO years come to equal or even exceed the 10 OOO year period of the precessional hémicycle.

426 John Chappell

P n a ! ! i ! ; ! ! ! ! i i ' i i i ! Terminer t + t t t t t t t t t t + t t t + t t

r • i ' i 1 1 1 1 1 i 1 1 1 1 _ 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Fig. 4 Comparison of three different sea level and climate records for last 700 000 years. Top: sea levels from Huon Peninsula and Atauro Island, Timor. Centre: equatorial Pacific core V28-238. Bottom: Loess and palaeosoils from Czechoslovakia, after Kukla (1970). The tall vertical bars represent times of forest soil or brown earth soil development, the low intervening strips represent times of loess accumulation. The time scale is given by the Brunhes-Matuyama magnetic reversal at unit 11, the Blake magnetic event between B1a and B1b, and C dates in the youngest soils. Times of solar radiation "glacial initiation" and "glacial termination" conditions are shown below.

Proof of the Milankovitch theory in detail, and exploration of theoretical models of orbital effects on climate and glaciation (e.g. Budd, 1981), requires further refinement of the relative sea level records', and their successful reduction to a reliable curve of oceanic water volume. Beyond this, it now has been shown that joint sea level and 0 studies of indicators such as raised coral reefs offers a tantalizing prospect of testing theories for rapid climatic and other changes (Mercer, 1981; Aharon et al., 1980). This appears to be a major thrust for the future, following resolution of the question of global epigenic changes.

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