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6 Graphic Analysis of Dislocated Quaternary Shorelines ARTHUR L. BLOOM Cornell University NOBUYUKI YONEKURA University of Tokyo ABSTRACT If a sequence of dated emerged shorelines is found at different heights on several transects, the heights of the intermediate shorelines can be plotted against the height of the highest one in each sequence. The regressions of the intermediate shoreline positions yield a set of equations: H. = a.H + b., `, ~z m, ~ (6.1) where Hm ~ is the height of the highest shoreline in the sequence on transect t, Hi ~ is the height of an intermediate shoreline i on the same transect, al is the regression coefficient, and bi is the intercept. By assuming or establishing the position of sea level at the time of formation of the highest shoreline, the initial heights of the intermediate shorelines can be calculated by substitution in Eq. (6.1) without the previously necessary assumption that the dislocation on any transect had been at a constant rate. A similar procedure gives the initial level of any shoreline in a sequence of submerged features, if their depths are plotted against the depth of the deepest shoreline in the sequence. Correlation coefficients for typical calculations are inflated because Hm' always incorporates all subsequent movements on that transect. Nevertheless, realistic and testable results can be achieved for intervals of the last 8000 yr and for the last 125,000 yr. INTRODUCTION This chapter is about an analytical technique for deter- mining the original altitude of shorelines that are now displaced by tectonic movements. Before we can evaluate the causes of sea-level changes, we must be able to mea- sure them with reasonable confidence. The literature of sea-level change is a jungle of generalizations, misconcep- tions, faulty interpretations, unsubstantiated age estimates, 104 and just poor science. R. Stuckenrath, Jr. (University of Pittsburgh, personal communication), estimated that fully half of the researchers who received radiocarbon dates from the Smithsonian Institution radiocarbon laboratory do not "believe" the results, meaning that if their a priori conclusions were not confirmed by the dates, the dates were doubted. A great amount of experience and judg- ment is required just to know which published data are valid.
GRAPHIC ANALYSIS OF DISLOCATED QUARTERNARY SHORELINES A basic problem is that sea level is constantly changing on many temporal and special scales (Figure 6.1~. Waves cause the water surface to rise and fall a meter or more every few seconds, providing a high-frequency background noise to the secular change of millimeters per year that we hope to document. The diurnal or semidiurnai tide is commonly the largest magnitude sea-level change on any time scale short of 104 yr, barring tsunamis and hurricanes. We consider sea-level changes on two time scales. The first is the scale of glacially controlled worldwide ("eu- static") sea-level fluctuations of 104 to 105 yr, which cov- ers the time scale of complete glacial-interglacial cycles and their major subdivisions. The second is the scale of 103 to 104 yr, primarily considering the deglacial hemi- cycle of the last ice age and the most recent 10,000 yr of geologic history called the Holocene Epoch. On this time scale, glacial eustatic sea level has risen some 120 m as the Northern Hemisphere ice sheets disintegrated, and then it has stabilized or fluctuated slightly with the subsequent isostatic adjustments of the solid earth to the postglacial distribution of surface ice and water masses. Doubts, uncertainty, and unresolved problems are noted in each section, for our present inquiry is to determine what we know with confidence and what we still need to learn. The chapter ends with some suggestions for resolving some of our uncertainties. THE GLACIAL CYCLE: 104 TO 105 YEARS Graphs of sea level for the last 140,000 yr or more are drawn primarily from two sources: (1) coral-reef terraces on tropical coasts that have been uplifted by tectonic forces, ICC A9B ~Joriations Meteorologicol Annual Tend TsURD : Gravity Wolves resect) ~ _;. ~,c ~°c 105 and (2) the deviations from a standard of the i8O/~6O ratios in foraminiferal tests from deep-sea cores. The methods and their premises are very different, and yet the superfi- cial agreement is good, especially concerning the time scale. The amplitude of interstadial-stadial sea-level change inferred from the oxygen-isotope record of the last ice age is significantly greater than that inferred from the coral- reef record, and we need to understand the cause of that difference. Nevertheless we should stress the overall similarities of the graphs more than their differences (Figures 6.2 and 6.3; see also Matthews, Figure 5.3, this volume). For coral-reef terraces, a reasonable time control (26 = 5 percents is available for 200,000 yr by the uranium- series dating method (Harmon et al., 1979~. For reefs, the problem is to convert a known age and a present height into both an initial level and an uplift rate. In the equation H = ax + b, present height H is the result of uplift rate a times x (time), and b is the desired initial or starting level. Clearly, the equation cannot be solved knowing only the present height and age of a terrace. However, by assuming that some older reef terrace, such as the last interglacial one that is about 125,000 yr old, formed at an assumed level such as +6 m, and assuming that tectonic uplift is reasonably constant on the time scale of 105 yr, the initial level b, and the uplift rate a can be inferred. The results from various islands, where uplift rates vary from 0.1 mm/ yr to more than 5 mm/yr, have been reasonably consistent. For example, one can confidently predict that if a 28,000- yr-old reef terrace is found above present sea level, the uplift rate is greater than about 1.5 mm/yr, because sea level at that time of reef formation was probably at -35 to FIGURE 6.1 Schematic diagram of the spectral distribution of sea level (Stom- mel, 1963, Figure 11.
106 3.0- V29-179 North Atlantic 3.5: ~ D e pt h '55tm) . ~ ~ 20 4060 80 100 120 140 Age (Ka) +3 ^ 12392-1 Canary Islands '~ °~4 \i~t _ ~4 6 8 ~10 cePth(m) . . . . . . ~ ) 20 40 60 80 100 120 140 Age (Ko) +3 V19-29 E. equatorial Pacific 0 a\ 60 +4 ~8 ~ 10 Depth(m) 65 . . . ~ .. ... ) 20 40 60 80 100 120 140 Age(Ko) FIGURE 6.2 Three oxygen-isotope curves from high-sedimentation-rate deep-sea cores that show details of the variations of oxygen-isotope ratios for the past 140,000 yr (see text for references). =0 m and the reef had to rise above present sea level by at least 5000 yr ago to escape the postglacial rise of sea level. Such a prediction can then be tested by using it to predict the height of older and younger terraces in the sequence: for instance, if the uplift rate were 1.5 mm/yr (or the equivalent 1.5 m/ka, or kiloanno), the last intergla- cial terrace should be nearly 200 m above sea level (125 ka x 1.5 m/la + 6 m, which is the assumed height of sea level in the last interglacial). Note that the assumed height of 6 m for the last interglacial sea level is only a few percent of the present terrace height on coasts where tectonic move- ments are rapid, that is, on the order of millimeters per year. As many islands on tropical island arcs have coral- reef terraces measured in hundreds of meters above sea Thousand years B. ~ 150 140 130r120 ~ 90 80 70 60 50 40 30 20 10 0 t ''J \t 'I \.,'''. ~ ? T 120 1~o l-60 \ ;-80 ~ 1-1°° ~ 1 -120 -140 Papua New Guinea paleo-sea levels. . . {Yoneluro and Bloom,ln prep., \ I T Papau New Guinea sea-level minima. ~ {Chappell,l974) ~ Barbados sea-level minima, a'.~973) FIGURE 6.3 Revised sea-level curve for the coral-reef terraces on the Huon Peninsula, Papua New Guinea (Bloom and Yo- nekura, 1985, Figure 6.4). ARTHUR L. BLOOM AND NOBUYUKI YONEKURA level, the calculation of the initial position of sea level during the last interglacial is not critical. A range of "a few meters" or "5 to 10 m" above the present gives satis- factory results that lie within the typical survey errors for terrace heights. Figure 6.4 graphically illustrates the assumptions of the above method: given a sea-level curve and an assumed uplift rate, reefs grow at times of tangency when land and sea level are rising at the same rate, just prior to a sea-level maximum. Sea level then drops while the constructional reef terraces are carried upward to their present position on a hillside. In principle, similar reef-building events should occur just after each sea-level minimum, but so far, only one such reef has been identified. As noted in the In ~ - . - 200 - ~ 160 lo ~0 40 MSL O I 120 40 80 160 o am ( predicted ) ~\~\\+6m Fumed ~ 20 40 60 80 100 120 140 AGE (x10005.) FIGURE 6.4 The hypothetical coral-terrace sequence generated by uniform tectonic uplift of 2 m per 1000 yr superimposed on sea-level oscillations modeled from the oxygen-isotope record of benthic foraminifera in core V29-179 (Streeter and~Shackleton, 1979~. final section of this chapter, if others could be found by drilling or in natural exposures, stadial and glacial minima as well as interstadial and interglacial maxima sea level could be dated and the amplitudes of sea-level fluctuations could be defined. In addition to the value of coral-reef models for predict- ing sea levels of the last 125 ka, an excellent predictive model of the ages of middle Pleistocene and older terraces can be made if one of the lowest terraces in the sequence can be proved to be of last interglacial age, about 125,000 yr old. This situation is very common, because sea level was probably a few meters higher than at present in last interglacial time (+6 m is the widely accepted estimate). On stable coasts or coasts with slow uplift, the last inter- glacial terrace is always prominent. Downtown Honolulu
GRAPHIC ANALYSIS OF DISLOCATED QUARTERNARY SHORELINES is built on it. The height of the last interglacial terrace above or below the assumed initial level of +6 m estab- lishes a long-term (105 yr) vertical tectonic rate, which can be cautiously extrapolated by an order of magnitude to predict the ages of higher and older terraces dating back to the early Pleistocene. In the few places where a tephro- chronology is available, as in New Zealand (Pillars, 1983) or Japan (Machida, 1975~; or where amino acid racemiza- tion methods can be applied, as in California (Muhs, 1983), or reasonably inferred, as in Baja California, Mexico (Ortlieb, 1980), the predicted ages of older terraces are reasonably well supported. Correlations are often sug- gested with the odd-numbered oxygen-isotope stages of the deep-sea record, but those correlations should be used with caution, because the earlier oxygen-isotope stages are themselves dated only by interpolation between the time of the last interglacial high sea level and the Brunhes- Matuyama paleomagnetic epoch boundary that is about 730 ka old (Imbrie et al., 1984, p. 282~. TERRACES OF SEA-LEVEL MAXIMA DURING AND SINCE THE LAST INTERGLACIAL The coral-reef terraces of the Huon Peninsula, Papua New Guinea (Figure 6.3) preserve an exceptionally com- plete chronology of sea-level fluctuations for at least the last 140,000 yr (Bloom et al., 1974; Chappell, 1974, 1983; Aharon and Chappell, 1986~. A great fault splinter on the northern coast of the Huon Peninsula has been rising and tilting in late Quaternary time, broken by numerous minor faults but maintaining its overall morphotectonic integrity. A succession of coral-reef terraces has been built on this block. Where the substrate surface was steep or the ter- race-building interval was brief, the reefs were fringing. Where the preexisting slope was gentle or sea level stayed in the same position for a relatively long time (or repeat- edly occupied the same level), the reef grew as a barrier seaward of a lagoon that was up to 1 km in width. The internal structure of the reefs shows that they built upward and outward over their own fore-reef taluses (Figure 7 of Chappell, 1974; Chappell and Polach, 19761. Their upper surfaces are usually level and composed of typical Indo- Pacific shallow-water corals and algae. Behind the reef crests, mollusc-rich back-reef and lagoon carbonate sand accumulated. Reef crests as old as 124,000 yr to 140,000 yr have no more than about 1 m of karst relief. Swallow holes and sinking streams mark the lagoon floors. Younger reef terraces show minor gully dissection and dripstone curtains down their fronts. Less than a meter of soil in weathered volcanic tephra has accumulated on the terrace treads. Previous analyses of the terraces' ages and heights used 107 an assumed constant uplift history and an assumed sea level of +6 m for the last interglacial stage (124,000 yr) to derive the paleosea-level positions during the multiple interstadials of the latest glaciation. The resulting esti- mates of interstadial sea levels (Bloom et al., 1974) were consistent with similar estimates for terraces on Barbados, where the average rate of tectonic uplift was only about 10 percent of the rate in Papua New Guinea. The converging estimates were regarded as reasonable for the interstadial sea-level maxima, so that the height of a terrace of similar age elsewhere could be converted into an uplift rate by adding the present terrace height above sea level to its estimated original height (which is for all interstadial ter- races at or below present sea level) and dividing the change in height by the age of the terrace. The weak point of the argument was obviously the assumption that on the time scale of 104 to 105 yr, tectonic uplift was at a uniform rate. A new graphic approach to this problem has been derived, based on previous Japanese work (e.g., Ota et al., 1968) but similar to the shoreline relation diagnosis of Scandinavian workers (e.g., Donner, 1965~. Most of the following paragraphs are directly from Bloom and Yonekura (1985~. For illustration, we can use data on the ages of terraces and their heights along six transects on the Huon Peninsula, Papua New Guinea (Tables 6.1 and 6.2~. Arbitrary errors of 5 m, 2 m, and 1 m were assigned to the reported terrace heights to permit least- squares error evaluation. The last interglacial terrace, known as terrace VIIb, has an assumed age of 124,000 yr and was formed when the sea was 6 m higher than at present. For each of the six transects, the heights of all lower and younger terraces are plotted against the height of terrace VIIb (Figure 6.5~. The data points on Figure 6.5 are drawn as closed circles large enough to include the probable errors of age and height. If we assume that at a critical location terrace VIIb main- tained a height of 6 m for the past 124,000 yr (ignoring erosion), then at this location there would have been no uplift and differences in the elevation of the terraces would reflect changes in sea level only. Thus where the line of best fit for a given terrace intersects the vertical dashed line at HV~b = 6 m, it gives the height of sea level at the time of formation of that terrace. A regression may be performed to give a better estimate of the relationship. This yields the equation: Hit = aiHvIlb,! + bi, (6.2) where HVl[~b ~ is the height of terrace VIIb on transect t; H is the height of an intermediate terrace i on the same intersect; ai is the regression coefficient; and bi is the intercept. The intercept gives the elevation of terrace i assuming the elevation of terrace VIIb is zero. However, as we have seen, to calculate sea level at the time each
108 ARTHUR L. BLOOM AND NOBUYUKI YONEKURA TABLE 6.1 Measure of Reef-Crest Elevations (in meters) for Six Transects Along the Huon Peninsula, Papua New Guinea Transects Terrace Age (ka) Kanzarua Blucher Kwambu Nama Sambero Kambin VIIb 124 30 80 15 60 50 120 VI 105 50 15 60 15 10 93 V 82 190 55 17 90 80 60 IV 60 125 70 48 - 28 IIIa 50~0 90 65 42 IIIb 40 70 41 28 10 10 II 28 30 18 7 - I 6 15 10 6 5 5 2.5 NOTE: Data include arbitrary errors that were assigned to permit least squares evaluation, as follows: _5 m for elevations H 2 60 m; _2 m for 60 m > H 2 5 m; and _1 m for H < 5 m (Bloom and Yonekura, 1985, Table 6.1~. terrace i was formed we must predict its value based on an elevation of 6 m for the terrace VIIb. This is analogous to predicting the heights of the terraces along a transect at the critical location where Ham has remained at 6 m and uplift has therefore been zero. Thus, by substituting the regression-line values of al and hi in the equation with Herb = 6 m, the paleosea-level estimates for the several Wisconsin-age interstadials can be calculated (Figures 6.3 and 6.5, Table 6.2) without the further assumption of a constant uplift rate. The justifica- tion for the method is the very high correlation coeffi- cients for the regression equations (Table 6.2~. Only ter- race IIa (50,000 to 40,000 yr ago), which has only three measured heights, has a least-squares predicted height error that is significantly greater than the errors arbitrarily as- signed to the measured terrace heights on the transects (Table 6.1, Figure 6.31. The calculated values for terraces I to IV are similar to those listed in Bloom et al. (1974, Tables 3 and 4), where they were calculated on the assumption of constant uplift. However, the newly calculated height of sea level during the formation of terrace VI (105,000 yr ago) is 0 m instead of the former value of-15 m, and the calculated height of sea level at the time of terrace V (82,000 yr ago) is -7 m instead of the former value of -13 m. The purpose of this chapter is to review the method, not the results, and so further discussion is deferred. However, the demonstra- tion that a valid mathematical regression technique gives sea-level estimates that are quite similar to those estimates made by assuming constant uplift rate is justification for the assumed constancy of uplift at the time scale of 105 yr. The last interglacial surface is widespread at depths of 6 to 10 m below a Holocene coral veneer on many atolls. If no more than 1 m of limestone has been lost from the reef surface during subaerial exposure, a lowering of the last interglacial reef surface from its assumed original height of +6 m to (for example) a present height of-6 m in about 120,000 yr implies an atoll subsidence rate of about 0.1 m per 1000 yr, a rate appropriate for subsidence of oceanic lithosphere during cooling (Sclater et al., 1971; Bloom, 1980, p. 512~. TECTONIC MOVEMENTS ON THE TIME SCALE OF 20,000 YEARS The straight-line regression equations with high corre- lation coefficients demonstrated above justify the assump- tion of constant uplift rate on the time scale of 105 yr, but do not require it on shorter times. Careful analysis of TABLE 6.2 Regression Equations for Elevations (Hi) of Terrace i against Elevations of Terrace VIIb Hi Age (ha) a r SL (m) VIIb HVI 105 HV 82 HIV 60 HIIIa 50 - 40 HIIIb 40 HII 28 HI 6 1.00 0.77 0.60 0.46 0.41 0.32 0.20 0.05 0.00 1.000 - .55 0.999 -10.16 0.999 -26.80 0.999 -48.26 0.995 -40.21 0.981 -36.25 0.995 -3.93 0.969 +6.0 (assumed) +0.1 -6.6 -24.0 -45.8 -38.3 -35.1 -3.6 NOTE: These equations are used for determination of paleo- sea levels (SL). See Table 6.1 and Figure 6.5 (Bloom and Yonekura, 1985, Table 6.2).
GRAPHIC ANALYSIS OF DISLOCATED QUARTERNARY SHORELINES Huon Peninsula, Popua New Guineo JO m 300 200 100 O - 100 1 24ka Vl lb: + 6m (ossu med ) 105 Vl . O (calculated) 82 ~ -T 60 IV . - 24 50-40: Itla: -46 40 111b:-38 28 11 - 35 6 1 - 4 ~ V / / ~ /Vl ^250 //6~/o''": IV I /J/ grout / in ll a 200 :^ Liz O 3 33 ~ cr cr 0 3 300 m ;K c. 2 o o FIGURE 6.5 Regression of height of terrace i (Hi) as a function of height of terrace VIIb (Habib) based on six transects on the Huon Peninsula, Papua New Guinea (Bloom and Yonekura, 1985, Figure 6.31. Table 6.2 shows that there was variation in the Huon Peninsula uplift rate on the time scale of the 20,000-yr sampling interval. For instance, terrace VI is 85 percent the age of terrace VIIb (105 versus 124 ka), but is only 77 percent as high (the value of coefficient al, ignoring bi). Uplift in the interval between 124,000 and 105,000 yr ago was somewhat greater than the long-term average. Another way of modeling uplift rates on the 20,000-yr time scale is to accept sea-level estimates such as those in Table 6.2 and Figure 6.3 and from them calculate uplift rates for successive increments of dated uplift history. This method is appropriately called bootstrapping in that each increment of uplift is used as the basis for calculating the next older increment. An example (Figure 6.6) is drawn from work by Urmos (19853. The site is Araki Island, a small reef-terraced island 5 km south of the south coast of Santo Island, Vanuatu. On Araki, a Holocene reef terrace about 5500 yr old is 26 m above present sea level. Assuming, for simplifica- tion, that sea level in the region has not changed in the last 5500 yr, the average late Holocene uplift rate is 4.75 m per 1000 yr. Above the large Holocene reef terrace on Araki 109 is a succession of small stair-step terraces up to the flat reef-capped summit at 237 m. The next dated terrace on ,VIlb the hillside is about 38,000 yr old and is now at a height of about 40 m. Since the rate of uplift for the last 5500 yr is known, and assuming an original paleosea level of - 1 m (Figure 6.3), the increment of uplift between 5500 and 38,000 yr is calculated to be about 1.75 m per 1000 yr. The process is repeated for each step back in time, using the previously established estimates of sea level at the time of reef growth. The reef at the top of Araki Island is 105,000 yr old and is now at 237 m. If sea level at the time of origin was at present level (see above), then the average uplift rate for 105,000 yr has been 2.26 m per 1000 yr. However, successive increments of uplift rate range from 1.67 to as high as 4.75 m per 1000 yr (Figure 6.6), a factor of 2.8. In particular, the late Holocene uplift rate has been faster than at any prior time in the last 105,000 yr. We do not know if this is evidence of accelerated Holocene tec- tonic movement, or an artifact of the short sampling inter- val. We believe that the tectonic uplift of the region has accelerated in the Holocene, because nowhere in Vanuatu have we found a reef 28,000 to 30,000 yr old, such as has been found on the Huon Peninsula of Papua New Guinea (Chappell and Veeh, 1978~. If the rapid uplift of the last 5500 yr had continued for as long as the last 28,000 yr, the interstadial terrace of that age would be far above the Holocene terrace, even though it started at a sea-level position 35 m below sea level (Table 6.2, Figure 6.3~. However, the extreme size of the Holocene terrace on Araki Island could be caused by a relatively thin Holocene veneer over an older reef-terrace substrate, as suggested by the dashed trajectory of inferred uplift for a hypotheti- cal 28,000-yr-old terrace on Figure 6.6. The presence of such a substrate under a Holocene veneer has been hy- pothesized from morphologic evidence in other parts of south Santo Island (Strecker et al., 1984), but will be verified only by future drilling. The accelerated uplift during the last 5500 yr on Araki Island cannot be directly compared either to the long-term average rate or to older dated increments of uplift. It is possible that during any previous 20,000-yr interval be- tween interstadial high sea levels, much of the total move- ment was concentrated in brief intervals of 5500 yr or less. There is no way that such jerkiness could be detected with the 20,000-yr sampling interval that is provided by the average spacing of interstadial sea-level oscillations. Therefore, we can only conclude that the last 5500 yr of uplift, during which postglacial sea level has been at its approximate present level, has been unusual by compari- son to the 20,000- and 100,000-yr average rates, although we cannot disprove that those average rates consisted of shorter intervals of alternately fast and slow vertical movements.
110 CONTRADICTORY SEA-LEVEL EVIDENCE FROM OXYGEN ISOTOPES AND CORAL REEFS Figures 6.2 and 6.3 illustrate the currently unresolved contradiction between sea level interpreted from the oxy- gen-isotope record and sea level calculated by regression equations on emerged coral-reef terraces. The heights of the several Wisconsin-age interstadial high sea levels in the coral record range from near present sea level to no lower than -46 m, or no more than one-third of full-glacial sea-level lowering. At comparable time intervals, the deep- sea oxygen-isotope record shows enrichment of INTO in the range of 50 to 70 percent of the values for full-glacial time. If the INTO value is interpreted as primarily con- trolled by ice volume (Shackleton and Opdyke, 1973; Shackleton, 1977~' then the sea-level maxima during the several interstadial intervals would have been twice as low as the values derived from coral-reef studies. Only if cooler temperatures of evaporation caused about 70 per- cent of the observed INTO increase and ice volume caused about 30 percent would the results of the coral-reef regres- sion analysis be similar to the predictions from the oxy- gen-isotope record. SEA LEVEL DURING FULL-GLACIAL AND STADIAL INTERVALS The Pleistocene epoch can be subdivided into glacial and interglacial ages, although the four glacial and three interglacial of the traditional classification is certainly wrong. Within each glacial age, lesser times of ice ad- vances are called stadials and times of retreat are called interstadials (or interstades). It is notable that no stratigra- pher has ever offered a subdivision of interglacial ages. It was never found necessary, because interglacial intervals have made up only about 10 percent or less of Pleistocene time and their duration was less that the inherent errors of the methods used to date them. Glacial ages, however, are rich in climate and sea-level detail, whether it is derived from oxygen-isotope or coral-reef studies. An important question concerns the drop of sea level between the various interstadial sea-level maxima. As should be clear from previous sections of this report, emerged coral-reef terraces record only times just prior to interstadial and interglacial high sea levels, when tectonic uplift was briefly equal to rising sea level. With only two known exceptions, the theoretically possible reefs that should grow during the interval of tangency between tec- tonic uplift and rising sea level just after each stadial or glacial sea-level minima are unknown. Chappell (1974, 1983) described a cut-and-fill cycle in the uplifted deltaic foreset beds of the Tewai River delta on the Huon Penin- sula of Papua New Guinea. Coral reefs that grew on the ARTHUR L. BLOOM AND NOBUYUK! YONEKURA nearshore topset beds of this gravel delta are now uplifted as much as 400 m. A coral cap on a terrace at 390 m is correlated with terrace VIIb (Figures 6.3 and 6.5, Table 6.1) with an age of about 125 ka. Following this reef- building event at a relatively high sea level, the underlying deltaic foreset beds were eroded down to a present height of 320 m, representing an abrupt drop of sea level of about 70 m. Subsequently, the eroded section was reburied by aggradation up to a present height of about 300 m, and the new deltaic gravels were capped by a reef of series VI, with an age of 105 ka. In a later paper, Chappell (1983, p. 24) expressed some reservation or ambiguity about the inferred 70-m drop and rise of sea level between 125 ka and 105 ka, but as the record now stands, this single locality may record an abrupt but extreme sea-level drop of 70 m, 50 to 60 percent of a full-glacial cycle, within the 20,000-yr interval between Papua New Guinea reefs VIIb and VI, which correlate very well with oxygen isotope stages Se and Sc. A similar event in the Barbados record was reported by Steinen et al. (1973), in which sea level would have dropped to -71 + 11 m in relation to present sea level between Barbados reef stages III (125 ka) and II (105 ka). This was included in a much-cited summary figure of the Papua New Guinea terrace chronology and sea-level record (Bloom et al., 1974; Figure 6.5) but was subsequently shown to be erro- neous (Fairbanks and Matthews, 1978, p. 185~. The ero- sional disconformity under Barbados reef II (105 ka) was shown not to separate reef II from underlying reef lime- stone of stage III (125 ka) but rather from an underlying limestone that was much older. Thus, the inferred sea- level drop between 125 ka and 105 ka could not be dem- onstrated in Barbados. The only other place where coral limestone from a low sea level can be shown to have a rational place in a terrace sequence is on Araki Island, near Santo Island in the Republic of Vanuatu. The details of uplift history of this island are not yet published, but Urmos (1985) described a coral sample with an age of 153 ka and a very heavy 6~8O ratio appropriate for full-glacial oxygen-isotope stage 6, in an eroded reef section 180 m above sea level on Araki (Figure 6.6~. The height and position of the sample local- ity would be appropriate for a reef that would have grown during the interstadial sea-level maxima V in the Papua New Guinea sequence, or oxygen-isotope stage Sa (83 ka). However, neither the age nor the isotopic ratio of the sample support this interpretation. As sketched on Figure 6.6, this coral apparently grew during the low sea-level minimum of the penultimate ice age when sea level was estimated at-165 m (oxygen-isotope stage 6; 140 to 160 ka). The island of Araki had then not yet emerged from the sea, but was a rising submarine tectonic block on which reef growth began when the extreme full-glacial
GRAPHIC ANALYSIS OF DISLOCATED QUARTERNARY SHORELINES low sea level exposed it to the photic zone. The reef was subsequently drowned by the rapid rise of sea level to isotope stage Se, and was buried by younger reef growth. However, the tectonic uplift of Araki Island caused this ancient, low-sea-level reef to be exposed by erosion dur- ing the relatively high sea level about 80,000 yr ago (Figure 6.6~. Scattered in the gray literature of uranium-series dates from various coral regions are other anomalously old dates from relatively high inferred sea-level positions. Figure 6.6 offers a valid theory of how such samples can be explained rather than disregarded as erroneous. An inten- sive drilling program on selected emerged coral-reef ter- race sequence, combined with U-series dating and oxy- gen-isotope analyses of suitable corals, could conceivably gain some other valid data points for full-glacial and sta- dial sea-level minima to supplement the current documen- tation of the intervening interglacial and interstadial maxima. It can be predicted that the amplitude of the oscillations would be greater than the amplitude shown by 1 240 A, 200 in 16O - 120. 3 in o HI ~ -40 if: z -80- O c -120 ~3 -160- :.75 ~'N'`1' 80 40 - ARAb<, UPLIFT INFERRED UPLIFT --- NEW GUINEA SEA LEVEL COMPILATION , \, ~ ~ :\ () 20 40 60 80 10~) 12C) 140 160 AGE(xlO3yrBP) FIGURE 6.6 One model of incremental uplift rates (meters per 1000 yr) for the past 105,000 yr on Araki Island, Vanuatu. Note the unusually rapid late Holocene rate (Urmos, 1985~. the deep-sea INTO record, which is inevitably decreased by even the least amount of bioturbation. The climatic impli- cations of such a record would be substantial. For ex- ample, the inferred 70 m drop of sea level recorded in the Tewai River delta section of Papua New Guinea suggests a dramatic initial and perhaps brief pulse of ice-sheet growth during oxygen isotope stage 5d that is not seen in the deep-sea record, perhaps because of its brevity. A sharp temperature minimum, corresponding to extreme cold in the areas of snow accumulation, is seen in the Vostok ice core (Lorius et al., 1985~. Another hint of the brief, sharp climatic deterioration in stage 5d is recorded in one exceptional palynologic record from France (Woil- lard and Mook, 1982~. If climate can be shown to have changed from a warmer-than-present interglacial 125 ka to an ice volume equivalent to one-half or two-thirds of an ice age within lO,000 yr, the implications for an abrupt onset of the next ice age are serious. Andrews and Ma- haffy (1976) showed how difficult it would be to generate large ice sheets and even 5 m of sea-level lowering within a few thousand years as a test of the "instant glacieriza- tion" hypothesis, although Mix and Ruddiman (1984) suggested how very rapid ice-sheet growth could occur around the margins of the North Atlantic if a relict mass of warm surface water remained in the region adjacent to abruptly cooled adjacent land masses. LATE-GLACIAL AND HOLOCENE SEA LEVELS: THE 103- TO 104-YEAR TIME SCALE Numerous Holocene sea-level graphs have been pub- lished (see Bloom, 1977~. Good Holocene sea-level curves are well constrained by radiocarbon dates and have accu- rate depth or height measurements. However, the search for the elusive postglacial eustatic sea-level curve of global applicability has proved fruitless. Every coastal site seems to have its own unique sea-level history. Regional trends are obvious and predictable (Figure 6.7), but local effects such as the size and harmonic shape of an estuary or bay and their tidal heights change with continued rise of sea level. In the event of a future sea-level rise that averages several meters, similar regional and local deviations can be expected. The role of isostatic warping of continental margins and ocean basins under the waterload of rising sea level has become a major topic of research (Cathles, 1975; Clark et al., 1978; Pettier, 1982, Chapter 4, this volume). By their nature, coasts are at the margins of the oceans, where differential flexure in late-glacial and postglacial time has been maximal. The graphic analysis or regression method of determin- ing late Quaternary sea-level maxima (Figure 6.5) can also be applied to late-glacial and Holocene sea-level research
112 o -2 _. ~0 ,~ so fir 2O tC, o r / EV514TIC | SEA- LEVEL RISE 6 100O YR BP O ~ - 2 a) - ,. As' ~ _.~ / ~ / m ~ ~ i 1000 YR BF . ', .-. 2 0 ~00 Ye ~ FIGURE 6.7 Distribution of the six predicted sea-level zones resulting from retreat of Northern Hemisphere ice sheets. Within each zone the form of the sea-level response is similar. Typical relative sea-level curves predicted for each zone are included, (Figure 6.8~. Results are tentative and preliminary, but are presented here because of their interesting implications about the relations between sea levels and postglacial isostatic adjustments (Peltier 1982, Chapter 4, this vol- ume). Thirteen well-documented and representative sea- level curves~were selected from the published literature to compile Figure 6.8. All have records of 8000 radiocarbon years or more. Some were chosen from Arctic regions where postglacial emergence due to isostatic uplift is the dominant process; others are from the Atlantic coast of the United States and Europe, which are generally areas of postglacial subsidence (zone II of Figure 6.7~. Each se ARTHURL.BLOOM AND NOBUYUKIYONEKURA · ~ ~50 \~ 20 v, JO ~ . , ToO lODO YR BF10 ~0 \ ~R~^srno~ \ ,0' \I -a ~, 0a 1 ~ -5 C ~ 2 0c,, ~10 lD00 YR BP_~5 t0~0 USES. ~.~ Y , ~ ~ ~ ~. ~ 2 - 07R 2 YR BP - 2. -2. ,1 ~ ~ ~ . . t ~ - - ~ 2 O K)OO OR 8P and they show the wide variety of sea-level expressions possible despite the assumption of no eustatic change since 5000 yrBP (Clark and Lingle, 1979). lected sea-level curve was sampled at 1000-yr intervals, recording the age and vertical height or depth of the sample locality. The height of sea level at each 1000-yr interval was then plotted as a function of the height (or depth) of the 8000-yr-old sea level at that locality (Figure 6.8) and the linear regressions were calculated for each 1000-yr trend relative to the 8000-yr-old trend line (Table 6.3?. Unlike Eq. (6.2) and Figure 6.5, in which a value of b, initial sea level, is significant, all of the regression lines in Figure 6.8 pass within a fraction of a meter of the origin. That seems to imply that all 13 samples of each 1000-yr time interval were proportionately spaced; that is, the
GRAPHIC ANALYSIS OF DISLOCATED QUARTERNARY SHORELINES amount of emergence or submergence recorded for each 1000-yr interval in the last 8000 yr is proportional to the total amount of emergence or submergence at that place. This statement is true even though the total vertical change of relative sea level at the various localities in the past 8000 yr ranges from relative uplift of 70 m at Oslo, Nor- way, to relative drowning of 20 m on the Delaware coast. Is it possible that mean sea level has not changed its absolute level (relative to the center of the Earth, for in- stance) for the last 8000 yr, and that the observed submer- gence at U.S. and western European ports is due to isostatic adjustments downward that are analogous and proportional to the uplift farther north in more recently deglaciated reasons? The near-zero intercept of all the regression lines in Figure 6.8 would seem to indicate that. If so, the best- documented sea-level histories, those of eastern United States and western Europe, are not directly reporting addi- tional water into the world ocean, but local isostatic re 70 60 50 _ Hi (m) 40 30 20 ~10 ~i210y /;L_ /^, 113 sponse to water added 8000 yr or more ago. Postglacial isostatic uplift has a half-life of 1000 yr or more, and would give the observed exponentially decreasing sub- mergence observed at all the stations in the lower left side of Figure 6.8, but it is provocative that the submergence should be so rigorously proportional to postglacial uplift elsewhere. Perhaps future sea-level change will be con- trolled mostly by residual isostatic response to the loading and unloading that was completed as much as 8000 yr ago. Any climatically induced change of real sea level would be added to or subtracted from the trends documented by Figure 6.8. CONCLUSIONS 1. Paleosea levels for the last 125,000 yr can be deter- mined from tectonically uplifted coral-reef regions, but thus far only for the interglacial and interstadial sea-level Us ~o,,- H29 H48 H6' H8 ~ Hl' H3IH5lH7: ~TO ~'63 HO f ~/ ~ ~ 10 20 30 40 50 60 70 -10 He (m) H7 = 0.66 H8 -.67 -20 H6=0.43H8-.5O HE = 0.32 H8 +.25 H. =0.21 H8 + .02 H3-0.13H8~05 HE =0.08H8+.10 Hl =0.05H8-.06 (r7=0.974) ~ r6 = 0.956 ~ (rS=0.923) (~4 = 0.938 ~ (r3=0.939) (r2=0.910) (rl ~ 0.947 FIGURE 6.8 Regression analysis of sea- level curves from 13 sites (Table 6.11. As in Figure 6.5, height or depth of sea level at each younger 1000-yr interval (H7, H6, etc.) is plotted relative to the height or depth of the 8000-yr-old shoreline (H8) at the site. Odd- and even-numbered incre- ments are plotted by different symbols only to permit easy visual evaluation of the re- gression lines. Method, equations, and coefficients are similar to those used in constructing Figure 6.5. The important difference here is that all regression lines pass through or very close to the origin, as shown by the small values of coefficient b (+0.25 to-0.67 m).
114 TABLE 6.3 Sea-Level History for the Past 8000 Yr, Northern Hemisphere Sites ARTHUR L. BLOOM AND NOBUYUKI YONEKURA Relative Sea Level at 1000-yr Intervals since 8000 yrBP Location 1000 yr2000 yr3000 yr 4000 yr5000 yr6000 yr7000 yr8000 yr Ellesmere, Canada 2 + 14+ 18 + 1 13 + 118 + 126+ 147+257+2 NE Greenland 1 + 12+ 12+ 1 2+ 13 + 14+ 112+ 134+2 Oslo, Norway 4 + 17 + 110 + 1 16 + 126 + 132 + 146 + 167 + 1 Jaeren, Norway 0 + 11 + 11 + 1 3 + 16 + 13 + 17 + 14 + 1 Clinton, Conn. -1.0 + 0.5-1.7 + 1-2.7 + 1 -3.6 + 1-5.0 + 1-6.2 + 1-7.3 + 1-8.5 + 1 Plum Island, Mass. -0.7 + 0.2-1.3 + 1-2.1 + 1 -3.2 + 1- .6 + 1-7.0 + 1-9.5 + 1-12.2 + 1 NW England 1 + 11 + 10+ 1 -2+ 1-2+ 1- + 1-6+ 1-14+ 1 San Francisco -2 + 1-3 + 1~ + 1 -6 + 2-7 + 2-10 + 2-12 + 2-17 + 3 U.S. Gulf Coast -0.3 + 0.50.6 + 0.5-1.4 + 0.5 -2.5 + 0.5-3.6 + 1- .8 + 1-7.6 + 2-12 + 2 Delaware -1 + 1-3.5 + 1-5 + 1 -8 + 1-11 + 1-14.5 + 1-17 + 1-20 + 2 Carribean Islands -1 + 2-2 + 2-2.5 + 2 -3.5 + 2-5.3 + 2-7.7 + 2-11.5 + 2-15.5 + 2 SW England 0 + 0.5-0.8 + 0.5-1.3 + 0.5 -1.9 + 0.5-3.4 + 0.5-5.6 + 0.5-11 i 1-17.2 + 1 Panama Republic -2 + 2-2 + 2-5 + 2 -8 + 3-12 + 3-12 + 3-13 + 3-15 + 4 NOTE: For original sources, see Bloom (19771. Error estimates 6.8, were estimated from statements in the original citations. maxima that recurred at approximately 20,000-yr inter- vals. The intervening full-glacial or stadial times of gla- cier expansion and sea-level minima probably could be determined from coral islands by a future program of drilling and sampling. In addition, it is possible that drowned reefs of former stadial and full glacial times are still exposed on the steep flanks of suitable atolls or shelf reefs at depths on the order of 120 m. These could be studied and sampled from existing submersible research vessels. 2. A few data points now available from coral-reef studies suggest that the amplitude of multiple sea-level oscillations within the last ice age was substantial; perhaps equal to 50 percent or more of the maximum sea-level lowering that occurred about 1 8,O00 yr ago. In particular, a shard drop of sea level of 70 m may have occurred within about 10,000 yr after the last interglacial (isotope stage Se) sea-level maximum. To the extent that such a sharp sea- level drop might occur at the beginning of the next ice age, it should be better documented. Only one place on the Huon Peninsula of Papua New Guinea is thought to record the event, but other regions of rapidly uplifted Quaternary coral reefs could be investigated. 3. The deep-sea oxygen-isotope record probably does not display the full amplitude of sea-level or temperature oscillations because of bioturbation. Furthermore, an obvious discrepancy persists between the absolute heights of interstadial sea-level maxima as interpreted from the deep-sea oxygen-isotope record and those calculated from uplifted coral reefs. The oxygen-isotope record of corals agrees with the record obtained from pelagic and benthic foraminifera, and so the problem is not to resolve a contra added for evaluating the correlation coefficients shown on Figure diction between the coral-reef and the deep-sea isotopic record, but to determine whether the oxygen isotope ratios document ice volumes or temperature, and whether tec- tonic uplift models of coral coasts are valid. No new recommendation on this question can be made since the dilemma is well known and vigorously debated. Only the cliche, "More work is needed," can be offered. 4. With the last 10,000 yr, proportional rates of emer- gence or submergence at a selection of North America and European coastal sites seem to have been remarkably uniform. One interpretation could be that most of the glacier meltwater had returned to the sea by 8000 yr ago, and that the observed sea-level changes at various stations since then do not involve changes in the mass of ocean water, but only continuing isostatic adjustments to dis- placed ice and water loads (and possibly thermal expan- sion). REFERENCES Aharon, P., and J. Chappell (19861. Oxygen isotopes, sea level changes, and the temperature history of a coral reef environ- ment in New Guinea over the last 105 years, Paleogeogr. Paleoclimat. Paleoecol. 56, 337-379. Andrews, J. T., and M. A. W. Mahaffy (1976~. Growth rate of the Laurentide ice sheet and sea level lowering (with emphasis on the 115,000 BP sea level low), Quat. Res. 6, 167-183. Bloom, A. L., compiler (19771. Atlas of Sea-Level Curves, Int. Geol. Correlation Prog., Project 61, Dept. Geol. Sci., Cornell Univ., Ithaca, N.Y. (litho.), vii + 114 pp. Bloom, A. L. (19804. Late Quaternary sea level change on South Pacific coasts: A study in tectonic diversity, in Earth Rheol
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