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OCR for page 88
Quaternary Sea-Level Change
ROBLEY K. MATTHEWS
Brown University
ABSTRACT
Attempting to understand Quaternary sea-level history provides a vigorous intellectual workout.
After negotiating a long path through data and concepts of mixed quality, one finds global ice
volume has fluctuated by tens of meters sea-level equivalent at rates that are difficult to resolve.
Satellite monitoring of global ice volume appears only prudent.
INTRODUCTION
From a geologist's point of view, the entire history of
modern civilization is written within the context of an
anomalously high stand of sea level. While 10,000 yr
seems like an extremely long time in the history of man, it
is an extremely brief time in the history of glacio-eustatic
sea-level fluctuations. While late Quaternary sea-level
events revealed by the geologic record are not likely to
influence short-term decision making, they do serve as an
awesome reminder of the dynamic geologic context of
mankind. We will not spend vast sums planning for an
event that is perhaps thousands of years into the future.
However, it is equally certain that we shall not simply
acknowledge the imminent demise of the human race.
Knowledge must come first; planning can come later.
Acquisition of knowledge in sedimentary geology is an
interesting subject in itself. Historically, this field is driven
by the inductive mode of investigation; more recently, the
deductive mode is taking on new importance. In the in
88
ductive mode, one is driven by necessity or by curiosity.
The task is begun with virtually zero understanding, but
with faith that some new truth will emerge if one works at
it hard enough. The emphasis is on data-gathering first,
followed by ad hoc explanation of the data later. Con-
versely, in the deductive mode one attempts to begin with
a general a priori model and then add sparse amounts of
carefully chosen new data to solve problems where the
model is in obvious need of refinement.
Intellectual conflict arises when the scientist in a de-
ductive mode balks at "explaining" vast amounts of sub-
stantially irrelevant information compiled by the scientist
operating in an inductive mode. The inductive scientist
does not like being told that his study area is too compli-
cated to be fit to a general model; the deductive scientist
does not wish to make his model unnecessarily compli-
cated simply to fit every random scrap of data.
By way of example, classic inductive investigation of
late Cenozoic sedimentary geology is rooted in the winter
vacations of wealthy Englishmen. Nevermind that the
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QUARTERNARY SEA-LEVEL CHANGE
Mediterranean region is an extremely complicated tec-
tonic area; it was a great place to spend the cold winter
months. The local fossiliferous strata constituted a very
interesting hobby. Surely if one described enough strati-
graphic sequences in enough detail, some new truth would
emerge. To this day, a "classic locality syndrome" re-
mains in force. Because much is known about these strata,
some workers continue to return to them to learn still
more, regardless of the fact that we do not understand their
regional tectonic history.
More recently, we find a lot of inductive geologic in-
vestigation driven by government edict. The geologic
mapping of a nation is substantially an inductive process.
One does it because the government is paying one to do it.
More to the point at hand, one cannot fault a government
for inductive investigation of coastal stratigraphy and
neotectonic history with regard to the siting of nuclear
power stations, for example. However, the fact that these
inductive investigations needed to be carried out by no
means ensures that the resultant data will be worthy of
incorporation into a priori models of Quaternary sea-level
history.
This chapter takes a deductive approach to the question
of Quaternary sea-level change. The 6~80 record in deep-
sea cores provides the best opportunity to obtain a con-
tinuous record of Quaternary (perhaps even Cenozoic)
sea-level change. There is elegant simplicity to the state-
ment that 6~80 variation in deep-sea cores reflects changes
in the isotopic composition of sea water and thereby re-
flects changes in continental ice volume. The major focus
of this chapter will be to add simple refinements to this a
priori model. The prospects for future ice-volume vari-
ation and the prospects for a more predictive approach to
stratigraphy will then be examined.
APPROPRIATE (AND INAPPROPRIATE)
TECHNOLOGY
There are several technologies that are appropriate to
attacking problems in the late Quaternary. These are re-
viewed below. Further, the inductive mode (and the need
to come to some conclusion concerning a practical prob-
lem) often leads the geologist to use inappropriate technol-
ogy. One's mind becomes set on gathering all the data one
possibly can; one loses sight of the fact that much of the
data may be seriously flawed. These situations are like-
wise reviewed below.
Stratigraphic Context of Study Materials
To obtain a local sea-level history on the 103- to 104-yr
time scale, one must have a stratigraphy that can be related
89
to sea level and one must be able to date events within that
stratigraphy. There are lots of good types of data to work
with and there are lots of pitfalls.
Morphostratigraphic Units To work with the strati-
graphic record of sea-level events, it is highly desirable to
establish formal morphostratigraphic units. Coral-reef
terraces and former strand-line complexes in elastic sedi-
ments are examples of morphostratigraphic units. They
can be mapped as a physical topographic or bathymetric
feature and exist as stratigraphic units regardless of age or
age relationships among units. A reasonably formal defi-
nition of morphostratigraphic units has been followed with
regard to coral-reef terrace sequences on Barbados (Bender
et al., 1979), New Guinea (Bloom et al., 1974), and Haiti
(Dodge et al., 19833. To a lesser extent, a similar formal-
ism has been applied to fossil strand-line deposits in clas-
tic environments of the U.S. Gulf coast and east coast (see
Cronin, 1983, for an excellent review).
Layer-Cake Stratigraphy In the absence of morpho-
stratigraphic units, geologists often fall back to physical
stratigraphic relationships defined in vertical profile such
as a core or borehole. In some cases, solid inference can
be derived from age dating within such a physical strati-
graphy. A good example of this is the dating of Holocene
peat deposits that immediately overlie late Pleistocene
subaerial exposure surfaces (e.g., Nelson and Bray, 1970~.
Bad examples of this practice exist especially where ge-
ologists correlate on the basis of questionable radiometric
dates rather than on the basis of physical characteristics or
stratigraphic continuity.
Deep-sea cores from many portions of the world ocean
sample an extremely reliable "layer-cake stratigraphy."
Sedimentation rates commonly can be assumed to be nearly
constant for long periods of time. Magnetic stratigraphy
and 6'80 variation can be correlated globally (e.g., Imbrie
et al., 1984).
Peats Holocene brackish water peat deposits in close
proximity to late Pleistocene subaerial exposure surfaces
provide quite good stratigraphic indicators of paleosea
level, which can be easily dated using the 14C method.
Importantly, it is difficult to move peats around as sea
level moves up and down. The peats would simply disin-
tegrate if moved at all. Thus, to find peat resting on
former subaerial exposure surfaces provides good indica-
tion of the time at which sea level first inundated the
former subaerial landscape.
Care must be taken in interpreting the sea-level signifi-
cance of peat dates taken from within thick sections of
peat. Time lines within peat accumulation may be hard to
define. Correction of present stratigraphic elevation of the
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90
sample with regard to compaction of peat below may prove
difficult. In the discussion that follows, only peat samples
from close proximity to subaerial exposure surfaces below
are used to establish sea-level history.
Mollusc Shells Mollusc shells from dredge hauls are by
far the most abused "datable material." If mollusc samples
are to have stratigraphic integrity, one must be able to
demonstrate that the shells have not been transported by
rising sea level, subsequent current activity, seagulls, or
man. Most commonly, this means recovering articulated
pelecypod valves (e.g., Curray, 19601. Numerous dates in
the literature undoubtedly represent shells that were trans-
ported landward by rising sea level (see MacIntyre et al.,
1978, for a good discussion). Still other mollusc shells
dredged from bathymetric lineations are species that are
known to live in tens of meters water depth. Thus, old
shells are washed upwards to shallower depth than where
they actually grew and younger shells represent deep
subtidal environments where offshore molluscs thrive.
A further problem, rampant in the '4C dating of mollusc
shell, is the exposure of sea-bottom samples to Holocene
diagenetic processes, such as boring and infilling by micro-
organisms. A small amount of contamination by Holocene
calcium carbonate can makeavery old mollusc sample look
like 20,000- or 30,000-yr-old material when dated by ]4C.
Corals Reef-crest corals (A. palmata in the Atlantic)
can be outstanding indicators of paleosea level. Such
corals live within a few meters of sea level and are not
subject to extensive transport. They provide excellent
material for }4C dating or for 230Th dating (e.g., Lighty et
al., 1978, 1982; Mesolella et al., 1969~. Virtually all
corals other than the reef-crest species have the obnoxious
habit of living either in the shallow back-reef or the deep
fore-reef environment. Thus, while they do contain dat-
able calcium carbonate, their relationship to past sea level
must be inferred from paleoenvironmental reconstruction.
Clearly, the coral did not live above sea level; but beyond
that, things get complicated.
"Beach Rock" Some of those who work on submerged
late Pleistocene and Holocene deposits take cementation
of sandy sediments to be an indication of deposition and
cementation in the beach environment. None of these
studies offer sufficient documentation of the beach envi-
ronment as the source of cementation in these materials.
Submarine cementation is equally capable of producing a
lithified sandstone (e.g., Land and Moore, 1980~. The
claim that "beach rock" identifies a former intertidal envi-
ronment must be held to close scrutiny. Further, all of the
datable materials to be found in such sediments are subject
to the problems noted above.
ROBLEY K. MATTHEWS
Geochronologyl Chronostratigraphy
Given good stratigraphic context of study materials,
there are several dating methods available for the study of
Quaternary sea-level change.
]4C Dating Back to the last glacial maximum (approxi-
mately 18,000 yr before present (18 kaBP)), the geologist
can virtually "date at will" with regard to ice margin ad-
vance on the land, with regard to sea-level fluctuations at
the shoreline and with regard to downcore stratigraphy in
deep-sea cores. In the best of worlds, one would hope that
all the material ]4C dated would have met selection criteria
noted above. In fact, this is not the case, and much of the
existing data must simply be regarded as irrelevant to the
questions under consideration.
Uranium-Series Dating 230Th and 23iPa dating of arago-
nite corals constitutes the backbone of late Pleistocene
reef-terrace geochronology. Unfortunately, a large amount
of literature provides only one or two dates per terrace.
When one takes the time to make multiple analyses on
multiple samples from the same terrace, one learns that
there is a considerable spread to these data. Presumably,
part of this spread is due to the complicated nature of the
analysis (e.g., Harmon et al., 1979~. Undoubtedly, some
of the spread is due to slight diagenetic alteration of sample
in the subaerial environment (Bender et al., 1979~. Even
more outrageous numbers are obtained when one tries to
date materials from within the present-day phreatic lense.
Uranium mobility is rampant, and such dates have no
. . . ,%.
geocnronolog~c s~gn~cance.
The summary data of Harmon et al. (1979) with regard
to three particular samples exemplify the problem faced
with regard to geochronologic significance of 230Th dates
within the time interval of the last interglacial. On the
basis of impeccable geologic arguments, Rendezvous Hill
(Barbados III) terrace, the Curacao +6 m terrace, and the
Key Largo limestone of South Florida all represent deep-
sea isotope stage Se (event 5.5 in the SPECMAP terminol-
ogy; taken to be centered around 122 kaBP; Imbrie et al.,
1984~. However, on the basis of 13 analyses of each
sample, the 230Th coral-dating community came up with a
pooled age estimate of 118 + 9, 124 + 12, and 139 + 19
kaBP for these samples, respectively (see Harmon et al.,
1979~.
Two points are worthy of note. First, there appear to be
systematic differences among these three samples as re-
flected by the respective means. Further, Barbados is
significantly uplifted whereas Curacao and Key Largo
limestone samples occur at an elevation presumed to rep-
resent the actual high stand of the sea. If anything, the
Barbados sample should appear somewhat older than the
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QUARTERNARY SEA-LEVEL ClIANGE
other two because an uplifting island would experience a
null point with rising sea level earlier in the history of sea-
level rise and subsequent high stand of the sea. In fact, the
reverse is true; the Barbados date is the youngest of the
three examples. Very likely, some small amount of postde-
positional alteration has affected at least two of three of
these samples. Indeed, x-ray data indicate considerable
calcite in the Key Largo sample; but the other samples
appear quite satisfactory.
Next, consider precision of these age determinations.
Even with 13 analyses of aliquots of the same sample,
precision (1~) remains in the range of 10,000 yr. This is
quite large in comparison with the chronostratigraphic
precision attainable from deep-sea cores by stacking and
spectral tuning of the INTO record (discussed below).
Thus, even under the best of circumstances uranium-
series dating in the range of 125 kaBP (an age range
extremely important in the geologic record)-uranium-
series dating affords precision and accuracy that are mar-
ginal in comparison with the chronology available through
the use of deep-sea cores. Where a sufficient number of
analyses are available on individual terraces, the dates
may serve as a crude constraint on correlations to deep-sea
cores. However, discussion of whether "the 120-kaBP
terrace" of one sequence is the same or a different glacio-
eustatic event from "the 130-kaBP terrace" in another
sequence must rest on geologic arguments, not on ura-
nium-series dating.
Further, one simply cannot trust dates on sparse samples
from a single or a few terraces. Such reports are nothing
more than a potentially interesting lead suggesting that
more work might be desirable. Finally, one simply should
not trust uranium-series dates on materials that were col-
lected from within the modern freshwater phreatic tense or
that can be demonstrated to have resided in a paleophreatic
tense at some time in the history of the sample.
Magnetic Stratigraphy Identification of the Brunhes/
Matuyama magnetic boundary in deep-sea cores is funda-
mental for late Quaternary chronostratigraphic studies. The
independent chronology provided by identification of the
Brunhes/Matuyama boundary and the continuous nature
of the deep-sea record are the prime reasons for attempting
to correlate terrace and shoreline stratigraphy to the deep-
sea record. One will never get the simple stratigraphic
continuity of deep-sea cores in the complicated nearshore
environment. Thus, it is important to develop a deductive
research strategy in which the deep-sea record and chro-
nology are the a priori model that one takes to the nearshore
environment.
The SPECMAP Time Scale The SPECMAP group has
generated a continuous deep-sea chronology for the last
91
780 ka, which claims a precision on the order of 2 ka (95
percent confidence level) for major events in the deep-sea
GINO ice-volume record. The technique utilizes a formal
taxonomy for recognizable events in the INTO stratigraphy
and utilizes orbital tuning to make local adjustments to
sedimentation rate (Imbrie et al., 1984; Prell et al., 1986~.
The fact that numerous individual records can be incor-
porated into the final grand chronology promises continu-
ing improvement of both chronology and the resultant
INTO composite estimate of the glacio-eustatic signal.
Further, application of the technology to the entire Ceno-
zoic is technologically feasible. In my opinion, this is the
chronostratigraphy that shall prevail.
The ~80 Ice VolumelTemperature Relationship
Calcium carbonate precipitated from sea water records
some combination of information concerning the isotopic
composition of the water and the temperature at which the
calcium carbonate grew. Sizable variation in the isotopic
composition of the sea water results from variation in
global ice volume. If one can constrain global ice volume,
one can read local temperature. Conversely, if one can con-
strain local temperature, one can read global ice volume.
This technology is applicable to planktonic foraminif-
ers, benthic foraminifers, corals, and molluscs. In order to
obtain useful results, one must further constrain strati-
graphic variability by working with single taxa. Time-
series data based on "mixed assemblages" of planktonic
foraminifers, for example, may reflect variation in propor-
tions of deep dwelling and shallow dwelling taxa with
time. Similarly, there is considerable variation among
benthic foraminifer taxa (Graham et al., 1981~. Records
based on "mixed benthics" or on several different benthics
strung together in time series must be considered suspect.
Similarly, some taxa can be shown to have considerable
variation in isotopic composition as function of size of the
specimen (Curry and Matthews, 1981~. Whether this re-
flects seasonal variation or variation in habitat among
juveniles and adults remains to be unraveled. Finally,
variation in degree of dissolution of deep-sea sediments as
a function of proximity to the lysocline can vary the iso-
topic composition of individual taxa by as much as 0.5 per
mil (Peterson and Prell, 19851.
The above-cited difficulties notwithstanding, many deep-
sea cores yield a remarkably similar global INTO signal.
This signal, taken in conjunction with magnetic stratigra-
phy, provides the data base for stacking and spectral tun-
ing, which produces a remarkably precise a priori model
of the global INTO signal (Imbrie et al., 1984; Prell et al.,
19861. The isotope-stage terminology of Imbrie et al.
(1984) is adopted here. Whether this signal should be
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92
interpreted as a global ice-volume signal or a temperature
signal remains a main point for discussion below.
d'80 and ~3C Relationships to Subaerial Exposure
Surfaces
Stable isotope data also provide incontrovertible identi-
fication of subaerial exposure surfaces within stratigraphic
sequences of bank margin carbonates (Allen and Mat-
thews, 19821. Recrystallization of unstable aragonite and
high-magnesium calcite in proximity to soil gas imparts a
highly negative 6~30 signal to sediments immediately be-
low subaerial exposure surfaces. Similiarly, recrystalliza-
tion of various unconformity-bounded packages of car-
bonate sediment generally occurs under slightly different
conditions of global ice volume and/or climate. Thus,
unconformity-bounded packages of carbonate sediment
commonly display slightly different SILO values from those
sediments above and below.
A detailed stratigraphy of sedimentation at or near sea
level, followed by subaerial exposure, makes the bank
margin carbonate environment a veritable gold mine of
information concerning glacio-eustacy (e.g., Major and
Matthews, 19831.
THE 18 kaBP TO PRESENT TIME INTERVAL
This time interval encompasses the transition from the
last glacial maximum to the present and is within the range
of convenient ]4C dating.
Latest Pleistocene Maximum Continental Ice Volume
The 18-kaBP perimeter of continental ice sheets is
reasonably well known. Calculations based on this pe-
rimeter and assumptions concerning ice mechanics of
equilibrium glaciers allow estimation of the volume of ice
tied up on land during times of maximum glaciation (Demon
and Hughes, 1981~. Estimates for ice-volume sea-level
equivalent tied up in equilibrium ice sheets range from as
high as 163 m to as low as 102 m. Importantly, all of these
calculations presume the ice sheets were at equilibrium. It
is possible for ice sheets to exist for long periods of time
at less than these theoretical equilibrium ice volumes. Thus,
the various estimates constitute formal statements of the
maximum ice volume given different assumptions. It is
important that confirmation be sought elsewhere for the
amount of ice tied up in continental glaciers.
Lowest, Low Sea-Level Estimates for the 18-kaBP
Shoreline
When we seek to confirm maximum sea-level lowering
caused by the maximum extent of the 18-kaBP continental
ROBLEY K. MATTHEWS
ice sheets, we find few high-quality data points below 40
m. Two data points from the Texas Gulf Coast at -57 m
and 12,900 + 400 yrBP for paired valves of the brackish
water bay clam, Rangia cuneata, tell a very clear story
(Curray, 1960~. Similarly, a freshwater peat off southern
New England at-56.5 m and 12,320 + 350 yrBP appears
trustworthy (Oldale and O'Hara, 1980~.
Thus, we are left with a high-quality number for maxi-
mum sea-level lowering at 18 kaBP that is both small and
young in comparison with the numbers suggested by con-
tinental ice-volume calculations cited above. There is a
lot of bathymetry suggestive of a sea stand at around -120
m (e.g., Curray, 1960), but it is poorly dated. These
features certainly look like good candidates for 18-kaBP
shorelines, but this common assertion has not been satis-
factorily proven.
We definitely want to know maximum sea-level lower-
ing during the time of maximum ice volume on the conti-
nents. This question will be examined below with regard
to the transition from isotope stage 6 to isotope stage 5.5
(Se).
Submergence Curves for 12 kaBP to Present
High-quality data are more readily available for the
shallow end of the sea-level rise curve. Key papers (Gould
and McFarland, 1959; Curray, 1960; Coleman and Smith,
1964; Scholl, 1964; Redfield, 1967; Scholl and Stuiver,
1967; Schnable and Goodell, 1968; Baltzer, 1970; Nelson
and Bray, 1970; Thom and Chappell, 1975; Hopley et al.,
1978; Lighty et al., 1978; Field et al., 1979) have been
reviewed with regard to criteria described above under
stratigraphic context of sedimentary materials. With re-
gard to Bermuda, 12 data points meet these criteria and
range back to -21 m at 9100 yrBP; Florida, 22 data points
ranging back to -27 m at 9400 yrBP; Texas/Louisiana
Gulf coast, 15 data points ranging back to -58 m at 13
kaBP; and South Pacific (Australia and New Caledonia),
11 data points ranging back to -37 m at 9700 yrBP. All of
these curves show sea level to be within less than 5 m of
present sea level by approximately 5000 yrBP. Differ-
ences among these curves in this time range are taken to
reflect continuing isostatic adjustment to unloading of late
Pleistocene glacial ice (e.g., Peltier, Chapter 4, this vol-
ume).
Calibration of Deep-Sea ~80 Signal as Continental
Ice Volume
The deep-sea 3~8O record from 18,000 yr to the present
is well known and well studied (see Mix and Ruddiman,
1985, for examples). The amplitude of the planktonic
signal in high-sedimentation-rate cores averages 1.7 + 0.1
per milt Inasmuch as there is no well-dated confirmation
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QUARTERNARY SEA-LEVEL CHANGE
of the 18-kaBP shoreline (discussed above), estimates of
calibration between the deep-sea 8~80 record and the shore-
line submergence record must be based upon i4C correla-
tion of the few good shoreline data (cited above) with the
high-quality deep-sea isotopic stacked record of Mix and
Ruddiman (1985, Table 41. The four independent esti-
mates thus resulting yield a value of 0.025 per mil per
meter for the calibration between deep-sea INTO fluctua-
tion and shoreline fluctuation during the Holocene sea-
level rise. If this change in 6~80 value as a function of
changing sea level were entirely an ice-volume effect, it
would require that glacial meltwater be returned to a well-
mixed ocean with an isotopic composition on the order of
-9S per milt This is a seemingly outrageously negative
number; glaciers today range from -20 to -50 per milt To
escape this dilemma, we might also pursue the possibility
that (1) there is systematic offset in the two age models;
(2) the shape of the geoid has changed, thus affecting the
observed depth difference between paleoshorelines and
present sea level; (3) sea-surface warming explains part of
the isotopic curve from 13 kaBP to the present; or (4) that
surface water of the world ocean (or specifically, the tropi-
cal Atlantic study area of Mix and Ruddiman) was con-
taminated by unusually negative precipitation at the time
of glacial melting.
THE 180- TO 18-kaBP TIME INTERVAL
Interest in this time interval stems from two facts; one
technological and one scientific. This is the time interval
for application of the uranium-series dating techniques.
This time interval also encompasses the second to last
major glaciation (isotope stage 6) and the last interglacial
(isotope stage S.S, formerly Se). In examining this time
interval, we will pick up further information concerning
maximum sea-level fluctuation from glacial to interglacial
conditions and we will pick up important calibration con-
cerning the relative importance of ice-volume variation
and temperature variation in the observed 6~80 curve from
deep-sea cores.
Coral-Reef Terrace Sequences
Absolutely essential to the calibration of the deep-sea
6~80 record as an ice-volume signal is the stratigraphy and
geochronology of coral-reef terraces on tectonically emerg-
ing islands. On the basis of the record within the range of
]4C dating, it appears that only the 6~80 signal in deep-sea
cores covaries with the sea-level signal from submerged
shorelines. Within the time interval from 180 to 70 kaBP,
a number of coral-reef terraces are observed that are in
substantial agreement with a well-defined 6~80 signal in
deep-sea cores.
93
Well-Dated Morphostratigraphic Units At least three
morphostratigraphic sequences have been relatively well
dated using uranium-series methods. These are the up-
lifted coral-reef terraces of Barbados (Mesolella et al.,
1969; Bender et al., 1979), New Guinea (Bloom et al.,
1974; Aharon, 1983), and Haiti (Dodge et al., 1983~. In
each of these areas, distinct morphostratigraphic units can
be recognized, which correspond to the Barbados terraces-
Rendezvous Hill, Ventnor, and Worthing (approximately
125, 105, and 82 kaBP, respectively). In each of these
study areas, the morphostratigraphic units stand indepen-
dent of radiometric dating. Additionally, the New Guinea
terrace sequence contains several younger terraces that are
not represented on Barbados or Haiti because the uplift
rate of Barbados and Haiti is considerably less than the
uplift rate of the New Guinea terrace sequence.
The Constant Uplift Rate Hypothesis Historically, a
great deal of literature concerning relative sea level within
the time interval 140 to 70 kaBP hinges on application of
the "constant uplift rate hypothesis" (Broecker et al., 1968)
to Barbados, New Guinea, Haiti, and elsewhere. The "125-
kaBP terrace" has been considered to represent a +6-m
high stand of the sea. With this te~Tace as the "known"
datum, uplift rates for the various regions were calculated.
Most such calculations indicate that sea level at 105 and
82 kaBP was considerably below present sea level. Mat-
thews ~ 1973) showed that relative sea-level estimates can
be made independent of the 125-kaBP sea-level assump-
tion where terrace sequences with differing (but constant)
uplift rates can be combined into a calculation.
Unfortunately, the constant uplift rate hypothesis is
substantially ad hoc. With the work of Stockmal (1983),
this assumption seems reasonable for a relatively simple
tectonic setting such as Barbados, but this assumption is
not easily justified for such complex areas as New Guinea
or Haiti. Here, scientific strategy bifurcates.
Historically, I have argued to simply consider tectonic
uplift a convenient mechanism for delivering coral-reef
terraces into the subaerial environment where they may be
easily sampled for geochronologic and isotopic studies. If
we had to rely on submarine observations for our knowl-
edge concerning sea-level stands at 105 and 82 kaBP, we
would still be groping around in the dark ages of Pleisto-
cene geochronology. We should be thankful for the uplift-
ing of these terraces; but should not rely on the "constant
uplift hypothesis" to tell us anything about relative sea
level.
Alternatively, some scientists argue that sea-level esti-
mations based on the constant uplift hypothesis constitute
an independent check on the isotopic estimates. In recent
years, this approach has taken on still greater appeal be-
cause of possible uncertainty in isotopic sea-level esti-
mates introduced by the possible existence of thickened
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94
Arctic ice pack (e.g., Broecker, 1975; Williams et al.,
1981~. In deference to this latter uncertainty, I here give
serious consideration to sea-level estimates based on the
constant uplift hypothesis. Note, however, that this hy-
pothesis remains substantially ad hoc and can be consid-
ered valid at this time only on an empirical, statistical
basis.
Dodge et al. (1983) provided a detailed comparison
between sea-level estimates derived from terrace elevation
data in tectonically active areas and the isotopic record
from deep-sea benthic foraminifers. They concluded that
the deep-sea benthic 6~80 record is two-thirds a bottom-
water temperature signal and only one-third a 6~80 ice-
volume signal.
There are two things wrong with this conclusion. First,
it is misleading and erroneous to compare sea-level infor-
mation to the benthic 6~80 record (e.g., Prentice and Mat-
thews, 19881. Second, the isotope data of Fairbanks and
Matthews (1978) for Barbados terraces (Worthing, Vent-
nor, and Rendezvous Hill) are slightly inconsistent with
data on new samples collected specifically to test the re-
sult concerning paleosea-level elevation indicated for
Barbados terraces Worthing and Ventnor. These two
problems are examined in more detail below.
The DO Record from Coral-Reef Terraces and
Deep-Sea Cores
Numerous deep-sea cores reveal a remarkably consistent
6~80 time series from planktonic and benthic foraminifers
within this time interval. With the approximate chronol-
ogy of the deep-sea record established from constant sedi-
mentation rate assumptions and the Brunhes/Matuyama
boundary, and with approximate chronology of coral-reef
terraces established from uranium-series dating, it is pos-
sible to undertake comparison of the INTO record from
deep-sea cores with the 6~80 record from coral-reef ter-
races.
Such an effort has been quite successful on Barbados
(Fairbanks and Matthews, 1978) and on New Guinea
(Aharon, 1983~. Both attempts yield similar results; the
Barbados record is stressed here because it contains more
information concerning maximum glacio-eustatic sea-level
fluctuation in the time interval of 170 to 130 kaBP. After
discussion of the coral-reef terrace 6~80 record, I will
return to integration of these data with the deep-sea 6~80
record and with sea-level estimates based on the coral-reef
terrace constant-uplift hypothesis discussed above.
The Barbados Coral-Reef Terrace ~ ~ SO Record Figure
5.1 presents a cross section of the Christ Church region of
Barbados indicating the location of numerous cored bore-
holes and the position of a subaerial exposure surface
ROBLEY K. MATTHEWS
encountered in these boreholes. Oxygen isotope data have
been obtained on numerous samples of reef-crest coral (A.
palmata) from terraces and from borehole materials above
and below the prominent subaerial exposure surface indi-
cated in Figure 5.1. Figure 5.2 presents a plot of INTO data
versus actual present-day elevation of the A. palmata
samples.
We presume that uplift of the island of Barbados is
relatively small in comparison to sea-level fluctuations
that have occurred within the time interval under consid-
eration. Thus, uplift rate, if present, may be considered as
"noise" with reversed sign for rising versus falling sea
level. Note that the slope of the 6'8O versus elevation
relationship for Kendal Hill to borehole 20 (regressive) is
quite similar to the slope for the borehole 20-Rendezvous
Hill transgressive relationship. The data of Wagner (1983)
confirm this relationship of 6~80 versus elevation for re-
crystallization products of phreatic lenses related to vari-
ous late Pleistocene stands of sea level.
Figure 5.3 plots INTO variation versus age for Barbados
A. palmata data and for data on planktonic foraminifers
from deep-sea cores. The general agreement between these
two records is unquestionable; there remains only some
small points concerning detail. These are dealt with below.
CHRIST CHURCH RIDGE
C ROS S S E C T I ON KINGS. AND
( BARBADOS
RENDE2V~
o
>
120:
h
h
J
;2 6 0
At:
en 40
_
100
J
>
8 0
IS
J
~ O ~
O
J
al ,_
-40 _
2, ~
,' PLEISTOCENE
LIMESTONE
TERTIARY
LIMESTONE
VERT EXAC. I5X
FIGURE 5.1 Cross section of the southern flank of Christ Church
Ridge, Barbados, indicating coral-reef terrace relationships, lo-
cation of cored boreholes, and the existence of a buried caliche
profile (dashed line) that records subaerial exposure during deep-
sea isotope stage 6. Samples of the surf zone coral (A. palmata)
were taken from immediately beneath the subaerial exposure
surface in RKM 20, 22, and 23 and from immediately above the
subaerial exposure surface in RKM 20. Isotopic data for these
materials are plotted against elevation in Figure 5.2. (From
Fairbanks and Matthews, 1978, with modification.)
OCR for page 95
QUARTERNARY SEA-LEVEr CHANGE
-~ u
-20 ~
T ~ I I
( BARBADOS Ill ) I
R E ~ D E Z V O U S ~ I L L |
(-0.012 ) KENDAL HILLY
1
1
1 (-0 010)
R K M # 20 Tat-
-
- (- 0 022 )
~M#20R
(-0 048)
1/
fRKM # 22 R
_ -I
, , , , t ,
-40 -30 -20 -10 0 10 20
ELEVATION RELATIVE TO PRESENT SEA LEVEL (M)
Frozen Bottom-Water Hypothesis The use of the benthic
6~80 record as a possible indicator of ice-volume fluctua-
tions is rooted in the clever argument of Shackleton (19671.
At a time when Emiliani (1966a, for example) was talking
about a 4° to 6°C colder tropical sea-surface temperature
(SST) during glacial times, and small ice-volume compo-
nent to the bi80 signal, Shackleton took the trouble to
analyze relatively scarce benthic foraminifers. The result
was astounding. The benthic 6~80 curve was similar in
shape and amplitude to the planktonic 6~80 curve of Emili-
ani. Bottom water could not possibly have gotten 4° to
6°C colder during glacial conditions because this would
put the water below the freezing point. The logical conclu-
sion was that Emiliani had greatly overestimated the tem-
perature effect and greatly underestimated the ice-volume
effect in both of these signals. Shackleton proposed that
the benthic bi80 signal was more like two-thirds ice-vol-
ume signal and one-third temperature signal, although the
precise relation could not be determined from the data at
hand.
Importantly, there is no a priori reason to assume con-
stant bottom water temperature during the glacial/intergla-
cial cycle. A priori, the bottom water shall not freeze, but
that is about it. Beyond this, there are no "magic num-
bers" for bottom-water production. Bottom water is sim-
ply that combination of temperature and salinity that is the
most dense water. With glacial/interglacial changes in
geometry of high-latitude marginal seas, all bets are off
concerning precisely what temperature/salinity combina-
tion will end up being the glacial world's bottom water.
Thus, the initial success of Shackleton (1967) must be
viewed as substantially empirical and semiquantitative. If
95
30 40 50
~ BARBADOS DATA
~ ECENT
-3.0
._
a, -2.0
Q
o
0
a)
n n
FIGURE 5.2 Plot of mean isotopic com-
position of Barbados A. palmata samples
against elevation relative to present sea
level. Assuming the rate of tectonic uplift
to be small in relation to the rate of glacio-
eustatic change, these data constitute a
calibration of 6~80 as a function of glacio-
eustatic sea-level fluctuation. A relation-
ship of 0.011 per mil per meter is taken as
the glacio-eustatic effect. More positive
6~80 values at RK 20 and 22 are satisfied
by CLIMAP estimates that this region was
approximately 2°C cooler than modern
during full interglacial conditions. (After
Fairbanks and Matthews, 1978.)
RENDEZVOUS HILL KINGSLAND
VENTNOR-t KENDAL HIL+BERDARE
WORTHING: ~ / \ if, '
RKM 2)T ~RKM 23R
~ ~RKM 20R
DEEP SEA RECORD ~RKM 22R
THOUSANDS OF YEARS BEFORE PRESENT
FIGURE 5.3 Comparison of Barbados coral isotope record with
the stacked deep-sea planktonic isotope curve of Prell et al.
(1986). Error bars on isotopic data indicate statistical certainty
about the mean at the 95 percent confidence level. Coral isotope
data for Worthing, Ventnor, and Rendezvous Hill are from Table
5.2; the remainder are from Fairbanks and Matthews (1978).
Note peak-for-peak correlation; Aberdare equals 7.5, and so
forward to Worthing equals 5.1. (Similar to Fairbanks and
Matthews, 1978, Figure 7; modified to include new data and a
more up-to-date deep-sea planktonic 6~80 record.)
OCR for page 96
96
ROBLEY K. MATTHEWS
one wishes to constrain temperature to be constant, one lated by many working on materials within Tertiary time
must hook up to a 6~80 recording system other than hen- intervals.
thic foraminifers. There are a priori arguments to suggest
that the low-latitude sea surface may provide such a re
cording system.
Low-Latitude Constant Sea-Surface Temperature Hy
pothesis Sea-surface temperature estimates based on total
faunal census data and core top regression equations
(CLIMAP, 1976, 1984) provided the (seemingly) final
death blow to the Emiliani concept of a substantially cooler
global SST during glacial times. Application of this tech
nology to the 18-kaBP maximum glacial reconstruction
indicates that some regions of the low-latitude ocean were
slightly warmer at 18-kaBP maximum glacial time than
they are today. Other large areas were only slightly cooler.
Only where major shifts in upwelling occurred are low
latitude SST greatly cooler at 18 kaBP than today. Prell
(1985) applied alternative methods and basically confirmed
the CLIMAP (1984) results, suggesting, if anything, still
less tropical cooling than CLIMAP.
Further, there are physical climatologic/oceanographic
reasons to predict relatively constant SST of low-latitude
ocean surface. The balance between radiation and latent
heat flux dictates a tropical SST of about 28°C (Done and
Shaw, 1977; Newell et al., 1978; among others). A1
though mechanisms can be envisioned that might cause
this number to increase slightly (increased atmospheric
CO2, for example), such mechanisms generate negative
feedback (increased frequency of hurricanes, for example)
that is poorly understood. At the present state of the art,
calculations of atmospheric circulation arrive at 28°C tropi
cal SST; this temperature is in generally good agreement
with the CLIMAP empirical estimates for 18 kaBP. In
light of existing data and experience, it would be difficult
to justify an hypothesis of runaway warmer tropical SST.
The possibility of cooler tropical SST is a different
matter. Variation in planetary albedo, atmospheric CO2,
or perhaps even solar insolation could result in a generally
cooler planet and with it generally cooler tropical SST.
Indeed, Manabe and Broccoli (1985) simulated a glacial
world with higher albedo and lower atmospheric CO2 and
thereby calculated a tropical SST on the order of 1° to 2°C
cooler than modern. Importantly, this model result is
opposed by the empirical temperature estimates of CLI
MAP (1984) and Prell (1985) discussed above.
Regardless of the ultimate resolution of this discrep
ancy between model and empirical results, all global
atmospheric circulation model experiments to date sug
gest that a colder SST in the overall tropical regions would
be accompanied by still yet colder polar regions. While
this relationship is generally proposed by most working on
late Pleistocene materials, the relationship is grossly vio
The validity of the constant tropical SST hypothesis is
an ongoing research problem. There is room for change.
However, tropical SST is a profoundly important number
with truly global ramifications. Change should not be
proposed lightly. If at some time in the future modeling
results move forward to still higher SST under certain
conditions, these deviations from constant temperature
hypothesis will be based on a priori models. Likewise, if
the disagreement between model and empirical data con-
cerning 18-kaBP SST shall be resolved in favor of cooler
temperatures, these estimates of cooler temperatures will
likewise be based on a priori models and thoroughly well-
reasoned discussion. These new numbers would then
become the basis for ice-volume calculations, and so on
toward a perfect understanding of Earth history.
With regard to isotopic estimation of sea-level history,
the point here is that the low-latitude tropical sea-surface
environment offers the best opportunity of constraining
temperature variation and thereby reading out the glacio-
eustatic ice-volume signal. This water mass is sampled by
reef-crest corals and by surface-dwelling planktonic fo-
raminifers. Indeed, if Dodge et al. (1983) had chosen to
compare their terrace elevation data to the planktonic INTO
record instead of the benthic INTO record, they would have
arrived at a much more favorable comparison (a point to
which I shall return below).
Maximum Glacio-Eustatic Sea-Level Lowering at Iso-
tope Stage 6 The data presented in Figures 5.2 and 5.3
afford an estimation of the amplitude of the glacio-eustatic
signal in late Pleistocene time. The deep-sea INTO record
clearly demonstrates that isotope stage 2 and isotope stage
6 are comparable examples of maximum glaciation during
late Pleistocene time. The Barbados borehole data capture
1.7 per mil 6~80 variation within an elevation variation of
approximately 80 m. Allowing for the fact that the Kendal
Hill INTO value is somewhat heavier than Recent and al-
lowing for local faulting (Wagner, 1983), the amplitude of
sea-level variation from full interglacial to data point "RKM
22 (regressive)" could be estimated at 108 m. Making
further allowance for the fact that local INTO amplitude in
nearby deep-sea cores is approximately 2.0 per mil, one
has an additional 0.3 per mil to play with as either local
temperature signal or global ice-volume sea-level signal.
Note in Figure 5.2 that the isotope versus elevation
variation increases dramatically below the data point "RKM
20 (regressive)." This reflects a still more negative INTO
value for late-stage continental ice, or a larger proportion
of floating ice, or a local temperature decline during ex-
treme glaciated conditions. Assuming the effect from data
points "RKM 20 (regressive)" to "RKM 22 (regressive)"
OCR for page 97
QUARTERNARY SEA-LEVEL CHANGE
to be solely an isotopic effect, the full 2.0 per mil ampli-
tude reflects 114-m sea-level lowering relative to present.
Alternatively, CLIMAP-18 kaBP AT estimates for this
area are approximately nil for summer and approximately
-2°C for winter. Assumption of a-2°C temperature dif-
ference from interglacial to glacial conditions and a 1 20-m
sea-level difference from present to maximum glacial
conditions satisfies the isotopic maximum amplitude data
in this area. Assuming the 108-m amplitude figure cited
above for actual Barbados data with some straightforward
corrections, a temperature difference of-3.3°C from inter-
glacial to glacial conditions is required. All three of these
estimates are probably within the uncertainty of the data.
Comparison of Isotope Stage 5 Data for Terraces and
Deep-Sea Cores As noted above, Dodge et al. (1983)
erroneously compared terrace elevation data to the benthic
rather than the Planktonic 6~80 record. A second source of
error in the Dodge et al. (1983) calculation concerns the
isotopic data reported by Fairbanks and Matthews (1978)
for Barbados terraces Worthing, Ventnor, and Rendezvous
Hill. The isotopic data contained in Fairbanks and Mat-
thews (1978) for Barbados surface samples were taken on
scraps of sample left over from radiometric dating and
petrographic studies. In many cases, these were relatively
small samples. Relatively small samples of A. palmata
present a special problem in that there is strong seasonal
banding within this coral. Given a small sample and
nondescript geometry of banding, it would be easy to take
a nonrepresentative sample for oxygen isotope analysis.
To evaluate this problem, an entirely new set of dia-
mond-drilled, 2-inch-diameter core samples were collected
97
expressly for isotopic study. The middle columns of Table
5.1 present Fairbanks and Matthews data and new data
(from Table 5.2) concerning the isotopic difference among
Barbados terraces Rendezvous Hill (isotope stage 5.5) as
compared to Worthing (5.1) and Ventnor (5.3~. The left-
hand columns present similar information from Dodge et
al. (1983) estimates of elevation differences converted to
INTO values by the calibration of Fairbanks and Matthews
(19781. The right-hand column presents observed isotopic
differences among Planktonic foraminifers based on the
average of low-latitude deep-sea cores.
With regard to Barbados isotope data, note that the new
data indicate slightly less difference between the terraces.
At a glance, the difference between the Fairbanks and
Matthews (1978) data and the new data is not that excit-
ing. Both data sets put the younger terraces low relative to
Rendezvous Hill; the differences among means are within
the range of overlapping confidence intervals; in 1978, the
situation looked well under control. However, over the
years there has been harping criticism concerning the
Fairbanks and Matthews estimate that sea level repre-
sented by the younger Barbados terraces might have been
as much as 45 m below present sea level. To me, the
situation was "close enough; move on to other things." To
those inclined to be more fastidious, the new data will be
somewhat reassuring.
When one compares the coral-reef terrace elevation
data of Dodge (left-hand column of Table 5.1) with the
Barbados terrace isotope data (middle columns) and with
the deep-sea record (right-hand column), one notes re-
markable agreement between the new Barbados isotope
data and the deep-sea cores. Estimates based on the ad hoc
TABLE 5.1 Comparison of Terrace Elevation Data to Isotope Data from Corals and
Low-Latitude Planktonic Foraminifers
Terrace Elevation Barbados Corals Low-Latitude
Data, 6~80 Fairbanks and Planktonic
Isotope Stage equivalent,a Matthews (1978), New Data,b Foraminifer,C
Comparison 6~80 (per mil) 6~80 (per mil) 6~80 (per mil) 6~80 (per mil)
5.1-5.5 +0.24 + 0.1 (3) +0.54 + 0.4 (7) +0.35 + 0.2 (9) +0.34 + 0.2 (5)
5.3-5.5 +0.21 + 0.1 (3) +0.52 + 0.1 (15) +0.35 + 0 2 (9) +0.30 + 0.1 (5)
aElevation data from Barbados, New Guinea, and Haiti converted to 6~80 via Fairbanks and
Matthews (1978~.
bSee Table 5.2.
CPlanktonic 6~80 data from the following cores and their respective sources were used: Core 280
(Emiliani, 1958), Core P6304-9 (Emiliani, 1966b), Core P6408-9 (Emiliani, 1978), Core V28-238
(Shackleton and Opdyke, 1973', and Core V22-174 (Thierstein et al., 19771.
NOTE: Data are reported as (mean /~) + (confidence about mean, 95% C.L.~; number of independent
estimates are in parentheses.
OCR for page 98
98
TABLE 5.2 Isotope Data on Barbados Corals
ROBLEY K. MATTHEWS
Sample Number 6,80 613C Sample Number 6~80 613C
and Description (per mil) PDB and Description (per mil) PDB
Worthing (Barbados I)
FS-SOA
Duplicate I
Ave. FS-50A
-3.47
-3.31
-3.39
+0.19
+0.08
+0.13
Maxwell ("New" terrace between Ventnor and Rendezvous Hill;
basically part of Rendezvous Hill)
AEJ-20 -3.65 +0.01
FS-51-3.420 96 AEJ-21 -3.65 -0.58
Duplicate B3.16+0.92 Duplicate G -3.82 0.52
FoSC5520A-3 61-1 19 AEJ--2120 -3 821 +0 28
OC-51-3 22+020 AGP 1l _3 50 +0.49
OC-53-3 20+0 64 -3.46
Sandy Lane- 1-2 94+0.55 Dupllcate D -3.61 +0.38
Sandy Lane-2-3.27-1.14 Duplicate D -3.39 +0.55
Sandy Lane-4-3.64+0.53 Ave. AGP-11 -3.49 +0.50
Duplicate H-3.61+0.62 AGP-12 -3.63 +0.15
Ave. Sandy Lane-4-3.62+0.58
Mean-3.280 17 Mean _3 57 +g 19
c,0.230.81 0.25 0.65
C 95 O. 8 0.6 r = ~.40 (not significant)
r = +0.45 (not significant) n = 6
n = 9
Rendezvous Hill (Barbados III)
Ventnor (Barbados 11) AFM-20A -3.74 +0.38
ANM-20 _3 03 -0 60 AFM-22A -3.75 +0.20
ANM-22 -3.30 0 37 R-50 -
Duplicate E -3.30 -0.37 Duplicate J 3.56 +0.29
Ave. ANM-22 _3 30 -0 37 3.56 +0.46
50 3 Z3 +070 R-5 -344 +0.32
Dopllca e A 3 33 +o 44
D pl ca e F 3 3 AFS- O -3 60 +0.77
Ave. FT-50 ~3 33 +0-49 AFS
FT-51 -3.65 -1.30 AFS- 12 -3.76 +0.10
FT-53 -3.55 -1.88 Mean -3.63 +0.1 l
Duplicate C -3.25 -1.57 ~0.11 0.6~
Ave. FT-53 -3.40 -1.73 CLgs 0.08 0.4E
BAB-10 -3.46 +1.18 r = -0.04 (not significant:
BAB-11 -3.00 _0.75 n 9
BAB-13 -3.24 -0.18
Mean -3.28 -0.40
~0.21 0.87
CL,s 0.16 0.67
r = +0.025 (not significant)
n = 9
NOTE: During the summer of 1977 A. palmata was recol-
lected on Barbados Terraces I, II, and III expressly to confirm the
results reported by Fairbanks and Matthews (1978~. Whereas
their subsurface data were taken on high-quality core materials,
the terrace materials were leftover scraps from other projects. As
such, samples were rather small and cut in undetermined relation
to growth lines. For this reason, it was highly desirable to
resample.
Each of the three terraces was sampled three times at each of
the three localities. Samples were taken with a 2-inch-diameter
diamond core drill. Each core was approximately one foot in
length. Samples for isotopic study were taken perpendicular to
growth bands to represent the average of the whole coral.
Isotopic data were collected on the Benedum Stable Isotopes
Laboratory VG 602D over four days from November 20, 1978
to February 20, 1979. Precision (1~) on standards (6) was +0.00
for 6~80 and +0.18 for 6~3C. Precision (half range) on random
duplicates (9) was +0.06 for both 6~80 and 6~3C. Averages were
used in calculation of terrace statistics.
OCR for page 99
QUARTERNARY SEA-LEVEL CHANGE
constant uplift hypothesis for terrace elevation (Dodge et
al., 1983) are only slightly smaller than the actual isotope
data for Barbados terraces and the deep-sea cores. The
difference is well within the confidence interval for the
various data subsets. To bring the means into literal agree-
ment requires tropical sea-surface cooling on the order of
only 0.5°C.
Thus within the uncertainty of the data, the low-latitude
planktonic INTO record is precisely a measure of global
ice-volume fluctuation within the sea-level fluctuation range
represented among the three younger Barbados terraces.
On the basis of isotope data from boreholes, this precise
relationship of INTO variation to change in elevation of sea
level may be extended to the depth of 50 to 70 m below
present sea level. Below this depth, the scant data that are
available (Figure 5.2) suggest that a local temperature
effect may become important in the depths of the glacial
portion of cycles.
The Deep-Sea I;'80 Record as an Ice-Volume Signal
In summary then, terrace elevation data, Barbados iso-
tope data, and deep-sea isotope data are in close agreement
within the time interval of Barbados terraces Worthing,
Ventnor, and Rendezvous Hill (82 to 125 kaBP). Given
the uncertainty within each data set, no temperature effect
is needed to bring these data into agreement; likewise, no
hidden floating ice is required. The calibration of Fair-
banks and Matthews (1978) is sufficient to the task in and
of itself. Further, the data on which that calibration is
based (namely, data points "RKM 23 regressive" and "RKM
20 transgressive" of Figure 5.2) indicate that ice volume
alone is a sufficient explanation of the deep-sea b'8O vari-
ation to a depth of sea-level lowering of approximately 50
to 70 m below present sea level, accounting for some-
where between 0.5 and 0.8 per mil of global ocean INTO
enrichment from interglacial conditions toward glacial
conditions.
In the region of Barbados, the lower half of the INTO
record in coral materials and in nearby deep-sea cores is
satisfied by a combination of continued ice-volume buildup
and a local temperature lowering of approximately 2°C (a
INTO effect of 0.5 per mil). Thus, although this combina-
tion of numbers arrives at the classical Shackleton (1967)
estimate of one-third AT effect and two-thirds /\ice-vol-
ume effect, it is important to note that the temperature
effect (if valid at all) is concentrated in the glacial half of
the signal. Importantly, to the extent that this temperature
effect may represent global cooling, it appears to be decid-
edly the result of ice-volume buildup (presumably an al-
bedo feedback mechanism); strikingly, this is the reverse
of the common wisdom scenario that global cooling initi-
ates continental glaciation.
99
THE PAST AS THE KEY TO THE FUTURE
The discussion above has dealt largely with geologic
information concerning "the numbers" regarding late
Quaternary sea-level history. What was the level of the
sea at what date in the past; how good are the data; how
shall we resolve apparent conflicts? Geologists also have
insight and opinion concerning the workings of the Earth
as a dynamical system. Two possible scenarios deserve
consideration here with regard to the possible ice-volume
effects of future climatic warming.
West Antarctic Ice Surge Accompanying Warm
Interglacial High Stand
Among the world's major ice sheets, the West Antarc-
tic ice sheet seems to be especially precariously situated
today. It is substantially a marine-based ice sheet, and the
present-day topography over vast areas lies below the
theoretical equilibrium profile. It can be argued that this
ice sheet is capable of disintegrating over the next few
hundred years. If such a glacial surge should occur, world
sea level would rise by approximately 6 m. The geologic
question is whether or not such a surge event has occurred
in the past.
There is no doubt that sea level at isotope stage 5.5
stood at around present sea level or slightly higher. The
question with regard to possible West Antarctic ice surge
is whether or not we can be confident that it stood 6 m
above present sea level. Moore (1982) provided a conven-
ient summary of 230Th dating from both tectonically active
and tectonically stable regions. Seemingly tectonically
stable regions giving seemingly reliable estimates for iso-
lope stage 5.5 sea level above present sea level include the
Bahamas, Bermuda, western Australia, Aldabra Atoll, and
the Yucatan Pennisula, Mexico. Estimates of sea level
relative to present range from +2 to +6 m.
Another approach to the relative position of sea level at
isotope stage 5.5 would be to compare 6~80 data to Holo-
cene values. However, the differences that we seek to
quantify are relatively small in comparison to analytical
and geologic precision of the isotopic data, the isotopic
difference between modern and +6 m sea-level high stand
being only approximately 0.07 per milt With regard to
deep-sea cores, the problem is compounded by nagging
problems concerning the reliability of core-top materials.
CLIMAP (1984, p. 206) notes the lack of consistency in
their data and refers the question to dated coral reefs.
With regard to Barbados coral data and New Guinea mollusc
data, modern and isotope stage 5.5 equivalent materials
yield comparable values within the uncertainty of the esti-
mates. Encouragingly, the New Guinea data do indicate
brief number VIIa (isotope stage 5.5 equivalent) to be 0.03
per mil lighter than modern; this is in the right direction,
OCR for page 100
100
but a difference of only about half the uncertainty of the
two estimates.
The isotopic data concerning New Guinea VIIb (Aharon,
1983) are especially troublesome with regard to the surge
hypothesis. On the basis of geologic correlation and on
the basis of reef-crest 6~80 values, New Guinea reef VIIa
is surely isotope stage Se equivalent. (The fact that its date
is slightly older than isotope stage 5.5 must be considered
subordinate to these other arguments; see discussion above
concerning problems with dating technology.) Aharon
(1983) continued to regard New Guinea reef VIIb as a
surge event that follows the deposition of reef VIIa. The
interesting problem is that reef-crest bi80 values for VIIb
are 0.5 per mil heavier than the values for VIIa. Aharon
explained these highly anomalous 6~80 values by calling
on dramatic global cooling accompanying a Wilson hy-
pothesis surge event (see Flohn, 1979, for a convenient
review of this and similar topics). It is important to note
that materials similar to New Guinea reef VIIb have not
been observed in any other terrace sequence. Therefore, it
is possible that these unusual data have a local explanation
and do not require integration into a global scenario.
Nevertheless, a scenario such as set forward by Aharon
(1983) could be consistent with the deep-sea 6~80 record.
Inasmuch as rapid cooling accompanies the surge of iso-
topically light water into the world ocean, the proposed
high stand of the sea is lost somewhere in the transition
from isotope stage 5.5 to 5.4.
Taking all of these data literally, one is left with the
unsatisfactory proposition that isotope stage 5.5 (equiva-
lent to New Guinea reef VIIa) achieved a level somewhere
between +2 and +6 m relative to present sea level by
removal of an ice sheet in a manner inconsistent with the
Wilson hypothesis, whereas isotope stage 5.5 was fol-
lowed closely by a glacial surge (and resultant New Guinea
reef VIIb) that was consistent with the Wilson hypothesis.
At this point, belief or disbelief becomes largely a matter
of taste. I would prefer to see the New Guinea reef VII
data set enlarged and the result reproduced within some
other terrace sequence before I proceed to make choices.
At this stage in the science, the simplest choice remains
that isotope stage 5.5 sea level is the same or slightly
higher than modern for reasons that are not fully under-
stood. While the West Antarctic surge hypothesis remains
exciting, the observation could equally be explained by
slight deviations of East Antarctic ice sheet from equilib-
rium conditions by either more rapid flow or insufficient
. . .
precipitation.
Ice Growth Adjacent to a Warm Ocean
An equally credible scenario for the end of an intergla-
cial can be written around rapid growth of continental ice
ROBLEY K. MATTHEWS
fed from a nearby warm ocean moisture supply. This
scenario is best documented with regard to growth of
North American ice sheets at the end of the last intergla-
cial (isotope stage 5.5/5.4 transition) (Ruddiman et al.,
1980~. By generating both planktonic faunal data and
planktonic isotopic data at close spaced sample intervals,
it is clearly demonstrated that the isotopic values become
relatively heavy while the fauna continues to indicate rela-
tively warm water. The simplest explanation for these
observations is that North American ice sheets grew to
considerable size (hence, relatively positive oceanic INTO
values) while the nearby North Atlantic remained rela-
tively warm.
A similar scenario can be envisioned for the Southern
Ocean at times throughout the Tertiary. In this case,
relatively warm water would be provided to the surface
ocean by upwelling to the south of the polar front. Such a
mechanism might be similar to the occasional occurrence
of ice-free conditions among winter pack ice around Ant-
arctica today (the Weddell polynya; see e.g., Gordon, 1982;
Gordon and Huber, 19841. The fact that the polynya
remains ice free throughout winter is ample demonstration
that upwelling water is transferring large amounts of latent
heat to the high-latitude southern atmosphere.
At times in the past, this effect has almost certainly
been much larger. For example, it is likely that the deep
ocean was approximately 8°C in Oligocene time. Upwelling
of such warm water around Antarctica may have provided
the local moisture supply for substantial growth of the
Antarctic ice sheet at approximately the Oligocene-Eo-
cene boundary (Matthews and Poore, 1980~.
Looking to the future, if man's warming of the planet
tends to warm intermediate and deep water (Roemmich,
Chapter 13, this volume), then one might script an Antarc-
tic ice-growth scenario such as outlined above. However,
modern Antarctica has the further complication of large
ice shelves that serve to buttress the glacial ice (tingle,
1984, for example). If these ice shelves were removed by
upwelling of warmer water, the initial effect would surely
be to increase the flow rate of ice streams. The new
equilibrium between these two competing effects (increased
precipitation and increased flow rates) is conjectural. The
fact that both effects are plausible emphasizes the need to
observe Antarctic surface-elevation changes from polar-
orbiting satellites.
GLACIO-EUSTACY IN STRATIGRAPHIC
PREDICTION
Given the high integrity of the deep-sea, low-latitude
planktonic INTO record as a glacio-eustatic signal, there
exists a significant opportunity for stratigraphy to move
beyond a descriptive mode to a predictive mode. The
OCR for page 101
QUARTERNARY SEA-LEVEL CHANGE
PREDICTED
STRATIGRAPHY
E
2
SEA LEVEL HISTORY
1'' __ -
~~ 5~7e~ 93
1
100 200 300 400 500
THOUSANDS OF YEARS BEFORE PRESENT
BASIN SUBSIDENCE
1 00m,'my SUBSIDENCE RATE
( ISOSTATICALLY COMPENSATED AT 2:1
1 ~
FIGURE 5.4 Stratigraphy predicted for bank-margin carbonates
where a well-known (or presumed) sea-level history is superim-
posed on basin subsidence. Sea-level rise deposits new bank-
margin carbonate sediment. Sea-level fall produces subaerial
exposure surfaces. Diagonal dashed lines from sea-level high
stands indicate subsidence that occurs before the next high stand
of the sea. This sort of forward modeling holds great promise for
bringing a new level of understanding to stratigraphy and thereby
petroleum exploration.
opportunity is before us to model an empirically derived
glacio-eustatic signal with mathematical formulations of
basin subsidence and isostasy, for example.
By way of example, Figure 5.4 presents a proposed
stratigraphy of subaerial exposure surfaces within bank-
margin carbonate rocks. The model is generated by inter-
action of the late Pleistocene average INTO curve (Prell et
al., 1986) with a regional driving subsidence of 100 m per
million years. In such a model, deposition occurs only
when rising sea level crosses the subaerial exposure sur-
face of previously existing bank-margin carbonate sedi-
ments. As the sea-level curve tops out and heads once
again toward glacial low-stand, yet another subaerial ex-
posure surface is formed. Such a diagram is, then, a
prediction of the stratigraphy that one would find if one
drilled a borehole through a subsiding passive margin (such
as the modern reef tract of Belize, Central America) that
was subsiding at approximately this rate. While such a
prediction in the late Pleistocene is academic, such predic-
tions in older rocks could have extreme importance in
petroleum exploration. Indeed, I join Christie-Thick et al.
(Chapter 7, this volume) in criticizing the alleged seismic
stratigraphy record of sea-level changes. This famous
curve is nothing more than an ad hoc explanation of the
data growing out of enumerable inductive mode studies of
seismic profiles. However, whereas Christie-Thick et al.
propose still further inductive mode investigations to
reevaluate the ages of sequence boundaries, the approach
101
outlined in this chapter would deductively compare inde-
pendent models for eustatic history (also see Harrison,
Chapter 8, this volume) with models for basin subsidence.
Initial attempts at applying a similar research strategy
to Tertiary materials have been at least partially success-
ful. Major and Matthews (1983) suggested that the shape
of the Vail et al. (1977) sea-level curve for middle Mio-
cene is approximately correct, but that the amplitude of
their proposed sea-level fluctuations is probably exagger-
ated by a factor of three.
CONCLUSIONS
1. The low-latitude planktonic INTO record surely of-
fers the most promise for delivering a continuous record of
glacio-eustatic sea-level fluctuations back through time.
On the basis of existing data, it appears that ice-volume
fluctuations are sufficient to account for the structure in
approximately the more interglacial half of the deep-sea
INTO signal. The claim that this relationship holds true to
a sea-level lowering of approximately 20 or 30 m relative
to present is confirmed by isotopic data on surficial samples
from coral-reef terraces of Barbados, New Guinea, and
elsewhere. The claim that this relationship extends to a
sea-level lowering of 50 to 70 m relative to present rests
on two data points obtained by Barbados core drilling.
These are very important numbers; similar drilling needs
to be carried out in other parts of the world to confirm or
reject the Barbados data.
2. With regard to the more glacial half of the deep-sea
INTO signal, we face still further problems with scarcity of
high-quality information. Estimates concerning the gla-
cio-eustatic amplitude of isotope stage 6 rest solely on a
single cored interval in Barbados RKM 22. Even here,
there is no claim that this data point represents maximum
low stand of the sea. The amplitude of the glacio-eustatic
signal is surely a fundamental property of the dynamic
system. We really should know that number quite well; if
it turns out to be 80 m as opposed to 130 m, the result
would significantly narrow uncertainties concerning con-
tinental ice sheets. Once again, Barbados style core drill-
ing in other parts of the world would seem to be indicated.
Similarly, renewed attempts to sample and date strati-
graphically significant material representing isotope stage
2 from submerged continental margins would appear to be
worth unusual effort.
3. With regard to future glacio-eustatic sea-level fluc-
tuations, it is noteworthy that the entirety of the civiliza-
tion of man has occurred within a single eustatic high
stand of the sea that is indeed quite anomalous to the
average late Pleistocene condition of the Earth. Without
human intervention, surely sea level shall once again fall
to intermediate levels. With inadvertent human interven
OCR for page 102
102
lion, the system might go either way; sea-level rise in
accordance with the West Antarctic ice surge hypothesis;
or sea-level lowering in accordance with the warm ocean
ice growth hypothesis. The precise nature of the geologic
record with regard to these two possibilities is substan-
tially i~Televent to planning concerning possible sea-level
consequences of future climatic warming. Both scenarios
are conceivable; we really do not know the time scales
involved; best we simply plan ahead to monitor the Earth's
ice budget henceforth and forthwith.
4. The advent of predictive stratigraphy would appear
to be at hand. It is technologically feasible to construct a
detailed glacio-eustatic sea-level curve throughout the
Cenozoic. Other long-term eustatic effects can easily be
added to this curve. The interaction of a detailed eustatic
sea-level curve with basin subsidence models awaits pri-
marily the awakening of the stratigrapher and exploration-
ist to the opportunity at hand.
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Representative terms from entire chapter:
isotope stage
