Geochimica et Cosmochimica Acta, Vol. 65, No. 16, pp. 2757–2770, 2001
Copyright © 2001 Elsevier Science Ltd
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Pergamon
PII S0016-7037(01)00621-4
U-Th dating of carbonate platform and slope sediments
GIDEON M. HENDERSON,1,* NIALL C. SLOWEY,2 and MARTY Q. FLEISHER3
1
Department of Earth Sciences, Oxford University, South Parks Road, Oxford, OX1 3PR, England
Department of Oceanography, Texas A&M University, College Station, Texas 77843-3146, USA
3
Lamont-Doherty Earth Observatory of Columbia University, Route 9W, Palisades, New York 10964, USA
2
(Received January 5, 2000; accepted in revised form February 26, 2001)
Abstract——Absolute chronology of marine sediment beyond the 14C age range provides a test for models
of climate change and has many other applications. U-Th techniques have been used for such chronology by
dating corals, but extending these techniques to marine sediment is complicated by the presence of significant
initial 230Th— both in detrital material and scavenged from seawater. In this study, we investigate four
methods of solving the initial 230Th problem for a particular type of marine sediment—the aragonite-rich
sediments of carbonate platforms and slopes. Bulk sediment U-Th analyses can be corrected for initial Th to
yield ages with ⬇2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly
high. Uncertainty on the corrections causes inadequate precision for sediment from other periods, however.
Removal of scavenged Th before analysis would enable a dramatic increase in this precision but has not
proved successful despite a range of chemical leach approaches. Using heavy liquids to separate the various
carbonate minerals found in Bahamas sediment enables an isochron approach to correct for initial Th, but the
presence of initial Th from two sources requires correction or removal of one source of initial Th before the
other is deconvolved by the isochron. Quantitative removal of detrital material before isochron analysis proves
a successful approach. Such isochron data demonstrate that, although sediment remains closed to U-Th on a
centimetre scale, nuclides are moved from grain to grain by ␣-recoil. Such intergrain exchange is expected to
be observed in all sediments containing mineral grains with different U concentrations. Measured 234U/238U
allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as
3 kyr. The accuracy of this approach has been proved by dating samples within the 14C age range. Sediments
spanning the penultimate deglaciation have also been dated. After a small correction for bioturbation, the age
for this event is found to be 135.2 ⫾ 3.5 ka. This date is ⬇8 kyr before the peak in northern hemisphere
insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics.
This isochron approach shows considerable promise for dating of sediments older than this event, which will
provide further information about the timing and mechanisms of global climate change. Copyright © 2001
Elsevier Science Ltd
date corals (e.g., Broecker and Thurber, 1965). Corals have a
high preservation potential only when they form at sea-level
highstands, however, and are difficult to recover from other
periods. Corals are also prone to a progressive diagenesis,
probably related to their subaerial exposure (Bard et al., 1991;
Hamelin et al., 1991; Henderson et al. 1993). This diagenesis
has made it difficult to accurately date even the last interglacial
(MIS 5e; Stirling et al., 1998) and reliable dates from the one
before that (MIS 7) are sparse (Gallup et al., 1994).
Submarine sediments offer advantages over corals because
they provide a continuous record and have not been subaerially
exposed. Unlike corals, however, such sediments contain significant amounts of initial 230Th and 231Pa, which makes dating
difficult. Early attempts to date such sediments made use of this
initial 231Pa and 230Th but were found to be unreliable largely
because the different behavior of these two elements in seawater (Broecker and Van Donk, 1970; Rosholt et al., 1960).
Although excess 230Th has continued to be widely used in the
deep-sea environment to provide information about sedimentation rates, it is only accurate to ⬇30% (Henderson et al.,
1999a).
Aragonite-rich sediments found on the banks and slopes of
tropical carbonate platforms are unusual because they come
close to the assumption of zero initial 230Th that makes corals
so suitable for dating. In periods of particularly high aragonite
1. INTRODUCTION
Marine sediments contain a continuous record of the global
environment, but interpretation of these records is often hindered by lack of good age control. Carbon-14 analyses have
enabled accurate and precise dating of the last ⬇40 kyr, but for
preceding portions of the Pleistocene, no such direct dating tool
has proved applicable. The best existing chronologies are constructed instead by tuning records of global ice-volume, recorded by marine oxygen isotopes, to changes in the Earth’s
orbit. Chronologies derived from tuning in this way, such as the
commonly used SPECMAP chronology (Imbrie et al., 1984;
Martinson et al., 1987), have been fundamental for comparison
of marine records from core to core. By their very nature,
however, such tuned records are dependent on the assumed
model of climate change. If the model of change is incorrect,
then the tuned chronology is also incorrect. Direct radiometric
dating of marine sediments is therefore an important goal both
to test these models and to date sediment that does not have
continuous stratigraphies.
Several U-series nuclides have appropriate half-lives to extend dating beyond the 14C age range and have proved useful to
*Author to whom correspondence should be addressed (gideonh@earth.
ox.ac.uk).
2757
2758
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
production (typically marine highstands), it has proved possible
to date bulk sediment from such an environment using 230Th
ingrowth with only small corrections for initial 230Th (Slowey
et al., 1996). If U-Th dating of such slope sediments could be
extended to the whole Pleistocene at a precision of a few
thousand years, it would represent an important step toward
putting marine records onto an absolute time scale.
The major problem to overcome to achieve this goal is to
distinguish the three sources of 230Th found in the sediment.
Radiogenic 230Th, of interest for dating, must be separated
from detrital230Th and from 230Th scavenged from seawater.
The latter two sources of 230Th also introduce 232Th to the
sediment, which suggests the possibility of using an internal
isochron technique to deconvolve radiogenic from other
sources of 230Th. Detrital and seawater Th have dramatically
different 230Th/232Th ratios, however, so these two contaminants must first be distinguished before an isochron technique
will yield accurate age information. This article reports results
of four approaches to deconvolve the three sources of 230Th:
1. Bulk sediment U-Th analysis with correction for detrital Th
by Al measurement and for seawater Th by remaining 232Th
concentration
2. Chemical pretreatment of samples to remove seawater Th
3. U-Th analyses on density separates from the sediment followed by correction for detrital Th using Al, and seawater
Th using an isochron
4. Physical pretreatment to remove detrital Th followed by
isochron analyses on density separates of the sediment to
correct for seawater Th
The first approach is successful for highstand sediments but not
for sediments from other periods. The second and third approaches are not satisfactory and are discussed only briefly. The
final approach shows significant promise and is the major focus
of this article.
2. REGIONAL SETTING AND SAMPLE CHOICE
All work reported in this article was conducted on sediment
from the leeward slopes of the Little Bahama Bank (LBB).
Sediment here, and on other Bahamian slopes, is dominated by
carbonate and shows regular sea-level-related cycles in mineralogy. During sea-level highstands, the Bahamas banks are
flooded and high aragonite production occurs in the resulting
shallow seas. This aragonite is washed from the banks by tides
and storms and settles on the slopes to form high sedimentation
rate, aragonite-rich sediment packages. At other times in the
sea-level cycle, the banks are exposed, aragonite production is
much lower, and slope sedimentation is slower with a higher
proportion of calcite. This cyclicity also affects grain-size with
high fractions of banktop material leading to finer grained
sediment during the highstands.
Immediately off bank, slopes are steep, and downslope sediment transport is dominant. At depths of greater than ⬇200 m,
slope angles decrease, and downslope transport is often absent.
The leeward slope of the LBB does not experience significant
downward transport beneath 200 m water depth (Burns and
Neumann, 1987) and is the site of continuous sedimentation
without visible turbidites or slumping in cores. Oxygen isotope
records for cores from this area (Slowey and Curry, 1995) and
from elsewhere in the Bahamas (Kroon et al., 2000) exhibit a
complete sequence of marine isotope stages further demonstrating the continuous nature of sedimentation. Cores analyzed in
this study were taken during RV Oceanus cruise 205–2 from
water depths of 400 to 600 m (Slowey and Curry, 1995).
3. GENERAL ANALYTICAL METHODS
U and Th were separated from dissolved samples by coprecipitation with iron followed by conventional anion exchange. Samples were spiked with a mixed 229Th/236U spike.
The 236U concentration of the spike (supplied by Harwell) was
checked immediately after dilution to a level of 1.2‰ (2 SD
n ⫽ 3. Note that here, and throughout the article, 2 sigma errors
are quoted) by analysis against the NIST-traceable concentration standard, CRM-145 (certified to 0.1‰). The concentration
was within error of that expected gravimetrically based on the
concentration of the Harwell 236U solution. Spike 236U/229Th
was calibrated against the secular equilibrium uraninite standard, HU-1 (Ludwig et al., 1992) and is known to a precision
of 1.6‰ (2 SD n ⫽ 4). Both spike isotopes are isotopically
clean (229Th ⬎99.99%; 236U ⫽ 99.97%) and, for U, the concentration of minor isotopes is well known. This enables spiking to a 236U/235U ratio of ⬇10 with only a small (⬇2‰) and
well-constrained correction to the 235U/234U measurement performed in the same run. This level of spiking enables in-run
correction for Daly/Faraday gains and increases the flexibility
of analysis.
U and Th were analyzed by thermal ionization mass spectrometry at the Lamont-Doherty Earth Observatory on a VG
54 –30 equipped with low-background ion-counting (Edwards
et al., 1986). 234U, 235U, and 236U were analyzed by peakswitching on the Daly with 235U, 236U and 238U also monitored
in high-side Faradays. For runs where the beam size was
sufficiently large, the 238U/235U ratio measured in the Faradays
was used directly to correct for mass fractionation. More typically, beam sizes were smaller and uncertainty on the Faraday
235
U became significant. In this case, 236U was used to assess
the Daly/Faraday gain and the Daly 235U measurement used to
assess 238U/235U and correct for mass fractionation.
For low 232Th/230Th samples, 229Th, 230Th, and 232Th were
analyzed on the Daly by peak switching. For samples with a
232
Th/230Th atom ratio higher than ⬇5000, 229Th was used to
assess the Daly/Faraday gain and this used to ratio Daly 230Th
against Faraday 232Th.
Mass spectrometry performance was routinely assessed by
measurement of the NIST U-500 standard. Standards run between 8/95 and 10/98 over a range of intensity give a long-term
average 235U/234U of 95.63 ⫾ 0.90 (2 SD; n ⫽ 57) within error
of the certified value. This level of precision incorporates
variability because of slight nonlinearity in the ion-counting
system, which led to 235U/234U ratios that vary with beam size.
To improve the repeatability, samples from each subsection of
this study were run in a limited intensity window and sufficient
U-500 standards run in the same window to assess the nonlinearity and the external repeatability. Two-sigma external repeatability assessed in this way was between 3 and 4‰ for each
batch of standards (typically n ⬇ 8).
Before analysis of the final isochron samples, machine performance was also assessed by analysis of 6 spike-CRM145
U-Th dating of carbonate platform and slope sediments
mixtures. These runs gave an external repeatability on the U
concentration of CRM-145 of 1.9‰ (2 SD n ⫽ 6). The implied
concentration of CRM-145 was 2.8‰ lower than its gravimetric value, which is consistent with slight evaporation from the
working spike bottle in the 19 months between initial spike
calibration and this experiment. After correction for Daly nonlinearity assessed using U-500 measurements, the (234U/238U)
(where round brackets signify an activity ratio) was 0.9632 ⫾
0.0031 (2 SD n ⫽ 6), in agreement with its measured value in
other labs. The external repeatability of U concentration and
isotope ratio assessed from these six standards is taken to be a
good representation of the uncertainty on individual sample
measurements for this study.
Throughout this study, decay constants used are ␭234 ⫽
2.835 ⫻ 10⫺6 (Lounsbury and Durham, 1971) ␭238 ⫽
1.551⫻10-10 (Jaffey et al., 1971) and ␭230 ⫽ 9.195 ⫻ 10-6
(Meadows et al., 1980). A recent study by Cheng et al. (2000)
assessed slightly different half-lives for 229Th and 234U. Although these new half-lives are preferred for future work, we
have used the earlier half-lives to maintain consistency with
previously published results from this same study (e.g., Slowey
et al., 1996; Henderson and Slowey, 2000). Resulting differences in ages are not significant at the level of precision quoted
in this article.
4. BULK SAMPLE DATING
At sea-level highstands, Bahamas slope sediment can be up
to 95% aragonite and have very low detrital contents (⬍0.5%).
Such sediments can be dated by U-Th with little or no pretreatment (Slowey et al., 1996). Bulk sediment is fully dissolved
and analyzed (the ⬍63␮m fraction can be used instead of bulk
sediment if foraminifera are required for other measurements).
Bulk-sediment ages require correction for U and Th incorporated in the small amount of detrital material present, and for
Th scavenged from seawater.
Detrital corrections are performed by analysis of Al concentrations in the analyzed sediment (either by neutron activation
before dissolution or by directly coupled plasma analysis on an
aliquot of the dissolved sample taken before U-Th chemistry).
This requires assuming U, Th, and Al concentrations for the
detrital fraction and that the U and Th isotopes are in secular
equilibrium. Uncertainty in the detrital U, Th, and Al concentrations are large (on the of order 20%, see Table 1), which
introduces error to the final corrected ages.
Any 232Th that is not accounted for by the detrital correction
is assumed to be scavenged from seawater. This allows a
correction for scavenged 230Th using the (230Th/232Th) of
modern seawater. A (230Th/232Th) of 18 ⫾ 9 (equivalent to a
232
Th/230Th atom ratio of 10000 ⫾ 5000) is consistent with
shallow seawater measurements close to the Bahamas (Hoff et
al., unpublished data) and with estimates from young Bahamian
sediment (Slowey et al., 1996). The large uncertainty that must
be assumed on this value contributes a similarly large uncertainty to any final ages that require a significant correction for
scavenged Th. For non-highstand periods the validity of using
the modern seawater (230Th/232Th) is also open to question.
At the peak of MIS 5e, the high sedimentation rate and low
detrital contents lead to age corrections of as little as 4.5 kyr
and corrected age errors (propagated to include analytical error
2759
and uncertainty due to both corrections) of as low as 2.4 kyr
(Slowey et al., 1996). This allows relatively precise dating of
the peak of the last interglacial using bulk sediments. Results of
five new bulk sediment samples analyzed in this way are shown
in Table 1. One of these (215 cm) is also from the peak of the
interglacial and is in agreement with published ages. Other
samples are from less favorable portions of the sea-level cycle
where aragonite concentrations are lower and corrections become larger. This leads to errors that are unacceptably large
and, in extreme cases, to ages that are clearly in error (Fig. 1).
For instance, a sample from the very beginning of MIS 5e
yields an age of 128.7 ka but to an error of 8.4 kyr, which is too
large to make it particularly useful. Two samples from MIS 6,
where detrital concentrations are ⬎2% and 232Th contents are
high, yield ages that are clearly too young, despite the large
calculated errors. This inaccuracy probably reflects a difference
between the highstand and lowstand seawater (230Th/232Th).
Although a value of 18 ⫾ 9 is reasonable for highstands, it may
well be in error during lowstands because the higher dust flux
and lower sea level during these periods both serve to decrease
seawater (230Th/232Th). If the (230Th/232Th) of seawater was
approximately half that of today during MIS 6, this would bring
the ages close to those expected. Unless we can assess the
seawater (230Th/232Th) in the past, however, we cannot reliably
date bulk samples from periods too dissimilar to today, even at
poor precision (Fig. 1).
In summary, bulk sediment dating is a successful approach
for interglacial periods in which corrections for initial 230Th are
low and well constrained. In other periods, higher initial 230Th
and an uncertain (230Th/232Th) make this approach unworkable.
5. CHEMICAL PRETREATMENT
The problem of uncertainty in past seawater (230Th/232Th)
would be solved if it were possible to remove seawater Th from
samples before analysis using a chemical pretreatment. In natural waters with high carbonate concentrations, Th is complexed as a carbonate ion and is soluble (Anderson et al., 1982;
Simpson et al., 1982). This is in marked contrast to the situation
in seawater where Th is complexed as a hydroxyl ion and is
very insoluble (e.g., Bacon and Anderson, 1982). Based on this
observation, we tested whether Th absorbed on the surface of
calcite grains could be removed by immersion in a high carbonate concentration solution. 228Th was first scavenged onto
aliquots of calcite which were then treated with 2M Na2CO3
solution for varying lengths of time. The 228Th was seen to be
removed from the calcite surfaces over a few weeks (Fig. 2).
Sadly, similar treatment with Na2CO3 solutions did not remove scavenged Th from the surface of Bahamian slope sediments. A 0.3-g aliquot of sediment from 159 cm in core
152JPC was shaken for 2 weeks in 50 mL of 2M Na2CO3and
its U and Th isotope then analyzed. Measured U and Th
concentrations and isotope ratios were very similar to those of
an aliquot of the same sample that had not been treated with
Na2CO3 solution (Table 2). In fact, (230Th/232Th), which
should have increased significantly if scavenged Th had been
removed, was slightly lower (5%)—probably because of slight
sample heterogeneity. Seawater Th in this natural sample is
clearly not passively adhering to the surface so that it can be
238
U
(ppm)
Th
(ppt)
230
232
Th/230Th
(atom ratio)
OC205-2-152JPC (26.2267°N, 77.6708°W, 577 m water depth)
159
4.974 ⫾ 0.012
62.8 ⫾ 0.3
3680 ⫾ 15
215
5.471 ⫾ 0.010
70.7 ⫾ 0.1
1197 ⫾ 2
309
4.824 ⫾ 0.016
65.3 ⫾ 0.6
3622 ⫾ 151
340
1.308 ⫾ 0.003
21.5 ⫾ 0.2
29779 ⫾ 286
360
0.850 ⫾ 0.003
15.6 ⫾ 0.2
39671 ⫾ 670
Sample
(cm)
1.111 ⫾ 0.003
1.107 ⫾ 0.004
1.106 ⫾ 0.005
1.112 ⫾ 0.003
1.096 ⫾ 0.004
(234U/238U)
measured
0.7741 ⫾ 0.0052
0.7924 ⫾ 0.0027
0.8308 ⫾ 0.0093
1.0067 ⫾ 0.0106
1.1232 ⫾ 0.0154
(30Th/238U)
(activity ratio)
125.9 ⫾ 1.7
132.4 ⫾ 1.3
145.8 ⫾ 3.7
232.4 ⫾ 8.8
499.4 ⫾ 201.0
Raw Age
(ka)
662
304
1188
2929
2392
Al
(ppm)
0.57
0.26
1.03
2.53
2.07
Detrital
%
1.154 ⫾ 0.005
1.156 ⫾ 0.004
1.161 ⫾ 0.006
1.210 ⫾ 0.031
1.196 ⫾ 0.037
(234U/238U)
Corrected Initial
107.0 ⫾ 8.2
127.0 ⫾ 3.6
128.7 ⫾ 8.4
75.8 ⫾ 37.7
49.3 ⫾ 47.3
Corrected Age
(ka)
Table 1. Bulk sediment U-Th data and ages. Errors are 2␴ and incorporate statistical error on the collected ratios and weighing error. Al concentrations were measured by neutron activation and are
used to calculate a detrital percent by assuming 11 ⫾ 2% Al in the detritus. These detrital values are used to correct the isotope data for the presence of U and Th in the detritus by assuming it contains
2.8 ⫾ 0.4ppm U, and 13 ⫾ 3ppm Th, both at secular equilibrium. 232Th not accounted for by this detrital correction is assumed to be scavenged from seawater with (230Th/232Th) ⫽ 18 ⫾ 9. The error
on the final age incorporates analytical error and uncertainty due to both of these corrections for initial Th.
2760
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
U-Th dating of carbonate platform and slope sediments
2761
Fig. 1. Bulk sediment U-Th ages from core JPC152. The lower curve shows the ␦18O stratigraphy measured on at least
two replicates of 3 G. sacculifers. This is used to ascribe marine oxygen isotope stages to the core, as shown at the bottom
of the figure. U-Th ages published in Slowey et al. (1996) are open circles and new dates are solid squares (Table 1). Errors
incorporate the 2␴ analytical error as well as error due to corrections for both detrital and scavenged initial Th. Note that
although good precision can be achieved during highstand periods, it is significantly worse during lowstands. The ages from
MIS 6 are also inaccurate, even at their poor level of precision. This demonstrates that a correction assessed with modern
conditions cannot be extrapolated into glacial periods.
complexed by carbonate-rich solutions and must instead be
incorporated into one of the sediment constituents.
A series of further experiments were conducted to selectively
break down various possible sediment constituents before
Na2CO3 complexation. These included attacking organic material with sodium hypochlorite, removing opal with hot
Na2CO3, removing fine clay material by repeated settling, and
removal of ferromanganese oxyhydroxides with a buffered
hydroxylamine hydrochloride leach. Regardless of their relative ferocity, none of these treatments was successful at increasing the (230Th/232Th) by more than 5% from the untreated
aliquot (Table 2). It therefore appears that much of the seawater
Th present in Bahamas slope sediment is bound within the
carbonate grains themselves, preventing the possibility of a
simple chemical pretreatment to remove it.
6. ISOCHRONS CORRECTED FOR DETRITAL THORIUM
The basic assumptions of isochron dating are that all subsamples have the same age and started with the same isotope
ratios. U-Th isochron subsamples must therefore have the same
initial (230Th/232Th) if they are to yield an accurate age. Where
there is only one source of initial Th, this assumption is valid.
But where there are two sources of Th with different isotope
ratios, such as the detrital and scavenged Th found in marine
sediments, this assumption may well be incorrect (Lin et al.,
1996). If the ratio of detrital to scavenged Th varies between
subsamples, then so will the initial (230Th/232Th), and isochrons cannot be used. One way around this problem is to
measure the total U-Th in subsamples separated from a sediment sample and then correct for one of the initial sources of
Th before plotting an isochron. In this section we report results
of such an approach for samples from the Bahamas.
Sediment samples were first sieved at ⬎125␮m to remove
the coarse fraction containing forams and other low U grains.
To create the range in U/Th required for isochron dating,
⬍125␮m separates were then divided into constituent carbonate minerals using heavy liquids. Sodium polytungstate solutions of 2.82, 2.75, and 2.70 g/cm3 were used to separate four
2762
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
Fig. 2. The activity of 228Th attached to the surface of calcite
samples after various lengths of immersion in 2M Na2CO3 solution.
0.1 g aliquots of calcite in 50-mL centrifuge tubes were exposed to
water containing 228Th spike for 3 days. One aliquot was then centrifuged, and the 228Th concentration of the calcite analyzed by ␣-counting. Other tubes were also centrifuged, the water discarded, and the
calcite rinsed into 125-mL polypropylene bottles using three rinses of
2M Na2CO3 solution totaling 50 mL. Each of these four calcites was
left in this high carbonate solution for a different length of time to
investigate the time scale of release of 228Th from the calcite surface to
the solution. Measured 228Th concentrations for the calcite demonstrate
gradual removal of scavenged 228Th from the mineral surfaces.
density fractions from the sieved sediment. The composition of
these separates was assessed using x-ray diffraction (XRD)
(Fig. 3). Separation of the carbonate minerals is surprisingly
good and results in a pure aragonite fraction (D ⬎ 2.82 g/cm3)
and fractions which greatly concentrate either high-Mg calcite
or low-Mg calcite.
Such heavy-liquid separation was performed on three samples from core JPC152 (Fig. 1). These samples (311 cm, 331
cm, and 341 cm) were chosen to span the penultimate deglaciation in an attempt to date this event. These samples also
allow this dating approach to be tested in the full range of
sediment composition seen at this site. Subsamples were fully
dissolved with nitric and hydrofluoric acid and analyzed for U
and Th. Before U-Th chemistry, an aliquot of each subsample
was removed and used to measure the Al/Ca ratio by directly
coupled plasma. Resulting Al/Ca ratios were used to assess the
percentage of detrital material by assuming that carbonate
contained 40% Ca and detrital material 10 ⫾ 3% Al (Table 3).
Detrital contents, particularly in the least dense fraction, are
Fig. 3. X-ray diffraction results from four density separates from a
single aliquot of Bahamas sediment initially containing 20% aragonite.
Density separation was performed using sodium polytungstate at the
densities shown by each curve (g/cm3). The major peak for aragonite,
low-Mg and high-Mg calcite are indicated by the gray bars. Note that
the densest separate yields a very clean aragonite concentrate and that
the calcites are also reasonably well separated from one another.
relatively high and range up to 3% of total mass (Table 3).
These values are used to correct the measured U and Th
concentrations and isotope ratios for initial detrital contamination by assuming that the detrital material is in secular equilibrium and has a U concentration of 2.8 ⫾ 0.6 ppm and Th
concentration of 10 ⫾ 5 ppm. The uncertainty on the Th and Al
concentrations are somewhat larger from those used for bulk
samples (Slowey et al., 1996). This reflects an allowance for
possible changes in typical detrital composition during glacial
times.
Corrected (238U/232Th) and (230Th/232Th) ratios are sometimes negative and show very large uncertainties due to the
uncertainty in the correction, principally the result of uncertainty in the detrital Al concentration. Regrettably, the size of
this uncertainty means that precise age information cannot be
Table 2. Results of various chemical pretreatment investigations on 152JPC-159cm. Details are discussed in “Chemical Pretreatment.”
Chemical Pretreatment
Untreated
Cold Na2CO3
Bleach, cold Na2CO3
Hot Na2CO3
Settling, cold Na2CO3
Bleach, settling, hot Na2CO3
Reducing solution, cold Na2CO3
Acetyl acetone
238
U (ppm)
4.974 ⫾ 0.012
4.770 ⫾ 0.017
4.432 ⫾ 0.005
1.732 ⫾ 0.003
4.531 ⫾ 0.008
1.804 ⫾ 0.002
4.651 ⫾ 0.022
(234U/238U)
(230Th/232Th)
1.111 ⫾ 0.003
1.110 ⫾ 0.003
1.110 ⫾ 0.002
1.113 ⫾ 0.004
1.114 ⫾ 0.002
1.110 ⫾ 0.003
1.091 ⫾ 0.007
50.50 ⫾ 0.2
47.85 ⫾ 0.2
49.54 ⫾ 1.5
42.46 ⫾ 1.5
51.08 ⫾ 1.0
41.33 ⫾ 0.6
53.42 ⫾ 0.7
52.11 ⫾ 1.3
230
Th (ppt)
62.8 ⫾ 0.3
61.0 ⫾ 0.3
61.4 ⫾ 2.2
42.5 ⫾ 1.3
58.2 ⫾ 1.3
40.7 ⫾ 0.6
58.6 ⫾ 0.7
U-Th dating of carbonate platform and slope sediments
2763
Table 3. U and Th concentrations and isotope ratios for density separates from bulk Little Bahama Bank sediment with no prior removal of detrital
material. Isotope errors are 2␴ and incorporate mass spectrometry error on the collected ratios and weighing error. Dust percentages were calculated
from Directly Coupled Plasma analysis of Al/Ca for each fraction by assuming an Al concentration in the dust fraction of 10 ⫾ 3 %. Errors in the
dust percent reflect this range in possible Al concentration. Activity ratios in the final two columns are corrected for U and Th incorporated in this
dust fraction by assuming a Th concentration of 13 ⫾ 5 ppm Th, a U concentration of 2.8 ⫾ 0.6 ppm, and secular equilibrium.
238
Sample
U conc
(ppm)
232
(234U/238U)
Th conc
(ppm)
OC205-2-152JPC (26.2267°N, 77.6708°W, 577m water depth)
311 cm depth
2.70–2.75 g/cm3
3.258 ⫾ 0.010
1.144 ⫾ 0.007
0.336 ⫾ 0.002
2.75–2.80 g/cm3
4.566 ⫾ 0.008
1.129 ⫾ 0.004
0.434 ⫾ 0.002
3
⬎2.80 g/cm
4.103 ⫾ 0.008
1.107 ⫾ 0.005
0.210 ⫾ 0.001
331 cm depth
2.70–2.75 g/cm3
2.460 ⫾ 0.003
1.115 ⫾ 0.005
0.615 ⫾ 0.005
2.75–2.80 g/cm3
2.313 ⫾ 0.003
1.119 ⫾ 0.005
0.603 ⫾ 0.005
3
⬎2.80 g/cm
3.612 ⫾ 0.007
1.097 ⫾ 0.003
0.351 ⫾ 0.001
341 cm depth
2.70–2.75 g/cm3
0.766 ⫾ 0.002
1.127 ⫾ 0.006
0.489 ⫾ 0.006
2.75–2.80 g/cm3
0.903 ⫾ 0.001
1.108 ⫾ 0.006
0.419 ⫾ 0.004
3
⬎2.80 g/cm
2.988 ⫾ 0.004
1.093 ⫾ 0.004
0.224 ⫾ 0.001
derived from these analyses. It is clear that, rather than correct
for the detrital material, it must instead be quantitatively removed before dissolution and analysis.
7. ISOCHRONS FOR PHYSICALLY PRETREATED
SAMPLES
7.1. Sample Selection and Physical Pretreatment
Two samples were selected from a gravity core (GGC33;
Fig. 4) with a complete Holocene and last glacial maximum.
Samples from near the core top and from close to the midpoint
of the deglaciation were selected to test U-Th isochron dating
against 14C. Three samples were also selected from piston core
JPC152 to span the penultimate deglaciation (Fig. 4). To fully
remove detrital clays, ⬃10 g of each sample was sieved to yield
(230Th/232Th)
Measured
Dust (%)
(238U/232Th)
Corrected
(230Th/232Th)
Corrected
30.49 ⫾ 0.28
32.09 ⫾ 0.17
50.02 ⫾ 0.18
2.1 ⫾ 0.4
2.7 ⫾ 0.6
1.1 ⫾ 0.2
⫺214 ⫾ 226
⫺209 ⫾ 223
823 ⫾ 430
⫺220 ⫾ 233
⫺208 ⫾ 223
689 ⫾ 359
10.95 ⫾ 0.06
10.46 ⫾ 0.08
26.49 ⫾ 0.28
2.9 ⫾ 0.6
2.6 ⫾ 0.5
0.6 ⫾ 0.1
83 ⫾ 40
55 ⫾ 24
46 ⫾ 6
74 ⫾ 36
49 ⫾ 21
39 ⫾ 5
5.57 ⫾ 0.07
7.19 ⫾ 0.10
35.68 ⫾ 0.24
1.7 ⫾ 0.4
1.4 ⫾ 0.3
0.3 ⫾ 0.1
13 ⫾ 4
15 ⫾ 4
52 ⫾ 4
15 ⫾ 5
17 ⫾ 5
45 ⫾ 4
a 63- to 250-␮m fraction. New plastic sieve mesh was used for
each sample and discarded after use. Samples were washed
repeatedly on the sieve in distilled water and methanol. Samples were then transferred to beakers and washed in distilled
water with ultrasound treatment and the water pipetted repeatedly until it was completely clear.
Detritus-free 63- to 250-␮m fractions were then divided into
constituent carbonate minerals using heavy liquids as in the
previous section. Density fractions of ⬎2.82, 2.75 to 2.82, and
2.70 to 2.75 were dissolved for further analysis. Before dissolution, aliquots of the subsamples from the last deglaciation
were taken for 14C analysis to compare the ages of the various
constituents. A similar 14C age comparison of different mineral
species was made for a full glacial sample. Subsamples surviv-
Fig. 4. ␦18O and aragonite stratigraphies spanning the last deglaciation in core GGC33 and the penultimate deglaciation
in core JPC152. Black dots are replicated analyses of three G. sacculifer individuals. The dark gray line is a fit to this data
derived by a model to account for bioturbation, and the pale gray line is the unmixed ␦18O signal input to this model. Marine
isotopes stages are marked above each figure, and kite marks represent depths from which samples were selected for U-Th
isochron dating. Aragonite percentages are shown by crosses on the lower curve on each figure.
2764
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
Table 4. U and Th isotope data with 2␴ errors for thoroughly cleaned, detrital free 63–250␮m separates from Bahamas sediment. f234 is the fraction
of radiogenic 234U that remains in the subsample (i.e., values ⬍1 mean that some 234U is lost by recoil, whereas values ⬎1 mean addition). Corrected
(230Th/232Th) values are adjusted for recoil effects on 230Th using f234. See “Alpha Recoil Effects” for full discusion.
232
Subsample
238
U (ppm)
(234U/238U)
OCE205-2-33GGC (26.221°N, 77.691°W, 770 m water depth)
6 cm core depth
2.70–2.75 g/cm3
1.8946 ⫾ 0.0026
1.1520 ⫾ 0.0031
2.75–2.80 g/cm3
4.1461 ⫾ 0.0076
1.1445 ⫾ 0.0030
⬎2.80 g/cm3
3.4482 ⫾ 0.0022
1.1480 ⫾ 0.0025
67 cm core depth
3
2.70–2.75 g/cm
0.7073 ⫾ 0.0006
1.1540 ⫾ 0.0039
2.75–2.80 g/cm3
2.4603 ⫾ 0.0019
1.1457 ⫾ 0.0032
⬎2.80 g/cm3
2.6491 ⫾ 0.0019
1.1511 ⫾ 0.0022
OC205-2-152JPC (26.226°N, 77.670°W, 577 m water depth)
311 cm core depth
2.70–2.75 g/cm3
2.7746 ⫾ 0.0023
1.1628 ⫾ 0.0033
2.75–2.80 g/cm3
4.5767 ⫾ 0.0037
1.1037 ⫾ 0.0028
⬎2.80 g/cm3
3.8656 ⫾ 0.0041
1.0975 ⫾ 0.0030
Bulk sediment
4.7984 ⫾ 0.0036
1.1012 ⫾ 0.0027
331 cm core depth
2.70–2.75 g/cm3
0.7203 ⫾ 0.0004
1.1345 ⫾ 0.0034
2.75–2.80 g/cm3
1.1456 ⫾ 0.0010
1.1203 ⫾ 0.0032
⬎2.80 g/cm3
2.6079 ⫾ 0.0019
1.0855 ⫾ 0.0030
Pteropods
0.5645 ⫾ 0.0003
1.1353 ⫾ 0.0027
Bulk sediment
2.2356 ⫾ 0.0014
1.1134 ⫾ 0.0027
341 cm core depth
2.70–2.75 g/cm3
0.5037 ⫾ 0.0002
1.1221 ⫾ 0.0032
2.75–2.80 g/cm3
1.0329 ⫾ 0.0007
1.1178 ⫾ 0.0034
3
⬎2.80 g/cm
2.5105 ⫾ 0.0014
1.0900 ⫾ 0.0035
pteropods
0.5112 ⫾ 0.0002
1.1247 ⫾ 0.0029
Bulk sediment
1.1827 ⫾ 0.0006
1.0995 ⫾ 0.0037
ing the cleaning and separation protocols and used for U/Th
analysis were between 0.18 and 1.20 g. For two of the samples,
there were sufficient pteropods in the ⬎250-␮m fraction that
they could be handpicked, thoroughly rinsed in distilled water
with ultrasound, and also analyzed. For the three old samples,
bulk sediment was also analyzed to assess the long-term diagenesis of the sediment. Samples were dissolved under water
by dropwise addition of nitric acid. Samples dissolved completely (except the bulk samples) indicating that they were pure
carbonate.
7.2. Results
Uranium concentrations in the subsamples range from 0.5 to
4.6 ppm and typically anticorrelate with thorium concentrations
that range from 35 to 300 ppb (Table 4). This gives a wide
range of (238U/232Th) ratios, ideal for the isochron approach
(Fig. 5). Initial (230Th/232Th) ratios are significantly higher than
typical crustal values (⬇0.7), and the three interglacial samples
yield values of 9.9, 9.6, and 11.9, which are within the modern
observed seawater value (18 ⫾ 9; see “Bulk Sample Dating”
section above). This suggests that the isochrons reflect twocomponent mixing of radiogenic Th and scavenged Th and do
not include detrital Th. Lower initial (230Th/232Th) value for
the two glacial samples are to be expected as the higher dust
flux in the glacial will increase the 232Th concentration.
(234U/238U) for subsamples from a single sample generally
do not agree with one another. This is surprising because all
grains precipitated from seawater and would therefore have had
an identical initial value. (234U/238U) anticorrelates with (238U/
Th
(ppb)
(230Th/232Th)
Measured
f234
(230Th/232Th)
Corrected
72.8 ⫾ 0.4
65.3 ⫾ 0.3
34.8 ⫾ 0.1
13.80 ⫾ 0.33
18.20 ⫾ 0.23
18.55 ⫾ 0.20
1.572 ⫾ 0.558
0.772 ⫾ 0.305
1.148 ⫾ 0.353
8.32 ⫾ 2.35
24.67 ⫾ 7.82
15.84 ⫾ 4.07
249.2 ⫾ 1.1
198.3 ⫾ 0.7
39.0 ⫾ 0.1
10.44 ⫾ 0.10
17.12 ⫾ 0.11
44.32 ⫾ 0.26
1.312 ⫾ 0.121
1.084 ⫾ 0.091
1.233 ⫾ 0.068
7.67 ⫾ 0.72
15.62 ⫾ 1.34
34.91 ⫾ 2.03
210.8 ⫾ 0.7
148.7 ⫾ 0.3
102.4 ⫾ 0.4
42.01 ⫾ 0.22
80.65 ⫾ 0.33
91.03 ⫾ 0.39
1.195 ⫾ 0.011
1.006 ⫾ 0.007
0.986 ⫾ 0.008
34.45 ⫾ 0.42
80.07 ⫾ 0.73
92.45 ⫾ 0.89
222.9 ⫾ 1.2
291.1 ⫾ 2.7
62.8 ⫾ 0.1
242.6 ⫾ 1.1
10.03 ⫾ 0.10
11.98 ⫾ 0.16
97.39 ⫾ 0.51
7.84 ⫾ 0.09
1.106 ⫾ 0.010
1.062 ⫾ 0.009
0.953 ⫾ 0.007
1.108 ⫾ 0.008
8.96 ⫾ 0.13
11.20 ⫾ 0.19
102.79 ⫾ 0.96
6.99 ⫾ 0.11
237.8 ⫾ 0.9
299.7 ⫾ 2.0
69.1 ⫾ 0.1
239.3 ⫾ 1.0
7.45 ⫾ 0.07
11.46 ⫾ 0.20
90.11 ⫾ 0.32
7.40 ⫾ 0.10
1.071 ⫾ 0.009
1.058 ⫾ 0.009
0.975 ⫾ 0.008
1.079 ⫾ 0.008
6.89 ⫾ 0.09
10.76 ⫾ 0.22
92.70 ⫾ 0.88
6.80 ⫾ 0.12
232
Th), which rules out the possibility of the variation being
due to detrital contamination, even if some clays had survived
the cleaning protocol [because the detrital material would have
low (234U/238U) and low (238U/232Th)]. The variability of
(234U/238U) therefore implies a postdepositional redistribution
of the U isotopes. Bulk sediment values, however, are close to
those expected for samples of these ages. As pore water (234U/
238
U) are elevated (Henderson et al., 1999c), bulk sediment
(234U/238U) would be perturbed if there was addition of U from
elsewhere. Bulk sediment measurements therefore imply that
the sediment has remained closed to U addition at the centimetre scale and that subsample (234U/238U) values represent
local internal reorganization. This interpretation is also consistent with the fact that pore-water U concentrations are three
orders of magnitude lower than the solid-phase U concentrations (Henderson et al., 1999c), making significant movement
of U within the sediment difficult. In fact, simple mass-balance
modeling of pore-water results implies that less than 2‰ of the
solid phase U is mobile in these sediment, even at ages greater
than those measured here.
7.3. Alpha Recoil Effects
The systematic variation of (234U/238U) is well explained by
internal reorganization of 234U due to ␣-recoil. Subsamples
with high U/Th values preferentially lose 234U, whereas low
U/Th subsamples gain 234U. The rate of change of 234U in each
subsample at time t, allowing for loss or gain of 234U by
␣-recoil, is given by
U-Th dating of carbonate platform and slope sediments
2765
Fig. 5. U-Th isochrons from the five core depths shown in Figure 4. Sediment separates were performed by heavy-liquid
separation on samples from which detrital material had previously been removed. Full results are presented in Table 4.
Lower panels in each case show the (234U/238U) and illustrate that ␣-recoil has redistributed 234U from U rich grains to U
poor grains during decay (see text for a full discussion of this process). This ␣-recoil redistribution must be corrected for
in the isochron plots. Gray data and lines are uncorrected, whereas black data and lines are ␣-recoil corrected. Note that the
initial Th isotope ratio for highstand samples is within the range of modern values (18 ⫾ 9), illustrating that these isochrons
represent mixing between radiogenic and seawater Th and do not include detrital Th.
⭸(U234)
⫽ f234 䡠 ␭238 䡠 U238 ⫺ ␭234 䡠 U234
⭸t
(1)
where f234 is the fraction of 234U that remains in the sample
after recoil (f234 is ⬎1 if the subsample gains 234U due to recoil
from its surroundings). Allowing for the decay of 238U since
t ⫽ 0 yields
⭸(U234)
234 238
⫽ f234 䡠 ␭238 䡠 UI238 䡠 e(⫺f .␭ .t) ⫺ ␭234 䡠 U234
⭸t
(2)
where UI238 is the initial 238U concentration. Integrating with
respect to t (Faure, 1986) gives
U234 ⫽
f234 䡠 ␭238
234
238
234
䡠 U238 䡠 (e(⫺f 䡠 ␭ 䡠 t) ⫺ e(⫺␭ 䡠 t))
␭ ⫺ f234 䡠 ␭238 I
234
⫹ UI234 䡠 e(⫺␭
234
䡠 t)
(3)
Converting 234U and 238U to activities and assuming that 238U
is unchanged with t due to its long half-life yields
冉 冊
U234
U238
f234 䡠 ␭234
234
238
234
⫽ 234
䡠 (e(⫺f 䡠 ␭ 䡠 t) ⫺ e(⫺␭ 䡠 t))
␭ ⫺ f234 䡠 ␭238
meas
⫹
冉 冊
U234
U238
䡠 e(⫺␭
234
䡠 t)
(4)
initial
Simplifying by assuming that ␭234 ⬎⬎ ␭238 and rearranging
yields
f234 ⫽
(234U/234U)meas ⫺ (234U/238U)initial 䡠 e⫺␭
234
1 ⫺ e⫺␭ t
mobility of 234U (from 238U decay) tells us directly about the
recoil mobility of 230Th (from 234U decay). The process of
recoil exchange is dependent only on the rate of decay of the
two U nuclides. It should also be noted that the initial decay
product is a Th isotope in both cases (234Th and 230Th). So any
exchange that occurs via pore waters rather than by direct grain
exchange will be chemically identical and fast (because of the
strong tendency for Th to adhere to particles). Values of f234
calculated above can therefore be used to make an entirely
self-consistent correction to the 230Th concentrations in the
subsamples. The average energy of 238U decay (4.184 ev:
Ivanovich and Harmon, 1992) is not identical to 234U decay
(4.754 ev). A correction is therefore made by assuming that the
amount of recoil is proportional to the energy of decay causing
the recoil:
1 ⫺ f234 ␣234MTh
⫽
234
1 ⫺ f230 ␣230MTh
230
(6)
where ␣ is the energy released on formation of the two nuclides
and M is the mass of the recoiled nuclides.
To calculate the ingrowth of 230Th allowing for the effects of
␣-recoil mobility, we calculate 230Th growth in two portions.
First, the 230Th growth assuming that 234U and 238U are in
secular equilibrium is calculated
230
⫽
Ths.e.
f230 䡠 ␭234
230
234
230
䡠 U234(e(⫺f 䡠 ␭ 䡠 t) ⫺ e(⫺␭ 䡠 t))
␭ ⫺ f230 䡠 ␭234 I
230
234t
(7)
(5)
If the energetic decay of 238U redistributes 234U, then the
approximately equally energetic decay of 234U will cause redistribution of the resulting 230Th. We are fortunate that both
234
U and 230Th form from the decay of U, which is initially
incorporated with a uniform (234U/238U) ratio. This means that,
regardless of the distribution of U within the grains, the recoil
At secular equilibrium, the activity of 234U is controlled by that
of 238U and, because ␭238 is small, Eqn. 7 reduces to
230
(Ths.e.
⫽ f230 䡠 (U238) 䡠 (1 ⫺ e(⫺␭
230
䡠 t)
)
(8)
Now considering the additional 230Th derived from decay of
234
U in excess of secular equilibrium
2766
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
230
Thxs
⫽
measurement of the bulk sediment (234U/238U) and the use of
the approach outlined above.
f230 䡠 ␭234
230
234
230
䡠 U234(e(⫺f 䡠 ␭ 䡠 t) ⫺ e(⫺␭ 䡠 t))
␭230 ⫺ f230 䡠 ␭234 I,xs
(9)
Converting to activity
f 䡠␭
230
234
230
234
䡠 (UI,xs
) 䡠 (e(⫺f 䡠 ␭ 䡠 t) ⫺ e(⫺␭ 䡠 t))
␭230 ⫺ f230 䡠 ␭234
230
230
)⫽
(Thxs
230
(10)
The full expression for the (230Th/238U) ratio with time, allowing for ␣-recoil, is therefore given by summing Eqn. 8 and Eqn.
10:
冉 冊
230
Th
f230␭230
230
⫽ f230 䡠 (1 ⫺ e⫺␭ t) ⫹ 230
U
␭ ⫺ f230␭234
238
䡠
冉 冊
234
UI,xs
230
⫺f230␭234t
⫺ e⫺␭ t)
238 䡠 (e
U
(11)
Eqn. 11 has three unknowns: (234U/238U)initial, t, and f230. For
marine samples, such as those in this study, we can assume that
all grains have (234U/238U)initial equal to the modern seawater
value of 1.148 because studies have demonstrated constancy of
this value over the last 200 kyr (Gallup et al., 1995; Henderson
et al., 1993). This leaves two unknowns and two measured
quantities (230Th/238U and 234U/238U), so we can solve for t
and f230.
The solution is arrived at iteratively: f230 is initially set to 1,
t is calculated for each sample using an isochron, and (234U/
238
U)initial ⫽ 1.148; deviations of (234U/238U) from that expected for a sample of age t are used to calculate f234 for each
subsample (Eqn. 5); f230 is recalculated for each subsample
(Eqn. 6); 230Th values are recalculated using this new f230
(Eqn. 11); t is recalculated with an isochron; and the process
repeated until t and f are stationary.
f234 values calculated in this way for all subsamples analyzed
in this study range from 0.77 to 1.57 (Table 4). Given that
recoil in minerals occurs at a length scale of 10⫺8 to 10⫺7 m,
the size of the recoil effect is larger than expected if U is
distributed uniformly within 63 to 250 ␮m grains. However,
much of the U in Bahamas sediments is contained in an organic
phase that coats the grains (Henderson et al., 1999c), and the
size of the effect is well explained if U is more concentrated on
the surface of grains.
The isochrons from the penultimate deglaciation are those
most effected by ␣-recoil because of their greater age. The
correction for ␣-recoil has the effect of steepening these three
230
Th/238U isochrons by 15%, 7% and 4% with the greatest
change for the highstand sediment (311 cm) and the smallest
for the lowstand (341 cm). This pattern is well explained by the
grain size of the sediment, which is significantly finer during
highstands than it is during lowstands, increasing the extent of
␣-recoil exchange.
The effects of intergrain ␣-recoil observed in this study are
likely to be seen in other environments. Whenever fine-grained
sediment contains minerals with significant variation in U
concentration, redistribution of daughter isotopes is to be expected. Such samples will be datable using isochrons only by
7.4. Random Errors
Assessing realistic errors on this dating approach is vital to
the correct interpretation of the results. In this section, we
discuss the propagation of random errors. In the next section,
we assess the sensitivity of the ages to possible systematic
errors related to the assumptions underlying the approach.
In each sample, the dense, aragonite separate provides the
majority of the age information because it has high U/Th ratios.
To retrieve an accurate age from this separate, its U/Th data
must be corrected for the effects of both ␣-recoil and initial
230
Th. The error on the final age therefore comes from three
sources: analytical uncertainty on the measured isotope ratios
and uncertainty due to both of the two corrections. We will
illustrate how these final errors are assessed with reference to
the three Termination 2 samples from depths of 311 cm, 331
cm, and 341 cm in core JPC152 (Table 4).
Analytical errors are straightforward, and those listed in
Table 4 contain uncertainty due to weighing, mass spectrometry, and blanks. Age uncertainties for the three Termination 2
aragonite fractions, due to only these analytical errors, are 1.8,
1.7, and 1.7 kyr (2␴). Error due to ␣-recoil is assessed by
allowing the ␦234U of each isochron subsample to vary within
analytical error until the age is at a maximum. The difference
between the age thus derived, and the best-fit isochron age,
establishes the random error due to ␣-recoil as 2.3, 2.2, and 2.8
kyr for the three Termination 2 samples. This is quadratically
summed with the analytical error to provide ages and errors for
three aragonite separates, before correction for the initial 230Th,
of 137.0 ⫾ 3.0, 143.4 ⫾ 2.8, and 149.9 ⫾ 3.3 ka.
Finally, these ages must be corrected for initial 230Th—a
relatively small age correction averaging only 4.6 kyr. In propagating uncertainty through this correction, we note that the
aragonite fraction provides the age information in each of the
isochrons, whereas the other points generally contain little age
information, although they allow for correction of initial Th.
We therefore assess the uncertainty on the correction by using
each low U/Th point in turn to correct for initial 230Th in the
aragonite separate. This gives two or three assessments of the
required correction for each isochron, the scatter of which
allow an assessment of the uncertainty due to this correction.
This approach does not take into account the error correlation
in the use of the aragonite point in each of the assessments of
initial Th, but it is considered more appropriate than more usual
isochron techniques because the low U/Th points in this study
are not expected to yield reasonable age information on their
own. This approach differs from that followed in Henderson
and Slowey (2000) in that it uses the scatter of the low U/Th
samples to assess the probable scatter in the initial value and it
is therefore a more conservative estimate of the uncertainty.
Calculated uncertainty on the Th correction is large for the
311-cm sample (24 ka), reflecting the scatter of the two low
U/Th points. The other two samples from the penultimate
deglaciation, however, have uncertainties in the corrections of
1.2 and 2.0 kyr, respectively. Combining uncertainty in the Th
correction with the analytical and ␣-recoil error yields final
ages and 2␴ random errors of 132.2 ⫾ 25.0 ka (311 cm),
U-Th dating of carbonate platform and slope sediments
136.9 ⫾ 3.0 ka (331cm), and 144.3 ⫾ 3.9 ka (341cm). Ages
and uncertainties calculated in this way are plotted on the
isochrons in Figure 5 and are quoted throughout the rest of this
paper.
7.5. Systematic Error
In this section, we consider the effect on the ages of flaws in
the underlying assumptions behind the dating approach. We
consider three such sources of systematic error: incomplete
removal of the detrital fraction before analysis, changes in past
seawater (234U/238U), and problems with the ␣-recoil model.
We do not add uncertainty due to these three assumptions into
our final age error because we believe that they are reasonable
assumptions. This is a similar approach to quoting coral U/Th
ages without adding possible systematic error due to diagenesis
to the quoted random error. Nonetheless, it is important to
assess the sensitivity of the ages to problems with these assumptions in case future work calls them into question.
First, we consider the effect of detrital material in isochron
subsamples. All subsamples dissolved in weak acid and yielded
no residue after centrifuging implying that no detrital material
survived the physical pretreatment. In addition, interglacial
isochrons yield (230Th/232Th)initial within the modern seawater
range implying little or no detrital Th (which would have a very
different Th isotope ratio). But what would happen if some
detritus did survive the pretreatment? If it occurred in all
subsamples of an isochron at an equal ratio to scavenged Th, it
will not bias the age. If it occurs only in the less dense fractions,
however, it will cause a non-zero initial age. The magnitude of
this effect was investigated for the JPC152 to 341-cm isochron
by assuming that 5% of the initial 232Th was detrital for the
three low U/Th subsamples. This reduces the age by only 0.25
kyr, indicating that the isochron ages are robust to small
amounts of detritus in the subsamples.
Temporal variability in seawater (234U/238U) would influence the ␣-recoil correction and therefore the age. Such variation has been the subject of much debate (e.g., Hamelin et al.,
1991; Esat and Yokoyama, 1999). The value during the last
interglacial, however, is reasonably well constrained. For example, studies on corals (Henderson et al., 1993) and Nicaragua
Rise sediments (Gallup et al., 1995) place it within error of the
modern value. In addition, the 10 bulk sediment Bahamas
samples of Slowey et al. (1996) average 1.152, again within
error of the modern value. The long residence time of U in the
ocean (⬇200 kyr) also makes rapid changes during the deglacial immediately before this interglacial unlikely. Such data
therefore constrain the seawater value to within a few permil of
the modern value at this time. The effect of 2‰ higher seawater
(234U/238U) is to decrease the age of the Termination 2 isochrons by 0.35 ka. Substantial changes in the seawater ratio are
therefore required to shift the ages by a significant amount.
Care must be taken, however, before using this isochron technique in other time windows where the seawater (234U/238U) is
not so well constrained.
Assessing the validity of the ␣-recoil model is more difficult.
The average change in age for the Termination 2 samples
induced by the correction is 5 kyr. The ␣-recoil correction
would therefore need to be seriously in error if it were to lead
to systematic age errors on the same order as the quoted
2767
Table 5. Accelerator Mass Spectrometry 14C ages of density subsample from 63–250 ␮m separates from two horizons in core GGC33.
The 67-cm samples are aliquots of those used to construct the isochrons
shown in Fig. 6. C-14 ages are not corrected for the age of surface
water, and errors are 1␴. C-14 ages were reduced by 400 years before
calculating calendar ages to account for the ages of surface water.
Calendar ages are 2␴ and were calculated using the Intcal98 programme (Bard et al. 1998; Stuiver et al, 1998).
14
Sample
C age
(years)
Calendar Age
(years)
OCE205-2-33GGC (26.221°N, 77.691°W, 770-m water depth)
67 cm depth
2.70–2.75 g/cm3
14350 ⫾ 80
16750 ⫾ 500
2.75–2.80 g/cm3
12450 ⫾ 45
14520 ⫾ 700
⬎2.80 g/cm3
11000 ⫾ 65
12630 ⫾ 310
87 cm depth
2.70–2.75 g/cm3
24100 ⫾ 160
26781 ⫾ 360
2.75–2.80 g/cm3
25200 ⫾ 190
28024 ⫾ 430
⬎2.80 g/cm3
25300 ⫾ 90
28137 ⫾ 210
random errors. Because the pattern of (234U/238U) in the subsamples is very difficult to explain with any other mechanism
than ␣-recoil, large problems with this correction are not expected. Nonetheless, because this is the first study to address
this situation, further work will be required to better understand
the intergrain effect of ␣-recoil and the limits to its correction.
7.6. Effect of Sediment Mixing
Bioturbation in these sediments must be considered for two
reasons. First, the rapid changes in sediment composition seen
at deglaciations will lead to differential mixing of aragonite and
calcite so that minerals at the same core depth have different
ages. Second, rapid changes in sedimentation rate will offset
the ␦18O features of bulk sediment.
Mixing is relatively shallow in Bahamas slope sediments
extending to a maximum of 8 cm (Henderson et al., 1999b).
More than 8 cm from a change in sediment composition,
different grain types are therefore expected to have similar ages
to one another. This expectation is confirmed by mineral separates from the full glacial (85 cm) in GGC33 because three
density separates have 14C ages that differ by only 1.3 ka—a
deviation that can probably be explained by analytical uncertainty and small amount of modern carbon contamination (Table 5). Closer to a change in sediment composition minerals
will experience differential mixing, however. At the deglaciations, for instance, aragonite will be preferentially mixed downward and calcite upward, so that aragonite at a single core depth
will be younger than calcite. This is the case for mineral
separates from the deglaciation (67 cm) in GGC33, which show
aragonite with a 14C age 3.4 kyr younger than the low-Mg
calcite from the same core depth (Table 6). This degree of age
offset is readily explained by simple mixing models (e.g.,
Henderson et al., 1999b).
The effect of the difference in age between the subsamples
makes only a small difference to the isochrons of this study
because the vast majority of the age information is provided by
the aragonite fraction with its high U/Th value. When calcite is
older than aragonite, as expected at deglaciations, isochrons
will be too old if the calcite is enriched in 230Th relative to
2768
G. M. Henderson, N. C. Slowey, and M. Q. Fleisher
Fig. 6. Theoretical isochron for sample GGC33 to 67 cm based on the 14C ages of the individual mineral separates.
Separates are assumed to start with their measured (238U/232Th); the modern seawater (234U/238U); and the seawater
(230Th/232Th) ratio (horizontal line) derived from the measured isochron. Theoretical points are calculated from these initial
conditions and the 14C-derived calendar ages for each subsample (Table 6). The theoretical isochron returns an age of 12.5
ka, within error of the measured isochron (shown by the dotted line and triangles for comparison). Despite a 4-kyr spread
in subsample ages, the isochron returns an age within error of the true age for the high U/Th (aragonite) separate.
secular equilibrium and too young if the calcite has less 230Th
than secular equilibrium. The magnitude of this effect can be
quantified for the 67-cm sample. Using measured (230Th/
232
Th)initial, (238U/232Th) and 14C age, a hypothetical isochron
can be calculated (Fig. 6). This isochron yields an age of 12.5
ka, within error of the 14C age derived calendar age for the
aragonite separate (12.6 ⫾ 0.3 ka), confirming the expectation
that the aragonite controls the isochron age. This hypothetical
age is also within error of the measured U/Th isochron age
(13.0 ⫾ 2.6 ka). Although the calcite separates are 4 kyr older
than the aragonite separate, they are sufficiently close to secular
equilibrium that this age difference makes an insignificant
difference to the isochron age and the isochron provides a good
measure of the age of the aragonite in the sediment. Termination 2 samples have calcite fractions similarly close to secular
equilibrium and therefore will not be significantly effected by
the age difference between subsamples.
Although the isochron approach provides accurate ages for
the aragonite portion of the sediment, this age may not be the
same as foraminifera at the same horizon. Foraminifera concentrations downcore do not show the same extreme variations
as do calcite/aragonite ratios. This, coupled with their larger
grain size, suggests that they will not be so perturbed by mixing
as mineral grains and should have an age intermediate between
that of the aragonite and calcite. This argument is supported by
an age model for GGC33 based on foraminiferal 14C ages,
which puts the foraminiferal age at 67 cm as ⬇13.8 calendar ka
(Slowey and Henderson, in preparation). During a deglaciation,
when the effects of differential mixing are at their most extreme, the age returned by the isochron may therefore be ⬇1 ka
younger than the age of foraminifera on which the ␦18O curve
is measured. In summary, the size of the uncertainty introduced
by differential grain mixing is expected to be significantly less
than the quoted random error.
Bulk sediment mixing must also be considered because ␦18O
across a deglaciation changes abruptly, whereas the age of the
sediment changes continuously. If sedimentation rate was constant, this difference would not make a significant change to the
midpoint of the deglaciation and would simply increase the
apparent duration of the change in the sediment. In the Bahamas, however, where sedimentation rates are dramatically
lower in the lowstand periods, such mixing also has the effect
of increasing the apparent age of the deglacial midpoint. The
effects of this were investigated in a simple mixing model in
Henderson et al. (1999b) and in a slightly more refined model
in Henderson and Slowey (2000). Here we use the latter model
(Fig. 4), which adds sediment in 0.2-cm increments to the top
of a sediment pile and mixes it efficiently through an 8-cm
mixed layer. Sediment exiting the mixed layer in 0.2-cm increments is the output preserved in the sediment record. With only
three age tie points across the deglaciations, we cannot perform
reverse modeling as in some studies (e.g., Bard et al., 1987).
U-Th dating of carbonate platform and slope sediments
Instead, we adjust the sediment input ␦18O with age to give a
good fit to the observed ␦18O data. We assume a simple linear
change in ␦18O from glacial to interglacial conditions and a
sedimentation rate which changes abruptly when full interglacial conditions are reached (i.e., when the carbonate producing
banks are flooded). Altering input to the model, such as the
abruptness of the ␦18O change, leads to changes in the duration
of the deglaciations. Nonetheless, the assessment of the midpoints of deglaciation are rather robust to such alterations.
Best fit input ␦18O to GGC33 suggests little or no offset for
the midpoint of the last deglaciation relative to that measured.
The best fit for JPC152, however, suggests that the apparent
midpoint of the penultimate deglaciation is ⬇1 kyr older than
the true midpoint age (Fig. 4).
7.7. The Absolute Timing of the Penultimate Deglaciation
The timing of the penultimate deglaciation is controversial
and has important implications for the mechanism causing
global climate change on glacial-interglacial time scales. If
climate is controlled by insolation changes in the northern
hemisphere summer, as suggested by Milankovitch, then deglaciation should occur close to ⬇127 ka when this insolation
reaches its peak. Several lines of evidence have suggested that
climate change occurs earlier than this. An example is the cave
deposit record at Devils Hole in Nevada (Winograd et al.,
1997), which is extremely well dated (Edwards et al., 1997;
Ludwig et al., 1992) and shows warming in this region centered
at ⬇142 ka. Several studies of highstand corals have also
suggested that sea level was at or above its modern level
significantly before 127 ka (Bard et al., 1996; Esat et al., 1999;
Gallup et al., 1994; Stein et al., 1993; Stirling et al., 1998;
Szabo et al., 1994; Zhu et al., 1993). Individual coral dates must
be treated with some caution because of the diagenetic problems of corals. But the large number of coral ages before 127
ka is evidence that deglaciation occurred sometime before the
peak in northern hemisphere deglaciation.
The isochron ages in this study enable us to date the midpoint of the local ␦18O change associated with this deglaciation
(Henderson and Slowey, 2000). The measured ␦18O curve is
pegged to the three isochron ages spanning the deglaciation.
The 331-cm age is pivotal to this process because it is at the
midpoint of the transition; therefore we are fortunate that this is
the most precise of the three isochron ages. The resulting age
model for the ␦18O curve is then corrected for bulk sediment
bioturbation by ⬇1 kyr as discussed above. The resulting age
for the midpoint of the change in ␦18O is found to be 135.2 ⫾
3.5 ka.
Deglaciation at the end of the penultimate ice age is partially
responsible for the observed change in ␦18O, but the full MIS6
to MIS5 ␦18O shift in this core is 2‰ and must therefore
contain an additional signal due to local temperature change.
Ideally, local T and ice-volume should be separated from one
another by the use of an independent paleothermometer on the
same samples. Unfortunately, Mg/Ca, the most widely used of
such paleothermometers, is not likely to provide reliable T
information in the Bahamas environment because of the presence of high-Mg calcite (Rosenthal et al., 1997). Instead, we
must turn to comparisons of the timing of local T changes and
global ice-volume changes in other cores from nearby areas.
2769
The CLIMAP reconstruction of the penultimate deglaciation
presented a summary of many such phasing relationships (CLIMAP, 1984). Sites in the far northern Atlantic exhibit a lag of
T change relative to ice-volume, whereas those in the South
Atlantic exhibit a lead. The Bahamas are close to the hinge
point between these two extremes, and it is likely that there is
no significant lead or lag of T at the sites investigated in this
study. The closest CLIMAP sites to those of this study are from
the Gulf of Mexico and show a slight T lag. The CLIMAP
results therefore suggest that the timing of ␦18O change at the
Bahamas is a good indicator of global ice volume or, if anything, slightly lags global ice-volume changes.
This study therefore indicates collapse of the Northern
Hemisphere ice sheets at around 135 ka, some 8 kyr earlier than
the peak in Northern hemisphere summer insolation. This is
difficult to reconcile with direct insolation forcing as the trigger
for collapse of the northern-hemisphere ice sheets. This age is,
however, consistent with deglaciation forced by insolation in
either the Southern hemisphere or in the tropics (Henderson
and Slowey, 2000).
8. CONCLUSIONS
Aragonite-rich marine sediments deposited on the slopes of
carbonate platforms are amenable to U-Th dating and therefore
offer considerable promise as a directly datable archive of
climate change over the last 400 ka. Bulk sediment U-Th ages
on such sediment are reasonably precise for sediment with
particularly high aragonite contents (generally highstand sediment). Nonetheless, errors introduced to such ages by corrections for initial 230Th become large for sediments with lower
aragonite contents. For such sediments, ages precise to ⬇3 ka
can be achieved by removal of fine-grained detrital material
and subsequent isochron dating. This technique demonstrates
the presence of some relocation of U daughter nuclides due to
␣-recoil from high-U to low-U grains—a feature that is expected to be general in fine-grained sediments but that can be
corrected. This isochron technique has been proved accurate on
sediments from within the 14C age range. It has then been used
to date sediments spanning the penultimate deglaciation and to
derive a midpoint age for that event of 135.2 ⫾ 3.5 ka. Unlike
corals, which are sporadically preserved and show progressive
diagenesis, marine aragonite–rich sediments are continuously
deposited and have concordant (234U/238U). Such sediments
therefore offer an opportunity to date marine climate change
from periods where corals are not preserved and to extend such
dating beyond the last interglacial into the period where corals
are generally diagenetically altered.
Acknowledgments—This work was funded by the Earth System History
programme of the National Science Foundation. We thank Bob Anderson for discussion on the behavior of the U-series nuclides. Ken
Ludwig, Tezer Esat, Edouard Bard, and an anonymous referee are
thanked for full and constructive reviews that improved the manuscript.
Associate editor: M. A. McKibben
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