Geology Reference
In-Depth Information
(b)
(a)
1590
±
47 years
Fig. 7.
Columnar stromatolite
from Site 1 dated in three
horizons. (a) Bottom horizon:
laminae overlying coarse grain-
stone; middle horizon: trapped
and bound grains; top horizon:
trapped and bound grains. Large
cavities due to boring clams; top
of stromatolite colonized by mac-
roalgae
Batophora.
(b) Remainder
of sawed stromatolite in the
back reef lagoon - stromatolite
nucleation on coarse grainstone
overlying basal surface.
1548
±
55 years
884
29 years
±
Table 1.
U/Th series dating results.
Activity Ratio
234
U/
238
U (dpm)
238
U conc (
232
Th conc (
Sample ID
Material
μ
g pm)
μ
g pm)
Ages (yr)
H-0410-bsc-1.1
Basal brain coral
1.1478
0.0013
3.048
0.002
0.730
0.007
790
11
H-0410-bsc-1.2
Encrusting coral
1.1471
0.0014
2.983
0.002
0.509
0.007
566
11
H-0410-bsc-1.3
Basal brain coral
1.1487
0.0017
3.138
0.003
0.440
0.006
667
12
H-0410-bsc-1.4a
Basal brain coral
1.1477
0.0017
2.769
0.003
0.487
0.005
972
14
H-0310-1-5a
Trapped & bound grains
1.1471
0.0016
2.362
0.003
12.26
0.130
1,590
47
H-0310-1-4a
Trapped & bound grains
1.1437
0.0027
2.439
0.002
14.00
0.150
1,548
55
H-0310-1-6a
Micritic laminae
1.1460
0.0021
2.464
0.003
7.67
0.080
884
29
U/Th series dates for coral and stromatolites samples, Highborne Cay, Bahamas. In coral samples the detrital
230
Th was corrected with a
230
Th/
232
Th
activity ratio of 0.6
0.2.
column standing in the lagoon (Fig. 7). Samples
were collected from three horizons; 10 and 20 and
30 cm below the surface, the latter representing an
initial lamination (Fig. 6a). Trapped and bound
ooids and skeletal grains dominate samples from
10 and 20 cm depths, whereas the sample from
30 cm depth was a micritic lamina comprised
mainly of authigenic microcrystalline aragonite.
at the GEOMAR mass spectrometry facilities on
a Finnigan MAT 262 RPQ+ multicollector mass
spectrometer. Measured
234
U/
238
U activity ratios
were 1.1478
0.0016 disintegrations per minute
(dpm) for corals and 1.1456
0.0021 dpm for
stromatolite samples, respectively, indicating a
clear seawater signal.
Uranium-thorium dating
Chronology
Four coral samples and three stromatolite sam-
ples, on average 510 mg, were chemically purifi ed
following the published procedures (Cheng
et al.
,
1986, 2000). The U/Th isotopes were measured
U/Th ages of corals and stromatolites are younger
than 1000 years, ranging from 970 to 560 calendar
years (Table 1). The oldest coral (H-0410-bsc-1.4a)
is from the most shoreward location within
