Chemical Geology (Isotope Geoscience Section), 101 (1992) 223-233
223
Elsevier Science Publishers B.V., Amsterdam
Stable carbon isotope variations in sediment from
Baffin Bay, Texas, U.S.A.: Evidence for cyclic changes in
organic matter source
Brian Anderson, R.S. Scalan, E.Wm. Behrens" and P.L. Parker
Marine Science Institute, The University of Texas at Austin, Port Aransas, TX 78373, USA
(Revised and accepted May 23, 1991 )
ABSTRACT
Anderson, B., Scalan, R.S., Behrens, E.Wm. and Parker, P.L., 1992. Stable carbon isotope variations in sediment from
Baffin Bay, Texas, U.S.A.: Evidence for cyclic changes in organic matter source. In: S.A. Macko and M.H. Engel (GuestEditors), Isotope Fractionations in Organic Matter: Biosynthetic and Diagenetic Processes. Chem. Geol. (Isot. Geosci.
Sec. ), 101: 223-233.
Baffin Bay, Texas, a drowned Pleistocene river valley, is filled with up to 20 m of exceptionally well-preserved Holocene
and recent sediments. Piston cores from the upper 4 m have been described and analyzed for elemental composition and
t~'3C of the total organic carbon and carbonate. Strong cyclic patterns were observed for all of these parameters. The total
organic carbon (TOC) level varied between 1% and 6%. (C/N ratios varied between l0 and 14. 8'3C showed a cyclicity
with depth and a slight shift toward more positive values with dep,hs. These variations are interpreted as being due to
different relative inputs of seagrass (5 ' 3C - = 10% ) and phytoplankton (g 13C _ ~, 20%0 ) to the sediment. The cores contained fine-grained carbonate and some massive dolomite, t~' 3C of the carbonate varied between - 3% and + 1o/o0except
for two more positive values. The lack of a trend toward light carbonate with depth was taken to mean that little ( < 5%)
CO2 from the oxidation of organic matter is present in the carbonates. Overall the changes in the relative intensity of these
sources is thought to reflect regional climatic and weathering cycles.
1. Introduction
The Laguna Madre of Texas is a shallow estuarine system bounded by the mainland and
by a barrier island, Padre Island (Fig. l ). The
Laguna Madre stretches from Corpus Christi
Bay to the Rio Grande River. Despite the fact
that the Laguna has historically been hypersaline, with salinities commonly of 45-65 ppt and
rarely of 85 ppt, it is highly productive (Hedgepeth, 1953; Odum and Wilson, 1962; Behrens, 1966). More than 105 ha of seagrass form
Correspondence to: P.L. Parker, Marine Science Institute,
The University of Texas at Austin, Port Aransas, TX
78373, USA.
"Present address: Institute for Geophysics, The University of Texas at Austin, Austin, TX 78712, USA.
vast submerged meadows in the Laguna Madre.
The Laguna-Baffin Bay complex is a major
producer of finfish on the Texas coast and the
seagrass community is probably responsible for
this important resource. This study was undertaken to gain insight into natural variations in
seagrass productivity over the past few thousand years. Our interest centered on the isotopic record of seagrass-derived carbon preserved in sedimentary organic matter.
Baffin Bay, an arm of the Laguna, is a
drowned and partially filled Pleistocene river
valley 37 km in length and 1.85-7.4 km wide.
An earlier survey of the bay with a sonoprobe
showed that the channels of the Pleistocene
rivers are filled with up to 20 m of sediment in
the bay centers (Behrens, 1963). The mouth
0168-9622/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
224
B. ANDERSON ET AL.
2. Experimental
PORT
HOUSTON
~: i"k
~i!:
- TEXAS
GALVESTON
BY
,~IU..E
GULF
OF
MEXICO
BA Y
.
'
~CORPL
tTI BAY
••:•~•::.......
¸¸•:: i:~:):!i:i
BAFFIN
BROWNSVILLE L [
~
~
BAY
(LOWER)LAGUNA MADRE
Fig. 1. Map of Texas coast showing Laguna Madre and
Baffin Bay.Coring locations are × (core 2) and O (core
3).
of the bay is partially silled by a fossil worm
reef. Because the bay is hypersaline and somewhat oxygen depleted there is an almost total
absence of a macro-infauna in the bay center,
so that the sediments are largely undisturbed.
Examination of short cores ( l0 cm) from the
mouth of the bay showed that the fine-grained
sediment contained from 1.0% to 2.5% organic matter, the ~3C of which ranged between - 18.5 and - 14.5%0 (Macko, 1981 ).
Based on these data and Behren's (1963) sonoprobe survey it was felt that the 20-m section
of sediment in Baffin Bay provides an exceptional opportunity for a geochemical and isotopic time-series study. An earlier core study
provided a few ~4C dates which allowed an estimation of sedimentation rate (Behrens and
Land, 1972). This paper reports on results
from coring the upper part of the section and
using stable isotope ratio variations to characterize the sources of organic matter in the
sediments.
The coring site was selected where the sediment was thickest (Fig. l ). Since the water
depth in Baffin Bay is relatively shallow, ~ 2
m, a decision was made to construct a portable
coring platform. Due to a relatively unobstructed long fetch of the winds the bay waters
can be rough, making it quite difficult to use a
small ship as a coring platform. The coring
tower shown in Fig. 2 was built from recycled
3-in ( ~ 7.6 cm) oil-well drill pipe by the crew
of the R / V "Longhorn" and was designed so
that it could be transported by that ship. Once
in place it provided a stabl¢ platform which allowed re-entry into the core hole and retrieval
of multiple piston cores. Aluminum irrigation
pipe (20 ft × 3 in or ~ 6.2 m × ~ 7.6 cm) was
used as a core barrel. Handles, which clamped
around the pipe, aided in inserting and removing the pipe. In general a crew of eight was able
to take 5-m core sections. The core hole was
L65 m
Fig. 2. Portable coring tower.
STABLE CARBON ISOTOPE VARIATIONS IN SEDIMENT FROM BAFFIN BAY, TEXAS
kept open with AI pipe casing just slightly larger
than the core barrel. Using this approach several cores of ~ 5 m length were taken.
Two cores were taken at each of two sites in
May and June of 1989. The sites were within
200 m of each other (Fig. l ). Three of the four
cores reached a hard dolomite layer, one of
several which were described by Behrens and
Land (1972). They assigned it a 14C age of
2300 yr BP. Although more than half of the
post-Pleistocene sediment lies below this
marker no effort was made to core beyond it.
Cores 2 and 3, one from each site, were used
for the chemical and isotopic studies reported
here.
The fresh core was returned to the laboratory while still in the A1 core barrels. The core
was extruded with the help of a hand-operated
winch and the fresh sediment caught on A1 foil.
The core was split using a large spatula which
was maintained at 30 V d.c. relative to the AI
core barrel and mud. The voltage was maintained by attaching one wire from a d.c. power
supply to the spatula and one wire to the A1 foil
onto which the core was extruded. The voltage
repelled the wet clay from the blade so that a
smooth core surface was obtained. The fresh
cores, which showed clear laminations, were
described and subsampled at 5- or 0.5-cm intervals for analysis. The subsamples were held
in plastic bags in a deep freeze until processed.
Sub-samples of the frozen samples were
treated with 5% phosphoric acid to remove
carbonates, the acid decanted through a glass
fiber filter and the residue washed twice with
distilled water. The sediment was collected on
the filter, dried at 50°C and ground to a powder with a mortar and pestle. These samples
were used for 613C and C, N analysis of total
organic matter. A second sub-set of the frozen
sample was processed for carbonate analysis.
These samples were freeze-dried and reduced
to a powder with mortar and pestle.
Samples of the carbonate-free, dried sediment were weighed and combusted using the
sealed tube method of Sofer (1980) to pro-
225
duce C O 2 suitable for isotopic analyses. Pyrex ®
tubes containing the sediment and copper oxide were heated at 590°C for 3 hr and allowed
to cool overnight. The volume of CO2 produced was measured man®metrically and the
% TOC (total organic carbon ) calculated. The
CO2 was then set aside for mass spectrometry.
The CO2 from the carbonate containing sample sub-set was recovered by reacting a weighed
subsample with 85% phosphoric acid for 1 hr.
The sediments were rich in carbonate so that
quantitative manometer readings were made to
permit a calculation of percent carbonate.
All C O 2 samples were analyzed on a VG ®
602E isotope ratio mass spectrometer using the
correction procedures of Craig (1957). A laboratory standard of vacuum pump oil (AER ® )
was combusted to produce a working CO2
standard. The AER ® oil was calibrated relative to the Chicago PDB standard by comparing it to NBS 22 (Northam et al., 1981 ). The
6~3C of NBS 22 is taken to be -29%o vs. PDB;
AER ® is - 27.32%o. The working standard for
carbonate is the Woods Run Travertine, having a 613C of --9.17%o. All data are reported
relative to the PDB standard.
Carbon to nitrogen ratios were measured on
the acidified samples using a Perkin-Elmer ®
model 240 Elemental Analyzer. Although some
organic matter is lost in the acidification process as soluble material, it is unlikely that the
C / N ratio is significantly altered.
3. Results and discussion
The concentration of total organic carbon,
TOC, in the 5-m cores varies between 1% and
6% on a carbonate-free basis (Figs. 3 and 4;
Table 1 ). A few samples composed of sand approached zero while a few which contained
plant remains exceeded 7%. The small-scale
variations seen in both cores are real. The TOC
content is distinctly hig~er below the 4-m level
in both cores. For comparison 33 surface sediment samples from nearby Corpus Christi Bay
varied between 0.1% and 2.0%. The higher
226
B. ANDERSON ET AL.
-50-50
150-150
+
..l
CA
250
-250
_e
"I@
CA
350-
o
-350
a~
G)
(2
450-450
-550
-550
11'lIII'
l,,,~I,r-
I J I i ]--~FSTi,l,,
0
5
-16
% Organic Carbon
-15 -14 -13
Del !3-C
i,
10
15
CIN Atom Ratio
Fig. 3. The variations with depth in core 2 of total organic
matter (TOC) on a carbonate-free basis, J ' 3C of TOC,
and the carbon to nitrogen ratio of organic matter. No data
was obtained between 340 and 390 cm due to coring
problems. The sample intervals were 5 cm.
values for Baffin Bay may reflect better preservation due to a restricted water circulation,
caused by the sill, which lowers oxygen and increases salinity. The C/N values generally
range between 10 and 14 and like TOC cycle
with depth (Figs. 3 and 4; Table 1 ). There is a
gradual drift toward higher C / N ratios with
depth. Fresh phytoplankton typically have C/
N ratios ~5.5-7.8 (Biggs et al., 1983; Hedges
et al., 1988). Fresh seagrasses, on the other
hand, are rather low in nitrogen, with C / N ratios near 20 (Farfan and Alvarez-Borrego,
1983). Assuming that C/N does not significantly change with diagenesis, the pattern of the
C/N ratios is consistent with a model wherein
the TOC is composed of mixtures of phytoplankton and seagrass. If so, then the relative
l'
0
5
-16
% Organic Carbon
l i
I'
I'
-15 -14 -13
De113-C
I~I'~I'
10
15
CIN Atom Ratio
Fig. 4. The variations with depth in core 3 of total organic
carbon (TOC) on a carbonate-free basis, ~'3C of TOC,
and the carbon to nitrogen ratio of organic matter. The
sample intervals were 5 cm.
amount of plankton and seagrass must change
on a time scale of decades based on the 14Cdata
of Behrens and Land (1972). Such a change
would reflect strong environmental changes
such as salinity or temperature.
The j~3C vs. depth curves (Figs. 3 and 4)
show two general trends, the cyclicity seen in
the TOC curves, and an overall shift to more
positive J'3C-values with depth. In core 3 the
upper section was lost so this shift is not seen,
but it is present in the other three cores. The
jl3C cyclicity, covering 4%0 in 4 m, is remarkable for an environment which at first glance
seems to be constant. By contrast Gulf of Mexico cores of Pleistocene sediment showed 12%0 variations over 8 m of sediment (Newman et al., 1973). The analytical error associated with these data was evaluated in two ways.
Repeated analyses of sub-sets of sediment
STABLECARBONISOTOPE VARIATIONSIN SEDIMENT FROM BAFFINBAY,TEXAS
227
TABLE 1
TABLE 1
(continuea)
Baffin Bay: cores 2 and 3
Depth
(cm)
Core 2
Depth
Core 2
C/N
Core 3
Core 3
%C
C/N
15
20
24
30
35
40
45
50
55
60
65
70
75
80
85
90
96
100
105
110
115
120
125
130
135
140
145
150
155
160
165
170
175
180
185
190
195
200
205
210
215
220
225
230
235
250
245
250
255
260
J'3C
C/N
%C
(%o)
(cm)
11.05
10.88
10.82
10.57
10.48
17.59
10.93
11.29
11.56
11.92
11.67
12.56
11.49
11.32
11.24
12.02
10.70
11.44
11.82
11.07
12.46
12.35
11.58
12.39
11.55
11.75
12.44
12.30
11.60
12.20
11.88
20.01
12.09
12.88
12.75
12.17
11.92
11.65
11.85
12.41
12.62
12.58
13.00
12.14
12.30
12.98
13.57
13.73
13.44
12.80
%C
2.52
2.35
3.56
2.70
2.64
3.43
2.42
3.77
3.14
4.03
2.28
4.21
2.58
2.60
2.30
3.58
2.72
2.35
3.28
2.76
3.46
4.31
3.14
3.19
2.03
1.95
4.08
3.97
2.53
1.37
2.10
2.12
1.86
2.00
1.38
0.12
2.78
3.08
3.05
3.11
2.58
3.49
2.46
1.95
2.08
3.68
3.25
3.95
3.64
3.36
c~13C
C/N
%C
jl3C
(%o)
jI3C
(%0)
(%0)
- 1 4 . 4 3 lostcore
-15.34
- 14.73
-15.97
- 15.93
-15.81
- 1 4 . 8 4 10.57 3.96
- 1 4 . 7 6 10.35 3.63
- 1 5 . 3 8 13.92 3.66
-14.31
10.23 2.45
- 1 6 . 5 3 10.50 4.08
- 15.19 11.02 3.85
- 1 6 . 5 2 12.39 3.82
- 1 5 . 8 8 11.60 3.62
-15.99
9.50 2.81
- 1 6 . 0 5 11.66 2.86
- 1 4 . 5 6 10.45 2.79
- 1 5 . 5 3 11.56 2.71
-i;.19
14.37 2.19
- 1 4 . 5 2 11.1" 2.50
- 1 5 . 6 4 10.38 3.04
- 14.47
9.09 1.58
- 1 5 . 2 5 11.79 4.01
- 1 5 . 0 6 12.08 3.07
- 1 6 . 4 6 11.64 2.91
- 1 5 . 8 2 10.19 3.61
- 1 5 . 3 0 11.97 1.66
- 14.96 12.95 2.19
- 1 5 . 7 8 13.03 3.36
- 1 7 . 3 4 13.21 3.74
- 17.46
8.55 1.13
-16.01
13.27 2.64
-15.91
13.21 2.73
- 1 5 . 6 6 13.71 2.50
- 1 6 . 2 6 13.45 3.61
- 1 6 . 8 4 13.11 2.72
- 1 4 . 9 5 12.36 2.67
-14.61
12.24 2.39
- 15.60 13.12 3.37
-15.31
12.82 3.12
- 15.58 12.89 3.05
- 1 4 . 6 3 13.36 3.85
- 14.84 13.18 2.07
- 1 5 . 8 5 12.54 1.66
- 1 5 . 8 7 12.10 1 . 7 1
- 1 4 . 7 3 11.78 1.67
- 1 5 . 2 4 13.69 2.96
- 1 4 . 4 2 13.43 2.34
- 1 4 . 7 5 13.34 3.63
-15.61
12.84 2.81
- 13.61
-14.97
-14.44
-14.90
-13.85
- 13.85
- 14.61
-14.68
-15.09
-15.06
-14.26
-14.53
-15.18
-13.76
-14.86
-14.89
-15.22
-15.59
-14.60
-15.88
-16.58
-16.30
-15.26
-15.74
-16.26
-16.61
-16.16
-16.76
-15.41
-17.00
-15.67
-15.74
- 15.40
-14.92
-15.56
-15.57
-15.02
-15.38
-15.79
-15.93
-14.32
-14.64
-14.93
-15.33
265
270
275
280
285
290
295
300
305
310
314
315
320
323
325
326
330
335
340
345
350
355
360
365
370
374
378
380
385
390
395
400
405
410
415
420
425
430
435
440
445
450
455
460
461
465
470
475
480
485
490
495
12.79
13.11
12.88
12.78
12.71
13.35
13.71
12.93
13.15
12.99
13.57
12.41
12.14
2.54
3.03
2.54
2.00
2.09
3.59
4.44
3.03
3.15
3.67
4.15
3.84
4.31
-15.24
-13.97
- 14.80
- 14.94
- 15.29
- 14.77
-13.74
-15.45
-15.14
-14.71
- 14.62
- 14.60
-14.05
12.06
2.52
- 14.93
18.02
2.48
11.39
3.65
12.17
2.88
incomplete core
-15.56
-15.90
-15.30
section
12.74
13.10
13.02
12.83
13.60
14.04
13.10
13.85
13.70
13.26
13.53
11.22
3.35
2.85
2.26
1.55
4.94
4.79
3.60
4.58
4.32
3.35
3.36
0.17
-15.10
-15.24
- 15.41
-16.04
-13.19
- 14.40
-14.51
-14.85
-14.86
-15.97
-15.33
-13.53
13.33
14.29
14.20
14.41
14.54
13.25
12.90
13.27
3.45
3.77
3.74
4.69
5.80
4.97
4.72
4.87
- 14.13
-14.52
-14.56
-13.85
- 13.10
- 13.59
- 15.14
- 13.78
13.03
11.99
11.98
13.44
12.53
13.10
12.47
12.76
12.79
12.84
2.76
1.53
2.07
3.66
2.30
4.60
3.37
4.21
3.29
3.91
--15.61
-15.05
-14.56
-12.78
-14.71
- 14.79
-15.26
-15.67
-14.85
- 14.41
12.51
12.39
12.38
3.65
2.89
2.92
-13.79
- 15.02
- 14.91
12.18
11.25
11.42
13.08
12.83
12.97
12.91
13.35
11.72
12.59
13.46
12.96
11.29
10.67
11.50
15.42
12.63
13.68
13.27
13.79
13.47
12.32
13.45
15.04
13.42
13.37
13.63
13.21
13.02
2.91
1.88
1.75
2.85
2.95
4.15
3.65
3.44
1.72
2.85
3.99
3.22
1.70
1.50
2.02
2.70
0.69
3.40
4.03
5.75
4.70
3.28
5.02
4.08
3.16
4.89
6.93
7.23
4.24
- 14.59
-15.02
-15.35
-13.90
- 15.06
- 13.21
- 14.90
- 15.12
- 15.92
- 15.22
- 15.01
- 14.06
-15.15
- 15.59
-14.57
- 15.92
-16.72
-14.32
- 14.51
-14.51
-14.81
-15.56
-15.71
-15.17
-14.40
-14.33
-14.29
-12.96
- 12.82
13.32
12.70
13.30
14.14
14.05
4.71
4.09
0.70
5.58
7.34
-15.03
-15.67
-14.87
-14.68
-15.18
228
B. ANDERSON ET AL.
TABLE 1 (continued)
Depth
(cm)
Core 3
Core2
C/N
%C
~13C
C/N
%C
disC
(~)
500
505
510
515A*
515B*
520
525
530
535
540
12.19
11.87
12.74
5.91
12.56
14.25
13.07
13.77
13.64
12.65
4.12
3.99
6.42
0.34
3.28
5.53
5.41
10.34
5.23
5.92
- 14.63
-15.15
- 14.62
-13.58
- 13.54
-14.38
- 13.20
-14.41
- 14.74
-14.13
(~)
incomplete core
The sample depths are distance (cm) below the water-sediment interface; carbon to nitrogen ratios are atomic ratios;
percent total organic carbon (TOC) is on a carbonate-free
basis; J' 3C-values are relative to the PDB standard.
*Core 2 contained thin, well-resolved layers of mud and sand
at ~ 515-cm depth. Sample 515A is sand and 515B is mud.
Dolomite rock was also present at 515 cm, but was not sampled here.
samples gave a reproducibility of +_0.1%0 over
a short time (days) and +_0,2%0 over a long
time (weeks). The difference is due to the repeated preparation of standard CO2. In a second control experiment, subsamples were taken
for which the entire procedure, beginning with
subsampling the core, was repeated. Each of
these was acidified, dried and burned separately. Excluding a sand sample with low TOC,
the average standard deviation was +_0.3%0 for
all samples and +_0.15%o for samples analyzed
with the same working standard. Based on
these data it seems safe to consider absolute
variations of +_0.5%0 as real although trends
are probably real to +_0.2%0 (B.R. Anderson,
1990). The cyclic variations are real within
+_0.2%o based on the control experiments discussed earlier. This extra care to establish a
reasonable level of confidence in the ~ 3 C data
was necessary in order to insure that the sharp
variations in ~ a c with depth are not experimental or sampling artifacts.
In order to evaluate the influence of sample
spacing on the pattern seen in Figs. 3 and 4 a
set of subsamples was taken from core 3 using
0.5-cm intervals rather than the usual 5-cm
ones. The results show considerable fine structure for ~3C enriched and depleted sections of
the core (Fig. 5; Table 2). While the 0.5-cm
data are the most interesting it appears that the
5-cm samples do not mask the major cycles and
trends. Using Behrens' (1963) estimate of a
sedimentation rate of 0.8 c m / l 0 yr, the series
of values in Table 3 from 187 to 191.5 cm with
~aG-values of - 16.85+_0.06%o represent a
50-yr period of deposition with a fairly constant source of organic matter. On the other
hand, the peak at 280 cm with a t~ac of
12.800 reflects a strong shift to a new ratio
of source materials in ~ 1000 yr. The cyclic
pattern of ~aC-values suggests that events of
~ 10-20-yr duration are resolved in the data.
This is consistent with the historical frequency
-
-15.41
A.
190
-15.67
20O
•I $,40
-17
-16
-15
-14
lJJllJl
-16
I
260
-15
-14
Del 13-C
-13
~1[
-1533
Q 2zo
280
290
Fig. 5. A comparison o f ~ 3 C of TOC for core 3 using 5.0and 0.5-cm sampling intervals. The 5.0-cm data points are
not plotted to the 0.5-cm t~3C scale, but are shown
numerically.
STABLE CARBON ISOTOPE VARIATIONS IN SEDIMENT FROM BAFFIN BAY,TEXAS
TABLE 2
Baffin
TABLE 2
Bay: core
3
229
(continued)
%C
Depth
t~' 3C
(cm)
Depth
%C
(cm)
181
181.5
182
182.5
183
183.5
184
184.5
185
185.5
186
186.5
186.5
186.5
187
187.5
188
188.5
189
189.5
190
190.5
191
191.5
192
192.5
193
193.5
194
194.5
195
195.5
196
196.5
197
197.5
198
199
199.5
200
200.5
201
201
201.5
202
203
203.5
204
262
263
264
264.5
265
2.86
2.75
2.68
2.18
2.48
1.84
2.87
3,28
3.38
3.74
3,78
2,56
2.82
2.11
1.57
2.93
3.40
3.32
3.35
2.69
2.84
3.21
3.22
3.09
2.58
3.33
2.62
2.40
3.44
3.42
3.20
3.87
3.15
4.15
4.52
3.82
4.19
3.82
3.71
3.52
3.17
3.50
3.44
3.46
3.44
3.34
3.05
1.40
2.55
2.54
2.40
2.54
t~ ~3C
(%o)
16.80
16.80
- 16.75
- 16.98
- 16.60
- 17.23
-16.18
-15.91
- 15.35
-15.17
- 15.34
- 15.87
- 15.90
-
-
16.84
16.67
16.90
16.77
16.90
16.93
- 16.86
-16.88
-16.81
-16.89
-16.75
- 16.62
- 16.54
- 16.43
- 16.02
- 15.62
-15.15
-16.13
- 15.90
-16.12
-16.15
- 16.48
- 16.84
- 16.87
- 16.84
-16.70
- 16.93
- 16.95
- 16.98
- 16.88
- 16.82
- 16.89
-16.91
- 16.98
- 15.50
- 15.39
- 15.83
-16.11
- 16.25
-
-
(%o)
265.5
266
266.5
267
267.5
268
268.5
269
269.5
270
270.5
271
271.5
272
272.5
273
273.5
274
274.5
275
275.5
276
276.5
277
250.5
278
278.5
279
279.5
280
280.5
281
281.5
282
282.5
282.5
283
283
283.5
285.5
t~13Co f
2.91
1.96
2.70
2.65
2.55
2.06
1.46
2.33
2.21
2.06
2.99
2.55
2.53
2.30
2.18
2.68
2.78
3.00
2.85
2.61
2.21
2.10
2.00
1.87
1.94
1.98
2.39
3.86
4.33
4.28
3.35
2.53
3.49
2.63
1.82
1.66
2.15
2.16
2.90
3.03
total organic
in sediment
using
-15.95
- 14.62
- 15.13
- 15.00
- 15.22
- 14.91
- 14.60
- 14.75
- 14.76
- 14.80
- 14.80
- 14.95
- 15.00
- 14.95
-15.19
- i 5.30
- 15.24
- 14.74
- 14.66
- 14.51
- 14.52
- 14.72
- 15.17
- 14.09
- 15.16
- 15.03
- 14.09
- 12.94
- 12.86
- 12.81
- 13.92
- 14.36
- 14.94
- 15.14
- 15.08
- 15.09
- 14.94
- 14.86
- 14.60
--14.50
matter
0.5-cm
relative
sample
to PDB
and percent
TOC
spacing.
of droughts and hurricanes on the Texas coast.
Jasper and Gagosian (1990) reported variations in t~3C and C / N for a 208.7-m core
from the northern Gulf of Mexico. They explained the variations as being the result of
mixing of two end-members, terrigenous and
marine organic matter. This resembles the
present stuay except that our end members are
230
B. ANDERSON ET AL.
(continued)
TABLE 3
TABLE 3
Baffin Bay: core 3
Depth
(cm)
%C
J t 3C
(%0)
355
360
365
370
374
378
380
385
390
395
400
405
410
415
420
430
435
440
445
450
455
460
465
470
475
480
485
495
0.86
1.27
0.17
1.70
1.24
1.22
0.48
0.63
1.08
1.02
1.24
1.25
1.11
1.14
1.64
1.18
2.01
0.97
0.92
1.05
0.87
0.88
0.89
8.04
-0.24
-0.81
-0.47
-0.87
+ 1.22
-0.30
-2.67
-0.22
- 1.09
- 1.01
- 1.24
- 1.74
- 1.36
-2.06
-0.96
- 1.13
-0.68
-0.71
- 1.48
- 1.58
- 1.96
- 1.29
-0.99
- 1.51
+2.92
Depth
(cm)
%C
50
55
60
65
70
80
85
90
95
100
105
I 15
120
125
130
135
140
145
150
160
170
175
180
185
190
195
200
205
210
220
225
230
240
245
250
255
260
265
270
275
280
285
290
295
300
305
320
323
326
330
335
345
350
1.35
1.31
1.44
1.55
1.07
0.37
0.66
0.83
0.50
0.45
0.44
0.97
1.05
0.89
0.21
0.93
0.72
0.35
0.99
0.74
0.30
0.30
0.53
1.07
0.29
0.55
I. ! 5
i.04
0.20
0.64
I. l I
1.96
1.40
1.10
1.14
0.95
1.16
0.45
0.45
1.32
2.24
j ~3
(%0)
+ 0.83
+0.19
+ 0.68
- 0.19
+ 0.41
- 0.90
- 1.81
- 0.64
- 2.00
- 0.02
- 0.79
- 1.41
- 0.44
-0.81
- 2.54
-0.58
- 0.68
-0.13
- 1.55
- 0.41
- 0.03
+ 0.60
- 0.03
+ 0.64
- 0.31
- !.26
- 1.39
- 1.00
-0.18
- 2.28
- 0.71
+ 0.04
- 2.24
- 0.97
- 1.04
- 0.64
- l.O0
- 1.09
+ 0.24
+ 0.25
+0.58
+0.58
- 1.02
-2.11
- 1.82
-2.89
-0.91
-0.58
-2.20
-0.97
- 1.33
- 1.45
+0.35
T h e precent o f carbonate carbon a n d •13C o f carbonate carb o n relative to PDB; the d e p t h intervals ( c m ) are the distance b e l o w the seawater s e d i m e n t interface.
plankton and seagrass. However, the time
frame is very- different, 2000 vs. 100,000 yr. For
this reason their sampling frequency did not
show the fine-scale variations seen in Baffin
Bay. Nevertheless, it is interesting that mixing
processes are preserved in two such different
environments and time spans.
Behrens and Land (1972) reported jlaCvalues for carbonates in the Baffin Bay cores
from - 3 . 0 to + 4.2%o. The carbonate sample
in core 3 at 495 cm (Table 3) with a jI3C of
+ 2.92%0 is probably the dolomite layer that
Behrens and Land (1972) encountered. In
their core two dolomite layers only 1 cm apart
had jl3C-values of + 1.9 and ~-4.2°/o0. They
conclude that the dolomite is marine in origin
and probably is the result of direct precipita-
STABLECARBON ISOTOPE VARIATIONSIN SEDIMENT FROM BAFFIN BAY,TEXAS
tion. They suggested that the more 13C-depleted samples might be the result of organicderived CO2. Because the sediment was rich
and variable in TOC we expected to observe
some rather negative carbonates. For core 3 the
t~3C-values fell between - 3 and + 1%o except
for two more positive samples (Fig. 6). The
slight dominance of negative values may be the
result of post-depositional oxidation of organic matter. Such a process is not a major
pathway for carbon in this system as compared
to fine-grained carbonates from Gulf of Mexico cores wherein values as light as -22.7%o
were reported (R.K. Anderson et al., 1983).
The absence of a trend toward more negative
values with depth indicates that over the 2000
yr represented by core 3, bacterial remineralization of the TOC has been minimal.
We suggest that a simple model which con-
g,
,
~
T
-350
g
-450
-550
'
0
I
1
'
~ '
2
'
I ' I
-1
0
Weight Percent
'
( '
1
I
2
'
I
3
De113-C
Fig. 6. The percent carbonate carbon and ~3C of total
carbonate for core 3.
231
siders two sources of organic matter can explain the variations in the isotopic and elemental data in these sediment cores. In the
model, Baffin Bay TOC is a mixture of material derived from seagrass and phytoplankton,
the two major plant types in the system.
Changes in the relative amounts of these two
sources can account for the cyclic patterns and
the long term drift of t~13C of TOC. The extensive seagrass meadows of the Laguna Madre
provide a source of organic matter which is 13C
rich (t~ ~3C~, - 10%o) (Parker, 1964). Baffin
Bay is too deep for seagrass growth except along
bay edges. However, the bay supports a phytoplankton community which is highly
productive.
During the course of this study we attempted to obtain pure phytoplankton from the
bay by size fractionation with only limited success. By luck, in the summer of 1990, the whole
Laguna Madre system including Baffin Bay was
subject to a massive phytoplankton bloom. The
bloom persists to this time, May 1991. The
standing crop was so high that a 99% pure culture could be obtained by simply filtering 1 I of
the brown water. The blooming organism has
not been fully identified, but it is a small,
golden Chrysophyte. The t~]aC-value of the
bloom is -20.3_+2.0%0. The cause of the
bloom is not known but it may be associated
with a series of cold winters which caused a fish
kill. Fortunately, the organism has not proved
to be toxic; however, the bloom is so intense
that there is concern that it may impact the
seagrass beds by depriving them of light. It is
this kind of environmental stress favoring one
plant type or the other which could cause the
variations seen in these cores.
If one uses these two end members, seagrass
( - 10%o)and phytoplankton ( - 20%o) then
it is understandable that all of the data points
in Figs. 3 and 4 fall between these values. A
value of - 150/00for TOC would correspond to
a 50:50 mixture of seagrass and phytoplankton. There is no doubt that a strong seagrass
signal is seen in several sections of the core.
232
Changes in source TOC such as those which affect a whole bay and persist for tens to
hundreds of years must reflect substantial
changes in the local environment. At the present time the J~ac of sediment TOC can be seen
to reflect a decreasing seagrass signal as one
moves from grass meadows toward the open,
plankton dominated, Corpus Christi Bay (Fry
et al., 1977).
Phytoplankton grow under most conditions
of salinity and temperature, depending mostly
to the availability of light and nutrients. Thus
phytoplankton input can be considered to be
fairly constant, but subject to intense blooms
such as was seen in the 1990 bloom. Seagrasses
on the other hand grow best at seawater salinities. They are doing well in the hypersaline lagoon now. However, prior to the dredging of
the Intracoastal Canal in 1948, local commercial fishermen maintain that little seagrass grew
in the Laguna Madre because it was too saline.
Breuer (1957) argues that as recently as 1850
Baffin Bay enjoyed a sustained period of lower
salinity, citing evidence of oyster collection in
the Bay by the native Karankawa Indians. If so
it would have been a period of flourishing seagrasses. Large hurricanes breach the barrier island, Padre Island, forming passes which can
stay open for years and which allow seawater
to flow into the system thus preventing hypersalinity and stimulating seagrass productivity.
Cyclic events such as hurricanes and massive
plankton blooms can account for the shortterm cycles in the J~ac curves. The drift toward more seagrass-rich values deep in the core
must reflect a more general change such as a
long period of higher rainfall in the drainage
basin. This would also freshen the Laguna and
stimulate seagrass growth. We have no independent evidence for this but it is mentioned
to identify topics worth further study.
The picture described here is, no doubt,
overly simple, but it is consistent with the J~ac,
C / N and percent TOC patterns. Certainly
modest land plant input is present but it may
be isotopicaUy leutral in that the local grass-
B. ANDERSON ET AL.
lands are a mixture of plants which fix carbon
by the C3 (j'3C~-26°/oo) and C4 (J~3C
- 10°/oo) pathways (Fry and Sherr, 1984).
Blue-green algae (j~3C~, - 13%o) may be important in periods of extreme salinity (Behrens and Frishman, 1971 ). There is no evidence in the sediment for substantial C3 plant
input, unlike estuaries which border on highrainfall landscapes (Sackett and Thompson,
1963).
4. Conclusions
Baffin Bay, Texas, sediments retain a record
of the isotopic and chemical history of the region, which reflects local cyclic events and climate for the last 2000 yr and probably for the
last 8000 yr. This study of the total organic
matter of cores indicates that at least two major sources of organic matter have varied in
their relative contribution to TOC:
( 1 ) J~aCToc-values cycle over 4%o in a 4-m
interval of core (Figs. 3-5 ). This range is consistent with varying quantities of phytoplankton ( - 20%o) and seagrasses ( - 10%o) being
the source of the TOC. No J~ac data was as
negative as -25%0, which would indicate a
terrestrial source, was found.
(2) Total carbonate J~aG-values fell in a
range of - 3 to + 1o/ooexcept for two more positive values. There was no trend toward more
negative carbonate with depth (Fig. 6). This
suggests that bacterial oxidation of TOC is not
a dominant process in the time period the~e
cores represent. It is certain that the small carbonate lenses seen in the core are not the result
of intense oxidation of TOC but are inorganic
precipitates.
A final conclusion is that restricted coastal
environments like Baffin Bay hold considerable geochemical information on past regional
climate and weather variations which would be
of interest.
STABLE CARBON ISOTOPE VARIATIONS IN SEDIMENT FROM BAFFIN BAY, TEXAS
Acknowledgements
This work was supported by Grant No. 4541
from the Texas Higher Education Coordinating Board, Advanced Technology Program. We
thank Capt. Don Gibson and the crew of the
R/V "Longhorn" for constructing and transporting the coring tower and for their help with
coring. We thank Dr. Dave Jeffries of Chevron
Oil Co. for the Woods Run Travertine
standard.
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