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Age Mixing Among Sympatric Bivalves
and Brachiopods from the Brazilian
South Atlantic
Richard A. Krause Jr.1, Susan L. Barbour-Wood2, Michał
Kowalewski1, Marcello G. Simões3, Darrell Kaufmann4,
Christopher S. Romanek5, and John F. Wehmiller6
1Virginia
Polytechnic Institute and State University
2Colby College
3Universidade Estadual Paulista
4Northern Arizona University
5Savannah River Ecology Laboratory, University of Georgia
6University of Delaware
Introduction
How does time averaging compare among two very different organisms
collected from the same environment?
Part 1:
Comparisons of age-frequency distributions among brachiopods and bivalves
Part 2:
Investigation of the relationship between depth and time averaging duration
Locality & Methods
•
Shells dredged from two offshore sites
(10m, 30m)
Each site is similar in sedimentological and
other physical characteristics
60
Weight Percent
•
Grain size distribution by site
Site 1: 30 m
Site 9: 10 m
50
40
30
20
30 m
Mud (%)
Very fine sand (%)
10 m site
30 m site
% carbonate
25
25
Temperature (°C)
21.4*
21.2*
Salinity (‰)
35*
34*
*Mean annual measurements
Barbour Wood et al. (2006) Quaternary Research
Fine sand (%)
Medium sand (%)
Coarse sand (%)
10 m
Very coarse sand (%)
0
Gravel (%)
10
Physical Characteristics
Bouchardia rosea
Semele casali
10 mm
Semele casali
Bouchardia rosea
- thin shell
- low organic content
- aragonitic
*infaunal life habit
- robust shell
- high organic content
- calcitic
*epifaunal life habit
Amino Acid Racemization Dating
• 178 shells dated in this study
• D/L aspartic acid ratios calculated
in several replicates for each shell
• Calibrated with 19 AMS
radiocarbon dates
Samples taken from hinge
area to minimize intrashell
variability (Brigham, 1983; Carroll et
al., 2003)
5 14C dated shells
(D/L Asp2.7)
Age Calibration
2
0.200 Adj. r = 0.970
p = 0.0014
0.100
Brachiopods 10 m
2.7


Age   Asp  0.0044
0.0000487 

0.000
0
2
4
6
4 14C dated shells
(D/L Asp2.7)
Calibrated kyrs
0.200
Adj. r2 = 0.915
p = 0.0287
Brachiopods 30 m
2.7


Age   Asp  0.0225
0.0000232 

0.100
0.000
0
2
4
6
8
4 14C dated shells
(D/L Asp2.7)
Calibrated kyrs
0.020 Adj. r2 = 0.971
p = 0.0096
Bivalves 10 m
2.7


Age   Asp  0.0019

0
.
0000084


0.010
0.000
0
1
2
3
6
14C
dated shells
(D/L Asp2.7)
Calibrated kyrs
0.040
Adj. r2 = 0.978
0.030 p = 0.0001
0.020
0.010
0.000
0
1
2
Barbour Wood et al. (2006) Quaternary Research
Calibrated kyrs
Bivalves 30 m
 Asp 2.7  0.00005

Age  

0
.
0000106


3
Age-Frequency Distribution Comparisons
25
Frequency
Brachiopods, n=103
20
15
10
5
Wilcoxon two-sample test
0
0
1
2
3
4
5
6
7
8
9
10
Age (kyrs)
Kolmogorov-Smirnov test
20
Frequency
Bivalves, n=75
15
10
5
0
0
1
2
3
4
5
Age (kyrs)
Z=-1.89, p=0.0582
6
7
8
9
10
D=0.186, p=0.0996
Frequency
Age-Frequency Distribution Comparisons
5
30 m site, n=69
Wilcoxon two-sample test
0
0
1
2
3
4
5
6
7
8
9
10
Age (kyrs)
Kolmogorov-Smirnov test
35
10 m site, n=109
30
Frequency
25
20
15
10
5
0
0
1
2
3
4
5
Age (kyrs)
6
Z=5.04, p<0.0001
7
8
9
10
D=0.409, p<0.0001
Age-Frequency Distribution Comparisons
20
Brachiopods 10 m
n = 71
15
Brachiopods, between-sites
Wilcoxon two-sample test
Frequency
Z=5.49, p<0.0001
Kolmogorov-Smirnov test
10
D=0.625, p<0.0001
5
30m site, between-species
Wilcoxon two-sample test
0
5
Brachiopods 30m, n = 32
Z=4.21, p<0.0001
Kolmogorov-Smirnov test
0
0
1
2
3
4
5
6
7
8
9
10
Age (kyrs)
20
Bivalves 10 m
n = 38
15
D=0.625, p<0.0001
Bivalves, between-sites
Wilcoxon two-sample test
Frequency
Z=2.38, p=0.017
10
Kolmogorov-Smirnov test
D=0.472, p=0.0005
5
0
5
Bivalves 30 m
n = 37
0
0
1
2
3
4
5
Age (kyrs)
6
7
8
9
10
10m site, between-species
Wilcoxon two-sample test
Z=-1.04, p=0.300
Kolmogorov-Smirnov test
D=0.188, p=0.344
Summary of the Data
Brachiopods 10m (n=71)
Brachiopods 30m (n=32)
Bivalves 10m (n=38)
Bivalves 30m (n=37)
10m site (n=109)
30m site (n=69)
Bivalves (n=75)
Brachiopods (103)
0
1
2
3
4
Semi-Quartile Range (kyrs)
SQR  Q3  Q1 2
95% Confidence intervals
from separate 5000 (SQR) and
1000 (SD) iteration bootstrap
simulations.
0
1
2
3
4
Standard Deviation (kyrs)
 x  x 
2
SD 
n  1
-Brachiopods and bivalves exhibit similar duration of time averaging when sites are pooled
- Site-to-site variation can impose significant differences, even in the same oceanographic
province
Exploring the Relationship Between Time Averaging Magnitude
and Depth
Depth (m)
40
40
Mean
n = 21
30
30
40
Standard Deviation (SD)
n = 21
30
20
20
20
10
10
10
0
0
1
2
3
kyrs
4
5
6
0
0
1
2
3
4
0
0
Semi-quartile Range (SQR)
n = 21
1
kyrs
2
3
kyrs
An increase in time averaging duration with increasing depth?
Possible Factors: sea level history; sedimentation rate; many others...
Meta-analysis restricted to siliciclastic-dominated inner-shelf settings, but a variety of depositional systems
and oceanographic settings were included
Meta-analysis data sources:
Bahia la Choya, Gulf of California, Mexico (Flessa et al. 1993)
Bahia Concepcion, Gulf of California, Mexico (Meldahl et al. 1997)
Colorado Delta, Gulf of California, Mexico (Kowalewski et al. 1998)
Ubatuba Bay, Brazil (Carroll et al. 2003; This study)
Caribbean Coast of Panama (Kidwell et al. 2005)
4
Exploring the Relationship Between Time Averaging
Magnitude and Depth
40
Mean
n = 21
30
30
40
Standard Deviation (SD)
n = 21
30
20
20
20
10
10
10
0
1
2
3
4
5
6
0
0
1
kyrs
2
3
4
0
Semi-quartile Range (SQR)
n = 21
1
0
kyrs
2
3
kyrs
Null Models
Indirect Relationships
Direct Relationships
Logarithmic
Linear
Age
Directional Trend
Age
Passive Trend
Depth
Depth
Depth
0
Depth
Depth (m)
40
Age
Age
4
Determination of adequate sample size using regression
1
Adj. r2
1
Mean
p
0.5
0.1
0.01
0
0
Adj.
10
20
30
SD
0.5
0
-0.5
-1
0
1
Adj. r2
0
40
1
1
r2
Mean
10
20
30
40
Preferred
Threshold
Sample
Size = 4
p
20
30
40
10
20
30
40
10
20
30
SD
0.1
0.01
0
1
SQR
0.5
10
p
SQR
0.1
0
0.01
-0.5
0
10
20
30
40
Threshold sample size
0
40
Threshold sample size
Direct Relationships
40
Mean
n = 14
30
30
40
Standard Deviation (SD)
n = 14
30
20
20
20
10
10
10
0
0
1
2
3
4
5
6
0
0
1
Age (kyrs)
2
3
4
Age (kyrs)
0
0
Semi-quartile Range (SQR)
n = 14
1
2
Age (kyrs)
Linear
Depth
Mean
Adj. r2 = 0.326
p = 0.019
SD
Adj. r2 = 0.245
p = 0.041
SQR
Adj. r2 = 0.332
p = 0.018
Age
Logarithmic
Mean
Depth
Depth (m)
40
Adj. r2 = 0.296
p = 0.026
Age
SD
Adj. r2 = 0.159
p = 0.088*
SQR
Adj. r2 = 0.373
p = 0.016
3
4
Indirect Relationships
Depth (m)
40
40
Mean
n = 14
30
30
40
Standard Deviation (SD)
n = 14
30
20
20
20
10
10
10
0
0
1
2
3
4
Age (kyrs)
5
6
0
0
1
2
3
4
0
0
Age (kyrs)
Semi-quartile Range (SQR)
n = 14
1
2
3
Age (kyrs)
More difficult to test for these models
More data are needed from a variety of environments
Passive Trend
Depth
Depth
Directional Trend
Age
Age
4
Conclusions: Part 1
1.
Brachiopod and bivalve age-frequency distributions vary between sites
1.
No clear trend in differences between sites: indicates stochastic variation in taphonomic
processes
2.
When pooled, brachiopods and bivalves have very similar duration of time
averaging
3.
Biological properties (shell mineralogy, life habit etc.) may not be as
important as the frequency and intensity of taphonomic processes in
determining time averaging duration for these two groups
Conclusions: Part 2
1.
For pooled data, there is a suggestion of a relationship between time
averaging duration and depth
1.
Time averaging duration generally increases with depth
2.
This putative relationship holds for bivalves and brachiopods
3.
Relationship may be direct or indirect, more data is needed