A Comparative Analysis of A Comparative Analysis of Time Averaging for Bivalves Time Averaging for Bivalves and Brachiopods from a Modern and Brachiopods from a Modern Tropical Shelf Tropical Shelf R.A. Krause Jr. 1 , S.L. Barbour Wood 1 , J.F. Wehmiller 2 , M. Kowalewski 1 , M.G. Simões 3 1 Virginia Tech, Dept. of Geosciences, Blacksburg, VA 2 Univ. of Delaware, Earth Sciences, Newark, DE 3 Universidade Estadual Paulista, Instituto de Biociências, Sao Paulo, Brazil Geobiology Geobiology Group Group www.geol.vt.edu/paleo
20
Embed
A Comparative Analysis of Time Averaging for Bivalves and Brachiopods from a Modern Tropical Shelf
Geobiology Group. www.geol.vt.edu/paleo. A Comparative Analysis of Time Averaging for Bivalves and Brachiopods from a Modern Tropical Shelf. R.A. Krause Jr. 1 , S.L. Barbour Wood 1 , J.F. Wehmiller 2 , M. Kowalewski 1 , M.G. Sim ões 3. 1 Virginia Tech, Dept. of Geosciences, Blacksburg, VA - PowerPoint PPT Presentation
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
A Comparative Analysis of A Comparative Analysis of Time Averaging for Bivalves Time Averaging for Bivalves and Brachiopods from a Modern and Brachiopods from a Modern
• NSF Geology & Paleontology (MK & JFW)• ACS-Petroleum Research Fund (MK)• David R. Wones Geoscience Scholarship, Dept. of Geosciences, Virginia Tech (RAK)
• Graduate Research Development Grant, Virginia Tech (RAK)
Introduction• Time averaging = Temporal mixing• Duration of temporal mixing determines resolution• Quantitative estimates of time averaging are
increasingly available, although studies are biased toward mollusks
Importance• First study to investigate duration of time
averaging on two very different shelled invertebrates from the same environment
• Allows more accurate interpretation of polytypic shell beds
Outline
• Age-Frequency Distributions (AFD):– Comparison of scale of time averaging– Are there differences between brachiopods and bivalves?
• Analysis of Completeness:– How complete is the record for each taxon?– With 100% completeness, what would AFD look like?
10 m
30 m
Locality & Methods
• Shells dredged from two offshore sites (10m, 30m)• Dated using amino acid racemization
– D/L ratios calibrated with AMS radiocarbon dates
• Comparison of Age-frequency distributions• Analysis of completeness of each sample
10 m 30 28 5830 m 36 36 72 66 64 130
Brach
iopods
Bival
ves
Totals
Physical Characteristics
10 cm
Semele casali
Bouchardia rosea
Semele casali
- thin shell- low organic content- aragonitic*infaunal life habit
Bouchardia rosea
- robust shell- high organic content- calcitic*epifaunal life habit
Amino Acid Racemization Dating
• D/L aspartic acid ratio determined with gas chromatography
• Calibrated with 19 AMS radiocarbon dates
0
0.1
0.2
0.3
0.4
0 2000 4000 6000Age (Years BP)
(D/L
As
pa
rtic
)2
r2= 0.96
Brachiopods
0
0.02
0.04
0.06
0.08
0.1
0.12
0 1000 2000 3000
Bivalvesr2=0.73
(D/L
As
pa
rtic
)2
Age (Years BP)
• Ratio of 'D' to 'L' form of aspartic acid predicts well age of shell
• Ratios of many shells can be calculated for the cost of one radiocarbon date
Wilcoxon Two-Sample TestBetween-taxa comparisons of central tendencyα=0.05 10 m 30 m
Z=4.0p<0.001
Z=-0.26p=0.79
Wilcoxon Two-Sample TestBetween-site comparisons of central tendencyα=0.05 Brachiopods Bivalves
Z=0.08p=0.94
Z=4.12p<0.001
Kolmogorov-Smirnov TestBetween-taxa comparisons of distribution shapeα=0.05 10 m 30 m
D=0.5p<0.001
D=0.22p=0.43
Kolmogorov-Smirnov TestBetween-site comparisons of distribution shapeα=0.05 Brachiopods Bivalves
D=0.25p=0.21
D=0.48p<0.001
Scale of Time AveragingScale of Time Averaging
• Dispersion metrics– Range: sensitive to sample size– Shell half-life: assumes continuous input of shells– Standard deviation: less sensitive to sample size, no
restrictive assumptions
• Confidence intervals around SD– estimated using independent 1000 iter. bootstrap
simulations– 95% and 99% confidence intervals calculated from 0.5,
2.5, 97.5, and 99.5 percentiles of sampling distribution
0
1000
2000
3000
BrachiopodsBivalves
Yea
rs
10
m
30
m
10
m
30
m
4
8
02
6
10
024
02468
10121416
02468
Brachiopods: 30 m
Brachiopods: 10 m
Bivalves: 30 m
Bivalves: 10 m
Confidence Intervals for SDConfidence Intervals for SD
Comparison With Other StudiesComparison With Other Studies
0
1000
2000
3000
This Study Carroll et al., 2003
Ubatuba Bay, Brazil:mixed carbonate-siliciclastic shelf
Sta
nd
ard
De
via
tio
n o
f S
he
ll A
ge
Kowalewski et al., 1998
Colorado River Delta:beach ridges
10 m 30 m
10
m1
6 m
23
m6
m
Flessa et al., 1993
Bahía la ChoyaGulf of California:intertidal, low sed.
Inner tidal flat
Tidalchannel
core co
re
fan deltas pocket
bays
Bahía ConcepcíonGulf of California:shallow, high sed.
Meldahl et al., 1997
40005000600070008000
900010000
Flessa & Kowalewski, 1994
nearshore
shelf
fossilassemblages
inactive beach ridges
*95% & 99% confidence intervals calculated by bootstrapping
BrachiopodsBivalves
Temporal CompletenessTemporal Completeness
• Completeness is scale-dependant– decreases with increasing resolution and/or range– increases with increasing sample size, generally
speaking
• High incompleteness suggests discontinuous time averaging
• However, most distributions have gaps due to sampling– With 100% complete fossil record, how likely is it to
get samples as complete as ours?
# of time intervals
# of time intervalswith paleontologicalrecord
Completeness (%) = X 100
Completeness Simulations
Monte Carlo Simulations: Randomly sample 100% complete distributions