Age Mixing Among Sympatric Bivalves Age Mixing Among Sympatric Bivalves and Brachiopods from the Brazilian and Brachiopods from the Brazilian

South AtlanticSouth AtlanticRichard 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 University2Colby College

3Universidade Estadual Paulista4Northern Arizona University

5Savannah River Ecology Laboratory, University of Georgia6University 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

10 m

30 m

Grain size distribution by site

0

10

20

30

40

50

60

Gra

vel (

%)

Ver

y co

arse

san

d (%

)

Coa

rse

sand

(%)

Med

ium

san

d (%

)

Fine

san

d (%

)

Ver

y fin

e sa

nd (%

)

Mud

(%)

Wei

ght P

erce

nt

Site 1: 30 m Site 9: 10 m

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

Physical Characteristics

10 mm

Semele casaliBouchardia 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 DatingAmino 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)

Age CalibrationAge Calibration

Brachiopods 30 m

0.000

0.100

0.200

0 2 4 6 8

Brachiopods 10 m

0.000

0.100

0.200

0 2 4 6

Bivalves 10 m

0.000

0.010

0.020

0 1 2 3

Bivalves 30 m

0.0000.0100.0200.0300.040

0 1 2 3

(D/L

Asp

2.7 )

(D/L

Asp

2.7 )

(D/L

Asp

2.7 )

(D/L

Asp

2.7 )

Calibrated kyrs

Calibrated kyrs

Calibrated kyrs

Calibrated kyrs

Adj. r2 = 0.970 p = 0.0014

Adj. r2 = 0.915p = 0.0287

Adj. r2 = 0.971 p = 0.0096

Adj. r2 = 0.978 p = 0.0001

0000232.0

0225.07.2AspAge

0000084.0

0019.07.2AspAge

0000106.0

00005.07.2AspAge

0000487.0

0044.07.2AspAge5 14C dated shells

4 14C dated shells

4 14C dated shells

6 14C dated shells

Barbour Wood et al. (2006) Quaternary Research

Wilcoxon two-sample testZ=-1.89, p=0.0582Kolmogorov-Smirnov testD=0.186, p=0.0996

0

5

10

15

20

25

0

5

10

15

20

1 2 3 4 5 6 7 8 9 100Age (kyrs)

1 2 3 4 5 6 7 8 9 100Age (kyrs)

Freq

uenc

yFr

eque

ncy

Age-Frequency Distribution ComparisonsAge-Frequency Distribution Comparisons

Brachiopods, n=103

Bivalves, n=75

Wilcoxon two-sample testZ=5.04, p<0.0001Kolmogorov-Smirnov testD=0.409, p<0.0001

0

5

0

5

10

15

20

25

30

35

30 m site, n=69

1 2 3 4 5 6 7 8 9 100Age (kyrs)

1 2 3 4 5 6 7 8 9 100Age (kyrs)

Freq

uenc

yFr

eque

ncy

Age-Frequency Distribution ComparisonsAge-Frequency Distribution Comparisons

10 m site, n=109

0

5

0

50

5

10

15

20

Brachiopods 30m, n = 32

0

5

10

15

20Brachiopods 10 m

n = 71

Age-Frequency Distribution ComparisonsAge-Frequency Distribution ComparisonsBrachiopods, between-sitesWilcoxon two-sample testZ=5.49, p<0.0001Kolmogorov-Smirnov testD=0.625, p<0.0001

Bivalves, between-sitesWilcoxon two-sample testZ=2.38, p=0.017Kolmogorov-Smirnov testD=0.472, p=0.0005

Bivalves 10 mn = 38

Bivalves 30 mn = 37

1 2 3 4 5 6 7 8 9 100

1 2 3 4 5 6 7 8 9 100Age (kyrs)

Age (kyrs)

Freq

uenc

yFr

eque

ncy

30m site, between-speciesWilcoxon two-sample testZ=4.21, p<0.0001Kolmogorov-Smirnov testD=0.625, p<0.0001

10m site, between-speciesWilcoxon two-sample testZ=-1.04, p=0.300Kolmogorov-Smirnov testD=0.188, p=0.344

Brachiopods 30m (n=32)Brachiopods 10m (n=71)

Bivalves 30m (n=37)Bivalves 10m (n=38)

Brachiopods (103)Bivalves (n=75)

0 1 2 3 4Standard Deviation (kyrs)

0 1 2 3 4Semi-Quartile Range (kyrs)

10m site (n=109)30m site (n=69)

Summary of the Data

-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

95% Confidence intervals from separate 5000 (SQR) and 1000 (SD) iteration bootstrap simulations.

1

2

nxx

SD 213 QQSQR

Exploring the Relationship Between Time Averaging Magnitude and Depth

0

10

20

30

40

0 1 2 3 4 5 60

10

20

30

40

0 1 2 3 40

10

20

30

40

0 1 2 3 4kyrs

Dep

th (m

) Meann = 21

Standard Deviation (SD)n = 21

Semi-quartile Range (SQR)n = 21

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)

kyrskyrs

Possible Factors: sea level history; sedimentation rate; many others...

An increase in time averaging duration with increasing depth?

Exploring the Relationship Between Time Averaging Magnitude and Depth

0

10

20

30

40

0 1 2 3 4 5 60

10

20

30

40

0 1 2 3 40

10

20

30

40

0 1 2 3 4kyrs

Dep

th (m

)

Age

Dep

th

Age

Dep

th

Age

Dep

th

Age

Dep

th

kyrskyrs

Null Models

Linear Logarithmic Directional Trend Passive TrendDirect Relationships Indirect Relationships

Meann = 21

Standard Deviation (SD)n = 21

Semi-quartile Range (SQR)n = 21

0

0.5

1

0 10 20 30 40

Threshold sample size

-1-0.5

00.5

1

0 10 20 30 40

-0.5

0

0.5

1

0 10 20 30 40

0.01

0.1

1

0 10 20 30 40

0.01

0.1

1

0 10 20 30 40

0.01

0.1

1

0 10 20 30 40

Threshold sample size

Adj. r2

Mean

SD

SQR

Adj. r2

Adj. r2

pMean

SD

SQR

p

p

Determination of adequate sample size using regression

PreferredThreshold Sample Size = 4

Age (kyrs)

Dep

th (m

)

Age

Dep

th

Age (kyrs) Age (kyrs)

0

10

20

30

40

0 1 2 3 4 5 60

10

20

30

40

0 1 2 3 40

10

20

30

40

0 1 2 3 4

Direct Relationships

Mean SD SQR

Linear

Adj. r2 = 0.326p = 0.019

Adj. r2 = 0.245p = 0.041

Adj. r2 = 0.332p = 0.018

Mean SD SQR

Adj. r2 = 0.296p = 0.026

Adj. r2 = 0.159p = 0.088*

Adj. r2 = 0.373p = 0.016

Age

Dep

th

Logarithmic

Meann = 14

Standard Deviation (SD)n = 14

Semi-quartile Range (SQR)n = 14

Age (kyrs)

Dep

th (m

)

Age (kyrs) Age (kyrs)

0

10

20

30

40

0 1 2 3 4 5 60

10

20

30

40

0 1 2 3 40

10

20

30

40

0 1 2 3 4

Age

Dep

th

Age

Dep

th

Directional Trend Passive Trend

Indirect Relationships

More difficult to test for these models

More data are needed from a variety of environments

Meann = 14

Standard Deviation (SD)n = 14

Semi-quartile Range (SQR)n = 14

Conclusions: Part 1

1. Brachiopod and bivalve age-frequency distributions vary between sites1. 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