Age Mixing Among Sympatric Bivalves and Brachiopods from the Brazilian South Atlantic Richard A. Krause Jr. 1, Susan L. Barbour-Wood 2, Michał Kowalewski 1, Marcello G. Simões 3, Darrell Kaufmann 4, Christopher S. Romanek 5, and John F. Wehmiller 6 1 Virginia Polytechnic Institute and State University 2 Colby College 3 Universidade Estadual Paulista 4 Northern Arizona University 5 Savannah River Ecology Laboratory, University of Georgia 6 University 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 Gravel (%) Very coarse sand (%) Coarse sand (%) Medium sand (%) Fine sand (%) Very fine sand (%) Mud (%) Weight Percent Site 1: 30 m Site 9: 10 m 10 m site30 m site % carbonate25 Temperature (°C)21.4*21.2* Salinity (‰)35*34* *Mean annual measurements Barbour Wood et al. (2006) Quaternary Research

Physical Characteristics 10 mm 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 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 Calibration Brachiopods 30 m Brachiopods 10 m Bivalves 10 m Bivalves 30 m (D/L Asp 2.7 ) Calibrated kyrs Adj. r 2 = p = Adj. r 2 = p = Adj. r2 = p = Adj. r 2 = p = C dated shells 4 14 C dated shells 6 14 C dated shells Barbour Wood et al. (2006) Quaternary Research

Wilcoxon two-sample test Z=-1.89, p= Kolmogorov-Smirnov test D=0.186, p= Age (kyrs) Age (kyrs) Frequency Age-Frequency Distribution Comparisons Brachiopods, n=103 Bivalves, n=75

Wilcoxon two-sample test Z=5.04, p< Kolmogorov-Smirnov test D=0.409, p< m site, n= Age (kyrs) Age (kyrs) Frequency Age-Frequency Distribution Comparisons 10 m site, n=109

Brachiopods 30m, n = Brachiopods 10 m n = 71 Age-Frequency Distribution Comparisons Brachiopods, between-sites Wilcoxon two-sample test Z=5.49, p< Kolmogorov-Smirnov test D=0.625, p< Bivalves, between-sites Wilcoxon two-sample test Z=2.38, p=0.017 Kolmogorov-Smirnov test D=0.472, p= Bivalves 10 m n = 38 Bivalves 30 m n = Age (kyrs) Frequency 30m site, between-species Wilcoxon two-sample test Z=4.21, p< Kolmogorov-Smirnov test D=0.625, p< m site, between-species Wilcoxon two-sample test Z=-1.04, p=0.300 Kolmogorov-Smirnov test D=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) Standard Deviation (kyrs) Semi-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.

Exploring the Relationship Between Time Averaging Magnitude and Depth kyrs Depth (m) Mean n = 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) kyrs 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 kyrs Depth (m) Age Depth Age Depth Age Depth Age Depth kyrs Null Models Linear Logarithmic Directional Trend Passive Trend Direct Relationships Indirect Relationships Mean n = 21 Standard Deviation (SD) n = 21 Semi-quartile Range (SQR) n = 21

Threshold sample size Threshold sample size Adj. r 2 Mean SD SQR Adj. r 2 p Mean SD SQR p p Determination of adequate sample size using regression Preferred Threshold Sample Size = 4

Age (kyrs) Depth (m) Age Depth Age (kyrs) Direct Relationships MeanSDSQR Linear Adj. r 2 = p = Adj. r 2 = p = Adj. r 2 = p = MeanSDSQR Adj. r 2 = p = Adj. r 2 = p = 0.088* Adj. r 2 = p = Age Depth Logarithmic Mean n = 14 Standard Deviation (SD) n = 14 Semi-quartile Range (SQR) n = 14

Age (kyrs) Depth (m) Age (kyrs) Age Depth Age Depth Directional Trend Passive Trend Indirect Relationships More difficult to test for these models More data are needed from a variety of environments Mean n = 14 Standard Deviation (SD) n = 14 Semi-quartile Range (SQR) n = 14

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