Quantitative Estimates of Time-Averaging in Brachiopod Shell Accumulations from a Holocene Tropical Shelf (SW Brazil)

Time-averaging, the mixing of fossils of different ages within a single bed, defines the limit of temporal resolution of the fossil record. Quantitative estimates of this resolution threshold have not been acquired for any group other than mollusks. This study provides the first quantitative estimates of time-averaging for brachiopods, extending our understanding of intrinsic, or group specific controls on this process. Estimates were obtained by direct dating of individual terebratulid brachiopod shells Bouchardia rosea (Mawe) collected from modern surficial shelly accumulations in the Southeast Brazilian Bight (SW Atlantic). Using amino acid racemization dating calibrated with radiocarbon, 82 individual brachiopod shells, collected from four nearshore localities, were dated. The shells vary in age from modern to 3000 years, standard deviation = 680 years. The age distribution is significantly right-skewed (K3=2.48). At 50-year resolution, the temporal completeness is 75% for the last 1000 years and declines to 20% completeness for 1000-2000 yr. BP. Preservational quality (taphonomy) of modern (<50 yr.) shells is statistically indistinguishable from that of older shells, demonstrating that shell taphonomy is not a good predictor of within-assemblage relative age. These results conform to previously published results for mollusks. Therefore, brachiopods can show considerable time-averaging and this time-averaging can be on a scale similar to aragonitic mollusks despite the apparent lack of robustness of calcitic brachiopod shells. This suggests that the brachiopod fossil record can be notably time-averaged, but estimates of this mixing cannot be reliably deciphered from the taphonomic condition of shells.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Geological Sciences

Department dc:contributor.department

Geological Sciences

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2001

Author dc:creator

• Carroll, Monica

Chair dc:contributor.committeechair

• Kowalewski, Michal

Committee members dc:contributor.committeemember

• Bambach, Richard K.
• Schreiber, Madeline E.

• Bouchardia rosea
• Terebratulid Brachiopods
• Fossil record
• Amino Acid Racemization Dating
• Taphonomy

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-07312001-115131

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/34270