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Date Permissions Signed


Date of Award

Summer 2021

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Dahl, Robyn

Second Advisor

Clark, Douglas H., 1961-

Third Advisor

Foreman, Brady


Fossil records have the potential to extract important paleoenvironmental records, and by ground truthing our assumptions with modern mollusks we can improve our interpretations of the fossil record. Modern molluscan death assemblages from Rosario Strait were analyzed to: 1) determine to what extent the molluscan communities were controlled by grain size or depth; and 2) determine the extent to which age mixing was occurring in the death assemblage. Twenty-eight Van Veen grab samples were collected in Rosario Strait to represent range of depth and grain sizes. All samples were wet sieved to isolate mature mollusks (> 2.00 mm), and sediment samples were freed of biologic material and grain size determined using the Mastersizer. The proportions of mud, silt, and sand were recorded to determine grain size of the benthic habitat. Northern Arizona University Amino Acid Geochronology Laboratory determined the D/L ratios of several amino acids for the selection of bivalves sent to them using High-Performance Liquid Chromatographic Analysis.

D/L concentrations of aspartic acid (Asp) and alanine (Ala) were relatively consistent for Cyclocardia (Cy), Chlamys (Ch), and Macoma (Ma) both within a single genus at a single site (intra-site) and between a single genus at multiple sites (inter-site). Intra-site variability of Asp was on average 0.031 (Cy), 0.007 (Ch), and 0.006 (Ma); the intra-site variability of Ala was on average 0.008 (Cy), 0.004 (Ch), and 0.024 (Ma). Inter-site variability of Asp was on average 0.007 (Cy), 0.068 (Ch), and 0.085 (Ma); the inter-site variability of Ala was on average 0.013 (Cy), 0.036 (Ch), and 0.081 (Ma). This indicates that there is some amount of age mixing occurring at the sample sites. Nuculana displayed relatively higher intra-site variability with the average difference 0.062 for Asp and 0.064 for Ala but had inter-site variability similar to the other genera with the average difference being 0.063 for Asp and 0.042 for Ala. This indicates that the genus Nuculana has larger intraspecies D/L concentration variability and is therefore less suitable for AAR analysis; samples that have an exact age through 14C or 210Pb dating would be useful.

The molluscan death assemblages of Rosario Strait were dominated by Macoma (up to 77%), Nutricola (up to 41%), and Calyptraea (up to 12%). Grain size was not correlated with the presence/absence of genera nor their abundance. These results suggest that grain size played a smaller role than initially hypothesized in the molluscan community composition of Rosario Strait. Previous studies that assumed depth was the primary control on species composition used species composition to estimate Pleistocene paleodepth. The paleodepth was then used to calculate ice thickness, which was used to calculate changes in sea level. I found no basis in the death assemblage data for this proxy and that mollusks should not be used as the primary evidence for paleodepth interpretations.




AAR, Nuculana, Rosario Strait


Western Washington University

OCLC Number


Subject – LCSH

Mollusks--Research--Washington (State)--Rosario Strait; Mollusk remains (Archaeology)--Environmental aspects--Washington (State)--Rosario Strait; Thanatocoenoses--Washington (State)--Rosario Strait

Geographic Coverage

Rosario Strait (Wash.)




masters theses




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Abundance.xlsx (228 kB)
PNW_molluscs.pdf (11223 kB)
PNW_molluscs.pdf (11223 kB)

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