Publication Date

2019

Document Type

Honors Project

Degree Name

Bachelor of Arts

Department

Geosciences

Advisors

Sara B. Pruss

Keywords

Cladophora, Cyanobacteria, Thrombolite, Nama

Abstract

Microbialites, such as stromatolites and thrombolites, are preserved throughout the geologic record. Stromatolites are a preserved laminated microbial fabric, whereas thrombolites have a distinctly clotted texture. There is still debate about how thrombolites form. Theories of the formation process of thrombolites include the calcification of coccoidal microbial communities and the deformation of already formed stromatolites. This research aims to explore how the presence of other organisms, such as metazoans and algae, with cyanobacteria impact the microbial fabric created. The presence of other organisms disrupted the formation of the fine lamination formed by cyanobacteria and extracellular polymeric substances (EPS) creating microbial mats, to create more complex microbial fabrics with different textures. Three reefs sites in the Nama Group from the Ediacaran were examined to determine if the early appearance of stem calcifying metazoans have a preference for stromatolitic or thrombolitic fabrics. The abundance of metazoans in each of the fabrics could indicate that their presence impacts the microbial fabric present. The calcifying metazoans were present in both stromatolitic and thrombolitic fabrics in the Nama Group. There was a greater abundance and diversity of metazoans present in the thrombolitic fabric than the stromatolitic fabric. In thrombolitic build-ups, skeletons account for 2.3% of points counted, in contrast to stromatolites (1.4%). The fill (grainstone/packstone) between and in the interstices of the mounds had 6.3% of skeletal material on average. This indicates a preference for the environments where thrombolites form. The interaction of the metazoans with cyanobacteria and sediment could have impacted microbial fabric formation. Modern microbial mats were grown to examine the texture of mats composed of cyanobacteria and Cladophora algal fragments. Coccoidal and filamentous cyanobacteria were identified in the modern microbial mats. The modern mats appear to be more similar to the stromatolitic fabric due to the presence of distinct layers separated by cyanobacteria and organic material. Metazoans appear to impact the type of microbial texture formed, whereas algal fragments do not disrupt the formation of modern microbial mats. The algae act as a stabilizing feature in the microbial mat, and while they could form a clotted texture after preservation, examination of the modern mats grown indicate that the algal fragments did not impact the growth of the cyanobacteria. The metazoans have a clear preference for thrombolites, indicating that they may play a role in the formation of thrombolite texture. The metazoans may play a bigger role in the formation of textures than algal fragments because of their size and life habit.

Rights

©2019 Molly Megan. Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.

Language

English

Comments

74 pages : color illustrations. Includes bibliographical references (pages 66-68)

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