Date of Thesis

Spring 2020

Description

Coral skeletal density banding (CSDB), composed of alternating high density band (HDB) and low density band (LDB) layers that comprise the CaCO3 (aragonite) skeleton of scleractinian corals, are used as chronometers for global paleoclimatic reconstructions of sea surface temperature (SST). Scleractinian coral skeletons have been intensively studied for centuries with detail analysis of the macro- and microscale skeletal structure to establish taxonomic and evolutionary relationships of coral species, mechanisms of biomineralization, and seafloor physical, chemical and biological alteration (diagenesis) of the skeleton. This study is the first to determine the crystalline architecture of HDBs and CSDB stratigraphic sequences in original unaltered scleractinian coral skeletons and how this might impact CSDB-derived SST reconstructions. High-resolution optical microscopy, microcomputed tomography (microCT) scans, and x-radiography analyses are presented of CSDB in Orbicella annularis collected in 2006 and 2019 from the fringing reefs at Playa Kalki (PK) and Snake Bay (SB) on the southern Caribbean island of Curaçao. Results indicate that HDB layers are formed by the thickening of skeletal components (thecal walls, costae walls, and exothecal dissepiments) outside the margin of individual skeletal cups (corallites) that house living coral polyps. Conversely, skeletal elements within each corallite (septal walls and endothecal dissepiments) do not exhibit crystalline thickening. In addition, minor seafloor physical, chemical and biological alteration (diagenesis) is observed in both HDBs and LDBs. CSDB stratigraphic sequences exhibit down lap, condensed sections, and cross-cutting relationships that represent changes in coral ecology and growth history. Multiple HDB-LDB couplets were observed to be thickness, which combined with previously recorded O. annularis growth rates, indicate that these couplets represent less than one year of skeletal growth and do not reflect annual changes in SST. The PK2006 head records a hiatus event where coral growth shifted laterally, exposed part of the outermost coral skeletal surface to fungal borings and encrusting organisms, and was then later overgrown by the same coral colony. No specific HDB, LDB, or other stratigraphic intervals could be successfully correlated using microCT and x-radiograph line profiles between the PK2006, PK2019 and SB2019 coral heads. These results suggest that CSDB formation is strongly ecologically influenced by the host coral, symbiotic zooxanthellae, and resident microorganisms (collectively called the coral holobiont). Therefore, the influence of these biotic processes needs to be factored into CSDB-derived SST paleothermometry to create accurate predictions of future climate change.

Keywords

coral skeleton, coral skeletal density banding, sea surface temperature, geobiology, climate change

Access Type

Honors Thesis

Degree Type

Bachelor of Science

Major

Geology & Environmental Geosciences

First Advisor

Jeffrey M. Trop

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