Deep reef bioerosion and deposition: Sedimentology of mesophotic coral reefs in the U.S. Virgin Islands

The structural complexity and geomorphic diversity of coral reefs are vital foundational characteristics responsible for the many ecological and economic benefits these ecosystems provide. Shallow-water coral reef geomorphology and structural sustainability is mostly determined by varying reef sedimentary components including: (1) sediment production (matrix) and deposition, (2) framework production and secondary carbonate accretion; (3) bioerosion; and (4) cementation. However, little is known regarding the variability and influence of these sedimentary processes in mesophotic coral ecosystems (MCEs), deep reef communities 30--150 m below sea-level. Despite recent increases in biological and ecological MCE studies, many crucial sedimentological research questions remain unaddressed. These unaddressed questions impede a greater understanding of mesophotic reef structural sustainability and potentially related habitat heterogeneity, carbonate reef shelf development and variability in mesophotic depths, and the general origins of modern coral reef biodiversity.;Critical gaps in knowledge of mesophotic coral reef geomorphology and structural sustainability were addressed in this dissertation by conducting one of the first extensive sedimentological analyses of a mesophotic coral reef ecosystem. Beyond a general exploration of MCEs, the overall research goal was to identify basic sedimentary processes integral to the development, modification, and sustainability of mesophotic coral reef structure. The goal was also to determine the variability of the identified processes at different mesophotic reef habitats and investigate how these processes and potential variability impact shelf-wide habitat heterogeneity and long-term accretion. To address these goals, sedimentary analyses and ecological surveys were conducted at mesophotic coral reef habitats with distinct structurally characteristics, and neighboring shallow-water reef counterparts in the northern U.S. Virgin Islands (USVI). Analyses at all reefs were designed to address four specific aims: (1) categorization and comparison of various mesophotic reef sediment and cement attributes; (2) determination of exposed consolidated substrate reef bioerosion rates, and the distribution and variability of bioeroding groups; (3) quantification and determination of primary coral mesophotic reef framework builder linear growth and calcification rate variability, and comparison to live mesophotic framework bioerosion and secondary accretion rates; and (4) application of study results for carbonate budget analysis and assessment of geomorphic carbonate production status.;Sediment and cement analysis (first aim) indicated that distinct MCE habitats produce subfacies. The interpreted hydrodynamic and biological interactions controlling mesophotic USVI subfacies have implications towards paleoenvironmental interpretations of ancient mesophotic reef deposits with similar sediment and cement characteristics. Significant differences in exposed consolidated substrate bioerosional processes were discovered between the analyzed habitats. These differences were found to primarily result from variation in parrotfish biomass and related controls on substrate exposure time and location in macroboring succession. Results also broadly confirm pervious hypothesizes that bioerosion decreases with depth along a carbonate shelf and have implications leaning toward rejection of traditional reef accretion theories. Analysis of coral growth identified statistically significant differences in mesophotic coral reef calcification rates, implying another potential long-term mechanism for enhancing mesophotic reef structural heterogeneity. However, on a larger scale, linear extension rates were found to fit within previously proposed models of decreasing coral growth rate with increasing depth.;Mesophotic coral reef sedimentary analyses were compared in a newly developed carbonate budget model to analyze structural sustainability and consider implications of these analyses on mesophotic reef habitat heterogeneity and Holocene carbonate shelf accretion. All USVI mesophotic habitats examined were identified with net positive carbonate production despite significant variability in geomorphic production states. Additionally, comparisons with earlier benthic surveys suggest higher net USVI mesophotic reef carbonate production in the recent past, potentially implying these deeper reefs are not fully immune to modern global stressors impacting shallow-water reefs. Results indicated that mesophotic reef accretion was not the main driver of shelf-scale topographic relief. However, mesophotic carbonate production variability substantially contributes to habitat-scale structural relief and complexity and relatedly to overall ecosystem diversity. Specific mesophotic reef sedimentology research methods and the need for similar studies at other mesophotic reef habitats were suggested. Comprehensive sedimentology analysis of mesophotic coral reefs in the USVI provide new insight into reef structural sustainability, geomorphic status, and potential impacts from global stressors, and should be considered when developing specific reef sustainability models and management strategies.