Abstract
This thesis presents a two-part study on the use of diatoms as paleoenvironmental recorders in the Ross Sea region of the Southern Ocean. Part one of the study focuses on the 11.75 m sediment core RS15-LC42, collected by the Korea Polar Research Institute. The core was obtained from continental rise in the Central Basin, downslope from the mouth of the Joides Trough. An age model was derived for the core based on the magnetic reversal stratigraphy, diatom biostratigraphy, and relative paleointensity records. The favoured age model, supported by the FO of Thalassiosira antarctica (FAD = 0.23 – 0.56 Ma) between 5.40 and 5.65 mbsf indicated a largely continuous sedimentation record back to 1.34 Ma, with a possible ~80 kyr hiatus at ~9.4 mbsf from 0.99 to 1.07 Ma. Throughout this interval, glacial-interglacial cycling changed from obliquity-forced, 40-kyr cycles to eccentricity forced 100-kyr cycles, with an increase in amplitude at ~400 – 600 ka. Analysis of diatom assemblages revealed that the upper 3.25 mbsf (~300 ka) has very few reworked diatom taxa present. Laminated silty intervals displayed diverse assemblages of reworked diatom taxa, providing evidence of extensive regional reworking from the continental shelf below 3.25 mbsf. Paleocurrent analysis using Anisotropy of Magnetic Susceptibility was attempted to clarify these results. However, the presence of coarse-grained sediments and IRD prevented the reliable interpretation of results. The results from diatom analysis in light of sedimentological and seismic studies completed prior, suggest that periods of elevated reworking from 0.3 – 1.3 Ma (low energy intervals) are the result of sediments being deposited under weak bottom current influence as low energy drift deposits behind a topographic high. Coarse grained intervals with low reworked taxa abundance and low absolute diatom abundance are suggested to be the result of enhanced bottom currents resulting in the winnowing of fine-grained sediments, including diatomaceous material.
The second part of the study is an analysis of modern diatom ecology and related stable isotope geochemistry. Samples were obtained from hydrocasts and sediment trap arrays deployed during the NBP18-01 expedition in the Ross Sea and Terra Nova Bay. Seventy-seven paired analyses were undertaken for diatom assemblage and stable isotopes from hydrocast samples, displaying a strong positive correlation between the absolute diatom abundance and δ13C of suspended particulate organic matter. Cluster analysis grouped the sites by diatom assemblages into four distinct spatial and temporal groups. Samples from the two most populated clusters were observed to group distinctly in the northern and southern Terra Nova Bay regions respectively, generally defined by a latitudinal boundary at 74.95˚S. South of this boundary, sustained katabatic winds influence surface waters and result in intense mixing and sea ice formation, while north of 74.95˚S the embayment was relatively sheltered over the study period. Carbon isotope fractionation mirrors the clusters, with elevated values (from -24 to -21) in northern Terra Nova Bay reflective of increased absolute diatom abundance and the relative abundance of Fragilariopsis curta, and more negative values (-23 to -29) observed in southern Terra Nova Bay related to increased relative abundances of Chaetoceros resting spores and decreases in F. curta abundance and absolute diatom abundance. Carbon isotope fractionation appears to be most significantly driven by primary productivity in the region, with significant regional enhancement as a result of sea ice melt, water column stratification, and taxon-specific fractionation effects – namely that of F. curta.