Planktonic foraminifera as ecological and geochemical palaeoclimate proxies : evaluation from Southern Ocean sediment traps

• Main Author: King, AL
• Format: Thesis
• Language: English
• Published: 2003
• Subjects: Foraminifera; Marine zooplankton; Paleoecology; Southern Ocean; New Zealand; Neogloboquadrina pachyderma; Planktonic foraminifera
• Online Access: https://eprints.utas.edu.au/20575/; https://eprints.utas.edu.au/20575/6/whole_KingAlexandraLouise2001.pdf

The analysis of planktonic foraminifera obtained from Southern Ocean sediment traps allows the calibration of several important paleoclimate proxies in this thesis. Foraminifera are analysed to determine the spatial and temporal changes in faunal and isotopic composition. Annual faunal assemblages collected from each sediment trap are compared to a global core top data base to assess the validity of the Modern Analog Technique in estimating sea-surface temperatures. The oxygen isotopic composition of several foraminiferal species also allows calibration of paleotemperature equations for the first time in Southern Ocean settings. The utility of the foraminiferal carbon isotopic record as a nutrient tracer is also tested. The sediment traps represent two separate deployments. The first deployment consists of two sediment traps moored north and south of Chatham Rise east of New Zealand at 42° and 44°S respectively. These traps sampled Subtropical and Subantarctic waters over an annual cycle. The second deployment consists of an array of traps moored south of Tasmania at 47°S in the Subantarctic Zone, 51°S at the Subantarctic Front, and 54°S in the Polar Frontal Zone. The timing of this second deployment coincides with the dominant period of biological production between September and March. The trap at 47°S was subsequently redeployed for a following year. Planktonic foraminifera collected in the traps exhibit distinct seasonal and latitudinal distributions. Diversity of foraminiferal species decreases to the south, with water mass boundaries associated with sharp changes in foraminiferal assemblages. The distinct change in species composition is particularly striking north and south of the Subtropical Front. Subtropical waters are dominated by the species Globorotalia inflata, while Subantarctic waters (at the 44°S and 47°S traps) are dominated by Globigerina bulloides. G. bulloides remained dominant at 51°S, but is replaced by Neogloboquadrina pachyderma (sinistral coiling) and Globigerina quinqueloba in Polar Frontal waters at 54°S. Despite the significant differences in species abundances and dominance between the sites, seasonal flux patterns appear constant for each of the major species. G. bulloides and G. injlata are most productive during spring and early summer at all sites, while N pachyderma (s.) and N pachyderma (dextral coiling) tend to be most abundant during summer to autumn. These seasonal succession patterns are associated with changes in thermal stratification, which drives the depth of the chlorophyll maximum in these regions. Analysis of the planktonic foraminifera from these sediment traps provides important insights for the interpretation of past climates in sedimentary records. Comparison between the total assemblages from each trap site and regional core tops confirms the relationship between core top material and modern environments, a relationship which is often extrapolated to the interpretation of down core material. Further comparison to core top assemblages, using the Modern Analog Technique, reveals a good consistency between foraminiferal assemblages and sea-surface temperatures in a global core top data base. These results indicate that the Modern Analog Technique allows the estimation of sea-surface temperatures to within 1.2° to 4.5°C of observations in these Southern Ocean environments. The isotopic composition of G. bulloides, G. inflata and N. pachyderma (s.) is analysed for each sediment trap. The δ¹⁸O content is compared to predictions from various temperature equations, revealing greatest consistency to the equation of Epstein et al. (1953). The depth range for each species is also determined by analysis of their δ¹⁸O content. G. bulloides and N pachyderma (s.) dwell at near-surface depths, while a deeper habitat is inferred for G. inflata. Similarity between flux weighted sediment trap values and regional core tops reveals the preservation of seasonal flux patterns in the sedimentary records. Changes in seasonal flux patterns down the core and between water masses may therefore overprint temporal or latitudinal gradients in δ¹⁸O. The δ¹³O composition of the three foraminiferal species reveals latitudinal trends in disequilibrium. Foraminiferal δ¹⁸O increases from north to south, while the δ¹³O of dissolved inorganic carbon decreases across these latitudes. Disequilibrium in G. bulloides can be accounted for by changes in temperature and carbonate ion content ([CO₃²⁻]). The relationship between disequilibrium and [CO₃²⁻] derived for whole shells of G. bulloides is consistent with the slope established from individual chambers in laboratory measurements. Corrected δ¹³O for G. bulloides is closely correlated to changes in nutrients at each site, indicating the utility of G. bulloides as a nutrient tracer in Southern Ocean environments. Comparison between flux weighted sediment trap values and nearby core tops indicates a modern depletion in δ¹³O, attributable to the oceanic Suess effect. The large magnitude of this effect signifies considerable equilibration between the surface waters and the atmosphere in the Subantarctic Zone.