Environmental Controls on Coccolithophore Blooms in the Southwest Pacific Ocean during Marine Isotope Stages 5e (125 ka) and 7a (210 ka)
Authors:   , Duncan Bella Jane 
Published: November 2021

<p>Coccolithophores play a key role in the ocean carbon cycle, regulating the uptake and release of CO2. Satellite observations over the past few decades show ocean change in a warming world is accompanied by changes in the latitudinal distribution of coccolithophore blooms. Despite their importance in the carbon cycle, knowledge of the causes of coccolithophore blooms, and how they may respond to future climate change is limited. In this study evidence from marine sedimentary cores is used to derive longer, more complete records of past coccolithophore productivity, and the factors that potentially caused enhanced coccolithophore productivity in previous interglacials. Carbonate-rich marine cores; subtropical P71 from north of New Zealand (33°51.3‟S, 174°41.6‟E) and subantarctic Ocean Drilling Project (ODP) 1120 from the Campbell Plateau (50°3.803‟S, 173°22.300‟E) show abrupt changes between foraminiferal-rich sediments during glacials to coccolith-rich sediments during interglacials. Both cores encompass the last two complete interglacial cycles, Marine Isotope Stage (MIS) 5 (71-130ka) and MIS 7 (191- 243ka). While MIS 5 has been well-studied in the Southwest Pacific Ocean, research on MIS 7 is limited. From the literature, and data from this study, new insights are presented into the climatic and oceanographic conditions during MIS 7. Sea surface temperatures in the subtropical Tasman Inflow were comparable to present during MIS 7a (191-222ka), but were cooler in MIS 7c (235-243ka), implying a change in flow regime potentially related to the dynamics of the South Pacific Gyre. During MIS 7a and 7c the temperature gradient across the Subtropical Front (STF), which separates subtropical and subantarctic waters, was greater than present on the Chatham Rise, at >2°C per 1° latitude. In the Tasman Sea, the STF moved northwards by ~2° latitude. This thesis employs grain size data and scanning electron microscope images to show that significant coccolithophore blooms occurred during MIS 7a at subtropical core P71, but not during interglacial peak MIS 5e (117-130ka), whilst the reverse is true at subantarctic core ODP 1120. A range of paleo-environmental proxies are used to determine the potential conditions that caused these coccolithophore blooms. This includes mass accumulation rates of CaCO3 and % of <20μm grain size that texturally identifies coccoliths, to determine relative rates of coccolithophore productivity. Oxygen isotopes (δ18O) of multiple planktic and benthic foraminifera provide age models, with the former also helping to identify upper water column stratification. Mg/Ca ratios in planktic foraminifera, Globigerinoides ruber, and Random Forest modelling of planktic foraminifera assemblages have been used to derive paleo-temperature estimates. These methods, coupled with trace element data from G. ruber as a productivity proxy, foraminifera assemblages, data on solar insolation and scanning electron microscope images, collectively determine the oceanic conditions at the time of coccolithophore blooms at each core site. The results suggest that no one factor was responsible for blooming, rather it was the combination, and interactions between different environmental processes, that were important. At P71, key factors for bloom formation in MIS 7a were high insolation, thermal stratification of the uppermost ocean, and well-mixed source waters from the Tasman Inflow. At ODP 1120, blooms in MIS 5e resulted from decreased windiness, warmer sea surface temperatures and reduced oceanic circulation over the Campbell Plateau, resulting in marked thermal stratification. It is likely that coccolithophore blooms further enhanced stratification at each core site, and restricted productivity further down the water column. At P71, modern oceanic trends suggest that conditions that caused blooms during MIS 7a will not be met in the near future, and blooming is unlikely to increase at this core site. At ODP 1120, modern trends are less clear, but future conditions are projected to be comparable to MIS 5e, suggesting that coccolithophore blooming may increase in the future in subantarctic waters.</p>