A Late Quaternary history of interior ocean circulation and ventilation from the Indian Sector of the Southern Ocean
Creators: name: Glaubke, Ryan Hunter
Publisher: name: No Publisher Supplied
Year: 2024

description: The Southern Ocean is an important player in Earth’s late quaternary climate history due in part to its role in mediating the exchange of carbon dioxide between the atmosphere and deep ocean on glacial-interglacial timescales. Reconstructions from the Atlantic and Pacific sectors of the Southern Ocean reveal that changes in ocean circulation and deep-sea ventilation at the end of the Last Ice Age were not synchronous across the Southern Ocean, emphasizing the importance of regional dynamics on deglacial climate change. Knowledge of circulation and ventilation changes in the Indian sector is limited by comparison, owing to the difficulty of recovering sediment cores with sufficient temporal resolution and spatial coverage in the region. Resolving this history is crucial to developing a unified theory of Southern Ocean dynamics as Earth transitioned out of the Last Ice Age. The central aim of this dissertation is to bring modern and paleo perspectives to bear on the circulation and ventilation history of the interior Indian Ocean. The first half of the dissertation (Chapters 2 and 3) describes the hydrography and stable isotope geochemistry of the modern Indian Ocean and refines proxy relationships for salinity and temperature used to reconstruct past ocean conditions in later chapters. The second half (Chapters 4 and 5) presents new high-resolution down-core records of circulation and ventilation change from upper and lower cells of the Indian sector’s overturning circulation. Each chapter makes extensive use of new sediment and seawater samples collected from the southeast Indian Ocean during the Coring to Reconstruct Ocean Circulation and Carbon dioxide across 2 Seas (CROCCA-2S) expedition in 2018. Chapter 2 combines 234 new seawater samples from the CROCCA-2S study region with historical data to present a broad-scale hydrographic survey of the south Indian Ocean. Temperature, salinity, and stable oxygen isotope data (δ18O) suggest the south Indian Ocean west of ~85°E is strongly influenced by eddy turbulence induced by the southward-flowing Leeuwin Current. The resulting mixing between subtropical and subantarctic waters yields a regional δ18O – salinity relationship distinct from the one dictated by hydroclimatic processes throughout the rest of the Indian subtropics, carrying important implications for paleosalinity reconstructions in the region. Chapter 3 presents a newly compiled database of paired foraminiferal Mg/Ca-δ18O observations from 115 Indian Ocean surface sediment samples to develop regional calibrations of the magnesium-to-calcium (Mg/Ca) temperature proxy and explore its sensitivity to salinity and carbonate chemistry in four planktic species. Multivariate regression and Bayesian factor analysis suggest temperature-only calibrations offer sufficiently robust parameterizations to reconstruct past temperatures with surface- dwelling species Globigerinoides ruber and Globigerina bulloides, despite some sensitivity to non-thermal parameters. Data from subsurface-dwelling Globorotalia inflata and Globorotalia truncatulinoides were restricted to the well-mixed water column in thesoutheast Indian Ocean, and therefore did not exhibit a broad enough dynamic range to yield robust calibrations. Chapter 4 features a reconstruction of the temperature and salinity history of Indian- sourced Subantarctic Mode Water (SAMW), an important shallow interior water mass that contributes substantially to the return flow of the global overturning circulation. Results from two foraminiferal species (G. bulloides and G. truncatulinoides) in core TT1811- 50GGC located within the region’s SAMW production center reveals an abrupt increase in salinity (~2 – 2.6 PSU) at the onset of deglacial warming (~20 ka). This high salinity event occurs synchronously (within error) with an increase in water mass age, according to radiocarbon analyses from the same core. These data capture the incorporation of salty, aged glacial bottom waters upwelled into SAMW from the Southern Ocean. Estimates of the salt load associated with this abyssal “salty blob” suggests it was a potentially important yet under-acknowledged source of negative buoyancy to the North Atlantic that could have helped reinitiate deep water production during the last deglaciation. Finally, Chapter 5 presents three new sub-millennially resolved records of deep-sea ventilation from the Indian sector of the Southern Ocean based on the stable carbon isotope composition (δ13C) of epifaunal benthic foraminifera. These new data are combined with 17 other high-resolution benthic records to develop composite δ13C stacks that reflect the underlying basin-wide ventilation signal for the Atlantic, Indian, and Pacific sectors of the Southern Ocean. A comparative analysis reveals distinctly different ventilation histories for each region. The onset of ventilation was time-transgressive, beginning in the Pacific (~20.9 ka) and moving to the Indian (~18.7 ka) and then the Atlantic (~15.3 ka). The earlier ventilation of the Indo-Pacific appears to be a dynamic response to changing southern hemisphere temperature, Antarctic sea ice, and southwesterly wind intensity, whereas the late ventilation of the Atlantic coincides with the resumption of deep water production in the North Atlantic. Together, this dissertation clarifies the paleoceanographic history of the southern Indian Ocean, underscores the global impact of localized dynamics in the region, and contributes to a more comprehensive view of Southern Ocean dynamics across the last major example of global climate change.