Sources of Element/Ca Variability in Foraminifera: From Genesis to Diagenesis to Signal Extraction

Abstract

Foraminifera are unicellular marine organisms that build calcium carbonate shells, or tests, dominantly made of the polymorph calcite, incorporating different ions into the test lattice based on ambient conditions. As such, foraminiferal Element-to-calcium (El/Ca) ratios from the sedimentary record are often invoked to explain past environmental changes associated with climate, ocean circulation and evolving biogeochemical cycles. In this dissertation, I explore the impact of calcification, diagenesis, and signal extraction methods on El/Ca ratios in foraminifera, principally focusing on the iodine to calcium (I/Ca) ratio, which is often used as an indicator of oceanic dissolved oxygen (DO) concentrations in the past. Paleo-reconstructions using El/Ca ratios in foraminiferal tests are fundamentally connected to the conditions under which unique elements become incorporated within the calcite crystal and the contaminants that adhere to the tests. Chapter 2 explores the thermodynamics of coupled monovalent iodate (IO3-) and iodide (I-) ion incorporation into calcite alongside alkali partner cations Li+, Na+, and K+ to determine the likely incorporation mechanism of trace monovalent ion impurities and the identity of the iodic ion embedded in the mineral host. The method by which El/Ca signals are extracted from foraminiferal samples and the pretreatment procedures used have wide ranging impacts on measured El/Ca, and therefore paleoclimate interpretations. In Chapter 3, we present a high precision method for the solution-based analysis of thirteen elements (Li, Na, Mg, Al, Mn, Fe, Zn, Sr, Cd, I, Ba, U, and Ca) on a collision-reaction cell equipped quadrupole ICP-MS in the acidic pH range, enabling measurement of iodine alongside other elements of paleoclimate significance. Using the method established in Chapter 3, in Chapter 4 we present the impact of sample pretreatment on measured El/Ca ratios in four populations of foraminifera from endmember depositional environments. Most of the observed El/Ca variability results from the foraminiferal depositional environment and differences in seawater and porewater chemistry, while the choice of sample pretreatment, or ‘cleaning’ procedure prior to analysis is the second largest driver of El/Ca variability. These results demonstrate the need for careful selection of appropriate cleaning procedures for samples from different oceanographic and sedimentary environments. Distinct depositional environments are subject to unique contaminants, which impact foraminiferal El/Ca and may bias paleoclimate interpretations. Finally, in Chapter 5, we leverage existing records of paleo-redox and paleo-productivity change in Santa Barbara Basin, California, to assess the relationship between planktic (surface dwelling) and benthic (bottom dwelling) foraminifera El/Ca ratios (Mn/Ca, Fe/Ca, Cd/Ca, I/Ca, Ba/Ca, and U/Ca) with bulk sediment proxies during the Common Era (CE). The California margin oxygen minimum zone (OMZ) is highly sensitive to climate, expanding and contracting in response to changes in marine productivity (via increased rates of organic matter oxidation) and ocean circulation. Our paleo-reconstruction targets the last millennium (~900 – 1900 CE), during which there were known variations in California Margin Oxygen Minimum Zone extent/ventilation mechanisms, as well as changes in productivity. Foraminiferal Cd/Ca ratios corroborate previously established periods of increased coastal upwelling, while Ba/Ca ratios support the use of Ba as an indicator of freshwater flux into coastal ocean environments. Mn/Ca ratios, however, are inconsistent with expectation, with little difference between planktics and benthics despite the strong oxycline in SBB, while U/Ca and Fe/Ca appear to record redox change. Additionally, I/Ca ratios are most strongly correlated with bulk sedimentary productivity indicators, not redox, counter to expectation.