Stable oxygen isotope measurements (δ18O) are important tools in studies of climate, oceanography, and hydrology of today and the past. δ18O in fossil marine carbonates (δ18Ocarb) is used to reconstruct past ocean temperatures, and seawater δ18O (δ18Ow) tracks with salinity and is thus used to interpret hydrological variability. However, δ18Ocarb-temperature estimates rely on assumptions of how δ18Ow varied in the past, which remains uncertain. ‘Clumped’ isotope thermometry (Δ47) measurements are independent of δ18Ow, allowing us to circumvent this uncertainty and to simultaneously reconstruct past ocean temperatures and δ18Ow when combined with δ18Ocarb. This dissertation explores innovations in the utility of these isotopic tools. First, we investigated the viability of Δ47 paleothermometry in marine gastropods - an archive largely unexplored for Δ47-paleotemperature reconstructions (Chapters 2 & 3). Second, we explored the utility of δ18Ow and Δ47-derived paleo-δ18Ow as tracers of ocean currents (Chapters 4 & 5) and ice sheet melt (Chapter 6).

Chapter 2 documents Δ47-T’s from a suite of modern marine gastropods collected from localities spanning the globe. Our findings suggest that many gastropods precipitate in isotopic equilibrium and accurately record environmental temperatures. However, some gastropods - notably Caviturritella/Turritella (turritellids) and Campanile - appear to precipitate out of equilibrium (“vital effects”) and thus exhibit Δ47-T’s cooler than actual environmental temperatures.

Chapter 3 expands on the finding from Chapter 2 that turritellids appear to exhibit a vital effect-induced cold bias of ~6°C, reporting Δ47-temperatures from 2 modern and 7 fossil (Mid-Miocene, ~18-14 Ma) turritellids from Colombia. The Δ47 data from the modern turritellids corroborate the previous finding of a ~6°C cold bias. After a +6°C adjustment is applied to Mid-Miocene Δ47-temperatures, adjusted Δ47-temperatures align with previous paleotemperature estimates in the region from other studies, highlighting the potential of vital effect-based corrections.

Chapter 4 explores the utility of δ18Ow as a tracer for the Labrador and Gulf Stream boundary currents in the North Atlantic. We collected 358 new salinity and δ18Ow measurements along the Eastern Seaboard, doubling the number of existing data in the region. Our paired salinity and δ18Ow data show distinct signatures for the Labrador Current and Gulf Stream roughly separated at Cape Hatteras, North Carolina, creating the framework for using δ18Ow as a tracer of these currents.

Chapter 5 expands on the findings from Chapter 4, as we discuss the ability of Δ47-derived paleo-δ18Ow to track the Labrador Current and Gulf Stream in the past. We measured fossil quahogs (Mercenaria sp., ~130-115 ka and ~80 ka) from localities along the Eastern Seaboard and found that, despite temporal differences, paleo-δ18Ow latitudinal trends generally align with the modern δ18Ow latitudinal gradient. Thus, these findings showcase the potential of Δ47-derived paleo-δ18Ow to track water masses from the past.

Chapter 6 builds on the framework of using Δ47-derived paleo-δ18Ow to interpret paleohydrological variability, as we infer substantial ice sheet melt at Nantucket, Massachusetts during the Last Interglacial (~130-115 ka). We measured 7 Mercenaria across 2 units at this locality and observed paleo-δ18Ow values significantly depleted relative to modern values, which is consistent with substantial freshwater input (i.e., ice sheet melt).

Altogether, this dissertation builds on the existing framework of Δ47 and δ18Ow as paleoclimatic and paleoceanographic tools by investigating the viability of marine gastropods for Δ47-paleotemperature reconstructions and exploring Δ47-derived paleo-δ18Ow as a tracer of ocean circulation and ice sheet melt in the past.