Search for: All records

Abstract The production and export of organic matter to deep‐sea sediments is a key driver in modulating glacial‐interglacial carbon cycles. Yet, it remains unsettled whether productivity has increased or decreased over glacial‐interglacial transitions, in part because productivity proxies may be complicated by sediment re‐deposition and diagenetic alterations. Here, we explore using non‐spinose foraminifera Ba/Ca ratios as a proxy for surface ocean productivity. We analyze foraminifera Ba/Ca ratios since the Last Glacial Maximum in cores that span a productivity gradient along the equatorial Pacific. Ba/Ca is low and invariable in the spinose speciesTrilobatus sacculifer. In contrast, Ba/Ca is higher and more variable in the non‐spinose speciesNeogloboquadrina dutertreiandPulleniatina obliquiloculata. Ba/Ca enrichment in non‐spinose species is hypothesized to be linked to the degradation of organic matter within the species' particulate microhabitat and reflects surface ocean particulate organic matter productivity at the time of shell calcification (Fehrenbacher et al., 2018,https://doi.org/10.1016/j.gca.2018.03.008). Ba/Ca in core‐top and sediment trap derived non‐spinose foraminifera correlate with organic matter productivity. We reconstruct an increase in non‐spinose species Ba/Ca during the deglacial in the western and eastern equatorial Pacific and suggest this may be linked to an increase in productivity, as observed in several other regional records. The 16–17 ka BP peak in non‐spinose foraminifera Ba/Ca is evident in specimens obtained from a deep ocean core and from regions that experience sediment focusing, suggesting the Ba/Ca proxy may be useful even in regions where samples are poorly preserved or complicated by sediment re‐deposition. 

The production of carbon and export to deep ocean sediments is linked to carbon partitioning between the ocean and atmosphere and is a key driver of climate change over the glacial-interglacial transition. Yet conflicting reconstructions create barriers to understanding changes to the carbon system over this important climate transition. Production reconstructions conflict in part because commonly used production proxies may be subject to water column and seafloor diagenetic alterations that overprint primary oceanographic signals. In addition, reconstructions of deep ocean carbonate chemistry are complicated by the variable ways that dissolution/preservation affects the proxy. This dissertation explores the utility of new proxies recorded in the shells of planktic foraminifera that have the potential to reconstruct parameters of the carbon system and can be carefully assessed for signs of diagenesis. Proxy developments and reconstructions are made using foraminifera from equatorial Pacific Ocean sediments that span a gradient in surface ocean carbon production and deep ocean carbon preservation. 

Plan language summary: Elements (including barium and magnesium) measured inside microscopic fossil shells are higher than expected and could reflect contamination or could reflect key information about the ancient surface ocean. For example, barium may reflect how much organic carbon is produced in the surface ocean, some of which is sunk to the deep ocean and thus removed from exchange with the atmosphere, effecting climate. Previous studies have attributed higher concentrations of these elements to contamination that forms on shells and have established harsh chemical cleaning techniques to remove the contamination. However, these cleaning techniques also remove parts of the shell that contain key information about past ocean conditions. We determine that high concentrations of these elements are not due to contamination in our samples and likely reflect conditions of the surface ocean. In future work, we suggest using techniques that measure at the microscale to confirm that there is no contamination and we suggest using more careful cleaning that preserves these elements. Preserving these elements in fossil shells may provide key information about organic carbon production and cycling in the surface ocean, which is critical to reconstruct carbon exchange between the atmosphere and ocean.