Shell Reworking Impacts on Climate Variability Reconstructions Using Individual Foraminiferal Analyses

Particle mixing by benthic fauna beneath the sediment-water interface (or bioturbation) fundamentally challenges the proxy based retrieval of past climatic conditions from deep-sea sediment cores. Previous efforts targeted the impacts of bioturbation on the nature of paleoceanographic changes gleaned from the proxy record, whereas impacts on seasonal and/or interannual variability reconstructions have received less attention. We present TurbIFA (Tracking uncertainty of reworking & bioturbation on IFA), a software that adapts and combines existing algorithms to quantitatively estimate the impact of sediment reworking and other uncertainties and assess significance of ocean and climate variability reconstructions based on individual foraminiferal analyses (IFA). Building upon previous idealized investigations of bioturbation using hydroclimate-sediment simulations, TurbIFA advances the IFA proxy system modeling such that users may directly assess the sensitivity of their data to various local parameters related to shell reworking across the global ocean. Using the output of state-of-the-art coupled atmosphere-ocean general circulation models, TurbIFA simulates planktic foraminiferal delta 18O or Mg/Ca-temperature signal carriers and evaluates uncertainties in the sample size, analytical protocols along with as those arising from bioturbation. Application of TurbIFA to synthetic and existing data sets indicates that the significance of IFA-based reconstructions can be assessed once the impacts of sediment accumulation rates, sediment mixed layer depths, length of time integrated by the chosen IFA sampling interval, and changes in the amplitude of climate variability (i.e., the targeted environmental signal) are comprehensively evaluated. We contend that TurbIFA can aid quantitative assessments of past seasonal and interannual variability gleaned from the paleoceanographic record. Planktic foraminifera are marine zooplankton with a life cycle of approximately a month. They dwell in the upper ocean, where they precipitate calcium carbonate shells that, postmortem, settle through the water column and become a main constituent of deep-ocean sediments. The geochemical composition of individual planktic foraminiferal shells provides snapshots of past surface ocean temperatures during their lifespan. Statistical distributions of these data inform on the past seasonal and/or interannual climate variability, but post-depositional processes may bias these reconstructions. Here we quantify the effects of the vertical displacement of foraminiferal shells in sediments by benthic microfauna, which contaminates sediment core samples with older shells deposited under colder or warmer climates. The code routines are available in an open-source repository. The final user can define their study-specific variables as inputs, thereby allowing the application of this method to a broad spectrum of climate and sedimentary settings. A new software (TurbIFA) quantifies the uncertainties in IFA data sets tied to sampling, analytical errors, and bioturbation TurbIFA assesses the sensitivity of IFA reconstructions to bioturbation parameters and propagates analytical and sampling uncertainties Application to existing data sets showcases its ability to assess statistical significance of IFA-based climate variability reconstructions