Calibrating Benthic Individual Foraminiferal Analyses (BIFA): Bottom Water Variability or Bioturbation Proxy?

Abstract

Previous studies have investigated the usage of individual foraminiferal analyses (IFA) on planktic species to examine sub-decadal variability of surface and thermocline depth waters. However, fewer studies have focused on IFA using benthic foraminiferal species. To investigate controls on benthic individual foraminiferal analyses (BIFA), we measured δ¹⁸O and δ¹³C in individuals of the epifaunal species Planulina wuellerstorfi (n≥20) and infaunal species Uvigerina peregrina (n≥20) across various core-top sediments taken from sites (n=12) across the Atlantic (n=6), Pacific (n=4), and Indian (n=2) Ocean basins. We hypothesize that the sub-annual calcification of benthic individuals can facilitate reconstructions of sub-annual advection variability of different abyssal water masses mixing over a given site. Accordingly, inter-individual δ¹⁸O and δ¹³C BIFA variability should capture the variance of distinct water mass intrusions to a given site within the decades to millennia represented by a sedimentary horizon. In the northern Atlantic Ocean, where bottom water variability and sedimentation rates are higher than in the Pacific, core-top means of BIFA δ¹⁸O values agree reasonably well with forward-modeled reanalysis datasets of bottom water hydrography, albeit with more considerable variability (1σ = 0.045‰ expected versus 1σ = 0.11‰ measured). Alternatively, we find that the BIFA signal is more challenging to interpret in the northern Pacific, where we observe δ¹⁸O with bimodal distributions and ranges far exceeding estimates from reanalysis ocean datasets (1σ = ~0.005‰ expected versus 1σ = 0.63‰ measured). We attribute this larger variability to the combination of bioturbation and low accumulation rates mixing ancient and modern individuals within core-top sediments. Overall, our results support that BIFA can accurately track bottom water changes in regions with relatively high accumulation rates. However, we posit that BIFA statistics could also provide key insights into bottom-water oxygenation (or lack thereof), given their propensity to be strongly modulated by bioturbated individuals in oxygenated sediments.