Reconstruction of CO2 oscillations through the study of marine fossil records during the Late Cenozoic: Boron isotopes in planktonic Foraminifera as a tracer for paleo-pH and pCO2.

Understanding the evolution of atmospheric CO2 over geological timescales is critical for assessing Earth's climate system dynamics and anticipating future changes. This thesis investigates atmospheric CO2 variability during the Late Cenozoic, with particular focus on the Pliocene and Pleistocene epochs, by reconstructing past pCO2 levels using boron isotopes (δ11B) analyses in planktonic foraminifera. This proxy serve as an indicator of paleo-pH and, consequently, atmospheric CO2. The study focuses on two climatically significant intervals: the Mid-Pleistocene Transition (MPT) and the Pliocene Epoch, including the Zanclean stage and mid-Pliocene Warm Period (mPWP), which are recognized as analogs for future climate scenarios. High-resolution δ11B records were generated from two distinct sites: ODP Site 668B in the equatorial Atlantic Ocean, covering the Pleistocene, and key outcrop sections in Sicily (Italy) encompassing the Zanclean and Piacenzian stages. Analytical work was carried out using state-of-the-art MC-ICPMS technology in ultra-clean laboratory environments, ensuring the precision required to detect subtle variations in seawater pH and CO2 levels. Results from ODP Site 668B offer a high-resolution reconstruction of atmospheric pCO2 across the Mid-Pleistocene Transition, revealing a significant long-term decline in glacial CO2 levels, approximately 23 µatm from the Early to Late Pleistocene. Notably, the study captures the "900-ka event", caracterized by a sharp drop in pCO2 (~82 µatm), which is associated with enhanced climate sensitivity and feedback mechanisms involving ice sheet dynamics and ocean circulation changes, such as a weakening of the thermohaline circulation (THC) and an increased intrusion of Antarctic Bottom Water into the Atlantic. In the Mediterranean Pliocene sections, δ11B and Mg/Ca analyses show relatively high atmospheric CO2 concentrations during the early Zanclean (~400-450 ppmv), consistent with a globally warmer conditions. A progressive cooling trend, marked by decreasing sea surface temperatures (SSTs) and pCO₂ levels, is evident toward the Late Pliocene (~3 Ma), culminating in values closer to 300-320 ppm. While these trends align with the onset of Northern Hemisphere glaciation, the persistently high pCO₂ values during the Zanclean suggest a strong influence of regional carbon cycle dynamics. In particular, the data indicate that obliquity-driven changes in primary productivity played a central role in modulating seawater CO₂: high obliquity phases enhanced productivity and CO2 uptake, making the Mediterranean a net CO2 sink, while low obliquity phases reduced productivity and favored CO2 outgassing. These findings underscore the Mediterranean's sensitivity to astronomical forcing and validate the δ11B proxy’s applicability to marginal marine settings and under varying diagenetic and post-diagenetic conditions. In addition to the geochemical reconstructions, this thesis presents a statistical analysis of the response of Mediterranean planktonic foraminiferal assemblages during the Pliocene to key environmental parameters reconstructed herein (temperature, salinity, pH, and pCO2). Salinity emerged as the primary driver of species richness, particularly during periods of hydrographic instability linked to freshwater inputs. In contrast, temperature and atmospheric pCO2 had a stronger influence on community evenness, modulating niche partitioning through their effects on water column stratification. Three distinct phases of community structure were identified: an initial phase of post-Messinian stability, a transitional phase with increasing environmental perturbations, and a final phase marked by a major biotic turnover coinciding with the intensification of Northern Hemisphere glaciation. These results emphasize the sensitivity of marine microplankton to climatic and hydrographic variability and provide valuable analogs for understanding biotic responses to current and future ocean changes.