High-frequency Quaternary climate changes: a key to understanding present and future coastal scenarios

This thesis provides an integrated, multi-archive reconstruction of coastal sedimentary sequences in Sardinia, spanning the Middle–Late Pleistocene to historical timescales, and addressing the gap between long-term sea-level trends and high-frequency coastal responses. In particular, it explores how coastal sequences respond to interactions among tectonics, climate, and eustatic fluctuations over timescales ranging from hundreds to thousands of years, focusing on successions from different areas of Sardinia (southern, western, and northwestern sectors). The coastal successions of southern Sardinia record repeated phases of marine transgression, shoreline progradation, and regression within unconformity-bounded units correlated with MIS 7, MIS 5e, and MIS 5c highstands. The study sites display contrasting stacking patterns, with composite terrace sequences preserving either MIS 7–MIS 5e or MIS 5e–MIS 5c marine units. These results indicate that composite marine terraces are a recurrent feature along the southern Sardinian coast. Moreover, the distribution of marine terrace sequences cannot be explained solely by glacio-eustatic sea-level fluctuations; instead, it reflects dominant tectonic control. The data support segmentation of southern Sardinia into crustal blocks affected by spatially and temporally variable uplift since at least the Middle–Late Pleistocene, where even modest uplift generates complex depositional architectures characterized by unconformities and re-occupation processes rather than simple staircase sequences. These sequences therefore record the interplay among variable uplift rates, climate change, and eustatic fluctuations at Milankovitch-frequency cyclicities (e.g., ~100 and ~20 ka). In this context, composite marine terraces, when integrated with stratigraphic and chronological data, enable reconstruction of sea-level–tectonic interactions and reveal subtle tectonic signals in regions traditionally considered stable. The combined effects of regional-scale tectonic activity and Holocene sea-level rise governed the infill of the Sal’e Porcus coastal pond on the Sinis Peninsula (western Sardinia). In particular, the alternation of back-barrier, lagoonal, and coastal-pond deposits developed over the last ~6 ka in response to this variability, likely linked to high-frequency climatic variability (e.g., Roman Warm Period, Medieval Climate Anomaly, Little Ice Age). Finally, saline surface crusts at Sal’e Porcus are currently reworked by seasonal processes, dissolving during runoff events and precipitating during the dry summer period. Intertidal algal rims at Capo Caccia record sub-centennial relative sea-level fluctuations over the last millennium, characterized by high-frequency, low-amplitude (decimetre-scale) oscillations linked to climatic variability. Four main phases of sea-level rise promoted rim growth, separated by three drops that caused subaerial exposure. Relative sea level fell by ≥ 0.5 m until ~1600 AD, followed by renewed rises (~1600–1630 and ~1701–1740 AD), interrupted by minor drops (e.g., Maunder Minimum) and a final decrease during the Dalton Minimum. Since ~1860 AD, sea level has risen, with acceleration in the late 20th century driven by anthropogenic warming, leading to partial drowning of the rims and replacement of Lithophyllum byssoides by subtidal coralline algae. The integration of these depositional archives demonstrates how climatic processes influence coastal evolution across different timescales and how larger-scale processes, such as tectonics, can complicate the stratigraphic record. Future projections of ongoing global warming and sea-level rise indicate significant socio-economic impacts, particularly for populations in low-lying coastal areas (e.g., Calvin et al., 2023). Placing current trends in the context of high-resolution past variability is therefore essential for informing effective adaptation strategies.