Late Holocene climate changes in the Northwest Iberian Peninsula reflect fluctuations in the relative dominance of Atlantic and Mediterranean climate zones

Abstract

The northwestern Iberian Peninsula lies at the ecotonal transition between the Atlantic and Mediterranean bioclimatic regions, and mountain lakes in this region are sensitive to climate variability, preserving sedimentary records that are useful for studying past climate dynamics. In this paper, we reconstruct late Holocene climate through a high-resolution multiproxy analysis of a sediment core from Lake Ocelo, a high elevation lake. Analysis of sedimentological, geochemical, and biological proxies reveal regional hydroclimatic variability over the past three millennia, and allow the identification of climate phases that reflect the complex interplay between Atlantic and Mediterranean influences. The Subatlantic Period (c. 890–200 BCE) marks a transition to wetter conditions after the 2.8 ka climate event. The Roman Warm Period (c. 200 BCE–300 CE) shows increased aridity, suggesting an expansion of Mediterranean influence, and the wetter Dark Ages Period (c. 300–750 CE) indicates renewed Atlantic dominance. The drier Medieval Climate Anomaly (c. 750–1100 CE) reflects greater Mediterranean influence. The transition from the Medieval Climate Anomaly to the Little Ice Age (c. 1100–1300 CE), defined as a distinct climatic phase, marks a shift to wetter conditions. The Little Ice Age (c. 1300–1900 CE) exhibits alternating wet-dry subphases and prolonged ice cover, leading to stronger denitrification under a stronger regional Atlantic influence. The Industrial Era (c. 1850 CE–present) is characterized by a warming trend, increased lake productivity, and reduced terrigenous input. These findings highlight the sensitivity of the Atlantic–Mediterranean ecotonal boundary to climate shifts and provide insights for predicting future dynamics, including potential “Mediterraneanization” of the Atlantic zone.