Late Holocene Climate Contrasts in NW Iberian Lakes

This study explores Late Holocene climate variability and environmental transitions in the NW Iberian Peninsula by analyzing organic biomarkers (n-alkanes) and their isotopic signatures (δ¹³C and δD) from two contrasting lake systems: Lake Ocelo, a mountain lake (1517 m a.s.l.) located at a crucial point between the Atlantic and Mediterranean bioclimatic regions, and Lake Doniños, a coastal back-barrier perched lake (2.5 m a.s.l.) within the sub-Atlantic climatic domain. Lakes at varying altitudes provide complementary paleoenvironmental records that capture diverse ecosystem responses to past climate changes across vertical gradients.

In both Lake Ocelo and Lake Doniños, δD values reflect hydrological variability. In Ocelo, long-chain n-alkanes indicate terrestrial vegetation as the main source of organic matter. δD values (~-190‰ to ~-160‰) reflect wetter and cooler conditions during the Older Subatlantic (OSA; ca. 800-200 BCE), the Dark Ages (DA; ca. 300-750 CE) and Little Ice Age (LIA; ca. 1300-1900 CE), contrasted with warmer and drier conditions during the Roman Warm Period (RWP; ca. 200 BCE-300 CE), Medieval Climate Anomaly (MCA; ca. 750-1100 CE) and the Industrial Era (IE; ca. 1850 CE-present). Similary, in Doniños, δD values during the MCA became more positive, suggesting drier conditions. The MCA-LIA transition (ca. 1100–1300 CE) in Ocelo shows a shifts to wetter and cooler conditions, with δD and δ¹³C values declining. In Doniños, between 1200 CE and 1585 CE, δD became more positive, suggesting episodic drying and marine influence, likely linked to increased storminess. Also, δ¹³C rose sharply between 1400 CE and 1550 CE, during the LIA, possibly indicating increased nutrient input associated with climatic fluctuations or marine incursions. After a hiatus spanning 1585-1700 CE, δD stabilized at its most positive levels, marking reduced hydrological variability and arid conditions. Post-1850 CE, δD and δ¹³C trended toward more negative levels, reflecting increased meteoric water input due to wetter climate conditions or anthropogenic watershed. Additionally, the lowering of δ¹³C values during the IE may also reflect the Suess effect from fossil fuel combustion.

In Ocelo, δ¹³C data align with δD trends, reflecting shifts in vegetation composition and water stress, with relative enrichment in δ¹³C during the RWP and MCA suggesting warmer and drier conditions, while more negative δ¹³C during the LIA and DA reflects cooler and wetter conditions.

These findings emphasize the utility of biomarkers in reconstructing regional climate variability and the contrasting responses of mountain and coastal lakes to Late Holocene transitions. Despite their geographical proximity (185 km), both lakes reflect different climatic influences: Lake Ocelo records broader fluctuations linked to its bioclimatic position, while Lake Doniños is influenced by local processes, including marine intrusions and anthropogenic impacts. Similarities include drier conditions during the RWP and MCA in both lakes, while differences emerge during the LIA, where marine influence and storminess played a larger role in Lake Doniños. 

This work is supported by Grants PID2019-107424RB-I00 and PID2022-139775OB funded by MCIN/AEI/10.13039/501100011033, with the latter also co-funded by “ERDF A way of making Europe”. Xunta de Galicia also supports this work through projects ED431F 2022/18 and ED431B 2024/03.