Recent global warming induces the coupling of dissimilar long-term sedimentary signatures in two adjacent volcanic lakes (Azores Archipelago, Portugal)

Paleoclimatological information derived from the study of lacustrine sedimentary records is not only biased by taphonomical processes but also by potential differences in the expression of climate variability in the sediments due to site-specific factors. Using a multiproxy approach (the elemental and isotopic compositions of organic matter, diatom assemblages, and marker pigments of algae and cyanobacteria), we study the different environmental signatures recorded since the Little Ice Age (LIA) in the sediments of two volcanic lakes located within the same caldera on São Miguel Island (Azores Archipelago). Lake Santiago is a crater lake whose eutrophic status in the last stage of the LIA was linked to external nutrient inputs associated with this humid period. Its post-LIA evolution was forced by changes in the thermal structure of the water, which determined its degree of mixing and therefore nutrient availability through recycling from the hypolimnion. In contrast, the decadal to centennial limnological evolution of Lake Azul, a caldera lake 2.5 km from Lake Santiago, shows geochemical and micropaleontological signatures disconnected from climate variability until 1980/1990 CE due to its greater exposure to the fallout of tephra after a catastrophic eruption in c. 1290 CE. Only after 1980/1990 CE did a global warming scenario induce a common ecological restructuring of both lakes, involving the replacement of turbulence-loving algal taxa by species adapted to strengthening water column stratification. Nevertheless, this shift was relatively gradual in Lake Azul but more sudden in Lake Santiago, indicating that the local site-specific components still had an effect on the expression of climate change in the sediments. Despite the short history of anthropogenic pressure (compared to their continental counterparts) and the large atmospheric patterns operating over the Azores Archipelago, the sedimentary records of these two adjacent oceanic volcanic lakes reacted quite differently to climate changes.