Tracking Hydroclimate Changes and Human Impacts over the Last Millennium in Lake Sediment Records from Flores Island, the Azores

Abstract:

Since the Portuguese settlement of the Azores Archipelago in the 15^th century humans have extensively modified the Azorean landscape, with exotic plants dominating the present-day vegetation and eutrophication in numerous lakes. However, it is uncertain whether hydroclimate variability contributed to changes in terrestrial and aquatic ecosystems and helped facilitate the arrival of humans in the Azores. To assess the role of climate in these processes, we developed complementary paleoecological and paleoclimate records from Lake Funda on Flores Island that span the last millennium. Low variability in precipitation amount, as recorded by leaf wax hydrogen isotopes (δD_wax) from C₃₀ fatty acids, suggests that the hydroclimate on Flores Island was relatively stable between c. 1000-1350 CE and drier between c. 1350-1450 CE when Portuguese explorers arrived in Flores Island (1452 CE). More depleted δD_wax values between c. 1450-1650 CE suggest wetter climate conditions may have contributed to the slow establishment of permanent settlements on Flores Island. Major changes in n-alkane average chain lengths (ACL_27-33) and 5β-stigmastanol indicate that major shifts in vegetation composition and the widespread introduction of livestock, respectively, took place over c. 100-years between c. 1500-1600 CE. Landscape changes were followed by a decrease in δ¹⁵N as N₂-fixation by cyanobacteria increased in Lake Funda. Around the same time, increases in the ratio of 5α(H)-stanols/Δ⁵-sterols suggest that the lake bottom water was more frequently experiencing reducing, i.e. hypoxic, conditions, thereby enhancing in-lake phosphorous cycling and increasing the lake trophic state. Evidence of eutrophication in Lake Funda persisted during the Little Ice Age despite increased variability in δD_wax between c. 1650-1850 CE, highlighting continued human impacts on the lake ecosystem. Reforestation efforts on Flores Island and a drier climate in the 20^th century most likely reduced nutrient leaching and soil erosion in the catchment area of Lake Funda, yet eutrophication continues to be a problem. This research is funded by through the research projects PaleoModes (CGL2016-75281-C2) and DISCOVERAZORES (PTDC/CTA-AMB/28511/2017) and the Luso-American Foundation.