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Aerosol radiative forcing is an important but often poorly understood component of regional climate. While glacier ice contains the most detailed archives of past atmospheric aerosol composition and temperature, no well-preserved ice records extending into the last climatic transition have been reported for the historically important European region. Here, we use an Alpine ice core to document changes in European aerosols and climate from the end of the last glacial age (LGA) through the Holocene. The core was drilled on a glacier dome in the French Alps called the Dôme du Goûter (DDG), and it provides a stratigraphically intact record of aerosol and climate extending to at least 12 kyears (ky) before present. Although dating near the base of the glacier is not well constrained, the oldest DDG ice layers reflect glacial conditions in western Europe during the LGA. In addition to changes in atmospheric transport, increased sea-salt and dust deposition in western Europe recorded in the LGA ice suggest enhanced westerly winds and more active dust sources, possibly including North Africa. Deposition of terrestrial biogenic indicators during the cold LGA climate was lower, however, consistent with strongly reduced European vegetation. The DDG record of terrestrial biogenic emissions also suggests a decline of European forests throughout the Holocene, resulting from deterioration of climatic conditions and more recently from establishment of the first agricultural societies. The pronounced changes in atmospheric aerosol recorded in Alpine ice imply large variations in aerosol radiative forcing in western Europe during the last 12 ky. 

Abstract Restoring wetlands will reduce nitrogen contamination from excess fertilization but estimates of the efficacy of the strategy vary widely. The intervention is often described as effective for reducing nitrogen export from watersheds to mediate bottom-level hypoxia threatening marine ecosystems. Other research points to the necessity of applying a suite of interventions, including wetland restoration to mitigate meaningful quantities of nitrogen export. Here, we use process-based physical modeling to evaluate the effects of two hypothetical, but plausible large-scale wetland restoration programs intended to reduce nutrient export to the Gulf of Mexico. We show that full adoption of the two programs currently in place can meet as little as 10% to as much as 60% of nutrient reduction targets to reduce the Gulf of Mexico dead zone. These reductions are lower than prior estimates for three reasons. First, net storage of leachate in the subsurface precludes interception and thereby dampens the percent decline in nitrogen export caused by the policy. Unlike previous studies, we first constrained riverine fluxes to match observed fluxes throughout the basin. Second, the locations of many restorable lands are geographically disconnected from heavily fertilized croplands, limiting interception of runoff. Third, daily resolution of the model simulations captured the seasonal and stormflow dynamics that inhibit wetland nutrient removal because peak wetland effectiveness does not coincide with the timing of nutrient inputs. To improve the health of the Gulf of Mexico efforts to eliminate excess nutrient, loading should be implemented beyond the field-margin wetland strategies investigated here.