Black Carbon Nanoparticles in Paleo-Records: A Combustion Proxy
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Emitted to the atmosphere through fire and fossil fuel combustion, refractory black carbon nanoparticles (rBC) impact the global climate, atmospheric chemistry, human health, and the carbon cycle. In the Southern Hemisphere (SH), rBC is transported in the atmosphere from low latitudes to Antarctica and deposited to the polar ice sheet, preserving a history of the variability of emissions and atmospheric transport. Here we present a total of eight records of rBC over the polar continent, covering a period ranging from the pre-industrial era to the modern era. These new records represent one of the first surveys of rBC deposition variability over Antarctica for that period of time. Firstly, two high-resolution rBC records from the West Antarctic Ice Sheet divide (WAIS) and Law Dome, on the periphery of the East Antarctic ice sheet, spanning calendar years 1850-2001, are investigated. Highly correlated over the past 60 years, the records show that coherent large-scale changes in rBC deposition to Antarctica occurred at decadal to inter-annual time scales, notably in ENSO-like periodicities. Since about 1940, the records show decadal trends similar to the inventories of SH rBC emissions from grass fires and biofuels. The two records suggest a large-scale reduction in rBC deposition from 1950 to 1990 to WAIS and Law Dome. In order to better understand the spatial and temporal variability in rBC deposition to the polar continent, those high resolution records are compared to six other rBC ice core records from the East-Antarctic Plateau. Located on the Dronning Maud Land, those additional records are characterized by a lower accumulation and thereby lower temporal resolution compared to the WAIS and Law Dome sites. Temporal and spatial analysis of rBC concentrations and fluxes in the East-Antarctic ice cores for the period 1800-2000 reveals high variability of concentrations, and a slightly increasing general trend. Some of the variability recorded at those sites is similar to the high resolution records and suggests a link with short and long-term variability of ENSO.Once emitted to the atmosphere, rBC particles also enter aquatic environments, where they may affect the fate of other pollutants. However, measurements of very low rBC-concentrated waters have almost never been investigated for that purpose. Here, a study determining rBC in waters of the Lake Tahoe watershed in the western United States from 2007 to 2009 are presented. The study period spanned a large fire within the Tahoe basin, seasonal snow-melt, and a number of storm events, which injected urban runoff into the Lake with rBC concentrations up to four orders of magnitude higher than background concentrations. The results show that rBC pulses from both the wildfire and urban runoff were rapidly attenuated within the lake, suggesting unexpected removal from the water column or aggregation to sizes outside of analysis detection capability. Those processes prevent rBC concentrations from building up in the clear and oligotrophic Lake Tahoe. Results obtained for the Tahoe study are compared to similar measurements performed in other oligotrophic lakes, and interpreted to characterize rBC transfer to sediments. Additionally, we measure rBC concentrations in a sediment core from Lake Tahoe, to evaluate the potential of this archive as a combustion record. Despite loss of rBC particles observed in the water column, rBC is transferred to sediments which preserve a local-to-regional scale history of its emissions as revealed by comparison with other pollutant records.