Résumé
Paleoecological study of the historical period in coastal areas is often hampered by the limitation of constructing reliable chronologies in recent sedimentary records. The coupling of radionuclide dating ( (super 210) Pb, (super 239+240) Pu) with stratigraphic changes of paleoecological markers indicative of land-usage can be an effective alternative to radiocarbon dating to build historic chronologies. This approach has been especially useful in North American coastal areas, which have been heavily impacted by historical human activities. We present results drawn from four sedimentary records studied along the California coast in the Point Reyes Peninsula north of San Francisco and from Santa Barbara County. We employed a varied suite of independent paleoecological proxies including pollen, non-pollen palynomorphs (NPP), sedimentary charcoal analyses, and spheroidal carbonaceous particles (SCP) to identify historically documented land-use events spanning the last approximately 250 yr. Coupling this novel combination of proxies with (super 239+240) Pu dating and historical accounts we generated robust chronologies which included distinct dating horizons such as: 1) the introduction of agriculture following the Spanish settlement along the coast in the late 18th century, identified by means of pollen and NPP markers of exotic plants, crops and grazing; 2) the 1793 Spanish fire suppression banning traditional Indian burning in Alta California, evidenced in the decline in sedimentary charcoal; and 3) the post-1850 introduction of distinct exotic plants -i.e. blue gum (Eucalyptus) trees- and of new land-uses following the American settlement. Following this we could track the development and expansion of the oil industry in southern California by coupling SCPs profiles with regional and local data-sets of oil production. This combination of proxies has been particularly useful in coastal California, while other suites of proxies might be employed in other areas as well.