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The term Anthropocene has been suggested to describe the epoch in which changes in Earth systems can be clearly attributable to human activities. The oasis ecosystems of the Baja California Peninsula provide a good example of the transformations that human activities can produce in an ecosystem over centuries of use. These sites, which are located in the northwestern desert of Mexico, are a refuge for a great diversity of species. At the same time, they are important for human subsistence, which has exerted pressure on the water resources of these ecosystems since humans settled in these locations following the arrival on the American continent. To reconstruct the changes in hydrological conditions in these ecosystems and their relationship with climatic patterns and human activities during the last century, we measured delta O-18 isotopes in the stem cellulose of the long-lived palm Washingtonia robusta in three Baja California Sur oases. Samples were collected at different heights from the base to the top (i.e., from the oldest part of the stem to the youngest) of each palm and from different water sources. The oxygen isotope values of W. robusta appear to be influenced mainly by intense climatic anomalies. Some El Nino Southern Oscillation (ENSO) events were recorded in the isotope signals of the palms, such as La Nina drought event that occurred between 1942 and 1957 and caused an evident increase in the delta O-18 of the palms of the San Ignacio oasis. However, other ENSO events did not produce isotope changes in the palms.

We propose that the hydrological characteristics of the oases, as well as agricultural irrigation, likely maintained constant moisture conditions that caused the observed stability of the O-18/O-16 isotope ratios in the palms and limited the evidence of climatic anomalies that these palms provide. Although W. robusta can serve as a reasonable proxy for climate reconstructions, improvements in our understanding are needed; e.g., more accurate methods for estimating the ages of palms and more knowledge of the effects of other factors related to the hydrological cycles of oases.