California’s La Brea Tar Pits is one of the richest Ice Age fossil localities in the world and offers unmatched potential for reconstructing environmental conditions and ecological relationships in the Late Pleistocene. Plant macrofossils are abundant at the site, but their accurate interpretation requires a better understanding of the processes behind their deposition in hydrocarbon seeps and potential spatial resolution, a topic not yet addressed in asphalt pit related research. In this study we monitor temperature, phenology, and seep activity of two hydrocarbon seeps in southern California over the span of a year. For each seep we surveyed surrounding vegetation within 10 meters and sampled seep surfaces for analysis of trapped plant material. We find hydrocarbon seeps to be highly localized with 77–91% of plant material originating from species found within five meters of the seep. Temperature and time-lapse imagery indicate a greater likelihood of transportation of plant material by wind or animal than by water in these particular seeps. Animal transport through fecal matter or entrapment was the most likely candidate for the species Sambucus mexicana to be transported outside the immediate vicinity of the monitored seeps. Variation in observed viscosity of liquid asphalt correlated with seasonal temperature changes, with higher temperatures coinciding with decreased viscosity and increased seep flow. We find observations of seasonal seep flow connected to a warm season bias of plant material, making phenology an important consideration in the interpretation of plant presence and absence in asphaltic fossil contexts.

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