A 55,000 Year History of California Plant Community and Ecological Response to Climate

Plant macrofossils can be powerful proxies for environmental change, but their preservation in the fossil record does not always allow for long-term reconstructions. The La Brea Tar Pits are unique not just for the sheer abundance of fossils they preserve from the Late Pleistocene and Holocene, but also for the particular taphonomic conditions that allow for continuous preservation of an ecologically diverse fossil assemblage across a 55 ka timespan (Chester Stock, 1992). This proves especially valuable in the Mediterranean region of coastal Southern California, where long-term terrestrial records are spatially scarce (Glover et al., 2020; Kirby et al., 2018). Hydrocarbon preservation of cellulose and lignin allows for the preservation of plant macrofossils (fossils visible with the naked-eye and identifiable using low-magnification microscopy) which have the potential to provide species specific data on plant presence through time (S. T. Jackson et al., 1997). Rich ecological data through the Late Pleistocene and Holocene provides much needed information on how plant communities were responding during a climatically active time of glacial growth and decline, and millennial-scale shifts in atmospheric and sea surface temperatures (SST), as well as important environmental context for development and the extinction of Ice Age megafauna and migration of human populations (Barnosky et al., 2017; Meltzer, 2020).This dissertation explores the viability of La Brea Tar Pit plant macrofossils as environmental and plant community proxies. Through active seep observation and exploration of taphonomic processes of plant deposition in modern hydrocarbon seeps, multi-method approaches to identification, and direct accelerator mass spectrometry (AMS) radiocarbon dating of preserved plant material, this project constructs a high taxonomic resolution, 55 ka long, record of vegetation change for the Los Angeles Basin. This dissertation finds plant material deposition in asphalt seeps to be highly localized and representative of flora fruiting in warmer months. Through the La Brea Tar Pits vegetation record we have found species response in the Los Angeles Basin to be sensitive to climatic changes driven by orbital forcings, and potentially variations in the Northeast Pacific and North Atlantic sea surface temperatures. Substantial changes in the flora and vegetation cover are recorded in the LBTP record and some of these shifts are contemporaneous with local extinction of mega-fauna both during the transition from the Pleistocene to the Holocene (Marine Isotope Stages MIS2 to MIS1 - ~12 ka) and in the earlier Pleistocene. The disappearance of large-bodied herbivores at the start of the Holocene, coupled with increased fires, possible increasing in frequency due to the arrival of humans, potentially also played a role in the vegetation changes observed, most notably the extirpation of juniper from the Los Angeles Basin. This local-scale record of environmental shifts from the Late Pleistocene into the Holocene at the LBTP’s provides a dynamic baseline of plant species presence for future environmental and ecological studies of paleo-coastal Southern California, and possibly assessing the sensitivity of current vegetation to ongoing and future climatic changes