Preliminary analyses of Tar Pit insects provide evidence for more aridity and warmth than previously proposed for the transition from late Ice Age to modern times.
Establishing Paleoclimate as a Means to Understand Current Climate Change
Scientists from the American Museum of Natural History, La Brea Tar Pits and Museum, UC Berkeley, UC, Irvine, and the California Academy of Sciences (CAS) are joining forces in an effort to use information from fossil insects to help establish what the Ice Age climate was like in southern California. This project is intended to help us answer questions such as: “To what degree can we predict the possible future climate of California from analogous, prehistoric data?” and “How have the flora and fauna responded to climatic changes over time?”
In California, we are currently experiencing one of the worst prolonged droughts in our recorded history (National Climate Assessment). While anthropogenic climate change may be one force driving the current episode, paleoenvironmental records indicate that this drought is not unique in the history of the region.
Although climate change impacts in southern California are well documented, our records only go back a century and a half at most. Deeper paleoenvironmental patterns must be reconstructed using other evidence. The Rancho La Brea (RLB) Tar Pits in southern California offers a wealth of fossils that can shed light on how the local environment has changed through time and these reach much further back than human records.
Leading the project, Anna Holden, Ph.D. candidate at the Richard Gilder Graduate School, American Museum of Natural History and Research Associate, La Brea Tar Pits and Museum
Insects as Paleoclimate Indicators
Though there is an abundance of insect material in the RLB deposit, this potential source of data has seen little use. Thanks to recent advances in reliable radiocarbon dating of disarticulated, identifiable beetle fragments from RLB (Holden and Southon, 2016), we are now in a position to date radiocarbon time as far back as 50,000 years or more, and are also able to date sufficient numbers of insect fossils such that they can provide a significant density of data for this period.
These developments have now been combined with methods using carabid and tenebrionid beetles as climate indicators. We can use these beetles as their present-day life-cycles, climate restrictions, and geographic distributions of the selected species are relatively well documented, and unlike migrating and often wide-ranging, mammals and birds, they offer crucial information about very local environments. Beetle species known from the Quaternary and even Tertiary are still extant and can reasonably be assumed to have flourished under the same climatic conditions that they do today.
Live Dicheirus dilatatus imaged by Joyce Gross, UC Berkeley
A number of species of carabids and some tenebrionids have been used as paleoclimate indicators over the past decades. In particular, these predatory and scavenging species, whose behavior is reasonably well-documented, are known to find food in a variety of habitats and are not tied to specific types of vegetation. They are able to migrate into suitably warm or cool, wetter or drier habits, independent of the flora, as necessary. Therefore, they are expected to typically respond to fluctuations faster than plants (Elias, 2010).
Prothorax of fossil Dicheirus dilatatus imaged by Carrie Howard, La Brea Tar Pits and Museum
Give this, generalist predator and scavenging insects may serve as better local indicators of change—in contrast, there may be a lag of hundreds of years for evidence of temperature changes to appear in pollen records (Elias, 2010). We expect that the rapid changes in populations of insects such as carabids and tenebrionids can reflect climate change that takes place within a matter of decades. Already entomologists are tracking the distribution of present day carabids and tenebrionids to record how they respond to current climate change.
Advances in Radiocarbon Dating Circumvent the Lack of Biostratigraphy* at RLB
Dr. John Southon, collaborator and co-founder of the UC Irvine Keck Carbon Cycle Mass Spectrometry Facility
For our initial step, we established a protocol to remove the asphalt matrix and other fossil preparation contaminants from the insect cuticle. This protocol works so effectively that it is now considered the most accurate procedure for radiocarbon dating insect cuticle from any Late Pleistocene deposit.
This method is invaluable for material from the RLB which lacks biostratigraphy as a means of establishing dates by association. Asphalt flows are characteristically intermittent and discontinuous and the result is mixing of fossils from different ages into the same layer.
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*Biostratigraphy: Layers, or “strata,” in which the fossils are assumed to correspond to the same age. Strata form and superimpose over time and are one of many layers.
Selecting the Right Insects for Analysis
The next step was to identify disarticulated fragments of carabids and tenebrionids with well-documented, narrow climate restrictions. Species also must be abundant enough in the RLB for destructive sampling and have enough mass to survive the chemical baths and then still have sufficient material remaining for radiocarbon combustion.
Rolf Aalbu, searching for tenebrionid beetles in California.
Kip Will (UC, Berkeley) and Rolf Aalbu (CAS associate) identified and contributed critical information on the ecology of the selected carabid and tenebrionid species. Systematists such as Kip and Rolf are fundamental to the success of this project because of their expertise in California species. They spend an inordinate amount of time in the field—thus, their experience on the ground, observing the behavior and the environments where their focal taxa live, provides the necessary, first-hand, detailed habitat information.
Extracting Climate data
We then associated a suite of informative bioclimatic variables from weather station data at Worldclim.org with hundreds of georeferenced natural history records for each species from CalBug, an open-access repository of occurrence data for California arthropods with contributions from thousands of collectors and data entry personnel. Selecting suitable records from those available for each of the target species resulted in a high-resolution data set.
Preliminary Results
So far, we have radiocarbon dated approximately 200 carabids and will soon have dates for 55 tenebrionids. The insect-based results are similar to what was found by Heusser et al. (2015) but notably different in one regard- there is no evidence for an earlier mesic interval. What might explain the difference?
A: A concatenation diagram of radiocarbon dates based on insect data. All the sample so far have dates between the Late Pleistocene and Holocene, except for the Last Glacial Maximum (LGM) period, which is unrepresented. This is interpreted as a case of consistent aridity and warmth except during the LGM period. Absence during the LGM is likely due to climate cooling. B: A concatenation diagram of radiocarbon dates based on pollen data from the sites nearest to RLB, including terrestrial pollen cores from lake Elsinore in Riverside County analyzed by Heusser et. al (2015), which is closer than the frequently cited Santa Barbara marine cores (Heusser et al., 1998).
Can distant pollen data adequately capture conditions at or near RLB and nearby locales? If we look at current conditions, it suggests that this may not be the case. Lake Elsinore is approximately 70 miles away, significantly farther from the ocean, and sits at nearly 400 meters above sea level compared to only about 50 meters above sea level at RLB. Therefore, assuming the two sites were equally dissimilar in the past, these paleoclimatic differences can easily be accounted for by microclimate differences between Elsinore and RLB.
Map showing terrestrial pollen core site near Lake Elsinore and the location of the Rancho la Brea
How are these results novel?
These results challenge the widely-accepted interpretation of the paleoenvironment at RLB during the Late Pleistocene as being a cool, mesic climate, similar to that of modern-day Monterey, California (which is 450km north of RLB), or having approximately twice the amount of rainfall as typically falls at RLB now.
The standard story is largely based on a remarkably small number of plant identifications that have problematically been associated with 26-29,000 year old bone collagen dates (Friscia et al., 2008; Coltrain et al., 2004) from one RLB deposit, “Pit 91.”
Surprisingly, the dated insects from Pit 91 show a broad range from late Pleistocene to Holocene in the same grids where they are associated with the much narrower bone collagen-based date range; a clear demonstration of the RBL’s lack of biostratigraphy. This emphasizes the need to date individual samples and be very cautious about relying on dates based solely on associated samples. Reliance on such associations has led to persistent, and apparently mistaken paleoenvironmental and taphonomic inferences.
Why do our samples of insects show such a wide spread of ages? Intermittent and shallow asphalt pooling, not deep enough to immobilize larger animals, may have entrapped smaller organisms such as insects just like a pitfall trap that was opened sporadically to sample active and abundant species.
Previous investigators have provided some evidence for the paleoenvironment of southern California during the late Quaternary, but we still lack important details. Using the insects of RLB as climate and taphonomic indicators we can more clearly evaluate RLB’s significance for understanding other events such as the timing and agency of the megafaunal extinctions.
Stay tuned for even more results and other studies which use RLB insect fossils to better understand changes in southern California flora and fauna over time.
Acknowledgements
Many thanks to the scientists, staff and volunteers who have collected and input CalBug data required for this study. We also thank the scientists and staff at the La Brea Tar Pits and Museum and the American Museum of Natural History.
References
Coltrain, J., Harris, J.M., Cerling, T.E., Ehleringer, J.R., Dearing, M., Ward, J. and J. Allen. 2004. Rancho La Brea stable isotope biogeochemistry and its implications for the palaeoecology of late Pleistocene, coastal southern California. Palaeogeography, Palaeoclimatology, Palaeoecology 205(3–4):199-219.
Elias S. 2010. Advances in Quaternary Entomology, Amsterdam: Elsevier, 3017 p.
Friscia A.R., Van Valkenburgh B., Spencer L., Harris J.M. 2008. Chronology and spatial distribution of large mammal bones in Pit 91, Rancho La Brea. Palaios 23:35-42.
Heusser, L.E., Kirby, M.E., Nichols, J.E. 2015. Pollen-based evidence of extreme drought during the last Glacial (32.6–9.0 ka) in coastal southern California. Quaternary Science Reviews 126:242-253.
Heusser, L.E. 1998. Direct correlation of millennial‐scale changes in western North American vegetation and climate with changes in the California Current system over the past∼ 60 kyr. Paleoceanography 13.3: 252-262.
Holden, A. R., and J. R. Southon. 2016. Radiocarbon Dating and Stable Isotopic Analysis of Insect Chitin from the Rancho La Brea Tar Pits, Southern California. Radiocarbon 58(1): 99-113.
