IMPROVED PREPARATION METHODS FOR ASPHALT-PRESERVED FOSSILS FROM RANCHO LA BREA, CALIFORNIA

Rancho La Brea (RLB) is a Late Pleistocene asphaltic locality preserving millions of fossils. Preparation of RLB fossils requires degreasing solvents to remove hardened asphaltic sedimentary matrix. Previous preparation methods included submersion by soaking fossils in n-propyl bromide (nPB) to dissolve matrix. Soaking removes much of the matrix, resulting in dissociation of fragile material and loss of supportive internal matrix. To improve RLB preparation methods, alternative solvents and an exclusively manual preparation technique were tested.

Six solvents were selected based on sustainable principles, representing a range of chemical properties: Aerotron, 2-butoxyethanol, d-limonene, Ecolink 1171, Novec 73DE, and nPB as the control. Representative canid and avian fossils were soaked (n=24) and manually prepared (n=24). While all solvents removed matrix, only Novec 73DE and nPB were sufficiently effective. Changes in fossil integrity were observed 9 months post-preparation, including oozing asphalt, drying, cracking, flaking, and crumbling. Soaked fossils exhibited greater degradation than manually-prepared fossils for all solvents. Aerotron and Novec 73DE presented the least degradation, whereas the low volatility of 2-butoxyethanol, d-limonene and Ecolink 1171 left residual solvent, and dehydration from nPB led to cracking.

Manual preparation was assessed with canid cranial specimens, as they are morphologically complex with large internal cavities. This technique removes only external matrix through targeted application of solvent. Manually prepared (n=5) and soaked (n=13) crania were compared for internal matrix retention and fragmentation. Manually prepared crania retained matrix in the braincase and nasals, while soaked crania retained minimal matrix. Cavities retained matrix in over 70% of manually prepared crania, but less than 10% in soaked specimens. Manually prepared fossils were less fragmented (median 2 repairs) than soaked (median 13 repairs), requiring less conservation.

Manual preparation with Novec 73DE proved most effective for external asphaltic matrix removal, internal matrix retention, and fossil integrity. Based on results of this research, RLB has adopted Novec 73DE and manual techniques as best practices for asphaltic paleontological preparation.