Tracing the intrusion of fossil carbon into coastal Louisiana macrofauna using natural 14 C 1 and 13 C abundances 2 3 Rachel M. Wilson 1* , Jennifer Cherrier 2 , Judith Sarkodee-Adoo 2 , Samantha Bosman 1 , 4 Alejandra Mickle 1 , Jeffrey P. Chanton 1 5 1 Department of Earth Ocean and Atmospheric Science, Florida State University, Tallahassee, FL 6 32306, USA 7 2 School of the Environment, Florida A&M University, Tallahassee, FL 32307, USA 8 * corresponding author, rachelmywilson@gmail.com 9 February 25, 2015 10 Keywords: petroleum hydrocarbon assimilation, radiocarbon, stable carbon, macrofauna, 11 12 ABSTRACT 13 The Deepwater Horizon oil spill released a large volume of 13 C and radiocarbon depleted 14 organic matter to the northern Gulf of Mexico. Evidence of petroleum-derived carbon entering 15 the offshore planktonic foodweb, as well as widespread oiling of coastal areas documented in 16 previous studies suggests that hydrocarbons could have entered the near shore foodweb. To test 17 this hypothesis, we measured radiocarbon (∆ 14 C‰) and stable carbon isotopes (δ¹³C) in an 18 assortment of fish tissue, invertebrate tissue and shell samples collected within a year of the spill 19 at seven sites from Louisiana to Florida USA across the northern Gulf of Mexico. We observed 20 a west-east gradient with the most depleted radiocarbon values found in Terrebonne Bay, 21 Louisana and increasingly enriched radiocarbon values in organisms collected at sites to the east. 22 Depleted radiocarbon values as low as −10‰ in invertebrate soft tissue from Terrebonne suggest 23 assimilation of fossil carbon (2.8 ± 1.2%), consistent with the hypothesis that organic matter 24 from petrochemical reservoirs released during the Deepwater Horizon spill entered the coastal 25 food web to a limited extent. Further there was a significant correlation between radiocarbon 26 and δ 13 C values in invertebrate tissue consistent with this hypothesis. Both oyster tissue and 27 hard head catfish tissue collected in impacted areas of coastal Louisiana were significantly 28 depleted in 14 C and 13 C relative to organisms collected in the unaffected Apalachicola Bay, 29 Florida (p<0.014). Alternative explanations for these results include the influence of chronic 30 hydrocarbon pollution along the western gulf coast or that the organisms ingest carbon derived 31 1