Distribution and sources of polycyclic aromatic hydrocarbons (PAHs) in laminated Santa Barbara Basin sediments

The Santa Barbara Basin (SBB) preserves a high-resolution sediment record that is varved down to ca. 1700 CE and laminated for at least 2500 years. The distributions of 25 polycyclic aromatic hydrocarbons (PAHs) were investigated in a suite of samples from sediment cores representing different depositional scenarios - varves/laminae, terrigenous flood layers, and marine turbidites. The concentrations of these PAHs were determined using gas chromatography-mass spectrometry in selected ion mode and evaluated in terms of their distributions, diagnostic ratios, and loadings recognized by principal component analysis (PCA). Two biogenic PAHs, perylene and retene, provided the greatest variation in their downcore concentration values. Perylene concentrations increased downcore exhibiting markedly higher abundance in sediments older than 300 CE, consistent with the interpretation that perylene is formed diagenetically in reducing sediments. A modest increase in perylene concentrations in sediments from the late 1960s may be associated with flood-derived transport of higher amounts of degraded terrestrial organic matter from fire-denudated landscapes into the coastal environment. The markedly higher concentrations of retene in flood layers and turbidites relative to varves/laminae suggest that these lithologies are more effective in transporting pyrogenic PAHs and can be interpreted as a proxy for wildfires in SBB. Changes in key diagnostic ratios reveal the shift from a dominance of petrogenic PAHs in older sediments to a strong pyrogenic signal beginning in the latter half of the 20th century. Statistical analysis using PCA of the PAH distributions in the samples revealed that modern sediments showed a strong, positive relationship with PC1, which represents the greatest variation among the samples, whereas flood layers showed a strong relationship with the second major variable (PC2). Thus, statistical approaches provide the capability to elucidate modern, anthropogenic signatures within the complex PAH distributions of SBB and also establish a PAH profile that characterizes the lithological signature of flood layers.