Correspondence Comment on “Natural Hydrocarbon Background in Benthic Sediments of Prince William Sound, Alaska: Oil vs Coal”
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 10, 2000
FIGURE 1. (A) Our plot of oleanane to C30-hopane ratio (OI) versus C2-dibenzothiophene to C2-phenanthrene ratio (DPI) for data presented in Short et al. (1). (B) Our plot of our OI versus DPI data. River/lake sediment (O); seep oils (b); Tertiary shale (0); PWS marine sediment (]); GOA terrestrial sediment (+); Bering River coalfield coal (º). KB ) Katalla Beach “coaly” sediment (*).
SIR: We take issue with the conclusion reached by Short et al. (1) that coal is the dominant source of the natural polycyclic aromatic hydrocarbon (PAH) background in offshore sediments of Prince William Sound, AK (PWS). The authors cite the Bering River coalfield as the possible source of the coal (1, 2). Their own data, on PAHs and biomarkers, however, argue against coal (Figure 1A). The coals reported in Table 1 of Short et al. (1) cannot be anything more than very minor contributors to the hydrocarbon background of PWS as evidenced by the PAHs and the saturate and aromatic biomarkers. Our more comprehensive sampling program indicates that seep oils and petroleum source-rock shales are better source candidates (Figure 1B). Had they sampled other possible sources that we suggested earlier, they would have discovered that seep oils and petroleum source-rock shales are far better source candidates than are the Bering River coals (Figure 1B). Short et al. (1) contend that a refractory index (RI) of triaromatic steranes to methyl chrysene shows that PWS sediment (with RI values of 0-0.2) is more closely matched by Bering River and Katalla Beach coals (0-0.02) than Katalla seep oil. However, their limited study did not include Gulf of Alaska (GOA) shales and seep oils that also have low RI values (0.001-1.0). Coal from the Bering River coalfield has RI values of zero and thus cannot explain the RI values we measured in PWS sediment (0.05-0.07). Katalla Beach sediment, listed as “coal” by Short et al. (1), is not a true source sample in that it contains varying proportions of Bering River coal, kerogen, coke, bitumen, mica, shale, and seep oil remnants. The dark appearance of the beach strand lines (1) stems primarily from their mica content not coal. Beach sediment RI values (0.02-0.14) reflect shale and seep oil residue input rather than coal. Short et al. (1) failed to consider mass balance constraints on the source of the background (4). If all of the total organic carbon (TOC) in deepwater PWS sediments (average TOC ∼0.6%) came from Bering River coals (TOC ∼80%), the total PAH levels of PWS sediments would be
