Carbon and hydrogen isotopic compositions of sterols from riverine and marine sediments

The sources (marine algae, terrestrial C3 and C4 plants) of sterols deposited to sediments along a riverine-marine transect from Ohtsuchi River in Iwate Prefecture, Japan, to the northwestern Pacific Ocean were estimated using carbon (δ¹³C) and hydrogen isotopic compositions (δD) in surface sediments. In marine sediments, algal sterols such as 24-methylcholesta-5,22-dien-3β-ol had δ¹³C values of -22.7 ± 0.4‰ and δD values of -292 ± 3‰. In contrast, sterols derived from multiple potential sources, such as 24-methylcholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol, were gradually enriched in ¹³C from riverine (-30.7‰ and -30.5‰, respectively) to marine sediments (-22.3‰ and -24.4‰, respectively), but showed little variation in δD values (-262 ± 1‰). These isotopic signatures suggest that 24-methylcholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are derived from C3 plants (∼90%) and C4 plants (∼10%) in the riverine sediments, and marine algae (∼30%), C3 plants (∼30%), and C4 plants (∼40%) in the open marine sediments. Thus, dual isotopic compositions (δ ¹³C-δD) of sterols allow determinations of the proportions of three biological sources, in which the relative contribution of C3 plants decreases while that of marine algae and C4 plants increases from riverine to marine sediments. These results indicate that δ¹³C-δD signatures of sterols provide a useful tool for interpreting the sources of sedimentary organic matter and for understanding the transport and mixing processes of distinct biological sources in riverine-marine environments.