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

Abstract

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 (d 13 C) and hydrogen isotopic compositions (dD) in surface sediments. In marine sediments, algal sterols such as 24-methylcholesta-5,22-dien-3b-ol had d 13 C values of 222.7 6 0.4‰ and dD values of 2292 6 3‰. In contrast, sterols derived from multiple potential sources, such as 24-methylcholest-5-en-3 b-ol and 24-ethylcholest5-en-3b-ol, were gradually enriched in 13 C from riverine (230.7‰ and 230.5‰, respectively) to marine sediments (222.3‰ and 224.4‰, respectively), but showed little variation in dD values (2262 6 1‰). These isotopic signatures suggest that 24-methylcholest-5-en-3b-ol and 24-ethylcholest-5-en-3b-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 (d 13 C-dD) 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 d 13 C-dD 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. Quantitative assessment of organic matter inputs from marine algae and terrestrial plants to riverine and marine sediments is important for understanding the burial and mixing processes affecting organic matter at continental margins and for estimating marine primary productivity and carbon flux associated with biogeochemical cycles in surface environments. Marine algae and terrestrial C3 and C4 plants are major primary producers and potential sources for sedimentary organic matter in river-dominated coastal environments. Molecular and isotopic compositions of sedimentary organic materials should reflect both variations in the input of marine algae relative to terrestrial plants and in the proportions of C3 and C4 terrestrial plant inputs. Many previous studies have estimated the relative contribution between two biological sources (marine algae vs. terrestrial plants or terrestrial C3 vs. C4 plants) for sedimentary organic materials from riverine (or coastal) to marine environments based on biomarker assessments (e.g., Laureillard and Saliot 1993; Mannino and Harvey 1999). Carbon isotopic compositions of bulk organic carbon (e.g., Wada et al. 1987; Carreira et al. 2002) and specific biomarkers including lignin-derived com1