Analytical pyrolysis for determining the molecular composition of contemporary monosulfidic black ooze

• Comparison of pyrolysis techniques to characterise a recent S-rich sediment.
• First organic geochemical study of monosulfidic black ooze, with focus on organic S.
• MBO consists of predominantly terrestrially sourced organic matter.
• MSSV-Py is more sensitive to S-analytes of the MBO than flash-Py.
• Absence of S compounds in hydropyrolysates suggests org-S is intermolecularly bound.

On-line flash pyrolysis, micro-scale sealed vessel (MSSV) pyrolysis and catalytic hydropyrolysis (HyPy) were used to characterise the insoluble, macromolecular organic component of monosulfidic black oozes (MBO) which have accumulated within the contemporary eutrophic environment of the Peel–Harvey estuary system (Geographe Bay, Western Australia). Pyrolysates were analysed by gas chromatography–mass spectrometry (GC–MS) and the relative characterisation potential of the three pyrolysis techniques were evaluated with a particular interest in their sensitivity to organic sulfur compounds (OSCs). A similarity of results obtained from three different parts of the ∼1 m cores sampled suggests a largely homogenous organic composition throughout the unit. The different pyrolysis techniques did, however, show several notable product differences, particularly the gaseous products detected by the on-line methods of flash- and MSSV-pyrolysis. The very high (i.e., ballistic) heating rate of flash-Py produced very high proportions of gaseous products (e.g., CO2, H2S and SO2). A strong terrestrially sourced product component was reflected in all three pyrolysates profiles: flash-Py showed relatively high concentrations of lignocellulose products, while MSSV-Py and HyPy produced an abundance of plant wax (>n-C20) n-alkanes. Additionally, quantitatively significant levels (7–14% of total GC product signal) of several higher plant derived terpenoids (e.g., cadalene, p-cymene and calamanene) were identified by MSSV-Py. MSSV-Py also produced far greater overall concentrations of GC-amenable products than flash-Py, including a 14-fold increase of OSCs, which comprised nearly 10% of the total sulfur signal (i.e., organic plus ‘inorganic’ sulfur; cf. 4% of flash pyrolysis). An extended series of alkyl (