Characteristics of organic matter in lagoonal sediments from the Great Barrier Reef

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We have applied organic carbon and total nitrogen analyses, Rock-Eval pyrolysis, and gas chromatography-mass spectrometric lipid analysis with in situ methylation with tetramethylammonium hydroxide (TMAH-pyrolysis-GC/MS) to characterize the organic matter in recent lagoonal sediments from the Great Barrier Reef (GBR). On a pyrogram, GBR sediments showed the unique feature that S₁ and S₃ values were anomalously high. The high S₁ value presumably reflects a higher proportion of hydrolyzed lipids, and the high S₃ value is due to the degradation of carbonate minerals in a low temperature range. Associated with a CaCO₃ increase towards the offshore area, the organic carbon/total nitrogen ratio (C/N ratio) decreased, S₁ and S₂ values increased, and total fatty acid and phytol concentrations increased. These changes have negative correlations with the branched/total fatty acid ratio, implying that the changes reflect the degree of eubacterial activity. The higher preservations of lipids and lignin phenols in CaCO₃-rich sediments are attributable to their higher preservation effect resulting from an impermeable carbonate matrix. The anteiso-/iso-fatty acid ratio has a significant negative correlation with the total fatty acid concentration and a positive correlation with the branched/total fatty acid ratio, indicating that the ratios are closely related to the degree of eubacterial degradation of organic matter.