Characteristics of organic matter in lagoonal sediments from the great barrier reef

Abstract

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 preservations 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.