Distribution and Composition of Terrigenous Organic Matter in Surface Sediments from the Fly River Delta and Clinoform (Papua New Guinea)

The distribution of terrigenous organic matter in surface sediments from the delta and inner clinoform region off the Fly River was investigated by measuring organic carbon:nitrogen (C/N) and stable carbon (d13C) isotopic ratios, as well as the concentrations and stable isotopic compositions of lignin-derived CuO oxidation products. The average C/N and d13C ratios of the sediments from this region ranged from 11 to 70 and from -23 to -27 per mil, respectively. Lignin phenol yields ranged from 0.2 to over 9 mg/100 mg OC, with an average of 3.4 mg/100 mg OC. Most of the differences in C/N ratios and lignin yields occur within a narrow range of d13C compositions (-27 to -25 per mil), which suggest that most of the variability encountered within the surfaces sediments is due to variations in the relative abundance of vascular plant detritus (high C/N ratios and high lignin yields) and soil organic matter (lower C/N ratios and lower lignin yields). Our results suggest negligible contributions from marine organic carbon in surface sediments of the northeast region of the delta. In contrast, the most enriched d13C values and lowest lignin yields, both indicative of marine inputs, were measured in coarse sands collected in the southwest region of the Fly River delta and inner clinoform. The relative abundances of different lignin phenol classes are consistent contributions from woody and non-woody angiosperm sources. The stable carbon isotopic compositions of individual lignin phenols indicate a predominant C3 source, with no evidence for contributions from C4 vascular plant sources. Overall, these results point to C3 angiosperm vegetation from the tropical forest in the drainage basin and delta region of the Fly River as the predominant sources of terrigenous organic matter in this part of the Gulf of Papua. The spatial distribution of these two pools of organic matter will be interpreted in the context of sediment texture, mineral surface area and particle distribution processes in this region of the gulf.