Distribution and Sources of Organic Matter in Surface Sediments of the Karachi Coast, Pakistan

Under the steadily intensifying human activities in coastal areas, it is crucial to distinguish the sources of organic matter (OM) in sediments for better understanding of the environmental change. In this study, 25 surface sediments collected along the harbor and creeks areas off Karachi coast were used for total organic carbon (TOC), total nitrogen (TN), stable carbon (delta 13C) and nitrogen (delta 15N) isotopic compositions, phytoplankton biomarkers (brassicasterol, dinosterol and cholesterol) and glycerol dialkyl glycerol tetraethers (GDGTs) analyses, to trace the distribution of OM and identify its sources. TOC, TN, delta 13C, and delta 15N are in the range of 0.04% to 5.28%, 0.01% to 0.57%, - 17.55 parts per thousand to - 26.31 parts per thousand, and 2.83 parts per thousand to 6.04 parts per thousand, respectively. C/N values range from 4.24 to 21.04. The delta 13C, delta 15N, C/N, BIT (Branched Isoprenoid Tetraether) index together with phytoplankton biomarkers show both marine and terrestrial OM input to the creeks and Harbor stations. In contrast, the stations from Western Coast indicate a marine source predominance. A clearly impact of human activities is seen from sediments near river mouths.