Geochemical Indication Of Biomarkers In The Sediments From The Northern Arabian Sea

Studying on the origin,pathway and fate of sedimentary organic matter in estuarine and shelf areas are essential for marine geochemistry.Since the geochemical markers are characterized by their source specificity and molecular stability,they are widely used to interpret changes that occurs within terrestrial and shelf ecosystems and to explore the possibility of short-term paleo environmental changes and geochemical process.In this study,groups of organic geochemical and sterol biomarkers were used to study the distribution,composition of organic matter,to trace the residue of polycyclic aromatic hydrocarbons(PAHs)in surface sediment of Karachi coast and to study as potential tracers of past hypoxic events and paleoproductivity.Our findings highlighted the spatial distributions of total organic carbon(TOC),total nitrogen(TN),nitrogen and carbon isotopic ratios(?15N and ?13C)in surface sediments are broadly similar with their average values being higher in harbor,creeks sediments than in offshore areas.There was a good linear relationship between TOC%and TN%,suggesting they were derived from the same source.The glycerol dialkyl glycerol tetraether(GDGT)distribution in sediments shows that isoprenoid GDGTs(iGDGTs)are the predominant type of GDGTs.The results suggest that domestic and industrial urban discharge may have a major impact on archaeal lipids in sediments.The highest BIT value was found in the Karachi harbor.The similar distributions of dinosterol,brassicasterol and cholesterol indicate that the relative contributions of the dominant members of the phytoplankton and zooplankton community.A 7.6 m long sediment core(67°16.31'E,23°36.66'N,?70 m water depth)recovered from the Western Indus Shelf of northern Arabian Sea adjacent to Indus River mouth and canyon spanning early Holocene to Recent(?14 ka)was used to distinguish between marine versus terrestrial processes involved in organic carbon production.The results provides a contribution to the interpretation of geochemical biomarkers in respect to paleoproductivity reconstructions,as identified by TOC and?13C and TOC was originate to be predominantly of marine source in Indus-23AP core,as confirmed by the TOC:TN ratio,?13Corg,and the branched and isoprenoid tetraether(BIT)index during the Holocene,even though begin located within 20 km of the mouth of Indus River,one of the main source of terrestrial sediment to the Western Indus shelf.TOC and nitrogen loading indicated that increased phytoplankton production likely caused the decline in bottom-water oxygen levels during the late Holocene.Phytoplankton abundance increases with vertical mixing and river discharge of nutrients during summer monsoon.Alkenone was not detected at the Shelf due to oxic degradation might be decreased the concentration of alkenone.Initial results of three sterol biomarkers(brassicasterol,dinosterol and cholesterol)show concurrent enrichments.GDGT crenarchaeol enrichment was controlling BIT indices.TEX86 records of sea surface temperature(SST)in the area are mainly controlled by the Southern Hemisphere climate dynamics during the SW monsoon.U37K SST records from the same cores are different due to unusual growing seasons of the biomarker source organisms,and to a change in the upwelling dynamics and monsoon strengths between the last glacial maximum(LGM)and the Holocene.The diagnostic ratios revealed pyrogenic and a mixture of pyrogenic and petrogenic PAHs in marine surface sediments along the Karachi coast,Pakistan,11 years after the Tasman Spirit oil spill was detected.The dominating three-ring PAHs(60.57±14.07%).Mixed sources of spill of petroleum-related products(42.34%),traffic emission(26.18%),Gasoline emission(24.23%),followed by natural origin(7.25%)might be the main sources by using principal component analysis-multiple linear regression(PCA-MLR)model.The ?16PAHs concentrations in surface sediment collected along the Karachi coast in 2014 were lower than the ?16PAHs concentrations in 2003 after the Tasman Spirit oil spill and at moderate levels compared to marine sediments from different parts of the world.The distribution of these geochemical biomarkers can be inferred to confirm the spatial distributions of TOC,TN and their isotopic signatures in surface sediments are mainly controlled the mixing inputs of terrigenous and marine organic matter.Moreover,the diagnostic ratios revealed pyrogenic and a mixture of pyrogenic and petrogenic PAHs in surface sediment was identified.This study will have important geochemical implication of biomarkers concerning the interpretation of biogeochemical cycle,sources of organic matter,TOC,PAHs,phytoplankton community,SST and oxygen depletion condition in future studies from the Shelf and surface sediments.