Historical Evolution of Organic Matter Accumulation in a Coastal Bay in the SW Atlantic, Brazil: Use of Sterols andn-Alcohols as Molecular Markers

Eduardo C Richard,Michelle A Passos,Cássia O Farias,José M Godoy,Plinio F Martinho,Renato S Carreira,Cláudia Hamacher,Natália C M Ribeiro,Marina P Dore

doi:10.5935/0103-5053.20140120

Abstract

The deposition processes and preservation of organic matter (OM) in the sediments of Ilha Grande Bay, SW Atlantic, were evaluated based on elemental composition (C and N) and molecular markers (sterols and n-alcohols). Samples from four dated sediment cores, representing distinct levels and type of human influence, were analyzed. The concentration of total sterols ranged from 71 to 9,320 ng g-1, with 24-ethyl-cholesta-5,22-dien-3β-ol, 24-ethyl-cholesta-5-en-3β-ol and 24-ethyl-5a-cholesta-3β-ol as the most abundant compounds. The n-alcohols, with a total concentration between 130 and 15,407 ng g-1, were dominated by long-chain compounds (> C22). Assignments of the selected markers to terrestrial or marine sources were evaluated using principal component analysis (PCA). The trends and events that influenced the OM accumulation in the last decades were also revealed by the PCA, as the increasing human settlement in the region and the removal of a mangrove forest occurred between 1940 and 1960. The absence or low level of sewage contamination was indicated by the sterol coprostanol and associated indexes.

Highlights

Coastal ecosystems are essential components for understanding the global carbon cycle.[1]
The distinct reactivities of organic matter (OM) according to its source is another relevant factor in this context, whose effect on the preservation of OM in the sedimentary record has been addressed in detail in the last years.[7,8]
The four sediment cores could be clearly differentiated into two groups, one with high organic carbon content, high proportions of fine sediments and higher concentrations of molecular markers, whereas the other group is poor in organic carbon, fine sediments and molecular markers

Summary

Introduction

Coastal ecosystems are essential components for understanding the global carbon cycle.[1]. Four sediment cores were collected in the Ilha Grande Bay, SW Atlantic, Brazil (Figure 1) to evaluate the historical evolution, deposition and transformation of OM in this environment. A lead carrier was added to the solution, and the mixture was transferred to an ion-exchange column containing Dowex 1X8, 50‐100 mesh This procedure was followed by a cleaning step with 0.5 mol L–1 HBr and 1.0 g hydroxylamine hydrochloride and further elution with 1 mol L–1 HNO3. Quantification was performed using a calibration curve (six points between 100 ng mL–1 and 20,000 ng mL–1) with commercial standards (n-C18OH, n-C19OH, cholest-5-en-3β-ol, 5β-cholestan-3β-ol, 5a-cholestan-3-one, 5b-cholestan-3-one, 5α-cholestan3β-ol, 24-methylcholest-5-en-3β-ol, 24-ethylcholest5,22E-dien-3β-ol and 24-ethylcholest-5-en-3β-ol) and by considering the peak areas of key ions (m/z 129 or 215 for sterols and m/z 103 for n-alcohols) and response factors relative to the internal standard (5α-cholestane, m/z 217). Varimax rotation was selected to represent the planar projection of the loadings (variables) and scores (samples) for the two principal components, using the Statistica® v7.0 package

Results

Discussion

Conclusions