Geoquímica orgânica molecular em sedimentos marinhos do sistema de ressurgência de Cabo Frio (RJ)

Abstract

The ocean at the Cabo Frio region is known for the upwelling phenomena, characterized by the emergence of cold, nutrient-rich waters from the ocean bottom to the surface, due, among other events, to the drag of surface waters by winds. These characteristics make this region a unique environment in the country and one of the few in the world. The increase in nutrients on the surface leads to greater primary productivity and, consequently, an increase in organic matter in this environment. Aiming paleoenvironmental reconstruction, this work targets to characterize molecular biomarkers in a 4.15 m core from the middle platform at the Cabo Frio, RJ, identifying the different contributions and the upwelling influence. The core was sectioned every centimeter. 15 sections were selected, in cm: 3-4, 11-12, 17-18, 24-25, 31-32, 43-44, 54-55, 63-64, 73-74, 83-84, 92-93, 108-109, 116-117, 124-125 and 133-134. Sections were initially freeze-dried. The organic matter was extracted at ultrasonic bath. The extracts were saponified with 5% aqueous KOH solution. The neutral fraction was fractionated into aliphatic and aromatic hydrocarbons and polar compounds by liquid chromatography. Aliphatic hydrocarbons were analyzed via gas chromatography with flame ionization detector (GC-FID) and gas chromatography coupled to mass spectrometry (GC-MS), while the aromatic hydrocarbons, polar compounds, and fatty acids fractions were analyzed via GC-MS only. The carboxylic acid fraction was previously derivatized. In the aliphatic hydrocarbon fraction, the following analytes were detected: n-alkanes in the C14-C35 range. The lighter ones (≤ C20) represent contributions from algae and bacteria, while the heavier ones (> C25) represent the contribution from land plants; tricyclic terpanes were detected in the most superficial sections; hopanoids, which represent contributions from bacteria, were detected in all sections. In the aromatic hydrocarbon fraction, several polycyclic aromatic hydrocarbons (PAHs) were identified, such as phenanthrene and alkylated derivatives, fluoranthene, pyrene, chrysene, and benzofluoranthene, with higher concentration of unsubstituted PAHs, indicating a pyrogenic contribution. In the polar compound fraction, steroids C27, C28, and C30, which represent contributions from planktonic material, were detected at higher concentrations. Other polar biomarkers were identified: a homologous series of linear ketones in the range of C12-C33 range, polar terpenoids (taraxerol, α- amyrin, and α-amirone), reinforcing the contribution of land plants; a homologous series of linear alcohols in the C10-C24 range, with a higher concentration of those with a short chain (≤ C20), indicating a greater contribution from planktonic material, but also the contribution of higher plants in the C20-C24 range; linear amides C16, C18, C18:1, C22 and C22:1, being the most abundant analytes at the samples and rarely found in marine sediments, which can come from different sources, such as vascular plants, algae, fungi and even burning of biomass; carboxylic acids were detected in the C8-C32 range, indicative of marine and terrestrial contributions. All analytes were detected at higher concentrations during the last 200 years from the period studied. With the different biomarker classes identified, verifying different contributions to the organic matter of the ecosystem under study was possible, highlighting the role of bacteria, planktonic material, and terrestrial plants. The higher biomarker concentration in the superficial sections indicates a greater upwelling effect and greater contribution from allochthonous sources in the last years of the Holocene, particularly from 1000 to 1500 years before the present. The profile observed for the 1500-3000 years before the present indicates little change in the upwelling effect and that the diagenetic process was not intense during these years.