Changes in the sedimentary record related to climate oscillations over the last ∼2500 years BP in Guanabara bay (SE Brazil): phytolith records

The study of the record of paleoclimatic oscillations in coastal areas is crucial because it helps us understand past climate changes, how they impacted these vulnerable regions, and draw lessons about what might happen in the future. This study presents the first application of phytolith analysis in a multiproxy paleoenvironmental reconstruction in a coastal marine area in Brazil. It investigates the natural and anthropogenic influences on the evolution of Guanabara Bay (GB), one of the most urbanized regions in Southeastern Brazil. To achieve this, grain size, geochemical, mineralogical, and phytolith data were analyzed on a short piston core. The analysis revealed changes in the continental input of lithogenic materials and variations in the quantity and type of organic matter, linked to alterations in vegetation and sediment supply from the rivers over the last ∼2500 years. The general marine regression may have favored the sediment transfer from the rivers to GB, while the low hydrodynamic regime contributed to the accumulation and preservation of the sedimentary record. Sedimentological changes related to shifts in rainfall patterns and the degree of mineral weathering were observed. Three main phases of dryness have been identified. The earliest was recorded between ∼415 and 50 BC, during the Late Holocene Dry Period (or the Late Holocene Neoglacial Anomaly); the second one was registered between ∼800 and 1300 AD, during the Medieval Climate Anomaly; and the third one happened ∼ between 1550 and 1900 yrs AD, during the Little Ice Age (LIA). These three dryness events have been caused by shifts in the oceanic Intertropical Convergence Zone (ITCZ) and the South Atlantic Convergence Zone (SACZ). Although phytolith analysis has been conducted in a few marine environments, no previous studies have been conducted in Guanabara Bay or Brazilian marine systems. The multiproxy analysis, including phytoliths, enabled the identification of changes in primary vegetation types, changes in rainfall and sediment supply and anthropogenic impacts in Guanabara Bay.