Morfologia e sedimentação em um estuário de macromaré e plataforma adjacente, margem equatorial brasileira

Abstract

Understanding morphosedimentary dynamics in macrotidal estuaries is a challenging task, particularly in large-scale systems where seabed morphology and sediment distribution are poorly constrained. This study presents an unprecedented investigation into the morphological changes and the spatial and seasonal variability of surface sediments in the São Marcos Estuary and its adjacent continental shelf. The objective was to assess multidecadal morphobathymetric evolution (1977–2022) and short-term sedimentary pattern modifications associated with climatic seasonality, characterized by a dry season and a rainy season (2020–2021). The 45-year analysis, based on comparisons of nautical charts, revealed a sediment transport pattern primarily controlled by ebb currents, which governs the evolution of key morphological features. A highly deflected subaqueous delta, developed in a marine environment dominated by macrotidal oceanographic processes, was identified on the inner continental shelf—this configuration deviates markedly from classical models of tidedominated deltas. The estuary exhibited morphological instability in the area seaward of the estuarine funnel—proximal to the São Luís Port Terminal—while the region landward of the funnel, including the lower estuary and the inner continental shelf, displayed relative morphological stability. Sediment budget analyses indicated a clearly negative sediment balance for both environments over the study period. The estuary experienced a net volume increase of 1.1 × 10⁹ m³, equivalent to 2.91 × 10¹² tm⁻³ of eroded sediments, with estimated erosion rates of 6.4 × 10¹⁰ tm⁻³/year and a volumetric erosion rate of 2.4 × 10⁷ m³/year. Similarly, the inner continental shelf recorded a net volume increase of 1 × 10⁹ m³, corresponding to 2.65 × 10¹² tm⁻³ of eroded sediments, with annual rates of 2.2 × 10⁷ m³/year for volume and 5.9 × 10¹² tm⁻³/year for sediment mass. Together, both environments accounted for a total net volume increase of 2.1 × 10⁹ m³, representing a total of 5.56 × 10¹² tm⁻³ of eroded sediments, with combined net erosion rates of 4.6 × 10⁷ m³/year and 1.23 × 10¹¹ tm⁻³/year, respectively. Sediment budget calculations across isobaths showed that the estuary experienced substantial sediment losses down to the -30 m isobath. Below this depth, however, sediment behavior was inverse, indicating sediment retention or accumulation. In contrast, the inner continental shelf exhibited a significant net volume increase up to the -25 m isobath, followed by a depositional regime with attenuated sediment accumulation down to the -45 m isobath. For sedimentary deposit characterization, 284 surface sediment samples were collected during the dry season and 281 during the rainy season. These samples were analyzed and spatially interpolated to quantify the percentages of mud (silt and clay), sand, organic matter, and calcium carbonate in both seasonal periods. Textural classifications of surface sediments were assigned based on the schemes proposed by Wentworth (1922), Shepard (1954), and Larsonneur (as modified by Dias, 1996). During the dry season, estuarine sediments were predominantly composed of fine and medium sand, while the adjacent continental shelf was mainly composed of medium sand. Sand accounted for approximately 70% of all dry season samples. In the rainy season, the estuary maintained a similar sediment distribution, with sand again dominant—fine sand being most abundant, followed by medium sand. The adjacent shelf showed a predominance of fine and medium sand, indicating that climatic seasonality exerted influence primarily over the continental shelf region. The intense tidal activity within the study area is the principal driver of the morphosedimentary dynamics of the São Marcos Estuary and its adjacent continental shelf.