Paleoecologia dos moluscos bivalves da formação La Meseta (Ilha Seymour, Antártica): relação com as fácies locais ou mudanças climáticas do eoceno?

Abstract

Antarctica records glaciation events linked to the global cooling that occurred at the end of the Eocene and the beginning of the Oligocene, which corresponds to the last Greenhouse-Icehouse shift of the planet. In this context, the La Meseta Formation (Seymour Island/Marambio) preserves an important record of local environmental changes that occurred previously to the Cenozoic intense cooling. The unit contains fossiliferous rocks from almost the whole Eocene, with high diversity of marine macroinvertebrates, especially bivalve molluscs. This work used ecological data from Present-Day bivalves, considered here as representatives of the bivalve taxa recorded in the La Meseta Formation, in order to assess changing of those parameters throughout the stratigraphy of the unit, and evaluate whether these changes may be related to local environmental changes, associated to sedimentary facies, or global changes, linked to climate change. We run correspondence analyses and chi-square tests using environmental parameters such as guilds, substrate, bathymetry, climatic zone and geographic distribution to understand the significance and correspondence of the variables with the intervals or stratigraphic subunits where bivalves are recorded. All intervals and variables are associated in a statistically significant way. The variable "climatic zone" represented 76.6% of significance in relation to the subunits, being the parameter with the highest percentage and greatest association, mostly with "Tropical" and "Subtropical" climates. The cooler climate categories ("Polar" and "Temperate") are absent in the lower subunits, and those related to the "Temperate" climate category are recorded upwards the half of the unit. The other parameters showed minor changes ("Guilds" 61%, "Substrate" 55.8%, "Bathymetry" 62.9% and "Geographic Distribution" 68.3%). However, these variables exhibited relevance and local influence on the ecology of the bivalves. Thus, we observed that the paleoecology of the bivalve community of the La Meseta Formation presents changes that suggest a response to the cooling trend at the end of the Eocene. These data may suggest that bivalves may migrate and eventually become extinct in response to environmental changes linked to temperature and climate. In this way, our data can contribute to discussions on how current climate change can affect benthic communities locally and globally.