Paleobiogeoclimatic scenarios of the Late Quaternary inferred from fluvial deposits of the Quadrilátero Ferrífero (Southeastern Brazil)

Abstract

The Quadrilátero Ferrífero is an important mineral province in Southeastern Brazil and has one of the largest iron ore reserves in the world. Previous work in this region has indicated that the formation of fluvial successions with duricrusts coincided with drier/cooler climatic phases alternating with moister/warmer periods during which the formation of fluvial successions without duricrusts occurred. For the construction of this proposal, ages of fluvial sediments obtained through Optically Stimulated Luminescence (OSL) were associated with data from the literature on paleobioclimatic scenarios. Therefore, using these observations as a starting point, this paper aims to investigate evidence of bioclimatic oscillations obtained directly from the fluvial successions and discuss its influence on the geomorphogenis of local river valleys. For this purpose, phytolith, carbon isotope and granulometric analyses were carried out, as well as dating of sediments using OSL and of soil organic matter through radiocarbon. The results show that in the oldest depositional succession (DS1 — about 34ka) the predominant phytoliths are those of bulliform polyedric, elongate, acicular and globular granulate types and δ13C values are typical of C3 plants. On the other hand, despite having a similar phytolith assemblage (abundance of bulliform polyedric, elongate, bulliform cuneiform, acicular, globular psilate and bilobate flat/concave types), the fluvial successions associated with significant conglomeratic duricrusts (DS2 and DS3) present a dominance of δ13C values characteristic of C4 plants. The Bi index indicates water stress in all the successions, and the Ic index suggests decreasing temperatures with depth in DS3. Thus, the three fluvial successions indicate a savanna-like environment, but depositional successions DS2 (∼27ka) and DS3 show drier/cooler climatic conditions when compared to DS1 and to the present-day regime. Both scenarios evolved under conditions of the Last Glacial period, but DS2 and DS3 were formed closer to the Last Glacial Maximum, and therefore under the strong influence of the lower temperatures during this period. These drier/cooler conditions in steep valleys with unprotected hillslopes may have been decisive for the formation of relatively thicker layers of gravel and sand, which later became duricrusts. The results indicate that climate has also played an important role in the regional hydrosedimentological dynamics, given the variations in vegetation influencing the abandonment of fill terraces and formation of nested floodplains.