Pliocene weathering processes recorded at mid-latitude in Southern Brazil

Paleoclimatic reconstructions suffer from difficulties in reconciling the more complete oceanic paleoclimatic record with the fragmentary nature of continental paleoclimatic proxies. Soils and weathering profiles potentially record paleoclimatic information, but retrieving this information requires precise geochronology for soils and weathering profiles at the time scales of interest. 40Ar/39Ar geochronology of randomly selected pedogenic K-bearing manganese oxyhydroxides (32 grains from 16 hand samples) from deep saprolites in the Upper Ribeira River Valley, southern Brazil (25°S lat.), reveals a history of weathering and mineral precipitation spanning the Pliocene (4.1 ± 0.1 to 2.0 ± 0.2 Ma), suggesting that weathering-prone conditions persisted in this region during this period. Since weathering processes are driven by water-rock interaction, the abundance of weathering minerals precipitated in this time interval suggests high rates of meteoric precipitation in this region during the Pliocene. This interpretation is consistent with evidence from the oceanic record indicating that the Pliocene was a time of warm sea surface temperatures at this latitude. Humid conditions for this mid-latitude zone are also consistent with paleoclimatic models suggesting a permanent cyclone along the eastern margin of South America at the time, where the westward prevailing winds would bring excess moisture from the warm coastal seas. A probability density plot for the Mn oxide precipitation record for this site also reveals that the frequency of mineral precipitation during the Pliocene was not constant, and two periods (3.2-2.9 and 2.5-2.1 Ma) witnessed enhanced weathering. The first period corresponds to the Pliocene climate optimum, consistent with our interpretation that warm and more humid conditions would result in enhanced weathering. The second period of enhanced mineral precipitation does not correspond to any previously identified climatic excursion. More complete paleoclimatic records for the South America may contribute to a better reconciliation of the continental and oceanic records for the region.