Paleoclimate Reconstructions from a Miocene-Aged Soil In the Atacama Desert

The Atacama Desert in Chile is presently one of the driest places on earth and contains broad expanses of well preserved Miocene soils.  Here, we examine a Miocene soil at the southern periphery of the Desert.  The present climate has a MAT of 15.5^oC and a MAP ranging from 11-50mm with sparse vegetative cover, yet the long term climate history is highly debated, and little direct evidence exists for post Miocene climate change.  The profile reveals a history of progressive aridification, based on: 1) formation of a clay-rich (>35%) Bt horizon in a gravelly silicate matrix, 2) the subsequent precipitation of a stage III CaCO₃ horizon over and above the Bt horizon, and 3) a recent, Holocene eolian sand cap overlying the surface.  The stable C isotope values of the bulk carbonate below the surface horizon decrease exponentially with depth.  Diffusion modeling suggests that the carbonate formed under a soil respiration rate of ~40 g C/m²/yr.  This respiration rate is equal to or higher than other soil respiration rates for the present ecosystem based on projections from other arid ecosystems.  Additionally, preliminary measurements of the C isotope signature of the finely laminated rinds on cobbles in the soil show significant increases (up to 4 ‰) with time, suggesting that a decline in soil respiration rates has occurred during the carbonate forming phase of the soil's formation. The C isotopes in carbonate in first two horizons have d¹³C values that indicate that these near surface CaCO₃ inputs have not been weathered and isotopically reset to soil conditions (reflecting the present hyperarid conditions).  Taken as a whole, all these data record a drastic decline in moisture since the Miocene, consistent with previous work suggesting a Pliocene onset of hyperaridity in this region.