Application of luminescence dating to the study of archaic age anthropogenic mounds from the Lower Mississippi River Valley.

摘要

Reduced luminescence intensity in soils is a widespread phenomenon but is not well understood. I examine the distributions of equivalent dose (De) values of quartz from surface and buried soils from earthen mounds in Louisiana, where pedoturbation is the likely cause of luminescence resetting. I first investigate the effectiveness of optical bleaching in surface samples similar in composition and environment to those at depth. Resulting De distributions from the top 10 cm of the topsoils approach 0 Gy. The concentration of zero-age grains decreases rapidly with depth. Paleosol ages based on minimum age analysis of the De distribution demonstrate good agreement with previously published ages. I propose a three-zone model for the explanation of surface bleaching in which the soil surface is continuously stressed and loaded by subsurface turbation mechanisms creating surface relief. Surface relief is reduced by subaerial impact- and transport-forces causing disaggregation and single-grain dispersion. This results in a surface bleaching zone where soil particles and topography are in constant transition, suggesting that many soils may offer frequent and thorough presentation of mineral grains to surface insolation, and thus high concentrations of zero-dose grains.