We measured 14C/12C in density fractions from soils collected before and after atmospheric thermonuclear weapons testing to examine soil organic matter (SOM) dynamics along a 3 million year California soil chronosequence. The mineral-free particulate organic matter (FPOM; 2.2 g cm-3) consists of relatively OM-free sand and OM-rich clays. Three indicators of decomposition (C:N, δ13C, and δ15N) all suggest increasing SOM decomposition with increasing fraction density. The Δ14C-derived SOM turnover rates suggest that ≥90% of FPOM turns over in <10 years. The four mineral-associated fractions contain 69--86% stabilized (decadal) SOM with the remainder assumed to be passive (millenial) SOM. Within each soil, the four mineral-associated fractions display approximately the same residence time (34--42 years in 200 kyr soil, 29--37 years in 600 kyr soil, and 18--26 years in 1--3 Myr soils), indicating that a single stabilized SOM pool exists in these soils and may turn over primarily as a result of soil disruption.