RADIOCARBON DATING OF SOIL ORGANIC MATTER FRACTIONS IN ANDOSOLS IN NORTHERN ECUADOR

Abstract

Volcanic ash soils (Andosols) may offer great opportunities for paleoecological studies, as suggested by their

characteristic accumulation of organic matter (OM). However, understanding of the chronostratigraphy of soil organic matter

(SOM) is required. Therefore, radiocarbon dating of SOM is necessary, but unfortunately not straightforward. Dating of fractions

of SOM obtained by alkali-acid extraction is promising, but which fraction (humic acid or humin) renders the most accurate

14C dates is still subject to debate. To determine which fraction should be used for 14C dating of Andosols and to evaluate

if the chronostratigraphy of SOM is suitable for paleoecological research, we measured 14C ages of both fractions and related

calibrated ages to soil depth for Andosols in northern Ecuador. We compared the time frames covered by the Andosols with

those of peat sequences nearby to provide independent evidence. Humic acid (HA) was significantly older than humin, except

for the mineral soil samples just beneath a forest floor (organic horizons), where the opposite was true. In peat sections, 14C

ages of HA and humin were equally accurate. In the soils, calibrated ages increased significantly with increasing depth. Age

inversions and homogenization were not observed at the applied sampling distances. We conclude that in Andosols lacking

a thick organic horizon, dating of HA renders the most accurate results, since humin was contaminated by roots. On the other

hand, in mineral soil samples just beneath a forest floor, humin ages were more accurate because HA was then contaminated

by younger HA illuviated from the organic horizons. Overall, the chronostratigraphy of SOM in the studied Andosols appears

to be suitable for paleoecological research.