科研之友
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摘要
Lacustrine sediments in deep lakes are suitable for establishing radiocarbon chronology. Lacustrine sediments in shallow lakes are not suitable for establishing radiocarbon chronology, simply because younger plant roots can readily penetrate into older lacustrine sediments. Three features are warning signs for shallow-lake sediments: (1) high concentration of salts, (2) abundant vertically-oriented plant residues, and (3) organic matter-enriched sand layer. Field and laboratory investigations of eolian and water-reworked eolian sequences suggest that fluvial addition and fluvial alteration on terraces may produce reversed chronologies. Fluvial layers interbedded within eolian sequences should be avoided in obtaining dating targets, and eolian layers isolated by fluvial or lacustrine layers should be the targeting layers from which dating samples should be obtained. Bulk sediment C-14 dates of a paleosol at Yuanbao loess section in the western Chinese Loess Plateau are distinctively older than charcoal dates, suggesting that bulk sediment samples contain older organic carbon that was likely brought upward by animal burrowing. In contrast, bulk sediment dates were found to be systematically younger than snail and charcoal dates at the Romantic section in Eastern Kazakhstan. The younger dates likely resulted from the downward intrusion of plant roots and/or translocation of mobile organic compounds into deeper layers. The only suggestion is to avoid bulk sediments as dating targets in eolian-paleosol sequences. The misleading chronology for the upper loess-paleosol complex at Lujiagou section indicated that even continuous loess deposits may yield reversed chronologies where loess has inherited older organic carbon from deflated floodplains and from loess hills. All kinds of dating targets (including charcoals) may produce unreliable chronologies.
