Abstract
Floodplain sedimentation removes particles from fluvial transport and constructs stratigraphic records of flooding, biogeochemical sequestration and other aspects of the environmental history of river basins-insight that is enhanced by accurate geochronology. The natural fallout radionuclide (210)Pb, often employed to date lacustrine and marine sediments, has previously been used to determine floodplain accumulation rates over decadal-to-century time scales using the assumption that both input concentration and sediment accumulation rates are constant. We test this model in approximately 110 cores of pristine floodplains along approximately 2000 km of the Rios Beni and Mamore in northern Bolivia; over 95 per cent of the (210)Pb profiles depict individual episodic deposition events, not steady-state accumulation, requiring a revised geochronological methodology. Discrete measurements of down-core, clay-normalized adsorbed excess (210)Pb activity are coupled with a new conceptual model of (210)Pb input during floods: constant initial reach clay activity, unknown sedimentation (CIRCAUS). This enhanced methodology yields (210)Pb dates that correspond well with (i) dates determined from meteoric caps, (ii) observed dates of river bar formation, (iii) known flood dates, and (iv) dates from nearby cores along the same transect. Similar results have been found for other large rivers. The CIRCAUS method for geochronology therefore offers a flexible and accurate method for dating both episodic (decadal recurrence frequency) and constant (annual recurrence) sediment accumulation on floodplains.
Highlights
River histories run from millions of years to hours: exceptional floods can transform a valley through avulsion, regular floods orchestrate channel migration, and even the smallest flood results in some sediment accretion
Itotal − Iatm Acatch where lPb is the 210Pb decay constant (0.031 yr−1) and Acatch is the assumed constant concentration of 210Pb in catchment-derived sediment that can be collected and measured. The reliability of this CICCS model depends on several important assumptions: (i) the mean 210Pb activity of river-derived sediment is relatively constant during floods, (ii) the accumulation rate is roughly constant over decadal time scales, (iii) atmospheric fallout is constant over decadal time scales, and (iv) the total depth of coring is sufficient to reach background supported 210Pb activity, such that
The observed activity profile can be reconstructed using the discrete-depth approach, given the following assumptions: (i) sediment accumulation rate is constant, (ii) the 210Pb activity of clay in freshly deposited river sediment is constant over time for that particular reach of channel (6.5 dpm g−1 clay here), and (iii) the meteoric fallout of 210Pb is constant (44 ± 2 dpm cm−2 documented for many locations throughout the Beni Foreland), and is adsorbed by the surface sediment within a depth interval of 5 cm
Summary
River histories run from millions of years to hours: exceptional floods can transform a valley through avulsion, regular floods orchestrate channel migration, and even the smallest flood results in some sediment accretion. Unlike studies that assume constant initial concentration and a constant sedimentation rate (the CICCS model) from bulk-averaged 210Pb concentration (and sometimes grain size) for entire cores, we sampled our cores at discrete intervals and at homogenized depths of a few centimetres so that our results yield excess adsorbed 210Pb activity profiles that are normalized to clay abundance (the carrier of 210Pb). Such detailed profiles document dates and thicknesses of sediment accumulation [2] and facilitate the evaluation of the CICCS model for large, dynamic and heterogeneous fluvial systems. We recommend revised laboratory and analytical procedures to determine clay-normalized absorbed excess (CNAXS) 210Pb activities and a more flexible set of assumptions for dating
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