ViewpointThere is no 1954 in that core! Interpreting sedimentation rates and contaminant trends in marine sediment cores
Highlights
► Biomixing smears sediment contaminant records, so depths do not correspond to individual years. ► Without modeling it is possible to interpret contaminant inventories and qualitative trends. ► Modeling permits the determination of sedimentation rates and contaminant histories in marine sediment cores.
Section snippets
Approach to dating biomixed cores
The steady-state radiotracer, 210Pb, is a common sediment dating tool. 210Pb is produced in the atmosphere and arrives at the sediment surface in association with particles. In addition, it is produced in situ in the sediments from the decay of 226Ra. To apply 210Pb to dating sediments, the portion that is supported by the decay of 226Ra must first be subtracted, leaving the excess 210Pb, 210PbEx 210PbEx is usually applied by assuming that a constant flux of 210Pb to the sediment surface is
An approach to interpreting contaminant and other profiles in biomixed cores
Biomixing may be an irreversible process that degrades all historical records of contaminants in sediment cores, but if it is recognized and accounted for properly, there remains much that can be learned from contaminant records in such sediments, depending on how much effort is applied.
Final comments
Mixed cores abound in the coastal marine environment, and these often provide a crucial means, sometimes the only means, to evaluate contaminant histories and pathways. Our discussion here has been primarily to illustrate that one cannot interpret a contaminant record in a mixed core without accounting for mixing. Indeed, one unstated benefit of conducting modeling of transients in sediment cores is that afterward one can never again look at a contaminant profile the same way (see, e.g., the
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Cited by (41)
Fidelity of the <sup>210</sup>Pb dating method, a subaquatic sediment perspective
2023, Science of the Total EnvironmentUsing stable isotopes and chemical markers to understand the history of Negombo Lagoon, Sri Lanka
2022, Organic GeochemistryCitation Excerpt :The resolution at which the 210Pb analyses were conducted precludes assessing the degree of potential mixing due to bioturbation in this core. However, with the relatively high sedimentation rate at this site associated with the poor hydrodynamic circulation and weak tide in the area, it can be considered that mixing on the surface of the core should not impact the observed patterns of the proxies to support the historical changes of the Negombo Lagoon (Johannessen and Macdonald, 2012). The subsamples sent to Monaco were pulverized using an oscillating Mixer-Mill (MM400, Retsch).
Recent coastal anthropogenic impact recorded in the Basque mud patch (southern Bay of Biscay shelf)
2020, Quaternary InternationalMixing depths and sediment accumulation rates on an arid tropical shelf based on fine-fraction <sup>210</sup>Pb analysis
2020, Marine GeologyCitation Excerpt :Arid weathering further produces scant fine-grained siliciclastic particles, delivered only episodically (along with sand) via floods, with potential to undercut assumptions of constant sedimentation and 210Pb delivery. In addition, the complicating effects of biogenic mixing on 210Pb profiles (e.g., Johannessen and Macdonald, 2012) should be magnified in tropical seabeds. Bioturbators capable of completely mixing the surface layer are active year-round rather than seasonally, and deep non-diffusive burrowers such as callianassid shrimp are known to rework sediment from ≥100 cm below the sediment-water interface as opposed to typically shallower depths in temperate settings (Fishelson, 1971; Griffis and Suchanek, 1991), creating potential for a zone of incomplete mixing below the surface mixed layer (e.g. Walbran, 1996; Tomašových et al., 2019).