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We explore time series of magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM) in settings of rapid sediment accumulation rate (SAR) with the goals of partitioning exogenic forcings from autogenic processes and to better understand how these magnetic signals are encoded in sedimentary archives. Environmental signals of periodic external forcings commonly operate at Milankovitch frequencies, but in rapid SAR settings autogenic processes including channel avulsions and delta lobe switching both shred high-frequency external forcings, or even impart their own quasi-periodic signals. We measure χ using both a hand-held KT-10 magnetic susceptibility meter and a lab-based Kappabridge KLY-3s, and ARM in the < 2 mm size fraction using a GSD-5 alternating-field and a 2G superconducting magnetometers, with all results mass normalized to SI units. We focus on 40 samples collected at 25 cm intervals from 10 m of propagating foresets in a Gilbert delta of the Provo stage of Lake Bonneville at High Creek, Utah. A luminescence-based age model in this delta establishes a mean SAR of 8 cm/yr and terrestrial cosmogenic nuclide concentrations of both delta sediment and alluvium in the source indicates modern and paleoerosion rates (E) ranging from ~60-100 m/Myr (0.006-0.01 cm/yr). Periodicities of 18 and 33 yrs in the rock magnetic time series are greater than twice the compensation time for these foresets where peaks in χ and ARM are positively correlated with fine-grained matrix. We interpret a positive correlation between E and χ as driven by stripping of soil-mantled hillslopes that harbor greater concentrations of magnetic minerals than the underlying bedrock. The encoding of the environmental signal, here interpreted as autogenic cascading of sediment on foreset surfaces, is primarily set by the SAR and depositional processes, which are decoupled from E. Nevertheless, the strength of the magnetic signal in our sedimentary archive varies with E which can be more widely explored as a E-proxy when locally calibrated. These results offer insight into how to isolate the impact of quasi-periodic tectonic forcings on stratigraphic archives at sub-Milankovitch frequencies, where autogenic processes dominate depositional processes but which also encode critical human-dimension natural hazard information.
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Exogenic Forcing Encoded in the Lithostratigraphy of Sand-Quarry Deltas
We analyze sediment texture, rock-magnetic, and depth-rank time series of meso-scale deltas and interpret the results in terms of autogenic depositional processes and exogenic forcings. As an analogue for natural deltas, this study leverages a semi-controlled environment where the deltas prograde rapidly into quarry settling ponds over decadal time spans and have compensation times on the order of ~2.4 months. The distal parts of two deltas were cored with a Geoprobe to a depth of 8.4 m. Recovery ranged from 30% to 70%, that we model as either sediment compaction or missing section (unconformities). The compaction model allows us to generate a complete time series of a decompacted section whereas the unconformity model allows us to consider the impact of significant periods of missing time. The cores were analyzed every 2 cm for magnetic susceptibility as well as grain size, texture, color, pebble content, and organic content, all of which contribute to an overall relative depth and textural ranking. Multi-taper method red-noise modeling of the time series using Astrochron identifies frequencies which rise above a 99% confidence level. The power spectra show a range of peaks, many of which fall below the compensation time and are disregarded. A significant periodicity of 2.6 months emerges in the compaction model. In the unconformity model, a peridocity of 2.6 months and also longer periodicities of 3.5 and 6 months emerge. These seasonal-scale periodicities are similar to those in regional precipitation data (4.4, 3.1, and 2.6 months) and suggest that exogenic forcing, in this case from precipitation that impacts both discharge in the source and water depth of the settling pond, are strong enough to be encoded in the sediments. Meso-scale studies of depositional systems such as these quarry deltas provide a bridge between small-scale analogue models and natural source to sink systems that we are in the process of sampling to further test our approach.
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- Award ID(s):
- 1904262
- PAR ID:
- 10231577
- Date Published:
- Journal Name:
- AGU Fall Meeting
- Page Range / eLocation ID:
- EP019-0017
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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