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Periodic external environmental forcings, typically operating at Milankovitch frequencies, are known to be encoded in sediments and sedimentary rocks, but sub-Milankovitch frequencies are more difficult to recognize, in part because of temporal overlap with autogenic processes. Here we use luminescence geochronology and rock-magnetic and lithostratigraphic-based cyclostratigraphy to investigate sub-Milankovitch periodicity in three high sedimentation rate deltas located in diverse tectonic and climatic settings. The Sciota kame delta in tectonically stable eastern Pennsylvania (USA) was deposited at a rate of ~2.5 cm/yr over ~1 kyrs and has a concentration of magnetic spectral power at ~53 cm, corresponding to a ~22 yr period. Lacking a subaerial fluvial catchment, periodicity in this delta is necessarily restricted to depositional processes or ice-wasting discharge fluctuations. Similarly, the Provo-stage Lake Bonneville High Creek delta (Utah, USA) was deposited at a rate of ~3 cm/yr over ~3 kyrs. It displays meter-scale coarsening-up bedsets interpreted as decadal-scale discharge variations during foreset propagation. Unlike the Sciota kame, the High Creek delta is fed by a subaerial catchment with little storage that supported a small cirque glacier during the LGM. The High Creek delta also aggraded in the subsiding hanging wall of the East Cache Valley fault and experienced at least one syn-depositional earthquake. Lastly, The Pagliara delta (northeast Sicily, Italy) was deposited at a rate of ~3 cm/yr in the rapidly uplifting footwall of a coastal seismogenic normal fault. This delta has a concentration of magnetic susceptibility spectral power at ~60 cm corresponding to a ~20 yr period. A multi-decadal periodicity emerges in all three deltas, despite the variable catchment, climate, and tectonic setting. We interpret this as representing a quasi-periodic, autogenic depositional process possibly related to distributary channel switching. However, differences in the rock-magnetic power spectra and lithostratigraphy of the Pagliara and High Creek deltas respectfully, indicates that sub-Milankovitch external forcings in the catchments, including tectonics, are also encoded in the delta stratigraphy. 

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. 

Interactions between deep Earth geodynamics and Earth surface processes are well documented at various scales, but many challenges remain in how inversion of a fluvially incised landscape should be interpreted in terms of long-term geodynamics or how deep Earth dynamics impact natural hazards. Here, we present results from geomorphic stream channel metrics and modeling of long profiles of streams draining the Tyrrhenian (northern) flank of Sicily (Italy), to assess the inferred, rapid, west-to-east horizontal translation of the Calabrian forearc. A detachment-limited stream power model-based determination of landscape response time and knickpoint migration provides an independent prediction for transient base level fall associated with the sweeping forearc over the past ~4 Ma. The model shows that two pulses of time-transgressive, west-to-east propagating base level fall occurred in the drainages of northern Sicily, where parallel north-flowing streams are arranged across the migrating path of the forearc. The long profile analysis indicates that the paired uplift pulses last ~1 Ma and are separated in time by ~1.5 Ma, consistent with the west-to-east passage first of the forearc high, followed by dynamic uplift in its wake due to sub-lithospheric mantle flow, as proposed in other plate boundary settings. The ongoing surficial response to these dynamics is represented by river incision, knickpoint migration, and drainage divide migration. Furthermore, these processes steepened the landscape, leading to an increase in active landsliding and contributing to the natural hazards in this region.