Delayed sedimentary grains

<p>River sediment grains are transported and stored episodically in different reservoirs (terraces, alluvial fans, foreland basin, etc.). The residence time of sediment grains in each reservoir has important implications for the paleo-environmental interpretation established from these grains, and their stratigraphic record, as well as for soil contamination, when these grains come from contaminated sources. The recycling of old sediments, via erosion of an old reservoir (e.g. foreland basin erosion), is a known problem. What is less well recognised is that the recycling of a minority of very old grains can strongly bias the average residence time of a grain population deposited in a stratum. In this case, the time-dependent paleo-environmental properties of a population of grains, such as the degree of weathering, or the concentration of cosmogenic isotopes, can then be biased. Several lines of evidence for this phenomenon, inherent to fluvial transport processes, have emerged, though reconstructing the residence time distribution of a grain population over long times (>>ka) remains a challenge. Using a landscape evolution model coupled with grain transport, we show that at the scale of a piedmont, grains can remain several hundred ka before being evacuated. At the scale of a river in Northern Chile, we used the concentration of 10Be in individual pebbles to show that some pebbles remain stored for several tens of ka before being evacuated to the river outlet. In addition, the distribution of residence times can also provide information on the nature of the diffusive processes that control the fluxes of exported sediment. These results suggest that the characterisation of grain-by-grain properties in a grain population can not only help to avoid possible interpretation biases but also provide constraints for models of long-term fluvial sediment outfluxes.  </p>

A 45 kyr laminae record from the Dead Sea: Implications for basin erosion and floods recurrence

<p>Recording and analyzing how climate change impacts flood recurrence, basin erosion, and sedimentation can improve our understanding of these systems. The aragonite-detritus laminae couplets comprising the lacustrine formations that were deposited in the Dead Sea Basin are considered as faithful monitors of the freshwater supply to the lakes. We count a total of ~5600 laminae couplets deposited in the last 45 kyr (MIS3-MIS1) at the Dead Sea depocenter, which encompass the upper 141.6 m of the ICDP Core 5017-1. The present study shows that aragonite and detritus laminae are thinner and occur at high frequency during MIS 3-2, while they are much thicker and less frequent during MIS 1. By analyzing multiple climate-connected factors, we propose that significant lake-level drops, enhanced dust input, and low vegetative cover in the drainage basin during the last deglaciation (22-11.6 ka) have considerably increased erodible materials in the Dead Sea watershed. We find a decoupling existed between the significant lake-level drop/lake size reduction and lamina thickness change during the last deglaciation. We argue that during the last glacial and the Holocene, the variation of lamina thickness at the multiple-millennium scale was not controlled directly by the lake-level/size change. We interpret this decoupling implying the transport capacity of flash-floods is low and might be saturated by the oversupply of erodible materials, and indicating a transport-limited regime during the time period. We suggest that the observed thickness and frequency distribution of aragonite-detritus laminae points to the high frequency of small-magnitude floods during the last glacial period, in contrast to low frequency, but large-magnitude floods during the Holocene.</p>

Pennypack Creek Drainage Basin Erosion History: Bucks, Montgomery, and Philadelphia Counties, PA, USA

Topographic map evidence is used to interpret Pennypack Creek drainage basin erosion history in and north of the City of Philadelphia, Pennsylvania (USA). Southwest and west-southwest oriented through valleys crossing the south oriented Pennypack Creek drainage basin, barbed Pennypack Creek tributaries, and significant valley direction changes are used to determine that the Pennypack Creek valley eroded headward across massive southwest oriented floods. Initially floodwaters flowed on a low gradient topographic surface at least as high, if not higher, than the highest Pennypack Creek drainage basin elevations today. Shallow low gradient diverging and converging flow channels were eroded into the underlying bedrock surface predominantly along fault lines and other zones of easier to erode materials. Headward erosion of the much deeper Pennypack Creek valley across this anastomosing channel complex captured southwest oriented floodwaters and flow on northeast ends of beheaded channels was reversed so as to move toward the newly eroded and deeper Pennypack Creek valley. These reversed flow channels captured southwest oriented floodwaters still moving north of the actively eroding Pennypack Creek valley head. This captured water then moved in a northeast direction and eroded deep northeast oriented valleys headward from the newly eroded Pennypack Creek valley. These valleys today account for northeast and east oriented Pennypack Creek valley segments and northeast oriented (barbed) tributaries flowing to south oriented Pennypack Creek. The floodwater source cannot be determined from Pennypack Creek drainage basin evidence, but was from the northeast. Melting of a continental ice sheet could produce floods of sufficient volume and duration to overwhelm whatever drainage system previously existed and to erode new drainage basins in a manner similar to how the Pennypack Creek drainage basin was eroded.