We report detailed depth profiles of the particle-reactive radionuclides 210Pb, 137Cs, and 7Be in sediment cores collected at five different sites from a salt marsh near North Inlet, South Carolina. Except for creekbank sites where bioturbation is intense, average recent accumulation rates determined by the 210Pb method (1.4-4.5 mm yr-I) agreed with accumulation rates over the past 20 yr (1.3-2.5 mm yr-I) with 137Cs as a stratigraphic marker. The mean rate of sea level rise over the past 50 yr determined from tide gauge records (-3.0 mm yr-I) is about the same as the sedimentation rate.Calculated 210Pb fluxes (0.93-l .55 dpm cm-2 yr-l) in back-marsh areas are in good agreement with atmospheric measurements at New Haven, Connecticut, and 7Be fluxes (4.7-6.8 dpm cm-2 yr-I) are lower by a factor of about three. Along creekbanks, however, both nuclides are about an order of magnitude greater than in back-marsh areas due to the greater intensity of fiddler crab burrowing on creekbanks. We present a mathematical model for regeneration of 210Pb at depth that can account for the observed difference in inventories between back marsh and creekbank by crab burrowing. We further suggest that intense crab burrowing is at least partly responsible for the enhanced growth of Spartina along creekbanks by virtue of its impact on the turnover of iron and sulfur in creekbank sediments.Two assumptions underlying the study of sediment mixing and accumulation rates with particle-reactive tracers are that the flux of a radionuclide is constant at the sediment-water interface (the so-called constant rate of supply or CRS model) and that the radionuclide is chemically immobile in the sediment column. In the case where sediment of unvarying composition is deposited at a constant rate, the CRS model is equivalent to the CIC (constant input concentration) model. However, if the sediment that is deposited at the interface is rapidly mixed via bioturbation with material from deeper in the column, the concentration of the radionuclide in the sediment at the top of the column will be somewhat less than its concentration in the sediment arriving at the interface via deposition, even if the assumptions of the CIC model are in 1 Financial support for this was provided by NSF

This study presents the first direct comparison of the influence of liquid-crystal order during synthesis on the thermo-mechanical behaviors of main-chain liquid-crystal elastomers (LCEs) in thiol-acrylate networks. Six polydomain nematic elastomer (PNE) chemistries were compared directly by synthesizing with the mesogens in either an isotropic state (i-PNE) or a nematic state (n-PNE). The i-PNE networks were created in the presence of solvent, which disrupted any liquid-crystal order during network formation. Conversely, the n-PNE networks were created without the presence of solvent below the isotropic transition (T). Differential scanning calorimetry (DSC) was first performed, and it showed that i-PNE networks experienced a clearly defined nematic-to-isotropic transition upon heating, whereas the transition in n-PNE networks was unable to be identified, which may be the result of a nematic-to-paranematic phase transition. Dynamic mechanical analysis (DMA) tests revealed that while both networks maintained elevated loss tangent in the nematic region, only i-PNE networks prominently displayed dynamic soft elasticity behavior. The two-way shape switching behaviors of LCE networks were examined using actuation tests under a 100 kPa bias stress. It showed that the strain amplitude strongly depends on synthesis history; it ranges from 66% to 126% in i-PNE samples and 3% to 61% in n-PNE samples. To help interpret the different actuation strain behaviors between i-PNEs and n-PNEs, wide-angle X-ray scattering (WAXS) was then performed where the LCE samples were strained to 40%. The results showed that order parameter (S) in n-PNE samples (ranging from 0.37 to 0.50) is lower than that in i-PNE samples (0.54 for all cases), and the parameter decreased as the cross-linking density increased. The stress-strain behaviors of the LCE networks measured from uniaxial tension tests revealed that all i-PNE samples had a lower soft-elasticity plateau during loading compared to the n-PNE samples. Finally, free-standing strain recovery of LCE samples after being strained to 100% was investigated. Immediately after removing stress on the samples, i-PNE and n-PNE samples recovered 14% to 38% and 27% to 73% of strain, respectively. We discuss the advantages and disadvantages of the different synthetic histories on LCE design.