Most organisms maintain a nearly 24-hour circadian rhythm which allows them to anticipate daily events in the Earth’s solar day. Circadian clocks can be regulated by external time cues such as light/dark (LD) cycles, allowing nocturnal organisms to synchronize their rhythms around dusk in a process known as entrainment. In the absence of external time cues, these rhythms persist, revealing the period of the organism’s internal clock. In nature, day-night cycles have gradual transitions at dawn and dusk, yet these transitions often are not reflected in laboratory studies. Most utilize instantaneous transitions from light to dark, including previous work on spider circadian behavior. To mimic natural conditions, the next logical step is to evaluate how a gradual, dusk transition affects activity patterns and entrainment in nocturnal spiders. Two spider species were used: Pholcus phalangioides and Metazygia wittfeldae. Three hypotheses were identified: 1) spider circadian behavior is unaffected by the type of dusk transition; 2) spider activity entrains to a proportional decrease in light intensity; and 3) spider activity entrains to a specific dimness threshold. Spiders were placed in activity monitors and exposed to 12:12 LD cycles with high (8934 lux) and low (2918 lux) light intensities. There were two groups within each light intensity: one with instantaneous, dusk transitions and one with linear 4-hour transitions. In all experiments, spiders were switched to constant darkness after 5 days of LD cycles to identify the phasing of the circadian clock. In P. phalangioides, there were no significant differences among onsets of activity for high and low light intensities with ramped transitions, nor high and low intensities with instantaneous transitions. The average onset of activity was 0.2 ± 0.1 hours after absolute darkness. For M. wittfeldae, onsets of activity were significantly earlier in ramped than instantaneous LD cycles. However, there was no observable difference in activity onset associated with absolute light intensity for either transition type. The average onset of activity for ramped transitions was 1.9 ± 0.5 hours before complete darkness, while the average onset for instantaneous transitions was 0.5 ± 0.1 hours after complete darkness. Pholcus phalangioides requires complete darkness before activity begins, supporting the hypotheses that either the activity onset is unaffected by the type of dusk transition or requires a very low threshold of light. However, M. wittfeldae began activity about 2 hours earlier or around when light decreases by 50%, supporting the hypothesis that this species entrains to a proportional decrease in light intensity. These activity patterns match their ecologies, as P. phalangioides typically sits and waits in a permanent web while the orbweaver M. wittfeldae begins building a new web during the dusk transition. Overall, these results suggest that ramping light cycles should be considered when circadian rhythms are evaluated in the lab.

Paleosols contain information about the rates of soil organic carbon turnover when the soil was actively forming. However, this temporal information is often difficult to interpret without tight stratigraphic control on the age of the paleosol. Here we apply ramped pyrolysis/oxidation (Ramped PyrOx) C-14 analyses to evaluate age spectra of transgressive early Holocene paleosols from the Mississippi Delta in southeastern Louisiana, USA. We find C-14 age spectra from soil organic matter (SOM) in both paleosols and overlying basal peats that represent variability in age that is close to, or only slightly greater than, analytical uncertainty of C-14 measurements, despite different sources of carbon with likely disparate ages. Such age spectra have not previously been observed in the sedimentary record. Here they indicate vigorous soil carbon turnover prior to burial, which homogenized C-14 ages within SOM across the entire thermochemical spectrum. The weighted bulk C-14 ages from Ramped PyrOx of paleosols and overlying peats are identical within analytical and process-associated uncertainty, and corroborate C-14 ages from charcoal fragments and plant macrofossils from the overlying peat. The youngest ages from Ramped PyrOx age spectra may also potentially be applied as chronometers for stratigraphic burial ages. Our results suggest rapid turnover (<<300 years) of carbon in these soils relative to input of allochthonous carbon, indicating that the C-14 age of different soil components is decoupled from thermochemical stability and instead reflects vigorous turnover processes. The concurrence of paleosol and peat C-14 ages also suggests that pedogenic processes were linked with the development of coastal marshes, and that the priming effect potentially masked the signal of allochthonous carbon inputs during sea level rise.

In order to determine the timing of Antarctic ice sheet retreat and advance during the Late Quaternary, various tools are used to measure the age of marginal marine sediments. Carbonate 14C dating is a well-established approach, but requires foraminiferal microfossils, shells or other carbonate materials that are rare in most Antarctic regions, and may also suffer from vital effects, which can result in variability of up to 500 years in living organisms. Bulk acid insoluble organic (AIO) 14C dates are frequently as an alternative, but this approach works best where high productivity and sedimentation rates reign, and not too well in condensed sequences where high proportions of detritus are present. Compound specific dating methods have also been employed, but these may still yield an average age from a mixture of components and require very large sample sizes. Alternate methods of applying a chronology have also been used, such as magnetic intensity dating, or regional correlation with well-dated cores, but these may not always provide accurate and precise dates. Here I present work, some published with co-authors, of progressive improvements of Ramped PyrOx 14C dating, which utilizes the thermochemical degradation of components within a bulk AIO sediment sample. This dissertation focuses on the study, improvement and application of advanced Ramped PyrOx techniques. These improvements include novel techniques, such as compositing and isotope dilution that I use to date sediments where the proportion of contemporaneously deposited carbon is very small relative to other detrital components, and maximize the accuracy of resulting dates while minimizing costs in precision from utilizing ultra-small fractions of the bulk sample. Ramped PyrOx 14C dating techniques allows us to generate chronologies for cores that would otherwise go undated. Furthermore, these techniques can be used to push the limits of radiocarbon dating not only to regions where accurate core chronologies have been difficult to come by, but also further back in time, into marine sediment horizons deposited at or before the last glacial maximum (LGM), where highly detrital material has precluded radiocarbon dating in the past. Wider use of these techniques can enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming.