Resultados

Hughes et al. (2003) conducted a small-scale spectral analysis study of 256 rhythmites from Outcrop A, denoted by them as the Wharncliffe argillite. Noting that it was not possible to determine the period of couplet deposition from field evidence, yet

recognizing dropstones in beds of laminated couplets of siltstone and claystone, Hughes et al. (2003) proposed that the couplets formed in a glaciolacustrine environment in association with recurring, annual, freeze-thaw cycles. Within the framework of this assumption of annual control, the authors proposed that variations in the thicknesses of the couplets in the measured section resulted from variations in climate on a year-over- year basis. Hughes et al. (2003) measured the couplets from photographs of the outcrop, and used both the DFT and the maximum entropy method to assess the data. Both of

these methods provided profiles of average spectral power at a given frequency using the complete data set. In order to allow for the assessment of variations in spectral power within the data set, Hughes et al. (2003) also conducted a coherent state analysis using a window of 20 layer couplets. A coherent state analysis seeks correlation amongst spectral peaks, and effectively identifies which peaks recur, and can thus identify whether or not a spectral peak is persistent throughout the dataset. In the Hughes et al. (2003) study, the most prominent peak in the couplet pattern was found to be at 14.3 layer couplets, with lesser peaks found at 27.8, 10.7, and 9.9 layer couplets. Hughes et al. (2003) connected the peak they found at 14.3 layer couplets with a peak period of 14.2 years found in the spectral analysis results of a regional temperature time series taken from central England. The peak at 14.3 layer couplets and the peak at 27.8 layer couplets of the Hughes et al. (2003) study also correlated well with the range (10.4 years – 27.3 years) of spectral peaks found in a study of the historical rainfall records of the Los Angeles Basin in California, USA. The periodicities of 10.7 and 9.9 layer couplets were interpreted as belonging to a single coherent mode, which the authors correlated to variability in the historical sunspot cycle record. Hughes et al. (2003) postulated that solar cycle variations could influence temperature and precipitation levels on an interannual time scale, citing sedimentary examples from the Modern, Eocene and Proterozoic. From these results, the authors concluded that both temperature and rainfall could have potentially influenced the thickness of the couplet layers found in the Gowganda Formation.

Hughes et al. (2003) also described a number of spectral peaks as holding less statistical significance due to a lack of coherence throughout the entire sequence. Describing these signals as “quasi-periodic”, the authors noted that quasi-periodic signals have been observed in modern climatic studies. For example, the El Niño Southern Oscillation (ENSO) is described as having a quasi-periodic cycle that ranges from 3 to 7 years (e.g. Rittenour et al., 2000; Breckenridge, 2007). In the Hughes et al. (2003) study the following quasi-periodic peaks were observed:

a) 2.97 and 3.17 layer couplets – peaks in the range of 2 to 70 months resemble the Quasi Triennial Oscillation, which involve modelled variances in atmospheric pressure

b) 6.10 layer couplets – similar to a 6.24 year peak in the atmospheric pressure spectrum associated with the Atmospheric Pole Tide (Hameed et al., 1995),

c) 4 – 5 layer couplets – several small, non-coherent peaks within this range are similar to peaks found in the modern record that result from the influence of ENSO (3 – 7 years) and the North Atlantic Oscillation (NAO, 5 – 7 years) (Appenzeller et al., 1998), d) 77.2 layer couplets – perhaps associated with an NAO spectrum in the 80 – 90 year range (Appenzeller et al., 1998).

In connection with (a) and (b) above, Hughes et al. (2003) note that variances in sedimentation rates, and thus in layer couplet thickness, do not result directly from changes in atmospheric pressure variances. Changes in atmospheric pressure result from variations in solar flux, which is implied to lead to variances in layer couplet thickness. Hughes et al. (2003) acknowledge that all the above-noted periodicity correlations assume annual control for the Gowganda rhythmites.

Within this context, Hughes et al. (2003) identify events that may lead to discrepancies within the sedimentary record, and thus in the spectral analysis results of the rhythmite measurements. First, turbidite deposits may result in the deposition of one or more rhythmites that are not associated with the inferred annual freeze-thaw cycle. Second, extended periods of warm climate could lead to the deposition of multiple interannual couplets, and extended periods of cold climate could lead to periods of no deposition at all. Irregular variations between warm and cold periods could also confound attempts to diagnose interannual periodicities. Hughes et al. (2003) demonstrate that minor

irregularities in the depositional process will not significantly change the results of the spectral analysis; however larger temporal or depositional anomalies could significantly alter the spectral profile, diluting the accuracy of the results. In conclusion, Hughes et al. (2003) suggest that the rhythmic deposition of the Gowganda Formation couplets

warrants the interpretation of annual deposition in a glaciolacustrine environment, and that the spectral profile obtained from the sequence measured provides a moderate to robust signal of Paleoproterozoic climate. It should be noted however that not all authors agree with this interpretation. Citing the paleomagnetic results of Williams and Schmidt

(1997), which place the deposition of Huronian rocks at between 4° and 11° latitude (where seasonal freeze-thaw cycles generally do not exist in modern time), Young (2013) suggests that an alternate, non-seasonal mechanism may have produced the varve-like rhythmites.