surface temperatures over North America and the North Pacific (Minobe, 1997; Biondi et al., 2001; Chavez et al., 2003), and also in reconstructions of SST’s from tree ring data (D'Arrigo et al., 1999). Similar frequency peaks are also present in spectral estimates from varved sediments from the Santa Barbara Basin (Bull et al., 2000), Saanich Inlet (Dean & Kemp, 2004), Arabian Sea (Berger & von Rad, 2002), Effingham Inlet (Patterson et al., 2004) and Gulf of California (Pike & Kemp, 1997) (Table 5.3). Dean and Kemp (2004) interpreted the 42.2 year peak identified in the biogenic varve segment from Saanich Inlet to relate to the occurrence of strong or modulated PDO events. Pike and Kemp (1997) similarly interpreted the 50 year peak recorded in the occurrence of Thalassiothrix mats to relate to the 50-70 year PDO oscillation, further surmising that the
periodicity may be forced by solar variability. Patterson et al. (2004) also interpreted the ~45 year cycle in varve thickness to relate to the PDO, but found it not to relate to solar forcing. The 44 year cycle identified in the occurrence of turbidites in sediments from the Arabian Sea was interpreted by Berger and von Rad (2002) to be a multiple of the tidal Perigee cycle.
An oscillation similar in character and frequency to the modern 50-70 year PDO oscillation could explain the 44 year peak in terrigenous laminae thickness present in the Marca Shale. Alternations between a warm (El Viejo) and cool (La Vieja) phase of such a cycle would result in long term wet and dry episodes along the proto Californian margin similar to the positive and negative phases of the PDO along the modern Californian margin (Fig. 5.18). Alternatively, the 44 year peak may be tidally forced, as proposed by Berger and von Rad (2002), or may be a harmonic of the sunspot or Hale cycle. However, it is difficult to reconcile the presence of a multiple of the Perigee cycle in a spectrum devoid of peaks relating to the Perigee cycle itself (4.425 years) and its other prominent multiples (e.g. 17.7 years). The 44 year peak is therefore interpreted to relate to a PDO-like oscillation similar to that effecting the modern North Pacific and North America (Minobe, 1997; Biondi et al., 2001; Chavez et al., 2003), the frequency of which may have been modulated by solar variability.
5.4.5 The Bjerknes feedback and a permanent El Niño state
As discussed in Chapter 5.4.1.3 the Maastrichtian climate state and global configuration should theoretically have played host to a severe weakening of the “Bjerknes” feedback loop and caused a severe decline or cessation of ENSO variability. However, data presented here demonstrates that an oscillation analogous to ENSO was a robust feature of the Maastrichtian climate and consequently the Maastrichtian was not characterised by a permanent El Niño climate state, consistent with a fully coupled ocean-atmosphere simulation for the Campanian (Otto-Bliesner et al., 2002). The concept of a permanent El Niño climate state was recently questioned by Huber and Caballero
Chapter 5 Time-series Analysis of the Marca Shale
frequencies (Ripepe et al., 1991; Mingram, 1998) and have been supported by fully coupled climate model simulations, collectively demonstrating that ENSO variability was a robust feature during this “hothouse” climatic period (Garric & Huber, 2003; Huber & Caballero, 2003). This research therefore adds to evidence that warm end-member climate states do not tend to collapse into a permanent El Niño state and that ENSO variability is not only confined to the dynamic climate system of the Quaternary. Although the theory of a permanent El Niño does explain the characteristics of many past climates very satisfactorily, it cannot be used as an explanation for the Maastrichtian climate state.
5.5 CONCLUSIONS
• Spectral analysis of bioturbation index records and thickness variations in diatomaceous and terrigenous laminae reveal numerous significant spectral peaks within well known frequency bands of modern climate cycles.
• All parameters analysed demonstrate strong variance in the ENSO frequency band, which together with other sedimentological evidence indicates that similar to the later greenhouse climate of the Eocene, the latest Maastrichtian climate system was host to robust ENSO-like variability, adding to the body of evidence conflicting with the theory of permanent El Niño climate states.
• Mean ENSO periodicity in laminae thickness variations was 5.3 years and that of strong events 9.3 years (although strong events are only recorded in diatomaceous laminae).
• Bioturbation index records show an ENSO peak at 6.2 years and strong ESNO peaks of 10.3 and 10.8 years. The periodicity of the strong ENSO peaks in bioturbation index may have been modulated by solar variability. Similarities in the bioturbation index and diatomaceous laminae thickness records adds to the evidence that benthic oxygen levels were largely controlled by diatom export flux and that variability at decadal/ sub-decadal periods in the two records were forced by the same mechanism (ENSO).
• Pervasive quasi-biannual peaks are present in laminae thickness records, inferred to be related to a cycle analogous to the QBO. The mean periodicity of quasi-biennial peaks in the Marca Shale (27.4 months) and the modern QBO (28 months) is striking.
• Several multi-decadal peaks were identified in laminae thickness variations at 20.0, 26.5 and 44 years and inferred to relate to an oscillation analogous to the PDO. The 20.0 and 26.5 year peaks relate to the shorter period component of the PDO, whilst the 44 year peak may be a manifestation of the longer period component of the PDO. Both the ~20 year peaks and the longer 44 year peak may have been modulated by solar variability.