Numerous peaks within the quasi biennial frequency band are present in laminae thickness records from several sample sets in the Marca Shale (Table 5.1 and Fig. 5.15), equally common in
diatomaceous and terrigenous laminae records. Such peaks in modern climate records may be related to the biennial component of ENSO variability (Rasmusson et al., 1990). However, in the Marca Shale solitary biennial signals are present without a concurrent ENSO frequency band signal (Fig. 5.8) indicating they must have an independent origin, similar to solitary QBO signals
recorded in varved sediments from Saanich Inlet (Dean & Kemp, 2004). The only known biennial periodic forcing mechanism is the QBO and thus significant spectral peaks with quasi-biennial frequencies are interpreted to relate to a QBO-like oscillation. If the stratospheric QBO results from equatorially trapped Kelvin waves and Rossby-gravity waves (Dunkerton, 1997) there is little reason to expect an analogous oscillation to be absent from the Maastrichtian climate. An oscillation similar in nature to the modern QBO is also likely to have been associated with
planetary waves in both the atmosphere and troposphere (Labitzke & van Loon, 1990), resulting in intensification of northern high latitude low pressure systems on biennial scales (Bull, 1998), causing biennial variations in rainfall intensity along the proto east Pacific coast. The Holocene varved record from Saanich Inlet shows that the QBO causes consecutive years to be slightly drier/ wetter relative to each other (Dean & Kemp, 2004). Alterations in the intensity of a northern high pressure system are also likely to have influenced water column stability and hence effected biological populations, including diatoms, as evident from Saanich Inlet (Dean & Kemp, 2004). The similarity between the average ~28 month period of the modern QBO and the inferred QBO
Chapter 5 Time-series Analysis of the Marca Shale
displays significant spectral peaks at 2.3 years in both the terrigenous and diatomaceous laminae that display a significant coherency and phase relationship of 89.8° (± 25.1°). Thus, over the 2.3 year cycle, perturbations in the diatomaceous flux were followed by perturbations in terrigenous flux 6.77 (± 1.90) months later. The 6.77 month lag may relate to the seasonal timing of the QBO forced perturbations, with diatom flux responding in autumn (e.g. November) and fluvial runoff during the summer (e.g. June).
5.4.4.2 Sub-decadal and quasi-decadal peaks
Sub-decadal peaks are present in four out of the five samples sets and the greatest numbers of peaks are present in this frequency band (Table 5.1 and Fig. 5.15). Variability in the sub-decadal frequency band is dominated by ENSO in the present climate system, although has also been attributed to tidal forcing. However, the ENSO-type oscillation inferred to be responsible for forcing sub-decadal to decadal variations in the level of bottom water anoxia, would also have forced laminae thickness variations on similar frequencies. Based on the frequency bands of modern ENSO (Fig. 5.15), the mean ENSO frequency band in laminae thickness variations is 5.3 years, whilst that of strong ENSO variability is 9.3 years, although is only recorded in
diatomaceous laminae.
Although the ~11 year cycle is not well expressed in laminae thickness records there are similarities between the peaks in the bioturbation index and laminae thickness records. The bioturbation index record from sample set H displays significant peaks at 6.2 and 10.3 years. Records of diatomaceous laminae thickness from the same sample set display significant peaks at 6.1, 6.7 and 10.0 years. The 6.1 and 10.0 year peaks are from the short records and therefore have a large bandwidth error. Further to this, although not reaching the 95% significance interval, power is expressed in the ~11 year frequency band in the long record of diatomaceous laminae thickness in sample set O (Fig. 5.9), consistent with the 10.8 year peak in the bioturbation index record from that sample set. These similarities add to the evidence that variations in diatom export flux were responsible for changes in benthic oxygen levels and that diatom export flux and changes in the bioturbation index were forced by the same mechanism.
Variations in the thickness of diatomaceous and terrigenous laminae from the Santa Barbara Basin, forced by ENSO, are distinctly out of phase by ~180° (Bull et al., 2000). Despite analogous out of phase short-term variability identified in the Marca Shale microfabric (see Fig. 4.21) and an antiphasing in the maximal diatom and terrigenous export flux in relation to benthic oxygen levels, no such phase relation was evident in the laminae thickness spectral estimates. The ~180° relation
Chapter 5 Time-series Analysis of the Marca Shale
successive El Niño and La Niña events in these Pleistocene varves. The lack of an ~180° phase relationship may therefore relate to the fact ENSO-type variability in the Maastrichtian did not always result in paired phases of El Niño and La Niña, as defined by Trenberth (1997). Tsonis et al. (2005) provide evidence that in a warming climate, El Niño events are liable to be more frequent than La Niña events. Similar, to results from the Marca Shale, spectral analysis from Saanich Inlet found no phase relationship between terrigenous and biogenic varve segment thickness, despite strong ENSO peaks present in both (Dean & Kemp, 2004). Even though ENSO has a profound effect on SST, atmospheric conditions and diatom community structure in the modern Gulf of California, laminae thickness records form the Gulf do not even contain significant spectral peaks in the ENSO band (J. Pike, personal communication). It therefore appears that the sub-decadal ~180° phase relationship seen in the Pleistocene varves from Santa Barbara Basin is an exception and not the rule.
5.4.4.3 Multi-decadal signals
Although uncommon, multi-decadal peaks were identified in both diatomaceous (26.5 years) and terrigenous laminae (20.0 years). It is feasible that within the bandwidth error, both peaks relate to the ~22 year Hale magnetic solar cycle, which is commonly inferred to be recorded in proxy climate records (Pike & Kemp, 1997; Ram & Stolz, 1999; Black et al., 2004). Both the 26.5 and 20.0 year peaks lie within the frequency band of the modern PDO (Fig. 5.15) and many proxy climate data sets have identified similar periodicities attributed to the PDO. These include records of terrigenous and diatomaceous laminae thickness from Saanich Inlet (Dean & Kemp, 2004), total varve thickness from Effingham Inlet (Patterson et al., 2004) and summer air temperature (Briffa et al., 1992) and SST (D'Arrigo et al., 1999) reconstructions from tree rings (Table 5.3). A PDO-like oscillation could explain the multi-decadal peaks in both the diatomaceous and terrigenous laminae thickness records in the Marca Shale. Positive phases of a cycle analogous to the PDO would result in anomalously wet conditions, accompanied with reduced upwelling, nutrient availability and hence reduced diatom export production, analogous to El Niño conditions (Fig. 5.18). Negative phases would have been anomalously dry, with increased diatom productivity and export flux, analogous to La Niña conditions. Solar variability, in this case the ~22 year Hale cycle, may have forced the periodicity of the inferred PDO-like phenomenon, as suggested by Enfield and Cid (1991).
Along with the ~20 year spectral peaks, a solitary significant multi-decadal 44 year peak was recorded in terrigenous laminae thickness. Information on analogous oscillations in the modern ocean/atmosphere system is scarce due to the lack of sufficiently long instrumental data sets.
Chapter 5 Time-series Analysis of the Marca Shale
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