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, 97 (8), 3814-9

The 1,800-year Oceanic Tidal Cycle: A Possible Cause of Rapid Climate Change

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The 1,800-year Oceanic Tidal Cycle: A Possible Cause of Rapid Climate Change

C D Keeling et al. Proc Natl Acad Sci U S A.

Abstract

Variations in solar irradiance are widely believed to explain climatic change on 20,000- to 100,000-year time-scales in accordance with the Milankovitch theory of the ice ages, but there is no conclusive evidence that variable irradiance can be the cause of abrupt fluctuations in climate on time-scales as short as 1,000 years. We propose that such abrupt millennial changes, seen in ice and sedimentary core records, were produced in part by well characterized, almost periodic variations in the strength of the global oceanic tide-raising forces caused by resonances in the periodic motions of the earth and moon. A well defined 1,800-year tidal cycle is associated with gradually shifting lunar declination from one episode of maximum tidal forcing on the centennial time-scale to the next. An amplitude modulation of this cycle occurs with an average period of about 5,000 years, associated with gradually shifting separation-intervals between perihelion and syzygy at maxima of the 1,800-year cycle. We propose that strong tidal forcing causes cooling at the sea surface by increasing vertical mixing in the oceans. On the millennial time-scale, this tidal hypothesis is supported by findings, from sedimentary records of ice-rafting debris, that ocean waters cooled close to the times predicted for strong tidal forcing.

Figures

Figure 1
Figure 1
Varying strength in an estimate of the tide raising forces, derived from Wood (ref. , Table 16). Each event, shown by a vertical line, gives a measure of the forcing in terms of the angular velocity of the moon, γ, in arc degrees per day, at the time of the event. Arcs connect events of strong 18.03-year tidal sequences. Centennial maxima are labeled, with the final one, “D”, occurring in A.D. 2151.
Figure 2
Figure 2
Extension of Fig. 1 to illustrate millennial periodicity in tide raising forces since 500 B.C. The angular velocity, γ, was computed from functions listed in Table 2. Events of a 180-year cycle, all at full moon, are labeled with times of occurrence (B.C. or A.D.). The 1,800-year cycle is evident as a progression of solar-lunar declination difference, listed at the top of the figure in degrees of arc of the moon above (or below) the ecliptic.
Figure 3
Figure 3
Varying strength of the global tide raising forces (bottom plot), as in Figs. 1 and 2, together with parameters (top and middle plots) that reveal the basis for the 1,800- and 5,000-year tidal cycles, as described in the text. The plots are for a hypothetical 110-kyr sequence of tidal events beginning with the moon, sun, and earth in perfect alignment and closest approach (zero separation-intervals), producing a maximum γ of 17.165° per day never again attained. Tidal events occurring near peaks in the 5,000-year cycle (near zero crossings of top plot) are connected by straight lines to reveal their pattern (which includes a 23-kyr cycle not discussed in the text).
Figure 4
Figure 4
Multitaper spectral analysis of glacial-Holocene petrologic events from cores VM 29–191 and VM 23–81 (reproduced from ref. , Fig. 7), compared with periodicities in tidal forcing. Overlain in red are the averages (calculated from 0 to 31.1 kyr BP) of the 1,800- and 5,000-year tidal periods (A) and times of peak forcing of the former cycle (B). Tidal timing and periodicity assume invariant orbital parameters, except for the 5,800-year period that is based on assuming secular variability of climatic precession, as described in the “Secular Variations in Tidal Forcing” section of the text.
Figure 5
Figure 5
Glacial-Holocene deep-sea core record of ice-rafted debris, petrology, and isotopes of the North Atlantic Ocean basin (from ref. , Fig. 6; BP in kyr before A.D. 1950), which shows evidence of pervasive millennial-scale fluctuations in climate. Overlaid in red are times of peak forcing in the 1,800-year tidal cycle.
Figure 6
Figure 6
Profile of mass accumulation rate of aluminum (AL-MAR in mg/cm2/yr) in sediments of Elk Lake, Minnesota [from Dean (ref. , Fig. 2)]. Overlain in red are the times of peak forcing in the 1,800-year tidal cycle (at 4,239, 5,921, and 7,744 yr BP).
Figure 7
Figure 7
Comparison of late-glacial and Holocene sedimentary core chronology of the North Atlantic Ocean basin with the times of tidal forcing. Tidal events are shown as in Figs. 1 and 2 with times of 1,800-year climactic events (in kyr BP) listed below. These climactic events are connected by line segments. Those that contribute to the 5,000-year tidal cycle are marked with asterisks. Immediately above this plot are vertical lines indicating cool periods inferred from the deep-sea core records, labeled as in Fig. 5. Their timing is derived from Fig. 5, except for Event 1 and Events 3–8, which are dates quoted in the text of ref. , and Event 2, inferred from Bond et al. (ref. , Fig. 14). Plotted as arrows are the times of dust layers in Elk Lake, Minnesota, the beginning of the Akkadian drought [as reported by Kerr (9)], the Little Ice Age, and the end of the Younger Dryas (YD) [Bond et al. (2)]. Events consistent with the hypothesis of tidal forcing of climate are shown as solid lines, exceptions as dashed lines.

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