Do super-tides kick start interglacials ?

Guest essay by Clive Best

It is proposed that for the last 800,000 years  super-tides caused by maxima in orbital eccentricity have been the key factor needed to break up large northern ice sheets to enable the 41,000 year insolation cycle to initiate an interglacial. Insolation alone was sufficient to melt back the ice sheets over the previous 4.4 million years, as observed by the long series of 41,000 year glaciation cycles in the LR04 Do18 stack[1]. The obliquity cycle was broken once an underlying cooling trend had increased glacial ice sheet extent beyond a threshold for “Milankowitch” summer melting.

Since that time huge tidal forces amplified by increased eccentricity,  have been required to bring a glacial cycle to an end by carving and shelving the ice sheets. Once initiated a rapid deglaciation proceeds due to enhanced insolation with positive albedo feedback, resulting in a sawtooth shape. The most exceptional tides occur when the perihelion of the sun and the moon coincide and both orbits are at maximum eccentricity. This process can explain both the origin of the 100,000y cycle of ice ages and  the transition from  earlier 41,000y glaciation cycles which have so far remained a mystery[2].

Fig 1a. 5 million years of benthic foram dO16 data. The blue curve is a fit to Milankovitch harmonic data described inPhenomenology of Ice Ages.

Fig 1b. Correlation of inter-glacials with maximum eccentricity of Earth’s orbit

Fig 1c. Correlation of larger obliquity and warmer temperatures. A calculation of the insolation at the poles that demonstrating the dominance of the 41,000 year cycle is shown in Fig 2.

Fig 2. Maximum and total solar insolation calculated at the poles during last 600,000 years. The total annual insolation and the N-S asymmetry show the underlying effect of the 41,000 obliquity signal.

The basic hypothesis behind this proposal is the following.

1. 5 million years ago a gradual cooling of the climate began (Fig 1a). This was most likely due to plate tectonics. First Antarctica moved further south to sit over the South Pole isolating the Southern Ocean. Second the Panama isthmus closed cutting off circulation between the Atlantic and Pacific.
2. A regular glacial cycle began driven by the 41,000 year change in obliquity of  the earth’s axis. Higher obliquity brings higher insolation to both poles modulated by the precession of equinoxes. The 41,000y signal dominates glaciation cycles from 5 million years ago until 1 million years ago. Meanwhile the intensity of glacial periods was slowly increasing as  global cooling due to plate tectonics continued.
3. 900,000 years ago this general cooling reached a critical stage because the  increase in spread of ice sheets in the Northern hemisphere became too large to fully melt back during the next peak in obliquity. The cycle of 41,000y ice ages was broken.
4. Something else was now needed to trigger ice ages and that something was extreme tidal forces caused by maximum orbital eccentricity. When these coincided with peak insolation in the Arctic Circle the breakup of the northern ice sheets could begin and they collapsed rapidly within one precession cycle.

To understand  these tidal forces we need to understand what perigee spring tides are. These are exceptional tides that occur when the new moon coincides with the lunar perigee (closest distance of approach to the earth). These tides are typically 20% larger than normal, because tides are tractional forces that depend  on 1/R^3. Perigean tides occurs every 411.78 days (spring tide at lunar perigee). However there are a series of even rarer and more extreme perigean tides:

§ Perigean Eclipse Tides (PET) which occur every 2.99847 years which is when a Perigean spring tide coincides with the Earth-Sun-Moon all aligned in the ecplitic plane. The lunar and solar tides then pull directly together on the earth rather than through  a cosine(declination) offset.

§ Finally there are Super Perigean Eclipse Tides(SPET) which occurs every 1832 years. This super tide occurs when a Perigean Eclipse Tide coincides with the  earth also at perigee in its orbit around the sun so that the solar tide is also at its maximum value possible. These rare events cause tidal forces some 30% above normal. There is also a 5000 year modulation in the strength of SPET.[3]

Now consider what additional effects variations in the “Milankovitch” cycle of eccentricity would have on these Perigean tides.

The minimum distance of approach at perigee depends on the orbital eccentricity both for the moon and the earth. Tides are a tractional force whose greatest  effect is felt near the poles. During both the Arctic and Antarctic winters with zero insolation there are clear signals of tidal effects on temperature (4). Furthermore tides have also a direct effect on sea ice. Postlethwaite et al.[5] write

Tidal mixing within the water column and at the base of the sea ice cover can increase the heat flow from deeper water masses towards the surface causing decreased freezing and increased melting of sea ice and possibly the formation of sensible heat polynyas (Morales-Maqueda et al., 2004; Willmott et al., 2007; Lenn et al., 2010). The tidal currents can additionally increase the stress and strain on the sea ice and cause leads to open periodically within the sea ice cover (Kowalik and Proshutinsky, 1994).

Tidal forces therefore  act to break up ice sheets and change ocean heat flows. Fortnightly changes of 20% in ice stream flow have also  been observed in Antarctica due to spring tides. [6]

The  100,000 and 400,000 year cycles  in the ellipticity of the Earth’s orbit are caused by regular gravitational effects of the other planets as they orbit the sun,  particularly those of Jupiter and Saturn. Every 100,000 years the orbits of Jupiter and Saturn align themselves so that their net gravity perturbs the Earth’s orbit causing it to elongate and become more elliptical. This cycle reaches a maximum every 400,000 years in  regular fashion. The gravitational force of the sun on the moon is more than twice that of the Earth. For an observer  in outer space the moon appears to orbit the sun just like any other planet. Its orbit is perturbed by the Earth’s gravity making it slightly concave.  It is only from Earth that it appears to us to be in an elliptical orbit around the Earth. The moon’s orbit is therefore also affected by the gravitational pull of the other planets inducing a similar (Milankovitch) variation of eccentricity in its orbit around the sun. However this also causes an increased elliptical orbit of the moon around the earth because they have different mass.

How large can the tides get during 100,000y cycles of maximum eccentricity? Figures 4 and 5 show calculations of the change in tidal forces due to the sun and the moon for various values of orbital eccentricity. These calculations are based on the distance to the earth for different times in the year for the sun, and in the sidereal month for the moon. Tides are tractional forces which depend on 1/R^3 which explains why the moon has a larger tidal pull on the oceans than does the much more massive sun. At spring tides the two tidal forces are superimposed:

Fig 4: Relative strength of the solar lunar tidal force – proportional to 1/R^3

The largest solar tides are up to 20% higher than those we experience today. I have been unable to find any information about Milankovitch calculations of effects of the lunar orbit but I will assume a proportional increase to that of the earth. Given that assumption we can look at the more important lunar tide.

Fig 5: Variation in strength of lunar tides with orbital eccentricity relative to today.

We see that spring lunar tides for a lunar orbit twice the current eccentricity would be about 60% higher than they are today. Lunar tides are about twice the strength of solar tides so overall spring tides would have been at least 50% stronger than they are today, and Super Perigean tides would have been 20% stronger again.

Are  these  super-tides the catalyst to break up the large northern ice sheets and exit ice ages once every 100,000 years ?

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References

1. Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records, Paleoceanography, 20, PA1003

2. Maureen Raymo & Peter Huybers, Unlocking the mysteries of the ice ages, Nature Vol 451/17 P. 284, 2008

3. The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change, Charles D. Keeling and Timothy P. Whorf, PNAS (2000) 3814-3819

4. The influence of the lunar nodal cycle on Arctic climate, Harald Yndestad, ICES Journal of Marine Science, 63(3) 401, 2005

5. The effect of tides on dense water formation in Arctic shelf seas, C. F. Postlethwaite, M. A. Morales Maqueda, V. le Fouest,*, G. R. Tattersall1,**, J. Holt, and A. J. Willmott, Ocean Sci., 7, 203–217, 2011

6. Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica, G.H. Gudmundsson, Nature 444, 1063-1064, 2006

7. On the factors behind large Labrador Sea tides during the last glacial cycle and the potential implications for Heinrich events, Brian K. Arbic,1 Jerry X. Mitrovica,2 Douglas R. MacAyeal,3 and Glenn A. Milne, PALEOCEANOGRAPHY, VOL. 23, PA3211