by Alan Longhurst
I think this paper on on ocean tides, sea-floor volcanoes and Milankevitch cycles is a game changer.
Mid-ocean ridge eruptions as a climate valve
Maya Tolstoy
Abstract. Seafloor eruption rates and mantle melting fueling eruptions may be influenced by sea level and crustal loading cycles at scales from fortnightly to 100 kyr. Recent mid-ocean ridge eruptions occur primarily during neap tides and the first 6 months of the year, suggesting sensitivity to minor changes in tidal forcing and orbital eccentricity. An ~100 kyr periodicity in fast-spreading seafloor bathymetry and relatively low present-day eruption rates at a time of high sea level and decreasing orbital eccentricity suggest a longer-term sensitivity to sea level and orbital variations associated with Milankovitch cycles. Seafloor spreading is considered a small but steady contributor of CO2 to climate cycles on the 100 kyr time scale; however, this assumes a consistent short-term eruption rate. Pulsing of seafloor volcanic activity may feed back into climate cycles, possibly contributing to glacial/interglacial cycles, the abrupt end of ice ages, and dominance of the 100 kyr cycle.
M. Tolstoy, Mid-ocean ridge eruptions as a climate valve, doi:10.1002/2014GL063015, Geophys. Res. Lett. 2015 [abstract] [manuscript]
A post at WUWT includes the press release from Columbia University
The AGU also issued a press release [link]
Discussion
Over millennial time scales, even stronger tidal strengths must occur, so it is natural to enquire if the 41,000-year variation of the obliquity of the ecliptic (the key to the Milankovic theory of ice ages) may also have consequences for tidal forcing of climate state comparable to those tought to be forced by changing solar radiation. Indeed, computation shows that the deep ocean tide could act as a pacemaker to terminate ice sheets at every second or third obliquity; if correct, this would solve the difficulty of reconciling the 40-ka obliquity period with the 100-ka period of glaciations.
Further, while solar radiation simply delivers heat to the earth’s surface, the perturbation of ocean heat content and distribution by tide-producing forces modifies the poleward transfer of heat associated with the meridional overturning circulation. During glaciations, North Atlantic tides were twice as high as today and pelagic dissipation three times as strong, and ‘these feedbacks dwarf the astronomic forcing’ of Milankevitch, according to Munk and Bills, who further note that, although there are complications not yet considered such as the increased depth of the shallow seas due to ice melt, and the consequent increase in tidal dissipation…..but, they note, “the numbers will not go away”.[1]
Finally, a new study from Lamont-Doherty demonstrates a very clear relationship between variable tidal loading of the deep sea floor with mid-ocean ridge eruptions at the present time: these occur primarily during neap tides (when loading is reduced) and in the first six months of each year, suggesting a response to orbital eccentricity. Eruption of sea-floor volcanoes will contribute to atmospheric CO2 levels and act, according to the author of this act “as a climate valve“.
This study was based on analysis of 9 mid-ocean ridge eruption/dyking events of which 8 occurred during neap tides which form lows in the fortnightly tidal modulation and also preferentially between months when the Sun-Earth distance is progressively increasing each year. As the author suggests, this sensitivity to tidal loading of the crust at mid-ocean ridges should reflect eccentricity in Earth’s orbit; the 100kyr cycle must therefore have the strongest consequences for sea floor volcanic activity. Further, decreases in sea-floor loading during glacial periods must occur when much seawater is transferred as ice onto continental surfaces. All of this will enhance oceanic mantle spreading and increases in sea floor vulcanism.
I suggest that this study is a game-changer of which we shall hear much more: as so often with critical contributions, the paper is single authored a fact that should suggest some reflections on the current state of science.
[1] Munk, W. and B. Billins (2007) J. Phys. Oceanogr. 37, 135-147
Filed under: Oceans