Australia: The Land Where Time Began

A biography of the Australian continent 

Seafloor Grooves Record Sea Level Changes During Ice Ages

It has been found that grooves on the floor of the ocean thousands of metres below the surface were left by ancient cycles of ice. Seafloor spreading centres at the mid-ocean ridges, where oceanic crustal plates move apart and magma erupts in the expanding gap builds new oceanic crust on the trailing edge of the plates as they move away. There are long “abyssal hills”, 100 m high ridges on the diverging plates which are separated by valleys that are parallel to these spreading centres. Maps of seafloor topography show they have the appearance of grooves on a record.  These grooves have recently been shown to preserve a record the episodes of ice ages through time.

According to Hand the ice ages are driven mainly by rhythmic variations in the orbit and spin of the Earth that alter the sunlight reaching the Northern Hemisphere, resulting in the growth of ice caps and glaciers which trap so much of the Earth’s water as ice that global sea levels are lowered by 100 m or more. The reduced the amount of water in the world ocean reduces pressure on the seafloor, which allows magma to erupt more easily at the spreading centres, with the result that the oceanic plates are thickened to form abyssal hills, as is suggested by 2 new studies, one in Science and another in Geophysical Research Letters.

Oceanic crust production driven by variation in glacial cycles²

Water is redistributed between oceans and continents by glacial cycles, and these resulting variations in pressure on the upper mantle have consequences for melting on the interior of the earth. Theoretical models of the dynamics of mid-ocean ridges which include melt transport predict temporal variations of hundreds of metres in crustal thickness. It has been shown by new bathymetry from the Australian-Antarctic Ridge that are statistically significant spectral energy near the Milankovic periods of 23,000, 41,000 and 100,000 years, which is consistent with the predictions of the models. It is suggested by these results that abyssal hills, which are among the most common bathymetric features of the Earth, record magmatic responses to sea level changes. A link between glacial cycles at the surface and melting of mantle at depth is recorded in the bathymetric fabric of the sea floor.

Mid-Ocean ridge as a climate valve³

It appears rates of eruption on the sea floor and mantle melting may be influenced by the sea level and crustal loading cycles at scales from fortnightly to 100,000 years. Sensitivity to minor changes in tidal forcing and orbital eccentricity is suggested by recent eruptions on the mid-ocean ridges which occur primarily during neap tides and the first 6 months of the year.  In fast-spreading seafloor bathymetry and eruption rates of the present which are relatively low at times of high sea level and decreasing orbital eccentricity, with a periodicity of about 100,000 years, suggest there is a long-term sensitivity to variations in sea level and orbital variations associated with the Milankovic cycles. It is considered that seafloor spreading is a small, steady contributor of carbon dioxide to climate cycles on the 100,000 years time scale; though this assumes there is a consistent short-term eruption rate.  Tolstoy suggests that the pulsing of volcanic activity on the seafloor may feed back into the climate cycles and so possibly be contributing to cycles of glacial/interglacial activity, the abrupt end of ice ages, and dominance of the 100,000 year cycle.

Sources & Further reading

1. Hand, E. (2015). "This week's section Seafloor grooves record the beat of the ice ages." Science 347(6222): 590-592.

2. Crowley, J. W., R. F. Katz, P. Huybers, C. H. Langmuir and S.-H. Park (2015). "Glacial cycles drive variations in the production of oceanic crust." Science 347(6227): 1237-1240.

3.  Tolstoy, M. (2015). "Mid-ocean ridge eruptions as a climate valve." Geophysical Research Letters 42(5): 2014GL063015.