New Discovery: Record of Glacial Cycles on Sea Floor

The history of the world’s hot and cold periods can be read on the ocean floor, according to a new study.

The Earth has gone through cycles of glacial periods, when the great continental ice sheets advanced during colder periods, and interglacial periods – warmer climate cycles like like present day. These glacial-interglacial cycles are caused by the slow cycle of shifts in the Earth’s orbit, called Milankovitch cycles, which affect the amount of sunlight reaching the Earth.

Now, scientists discovered that these ancient glacial-interglacial cycles are recorded on the sea floor through ocean ridges. A recent study by Crowley et al. in Oceanography demonstrated that new mapping of the sea floor from the Australian-Antarctic ridge showed statistically significant match with the Earth’s glacial cycles.

“Step back and think about this: Small variations in the orbital parameters of the Earth—tilt and eccentricity and wobble—are recorded on the sea floor, it kind of blows my mind.” says Richard Katz, one of the researchers told Science Magazine.

Each time the world passed between glacial and interglacial periods, global water distribution shifted. Specifically, when the Earth enters an ice age, the cold temperature freezes the sea water into glaciers, causing sea level to drop significantly. During the last glacial cycle, sea level dropped by 35 meters, which is more than twice the volume of the Greenland and West Antarctic ice sheets. As sea level drops, the pressure on the sea floor decreases accordingly. The ease in pressure allows magma beneath the seafloor to erupt and break the Earth’s surface, leading to divergence of the oceanic plates and forming the seafloor spreading center. This eruption thickens the crusts on teh sea floor and forms the abyssal hills – elevated landforms along the seaward margin parallel to mid-ocean ridges.

The sea floor spreading mechanism. The upwelling of mantle pushes the plate away and forms the abyssal hills. (Source: Conrad/Science).
The sea floor spreading mechanism. The upwelling of mantle pushes the plate away and forms the abyssal hills. (Source: Conrad/Science).

So how did the researchers discover the record of glacial cycles on sea floor? The answer lies in the variations in crustal thickness at the seafloor spreading center. They examined the crustal thickness response to sea level change by computing the physical mechanisms beneath the sea floor, such as mantle flow, thermal structure, melting, and pathways of melt transport. The model they used, which predicts the time series of crustal thickness caused by sea level change, was used to simulate the dynamics of mid-ocean ridge.

Map of the ocean floor. The mid-ocean ridges are shown as dark bands (Source: Earthguide).
Map of the ocean floor. The mid-ocean ridges are shown as dark bands (Source: Earthguide).

The numerical model results from this study contradict previous findings. Earlier research indicated an inverse relationship between variations in crustal thickness and spreading rate, which did not include the effects of sea level change. However, the new study reveals that melting or sensitivity to sea level variation does not simply decrease with increasing spreading rate. Instead, the crustal thickness response depends on changes in sea level and how long it took for melted magma to reach the surface. This newly discovered crustal response illustrates the link between glacial cycles and sea floor, contrary to findings from previous studies.

 

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