20.3: Evidence of low-latitude of glacial deposits

We have now established that there is plenty of evidence of glacial activity preserved in the rock record from the Neoproterozoic. But it is also true that there is plenty of glacial activity on the modern Earth, and no one would mistake our modern planet for a giant snowball. So the question becomes: why do we think that the Snowball Earth glaciations were global freeze-overs, not just local glaciations (as in the modern world)? The answer comes from examining paleomagnetic signatures within the sedimentary rocks.

Paleomagnetism

The study of ancient magnetic signatures within rocks is paleomagnetism. Geologists who pursue this important work are paleomagneticians, but they are a cheeky lot, and like to call themselves “paleomagicians.” They collect very-precisely-oriented samples of either volcanic or sedimentary rocks, and then place those samples within a machine that can measure the orientation of their faint remnant magnetic field very precisely. This yields several kinds of magnetic information, but for our purposes we will be focused on determinations of paleo-inclination, which in turn can speak to the paleo-latitude at which the sample formed.

How does this work?

Figure \(\PageIndex{1}\): The shape of Earth’s magnetic field is a torus. Lines of magnetic force are vertical at the poles, and horizontal at the equator, with intermediate values in between. (Callan Bentley cartoon)

The Earth’s magnetic field is generated due to flow in its molten outer core, which is made of magnetically-conducting iron/nickel alloy. The magnetic force penetrates through the overlying mantle, crust, ocean, and atmosphere, and even extends out into space around the planet. The shape of that magnetic field is the relevant issue here. It is shaped like a very fat doughnut, with a dimple-like “hole” over the north magnetic pole and the south magnetic pole. Technically, this shape is called a “torus.”

As expressed at Earth’s surface, lines of magnetic force are vertical at the poles, and horizontal at the equator. In the diagram at right, consider the angular relationship between the “shaft” of the arrows and the surface of the Earth. At the equator, they are parallel, and at the poles you will note a perpendicular relationship. In between, the angles vary systematically. This angle (ranging between 0\(^{\circ}\) at the magnetic equator and 90\(^{\circ}\)at the magnetic poles) is called the angle of inclination. Because it varies by north-to-south position on the planet’s surface, it is used as a record of ancient latitude (paleo-latitude).

Lava flows and sedimentary deposits form at Earth’s surface as more or less horizontal sheets. Prior to consolidation (cooling and crystallization in lava flows, settling and lithification within clastic sedimentary strata), magnetically-susceptible grains such as magnetite are free to move, and they align themselves with the surrounding, permeating planetary magnetic field. Once lithified or crystallized, the grains have no more freedom of movement, and are locked in place as a durable record of the orientation of the Earth’s magnetic field at the moment that layer formed.

Equatorial sea-level glaciers imply high albedo

Figure \(\PageIndex{2}\): Gently folded tidal rhythmites of the Elatina Formation of central Australia, strata that show both evidence of glaciation and paleo-latitudes as low as 4 degrees from the equator. (Photo by Galen Halverson; reproduced with permission.)

A series of papers in the mid-1980s to late 1990s established measurements of paleomagnetic inclination (and thus paleo-latitude) for Marinoan-aged sea-level marine strata in several locations, particularly central Australia. These strata formed about 635 Ma and include signatures of glacial influence (such as dropstones), but they also show tidal rhythms preserved as varves. The idea behind studying these special strata is that (1) we know they are glacial, but (2) they also show fine layers that we can rely on as representing “horizontal” at the time they formed. Collecting carefully-oriented samples from such rocks allows for the precise measurement of their subtle magnetic signatures. Luckily, these samples have also been undisturbed since the time that they formed: their magnetism hasn’t been reset by a metamorphic event, for instance. The quality of the paleomagnetic measurements is robust, meaning that they are trustworthy and have passed several tests validating the results. And those results are: the sediments formed within 10\(^{\circ}\) of the equator.

Figure \(\PageIndex{3}\): Marinoan-aged glaciogenic sedimentary rocks in central Australia show near-equatorial paleolatitudes. (Callan Bentley redrawing of a figure from Sohl, et al. (1999))

Now, this is a big deal, because the equator is usually pretty warm, and not a place where we would expect continental ice sheets to be flowing into ocean waters, shedding dropstone-laden icebergs.

Similar measurements of Sturtian (~710 Ma) sedimentary and volcanic rocks in ancestral North America show it too was positioned astride the equator during Neoproterozoic time.

This is important, because generally speaking the equator is a very warm part of the planet’s surface: it’s where the incoming rays of sunlight are most direct. As you move toward the poles from the equator, temperatures generally decrease. Therefore, if indeed there were glaciers flowing into the sea at such equatorial latitudes, it implies an exceptionally frigid climate for the planet as a whole.

Furthermore, because snow and ice are highly-reflective, such low-latitude ice would imply an overall very reflective planetary surface. In other words, an icy equator implies an icy planet, which would reflect away a lot of otherwise-warming incoming sunlight: it would have a high albedo. Among naturally occurring substances, snow and ice have the highest albedo (they are most reflective) and open ocean water has the lowest (it is most absorptive of incoming solar radiation).

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Why are paleomagnetic records of low-latitude glaciation important?

a. They imply very low levels of oxygen in sea water during the time of the Snowball Earth.

b. They show us the ability of glaciers to erode the landscape into U-shaped valleys.

c. They show the sudden timing of the post-glacial melt-off, and ensuing deposition of "cap carbonate" strata.

d. If it was cold enough for glaciers at sea level at the equator, then it must have been cold enough for glaciers everywhere else on Earth.

Answer

d. If it was cold enough for glaciers at sea level at the equator, then it must have been cold enough for glaciers everywhere else on Earth.

Exercise \(\PageIndex{2}\)

Which of the following has the highest albedo?

a. Snow

b. Ocean water

c. Rock

Answer

a. Snow

Exercise \(\PageIndex{3}\)

Consider these annotated photographs showing cross-sectional views of 3 hypothetical sedimentary layers. The arrows show the paleomagnetic inclination. Which of the strata formed at the paleo-equator?

a.

b.

c.

Answer

c.