How do we know about the Earth's past climate? Part I: The rock record.

We know that today, the climate is changing at an unprecedented rate due to anthropogenic (human) causes, shown by the abrupt rise in CO2 in the atmosphere since the industrial revolution in the 1850s, and it is the rate of change that is the important part. But how do we know how this compares to what the Earth has seen previously, before records began? Let’s dive into a few of the methods that climate scientists use to identify paleoclimate trends.

Part I: Rock record

Cross bedded aeolian sandstone: Navajo Sandstone, Utah, USA (Image: Elder, 2006 [1])

The rock record can tell you an awful lot about what previous climates were like in certain areas. For example, think of an arid area; you’ll probably think of a desert, like the Sahara, however deserts are classified by the amount of rainfall they receive. Therefore, other areas such as the tundra can also fall into this category. Let’s take the classic Sahara style desert. The sand here is blown by wind, which will give different structures to sand that is moved by water in a river or in the sea at the shoreface. I won’t go into too much detail here, but the types of rocks you find can provide lots of detail about the environment it was formed in, especially sedimentary rocks like we are discussing here.

Interbedded marine sandstone and shale sequence, bag for scale: Whitby, UK.

Logging how the rock record has changed over time in a particular region/location can provide critical information on several features, one of which is how the climate has changed over time in this area. This can be coupled with other geological tools, such as dating, and palaeomagnetism. The former is probably a familiar concept for most, simply put, this is using radioactive isotopes to measure the amount of decay of a certain isotope over time, and therefore perceiving a date for the rock. We can also compare this to any fossils preserved in the rock; certain species would have only have been alive for certain amounts of time, and therefore provide a time frame for the age of the rock; this works better with species that went extinct fairly quickly over geological timescales. Fossils can also provide an indicator of the climate, as each species would have been adapted to certain climates – one of the things that palaeontologists can do is to compare fossilised organisms to those that are alive today to better understand this.

Palaeomagnetism is an interesting concept which may be less familiar than dating to some; 95% new crust is created at mid ocean ridges, such as the Mid Atlantic Ridge. Basalt, an extrusive igneous rock, is most commonly produced at mid ocean ridges, and generally contains 5-14 wt% (weight percent) iron (Fe). As Fe is magnetic, it aligns with the Earth’s magnetic field upon formation of the basalt. Interestingly, the Earth’s magnetic field undergoes a phenomena whereby the current magnetic north flips to the south pole and vice versa, also known as a geomagnetic reversal. This happens every few hundreds of thousands of years, and we don’t fully understand why it happens other than changes in the Earth’s core. Check out the video below for more information.

We can therefore record reversals by examining basalt where the Fe is pointing to the current magnetic north, and seeing where it changes to where the Fe points to the current magnetic south. Assuming that the spreading rates at the Mid Atlantic Ridge have not changed much, we can work out the ages at which these reversals happened. Oceanic crust that we see today is generally less than 180 million years old, before it is recycled by geological processes. Reconstructions older than 180 million years old use other techniques to determine the placement of the continents. Therefore we can reconstruct the movement of the current continents we see on the Earth’s surface to see how they have moved over time.

Geologial reconstructions of the continental plates, 225 million years ago to present (Image: Encylopaedia Britannica, 2007 [2])

But how does this relate to climate?

By using this method and the evidence we see in the rock record, we can paint a good picture of where these continents were (palaeomagnetism) alongside what climates they were experiencing (rock and fossil record). For example, if the rock, fossil, and paleomagnetic record provide evidence that there were Sahara style deserts further from the equator than there are today, we can infer that the climate was warmer.

So far, we’ve only really explored relative changes to today. So how do climate scientists get accurate temperatures for the past climates? This will be covered in Part II, so stay tuned!

References

[1] Elder, J. (2016). Aeolian Terminology. [online] Smallpond.ca. Available at: http://smallpond.ca/jim/sand/overview/ [Accessed 18 Oct. 2019].

[2] van Andel, T. and Murphy, J. (2007). Plate tectonics - Development of tectonic theory. [online] Encyclopedia Britannica. Available at: https://www.britannica.com/science/plate-tectonics/Development-of-tectonic-theory#/media/1/463912/172046 [Accessed 18 Oct. 2019].