Earth scientists think on long timescales. They have to, because the changes in our planet have taken place over 4.5 billion years since its formation. The cycles of change occur on timescales which can be as short as a day, or as long as millions of years.
Most of the information about our planet is buried in its past and this week’s topics pick on the theme of how we can discover and understand what happened in the past, and what we can learn from it. They include timescales of decades, centuries — and billions of years.
Salt of the Earth
I smiled the other day to see someone posting on Twitter a bag of salt they’d bought. The salt, the label claimed, was millions of years old… and the sell-by date was about to expire.
I know — I’m easily amused. Obviously, that was a light-hearted post and the labelling was a legal requirement rather than a scientific assessment. But this week another salty article caught my eye. In this one, a study of a salt deposit dating back around two billion years has thrown new light on one of the most significant events in our planet’s history — the oxygenation of the atmosphere, known as the Great Oxidation Event.
We know from study of chemical isotopes that when the Earth formed, its atmosphere was lacking in oxygen, and that at some point between 2.45 and 1.85 billion years ago oxygen levels in both the atmosphere and the oceans increased dramatically as bacteria began to photosynthesise. This week, a study by a group of universities looked at a deposit of salt from Siberia and found new clues as to how quickly this process took place.
The chemical composition of the salt deposit is a telltale clue to the process. A high level of sulphates, which are produced by chemical reactions involving oxygen, indicates that the process of oxygenation took place much more rapidly than was previously thought. We still don’t know that much about the Great Oxidation Event, but this study is another piece in the jigsaw of knowledge of our planet.
The Massachusetts Earthquake of 1755
You probably haven’t heard of the Massachusetts Earthquake of 1755. In fact, you probably haven’t heard of any earthquakes in Massachusetts, and and if the year 1755 means anything to you in earthquake terms, you’re almost certainly thinking of the much more socially-significant event which struck in Lisbon in that year.
Modern seismometers, which record the location and magnitude of earthquakes, go back only to the beginning of the twentieth century, which makes understanding the causes and effects of earlier tremors difficult. It’s important, though, because a large earthquake hundreds of years ago in a usually seismically-quiet area may be a clue to future events — and a useful tool to help authorities mitigate and manage them.
What we know of the earthquake comes from contemporary accounts. The Massachusetts Historical Society notes that: “According to contemporary reports, as many as 1500 chimneys were shattered or thrown down in part, the gable ends of about 15 brick buildings were broken out, and some church steeples ended up tilted due to the shaking”.
The Society’s website goes on to say that: “From comparisons of damage reports and felt areas of the 1755 earthquake with those same observations for modern earthquakes, the magnitude of the 1755 earthquake is estimated to have been about 6.0 to 6.3”.
This week, a report in the journal Seismological Research Letters pinned down the event to a core of sediment taken from a pond in New England. Sediment cores give valuable clues to past environments and in this one, the scientists identified an unusual horizon indicating disturbance which could be tied to the 1755 event.
While this doesn’t teach us anything new about the 1755 event, the report notes that: “These results suggest that strong earthquakes in New England leave a record in the organic‐rich sediments of small ponds”. That means that similar occurrences, should they leave a similar trace, might in the future be used to piece together a seismic history going back long before recorded history in the area — and that will in turn help understand what might happen in the future.
Happy Birthday to the Keeling Curve
If you haven’t heard of the Keeling Curve, you’ll know about what it tells us. Among the gems on Twitter this week was a thread from the Scripps Institution of Oceanography celebrating the 60th anniversary of the Keeling Curve — the record of atmospheric carbon dioxide.
To get the full story of the curve, you’re best off looking up the thread as noted in the reference list below. As the first segment in this digest notes, we have ways of finding out what the composition of the atmosphere was in the past, although those methods inevitably give a generalised result with significant margins of error.
This is not the case with the Keeling Curve. It measures the current level of atmospheric carbon dioxide, which as a greenhouse gas is inextricably linked to global warming and to climate change. (The two are not necessarily the same.)
NASA’s Earth Observatory notes that: “The measurements shown in this curve represent the world’s longest continuous record of atmospheric carbon dioxide and were the first to confirm the rise of atmospheric carbon dioxide released from the burning of fossil fuels”.
The peaks and troughs on the curve represent seasonal variations but the trend is clear — and it’s upwards. According to Scripps: “In 1958, the average carbon dioxide concentration of the first measurement was 316.19 parts per million (ppm). Next month, it’s expected that weekly averages will exceed 410 ppm.”
If that doesn’t give you pause for thought, it should.