Have you ever thought about the implications of ocean anoxia in the past? Here to tell you more about the role anoxia has played in shaping the history of Earth is Libby Robinson…..
Hi, I’m Libby, a first year PhD student at NOC studying climates of the past, otherwise known as paleoclimates (paleo just meaning “very, very old” – and in this case, having nothing to do with the unprocessed, whole-food diet).
You can’t escape the media coverage and debate around the subject of climate change. You may, or may not be, sick of hearing about it but it is an important thing to consider, especially when it could have far-reaching consequences to our planet and the things that live on it. After all it is the only planet we have – for the foreseeable future anyway – and we should probably be nice to this hospitable lump of rock flying through the galaxy.
The famous geologist Charles Lyell (OK, OK maybe just famous in geological circles!) popularised the saying “the present is the key to the past”. That basically means that any process that happens today most likely happened during our geological past, so the study of what happened millions of years ago can ultimately help us understand what is going on today, and by extension, what may happen in the future. Humans have only been on this planet for 0.0000043% of the Earth’s history, so we have a pretty large history book to be looking back on.
A particular aspect of environmental change that interests me is the similarities between the time that we are living through now and some of the infamous mass extinctions that have occurred in the past, where whole species have been wiped out in a geological blink of an eye. In the case of the biggest recorded mass extinction at the Permian-Triassic boundary (252 million years ago), up to 96% of marine species died. As well as ‘mass extinctions’ these events are sometimes appropriately called ‘mass mortality’ events, and they represent a major overhaul of the types of flora and fauna – plants and animals – that lived on our planet.
Throughout geological history there has always been an overturning of species, where certain species die out and others take over (also known as extinction and origination), with new species often exploiting the ecological niches extinct species leave behind. However, these mass extinction events document extinction levels significantly, and catastrophically, above background levels. Perhaps chillingly for us humans living in a rapidly warming world today, these mass extinction events have all been associated with rapid climate change.
During many warming phases in Earth’s history an oceanic phenomenon has occurred whereby much of the ocean becomes anoxic – essentially devoid of dissolved oxygen for creatures to respire. This is thought to be a major contributor to the demise of marine species. Extreme anoxic events have been identified in Earth’s history, and given the title Oceanic Anoxic Events (OAEs). These are shown on the image below, where the thick arrows represent the main OAEs and the thinner arrows show other significant times of anoxia.
The cause of anoxia is debated but it is generally thought to be significantly enhanced during warm climates when oceanic currents can become sluggish and stratified, and oxygenated water and nutrients are less likely to be mixed throughout the ocean. It can also be exacerbated by particularly high levels of productivity in the surface waters of the ocean, as vast quantities of oxygen are used up in the waters below when these organisms die and decompose.
You can see from the figures here that low oxygen levels (i.e. anoxia) are associated with all but one of the major mass extinctions. How important then is anoxia as a ‘kill-mechanism’ in mass extinctions? Why do some major OAEs only have a minor drop in species diversity, whereas smaller OAEs correspond to major mass extinctions? These are questions that I hope to consider in the course of my research by studying core samples of sediments deposited in the oceans millions of years ago. I’ll be using geochemistry, sedimentology and paleoecology to investigate the productivity levels and chemical changes in the oceans during anoxic events, and how this relates to a changing climate.
As much as I enjoy studying rocks for the sake of figuring out what happened in the past, this work on anoxia also has implications for studies of the modern environment. As our climate warms today, scientists have documented many other changes across the world that bear stark similarities to what geoscientists have seen as being precursors to OAEs. These include the changes in the isotope signature of carbon dioxide (a geochemical technique we use to track changes in the carbon cycle), an increase in wind-driven upwelling, increases in monsoonal rainfall and the expansion of oxygen minimum zones across the globe. These changes have only been documented in the past 200 years or so, and so we do not have the thousands of years of data we often have in the rock record – but they are worrying trends that shouldn’t be ignored.