A new explanation for the cause of changes in the chemical makeup of the oceans through recent Earth history is put forward in a paper published in Nature.
Scientists from the Universities of Southampton and Bristol suggest that adjustments in ocean chemistry through recent geological time are driven by variations in the intensity of chemical breakdown of continental rocks by rain and ground water. These changes are, in turn, controlled by the profound changes in the Earth's climate, and in particular the Ice Ages, that have occurred over the past 2-3 million years.
The elements that give seawater its distinctive saltiness are mostly supplied in dissolved form by rivers. Rivers, in turn, receive these elements from runoff that has reacted with and partially dissolved rocks, a process known as chemical weathering. Another major source of dissolved material to seawater is submarine “black smoker” hydrothermal systems. Movement of seawater through young, hot rocks at the mid-ocean ridges causes leaching of some elements from sea-floor basalts, as well as the precipitation out of solution of some constituents of seawater. Thus, these hydrothermal systems are both a source of dissolved material to the oceans and also a means by which some others are lost. The other major output of dissolved material from the oceans is to marine sediments, which are principally made up of the shells of dead marine organisms. Imbalances in these inputs and outputs cause changes in the chemical make-up of the oceans through time.
The team, led by Dr. Derek Vance of the University of Bristol, draws on records of past ocean chemistry preserved in deep-sea sediments to point out that some aspects of the chemistry of seawater have been changing too slowly over the past 2-3 million years given what is known about the sizes of the inputs and outputs to the oceans. The paper challenges the prevailing notion that this inconsistency is caused by inaccuracies in estimates of the impact of submarine hydrothermal systems on ocean chemistry, or that we don’t have accurate measurements of river chemistry and run off. Instead, they point to changes in continental chemical weathering rates caused by profound climate change operating over the past 2-3 million years.
Dr Vance explains, “Chemical weathering rates have been periodically perturbed in recent Earth history because the ice-sheets and glaciers produced during the great ice ages have physically ground rock up to smaller and smaller grain sizes. In the succeeding hotter and wetter ‘interglacial’ periods, this ground up rock is very susceptible to chemical weathering.” All chemical reactions occur faster if the substrate is finer grained because there is more surface area for reaction to take place – this is why school chemical experiments use iron filings instead of a block of steel!
One of the main conclusions is that in the instant of geological time represented by, say, the last 100 years, landscapes remain significantly perturbed by this process. Co-author Professor Damon Teagle, of the University of Southampton’s School of Ocean and Earth Science, based at the National Oceanography Centre, Southampton, explains, “The Earth emerged from the last Ice Age only 10,000 years ago, and chemical weathering is still playing ‘catch-up’ with the massive production of reactive, fine-grained particles produced during the last Ice Age.” As a result the measurements of the chemistry of rivers that scientists are currently making, although an accurate estimation of the modern Earth, are not representative of the past few million years.
The team conclude the paper by assessing some of the implications. One of these is the potential impact on the natural greenhouse effect on planet Earth. Chemical weathering not only dissolves rocks, it reacts atmospheric CO2 with those rocks and takes CO2 out of the atmosphere. This carbon is also washed into the oceans in dissolved form, where it is incorporated into the calcium carbonate shells of marine organisms, which in turn die and accumulate in deep ocean sediment.
On very long timescales – longer than hundreds of thousands of years – the amount of CO2 in the atmosphere represents a balance between that emitted as volcanic gases versus that the amount taken up by chemical weathering. The conventional view of the long-term evolution of Earth's climate is that chemical weathering and CO2 act together to thermostatically regulate the Earth's surface temperature. If for some reason atmospheric CO2 increases, the resulting higher temperatures cause greater chemical weathering, which acts to reduce the CO2 concentration of the atmosphere.
The team further suggest that during Ice Ages, this thermostat could be over-ridden due to glaciers grinding up much greater quantities of rock, which results in much higher rates of chemical weathering, leading to increased removal of CO2 from the atmosphere. Bristol's Gavin Foster remarks "this means that in periods like the last 2-3 million years, higher chemical weathering rates could act to maintain ‘icehouse’ conditions once they have started". However, no one should make the mistake of thinking that these processes could extract us from the modern predicament of high and rising atmospheric CO2. The natural processes discussed in this article are slow and, although crucial on geological timescales of hundreds of thousands to millions of years, are not relevant to the short span of modern industrialised society.
Materials provided by National Oceanography Centre, University of Southampton. Note: Content may be edited for style and length.
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