̽��ֱ�� of Cambridge - river

Saving England's chalk streams

ta385 — Thu, 27 Apr 2023 07:00:00 +0000

A conference organised by Pembroke College, Cambridge Conservation Initiative and WildFish Conservation has mobilised activists working to save chalk streams - one of the world's rarest habitats - from pollution and over-abstraction.

Muddying the waters: rock breakdown may play less of a role in regulating climate than previously thought

cmm201 — Mon, 21 Dec 2020 20:00:00 +0000

̽��ֱ��findings, published in PNAS, suggest Earth’s natural mechanism for removing carbon dioxide (CO₂) from the atmosphere via the weathering of rocks may in fact be weaker than scientists had thought – calling into question the exact role of rocks in alleviating warming over millions of years.

̽��ֱ��research also suggests there may be a previously unknown sink drawing CO₂ from the atmosphere and impacting climate changes over long timescales, which researchers now hope to find.

Weathering is the process by which atmospheric carbon dioxide breaks down rocks and then gets trapped in sediment. It is a major part of our planet’s carbon cycle, shuttling carbon dioxide between the land, sea and air, and influencing global temperatures.

“Weathering is like a planetary thermostat - it’s the reason why Earth is habitable. Scientists have long suggested this is why we don’t have a runaway greenhouse effect like on Venus,” said lead author Ed Tipper from Cambridge’s Department of Earth Sciences. By locking carbon dioxide away in sediments, weathering removes it from the atmosphere over long timescales, reducing the greenhouse effect and lowering global temperatures.

̽��ֱ��team’s new calculations show that, across the globe, weathering fluxes have been overestimated by up to 28%, with the greatest impact on rivers in mountainous regions where rocks are broken down faster.

They also report that three of the largest river systems on Earth, including the neighbouring Yellow and Salween Rivers with their origins on the Tibetan Plateau and the Yukon River of North America, do not absorb carbon dioxide over long timescales - as had been thought.

For decades, the Tibetan plateau has been invoked as a long-term sink for carbon and mediator of climate. Some 25% of the sediment in the world’s oceans originate from the plateau.

“One of the best places to study the carbon cycle are rivers, they are the arteries of the continents. Rivers are the link between the solid Earth and oceans – hauling sediments weathered from the land down to the oceans where their carbon is locked up in rocks,” said Tipper.

“Scientists have been measuring the chemistry of river waters to estimate weathering rates for decades,” said co-author Victoria Alcock “Dissolved sodium is one of the most commonly measured products of weathering – but we’ve shown that it’s not that simple, and in fact sodium often comes from elsewhere.”

Sodium is released when silicate minerals, the basic building blocks of most of Earth’s rocks, dissolve in carbonic acid - a mix of carbon dioxide in the atmosphere and rainwater.

However, the team found not all sodium comes from this weathering process. “We’ve found an additional source of sodium in river waters across the globe,” said co-author Emily Stevenson. “That extra sodium is not from weathered silicate rocks as other studies assume, but in fact from very old clays which are being eroded in river catchments.”

Tipper and his research group studied eight of the largest river systems on Earth, a mission involving 16 field seasons and thousands of lab analyses in search of where that extra sodium was coming from.

They found the answer in a soupy ‘gel’ of clay and water – known as the cation exchange pool – which is carried along by muddy river sediment.

̽��ֱ��exchange pool is a reactive hive of cations – positively charged ions like sodium - which are weakly bonded to clay particles. ̽��ֱ��cations can easily swap out of the gel for other elements like calcium in river water, a process that can take just a few hours.

Although it has been described in soils since the 1950s, the role the exchange pool plays in supplying sodium to rivers has been largely neglected.

“ ̽��ֱ��chemical and isotopic makeup of the clays in the exchange pool tell us what they are made of and where they’ve come from,” said co-author Alasdair Knight. “We know that many of the clays carried by these rivers come from ancient sediments, and we suggest that some of the sodium in the river must come from these clays.”

̽��ֱ��clays were originally formed from continental erosion millions of years ago. On their journey downstream they harvested cations from the surrounding water – their exchange pool picking up sodium on reaching the sea. Today, after being uplifted from the seafloor, these ancient clays – together with their sodium - are now being eroded by modern rivers.

This old sodium, which can switch out of the clays in the exchange pool and into river water, has previously been mistaken as the dissolved remnants of modern weathering.

“Generating just one data point took a huge amount of work in the lab and we also had to do a lot of maths,” said Stevenson. “It’s like unmixing a cake, using a forensic approach to isolate key ingredients in the sediments, leaving behind the exchange pool and the clays. People have used the same methods for a really long time - and they work - but we’ve been able to find an extra ingredient that provides the sodium and we need to account for this.”

“It's thanks to the hard work of many collaborators and students over many years that our samples had the scope to get to grips with this complex chemical process at a global scale,” said Tipper.

Scientists are now left to puzzle over what else could be absorbing Earth’s carbon dioxide over geological time. There are no certain candidates – but one controversial possibility is that life is removing carbon from the atmosphere. Another theory is that silicate dissolution on the ocean floor or volcanic arcs may be important. “People have spent decades looking on the continents for weathering - so maybe we now need to start expanding where we look,” said Tipper.

Reference:
Edward T. Tipper et al. ‘Global silicate weathering flux overestimated because of sediment–water cation exchange.’ PNAS (2020). DOI: 10.1073/pnas.2016430118

̽��ֱ��weathering of rocks at the Earth’s surface may remove less greenhouse gas from the atmosphere than previous estimates indicated, says new research from the ̽��ֱ�� of Cambridge.

People have spent decades looking on the continents for weathering - so maybe we now need to start expanding where we look

Ed Tipper

Flowing Carbon - Cambridge Earth Sciences in the field

E Tipper

̽��ֱ��Khone waterfall, Mekong River

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Going with the slow flow

amb206 — Tue, 28 Aug 2012 12:30:58 +0000

Nothing deters those determined to swim. ̽��ֱ��winter of 1928-1929 saw Britain in the grip of icy weather. Waterways all over the country were reported frozen and skaters took to ponds and rivers. ̽��ֱ��Cambridge News of 16 February 1929 reported in a jocular tone: ‘Eight hardy spirits took the plunge at the Town Bathing Sheds though it took nearly half an hour to break the ice. Many of them have hardly missed a day.’

̽��ֱ��Town Bathing Sheds (wooden changing rooms built and maintained by the city corporation) which stood on Sheep’s Green have long been demolished along with the diving boards where local youngsters once performed spectacular swallow dives and daring somersaults. But along the banks of a half-mile stretch of the Cam, just upstream of the city, are small signs of a secret history of bathing that pre-dates the chlorine and concrete of municipal swimming pools.

In a walking tour taking place on 8 September as part of Open Cambridge, author and swimmer Jean Perraton will explore Cambridge’s historic bathing spots and look at the ways in which social attitudes shaped, and continue to shape, our enjoyment of rivers. In particular she will celebrate the joyful experience of swimming as “a chance to revel in the element that covers most of our planet” and look at the ways in which writers down the ages have drawn inspiration from the watery world.

Emmanuel and Christ’s Colleges have their own historic pools. But for centuries, until the public pool at Jesus Green was constructed in 1923, people swam in the River Cam. On a stretch of the river at Sheep’s Green, ten minutes’ walk from the city centre, in the 19^th and much of the 20^th century hundreds of local children took their first strokes in a side stream known at the Snobs.

“Children had to show they could swim confidently in the Snobs before they were allowed to swim in the main river where they would be out of their depth,” says Jean Perraton.

For more than 30 years swimming at Sheep’s Green was supervised by a man called Charlie Driver. He was custodian of the Sheds and a fine athlete who taught swimming and diving. Swimming galas and races took place on the river right up to the 1970s with one of the annual races being along the Backs right through the town and past many of the oldest colleges.

Next to the footbridge at Sheep’s Green is a little classical building in a walled garden. Hodson’s Folly was built in 1887 by local resident John Hodson so that he could keep a watchful eye on his daughters bathing in the river while overseeing his nearby fish hatchery. ̽��ֱ��land used was common land which Hodson, who was at one point a butler’s assistant at Pembroke College, had annexed without permission. As a speculative builder Hodson excelled: his building sports a coat of arms, showing a swan in a rain storm, and the summer house and its garden are in many ways a Cambridge college in miniature.

At the town bathing places men and women bathed at separate spots until well after the Second World War. Men and boys had long been able to swim on Sheep’s Green, but in 1896 the city authorities provided bathing sheds for women in a more secluded spot opposite Hodson’s Folly. ̽��ֱ��water there was never really deep enough but, although strong female swimmers were allowed to join the men earlier, it was not until 1966 that the Town Sheds became mixed.

All that remains of the Ladies Bathing Place today is a solitary handrail . While women bathed in voluminous costumes, before the 20^th century it was usual for men and boys to swim naked. As Gwen Raverat (grand-daughter of Charles Darwin) described in Period Piece, her account of a Cambridge childhood in the 1890s: “All summer, Sheep’s Green and Coe Fen were pink with boys, as naked as God made them; for bathing drawers did not exist then: or at least, not on Sheep’s Green.”

While the men and boys of the town swam at the Town Sheds, pupils from the Perse and the Leys School had their own bathing places on the other side of the river. Further upstream is the site of the ̽��ֱ��’s bathing place (now a private club run by a committee). Beyond it the river dreamily runs through Grantchester Meadows where in the early 1900s a group of Cambridge undergraduates and their friends, dubbed the neo-Pagans, spent their summers bathing in the river and camping under the stars. Among them were Rupert Brooke and Virginia Woolf who are said to have swum naked by moonlight at Byron's Pool, a place that’s disappointingly unromantic today with the M11 roaring past.

In a memoir published in 1935 the novelist Rose Macaulay, whose father was a Fellow of Trinity College, captured the sensuous pleasure of swimming in nature early one spring morning upstream of the city. She wrote: “I let the slow flow carry me gently along through shadow and light, between long weedy strands that slimily embrace me as I drift by, between the bobbing white and gold cups and the slippery juicy stems, between willows that brush my head with light leaves, beneath banks massed high with may, smelling sharp and sweet above the musky fragrance of the tall cow-parsley.”

̽��ֱ��early history of swimming is shrouded in mystery. While swimming was seen as a useful military skill, it’s thought that few people in Britain were good swimmers before the 16^th century. ̽��ֱ��danger of venturing into deep water prompted the Vice-Chancellor of the ̽��ֱ�� of Cambridge in 1571 to forbid undergraduates from entering the water: “..if any scholar should go into any river, pool or other water in the county of Cambridge, by day or night, he should .. be sharply and severely whipped publicly…”

But a change of attitude was afoot. In 1587 Everard Digby, a Fellow of St John’s College, published a treatise on swimming called De Arte Natandi (the Art of Swimming). Written in Latin, its purpose was to raise swimming from “the depth of ignorance and the dust of oblivion”. A few years later Christopher Middleton, also a scholar from St John’s, published a shorter version of Everard’s opus in English. A section devoted to practical tips advises against swimming when rain is washing dung into the water; a section on how to swim describes how to cut one’s toe nails while floating.

Many more books giving advice on swimming were published in the following two centuries, no doubt boosted by the discovery of the joys of sea bathing. Although the seaside became more accessible with the expansion of the railways in the mid-19th century, and then swimming baths and lidos became more widely available, river swimming – in the Cam as elsewhere – remained immensely popular.

̽��ֱ��final years of the 20^th century saw the closure of many outdoor swimming pools and increasing discouragement of swimming in rivers and lakes. But the past ten years have witnessed a quiet revolution as growing numbers of people have rediscovered the pleasures of swimming in rivers and lakes. Roger Deakin’s Waterlog (Chatto and Windus, 1990), an eclectic account of a swimming journey across Britain, lit the flame for a surge of books and films and inspired the formation of countless groups and websites.

Jean Perraton adds: “In the later years of the 20^th century official concerns about pollution increased, just as the river was getting cleaner, and Cambridge City Council’s practical support for swimming in the river turned to active discouragement. ̽��ֱ��Cam Conservator’s byelaws forbade swimming except at official bathing places – but by then there were none left. Now, the Conservators have agreed that the whole of the upper river, from the Mill Pond to Byron’s Pool, can be regarded as a bathing place. Now that wild swimming has become cool, more and more people are discovering, or re-discovering, this most simple and delightful of summer pleasures.”

Jean Perraton is author of Swimming Against the Stream (Jon Carpenter Publishing, 2005, available from bookshops or Amazon.

There are places still available for some Open Cambridge events – including a talk about Cambridge’s Civic Insignia and Historic Charters, tour of the English Faculty Library and tour of New Hall Art Collection. For details of how to book go to https://www.opencambridge.cam.ac.uk/. Some events are drop-in with no need to book.

A walking tour of historic swimming spots taking place during Open Cambridge (7-9 September) will celebrate Cambridge’s shifting relationship with the river that flows through it. ̽��ֱ��tour is fully booked but the places it explores are accessible to the public.

All summer, Sheep’s Green and Coe Fen were pink with boys, as naked as God made them; for bathing drawers did not exist then: or at least, not on Sheep’s Green.

Gwen Raverat, Period Piece (1952)

Jacqueline Garget

Hodson's Folly on the River Cam opposite Sheep's Green

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