Update on a classic British field site

English: Glacial erratic, Norber One of severa...
Glacial erratic at Norber Brown that sits nicely on a limestone plinth, dues to the erratic’s having protected the limestone underneath from erosion. (credit: Wikipedia)

Few expect Earth scientists to get all sentimental, but they do. My soft spot is for one of the most rewarding and least strenuous geological sites in Britain, Norber Brow near Austwick on the southern edge of the Yorkshire Dales National Park. As well as the famous glacial erratics of Silurian greywackes perched on Lower Carboniferous limestone, 250 m to the SE by a well-trodden path is the inverse, the Variscan unconformity at the base of the Carboniferous on the very same Silurian formation. I was lucky to be taken there at age 15 by Roy Happs who taught A-level Geology, and it decided my future, there and then.

The erratics don’t just site on the limestone, but are on pedestals up to 30 cm above the surrounding limestone surface as if carefully balanced by Beowulf’s assailant Grendel. Somehow, since the time glacial flow had deposited the Silurian boulders the underlying limestone had been dissolved away; but how fast was that? That is the key to the pace at which limestone pavement, to most general visitors such a stunning and unexpected feature of the Dales, might have formed. And such a delight to hear of its terminology: clints, redolent of the former Viking people of the Dales, that stand proud between deep fissures known as grikes, a suitably ominous term of unknown derivation. Such superbly fractal landforms are, of course, but one part of karst (from the eponymous region of limestone country in Slovenia).

English: Limestone Pavement at Twisleton Scar ...
A classic limestone pavement in the Yorkshire Dales National Park (credit: Wikipedia)

It is really satisfying to discover that a lot of cutting-edge science has recently been aimed at Norber from a substantial review in Earth Pages’ sister journal Geology Today (Wilson, P. et al. 2013. Dating in the Craven Dales. Geology Today, v. 29 (January-February Issue), p. 16-22). The length of time that the Norber erratics have been exposed to cosmic-ray bombardment has been determined from 10Be, 26Al and 36Cl analyses with a precision of ±1000 years to 17.9 ka, shortly after the last glacial maximum (LGM) when warming and glacial melting had just begun in this part of Yorkshire. That might seem to indicate an average of 330 mm of limestone had been dissolved over that period to form the pedestals, i.e. a dissolution rate averaging about 20 micrometres per year, which is extremely rapid, geologically speaking. In 1962 when I was show the site we were told that elsewhere the limestone pavement had formed since the first field systems (Iron Age) were laid out as now useless drystone walls crossed it. Roy Happs somewhat darkly suggested that they had formed since the start of the Industrial Revolution because of acid rain.

He was pretty much wrong on that score, but cosmogenic dating of the clints shows significant discrepancies between the age of deposition of the erratics and  and the exposure age of the clints. This suggests both chemical dissolution and also periods of frost shattering and gravel removal, perhaps by soil creep. Dating of other materials enlivens the history of local landform development. Another karstic feature is the presence of sinkholes or dolines that are often filled with yellowish silts that show clear textural evidence of being windblown sediments or loess. These aeolian sediments have long been regarded as post-LGM too, but optically stimulated luminescence dating of their quartz grains gives an age split between pre- (27.5 ± 2.6 ka) and post-LGM (16.5 ± 1.7 ka). Some loess elsewhere in Craven district comes out to be as young as 8.2 ka, to tally with evidence from Greenlandic ice cores for a sudden deterioration in North Atlantic seaboard climate during this early time in the Holocene.

Then there are the local caves, renowned in Victorian times for their cave bears and other mammal fossils. One bear skull from Victoria Cave in the Craven area gave a 14C age of 14.6 ± 0.4 ka which statistically coincides with that from a cut-marked horse vertebra. More than likely the bears were turfed out when humans reached Craven, but did they return when humans fled in the face of the Younger Dryas return to frigid-desert conditions? Probably not, as the YD would almost have sterilized what are now the Yorkshire Dales. Even earlier ages of 114 ka from U-Th dating of calcite flowstone that embeds hippo, elephant, rhino and hyena bones in Victoria Cave date to the previous Eemian interglacial. Indeed this speleothem has yielded ages as far back as the limit of the U-Th method (%00 ka). On a solo expedition in 1964 I had the chance to sleep-over in Victoria Cave, but pressed on with goose bumps to the nearby Youth Hostel.

Grand Canyon now the Grand Old Canyon?

Grand Canyon in Winter
Grand Canyon in Winter (credit: Wikipedia)

Among the best known and certainly the most visited topographic feature on the planet, the Grand Canyon resulted from erosion by the Colorado River keeping pace with uplift of the south-central United States. It is the archetype for what is known as antecedent drainage. Since that uplift is still going on, albeit slowly, the Grand Canyon has been assumed to be a relative young landform. By dating the first appearance of debris from the eastern end of the canyon in sediments at its western limit geomorphologists estimated that incision began around 6 Ma ago. Yet a range of other observations present puzzling contradictions. One means of settling the issue is to somehow to date the uplift radiometrically.

A long-used technique is to determine ‘cooling ages’ of crustal rocks exposed by uplift and erosion, exploiting the way in which rock temperature determines whether or not products of radioactive decay cab be preserved intact. One method uses the tracks of defects produced by electrons or helium nuclei from radioactive decay as they pass through various minerals that incorporate high amounts of elements such as uranium. Above a certain temperature the fission tracks anneal and disappear quickly, while below it they accumulate over time. Quantifying that build-up allows the date of cooling below the threshold temperature to be estimated. Similarly, gases produced by radioactive decay of some radioactive isotopes, such as argon from the decay of 40K or helium from uranium and thorium isotopes, can only stay in their host mineral if it remains cooler than a narrow range of temperatures. As rock rises towards the Earth’s surface, it starts out hot at depth but cools by conduction as it get closer to the surface. For the 1.8 km of uplift of the Grand Canyon and the relatively cool nature of the underlying crust, neither the fission-track nor the  40Ar/39Ar cooling-age methods give meaningful results. However, minerals lose helium at temperatures above about 70°C, so a method based on helium accumulation from uranium and thorium isotope decay is a possible means of assessing uplift timing. But there have been plenty of snags to overcome to make this approach reliable. In the case of the Grand Canyon analytical quality and careful sample collection has given a credible result (Flowers, R.M. & Farley, K.A. 2012. Apatite 4He/3He and (U-Th)He evidence for an ancient Grand Canyon. Science , doi 10.1126/science.1229390)

Flowers and Farley from the University of Colorado at Boulder and the California Institute of Technology, Pasadena, respectively, produced a result that completely overturns previous conceptions. The western end of the Canyon had been incised to within a few hundred metres of modern depths by 70 Ma ago; more than ten times earlier than previously thought. The eastern end has a more complex history that reveals cooling events in the Neogene as well as an end-Cretaceous initiation of uplift and erosion. Their data are consistent with early incision of the Grand Canyon by a Cretaceous river flowing eastward from the Western Cordillera, with a reversal of flow in the late-Tertiary as uplift of the Colorado Plateau began and western mountains subsided. Whether or not this fits with Cretaceous and later geological history of the SW US, is beyond my ken, but you can bet there will be a storm of comment from US geomorphologists once the paper appears in the print issue of Science.

Greening and changing the land

English: Liverwort Liverworts are small plants...
A very British liverwort mat. Image via Wikipedia

Evidence for the earliest colonisation of the continents by plants is in the form of spores and body fragments from terrestrial sediments of Middle Ordovician age (~470 Ma) (Rubinstein, C.  et al. 2010. Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana). New Phytologist, v. 188, p. 365-369)suggest that the first vegetation cover involved simple ground-hugging plants that lacked stems of roots, very like the liverworts that I struggle to deter from my gravel drive. Vinegar is the only solution, preferably boiling, but that does not harm their spores and inevitably they re-emerge. Rearranging the gravel, of a pale pink limestone, is one of a very few means of keeping fit that I can bear, and I suppose the liverworts spice that up a little: but I do detest them. Part of their irritation is that they form an impermeable coating to what once was a passable if minor aquifer that channelled rainfall that would otherwise repeat the house-flooding that greeted me within a day of my moving in. So it was with some solemnity that I read a paper on how these damnable organisms transformed the Ordovician continental surface and the geomorphological processes that shaped it (Gibling, M.R. & Davies, N.S 2012. Palaeozoic landscapes shaped by plant evolution. Nature Geocience, v. 5, p. 99-105).

Sedimentologists have shown that rivers of earlier times formed wide tracts of ephemeral braided channels that transported and reworked sands and gravels that were not hampered by any vegetable binding agent. Floods merely accelerated the braiding and spread coarse sediment across valley floors, repeated spates washing out almost of the fines to take them ultimately to the continental shelves: there are few if any relics of Cambrian and older muddy floodplains. Moreover, untrammelled by vegetation any remaining fine material would be picked up by wind, even in humid climates, to meet the same marine fate. Overbank deposits of silts and clays, unsurprisingly, demand banks over or through which floodwater  escapes from defined channels and is then delayed by low gradients away from the main flow, so to deposit the fines carried by its sluggish speed. Except in arid terrains where braided channels are still the rule, in succeeding geological time evidence grows for nowadays familiar channels, meanders with point bars and eroded opposite banks, levées and floodplains on every conceivable scale. Apparently, they became conspicuous in Silurian times and then forming 30% of all fluvial sediments by the Devonian.

Meanwhile, plants were diversifying though evolution of vascular systems that transport sap up supporting structures that emerged in parallel eventually to form trunks and branches. The consequent rise in volume and in area exposed to sunlight and photosynthesis of a plant’s tissues increased the potential to draw CO2 from the air, witnessed by changes in carbon isotopes that show carbon burial rising shortly after the mid-Ordovician from far lower values in earlier times. (Incidentally, it seems likely that such meagre colonisers as early liverworts thrived sufficiently to contribute to the cooling in the Upper Ordovician that led to sporadic glacial episodes).  Preservation of wood in peats – liverworts are not implicated in any kind of fossil-fuel production – helped to maximise carbon burial by the end of the Palaeozoic Era. But trees make logs and, carried by rivers, logjams. By the Upper Carboniferous effects of damming become common in fluvial sediments, which seemed to serve the formation of islands within wide river channels.

By the present day, vegetation has come to dominate all but the most arid river systems. Even in central Australia sturdy gums able only to get water from below ephemeral river beds end up defining the flow regime and stabilising it on low relief plains that would otherwise be ravaged by sheet floods every rainy season. The authors support stratigraphic observations through the use of scaled down models of channels in vegetated areas by the cunning use of alfalfa seeded to sprout during simulated dry conditions then resuming channel flow in a flume tank.

Gilboa Fossils - Gilboa, New York
Fossils tree stumps from Gilboa, New York (Photo credit: Dougtone)

The earliest substantial trees, represented by wood fragments rarely assignable to any particular structure, occur in the Middle Devonian (385-400 Ma). Although some groups can be differentiated, how their encompassing woodland ecosystems looked has been a mystery until recently . Being ‘priitive’ it has been assumed to be very simple, unlike the well-documented forests of the Carboniferous coal swamps. But, once in a while, a site of exceptional preservation is unearthed, one such being a palaeosol that clearly formed on the floor of a Middle Devonian woodland exposed by quarrying in New York state, USA (Stein, W.E. et al. 2012. Surprisingly complex community found in the mid-Devonian fossil forest at Gilboa. Nature, v. 483, p. 78-81). Once backfill accumulated during the quarry’s active life was removed it became possible to plot the arrangement of roots systems of the last trees to live at the site before inundation and preservation.  Together with other plant material found in the ancient soil, the growing sites have been reconstructed to assess the full ecosystem involved. It was a great deal more complex than previously thought possible, with a series of tiers formed by three large tree types: tall, lollipop-like Eospermatopteris; smaller lycopsid-like trees and subsurface propagators related to gymnosperms that sprouted to form an understorey that may have climbed the larger trees in the manner of vines. Its setting was akin to that of modern mangrove swamps – by the sea – subject to sea-level change that inundated, killed and preserved the coastal woodland.

Low-lying Tibet before India-Asia collision

The Tibetan plateau lies between the Himalayan...
The semi-arid Tibetan Plateau from spaceImage via Wikipedia

The vast Tibetan Plateau at an average elevation of 4500 m is a major influence on the climate of Asia, being central to the annual monsoons, as well as one the world’s largest continental tectonic features. When it formed is crucial in palaeoclimatic modelling as well as to geomorphologists and structural geologists. Whether or not it was present before the Indian subcontinent collided with Asia at 50 Ma has been the subject of perennial debate; it could have formed during the more or less continual accretion of terranes to southern Eurasia since the Jurassic Period. A novel approach to timing uplift of Tibet is obviously needed to resolve the controversies, and that may have been achieved (Hetzel, R. et al. 2011. Peneplain formation in southern Tibet predates the India-Asia collision and plateau uplift.  Geology, v.39, p.983-986). North of Lhasa is an area of coincident small plateaus at around 5200-5400 m into which are cut valleys a few hundred metres. It has the hallmarks of a peneplain stripped to the base level of erosion, and developed on Cretaceous granites. The German-Chinese-South African team applied a range of geochronological techniques to date the emplacement of the granites and their cooling history. U/Pb dating shows the granites to have crystallised between 120 to 110 Ma; U-Th/He dating of zircons in them indicate their cooling from 180° to 60°C between 90 and 70 Ma; apatite  U-Th/He and fission-track dating show that the granites experienced surface temperatures by around 55 Ma during a period of erosion at a rate of 200-400 m Ma-1. The clear inference is that an area >10 000 km2 became a peneplain by the end of the Palaeocene, to be unconformably overlain by Eocene continental redbeds.

By the Eocene the northern Lhasa Block had become a low-elevation plain from which a vast amount of sediment had been removed to be deposited elsewhere – Palaeocene and Eocene sediments are not common throughout the whole Tibetan Plateau. This is strong evidence that uplift of the Plateau only began after the India-Asia collision during the Eocene. Despite that and the erosion that would have taken place, much of the peneplain remains; given resistant bedrock peneplains can be very long-lived.

Threat to landscape from alien crayfish?

Pacifastacus leniusculus 5
Image via Wikipedia

The stealthy invasion of rivers in Europe by the tasty American signal crayfish Pacifastacus leniusculus poses a threat not only to the indigenous European species Astacus astacus (P. leniusculus carries a fungal infection as well as being formidably armed), but conceivably to the very landscape itself (Johnson, M.F. et al. 2010. Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus). Geomorphology, v. 123, p. 269-278). Johnson and colleagues from the University of Loughborough, UK used captive alien crayfish to model the effects of their bioturbation under controlled laboratory conditions, assessing their activity by the use of millimetre-resolution gravel-surface elevation data generated by a laser altimeter. The sturdy little beasts behave like frenzied bulldozers creating mounds and pits in the gravel substrate, displacing on average about 1.7 kg of gravel every day over an area of 1 m2 thereby completely disrupting the perfectly flat experimental substrate onto which they were introduced in about 3 days. By this activity they render the surface more prone to erosion by flowing water so that greater grain transport ensues; they could effect bother erosion and deposition by increasing transportation of grains. To my knowledge, this is the first experimental study of bioturbation in the context of hydrology. We can expect more now that the technology is available: the burrowers as well as the diggers of the animal world. While the Phanerozoic is best know for having begun with the Cambrian Explosion of multicellular life, a sometimes overlooked attribute is that it coincided with the start of bioturbation. That may well have had a profound effect on sediment transport as the American invader suggests.
See also: Newton, A. 2010. Crayfish at work. Nature Geoscience, v. 3, p. 592

Threat to landscape from alien crayfish?

The stealthy invasion of rivers in Europe by the tasty American signal crayfish Pacifastacus leniusculus poses a threat not only to the indigenous European species Astacus astacus (P. leniusculus carries a fungal infection as well as being formidably armed), but conceivably to the very landscape itself (Johnson, M.F. et al. 2010. Topographic disturbance of subaqueous gravel substrates by signal crayfish (Pacifastacus leniusculus). Geomorphology, v. 123, p. 269-278). Johnsson and colleagues from the University of Loughborough, UK used captive alien crayfish to model the effects of their bioturbation under controlled laboratory conditions, assessing their activity by the use of millimetre-resolution gravel-surface elevation data generated by a laser altimeter. The sturdy little beasts behave like frenzied bulldozers creating mounds and pits in the gravel substrate, displacing on average about 1.7 kg of gravel every day over an area of 1 m2 thereby completely disrupting the perfectly flat substrate onto which they were introduced in about 3 days. By this activity they render the surface more prone to erosion by flowing water so that greater grain transport ensues; they could effect bother erosion and deposition by increasing transportation of grains. To my knowledge, this is the first experimental study of bioturbation in the context of hydrology. We can expect more now that the technology is available: the burrowers as well as the diggers of the animal world. While the Phanerozoic is best know for having begun with the Cambrian Explosion of multicellular life, a sometimes overlooked attribute is that it coincided with the start of bioturbation. That may well have had a profound effect on sediment transport as the American invader suggests.

See also: Newton, A. 2010. Crayfish at work. Nature Geoscience, v. 3, p. 592

Catastrophic canyon formation

Huge canyons, such as the Grand Canyon and the Gorge of the Blue Nile, have generally been supposed to have resulted from steady-state erosion through resistant rocks, accelerating during annual floods. There are exceptions that produced spectacular gorges during emptying of proglacial lakes in North America and on a lesser scale in northern Britain. Just how efficient at erosion individual floods may be was demonstrated by release of reservoir water through a spillway in Texas for about 3 days in 2002 (Lamb M.P. & Fonstad, M.A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, v. 3, p. 477-481). The peak discharge was ~1500 m3s-1, which is not especially huge, yet up to 12 m of erosion occurred through bedrock to produce a sizeable canyon in what was previously a typical small stream valley. Although some erosion was by plucking of joint blocks a considerable amount occurred by potholes scoured by boulders swirling in the rapid currents. Small islands, resembling those preserved in glacial lake outburst floods, were sculpted mainly by suspended sediment rather than by boulder impacts. Another feature that forces a rethink of erosional processes is that waterfalls show no sign of headward retreat by undercutting, but seem to have formed as slabs were plucked by the hydraulic force and slid down stream to form tabular boulders. The implication is that canyons may form episodically during flood events, when the shear stress of the flow on its bed is sufficient to lift and slide joint-bounded slabs.

Catastrophic canyon formation

Huge canyons, such as the Grand Canyon and the Gorge of the Blue Nile, have generally been supposed to have resulted from steady-state erosion through resistant rocks, accelerating during annual floods. There are exceptions that produced spectacular gorges during emptying of proglacial lakes in North America and on a lesser scale in northern Britain. Just how efficient at erosion individual floods may be was demonstrated by release of reservoir water through a spillway in Texas for about 3 days in 2002 (Lamb M.P. & Fonstad, M.A. 2010. Rapid formation of a modern bedrock canyon by a single flood event. Nature Geoscience, v. 3, p. 477-481). The peak discharge was ~1500 m3s-1, which is not especially huge, yet up to 12 m of erosion occurred through bedrock to produce a sizeable canyon in what was previously a typical small stream valley. Although some erosion was by plucking of joint blocks a considerable amount occurred by potholes scoured by boulders swirling in the rapid currents. Small islands, resembling those preserved in glacial lake outburst floods, were sculpted mainly by suspended sediment rather than by boulder impacts. Another feature that forces a rethink of erosional processes is that waterfalls show no sign of headward retreat by undercutting, but seem to have formed as slabs were plucked by the hydraulic force and slid down stream to form tabular boulders. The implication is that canyons may form episodically during flood events, when the shear stress of the flow on its bed is sufficient to lift and slide joint-bounded slabs.

The Younger Dryas flood

In 2006 Wallace Broeker first suggested that the sudden interruption of emergence from the last glacial maximum by a frigid climate about 12.8 ka was due to a massive release of fresh water to the North Atlantic that shut down its thermohaline ‘conveyor’ (see The Younger Dryas and the Flood in June 2006 issue of EPN). He resurrected an earlier idea that a vast lake of glacial meltwater (Lake Agassiz) to the north-west of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. His hypothesis was that the resulting freshening of surface water in the North Atlantic and decreased density stopped the formation of cold dense brines that sink and drag warm water northwards. Setting aside the notion by some enthusiastic authors that a trigger for the Younger Dryas was an exploding comet and a kind of ‘nuclear winter’ (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008) Broeker’s hypothesis is widely accepted. However there are few signs, if any, of a catastrophic glacial-lake outburst through the Great Lakes region and down the St Lawrence. An alternative is that Lake Agassiz drained northwards towards the Arctic Ocean. (Since the North American ice sheet covered Hudson’s Bay that could not have been the destination.) At the end of the last last full glaciation there was a corridor with relatively little glacial cover between the main ice over the Canadian Shield and that mantling the Rocky Mountains, roughly along the course of the modern Mackenzie River. That route would serve the hypothesis well, and there is clear evidence that an outburst flood followed it (Murton, J.B. et al. 2010. Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature, v. 464, p. 740-743).

Sediments of the huge Mackenzie Delta of NW Canada contain a sharp erosion surface overlain by gravels that belie the low-energy of deposition today. Optically stimulated luminescence dating of sediment immediately below and above the erosion surface range from 13.4 (below) to 12.7 ka (above), the latter approximating the onset of frigid Younger Dryas conditions. The surface occurs all the way along the Mackenzie into its major tributary the Athabasca River. Near Fort MacMurray, 20 km north of what was the northern shore of Lake Agassiz, there is a terrace composed of massive boulders. Further evidence comes from the apex of the Mackenzie delta in the form of a 25 km long, 2 km wide spillway scoured of all loose sediment and with topographic features reminiscent of the famous Channelled Scablands of Washington State in the NW USA. Numerous beach lines record the drainage of Lake Agassiz, the highest being dated at the start of the Younger Dryas and giving a clue to the volume involved in the initial outburst flood: around 9500 km3. Dating of other features suggest that a second flooding into the Arctic Ocean occurred during the Younger Dryas around 11.5 ka, during its last stages, and a third at 9.3 ka. One effect of the Younger Dryas was a regrowth of the main ice sheet that allowed Lake Agassiz to refill periodically perhaps allowing quieter flooding events down the Mississippi and through the Great Lakes. There are no signs in the climate record of any major perturbation at 9.3 ka.

Broeker received the news graciously, commenting that a freshening of the Arctic Ocean would have been more effective at shutting down North Atlantic thermohaline circulation than a spillway down the St Lawrence, because the sites of modern day sinking of dense cold brine lie well to the north of its outlet. The only way additional water in the Arctic Ocean could escape would have been into the northernmost North Atlantic.

See also: Schiermeier, Q. & Monastersky, R. 2010. River reveals chilling tracks of ancient flood. Nature, v. 464, p. 657.

Mystery of the sands unmasked

One of the delights of Google Earth is to commit a little Thesigery in the comfort of your front room and traverse the Sahara, the Empty Quarter of Arabia, the Namib or the Gobi. Not only are there dunes on gargantuan scales, but zooming-in from 30 m Landsat to 65 cm Quickbird images on Google Earth reveals a dune hierarchy down to largish ripples. And not all dunes are classic in shape. In the same issue of Nature as a retrospective review of Ralph Bagnold’s classic The Physics of Blown Sand and Desert Dunes, French, Algerian and US workers give a clue to the fundamental controls over dunes systems, that was not available to early researchers (Andreotti, B. et al 2009. Giant aeolian dune size determined by the average depth of the atmospheric boundary layer. Nature, v. 457, p. 1120-1123). They conclude that the general dynamics are analogous to those in flowing water; i.e. like a river, the wind has a capping surface that is the thermal inversion in the atmosphere marked by the tropopause. Flow that is  physically bounded involves a series of resonances (as in a flute), which help to explain the tiered nature of dune systems and also their maximum size in a particular area of desert. Together with seasonal shifts in wind direction, fluctuations in the ‘depth’ of the wind combine together to produce the hypnotically addictive disorganised order that makes big sand deserts so attractive, despite their dangers.