Author Archives: derasa

Whizz-bang hypothesis for the Younger Dryas bites the dust

Such has been the urge to leap on the impact theory of Earth system change, that virtually every drastic event recorded in the geological timescale has been linked by someone or other to the effects of bombardment by extraterrestrial objects. The most recent concerns the Younger Dryas and the extinction of the mammoths (see Whizz-bang view of Younger Dryas and Impact cause for Younger Dryas draws flak in EPN July 2007 and May 2008). The hypothesis stemmed from reports of an association of tiny magnetic spherules, soot and purported nanodiamonds and fullerenes (carbon molecules bonded into ‘geodesic’ spheres) with the onset of the Younger Dryas, the roughly coincident disappearance of Clovis tools and the demise of several large North American mammal species, including mammoths. Regular columnist for Science magazine, Richard Kerr,  reports that independent searches for all the evidential materials at the sites where they were said to occur have drawn unrelieved blanks (Kerr, R.A. 2010. Mammoth-killer impact flunks out , Science, v. 329, p. 1140-1141). Nonetheless, the core supporters of the hypothesis are clinging to their guns.

Phosphorus, Snowball Earth and origin of metazoans

As any gardener knows, the element phosphorus is an essential plant nutrient or fertiliser, along with potassium and nitrogen plus a host of minor elements that are rarely mentioned as sufficient amounts are generally available in soils. The same necessities for life apply to oceans too, in which amounts vary a great deal from place to place and whose relative proportions have changed through geological time. For the oceans the main source of phosphorus is the continental crust, where the element resides mainly in the mineral apatite (Ca5(PO4)3(F,Cl,OH)). This is not an easily dissolved mineral, which is why for agricultural fertiliser it is generally made available in the soluble form of calcium superphosphate (Ca(H2PO4)2) that is produced by reaction between apatite and sulfuric acid. Since the land surface was colonised by plants about 450 Ma ago, biological processes made phosphorus more readily available to solution in river water by their break-down of apatite; supply by rivers to the ocean nowadays is of the order of 109 kg y-1. Ups and downs of P dissolved in ocean water though geological time would be expected to have influenced its overall biological productivity, controlled by photosynthetic phytoplankton and prokaryotes. Variations of carbon isotopes (δ13C) in organic and carbonate sediments are know to have occurred episodically since Archaean times, suggesting wide fluctuations in both bioproductivity and burial of dead organic matter. However, it has been hard to judge any geochemical reasons underpinning such variations. Since it is now clear that the common iron mineral goethite (FeOOH) ‘mops up’ many chemical species including phosphate ions by adsorption on its grain surfaces, measuring the P/Fe ratios in marine ironstones containing these minerals is a potential guide to the changing phosphorus concentration in the oceans (Planavsky, N.J. et al. 2010. The evolution of the marine phosphate reservoir. Nature, v. 467, p. 1088-1090).

The US-French-Canadian researchers charted P/Fe ratios in banded iron formations and ironstones precipitated around ocean-floor hydrothermal vents since the Archaean. What emerged were four episodes: from 2900 to 1700 Ma with generally low ratios; the Neoproterozoic from 750 to 635 Ma with much higher ratios; the Phanerozoic from Cambrian to Jurassic with low ratios and post-Cretaceous high ratios. There are several significant gaps in the record of ocean phosphate levels, notable one a billion years long from 750 to 1700 Ma. One factor that probably affected the variation is the way that dissolved silica (SiO2) drives down the proportion of phosphate adsorbing onto goethite. The rapid evolution and expansion since the Cretaceous of diatoms that secrete silica probably reduced SiO2 concentration in ocean water as their remains rained down to be buried on the ocean floor; that explains the high P/Fe ratios since about 100 Ma. Earlier Phanerozoic oceans are estimated to have had as much as seven times the present concentration of dissolved SiO2, thereby explaining the low values of P/Fe in ironstones deposited in the 100-540 Ma range. From 1700 to 3000 Ma the low P/Fe suggests oceanic phosphorus levels equivalent to those from the Jurassic to Cambrian (but perhaps up to 4 times that, depending on the poorly constrained SiO2 concentrations).

The Neoproterozoic phosphorus ‘spike’, at a time when dissolved SiO2 would have been no different from that in earlier times, suggests a massive influx of phosphate to the oceans at that time. It coincides with the two greatest glacial epochs the Earth has experienced: ‘Snowball’ Earth when glacial ice existed at tropic latitudes. In themselves the massive glaciations offer an explanation for high phosphorus delivery from the continents through glacial erosion and massive run-off during melting. More exciting is that the P/Fe ‘spike’ occurred at a time of massive perturbations in stable carbon isotopes ascribed to huge explosions of phytoplankton and organic carbon burial, which would have been permitted by high dissolved phosphate in the oceans. A large increase in primary biological productivity, i.e. photosynthesis, would have boosted oxygen levels; a necessity for the emergence of metazoan life forms soon after the end of ‘Snowball’ Earth conditions. But that begs the question of how glacially ground-up apatite, abundant as it would have been together with vast amounts of other rock debris, came to be dissolved. In today’s oceans crystalline apatite is barely soluble. It seems that apatite’s solubility decreases as temperature rises, and increases with pH – in alkaline conditions. As well as being cold, Neoproterozoic ocean water around the time of the ‘Snowball’ Earths was saturated with carbonate ions that helped thick, almost pure limestones to form globally after each glaciation. That spells alkaline conditions favouring phosphate solution. The authors speculate that global geochemical conditions during the Cryogenian Period (850-635 Ma) may have fostered the origin of the metazoans. Maybe, but their data have a billion-year gap immediately before that Period, and genomic molecular clocks suggest that the root metazoans emerged as much as half a billion years earlier.

See also: Filippelli, G.M. 2010. Phosphorus and the gust of fresh air. Nature, v. 467, p.1052-1053.

Hard-core continental lithosphere

The oldest and most stable parts of the continents are known as cratons, after the Greek word for strength κράτο (kratos). All the present continents have at least one craton (Africa and South America have 4 each, and Eurasia 6 or 7). Each has remained unaffected by major deformation for a billion years or more, even during continent-to-continent collisions in which they participated. Almost all cratons began to form during the Archaean Eon before 2500 Ma, but most became rigid long after. Several theories have been suggested to account for their durability, one commonly accepted being that somehow the crust ‘ripened’ so that most of the heat-producing radioactive isotopes of U, Th and K were moved by igneous and metamorphic processes to the uppermost crust, along with water; most cratons expose fragments of anhydrous granulites of tonalitic composition. These bear evidence of having formed at the base of the continental crust and have been heavily depleted in “granitophile” trace elements. As a result they cannot undergo partial melting under normal geothermal conditions and where they remain at great depth are much cooler than younger, deep crust. The other dominant feature of cratonic lithosphere is a mantle portion that is anomalously thick (sometimes down to 250 km); in some cases there is little if any sign of asthenosphere beneath such ‘keels’. Research on rocks brought up from the ‘roots’ of cratons by the kimberlite magmas famous for their diamonds points to that deep mantle itself having conferred great rigidity and thus longevity (Peslier, A.H. et al. 2010. Olivine water contents in the continental lithosphere and the longevity of cratons. Nature, v. 467, p. 78-81).

The presence of water in minerals that make up igneous and metamorphic rocks enables them to begin melting at lower temperatures than their dry equivalents, and also to behave in a more plastic fashion under stress. Anne Peslier of NASA in Houston and her US and German colleagues analysed the minerals in ultramafic mantle rocks dragged upwards by kimberlites that punched through the Kaapvaal craton in southern Africa long after it formed. The dominant mantle mineral is olivine (50-80%), generally thought of as anhydrous but typically containing a few hundred parts per million by weight. Olivines in the Kaapvaal mantle xenoliths become drier with increasing depth of their formation (determined from their mineralogy in which garnet is stable at the deepest levels). At depths around 150-250 km low water content in olivine makes it and the mantle itself 20 to 3000 times stronger than the asthenosphere, which protects it from the underlying flow associated with tectonic motions.

How such root zone of continents may have formed has been addressed by two papers on seismic structure beneath the best studied craton; that of the Canadian Shield (Yuan, H. & Romanowicz, B. 2010. Lithospheric layering in the North American craton. Nature, v. 466, p. 1063-1068; Miller, M.S. & Eaton, D.W. 2010. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL044366).In the first, Yuan and Romanowicz of the Berkeley Seismological Laboratory, California use a form of seismic tomography to map anisotropy in the mantle along transects that cross the ancient core of the North American continent. Their results chart the depth of the base of the lithosphere and also define two layers in the lithospheric mantle. The upper layer (down to 150 km) only occurs beneath the Archaean craton, and the top of the asthenosphere ranges from 100-240 km down: at its deepest beneath the craton. The sub-craton mantle they ascribe to chemical depletion of its upper part during early lithospheric evolution, and later addition of the less chemically evolved deeper layer. Miller and Eaton of the Universities of California USA and Calgary Canada used S-wave data from eight seismic stations extending from WSW to ENE over the western cordillera and the Canadian Shield to the Arctic islands of Canada. Their results show a similar variation in dept of the base of the lithosphere and resolve several roughly eastward-dipping boundaries in the sub-craton lithospheric mantle, which they link to Precambrian volcanic arcs preserved in the upper crust above them; i.e. suggesting that the upper layer in the first paper stems from a major episode of arc accretion that built the Canadian Shield.

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

Antipodean glaciers confirm complementary southern warming during the Younger Dryas

Studies of air-temperature proxies in cores from the Antarctic ice cap show a roughly mirrored climate record to that found in the Greenland ice. While the Northern Hemisphere underwent a sudden climate collapse into almost full-glacial conditions around 12.9 ka and an equally dramatic warming around 11.7 ka, Antarctica steadily warmed over the same period to reach full interglacial conditions by 11.5. That this climatic inversion reached relatively low southern latitudes is confirmed by a record of the changing size of glaciers on mountains in New Zealand’s South Island (Kaplan, M.R. and 9 others 2010. Glacier retreat in New Zealand during the Younger Dryas stadial. Nature, v. 467, p. 194-197). The US-New Zealand-Norwegian-French partnerships used detailed geomorphological mapping, and cosmogenic isotope studies of exposed rock fragments to show that after about 13 ka glaciers retreated by more than a kilometre in the succeeding 1500 years in contrast to the dramatic glacial advances in northern areas such as the Scottish Highlands.

Record of rising sea-level in the tropics

Areas beyond the zones of isostatic depression by ice-loading and recovery during glacial-interglacial cycles passively undergo sea-level fall and inundation. They best record the progress of Holocene ice-sheet melting and sea-level rise since 11.5 ka, especially if they are tectonically stable. The island state of Singapore, 1.5 º north of the Equator, is a near-ideal place for study (Bird, M.I. et al. 2010. Punctuated eustatic sea-level rise in the early mid-Holocene. Geology, v. 38, p. 803-806). The Australian and British geoscientists analysed a core through sediments in a mangrove swamp now just below sea level. The top 14 m penetrated a uniform though laminated sequence of marine muds, calibrated to time by radiocarbon dating of mollusc shells, mainly focused on the period from 9 to 6ka period that the global oxygen-isotope record of ice volume suggests to have been the main period of final melting after the Younger Dryas.

Sedimentation was very rapid (~1 cm y-1) from  8.5 to 7.8 ka, probably as sea level rose too rapidly for the coast to be protected by mangrove growth.  Then for 400 years it slackened off to ~0.1 cm y-1 to rise again to 0.5 cm y-1 by 6.5 ka. The last date is the time of the mid-Holocene sea level highstand, after which sedimentation rate soon declined to 0.05 cm y-1, when mangroves became established at the site. Stable isotopes of carbon in the core (δ13C) show how the relative input of marine and terrestrial (mainly mangroves) organisms shifted over the period and are a proxy for the distance to the coastline and hence sea level. From 8.5 to 6.5 ka this was erratic from a starting point about 10 m lower than nowadays, showing rapid rises and falls that culminated in a sea level in Singapore about 3 m above present during the mid-Holocene sea level highstand that slowly declined to that of the present.

The team’s findings tally with evidence for the melting record of the North American ice sheet. An interesting aspect is that they also cover the period when rice cultivation in swampy areas of SE Asia got underway (~7.7 ka). Very rapid sedimentation would have encouraged development of the substrate for the highly fertile delta plains that now support the largest regional population densities on Earth. In turn they culminated in a series of early south and east Asian civilisations based on class societies.

Correction to marine biodiversity record and mass extinctions

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.

One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos?

Comet impacts’ candidature for origin of life

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules.  Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

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 anatomy of a small landslip

At the centre of the Peak District National Park in England is a small mountain called Mam Tor, at the summit of which is a large Iron Age fort complete with defensive ramparts and ditches. Complete, that is, except for its southern parts, which are chopped through by a large arcuate cliff. Below that is hummocky ground typical of landslips, but such disturbed ground is common over large tracts in the Peak District that lie below hills, especially those underlain by Lower Namurian shales of the region. Mam Tor is the only one of these that has an active landslip. Since my early childhood the local authority has tried to keep trafficable a once major road linking the cities of Sheffield and Manchester, but to no avail; most winters it was buckled and cracked by continued motion. The road was abandoned in 1979 and is now a magnificent laboratory for judging the kind of motion involved in the Mam Tor slip. The Iron Age people had much the same problem, as the slip began around 1500 BC long before the fort was built. Clearly, they were not engineering geologists, though the unclimbable scar was maybe a defensive bonus, provided the old, the bewildered and the very young were kept well away from it, as they are today.

Records of the movement have been kept since the road was constructed in 1820, and one milestone has moved 50 m in 190 years at a constant annual rate, but just how it moves has only become clear since Manchester University geologists installed tilt and creep meters, and 50 survey stations in 2004-5. Their preliminary results are just in (Green, S. et al. 2010. The effects of groundwater level and vegetaion on creep of the Mam Tor landslip. Geology Today, v. 26, p. 134-139). The creep rate is clearly governed by groundwater level beneath the slip, and has risen as high as 19.5 mm per day. From the logarithmic plot between the two variables it is possible to estimate the creep rate with completely saturated ground, which would be an ominous 0.6 m per day. Thankfully, drainage through the slip is good, as beneath lie highly unstable mudstones; but things could change. The team has also monitored local rainfall, and precipitation underwent a marked increase from 2000 onward (1.64 m per year) compared with the average since 1930 of 1.3 m per year. Fortunately, spring and summer rains are quickly returned to the atmosphere by vigorous evapotranspiration by the lush grasses and ferns on the slipped mass. The greatest creep takes place in the winter when vegetation has died back.  Mam Tor is indeed highly instructive, but at present poses no great hazard, yet it might become less predictable should annual rainfall increase. It is unlikely to attain the awesome pace of that in Calabria, southern Itaaly on 15 February 2010 at Maierato near Vibo Valentia (view

Wet Moon, dry Moon

Regular readers will remember my remarkable though very reluctant conversion to the notion that there may be water on Mars. My stubborn reaction had been against the background that shrouded the hypothesis with a certain desperation; the need of any future crewed mission to Mars for a water supply and thereby one of hydrogen fuel, plus the determination of the whole Mars-oriented community to justify such a mission by hyping ‘xenobiology’ on the ‘Red Planet’. A similar desperation claoked the search for surface water on the Moon, although one more dominated by the ‘Everest’ syndrome: since the boot prints and flags appeared, everyone wants to go. The Moons internal water is an entirely different kettle of fish. The hypothesis of the Moon’s formation by condensation from an incandescent mass flung into orbit after a planet – planet collision involving the Earth has the corollary that the lunar mantle ought to be bone dry: and so it seemed to be from bulk analyses of rocks brought back by the Apollo missions. In fact, there are a number of possibilities to explain vanishingly small amounts of internal water: the Moon is made of impactor that happened to be dry rather than terrestrial material; Earth and Moon are a mix of both and both Earth and impactor started out dry, but the Earth later received its water from comets; low pressure condensation of the Moon ruled out water entering itss silicate minerals and so on. Then water was found in apatite grains from lunar maria basalts (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). Within a couple of months we are back to the dry-as-an-alco’s-throat view (Sharp, Z.D. et al. 2010. The chlorine isotope composition of the Moon and implications for an anhydrous mantle. Science, v. 329, p. 1050-1053). Both terrestrial and meteoritic chlorine isotopes are in remarkably consistent proportions, but lunar rocks show an 25 times greater spread by comparison. To cut a long and complicated discussion short, such a range could only have formed if chlorides of a variety of metals were vaporised from lunar magmas each having its own effect on fractionation of Cl isotopes. In turn, combination of chlorine with metal ions requires virtually no hydrogen ions and therefore vanishingly little water in the moon, otherwise chlorine would have been combined in HCl and not subject to any fractionation when that volatilised on eruption. So that seems settled, then…

Carbonates on Mars

Ancient valley systems, huge water-carved gorges and sedimentary deposits signify with little room for doubt that early in its history Mars was wet; it must therefore have been warm. A thick CO2-rich atmosphere seems obligatory to give the kind of greenhouse warming that prevented Earth from freezing over when the young Sun was weaker than now. The question is, where did the CO2 go so that the planet became chilled? Gravity on Mars is sufficient to have retained the gas, unlike water vapour that dissociates to hydrogen and oxygen, of which hydrogen easily escapes even a much stronger gravitational field. A consensus is developing that it resides in carbonate minerals. The other likely greenhouse gas is sulfur dioxide, for whose drawdown there is ample evidence in the form of sulfates detected from orbit and by surface rovers. Carbonates have a relatively simple, and unique spectrum of reflected solar radiation, with an absorption feature at a wavelength around 2.3 micrometres. Carbonates have been detected on Mars using orbital hyperspectral imaging, but only in patches. The NASA rovers rely on serendipity for any discovery, yet Spirit did stumble on a large carbonate-rich outcrop identified by its on-board Mössbauer spectrometer (Morris, R.V. and 12 others 2010. Identification of carbonate-rich outcrops on Mars by the Spirit rover. Science, v. 329, p. 421-424). It appears to be a Fe-Mg variety in association with olivines, and carbonate makes up to 34 % of part of the outcrop. The texture is granular, yet the area abounds with evidence for hydrothermal activity in the form of sulfates and silica-rich materials, implying that some kind of circulation system deposited the carbonates. The associated olivine is odd, as that mineral is prone to rapid breakdown to serpentines in the presence of water.

The discovery of carbonate rock does help the CO2 early greenhouse theory and the fate of the warming gas, but aside from the fact the identification has been done at vast distance does it rank with geoscience that can be accomplished on Earth? It is a small piece in the jigsaw of Mars’s climatic evolution, but cannot resolve the issue of drawdown of greenhouse gas. The real drama there lay in the finding of abundant signs of water erosion on many scales set against today’s surface hyperaridity; evidence for glaciation and subsurface water ice in apparently large volumes. Earth had to have had a thick CO2-rich atmosphere at the same time as that of Mars, but we are still not sure where all that carbon ended up in the early Precambrian, despite limestones and carbon-rich mudstones dating back to 3.4 Ga: as we cannot quantify that aspect of Earth’s history, neither can we expect an early answer for Mars. Indeed, what is the benefit set against the cost?

See also: Harvey, P. 2010. Carbonates and Martian climate. Science, v. 329, p. 400-401.

The vestige of a beginning

Geoscientists take it for granted that the Earth has a certain age (currently estimated at 4.54 Ga), but it is one divined from indirect evidence, lead isotopes in meteorites and ancient ores of lead derived from uranium. If ever geoscientists are to grasp the nature of the early planet the evidence would be geochemical, yet also second-hand because relics must lie somewhere in the mantle as the crust is constantly being changed. For decades it has been known that the mantle shows geochemical heterogeneity as a result of episodes of partial melting from which the oceanic and continental crust emerged. Even with such an ancient origin it seems intuitively likely that there should be some mantle that has not been interfered with. Now a group of geochemists from the US and Britain have presented evidence for just such ur-mantle (Jackson, M.G et al. 2010. Evidence for the survival of the oldest terrestrial mantle reservoir. Nature, v. 466, p. 853-856). Their data come from Cenozoic lavas collected on Baffin Island and in West Greenland, which gave an earlier clue for having melted from a truly antique source: they contain the highest ratio of helium produced in the Big Bang (3He) to that released by radioactive decay (4He). Repeated melting of the mantle gradually drives off, yet radioactive decay continually replenishes its complement of 4He, so the more reworked a mantle source for lavas is the lower its 3He/ 4He ratio. This notion is backed up by the lead and neodymium isotopes in the Baffin Island and West Greenland lavas and they suggest an age of formation of the mantle source between 4.45-4.55 Ga.

Convection over billions of years ensures a degree of mixing in the mantle, but such is the viscosity of the Earth that there is a good chance that some areas have remained unchanged, the more so if the bulk of magmatism involving deep mantle has been linked to narrow rising plumes. But what emerges from the rest of the geochemistry of these lavas? Provided they have not been contaminated by continental crust through which they have passed, it should be possible using models for the way different elements are contributed to or withheld from magma by mantle minerals to estimate the source mantle’s overall composition. The team did this, bearing in mind the uncertainties. Plotted relative to a ‘guestimate’ of the original bulk Earth based on the geochemistry of chondritic meteorites they sshoww a very good fit for those elements that are likely to be retained by mantle minerals during partial melting: the so-called ‘compatible’ elements. But the estimated source for the lavas seems to have been depleted in the ‘incompatible’ elements that are highly likely to enter magma as soon as partial melting starts. This pattern would be expected if the early mantle had undergone some kind of differentiation as a whole, and that would be a consequence of the entire mantle having been molten and then crystallising: some low-density minerals could preferentially have taken in incompatible elements and floated upwards to deplete those elements in the deep mantle. That is compatible with the idea of Moon formation as a result of a collision between the proto-Earth and a Mars-sized planet, which could have released sufficient energy in the form of heat tp completely melt the outermost Earth.

So the data reveal a great deal, especially that this ancient mantle may well have been the parent for all later mantle compositions as the Earth evolved by dominantly igneous processes. But they do not resolve the perennial debate as to whether the Earth accreted from a uniform mix of nebular material of which meteorites are relics, roughly the composition of chondrites, or heterogeneously from different materials that had condensed from incandescent vapour at different nebular temperatures at different times. Moon formation would have mixed up the latter efficiently in a mantle-wide magma ocean, so we may never know. However some of the oldest meteorites contain fragments of condensates that did form at different temperatures.