A full list of publications can be found in the NIGL Annual Reports. This section
highlights some of our high-impact papers as they go to press. A full list of NERC staff publications and outputs can be found in the NERC Open Research Archive (NORA)
Journal of Paleolimnology – May 2013
Isotope geochemistry is an essential part of environmental and climate change research and over the last few decades has contributed significantly to our understanding of a huge array of environmental problems, not least in palaeolimnology and limnogeology. Here we describe some of the recent developments in the use of stable isotopes in palaeo-lake research. These are: better preparation, analysis, and interpretation of biogenic silica oxygen and silicon isotopes; extraction and characterisation of specific compounds such as leaf waxes and algal lipids for isotope analysis; determining the excess of 13C-18O bonds in
clumped isotopes; and the measurement of multiple isotope ratios in chironomid chitin. These advances have exciting prospects and it will be interesting to see how these techniques develop further and consequently offer a real advancement in our science over the next decade.
Leng, M.J. and Henderson, A.C.G. 2013. Recent advances in isotopes as palaeolimnological proxies. Journal of Paleolimnology, 49, 481-496.
Encyclopedia of Quaternary Science, 2nd Edition – April 2013
The second revised edition of the Encyclopedia of Quaternary Science, provides both students and professionals with an up-to-date reference work on this important and highly varied area of research. There are lots of new articles, and many of the articles that appeared in the first edition have been updated to reflect advances in knowledge since 2006, when the original articles were written.
Leng, M.J., Barker, P.A., Swann, G.E.A., Snelling, A.M. 2013. δ18O Records. In: Elias, S.A. (ed). The Encyclopedia of Quaternary Science, vol. 1, pp.481-488. Amsterdam: Elsevier.
Paleoceanography – April 2013
The development of a permanent, stable ice sheet in East Antarctica happened during the middle Miocene, about 14 million years (Myr) ago. The middle Miocene therefore represents one of the distinct phases of rapid change in the transition from the “greenhouse” of the early Eocene to the “icehouse” of the present day. Carbonate carbon isotope records of the period immediately following the main stage of ice sheet development reveal a major perturbation in the carbon system, represented by the positive δ13C excursion known as carbon maximum 6 (“CM6”), which has traditionally been interpreted as reflecting increased burial of organic matter and atmospheric pCO2 drawdown. More recently, it has been suggested that the δ13C excursion records a negative feedback resulting from the reduction of silicate weathering and an increase in atmospheric pCO2.
Here we present high-resolution multi-proxy (alkenone carbon and foraminiferal boron isotope) records of atmospheric carbon dioxide and sea surface temperature across CM6. Similar to previously published records spanning this interval, our records document a world of generally low (~300 ppm) atmospheric pCO2 at a time generally accepted to be much warmer than today. Crucially, they also reveal a pCO2 decrease with associated cooling, which demonstrates that the carbon burial hypothesis for CM6 is feasible and could have acted as a positive feedback on global cooling.
Badger M, Lear C, Pancost R, Foster G, Bailey T, Leng M, Abels H. 2013. CO2 drawdown following the middle Miocene expansion of the Antarctic Ice Sheet. Paleoceanography 28, 42–53.
Over the past 50 years, warming of the Antarctic Peninsula has been accompanied by accelerating glacier mass loss and the retreat and collapse of ice shelves. A key driver of ice loss is summer melting; however, it is not usually possible to specifically reconstruct the summer conditions that are critical for determining ice melt in Antarctic. Here we reconstruct changes in ice-melt intensity and mean temperature on the northern Antarctic Peninsula since AD 1000 based on the identification of visible melt layers in the James Ross Island ice core and local mean annual temperature estimates from the deuterium content of the ice. During the past millennium, the coolest conditions and lowest melt occurred from about AD 1410 to 1460, when mean temperature was 1.6°C lower than that of 1981–2000. Since the late 1400s, there has been a nearly tenfold increase in melt intensity from 0.5 to 4.9%.
The warming has occurred in progressive phases since about AD 1460, but intensification of melt is nonlinear, and has largely occurred since the mid-twentieth century. Summer melting is now at a level that is unprecedented over the past 1,000 years. We conclude that ice on the Antarctic Peninsula is now particularly susceptible to rapid increases in melting and loss in response to relatively small increases in mean temperature.
Nerilie J. Abram, Robert Mulvaney, Eric W. Wolff, Jack Triest, Sepp Kipfstuhl, Luke D. Trusel, Françoise Vimeux, Louise Fleet & Carol Arrowsmith. 2013. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geoscience. Online.
The Antarctic Peninsula is one of the fastest-warming regions on Earth, but its palaeoenvironmental history south of 63° latitude is relatively poorly documented, relying principally on the marine geological record and short ice cores. In this paper, we present evidence of late-Quaternary environmental change from the Marguerite Bay region combining data from lake sediment records on Horseshoe Island and Pourquoi-Pas Island, and raised beaches at Horseshoe Island, Pourquoi-Pas Island and Calmette Bay. Lake sediments were radiocarbon dated and analysed using a combination of sedimentological, geochemical and microfossil methods. Raised beaches were surveyed and analysed for changes in clast composition, size and roundness. Results suggest a non-erosive glacial regime could have existed on Horseshoe Island from 35,780 (38,650–33,380) or 32,910 (34,630–31,370) cal yr BP onwards.
There is radiocarbon and macrofossil evidence for possible local deglaciation events at 28,830 (29,370–28,320) cal yr BP, immediately post-dating Antarctic Isotopic Maximum 4, and 21,110 (21,510–20,730 interpolated) cal yr BP coinciding with, or immediately post-dating, Antarctic Isotopic Maximum 2. The Holocene deglaciation of Horseshoe Island commenced from 10,610 (11,000–10,300) cal yr BP at the same time as the early Holocene temperature maximum recorded in Antarctic ice cores.
Late Quaternary environmental changes in Marguerite Bay, Antarctic Peninsula,inferred from lake sediments and raised beaches. (2013) Dominic A. Hodgson; Stephen J. Roberts; James A. Smith; Elie Verleyen; Mieke Sterken; Minke Labarqueb; Koen Sabbe; Wim Vyverman; Claire S. Allen; Melanie J. Leng; Charlotte Bryant. Quaternary Science Reviews 68: 216–236.
Journal of Climate “ The Freshwater System West of the Antarctic Peninsula ” – April 2013
Climate change west of the Antarctic Peninsula is the most rapid of anywhere in the Southern Hemisphere, with associated changes in the rates and distributions of freshwater inputs to the ocean. Here, results from the first comprehensive survey of oxygen isotopes in seawater in this region are used to quantify spatial patterns of meteoric water (glacial discharge and precipitation) separately from sea ice melt. High levels of meteoric water are found close to the coast, due to orographic effects on precipitation and strong glacial discharge. Concentrations decrease offshore, driving significant southward geostrophic flows (up to ~30 cm s−1).
These produce high meteoric water concentrations at the southern end of the sampling grid, where collapse of the Wilkins Ice Shelf may also have contributed. As the regional freshwater system evolves, the continuing isotope monitoring described here will elucidate the ongoing impacts on climate and the ecosystem.
Michael P. Meredith, Hugh J. Venables, and Andrew Clarke, Hugh W. Ducklow and Matthew Erickson, Melanie J. Leng, Jan T. M. Lenaerts and Michiel R. van den Broek. 2013. The Freshwater System West of the Antarctic Peninsula: Spatial and Temporal Changes. Journal of Climate, 26, p. 1669-1684.
Special issue of the journal “Quaternary Science Reviews: Isotopes and Lakes” edited by Prof Melanie Leng – March 2013
Isotope geochemistry is increasingly an essential part of environmental and climate change research and now routinely contributes to our understanding of many critical environmental problems, which span the whole of Earth system science and not least in palaeolimnology and limnogeology. The International Association of Limnogeology organises an international conference every four years. The fifth International Limnogeology Congress, ILIC V, was held in Konstanz, Germany, from August 31st - September 3rd, 2011. During this congress we identified several papers where isotope methodologies were used in a particularly novel way, or provided an ‘added value’ data set.
Many of these are brought together in this volume as a series of state-of-the-art papers dealing with various aspects of isotopes in lake sediment archives. These papers are themed under isotopes in contemporary processes, isotopes in diatom silica from lake sediments, isotopes in organic materials in lake sediments, and isotopes in carbonates from lake sediments. The journal editors of this volume are Prof Melanie J. Leng (British Geological Survey and University of Leicester), Prof Phillip A. Barker (Lancaster University) and Prof Antje Schwalb (Universität Braunschweig,Germany).
The Early Jurassic Epoch was a predominantly greenhouse phase of Earth history, but a comprehensive understanding of its climate dynamics is hampered by a lack of high resolution multi-proxy environmental records. Here we report a geologically brief (approximately several hundred thousand years) negative carbon isotope excursion in both marine and terrestrial materials, recognised for the first time for the Late Sinemurian Substage (Early Jurassic, ~194 Ma) of eastern England. The Late Sinemurian carbon isotope excursion,
which is termed the S-CIE, is accompanied by peaks in the abundance of various pollen grains...
Riding, J.B., Leng, M.J., Kender, S., Hasselbo, S.P., Feist-Burkhardt, S. 2013. Isotopic and palynological evidence for a new Early Jurassic environmental perturbation. Palaeogeography, Palaeoclimatology, Palaeoecology, 374, 16-27.
The current issue of Elements Magazine is entitled ‘100 years of Isotope Geochronology’ and is co-edited by Dr Dan Condon (NIGL) and includes a several papers co-authored by Dan Condon and Matt Horstwood.
Earth and Planetary Science Letters – January 2013
Understanding the response of the Antarctic ice sheets during the rapid climatic change that accompanied the last deglaciation has implications for establishing the susceptibility of these regions to future 21st Century warming. A unique diatom δ18O record derived from a high-resolution deglacial seasonally laminated core section off the west Antarctica Peninsula is presented. By extracting and analysing single species samples from individual laminae, season-specific isotope records were separately generated to show
changes in glacial discharge (comprised of both glacial and iceberg sourced meltwater) to the coastal margin during spring and summer months...
Swann, G.E.A., Pike, J., Snelling, A.M., Leng, M.J., Williams, M.C. 2013. Seasonally resolved diatom δ18O records from the West Antarctica Peneinsula over the last deglaciation. Earth and Planetary Science Letters, 364, 12-23.
This paper describes a multi-proxy palaeoecological investigation of lake sediments undertaken in conjunction with an archaeological survey of the Upper Sangro Valley in the Abruzzo National Park, Central Italy. The data show how cultural factors had a profound effect on this mountainous region which, in this case, outweighed the effects of climatic fluctuations which are known to have occurred locally and across the wider region. These findings have positive implications for the
conservation of top predators which require large wooded ranges.
Brown, A.G., Hatton, J., Selby, K.A., Leng, M.J., Christie, N. 2013. Multi-proxy study of Holocene environmental change and human activity in the Central Apennine Mountains, Italy. Journal of Quaternary Science, 28, 71-82.
The Antarctic Peninsula Ice Sheet is the most northerly of the Antarctic ice sheets and the most vulnerable to climate warming. In light of recent instrumental records of warming along the Antarctic Peninsula, there has been much debate about what has influenced rising temperatures along the west Antarctica Peninsula (WAP) during the Holocene, with terrestrial and oceanic proxies often suggesting different, sometimes opposing, mean conditions. Here, we present a Holocene glacial (terrestrial ice) melt water record derived from marine δ18Odiatom from Palmer Deep, west Antarctic Peninsula. Our results provide a unique opportunity to assess atmospheric versus oceanic influence on melting along the WAP. We demonstrate that since approximately 5.0 ka the increasing occurrence of La Niña, as recorded in the lower latitude Pacific, and increasing summer insolation at 60ºS have had a stronger influence on glacial melt water inflow to the WAP margin than oceanic processes driven by the southern westerly winds.
Pike, J., Swann, G.A.E., Leng, M.J., Snelling, A. 2013. Glacial discharge along the west Antarctic Penninsula during the Holocene. Nature Geoscience. Online.
The late Mesoproterozoic Sveconorwegian orogen in southwest Baltica is traditionally interpreted as the eastward continuation of the Grenville orogen in Canada, resulting from collision with Amazonia, forming a central part in the assembly of the Rodinia supercontinent. We challenge this conventional view based on results from recent work in southwest Norway demonstrating voluminous subduction–related magmatism in the period 1050–1020 Ma, followed by geographically restricted high–T/medium–P metamorphism between 1035 and 970 Ma, succeeded by ferroan magmatism over large parts of south Norway in the period 990–920 Ma.
This magmatic and metamorphic evolution may be better understood as reflecting a long–lived accretionary margin, undergoing periodic compression and extension, than continent–continent collision. This study has implications for Grenville–Sveconorwegian correlations, comparisons with modern continental margins, Rodinia reconstructions and how we recognize geodynamic settings in ancient orogens.
Slagstad, T., Roberts, N.M.W., Marker, M., Røhr, T., Schiellerup, H. 2013. A Non-Collisional, Accretionary Sveconorwegian Orogen. Terra Nova 25, 30-37.
Precambrian Research – January 2013
We present a study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa, which yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern margin of the Tanzania Craton is a ca. 1960 Ma top-to-the-NW ductile thrust zone. Orthogneisses of the eastern part of craton were dated by U–Pb zircon at ca. 2.7 Ga. High grade paragneisses to the east (Mpwapwa Group) are also Archaean (>ca. 2.6 Ga) and thus do not belong to the Usagaran belt, but to the “Western Granulite” of the East African Orogen. Charnockites in the south have been dated at 2701 ±
21 Ma and point to a third Archaean assemblage. A post-Usagaran granite dated at 1873 ± 31 Ma shows Neoproterozoic metamorphic zircon rims that grew during the East African orogenic event.
A sliver of southern tonalitic orthogneiss, entrained in the craton margin shear zone, is an igneous rock of probable early Palaeoproterozoic or Archaean age (>ca. 2300 Ma) with a strong metamorphic overprint at ca. 1960 Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990 Ma, this is believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks.
Thomas, R.J., Roberts, N.M.W., Jacobs, J., Bushi, A.M., Horstwood, M.S.A., Mruma, A. 2013. Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania. Precambrian Research 224, 671-689.
Geochimica et Cosmochimica Acta - November 2012
Despite a growing body of work that uses diatom δ30Si to reconstruct past changes in silicic acid utilisation, few studies have focused on calibrating core top data with modern oceanographic conditions. In this study, a microfiltration technique is used to divide Southern Ocean core top silica into narrow size ranges, separating components such as radiolaria, sponge spicules and clay minerals from diatoms. Silicon isotope analysis of these components demonstrates that inclusion of small amounts of non-diatom material can significantly offset the measured from the true diatom δ30Si. Once the correct size fraction is selected (generally 2–20 μm), diatom δ30Si shows a strong negative correlation with surface water silicic acid concentration (R2°=°0.92), highly supportive of the qualitative use of diatom δ30Si as a proxy for silicic acid utilisation. The core top diatom δ30Si matches well with mixed layer filtered diatom δ30Si from published in situ studies,
suggesting little to no effect of either dissolution on export through the water column, or early diagenesis, on diatom δ30Si in sediments from the Southern Ocean. However, the core top diatom δ30Si shows a poor fit to simple Rayleigh or steady state models of the Southern Ocean when a single source term is used. The data can instead be described by these models only when variations in the initial conditions of upwelled silicic acid concentration and δ30Si are taken into account, a caveat which may introduce some error into quantitative reconstructions of past silicic acid utilisation from diatom δ30Si.
Egan, K., Rickaby, R.E.M., Leng, M.J., Hendry, K.R., Hermoso, M., Sloane, H.J., Bostock, H., Halliday, A.N. 2012. Silicon isotopes as a proxy for silicic acid utilisation: a Southern Ocean core top calibration. Geochimica et Cosmochimica Acta, 96, 174-192.
Geochemistry, Geophysics, Geosystems - September 2012
This study summarizes organic carbon isotope (δ13C) and total organic carbon (TOC) data from a series of tests undertaken to provide an appropriate methodology for pre-analysis treatment of mudstones from an Upper Carboniferous sedimentary succession, in order to develop a consistent preparation procedure. The main treatments involved removing both inorganic carbonate and hydrocarbons (which might be extraneous) before δ13C and TOC analysis. Overall we show that the most accurate assessment of bulk organic carbon isotopes and concentration in these samples can be achieved through decarbonating the material prior to measurement via the 'rinse method'. However, our results support recent findings that pre-analysis acid treatments can cause variable and unpredictable errors in δ13C and TOC values.
We believe that, despite these uncertainties, the findings presented here can be applied to paleoenvironmental studies on organic matter contained within sedimentary rocks over a range of geological ages and compositions.
Könitzer, S.F., Leng, M.J., Davies, S.J., Stephenson, M.H. 2012. An assessment of geochemical preparation methods prior to organic carbon concentration and carbon isotope ratio analyses of fine-grained sedimentary rocks. Geochemistry, Geophysics, Geosystems, 13, 12pp.0.
Earth and Planetary Science Letters - September 2012
Highlights: We identify the carbon isotope excursion from the North Sea in exceptional detail. We document changes to ocean stratification well before atmospheric carbon release.Precursor stratification may be associated with the trigger for carbon release. Increased precipitation and runoff likely occurred along with carbon release. We document a rapid change to regional vegetation during carbon release.
Kender, S., Stephenson, M.H., Riding, J.B., Leng, M.J., Knox, R.O'B., Peck, V.L., Kendrick, C.P., Ellis, M.A., Vane, C.H., Jamieson, R. 2012. Marine and terrestrial environmental changes in NW Europe preceding carbon release at the Paleocene-Eocene transition. Earth and Planetary Science Letters, 353-354, 108–120.
Rapid warming over the past 50 years on the Antarctic Peninsula is associated with the collapse of a number of ice shelves and accelerating glacier mass loss. In contrast, warming has been comparatively modest over West Antarctica and significant changes have not been observed over most of East Antarctica, suggesting that the ice-core palaeoclimate records available from these areas may not be representative of the climate history of the Antarctic Peninsula. Here we show that the Antarctic Peninsula experienced an early-Holocene warm period followed by stable temperatures, from about 9,200 to 2,500 years ago, that were similar to modern-day levels. Our temperature estimates are based on an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula. We find that the late-Holocene development of ice shelves near James Ross Island was coincident with pronounced cooling from 2,500 to 600years ago. This cooling was part of a millennial-scale climate excursion with opposing anomalies on the eastern and western sides of the Antarctic Peninsula. Although warming of the northeastern Antarctic Peninsula began around 600 years ago, the high rate of warming over the past century is unusual (but not unprecedented) in the context of natural climate variability over the past two millennia.
The connection shown here between past temperature and ice-shelf stability suggests that warming for several centuries rendered ice shelves on the northeastern Antarctic Peninsula vulnerable to collapse. Continued warming to temperatures that now exceed the stable conditions of most of the Holocene epoch is likely to cause ice-shelf instability to encroach farther southward along the Antarctic Peninsula.
Mulvany, R., Abram, N.J., Hindmarsh, R.C.A., Arrowsmith, C., Fleet, L., Triest, J., Sime, L.C., Alemany, O., Foord, S. 2012. Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history. Letter to NATURE.
Here we present Holocene organic C, N, S, C-isotope and macrofossil data from a small freshwater lake near Sisimiut in southwest Greenland. The lake was formed c. 11 cal. ka BP following retreat of the ice sheet margin and is located above the marine limit in this area. The data suggest a complex deglaciation history of interactions between the lake and its catchment, reflecting glacial retreat and post-glacial hydrological flushing probably due to periodic melting of local remnant glacial ice and firn areas between 11 and 8.5 cal. ka BP. After 8.5 cal. ka BP, soil development and associated vegetation processes began to exert a greater control on terrestrial-aquatic carbon cycling. By 5.5 cal. ka BP, in the early Neoglacial cooling, the sediment record indicates a change in catchment-lake interactions with consistent δ13C while C/N exhibits greater variability. The period after 5.5 cal. ka BP is also characterised by higher organic C accumulation in the lake.
These changes are most likely the result of increasing contribution (and burial of) terrestrial organic matter as a result of enhanced soil instability, as indicated by an increase in various macrofossil remains. The impact of glacial retreat and relatively subdued mid- to late-Holocene climate variation at the coast is in marked contrast to the greater environmental variability seen in inland lakes closer to the present-day ice sheet margin.
Leng, M.J., Wagner, B., Anderson, N.J., Bennike, O., Woodley, E., Kemp, S.J. 2012. Deglaciation and catchment ontogeny in coastal southwest Greenland: implications for terrestrial and aquatic carbon cycling. Journal of Quaternary Science, 27, 575-584.
Palaeogeography, Palaeoclimatology, Palaeoecology - August 2012
A stratigraphically and temporally ordered sequence of channel calcretes preserved along the Wadi Dana, Southern Jordan, records the Quaternary evolution of the formation and infilling of rock-cut channels and their subsequent incision in a tectonically subsiding basin. It is currently unknown under what palaeoenvironmental conditions these non-pedogenic (alpha) calcretes formed. Stable isotope analyses have been used to investigate whether any past topographical, hydrological, vegetational, diagenetic, temporal and/or climatologic signatures can be identified from the channel calcretes.
McLaren, S.J., Leng, M.J., Knowles, T., Bradley, A.V. 2012. Evidence of past environmental conditions during the development of a calcretised Wadi System in Southern Jordan using stable isotopes. Palaeogeography, Palaeoclimatology, Palaeoecology, 348-349, 1-12.
The mid-Holocene East Asian Monsoon history was reconstructed using bulk organic carbon isotopes and concentration data from the Pearl River estuary, southern China. The data are indicators of changes in monsoonal precipitation strength, e.g. sediments buried during a period of high precipitation exhibit a high proportion of terrigenous sediments, and have low δ13C and high C/N, and vice versa. Results suggest a general decreasing trend in monsoonal precipitation from 6650–2215 cal yr BP due to the weakening insolation over northern hemisphere most likely related to the current precession circle. Superimposed on this trend are apparent dry-wet oscillations at centennial to millennial timescales most likely in response to solar activity.
Yu, F., Zong, Y., Lloyd, J.M., Leng, M.J., Switzer, A.D., Yim, W, W-S., Huang, G. 2012. Mid-Holocene variability of the East Asian monsoon based on bulk organic δ13C and C/N records from the Pearl River estuary, southern China. The Holocene, 22, 705–715.
Boreal ecosystems are highly vulnerable to climate change, and severe ecological impacts in the near future are virtually certain to occur. We undertook a multiproxy study on an alpine lake at the modern tree-line in boreal, southern Siberia. Steppe and tundra biomes were extensive in eastern Sayan landscapes during the early Holocene. We conclude that lakes in continental, boreal regions undergo different models of lake ontogeny than oceanic boreal regions. Unlike other regions discussed, climate is a more important driver of ecosystem change than catchment changes. We also demonstrate that the start of the period coincident with the onset of the Little Ice Age resulted in important thresholds crossed in catchment vegetation and aquatic communities.
Mackay, A.W., Bezrukova, E.V., Leng, M.J., Meaney, M., Nunes, A., Piotrowska, N., Self, A., Shchetnikov, A., Shilland, E., Tarasov, P., Wang, L., White, D. 2012. Aquatic ecosystem responses to Holocene climate change and biome development in boreal, central Asia. Quaternary Science Reviews, 41, 119-131.
Science — 238U/235U Systematics in Terrestrial Uranium Bearing Minerals — 30th March 2012
Minerals, such as zircon, naturally capture uranium when they form, which in turn undergoes a chain of radioactive decays to other elements, ending with lead. Dating of zircon and other minerals underpins much if what we know about geological time. This new research has shown that, by more accurately measuring the relative amount of the uranium isotopes 238U and 235U, we now have a better understanding of how much time has passed since a mineral or rock has formed. We have measured, for the first time, 238U/235U in a suite of U-bearing minerals, including 45 zircon samples typical of those dated to quantify geological time in a wide range of rock types. A major impact of this work is an 'average zircon' 238U/235U to replace the 'consensus value' of 137.88, the accuracy of which could not be verified. Using this new value will decrease all previous zircon uranium-lead (U-Pb) age determinations, by up to 700,000 years for samples that are about 4.4 billion years old — the age of the Earth's oldest minerals.
The new 238U/235U ratio will allow geologists to place more accurate limits on the exact timing of a broad range of geological processes, from the initial formation of our planet, continents and economic mineral deposits, to past evolutionary events and climate changes. In addition to the implications for U-Pb zircon geochronology, this study provides insight into uranium isotopic fractionation processes at magmatic temperatures, the possible role of weathering to driving changes of 238U/235U in seawater, and comparison of the terrestrial 238U/235U database to extra-terrestrial meteorite samples.
The volume of Earth's continental crust depends on the rate of addition of continental crust from the mantle compared to the rate of continental loss back to the mantle, which at present is roughly balanced. Models for the growth rate of continental crust vary, with isotope data suggesting various episodes of increased growth rate throughout Earth's history; these episodes have been correlated with the supercontinent cycle, but may be a consequence of preferential preservation of continental crust during these cycles. The global balance between addition and loss of continental crust is controlled by: 1) the extent of internal orogens versus exterior orogens, with the latter favouring continental addition, and 2) the balance between exterior orogens in retreating mode versus those in advancing mode, with the latter favouring continental loss. A greater balance of continental addition versus loss should exist during supercontinent break-up, due to a high magmatic flux in retreating accretionary orogens, whereas the amalgamation of supercontinents should involve increased continental loss due to increased sediment subduction and tectonic erosion. Zircon U-Pb and Hf isotopes provide insight to models of crustal growth rate since they sample the continental crust at their time of formation. Using the distribution of data within εHf(t)-time space of a global zircon database, it is demonstrated that the data are in accord with the concept of increased continental loss during supercontinent amalgamation.
Periods featuring increased continental addition relative to continental loss, and hence increased continental crust growth rate, occur at ~ 1.7–1.2 Ga, ~ 0.85–0.75 Ga, and ~ 0.45–0.35 Ga, and follow the formation of the Columbia (Nuna), Rodinia and Gondwana supercontinents respectively. Distinct increases in continental loss compared to continental addition, i.e. decreased continental growth rate, occur at c. 1.0–0.9 Ga, and ~ 0.6–0.55 Ga, correlating with the periods of Rodinia and Gondwana amalgamation respectively. Formation of Pangea by introversion rather than extroversion, means that continental addition in exterior orogens was concurrent with continental loss in interior orogens; a similar process may have been responsible for formation of the Columbia supercontinent. Peaks in the compilation of U-Pb zircon ages correlate with the timing of supercontinent amalgamation, and are likely to be a consequence of preferential preservation of continental crust during this part of the supercontinent cycle.
Roberts, N. M. W., 2012. Increased loss of continental crust during supercontinent amalgamation. Gondwana Research 21, 994-1000.
Palaeogeography, Palaeoclimatology, Palaeoecology - February 2012
We aim to reconstruct the climatic and environmental conditions in the Valsequillo Basin during the deposition of the Valsequillo gravels between c. 40 000 and 8000 years ago, when large mega-fauna and potentially humans occupied the basin. Fossil freshwater (Fossaria sp. and Sphaeriidae (Family)) and terrestrial (Polygyra couloni, Holospira sp. and Cerionidae (Family)) snail shells from sections within the Barranca Caulapan were collected for oxygen and carbon stable isotope analysis. Oxygen and carbon isotopes in terrestrial and freshwater snail shells relate to local climatic parameters and environmental conditions prevailing during the lifetime of the snail. Whole shell isotope analysis showed that c. 35 000 years ago climate in the Valsequillo Basin was similar to the present day. Between c. 35 000 and 20 000 BP conditions became increasingly dry, after which conditions became wetter again, although this record is truncated. Intra-shell isotopic analyses show that the amount of precipitation varied seasonally during the late Pleistocene. If people did reach this part of the Americas in the late Pleistocene they would have experienced changing long-term and seasonal climatic conditions and would have had to adapt their life strategies accordingly.
Stevens, R.E., Metcalfe, S.E., Leng, M.J., Lamb, A.L., Sloane, H.J., Naranjo, E., Gonzalez, S. 2012. Reconstruction of late Pleistocene climate in the Valsequillo Basin (Central Mexico) through isotopic analysis of terrestrial and freshwater snails. Palaeogeography, Palaeoclimatology, Palaeoecology, 319-320, 16-27.
Rapid Communications in Mass Spectrometry - February 2012
Stable isotope analysis of cellulose is an increasingly important aspect of ecological and palaeoenvironmental research. Since these techniques are very costly, any methodological development which can provide simultaneous measurement of stable carbon and oxygen isotope ratios in cellulose deserves further exploration. A large number (3074) of tree-ring a-cellulose samples were used to compare the stable carbon isotope ratios (δ13C) produced by high-temperature (1400 °C) pyrolysis/gas chromatography (GC)/isotope ratio mass spectrometry (IRMS) with those produced by combustion GC/IRMS. Although the two data sets are very strongly correlated, the pyrolysis results display reduced variance and are strongly biased towards the mean. The low carbon isotope ratios of tree-ring cellulose during the last century, reflecting anthropogenic disturbance of atmospheric carbon dioxide, are thus overestimated. The likely explanation is that a proportion of the oxygen atoms are bonding with residual carbon in the reaction chamber to form carbon monoxide. The 'pyrolysis adjustment', proposed here, is based on combusting a stratified sub-sample of the pyrolysis results, across the full range of carbon isotope ratios, and using the paired results to define a regression equation that can be used to adjust all the pyrolysis measurements. In this study, subsamples of 30 combustion measurements produced adjusted chronologies statistically indistinguishable from those produced by combusting every sample.
This methodology allows simultaneous measurement of the stable isotopes of carbon and oxygen using high-temperature pyrolysis, reducing the amount of sample required and the analytical costs of measuring them separately.
Woodley, E.J., Loader, N.J., McCarroll, D., Young, G.H.F, Robertson, I, Heaton, T.H.E., Gagen, M.H. and Warham, J.O. 2012. High-temperature pyrolysis/gas chromatography/isotope ratio mass spectrometry: simultaneous measurement of the stable isotopes of oxygen and carbon in cellulose. Rapid Communications in Mass Spectrometry, 26, 109-114.
A shell of Gigantoproductus okensis shows twenty growth lines with marked changes of fabric, indicating periodical reduction of growth rates caused by environmental perturbations. The number of growth lines suggests a lifespan of 20 years in agreement with the survival rates of extant brachiopods, and with spiral deviation analysis. Geochemical analyses across the growth profile show a heterogeneous distribution of stable isotopes and trace elements. It is possible to distinguish primary from altered carbonate, and to interpret the isotopic data. The oxygen isotope signal in the unaltered parts is periodical and annual, with oscillation of 1.1 per mille. The higher values are at the growth lines (winter), and therefore most likely related to monsoon circulation during the Visean. The annual periodicity seems also present in the altered part of the shell, suggesting that diagenesis could have reset the primary values, but preserved their cyclicity.
Angiolini, L., Stephenson, M., Leng, M.J., Jadoul, F., Millward, D., Aldridge, A., Andrews, J., Chenery, S., Williams, G. 2012. Heterogeneity, cyclicity and diagenesis in a Mississippian brachiopod shell of palaeoequatorial Britain. Terra Nova, 24, 16-26.
This paper presents a significantly simplified method for in-situ U-Th-He dating removing the need to know any absolute concentrations. This is done by calculating the normalised U, Th, and He concentrations of a conventionally dated calibration standard from its measured Th/U ratio and known U-Th-He age, and scaling these concentrations to the raw U, Th, and He signals of the sample. The Th/U ratio of the standard can be determined from its measured 208Pb/206Pb ratio, removing the need to use NIST glass as a reference material. We introduce an LA-ICP-MS-based method to correct for variable ablation depths between the standard and the unknown, using the strength of the ablated 29Si signal. Finally, we propose a pseudo-depth profile method to assess the effects of compositional zoning on the accuracy of in-situ U-Th-He data.
The effectiveness of the proposed method has been demonstrated on three samples of gem-quality Sri Lanka zircon, which yield ages that are in agreement with previously published conventional U-Th-He measurements.
Vermeesch, P., Sherlock, S. C., Roberts, N. M. W., Carter, A., 2012. A simple method for in-situ U-Th-He dating. Geochimica et Cosmochimica Acta 79, 140-147.
Stable carbon isotope time-series (δ13C) from tree-rings are capable of providing valuable palaeoclimatic information, but analysis of individual tree-rings is time consuming and expensive. Pooling material from several tree-rings prior to isotopic analysis reduces costs, but does not allow the magnitude of uncertainty in the mean δ13C chronology to be calculated unless the pool is broken and each tree-ring measured individually at regular intervals. Here we use a comparison of pooled and mean individual (the arithmetic mean of isotopic data from tree series measured individually) δ13C records between AD 1650 and 2007, comprising cores from 21 Scots pine (Pinus sylvestris L.) trees growing in the western Highlands of Scotland. The aim is to determine whether the true error structure of the time series is better captured by using the overall mean error estimate for the entire time series or by linear interpolation between the equally spaced measurements.
We conclude that where autocorrelation exists within the error structure of a chronology, annual estimates of 95% confidence intervals, developed through linear interpolation at 5-year or 10-year intervals, are preferable to using the overall mean uncertainty. The method outlined increases the viability of pooled δ13C records for palaeoclimatic research by retaining error structure whilst reducing analytical time and costs. The method is applied here using tree-ring data, but could theoretically be applied to any well-replicated time-series.
Woodley, E. J., Loader, N. J., McCarroll, D., Young, G. H. F., Robertson, I., Heaton, T. H. E., Gagen, M. J., 2012. Estimating uncertainty in pooled stable isotope time-series from tree-rings. Chemical Geology 294-295, 243-248.
Levels of nitrate in Malta's groundwater are amongst the highest in Europe (median concentrations of 14 mg NO3-N L-1 in the main sea-level aquifer, and 37 mg NO3-N L-1 in the younger groundwaters of the perched aquifers). As part of a Rural Development Programme for Malta, the British Geological Survey was contracted to investigate the source/s of this nitrate, with specific emphasis on a combined 15N/14N and 18O/16O study. In addition to analysing a wide variety of groundwater samples, a special feature of the study was a determined effort to measure, rather than assume (as is common in many studies) the 15N/14N and 18O/16O compositions of the major potential sources of nitrate: fertilizers, septic and sewage wastes, animal manures, and soils. The data allowed the former two sources to be ruled out and, whilst some direct leaching of manure-derived nitrate could not be discounted, the data suggested that soil nitrification is the major source of nitrate in the groundwaters. Malta has a very long history of cultivation, during which time the soils may have developed high 15N/14N values reflecting greater mobility of nitrogen in soils with low C/N ratios.
The 15N/14N and 18O/16O values of nitrate in the groundwaters suggest that it is derived by microbial nitrification of organic N in these soils, with virtually no reduction in nitrate levels by denitrification.
Heaton, T.H.E., Stuart, M.E., Sapiano, M. and Micallef Sultana, M. 2012. An isotope study of the sources of nitrate in Malta's groundwater. Jounal of Hydrology, 414-415, 244-254.