NERC Isotope Geosciences Laboratory

PhD's awarded

December 2012 - Congratulations to Katie Egan, who has recently defended her Ph.D. entitled: New insights into Cenozoic silicon cycling in the Southern Ocean: refined application of silicon isotope ratios in biogenic opal".

The marine silicon and carbon cycles are intrinsically linked by a unique group of primary producers; the diatoms. Diatoms play a significant role in carbon export, making them a critical component of the global biological carbon pump with the power to affect climatic change. In this thesis, the silicon isotope composition (δ³⁰Si) preserved in diatom opal is used together with the δ³⁰Si of sponge opal, a powerful new proxy for deepwater silicic acid concentration, to document the Cenozoic Silicon cycle, shedding light on its role in carbon cycling and global climatic change. This study has developed a novel size-separation methodology to produce the first core top calibration of diatom δ³⁰Si. This calibration demonstrates that diatom &delta³⁰Si exhibits a strong negative correlation with surface water silicic acid concentration, supportive of its application as a proxy for silicic acid utilisation. The refined method is used to produce a diatom δ³⁰Si record, for the first time combined with sponge δ³⁰Si, to gain insight into the Southern Ocean silicon cycle over one of the largest Cenozoic

climatic shifts; the onset of Antarctic glaciation (~33.7Ma). The two proxy records yield the first geochemical footprint which demonstrates the coincident proliferation of diatoms with the onset of Antarctic circumpolar flow as a precursor event to the Eocene-Oligocene boundary.

Katie was supervised by Ros Rickerby at the University of Oxford and Melanie Leng at NIGL

November 2012 - Congratulations to Helen Cockerton, who has recently defended her Ph.D. entitled: "Late-glacial and Holocene variations in the Si cycle in the Nile Basin: multi-isotope evidence from modern waters and lake sediments".

Until recently the continental Si cycle at Quaternary (decadal to million-year) time scales has been largely neglected. Emphasis was placed on silicate-rock weathering and resulting CO₂ drawdown on geological time scales, rather than on shorter-term biogenic processes occurring along the land-ocean continuum. The ability of some terrestrial plants (e.g. tropical rainforest trees, savanna and wetland grasses, Papyrus) and aquatic organisms (e.g. diatoms in lakes, rivers and swamps) to take up, store and recycle significant amounts of Si is increasingly being recognised, although their impact on the continental Si cycle and Si export to the oceans under different climatic regimes remains unquantified. The main aim of this thesis was to reconstruct spatial and temporal patterns of Si cycling in the Nile Basin during the last 15,000 years.Seasonal variations in hydrology and Si cycling in the Nile Basin were investigated using stable isotope (H, O, and Si) compositions of surface waters, as a basis for interpreting lacustrine diatom sequences. Si- and O-isotope analysis of diatom silica in cores from Lakes Victoria and Edward, in the headwaters of the White Nile, were employed to reconstruct changes in biotic Si cycling and palaeohydrology, respectively. The relative abundances of lipid biomarkers (hydrocarbon-fraction) permitted major changes in terrestrial and aquatic ecosystems to be tracked. During drier conditions (e.g. the last glacial and late Holocene) (high δ¹⁸O_diatom), Si cycling was greatly reduced.

Diminished biomass, reduced biotic weathering, a declining soil stock of amorphous silica (ASi) and decreased run-off in the catchment resulted in biological demand for Si (high δ³⁰Si_diatom). In contrast, enhanced monsoon rainfall (low δ¹⁸O_diatom) during the early to mid-Holocene enabled the proliferation of vegetation in the catchment, which in turn accelerated silicate-rock weathering and the mobilisation of DSi in surface runoff, providing a plentiful supply of Si (low δ₃₀Si_diatom). Both the modern waters and palaeo-records indicate that the riverine flux of Si to the oceans on glacial \ interglacial time scales was not constant; resulting in important implications for the marine Si budget and consequently the global C cycle.

Helen was based at the University of Swansea

April 2012 - Congratulations to Holly Miller who recently passed her PhD entitled "The Origins of Nomadic Pastoralism in the Southern Levant: stable isotope, chipped stone and architectural analysis of archaeological evidence".

Herded sheep and goats were introduced to the steppic region of Eastern Jordan late in the seventh millennium cal BC. The evidence for initial pastoral activities in this arid area, some 500 years after domestic caprines are known to have been kept at large village sites in the neighbouring and more hospitable Mediterranean regions, suggests that these early herding groups began to develop a lifeway that pastoral nomads continue to live in some regions of the world today.

¹³C/¹²C and ¹⁵N/¹⁴N ratios from the bone collagen of animal remains from Mediterranean sites, where C₄ plants were less common and water availability was greater, was compared to those of the steppic sites, where C₄ vegetation was abundant and water scarce. The results of this study were used to pinpoint the foraging range of the herded animals, suggesting the regions in which they, and their accompanying herders, spent lengths of time. It was determined that differences in the isotopic ratios of animals from two sites in the arid region signify different herding strategies used to bring animals to the area. Animals from one site have indications of a diet that included Mediterranean vegetation, suggesting that their herders had more frequent contact with village groups. At the second site, animal diets, and thus movements, were restricted to the steppe, suggesting the development of a lifestyle based around frequent mobility within the arid regions, and allowing the rejection of a theory that stated early Neolithic Southern Levantine herding was village based.

In combination with chipped stone and architectural analysis, the results of the stable isotope study have allowed an investigation of the social and economic behaviours and practices of the communities that initially saw the potential of the Southern Levantine steppe as an area for pastoral activities.

Liverpool supervisors- Drs Jessica Pearson and Douglas Baird, NIGL- Dr Angela Lamb and Mrs Carolyn Chenery

December 2011 - Congratulations to Michael Wallace who successfully defended his PhD thesis: Crop watering practices in the Neolithic and Bronze Age: the stable carbon isotope approach

In the ancient past, agriculture was central to the daily routines of life and economic organisation. The major limiting factor on crop production in dry regions, such as Western Asia, is the availability of water. In these regions, rainfed crops are susceptible to drought-induced failure and, while farmers can water their crops artificially, this places demands on labour and water supplies. The effort and resources afforded to crops by farmers can indicate the scale of production, whereas the preferential treatment of certain crops over others offers insights into the cultural and economic role of different crops. Charred crop remains are ubiquitous at archaeological sites in dry regions, and this thesis assesses the utility of stable carbon isotope analysis as a means of inferring crop water status. Experiments were conducted to establish the relationship between 13C/12C ratios and water status in modern crops grown under known conditions. Laboratory tests were also undertaken to determine the extent to which the ratios may be altered post-mortem. In light of the findings from these experiments, isotope analysis was carried out on crop remains from nine Neolithic and Bronze Age sites, primarily located in Western Asia. The results showed that while the precision of the method is limited (by natural variations, unknown differences in growing conditions and plant physiology, and small post-mortem alterations), 13C/12C analysis can nevertheless provide a reliable indication of the water status of ancient crops.

On this basis it was possible to develop interpretations regarding agricultural arrangements at individual sites, and to identify regional trends in ancient crop production.

The thesis project was tied to a NERC Standard Grant, on which NIGL were co-investigators. Michael was supervised at Sheffield University by Glynnis Jones, and at NIGL by Tim Heaton and Chris Kendrick.

December 2011 - congratulations to Wendy Austin-Giddings on her PhD entitled "The Deposition and Reworking of Tsunami Sediments in Agaete, Gran Canaria"

Coarse-grained, polymict, deposits, draping hillslopes at high elevations on ocean island volcanoes, have been variously interpreted as sourced either from sea-level highstands or from tsunamis; their origin is thus controversial. A detailed facies analysis of coarse-grained, fossiliferous sediments located at Agaete, on the north-west coast of Gran Canaria, has been undertaken. Previously interpreted as the result of a sea-level highstand, these deposits have recently been re-interpreted as sourced from a tsunami; itself triggered by a volcano flank failure. The Agaete Fossiliferous Conglomerates occur at several locations in the Barranco de Agaete, up to 188m apsl. and 2 kilometres inland from the coast. They comprise seven facies, all of which are variably graded, matrix- and clast-supported, range from ~0.3 to 2 m thick, and contain a diverse assemblage of volcanic clasts, large beachrock boulders and a shallow marine fauna. All the facies have sharp, erosional bases. A facies at one site contains large palaeosol rip-up clasts up to 1.5 m across and at its base truncates plant roots. The upper facies of the conglomerates are all finer-grained, reverse-graded, clast-supported and better-sorted than the lower facies of the group. At one location the upper conglomerates comprise prograding beds that are interpreted as alluvial.

The lower facies of the conglomerates are interpreted as primary tsunami deposits, whereas the upper facies are interpreted as tsunami deposits that have been reworked. The alternative, marine highstand, interpretation of the coarse-grained deposits is discounted on the basis of (i) an absence of supporting geomorphological features such as a marine terrace and/or a wave cut platform; (ii) the composition of the sediments; (iii) diagenetic features; (iv) distance from the coast; and (v) elevation of the deposits. Gran Canaria is in its erosional post-shield stage of development and the north-western coastline has experienced 40-50 m of tectonic uplift over the past 1.75Ma. Thus uplift of highstand deposits cannot account for the occurrence of the Agaete sediments at elevations of up to 188 m apsl. The Güimar lateral collapse event on the neighbouring island of Tenerife, dated at around 0.8Ma BP, is presented as the most likely tsunami source.

Wendy was a BGS BUFI student and supervised by Prof Dave Tappin at BGS, Prof Bill McGuire at UCL, and Prof Randy Parrish at NIGL.

December 2011 - congratulations to Ronan Roche on his PhD entitled "A multi-proxy reconstruction of mid-Holocene environmental conditions at a nearshore Great Barrier Reef site: King Reef, Northern Queensland"

Major changes in sediment and nutrient inputs to Australia's Great Barrier Reef have been documented since European settlement of the region (~1840 A.D.) and the concurrent introduction of modern agricultural methods. This study focussed on King Reef, a mid-Holocene (~5800-4600 yr B.P.) age coral reef in the Wet Tropics region of Australia's Great Barrier Reef. Combined records of Sr/Ca and oxygen isotope ratios of coral skeleton were used to estimate sea- surface temperature and the oxygen isotopic value of the surrounding seawater (δ¹⁸O_sw ) which is strongly correlated with sea-surface salinity. At 4665 yr B.P. sea-surface temperatures indicated by Sr/Ca ratios were ~1 to 1.4°C higher than modern values. δ¹⁸O_sw values were slightly higher in winter (+0.1‰) and lower in summer (-0.1‰) resulting in an increased annual range. Overall mean seasonal mid-Holocene δ¹⁸O_sw values were similar to modern values, however, a comparison of δ¹⁸O_sw during drought periods suggests enrichment by 0.3 to 0.38‰.

This enrichment suggests that evaporation was enhanced due to higher sea-surface temperatures in the mid-Holocene, which would have resulted in a stronger Australian-Indonesian monsoon system. These results may indicate that riverine influence and associated sediment input at the study site was high during the mid-Holocene, and that nearshore reefs within Australia's Great Barrier Reef have previously experienced conditions of high sediment input during their phase of active accretion.

Ronan was supervised at NIGL by Tim Heaton and Melanie Leng.

November 2011 - Congratulations to Alistair Seddon who successfully defended his PhD entitled: Palaeoecology, Biogeography and Evolution of Benthic Littoral Diatoms from the Galápagos Islands

This thesis presents palaeoecological and biogeographic evidence to investigate whether the Galápagos Islands, famous for their biogeographical isolation in terrestrial organisms, are also isolated for diatoms. Are diatom assemblages principally controlled by environmental changes at the local and regional scale (abiotic processes), or does biogeographical isolation (reshuffling the nature of biotic processes) also play a role? Ten new species of diatom belonging to the genus Navicula sensu stricto were described from coastal lagoons in the Galápagos. Using a multi-proxy approach, these lagoons are dynamic, threshold environments which have exhibited non-linear responses to environmental change over the past 3,000 years. However, ecological turnover, rather than evolutionary processes such as rapid morphological speciation, were the main influences in the formation of Galápagos diatom biodiversity on Holocene timescales. Indeed, newly discovered Galápagos taxa to be restricted to the shorelines of cyanobacterial mats in assocation with a larger number of 'generalist' species. As yet, the true biogeographic extent of the novel taxa are unknown, but it is likely that they are simply 'niche specialists' with smaller global population sizes and, as a result, a limited global distribution.

Alistair was supervised at NIGL by Melanie Leng.

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July 2011 - Congratulations to Katy Wilson who successfully defended her PhD thesis entitled: Plio-Pleistocene Reconstruction of East African and Arabian Sea Palaeoclimate

Superimposed upon a long-term trend of aridification, the climate history of Africa was punctuated by episodes of extreme variability, characterised by the precessionally-forced appearance cycling of large lake systems within the East African Rift Valley. In order to investigate the nature of low-latitude climate variability during the Late Pliocene and Early Pleistocene, high-resolution analyses from one of the lake phases were combined with the reconstruction of long-term changes in the transport of wind-borne terrigenous dust to the Arabian Sea. Climate in both regions is strongly influenced by relative changes in the strength of the Indian Ocean monsoons, which determine rainfall distribution in equatorial East Africa and generate the low-level winds which transport dust offshore from the Arabian Peninsula. The Baringo-Bogoria basin in the East African Rift Valley contains a well-dated package of fluvio-lacustrine sediments and diatomite units documenting a major humid phase between 2.7 and 2.55 million years ago (Ma). Stable oxygen isotope measurements of diatom silica, combined with the XRF analysis of whole-sample geochemistry, reveal that the deep lake phase was characterised by wet-dry cycles lasting, on average, 1,400 years.

Over longer timescales, variations in the aeolian delivery of lithogenic matter to the Arabian Sea, reflected in the normalised flux of titanium, show that monsoonal circulation prior to 2.6 Ma was highly variable and primarily driven by orbitally-forced changes in tropical summer insolation, modulated by the 400,000 year cycle of orbital eccentricity. Millennial-scale fluctuations in the dust record also support the evidence of abrupt wet-dry cycles in East Africa. Such high-resolution cycles are rarely found in older records, thus giving a valuable insight to the nature of short-term fluctuations in Plio-Pleistocene climate.

Katy was supervised by Mark Maslin and Ans Mackay at UCL and Melanie Leng at NIGL

June 2011 - Congratulations to Gareth Preston who successfully defended his PhD these entitled: From Nomadic Herder-Hunters to Sedentary Farmers: the relationship between climate, environment and human societies in the United Arab Emirayes from the Neolithic to the Iron age

Owing to its archaeological and climatic history, the United Arab Emirates (UAE) acts as a natural laboratory in which to examine the relationship between climate, the environment and early human populations from the onset of the Neolithic to the termination of the Iron Age (~11 500 - 2300 cal. yr BP). Despite this, such studies have to date been hindered by the paucity of high-quality palaeoenvironmental data from the region. To help fill this gap in knowledge, this thesis reports the results of high-resolution, multi-proxy investigations (bulk physical, geochemical and isotopic) carried out on two palaeolake sediment sequences in the Emirate of Ra's al-Khaimah, UAE; Awafi (25° 42' 57" N; 57° 55' 57" E) and Wahalah (25° 38' 48˝ N, 55° 47' 26˝ E). The new data reveals that climate has varied greatly between ~8500 - 4200 cal. yr BP, with periods during which conditions were more pluvial than the present punctuated by phases of intense aridity. It is demonstrated that this variability had a considerable impact on the natural environment of the UAE, resulting in significant changes in landscape stability, water availability, and vegetation over both long and short-term timescales. Furthermore, it is suggested that abrupt increases in aridity recorded in the palaeolake sediments between ~8000 - 7800 cal. yr BP, from ~5900 cal. yr BP and at ~4200 cal. yr BP, reflect the response of the regional landscape to global climatic variations.

By placing the archaeological record of the UAE into this detailed framework of landscape evolution it is demonstrated that the region's early Holocene populations adapted with the prevailing climate by altering their subsistence strategies (e.g. from hunter-gatherer to animal husbandry and eventually sedentary agriculture) and settlement patterns (e.g. by moving between the interior, Arabian Gulf, and Gulf of Oman coasts), as well as utilising new technologies (e.g. irrigation). Indeed, this study provides compelling evidence for the potential of climatic-environmental change to influence the behaviour of early human societies. Understanding this relationship is made all the more imperative given the growing concerns over contemporary climate change.

Gareth was supervised by Adrian Parker and Martin Hodgson at Oxford Brookes and Melanie Leng at NIGL.

May 2011. Congratulations to Tom Broadbent who passed his PhD: Low latitude Pacific palaeoceanographic change across the Eocene/Oligocene boundary

Evidence from both terrestrial and marine environments indicates a cooling of Earth's climate across the Eocene/Oligocene boundary (EOB), with the likely development of continental scale glaciation. The most geographically widespread and persuasive line of evidence for the shift in Earth's climate comes from variation in stable isotopes measured from benthonic foraminiferal carbonate, with a geologically rapid and globally observed >1 ‰ increase in δ¹⁸O. Increasing foraminiferal δ¹⁸O reflects either a cooling in the deep ocean or an increase in the δ¹⁸O of seawater, which is related to removal of light oxygen isotopes through continental glaciation. However, despite the global recognition of an increase of >1.0 ‰ in benthonic foraminiferal δ¹⁸O and fundamental change in Earth's climate, the proportion of change, i.e. temperature decrease and/or ice volume development, is poorly constrained.

Deconvolution of the foraminiferal δ¹⁸O requires and independent proxy to isolate the temperature or ice-volume change, one such proxy is foraminiferal Mg/Ca ratios. Foraminiferal Mg/Ca ratios are a palaeotemperature proxy, however, their application has lead to the observation of bottom-water warming and thus suggesting bipolar glaciation; a scenario inconsistent with a warmer Earth and unsupported by sedimentary evidence. The warming observed in bottom-water Mg/Ca palaeotemperatures, however, was determined from a deep-ocean site that experienced significant deepening of the carbonate compensation depth (CCD) concomitant with Mg/Ca increase, leading to the hypothesis increasing carbonate ion saturation (Δ[CO32-]) caused enhanced foraminiferal uptake of Mg and thus the observed temperature increase. This study aimed to deconvolute the foraminiferal δ¹⁸O record using paired benthonic foraminiferal records from a site with minimal change in Δ[CO32-] amongst other proxies.

Tom was supervised at Bangor University by Leon Clarke and at NIGL by Melanie Leng and Ian Millar.

May 2011. Congratulation to Chris Brodie who successfully defended his PhD thesis: The effect of acid treatment methods on bulk organic materials, and a long-core geochemical palaeoenvironmental reconstruction from South China.

The first aim of this thesis was an investigation of the effect of acid treatment on C/N, δ13C and δ15N of organic materials. Three common acid pre-treatment methods were compared (capsule method; rinse method; fumigation method), using a range of acid reagents (HCl; H₂SO₃; H₃PO₄), on a range of terrestrial and aquatic, modern and geological sample materials. An inherent assumption of these acid treatment methods is that any offsets in C/N, δ¹³C and δ¹⁵N are linear and proportional, and that any bias is not greater than instrument accuracy and precision. The C/N, δ¹³C and δ¹⁵N values of OM are not just dependent upon environmental processes but also on acid treatment method, which adds significant non-linear biasing to the OM signal several orders of magnitude above instrument precision. This biasing is caused by the inefficient removal of IC from sample materials and the alteration of OM by the acid treatment process. Consequently, this can significantly alter the environmental interpretation of these proxies. The second aim of this thesis concerned a reconstruction of palaeoenvironmental change from Lake Tianyang, Leizhou Peninsula, south China (20°31'1.11" N, 110°18'43.02" E) using a suite of geochemical proxies (δ¹³C of bulk OM; XRF elemental ratios, magnetic susceptibility).

The lake sedimentary sequence is substantially older than previously reported, with the upper ~40 m representing ~30 ka BP. In addition, the δ¹³C (taken to be an aridity indicator where values are above known inaccuracy and imprecision) show a striking glacial - interglacial imprint. The sedimentological evidence suggest the lake has been continually silting up since Marine Isotope Stage 9. The Tianyang δ¹³C record and elemental ratios (MIS 9 - MIS 6 inclusive) show a strong glacial - interglacial imprint, though the elemental ratios lose this imprint during MIS 5 likely due to an increase in catchment erosion.

Chris was supervised at Durham University by James Casford, Jerry Lloyd and at NIGL by Melanie Leng, Tim Heaton with support from Chris Kendrick.

April 2011. Congratulations to Melanie Bugler who has successfully defended her PhD: An investigation into use of the freshwater gastropod Viviparus as a recorder of past climatic change

Through isotopic analysis of Viviparus lentus (V. lentus) a high resolution record of stepwise changes in δ¹⁸O and δ¹³C across the Eocene / Oligocene transition and Oi-1 glacial maximum has been produced for the continental Solent Group strata, Isle of Wight (UK). Comparison of this V. lentus δ¹⁸O_carb. record with high resolution marine δ¹⁸O_carb. records shows that similar isotopic shifts exist in the near coastal continental and marine realms. In order to calculate palaeotemperatures from this new continental record an investigation into the biology of modern Viviparus and its effect on the isotopic composition of its shell carbonate was undertaken. Experimental measurements of the ¹⁸O/¹⁶O isotope fractionation between the biogenic aragonite of Viviparus and its host freshwater were undertaken on samples derived from the Somerset Levels in order to generate a genus specific thermometry equation. The results from using this new Viviparus equation on fossil V. lentus shell fragments suggests that aquatic and terrestrial biota were being affected by climate change associated with the Late Eocene Event. This coincides with a decrease in mammal species richness in the Osborne Member, reaching its climax at the end of the Osborne / Seagrove Bay Members.

This event is followed by a brief warming in the Bembridge Limestone which was marked by a within-Europe mammal turnover involving dispersal from the south and an increase in species richness, concurrent with this is an increase in size of Harrisichara gyrogonites. An additional investigation into seasonal isotopic variability using whole well preserved V. lentus specimens has also revealed a shift from tropical /subtropical to temperate climatic zones occurring before the Eocene /Oligocene boundary and Oi-1 glacial maximum. Overall the evidence provided by these investigations would suggest that climatic change was already in progress prior to the build up of glacial ice on Antarctica.

Mel was supervised by Stephen Grimes at Plymouth and Melanie Leng at NIGL.