Daniel Schillereff

From mud to moai statue: lake sediments reveal new insights into Easter Island colonization

Daniel Schillereff December 17, 2013

The small landmass of Easter Island (164 km²), the southeasterly point of Polynesia in the Pacific Ocean, has achieved iconic status in the world today as people wonder how its colonisation was physically possible by settlers journeying through the vast ocean in tiny boats, how and why the enormous moai s were constructed and, most infamously, to what extent they contributed to their own downfall through severe environmental degradation. This story has received a lot of attention, featuring in Jared Diamond’s eye-opening book ‘Collapse’ as well as National Geographic. (Check out the neat video embedded in the National Geographic article in which researchers replicate their theory on ‘walking’ the s).

The location of Easter Island related to South America. Used by permission of the University of Texas Libraries, The University of Texas at Austin.

Image of Easter Island from the Earth Observatory, NASA.

Moai s at Rano Raraku, Easter Island. Source: WikiCommons

Statues at Rano Raraku, Easter Island. Source: WikiCommons

Substantial research efforts have tried to build up a detailed picture of the timing and rate of human settlement and changes in vegetation cover on the island, and most importantly whether there is a clear causal link between the two. Much of the evidence for environmental changes on Easter Island come from lake sediment sequences, a subject close to my heart (having invested 3 years and counting looking at lake sediments for my PhD).

An example of a sediment core extracted from a lake bed. Photo from WikiCommons, courtesy of Gary Rogers.

In particular, identifying pollen grains at different depths in lake sediment cores can be used to reconstruct what types of plants were growing on the island through time and identify the timing of shifts from a tree-dominated landscape to a more open grassland, for example. I recently found a paper in the journal Quaternary Science Reviews (Cañellas-Bolta et al. 2013, QSR; I apologise that it is not open-access) that I was immediately drawn to as it applied a multi-proxy (integrating different, independent techniques) analytical approach to a new sediment core from a lake on Easter Island. Now, I have thoroughly enjoyed my PhD (so far!); it has been exciting, interesting and the fieldwork has been in the picturesque landscapes of the English Lake District and southern Scotland. Nevertheless, reading this paper set me off on a series of day-dreams because, as lakes are pretty much ubiquitous around the world, maybe one day in the future as part of a successful academic career, I could drill a lake somewhere REALLY cool?!?

A number of hypotheses have been set out to try and explain when settlers first arrived on Easter Island and what triggered the demise of their undoubtedly complex civilisation. One common story is that increased demand for firewood and space for agriculture led to rapid deforestation, severe soil degradation and ultimately cultural collapse. Climatic fluctuations, the introduction of rats, contact with European explorers or simply living in such isolation have also been proposed as drivers of this collapse.

Panoramic view across Hanga Roa, Easter Island. The modern-day ‘natural’ landscape dominated by open grassland and largely devoid of palm trees is clear to see. Photo from WikiCommons courtesy of Makemake.

The new data from Cañellas-Bolta et al. include identification of a new pollen type Verbena litoralis and a more refined chronology for their sediment core based on radiocarbon dating. Using radiocarbon dates to identify the timing of events on Easter Island has been problematic due to inverted ages (i.e., a radiocarbon age higher in the sediment sequence returns an older date than samples lower down the core) and hiatuses, or gaps, in the sediment record. The hiatuses are particularly problematic here because sediment accumulation rates are very slow (100 years per cm), thus any gap in the record sadly means a significant chunk of the story has been lost. The authors use the BACON age-depth modelling software package to construct a more robust chronology, as the model incorporates known prior information on sediment accumulation rates and the identification of hiatuses within a Monte Carlo Markov Chain framework (Systematically running many millions of slightly-different simulations based on the same radiocarbon ages) in order to incorporate all possible age distributions, thus doing a better job of accounting for chronological uncertainties.

Verbena litoralis. Photo from WikiCommons, courtesy of Forest & Kim Starr.

Their vegetation reconstruction suggests Easter Island was dominated by palm trees until ~450 B.C., where the first evidence for a shift in vegetation cover is observed. At this time, grassland and the marker weed species V. litoralis increase in abundance while palm pollen numbers decline, likely suggesting a clearance episode. The replacement of palm-dominated vegetation by herbaceous taxa then continues in a stepped manner, with another major palm decline at ~1200 A.D. This is the most-commonly accepted date for the first Polynesian settlers to the island (~A.D. 800 – 1200) so fits in well with previous work. But the suggestion that human-induced changes in vegetation cover began ~450 B.C., approximately 1500 years earlier, has enormous implications for our understanding of the history of not only Easter Island but also the pattern of settlement across much of Polynesia.

The history of Easter Island has been a topic of intense debate and this paper is another entry into the story. One part of their paper I found particularly neat is that V. litoralis is a weed species native to North America; how then did it arrive on Easter Island nearly 2500 years ago? The authors, after careful consideration of natural dispersal mechanisms, strongly support an interpretation of human-driven spread. They mention other interesting examples such as evidence that sweet potato and bottle gourd, crops native to South America, arrived in Polynesia in prehistoric times and a contemporaneous introduction of Polynesian chickens to Chile.

The authors refrain from making sweeping statements about their findings in terms of re-thinking local and regional history, instead emphasising that many hypotheses remain plausible and there is tremendous scope for future work. I am now mentally planning my mud-extraction expedition as I type…

As a final point, I read Collapse several years ago and was quite taken by the author’s arguments but having read the paper and put together this blog post, I am inspired to conduct a more critical examination of all the information pertaining to Easter Island, especially as I am extremely concerned by the current state of natural environments around the globe and our role in their rapid degradation.

A DYNAMITE journey around western Ireland

Daniel Schillereff October 28, 2013

The four-year DYNAMITE project (DYNAmic Models in Terrestrial Ecosystems and Landscapes), a teaching and research cooperation programme between the School of Environmental Sciences, University of Liverpool, UK and the Departments of Geology and Physical Geography and Ecosystem Science at Lund University, Sweden, recently ended with an excursion for PhD students, postdocs and academic staff from both institutions to western Ireland in September 2013.

A brief report from the trip offers an excellent overview of the breadth of Quaternary Science as a discipline, illustrating how we integrate geomorphology, archaeology, geology and palaeoecology, to foster better understanding of local- to global-scale environmental change at varying temporal scales through the Holocene and Pleistocene.

Our route around western Ireland, including Co. Galway, Connemara and Co. Mayo.

Archaeology

Our trip began (Day 1) in The Burren, an extensive karstic landscape composed of remarkable limestone pavements and that supports many rare species.

Limestone pavements typical of The Burren. Photo: Uploaded to WikiCommons by HEireann.

Michael Gibbons talking about Oyster harvesting, meal preparation and shell waste deposition and modern day exposure of a Neolithic Oyster Midden. Photo: D. Schillereff

Michael Gibbons guided us around a number of fascinating archaeological sites, many of which feature in this detailed report from the Burren Landscale and Settlement Project. We visited impressive hill forts, court tombs and exposed oyster middens, many of them dating from Neolithic, and in some cases Mesolithic, age.

Many sites in the Burren have yet to be excavated, including these stone piles in the tidal zone; what was their purpose and when were they constructed remains to be discovered.

Stone piles in the tidal zone yet to be excavated. Unknown age and purpose. Photo: D. Schillereff

The trip also ended (Day 6) discussing archaeology, specifically the Céide Fields Neolithic complex at Ballycastle, County Mayo. These field systems enclosed by stone walls represent the most extensive Neolithic Stone Age monument in the world, dating to 5000 – 6000 years ago, and is today mostly covered by extensive blanket peat except for a few isolated areas currently undergoing excavation. The age of the walls is determined by applying radiocarbon dating to fossilized pine stumps preserved in the bog. Seamus Caulfield (Archaeology, University College Dublin) who has focused much of his research career on these sites led an extensive guided tour of the excavations, where the peat has been removed at various intervals revealing the abandoned stone walls.

Professor Seamus Caulfield describing an excavated stone wall section in the Céide Fields. Photo: D. Schillereff

While individually the piles of stone do not initially appear tremendously impressive, when the spatial extent (>10 km2) and perfectly parallel construction of the walls is considered, the enormous scale of Neolithic agriculture in the region is unveiled. What is also of great interest is the rarity or lack of preservation of a monument of similar age elsewhere in northwest Europe. It appears most likely that a regional decline in pine forests (indicated by pollen reconstructions) meant stone walls were constructed at great effort, instead of the log walls constructed from forest timber at the time elsewhere in Europe.

Palaeoecology

A short boat ride on Day 2 took us to Inis Oírr, the smallest of the Aran Islands, led by Karen Molloy (National University of Ireland, Galway). The small field boundaries struck me as unusual but apparently such land division has a long history in western Ireland (as we discovered at the Céide Fields). Karen presented the impressive lake sediment sequence of An Loch Mór; the unique setting of the lake means the >13 m of sediment deposited here records a fascinating story of palaeoecological change (e.g., Holmes et al. 2007, QSR) through the late-Glacial and Holocene periods, including insight into local ice retreat at the end of the last glaciation, sea-level and salinity changes, vegetation history and phases of exceptionally high windspeed due to its exposure to the Atlantic Ocean.

Later in the trip (Day 4) we tracked down a small exposed organic deposit exposed in a fluvial terrace at Derrynadivva that contained many large plant macrofossils. It turns out these deposits are not Holocene in age; rather, they are remnants of plants growing during a previous Pleistocene interglacial. It remains uncertain which interglacial is represented here however based on analysis of the pollen and plant macrofossils, the deposit possibly represents Oxygen Isotope Stage 11 (Hoxnian; e.g., Coxon et al. 1994 JQS).

Interglacial deposit containing many large plant macrofossils exposed in an alluvial terrace. Photo: D. Schillereff

Glacial Geology and Geomorphology

We visited a number of sites around Co. Galway, Co. Mayo and Connemara (Days 3 – 5) with Professor Peter Coxon (Geography, Trinity College Dublin) and Dr Richard Chiverrell (Environmental Sciences, University of Liverpool) to examine the complex, fascinating and still-unresolved history of Late Glacial ice-retreat in western Connemara. The stunning landscape of Connemara bears vast evidence of ice-sculpting during the last glacial period, including the elongated fjord of Killary Harbour, the Twelve Bens mountain massif that rises almost directly from the sea and the partly submerged drumlin field at Clew Bay.

View over the drowned drumlin field in Clew Bay. Photo by K. Campbell, Geograph.org, from WikiCommons

This exposed drumlin was particularly impressive as it is a rare example of coastal erosion revealing a length-wise cross-section through the middle of a drumlin. One can thus walk along the beach examining its internal sedimentology in great detail. The sharp contact to angular facies at the head of the drumlin, suggesting coarse sediments rapidly deposited by sub-glacial meltwater in a cavern beneath the ice, was especially neat.

Exposed internal structure of a drumlin. Photo: D. Schillereff

Angular facies at drumlin head related to deposition of coarse sediment from a sub-glacial meltwater stream. Photo: D. Schillereff

Angular facies at drumlin head related to deposition of coarse sediment from sub-glacial meltwater stream. Photo: D. Schillereff

We visited quarries at Tullywee cut into a subacqueous fan series related to ice retreat (~20 – 18 k years ago) that imply a water-surface of 60-65 m above IOD and the large ice-contact delta at Leenaun at the end of Killary Harbour that exhibits a classic Gilbert-style structure and also implies a high shore-level of 78 m IOD. The causal mechanism(s) for this high sea-level stand have yet to be fully deciphered, especially the question of whether the water at these ice-contact features was glacio-marine (much higher local sea-level than models or other reconstructions possibly suggest) or glacio-lacustrine (enormous ponds dammed by ice further seawards, requiring immensely complex ice-streaming configuration). More discussion of these implications can be found in Thomas & Chiverrell, 2005 QSR.

A series of eskers at Tullywee. Photo: D. Schillereff

Many pristine examples of glacial geomorphology were observed during the trip, for example the eskers at Tullywee, as well as much smaller features such as this ‘dropstone’ in a small exposure in the Leenaun delta. One could easily stroll past and not realise the significance of this cobble; the deformed sediments indicate we were adjacent to a calving margin and this cobble exited the iceberg as it floated seawards and was deposited in the soft sediments below.

A ‘dropstone’ deposited in the bottomset sediments of Leenaun delta. Photo: D. Schillereff

It was a wonderful trip, tremendously educational and certainly a place I’d love to visit again for its visual beauty and ideally for the purpose of research as there is much yet to be understood about the Quaternary environments of western Ireland.

For interested readers, the Quaternary of Central Western Ireland (edited by Professor Pete Coxon, 2005) contains a wealth of further information on many of these sites and other case studies.

Soliciting peer reviews from PhD students

Daniel Schillereff October 2, 2013

This post does not discuss science per se but the topic is important from a personal point of view and hopefully useful to others in a similar position (i.e., PhD student currently putting together their first papers and hoping to learn more about the peer-review process). Feedback from those at a similar stage as well as more experienced members of academia is most welcome.

The British Society for Geomorphology Annual Conference held last year in Nottingham featured a Meet the Editor workshop for Early Career Researchers, including PhD students, which was a resounding success. It was hosted by Editors from several prestigious Geomorphology-focused journals and attendees learned more about the process of submitting a paper, gleaned considerable advice on the Do’s and Don’ts of academic publishing as well as what Editors are seeking in a good manuscript review. Until this workshop I had not considered PhD students as viable reviewers, but the positive encouragement from Editors at the workshop led me to investigate further the potential to act as a reviewer for other Journals. The responses were intriguing and I feel they are worth sharing with the wider community.

My research area is reconstructing recent environmental change, particularly historic floods, using lake sediment records but I am interested in most branches of Quaternary Science so I set about contacting Editors of various relevant journals. I explained I was a PhD student, that I had recently attended a workshop where the possibility of acting as a reviewer was highlighted and enquired as to their feelings on soliciting reviews from PhD students.

Firstly, I can confirm each Editor kindly replied very promptly with a detailed response. The responses fell into three general categories:

Some thanked me for my interest and stated that, while they recognized it would be a tremendous opportunity for a PhD student to be involved in the peer-review process, they felt it was inappropriate for one major reason: our lack of breadth of knowledge of the literature outside our specific fields. I tend to agree; while I attempt to read extensively around all aspects of Quaternary Science and try to keep abreast of recent methodological developments, I have no doubt that my three years’ experience of reading as a PhD student has barely scratched the surface of relevant literature. A member of staff in my Department who acts as an Associate Editor for a peer-reviewed journal confirmed this as being their view too.

Secondly, some Editors replied saying they had simply never before considered asking PhD students to act as reviewers. They invited me to submit my contact details and indicated they’d perhaps be in touch in the future. This suggests it is rather uncommon?

The third theme was entirely positive; Editors replied confirming they had solicited reviews from PhD students in the past and, in fact, these reviews had turned out to be some of their highest-quality reviews. The student had clearly taken the responsibility extremely seriously and had spent a huge amount of time going through manuscript in great detail. In these cases, I was asked to provide my contact details and they would add me to the database and be in touch if an appropriate manuscript appeared, which I was delighted to do.

I have intentionally left this post rather open-ended. I’d love to hear from any PhD students who have acted as a reviewer in the past. Did the Editor contact you directly or did your Supervisor put forward your name? (I’ve heard of this before from other colleagues). Was it challenging? Fun?

I’m also keen to hear thoughts of those with much more peer-reviewed publishing experience on the appropriateness of PhD students as reviewers or from Editors in any field as to whether they have solicited reviews from PhD students in the past. If so, how did you go about finding them? I imagine the most common source is via an academic search engine (i.e., GoogleScholar, Scopus, etc.); this suggests once you have published your first paper, you are much more likely to receive requests to act as a reviewer. Is this indeed the case?

Contact me on Twitter (@dschillereff) if the information may be of relevance to a wider audience, or indeed in the Comments section below.

(Photo uploaded to the public domain on WikiCommons by M. Imran)

Reporting on a recent visit to the NERC Radiocarbon Facility (East Kilbride, Scotland)

Daniel Schillereff September 19, 2013

I (Daniel) recently had the opportunity to visit the Natural Environment Research Council (NERC) Radiocarbon Facility – Environment (NRCF-EK), hosted at the Scottish Universities Environmental Research Centre (SUERC), a collaborative facility between the Universities of Glasgow and Edinburgh. The lab is located in East Kilbride, a 30-minute train ride south of Glasgow city centre.

The opportunity arose via an application I submitted with my Supervisor (Dr Richard Chiverrell) to the NCRF-Steering Committee for funding towards a series of radiocarbon (¹⁴C) dates for our lake sediment sequence at Brotherswater, northwest England. The dating rationale was to augment our current chronology in order to confirm the local mining history recorded in the lake sediment sequence (EGU abstract) as well as more confidently temporally correlate individual palaeoflood laminations with known historical floods (EGU abstract). I was delighted to be awarded 14 radiocarbon dates in total and being invited to bring my samples to their lab and observe the preparation procedures and analytical equipment they use seemed an excellent opportunity.

Pauline Gulliver, a Research Associate at the NRCF-EK lab and the manager of our project, kindly picked me up from the train station on Monday morning (Aug 19th). My first morning involved reading detailed Health and Safety briefings, which were by some margin the most interesting paperwork I have ever completed. The repeated mentions of liquid nitrogen, for example, were intriguing as my prior knowledge of its properties was only from movies.

Photos: Daniel Schillereff

Monday afternoon was more ‘hands-on’ and in fact, by the end of my three-day visit, I had been lucky enough to observe and attempt first-hand each different stage of sample pre-treatment. Callum Murray, the lab technician with whom I was working, was brilliant throughout, explaining each step in detail and with admirable patience. The gas lines (see photo) used for extracting various gases and cryogenically capturing the CO2 were visually impressive (and initially daunting when Callum suggested I attempt a sample myself). But he clearly explained the order in which each valve is turned in order to check for leaks, move gases through the liquid nitrogen and water traps, and measure the total CO2 captured so, in the end, it was great fun and I spent much of Tuesday doing this procedure. Once the sample CO2 is captured it is turned into a form of elemental carbon called graphite and I also watched this being compressed into a graphite pellet. It is this pellet that is subsequently placed on a large diameter tray and inserted into the AMS, enabling the ¹⁴C to be measured. I also had the opportunity to put some of my own samples in their Mass Spectrometer, with the help of technician Josanne Newton, in order to measure the ratio of 12C to 13C isotopes, which is used to correct the radiocarbon data for isotopic fractionation.

An important ‘known unknown’ within my knowledge of radiocarbon sample preparation prior to visiting the lab was how the glass vials containing CO2 are kept absolutely sealed from surrounding air and the possibility of mixing is minimised. Their method is extremely effective: using a butane torch, the narrow stem of the silicon glass vial is made molten and sealed shut (see photo). I had a few attempts with mixed results; another fun yet effective task.

Wednesday lunchtime Pauline and I visited the SUERC AMS lab where their enormous, powerful Accelorator Mass Spectrometer for measuring the carbon isotopes (as well as other selected cosmogenic isotopes) is housed in a purpose-built facility. Upon arrival, Philippa Ascough kindly volunteered her time to provide me with a fascinating guided tour. (On a side note, this demonstrates the power of Twitter as I’ve had one or two Twitter conversations with Philippa in recent months). I honestly cannot recall which aspect was most impressive; the size of the accelerator or the complexity of wiring visible through the casing perhaps. The fact such a large machine is needed when the particles sought to be measured are so small seemed astounding but Philippa’s explanations made this very clear. Truly one of the most impressive machines I’ve ever seen.

The SUERC Accelorator Mass Spectrometer. Photos used with the kind permission of Philippa Ascough, SUERC AMS Laboratory

I am very grateful to the NRCF-E staff and Philippa Ascough for taking the time out of their undoubtedly busy schedules to provide such an educational experience. I’ve taken a number of lessons from my visit to the NRFC-E lab; for one, it has inspired me to greatly improve my knowledge base of standard chemistry and physics. While the staff provided helpful explanations of the various reactions taking place and equipment being used, a solid understanding of the underlying mechanics can only be acquired through a better personal understanding. My visit confirmed yet again that hands-on, visual experience is undoubtedly the most effective learning tool; I don’t think reading every issue of Radiocarbon plus relevant textbooks could replace the knowledge I acquired during my visit (ignoring the time needed for all that reading!). Most importantly, I feel much more confident to discuss the radiocarbon dating technique in published papers or my viva, when the time comes.

Visiting the NRCF-EK lab was an invaluable experience that I’d recommend to any Early Career Researcher who has successfully been awarded funding from the Steering Committee towards ¹⁴C dates.