Radiocarbon dating

Sharing my new discovery – aquatic plants are (sometimes) suitable for radiocarbon dating!

Daniel Schillereff December 4, 2014

Radiocarbon dating is probably the most well-known chronological technique regularly employed by archaeologists, geomorphologists and researchers analysing sediment cores. Palaeolimnologists (those who study lake sediments) hoping to determine the age of a sample extracted from a long sediment core traditionally seek terrestrial plant macrofossils (fragments of organic matter visible without a microscope), as these are considered most likely to return a reliable date. When I was searching for material to send to the radiocarbon lab during my PhD, finding bits of leaf from a tree, seeds or twigs made me especially excited.

Segment of a core extracted from Brotherswater, UK. Photo: D. Schillereff

A new paper written by James Marty and Amy Myrbol (Department of Earth Sciences, University of Minnesota) and published in Journal of Paleolimnology (MM2014) reviews the feasibility of acquiring robust ages from certain aquatic plant macrofossils. It was an interesting read, as I had not previously considered their viability, and I believe worthy of attention from the palaeolimnology community. The online Tool for Macroscopic Identification available from LacCore team at the University of Minnesota is a valuable resource too.

Examples of terrestrial plant macrofossils found in a sediment core extracted from Brotherswater, England. From left to right: Birch seed; leaf fragment; alder seed. Photos: D. Schillereff.

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A much shorter review of flood stratigraphies in lake sediments

Daniel Schillereff August 8, 2014

An example from Lake Superior of a sediment plume dispersing across a lake. A similar feature may occur during floods, although sometimes the high suspended sediment concentration causes the plume to plummet rapidly to the lake flood. (Photo courtesy of WikiCommons (Thoth), CC-BY 2.0, Image by David Dewitt).

Earlier this year my PhD supervisors and I (Daniel) had a paper accepted for publication in Earth-Science Reviews entitled ‘Flood stratigraphies in lake sediments: A review’ (Schillereff et al., 2014). It’s been fairly popular in terms of downloads but it occurred to me the other day that many of those prospective readers may be put off somewhat by its hefty word count. Thus, putting together a shortened version outlining the main points and conclusions seemed wise!

The review stems from my PhD research investigating whether sediment cores extracted from UK lakes contain distinct layers deposited by severe floods that occurred in past decades or centuries. It follows many neat papers illustrating similar case studies from every continent bar Antarctica. (We’ve included a KML GoogleEarth file in the Supplementary Info enabling users to fly to the sites of each published palaeoflood record mentioned in the text).

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Moraines in Costa Rica? Really?

Daniel Schillereff June 29, 2014

Tarns visible from the summit of Cerro Chirripo. Photo courtesy of Scott Schillereff.

During a recent trip to Costa Rica in May, I had a conversation with some family and friends in which I uttered those words: “Moraines in Costa Rica? Really?” as they were describing a trek they’d undertaken earlier this year to the summit of Cerro Chirripó. This is the highest peak in the country (3819 m a.s.l.), part of the Cordillera de Talamanca (9°30′ N, 83°30′ W) in southern central Costa Rica.

Relief of Costa Rica and location of Cerro Chirripó. Base map courtesy of Sting (WikiCommons CC BY-SA 3.0)

Map board at entrance to Parque Nationale Chirrippo. Photo courtesy of Scott Schillereff.

While the photographs looked stunning (on a clear day, both oceans are visible from the summit), I was especially intrigued by their description of the landscape surrounding the peak: curved valleys, moraines and other landforms often associated with glacial activity were visible as they were above the tree-line. There are certainly no glaciers there at present (apparently hail occurs occasionally at the summit) and Costa Rica can definitely be classified as a tropical environment today. However this inspired me to track down research confirming (or not) that these landforms are indeed glacial in origin and, if so, discover the timing and duration of this period of high-altitude tropical glaciation.

The summit of Chirrippo (trail with hikers on left for scale). Photo courtesy of Scott Schillereff.

Sunrise over the Caribbean from the summit of Cerro Chirripó. Photo courtesy of Scott Schillereff.

It turns out the first papers on episodic glaciations in Costa Rica and other Central American countries emerged in the 1950s and investigations have continued through to the present day (particularly by researchers at the University of Tennessee). Much of the peer-reviewed research I found for Cerro Chirripó in particular is based on geomorphic surveys as well as analysis of sediment cores extracted from lakes located on valley floors along the flanks of the mountain. Many of these lakes have formed behind what appear to be moraines (see photo). A particularly interesting feature near the base of these sediment cores is a distinct shift from light-coloured material dominated by mineral particles to much darker brown or black sediments rich in organic matter. This transition is observed in lacustrine sequences all over the world, and certainly here in the UK, and is commonly attributed to the transition from the Younger Dryas interstadial at the end of the last glacial period into the early Holocene. The dark, more organic sediments are typical of deposition through the Holocene as the climate warmed and vegetation cover expanded around the world. A series of radiocarbon dates confirm a similar timing for these sediment transitions in multiple lakes around Cerro Chirripó, ranging between 12, 360 and 9, 470 calibrated years Before Present (BP) within the dating uncertainties (Horn, 1990; Orvis and Horn, 2000). The span of these dates likely relates to the relative position of the each lake with respect to the retreating glacier during the period of deglaciation, and the timing corresponds nicely with the Younger Dryas event (12, 900 – 11, 600 cal. yr BP). In fact, evidence for a Younger Dryas re-advance has been reported elsewhere in the neotropics including the Columbian Cordillera, the Eastern Cordillera of Equador, the Cordillera Real in Bolivia and around the Malinche volcano in central Mexico (references are found in Lachniet, 2004).

Panoramic view from summit of Cerro Chirripó with lakes visible in the foreground that have formed behind what appear to be moraines. Photo courtesy of Peter Anderson (WikiCommons CC BY-SA 3.0)

In terms of geomorphic evidence, phases of glacial advance and retreat are recorded by the large medial (a ridge running through the middle of a valley where two glaciers meet), lateral (two parallel ridges on either side of a glacier) and terminal (ridges formed at the end of a glacier) moraines found in the valleys around Cerro Chirripó. These fingerprints are found up to four hundred metres below the summit (Lachniet, 2004). Other field evidence includes striated bedrock (smoothed and grooved rock formed as ice moves across the surface; see photo).

Striated bedrock near Cerro Chirripó. Photo courtesy of Scott Schillereff.

To date, researchers have been unable to find suitable organic material at the base of the moraines to attempt radiocarbon dating but they are assumed to represent the maximum extent of the last ice advance, equating to a total ice-covered area of 35 km2. Similar features found at lower elevations in other parts of the Cordillera may point towards even more extensive ice cover during stages earlier in the Pleistocene but no effort to date these landforms has yet been invested. A recent review paper (Lachniet and Roy, 2011) emphasised that obtaining further radiocarbon dates from the lake sediments and landforms is critical to better understand the timing and duration of local and wider tropical glaciations. They also suggest OSL may be suitable, but cosmogenic radionuclide dating has been largely unsuccessful due to intense weathering of rock surfaces in the humid tropical environment.

Looking into the research and browsing through photographs of Cerro Chirripó has certainly inspired me to aim to hike up the mountain on my next visit to Costa Rica. One of the amazing things about Costa Rica and other Central American countries that I have backpacked through is how much the climate and landscape and culture can vary across relatively short distances – but trying to imagine glaciers sweeping down the valleys is very difficult to imagine!!

References

Horn, S.P. (1990) Timing of deglaciation in the Cordillera de Talamanca, Costa Rica. Climate Research 1, 81-83. PDF

Lachniet, M. (2004) Late Quaternary glaciation of Costa Rica and Guatemala, Central America. In: Ehlers, J., Gibbard, P. (Eds.) Quaternary Glaciations – Extent and Chronology Part III: South America, Asia, Africa, Australasia, Antarctica. Elsevier, Amsterdam, 135-138. DOI: 10.1016/S1571-0866(04)80118-0

Lachniet, M. and Roy, A. (2011) Costa Rica and Guatemala. In: Ehlers, J., Gibbard, P., Hughes, P. (Eds.) Quaternary Glaciations: Extent and Chronology. Elsevier, Amsterdam, 843-848. DOI: 10.1016/B978-0-444-53447-7.00060-X

Orvis, K. and Horn, S.P. (2000) Quaternary glaciers and climate on Cerro Chirripó, Costa Rica. Quaternary Research 54, 24-37. DOI: 10.1006/qres.2000.2142

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.