Peter G. Langdon

Mire-development pathways and palaeoclimatic records from a full Holocene peat archive at Walton Moss, Cumbria, England.

Plant macrofossil data have been used to identify the successive mire communities occupying both central and marginal locations in the Walton Moss peatland complex, during the last 10 500 years. The reconstructed pathways of mire development indicate that early-Holocene fen and fen-carr communities were succeeded by species indicative of deep mire water tables and oligotrophic conditions. The character of the fen/bog transition (FBT) is compared with similar records of peatland development from Britain and Scandinavia and with independent climate data for the early Holocene. The ‘pseudohochmoor’ of central Europe is suggested as an approximate modern analogue for the dry pioneer oligotrophic mire type and alternative explanations for its presence are explored. The first major increase in ombrotrophic Sphagna occurred at c. 7800 cal. BP. Overlying Sphagnum peats provide a continuous record of climate change, inferred from fluctuations in raised mire surface wetness. The proxy palaeoclimate record, reconstructed using Detrended Correspondence Analysis, registers wet shifts commencing at c. 7800, c. 5300, 4410–3990 (2s range), c. 3500, 3170–2860 (2s range), 2320–2040 (2s range), c. 1750, c. 1450, c. 300 and c. 100 cal. BP. This climate record is compared with a similar one from Bolton Fell Moss and spectral analysis of the time-series gives periodicities of c. 1100 years and c. 600 years between wet shifts.

Flickering gives early warning signals of a critical transition to a eutrophic lake state.

There is a recognized need to anticipate tipping points, or critical transitions, in social–ecological systems. Studies of mathematical and experimental systems have shown that systems may ‘wobble’ before a critical transition. Such early warning signals may be due to the phenomenon of critical slowing down, which causes a system to recover slowly from small impacts, or to a flickering phenomenon, which causes a system to switch back and forth between alternative states in response to relatively large impacts. Such signals for transitions in social–ecological systems have rarely been observed, not the least because high-resolution time series are normally required. Here we combine empirical data from a lake-catchment system with a mathematical model and show that flickering can be detected from sparse data. We show how rising variance coupled to decreasing autocorrelation and skewness started 10–30 years before the transition to eutrophic lake conditions in both the empirical records and the model output, a finding that is consistent with flickering rather than critical slowing down. Our results suggest that if environmental regimes are sufficiently affected by large external impacts that flickering is induced, then early warning signals of transitions in modern social–ecological systems may be stronger, and hence easier to identify, than previously thought.

A 7500-year peat-based palaeoclimatic reconstruction and evidence for an 1100-year cyclicity in bog surface wetness from Temple Hill Moss, Pentland Hills, southeast Scotland

Analyses of plant macrofossils, peat humification and testate amoebae were used to reconstruct a proxy climate record spanning the last 7500 years from an ombrotrophic bog, Temple Hill Moss, in southeast Scotland. The plant macrofossil data were subjected to detrended correspondence analysis (DCA) which modelled effectively the significant wet shifts within the record. A mean water table depth transfer function was applied to the testate amoebae data to provide quantifiable changes. The three proxy records show coherent phase changes which are interpreted as variability in past effective precipitation. Two tephra horizons (Glen Garry and Lairg A) were used in conjunction with radiocarbon dates to construct an age/depth model, producing a robust geochronology from which a time series was calculated. The palaeoclimatic reconstruction identified major wet shifts throughout the Holocene, with specific events occurring around cal. 6650, 5850, 5300, 4500, 3850, 3400, 2800–2450, 1450–1350 and 250–150 BP. Spectral analysis of the plant macrofossil DCA and colorimetric humification data produced a millennial scale periodicity of 1100 years. The same periodicity has also been found in a palaeoclimatic reconstruction from a site in Cumbria (Walton Moss), and may be linked with millennial scale periodicities found in oceanic palaeoclimatic records.

Extending the timescale and range of ecosystem services through paleoenvironmental analyses, exemplified in the lower Yangtze basin

In China, and elsewhere, long-term economic development and poverty alleviation need to be balanced against the likelihood of ecological failure. Here, we show how paleoenvironmental records can provide important multidecadal perspectives on ecosystem services (ES). More than 50 different paleoenvironmental proxy records can be mapped to a wide range of ES categories and subcategories. Lake sediments are particularly suitable for reconstructing records of regulating services, such as soil stability, sediment regulation, and water purification, which are often less well monitored. We demonstrate the approach using proxy records from two sets of lake sediment sequences in the lower Yangtze basin covering the period 1800–2006, combined with recent socioeconomic and climate records. We aggregate the proxy records into a regional regulating services index to show that rapid economic growth and population increases since the 1950s are strongly coupled to environmental degradation. Agricultural intensification from the 1980s onward has been the main driver for reducing rural poverty but has led to an accelerated loss of regulating services. In the case of water purification, there is strong evidence that a threshold has been transgressed within the last two decades. The current steep trajectory of the regulating services index implies that regional land management practices across a large agricultural tract of eastern China are critically unsustainable.

Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland

Multi-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 14C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 14C yr B.P. and that July temperatures were around 4°C at ca. 9500 14C yr B.P. Temperatures then rose to ca. 8°C at the time of the deposition of the Saksunarvatn tephra (9100 14C yr B.P.), reaching ca. 10°C by 8500 14C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 14C yr B.P. There is some evidence for cooling immediately preceding 9100 14C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 14C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 14C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 14C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone.

Snapshots in time: precise correlations of peat-based proxy climate records in Scotland using mid-Holocene tephras

Mid-Holocene tephra layers have been located and geochemically analysed from seven ombrotrophic bogs in Scotland. The tephras found in these Scottish peats occur as stratigraphically discrete horizons, not visible to the naked eye, and originate from volcanic activity in Iceland. Identifying and geochemically typing the tephra layers can enable precise correlations between sites on regional scales, depending on the spatial extent of the airfall events. Dispersal of both the Glen Garry and Hekla-4 tephras is excellent over Scotland, enabling snapshot views of the past at the time of these tephra depositions. Palaeoecological analyses have also been undertaken on these bogs, providing detailed reconstructions of past changes in bog vegetation and surface wetness, a proxy for past climates. Correlations between the ombrotrophic bogs at the time of the Glen Garry tephra revealed significant differences between the proxy climate records in the north and the south of Scotland, suggesting asynchronous changes between northern Scotland and the rest of Great Britain.

Chironomids can be reliable proxies for Holocene temperatures. A comment on Velle et al. (2010)

Velle et al. (2010) discussed discrepancies between Scandinavian Holocene chironomid-inferred temperature estimates, which they attribute to the response of chironomids to environmental variables other than temperature and to taxonomic shortcomings. They suggest ways in which the reliability of chironomid-based paleotemperature reconstructions could be improved by taking into account ecological complexity. While we agree with many of their recommendations, based on the results of other work, we think their paper is unnecessarily pessimistic regarding the ability of existing chironomid-based temperature inference models to provide reliable estimates of past temperature. We offer a critique of the main points discussed by Velle et al. (2010) and provide evidence that chironomid-based temperature inference models can reliably reconstruct mean July air temperature in the Lateglacial and Holocene over millennial and centennial timescales.

Lacustrine evidence of early-holocene environmental change in northern Iceland: A multiproxy palaeoecology and stable isotope study

Early-Holocene warming in Iceland caused rapid glacial ice melt which led to exposed landscapes on which soils developed and floras quickly established. Our chironomid-based records from northern Iceland suggest temperatures were up to 2—2.5°C warmer than present throughout the first two millennia post deglaciation (~10 500 to 8500 cal. BP) while sedimentary and isotopic data indicate the development of soils within the local environment throughout this period before catchment conditions started to stabilise around 8400 cal. BP. The warming trend over this period was not uniform however, but punctuated by a series of relatively short-lived climatic events. Specifically inwash events are suggested by the δ13Corganic, %TOC and C/N data around 9600 cal. BP and 8250 cal. BP and are seen at two independent sites. There is also evidence from the δ18Ocarbonate and δ13Ccarbonate records which suggests that progressive evaporation of the study lakes occurred from ~8200 cal. BP, the timing of which accords well with other isotopic records of drier conditions from around the North Atlantic.

Social-ecological systems in the Anthropocene: The need for integrating social and biophysical records at regional scales

Understanding social-ecological system dynamics is a major research priority for sustainable management of landscapes, ecosystems and resources. But the lack of multi-decadal records represents an important gap in information that hinders the development of the research agenda. Without improved information on the long-term and complex interactions between causal factors and responses, it will be difficult to answer key questions about trends, rates of change, tipping points, safe operating spaces and pre-impact conditions. Where available long-term monitored records are too short or lacking, palaeoenvironmental sciences may provide continuous multi-decadal records for an array of ecosystem states, processes and services. Combining these records with conventional sources of historical information from instrumental monitoring records, official statistics and enumerations, remote sensing, archival documents, cartography and archaeology produces an evolutionary framework for reconstructing integrated regional histories. We demonstrate the integrated approach with published case studies from Australia, China, Europe and North America.

Dating the Glen Garry tephra : a widespread late-Holocene marker horizon in the peatlands of northern Britain

The distal Icelandic tephra known as the Glen Garry layer has been dated by AMS 14C at eight ombrotrophic peat bogs from northern England and Scotland. The geochemistry of all the tephra layers is consistent with there being only one Glen Garry tephra, rather than two or more from eruptions of the same volcanic source. At seven of the sites, all in Scotland, the tephra layer was bracketed by 1 cm contiguous samples below, at and above the horizon. At the one English site, Walton Moss, Cumbria, a suite of 16 AMS dates was applied to a 116 cm section of a peat core containing the tephra. The resulting dates were calibrated and wiggle-matched using the BCal and Bpeat programmes to give an estimated age for the Glen Garry tephra of 2176 cal. BP, with a 2σ range of 2210—1966 cal. BP. This 244 year range is an improvement on the 410 years calibrated date range for the date without the wiggle-match. This age estimate will allow future work on peat profiles and other sediments containing the tephra to use the date as a pinning-point in age/depth models and obviate the need for radiocarbon dating at this period.