Chironomid-inferred Temperature Reconstruction Research Articles

A temperature reversal within the rapid Younger Dryas-Holocene warming in the North Atlantic?

Abstract The onset of the Holocene has been generally considered rapid and uninterrupted in the circum-Atlantic region. Loss-on-ignition (LOI – an index of organic carbon) profiles from 18 lateglacial-aged lakes in Nova Scotia, Canada, together with chironomid-inferred temperature reconstructions at 5 sites, demonstrate that the rapid warming from the Younger Dryas (GS-1) to the Holocene was interrupted by a cooling of 1.6–6.4 °C in summer surface lake water temperature. The resulting inflection or reversal on the rising temperature curve has also been identified at 35 sites outside Nova Scotia from terrestrial and marine settings, indicating that this cool step is a robust feature throughout the North Atlantic and is likely the result of major oceanic and atmospheric reorganization of the Holocene climate system.

A model-data comparison of Holocene timberline changes in the Swiss Alps reveals past and future drivers of mountain forest dynamics.

Mountain vegetation is strongly affected by temperature and is expected to shift upwards with climate change. Dynamic vegetation models are often used to assess the impact of climate on vegetation and model output can be compared with paleobotanical data as a reality check. Recent paleoecological studies have revealed regional variation in the upward shift of timberlines in the Northern and Central European Alps in response to rapid warming at the Younger Dryas/Preboreal transition ca. 11 700 years ago, probably caused by a climatic gradient across the Alps. This contrasts with previous studies that successfully simulated the early Holocene afforestation in the (warmer) Central Alps with a chironomid-inferred temperature reconstruction from the (colder) Northern Alps. We use LandClim, a dynamic landscape vegetation model to simulate mountain forests under different temperature, soil and precipitation scenarios around Iffigsee (2065 m a.s.l.) a lake in the Northwestern Swiss Alps, and compare the model output with the paleobotanical records. The model clearly overestimates the upward shift of timberline in a climate scenario that applies chironomid-inferred July-temperature anomalies to all months. However, forest establishment at 9800 cal. BP at Iffigsee is successfully simulated with lower moisture availability and monthly temperatures corrected for stronger seasonality during the early Holocene. The model-data comparison reveals a contraction in the realized niche of Abies alba due to the prominent role of anthropogenic disturbance after ca. 5000 cal. BP, which has important implications for species distribution models (SDMs) that rely on equilibrium with climate and niche stability. Under future climate projections, LandClim indicates a rapid upward shift of mountain vegetation belts by ca. 500 m and treeline positions of ca. 2500 m a.s.l. by the end of this century. Resulting biodiversity losses in the alpine vegetation belt might be mitigated with low-impact pastoralism to preserve species-rich alpine meadows.

Impacts of changing climate and land use on vegetation dynamics in a Mediterranean ecosystem: insights from paleoecology and dynamic modeling

Forests near the Mediterranean coast have been shaped by millennia of human disturbance. Consequently, ecological studies relying on modern observations or historical records may have difficulty assessing natural vegetation dynamics under current and future climate. We combined a sedimentary pollen record from Lago di Massacciucoli, Tuscany, Italy with simulations from the LandClim dynamic vegetation model to determine what vegetation preceded intense human disturbance, how past changes in vegetation relate to fire and browsing, and the potential of an extinct vegetation type under present climate. We simulated vegetation dynamics near Lago di Massaciucoli for the last 7,000 years using a local chironomid-inferred temperature reconstruction with combinations of three fire regimes (small infrequent, large infrequent, small frequent) and three browsing intensities (no browsing, light browsing, and moderate browsing), and compared model output to pollen data. Simulations with low disturbance support pollen-inferred evidence for a mixed forest dominated by Quercus ilex (a Mediterranean species) and Abies alba (a montane species). Whereas pollen data record the collapse of A. alba after 6000 cal yr bp, simulated populations expanded with declining summer temperatures during the late Holocene. Simulations with increased fire and browsing are consistent with evidence for expansion by deciduous species after A. alba collapsed. According to our combined paleo-environmental and modeling evidence, mixed Q. ilex and A. alba forests remain possible with current climate and limited disturbance, and provide a viable management objective for ecosystems near the Mediterranean coast and in regions that are expected to experience a mediterranean-type climate in the future.

A last millennium temperature reconstruction using chironomids preserved in sediments of anoxic Seebergsee (Switzerland): consensus at local, regional and Central European scales

Determining if temperatures of the last century exceed natural variability necessitates the use of high-resolution paleo-reconstructions extending beyond the instrumental data (i.e. >150 years). Although syntheses using tree-ring, stalagmite, and borehole based reconstructions are available, biological proxies preserved in lake sediments have been neglected as a source of high-resolution information on climate change. Here, we present a decadal-scale mean July air temperature reconstruction covering the past millennium from varved Seebergsee, Switzerland. This reconstruction is compared to instrumental data at local, regional and European scales, to another high-resolution chironomid-inferred temperature reconstruction from Lake Silvaplana (Switzerland), to a composite of paleo-climate reconstructions from the Greater Alpine Region and to various millennial scale climate reconstructions from the northern hemisphere. When compared to local and regional instrumental records since ca 1760 AD significant (p < 0.01) relationships (rPearson > 0.5) are obtained, suggesting that chironomids accurately register the changes in temperature for the past ca 250 years. At European scale, the Seebergsee reconstruction correlates (rPearson = 0.40; p < 0.01) with instrumental and early instrumental data back to 1500 AD. On the millennial time scale, the chironomid reconstruction of Seebergsee provides a pattern of temperature changes mirrored by the chironomid reconstruction from Silvaplana (rPearson = 0.44; p < 0.01) and the Greater Alpine Region composite of reconstructions (rPearson = 0.40; p < 0.01). This includes warmer-than-the-last-century mean July air temperatures (+1.2 °C on average) during the end of the “Medieval Climate Anomaly” (MCA) and colder-than-the-last-century temperatures (−0.5 °C on average) during the “Little Ice Age”. Both chironomid reconstructions inferred a warming during the last decades, but this chironomid-inferred warming does not exceed the MCA temperatures. This result is not singular as many millennial temperature reconstructions in the northern Hemisphere do not show unprecedented warming of the last century at local/regional scale. However, the chironomid assemblages found in Seebergsee and Silvaplana since ca 1950 AD seem to be unique (i.e. they show unprecedented assemblage compositions) for the past 1000 years.

Regional consistency in Lateglacial chironomid-inferred temperatures from five sites in north-west England

High resolution Lateglacial ice-core records from Greenland show both millennial and centennial-scale change and have been used as a benchmark for Lateglacial climatic stratigraphy throughout the North Atlantic region and beyond. In this study we assess the local reflection in north-west Europe of climatic events recognised in ice cores and identify differences in the climate signature between Greenland, UK and sites in continental Europe. This study uses chironomid-inferred temperature analysis and loss-on-ignition data from five carbonate lakes in north-west England to determine the pattern of Lateglacial climatic change and demonstrates the reproducibility of chironomid-inferred temperature reconstructions both within a catchment and from sites within a small area. At both millennial and centennial scales, our results show statistical similarities between sites, giving a clear regional signal. Chironomid assemblages pick out the rapid shifts at the major event boundaries at all sites and indicate temperatures of between 12 and 13 °C for the region at the beginning of Greenland Interstadial 1, a rapid fall of around 4 °C into the Younger Dryas and a rise of around 5 °C in the Early Holocene. At a finer scale, four centennial cooling events are identified within the interstadial, together with an oscillation in the Early Holocene which we tentatively identify as the pre-Boreal event. The records were analysed statistically using redundancy analysis and sequence-slotting techniques. Differences in the impact of individual climatic events, demonstrate that subtle changes in the chironomid-inferred temperatures reflect the local geographical setting and morphology of the lakes; with more marked impact registered in higher altitude and more wind-exposed sites. Whilst the overall pattern of Lateglacial climate can be recognised from Greenland to central Europe, important regional differences, that can be attributed to marine influence and, perhaps, changes in atmospheric circulation, are beginning to emerge.

Paleolimnological evidence of the response of the central Canadian treeline zone to radiative forcing and hemispheric patterns of temperature change over the past 2000 years

Instrumental climate records from the central Canadian treeline zone display a pattern of variation similar to general Northern Hemisphere temperature trends. To examine whether this general correspondence extends back beyond the instrumental record, we obtained a sediment core from Lake S41, a small lake in the Northwest Territories of Canada at 63°43.11′ N, 109°19.07′ W. A radiocarbon-based chronology was developed for the core. The sediments were analyzed for organic-matter content by loss-on-ignition (LOI), biogenic-silica content (BSi), and chironomid community composition to reconstruct July air temperature and summer water temperature. The paleolimnological records were compared with records of atmospheric CO2 concentration, solar variability, and hemispheric temperature variations over the past 2000 years. The results of the analyses suggest that widely-documented long-term variations in Northern Hemisphere temperature associated with radiative forcing, namely the cooling following the medieval period during the Little Ice Age (LIA), and twentieth century warming, are represented in the central Canadian treeline zone. There is also evidence of a brief episode of warming during the eighteenth century. As evidenced by LOI and BSi, the twentieth century warming is typified by increased lake productivity relative to the LIA. Depending upon the measure, the increased productivity of the twentieth century nearly equals or exceeds that of any other period in the past 2000 years. In contrast, the rate of chironomid head capsule accumulation decreased and remained low during the twentieth century. Although the chironomid-inferred temperature reconstructions indicate cooling during the LIA, they present no evidence of greatly increased temperatures during the twentieth century. Warming during the twentieth century might have enhanced lake stratification, and the response of the chironomid fauna to warming was attenuated by decreased oxygen and lower temperatures in the hypolimnion of the more stratification-prone lake.

Validation of chironomid-inferred temperature reconstructions in Iceland : the potential for reconstructing quantitative changes in Holocene climate

Abstract. Subfossil chironomids in short cores from two lakes in western Iceland were analysed and a chironomidmean July air temperature transfer function applied to produce chironomid-inferred temperature reconstructions for the recent past. These reconstructions were compared with local meteorological data in order to evaluate the technique in Iceland. The chironomidinferred temperature reconstructions showed similar patterns and magnitudes of change to those recorded in the instrumental data, but the chironomid temperature reconstructions slightly underpredicted the observed temperatures. This suggests that the chironomids are able to reconstruct sequence trends of temperature change although the accuracy of the actual figures produced needs to be addressed. Subfossil chironomids in Iceland can be used to reconstruct the relatively small magnitude temperature changes of the last few hundred years, and also the larger magnitude changes experienced earlier in the Holocene.

Environmental controls on modern chironomid faunas from NW Iceland and implications for reconstructing climate change

Reconstructing climate change quantitatively over millennial timescales is crucial for understanding the processes that affect the climate system. One of the best methods for producing high resolution, low error, quantitative summer air temperature reconstructions is through chironomid analyses. We analysed over 50 lakes from NW and W Iceland covering a range of environmental gradients in order to test whether the distribution of the Icelandic chironomid fauna was driven by summer temperature, or whether other environmental factors were more dominant. A range of analyses showed the main environmental controls on chironomid communities to be substrate (identified through loss-on-ignition and carbon content) and mean July air temperature, although other factors such as lake depth and lake area were also important. The nature of the Icelandic landscape, with numerous volcanic centres (many of which are covered by ice caps) that produce large quantities of ash, means that relative lake carbon content and summer air temperature do not co-vary, as they often do in other chironomid datasets within the Arctic as well as more temperate environments. As the chironomid–environment relationships are thus different in Iceland compared to other chironomid training sets, we suggest that using an Icelandic model is most appropriate for reconstructing past environmental change from fossil Icelandic datasets. Analogue matching of Icelandic fossil chironomid datasets with the Icelandic training set and another European chironomid training set support this assertion. Analyses of a range of chironomid-inferred temperature transfer functions suggest the best to be a two component WA-PLS model with r2jack = 0.66 and RMSEP = 1.095°C. Using this model, chironomid-inferred temperature reconstructions of early Holocene Icelandic sequences show the magnitude of temperature change compared to contemporary temperatures to be similar to other NW European chironomid sequences, suggesting that the predictive power of the model is good.

What drives the peat-based palaeoclimate record? A critical test using multi-proxy climate records from northern Britain

Peatland palaeoclimate sequences produce bog surface wetness (BSW) reconstructions which are commonly interpreted as changes in summer effective precipitation, i.e. the net balance between precipitation and evapotranspiration, the latter being mainly governed by temperature. The relative roles of precipitation and temperature have been investigated previously, although over centennial and millennial timescales no conclusive relationships have as yet been established, but it has been suggested that summer temperature may play the dominant role. We aimed to test this by comparing a late-Holocene peat-based palaeoclimate record from Walton Moss, northern England, with a chironomid-inferred temperature (CI-T) reconstruction from a nearby lake, Talkin Tarn. Both records showed significant changes in inferred climate over the last 3000 years with lower temperatures corresponding to increases in BSW. The CI-T reconstruction, which covered the last 6000 years, was also compared to a longer BSW record, again from Walton Moss, and the same relationships were observed. Our evidence therefore suggests that over centennial timescales summer temperatures are important drivers of the peat-based palaeoclimate record.

Effects of within-lake variability of fossil assemblages on quantitative chironomid-inferred temperature reconstruction

We studied the effects of within-lake variation of subfossil chironomid assemblages on chironomid-inferred temperatures in five lakes in southern Norway. In each lake six replicate surface-sediment samples in the deepest part of the lake basin and two transects of seven samples each towards the littoral were analysed for subfossil chironomid assemblages. A quantitative chironomid–July air temperature inference model applied to the subfossil assemblages inferred temperatures with a standard deviation of 0.35–0.54°C for the six mid-lake samples and of 0.38–0.59°C for all 20 sediment samples. This variation seems high when compared with the cross-validated root mean square error of prediction (RMSEP) of the model of 1.04°C. However, we show that the variability of the replicate mid-lake samples can only account for a maximum of 15% of the model prediction error if the RMSEP is decomposed into a component due to uncertainties associated with sampling the chironomid assemblage of a lake and into a residual error component. Thus, according to our results analysing multiple samples in the model lakes to obtain better estimates of their subfossil assemblage composition may only slightly reduce the model prediction error. It was difficult to detect a clear pattern of chironomid-inferred temperatures with respect to water depth in any single lake due to the high variability of inferred values. However, a comparison of all five study lakes revealed that, relative to the mid-lake samples, cooler temperatures were inferred at intermediate depths and warmer temperatures in the shallowest parts of the lakes, although two shallow-water samples with very low inferred temperatures were exceptions in this respect. This depth-related bias of inferred temperatures in our relatively shallow study lakes indicates that chironomid–temperature models calibrated on mid-lake samples should only be applied with caution to near-shore sediments and that a systematic offset in the inferred temperatures may occur.

How many chironomid head capsules are enough? A statistical approach to determine sample size for palaeoclimatic reconstructions

Chironomid-inferred temperature reconstruction has been shown to be accurate but the method is tenuous and time consuming. With the increased importance of high-resolution studies, more samples should be analysed, increasing the time taken to obtain climate reconstructions. Here, the number of head capsules that should be used to be both time effective and statistically representative for climate reconstruction was tested. In 30 of the 33 samples, counting 50 head capsules was enough to obtain a temperature reconstruction similar to the one obtained by counting 150 or more head capsules. Counting 90 head capsules or more gave the lowest differences between temperature estimates with 200 head capsules. There was a significant increase in the number of new taxa found in relation with the number of head capsules counted. For ecological purposes, the more head capsules that are identified, the better is the representation of the population.