ALBERT

Description: In this study we investigate pollen and oxygen isotopes of moss cellulose from the peat bog ‘Dürres Maar’ in the Eifel low mountain range, Germany (450 m a.s.l.) to quantitatively infer mid-Holocene climate change for the period between ~7000 and 3800 cal. BP. Pollen was analysed on the same samples from which Sphagnum leaves were isolated to extract cellulose for the determination of its oxygen isotope composition ( 18 O Sphagnum ). To quantitatively estimate January and July temperature and annual precipitation from the pollen data, we applied a probabilistic indicator taxa method (‘pdf-method’). The pollen-based reconstructions indicate July temperatures ~1°C higher around 6000 cal. BP than after ~5500 cal. BP, which is consistent with a ~1 decrease in 18 O Sphagnum during the same period. While the pollen-based climate reconstructions indicate little variability in summer temperature after ~5500 cal. BP, winter temperature shows several pronounced cold excursions of ~2–4°C in this period, which was likely accompanied by changes in precipitation patterns. Test reconstructions leaving out specific taxa indicate that not only larger climate trends, but also relatively small-scale climate variability can robustly be reconstructed with the pdf-method. This is of particular importance for reliable reconstructions of climate variability not only during the Holocene, but also in former interglacials, for which archives are rare and pollen is often the only suitable proxy in terrestrial records. The stable isotope values agree with the reconstructions based on pollen for the time between ~4500 and 3500 cal. BP, but not for the period before 4500 cal. BP. We explain this difference by atmospheric circulation patterns being different in the mid and late Holocene, respectively.

Description: This paper presents a high resolution reconstruction of local growing season temperature (GST) anomalies at Dürres Maar, Germany, spanning the last two millennia. The GST anomalies were derived from a stable carbon isotope time series of cellulose chemically extracted from Sphagnum leaves (δ13Ccellulose) separated from a kettle-hole peat deposit of several metres thickness. The temperature reconstruction is based on the Sphagnum δ13Ccellulose /temperature dependency observed in calibration studies. Reconstructed GST anomalies show considerable centennial and decadal scale variability. A cold and presumably also wet phase with below-average temperature is reconstructed between the 4th and 7th century AD which is in accordance with the so called European Migration Period marking the transition from the Late Roman Period to the Early Middle Ages. At High Medieval Times above-average temperatures are obvious followed by a temperature decrease. On the contrary, a pronounced Late Roman Climate Optimum, often described as similar warm or even warmer as medieval times, could not be detected. The temperature signal of the Little Ice Age (LIA) is not preserved in Dürres Maar due to considerable peat cutting that takes place in the first half of the 19th century. The local GST anomalies show a remarkable agreement to northern hemispheric temperature reconstructions based on tree-ring data sets and are also in accordance with climate reconstructions on the basis of lake sediments, glacier advances and retreats, and historical data sets. Most notably, e.g. during the Early Middle Ages and at High Medieval Times, temperatures were not low or high in general. Rather high frequency temperature variability with multiple narrow intervals of below- and above-average temperatures at maximum lasting a few decades are reconstructed. Especially the agreements between our estimated GST anomalies and the NH temperature reconstructions derived from tree-ring chronologies indicate the great potential of Sphagnum leaves δ13Ccellulose time series from peat deposits for palaeoclimate research. This is particularly the case, given that a quantitative δ13Ccellulose/temperature relationship has been found for several Sphagnum species. Although the time resolution of Sphagnum δ13Ccellulose data sets certainly wouldn't reach the annual resolution of tree-ring data, reconstructions of past temperature variability on the basis of this proxy hold one particular advantage: due to often relatively high peat accumulation rates, especially in kettle-hole bogs accumulated on temperate latitudes over periods of up to several millennia, they allow extending temperature reconstructions based on tree-ring series into the past to enhance our knowledge of natural climate variability during the Holocene.

Description: This paper presents a high resolution reconstruction of local growing season temperature (GST) anomalies at Dürres Maar, Germany, spanning the last two millennia. The GST anomalies were derived from a stable carbon isotope time series of cellulose chemically extracted from Sphagnum leaves (δ13Ccellulose) separated from a kettle-hole peat deposit of several metres thickness. The temperature reconstruction is based on the Sphagnum δ13Ccellulose/temperature dependency observed in calibration studies. Reconstructed GST anomalies show considerable centennial and decadal scale variability. A cold and presumably wet phase with below-average temperature is reconstructed between the 4th and 7th century AD which is in accordance with the so called European Migration Period, marking the transition from the Late Roman Period to the Early Middle Ages. At High Medieval Times, the amplitude in the reconstructed temperature variability is most likely overestimated; nevertheless, above-average temperatures are obvious during this time span, which are followed by a temperature decrease. On the contrary, a pronounced Late Roman Climate Optimum, often described as similarly warm or even warmer as medieval times, could not be detected. The temperature signal of the Little Ice Age (LIA) is not preserved in Dürres Maar due to considerable peat cutting that takes place in the first half of the 19th century. The local GST anomalies show a remarkable agreement to northern hemispheric temperature reconstructions based on tree-ring datasets and are also in accordance with climate reconstructions on the basis of lake sediments, glacier advances and retreats, and historical datasets. Most notably, e.g., during the Early Middle Ages and at High Medieval Times, temperatures were neither low nor high in general. Rather high frequency temperature variability with multiple narrow intervals of below- and above-average temperatures at maximum lasting a few decades are reconstructed. Especially the agreements between our estimated GST anomalies and temperature reconstructions derived from tree-ring chronologies indicate the great potential of Sphagnum δ13Ccellulose time series from peat deposits for palaeoclimate research. This is particularly the case, given that a quantitative δ13Ccellulose/temperature relationship has been found for several Sphagnum species. Although the time resolution of Sphagnum δ13Ccellulose datasets certainly wouldn't reach the annual resolution of tree-ring data, reconstructions of past temperature variability on the basis of this proxy hold one particular advantage: often due to relatively high peat accumulation rates, especially in kettle-hole bogs accumulated on temperate latitudes over periods of up to several millennia, they allow extending temperature reconstructions based on tree-ring series into the past to enhance our knowledge of natural climate variability during the Holocene.

Description: Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declines over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands.

Description: Peatlands are a major terrestrial carbon store and a persistent natural carbon sink during the Holocene, but there is considerable uncertainty over the fate of peatland carbon in a changing climate. It is generally assumed that higher temperatures will increase peat decay, causing a positive feedback to climate warming and contributing to the global positive carbon cycle feedback. Here we use a new extensive database of peat profiles across northern high latitudes to examine spatial and temporal patterns of carbon accumulation over the past millennium. Opposite to expectations, our results indicate a small negative carbon cycle feedback from past changes in the long-term accumulation rates of northern peatlands. Total carbon accumulated over the last 1000 yr is linearly related to contemporary growing season length and photosynthetically active radiation, suggesting that variability in net primary productivity is more important than decomposition in determining long-term carbon accumulation. Furthermore, northern peatland carbon sequestration rate declined over the climate transition from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA), probably because of lower LIA temperatures combined with increased cloudiness suppressing net primary productivity. Other factors including changing moisture status, peatland distribution, fire, nitrogen deposition, permafrost thaw and methane emissions will also influence future peatland carbon cycle feedbacks, but our data suggest that the carbon sequestration rate could increase over many areas of northern peatlands in a warmer future.

Description: In 2009 the Alfred-Wegener-Institute organised an inter-laboratory comparison for analysing the oxygen isotope compositions of biogenic silica. The relationship of the oxygen isotope compositions of carbonate to climate-relevant parameters is widely utilized. However, challenges still exist in the use of biogenic silica. Problems arise during the sample preparation stage and during the isotopic analysis itself. The problems are commonly related to the removal of loosely bound exchangeable oxygen from the silica. Amorphous silica contains OH groups within the SiO2 skeleton as well as chemically combined water. The OH groups and chemically combined water must be removed prior to analysis as their oxygen is easily exchanged following silica formation and thus does not reflect the original isotopic composition of the water. Various methods have been established in the past 20 years for the dehydration and dehydroxylation of amorphous silica, including Controlled Isotopic Exchange (CIE) followed by fluorination, Stepwise Fluorination (SWF) and inductive High-Temperature carbon reduction (iHTR). A new method under consideration is Helium Flow Dehydration (HFD) followed by fluorination. These methods have never been compared in a comprehensive interlaboratory comparison.This inter-laboratory comparison should function as a method performance study as well as a material certification study. The studys goals are:1. To evaluate the agreement of δ18O results in terms of accuracy and reproducibility among different methods in different laboratories, and2. To provide the δ18OSi community with well-calibrated biogenic standards covering a large range of δ18O values.The second goal is related to the current existence of only one quartz standard (NBS-28) made available by IAEA as a reference material for analysis of oxygen isotopes from SiO2. There is a need for natural biogenic reference materials with oxygen isotopic compositions in the range of naturally occurring lacustrine and marine sediments. Six different samples so far only used as internal standards in the participating laboratories have been analysed by eight different laboratories using their typical analytical methods. The samples cover a wide-range of δ18O values (23 to 43 ) and originate from lacustrine and marine sediment deposits as well as from chemically precipitated amorphous silica. The samples consist of diatoms, phytoliths or synthetical nano-spheres. Each laboratory has analysed each sample at least ten times on various days to evaluate reproducibility. X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses have also been performed to verify the purity and the structure of the proposed standards. The XRF analysis of the six standards indicates SiO2 contents ranging from 94 to 99%. The XRD analyses showed that five of the standards have a broad, pure amorphous silica (opal-A) peak whereas one (Kieselguhr) is crystalline, with crystobalite bands resulting from pre-calcination performed by the supplying company.At the time of this abstract submission the laboratories were completing their analyses. Preliminary results will be presented at the meeting. Based on these results, we hope to recommend up to six biogenic silica standards for δ18OSi values with a reproducibility of better than ±0.3 . We will also evaluate the various methods in terms of the analytical uncertainties accompanying each approach.