Bas van Geel

Biomarkers as proxies for plant inputs to peats: an example from a sub-boreal ombrotrophic bog

Lipid distributions in modern peat-forming plants were determined and compared to biomarkers recovered from a ca. 2800 year old peat deposited in a Dutch ombrotrophic bog. The peat section spans a well-constrained shift in local hydrology and vegetation, and potential molecular proxies for plant inputs were directly tested against the macrofossil record. Based on the analyses of modern plants, low-molecular-weight (LMW) n-alkanes (C23, C25) and high-molecular-weight n-alkanes (C33) are proposed as potential biomarkers for Sphagnum and non-Sphagnum inputs, respectively. Consistent with this, in the Bargerveen peat core, LMW n-alkane abundances increase and HMW n-alkane abundances decrease coincident with the shift to Sphagnum dominated peat. Additional potential proxies include the abundances of C22 α,ω-alkanedioic acid and phytenyl phytenoate as Sphagnum indicators and the abundances of the triterpenoids taraxer-14-ene and taraxast-20-ene as biomarkers for Ericaceae rootlets.

A comparative study of lipids in Sphagnum species

Abstract The free lipid compositions of twelve species of Sphagnum were determined by capillary gas chromatography/mass spectrometry as part of a study to identify characteristic lipids for Sphagnum in peat bogs. Complex mixtures of lipids, comprised of C28–C29 sterols, C30 triterpenoids, C16–C30 fatty acids, C22–C30 fatty alcohols, C21–C33n-alkanes and isoprenoid and straight-chain wax esters, were identified and quantified. Sterols are dominated by the C29 sterols, 24-ethylcholesta-5,22-dien-3β-ol and 24-ethylcholest-5-en-3β-ol, whilst in some species C28 sterols are also abundant. Summed concentrations of triterpenoids varied widely (20–3500 μg/g dry weight), with only small concentrations present in the mesotrophic species, S. fimbriatum and S. palustre. Ursolic acid is always the major triterpenoid detected. Although absolute concentrations vary significantly, the carbon number distributions of fatty acids, fatty alcohols and n-alkanes are similar in all examined species. Thus, the distributions of these compounds, and especially the dominance of C23 and C25n-alkanes, are a useful chemotaxonomic fingerprint for Sphagnum species and can thus be used in compound-specific 13C and 14C studies of peat bogs.

Solar forcing of climatic change during the mid-Holocene: indications from raised bogs in The Netherlands

Two cores of mid-Holocene raised-bog deposits from the Netherlands were 14C wiggle-match dated at high precision. Changes in local moisture conditions were inferred from the changing species composition of consecutive series of macrofossil samples. Several wet-shifts were inferred, and these were often coeval with major rises in the Δ14C archive (probably caused by major declines in solar activity). The use of Δ14C as a proxy for changes in solar activity is validated. This paper adds to the increasing body of evidence that solar variability forced climatic changes during the Holocene.

High-resolution lacustrine record of the late glacial/holocene transition in central Europe

Abstract In this paper we present the high-resolution record of proxy climatic data in central Europe during the final stages of the last deglaciation, derived from the annually laminated sediments of Lake Gościaz (central Poland). The isotopic, palynological and other microfossil data confirm sudden changes of climate at the onset and termination of the Younger Dryas (completed within 150 and 70 years, respectively), in close agreement with the previous estimates derived from the polar ice cores and marine sediments. In the upper YD some amelioration of climate took place already about 600 years before the main YD/Preboreal transition. Counting of annual varves in the lake sediments allows a direct estimate of the duration of the Younger sDryas in central Europe; it lasted approximately 1640 years, substantially longer than suggested by previous estimates derived from laminated lake sediments and glacial varves, but agreeing with the radiocarbon calibration data obtained for Barbados corals. The calendar ages of the boundaries of the YD, 12,920 and 11,280 cal BP, are tentatively set.

Changes in solar activity and Holocene climatic shifts derived from 14C wiggle-match dated peat deposits

Closely spaced sequences of accelerator mass spectrometer (AMS) 14C dates of peat deposits display century-scale wiggles which can be fitted to the radiocarbon calibration curve. By wiggle-matching such sequences, high-precision calendar age chronologies can be generated which show that changes in mire surface wetness during the Bronze Age/Iron Age transition (c. 850 cal. BC) and the ‘Little Ice Age’ (Wolf, Spörer, Maunder and Dalton Minima) occurred during periods of suddenly increasing atmospheric concentration of 14C. Replicate evidence from peat-based proxy climate indicators in northwest Europe suggest these changes in climate may have been driven by temporary declines of solar activity. Carbon-accumulation rates of two raised peat bogs in the UK and Denmark record low values during the ‘Little Ice Age’ which reflects reduced primary productivity of the peat-forming vegetation during these periods of climatic deterioration.

Response of an ombrotrophic bog to a regional climate event revealed by macrofossil, molecular, and carbon isotopic data

We examined the variations in vegetation, organic carbon isotopic compositions and biomarker distributions spanning a well-constrained climatic shift in a Sub-Boreal Dutch raised bog. The macrofossils record a vegetation shift from a predominance of degraded Ericaceae rootlets to a predominance of well-preserved Sphagnum species, first S. cuspidatum, followed by S. imbricatum. This shift, as well as evidence from other macrofossils and fungal spores, indicates that the local climatic conditions changed from relatively dry to relatively wet, with the wettest local conditions occurring at the base of the transition. At this boundary, a positive shift occurs in the δ13C values of bulk peat (c. 4‰) and compounds derived from specific peat-forming plants (1 to 2‰). This is attributed to a combination of factors, including selective preservation of certain compound classes, changes in precipitation and its effect on the water table, and plant growth rates. The changes in peat preservation also indicate that redox changes occurred across the climatic change event. Indeed, biomarker transformations thought to reflect past redox conditions or bog water acidity – the 1,2,9-trimethyl 1,2,3,4-tetrahydropicene to ursolic acid ratio, diacid to hydroxy acid ratio, and 17β,21β(H)-homohopane to total homohopane ratio – all exhibited shifts spanning the climatic change transition. Although some of these trends indicate the need for more fundamental research on biomarker transformations in peat, some, such as the 17b,21b(H)-homohopane to total homohopane ratio, show promise as indicators of past bog water conditions that could complement macrofossil analyses.

δ13C values and radiocarbon dates of microbial biomarkers as tracers for carbon recycling in peat deposits

This paper describes the first application of compound-specific stable carbon isotope and radiocarbon isotope analyses to the investigation of microbial processes in peat deposits.

Mid- to Late-Holocene pollen-based biome reconstructions for Colombia

Abstract The assignment of Colombian pollen data to biomes allows the data to be synthesised at 10 ‘time windows’ from the present-day to 6000 radiocarbon years before present (BP). The modern reconstructed biomes are compared to a map of modern potential vegetation to check the applicability of the method and the a priori assignment of pollen taxa to plant functional types and ultimately biomes. The reconstructed modern biomes are successful in describing the composition and distribution of modern vegetation. In particular, altitudinal variations in vegetation within the northern Andean Cordilleras are well described. At 6000 BP the biomes are mainly characteristic of warmer environmental conditions relative to those of the present-day. This trend continues until between 4000 and 3000 BP when there is a shift to more mesic vegetation that is thought to equate to an increase in precipitation levels. The period between 2500 and 1000 BP represents little or no change in biome assignment and is interpreted as a period of environmental stability. The influence attributed to human-induced impact on the vegetation is recorded from 5000 BP, but is particularly important from 2000 BP. The extent of this impact increases over the Late-Holocene period, and is recorded at increasingly high altitudes. Despite these changes, a number of sites do not change their biome assignment throughout the analysis. This asynchronous vegetation response is discussed within the context of site location, non-linear response of vegetation to Late-Holocene environmental change, regionally differential signals, localised human impact and methodological artefacts.

Long-term effects of climate change on vegetation and carbon dynamics in peat bogs

Abstract Questions: What are the long-term effects of climate change on the plant species composition and carbon sequestration in peat bogs? Methods: We developed a bog ecosystem model that includes vegetation, carbon, nitrogen and water dynamics. Two groups of vascular plant species and three groups of Sphagnum species compete with each other for light and nitrogen. The model was tested by comparing the outcome with long-term historic vegetation changes in peat cores from Denmark and England. A climate scenario was used to analyse the future effects of atmospheric CO2, temperature and precipitation. Results: The main changes in the species composition since 1766 were simulated by the model. Simulations for a future warmer, and slightly wetter, climate with doubling CO2 concentration suggest that little will change in species composition, due to the contrasting effects of increasing temperatures (favouring vascular plants) and CO2 (favouring Sphagnum). Further analysis of the effects of temperature showed that simulated carbon sequestration is negatively related to vascular plant expansion. Model results show that increasing temperatures may still increase carbon accumulation at cool, low N deposition sites, but decrease carbon accumulation at high N deposition sites. Conclusions: Our results show that the effects of temperature, precipitation, N-deposition and atmospheric CO2 are not straightforward, but interactions between these components of global change exist. These interactions are the result of changes in vegetation composition. When analysing long-term effects of global change, vegetation changes should be taken into account and predictions should not be based on temperature increase alone.

Early human disturbance of the natural environment recorded in annually laminated sediments of Lake Gosciaz, central Poland

Palynological records from the annually laminated sediments of Lake Gosciaz are presented for the period ca. 7600–3900 conventional B.P. Indications in percentage and influx diagrams of Mesolithic and Early Neolithic human impact on the natural environment around the lake are discussed and compared with the archaeological evidence. The first indications of human disturbance start at ca. 6700 B.P. and in the following period four disturbance phases with interruptions are distinguished. A relationship between the Elm Decline and an increasing thickness of annual sediment layers as a consequence of increased soil erosion is suggested. Some hitherto unrecognized or incompletely known microfossils are identified and figured, and their palaeoecological significance is discussed. The records of akinetes of Aphanizomenon probably indicate increasing eutrophication of the lake. Charred fragments of grass epidermis are possibly indicative of openings in the forest.