If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical methods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14C calibration curve. In most dating applications, however, we have larger numbers of 14C measurements and we wish to relate those to events in the past. Bayesian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal software (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used "uniform phase" models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run simulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing possible problems with statistical models of this kind.

/pdf/bayesian-analysis-of-radiocarbon-dates-26ui63c9ae.pdf

Bayesian analysis of radiocarbon dates

Boreal and subarctic peatlands comprise a carbon pool of 455 Pg that has accumulated during the postglacial period at an average net rate of 0.096 Pg/yr (1 Pg = 1015g). Using Clymo's (1984) model, the current rate is estimated at 0.076 Pg/yr. Longterm drainage of these peatlands is estimated to be causing the oxidation to CO2 of a little more than 0.0085 Pg/yr, with conbustion of fuel peat adding °0.026 Pg/yr. Emissions of CH4 are estimated to release ° 0.046 Pg of carbon annually. Uncertainties beset estimates of both stocks and fluxes, particularly with regard to Soviet peatlands. The influence of water table alterations upon fluxes of both CO2 and CH4 is in great need of investigation over a wide range of peatland environments, especially in regions where permafrost melting, thermokarst erosion, and the development of thaw lakes are likely results of climatic warming. The role of fire in the carbon cycle of peatlands also deserves increased attention. Finally, satellite—monitoring of the abundance of open water in the peatlands of the West Siberian Plain and the Hudson/James Bay Lowland is suggested as a likely method of detecting early effects of climatic warming upon boreal and subarctic peatlands.

/pdf/northern-peatlands-role-in-the-carbon-cycle-and-probable-4bxfb9ejj8.pdf

Northern Peatlands: Role in the Carbon Cycle and Probable Responses to Climatic Warming.

Radiocarbon dating is routinely used in paleoecology to build chronolo- gies of lake and peat sediments, aiming at inferring a model that would relate the sediment depth with its age. We present a new approach for chronology building (called \Bacon") that has received enthusiastic attention by paleoecologists. Our methodology is based on controlling core accumulation rates using a gamma au- toregressive semiparametric model with an arbitrary number of subdivisions along the sediment. Using prior knowledge about accumulation rates is crucial and in- formative priors are routinely used. Since many sediment cores are currently ana- lyzed, using difierent data sets and prior distributions, a robust (adaptive) MCMC is very useful. We use the t-walk (Christen and Fox, 2010), a self adjusting, robust MCMC sampling algorithm, that works acceptably well in many situations. Out- liers are also addressed using a recent approach that considers a Student-t model for radiocarbon data. Two examples are presented here, that of a peat core and a core from a lake, and our results are compared with other approaches. Past climates and environments can be reconstructed from deposits such as ocean or lake sediments, ice sheets and peat bogs. Within a vertical sediment proflle (core), mea- surements of microfossils, macrofossils, isotopes and other variables at a range of depths serve as proxy estimates or \proxies" of climate and environmental conditions when the sediment of those depths was deposited. It is crucial to establish reliable relationships between these depths and their ages. Age-depth relationships are used to study the evolution of climate/environmental proxies along sediment depth and therefore through time (e.g., Lowe and Walker 1997). Age-depth models are constructed in various ways. For sediment depths containing organic matter, and for ages younger than c. 50,000 years, radiocarbon dating is often used to create an age-depth model. Cores are divided into slices and some of these are radiocarbon dated. A curve is fltted to the radiocarbon data and interpolated to obtain an age estimate for every depth of the core. The flrst restriction to be considered is that age should be increasing monotonically with depth, because sediment can never have accumulated backwards in time (extraordinary events leading to mixed or reversed sediments are, most of the time, noticeable in the stratigraphy and therefore such cores are ruled out from further analyses). Moreover, cores may have missing sections, leading to ∞at parts in the age depth models.

https://projecteuclid.org/download/pdf_1/euclid.ba/1339616472

Flexible paleoclimate age-depth models using an autoregressive gamma process

▶ Addresses a wide range of timely environment, economic and energy topics ▶ A forum to review, analyze and stimulate the development, testing and implementation of mitigation and adaptation strategies at regional, national and global scales ▶ Contributes to real-time policy analysis and development as national and international policies and agreements are discussed and promulgated ▶ 94% of authors who answered a survey reported that they would definitely publish or probably publish in the journal again

Mitigation and Adaptation Strategies for Global Change

Methane is the most abundant organic chemical in Earth's atmosphere, and its concentration is increasing with time, as a variety of independent measurements have shown. Photochemical reactions oxidize methane in the atmosphere; through these reactions, methane exerts strong influence over the chemistry of the troposphere and the stratosphere and many species including ozone, hydroxyl radicals, and carbon monoxide. Also, through its infrared absorption spectrum, methane is an important greenhouse gas in the climate system. We describe and enumerate key roles and reactions. Then we focus on two kinds of methane production: microbial and thermogenic. Microbial methanogenesis is described, and key organisms and substrates are identified along with their properties and habitats. Microbial methane oxidation limits the release of methane from certain methanogenic areas. Both aerobic and anaerobic oxidation are described here along with methods to measure rates of methane production and oxidation experimentally. Indicators of the origin of methane, including C and H isotopes, are reviewed. We identify and evaluate several constraints on the budget of atmospheric methane, its sources, sinks and residence time. From these constraints and other data on sources and sinks we construct a list of sources and sinks, identities, and sizes. The quasi-steady state (defined in the text) annual source (or sink) totals about 310(±60) × 1012 mol (500(±95) × 1012 g), but there are many remaining uncertainties in source and sink sizes and several types of data that could lead to stronger constraints and revised estimates in the future. It is particularly difficult to identify enough sources of radiocarbon-free methane.

/pdf/biogeochemical-aspects-of-atmospheric-methane-1j0pgr75la.pdf

Biogeochemical aspects of atmospheric methane

Methods for physical and chemical analysis of fresh waters

Not less than 2% of the Earth's land surface is peat-covered, so it is important to try to understand the dynamics of peat accumulation. Peat-forming systems (mires) accumulate peat because conditions within them impede the decay of the plant material produced by their surface vegetation. This paper concerns the rate of peat production and some unexpected consequences of the processes of decay. These consequences are likely to be of interest to those concerned with mire ecology and with the history of vegetation during Flandrian times. Most peat-forming systems consist of two layers: an upper 10-50 cm deep aerobic layer of high hydraulic conductivity, the acrotelm, in which the rate of decay is relatively high; and a thicker, usually anaerobic, lower layer, the catotelm, of low conductivity and with a much lower rate of decay. Plant structure at the base of the acrotelm collapses as a consequence of aerobic decay, and the hydraulic conductivity consequently decreases. As long as precipitation continues the water table therefore rises to this level, thus engulfing material at the base of the acrotelm. The rate, p $\_c$ , of this input to the catotelm is exactly analogous to the rate, p $\_a$ , of input to the acrotelm i.e. of primary productivity of the vegetation. During passage through the acrotelm the peat becomes richer in the more slowly decaying components. The depth of, and the time for transit through, the acrotelm thus control p $\_c$ . The catotelm, however, usually forms much the largest part of the peat mass. Selective decay may continue in the catotelm. The specific composition of the peat thus becomes a progressively poorer indicator of the surface vegetation that formed it, and to a degree that is not generally realized: reconstructions of the past surface vegetation may become very inaccurate. If p $\_c$ were constant and there were no decay in the catotelm then for the centre of a peat bog the profile of age against depth (measured as cumulative mass below the surface) would be a straight line. But if either or both these conditions is untrue then the profile would probably be concave. Most of the cases for which data exist are consistent with a concave profile and a value (constant over several thousand years) of p $\_c$ of about 50 g m $^{-2}$ a $^{-1}$ and a decay rate coefficient, $\alpha\_c$ , proportional to the amount of mass remaining, of about 10 $^{-4}$ a $^{-1}$ . This rate of input to the catotelm is about 10% of the primary productivity i.e. about 90% of the matter is lost during passage through the acrotelm. The relation seems to hold in spite of short-term fluctuations such as those represented by recurrence surfaces. Although 10 $^{-4}$ a $^{-1}$ seems a very slow rate, it has important consequences. (i) The peat mass tends towards a steady state in which the rate of addition of matter at the surface, p $\_a$ , is balanced by losses at all depths: the rate of accumulation is zero. This depth is, for the cases examined, about 5-10 m. (ii) The very concept of \`peat accumulation rate' thus needs careful consideration. To calculate it as the difference between two $^{14}$ C dates divided by the depth between the samples from which they were measured, as is commonly done, may be seriously misleading. The error is likely to increase with age, depth and time span. (iii) Progress in such studies can be made only if the easily measured profile of bulk density is known. The position of the profile in the peat bog must also be known. There is some evidence that peat contains, or comes to contain, about 1% or less of the original mass in a highly refractory state, so that the concept of a steady state is unlikely to be correct if times much greater than about 50 000 years are involved. Three more consequences of the continued very slow decay in the catotelm may be of interest to mire ecologists. (iv) Most of the mass that leaves the catotelm probably does so as methane gas. The concentration of methane increases with depth and may be as high as 5 $\mu mol$ cm $^{-3}$ at 5 m depth (about 10% by volume). Diffusion alone is able to remove mass at the necessary rate and would create concentration profiles similar to those observed. The solubility of methane in water is exceeded, however, and much of the methane may in practice be lost by mass flow of bubbles to the surface. (v) The amplitude of temperature fluctuations, as well as the mean temperature, may have a significant effect on the rate of peat decay, particularly in a cold climate. (vi) If this analysis is correct then the maximum depth of peat which can accumulate in 50 000 years is determined largely by the value of the quotient p $\_c$ / $\alpha\_c$ . The usual view that the maximum depth is determined directly by climate operating through hydrology may be incorrect, though hydrology may have an indirect effect on the value of p $\_c$ , the rate of input to the catotelm at the bog centre. Away from the centre p $\_c$ is probably variable p $\_c$ and determined by hydrology. Its dependence on distance from the centre and on time is complicated: p $\_c$ /p $\_c$ may be more than, equal to, or less than 1.0. The age against depth profile away from the bog centre may be directly affected by hydrology, though the effect is not large except near the edge of the bog or near the base of the peat. There may, of course, be catastrophic failure - a bog-burst or \`flow' - before the p $\_c$ / $\alpha\_c$ limit is reached in the centre, or slower but equally destructive development of gullies and erosion.

The Limits to Peat Bog Growth

Methods for chemical analysis of fresh waters , Methods for chemical analysis of fresh waters , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Methods for chemical analysis of fresh waters

It may be true of men; it is certainly true of Sphagnum. The plants grow at the apex, as do most other mosses. The apex produces initials which develop into branches of determinate growth, though in a few cases (S. cuspidatum var. plumosum for example), the branches may themselves branch. Whilst the branches are increasing in length the internodes of the main stem do not elongate. This results in the branches and attendant leaves forming a compact hemispherical head — the coma or capitulum. Later, after branch growth has finished, the internodes do elongate; it then becomes obvious that the branches are borne in groups (fascicles) on the central stem. Each branch bears 30 to 150 spirally arranged imbricate leaves. These leaves are one cell thick, with the unique and well known differentiation into porose hyaline cells with thickened hoops, and enclosed or chlorophyllose cells. The branches may be spreading or pendent and clasping the stem (Fig. 8.1). The individual leaves live for a year or two, but by that time the growth of branches above has put them into dense shade and they die. The only parts left alive at this distance below the apex seem to be the axillary buds. These usually remain inactive, and eventually die, but if the apex is destroyed, either artificially (for example, by cutting if off) or accidentally (for example, by drought) then one or more lateral buds may begin to grow again from as far as 10 cm below the apex.

The Ecology of Sphagnum

(1) Models of peat accumulation are developed that include constant, linear and quadratic decay of dry mass remaining. Profiles of dry bulk density of 795 peatlands distributed over Finland are used to infer cumulative carbon for each site. These values and basal ages are themselves used to infer rates of growth and decay of the peat. (2) A method, 'function parameter fitting' (FPF), is devised to estimate parameter values in non-linear functions when there are uncertainties in both variables, as there are in cumulative carbon and age. Where the data are highly variable then results with FPF are substantially different from those used hitherto that assume uncertainty in only the dependent variable. (3) For five regions in Finland and in Boreal Canada the inferred rate of addition, p* [M L -2 T -1 ], is related to degree-days above zero, and decay, a* [T -1 ] is related logarithmically to mean annual temperature. The present day rate of accumulation of carbon in northern peatlands is about 5.6 Tmol yr -1 or, as dry mass, 0.07 Gt yr -1 . (4) There are difficulties in the interpretation of LARCA (=LORCA=long term average rate of carbon accumulation). Understanding of peatland dynamics may result from the use of intrinsic models allowing decay: it is unlikely to emerge from the exotic models in common use.

R. S. Clymo

Papers

Methods for physical and chemical analysis of fresh waters

The Limits to Peat Bog Growth

Methods for chemical analysis of fresh waters

The Ecology of Sphagnum

Carbon accumulation in peatland