van der Johannes Plicht

The role of solar forcing upon climate change.

Evidence for millennial-scale climate changes during the last 60,000 years has been found in Greenland ice cores and North Atlantic ocean cores. Until now, the cause of these climate changes remained a matter of debate. We argue that variations in solar activity may have played a significant role in forcing these climate changes. We review the coincidence of variations in cosmogenic isotopes (14C and 10Be) with climate changes during the Holocene and the upper part of the last Glacial, and present two possible mechanisms (involving the role of solar UV variations and solar wind/cosmic rays) that may explain how small variations in solar activity are amplified to cause significant climate changes. Accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate. It implies that the climate system is far more sensitive to small variations in solar activity than generally believed.

The sharp rise of 14C around 800 cal BC: possible causes, related climatic teleconnections and the impact on human environments.

In this study we report on accelerator mass spectrometry (AMS) wiggle-match dating of selected macrofossils from organic deposits ca. 800 cal BC (ca. 2650 BP). Based on paleological, archaeological and geological evidence, we found that the sharp rise of atmospheric (super 14) C between 850 and 760 cal BC corresponds to the following related phenomena: 1. In European raised bog deposits, the changing spectrum of peat forming mosses and a sharp decline in decomposition of the peat indicate a sudden change from relatively dry and warm to cool, moist climatic conditions. 2. As a consequence of climate change, there was a fast and considerable rise of the groundwater table so that peat growth started in areas that were already marginal from a hydrological point of view. 3. The rise of the groundwater table in low-lying areas of the Netherlands resulted in the abandonment of settlement sites. 4. The contemporaneous earliest human colonization of newly emerged salt marshes in the northern Netherlands (after loss of cultivated land) may have been related to thermal contraction of ocean water, causing a temporary stagnation in the relative sea-level rise. Furthermore, there is evidence for synchronous climatic change in Europe and on other continents (climatic teleconnections on both hemispheres) ca. 2650 BP. We discuss reduced solar activity and the related increase of cosmic rays as a cause for the observed climatological phenomena and the contemporaneous rise in the (super 14) C-content of the atmosphere. Cosmic rays may have been a factor in the formation of clouds and precipitation, and in that way changes in solar wind were amplified and the effects induced abrupt climate change.

A 40,000-year varve chronology from Lake Suigetsu, Japan : Extension of the C-14 calibration curve

A sequence of annually laminated sediments is a potential tool for calibrating the radiocarbon time scale beyond the range of the absolute tree-ring calibration (11 ka). We performed accelerator mass spectrometric (AMS) (super 14) C measurements on >250 terrestrial macrofossil samples from a 40,000-yr varve sequence from Lake Suigetsu, Japan. The results yield the first calibration curve for the total range of the (super 14) C dating method.

High-resolution records of late-Holocene climate change and carbon accumulation in two north-west European ombrotrophic peat bogs

The peat stratigraphy (plant macrofossils, colorimetric humification, pollen/non-pollen microfossils, carbon/nitrogen ratios) of three replicate cores from a raised peat bog in the UK (Walton Moss) and a single core from a raised peat bog in Denmark (Lille Vildmose) were examined in an attempt to investigate the relation between long-term climate change and changes in species composition of the peat-forming vegetation. Nine wet-shifts identified in the monoliths from Walton Moss and Lille Vildmose mainly occurred during periods of increasing atmospheric C-14 content. Because fluctuations in Holocene atmospheric radiocarbon concentrations have been shown to be due to variations in solar activity, the Little Ice Age (LIA) climatic deteriorations registered in both peat bogs were probably driven by changes in solar activity. Climatic deteriorations in the three replicate cores from Walton Moss are not all consistently registered and may reflect the internal variability in the extent of hollow/lawn/hummock microforms within raised peat-bog ecosystems. The lowest carbon accumulation values for the Walton Moss monoliths between ca. cal AD 1300 and 1800 and between ca. cal AD 1490 and 1580 for Lille Vildmose occurred during the course of LIA climatic deteriorations. Peat-bog primary productivity may have been reduced during the LIA owing to lower spring-summer temperatures and shorter growing seasons for the peat-forming plants. Species-specific higher decomposition rates may have also caused the lower carbon accumulation rates in the peat bogs during the LIA

Improving the time control of the Subboreal/Subatlantic transition in a Czech peat sequence by 14C wiggle-matching

To achieve an optimal time-control for a late Subboreal to early Subatlantic peat sequence from Pancavska Louka in the Czech Republic, different strategies are applied to convert a series of radiocarbon dates into a calendar time-scale. The methods of selection and preparation of the samples for AMS 14C dating are presented. The results of calibrating single radiocarbon dates are compared with a 14C wiggle-match strategy. As the accumulation rate of the peat was not constant, the concentrations of arboreal pollen are used to estimate the accumulation rate changes and to correct for these changes. The resulting time-control represents the best solution for this peat sequence with the methods currently available.

14C AMS wiggle matching of raised bog deposits and models of peat accumulation

Abstract High-resolution Accelerator Mass Spectrometer (AMS) 14 C dates of selected plant macrofossils from the raised bog Engbertsdijksvenen (Eastern Netherlands) show century-scale wiggles analogous to the radiocarbon calibration curve. We used three relative time scales, viz., based on depth, mass, and pollen concentration, respectively, to match the peat AMS dates to the calibration curve. This procedure is repeated for one conventionally dated core. For each relative time scale, realistic confidence intervals are calculated. Depth appears to be the best time scale for certain stratigraphical units of a core. This justifies using depth for wiggle matching conventionally dated mire cores published by others, even when 14 C errors are larger. Our evidence shows four major sources of 14 C variation for mire deposits compared to treerings: (1) dating error, due to sample composition. This includes a reservoir effect demonstrated for many bulk peat samples; (2) hiatuses, causing a sudden `leap’ of peat 14 C age; (3) changing accumulation rates, apparent from a break in the slope of the peat 14 C ages; (4) sampling error. These results shed doubt on the assumed continuities in theoretical peat accumulation models. Both mire stratigraphy and changing accumulation rates can be explained in qualitative hydrological terms. More evidence for a previously discovered reservoir effect in bog deposits is presented, though the physical/biological mechanisms remain uncertain.

Dating of cremated bones

When dating unburnt bone, bone collagen, the organic fraction of the bone, is used. Collagen does not survive the heat of the cremation pyre, so dating of cremated bone has been considered impossible. Structural carbonate in the mineral fraction of the bone, however, survives the cremation process. We developed a method of dating cremated bone by accelerator mass spectrometry (AMS), using this carbonate fraction. Here we present results for a variety of prehistoric sites and ages, showing a remarkable success rate for this method.

A numerical approach to 14C wiggle-match dating of organic deposits: best fits and confidence intervals.

C-14 wiggle-match dating (WMD) of peat deposits uses the non-linear relationship between C-14 age and calendar age to match the shape of a sequence of closely spaced peat C-14 dates with the C-14 calibration curve. A numerical approach to WMD enables the quantitative assessment of various possible wiggle-match solutions and of calendar year confidence intervals for sequences of C-14 dates. We assess the assumptions, advantages, and limitations of the method. Several case-studies show that WMD results in more precise chronologies than when individual C-14 dates are calibrated. WMD is most successful during periods with major excursions in the C-14 calibration curve (e.g., in one case WMD could narrow down confidence intervals from 230 to 36 yr)

Evidence for solar forcing of climate change at ca. 850 cal BC from a Czech peat sequence

A climatic change to wetter and cooler conditions was recorded in the Pancavska Louka peat sequence in the Giant Mountains (Czech Republic) at ca. 850 cal BC. The curves of climate indicators were compared to detrended Δ14C, which is considered to be a proxy for solar activity, to assess if solar activity, through changes in, for example, irradiance, had caused the observed climatic change. Visual comparison of the curves showed that changes in the bog and in the regional vegetation occurred in correspondence with the start of the sharp rise in radiocarbon at ca. 850 cal BC, caused by a sudden shift to low solar activity. Cross-correlation of the curves of climate indicators with that of Δ14C is significant. The results of the sequence provide independent evidence for solar forcing of climate change.

Automatic AMS Sample Combustion and CO2 Collection

In Groningen, all organic samples for accelerator mass spectrometry (AMS) are combusted in an automatic Elemental Analyzer, coupled to an Isotope Ratio Mass Spectrometer and Cryogenic Trapping System. The Gas Chromatographic (GC) column, part of the Elemental Analyzer system, appeared to be the main cause for memory effects. Therefore we modified the Elemental Analyzer, such that the trapped CO 2 no longer passed the GC column. Our system modification reduced the memory effect significantly, as shown by lower radiocarbon concentration values for anthracite backgrounds, and a much smaller spread in these values. Our modified system can perform up to 40 combustions unattended in about 6 hr.