Solar Variability Over the Past Several Millennia

The Sun is the most important energy source for the Earth. Since the incoming solar radiation is not equally distributed and peaks at low latitudes the climate system is continuously transporting energy towards the polar regions. Any variability in the Sun-Earth system may ultimately cause a climate change. There are two main variability components that are related to the Sun. The first is due to changes in the orbital parameters of the Earth induced by the other planets. Their gravitational perturbations induce changes with characteristic time scales in the eccentricity (∼100,000 years), the obliquity (angle between the equator and the orbital plane) (∼40,000 years) and the precession of the Earth's axis (∼20,000 years). The second component is due to variability within the Sun. A variety of observational proxies reflecting different aspects of solar activity show similar features regarding periodic variability, trends and periods of very low solar activity (so-called grand minima) which seem to be positively correlated with the total and the spectral solar irradiance. The length of these records ranges from 25 years (solar irradiance) to 400 years (sunspots). In order to establish a quantitative relationship between solar variability and solar forcing it is necessary to extend the records of solar variability much further back in time and to identify the physical processes linking solar activity and total and spectral solar irradiance. The first step, the extension of solar variability, can be achieved by using cosmogenic radionuclides such as 10Be in ice cores. After removing the effect of the changing geomagnetic field, a 9000-year long record of solar modulation was obtained. Comparison with paleoclimatic data provides strong evidence for a causal relationship between solar variability and climate change. It will be the subject of the next step to investigate the underlying physical processes that link solar variability with the total and spectral solar irradianc

Coupled climate model simulation of Holocene cooling events: oceanic feedback amplifies solar forcing

The coupled global atmosphere-ocean-vegetation model ECBilt-CLIO-VECODE is used to perform transient simulations of the last 9000 years, forced by variations in orbital parameters, atmospheric greenhouse gas concentrations and total solar irradiance (TSI). The objective is to study the impact of decadal-to-centennial scale TSI variations on Holocene climate variability. The simulations show that negative TSI anomalies increase the probability of temporary relocations of the site with deepwater formation in the Nordic Seas, causing an expansion of sea ice that produces additional cooling. The consequence is a characteristic climatic anomaly pattern with cooling over most of the North Atlantic region that is consistent with proxy evidence for Holocene cold phases. Our results thus suggest that the ocean is able to play an important role in amplifying centennial-scale climate variability

Relative paleointensity (RPI) in the latest Pleistocene (10–45 ka) and implications for deglacial atmospheric radiocarbon

We report magnetic properties and relative paleointensity (RPI) proxies from a suite of 10 conventional piston cores and Kasten cores from the SW Iberian Margin collected during cruise JC089 of the RSS James Cook in August 2013. Mean sedimentation rates are in the 10-20 cm/kyr range. Age models were acquired by correlation of Ca/Ti and Zr/Sr XRF core-scanning data to L* reflectance from the Cariaco Basin that is, in turn, tied to the Greenland ice-core chronology. The natural remanent magnetization (NRM) is represented by a single magnetization component carried by a low-coercivity mineral (magnetite), although reflectance and bulk magnetic properties indicate the presence of a high-coercivity (hematitic) magnetic phase, possibly from eolian dust. The presence of fine-grained hematite means that the sediments are not ideal for RPI studies, however the detrital hematite does not appear to contribute to the NRM or anhysteretic remanent magnetization (ARM). In order to test the usefulness of the RPI data, we construct a stack of 12 RPI records from the SW Iberian Margin for the 0-45 ka interval and compare it with a stack of 12 globally distributed marine and lake records, chosen on the basis of mean sedimentation rates (>15 cm/kyr) and superior age models. The two stacks are similar, but different from published RPI stacks, particularly for the 10-30 ka interval, and imply a virtual axial dipole moment (VADM) high at ~15-18 ka followed by a drop in field strength from ~15 to 13 ka. A revised VADM estimate calculated from Greenland 10Be ice-core flux using a contemporary age model is remarkably consistent with the new overall RPI stack, based on Iberian Margin and global RPI records. The elevated atmospheric 14C levels of the last ice age cannot, however, be fully explained by this RPI stack although relative changes such as the long-term drop in atmospheric 14C from 30 to 15 ka are reproduced, supporting the hypothesis of a combined influence of production rate and ocean ventilation on 14C during the last ice age

Biases in radiocarbon dating of organic fractions in sediments from meromictic and seasonally hypoxic lakes

We present here radiocarbon dating results from two boreal lakes in Finland, which are permanently (meromictic) or seasonally stratified and contain continuous sequences of annually laminated sediments that started to form in the early Holocene. The radiocarbon dating results of different organic components were compared with the varve-based sediment chronologies. The deviation between the Lake Valkiajarvi varve chronology (8400 varve years 2-3% error estimate) and 33 C-14 dates taken from insoluble and soluble organic phases vary inconsistently throughout the Holocene. In extreme cases mean calibrated radiocarbon dates with 95.4% confidence levels (2 sigma) are -2350 and +2040 years offset when compared with the varve chronology. On average, the radiocarbon dates are offset by ca. +550 years. The deviation between the Lake Nautajarvi varve chronology (9898 varve years +/- 1% error estimate) and 26 C-14 dates analyzed with conventional and AMS methods indicates that radiocarbon dates are systematically older by 500-1300 years (about 900 years on average). This significant offset mean that radiocarbon dates obtained from organic bulk sediment of meromictic and seasonally hypoxic lakes must be cautiously interpreted because of the reservoir effect and carbon cycling at the sediment-water interface. Direct evidence was obtained from the dating of soluble fraction and insoluble organic matter from near bottom water in the monimolimnion of Lake Valkiajarvi, which yielded C-14 ages of 560 +/- 80 BP and 2070 +/- 140 BP, respectively. Our study reinforces previous results that age-depth models based on bulk sediment radiocarbon dates obtained on sediments of stratified lakes are of limited value for accurate dating of changes in land use and especially the commence of agriculture.Peer reviewe

Was 14C über Sonne und Erde verrät

Das radioaktive Kohlenstoffisotop 14C, auch Radiokarbon genannt, ist ein wichtiger Informationsträger für die Geowissenschaften. Es hilft, unser Verständnis von Klimaprozessen, der Sonne, des Geodynamos und des Kohlenstoffkreislaufs der vergangenen 55 000 Jahre zu verbessern. Moderne Messmethoden erlauben auch 14C-Messungen an Kleinstproben. Das verringert die Unsicherheiten in den aktuellen IntCal20-Alterskalibrationskurven. Dadurch zeigen sich klarere Strukturen in den Veränderungen im 14C-Gehalt der Atmosphäre und des oberflächennahen Meerwassers während der vergangenen 55 000 Jahre. Dieser Fortschritt ermöglicht eine verbesserte Datierung und Untersuchungen von Sonnenaktivität, Erdmagnetfeld und Kohlenstoffkreislauf. In allen diesen Gebieten der Erdsystemforschung kommen Radiokarbon-Messungen erfolgreich zum Einsatz

A varved lake sediment record of ¹⁰Be solar activity proxy for the Lateglacial-Holocene transition

Solar modulated variations in cosmogenic radionuclide production provide both information on past changes in the activity of the Sun and a global synchronization tool. However, to date the use of cosmogenic radionuclides for these applications is almost exclusively based on 10Be records from ice cores and 14C time-series from tree rings, all including archive-specific limitations. We present the first 10Be record from annually laminated (varved) lake sediments for the Lateglacial-Holocene transition from Meerfelder Maar. We quantify environmental influences on the catchment and, consequently, 10Be deposition using a new approach based on regression analyses between our 10Be record and environmental proxy time-series from the same archive. Our analyses suggest that environmental influences contribute to up to 37% of the variability in our 10Be record, but cannot be the main explanation for major 10Be excursions. Corrected for these environmental influences, our 10Be record is interpreted to dominantly reflect changes in solar modulated cosmogenic radionuclide production. The preservation of a solar production signal in 10Be from varved lake sediments highlights the largely unexplored potential of these archives for solar activity reconstruction, as global synchronization tool and, thus, for more robust paleoclimate studies

Heliospheric Magnetic Field 1835-2009

We use recently acquired geomagnetic archival data to extend our long-term reconstruction of the HMF strength. The 1835-2009 HMF series is based on an updated and substantiated IDV series from 1872-onwards and on Bartels' extension, by proxy, of his u-series from 1835-1871. The new IDV series, termed IDV09, has excellent agreement (R^2 = 0.98; RMS = 0.3 nT) with the earlier IDV05 series, and also with the negative component of Love's extended (to 1905) Dst series (R^2 = 0.91). Of greatest importance to the community, in an area of research that has been contentious, comparison of the extended HMF series with other recent reconstructions of solar wind B for the last ~100 years yields a strong consensus between series based on geomagnetic data. Differences exist from ~1900-1910 but they are far smaller than the previous disagreement for this key interval of low solar wind B values which closely resembles current solar activity. Equally encouraging, a discrepancy with an HMF reconstruction based on 10Be data for the first half of the 20th century has largely been removed by a revised 10Be-based reconstruction published after we submitted this paper, although a remaining discrepancy for the years ~1885-1905 will need to be resolved