Evidence from northwest European bogs shows 'Little Ice Age' climatic changes driven by variations in solar activity

Fluctuations in Holocene atmospheric radiocarbon concentrations have been shown to be due to variations in solar activity. Analyses of both Be-10 and C-14 nuclides confirm that production-rate changes during the Holocene were largely modulated by solar activity. Analyses of peat samples from two intact European ombrotrophic bogs show that climatic deteriorations during the 'Little Ice Age' are associated with transitions to increasing atmospheric C-14 content due to greater C-14 production. Both ombrotrophic mires, which are positioned c. 800 km apart, register reactions to globally recorded C-14 fluctuations between AD 1449 and 1464 and an almost identical reaction between AD 1601 and 1604.

Investigating the Interhemisphere 14C Offset in the 1st Millennium AD and Assessment of Laboratory Bias and Calibration Errors

Past measurements of the radiocarbon interhemispheric offset have been restricted to relatively young samples because of a lack of older dendrochronologically secure Southern Hemisphere tree-ring chronologies. The Southern Hemisphere calibration data set SHCal04 earlier than AD 950 utilizes a variable interhemispheric offset derived from measured 2nd millennium AD Southern Hemisphere/Northern Hemisphere sample pairs with the assumption of stable Holocene ocean/ atmosphere interactions. This study extends the range of measured interhemispheric offset values with 20 decadal New Zealand kauri and Irish oak sample pairs from 3 selected time intervals in the 1st millennium AD and is part of a larger program to obtain high-precision Southern Hemisphere 14C data continuously back to 200 BC. We found an average interhemispheric offset of 35 ± 6 yr, which although consistent with previously published 2nd millennium AD measurements, is lower than the offset of 55–58 yr utilized in SHCal04. We concur with McCormac et al. (2008) that the IntCal04 measurement for AD 775 may indeed be slightly too old but also suggest the McCormac results appear excessively young for the interval AD 755–785. In addition, we raise the issue of laboratory bias and calibration errors, and encourage all laboratories to check their consistency with appropriate calibration curves and invest more effort into improving the accuracy of those curves.

Flexible paleoclimate age-depth models using an autoregressive gamma process

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 autoregressive semiparametric model with an arbitrary number of subdivisions along the sediment. Using prior knowledge about accumulation rates is crucial and informative priors are routinely used. Since many sediment cores are currently analyzed, using different 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. Outliers 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.

Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds

Tree ring Delta C-14 data (Reimer et al., 2004; McCormac et al., 2004) indicate that atmospheric Delta C-14 varied on multi-decadal to centennial timescales, in both hemispheres, over the period between AD 950 and 1830. The Northern and Southern Hemispheric Delta C-14 records display similar variability, but from the data alone is it not clear whether these variations are driven by the production of C-14 in the stratosphere (Stuiver and Quay, 1980) or by perturbations to exchanges between carbon reservoirs (Siegenthaler et al., 1980). As the sea-air flux of (CO2)-C-14 has a clear maximum in the open ocean regions of the Southern Ocean, relatively modest perturbations to the winds over this region drive significant perturbations to the interhemispheric gradient. In this study, model simulations are used to show that Southern Ocean winds are likely a main driver of the observed variability in the interhemispheric gradient over AD 950-1830, and further, that this variability may be larger than the Southern Ocean wind trends that have been reported for recent decades (notably 1980-2004). This interpretation also implies that there may have been a significant weakening of the winds over the Southern Ocean within a few decades of AD 1375, associated with the transition between the Medieval Climate Anomaly and the Little Ice Age. The driving forces that could have produced such a shift in the winds at the Medieval Climate Anomaly to Little Ice Age transition remain unknown. Our process-focused suite of perturbation experiments with models raises the possibility that the current generation of coupled climate and earth system models may underestimate the natural background multi-decadal- to centennial-timescale variations in the winds over the Southern Ocean.

Understanding the variability in freshwater radiocarbon reservoir offsets: a cautionary tale

Freshwater resources in past diets can lead to inaccuracies when attempts are made to ascertain their radiocarbon ages or those of the consumers. Radiocarbon reservoir effects may lead to significant age offsets when the bones or other tissues of these consumers are radiocarbon dated. A number of recent studies have investigated freshwater reservoir offsets. However no study thus far has satisfactorily obtained a ubiquitous freshwater reservoir correction due to variability in the ecosystems analysed. This study tests the possibility of predicting freshwater reservoir effects from the carbonate alkalinity of the water with measurements on modern fish bone and water samples. A predictive capability would be especially valuable in the absence of well-preserved archaeological fish bone. We surveyed samples from lakes and rivers in varying geological settings in Britain and Ireland. Modern fish bone and water samples were analysed to investigate modern radiocarbon offsets from the atmosphere. Archaeological fish bone was also analysed to examine past reservoir offsets at selected sites. Stable carbon and nitrogen isotope values were measured to aid in interpretation of any variability in the offsets. Large freshwater reservoir offsets were measured in some modern and archaeological samples (maximum offset = 1638 14C years). The freshwater reservoir offsets in the fish bone were highly correlated with alkalinity of water in modern lake sites analysed. However, a high amount of variation within and between fish species was also evident in the results, precluding the possibility of providing regional corrections for freshwater reservoir offsets from alkalinity although this still may provide a general guideline. The variability is thought to be due to differences in the diet of individual fish.

Late-Holocene marine radiocarbon reservoir correction (Delta R) for the west coast of South Africa

In order to calibrate radiocarbon ages based on samples with a marine carbon component it is important to know the marine carbon reservoir correction or Delta R value. This study measured the Delta R on both known-age pre-bomb marine shells and paired marine and terrestrial samples from two regions on the west coast of South Africa: the southwestern Cape and Namaqualand. Pooling the data by region produces Delta R values that are similar enough to use a west coast weighted mean Delta R of 146 +/- 85 C-14 years to correctly calibrate marine shell or mixed marine and terrestrial C-14 ages. There are however temporal differences in Delta R throughout the Holocene, which we compare with proxy data for upwelling and sea surface temperatures.

INTCAL radiocarbon database

Radiocarbon data comprising the 2009 and 2013 internationally agreed radiocarbon calibration curves has been put in a format which makes it available to the climate and statistics research communities
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‘Old wood’ effect in radiocarbon dating of prehistoric cremated bones?

Numerous reports of successful radiocarbon dating of cremated bones have emerged during the last decade. The success of radiocarbon dating cremated bones depends on the temperature during burning and the degree of recrystallisation of the inorganic bone matrix. During cremation bones undergo major morphological and mineralogical changes which have raised some interesting questions and discussion on the origin of the carbon source in archaeologically cremated bones. Recent laboratory experiments reveal that the properties of the combustion atmosphere play a significant role regarding the source carbon in cremated bones. Thus radiocarbon dating cremated bones is potentially dating the wood used for the cremation fire. Here we compare a high precision radiocarbon dated human bone with an associated dendrochronological age from an oak coffin. We find that the age discrepancy between the dendrochronological age and the cremated bone of 73 ± 26 14C yr is best accounted for by the so called ‘old wood’ effect.

A revised age for the Kawakawa/Oruanui tephra, a key marker for the Last Glacial Maximum in New Zealand

The Kawakawa/Oruanui tephra (KOT) is a key chronostratigraphic marker in terrestrial and marine deposits of the New Zealand (NZ) sector of the southwest Pacific. Erupted early during the Last Glacial Maximum (LGM), the wide distribution of the KOT enables inter-regional alignment of proxy records and facilitates comparison between NZ climatic variations and those from well-dated records elsewhere. We present 22 new radiocarbon ages for the KOT from sites and materials considered optimal for dating, and apply Bayesian statistical methods via OxCal4.1.7 that incorporate stratigraphic information to develop a new age probability model for KOT. The revised calibrated age, ±2 standard deviations, for the eruption of the KOT is 25,360 ± 160 cal yr BP. The age revision provides a basis for refining marine reservoir ages for the LGM in the southwest Pacific.

Long-term mass balance of perennial firn and ice in an Alpine cave (Austria):Constraints from radiocarbon-dated wood fragments

Hundsalm ice cave located at 1520 m altitude in a karst region of western Austria contains up to 7-m-thick deposits of snow, firn and congelation ice. Wood fragments exposed in the lower parts of an ice and firn wall were radiocarbon accelerator mass spectrometry (AMS) dated. Although the local stratigraphy is complex, the 19 individual dates - the largest currently available radiocarbon dataset for an Alpine ice cave - allow to place constraints on the accumulation and ablation history of the cave ice. Most of the cave was either ice free or contained only a small firn and ice body during the 'Roman Warm Period'; dates of three wood fragments mark the onset of firn and ice build-up in the 6th and 7th century ad. In the central part of the cave, the oldest samples date back to the 13th century and record ice growth coeval with the onset of the 'Little Ice Age'. The majority of the ice and firn deposit, albeit compromised by a disturbed stratigraphy, appears to have been formed during the subsequent centuries, supported by wood samples from the 15th to the 17th century. The oldest wood remains found so far inside the ice is from the end of the Bronze Age and implies that local relics of prehistoric ice may be preserved in this cave. The wood record from Hundsalm ice cave shows parallels to the Alpine glacier history of the last three millennia, for example, the lack of preserved wood remains during periods of known glacier minima, and underscores the potential of firn and ice in karst cavities as a long-term palaeoclimate archive, which has been degrading at an alarming rate in recent years. © The Author(s) 2013.

New age estimates and climatostratigraphic correlations for the Borrobol and Penifiler Tephras: evidence from Abernethy Forest, Scotland

The emerging tephrostratigraphy of NW Europe spanning the last termination (ca. 15–9 ka) provides the potential for synchronizing marine, ice-core and terrestrial records, but is currently compromised by stratigraphic complications, geochemical ambiguity and imprecise age estimates for some layers. Here we present new tephrostratigraphic, radiocarbon and chironomid-based
palaeotemperature data from Abernethy Forest, Scotland, that refine the ages and stratigraphic positions of the Borrobol and Penifiler tephras. The Borrobol Tephra (14.14–13.95 cal ka BP) was deposited in a relatively warm period equated with Greenland Interstadial sub-stage GI-1e. The younger Penifiler Tephra (14.09–13.65 cal ka BP) is closely associated with a cold oscillation equated with GI-
1d. We also present evidence for a previously undescribed tephra layer that has a major-element chemical signature identical to the Vedde Ash. It is associated with the warming trend at the end of the Younger Dryas, and dates between 11.79 and 11.20 cal ka BP.

The latest LGM culmination of the Garda Glacier (Italian Alps) and the onset of glacial termination. Age of glacial collapse and vegetation chronosequence

In the deglacial sequence of the largest end moraine system of the Italian Alps, we focused on the latest culmination of the Last Glacial Maximum, before a sudden downwasting of the piedmontane lobe occupying the modern lake basin. We obtained a robust chronology for this culmination and for the subsequent deglacial history by cross-radiocarbon dating of a proximal fluvioglacial plain and of a deglacial continuous lake sedimentation. We used reworked dinocysts to locate sources of glacial abrasion and to mark the input of glacial meltwater until depletion. The palynological record from postglacial lake sediments provided the first vegetation chronosequence directly reacting to the early Lateglacial withdrawal so far documented in the Alps.

Glacier collapse occurred soon after 17.46 +/- 0.2 ka cal BP, which is, the Manerba advance culmination. Basin deglaciation of several overdeepened foreland piedmont lakes on southern and northern sides of the Alps appears to be synchronous at millennial scale and near-synchronous with large-scale glacial retreat at global scale. The pioneering succession shows a first afforestation step at a median modeled age of 64 years after deglaciation, while rapid tree growth lagged 7 centuries. Between 16.4 +/- 0.16 and 15.5 +/- 0.16 ka cal BP, a regressive phase interrupted forest growth marking a Lateglacial phase of continental-dry climate predating GI-1. This event, spanning the most advanced phases of North-Atlantic H1, is consistently radiocarbon-framed at three deglacial lake records so far investigated on the Italian side of the Alps. Relationships with the Gschnitz stadial from the Alpine record of Lateglacial advances are discussed

Presence of cave bears in western Austria before the onset of the Last Glacial Maximum: new radiocarbon dates and palaeoclimatic considerations

Tischoferhohle and Pendling-Barenhohle near Kufstein, Tyrol, are among the only locations where remains of cave bear, Ursus spelaeus-group, were found in the western part of Austria. One sample from each site was radiocarbon-dated four decades ago to ca. 28 C-14 ka BP. Here we report that attempts to date additional samples from Pendling-Barenhohle have failed due to the lack of collagen, casting doubts on the validity of the original measurement. We also unsuccessfully tried to date flowstone clasts embedded in the bone-bearing sediment to provide maximum constraints on the age of this sediment. Ten cave bear bones from Tischoferhohle showing good collagen preservation were radiocarbon-dated to 31.1-39.9 C-14 ka BP, again pointing towards an age underestimation by the original radiocarbon-dated sample from this site. These new dates from Tischoferhohle are therefore currently the only reliable cave bear dates in western Austria and constrain the interval of cave occupation to 44.3-33.5 cal ka BP. We re-calibrate and re-evaluate dates of alpine cave bear samples in the context of available palaeoclimate information from the greater alpine region covering the transition into the Last Glacial Maximum, eventually leading to the demise of this megafauna.

Sediment accumulation rates in subarctic lakes: insights into age-depth modeling from 22 dated lake records from the Northwest Territories, Canada

Age-depth modeling using Bayesian statistics requires well-informed prior information about the behavior of sediment accumulation. Here we present average sediment accumulation rates (represented as deposition times, DT, in yr/cm) for lakes in an Arctic setting, and we examine the variability across space (intra- and inter-lake) and time (late Holocene). The dataset includes over 100 radiocarbon dates, primarily on bulk sediment, from 22 sediment cores obtained from 18 lakes spanning the boreal to tundra ecotone gradients in subarctic Canada. There are four to twenty-five radiocarbon dates per core, depending on the length and character of the sediment records. Deposition times were calculated at 100-year intervals from age-depth models constructed using the ‘classical’ age-depth modeling software Clam. Lakes in boreal settings have the most rapid accumulation (mean DT 20 ± 10 years), whereas lakes in tundra settings accumulate at moderate (mean DT 70 ± 10 years) to very slow rates, (>100 yr/cm). Many of the age-depth models demonstrate fluctuations in accumulation that coincide with lake evolution and post-glacial climate change. Ten of our sediment cores yielded sediments as old as c. 9,000 cal BP (BP = years before AD 1950). From between c. 9,000 cal BP and c. 6,000 cal BP, sediment accumulation was relatively rapid (DT of 20 to 60 yr/cm). Accumulation slowed between c. 5,500 and c. 4,000 cal BP as vegetation expanded northward in response to warming. A short period of rapid accumulation occurred near 1,200 cal BP at three lakes. Our research will help inform priors in Bayesian age modeling.