14C contamination testing in natural abundance laboratories: a new preparation method using wet chemical oxidation and some experiences

Substances enriched with radiocarbon can easily contaminate samples and laboratories used for natural abundance measurements. We have developed a new method using wet chemical oxidation for swabbing laboratories and equipment to test for 14C contamination. Here, we report the findings of 18 months’ work and more than 800 tests covering studies at multiple locations. Evidence of past and current use of enriched 14C was found at all but one location and a program of testing and communication was used to mitigate its effects. Remediation was attempted with mixed success and depended on the complexity and level of the contamination. We describe four cases from different situations

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

Soil erosion plays a crucial role in transferring sediment and carbon from land to sea, yet little is known about the rhythm and rates of soil erosion prior to the most recent few centuries. Here we reconstruct a Holocene erosional history from central India, as integrated by the Godavari River in a sediment core from the Bay of Bengal. We quantify terrigenous fluxes, fingerprint sources for the lithogenic fraction and assess the age of the exported terrigenous carbon. Taken together, our data show that the monsoon decline in the late Holocene significantly increased soil erosion and the age of exported organic carbon. This acceleration of natural erosion was later exacerbated by the Neolithic adoption and Iron Age extensification of agriculture on the Deccan Plateau. Despite a constantly elevated sea level since the middle Holocene, this erosion acceleration led to a rapid growth of the continental margin. We conclude that in monsoon conditions aridity boosts rather than suppresses sediment and carbon export, acting as a monsoon erosional pump modulated by land cover conditions

Permafrost-carbon mobilization in Beringia caused by deglacial meltwater runoff, sea-level rise and warming

During the last deglaciation (18–8 kyr BP), shelf flooding and warming presumably led to a large-scale decomposition of permafrost soils in the mid-to-high latitudes of the Northern Hemisphere. Microbial degradation of old organic matter released from the decomposing permafrost potentially contributed to the deglacial rise in atmospheric CO2 and also to the declining atmospheric radiocarbon contents (Δ14C). The significance of permafrost for the atmospheric carbon pool is not well understood as the timing of the carbon activation is poorly constrained by proxy data. Here, we trace the mobilization of organic matter from permafrost in the Pacific sector of Beringia over the last 22 kyr using mass-accumulation rates and radiocarbon signatures of terrigenous biomarkers in four sediment cores from the Bering Sea and the Northwest Pacific. We find that pronounced reworking and thus the vulnerability of old organic carbon to remineralization commenced during the early deglaciation (~16.8 kyr BP) when meltwater runoff in the Yukon River intensified riverbank erosion of permafrost soils and fluvial discharge. Regional deglaciation in Alaska additionally mobilized significant fractions of fossil, petrogenic organic matter at this time. Permafrost decomposition across Beringia's Pacific sector occurred in two major pulses that match the Bølling-Allerød and Preboreal warm spells and rapidly initiated within centuries. The carbon mobilization likely resulted from massive shelf flooding during meltwater pulses 1A (~14.6 kyr BP) and 1B (~11.5 kyr BP) followed by permafrost thaw in the hinterland. Our findings emphasize that coastal erosion was a major control to rapidly mobilize permafrost carbon along Beringia's Pacific coast at ~14.6 and ~11.5 kyr BP implying that shelf flooding in Beringia may partly explain the centennial-scale rises in atmospheric CO2 at these times. Around 16.5 kyr BP, the mobilization of old terrigenous organic matter caused by meltwater-floods may have additionally contributed to increasing CO2 levels

Using an independent geochronology based on palaeomagnetic secular variation (PSV) and atmospheric Pb deposition to date Baltic Sea sediments and infer 14C reservoir age

Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the C-14 reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the C-14 reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, C-14 determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 mu g C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for these sediments. An inferred marine reservoir age offset (Delta R) is calculated by comparing the foraminifera C-14 determinations to a PSV & Pb age model. This Delta R is found to trend towards younger values upwards in the core, possibly due to a gradual change in hydrographic conditions brought about by a reduction in marine water exchange from the open sea due to continued isostatic rebound. (C) 2012 Elsevier Ltd. All rights reserved

Evidence of a dynamic ice sheet system in Filchner Trough until the early Holocene

The past ice sheet conditions in the southern Weddell Sea Embayment (WSE) are only poorly known. Studies from this area have led to two contradicting scenarios of maximum ice extent during the Last Glacial Maximum (LGM). The first scenario is mainly based on terrestrial data indicating only very limited ice sheet thickening in the hinterland and suggests a grounding-line position on the inner shelf. The alternative scenario is based on marine geological and geophysical data and concludes that the LGM grounding line was located on the outer shelf, about 650 km further offshore than in the other scenario. Three hypotheses have been brought forward to explain these two apparently contradictory scenarios. A) An ice plain was present on the shelf that enabled a large ice extent while maintaining little ice thickness in the hinterland. B) The maximum grounded ice advance lasted for a short period only and was probably caused by a short-termed touch down of an ice shelf on the outer shelf, which did not cause sufficient ice sheet thickening in the hinterland to be traced today. C) Due to an ice flow switch, Filchner Trough was fed by an area further to the west where ice had thickened at the LGM. Besides the poorly constrained LGM ice extent, studies suggest a complex development of its retreat speed and drainage pattern in succession of the LGM that needs to be further constraint. For example, radar data from ice rises in the southwestern hinterland of the WSE suggest that ice flow switches occurred as late as the Mid-Holocene and cosmogenic exposure ages indicate an early Holocene ice sheet thickness in the Ellsworth Mountains comparable to that of the LGM. We investigated multibeam bathymetry data (ATLAS Hydrosweep DS3), acoustic sub-bottom profiles (ATLAS Parasound P-70) and marine sediment cores collected from Filchner Trough during RV “Polarstern” expedition PS96 in Dec 2015-Feb 2016. Our key finding is a previously unknown stacked grounding zone wedge (GZW) located on the outer shelf. This GZW shows that the Filchner palaeo-ice stream stabilized at this position at least two times. Two sediment cores were recovered seaward of the GZW and on top of the lower part of the GZW, respectively. Radiocarbon dates from these cores indicate that (i) the GZW was formed in the Early Holocene and (ii) grounded ice did not extend seaward of the GZW at the LGM. Hence, our data provide evidence that the grounding line in Filchner Trough experienced dynamic changes in the Holocene and that no linear ice sheet retreat occurred within this trough after the LGM

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 11997, doi:10.1038/s41598-018-30091-8.The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.We acknowledge funding support from the Agouron Institute Postdoctoral Fellowship (K.L.F), the US National Science Foundation (Awards: OCE-1333387 and OCE-13333826), the Investment in Science Fund given primarily by WHOI Trustee and Corporation Members, and the Swiss National Science Foundation (Award: 200020_163162)

What on Earth have we been burning? Deciphering sedimentary records of pyrogenic carbon

Humans have interacted with fire for thousands of years, yet the utilization of fossil fuels marked the beginning of a new era. Ubiquitous in the environment, pyrogenic carbon (PyC) arises from incomplete combustion of biomass and fossil fuels, forming a continuum of condensed aromatic structures. Here we develop and evaluate 14C records for two complementary PyC molecular markers, benzene-polycarboxylic-acids (BPCAs) and polycyclic-aromatic-hydrocarbons (PAHs) preserved in aquatic sediments from a sub-urban and a remote catchment in the United States (U.S.) from mid-1700s to 1998. Results show that the majority of PyC stems from local sources and is transferred to aquatic sedimentary archives on sub-decadal to millennial time scales. Whereas a small portion stems from near-contemporaneous production and sedimentation, the majority of PyC (<90%) experiences delayed transmission due to ‘pre-aging’ on millennial timescales in catchment soils prior to its ultimate deposition. BPCAs (soot) and PAHs (precursors of soot) trace fossil fuel-derived PyC. Both markers parallel historical records of the consumption of fossil fuels in U.S., yet never account for more than 19% total PyC. This study demonstrates that isotopic characterization of multiple tracers is necessary to constrain histories and inventories of PyC, and that sequestration of PyC can markedly lag its production

Annual Variation in Atmospheric 14C between 1700 BC and 1480 BC

In 2018 Pearson et al. published a new sequence of annual radiocarbon (14C) data derived from oak (Quercus sp.) trees from Northern Ireland and bristlecone pine (Pinus longaeva) from North America across the period 1700–1500 BC. The study indicated that the more highly resolved shape of an annually based calibration dataset could improve the accuracy of 14C calibration during this period. This finding had implications for the controversial dating of the eruption of Thera in the Eastern Mediterranean. To test for interlaboratory variation and improve the robustness of the annual dataset for calibration purposes, we have generated a replicate sequence from the same Irish oaks at ETH Zürich. These data are compatible with the Irish oak 14C dataset previously produced at the University of Arizona and are used (along with additional data) to examine inter-tree and interlaboratory variation in multiyear annual 14C time-series. The results raise questions about regional 14C offsets at different scales and demonstrate the potential of annually resolved 14C for refining subdecadal and larger scale features for calibration, solar reconstruction, and multiproxy synchronization