Sulfur-isotope anomalies recorded in Antarctic ice cores as a potential proxy for tracing past ozone layer depletion events

Changes in the cosmic-ray background of the Earth can impact the ozone layer. High-energy cosmic events [e.g. supernova (SN)] or rapid changes in the Earth's magnetic field [e.g. geomagnetic Excursion (GE)] can lead to a cascade of cosmic rays. Ensuing chemical reactions can then cause thinning/destruction of the ozone layer-leading to enhanced penetration of harmful ultraviolet (UV) radiation toward the Earth's surface. However, observational evidence for such UV "windows" is still lacking. Here, we conduct a pilot study and investigate this notion during two well-known events: the multiple SN event (≈10 kBP) and the Laschamp GE event (≈41 kBP). We hypothesize that ice-core-Δ³³S records-originally used as volcanic fingerprints-can reveal UV-induced background-tropospheric-photochemical imprints during such events. Indeed, we find nonvolcanic S-isotopic anomalies (Δ³³S ≠ 0‰) in background Antarctic ice-core sulfate during GE/SN periods, thereby confirming our hypothesis. This suggests that ice-core-Δ³³S records can serve as a proxy for past ozone-layer-depletion events.

Tree-rings reveal two strong solar proton events in 7176 and 5259 BCE

The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14C, 10Be and 36Cl have been found. Analyzing annual 14C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric 14C recorded for both events exceed all previously known 14C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure.

Cosmogenic radionuclides reveal an extreme solar particle storm near a solar minimum 9125 years BP

During solar storms, the Sun expels large amounts of energetic particles (SEP) that can react with the Earth's atmospheric constituents and produce cosmogenic radionuclides such as 14C, 10Be and 36Cl. Here we present 10Be and 36Cl data measured in ice cores from Greenland and Antarctica. The data consistently show one of the largest 10Be and 36Cl production peaks detected so far, most likely produced by an extreme SEP event that hit Earth 9125 years BP (before present, i.e., before 1950 CE), i.e., 7176 BCE. Using the 36Cl/10Be ratio, we demonstrate that this event was characterized by a very hard energy spectrum and was possibly up to two orders of magnitude larger than any SEP event during the instrumental period. Furthermore, we provide 10Be-based evidence that, contrary to expectations, the SEP event occurred near a solar minimum.

10Be depositional flux variation in the central Indian Ocean during the last 43 ka

The advent of Accelerator Mass Spectrometer (AMS) enhanced the application of meteoric ¹⁰Be (half-life of 1.39 Ma) as a tracer for understanding earth surface processes on thousand to million-year time scales. However, for the majority of applications, an adequate understanding of the ¹⁰Be depositional flux is a prerequisite. A number of efforts have been made to understand both spatial and temporal variation of ¹⁰Be depositional flux. Yet, due to the limited globally distributed dataset and modulation of the ¹⁰Be signal by local processes, a significant offset is observed between model-derived and measured deposition rates of ¹⁰Be. In this study, an attempt has been made to determine the ¹⁰Be depositional flux from a marine sediment core from the central Indian Ocean chronologically constrained with the AMS radiocarbon dating and ¹⁰Be concentration measured with AMS. The ¹⁰Be depositional flux estimates using weak leaching method are found to be nearly 44% lower compared to the strong leaching method. The calculated ¹⁰Be depositional flux during the Holocene varies between 9.63 and 13.01 × 10⁵ atoms/cm²/yr, which is 2-28% lower compared to the modeled depositional flux for the region. The difference observed in ¹⁰Be depositional flux could be due to the local processes (such as boundary scavenging, changing rate of sediment deposition at the location) affecting ¹⁰Be deposition into the sediment column or offset associated with the model estimations. The changes in ¹⁰Be depositional flux and the ¹⁰Be/⁹Be ratio have been reconstructed up to 43 ka. An increase in the ¹⁰Be/⁹Be ratio during 28 to 43 ka is observed due to the lower geomagnetic field intensity during the period. A high-resolution ¹⁰Be/⁹Be ratio reconstruction shows a peak at 41.2 ka, which can be attributed to the Laschamp event.

Multiradionuclide evidence for an extreme solar proton event around 2,610 B.P. (∼660 BC)

Recently, it has been confirmed that extreme solar proton events can lead to significantly increased atmospheric production rates of cosmogenic radionuclides. Evidence of such events is recorded in annually resolved natural archives, such as tree rings [carbon-14 (¹⁴C)] and ice cores [beryllium-10 (¹⁰Be), chlorine-36 (³⁶Cl)]. Here, we show evidence for an extreme solar event around 2,610 years B.P. (∼660 BC) based on high-resolution ¹⁰Be data from two Greenland ice cores. Our conclusions are supported by modeled ¹⁴C production rates for the same period. Using existing ³⁶Cl ice core data in conjunction with ¹⁰Be, we further show that this solar event was characterized by a very hard energy spectrum. These results indicate that the 2,610-years B.P. event was an order of magnitude stronger than any solar event recorded during the instrumental period and comparable with the solar proton event of AD 774/775, the largest solar event known to date. The results illustrate the importance of multiple ice core radionuclide measurements for the reliable identification of short-term production rate increases and the assessment of their origins.

Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial

Contrasting Greenland and Antarctic temperatures during the last glacial period (115,000 to 11,650 years ago) are thought to have been driven by imbalances in the rates of formation of North Atlantic and Antarctic Deep Water (the 'bipolar seesaw'). Here we exploit a bidecadally resolved ¹⁴C data set obtained from New Zealand kauri (Agathis australis) to undertake high-precision alignment of key climate data sets spanning iceberg-rafted debris event Heinrich 3 and Greenland Interstadial (GI) 5.1 in the North Atlantic (~30,400 to 28,400 years ago). We observe no divergence between the kauri and Atlantic marine sediment ¹⁴C data sets, implying limited changes in deep water formation. However, a Southern Ocean (Atlantic-sector) iceberg rafted debris event appears to have occurred synchronously with GI-5.1 warming and decreased precipitation over the western equatorial Pacific and Atlantic. An ensemble of transient meltwater simulations shows that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train.A challenge for testing mechanisms of past climate change is the precise correlation of palaeoclimate records. Here, through climate modelling and the alignment of terrestrial, ice and marine ¹⁴C and ¹⁰Be records, the authors show that Southern Ocean freshwater hosing can trigger global change.

Anchoring historical sequences using a new source of astro-chronological tie-points

The discovery of past spikes in atmospheric radiocarbon activity, caused by major solar energetic particle events, has opened up new possibilities for high-precision chronometry. The two spikes, or Miyake Events, have now been widely identified in tree-rings that grew in the years 775 and 994 CE. Furthermore, all other plant material that grew in these years would also have incorporated the anomalously high concentrations of radiocarbon. Crucially, some plant-based artefacts, such as papyrus documents, timber beams and linen garments, can also be allocated to specific positions within long, currently unfixed, historical sequences. Thus, Miyake Events represent a new source of tie-points that could provide the means for anchoring early chronologies to the absolute timescale. Here, we explore this possibility, outlining the most expeditious approaches, the current challenges and obstacles, and how they might best be overcome.

The Laschamp geomagnetic excursion featured in nitrate record from EPICA-Dome C ice core

Here we present the first direct comparison of cosmogenic ¹⁰Be and chemical species in the period of 38–45.5 kyr BP spanning the Laschamp geomagnetic excursion from the EPICA-Dome C ice core. A principal component analysis (PCA) allowed to group different components as a function of the main sources, transport and deposition processes affecting the atmospheric aerosol at Dome C. Moreover, a wavelet analysis highlighted the high coherence and in-phase relationship between ¹⁰Be and nitrate at this time. The evident preferential association of ¹⁰Be with nitrate rather than with other chemical species was ascribed to the presence of a distinct source, here labelled as “cosmogenic”. Both the PCA and wavelet analyses ruled out a significant role of calcium in driving the ¹⁰Be and nitrate relationship, which is particularly relevant for a plateau site such as Dome C, especially in the glacial period during which the Laschamp excursion took place. The evidence that the nitrate record from the EDC ice core is able to capture the Laschamp event hints toward the possibility of using this marker for studying galactic cosmic ray flux variations and thus also major geomagnetic field excursions at pluri-centennial-millennial time scales, thus opening up new perspectives in paleoclimatic studies.

Time series study of a 17-year record of (7)Be and (210)Pb fluxes in northern Taiwan using ensemble empirical mode decomposition

Using the ensemble empirical mode decomposition (EEMD) and a significance test method, we have analyzed time series data on the fluxes of (7)Be and (210)Pb collected over a span of 17 y in Northern Taiwan. Among nine intrinsic mode functions (IMFs) extracted from the method five (IMF4-8) are non-trivial for (210)Pb and have adequate S/N with significant power in localized windows around the periodicities of 0.5 y, 1 y, 2 y, 5 y, and 11 y, respectively. For (7)Be, IMF5 and IMF8 with periods around 1 y and 11 y, respectively, have adequate S/N. The semi-annual and annual cycles represented by IMF4 and IMF5, respectively, are dominated by East Asian monsoon. The sum of IMF6 and IMF7 reveals an inter-annual cycle where both (7)Be and (210)Pb fluxes are well-correlated with the East Asian winter monsoon index (EAWMI). The close tracking of the (210)Pb and (7)Be in IMF8 cases may reflect an 11 y cycle; implying that it is caused by common climatologic factors, likely related to solar cycle, rather than their distinct production modes.

10Be climate fingerprints during the Eemian in the NEEM ice core, Greenland

Several deep Greenland ice cores have been retrieved, however, capturing the Eemian period has been problematic due to stratigraphic disturbances in the ice. The new Greenland deep ice core from the NEEM site (77.45 °N, 51.06 °W, 2450 m.a.s.l) recovered a relatively complete Eemian record. Here we discuss the cosmogenic (10)Be isotope record from this core. The results show Eemian average (10)Be concentrations about 0.7 times lower than in the Holocene which suggests a warmer climate and approximately 65-90% higher precipitation in Northern Greenland compared to today. Effects of shorter solar variations on (10)Be concentration are smoothed out due to coarse time resolution, but occurrence of a solar maximum at 115.26-115.36 kyr BP is proposed. Relatively high (10)Be concentrations are found in the basal ice sections of the core which may originate from the glacial-interglacial transition and relate to a geomagnetic excursion about 200 kyr BP.

9,400 years of cosmic radiation and solar activity from ice cores and tree rings

Understanding the temporal variation of cosmic radiation and solar activity during the Holocene is essential for studies of the solar-terrestrial relationship. Cosmic-ray produced radionuclides, such as (10)Be and (14)C which are stored in polar ice cores and tree rings, offer the unique opportunity to reconstruct the history of cosmic radiation and solar activity over many millennia. Although records from different archives basically agree, they also show some deviations during certain periods. So far most reconstructions were based on only one single radionuclide record, which makes detection and correction of these deviations impossible. Here we combine different (10)Be ice core records from Greenland and Antarctica with the global (14)C tree ring record using principal component analysis. This approach is only possible due to a new high-resolution (10)Be record from Dronning Maud Land obtained within the European Project for Ice Coring in Antarctica in Antarctica. The new cosmic radiation record enables us to derive total solar irradiance, which is then used as a proxy of solar activity to identify the solar imprint in an Asian climate record. Though generally the agreement between solar forcing and Asian climate is good, there are also periods without any coherence, pointing to other forcings like volcanoes and greenhouse gases and their corresponding feedbacks. The newly derived records have the potential to improve our understanding of the solar dynamics and to quantify the solar influence on climate.

Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years

A high-resolution deuterium profile is now available along the entire European Project for Ice Coring in Antarctica Dome C ice core, extending this climate record back to marine isotope stage 20.2, approximately 800,000 years ago. Experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation. We assessed the general correspondence between Dansgaard-Oeschger events and their smoothed Antarctic counterparts for this Dome C record, which reveals the presence of such features with similar amplitudes during previous glacial periods. We suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records.

10Be evidence for the Matuyama–Brunhes geomagnetic reversal in the EPICA Dome C ice core

An ice core drilled at Dome C, Antarctica, is the oldest ice core so far retrieved. On the basis of ice flow modelling and a comparison between the deuterium signal in the ice with climate records from marine sediment cores, the ice at a depth of 3,190 m in the Dome C core is believed to have been deposited around 800,000 years ago, offering a rare opportunity to study climatic and environmental conditions over this time period. However, an independent determination of this age is important because the deuterium profile below a depth of 3,190 m depth does not show the expected correlation with the marine record. Here we present evidence for enhanced 10Be deposition in the ice at 3,160-3,170 m, which we interpret as a result of the low dipole field strength during the Matuyama-Brunhes geomagnetic reversal, which occurred about 780,000 years ago. If correct, this provides a crucial tie point between ice cores, marine cores and a radiometric timescale.

Consistent simulations of multiple proxy responses to an abrupt climate change event

Isotope, aerosol, and methane records document an abrupt cooling event across the Northern Hemisphere at 8.2 kiloyears before present (kyr), while separate geologic lines of evidence document the catastrophic drainage of the glacial Lakes Agassiz and Ojibway into the Hudson Bay at approximately the same time. This melt water pulse may have been the catalyst for a decrease in North Atlantic Deep Water formation and subsequent cooling around the Northern Hemisphere. However, lack of direct evidence for ocean cooling has lead to speculation that this abrupt event was purely local to Greenland and called into question this proposed mechanism. We simulate the response to this melt water pulse using a coupled general circulation model that explicitly tracks water isotopes and with atmosphere-only experiments that calculate changes in atmospheric aerosol deposition (specifically (10)Be and dust) and wetland methane emissions. The simulations produce a short period of significantly diminished North Atlantic Deep Water and are able to quantitatively match paleoclimate observations, including the lack of isotopic signal in the North Atlantic. This direct comparison with multiple proxy records provides compelling evidence that changes in ocean circulation played a major role in this abrupt climate change event.

Use of AMS in the marine environment

In recent years, the field of AMS has expanded into many areas of science. This paper reviews a variety of applications of AMS in the marine environment, focusing particularly on recent developments and applications. Following a brief summary of the three main isotope techniques used in environmental studies: dating, tracing and source identification, a number of applications are considered. Traditional (14)C-dating is no longer the dominant application of AMS measurements, and together with measurements of (10)Be, (26)Al and (36)Cl, much of the research is now directed towards an understanding of global climate change via studies of oceanic circulation, atmospheric processes and past climates by cosmic ray exposure dating. Profiles of long-lived cosmogenic radionuclides in sediments and ice cores, as a function of depth and, thus, age, provide key information on past solar variability, production rate changes and atmospheric transport and deposition mechanisms. Useful paleoclimatic information may be derived from these archives both because deposition is influenced by climate and because solar activity (which influences production) and solar radiance (which influences climate) are correlated. In recent years, emphasis has been put on the development and application of AMS techniques for the measurement of heavier long-lived isotopes, including (99)Tc, (129)I, (236)U and other actinide isotopes. AMS combines ultra low detection limits and the possibility to analyse isotope ratios that can be difficult with traditional instruments and has been used in a number of applications on the consequences and uses of releases from nuclear energy. Finally, the use AMS in environmental sciences is expected to expand further in the foreseeable future with long-lived cosmogenic radionuclides contributing to a large body of knowledge on processes involving atmosphere, oceans, ice sheets, biosphere, soils and sediments.

Cyclic variation and solar forcing of Holocene climate in the Alaskan subarctic

High-resolution analyses of lake sediment from southwestern Alaska reveal cyclic variations in climate and ecosystems during the Holocene. These variations occurred with periodicities similar to those of solar activity and appear to be coherent with time series of the cosmogenic nuclides 14C and 10Be as well as North Atlantic drift ice. Our results imply that small variations in solar irradiance induced pronounced cyclic changes in northern high-latitude environments. They also provide evidence that centennial-scale shifts in the Holocene climate were similar between the subpolar regions of the North Atlantic and North Pacific, possibly because of Sun-ocean-climate linkages.

Precise dating of Dansgaard-Oeschger climate oscillations in western Europe from stalagmite data

The signature of Dansgaard-Oeschger events--millennial-scale abrupt climate oscillations during the last glacial period--is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of (14)C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 +/- 0.6 and 67.4 +/- 0.9 kyr ago.

Persistent solar influence on North Atlantic climate during the Holocene

Surface winds and surface ocean hydrography in the subpolar North Atlantic appear to have been influenced by variations in solar output through the entire Holocene. The evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon-14 and beryllium-10 and centennial to millennial time scale changes in proxies of drift ice measured in deep-sea sediment cores. A solar forcing mechanism therefore may underlie at least the Holocene segment of the North Atlantic's "1500-year" cycle. The surface hydrographic changes may have affected production of North Atlantic Deep Water, potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally.

Extremely Large Variations of Atmospheric 14C Concentration During the Last Glacial Period

A long record of atmospheric 14C concentration, from 45 to 11 thousand years ago (ka), was obtained from a stalagmite with thermal-ionization mass-spectrometric 230Th and accelerator mass-spectrometric 14C measurements. This record reveals highly elevated Delta14C between 45 and 33 ka, portions of which may correlate with peaks in cosmogenic 36Cl and 10Be isotopes observed in polar ice cores. Superimposed on this broad peak of Delta14C are several rapid excursions, the largest of which occurs between 44.3 and 43.3 ka. Between 26 and 11 ka, atmospheric Delta14C decreased from approximately 700 to approximately 100 per mil, modulated by numerous minor excursions. Carbon cycle models suggest that the major features of this record cannot be produced with solar or terrestrial magnetic field modulation alone but also require substantial fluctuations in the carbon cycle.

Atmospheric radiocarbon calibration to 45,000 yr B.P.: late glacial fluctuations and cosmogenic isotope production

More than 250 carbon-14 accelerator mass spectrometry dates of terrestrial macrofossils from annually laminated sediments from Lake Suigetsu (Japan) provide a first atmospheric calibration for almost the total range of the radiocarbon method (45,000 years before the present). The results confirm the (recently revised) floating German pine chronology and are consistent with data from European and marine varved sediments, and combined uranium-thorium and carbon-14 dating of corals up to the Last Glacial Maximum. The data during the Glacial show large fluctuations in the atmospheric carbon-14 content, related to changes in global environment and in cosmogenic isotope production.