Cosmogenic Production Rates Research Articles

Cosmogenic production rates and recoil loss effects in micrometeorites and interplanetary dust particles

AbstractWe present a purely physical model to determine cosmogenic production rates for noble gases and radionuclides in micrometeorites (MMs) and interplanetary dust particles (IDPs) by solar cosmic‐rays (SCR) and galactic cosmic‐rays (GCR) fully considering recoil loss effects. Our model is based on various nuclear model codes to calculate recoil cross sections, recoil ranges, and finally the percentages of the cosmogenic nuclides that are lost as a function of grain size, chemical composition of the grain, and the spectral distribution of the projectiles. The main advantage of our new model compared with earlier approaches is that we consider the entire SCR particle spectrum up to 240 MeV and not only single energy points. Recoil losses for GCR‐produced nuclides are assumed to be equal to recoil losses for SCR‐produced nuclides. Combining the model predictions with Poynting‐Robertson orbital lifetimes, we calculate cosmic‐ray exposure ages for recently studied MMs, cosmic spherules, and IDPs. The ages for MMs and the cosmic‐spherule are in the range <2.2–233 Ma, which corresponds, according to the Poynting‐Robertson drag, to orbital distances in the range 4.0–34 AU. For two IDPs, we determine exposure ages of longer than 900 Ma, which corresponds to orbital distances larger than 150 AU. The orbital distance in the range 4–6 AU for one MM and the cosmic spherule indicate an origin either in the asteroid belt or release from comets coming either from the Kuiper Belt or the Oort Cloud. Three of the studied MMs have orbital distances in the range 23–34 AU, clearly indicating a cometary origin, either from short‐period comets from the Kuiper Belt or from the Oort Cloud. The two IDPs have orbital distances of more than 150 AU, indicating an origin from Oort Cloud comets.

Estimating the Amount of 14CO2 in the Atmosphere During the Holocene and Glacial Periods

Radiocarbon has been used to define parameters for modeling past, recent, and future CO2/carbon amounts in the atmosphere and in other environmental compartments. In the present paper, we estimate the amount of 14C in the atmosphere by calculating the molar activity of 14CO2 (quantity of 14CO2 molecules per mol of air). Data on the reconstruction of the past concentration of atmospheric CO2 from Antarctic ice cores and Δ14C activities from the IntCal09 calibration curve were applied. The results obtained indicate that cosmogenic production had a dominant influence on the 14C amount in the atmosphere between 50 and 20 ka BP, when the CO2 concentrations were relatively stable, with a slowly decreasing trend. The decreasing 14C activity (Δ14C) between 20 and 2 ka BP seems to be caused predominantly by a dilution of atmospheric 14CO2 by input of CO2 with a depleted amount of 14C (probably from deeper oceanic layers), which is evident from a comparison with the Δ14C and molar activity time series. A strong linear relation was found between the 14C activity and CO2 concentration in the air for the period 20–2 ka BP, which confirms a dominant influence of atmospheric dilution of 14CO2. The observed linear relation between the CO2 and Δ14C levels persists even in the prevailing part of the Holocene. Likewise, the quantity of 14CO2 in the atmosphere (calculated as molar activity) during the prevailing part of the deglacial period (20–11 ka BP) was surprisingly increasing, although a decreasing trend in the 14C cosmogenic production rate could be expected.

A regional cosmogenic 10Be production rate calibration using two 12 kyr radiocarbon-dated rock avalanches at 69° N, Norway

A regional cosmogenic 10Be production rate calibration using two 12 kyr radiocarbon-dated rock avalanches at 69° N, Norway

In situ cosmogenic radiocarbon production and 2‐D ice flow line modeling for an Antarctic blue ice area

Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic 14C production in ice, and this component must be well understood before useful information can be extracted from 14C data. We combine cosmic ray scaling and production estimates with a two‐dimensional ice flow line model to study cosmogenic 14C production at Taylor Glacier, Antarctica. We find (1) that 14C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface 14C activity that differs by up to 25% from the activity calculated using an ablation‐only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two‐dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future 14C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric 14C content of methane.

Regional 10Be production rate calibration for the past 12 ka deduced from the radiocarbon-dated Grøtlandsura and Russenes rock avalanches at 69° N, Norway

Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1 σ), respectively. Quartz samples ( n = 6) from these two sites contained between 4.28 × 10 4 and 5.06 × 10 4 at 10Be/g using the 1.387 Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12 ± 0.19 at/g/yr (1 σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96 ± 0.16 at/g/yr (1 σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1 σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation.

Fast-neutron activation of long-lived isotopes in enriched Ge

We measured the production of \nuc{57}{Co}, \nuc{54}{Mn}, \nuc{68}{Ge}, \nuc{65}{Zn}, and \nuc{60}{Co} in a sample of Ge enriched in isotope 76 due to high-energy neutron interactions. These isotopes, especially \nuc{68}{Ge}, are critical in understanding background in Ge detectors used for double-beta decay experiments. They are produced by cosmogenic-neutron interactions in the detectors while they reside on the Earth's surface. These production rates were measured at neutron energies of a few hundred MeV. We compared the measured production to that predicted by cross-section calculations based on CEM03.02. The cross section calculations over-predict our measurements by approximately a factor of three depending on isotope. We then use the measured cosmic-ray neutron flux, our measurements, and the CEM03.02 cross sections to predict the cosmogenic production rate of these isotopes. The uncertainty in extrapolating the cross section model to higher energies dominates the total uncertainty in the cosmogenic production rate.

Studies on cosmogenic production as a background for rare physics processes

We present the cosmogenic production rates of several long-lived isotopes estimated by using updated cosmic-ray neutron flux measurements and excitation functions from TALYS code 1.0. We find that the cosmogenic isotopes produced inside different detector target materials can be potential sources of background in searching for rare physics processes such as the detection of dark matter and neutrinoless double-beta decay.

Cosmogenic nuclides in stony meteorites revisited

Abstract— We present new model calculations for depth and size dependent cosmogenic production rates in ordinary and carbonaceous chondrites by galactic cosmic rays. This model, essentially that of Leya et al. (2000a), folds together particle spectra and cross sections for the relevant nuclear reactions, but has been significantly improved due to major improvements in the neutron cross section database and better Monte Carlo modeling of the primary and the secondary particle spectra. The data presented here replace (and extend) the results of our earlier model predictions. Here we give for ordinary and carbonaceous chondrites elemental production rates for the cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, 41Ca, 53Mn, 60Fe, and 129I as well as for the noble gas isotopes 3He, 4He, 20Ne, 21Ne, 22Ne, 36Ar, and 38Ar. Using the new data and expressing size and depth scales to the unit [g/cm2], we are able to demonstrate that the matrix effect for both chondrite types is negligible for all target product combinations, except for those which are dominated by thermal or very low energy neutron reactions. Based on the new model predictions, we present a variety of elemental and isotopic production rate ratios allowing for a reliable determination of preatmospheric sizes, shielding depths, cosmic‐ray exposure ages, and diffusive losses.

Relative timing of last glacial maximum and late-glacial events in the central tropical Andes

Whether or not tropical climate fluctuated in synchrony with global events during the Late Pleistocene is a key problem in climate research. However, the timing of past climate changes in the tropics remains controversial, with a number of recent studies reporting that tropical ice age climate is out of phase with global events. Here, we present geomorphic evidence and an in-situ cosmogenic 3He surface-exposure chronology from Nevado Coropuna, southern Peru, showing that glaciers underwent at least two significant advances during the Late Pleistocene prior to Holocene warming. Comparison of our glacial-geomorphic map at Nevado Coropuna to mid-latitude reconstructions yields a striking similarity between Last Glacial Maximum (LGM) and Late-Glacial sequences in tropical and temperate regions.Exposure ages constraining the maximum and end of the older advance at Nevado Coropuna range between 24.5 and 25.3ka, and between 16.7 and 21.1ka, respectively, depending on the cosmogenic production rate scaling model used. Similarly, the mean age of the younger event ranges from 10 to 13ka. This implies that (1) the LGM and the onset of deglaciation in southern Peru occurred no earlier than at higher latitudes and (2) that a significant Late-Glacial event occurred, most likely prior to the Holocene, coherent with the glacial record from mid and high latitudes. The time elapsed between the end of the LGM and the Late-Glacial event at Nevado Coropuna is independent of scaling model and matches the period between the LGM termination and Late-Glacial reversal in classic mid-latitude records, suggesting that these events in both tropical and temperate regions were in phase.

Cosmogenic production as a background in searching for rare physics processes

We revisit calculations of the cosmogenic production rates for several long-lived isotopes that are potential sources of background in searching for rare physics processes such as the detection of dark matter and neutrinoless double-beta decay. Using updated cosmic-ray neutron flux measurements, we use TALYS 1.0 to investigate the cosmogenic activation of stable isotopes of several detector targets and find that the cosmogenic isotopes produced inside the target materials and cryostat can result in large backgrounds for dark matter searches and neutrinoless double-beta decay. We use previously published low-background HPGe data to constrain the production of 3H on the surface and the upper limit is consistent with our calculation. We note that cosmogenic production of several isotopes in various targets can generate potential backgrounds for dark matter detection and neutrinoless double-beta decay with a massive detector, thus great care should be taken to limit and/or deal with the cosmogenic activation of the targets.

New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are 53Mn and 60Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for 3He, 21Ne, and 38Ar are in agreement. For 4He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled 36Ar/38Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm3STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply 3,4He, 21Ne, or 36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the 41K‐40K‐and the 36Cl‐36Ar dating methods with the new model. Note that we distinguish between 36Ar ages (calculated via 36Ar production rates only) and 36Cl‐36Ar ages. Exposure ages for Grant and Carbo, calculated with the revised 41K‐40K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using 3He, 21Ne, and 38Ar but higher than the 36Ar‐ and 36Cl‐36Ar ages by ˜30%. For Carbo the 41K‐40K age is ˜40% lower than the ages obtained using 3He, 21Ne, and 38Ar but equal to the 36Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR.

A late Holocene slip rate for the central North Anatolian fault, at Tahtaköprü, Turkey, from cosmogenic 10Be geochronology: Implications for fault loading and strain release rates

Measurement of a stream offset and cosmogenic dating (10Be) of the alluvial surface into which the stream incised yield a preferred late Holocene slip rate of 18.6 +3.5/−3.3 mm a−1 for the central part of the North Anatolian fault (NAF) at Tahtaköprü, Turkey; use of variable cosmogenic production rate (VPR) models yields a slightly slower rate of ∼16.4 +6.4/−4.5 mm a−1. The offset drainage (Karanlık Dere), which flows southward almost perpendicular to the east‐west trace of the NAF at the site, is displaced right laterally by 55 ± 10 m, with no vertical displacement. A 10Be age of ∼3 ka (∼3.5 ka VPR) from the top of a boulder on the best preserved part of the incised alluvial surface provides the most reliable maximum age for the onset of incision; 11 10Be ages from cobbles collected from the cultivated surface to the south yield younger ages, consistent with exhumation, erosion, and mechanical mixing during plowing. Our 18.6 +3.5/−3.3 mm a−1 rate is similar to other geologic slip rates measured along the NAF, all of which cluster between 15 and 20 mm a−1 over a wide range of (103–105) timescales. All of these geological rates, however, are slower than the ∼25 ± 2 mm a−1 short‐term rate of elastic strain accumulation measured geodetically. This disparity suggests the possibility that the NAF is experiencing a strain transient in which the lower crust beneath the fault is deforming faster than its long‐term rate.

Cosmogenic beryllium-10 and neon-21 dating of late Pleistocene glaciations in Nyalam, monsoonal Himalayas

We present cosmogenic 10Be and 21Ne chronologies from 21 erratic boulders on three moraine sequences in Nyalam county, monsoonal Himalayas, southern margin of the Tibetan Plateau. The surface exposure ages provide evidence for at least two distinct glacial advances during the late stage of the last glacial cycle and for one or more significantly older glaciations. The distribution of cosmogenic ages from the three ridges of the old moraine sequence is inconsistent with their stratigraphic order. Because exposure periods of the erratics prior to deposition on the moraine surface is shown to be small, the chronology–stratigraphy mismatch suggests that the cosmogenic ages do not date moraine deposition but most likely significant moraine ridge denudation and related boulder exhumation after initial deposition of the moraines during the penultimate glacial cycle or earlier. The surface exposure ages based on various currently accepted production rate scaling protocols yield age differences of up to 35% reflecting the poor knowledge of terrestrial cosmogenic production rates at low latitude/high altitude sites. Even within this conservative uncertainty range, our results do not yield evidence for late Pleistocene glaciations in monsoonal Tibet to be asynchronous to those in mid-latitudes on both hemispheres. There is an urgent need to improve the knowledge of terrestrial cosmogenic nuclide production rates and their scaling to low latitude regions to fully exploit the climate information archived in tropical moraine sequences.

Fossil cosmogenic 3He record from K–Ar dated basaltic flows of Mount Etna volcano (Sicily, 38°N): Evaluation of a new paleoaltimeter

This study investigates the reliability of a paleoaltimeter based on the elevation dependency of the cosmogenic nuclides production rate. The proposed method relies on “fossil” cosmogenic record. Indeed, measuring the cosmogenic nuclide concentration in an exposed and then buried lava flow combined with an independent estimate (by K–Ar or 40Ar / 39Ar dating) of the past exposure duration theoretically allows recording “fossil” cosmogenic production rates, which can thus be converted in paleoelevations. This approach was tested by measuring cosmogenic 3He ( 3He c) in olivines and clinopyroxenes of Quaternary K–Ar dated basaltic flows of Mount Etna volcano (Sicily, 38°N). The agreement within uncertainties (1 σ ≤ 500 m) between current and cosmogenic-derived elevations demonstrates the paleoaltimeter reliability. However, dating imprecision, Earth's paleomagnetic fluctuations and erosion contemporary to the flow exposure are parameters that may affect the accuracy and the precision of the method on older geological scales. Their respective influences were thus carefully evaluated and discussed to conclude that the paleoaltimeter can potentially reach resolutions better than ∼1000 m, even for Cenozoic records.

Monte Carlo simulations of low-energy cosmogenic neutron fluxes near the bottom of cliff faces

Cosmogenic radionuclide analysis for determining exposure ages and erosion rates is becoming more extensively utilized. Due to the complexity of the neutron absorption production avenue, the interpretation of 36Cl data is a complex problem, particularly for non-trivial landform geometries. This work provides a numerical, Monte Carlo simulation analysis of variations in in situ cosmogenic 36Cl production rate near the bottom of vertical cliffs. It is found that in comparison with production at height (more than a thermal neutron mean free path length in air) the production rate in the cliff face increases with decreasing height, but within a thermal neutron mean free path length in rock near the bottom of the cliff, the production rate exhibits a sharp decrease.

On scaling cosmogenic nuclide production rates for altitude and latitude using cosmic-ray measurements

The wide use of cosmogenic nuclides for dating terrestrial landforms has prompted a renewed interest in characterizing the spatial distribution of terrestrial cosmic rays. Cosmic-ray measurements from neutron monitors, nuclear emulsions and cloud chambers have played an important role in developing new models for scaling cosmic-ray neutron intensities and, indirectly, cosmogenic production rates. Unfortunately, current scaling models overlook or misinterpret many of these data. In this paper, we describe factors that must be considered when using neutron measurements to determine scaling formulations for production rates of cosmogenic nuclides. Over the past 50 years, the overwhelming majority of nucleon flux measurements have been taken with neutron monitors. However, in order to use these data for scaling spallation reactions, the following factors must be considered: (1) sensitivity of instruments to muons and to background, (2) instrumental biases in energy sensitivity, (3) solar activity, and (4) the way of ordering cosmic-ray data in the geomagnetic field. Failure to account for these factors can result in discrepancies of as much as 7% in neutron attenuation lengths measured at the same location. This magnitude of deviation can result in an error on the order of 20% in cosmogenic production rates scaled from 4300 m to sea level. The shapes of latitude curves of nucleon flux also depend on these factors to a measurable extent, thereby causing additional uncertainties in cosmogenic production rates. The corrections proposed herein significantly improve our ability to transfer scaling formulations based on neutron measurements to scaling formulations applicable to spallation reactions, and, therefore, constitute an important advance in cosmogenic dating methodology.

Determination of cosmogenic production rates of 10Be, 3He and 3H in water

To improve our understanding of present-day cosmogenic production systematics of 10Be, 3H and 3He, we exposed three sets of targets of purified water at altitudes of 620, 3810 and 4745 m in the Mont Blanc Massif of the French Alps. In addition, tanks were stored 1780 m underground to quantify 3He contributions from decay of “inherited” tritium initially present in the water. After analyses of 3He and 10Be, both the summit and tunnel 3H– 3He tanks were re-degassed and stored underground for an additional year. The stored summit tanks were then analyzed to determine cosmogenic 3H levels by the 3He ingrowth method, and the tunnel tanks used to re-determine inherited tritium. Production rates (in atoms per g H 2O per year) for direct production of 3He and 10Be were 1824±52 and 112±9; 1013±16 and 70±5; and 134±58 and 5.9±0.7 at the three elevations, respectively. We determined production ratios of 0.32±0.08 for 3H: 3He and 20.2±1.5 for ( 3H+ 3He): 10Be. Our 10Be production rates, when normalized for inter-laboratory calibration and for differences in geomagnetic latitude of exposure, are somewhat lower than results of a similar experiment undertaken by Nishiizumi et al. (1996). Our 3H: 3He ratio is consistent with theoretical and meteorite estimates (Kruger and D. Heymann, 1968), but considerably lower than values assumed in many exposure age studies of igneous rocks (e.g., (Kurz, 1986; Trull et al., 1995)).

Cosmogenic nuclide production rate systematics in terrestrial materials: Present knowledge, needs and future actions for improvement

The growing applications of cosmogenic nuclides produced in the Earth’s atmosphere, and in situ in a variety of terrestrial materials, as tracers in a wide ranging Earth science problems, has put a greater demand on accurate determination of the nuclide production rates in a variety of targets exposed in different settings on the Earth in the troposphere. The present state of our knowledge of cosmogenic production rates is reviewed briefly, in conjunction with the phenomenological and theoretical framework for: (i) the cosmic ray flux incident in the near Earth environment, (ii) the nucleonic cascade set off in the Earth’s atmosphere by the primary cosmic radiation, and (iii) the rate for production of nuclides in terrestrial materials, in widely different settings. These considerations set the stage for the diverse questions, which must be taken into account for determining the source functions of isotopic changes in terrestrial materials. We discuss the different approaches which have been adopted earlier to obtain the source strengths of nuclear interacting particles of the cosmic radiation, and direct measurements of nuclide production rates made by exposing targets to cosmic radiation at sea level and at mountain altitudes. We show that rapid progress in determining nuclide source functions with sufficient information on temporal variability is indeed expected in the near future as a result of: (i) dramatic improvements in the past 2–3 decades in our understanding of the character of propagation of cosmic radiation within the heliosphere, (ii) experiments now being conducted by a few groups to determine the source strengths of cosmic ray slow neutrons, and nuclide production rates in cosmic ray exposed targets, and finally (iii) the emergence of better nuclear codes which deal with the development of nucleonic cascades in the Earth’s atmosphere.

Direct measurement of the production rate of 14C near Earth's surface

Environmental 14C is well known for its application in carbon dating organic material, as well as its use as a geochemical tracer in the oceans and atmosphere. More recently, 14C in the form of atmospheric 14CO has been used to estimate the concentration of hydroxyl radical, which is the primary oxidant of the atmosphere. This latter application is dependent upon accurate knowledge of the cosmogenic production rate, which has been calculated, but not directly measured. We have measured the atmospheric production rate of 14C near the earth's surface on a 300‐meter research tower in Colorado, at the South Pole, at Scott Base, Antarctica, and near Mount Cook, New Zealand. The observed production rate near the surface was 50% of the production rate calculated by an earlier study. We also find that the ‘ground effect’, which causes an increase in the thermal neutron flux resulting in an enhancement of the production rate of 14C, is significantly smaller than calculated in earlier studies.

New evidence for enhanced cosmogenic isotope production rate in the atmosphere ≈ 37 kaBP from the Guliya ice core

A 36CI peak was found in the predicted section of Guliya ice core, from the Qinghai-Tibetan Plateau, at about 37 kaBP. This cannot be interpreted by means of changes in the accumulation rate, but by the enhanced cosmogenic isotope production rate in the atmosphere. Compared with the records of 10Be and 36CI in the other regions, the peaks of the cosmogenic isotopes are global and can be considered as time marks. An intriguing fact is that the peaks coincided with cold periods.