Semi-automated counting of complex varves through image autocorrelation

Annual resolution sediment layers, known as varves, can provide continuous and high-resolution chronologies of sedimentary sequences. In addition, varve counting is not burdened with the high laboratory costs of geochronological analyses. Despite a more than 100-year history of use, many existing varve counting techniques are time consuming and difficult to reproduce. We present countMYvarves, a varve counting toolbox which uses sliding-window autocorrelation to count the number of repeated patterns in core scans or outcrop photos. The toolbox is used to build an annually-resolved record of sedimentation rates, which are depth-integrated to provide ages. We validate the model with repeated manual counts of a high sedimentation rate lake with biogenic varves (Herd Lake, USA) and a low sedimentation rate glacial lake (Lago Argentino, Argentina). In both cases, countMYvarves is consistent with manual counts and provides additional sedimentation rate data. The toolbox performs multiple simultaneous varve counts, enabling uncertainty to be quantified and propagated into the resulting age-depth model. The toolbox also includes modules to automatically exclude non-varved portions of sediment and interpolate over missing or disrupted sediment. CountMYvarves is open source, runs through a graphical user interface, and is available online for download for use on Windows, macOS or Linux at https://doi.org/10.5281/zenodo.4031811.

The First Holocene Varve Chronology for the UK: based on the integration of varve counting, radiocarbon dating and tephrostratigraphy from Diss Mere (UK).

<p>This paper reports the first Holocene varved chronology for the lacustrine sediment record of Diss Mere in the UK. The record of Diss Mere is 15 m long, and shows 4.2 m of finely-laminated sediments, which are present between ca. 9 and 13 m of core depth. The microfacies analysis identified three major seasonal patterns of deposition, which corroborate the annual nature of sedimentation throughout the whole interval. The sediments are diatomaceous organic and carbonate varves with an average thickness of 0.45 mm. A total of 8473 varves were counted with maximum counting error of up to  40 varves by the bottom of the varved sequence. To tie the resulting floating varve chronology to the IntCal 2020 radiocarbon timescale, we used a Bayesian Deposition model (P_Sequencewith outlier detection) on all available chronological data from the core. The data included five radiocarbon dates, two known tephra layers (Glen Garry and OMH-185) with calendar ages based on Bayesian modelling of sequences of radiocarbon ages, and the relative varve counts between dated points. The resulting age-depth model (DISSV-2020) dates the varved sequence between ca. 2100 and 10,300 cal BP and age uncertainties are decadal in scale (95% confidence). </p>

Reanalysis of the Atmospheric Radiocarbon Calibration Record from Lake Suigetsu, Japan

ABSTRACTTerrestrial plant macrofossils from the sedimentary record of Lake Suigetsu, Japan, provide the only quasi-continuous direct atmospheric record of radiocarbon (14C) covering the last 50 ka cal BP (Bronk Ramsey et al. 2012). Since then, new high precision data have become available on U-Th dated speleothems from Hulu Cave China, covering the same time range (Cheng et al. 2018). In addition, an updated varve-based chronology has also been published for the 2006 core from Lake Suigetsu (SG06) based on extended microscopic analysis of the sediments and improved algorithms for interpolation (Schlolaut et al. 2018). Here we reanalyze the radiocarbon dataset from Suigetsu based on the new varve counting information and the constraints imposed by the speleothem data. This enables the new information on the calendar age scale of the Suigetsu dataset to be used in the construction of the consensus IntCal calibration curve. Comparison of the speleothem and plant macrofossil records provides insight into the mechanisms underlying the incorporation of carbon into different types of record and the relative strengths of different types of archive for calibration purposes.

Tephra anchored floating varve chronology covering ca. 19.0-11.0 ka BP in new core from Lake Lago Grande di Monticchio: preliminary results

<p>The high-resolution Monticchio (MON) sediment record has been demonstrated to be a key archive for reconstructing climate and environmental changes in the central Mediterranean for the last glacial-interglacial cycle. New sediment cores have been retrieved in April 2016 to investigate particularly the transition from the Last Glacial Maximum into the Holocene with a new high-resolution methodological approach. A floating varve chronology spanning ca. 8,000 years has been established by varve counting on thin sections using a petrographic microscope and layer thickness based sedimentation rate estimates for non- or poorly varved intervals. Varve counting is based on detailed seasonal deposition models of five different varve types. The resulting floating chronology consist of 66.6% individually counted varves and 33.4% interpolated years. The uncertainty estimate of the floating chronology has been determined by double counting and amounts to ±5.8%.</p><p>The floating chronology is anchored to an absolute chronology using the Agnano Pomici Principali tephra, dated at 11,999±52 cal yrs BP from paleosols overlying proximal tephra (Bronk Ramsey et al. 2015), is a suitable anchoring point to cross correlation. The resulting varve-based chronology has been compared with several other marker tephras dated elsewhere including the Soccavo 4 tephra (11,700±150 cal yrs BP), the Neapolitan Yellow Tuff (NYT; 14,194±172 cal yrs BP) and the Greenish tephra (19226±104 cal yrs BP). Further comparison with published (Hajdas et al. 1997) and new radiocarbon dates from different terrestrial macro remains are discussed in this paper. This study presents an independent chronology for the last glacial/interglacial transition for a comparison of MON data with high-resolution lake records western and central Europe.</p><p>References</p><p>Bronk Ramsey, C., P. G. Albert, S. P. E. Blockley, M. Hardiman, R. A. Housley, C. S. Lane, S. Lee, I. P. Matthews, V. C. Smith & J. J. Lowe (2015) Improved age estimates for key Late Quaternary European tephra horizons in the RESET lattice. Quaternary Science Reviews, 118<strong>,</strong> 18-32.</p><p>Hajdas, I., G. Bonani, B. Zolitschka, A. Brauer & J. Negendank (1997) 14C Ages of Terrestrial Macrofossils from Lago Grande Di Monticchio (Italy). Radiocarbon, 40<strong>,</strong> 803-807.</p>

Eastern Mediterranean climate reconstruction over the last 12,000 yrs based on a non-continuous varve sediment record from Vouliagmeni lake, Gulf of Corinth (Greece)

<p>Understanding the mechanisms that formed past climatic and environmental changes is essential in order to produce models of future climatic trends. Climatically highly sensitive areas like the Eastern Mediterranean are characterized as key sites for those studies and can imprint regional and large-scale atmospheric patterns as well as the impact of those changes into early human societies. Still, the distinction between regional and global climatic signals is challenging, due to variations between chronological control and environmental factors occurring in each study site. Annually laminated sediments assist to that problem since they can provide information on temperature fluctuations, precipitation, volcanism, solar activity etc, up to seasonal scale through coherent varve chronology. In this study, we present a multi-proxy climate reconstruction of eastern Mediterranean over the last 12000 years, based on a non-continuous varved sediment core from Vouliagmeni lake, located in the eastern part of Gulf of Corinth, Greece. The compiled dataset consists of: (a) grain size analysis and magnetic susceptibility measurements, (b) high-resolution X-ray fluorescence data, (c) mineralogical analysis, (d) Computed Tomography (CT) and μCT analysis (e) AMS radiocarbon dating correlated with varve counting, (f) isotopic composition (δ<sup>18</sup>O, δ<sup>13</sup>C) on selected samples and (g) diatom analysis. For the determination of lamination boundaries and thickness, standard Computed Tomography was conducted with the highest possible resolution (0.3 mm) and combined with μCT results from selected sections from the core. Sedimentological changes were also documented through the 2D Haunsfield model that was constructed for the core and further correlated with the other studied proxies. The chronological framework of the core was established at 12,500 cal BP through a combination of Bayesian age-depth modelling and varve counting. Accumulation rates change drastically at around 3000 cal BP at the approximate time of extensive urban growth in the study area, as indicated from historical reports. Different precipitation/temperature and runoff patterns for the catchment area were recognized, through the different proxies examined, providing signals of long scale and regional climatic anomalies. The covariation of δ<sup>13</sup>C<sub>carb</sub> and δ<sup>18</sup>O characterizes the lake system as evaporatively dominant, whereas elemental ratios and isotopic data determine wet (1200-1800 cal BP, 2200-300 cal BP, 4500-4800 cal BP, 5800-6600 cal BP, 9500-10500 cal BP) and arid phases (3000-3200 cal BP, 3800 cal BP, 7100 cal BP) that correspond also to changes in water level, stratification and the formation of laminations. Addressing the fact that laminated sediments in Eastern Mediterranean that extend in 12 ka years are scarce, Vouliagmeni lake seems to present a very promising geoarchive.</p>

Varve microfacies and varve preservation record of climate change and human impact for the last 6000 years at Lake Tiefer See (NE Germany)

The Holocene sediment record of Lake Tiefer See exhibits striking alternations between well-varved and non-varved intervals. Here, we present a high-resolution multi-proxy record for the past ~6000 years and discuss possible causes for the observed sediment variability. This approach comprises microfacies, geochemical and microfossil analyses and a multiple dating concept including varve counting, tephrochronology and radiocarbon dating. Four periods of predominantly well-varved sediment were identified at 6000–3950, 3100–2850 and 2100–750 cal. a BP and AD 1924–present. Except of sub-recent varve formation, these periods are considered to reflect reduced lake circulation and consequently, stronger anoxic bottom water conditions. In contrast, intercalated intervals of poor varve preservation or even extensively mixed non-varved sediments indicate strengthened lake circulation. Sub-recent varve formation since AD 1924 is, in addition to natural forcing, influenced by enhanced lake productivity due to modern anthropogenic eutrophication. The general increase in periods of intensified lake circulation in Lake Tiefer See since ~4000 cal. a BP presumably is caused by gradual changes in the northern hemisphere orbital forcing, leading to cooler and windier conditions in Central Europe. Superimposed decadal- to centennial-scale variability of the lake circulation regime is likely the result of additional human-induced changes of the catchment vegetation. The coincidence of major non-varved periods at Lake Tiefer See and intervals of bioturbated sediments in the Baltic Sea implies a broader regional significance of our findings.