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>

A new Last Glacial-Interglacial Transition varve record from Chilean Patagonia

<p>High-resolution sedimentary archives, such as glaciolacustrine varve sequences formed in proglacial lakes, can enable detailed reconstructions of past glacier dynamics, assess regional to global climate (a)synchronicity and disentangle oceanic and atmospheric climatic forcing mechanisms. Specifically, glaciolacustrine varved sequences can be utilised to investigate, at an annual resolution, the rates and/or duration of change during deglaciation of a former glacial basin, significantly refining models of ice-sheet deglaciation. Additionally, the identification of tephra layers deposited within varved sequences offer the potential to provide isochronous marker layers, enabling high-precision correlations between sites and palaeoenvironmental archives, and an independent means of generating an absolute age for the varve chronology.</p><p>We present new varve data obtained from sediment sections in Chile Chico (-46.53<sup>o</sup>S, -71.73<sup>o</sup>W) in the Lago General Carrera (Chile)/Buenos Aires (Argentina) basin in central Patagonia. Here, an ice-contact, proglacial lake formed during the recession of an ice lobe of the former Patagonian Ice Sheet from its local Last Glacial Maximum position (18,778 ± 615 to 18,086 ± 214<sup>1</sup>). Sequences of laminated glaciolacustrine sediment accumulated in the palaeolake and have subsequently been exposed following lake drainage. We report on the detailed macro- and micro-facies of the Chile Chico sediments as well as an updated tephrostratigraphy for the region. In particular we (1) develop a process model for the formation of melt season and non-melt season components that suggest a varved origin; (2) present a ~3.5 kyr varve thickness record for Chile Chico that post-dates (<17 ka BP) the Fenix Chico Master Varve Chronology, and (3) detail pilot tephrochronological assessment of the sequences. Consideration is also given to the challenges associated with developing both the varve chronology, where detection of varves is variable and hiatuses and deformation exist within the sequences, and tephrochronology, where distinguishing between eruptive events with this level of stratigraphic resolution is unparalleled in the region.</p><p><em><sup>1</sup>Bendle, J.M., Palmer, A.P., Thorndycraft, V.R., Matthews, I. P. (2019) Phased Patagonian Ice Sheet response to Southern Hemisphere atmospheric and oceanic warming between 18 and 17 ka. Sci Rep 9, 4133</em></p>

Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10Be fallout, but 9Be and 10Be concentrations considered together show that the majority of 10Be is present in glacial sediment when it enters the lake, and only a minority of total 10Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10Be fallout that have similar properties as coeval 10Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10Be fallout variations. We attempted to match centennial-period variations in reconstructed 10Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10Be measurements on targeted sections of the NAVC.

A new glacial varve chronology along the southern Laurentide Ice Sheet that spans the Younger Dryas–Holocene boundary

Glacial varves can detail ice-margin positions and provide a proxy for meltwater discharge at resolutions comparable to those of the Greenland ice core archives, and thus they can be critical paleorecords for assessing the response of both ancient and modern ice sheets to climate change. Here we provide an ~1500 yr varve chronology straddling the Younger Dryas (YD)–Holocene boundary (11.65 cal. kyr B.P.), the first such chronology in North America. The varves are from glacial Lake Agassiz (central North America). The chronology is pinned on accelerator mass spectrometry radiocarbon-dated terrestrial macrofossils at the base of a widespread red-clay bed deposited during flooding from the Lake Superior basin. We illustrate the utility of this record by examining ice-margin retreat and melting through the late Younger Dryas and across the Holocene boundary. The ice margin receded at a constant rate, not only during the late YD, but for at least 300 yr after the onset of the Holocene. In contrast, varve thicknesses increased at the boundary, and a moraine formed over a 50 yr period, perhaps in response to the warming climate. Our expectation is that this time series will continue to be developed, expanded, and refined because it promises to be the longest and most geographically extensive glacial varve data set in North America.

Supplemental Material: A new glacial varve chronology along the southern Laurentide Ice Sheet that spans the Younger Dryas–Holocene boundary

Varve thickness data and ¹⁴C-date metadata.<br>

Supplemental Material: A new glacial varve chronology along the southern Laurentide Ice Sheet that spans the Younger Dryas–Holocene boundary

Varve thickness data and ¹⁴C-date metadata.<br>

Atmospherically-produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

We attempt to synchronize the North American Varve Chronology (NAVC) with the calendar year time scale by comparing records of atmospherically produced Be-10 fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18,200–12,500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same time scale to facilitate high-resolution correlation of climate events. Total Be-10 concentrations in NAVC sediments are within the range of those observed in other lacustrine records of Be-10 fallout, but Be-9 and Be-10 concentrations considered together show that the majority of Be-10 is present in glacial sediment when it enters the lake, and only a minority of total Be-10 derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial strategy to determine whether or not Be-10 fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in high-resolution sections of varve sequences was inconclusive: observed short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, we did observe centennial-period variations in Be-10 fallout that are replicated between separate varve sections and have similar magnitude and frequency as coeval Be-10 fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes, but are suppressed in paraglacial sections of the NAVC deposited in lakes lacking direct glacial sediment input, which leads us to conclude that proglacial lakes whose watershed likely includes a large portion of the ablation area of an ice sheet can effectively record Be-10 fallout. We matched observed centennial-scale Be-10 fallout variations in two segments of the NAVC to ice core Be-10 fallout records. Although the calibration of the NAVC to the calendar year time scale implied by these matches is similar to that proposed previously in independent calibrations based on radiocarbon data and correlation of climate events, matches for the two different segments disagree with each other and with the independent calibrations by 50–200 years. One of these matches is not consistent with independent evidence and is probably not valid, but the other is consistent with most, although not all, evidence and may be valid. This leaves several remaining ambiguities in whether or not Be-10 fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could likely be resolved by higher-resolution and replicate Be-10 measurements on targeted sections of the NAVC.