This study presents a summer temperature reconstruction using Scots pine tree-ring chronologies for Scotland allowing the placement of current regional temperature changes in a longer-term context. ...‘Living-tree’ chronologies were extended using ‘subfossil’ samples extracted from nearshore lake sediments resulting in a composite chronology >800 years in length. The North Cairngorms (NCAIRN) reconstruction was developed from a set of composite blue intensity high-pass and ring-width low-pass filtered chronologies with a range of detrending and disturbance correction procedures. Calibration against July–August mean temperature explains 56.4% of the instrumental data variance over 1866–2009 and is well verified. Spatial correlations reveal strong coherence with temperatures over the British Isles, parts of western Europe, southern Scandinavia and northern parts of the Iberian Peninsula. NCAIRN suggests that the recent summer-time warming in Scotland is likely not unique when compared to multi-decadal warm periods observed in the 1300s, 1500s, and 1730s, although trends before the mid-sixteenth century should be interpreted with some caution due to greater uncertainty. Prominent cold periods were identified from the sixteenth century until the early 1800s—agreeing with the so-called Little Ice Age observed in other tree-ring reconstructions from Europe—with the 1690s identified as the coldest decade in the record. The reconstruction shows a significant cooling response 1 year following volcanic eruptions although this result is sensitive to the datasets used to identify such events. In fact, the extreme cold (and warm) years observed in NCAIRN appear more related to internal forcing of the summer North Atlantic Oscillation.
Changes in penguin populations on the Antarctic Peninsula have been linked to several environmental factors, but the potentially devastating impact of volcanic activity has not been considered. Here ...we use detailed biogeochemical analyses to track past penguin colony change over the last 8,500 years on Ardley Island, home to one of the Antarctic Peninsula's largest breeding populations of gentoo penguins. The first sustained penguin colony was established on Ardley Island c. 6,700 years ago, pre-dating sub-fossil evidence of Peninsula-wide occupation by c. 1,000 years. The colony experienced five population maxima during the Holocene. Overall, we find no consistent relationships with local-regional atmospheric and ocean temperatures or sea-ice conditions, although the colony population maximum, c. 4,000-3,000 years ago, corresponds with regionally elevated temperatures. Instead, at least three of the five phases of penguin colony expansion were abruptly ended by large eruptions from the Deception Island volcano, resulting in near-complete local extinction of the colony, with, on average, 400-800 years required for sustainable recovery.
The Amundsen Sea Embayment (ASE) drains approximately 35% of the West Antarctic Ice Sheet (WAIS) and is one of the most rapidly changing parts of the cryosphere. In order to predict future ice sheet ...behaviour, modellers require long-term records of ice-sheet melting to constrain and build confidence in their simulations. Here, we present detailed marine geological and radiocarbon data along three palaeo-ice stream tributary troughs in the western ASE to establish vital information on the timing of deglaciation of the WAIS since the Last Glacial Maximum (LGM). We have undertaken multi-proxy analyses of the cores (core description, shear strength, x-radiographs, magnetic susceptibility, wet bulk density, total organic carbon/nitrogen, carbonate content and clay mineral analyses) in order to: (1) characterise the sedimentological facies and depositional environments; and (2) identify the horizon(s) in each core that would yield the most reliable age for deglaciation. In accordance with previous studies we identify three key facies, which offer the most reliable stratigraphies for dating deglaciation by recording the transition from a grounded ice sheet to open marine environments. These facies are: i) subglacial, ii) proximal grounding line, and iii) seasonal open marine. In addition, we incorporate ages from other facies (e.g., glaciomarine diamictons deposited at some distance from the grounding line, such as glaciogenic debris flows and iceberg-rafted diamictons and turbates) into our deglacial model. In total, we have dated 78 samples (mainly the acid insoluble organic (AIO) fraction, but also calcareous foraminifers), which include 63 downcore and 15 surface samples. Through careful sample selection prior to dating, we have established a robust deglacial chronology for this sector of the WAIS. Our data show that deglaciation of the western ASE was probably underway as early as 22,351 calibrated years before present (cal yr BP), reaching the mid-shelf by 13,837 cal yr BP and the inner shelf to within c.10–12 km of the present ice shelf front between 12,618 and 10,072 cal yr BP. The deglacial steps in the western ASE broadly coincide with the rapid rises in sea-level associated with global meltwater pulses 1a and 1b, although given the potential dating uncertainty, additional, more precise ages are required before these findings can be fully substantiated. Finally, we show that the rate of ice-sheet retreat increased across the deep (up to1600 m) basins of the inner shelf, highlighting the importance of reverse slope and pinning points in accelerated phases of deglaciation.
ABSTRACT
The offshore sector around Shetland remains one of the least well‐studied parts of the former British–Irish Ice Sheet with several long‐standing scientific issues unresolved. These key ...issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice‐Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km2, an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice‐sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi‐proxy suite of new isotopic age assessments, including 32 cosmogenic‐nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector‐wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP, this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km2, accompanied by several ice‐margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re‐growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km2 to ca. 15 000 km2 between 19 and 18 ka BP, stabilizing at ca. 2000 km2 by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP, and may have involved one or more subsidiary ice centres on now‐submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine‐influenced Shetland Ice Cap.
This contribution documents the process of assessing the quality of data within a compilation of legacy geochronological data relating to the last British-Irish Ice Sheet, a task undertaken as part ...of a larger community-based project (BRITICE-CHRONO) that aims to improve understanding of the ice sheet's deglacial evolution. As accurate reconstructions depend on the quality of the available data, some form of assessment is needed of the reliability and suitability of each given age(s) in our dataset. We outline the background considerations that informed the quality assurance procedures devised given our specific research question. We describe criteria that have been used to make an objective assessment of the likelihood that an age is influenced by the technique specific sources of geological uncertainty. When these criteria were applied to an existing database of all geochronological data relating to the last British-Irish Ice Sheet they resulted in a significant reduction in data considered suitable for synthesis. The assessed data set was used to test a Bayesian approach to age modelling ice stream retreat and we outline our procedure that allows us to minimise the influence of potentially erroneous data and maximise the accuracy of the resultant age models.
Glaciers flowing into the Amundsen Sea Embayment (ASE) account for >35% of the total discharge of the West Antarctic Ice Sheet (WAIS) and have thinned and retreated dramatically over the past two ...decades. Here we present detailed marine geological data and an extensive new radiocarbon dataset from the eastern ASE in order to constrain the retreat of the WAIS since the Last Glacial Maximum (LGM) and assess the significance of these recent changes. Our dating approach, relying mainly on the acid insoluble organic (AIO) fraction, utilises multi-proxy analyses of the sediments to characterise their lithofacies and determine the horizon in each core that would yield the most reliable age for deglaciation. In total, we dated 69 samples and show that deglaciation of the outer shelf was underway before 20,600 calibrated years before present (calyrBP), reaching the mid-shelf by 13,575calyrBP and the inner shelf to within ca. 150km of the present grounding line by 10,615calyrBP. The timing of retreat is broadly consistent with previously published radiocarbon dates on biogenic carbonate from the eastern ASE as well as AIO 14C ages from the western ASE and provides new constraints for ice sheet models. The overall retreat trajectory – slow on the outer shelf, more rapid from the middle to inner shelf – clearly highlights the importance of reverse bedslopes in controlling phases of accelerated groundling line retreat. Despite revealing these broad scale trends, the current dataset does not capture detailed changes in ice flow, such as stillstands during grounding line retreat (i.e., deposition of grounding zone wedges) and possible readvances as depicted in the geomorphological record.
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•We provide a comprehensive deglacial chronology for the eastern Amundsen Sea.•Data will help constrain ice sheet models.•Data show a consistent pattern of deglaciation across the Amundsen Sea Embayment.•Pattern of retreat highlights the importance of reverse bedslopes.
ABSTRACT
The findings of BRITICE‐CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice‐sheet marginal oscillation between ~31 and 16 ka, with seven distinctive ...former ice‐sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice‐sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice‐marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice‐dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice‐sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth.
The recent disintegration of Antarctic Peninsula ice shelves, and the associated accelerated discharge and retreat of continental glaciers, has highlighted the necessity of quantifying the current ...rate of Antarctic ice mass loss and the regional contributions to future sea-level rise. Observations of present day ice mass change need to be corrected for ongoing glacial isostatic adjustment, a process which must be constrained by geological data. However, there are relatively little geological data on the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene so the glacial isostatic correction remains poorly constrained. To address this we provide field constraints on the timing and rate of APIS deglaciation, and changes in relative sea-level (RSL) for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea-level fell from a maximum of c. 15 m above present at c. 8000 cal yr BP, at a rate of 3.91 mm yr
−1 declining to c. 2.11 mm yr
−1 between c. 6900–2900 cal yr BP, 1.63 mm yr
−1 between c. 2900–1800 cal yr BP, and finally to 0.29 mm yr
−1 during the last c. 1800 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic. It is in broad agreement with some glacio-isostatic adjustment models applied to this location, and with work undertaken elsewhere on the Antarctic Peninsula. These geological and RSL constraints from Beak Island imply significant thinning of the north-eastern APIS by the early Holocene. Further, they provide key data for the glacial isostatic correction required by satellite-derived gravity measurements of contemporary ice mass loss, which can be used to better assess the future contribution of the APIS to rising sea-levels.
► We provide new constraints on Holocene relative sea-level change for the Antarctic Peninsula. ► RSL on Beak Island fell from a maximum of 14.91 m at c. 8000 cal yr BP. ► There is no evidence of glacial still stands or advances on Beak Island. ► The Beak Island RSL curve constrains glacio-isostatic adjustment models for this location.
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