Biomarker proxy shows potential for studying the entire Quaternary Arctic sea ice history

A novel and promising biomarker proxy for reconstruction of Arctic sea ice conditions was developed and is based on the determination of a highly branched isoprenoid with 25 carbons (IP₂₅). IP₂₅ records have been restricted to the last 150 kyr BP. We present a biomarker record from Ocean Drilling Program (ODP) Site 912, going back to the Pliocene–Pleistocene boundary and indicating that sea ice of variable extent occurred in the Fram Strait/southern Yermak Plateau area at least since about 2.2 Ma. Furthermore, our data support the idea that a combination of IP₂₅ and open water, phytoplankton biomarker data (“PIP₂₅ index”) may give a more reliable and quantitative estimate of past sea ice cover (at least for the study area). The study reveals that the novel IP₂₅/PIP₂₅ biomarker approach has potential for semi-quantitative paleo-sea ice studies covering the entire Quaternary and could motivate further detailed high resolution research on ODP/IODP material using this proxy.     

Lateglacial and early Holocene climatic oscillations on the western Svalbard margin,European Arctic

A multi proxy sediment core record on the continental margin off western Svalbard, European Arctic, reflects large climatic and oceanographic oscillations at the Lateglacial–early Holocene transition. Based on studies of planktonic foraminifera, their stable oxygen and carbon isotopic composition and ice rafted debris, we have reconstructed the last 14 cal. ka BP. The period 14–13.5 cal. ka BP was characterized by highly unstable climatic conditions. Short-lived episodes of warming alternated with meltwater pulses and enhanced iceberg rafting. This period correlates to a regional warming of the northern North Atlantic. An overall decrease in meltwater took place during the deglaciation (14–10.8 cal. ka BP). The late Younger Dryas and subsequent transition into the early Holocene is characterized by a reduced flux of planktonic foraminifera and increased iceberg rafting. A major warming took place from 10.8 to 9.7 cal. ka BP, the influence of meltwater ceased and the flux of warm Atlantic Water increased. From 9.7 to 8.8 cal. ka BP, the western Svalbard margin surface waters were significantly warmer than today. This warm period, the thermal maximum, was followed by an abrupt cooling at 8.8. cal. ka BP, caused by an increased influence of Arctic Water from the Arctic Ocean. The results document that the European Arctic was very sensitive to climatic and oceanographic changes at the end of the last glacial and during the Holocene.     

Holocene climate and glacial history of the northeastern Antarctic Peninsula: the marine sedimentary record from a long SHALDRIL core

A high-resolution record of Holocene deglacial and climate history was obtained from a 77 m sediment core from the Firth of Tay, Antarctic Peninsula, as part of the SHALDRIL initiative. This study provides a detailed sedimentological record of Holocene paleoclimate and glacial advance and retreat from the eastern side of the peninsula. A robust chronostratigraphy was derived from thirty-three radiocarbon dates on carbonate material. This chronostratigraphic framework was used to establish the timing of glacial and climate events derived from multiple proxies including: magnetic susceptibility, electric resistivity, porosity, ice-rafted debris content, organic carbon content, nitrogen content, biogenic silica content, and diatom and foraminiferal assemblages. The core bottomed-out in a stiff diamicton interpreted as till. Gravelly and sandy mud above the till is interpreted as proximal glaciomarine sediment that represents decoupling of the glacier from the seafloor circa 9400 cal. yr BP and its subsequent landward retreat. This was approximately 5000 yr later than in the Bransfield Basin and South Shetland Islands, on the western side of the peninsula. The Firth of Tay core site remained in a proximal glaciomarine setting until 8300 cal. yr BP, at which time significant glacial retreat took place. Deposition of diatomaceous glaciomarine sediments after 8300 cal. yr BP indicates that an ice shelf has not existed in the area since this time.The onset of seasonally open marine conditions between 7800 and 6000 cal. yr BP followed the deglacial period and is interpreted as the mid-Holocene Climatic Optimum. Open marine conditions lasted until present, with a minor cooling having occurred between 6000 and 4500 cal. yr BP and a period of minor glacial retreat and/or decreased sea ice coverage between 4500 and 3500 cal. yr BP. Finally, climatic cooling and variable sea ice cover occurred from 3500 cal. yr BP to near present and it is interpreted as being part of the Neoglacial. The onset of the Neoglacial appears to have occurred earlier in the Firth of Tay than on the western side of the Antarctic Peninsula. The Medieval Warm Period and Little Ice Age were not pronounced in the Firth of Tay. The breadth and synchroneity of the rapid regional warming and glacial retreat observed in the Antarctic Peninsula during the last century appear to be unprecedented during the Holocene epoch.     

Holocene glacier and deep water dynamics,Adélie Land region,East Antarctica

This study presents a high-resolution multi-proxy investigation of sediment core MD03-2601 and documents major glacier oscillations and deep water activity during the Holocene in the Adélie Land region, East Antarctica. A comparison with surface ocean conditions reveals synchronous changes of glaciers, sea ice and deep water formation at Milankovitch and sub-Milankovitch time scales. We report (1) a deglaciation of the Adélie Land continental shelf from 11 to 8.5 cal ka BP, which occurred in two phases of effective glacier grounding-line retreat at 10.6 and 9 cal ka BP, associated with active deep water formation; (2) a rapid glacier and sea ice readvance centred around 7.7 cal ka BP; and (3) five rapid expansions of the glacier–sea ice systems, during the Mid to Late Holocene, associated to a long-term increase of deep water formation. At Milankovich time scales, we show that the precessionnal component of insolation at high and low latitudes explains the major trend of the glacier–sea ice–ocean system throughout the Holocene, in the Adélie Land region. In addition, the orbitally-forced seasonality seems to control the coastal deep water formation via the sea ice–ocean coupling, which could lead to opposite patterns between north and south high latitudes during the Mid to Late Holocene. At sub-Milankovitch time scales, there are eight events of glacier–sea ice retreat and expansion that occurred during atmospheric cooling events over East Antarctica. Comparisons of our results with other peri-Antarctic records and model simulations from high southern latitudes may suggest that our interpretation on glacier–sea ice–ocean interactions and their Holocene evolutions reflect a more global Antarctic Holocene pattern.     

Enhanced rainfall in the Western Mediterranean during deposition of sapropel S1: stalagmite evidence from Corchia cave (Central Italy)

A stable isotope record from a stalagmite collected from Antro del Corchia cave (Apuan Alps, Central Italy), supported by 17 uranium-series ages, indicates enhanced regional rainfall between ca 8.9 and 7.3 kyr cal. BP at the time of sapropel S1 deposition. Within this phase, the highest rainfall occurred between 7.9 and 7.4 kyr cal. BP. Comparison with different marine and lake records, and in particular with the Soreq Cave record (Israel), suggests substantial in-phase occurrence of enhanced rainfall between the Western and Eastern Mediterranean basins. There is no convincing evidence for major climatic change at the time of the “8.2 kyr event”.     

Calibrating a glaciological model of the Greenland ice sheet from the Last Glacial Maximum to present-day using field observations of relative sea level and ice extent

We constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. Our new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. Our results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.     

Holocene glacier history of Bjørnbreen and climatic reconstruction in central Jotunheimen,Norway, based on proximal glaciofluvial stream-bank mires

Holocene variations of Bjørnbreen, Smørstabbtinden massif, west-central Jotunheimen are reconstructed from the lithostratigraphy of two alpine stream-bank mires flooded episodically by meltwater. The approach uses multiple sedimentological indicators (weight loss-on-ignition, mean grain size, grain-size fractions, bulk density, moisture content and magnetic susceptibility), an a priori model of overbank deposition of suspended glaciofluvial sediments, a detailed chronology based on 56 radiocarbon dates, and a Little Ice Age sedimentological analogue. Rapid, late-Preboreal deglaciation was indicated by immigration of Betula pubescens by 9700 cal. BP. An interval of at least 3000 years in the early Holocene when glaciers were absent was interrupted by two abrupt episodes of glacier expansion around the time of the Finse Event, the first at ca 8270–7900 cal. BP (Bjørnbreen I Event) and the second at ca 7770–7540 cal. BP (Bjørnbreen II Event). Neoglaciation began shortly before ca 5730 cal. BP with gradual build-up to the maximum of the Bjørnbreen III Event at ca 4420 cal. BP. Later maxima occurred at ca 2750 cal. BP (Bjørnbreen IV Event) and at 1300, 1260, 1060 and 790 cal. BP (all within the Bjørnbreen V Event). Glaciers were smaller than today and possibly melted away on several occasions in the late Holocene (ca 3950, 1410 and 750 cal. BP). Minor maxima also occurred at ca 660 and 540 cal. BP, within the late Mediaeval Warm Period and the early Little Ice Age, respectively. The Little Ice Age maximum was dated to 213±25 BP (ca 205 cal. BP). The relative magnitudes of the main glacier maxima were determined: Erdalen Event>Little Ice Age Event (Bjørnbreen VI)>Bjørnbreen I (Finse Event) ≈ Bjørnbreen II>Bjørnbreen V⩾Bjørnbreen IV>Bjørnbreen III. These episodic events of varying magnitude and abruptness were used in conjunction with an independent summer-temperature proxy to reconstruct variations in equilibrium-line altitude (ELA) and a Holocene record of winter precipitation. Since the Preboreal, ELA varied within a range of about 390 m, and winter precipitation ranged between 40 and 160% of modern values. Winter precipitation variations appear to have been the main cause of these century- to millennial-scale Holocene glacier variations.     

Relative sea-level changes during the Holocene in Bangladesh

This paper presents a reconstruction of the Holocene paleo-environment in the central part of Bangladesh in relation to relative sea-level changes 200 km north of the present coastline. Lithofacies characteristics, mangal peat, diatoms and paleophysiographical evidence were considered to reconstruct the past position and C-14 ages were used to determine the time of formation of the relative sea level during the Holocene. With standard reference datum, the required m.s.l. at the surface of five sections was calculated. The relative sea-level (RSL) curve suggests that Bangladesh experienced two mid-Holocene RSL transgressions punctuated by regressions. The curve shows an RSL highstand at approximately 7500 cal BP, although the height of this highstand could not be determined because the transgressive phase was observed in a bioturbated sand flat facies. The curve shows a regression of approximately 6500 cal BP, and the RSL was considerably lower, perhaps 1–2 m, than the present m.s.l. The abundant marine diatoms and mangrove pollens indicate the highest RSL transgression in Bangladesh at approximately 6000 cal BP, being at least 4.5 to 5 m higher than the modern m.s.l. After this phase, the relative sea level started to fall, and consequently, a freshwater peat developed at approximately 5980–5700 cal BP. The abundant mangrove pollens in the salt-marsh succession shows the regression at approximately 5500 cal BP, when it was 1–2 m higher than the modern sea level. The curve indicates that at approximately 5000 cal BP and onwards, the RSL started to fall towards its present position, and the present shoreline of Bangladesh was established at approximately 1500 cal BP and has not noticeably migrated inland since.     

Testing models of mid to late Holocene sea-level change,North Queensland,Australia

Understanding the nature of global ice-equivalent eustatic sea-level changes during the mid to late Holocene is important to our understanding of how ice sheets will respond to future climate change. This study re-analyses the indicative meaning and age control of existing relative sea-level (RSL) data from Cleveland Bay, North Queensland, Australia and presents new RSL data from a foraminifera-based transfer function as a preliminary test of global geophysical models in this region during the mid to late Holocene. The foraminifera-based transfer function produces reliable RSL estimates, consistent through the mid to late Holocene at different locations in Cleveland Bay. Analysis of the combined RSL database reveals that RSL rose above present between 8 and 6.2 ka cal. BP, with the peak of the sea-level highstand c. 2.8 m above present at c. 5 ka cal. BP, remaining relatively stable above +1.5 m from 6.2 until at least 2.3 ka cal. BP, falling to present in the last millennia. This long period of sea level above present in the mid to late Holocene suggests a gradual rather than abrupt end to global ice melt, which must have continued into the late Holocene. This new analysis also shows no evidence for episodic fluctuations within the highstand, although they cannot be entirely ruled out by this study. This study demonstrates that more sea-level data needs to be collected from locations uncontaminated by glacio-isostasy, hydro-isostasy and tectonic effects, in order to better constrain the late Holocene melt histories of the large polar ice sheets.     

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.     

Out-of-phase evolution between summer and winter East Asian monsoons during the Holocene as recorded by Chinese loess deposits

We analyzed climate proxies from loessic-soil sections of the southern Chinese Loess Plateau. The early Holocene paleosol, S0, is 3.2 m thick and contains six sub-soil units. Co-eval soils from the central Loess Plateau are thinner (~ 1 m). Consequently higher-resolution stratigraphic analyses can be made on our new sections and provide more insight into Holocene temporal variation of the East Asian monsoon. Both summer and winter monsoon evolution signals are recorded in the same sections, enabling the study of phase relationships between the signals. Our analyses consist of (i) measurements of magnetic properties sensitive to the production of fine-grained magnetic minerals which reflect precipitation intensity and summer monsoon strength; and (ii) grain-size analyses which reflect winter monsoon strength. Our results indicate that the Holocene precipitation maximum occurred in the mid-Holocene, ~ 7.8–3.5 cal ka BP, with an arid interval at 6.3–5.3 cal ka BP. The winter monsoon intensity declined to a minimum during 5.0–3.4 cal ka BP. These results suggest that the East Asian summer and winter monsoons were out of phase during the Holocene, possibly due to their different sensitivities to ice and snow coverage at high latitudes and to sea-surface temperature at low latitudes.     

Ice-rafted detritus evidence from 40Ar/39Ar ages of individual hornblende grains for evolution of the eastern margin of the Laurentide ice sheet since 43 14C ky

During the last glacial interval, the North Atlantic ice sheets expanded and contracted in approximate synchronicity with orbitally forced global climate change. Variation in ice rafted detritus content in North Atlantic marine sediment cores record the waxing and waning of glaciers, as well as the abrupt temperature changes at millennial time scales. The background variations of ice rafting are punctuated by Heinrich layers, which appear to record the catastrophic collapse of the Laurentide ice sheet through the Hudson Strait. The objective of this paper is to document the evolution of glaciation on Laurentia during the last 43 ¹⁴C kyr. We present a provenance study based on ⁴⁰Ar/³⁹Ar dates of individual hornblende grains from 57 samples taken at 2 cm spacing between 4 and 134 cm from core V23-14 (43.4°N, 45.25°W, 3177 m). Sedimentation rates outside of the Heinrich layers are very low in this core, but the Heinrich layers are easily identified. Laurentide glaciation did not extend into the ocean south of 55°N until about 26 ¹⁴C kyr, and retreated to the coastline or beyond by 14 ¹⁴C kyr. Documenting the history of this major ice sheet has significant implications for understanding ice rafting sources in more distal locations where mixing among different ice sheets is likely.     

The role of the ocean feedback on Asian and African monsoon variations at 6 kyr and 9.5 kyr BP

The role of ocean feedback on monsoon variations at 6 and 9.5 kyr Before Present (BP) compared to present-day is investigated by using sets of simulations computed with the IPSL–CM4 ocean–atmosphere coupled model and simulations with the atmospheric model only with the SST prescribed to the present-day simulation for the coupled model. This work is complementary to the study by Marzin and Braconnot (2009) who have analyzed in detail the response of Indian and African monsoons to changes in insolation at 6 and 9.5 kyr BP using the IPSL–CM4 coupled model. The monsoon rainfall was intensified at 6 and 9.5 kyr BP compared to 0 kyr BP as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. In this paper, the impact of the ocean feedback is analysed for the Indian, East-Asian and African monsoons. The response of the ocean to the 6 and 9.5 kyr BP insolation forcing shares similarities between the two periods, but we highlight local differences and a delay in the response of the surface ocean between 6 and 9.5 kyr BP. The ocean feedback is shown to be positive for the early stage of the African monsoon. A dipole of SST in the tropical Atlantic favouring the earlier build-up of the monsoon in the 6 and 9.5 kyr BP coupled simulations. However, it is strongly negative for the Indian and East Asian monsoons, and of stronger amplitude at 9.5 than at 6 kyr BP over India. In these Asian regions, the convection is more active over the ocean than over the continent during the late monsoon season due to the ocean feedback. The results are consistent with previous studies about 6 kyr BP climate. In addition, it is shown that the ocean feedback is not sufficient to explain the relative amplifications of the different monsoon systems within the three periods of the Holocene, but that the mechanisms such as the effect of the precession on the seasonal cycle of monsoons as discussed in Marzin and Braconnot (2009) are more plausible.     

A 11,000 yr palaeotemperature reconstruction from the southern boreal zone in Finland

The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000 cal yr BP. At 8000–4500 cal yr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800 cal yr BP. From 4500 cal yr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ¹⁸O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.     

Synchronous climate anomalies in the western North Pacific and North Atlantic regions during the last 14,000 years

A peat cellulose δ¹⁸O record spanning around 14,000 years from the Hani peat mire in northeastern China reveals several abrupt temperature anomalies in the period from the last deglaciation through the Holocene. The timing of these anomalies coincides well with the notable cooling events recorded respectively using the GISP2 ice core and ice-rafted sediment of the North Atlantic Ocean, such as the Older Dryas, Inter-Allerød, Younger Dryas, and the nine ice-rafted debris events. The results demonstrate that this repeating pattern of abrupt temperature deterioration is not limited to the North Atlantic area at high latitude but also exists in the western North Pacific region at middle latitude. The synchronous temperature anomalies possibly are resulted from the joint effects of meltwater discharge into the North Atlantic Ocean and reduced solar activity. In the period from around 8600 to 8200 cal. yrs BP the Hani peat record shows a broad δ¹⁸O peak that may reflect compound climate signals resulting from the two kinds of forcing factors: the temperature drop related to reduced solar activity at around 8600–8250 cal. yrs BP, and the temperature anomaly attributed to the meltwater effect at around 8220 ± 70 cal. yrs BP. This result may provide palaeo-temperature evidence for existence of the sharp “8.2 k” event in the western North Pacific region. In addition, our results have revealed that in the period from the last deglaciation through the Holocene the synchronous temperature anomalies before and after the “8.2 k” event seem to be related to meltwater outflow and reduced solar activity, respectively. It is important that the all temperature anomalies—whether because of reduced solar activity in the late Holocene or from meltwater discharge in the early Holocene—are accompanied by an abrupt decline in the Indian Ocean summer monsoon and abrupt strengthening of the East Asian summer monsoon. It is likely that reduced solar activity and meltwater outflow appear to modulate Earth system changes in the same direction. The influences could be compounded. Reduced solar activity and meltwater outburst both appear to act as triggers for occurrence of the El Niño phenomenon in the equatorial Pacific Ocean, which may result in broad teleconnections between the temperature anomaly in the Northern Hemisphere and abrupt variation of the Asian monsoon.     

Diatom-inferred total phosphorus from dystrophic Lake Arapisto,Finland, in relation to Holocene paleoclimate

A sediment core from Lake Arapisto, Finland, was examined for fossil diatom assemblages to reconstruct changes in Holocene nutrient availability. Our aim was to investigate the long-term relationship between lake trophic status and climate by comparing the diatom-based phosphorus reconstruction with paleoclimatic proxies. Our results showed that the cold early Holocene was characterized by elevated nutrient conditions concurrent with newly exposed fertile ground. As the climate rapidly warmed and ice sheet further retreated, the catchment vegetation developed, which resulted in decreased nutrient flux into the lake. The Holocene Thermal Maximum (HTM), between ~ 8000 and 4000 cal yr BP, was characterized by oligotrophic conditions, which may have been caused by low effective precipitation and stable watershed vegetation. After the HTM, the lake became more productive. There was no particular increase in the trophic state that could be connected to more recent human influence. Although lake productivity has been shown to be affected by temperature, our record indicated that the nutrient dynamics were driven by complex interactions between changes in temperature, precipitation, catchment, and in-lake processes. Understanding of long-term nutrient dynamics and the associated processes can help in resolving relationships between lake productivity and climate during past and present climate changes.     

Major changes in ice stream dynamics during deglaciation of the north-western margin of the Laurentide Ice Sheet

Victoria Island lies at the north-western limit of the former North American (Laurentide) Ice Sheet in the Canadian Arctic Archipelago and displays numerous cross-cutting glacial lineations. Previous work suggests that several ice streams operated in this region during the last (Wisconsinan) glaciation and played a major role in ice sheet dynamics and the delivery of icebergs into the Arctic Ocean. This paper produces the first detailed synthesis of their behaviour from the Last Glacial Maximum through to deglaciation (～21–9.5 cal ka BP) based on new mapping and a previously published radiocarbon-constrained ice sheet margin chronology. Over 70 discrete ice flow events (flow-sets) are ‘fitted’ to the ice margin configuration to allow identification of several ice streams ranging in size from large and long-lived (thousands of years) to much smaller and short-lived (hundreds of years). The reconstruction depicts major ice streams in M'Clure Strait and Amundsen Gulf which underwent relatively rapid retreat from the continental shelf edge at some time between ～15.2 and 14.1 cal ka BP: a period which encompasses climatic warming and rapid sea level rise (meltwater pulse-1a). Following this, overall retreat was slower and the ice streams exhibited asynchronous behaviour. The Amundsen Gulf Ice Stream continued to operate during ice margin retreat, whereas the M'Clure Strait Ice Stream ceased operating and was replaced by an ice divide within ～1000 years. This ice divide was subsequently obliterated by another short-lived phase of ice streaming in M'Clintock Channel ～13 cal ka BP. The timing of this large ice discharge event coincides with the onset of the Younger Dryas. Subsequently, a minor ice divide developed once again in M'Clintock Channel, before final deglaciation of the island shortly after 9.5 cal ka BP. It is concluded that large ice streams at the NW margin of the Laurentide Ice Sheet, equivalent in size to the Hudson Strait Ice Stream, underwent major changes during deglaciation, resulting in punctuated delivery of icebergs into the Arctic Ocean. Published radiocarbon dates constrain this punctuated delivery, as far as is possible within the limits imposed by their precision, and we note their coincidence with pulses of meltwater delivery inferred from numerical modelling and ocean sediment cores.     

Lateglacial to early Holocene multiproxy record from Loch Assynt,NW Scotland

A core, recovered from a water depth of 53 m in Loch Assynt, North-West Scotland, has yielded a 9 m sequence comprising two distinct units, an upper, organic-rich unit (Unit I, ca. 6 m) overlying a sequence of laminated clays, silts and sands (Unit II, ca. 3 m). The upper unit is essentially Holocene in age based upon three bulk AMS radiocarbon dates while a fourth radiocarbon date from Unit II confirms a late-glacial age for that interval and supports a broadly linear age–depth relationship. Distinct variations in the magnetic susceptibility record of the lower unit can be visually correlated to major changes in the Greenland ice core (GISP2), this together with pollen evidence supports the radiocarbon dating suggesting an age of approximately 11,000 to around 17,000 cal. BP for Unit II, with evidence for the Younger Dryas (Loch Lomond) stadial and the Bolling–Allerød climatic phases. Variations in the magnetic susceptibility record of the late-glacial sediments are thought to relate to climatically driven changes in soil cover and erosion rates. The multiproxy record from Loch Assynt indicates relatively continuous, sub-aqueous sedimentation during the last ～17,000 years, providing an approximate age for the initiation of modern Loch Assynt and supporting recent dates of moraine retreat lines in the Loanan Valley from about 14–15 ka BP. Pollen and chironomid sampling provides further insights to the history of this relatively deep water body and compliment existing high-resolution palaeo-precipitation records for the mid to late Holocene interval from speleothem archives within the loch catchment.     

Holocene sea-level changes along the North Carolina Coastline and their implications for glacial isostatic adjustment models

We have synthesized new and existing relative sea-level (RSL) data to produce a quality-controlled, spatially comprehensive database from the North Carolina coastline. The RSL database consists of 54 sea-level index points that are quantitatively related to an appropriate tide level and assigned an error estimate, and a further 33 limiting dates that confine the maximum and minimum elevations of RSL. The temporal distribution of the index points is very uneven with only five index points older than 4000 cal a BP, but the form of the Holocene sea-level trend is constrained by both terrestrial and marine limiting dates. The data illustrate RSL rapidly rising during the early and mid Holocene from an observed elevation of ?35.7 ± 1.1 m MSL at 11062–10576 cal a BP to ?4.2 m ± 0.4 m MSL at 4240–3592 cal a BP.We restricted comparisons between observations and predictions from the ICE-5G(VM2) with rotational feedback Glacial Isostatic Adjustment (GIA) model to the Late Holocene RSL (last 4000 cal a BP) because of the wealth of sea-level data during this time interval. The ICE-5G(VM2) model predicts significant spatial variations in RSL across North Carolina, thus we subdivided the observations into two regions. The model forecasts an increase in the rate of sea-level rise in Region 1 (Albemarle, Currituck, Roanoke, Croatan, and northern Pamlico sounds) compared to Region 2 (southern Pamlico, Core and Bogue sounds, and farther south to Wilmington). The observations show Late Holocene sea-level rising at 1.14 ± 0.03 mm year^?1 and 0.82 ± 0.02 mm year^?1 in Regions 1 and 2, respectively. The ICE-5G(VM2) predictions capture the general temporal trend of the observations, although there is an apparent misfit for index points older than 2000 cal a BP. It is presently unknown whether these misfits are caused by possible tectonic uplift associated with the mid-Carolina Platform High or a flaw in the GIA model. A comparison of local tide gauge data with the Late Holocene RSL trends from Regions 1 and 2 support the spatial variation in RSL across North Carolina, and imply an additional increase of mean sea level of greater than 2 mm year^?1 during the latter half of the 20th century; this is in general agreement with historical tide gauge and satellite altimetry data.     

Holocene thermal maximum in the western Arctic (0–180°W)

The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0–180°W; north of ∼60°N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6±0.8°C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12–10 ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9 ka, about 4000 yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet.