Paul C. Knutz

New data for the Last Glacial Maximum in Great Britain and Ireland

Abstract Understanding the history of the British and Irish Ice Sheet (BIIS) at the Last Glacial Maximum (LGM) has been advanced by new approaches, in particular, by cosmogenic nuclide surface-exposure dating, aminostratigraphy of ‘shelly’ glacial deposits, AMS radiocarbon dating, and the evidence from continental margin marine cores, all of which supersede the previously weak geochronologic control. It was formerly believed that Great Britain and Ireland was largely ice free between the last interglacial (oxygen isotope sub-stage 5e) and the Late Devensian, when the LGM occurred. As such the BIIS was effectively out of phase with Laurentide and Scandinavian ice sheets, as well as inferences of ice volume from oxygen isotope stratigraphy. The BIIS during the Late Devensian maximum was also perceived as having been relatively stationary. New evidence shows that the LGM was an important event during the evolution of an earlier BIIS when the extent of ice was greater. Repeated iceberg rafting events over the past 50xa0ka are shown by marine cores, while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36 Cl ages on glaciated surfaces and glacial boulders, that are indicative of deglacial events between 40 and 12xa0ka. These appear to be associated with Heinrich events, the earliest being inferred as Heinrich 4 at about 40xa0ka. During this advance, the BIIS and Scandinavian Ice Sheet (SIS) were in contact and all of Ireland was glaciated. The ice sheet appears to have fluctuated several times between 40 and 25xa0ka, although evidence for this is poorly preserved. But the 36 Cl and 14 C evidence is clear that the BIIS reached its LGM maximum size about 22xa0ka, soon after Heinrich Event 2, when the BIIS and SIS were not in contact. One cluster of 36 Cl and 14 C ages, at 21.4±1.3xa0ka, records an initial pulse of deglaciation that was followed by extensive deglaciation about 17.4±0.4xa0ka just before Heinrich Event 1, when the ice sheet readvanced. Contrary to previous views, the BIIS probably existed throughout much of Devensian time as a mobile and sensitive ice sheet, during which the LGM advance was but one important event. In places, glacial deposits of the earlier Devensian glaciation have previously been incorrectly identified as products of the later LGM glaciation.

Millennial-scale depositional cycles related to British Ice Sheet variability and North Atlantic paleocirculation since 45 kyr B.P., Barra Fan, U.K. margin

Lithology, lithic petrology, planktonic foraminiferal abundances, and clastic grain sizes have been determined in a 30 m-long core recovered from the Barra Fan off northwest Scotland. The record extends back to around 45 kyr B.P., with sedimentation rates ranging between 50 and 200 cm/kyr. The abundance of ice-rafted debris indicates 16 glacimarine events, including temporal equivalents to Heinrich events 1–4. Enhanced concentrations of basaltic material derived from the British Tertiary Province suggest that the glacimarine sediments record variations in a glacial source on the Hebrides shelf margin. Glacimarine zones are separated by silty intervals with high planktonic foraminifera concentrations that reflect an interstadial circulation regime in the Rockall Trough. The results suggest that the last British Ice Sheet fluctuated with a periodicity of 2000–3000 years, in common with the Dansgaard-Oeschger climate cycle.

Accelerated drawdown of meridional overturning in the late‐glacial Atlantic triggered by transient pre‐H event freshwater perturbation

[1] Abrupt decreases of the Atlantic meridional overturning circulation (MOC) during the Late Pleistocene have been directly linked to catastrophic discharges of glacimarine freshwater, triggering disruption of northward marine heat transport and causing global climate changes. Here we provide measurements of excess sedimentary 231 Pa/ 230 Th from a high-accumulation sediment drift deposit in the NE Atlantic that record a sequence of sudden variations in the rate of MOC, associated deep ocean ventilation and surface-ocean climatology. The data series reveal a sequential decrease in the MOC rate at � 18.0 ka BP ago that coincides with only transient and localized freshwater inputs. This change represents a substantial, though not total, cessation in MOC that predates the major Heinrich (H1) meltwater event by at least 1,200 years. These results highlight the potential of targeted freshwater perturbations in promoting substantial MOC changes without a direct linking with catastrophic freshwater surges. Citation: Hall, I. R., S. B. Moran, R. Zahn, P. C. Knutz, C.-C. Shen, and R. L. Edwards (2006), Accelerated drawdown of meridional overturning in the late-glacial Atlantic triggered by transient pre-H event freshwater perturbation, Geophys. Res. Lett., 33, L16616, doi:10.1029/2006GL026239.

The Faroe-Shetland Gateway: Late Quaternary water mass exchange between the Nordic seas and the northeastern Atlantic

Abstract Thirteen piston and gravity cores from the Faroe–Shetland area were investigated for their planktic and benthic foraminiferal and oxygen isotopic distributions. Eight time-slices between 18 ka BP and the present were reconstructed to study variations in surface and deep water exchange between the SE Norwegian Sea and the northeast Atlantic Ocean. Today, a relatively strong northward flow of warm North Atlantic surface water is counterbalanced by a southward outflow of newly convected cold bottom water, the Norwegian Sea Overflow Water. During the last glacial maximum at 18 ka BP both the surface and bottom flows were slow and the climate conditions were Arctic. The convection north of the Faroe area was weak and unstable. The first indication of the deglaciation is a decrease in the planktic oxygen isotope values discernible southwest of the Faroe Islands at 15.5 ka BP. The deglaciation proceeded northeast and eastward synchronous with a gradual intensification of northward flowing warmer Atlantic Intermediate Water along the sea bottom. Meltwater fluxes increased between 14 and 13 ka BP producing cold surface waters, and the climatic cooling was extreme. There was no southward overflow of cold bottom water during this time period and the exchange of water masses between the Nordic seas and the North Atlantic Ocean was essentially reversed, i.e. estuarine. During the Bolling Interstadial at 12.5 ka BP northward flowing warm surface water was present to the east of the Faroe–Shetland Channel, wedged below a tongue of polar water spreading from the northwest and reaching into the Faroe–Shetland Channel. Convection in the Nordic seas and overflow of cold deep water started during the Bolling Interstadial. The polar water spread more eastward and southward during the following cold spell, the Younger Dryas, around 10.3 ka BP. The polar water was overlying the warmer, but more saline Atlantic water, which flowed northward below the cold surface water. The overflow of cold bottom water was supposedly only slightly weaker than during the Bolling Interstadial. Strong inflow of warm surface water took place during the Early Holocene at 9.5 ka BP and relatively dense cold water flowed southward along the bottom. The rate of water mass exchange reached a maximum at 6.5 ka BP, when both the inflow of warm Atlantic surface water and the outflow of cold dense bottom water appear to have been stronger than today.

Centennial-scale variability of the British Ice Sheet: Implications for climate forcing and Atlantic meridional overturning circulation during the last deglaciation

Evidence from paleoclimatic archives suggests that Earths climate experienced rapid temperature changes associated with pronounced interhemispheric asymmetry during the last glacial period. Explanations for these climate excursions have converged on nonlinear interactions between ice sheets and the oceans thermohaline circulation, but the driving mechanism remains to be identified. Here we use multidecadal marine records of faunal, oxygen isotope, and sediment proxies from the northeast Atlantic proximal to the western margins of the last glacial British Ice Sheet (BIS) to document the coupling between ice sheet dynamics, ocean circulation, and insolation changes. The core data reveal successions of short-lived (80–100 years), high-amplitude ice-rafted debris (IRD) events that were initiated up to 2000 years before the deposition of detrital carbonate during Heinrich events (HE) 1 and 2. Progressive disintegration of the BIS 19–16 kyr before present (B.P.) occurred in response to abrupt ocean-climate warmings that impinged on the northeast Atlantic during the early deglaciation. Peak IRD deposition recurs at 180–220 year intervals plausibly involving repeated breakup of glacial tidewater margins and fringing marine ice shelves. The early deglaciation culminated in a major meltwater pulse at ∼16.3 kyr B.P. followed by another discharge associated with HE1 some 300 years after. We conclude that temperature changes related to external forcing and marine heat transport caused a rapid response of the BIS and possibly other margins of the Eurasian Ice Sheet. Massive but short-lived meltwater surges influenced the Atlantic meridional overturning circulation thereby contributing to North Atlantic climate variability and bipolar climatic asymmetry.

Glacimarine slope sedimentation, contourite drifts and bottom current pathways on the Barra Fan, UK North Atlantic margin

The sedimentary record of a 30-m core (MD95-2006) from the Barra Fan in the eastern Rockall Trough has been correlated with high-resolution seismic profiles obtained across sediment drifts and large mass flow deposits. A series of sediment drifts, featuring upslope migrating wavy bedforms, has been identified with deposition focussed along topographic steps created by glacigenic debrite lobes. The most extensive drift accumulation, termed the Barra Fan Drift, is observed on the distal fringe of the fan where an 80-m-thick sequence of aggrading to migrating sediment waves onlap a mega-debrite scarp. Prior to 26 ka (14C yr) and during the late glacial to Holocene transition silty-muddy contourites were deposited by the northward-flowing Deep Northern Boundary Current. Between 26 and 18 ka and around 14 ka contourite deposition was replaced by distal glacimarine sedimentation featuring thin-bedded sandy turbidites that were triggered during shelf-edge advances of the British Ice Sheet. During the last glacial period sediments accumulated at rates of more than 40 cm/ka, as a consequence of the high flux of sediments from the shelf margin and winnowing of exposed mass flow deposits by along-slope currents. In contrast the Holocene is represented by a condensed interval of silty-sandy mud due to vigorous bottom circulation and a low terrigenous sediment supply. Seismic seafloor signatures suggest that the present morphology of the Barra Fan is shaped by two pathways of bottom currents, probably both related to the Deep Northern Boundary Current. One branch of this water mass follows the lower slope, causing pronounced erosion on the distal part of the Barra Fan Drift, while the other is directed across the debrite topography of the fan bulge.

Paleocurrent reconstruction of the deep Pacific inflow during the middle Miocene: Reflections of East Antarctic Ice Sheet growth

[1]xa0Today the deep western boundary current (DWBC) east of New Zealand is the most important route for deep water entering the Pacific Ocean. Large-scale changes in deep water circulation patterns are thought to have been associated with the development of the East Antarctic Ice Sheet (EAIS) close to the main source of bottom water for the DWBC. Here we reconstruct the changing speed of the southwest Pacific DWBC during the middle Miocene from ∼15.5–12.5 Ma, a period of significant global ice accumulation associated with EAIS growth. Sortable silt mean grain sizes from Ocean Drilling Program Site 1123 reveal variability in the speed of the Pacific inflow on the timescale of the 41 kyr orbital obliquity cycle. Similar orbital period flow changes have recently been demonstrated for the Pleistocene epoch. Collectively, these observations suggest that a strong coupling between changes in the speed of the deep Pacific inflow and high-latitude climate forcing may have been a persistent feature of the global thermohaline circulation system for at least the past 15 Myr. Furthermore, long-term changes in flow speed suggest an intensification of the DWBC under an inferred increase in Southern Component Water production. This occurred at the same time as decreasing Tethyan outflow and major EAIS growth between ∼15.5 and 13.5 Ma. These results provide evidence that a major component of the deep thermohaline circulation was associated with the middle Miocene growth of the EAIS and support the view that this time interval represents an important step in the development of the Neogene icehouse climate.

Multiple‐stage deglacial retreat of the southern Greenland Ice Sheet linked with Irminger Current warm water transport

[1]xa0There is limited knowledge pertaining to the history of the Greenland Ice Sheet (GIS) during the last glacial-interglacial transition as it retreated from the continental margins to an inland position. Here we use multiproxy data, including ice-rafted debris (IRD); planktonic isotopes; alkenone temperatures; and tephra geochemistry from the northern Labrador Sea, off southwest Greenland, to investigate the deglacial response of the GIS and evaluate its implications for the North Atlantic deglacial development. The results imply that the southern GIS retreated in three successive stages: (1) early deglaciation of the East Greenland margins, by tephra-rich IRD that embrace Heinrich Event 1; (2) progressive retreat during Allerod culminating in major meltwater releases (δ18O depletion of 1.2‰) at the Allerod–Younger Dryas transition (12.8–13.0 kyr B.P.); and (3) a final stage of glacial recession during the early Holocene (∼9–11 kyr B.P.). Rather than indicating local temperatures of ambient surface water, the alkenones likely were transported to the core site by the Irminger Current. We attribute the timing of GIS retreat to the incursion of warm intermediate waters along the base of grounded glaciers and below floating ice shelves on the continental margin. The results lend support to the view that GIS meltwater presented a forcing factor for the Younger Dryas cooling.

Multidecadal ocean variability and NW European ice sheet surges during the last deglaciation

A multiproxy paleoceanographic record from the Atlantic margin off the British Isles reveals in unprecedented detail discharges of icebergs and meltwater in response to sea surface temperature increases across the last deglaciation. We observe the earliest signal of deglaciation as a moderate elevation of sea surface temperatures that commenced with a weakly developed thermocline and the presence of highly ventilated intermediate waters in the Rockall Trough. This warming pulse triggered a series of multidecadal ice-rafted debris peaks that culminated with a major meltwater discharge at 17,500 years before present related to ice sheet disintegration across the NW European region. The impact of meltwater caused a progressive reduction in deep water ventilation and a sea surface cooling phase that preceded the collapse of the Laurentide Ice Sheet during Heinrich event 1 by 500–1000 years. A similar sequence of rapid ocean-ice sheet interaction across the European continental margin is identified during the Bolling-Allerod to Younger Dryas transition. The strategic location of our sediment core suggests a sensitive and rapid response of ice sheets in NW Europe to transient increases in thermohaline heat transport.

Seismic stratigraphy of the West Shetland Drift: Implications for late Neogene paleocirculation in the Faeroe-Shetland gateway

[1]xa0The morphology and anatomy of a late Neogene contourite drift on the British margin of the Faeroe-Shetland Channel is investigated in unprecedented detail using industrial two-dimensional and three-dimensional seismic data. The West Shetland Drift is constructed by a succession of mounded, asymmetric strata that in the basin and on the midslope has accumulated on a regional unconformity of early Pliocene age. The midslope component of the drift system forms a 400 m thick convex, elongate sedimentary body that can be traced from the outlet of the Norwegian Channel and 250 km southwest along the Shetland margin. Contourite drifts occupying the basin are evident as a succession of sheeted-mounded and wavy migrating units intercalated by three mega-debrite sequences. The seismic architecture of the West Shetland Drift and tentative age estimates of its bounding surfaces suggests that it formed rapidly (∼5–10 cm kyr−1) by alongslope transport of hemipelagic sediments derived from the NW European shelf. The change from erosion/nondeposition to enhanced contourite drift accumulation during the early Pliocene suggests that a moderate thermohaline current regime prevailed in the Faeroe-Shetland Channel prior to the main phase of Northern Hemisphere glaciation at 3.0–2.5 Ma. A reduction in meridional heat transport associated with a decrease in the flux of North Atlantic deep waters may have been an important factor for the growth of Northern Hemisphere ice sheets.