studies on the detailed crustal structure of the trench trench trench triple junction off southeast japan abstract the fine structure of the upper part of the crust around a unique trench trench trench triple junction 34 10 n 141 50 e fig 1 off the coast of southeastern honshu japan was analyzed on the basis of geophysical data the triple junction is characterized by a triangular shaped 9000 m deep basin with a strong negative free air gravity anomaly distorted magnetic lineaments and shallow angle sub bottom faults although there is a lack of refraction data simulation experiments in terms of gravity and magnetics suggest that the topmost part of the lithosphere of the northeastern corner of the philippine sea plate is stretched to form a wide basin in the triple junction area no attempt is made to discuss the dynamic structure of the triple junction

land uplift near vatnajokull iceland as observed by gps in 1992 1996 and 1999 summary war ming of the climate in the 20th century has been manifested by an ablation of europe s largest ice cap vatnajokull in iceland the thin elastic lithosphere and the low viscosity as thenosphere are responding to the reduction in mass by current land uplift in the vicinity of the ice cap suggested to be of the order of 5 10 mm yr 1 lithosphere thickness and asthenosphere viscosities compatible with these values have been inferred from our repeated gps epoch campaigns in 1992 1996 and 1999 uplift rates are estimated to be of the order of 5 19 mm yr 1 and the uplift rate is decreasing by 0 11 0 01 mm yr 1 km 1 with radial distance from the centre of the ice cap these results deviate from previous earth rheology models estimated for the region our data indicate that the lithosphere thickness might be of the order of 10 20 km and the asthenosphere viscosity may be as low as 5 10 17 pa s but these parameters need a careful fitting to the estimated uplift rates

the thin hot plume beneath iceland summary we present the results of a seismological investigation of the frequency dependent amplitude variations across iceland using data from the hotspot array currently deployed there the array is composed of 30 broad band passcal instruments we use the parameter t de ned in the usual manner from spectral ratios halderman davis1991 to compare observed s wave amplitude variations with those predicted due toboth anelastic attenuation and diiraction eiects four teleseismic events at a range of azimuths are used to measure t a 2 d vertical cylindrical plume model with a gaussian shaped velocity anomaly is used to model the variations that part of t caused by attenuation was estimated by tracing a ray through iasp91 then superimposing our plume model velocity anomaly and calculating the path integral of 1 oq that part of t caused by diiraction was estimated using a 2 d nite diierence code to generate synthetic seismograms the same spectral ratio technique used for the datawas then used to extract a predictedt thet variations caused by anelastic attenuation are unable to account for the variations we observe but those caused by diiraction do we calculate the t variations caused by diiraction for diierent plume models and obtain our best t plume which exhibits good agreement between the observed and measured t the best t plume model has a maximum s velocity anomaly of 12 per cent and falls to 1 e of its maximum at 100 km from the plume centre this is narrower than previous estimates from seismic tomography which are broadened and damped by the methods of tomography this velocity model would suggest greater ray theoretical traveltime delays than observed however we nd that for such a plume wave front healing eiects at frequencies of 0 03 0 175 hz the frequency range used to pick s wave arrivals causes a 40 per cent reduction in traveltime delay reducing the ray theoretical delay to that observed

a review of the environmental history of iceland 13 000 9000 yr bp a synthesis of the main environmental changes that are interpreted to have occurred in iceland during the period 13 9 ka bp is presented most of the evidence available relates to variations in the position of ice margins although some limited information on vegetation history and soil stability is also referred to only qualitative and limited climatic inferences can be made for this period because of the lack of detailed evidence a summary curve of the relative extent of ice cover is presented

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

older crust underlies iceland summary the oldest rocks outcropping in northwest iceland are 16 myr old and in east iceland 13 myr the full plate spreading rate in this region during the cenozoic has been 2c m a 1 and thus these rocks are expected to be separated by 290 km they are however 500 km apart the conclusion is inescapable that an expanse of older crust 210 km wide underlies iceland submerged beneath younger lavas this conclusion is independent of any considerations regarding spreading ridge migrations jumps the simultaneous existence of multiple active ridges three dimensionality or subsidence of the lava pile such complexities bear on the distribution and age of the older crust but not on its existence or its width if it is entirely oceanic its maximum age is most likely 26 37 ma it is at least 150 km in north south extent but may taper and extend beneath south iceland part of it might be continental a southerly extension of the jan mayen microcontinent this older crust contributes significantly to crustal thickness beneath iceland and the 40 km local thickness measured seismically is thus probably an overestimate of present day steady state crustal production at iceland

the lithosphere asthenosphere boundary in the north west atlantic region abstract a detailed knowledge of the thickness of the lithosphere in the north atlantic is an important parameter for understanding plate tectonics in that region we achieve this goal with as yet unprecedented detail using the seismic technique of s receiver functions clear positive signals from the crust mantle boundary and negative signals from a mantle discontinuity beneath greenland iceland and jan mayen are observed according to seismological practice we call the negative phase the lithosphere asthenosphere boundary lab the seismic lithosphere under most of the iceland and large parts of central greenland is about 80 km thick this depth in iceland is in disagreement with estimates of the thickness of the elastic lithosphere 10 20 km found from postglacial rebound data in the region of flood basalts in eastern greenland which overlies the proposed iceland plume track the lithosphere is only 70 km thick about 10 km less than in iceland which is located directly above the proposed plume at the western greenland coast the lithosphere thickens to 100 120 km with no indication of the iceland plume track identified below jan mayen the lithospheric thickness varies between 40 and 60 km

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

volume changes of vatnajokull ice cap iceland due to surface mass balance ice flow and subglacial melting at geothermal areas 1 we present observed changes in the geometry of western vatnajokull over a period of about ten years which are caused by the surface mass balance ice flow both during surges and quiescent periods and basal melting due to geothermal and volcanic activity comparison of two digital elevation models shows that from 1985 to 1998 the outlet glaciers have lost 14 5 km3 on the average 1 m yr 1 evenly distributed over the area of 1360 km2 however the marginal areas of four outlets have gained up to 80 m in elevation and the uppermost parts have subsided similarly during surges altogether 26 3 km3 were transported from the accumulation areas to the ablation areas of which 19 3 km3 are attributed to surges comparison of dem s from august 1997 and august 1998 revealed 10 ice cauldrons produced by subglacial geothermal activity of which some were previously unknown

contribution of the patagonia icefields of south america to sea level rise digital elevation models of the northern and southern patagonia icefields of south america generated from the 2000 shuttle radar topography mission were compared with earlier cartography to estimate the volume change of the largest 63 glaciers during the period 1968 1975 2000 these glaciers lost ice at a rate equivalent to a sea level rise of 0 042 0 002 millimeters per year in the more recent years 1995 2000 average ice thinning rates have more than doubled to an equivalent sea level rise of 0 105 0 011 millimeters per year the glaciers are thinning more quickly than can be explained by warmer air temperatures and decreased precipitation and their contribution to sea level per unit area is larger than that of alaska glaciers

glacio isostatic crustal movements caused by historical volume change of the vatnajokull ice cap iceland measurements of the lake level of lake langisjor at the sw edge of the vatnajokull ice cap indicate a tilt of 0 26 0 06 rad year away from the ice cap in the years 1959 1991 the tilt is too large to be explained as an elastic earth response to ice retreat this century or to be caused by change in the gravitational pull of the ice cap but it can be explained by sub lithospheric viscous adjustment regional subsidence in historical times in se iceland can similarly be attributed to viscous adjustment resulting from the increased load of vatnajokull during the little ice age the inferred sub lithospheric viscosity is 1 1018 5 1019 pa s

intracrustal derivation of na rich andesitic and dacitic magmas an example from volcan ollague andean central volcanic zone andesitic and dacitic lavas erupted from quaternary stratovolcanoes in the central volcanic zone cvz of the andes have major and trace element compositions similar to proposed melts of subducted mid ocean ridge basalt and na rich trondjhemites tonalites and granodiorites ttg in early archean high grade gneiss terrains suites of lavas from individual volcanoes have average la yb of 25 45 sr y of 30 65 yb contents of 1 5 1 1 ppm and na 2 o contents 3 8 wt with lavas erupted on the volcanic front displaying the most affinity to the early archean suites because slab melting is not involved in the derivation of the cvz magmas these data indicate that rocks with early archean ttg compositions can also be produced through intracrustal differentiation processes isotopic major element and ree data from volcan ollague are consistent with a model incorporating contamination of mantle derived basaltic magmas in garnet bearing deep continental crust to produce basaltic andesite magma followed

ice volcano interaction of the 1996 gjalp subglacial eruption vatnajokull iceland volcanic eruptions under glaciers can cause dangerous floods and lahars1 2 3 and create hyaloclastite fragmented glassy rock mountains4 5 6 7 8 but processes such as the rate of heat transfer between ice and magma edifice formation and the response of the surrounding glacier are poorly understood because of the lack of data here we present observations from the fissure eruption at vatnajokull ice cap iceland in october 1996 in the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6 7 km long and 200 300 m high under 500 750 m of ice meltwater of temperatures of 15 20 c flowed along a narrow channel at the glacier bed into the grimsvotn subglacial lake for five weeks before draining in a sudden flood or jokulhlaup subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path whereas elsewhere the glacier surface remained intact suggesting that subglacial eruptions do not trigger widespread basal sliding in warm based glaciers

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

glacial and climatic events in iceland reflecting regional north atlantic climatic shifts during the pleistocene holocene transition abstract this paper presents a summary of the evidence for glacial and climatic changes during the late pleistocene early holocene transition in iceland the deglaciation during the bolling allerod event was interrupted by a short lived older dryas glacial advance a biostratigraphical record from northern iceland shows significant climate warming in late allerod when mean july temperatures were at least as warm as those of today an abrupt cooling marked the beginning of the younger dryas event it was characterised by a cold and stable polar climate and an extensive glaciation before the postglacial warming of climate set in the icelandic paleoclimatic record is discussed in the light of climatic oscillations recorded from the grip ice core from the greenland inland ice and with reference to major shifts in the oceanic front systems recorded in the troll 8903 marine sediment core from the north sea the vedde ash gives a unique opportunity to address the chronological problems and correlate event stratigraphies of the different proxies it is concluded that the icelandic record of glacial and climatic changes during the late pleistocene early holocene transition largely reflects the climatic development in the north atlantic region

glacial isostatic adjustment and the viscosity structure underlying the vatnajokull ice cap iceland we examine the dependence of glacial isostatic adjustment gia due to changes in the vatnajokull ice cap iceland on the underlying viscosity structure iceland offers a unique case study for gia research with a thinner elastic lithosphere underlain by a low viscosity zone or asthenosphere as opposed to regions such as fennoscandia or north america described by a thicker lithosphere while not necessarily featuring an asthenosphere

rapid viscoelastic uplift in southeast alaska caused by post little ice age glacial retreat our observations show that extreme uplift in southeast alaska began about 1770 ad with relative sea level rsl change to 5 7 m and current uplift rates to 32 mm yr this region experienced widespread glacial melting following the little ice age lia with the collapse of the glacier bay icefield alone equivalent to 8 mm of global sea level rise geodynamic modelling links the uplift to post lia isostatic rebound with the extreme uplift signal and a priori knowledge of ice load changes requiring the presence of a low viscosity asthenosphere 3 710 18 pa s these crustal deformations are triggered by climate change

postseismic deformation following the june 2000 earthquake sequence in the south iceland seismic zone 1 we observe postseismic deformation on two spatiotemporal scales following mw 6 5 earthquakes in the south iceland seismic zone on 17 and 21 june 2000 we see a rapidly decaying deformation transient lasting no more than 2 months and extending about 5 km away from the two main shock ruptures this local month scale transient is captured by several radar interferograms and is also observed at a few campaign gps sites located near the faults a slower transient with a characteristic timescale of about a year is detected only by gps measurements the month scale deformation pattern has been explained by poroelastic rebound due to postearthquake pore pressure changes in contrast the year scale deformation can be explained by either afterslip at 8 14 km depth or viscoelastic relaxation of the lower crust and upper mantle in response to the coseismic stress changes the optimal viscoelastic models have lower crustal viscosities of 0 5 1 1019 pa s and upper mantle viscosity of 3 1018 pa s because of the limitations of our gps campaign data we consider both afterslip and viscoelastic relaxation as plausible mechanisms explaining the deformation field both types of postseismic deformation models suggest that the areas of large coseismic stress increase east of the 17 june and west of the 21 june ruptures continue to be loaded by the postseismic deformation

the 20th century retreat of ice caps in iceland derived from airborne sar w vatnajokull and n mýrdalsjokull we present observations of the long term recession of surging outlets of icelandic ice caps in response to 20th century climate in august 1998 synthetic aperture radar sar data covering the western part of vatnajokull and the northern part of m rdalsjokull in southern iceland were acquired with the danish airborne emisar radar system polarimetric and interferometric sar data reveal the margins of the present ice caps as well as a series of terminal moraines in the fore field these moraines date back to the maximum neoglacial extent at the end of the 19th century and the outermost allow reconstruction of the margin at that time the data offer a unique opportunity to estimate the area decrease of these ice caps in the 20th century the influence of the fluctuations of the surge type outlets constituting most of w vatnajokull area and a good part of n m rdalsjokull area is minimal since they had all recently surged in 1998 as was presumably the case when the outermost moraines were formed the major contributor to the area decrease is therefore climate changes in the 20th century even though the glacier retreat has been interrupted by short lived surges moraines associated with most of the surges in w vatnajokull in the 20th century are observed in the sar data including the most recent surges in the 1990s interestingly no push moraines were observed in front of the surge advance but the moraines appear when the glaciers start retreating we estimate that the collective decrease of the outlets of western vatnajokull since maximum neoglacial extent of each outlet is 109 km2 6 7 corresponding to an average retreat of 850 m over a 130 km long margin in the same period the outlet slettjokull in n m rdalsjokull has decreased by 33 km2 20 corresponding to an average retreat of 1500 m over a 20 km long margin

glacier volcano interactions deduced by sar interferometry glacier surface displacements produced by geothermal and volcanic activity beneath vatnajokull ice cap in iceland are described by field surveys of the surface topography combined with interferograms acquired from repeat pass synthetic aperture radar images a simple ice flow model serves well to confirm the basic interpretation of the observations the observations cover the period october 1996 january 1999 and comprise a the ice flow field during the infilling of the depressions created by the subglacial gjalp eruption of october 1996 b the extent and displacement of the floating ice cover of the subglacier lakes of grimsvotn and the skafta cauldrons c surface displacements above the subglacier pathways of the jokulhlaups from the gjalp eruption site and the grimsvotn lake d detection of areas of increased basal sliding due to lubrication by water and e detection of spots of temporal displacement that may be related to altering subglacial volcanic activity at the depression created by the gjalp eruption the maximum surface displacement rate away from the radar decreased from 27 cm d 1 to 2 cm d over the period january 1997 january 1999 the observed vertical displacement of the ice cover of grimsvotn changed from an uplift rate of 50 cm d 1 to sinking of 48 cm d 1 and for skafta cauldrons from 2 cm d 1 to 25 cm d 1

bedrock response to llanquihue holocene and present day glaciation in southernmost south america 1 modern geodetic techniques such as precise global positioning system gps measurements and high resolution space gravity mapping make it possible to measure the present day rate of viscoelastic gravitational earth response to present and past glacier mass change patagonia is a rapidly evolving glacial environment over the past decade the local rate of surface stress unloading may be the largest anywhere on the planet we compute the present day land uplift rate that could be observed using bedrock gps measurements the little ice age lia of the past half millennium generates large vertical rates since the underlying mantle material is likely to have anomalously low viscosity owing to late neogene ridge subduction

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

a review of the holocene environmental history of iceland abstract the purpose of this paper is to review the present knowledge of the holocene environmental history of iceland focusing on glacier fluctuations the ideas of the holocene being a relatively flat epoch in terms of environmental changes have been superseded by new studies reflecting active fluctuations the present paper promotes a revised model of the holocene glacier fluctuation history of iceland and recognises a long period of neoglaciation present data suggests an onset of glacier expansion in iceland around 5 ka bp which can be roughly correlated between different parts of the country and coincides with wider environmental changes specific glacier advances have been bracketed to 5 4 5 ka bp ca 4 2 ka bp ca 3 ka bp 2 ka bp and 1 5 1 2 ka bp during the little ice age lia glaciers in iceland begun to advance in the late 18th century and were biggest in the late 19th century

rapid wastage of alaska glaciers and their contribution to rising sea level we have used airborne laser altimetry to estimate volume changes of 67 glaciers in alaska from the mid 1950s to the mid 1990s the average rate of thickness change of these glaciers was 0 52 m year extrapolation to all glaciers in alaska yields an estimated total annual volume change of 52 15 km 3 year water equivalent equivalent to a rise in sea level sle of 0 14 0 04 mm year repeat measurements of 28 glaciers from the mid 1990s to 2000 2001 suggest an increased average rate of thinning 1 8 m year this leads to an extrapolated annual volume loss from alaska glaciers equal to 96 35 km 3 year or 0 27 0 10 mm year sle during the past decade these recent losses are nearly double the estimated annual loss from the entire greenland ice sheet during the same time period and are much higher than previously published loss estimates for alaska glaciers they form the largest glaciological contribution to rising sea level yet measured

post glacial rebound and asthenosphere viscosity in iceland during the weichselian glaciation iceland was covered with an ice cap which caused downward flexure of the earth s surface the post glacial rebound in iceland was very rapid being completed in about 1 000 years the length of this time interval constrains the maximum value of asthenosphere viscosity in iceland to be 1 times 10 sup 19 pa s or less further clarification of the ice retreat and uplift history may reveal lower viscosity current changes in the mass balance of icelandic glaciers must lead to measurable elevation changes considering this low viscosity expected current elevation changes around the vatnajoekull ice cap are of the order of 1 cm per year due to mass balance change in this century

concurrent changes of relative sea level and glacier extent at the weichselian holocene boundary in berufjordur eastern iceland abstract a detailed study of raised shorelines and radiocarbon dated sublittoral sediments in the berufjordur area eastern iceland revealed a complex relationship between relative sea level changes and variations in extent of glaciers the marine limit shoreline at 58 m a s l is correlated with a glacier advance and formation of distinct moraines in late younger dryas time after a considerable retreat of the glaciers and concurrent lowering of relative sea level the glaciers readvanced in early preboreal time and relative sea level rose and culminated at about 9900 14c yr bp an extrapolation of a tentative minimum shoreline displacement curve indicates a minimum position of relative sea level at about 30 m around 8600 14c yr bp this suggests a total relative sea level change of about 90 m and a total glacio isostatic rebound of 120 m yielding 7 3 cm 14c yr 1 as a minimum rate of uplift between 10 300 and 9900 14c yr bp the glacier advances in eastern iceland are correlated with younger dryas and preboreal expansions of the icelandic inland ice radiocarbon dated elsewhere to about 10 300 and 9800 14c yr bp respectively these glacier advances and concurrent rise of relative sea level in iceland coincide in time with changes in north atlantic oceanic and atmospheric circulation revealed e g from studies of greenland ice cores and north atlantic deep sea and shelf cores