Karl Grönvold

High-resolution record of Northern Hemisphere climate extending into the last interglacial period

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5 °C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments

Four previously known ash layers (Ash Zones I and II, Saksunarvatn and the Settlement layer) all originating in Iceland, have been identified in the Central Greenland ice core GRIP. This correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash. This establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately. A precise connection with dates obtained by14C beyond the range of dendrochronology is established which provides an excellent confirmation of230Th-234U dates from corals. Four additional Icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits.

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid-ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8414 Volcanology: Eruption mechanisms; 8439 Volcanology: Physics and chemistry of magma bodies; 3655 Mineralogy and Petrology: Major element composition; KEYWORDS: lava flow, chemical heterogeneity, erupted volume, lava morphology, side-scan sonar

Helium, oxygen, strontium and neodymium isotopic relationships in Icelandic volcanics

Abstract 3He/4He ratios have been obtained for basaltic, intermediate and acid volcanic glasses from Iceland. Basaltic glasses exhibit a wide range of 3He/4He ratios (4 3He/4He does not correlate with either 87Sr/86Sr or 143Nd/144Nd ratio and radiogenic components of He, Sr and Nd have apparently been decoupled. Interaction of Icelandic magmas with hydrothermally altered and older Icelandic crust is the preferred explanation for variable and often low δ18O values. It is suggested that primary 3He/4He ratios may have been modified by incorporation of radiogenic helium developed within the Icelandic crust to impose a larger range of 3He/4He ratios on the erupted products than was actually inherited from the mantle beneath Iceland. Intermediate and acid samples have all been severely contaminated by atmospheric helium, most probably at very shallow levels within the crust.

Extreme 3He/4He ratios in northwest Iceland: constraining the common component in mantle plumes

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. 3He/4He ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently ˜37 RA (RA = air 3He/4He) — significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189–218]). The Sr, Nd and Pb isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517–520]) — in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275–24300]). This observation implies that Selardalur may represent the best estimate available to date of the He–Sr–Nd–Pb isotopic composition of the 5th mantle component common to many mantle plumes.

Postglacial eruptive history of the Western Volcanic Zone, Iceland

New field observations, age constraints, and extensive chemical analyses define the complete postglacial eruptive history of the 170-km-long Western Volcanic Zone (WVZ) of Iceland, the ultraslow-spreading western boundary of the south Iceland microplate. We have identified 44 separate eruptive units, 10 of which are small-volume eruptions associated with the flanking Grimsnes system. Overall chemical variations are consistent with very simplified models of melting of a source approximating primitive mantle composition. The 17 eruptions in the first 3000 years of postglacial time account for about 64% of the total postglacial production and are incompatible-element depleted compared to younger units, consistent with enhanced melting as a consequence of rebound immediately following deglaciation. Steadily declining eruption rates for the last 9000 years also correlate with changes in average incompatible element ratios that appear to reflect continued decline in melting extents to the present day. This result is not restricted to the WVZ, however, and may herald a decline in melting throughout all of western Iceland during later postglacial time. Lavas from the northern part of the WVZ are depleted in incompatible elements relative to those farther south at all times, indicating either a long-wavelength gradient in mantle source composition or variations in the melting process along axis. We find no evidence in the postglacial volcanic record for current failure of the WVZ, despite evidence for continued propagation of the eastern margin of the microplate. The dominance of lava shields in the eruptive history of the WVZ contrasts with the higher number of fissure eruptions in other Icelandic volcanic zones. WVZ shields represent long-duration, low-effusion rate eruptions fed by recharge magma arising out of the mantle. Average effusion rate is the key variable distinguishing shield and fissure eruptions, both within the WVZ and between different volcanic zones. High effusion rate, large-volume eruptions require the presence of large crustal magma reservoirs, which have been rare or absent in the WVZ throughout postglacial time.

Plume-driven upwelling under central Iceland

A suite of 70 basaltic samples from the Herdubreid region of the Northern Volcanic Zone (NVZ) in central Iceland has been analysed for major and trace element compositions. The average light rare earth element concentration of these basalts is more than a factor of 2 higher than that of basalts from the Theistareykir volcanic system near the northern end of the NVZ. Seismic surveys of the NVZ have shown that the crustal thickness increases from ∼20 km near Theistareykir to 32–40 km in central Iceland. The observed REE composition and crustal thickness of the Theistareykir area can be reproduced by a melting model where mantle with a potential temperature of ∼1480°C upwells under the spreading ridge and the mantle upwelling is driven by plate separation alone. However, plate-driven upwelling models cannot simultaneously reproduce the composition of the Herdubreid region lava and the observed crustal thickness. Forward and inverse techniques show that plate-driven models that match the crustal thickness underestimate the La concentration by more than a factor of 2, and models that reproduce the compositions underestimate the crustal thickness by a factor of 4–5. Therefore one of the assumptions involved in the plate-driven upwelling models is not appropriate for central Iceland. A new set of models was developed in which mantle upwelling rates are allowed to differ from those of plate-driven upwelling in order to investigate the role of plume-driven mantle upwelling. The lava composition and crustal thickness of the Herdubreid region can be reproduced by models where the upwelling rates near the base of the melting region (>100 km depth) are ∼10 times higher than those expected from plate-driven upwelling alone and the mantle potential temperature is 1480–1520°C. About half of the melt generation under central Iceland results from plume-driven upwelling, with the remainder caused by plate-driven upwelling of hot material. This result is in agreement with numerical models of ridge-centred plumes.

Crustal accretion under northern Iceland

Successful models of oceanic crustal accretion should be consistent with both geochemical and geophysical observations from active mid-ocean ridges as well as those from ophiolite sections. The major element concentrations of a set of basalt and picrite samples from the Krafla and Theistareykir volcanic systems of the Northern Volcanic Zone of Iceland show two distinct trends. The compositional variation in samples with MgO>9.5 wt% can be explained by addition/removal of a wehrlitic cumulate, while the major element variability in samples with 5–9.5 wt% MgO is dominated by gabbro removal. The results of thermobarometry based on the composition of the samples and the crystals found within them show that crystallisation took place at a range of temperatures (1160–1350°C) and pressures (<0.3–0.9 GPa) in the crust and uppermost mantle under Krafla and Theistareykir. The geochemical results are consistent with crustal accretion models where crystallisation takes place over a range of depths in the crust and uppermost mantle (<10–30 km). The geochemical observations allow estimates of the composition, mineralogy, pressure and temperature of material in the crust and shallow mantle under northern Iceland to be made and these estimates can be used to predict the seismic velocity of the material at the ridge axis. These P-wave velocity estimates are in agreement with the results of a seismic survey of the ridge axis at Krafla. The presence of temperatures of over 1000°C and magma chambers at depths greater than 10 km in the Icelandic crust cannot be ruled out using the available geophysical data from the Icelandic rift zones. Therefore both the geochemical and geophysical observations are consistent with models where crustal accretion takes place at a range of depths under northern Iceland.

Deglaciation effects on mantle melting under Iceland: results from the northern volcanic zone

Abstract A striking feature of Icelandic volcanism is the effect that the last ice age had on volcanic activity. After the final retreat of ice ∼11 kyr BP, the average eruption rate is estimated to have been 20–30 times greater than it is today. This increase has been attributed to the release of pooled magma through differential tectonic movements during the unloading of ice. However recent work has shown that deglaciation can account for the increase in mantle melting by decreasing the pressure in the upper mantle. We present geochemical data and volume estimates of erupted magmas from Icelands northern neovolcanic zone which show that the average composition of magmas erupted during the last glacial period in Iceland are significantly more enriched in incompatible trace elements than postglacial and interglacial lavas. The difference in light rare earth element concentrations cannot be accounted for by liquid–crystal fractionation. Averaging the compositions of glacial and postglacial magmas also eliminates the likelihood that the compositional change is due to variations in source composition. An increase in mantle melting from deglaciation can account for both the magma eruption rate and observed changes in trace element concentrations. Finite transport times for magma to travel from the source region to the surface can be estimated from the delay in timing of the increased eruption rates and the end of the last glacial period. This gives transport times of about 1–3 kyr and is consistent with estimates from (226Ra/230Th) activity ratios measured in ocean island and mid-ocean ridge basalts.

Melt mixing and crystallization under Theistareykir, northeast Iceland

Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is 0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of ~4 between Fo90 and Fo87. This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of ~50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (δ18O from 3.3–5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo89–90, and the low δ18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.