Joseph M. Licciardi

DEGLACIATION OF A SOFT-BEDDED LAURENTIDE ICE SHEET

We present a series of numerical reconstructions of the Laurentide Ice Sheet during the last deglaciation (18–7 14C ka) that evaluates the sensitivity of ice-sheet geometry to subglacial sediment deformation. These reconstructions assume that the Laurentide Ice Sheet flowed over extensive areas of water-saturated, deforming sediment (soft beds) corresponding to the St. Lawrence lowland, the Great Lakes region, the western prairies of the U.S. and Canada, and the Hudson Bay and Hudson Strait regions. Sediment rheology is based on a constitutive law that incorporates experimental results from late Wisconsin till deposited by the Laurentide Ice Sheet which suggest only mildly nonlinear viscoplastic behavior. By varying the effective viscosity of till, we produced four reconstructions for the ice sheet during the last glacial maximum 18 14C ka, and two reconstructions each of the ice sheet at 14, 13, 12, 11 and 10 14C ka. We also produced one reconstruction for 9, 8.4, 8, and 7 14C ka. Reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl-shaped depression over Hudson Bay and thin ice (<1000 m above modern sea-level) over the western and southern margins. Those reconstructions that assume a higher effective viscosity of till in the Hudson Bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over Hudson Bay and a more northerly position of the central ice divide. These two different geometries may represent ice-sheet orographic changes associated with a Heinrich event. Further increases in effective viscosity of till, approaching the effective viscosity of ice, would result in a high, monolithic ice dome centered over Hudson Bay, reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology. Modeled ice-surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds. Our reconstructions with higher effective till viscosities in Hudson Bay also agree with the ICE-4G reconstructions (Peltier, 1994), which are based on inversion of relative sea-level data, for the early part of the last deglaciation (18–13 14C ka), but then depart significantly from ICE-4G beginning at about 12 14C ka due to differing assumptions of the history of deglaciation. Modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50–55 m by a soft-bedded Laurentide Ice Sheet; this would increase as the effective viscosity of till increases. Subsequent ice-volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at Barbados, New Guinea, and Tahiti, but suggest that the Laurentide Ice Sheet was not the source of meltwater pulse 1A.

Origin of the first global meltwater pulse following the Last Glacial Maximum

Well-dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates. These very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets. The Laurentide Ice Sheet is commonly targeted as the source of the first, and largest, of the meltwater pulses (mwp-IA between ∼14,200 (12,200 14C years B.P.) and 13,700 years ago (11,700 14C years B.P.)). In all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review, only those from the Gulf of Mexico and the Bermuda Rise show evidence of low δ18O values at the time of mwp-IA, identifying the southern Laurentide Ice Sheet as a potential source for mwp-IA. We question this source for mwp-IA, however, because (1) ice sheet models suggest that this sector of the ice sheet contributed only a fraction (<10%) of the sea level needed for mwp-IA, (2) melting this sector of the ice sheet at the necessary rate to explain mwp-IA is physically implausible, and (3) ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the Mississippi River into the North Atlantic than is recorded. This leaves the Antarctic Ice Sheet as the only other ice sheet capable of delivering enough sea level to explain mwp-IA, but there are currently no well-dated high-resolution records to document this hypothesis. These conclusions suggest that reconstructions of the Laurentide Ice Sheet in the ICE-4G model, which are constrained to match the sea level record, may be too low for time periods younger than 15,000 years ago. Furthermore, δ18O records from the Gulf of Mexico show variable fluxes of meltwater from the southern margin of the Laurentide Ice Sheet which can be traced to the opening and closing of eastward draining glacial-lake outlets associated with surging ice sheet behavior. These variable fluxes through eastern outlets were apparently sufficient to affect formation of North Atlantic Deep Water, thus underscoring the sensitivity of this process to changes in freshwater forcing.

Calibration of cosmogenic 3He production rates from Holocene lava flows in Oregon, USA, and effects of the Earth's magnetic field

Abstract We have measured cosmogenic 3He production rates in olivine phenocrysts from four radiocarbon-dated Holocene lava flows in Oregon. The flows span the period between 2 and 7 ka when there were significant fluctuations in the intensity of the Earths dipole moment. Our individual 3He production rate determinations are consistent with previous estimates, and reinforce the feasibility of dating very young (late Holocene) surfaces with the cosmogenic 3He method. Integrated cosmogenic 3He production rates exhibit small temporal variations during the Holocene, supporting predictions that production rates at mid-latitudes are weakly affected by geomagnetic modulation of cosmic ray flux. However, the time-varying difference between geographic and geomagnetic latitude caused by secular variation of dipole axis position may represent an important source of error (as much as 5%) in Holocene surface exposure ages and production rate calibrations. The best value for the integrated Holocene production rate of cosmogenic 3He from calibration sites in this study is 116±3 atoms g−1 yr−1.

Numerical reconstruction of a soft-bedded Laurentide Ice Sheet during the last glacial maximum

We used a numerical ice-sheet model to reconstruct the North American Laurentide Ice Sheet during the last glacial maximum. Our model simulates ice-sheet conditions that can be specified experimentally as either a rigid substrate (hard bed) or a wet, deformable till (soft bed); basal sliding is excluded. We use geologic records of former basal ice-sheet processes to prescribe the distribution of hard and soft beds. Our reconstruction of the Laurentide Ice Sheet is significantly lower in ice-surface height and contains less ice volume than the CLIMAP (maximum) reconstruction. In contrast, our reconstruction agrees well with the ICE-4G reconstruction, both in height and volume. Because the ICE-4G reconstruction is based on the inversion of relative sea-level data, whereas our reconstruction is based on glacial geology and ice mechanics, this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the Laurentide Ice Sheet that is most consistent with observations of relative sea-level change and other geodynamic considerations.

Holocene Glacier Fluctuations in the Peruvian Andes Indicate Northern Climate Linkages

Togetherness Two of the most important questions in paleoclimatology are, how are the climates of the Northern and Southern Hemispheres linked, and what are the roles of the high latitudes and the tropics in driving and transmitting climate changes? Past investigations have concentrated on the study of large, rapid climate changes like deglaciations or the Younger Dryas because they are the easiest ones to see and to date. Licciardi et al. (p. 1677) expand the scope of these investigations by determining precise cosmogenic isotope ages for glacial moraines formed in the Peruvian Andes during the Holocene (the last 11,000 years). The precision of these data reveals a broad correlation between Peruvian glacial advances and climate in the North Atlantic region, revealing important climate linkages between the tropics and higher latitudes. Glacial advances in the southern Peruvian Andes during the Holocene are correlated with the climate of the North Atlantic region. The role of the tropics in triggering, transmitting, and amplifying interhemispheric climate signals remains a key debate in paleoclimatology. Tropical glacier fluctuations provide important insight on regional paleoclimatic trends and forcings, but robust chronologies are scarce. Here, we report precise moraine ages from the Cordillera Vilcabamba (13°20′S) of southern Peru that indicate prominent glacial events and associated climatic shifts in the outer tropics during the early Holocene and late in the “Little Ice Age” period. Our glacier chronologies differ from the New Zealand record but are broadly correlative with well-dated glacial records in Europe, suggesting climate linkages between the tropics and the North Atlantic region.

Cosmogenic 3He and 10Be chronologies of the late Pinedale northern Yellowstone ice cap, Montana, USA

Cosmogenic 3 He and 10 Be ages measured on surface boulders from the moraine sequence deposited by the northern outlet glacier of the Yellowstone ice cap indicate that the outlet glacier reached its terminal position at 16.5 6 0.4 3 He ka and 16.2 6 0.3 10 Be ka, respectively. Concordance of these ages supports the scaled production rates used for 3 He (118.6 6 6.6 atoms · g 21 ·y r 21 ) and 10 Be (5.1 6 0.3 atoms · g 21 ·y r 21 )( 62s at high latitudes at sea level). Two recessional moraines upvalley from the terminal moraine have mean ages of 15.7 6 0.5 10 Be ka and 14.0 6 0.4 10 Be ka, respectively, and a late-glacial flood bar was deposited at 13.7 6 0.5 10 Be ka. These cosmogenic chronologies identify a late Pinedale glacial maximum in northern Yellowstone that is significantly younger than previously thought, and they suggest deglaciation of the Yellowstone plateau by ;14 10 Be ka.

Assessing climatic and nonclimatic forcing of Pinedale glaciation and deglaciation in the western United States

New 10 Be surface exposure ages from adjacent valleys in the upper Arkansas River basin, Colorado (United States), indicate that Pinedale maxima culminated asynchronously at 22.4 ± 1.4, 19.2 ± 0.2, 17.8 ± 0.6, and 15.8 ± 0.4 ka, but that deglaciation initiated synchronously between ca. 16 and 15 ka. These data are combined with published glacial chronologies across the western United States, and indicate that although the ages of Pinedale terminal moraines vary within individual ranges as well as regionally, most western United States glaciers remained near their Pinedale termini until ca. 16 ka, at which time widespread deglaciation commenced. We hypothesize that the near-synchronous demise of glaciers across the western U.S. between ca. 15 and ca. 13 ka was driven by the first major Northern Hemisphere warming following the Last Glacial Maximum, but that some differences in Pinedale culmination ages can be explained by nonclimatic factors intrinsic to individual valleys. These results suggest the need for caution in focusing exclusively on climate forcings to explain apparent asynchrony in Pinedale maxima.

Surface‐melt driven Laurentide Ice Sheet retreat during the early Holocene

Received 21 September 2009; revised 3 November 2009; accepted 30 November 2009; published 30 December 2009. [1] To better understand mechanisms of ice-sheet decay, we investigate the surface mass balance of the Laurentide Ice Sheet (LIS) during the early Holocene, a period of known rapid LIS retreat. We use a surface energy-mass balance model (EMBM) driven with conditions derived from an equilibrium atmosphere-ocean general circulation model 9 kilo-years ago simulation. Our EMBM indicates a net LIS surface mass balance of 0.67 ± 0.13 m yr 1 , with losses primarily due to enhanced boreal summer insolation and warmer summers. This rate of loss compared to LIS volume reconstructions suggests that surface ablation accounted for 74 ± 22% of the LIS mass loss with the remaining loss likely driven by dynamics resulting in basal sliding and calving. Thus surface melting likely played a governing role in the retreat and disappearance of this ice sheet. Citation: Carlson, A. E., F. S. Anslow, E. A. Obbink, A. N. LeGrande, D. J. Ullman, and J. M. Licciardi (2009), Surface-melt driven Laurentide Ice Sheet retreat during the early Holocene, Geophys. Res. Lett., 36, L24502, doi:10.1029/ 2009GL040948.

Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation

Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible deglaciation triggers. Using boulder 10 Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO 2 supports the Milankovic hypothesis of orbital forcing of deglaciations.

Pleistocene to Holocene Growth of a Large Upper Crustal Rhyolitic Magma Reservoir beneath the Active Laguna del Maule Volcanic Field, Central Chile

The rear-arc Laguna del Maule volcanic field (LdM) in the Andean Southern Volcanic Zone, 36 S, is among the most active latest Pleistocene–Holocene rhyolitic centers globally and has been inflating at a rate of> 20 cm a since 2007. At least 50 eruptions during the last 26 kyr allow for a thorough interrogation of changes in the physical and chemical state of this large, 20 km diameter, silicic system. Trace element concentrations and Sr, Pb and Th isotope ratios indicate that the mafic precursors to the LdM rhyolites result from mixing between partial melts of garnet-bearing mantle and crust in Th-excess and partial melts of garnet-free crust in U-excess. The U/Th ratios of the LdM lavas are decoupled from the slab fluid signature, similar to several recently studied frontal arc volcanic centers in the Southern Volcanic Zone. A narrow range of radiogenic isotope compositions and increasing isotopic homogeneity with differentiation indicate that silicic magma is generated by magma hybridization and crystallization in the upper crust with limited involvement of older, radiogenic material. New Ar/Ar and Cl ages reveal a wide footprint of silicic volcanism during the early post-glacial (25–19 ka) and Holocene (c. 8–2 ka) periods, but focused within a single eruptive center during the interim period. Subtle temporal variations in trace element compositions and two-oxide temperatures indicate that these eruptions, issued from vents distributed within a similar area, tapped at least two physically discrete rhyolite reservoirs. This compositional distinction favors punctuated extraction and ephemeral storage of the erupted magma batches. Frequent mafic recharge incubates this long-lived, growing shallow silicic magma reservoir above the granite eutectic, which favors magma interactions over rejuvenation of nearto sub-solidus silicic cumulates. A long-term rate of mass addition—extrapolated from surface deformation accumulated over the past decade—is comparable with those that have produced moderateto largevolume caldera-forming eruptions elsewhere.