THE RELATION OF GREAT BASIN LATE QUATERNARY HYDROLOGIC AND CRYOLOGIC VARIABILITY TO NORTH ATLANTIC CLIMATE OSCILLATIONS L. Benson a, R. Spencer b, D. Rhode c, L. Louderback d, R. Rye e a U. S. Geological Survey, 3215 Marine Street, Boulder, CO 80303, USA b Department of Geology and Geophysics, University of Calgary, Alberta T2N 1N4, Canada c Desert Research Institute, Division of Earth and Ecosystem Sciences, 2215 Raggio Parkway, Reno, NV 89512, USA d Anthropology Department, University of Nevada,1664 North Virginia Street, MS 096, Reno, NV 89557, USA e U.S. Geological Survey, MS 963, Denver Federal Center, Lakewood, CO 80225, USA
HEINRICH AND DO EVENTS Heinrich events were first recognized in marine sediments as layers containing high percentages of carbonate fragments in the 180-µm to 3-mm sediment-size fraction (Heinrich, 1988). Broecker et al. (1992) interpreted the Heinrich layers as debris released during massive influxes of icebergs into the North Atlantic. The carbonate fragments in Heinrich events 1, 2, 4, and 5 indicate that much of the ice that transported the carbonate came from the LIS (Bond et al., 1992). Heinrich events occur during cold DO stadials. There are two principal theories regarding the relation of Heinrich events to variability in the strength of the AMOC. In one theory (SURGE FIRST), the Heinrich event surges ice from the LIS producing a freshwater cap on the North Atlantic which shuts down the production of NADW and shuts down the AMOC (MacAyeal, 1993). In the other theory (SURGE LAST), freshwater is released from ablation of the LIS or melting of pack ice in the North Atlantic, shutting down the AMOC, which through a variety of possible mechanisms leads to the ice surging associated with a Heinrich event (Clark et al., 1999; Calov et al., 2002). Both theories are consistent with the observation that abrupt climate change in almost all climate models is triggered by changes in the surface freshwater balance of the North Atlantic. During a non-heinrich DO stadial, air and water temperatures in the North Atlantic region also decline in concert with reductions in the strength of the AMOC (Boyle, 2000). Wunsch (2006) has suggested that DO oscillations observed in Greenland ice were not generated by shifts in North Atlantic ocean circulation, given the small contribution of heat from the high-latitude ocean to the overall meridional heat flux. Instead, Wunsch suggests that DO oscillations are a consequence of the interactions of the windfield with Northern Hemisphere ice sheets and that changes in ocean circulation also are a consequence of wind shifts.
Outburst floods: 8.2 ka event (8.4-8.0 cal ka) Preboreal Oscillation (11.3-10.7 cal ka) Younger Dryas (12.9-11.6 cal ka) Surging Ice: Heinrich 1 (15.8 ka) Heinrich 2 (23.9 ka) Heinrich 3 (30.0 ka) Heinrich 4 (38.5 ka) Melting Ice: Dansgaard-Oeschger interstadial 1-12
(A) Blue Lake/Bonneville core, (B) Wilson Creek Fm, (C, D) Owens Lake cores, (E) Pyramid Lake core. The age model for the Wilson Creek Formation (Fig. B) was constructed using the GISP2 ages of H1, H2, H3, H4 and their paleomagnetic secular variationbased locations in the sediment column together with the GISP2 age of the MLE (33.9 cal ka) and the calibrated 14 C age of the top of the stratigraphic section (14.14 cal ka) (Benson et al., 1998). All 14 C ages were assigned a zero reservoir age except those for Pyramid Lake which were assigned a 600-yr value before calibration (Fig. E). However, sediments in Bonneville associated with the Mazama tephra are 1800 14 C yr too old. Age-model calibration done using CALIB 5.0.1 (Stuiver et al., 2005) for 14 C ages <19,250 yr. Older ages calibrated by interpolation of Cariaco-based chronology discussed in Hughen et al. (2005).
(1298 m) 8-m sediment core 1-cm sampling interval 1-cm ~ 60 yr
The Bonneville climatic highstand/provo was terminated by the warming that followed H1. Periods of aridity occur during H3 and slightly before H2 and H4. If we invoke a 1000-yr reservoir correction then H2 and H4 are associated with aridity and H3 is not. The YD and PBO are associated with abrupt increases in lake size. The overall shape of the Bonneville δ 18 O record resembles the GISP2 δ 18 O record.
BONNEVILLE CARBONATE MINERALOGY Aragonite, high-mg calcite, low- Mg calcite form in sequence as salinity decreases. In general the data support the isotope and TIC records.
Cattails Typha latifolia Typha angustifolia Cyperaceae Sedges Potamogeton Spore A Pediastrum Botryococcus 1000 2000 3000 4000 6 5 Zones 5000 6000 7000 8000 9000 4 3b 10000 11000 12000 13000 14000 15000 16000 YD 3a 2 1 17000 Age (cal BP) 40 1000 2000 2000 4000 Pollen grains per cubic centimeter/years per sample 400 1000 200040 15 Appendix A. Aquatic pollen accumulation rates. Accumulation rates were calculated by dividing the pollen concentration by the number of years represented by the sample (Data of Lisbeth Louderback)
Summer Lake and GISP2 records tied together by the Mono Lake excursion (Zic et al., 2002). IRM record stretched to match the GISP2 δ 18 O record. Cold N. Atlantic temps. occur during low lake levels. Low IRM occurs during low-lake levels when magnetite is lost due to oxidative decomposition of TOC. M L E Summer Lake δ 18 O record indicates low lake levels during Heinrich events but GISP2 interstadials do not usually line up with Summer Lake δ 18 O maxima (Benson et al. 2003).
Mono Lake record indicates aridity during H2 and H1 and possibly during H4. Two highstands (δ 18 O minima): One during glacial maximum (20 cal ka) and another at 14.9 cal ka (terminated by BØA warm Interval). TIC values between 30 and 15.5 cal ka indicates that Tioga glaciation was also terminated by the BØA.
Pyramid Lake δ 18 O record indicates aridity during H1 and a highstand at 15.5 cal ka that was terminated by BØA. The Pyramid Lake TOC record indicates that the TIOGA glaciation began ~28 cal ka and probably ended during or after the BØA. Note that shape of Pyramid Lake δ 18 O record is similar to GISP2 δ 18 O record but shifted in time and that the frequency of TOC oscillations is similar to the frequency of GISP2 δ18o oscillations.
Owens Lake desiccated during H1. The TIC record indicates Tioga glaciation occurred between 29.5 and 15.5 cal ka and was terminated by BØA. The TOC record indicates that six pre-tioga Sierran glacier advances occurred during GISP2 stadials. The YD and PBO oscillations were associated with peaks in Owens Lake overflow (δ 18 O and TOC minima).
GLACIER RECESSION IN THE UINTA, WASATCH AND SIERRA NEVADA MOUNTAINS Well-constrained cosmogenic surface-exposure 10 Be ages for two LGM moraines in the southwestern Uinta Mountains have ranges, respectively, of 16.7 ± 1.5 to 19.9 ± 2.0 ka and 16.1 ± 1.5 to 18.0 ± 1.9 ka (Munroe et al., 2006). Bear Lake IRM data indicate glacial activity between 32 and 17.5 cal ka (Joe Rosenbaum, pers. comm.). 10 Be ages of moraines deposited after the Bonneville flood, downvalley of the mouths of Little Cottonwood and Bells canyons on the western flank of the Wasatch, have younger ages, ranging from 16.9 ± 0.4 to 15.2 ± 0.4 ka (Elliot Lips, pers. comm.). Sierran Tioga 4 glaciers retreated from Chiatovich Creek between ~15 and 14 36 Cl ka (Phillips et al.,1996).
SUMMARY H1 was a dry period in the Great Basin. H2 and H4 may also have been associated with dry periods. The YD and PBO were associated with abrupt increases in lake size. The overall shapes of the Lake Bonneville and Lake Lahontan δ 18 O records resemble the GISP2 δ 18 O record shifted in time. Alpine glaciers and highstand lakes were terminated by the warming that followed H1 (~15.5 cal ka) and which caused changes in the topography of the LIS. Between 39 and 29.5 cal ka, Sierran alpine glacial advances were associated with four DO stadials and two Heinrich events (which also occur during DO stadials). The lack of consistent coherence between the GISP2 and Great Basin hydrologicbalance records suggests that the Heinrich/DO signal may have been shifted in time and/or space before it reached the Great Basin, or that the precipitation field over the Great Basin was spatially inhomogeneous, or that the age models used to construct the hydrologic-balance records are flawed, or that older carbonate-containing sediment may have been reworked and transported to core sites during lowstand lakes that accompanied Heinrich events and may have confounded isotopic records of low-lake levels.
Chappell (2002) calculated that 10-15 m increases in sea level occurred during H4 and H5; Clark et al. (2007) recently modeled the response of sea level to changes in the mass balance of Northern Hemisphere ice sheets. These simulations, indicate that H4 through H6 were accompanied by 10-17 m of sea-level rise). An isotope-based calculation by Roche et al. (2004) suggested that H4 released only 2 ± 1 m sea-level equivalent of ice; Bintanja et al. (2005) has calculated the amount of water stored in the LIS during the past 1.2 Ma. We used the amount of water stored in the LIS just prior to H1, H2, and H4 and estimates of sea level change, ranging from 2-15 m, to calculate the percentage of LIS ice transported to the North Atlantic during a Heinrich event, assuming all the ice came from the LIS. H1 H2 H3 H4 Percent Reduction in Size of Laurentide Ice Sheet Heinrich event 2 m 3.1 3.3 4.0 5.0 Increase in sea level 5 m 7.7 8.3 10.0 12.5 10 m 15.4 16.7 20.0 25.0 15 m 23.1 25.0 30.0 37.5
HEINRICH EVENT FORCING OF THE WINDFIELD We offer the hypothesis that the change in size and shape of the LIS associated with warming after H1 and with Heinrich events, in general, caused a shift in the mean position of the PJS away from the catchment areas of one or more Great Basin lakes. For example, Dyke et al. (2002) have suggested that H1 probably drew down the entire central ice surface positioned over Hudson Bay. Such a change in topography should have affected the windfield, including the trajectory of the PJS, which is in agreement with the hypothesis of Wunsch (2006).
Changes In the Trajectory Of The Polar Jet Stream Many of the lakes discussed in this paper indicate increases in wetness at 20 ± 1 and 15.5 ± 0.5 cal ka. Mono Lake experienced highstands at 20.3 and 15.0 cal ka, and Lake Mojave to the south experienced prominent highstands centered at 21.7 and 14.8 cal ka. 16-15 ka These records indicate that the mean position of PJS reached its most southerly extent during the LGM (21-20 cal ka). At ~16 cal ka, Pyramid Lake/Lake Lahontan (~40ºN) experienced its maximum highstand; however, Mono Lake and Lake Mojave also experienced highstands, suggesting that the PJS still affected latitudes south of Pyramid Lake. 21-19 ka