|Abstract or Summary
- The goal of dissertation research was to use geochemical
- The goal of dissertation research was to use geochemical, statistical andgeological methods to constrain and understand climate variability over severaldifferent time scales. Specifically, I have addressed three questions regarding pastclimate change: (1) how does the record of Irish cirque glaciers constrain thedimensions of the Irish Ice Sheet during and since the Last Glacial Maximum (LGM);(2) what is the record of millennial-scale glacier variability in Ireland during the lastglaciation; and (3) how did variability of various components of the climate systeminteract to contribute to the evolution of climate over the last 800,000 years.The first chapter involves constraining the vertical and spatial extent of theIrish Ice Sheet (IIS). Reconstructions of the LGM IIS are widely debated, in large partdue to limited age constraints on former ice margins and due to uncertainties in theorigin of the trimlines used to identify vertical ice limits. The greatest differencesexist in southwestern Ireland where reconstructions either have complete coverage bya contiguous IIS that extends onto the continental shelf or a separate, southernsourcedKerry-Cork Ice Cap (KCIC) with more limited spatial and vertical extent.New ¹⁰Be surface exposure ages from two moraines in a cirque basin in this regionprovide a unique constraint on ice thickness for this region insofar as the presence ofa cirque glacier at a given time clearly indicates that the site was not covered by theIIS. My new ¹⁰Be ages from these two moraines show that the central mountains insouthwestern Ireland were not covered by the IIS or a KCIC since at least 24.5±1.4ka, thus supporting the more-limited reconstructions of the IIS at the LGM, indicatinga reduced contribution to sea-level change and a smaller loading of the solid Earth,which is consistent with models of glacial isostatic adjustment to the IIS.The second chapter presents research that has developed a record ofmillennial-scale variability in former Irish cirque glaciers between ~25 ka and 10 ka.Small alpine glaciers are sensitive to climate, and the paleo record of past smallglacierfluctuations offers an outstanding opportunity to use this glacier sensitivity fordeveloping centennial- to millennial-scale records of climate variability. BecauseIreland is immediately adjacent to, and downwind of, the North Atlantic, glacialrecords there are ideally located to record past climate changes associated withchanges in North Atlantic Deep Water (NADW) formation and attendant feedbacks. Ihave developed a high-precision ¹⁰Be surface-exposure chronology of multiplemoraines deposited by glaciers in eight cirque basins across Ireland to constrain thisvariability. The data show a remarkable record of persistent millennial-scalevariability between 24.5±1.4 ka and 10.8±0.7 ka. Several of these events areassociated with known climatic events during the last deglaciation such as onset ofthe Bølling-Allerød and end of the Younger Dryas. However, this persistent signalextends back to the Last Glacial Maximum (LGM), suggesting a previouslyunidentified mode of climate variability unrelated to large changes in the NADW.Multi-decadal to multi-centennial variability identified in Greenland ice cores presenta mechanism for the variability recorded in the Irish glaciers.The third chapter of this research involves characterizing and explainingclimate variability at orbital timescales across the mid-Brunhes Transition (MBT;~430 ka). The MBT involved a change in the amplitude of variability associated withcooler interglacials prior to 430 ka and warmer interglacials after. The key questions Iaddress include determining whether other components of the climate system changedat this time, and identifying the mechanism for the MBT. Statistical tests of multipleproxies (sea-surface temperature, ∂¹⁸O, ∂¹³C, CO₂, CH₄, and dust) indicate that theMBT was largely a reorganization of the global climate system perhaps driven by anincrease in interglacial CO2 concentrations. Changes in marine ∂¹³C may provideinsights into this change in the carbon cycle, perhaps associated with changes inglobal ocean circulation. In particular, there is a large positive ∂¹³C excursion duringthe interglacial immediately prior to the MBT, suggesting an enrichment of theAtlantic basin at this time relative to other interglacials of the past 800 kyr.Variability in the depth gradient of ∂¹³C from the North Atlantic show increasedcorrelation with South Atlantic ∂¹³C records after the MBT, indicating morehomogenous mixing of the northern- and southern-component water masses. This isaccompanied by a greater difference in the ∂¹³C latitudinal gradient in the Atlanticbasin prior to the MBT that is reduced afterwards.