Effect of early marine diagenesis on coral reconstructions of 20th century changes in surface-ocean carbonate saturation state

Ice sheets: landscape preservers or destroyers.

D. Fabel, J. Harbor1, A. Stroeven2, J. Kleman2, C. Hättestrand2, D. Elmore3 and D. Fink4

1 Department of Earth and Atmospheric Sciences, Purdue University, U.S.A.
2 Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden
3 PRIME Lab, Purdue University, U.S.A.
4 Australian Nuclear Science and Technology Organisation.

Critical tests of global climate models include their ability to reconstruct environmental conditions during former periods of distinctly different or rapidly changing climate, such as glacial maxima and periods of large-scale ice-sheet growth and decay. Global climate signals and ice volume estimates have been extracted from deep-sea, coral reef, and ice sheet proxies. Over the past 20 years, since the CLIMAP reconstruction of the Last Glacial Maximum (LGM), including LGM ice volumes, there has been growing concern that ice sheet thicknesses and volumes suggested by CLIMAP may have been overestimated, and that new reconstructions are needed. However, CLIMAP estimate of LGM ice volumes are broadly consistent with some recent new evidence: (i) far-field evidence of sea level low-stands imply more ice on earth than formerly thought, (ii) relative sea level curves from glaciated regions do not normally cover the late glacial period, and, hence, potentially underestimate the volume of ice at LGM, (iii) the Arctic ocean may have been covered by an ice shelf, and (iv) direct glacial-geological observations of ice sheet extent have modified prevailing perceptions of ice sheet volume (e.g. the Innuitian Ice Sheet is now commonly accepted). However, key aspects such as the height and evolution of individual paleo-ice sheets cannot be traced in proxy sea level data, yet have a significant impact on climate models. Potential records of the paleotopography of Northern Hemisphere ice sheets are found in formerly glaciated areas in northern Sweden, western Norway, Scotland, and eastern Canada. In these locations, features such as the upper limit of glacially eroded bedrock surfaces (trimlines) and differences in rock weathering (weathering zones) have been interpreted as indicators of former ice sheet height. This interpretation has been questioned by others who have argued that such features may reflect internal thermal boundaries between wet (warm-) based erosive ice and dry (cold-) based ice that is effectively non-erosive. Resolving this issue is critical because of differences in predicted ice sheet configurations. For example, a large and thick late Weichselian Fennoscandian ice sheet (FIS), as opposed to a much thinner, and possibly multidomed, late Weichselian ice sheet.

Over the past three years we have worked collaboratively to examine deglaciation chronology and patterns of erosion and landscape preservation in the northern Swedish mountains, the core area of the Fennoscandian ice sheet using in situ produced cosmogenic 10Be and 26Al. The accumulation of in situ produced cosmogenic 10Be (half-life = 1.51 ± 0.05 x 106 yr) and 26Al (half-life = 7.1 ± 0.2 x 105 yr) in quartz exposed to cosmic radiation provides a means of determining the amount of time the rock has been at or near the ground surface. Because nuclide production decreases with depth, removal of two or more meters of irradiated rock during one glacial event will create a zero age surface. In this context, areas known to have been ice covered should have exposure ages equivalent to deglaciation if they were significantly eroded by ice and older exposure ages if they suffered limited erosion or were completely protected. Ice cover of 10m or more shields the underlying rock surface from most cosmic radiation, so areas that undergo multiple cycles of ice sheet overriding but no erosion should accumulate 10Be and 26Al cosmogenic nuclide concentrations equivalent to the sum of the ice free periods, minus decay during periods of ice shielding.

Patches of relict landform assemblages were identified in the northern Swedish mountains through extensive field and air photo mapping. Quartz-rich samples for cosmogenic radionuclide analysis were collected from bedrock outcrops and erratics in mapped relict patches. Erratics confirm that the sites were overridden by ice and were dated to determine whether they were deposited during the last glaciation. Bedrock in relict patches was sampled to determine whether these sites were in fact eroded during the last glaciation (to give a deglacial exposure age) or whether they are relict (exposure ages reflect both post-glacial time and inheritance from one or more previous ice-free period).

Our results from erratics provide deglaciation ages and indicate that relict patches have been covered by ice during the last glaciation. Bedrock samples from the same relict patches provide much older cosmogenic ages suggesting insufficient glacial erosion and hence preservation of these patches during multiple ice overriding events. Relict patches therefore appear to represent areas of frozen bed conditions at the base of the FIS supporting the current swing in opinion toward the idea that, under given boundary conditions, ice sheets are landscape 'preservers' rather than 'destroyers'.