HR: 1340h
AN: H43C-0389    [Abstracts]
TI: The Effects of Bedrock age on Radiogenic Marine Isotope Records
AU: * Peucker-Ehrenbrink, B
EM: behrenbrink@whoi.edu
AF: WHOI, MS 25, Woods Hole, MA 02543 United States
AB: Since the discovery of secular changes in radiogenic isotope compositions of seawater in the second half of the last century, a variety of processes capable of forcing such changes has been proposed (glaciations, uplift of mountain ranges, changes in sea level, eruption of flood basalts, submarine hydrothermal activity). Processes affecting continental crust are particularly relevant because continental runoff dominates inputs of most radiogenic isotopes to seawater. Tributaries to the Mississippi and Fraser rivers define positive, linear correlations with different slopes between bedrock age and isotopic composition of the dissolved and particulate Sr, and negative, linear correlations with particulate Nd. These correlations indicate that processes exposing older bedrock to weathering will lead to more radiogenic Sr and less radiogenic Nd fluxes to the ocean. The different slopes are thought to be caused by the different parent/daughter ratios of the end members that define the trends in the respective drainage basins. Though not yet supported by data, similar correlations are likely to exist for other radiogenic isotope systems (Pb, Hf, Os). Based on these observations, I propose that secular changes in the average age of bedrock undergoing weathering are the root cause for changes in the radiogenic isotopic composition of continental runoff. The proposed forcing mechanisms contribute in various ways to changes in the average age of bedrock undergoing weathering. For instance, glaciations and mountain building expose older bedrock through erosion of overlying strata. In contrast, eruption of flood basalts blanket older bedrock with zero-age extrusives. Interestingly, processes that change the globally averaged age of bedrock undergoing weathering often also affect the detrital and dissolved inputs to to the ocean (e.g., glaciation, mountain building), thereby changing both the isotopic composition and the flux of elements to seawater. The proposed hypothesis implies that some processes rarely considered impacting marine isotope records are worthy of closer scrutiny. An example is the formation or destruction of large, internally drained basins, provided the average age of bedrock in such basins differs from the global average. A systematic investigation of global bedrock ages is under way to test this hypothesis and to work towards a reconstruction of changes in bedrock ages throughout the Phanerozoic. The present-day average age of mapped and dated continental crust (600 $\pm$ 125 Myr, 2 S.D.), calculated from large scale bedrock maps, serves as a first Holocene data point for such a reconstruction.
DE: 9699 General or miscellaneous
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (1625)
DE: 1030 Geochemical cycles (0330)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting