QUATERNARY TIDAL RHYTHMITES AS A PROXY FOR PALEOPRECIPITATION

There is much scientific debate over the impact of global warming on the Arctic and whether the observed recent climatic changes relate to the Arctic's natural variability or are an indication of the start of a major anthropogenic-induced climate shift in the north. The extent of climatic variability within Quaternary ice-free intervals is not fully known. Time resolutions in terrestrial and marine paleoclimatic records are largely at decadal or longer scales. Records documenting climatic changes on the order of seasonal scales and shorter are generally nonexistent.

There is a need for time-efficient ways to extract detailed high-resolution paleoclimatic records that document climatic variability at scales finer than decadal for comparison to modern records.

A recent study of 8,000- to 9,000-year-old tidal rhythmite successions in the fjords of northern Norway resulted in a technique for extracting paleoclimatic data that could impact Quaternary climatic investigations in much the same way that tree ring and ice core studies have.

Tidal rhythmites are vertically accreted, small-scale, thinly layered sands and silts deposited by tidal currents in coastal marine settings. The tidal influence on the origin of these features is indicated by the very regular progressive vertical thickening and thinning of individually accreted sets in response to changing current velocities associated with various tidal cycles.

We developed a technique whereby paleo-precipitation data with a resolution of 2 weeks can be extracted by measuring the thicknesses of the semimonthly tidal cycles. Deviations from the predicted tidal curve can be related to a seasonal fluvial discharge record superimposed on the reconstructed tidal records. The paleohydrographs show a close correlation to modern hydrographs associated with summer-winter discharge and can be used to infer stream discharge, discharge anomalies, and when certain time inferences are made, snow melt days. These data are much more precise in estimating yearly and seasonal precipitation than using pollen and spores. Our technique could potentially be applied to similar tidally influenced depositional systems in Chile, Alaska, and elsewhere.