MICROSCOPIC GROWTH BANDING IN A STALAGMITE FROM CENTRAL TEXAS: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTION

A stalagmite with fine-scale growth banding is an uncommon but valuable paleoclimate proxy because it might provide a high-resolution record of short-term changes in precipitation, temperature, and/or soil production. A stalagmite (DDS2) from a central Texas cave has sub-millimeter visible and fluorescent banding that previous research has suggested might reflect a seasonal timescale. Two intervals of bands were photographed under plane and cross-polarized light, UV light, and with SEM. Visible banding is composed of alternating inclusion-rich and translucent bands. Under UV light, the inclusion-rich bands fluoresced. Further, a pattern of alternating light and dark fluorescent bands invisible in plane light was observed. Light and dark UV bands range in thickness from about 13 to 357 μm. SEM analysis revealed an abrupt transition with the same orientation as banding between smooth calcite and calcite with an abundance of elliptical nodules. The nodules have diameters ranging from ~50 to 125 nm, which are similar to the dimensions of nanobacteria. If these nodules are nanobacteria, they might be the organic material causing fluorescence. On the basis of average growth rates determined from U-series ages, light-dark couplets from two intervals represent between 1.2 – 5.2 and 4.4 – 10.1 years of growth for the two respective intervals. Given this time scale, the couplet banding might result from a climatic cycle similar in frequency to ENSO. Alternatively, the fluorescent bands might be seasonal if 1) the bands were undercounted due to indistinct couplet boundaries, 2) unidentified depositional hiatuses are present, or 3) the stalagmite’s growth in the two intervals studied is faster than calculated U-series growth rates as neither interval is exactly bounded by U-series ages. Adjacent bands were noted to differ in thickness by as much as ~175 μm, suggesting that growth rates might shift markedly from season to season or from one climatic cycle to the next. The difference in the ratio of light/dark band thickness between the two intervals suggest there was stronger seasonality during the interval from 15,990 – 15,290 years before present (ybp) than during the interval from 23,890 – 21,870 ybp, which is consistent with inferences from faunal records in the region.