DEPOSITION AND EROSION OF AN APPALACHIAN TUFA DEPOSIT: HYDROLOGIC RESPONSE TO CLIMATE CHANGE?

Tufas, travertine, and marl are surficial carbonate deposits that yield information on Holocene climatic and hydrologic changes in temperate karst regions across the globe. In some karst areas of the Appalachian Mountains, such deposits are abundant, yet are greatly eroded. Here, we present results of a study of a tufa deposit along Spout Run in Clarke County, Virginia, where the stream has cut a channel through its own ancient tufa deposits via a process of headward erosion and incision. More than 8 m of tufa deposited in former waterfalls and pools are currently exposed along the cut banks of the stream. Augering into the floodplain shows that marl exists to depths of up to 6.0 m below the ground surface at least 100 m away from the stream.

Results of a diel study of geochemical dynamics during low flow in the summer of 2012 showed that calcite precipitation was controlled primarily by CO₂ outgassing driven by changing stream water temperature, and that the stream water was saturated with calcium carbonate throughout the 48 hr sampling period. We infer that water temperature exerts a primary geochemical control on calcium carbonate precipitation in tufa-depositing streams in general; however, hydrologic changes may control overall tufa accumulation or erosion.

Three calibrated carbon-14 ages on carbonate and shell material (corrected for a dead-carbon reservoir age of 1690 yr) show decreasing ages moving 0.5 km downstream of 3530±65 yr, 2750±55 yr, and 1350±65 yr BP, indicating progressive tufa accumulation during the early part of the late Holocene. The latest stage of tufa aggradation occurred at an ancient waterfall that once spilled into the Shenandoah River at the confluence with Spout Run approximately 1350 yr BP (600 CE), preceding a period of anomalous warming in the Mid-Atlantic region known as the Medieval Climate Anomaly (~800-1500 CE). Charcoal embedded in the uppermost part of a second tufa deposit located 11 km southwest along Borden Marsh Run yielded a similar radiocarbon age of 1405±35 yr (545 CE). Headward erosion of the two streams since ~1400 yr BP (550 CE) implies that the climatic shift out of a cooler regime into a warmer regime initiated stream incision, despite continued carbonate deposition. Thus, hydrologic changes may outweigh warming temperatures for controlling Holocene tufa preservation in this region.