Decreasing hydrothermalism at Pamukkale- Hierapolis (Anatolia) since the 7th century

The dating of travertine deposition and groundwater / hydrothermal seepages in relation to late Holocene climatic changes can be achieved using short-lived isotopes of the ²³⁸U decay series, as illustrated by the present study of the Pamukkale travertine system, at the northern edge of the Denizli and Baklan graben merging area (see Özkul et al., 2013; https://doi.org/10.1016/j.sedgeo.2013.05.018. The strongly lithified self-built channels and modern pools where analysed for their ²³⁸U, ²³⁴U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po contents, whereas ²³⁸U, ²³⁴U and ²²⁶Ra were measured in modern hydrothermal waters. When corrected for detrital contamination, ²³⁰Th-ages of travertine samples range from 1215±80 years, in the oldest self-built hydrothermal channels, to the Present (modern pool carbonate deposits) thus pointing to the inception of the existing huge travertine depositional systems during the very late Holocene, probably following the major Laodikeia earthquate of the early 7^th century (cf. Kumsar et al., 2016; DOI 10.1007/s10064-015-0791-0). So far, the available data suggest three major growth phases of the travertine system: an early phase (7^th to 8^th centuries CE), an intermediate phase (~ 14^th century CE) and a modern one, less than one century old. A more detailed survey of the travertine system would be needed to strengthen these age clusters, however, worth of mention is the fact they they broadly fit with the timing of humid episodes as reconstructed from a southern Turkey paleolake study (Jones et al., 2006; doi: 10.1130/G22407.1). Despite nearly identical ²³⁴U/²³⁸U activities in modern waters and travertines (1.132±0.006), the latter show decreasing ²²⁶Ra concentrations through time, from ~ 0.2 dpm/g in the oldest shelf-built channels to ~ 0.07 dpm/g in present day pool carbonates, thus pointing to a significant decrease in the hydrothermal activity since the inception of the travertine complex, and raising concerns about the response of the Pamukkale hydrothermal system to the present climatic trend. Indeed, the predicted decrease in precipitation of up to 30% in the forthcoming decades (Lelieveld et al., 2012; DOI 10.1007/s10584-012-0418-4) is likely to result in a lesser meteoritic water recharge of the aquifer system feeding the springs (Dilsiz, 2006; DOI 10.1007/s10040-005-0001-4).