U-series geochronology and geochemistry of vein and cave carbonate deposits in Turkey: the relationship between late Quaternary tectonic and climatic events

Abstract

This study is based on the use of vein calcites and speleothems to improve our understanding of late Quaternary tectonic activity and climatic variability in SW Turkey, and to investigate their effects on CO2 degassing events. To facilitate this, multi-generation carbonate veins and breccia within active faulting zones in Marmaris (Kumlubuk and Amos; SW Turkey) and three stalagmites from the Dim Cave (SW Turkey) were studied through thorough petrographical, geochronological (U-series dating), and geochemical (O-C-Sr isotope ratio and trace element concentration) analyses. The findings are presented as a series of four papers. The tectonic study is based on a 150-kyr long precisely-dated (U-series) record of 160 calcite vein/breccia samples from an extensional zone in SW Turkey. The age distribution of the carbonates is commensurate with periods of low to transitional summer insolation. Net moisture balance was positive during the last glacial period, which primarily reflected reduced evapotranspiration rather than increased precipitation. Host limestone sourced-CO2 accumulation at depth was enhanced by reduced infiltration of meteoric water from shallow depth; facilitating CO2 build up in the rupture zones and fault failures. This resulted in substantial CO2 degassing and carbonates (veins and breccias) formation near and at the surface. [Paper 1] Further investigations of the vein carbonates using d13C, d18O and Sr- isotope, and Rare Earth Element (and Yttrium) (REY) concentration data were undertaken to determine the sources of solutions responsible for the calcite deposition (272.6 to 20.5 kyr) and the genetic processes involved in vein formation. Based on geochemical characteristics of the veins, it is concluded that the water source for the fluids originated dominantly from meteoric sources with various degrees of interactions with the host limestone and siliciclastic basement rocks. CO2 is sourced mainly from dissolved host limestone. Microstructural/geochronological observations demonstrate that vein formation is controlled by a crack-seal mechanism, commonly accompanied by the brecciation of the host limestone. Stable isotope (O, C) records of the veins reveal a cyclic nature of CO2-degassing events with meteoric water infiltration through normal faults influencing growth-cycles of the veins. However, the overall control on faulting is the rate of regional tectonic activity. [Paper 2] The paleoclimatic work was underpinned by high-resolution stable isotope (O, C) profiles derived from three stalagmites (Dim-E2, Dim-E3, and Dim-E4) in Dim Cave (36 oN, 232 m a.s.l.), SW Turkey. The record is controlled by 44 high-precision U-series dates spanning from ~90 to 9 kyr, and covers the whole of the last glaciation. Changes in d18O can be best explained by the track changes in the trajectory of winter westerlies between a northerly (less depleted) and a more southerly (more depleted) air mass track. This change in track is attributed to the growth of the Fennoscandian Ice Sheet from MIS 4 to MIS 2, which gradually depressed winter westerly tracks southward. Glacial/interglacial swings in d13C appear to reflect changes in the vegetation/soil cover and, critically, interactions with the host limestone. A positive moisture water balance throughout the last glaciation is inferred, based on continuous speleothem growth of the Dim-E3 speleothem. In addition, isotopic responses to Heinrich events are recorded in the Dim Cave. They also reflect changes in the intensity of westerly flow and respond to the gradual change in the westerly track through the last glacial cycle. [Paper 3] Finally multi-proxy records of stable/radiogenic isotope (O, C, Sr), trace element/Ca (Mg/Ca, Sr/Ca, U/Ca, Ba/Ca, Y/Ca, and Zr/Ca), element concentration of speleothems of the Dim Cave (SW Turkey) were examined. Chemical changes in cave drip water and speleothems are attributed to alternating wet and dry periods in Dim Cave, and are consistent with changes in moisture source characteristics in SW Turkey. Trace elements are sourced from the host dolomitic limestone and display significant interactions with soil and groundwater. Higher values in Zr/Ca, U/Ca, and Y/Ca during periods of higher growth rates of Dim-E3 (during 51n40, 71n63, and 89n80 kyr) are associated with wet periods where more non-calcareous material is fed into the drip system. This may reflect either deposition of aeolian dust and/or increased dissolution of silica-rich rock. Covariance and increased values of Mg/Ca, Sr/Ca, and Ba/Ca are interpreted to be controlled by enhanced interactions between host limestone and drip waters during drier conditions (during times of lower growth rates; 40n18, 63n51, and 80n71 kyr), probably in response to the mechanisms of prior calcite precipitation and differential dissolution of dolomite in the host. [Paper 4]