A central Texas drying event identified at the Younger Dryas-early Holocene transition using coupled speleothem 𝛿¹³C-¹⁴C analysis

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James, Christina Danielle

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The Younger Dryas (YD, 12,900-11,700 years BP) is characterized by a return to near-glacial conditions in the northern hemisphere during the last deglacial period. Texas moisture proxy records support a general regional warming/drying trend from the YD through the Holocene, however, the timing and magnitude of changes in regional moisture and temperature conditions are poorly constrained. We use moisture proxies from a central Texas stalagmite record (McN-1) collected in the Edwards aquifer (an important human and ecological groundwater resource in central Texas) to assess how epikarst moisture conditions varied in the region during the YD-Holocene transition. The relatively high concentrations of ²³²Th (>1 ppb) in several horizons in this sample resulted in McN-1 U-series ages with high uncertainties (>10% of measured age). We use two isochrons and measured ²³⁰Th/²³²Th ratios from modern calcite grown in central Texas caves to estimate the McN-1 initial ²³⁰Th/²³²Th ratio and develop an age model with better constrained uncertainties. Strong correlations between speleothem 𝛿¹³C and ¹⁴C activities can result from changes in epikarst carbonate dissolution due to variable CO2 ventilation or pore space moisture. A decrease in the proportion of ¹⁴C-free carbon (the dead carbon proportion, DCP) to 0% is interpreted as a change from partial dissolution in a water-saturated closed system to dissolution in an open system, where carbon from limestone has no measurable effect on dissolved inorganic carbon ¹⁴C activities. A negative shift in 𝛿¹³C values of 2.8‰ coincident with a decrease in DCP from 7.5 to 0% occurs in McN-1 at the Younger Dryas-Holocene boundary in less than 230 years (as little as 50 years given age constraints). We attribute these parallel declines in 𝛿¹³C and DCP to changes in the epikarst dissolution system. If changes in the dissolution system are controlled by pore space moisture, the change to an open system at the YD-Holocene boundary indicates a rapid regional drying event. Application of a calcite dissolution model indicates that 25% of the negative 𝛿¹³C shift can be explained by change from a more closed to an open system in the epikarst dissolution region above the cave. We attribute the remaining shift in 𝛿¹³C values to reduced drip water pH, associated with an increased contribution of respired CO₂ to epikarst pCO₂. Speleothem growth rates decrease at the YD-Holocene boundary, consistent with our interpretation that carbon isotopes record a decrease in vadose zone moisture. This epikarst moisture interpretation is consistent with other Texas paleoclimate records, indicating a climate transition to drier early Holocene conditions. Compared with existing regional proxy records, the relatively high temporal resolution of the McN-1 𝛿¹³C record (inter-annual) indicates a rapid drying event concurrent with Greenland temperature increases, suggesting a contemporaneous climate response between regional and high latitude climate at end of the YD.


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