Biomass effects on stalagmite growth and isotopic ratios: A 20^th century analogue from Wiltshire, England

Increases in calcite deposition rates combined with decreases in δ¹³C and δ¹⁸O in three modern stalagmites from Brown’s Folly Mine, Wiltshire, England, are correlative with a well-documented re-vegetation above the mine. Increased soil PCO₂ resulted in greater amounts of dissolved CaCO₃ in the drip waters, which consequently increased annual calcite deposition rates. The absence of deposition prior to 1916 (28 years after the mine was closed) indicates that vegetation had not yet sufficiently developed to allow higher PCO₂ values to form in the soil. Lower δ¹³C values through time may reflect the increased input of isotopically light biogenic carbon to the total dissolved inorganic carbon (DIC). δ¹⁸O decreased synchronously with δ¹³C, reflecting the increased importance
of isotopically light winter recharge due to greater biomass-induced summer evapotranspiration. This is the first empirical demonstration that vegetation density can control stalagmite growth rates, δ¹³C, and δ¹⁸O, contributing critical insights into the interpretation of these climate proxies in ancient stalagmites.