Dados da Submissão

Título

STALAGMITES GROWTH RATES BETWEEN THE HOLOCENE AND LAST GLACIAL MAXIMUM

Texto do resumo

Stalagmites growth rates (GR) greatly vary through time, as the same
sample can sometimes double or triple its GR while active or cease to grow
during some periods. The rate of stalagmite formation depends on multiple
factors, being temperature, rainfall, soil processes and soil bioactivity the most
important on a millenary timescale. The complex interaction between these
factors and the host calcareous rock dictates the amount of dissolved carbonate
in the percolating solution, and consequently, the stalagmite growth rate.
This study proposes the analysis of multiple GRs from stalagmites in South
America, comparing the U/Th dating to their oxygen and carbon isotopic profiles
to better understand how growth rate varied throughout the last 23 ky and how
climatic and environmental variables affected it, specifically temperature, rainfall
and soil erosion/formation due to changes in vegetation in South America.
Eight different caves in South America were selected. The spatial distribution of
the sites chosen assures that diverse climatic and environmental conditions are
compared, facilitating the isolation and comparison between variables influence
on GR. As for the time frame selected, the transition from LGM to Holocene is
ideal as the temperature differences (up to 5-6 °C on average) between the two
periods isolates the most important variable on calcite dissolution and
reprecipitation, as the chemical kinectics of the H2O-CaCO3-CO2 system and
soil bioactivity are highly dependent on it.
The results of this study show firstly a clear temperature influence on GRs
throughout South America, as hotter regions presented much higher rates than
colder sites during the same climatic periods. Moreover, this temperature
related behavior is also seen on the GR when comparing the Holocene and
LGM periods, as all stalagmites, with exception to samples in one cave in
northeastern semi arid Brazil, have slower precipitation rates during the last 10
ky years due to very low water availability. An interesting pattern of lower rainfall
and soil cover stabilization occurs in most cases, as δ13C values decrease, δ18O
values increase and GR increases during the Holocene.
Overall, temperature is the main driver of stalagmites GR, as it impacts the
chemical kinectics of the H2O-CaCO3-CO2 system, with higher temperatures
accelerating the reprecipitation of the dissolved carbonates. Furthermore,
increased temperatures also result in higher soil bioproduction of CO2,
increasing the dissolution rates of the host rock, which combined with a more
stable soil cover can greatly increase GR. In very arid regions water availability
is the main driver, as even though these sites show the largest GRs, their
formation ceases to happen or decreases to rates like southeastern Brazil
during the Mid-Holocene.

Palavras Chave

Growth Rate; Stalagmites; Paleoclimate; Karst