Stable
isotope systematics of two cenotes from the northern Yucatan Peninsula,
Mexico
.
Richard A. Socki
Lockheed Martin, Astromaterials Research and Exploration Science,
Mail Code C-23, 2400 NASA Road, Houston, Texas 77058.
Eugene C. Perry, Jr.
Northern Illinois University, Department of Geology and Environmental
Sciences, DeKalb, Illinois 60115.
Christopher S. Romanek
University of Georgia, Savannah River Ecology Lab, Aiken, South Carolina
29802.
Abstract
Deep water-filled sinkholes, cenotes, are common in the northern Yucatan
Peninsula. At least five of these cenotes are deep enough to extend through
a freshwater lens of meteoric origin in which 8180 and aD follow the trend
aD = 8.11 X 8/80 + 10.4. Below this freshwater lies saline water that
originated as seawater and has retained its seawater isotopic identity.
Deep cenotes, characterized by input of variable amOunts of organic debris
from tropical vegetation and by poor circulation below the fresh-isaltwater
interface, provide excellent water columns in which to study sulfur redox
phenomena. Measurements include 0, H, and S isotope composition, conductivity,
sulfur speciation, and pH from two cenotes (Ucil,98 m deep, and Xcolac,
125 m deep). Strong 34S enrichment of sulfate and .~S depletion of sulfide
indicate anaerobic bacterial reduction of sulfate. A shift in the isotopic
composition of sulfur in Xcolac from a seawater value of +21.0%0 (CDT)
to +41.8%0 indicates conversion of sulfate to isotopically light sulfide.
Mass balance calculations indicate that escape of isotopically light sulfur
from the system is a slow process. At 80 m in Xcolac, a difference in
sulfur isotope composition between sulfate and sulfide (~J4S) of 63.2%0
is observed and could be the result of multiple sulfate reduction reactions.
Higher in the water column, sulfide oxidation occurs, probably the result
of bacterially mediated sulfide oxidation processes. A deep observation
well (lacking organic matter input) shows only a slight deviation in sulfur
isotope composition of sulfate from seawater values.
SREL Reprint #2742
Socki, R. A., E. C. Perry, Jr. and C. S. Romanek. 2002. Stable isotope systematics of two cenotes from the northern Yucatan Peninsula, Mexico. Limnology Oceanography 47:1808-1818.
