TY - JOUR
T1 - Isotope Geochemistry of Caliche Developed on Basalt
AU - Knauth, L. Paul
AU - Brilli, Mauro
AU - Klonowski, Stan
N1 - Funding Information:
This work was supported by NASA Exobiology grant NAG5-9430 and Astrobiology Grant NCC2-1051. Todd Luther collected and analyzed samples from the volcanic field in NW Arizona. We thank Michelle Moreno for sample preparation and assistance in the lab. Discussion with J. Quade and reviews by T. Cerling, K. Muehlenbachs, and 2 other anonymous reviewers greatly improved the initial manuscript. We salute Bob Clayton on this occasion of his retirement. Paul Knauth is humbly grateful for Bob’s guidance and introduction to the world of stable isotope geochemistry after being hired as an undergraduate hourly worker in the University of Chicago’s stable isotope lab. It was the opportunity of a lifetime which continues to reward in countless ways.
PY - 2003/1/15
Y1 - 2003/1/15
N2 - Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ18O and δ13C observed in terrestrial caliche. Within each volcanic field, δ18O is broadly co-variant with δ13C and increases as δ13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ13C characteristic of microbial soil CO2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a "warm, wet" early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche.
AB - Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ18O and δ13C observed in terrestrial caliche. Within each volcanic field, δ18O is broadly co-variant with δ13C and increases as δ13C increases. The most 18O and 13C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most 18O and 13C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ18O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ13C characteristic of microbial soil CO2. Subaerial caliche has formed from 18O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated 13C enrichment is due either to removal of 12C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a "warm, wet" early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ18O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche.
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U2 - 10.1016/S0016-7037(02)01051-7
DO - 10.1016/S0016-7037(02)01051-7
M3 - Article
AN - SCOPUS:0037438930
SN - 0016-7037
VL - 67
SP - 185
EP - 195
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 2
ER -