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Abstract
Ferric oxides (including oxyhydroxides) are common minerals under various surface depositional conditions. Ferric oxides contain in their structure oxygen atoms that are directly related to the oxygen atoms of the water from which the oxides precipitated. They have great potential as mineral proxies for paleohydrological and paleoclimatic reconstruction if the oxygen isotope fractionation-temperature (α-T) relationship for the ferric oxide-water system can be established experimentally and demonstrated to apply in recent natural settings.
Synthesis experiments under controlled chemical conditions resulted in slightly different α-T relationships for different phases [hematite (α-Fe2O3), goethite (α-FeOOH), and akaganeite (β-FeOOH)]. All of the α-T relationships show little sensitivity to T under near surface conditions (30°C–140°C). Study on the oxygen isotope compositions of recent ferric oxides from a variety of settings indicate that the oxygen isotope fractionation between ferric oxide and water in nature is consistent with the synthesis results, although interpretation of the water signal may be complicated.
Since the oxygen isotope composition of pedogenic ferric oxides tracks that of local surface water, which is strongly correlated to temperature and vapor transport dynamics, continental paleoclimate can be reconstructed through oxygen isotope analysis of ferric oxides. This approach was applied to the Paleocene-Eocene (P-E) transition in Wyoming and the Eocene-Oligocene (E-O) transition in South Dakota, the two most important climatic transitions in the Cenozoic. Ferric oxides in both cases were associated with vertebrate fossils in paleosol deposits. The hematite coatings on fossils in the Bighorn Basin (BHB), Wyoming, are pedogenic minerals that formed under surface conditions and experienced little burial alteration, as indicated by petrographic and isotope-geochemical evidence, and thus are ideal substrates for paleoclimatic reconstruction. Oxygen isotopes in hematite from the BHB reveal a significant cooling in the very early Eocene, closely tracking the rise and fall of paleotemperature indicated by independent proxies. Goethite was the dominant ferric oxide coating fossils in the E-O deposits in Badlands National Park. The oxygen isotope variation of goethite from latest Chadronian to late Whitneyan matches data from other land proxies as well as the marine climate record, indicating a dramatic cooling in the early Oligocene.
