Abstract:
Siderite wheat-grain crystals formed as early diagenetic products in peaty sediments may provide excellent isotopic tools for reconstructing the sources and evolution of paleoprecipitation for ancient non-marine basins. We examine a case study for lower Albian continental deposits from the mid-Cretaceous Kerguelen Plateau, then located 60 S in a mid-Gondwana setting. The samples derive from basal deposits over basalts of ODP Leg 120 Site 750. Siderite occurs in quartz-free, kaolinite-rich, ped-like clods of dark fissile claystone with abundant woody debris. Crystals show two habits, either coarse sand size oblong wheat grains with stubby terminations or as fine sand-size clear slender prismatic twin sprays. Carbon delta values of -10 to -20 per mil are consistent with the early diagenetic mechanism of formation within marshy environments with degrading organic matter. The oxygen isotopic delta values ranging from (super -) -6 (super -) to -3.5 per mil PDB appear quite positive for the paleogeography and could imply nearby seaways, or warm, equable climate of lowland areas. The latter is consistent with hypotheses of high latitude Albian warming based on pollen assemblages and dense conifer podocarp forest evidence, but the isotopic values remain puzzling unless moisture sources derive from lower latitudinal monsoons.