Osmium Isotopic Evidence for Eccentricity‐Paced Increases in Continental Weathering During the Latest Hauterivian, Early Cretaceous

Abstract

Abstract The 405‐kyr eccentricity cycle is a consistent orbital parameter throughout the Phanerozoic that is associated with long‐term variations in global continental weathering. However, a lack of reliable geological evidence has hampered the understanding of the relation between the 405‐kyr eccentricity cycle and continental weathering during the Cretaceous. Os isotopic ratios (187Os/188Os) of the sedimentary record reflect the balance between radiogenic Os derived from continental weathering and Os derived from unradiogenic sources (e.g., hydrothermal activity, weathering of mafic rocks, and extraterrestrial sources). This ratio is therefore considered as a good proxy for the evaluation of short‐term changes in continental weathering patterns. To trace orbital‐paced continental weathering, this study reconstructs the marine Os isotopic records in upper Hauterivian to lower Barremian (Lower Cretaceous) carbonate rocks in central Italy, where previous studies have reported that variations in clay mineral composition are paced by the 405‐kyr cycle. Our new Os isotopic record documents periodic oscillations of 187Os/188Os between 0.7 and 0.9 that correspond to the 405‐kyr Earth's eccentricity cycle. Because the sedimentary interval with radiogenic 187Os/188Os values (∼0.9) corresponds to a time interval characterized by a humid climate in areas surrounding the Tethys, variations in the 187Os/188O values likely reflect cyclic changes in continental weathering caused by eccentricity‐paced intensification of monsoonal activity at low latitudes. This variation could have been further amplified by increased input of radiogenic Os from Paleozoic shale and Precambrian crust at higher latitudes that resulted from a latitudinal shift of the intertropical convergence zone.

Keywords

• eccentricity
• Faraoni level
• Hauterivian
• monsoon
• osmium isotope