Understanding Valanginian continental climate using δ18O from sphaerosiderites as a proxy for precipitation

Terrestrial and marine carbon isotopes record a significant perturbation of the global carbon cycle during the Valanginian: the Valanginian positive Carbon Isotope Event (CIE). The CIE was associated with warm, invariant, low-latitude sea surface temperatures, increased humidity, and ocean fertilization. Although marine changes are well documented, terrestrial temperatures and hydrological response during the CIE are not yet fully understood. The non-marine Wealden Supergroup of Southern England (Berriasian–Hauterivian) provides an ideal record for reconstructing low-latitude continental palaeoclimate during the CIE. These strata contain abundant sphaerosiderites, spherical iron carbonate concretions (FeCO3), which formed mainly in wetland environments and are thought to record meteoric δ18Ogroundwater reflective of regional hydrology. Wealden δ18Osiderite varies between –3.5‰ and 0.6‰ VPDB, Δ47 temperatures are 33–39° C, and reconstructed groundwater values are isotopically heavy (–3.7‰ to 2.6‰ VSMOW). Compositional and morphological heterogeneity of the sphaerosiderites, comparison to experimental siderite syntheses, isotope transects of individual spheres, high Δ47 temperatures, and comparisons to climate models suggest that the sphaerosiderites are a seasonally, summer-biased palaeohydrological record. The δ18Osiderite values from sphaerosiderite horizons alternate between isotopically heavy and light values, interpreted as drier (less precipitation/higher evaporation) and wetter (higher precipitation/less evaporation) conditions, respectively. Correlation to marine records using carbon isotope stratigraphy suggests synchronous relatively arid periods at the onset of the CIE, and just proceeding the peak of the CIE. A peak in humidity observed in the Tethyan realm during the peak of the CIE cannot be assessed conclusively in the Weald basin, due to poor siderite and organic matter preservation in the Wadhurst Clay. A facies-specific δ13Corg shift and these poorly-preserved sphaerosiderites suggest a significant palaeoenvironmental change during deposition of the Wadhurst Clay, which needs further assessment. The geochemical records from the Weald Basin indicate that low-latitude terrestrial temperatures continued to be warm during the Valanginian CIE, supporting evidence from sea surface temperature reconstructions, and that there was periodic enhancement of the hydrological cycle during the Valanginian.