Insights into terrestrial hydrology during the Valanginian (Early Cretaceous) using sphaerosiderites from the Wealden beds of Southern England
Abstract
Terrestrial and marine carbon isotope records indicate a significant global carbon cycle perturbation during the Valanginian stage (the Valanginian positive Carbon Isotope Event, CIE)1. Although this event has typically been associated with decreased pCO2 and cooling2, evidence of warm and invariant low latitude sea surface temperatures suggests complexity in the climate response3. Therefore, the current constraints on temperature change and hydrological responses during the Valanginian CIE are poor. The non-marine Wealden strata of Southern England (Berriasian - Hauterivian) span the Valanginian CIE and provide an ideal archive for reconstructing low latitude continental palaeoclimate during this event. These strata contain abundant sphaerosiderites, spherical FeCO3 concretions, which formed in wetland environments and are thought to record the regional precipitation δ18O and atmospheric temperature signature. Sphaerosiderites from four cores spanning the entire Wealden stratigraphy were studied. Siderite clumped isotope compositions (Δ47) indicate Wealden terrestrial temperatures remained warm (32-38oC) throughout the Valanginian. Average siderite δ18O values are relatively enriched in the Late Berriasian (-0.8 to 0.2 ‰) and depleted in the Early Valanginian (-2 ‰). Siderite δ18O increases to 0.3 ‰ in the Mid Valanginian, just before the start of the Valanginian CIE, and decreases to -1 ‰ by the Late Valanginian. These periodic δ18O changes are decoupled from the CIE, and likely reflect regional changes in hydrology independent of the CIE. The geochemistry of pure siderites and more complex samples (containing more than one carbonate phase) were assessed in light of modern sphaerosiderite4 data and our own experimental results. This suggests the Wealden sphaerosiderites record a seasonally biased palaeoenvironmental signal within a dynamic wetland, correlating with sedimentological evidence5 and GCM models6. This provides a better understanding of the paleoclimatic changes through the Wealden and further constrains siderite as a proxy for palaeohydrology.
1. Erba E et al (2004) Geology 32, 149-152; 2. Price G et al (2010) Geology 28, 251-254; 3. Littler K et al (2011) Nat GeoSci 4, 169-172; 4. Wright V et al (2007) Sediment Res 70,619-632;5. Haywood A et al (2004) Cretac.Res 25, 303-311- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP44B..04S
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 4914 Continental climate records;
- PALEOCEANOGRAPHY;
- 4924 Geochemical tracers;
- PALEOCEANOGRAPHY;
- 4994 Instruments and techniques;
- PALEOCEANOGRAPHY