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| Titel |
High sea-surface temperatures in the Early Cretaceous (Berriasian – Barremian): a problem for ice? |
| VerfasserIn |
K. Littler, S. Robinson, P. Bown, A. Nederbragt, R. Pancost |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250021052
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| Zusammenfassung |
| The Early Cretaceous (~145 – 125 million years ago) is thought to have been a period of
elevated atmospheric CO2 and warm average global temperatures. Some sedimentological
evidence, such as the presence of high latitude glendonites, seems to suggest transient
glaciations despite the inferred high CO2 conditions. Previous geochemical studies of
Early Cretaceous palaeotemperatures have relied upon oxygen-isotopes and Mg/Ca
ratios of belemnites. However, species-specific responses and uncertainty relating
to the original isotopic and elemental-ratio composition of Cretaceous seawater
remain problematic. In this study, we apply the TEX86 organic palaeothermometer
to Early Cretaceous marine sediments from DSDP/ODP sites, thereby producing
sea-surface temperature estimates which are independent of the original seawater
chemistry. Our results show extremely high average annual temperatures in the tropical
Proto-North Atlantic of at least ~35˚ C, far in excess of modern values of ~25˚ C
at the same latitude. The temperature record in this region remained essentially
constant (~1.5˚ C variation), over a time period greater than 10Ma. In addition to the
tropical sites, a high-latitude site (~60˚ S palaeolatitude) yields a minimum average
temperature of ~25˚ C, which is also far above modern values for this latitude. The
temperature range at this site (~2.5˚ C) is slightly more variable than at the low-latitude
sites.
The TEX86 temperature record demonstrates that tropical regions are not limited to modern
temperature maximums, as has been suggested by some previous oxygen-isotope studies. The
global temperature gradient constructed from these records suggests excessively warm
temperatures at higher latitudes, which it is difficult to reconcile with the existence of large
ice sheets. The small range in temperatures recorded at these sites also suggests a high degree
of long-term temperature stability in the tropics, which is interesting in light of the large
positive carbon-isotope excursion in the late Valanginian (~137Ma). This event, which is
well documented in Tethyan, North Atlantic and Pacific sections, is often attributed
to large volcanically induced changes in temperature and weathering rates. The
lack of significant changes in our temperature record suggests that any temperature
perturbations during the Valanginian carbon-isotope event were either spatially
variable or shorter in duration than the resolution of our record. In either case, it
may be that the proposed model for the Valanginian event needs reassessment. |
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