Reconstructing Early to Mid-Cretaceous Climate Dynamics: A Continuous Marine pCO2 Record&#160;
Authors: Graham Olivia A. , Witkowski Caitlyn R. , Naafs B. David A. 
Published: March 2024

The Cretaceous period (145-66 Ma) experienced dramatic changes in climate, biogeochemistry, and biotic innovation. Climate varied between a super greenhouse and coolhouse world (O'Brien et al., 2017), multiple ocean anoxic events (OAEs) drove major changes in ocean chemistry and biodiversity (Jenkyns, 2010), and angiosperms became the most dominant land plant group on Earth (Lidgard and Crane, 1988, Condamine et al., 2020). However, we are unable to assess the role of pCO2 in driving these climatic, biogeochemical, and biotic changes because there is no continuous, marine based, pCO2 record for this period, mainly due to the lack of established marine-based proxies able to span this time interval.To address this issue, we measured the carbon isotopic composition of the general phytoplankton biomarker, phytane, in ~50 sediment samples from Deep Sea Drilling Project Site 398 that span the early and middle Cretaceous (Hauterivian to Cenomanian). Additionally, we reconstruct sea surface temperature (SST) using the TEX86 paleothermometer in the same sediments, providing a long continuous temperature record from a single site and thus bridging multiple important &#8216;gaps&#8217; in the current record (O'Brien et al., 2017). Together, our findings provide the first continuous marine pCO2 and temperature record of the early to mid-Cretaceous, spanning the Hauterivian to Cenomanian.Our results indicate SSTs around 30-35 &#176;C for most of the Hauterivian to Albian. There is a transient warming during OAE 1a (~120 Myr) followed by a more gradual warming into the Cenomanian. During the Cenomanian SSTs reach maxima of ~40 &#176;C at this mid-latitude site, consistent with other SST records from this period that indicate extreme warmth. pCO2 values during the Hauterivian to Albian vary between 1000 and 2000 ppmv, consistent with the elevated SSTs at this time. However, unexpectedly, we do not observe a rise in pCO2 during the Cenomanian when SSTs reach their maxima. These results suggest that pCO2 was not the main driver of the Cenomanian super hothouse.&#160;References:CONDAMINE, F. L., SILVESTRO, D., KOPPELHUS, E. B. &amp; ANTONELLI, A. 2020. The rise of angiosperms pushed conifers to decline during global cooling. Proceedings of the National Academy of Sciences, 117, 28867-28875.JENKYNS, H. C. 2010. Geochemistry of oceanic anoxic events. Geochemistry, Geophysics, Geosystems, 11.LIDGARD, S. &amp; CRANE, P. R. 1988. Quantitative analyses of the early angiosperm radiation. Nature, 331, 344-346.O'BRIEN, C. L., ROBINSON, S. A., PANCOST, R. D., SINNINGHE DAMST&#201;, J. S., SCHOUTEN, S., LUNT, D. J., ALSENZ, H., BORNEMANN, A., BOTTINI, C., BRASSELL, S. C., FARNSWORTH, A., FORSTER, A., HUBER, B. T., INGLIS, G. N., JENKYNS, H. C., LINNERT, C., LITTLER, K., MARKWICK, P., MCANENA, A., MUTTERLOSE, J., NAAFS, B. D. A., P&#220;TTMANN, W., SLUIJS, A., VAN HELMOND, N. A. G. M., VELLEKOOP, J., WAGNER, T. &amp; WROBEL, N. E. 2017. Cretaceous sea-surface temperature evolution: Constraints from TEX86 and planktonic foraminiferal oxygen isotopes. Earth-Science Reviews, 172, 224-247.