Climate and carbon-cycling in the Early Cretaceous

• Main Author: Littler, K.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: UCL (University College London) 2011
• Subjects: North Atlantic
• Online Access: https://discovery.ucl.ac.uk/id/eprint/1335899/1/1335899.pdf; https://discovery.ucl.ac.uk/id/eprint/1335899/

The Cretaceous (~145–65 Ma) is widely regarded as a greenhouse period with warm, equable climates and elevated atmospheric CO2 relative to the modern. However, the earliest Cretaceous (Berriasian–Barremian; 145–125 Ma) is commonly characterised as a relatively colder “coolhouse” interval, typified by lower global temperatures than the mid-Cretaceous. Unfortunately, the lack of absolute sea surface temperature (SST) estimates prior to the Barremian has hampered efforts to definitively reconstruct Early Cretacous climate. Here, the TEX86 palaeotemperature proxy, for which a detailed review is provided, has been used to generate a 13 myr record of SST estimates for the Early Cretaceous, based on sediments from assorted deep-sea drilling sites. A consistent offset in the TEX86 ratio between transported mudstones and pelagic carbonates in the low-latitude marine sediments (DSDP Sites 603 and 534) has been identified, which may be linked to post-burial diagenesis or a difference in organic matter type between lithologies. Mindful of these apparent lithological effects on TEX86, only the pelagic sediments were used to subsequently reconstruct Early Cretaceous SSTs. These TEX86 records demonstrate both elevated SSTs (>27 ºC) at low and mid-latitudes relative to the modern, and the apparent stability of these high temperatures over long timescales. This lack of SST variation in the low-latitudes during the Valanginian positive carbon-isotope event (CIE; ~135–138 Ma), casts doubt on the warming-weathering feedback model put forward to explain this major perturbation. Additionally, new paired bulk organic (δ13Corg) and bulk carbonate (δ13Ccarb) carbon-isotope records from North Atlantic DSDP sites, have been used to reconstruct relative changes in pCO2 across the CIE. These observed fluctuations in Δ13C imply changes in carbon-cycling and a possible drawdown in CO2, due to excess organic matter burial associated with the CIE.