Ocean Anoxic Events
The Cretaceous was one of the warmest periods in Earth history. Several proxy reconstructions suggest that atmospheric CO2 was several times higher than at present. The Cretaceous therefore provides a potential analogue for future warm climates, and studying this interval of time can test our understanding of how the climate system works under warm conditions.
Another fascinating feature of the Cretaceous is the occurrence of several Oceanic Anoxic Events (OAEs). OAEs are brief intervals (<106 yr) when large areas of the seafloor became anoxic. Evidence of OAEs can be found in Cretaceous marine sediments, including characteristic organic-rich sediments (“black shale”), carbon-isotope excursions, major marine faunal turnover, etc. OAEs are consequences of a set of complicated feedbacks operating both in the ocean and on land, and represent major perturbations in the carbon cycle. Investigating the causes of OAEs can offer insights into how different components of the Earth system interact with each other during periods of major climatic and environmental changes.
In this group, we develop and employ new geochemical tracers to investigate various aspects of the Cretaceous climate, with a particular focus on understanding the feedbacks and consequences of OAEs.
1. Ocean circulation in the Cretaceous and during OAE 2
Ocean circulation during the Cretaceous is poorly constrained, despite its importance in global heat transport, nutrient delivery and setting up favourable conditions for the development of OAEs. In this project, we employ Nd isotopes preserved in fossil fish debris as tracers to reconstruct upper ocean circulation in NW Europe from the mid- to Late Cretaceous. Nd isotopes have proven to be powerful tracers of ocean circulation. Fish debris (teeth and bones) acquire abundant Nd from ambient seawater on the seafloor, therefore always recording bottom-water Nd isotopes. Hence, Nd-isotope time-series from fish remains can track changes in bottom water masses in the past.
Samples are mainly collected from the English Chalk and Gault Clay, with high temporal resolution sampling across OAE 2 (Eastbourne section) and the Mid-Cenomanian Event (Dover). Chalk deposition represents a pelagic shallow shelf-sea environment, and can therefore provide information about the upper ocean. Evolution of the Nd-isotope record from the English Chalk reveals long-term variations in upper ocean circulation/ocean chemistry during the mid- and Late Cretaceous. The Eastbourne record shows intriguing excursions during OAE 2, which may indicate a tight coupling between ocean circulation and climate. Further data interpretation is underway.
Some photos of field work can be found here.
2. Quantifying continental weathering during Cretaceous OAEs with Ca isotopes
A negative feedback between temperature / carbon dioxide and silicate weathering is believed to help maintain a stable climate at Earth’s surface over long timescales. During periods of abrupt environmental change, such as the OAEs, it may be possible to observe and quantify this important effect. Identifying changes in the calcium (Ca) cycle is one of the most direct ways to observe such a weathering feedback, since the transfer of Ca from silicate to carbonate form is the primary reaction for sequestering atmospheric carbon dioxide in a geological reservoir. Ca-isotope ratios in seawater can be temporarily perturbed by large imbalances in Ca input fluxes, and negative isotopic excursions during OAE1a and OAE2 demonstrate that such imbalances were present during these intervals. The Ca-isotope data supplement radiogenic isotope systems such as Sr or Os to help quantify the size and effect of weathering pulses in response to the climatic changes of the OAEs.
- Blättler, CL, Jenkyns, HC, Reynard, LM and Henderson, GM, (2011) 'Significant increases in global weathering during Oceanic Anoxic Events 1a and 2 indicated by calcium isotopes', Earth and Planetary Science Letters. Vol. 309, pp. 77-88
- Zheng, X.-Y., Jenkyns, H.C., Gale, A.S., Ward, D.J., Henderson, G.M., (2013). Changing ocean circulation and hydrothermal inputs during Ocean Anoxic Event 2 (Cenomanian–Turonian): Evidence from Nd-isotopes in the European shelf sea, Earth Planet. Sci. Lett. 375, 338–348. http://dx.doi.org/10.1016/j.epsl.2013.05.053.