The Early Cretaceous epoch was characterized by a greenhouse climate with high atmospheric CO2 levels ocean temperatures, leading to a reduced latitudinal temperature gradient (Littler et al., 2011) . Superimposed on this greenhouse climate are Oceanic Anoxic Events (OAEs) during which a large part of the ocean became anoxic, impacting the global carbon cycle and other biogeochemical cycles (Jenkyns, 2010) . Aptian OAE 1a, taking place around 120 Ma, is one of the largest Mesozoic OAEs with black shale deposition occurring in all major ocean basins (Jenkyns, 2010) . The negative carbon isotope excursion (NCIE) together with a pronounced shift towards more unradiogenic (mantle-like) Osmium isotope values in different ocean basins suggest that volcanic CO2 outgassing from the Ontong Java plateau in the Pacific was the ultimate trigger for OAE 1a (Tejada et al., 2009; Bottini et al., 2012).
The main focus of this research revolves around :
1) testing the recent suggestion that changes in morphological features and palaeoflux of calcareous nannofossils during OAE 1a represent a biological response to surface water ocean acidification (Erba et al., 2010). The link between extinction, calcification crisis and ocean acidification for OAE1a is heavily debated as crucially the relative timing, rate and magnitude of CO2-input are unknown. I recently used compound specific carbon isotopes from expanded and well-dated sections in southern Spain to for the first time constrain the relative timing, rate, and magnitude of CO2-input during OAE 1a. The outcome of this study was published in Nature Geoscience in Jan. 2016 and demonstrated that the pCO2 increase across OAE 1a likely took much more than 100,000 years, too gradual to have caused widespread ocean acidification.
2) Reconstructing sea surface temperatures (SST) across OAE 1a. Using the organic paleothermometer TEX86, I am generating high-resolution SST records across OAE 1a to determine the response of SSTs to changes in pCO2 (see above) and to determine what the latitudinal SST gradient was on earth during OAE 1a. In October 2016 I published a paper in Geology that shows that SSTs varied in response to changes in pCO2 (see above) with an increase of 2-4 oC at the onset of OAE 1a to values as high as 42 oC. Despite these high SSTs we demonstrated that OAE 1a was characterized by a clear latitudinal temperature gradient, contrary to the generally accepted view that a nearly flat SST gradient existing during the Early Cretaceous. These results are more consistent with climate model simulation of the Cretaceous that have failed to produce flat SST gradients.
3) Reconstructing the response of the marine nitrogen cycle to OAE 1a. Oceanic anoxic events (OAEs) reflect the most dramatic changes in climatic and palaeoceanographic state of the planet of the last 250 Ma. It is generally accepted that the marine nitrogen (N)-cycle operated fundamentally different during OAEs. However, the precise response of the marine N-cycle to periods of widespread anoxia is subject to debate (Rau et al., 1987; Kuypers et al., 2004; Higgins et al., 2012; Zhang et al., 2014). Using a data – model comparison this study aims to provide detailed insights into the extent of anoxia during OAE 1a and response of the marine N-cycle.
This project in funded through a Rubicon Grant and is in collaboration with Rich Pancost, Daniela Schmidt, and J.M. Castro. I co-superise two PhD students:
– Layla Behrooz who uses biomarkers in marine sediments collected from ancient anoxic basins from around the world to study the dynamics and formation of anoxic oceans.
– Markus Adloff who is using the GENIE earth system model to study the response of the climate system to large C-cycle perturbations.
I also work together with Fanny Monteiro, Ann Pearson, and Andy Ridgwell to determine the impact of OAE 1a and widespread anoxia on marine biogeochemical cycles, in particular the marine Nitrogen cycle.