Establishing causes of sea-level change and oceanic anoxia in the Late Cretaceous: regional versus global patterns

Lead Research Organisation: Kingston University
Department Name: Fac of Science Engineering and Computing

Abstract

The Cretaceous, 146-66 million years ago, experienced high levels of atmospheric CO2, and the warmest climates and highest global sea-levels in the last 300 Ma. On several occasions, the oceans became abruptly depleted in oxygen, so-called oceanic anoxic events (OAEs), when organic matter accumulated in the oceans, producing widespread black shales that now act as oil source rocks. However, the mechanisms that caused these events remain hotly debated. This proposal aims to use a new multi-dynamic approach to better understand the mechanisms that caused the onset, duration and cessation of global carbon burial events during the Late Cretaceous. Burial of organic matter leads to the preferential removal of isotopically light carbon from the oceans, increasing the 13C/12C ratio of seawater and, via the atmospheric CO2 reservoir, the entire Earth surface system. Weathering releases 12C back to the surface carbon cycle. Carbonates and organic matter in rocks preserve changes in C-isotope ratios through time, providing a basis for C-isotope stratigraphy. Major changes are synchronous and global in extent, and we have proposed that C-isotope variation in the Late Cretaceous may be used as a proxy for global sea-level change; this remains to be tested. Osmium, a platinum group element with a short ocean residence time of <40 kyr, also shows isotopic variation in seawater through time, being controlled predominantly by two end-member components: weathering of crust and input from volcanic activity (mantle). These have drastically different ratios, so Os isotopes potentially may provide high-resolution stratigraphic control during times of palaeoenvironmental change, such as episodes of increased weathering or volcanic input. The modern oceans display a uniform Os-isotope ratio, but our new data for Os isotopes through an OAE at ~94 Ma indicate that the Atlantic displayed diachronous shifts in Os isotopes. This offers an exciting potential new tool for studying palaeocean-mixing. However, this OAE may be a unique event with regard to oceanic Os; further regions and OAEs need to be tested. This project will use C-isotope stratigraphy from organic matter to correlate global successions from diverse environments, palaeolatitudes and oceanic settings. The time interval to be investigated, 101-83 Ma, was characterized by two OAEs and other significant changes in the carbon cycle. We aim to answer the following: (1) Are secular C- and Os-isotope curves related to sea-level change? (2) Can Os-isotope stratigraphy be used for chemostratigraphy: is it synchronous or diachronous? (3) Do OAEs coincide with Os-isotope excursions, and what was the steady state of the oceans? (4) What are the relationships between sea-level change, climate and ocean anoxia; can we finally identify the key forcing mechanism for widespread ocean stagnation? Sites in Canada, France, Czech Republic, Far East Russia, Ecuador, South Atlantic, and offshore Australia will be studied. The relative sea-level histories for each basin, correlated using C-isotopes, will be used to test relationships between C-isotope stratigraphy and sea-level change. Key stratigraphic time intervals will be characterised for Os isotopes and trace-metals to: establish the evolution of Os isotopes in the Late Cretaceous oceans; evaluate possible regional variation in the Os-isotope composition of seawater; establish levels of seawater oxygenation in the associated water masses; and identify the causes of widespread ocean stagnation. Results from our Cretaceous extreme-greenhouse study will provide unique constraints for modelling interactions between, and the impacts of, sea-level and climate change, and perturbations of the global carbon cycle for an icecap-free Earth; the increasingly likely near-future for our planet. The proposed research will aid in understanding whether periods of ocean stagnation are a likely future consequence of present-day global warming.

Publications

10 25 50

 
Description • Our osmium isotope results from Late Cretaceous sections world-wide demonstrate that one of the largest perturbations of the global carbon cycle that has occurred in the last 100 million years, Oceanic Anoxic Event 2 (OAE2, ~94 million years ago), was exactly coincident with a massive volcanic episode lasting around 450 thousand years. The emplacement of the Caribbean Large Igneous Plateau offers a potential source for this volcanic event. Paired measurements of carbon stable isotopes in calcium carbonate and organic matter from limestones deposited during OAE2 indicate a trend of rising atmospheric carbon dioxide and global warming accompanying the volcanism. However, the warming trend was temporarily reversed by sharply increased deposition rates of organic matter-rich sediments in the oceans that led to falling atmospheric CO2. Cooling of ocean surface temperatures by more than 4°C occurred in Europe for a period of around 40 thousand years, and was accompanied by water mass changes and the southerly spread of higher latitude fauna - the Plenus Cold Event. The warming tend resumed as the volcanic CO2 flux again predominated.

• An episode of maximum climate warming followed Oceanic Anoxic Event 2 ~94 million years ago when, arguably, the highest global temperatures were reached for at least the last 600 million years. This was the acme of the so-called Cretaceous super-greenhouse. We document how, subsequently, an episode of Europe-wide cooling occurred around 91 million years ago, lasting for approximately 1 million years, and accompanied a period of long-term stepped sea-level fall and sea-level lowstand. Carbon isotopes provide evidence of a possible atmospheric CO2 decline accompanying cooling. Our work confirms that close linkages existed between sea level, climate and greenhouse gas concentrations, even under super-greenhouse conditions.

• Our new high-resolution geochemical studies of sediments deposited between 94 and 89 million years ago, following OAE2, confirm the viability of using temporal variations in carbonate and organic matter carbon isotopes for dating and correlation of sediments on a global scale. We have developed a new isotope stratigraphy correlation framework for marine sediments in Europe and North America, and tied this a terrestrial wood record from the NW Pacific (Japan). Our results show how carbon isotopes offer a means to significantly improve the correlation and dating of non-marine deposits, if some other age constraints are available.

• Comparison of carbon isotope variation to an interpreted sea-level record from the Czech Republic has not yielded a clear causal link. A long-term 'background' cyclicity in carbon isotope values shows a duration close to 2.4 million years. This corresponds to a "long-eccentricity cycle", a cyclic change in Earth's orbital geometry that is believed to force long-term climate and other palaeoenvironmental change. Shorter-term (1 million year scale) highs and lows in carbon isotope values appear to broadly correspond to intervals characterized by more pronounced short-term sea-level highs and lows, respectively, but on time scales below 1 million years, it is not possible to associate individual fluctuations in carbon isotopes with particular pulses of sea-level rise or fall.
Exploitation Route Integration of our new age-calibrated high-resolution organic and inorganic carbon isotope data with new organic walled dinoflagellate cyst and macrofossil records, and existing microfossil biostratigraphy, provide an improved stratigraphic framework for the Cenomanian - Coniacian. This framework can be used to improve the correlation and dating of strata in both conventional and unconventional petroleum basins. It is estimated that up to 30% of the World's oil and gas source rocks were deposited at that time.
Spectral analysis of our geochemical time series offers a mean of addressing fundamental questions concerning mechanisms linking Milankovitch-band orbital forcing of climate to the global carbon cycle, and relationships between climate change and changes in eustatic sea level.
Sectors Energy,Environment

 
Description Our refinement of the carbon-isotope chemostratigraphy for the Cenomanian - Coniacian is informing the dating and correlation of biostratigraphically poorly dated strata in petroleum basins, including the Eagle Ford shale gas play of North America, and the La Luna and Querecual Formation oil source rocks of Venezuela. The new results complement our original work on Upper Cretaceous chemostratigraphy published in 2006, and confirm the viability of using data obtained from either carbonate and/or organic matter hosts. The method is employed for specialist basin analysis and source rock characterisation studies in the petroleum industry.
First Year Of Impact 2006
Sector Energy
Impact Types Economic

 
Description Maersk Oil Research Fund
Amount £57,000 (GBP)
Funding ID 8600000694 
Organisation Maersk 
Department Maersk Oil Denmark
Sector Private
Country Denmark
Start 01/2012 
End 12/2012
 
Description Amateur geological society talk (Chalk, black shales, climate and sea level change: Unravelling Late Cretaceous history) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Talk sparked enthusiastic questioning and extensive discussion afterwards

audience members expressed new awareness of the utility of geochemistry for understanding past and future climate and sea-level change
Year(s) Of Engagement Activity 2013
 
Description Amateur geological society talk (Chalk, black shales, climate and sea level change: Unravelling Late Cretaceous history. Predicting our future) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Talk sparked extensive questions and enthusiastic discussion

Considerable interest was expressed in new geochemical methods for understanding Earth processes
Year(s) Of Engagement Activity 2014