Background: Accurately defining the timing, duration and rates of palaeoenvironmental changes in Earth history is a vital prerequisite for understanding the causes and consequences of these changes, and for pertinent comparison with other events, both ancient and modern. Members of the team investigating palaeoenvironmental change at the Open University employ a range of techniques for accurately defining time in the geological record.
Cyclostratigraphy is the study of cyclic changes in sedimentary successions attributable to slight, periodic variations in the Earth’s orbit around the Sun due to gravitational interaction between planetary masses in the solar system. These cyclic changes cause small, periodic changes in the Earth-Sun distance with periods typically between ~20,000 and 400,000 years. In climatically sensitive sedimentary rocks, these climate cycles will manifest themselves as cyclic changes in lithology or chemistry, and these cycles can thus be counted and used to construct high-resolution timescales for ancient sedimentary successions. Crucially, these timescales have a temporal resolution beyond that typically achievable using radiometric dating methods, biostratigraphy or magnetostratigraphy and are thus ideal for accurately defining the timing, rates and duration of ancient episodes of palaeoenvironmental change.
Previous work by the team has emphasized how astronomical cycles, although subtle, can actually pace large and severe changes in climate. Notably, our research has shown how astronomical forcing of climate played a leading role in the pacing and timing of abrupt climate changes through the Eocene and Jurassic (see also ocean deoxygenation).
Selected examples of the team’s work on astronomical pacing of palaeoenvironmental change:
- Sexton, Philip F.; Norris, Richard D.; Wilson, Paul A.; Pälike, Heiko; Westerhold, Thomas; Röhl, Ursula; Bolton, Clara T. and Gibbs, Samantha (2011). Eocene global warming events driven by ventilation of oceanic dissolved organic carbon. Nature, 471 pp. 349–352.
- Kemp, David; Coe, Angela L.; Cohen, Anthony S. and Weedon, Graham P. (2011). Astronomical forcing and chronology of the early Toarcian (Early Jurassic) oceanic anoxic event in Yorkshire, UK. Paleoceanography, 26(4)
- Kemp, David B.; Coe, Angela L.; Cohen, Anthony S. and Schwark, Lorenz (2005). Astronomical pacing of methane release in the Early Jurassic period. Nature, 437 pp. 396–399.
- Gilmour, Iain; Gilmour, Mabs; Jolley, David; Kelley, Simon; Kemp, David; Daly, Robert and Watson, Jonathan (2013). A high-resolution nonmarine record of an early Danian hyperthermal event, Boltysh crater, Ukraine. Geology, 41(7) pp. 783–786.
Astronomical timescales defined using cyclostratigraphy now constrain around a fifth of the geologic timescale, and the members of the team at the Open University are playing an active role to both improve and increase the length of these astronomical timescales throughout the Mesozoic.
- Kemp, David and Coe, Angela L. (2007). A nonmarine record of eccentricity forcing through the Upper Triassic of southwest England and its correlation with the Newark Basin astronomically calibrated geomagnetic polarity time scale from North America. Geology, 35(11), pp. 991–994.
- Weedon, G.P.; Coe, A.L. and Gallois, R.W. (2004). Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England. Journal of the Geological Society, 161(4), pp. 655–666.
A further important facet of the work on cyclostratigraphy carried out by David Kemp is the study of how astronomical cycles are preserved in the stratigraphic record and the how depositional environments affect the utility of the cyclostratigraphic method.
- Kemp, David B. and Sadler, Peter M. (2014). Climatic and eustatic signals in a global compilation of shallow marine carbonate accumulation rates. Sedimentology (Early Access).
- Kemp, David B. (2012). Stochastic and deterministic controls on stratigraphic completeness and fidelity. International Journal of Earth Sciences, 101(8) pp. 2225–2238.
- Kemp, David B. (2011). Shallow-water records of astronomical forcing and the eccentricity paradox. Geology, 39 pp. 491–494.
Re-Os radiometric dating
In the absence of volcanic ash layers, defining the age of marine sediments has for many years relied on correlation with geographically disparate radiometrically constrained successions, facilitated in the most part by age diagnostic fossils (biostratigraphy). In recent years, however, geologists have developed and refined a radiometric geochronometer based on the Re-Os isotope system. Rhenium and osmium are typically enriched in organic-rich deposits that accumulate in seawater. 187Re is radioactive and decays to 187Os with a half life of approximately ten times the age of the Earth, thus the proportion of 187Os in the marine mudrocks will increase as a function of time and of the Re/Os ratio. Providing certain other constraints are met, the analysis of the Os isotope composition of mudrocks can provide an estimate of their absolute age of deposition. (See also isotope geochemistry.)
Members of the team at Open University were among the first to develop and utilize Re-Os dating as a tool for assessing the age of ancient organic-rich mudrock successions. Team members involved in this research: Anthony Cohen, Angela Coe, David Kemp and Marc Davies.
Selected papers by team members on Re-Os dating:
- Cohen, Anthony S.; Coe, Angela L.; Harding, Stephen M. and Schwark, Lorenz (2004). Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering. Geology, 32(2), pp. 157–160.
- Cohen, Anthony S. (2004). The rhenium–osmium isotope system: applications to geochronological and palaeoenvironmental problems. Journal of the Geological Society, 161(4) pp. 729–734.
- Cohen, Anthony S.; Coe, Angela L.; Bartlett, Jessica M. and Hawkesworth, Christopher J. (1999). Precise Re–Os ages of organic-rich mudrocks and the Os isotope composition of Jurassic seawater. Earth and Planetary Science Letters, 167(3-4), pp. 159–173.
Magnetostratigraphy and GSSPs?
[information to come]