Timescales

working button timescales
Cyclic alternations between carbonate- and organic-rich mudrocks (Belemnite Marls Fm, Dorset, UK). These cycles (~40 cm thick) pertain to 20,000 year astronomical precession.

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

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).

Team members involved in this research: David Kemp, Angela Coe, Philip Sexton and Anthony Cohen.

Selected examples of the team’s work on astronomical pacing of palaeoenvironmental change:

COE St Audries cycles
Cyclic Triassic deposits, Somerset, UK that formed the basis of Kemp and Coe (2007) where we showed that these colour alternations represent eccentricity and precession cycles. David Kemp and Angela Coe are at the base of the cliff.

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.

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.

Re-Os radiometric dating

KEMP TJ low res
The organic-rich mudrocks at the base of the Jurassic, Somerset UK. These rocks have been used for Re-Os dating, cyclostratigraphy, biostratigraphy and palaeomagnetic 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:

Magnetostratigraphy and GSSPs?

[information to come]