Milankovitch and Tidal Cycle History (MATCH)

The Apollo 11 moon landing on July 20 1969, was a major achievement in the history of human civilization. The rock samples brought back showed that the moon was 4.5 billion years old, and so must have formed only 200 million years or so after Earth formed. The mirrors left on the surface of the moon during the Apollo missions allowed us to very accurately measure the Earth-moon separation, and the rate at which the Earth and the Moon are moving away from each other or receding. This recession rate is estimated to be 3.8 cm/year. However, if this recession rate was constant in time, the moon would only be 1.5 billion years old, otherwise it would have been torn apart by the Earth's gravitational field. Here lies an obvious paradox: the age of the moon and the present-day recession rate do not add up!

The reason the moon is receding is tidal friction: the loss of tidal energy into the ocean is gradually slowing the Earth's rotation rate and pushing the moon away from the Earth. If tidal friction is weak, the moon will recede more slowly, whilst if tidal dissipation is large, the moon will recede more quickly. However, we currently have very few reliable estimates of the dissipation of tidal energy over the history of Earth. Furthermore, these are restricted to a few time slices over the more recent Earth history (the past 250 million years). These estimates indicate that the tidal dissipation rate has not been constant over time. Work by team members have predicted the existence of a "super tidal cycle" - with a period of 400 million years - in which the tidal dissipation varies by a factor of four and is associated with continental drift.

The dawn of supercomputers has facilitated the development of high-accuracy global tidal models, which allow us to simulate the tidal dissipation rates in Earth's past. From the modelled dissipation, we can compute the past lunar recession rates for a large number of time slices. The recession rates from the model will be constrained for a few periods using data from bore holes. In bore holes there are signals of long-term climate cycles laid down in the sediments, but to analyse them you need the lunar recession rate. Using a new approach developed by the team, we will analyse data from a number of holes and provide a tool to confirm the model results. The novelty of our approach being that it does not assume a constant recession rate and so allows us to achieve a step change in our understanding of the evolution of the tides globally and quantify the evolution of Earth-Moon separation on geological time-scales.

Achieving this aim will allow us to better predict the evolution of the Earth-moon system by providing detailed estimates of the lunar recession rate over the past 600 million years. This has implications for ocean tides in the Earth system, for example how the tide provides energy for stirring the ocean and thus sustaining biological production and influencing the climate-controlling global ocean circulation patterns. The project results will also be important for any investigation in need of lunar recession or tidal dissipation rates, for example investigations of past climate cycles, sediment laminations, and for simulations of past climates.

This interdisciplinary programme fits in NERC's "Benefiting from natural resources" and "Managing Environmental Change" remits. It will lead to substantial outreach impact, satisfying NERC's charter commitment "to generate public awareness, communicate research outcomes, encourage public engagement and dialogue, disseminate knowledge and provide advice in relation to those activities".

1. Benefitting from natural resources. Extractive industries will make direct use of this research. Improved Milankovitch cycle estimates and improved techniques for identifying Milankovitch cycles will enhance understanding of sedimentary processes, timing and rates in commercially-valuable sedimentary deposits. In many periods of Earth history, there is a direct link between Milankovitch cyclicity and eustatic sea-level change which, in turn, influences oil exploration/production decisions via its impact on reservoir compartmentalization and, hence, the economic viability of a potential oil field. Also, understanding the timing and duration of geological events is crucial for understanding the formation, migration and trapping of hydrocarbons. All of this is relevant to exploration and production of hydrocarbons from the UK's continental shelf and will help maintain a healthy UK oil-industry in the long-term. These impacts will be immediately available to Industry at the end of the project-thanks to the release of an open-source, JavaScript program by the project.

2. Managing environmental change. Long-term climate change will result in substantial sea-level rise over the next few centuries, which affects tides. It is essential, for climate change mitigation, that we understand the possible consequences of this for shipping, fisheries and coastal defences. By validating models of the tidal consequences of ancient sea level changes, this project will enhance confidence in predictions of the tidal consequences of future sea level rise. These results will be shared with society through publications, through the project website, and via public and wider engagement activities.

3. Public Engagement. The team has demonstrated that there is a public appetite for understanding the links between Earth-Moon-system evolution, climate and planetary habitability. This is highlighted by the high level of interest in PI Green's paper on a tectonically-driven tidal cycle by New Scientist, The Conversation, and others. We will build on these foundations to contribute to the enhancement of the public understanding of many scientific fields; from climatology, through astronomy to biology and geology. The team will be engaged in a great deal of outreach activity (schools, local scientific societies, U3A, radio, science festivals, Open Days etc.) and the science uncovered during this exciting research programme will directly feed into those activities. Outreach activities are an ongoing commitment and so these impacts will be effective within the term of the project. We aim to participate in the Royal Society Summer Exhibition slot, and/or have exhibitions at the Science Museum London and National Museum of Wales.

4. Scientific beneficiaries beyond NERC's remit. This project will have impact outside of NERC's remit. An improved understanding of the lunar recession history is of interest to astronomers. It also links to our understanding of planetary habitability since the evolution of Earth-Moon-like systems influences climate stability. The PI has shown that tidally driven mixing must be appropriately implemented in paleo-climate models, and this project will provide dissipation fields that can easily be introduced into models. The CO-I is well placed to pass these advances onto the astronomy and astrobiology communities through presentations at conferences and publication of papers. These activities will be undertaken during the final stages of the project and so the impacts will be felt by the end of the project.