Environmental tipping points during supercontinent breakup

Throughout Earth history, supercontinents (united continental landmasses) assemble and break up in a repeating cycle of plate tectonics. It has been suggested that this 'supercontinent cycle' drives the chemistry of the ocean-atmosphere system, and in turn influences Earth's climate. However, this association is largely qualitative and the detailed mechanisms, feedbacks and chemical fluxes leading up to turning points in Earth history remain to be quantified. Understanding how these processes are interrelated requires integration of geochemical, climate and tectonic modelling tools and expertise. We propose to integrate our existing volcanic weathering models (NE/K00543X/1) with dynamic plate models developed by world-leading groups at Sydney and Adelaide, to quantify the geochemical footprint of supercontinent breakup.
Specifically, we will consider the breakup of Rodinia ~750 million years ago (Ma) and Gondwana at ~180 Ma. Both events were heralded by an apparently similar chain of events, including intensified volcanism and weathering, but resulted in fundamentally different climatic responses. Rodinia breakup culminated in a 'Snowball Earth' glaciation lasting tens of millions of years, whereas Gondwana breakup gave rise to the Cretaceous greenhouse world. We hypothesise that the climatic outcome of supercontinent breakup is governed to a first order by the arrangement of the resulting continents, or plate topology.
Supercontinent breakup spells chaos for the Earth system. Large Igneous Provinces release vast amounts of CO2 and aerosols, trapping heat inside the atmosphere making the planet warmer. Breakup also brings intensified chemical weathering, that is, rainwater reacting with rocks to flush dissolved elements via rivers into the oceans. This occurs because tearing up a landmass produces more seaways, resulting in closer proximity to the oceans for a higher proportion of the continental landmass. Our work has shown that forming new mid-ocean-ridges, an integral part of breakup, leads to intense weathering of fresh volcanic rocks. When rivers and volcanoes flush elements such as calcium into seawater, these combine with CO2 to form CaCO3, ultimately reducing atmospheric CO2 levels and potentially causing a net cooling effect. Continents near the equator are more prone to chemical weathering than those near the poles, so plate topology is a powerful driver of climate.
As illustrated by this example, slight changes in certain processes or states could tip the Earth into a long-lived greenhouse, or an icehouse phase, depending on a combination of background geological conditions (i.e. a 'butterfly effect'). Our current NERC-funded research demonstrates a major role for volcanic ash dispersal and ridge volcanism in affecting the Earth's carbon cycle and marine biological pump. Coupled with volcanic outgassing, these feedbacks might be sufficiently powerful to destabilise the climate system and tip the response in either direction.
Our models will use a Monte Carlo approach to resolve system complexity, and account for uncertainties in geological conditions and fluxes. As continental 'unzipping' strongly influences chemical fluxes from the continents and ocean crust, combining our simulations with dynamic plate motion models will significantly reduce uncertainties in weathering flux estimates. Bayesian models will allow us to deconvolve links between rates of volcano-tectonic processes and geochemical proxies of environmental change. Together with interrogation of Earth system models, this will allow us to better understand climate forcings, and thereby develop a framework for understanding 'tipping points' during supercontinent breakup. Although Snowball Earth and the Cretaceous world are often regarded as polar opposites, chemical weathering during these periods likely stimulated, respectively, the rise of complex life and radiation of planktonic organisms, which play a crucial role in regulating ocean chemistry.

Although the proposed research will primarily be of interest to the academic community, many aspects will attract interest from industry and the general public. The concept of 'Snowball Earth' has captured the public imagination, due to the unprecedented scale and duration of these glaciations, and the idea that Earth could once have had a similar appearance to Jupiter's ice-covered moon, Europa. We anticipate our study on climate forcings such as tectonic shifts and explosive volcanism during the Snowball Earth and Cretaceous greenhouse world will stimulate great public interest.

The interactions between plate tectonics and volcanism strongly influence ocean chemistry, and have on occasion stimulated the evolution of autotrophic organisms. Therefore, our coupled 5D models (incorporating space, time and uncertainty estimates) have potential to impact on many areas of Earth and planetary sciences, and potentially even unify groups focusing on traditionally disparate fields of Precambrian and Mesozoic geology. To this end we will engage with researchers working on environmental feedbacks and marine nutrient cycles during both Earth System transitions. Here, we intend to work closely with groups involved in the previous NERC programme, "Long-term Co-Evolution of Life and the Planet", especially the "Re-inventing the Planet" project (PI Tim Lenton), to examine how our collective efforts can be combined to generate maximum impact, and thus add value to NERC science.

Academic groups may use our models to study specific regions at high spatiotemporal resolution, or explore how volcanism and tectonics may have worked together at certain times to produce massive chemical 'dumps' in the ocean. For example, the study will provide a volcanic and tectonic framework for understanding Neoproterozoic iron formations, which will be of interest to the mining and exploration industry currently exploiting reserves of this age (BHP, Rio Tinto). Black shales formed in Mesozoic Ocean Anoxic Events (OAEs) are a major global source of hydrocarbons, and the widespread euxinia responsible for their formation has previously been linked to intensified volcanism during supercontinent breakup. Our time-dependent models will shed light on the tectonic, volcanic and geochemical precursors to such widespread depositional events, which will be highly relevant to the hydrocarbon exploration sector.

This collaboration and secondment with internationally leading groups at the forefront of developing innovative global models (earthbyte.org) will facilitate knowledge exchange and interactions with potential end-users of the research. During Müller and Collins' UK visit, we will host a workshop involving collaborators and beneficiaries, including Earth System scientists and climate-geochemical modellers. Whilst the workshop's primary goal will be developing a framework for integrating fluxes and exploring climate feedbacks, it will also be an opportunity to engage with end-users to explore how models and outputs could be used to test hypotheses in future studies. After the workshop, we will prepare a meeting summary for Eos magazine to raise awareness of the modelling initiative. Our code and simulation results will be made available via a dedicated website, ensuring their long-term use by other groups. We will actively engage with the mining and hydrocarbons industries, with which we already have strong links, to keep them informed of relevant developments.

This study has potential to generate high impact publications. To ensure maximum reach and impact, we will issue press releases and prepare contributions for independent news websites such as the Conversation, which often attract considerable attention from the mainstream and social media. Gernon has a strong track record in public outreach, and will engage with thousands of members of the public through NOCS Ocean and Earth Days, as well as local science festivals attended by many schools.