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Alternative Earths: Explaining Persistent Inhabitation on a Dynamic Early Earth

What would Earth look like if analyzed remotely over its long history? Specifically, what information about the tectonic, biotic, and extra-planetary processes that have combined to sustain Earth’s dynamic habitability over billions of years could we glean if we had snapshots of our evolving planet, tracked light-years away? Earth’s rock record provides a means to tackle this question via an array of diverse windows into past states of inhabitation—what we can think of as ‘Alternative Earths’. The key is to pinpoint traces of the biotic and tectonic events that differentiate one Earth state from the next, whether the beginnings of plate tectonics and the earliest formation and stabilization of continents, the origin of life and subsequent advent of oxygenic photosynthesis, or the oxidation of the oceans and atmosphere. It is conceivable that each of Earth’s widely varying planetary states translates to a particular atmospheric composition that could one day be detected on an extrasolar world.

Each Alternative Earth is defined by a suite of specific intrinsic and extrinsic controls and feedbacks that, when combined, permit (if not favor) life and its sustainability. It is the objective of the UC-Riverside team to explicitly characterize these systems and turn them into predictive models. Over billions of years, the history of life is one of persistence—long-term stability punctuated by dramatic transitions and milestones—all tied to the flow of nutrients and energy across Earth’s surface within a remarkably narrow surficial temperature range. Temperature-stabilizing feedbacks, many dependent on planetary oxidation state, saved the day despite myriad other changes to the Earth system. This journey takes one through the many and diverse temporal windows of life and life-sustaining conditions from the earliest production of oxygen by photosynthesis perhaps 3.0 billion years ago to the increase of oxygen roughly 2.5 billion years later. The latter was as a harbinger of the complex animal life that would follow soon after and the purview of other researchers working under the NAI umbrella. Through it all, Earth transitioned from a fundamentally reducing world to one of dynamically rising and falling oxygen concentrations against a backdrop of coupled tectonic and climatic change, including global-scale glaciation, massive volcanism, and the formation, growth, and decay of continents and supercontinents. Through it all, and in the face of a progressively warming sun, Earth remained in a ‘sweet spot’ of habitability, thanks in no small way to the stabilizing feedbacks that couple life to its planetary milieu.

We think of Earth’s atmosphere as the remotely detectable sum product of all the controlling factors on and within Earth that at any given time combine to sustain life. Photosynthetically produced oxygen emerges as a potentially powerful biosignature, but evolving atmospheric compositions speak far more broadly to complex, interrelated processes, both biotic/abiotic and aerobic/anaerobic, and the feedbacks among them. Those evolving compositions also capture evidence of great challenges to life—as during global-scale glaciations, major impact events, or severe nutrient limitation—and of life-sustaining tectonic processes. Without tectonic contributions to nutrient recycling through mountain building and weathering, uninterrupted life on any planet is unlikely, and volcanic/tectonic controls on cycling of CO2 and other volatiles are primary in driving Earth’s climate. Any checklist designed for planetary exploration, whether looking down through time on Earth or up to other worlds, must work to include tectonic processes and their feedbacks with the biosphere. All of these factors distill down to one simple question that drives the research of the UC-Riverside team:

How has Earth remained persistently inhabited through most of its dynamic history, and how do those varying states of inhabitation manifest in the atmosphere?

With an overall aim of detecting life and habitability beyond Earth, the UC-Riverside team sub-divides this theme into several clarifying questions: How and why has the redox state of the atmosphere (and ocean by association) changed through Earth’s history? Was planetary oxygenation inevitable on Earth? What ultimately controls the dynamics and stability of planetary atmospheric redox, using Earth’s long history as a series of case studies? What controls the flow and inventory of bioessential elements through time? How has ocean chemistry, as impacted by tectonics and external contingencies, controlled atmospheric chemistry through biological catalysis, and vice versa? Is Earth uniquely or unusually well poised through its stabilizing feedbacks to develop and sustain life; if so, why; and was increasing complexity among that life an inevitability or a serendipitous product of progress and contingency?

From these questions emerges the team’s key goal: to characterize Earth’s early oxygen history, its atmospheric evolution more generally, and the coupled drivers and consequences of this record. Indeed, a central theme of their research is the evolving redox state of the early atmosphere. It is known, for example, that atmospheric redox and thus the abundance of associated gases are fingerprints of the complex interplay of processes on and within a host planet that point both to the presence and possibility of life—redox-sensitive greenhouse gases, for instance, maintain warmth across a spectrum of solar output far wider than that predicted by the size of a planet’s star and its distance from that energy source alone. The expected outcomes of their team’s research include a series of modeled Earth states, each tied to very different forcings and feedbacks but linked through the common theme of persistent inhabitation. At its core, the UC-Riverside team is structured around plans for a comprehensive deconstruction of the geologic record from the earliest biological production of oxygen to its permanent accumulation in large amounts almost three billion years later. To focus their efforts, they have carefully selected three critical time intervals centered on a compelling question or controversy, each with its own primary objective:

• In the mid-late Archean (3.2 to 2.4 billion years ago, Ga) they will search for atmospheric traces of oxygenic photosynthesis and the reasons behind their timing in the rock record.
• In the mid-Paleoproterozoic (2.2 to 2.0 Ga) they will investigate whether Earth’s surface experienced a unidirectional oxygen rise or instead rose to near-modern levels, then crashed dramatically.
• In the mid-Proterozoic (1.9 to 0.7 Ga), they will explore the interplay among oxygen, the rise and increasing complexity of eukaryotes, and conditions that set the stage for the rise of metazoan life.

The UC-Riverside team is assembled around one central goal: the development and application of cutting-edge geochemical proxy approaches (surrogate indirect tracers—be they mineral, isotopic, elemental or molecular—of past, transient conditions, such as atmospheric compositions) for characterization of Earth’s oxygen history, its atmospheric evolution more generally, and the coupled drivers and consequence of this record. The team will support these efforts with tectonic and atmospheric modeling and biogeochemical experimentation that will both steer the initial research design and drive the final interpretations of the proxy data. They will emphasize refined applications of traditional methods (such as trace metal and carbon-sulfur-iron systematics and presence/absence of pyrite and other reduced minerals in detrital sediment loads) and novel proxies (such as metal isotopes, chromium and uranium in particular) applied to classic, well preserved stratigraphic sequences—many of which are already well understood.

The Alternative Earths team based at UC-Riverside has designed four working groups around the need to develop and apply novel proxies for: (1) ocean-atmosphere redox and associated nutrient landscapes in the ocean (Redox Tracers), (2) their cause-and-effect relationships with life (Earth-Life Interface), (3) tectonic changes that potentially underlie most of the first-order dynamics in redox evolution (Tectonic Drivers) and (4) thoroughly modeled Earth surface chemistries (Earth System Synthesis). Each group is broadly integrated in range of expertise represented, and each will inform, and is informed by, the others’ efforts.