[Paper Review] The Case for a Hot Archean Climate and its Implications to the History of the Biosphere

David Schwartzman|arXiv (Cornell University)|Apr 1, 2015

Paleontology and Stratigraphy of Fossils80 references6 citations

TL;DR

The paper argues that the Archean Earth had a significantly warmer climate than today, supported by oxygen isotope data in chert, protein stability reconstructions, high pCO2 levels, and seawater viscosity estimates. This hot climate, sustained despite a fainter sun, implies that temperature constraints played a crucial role in shaping the timing and pace of major evolutionary transitions, including the emergence of oxygenic photosynthesis and metazoans.

ABSTRACT

The case for a much warmer climate on the early Earth than now is presented. The oxygen isotope record in sedimentary chert and the compelling case for a near constant isotopic oxygen composition of seawater over geologic time support thermophilic surface temperatures prevailing in the Archean, with some support for hot conditions lasting until about 1.5 billion years ago, aside from lower temperatures including glacial episodes at 2.1-2.4 Ga and possibly an earlier one at 2.9 Ga. Other evidence includes the following: 1) Melting temperatures of proteins resurrected from sequences inferred from robust molecular phylogenies give paleotemperatures at emergence consistent with a very warm early climate. 2) High atmospheric pCO2 levels in the Archean are consistent with high climatic temperatures near the triple point of primary iron minerals in banded iron formations, the formation of Mn-bicarbonate clusters leading to oxygenic photosynthesis and generally higher weathering intensities on land. These higher weathering intensities would not have occurred if seafloor weathering dominated the carbon sink, pulling down the temperature, hence this empirical evidence supports a hot climate and high carbon dioxide levels. 3) The inferred viscosity of seawater at 2.7 Ga is consistent with a hot Archean climate. 5) A cold Archean is hard to explain taking into account the higher outgassing rates of carbon dioxide, significantly smaller land areas and weaker biotic enhancement of weathering than present in the context of the long-term carbon cycle, taking into account the fainter Archean sun in climate modeling. This evidence points to an important conclusion regarding biological evolution, namely to the critical role of a temperature constraint holding back the emergence of major organismal groups, starting with phototrophs, culminating with metazoans in the latest Precambrian.

Motivation & Objective

• To challenge the prevailing view of a cold or temperate Archean climate by synthesizing multiple geological and biochemical lines of evidence.
• To resolve the faint young sun paradox by proposing sustained high greenhouse gas concentrations, particularly CO2.
• To explore how thermal constraints influenced the timing of key evolutionary innovations, such as oxygenic photosynthesis and metazoan evolution.
• To demonstrate that high weathering intensities and mineral stability in banded iron formations are consistent with a warm early Earth.

Proposed method

• Analysis of oxygen isotope ratios in Archean sedimentary chert to infer surface temperature history.
• Resurrection and thermal stability testing of ancestral proteins using molecular phylogeny to estimate paleotemperatures at emergence.
• Evaluation of mineralogical constraints, such as the triple point of primary iron minerals in banded iron formations, to infer atmospheric CO2 levels.
• Assessment of seawater viscosity at 2.7 Ga as a proxy for temperature.
• Integration of long-term carbon cycle models to assess the balance between outgassing, weathering, and atmospheric CO2.
• Use of empirical data on glacial episodes (2.1–2.4 Ga and 2.9 Ga) to identify intervals of cooler climate within an otherwise warm Archean.

Experimental results

Research questions

• RQ1 What evidence supports a warmer Archean climate despite the faint young sun?
• RQ2 How do protein stability reconstructions from molecular phylogenies inform estimates of early Earth surface temperatures?
• RQ3 Why is high atmospheric pCO2 necessary to explain the persistence of certain mineral assemblages in banded iron formations?
• RQ4 How does the inferred viscosity of Archean seawater support a hot climate?
• RQ5 What role did temperature constraints play in regulating the timing of major evolutionary transitions in the biosphere?

Key findings

• Oxygen isotope data from Archean chert indicate a near-constant isotopic composition of seawater, supporting sustained thermophilic surface temperatures throughout the Archean.
• Proteins resurrected from ancestral lineages suggest paleotemperatures at emergence consistent with a warm early Earth, with some evidence extending to 1.5 billion years ago.
• High atmospheric pCO2 levels are required to explain the formation of Mn-bicarbonate clusters and the stability of primary iron minerals in banded iron formations, implying a warm climate.
• Seawater viscosity estimates at 2.7 Ga are consistent with a hot Archean climate, reinforcing the thermal interpretation of geochemical data.
• The absence of dominant seafloor weathering as a carbon sink implies higher continental weathering intensities, which are only feasible under warm conditions.
• The combined evidence contradicts models of a cold Archean, supporting instead a prolonged warm climate that influenced the tempo of biological evolution.