When and how did Earth’s earliest continents first emerge above the oceans?

The emergence of continents above sea-level marks a pivotal junction in Earth’s evolution that fundamentally changed the chemistry of the atmosphere and oceans, which was critical to establishing a habitable planet. However, when and how the first subaerial continental landmasses formed remains contentious. Abrupt changes in proportion of submarine vs subaerial volcanism and in the oxygen isotopic ratios of shales and zircons at the Archean-Proterozoic transition (2.5 billion years ago, Ga) are invoked to argue for global continental emergence around that time (e.g., Kump and Barley, 2007; Bindeman et al., 2018). However, direct evidence for an earlier episode of continental emergence comes from ~3.0-2.7 Ga paleosols (like the Nsuze paleosol) and terrestrial sedimentary strata that formed atop stable cratons (cf. Eriksson et al., 2013). This attests continental emergence > 2.5 Ga, at a time when the operation of modern plate tectonics is debated.

To help resolve these issues, we focussed on the cratons like the Singhbhum and Kaapvaal cratons since they host widespread Mesoarchean terrestrial to shallow marine clastic strata and paleosols, which suggests early (> 2.5 Ga) continental emergence on Earth. We studied how crustal thickness and composition of these cratons evolved through time leading to their emergence, by linking the Paleo-to-Mesoarchean sedimentary and magmatic records of these cratons (Chowdhury et al., 2021). First, we studied the conglomerate-sandstone-shale successions that are uncomforably lying on the cratonic basement and determined their depositional ages to constrain the timing of the continental emergence. Then we analysed the chemistry of the tonalite-trondhjemite-granodiorite (TTG) suite of felsic rocks and performed petrogenetic modelling to quantify the evolution of crustal thickness and P-T conditions of crust formation, which elucidated the underlying mechanism and tectonic environment of emergence.

Our results show that the studied cratons became emergent between ca. 3.3-3.1 Ga due to progressive crustal thickening and maturation driven by granitoid magmatism. The cratonic crust  became chemically mature and extremely thick (45-50 km) by 3.2-3.1 Ga, such that isostatic compensation led to their rise about the sea level. Modelling of the TTG chemistry further elucidated that these TTGs formed at hotter thermal conditions characteristic of a thickened Archean crust atop a zone of rising mantle. Hence, we propose that emergence of stable continental crust began at least during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened, SiO₂-rich crust without the help of plate tectonics (Chowdhury et al., 2021). We further surmise that such early episodes of emergence caused important changes in Earth’s early surficial environments including promoting transient atmospheric-oceanic oxygenation (O₂-whiffs) and CO₂ drawdown leading to glacial events.

Reference:

Bindman et al., 2018. Nature 557, 545–548.

Chowdhury et al., 2021. PNAS 118, e2105746118.

Eriksson et al., 2013. Gondwana Research 24, 468–489.

Kump and Barley, 2007. Nature 448, 1033–1036.