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When did oceans form on Earth? What evidence is preserved in the rock record?

Submitted by Aaron Cavosie, University of Puerto Rico

Why is this question important?

Most of us teach earth science using simple or sophisticated models and imagery that demonstrate to students that the Earth is essentially a blue marble whose surface is dominated by oceans. This vision of a blue Earth has even been espoused by popular science writers such as the non-fiction work 'Pale Blue Dot' by the late Carl Sagan. For many it would be difficult to envision an Earth without its blue blanket of oceans. However this is precisely what the early stages of our planet were like. An ocean-free Earth existed, perhaps for several hundred million years as a consequence of extremely high surface temperatures following planetary accretion. The formation of oceans on Earth represents no less than a global-scale cooling of Earth's surface to temperatures at which water is stable as a liquid phase.

That such a profound transition occurred from the highly energetic conditions of the newly accreted Earth whose surface was dominated by meteorite impacts and transient magma oceans to cooler conditions capable of supporting liquid water and eventually life is not in question. However, the timing of this transition - which has implications for when surface conditions necessary for the development were established - is poorly known. Part of the uncertainty of the timing of this transition is due to the fragmentary nature of the rock record for the first ~500 million years of Earth history. Simply put, not much is preserved in the rock record for this time. It is the 'not much' part of the rock record that hold the clues however. The most promising information has come in two forms: (1) preserved sediments up to ~3800 million years old and (2) oxygen isotope studies of detrital zircons up to 4400 million years old.

So what does the rock record tell us?

What we know...

The rock record: Isua BIF

The oldest known rocks on Earth are almost exactly 4 billion years old and are comprised of metamorphosed and deformed granitoids from northwestern Canada collectively called the Acasta gneiss. Direct radiometric dating using the U-Pb method on zircons has demonstrated that these rocks crystallized 4030 million years ago. However, these 'oldest rocks' do not record information on surface conditions at the time of their formation. The oldest direct evidence for the presence of surface waters are slightly younger ca. ~3800 million years old sedimentary rocks called banded iron formation (BIF) that are exposed in southwest Greenland at a location called Isua. The very existence of the Isua BIF requires the presence of stable surface water, at least locally for the chemical deposition of the sedimentary components at ca. 3800 Ma. These rocks were deposited in a somewhat analogous way to how limestones or cherts are deposited directly from seawater in modern marine environments.

The mineral record: Detrital zircons

The oldest known Earth materials are actually not rocks. Sand grains comprised of the mineral zircon (ZrSiO4) have been discovered that are almost 400 million years older than the oldest rocks in the rock record. In the Jack Hills of Western Australia detrital igneous zircons with U-Pb crystallization ages as old as 4400 million years occur in Archean clastic sediments deposited at ~3000 Ma. Zircon is a very useful mineral that is mechanically resistant to erosion, chemically resistant to fluids, and can be 'dated' with the U-Pb method owing to the ubiquitous presence of trace amounts of radioactive U and Th that are incorporated in most zircons at the time of crystallization. The very existence of these ancient zircons demonstrates that igneous rock (e.g. crust) was present starting at ca. 4400 Ma. But the evidence of oceans preserved in these grains comes in a different form.

Oxygen isotopes in geologic materials are affected by temperatures present during the formation and alteration of rocks and minerals. In basic terms the oxygen isotope ratio- the ratio of 18O-to-16O (usually expressed in a notation called 'delta', or δ18O, and reported relative to a standard material) of minerals from the mantle varies little due to the high temperatures in the mantle, and is usually around a value of ~5.5‰ (per mil or parts-per-thousand relative to a reference material). In contrast the δ18O composition of surface materials (e.g., minerals and rocks) varies much more widely and can range from values similar to mantle minerals if unaltered (e.g. 5-6‰) up to values of δ18O = >30‰ due to low temperature reactions of minerals with fluids, such as surface waters like oceans. In simple terms mantle materials have 'low' δ18O values, while sediments and other low-T altered materials have 'high' δ18O values.

Analysis of oxygen isotope ratio in zircon can address the nature of the reservoir of oxygen in the magma that is adopted by the zircon during crystallization. In other words δ18O(zircon) provides a reliable record of whether the parental δ18O(magma) was 'mantle-equilibrated' as all primary mantle-derived magmas are prior to interaction with crustal materials, or whether the parental δ18O(magma) was 'crustal' meaning that the magma inherited a component of its oxygen budget from assimilated crustal materials (like sediments or other altered rocks) which results in higher δ18O values in the bulk rock and constituent igneous minerals.

To address our question of when oceans first formed on Earth we can investigate the δ18O values of the ancient zircons to see if they record 'mantle-equilibrated' values, meaning no evidence of a crustal component is detectable in the oxygen or if they record 'crustal' δ18O values meaning that the early magmas assimilated crustal materials that were affected by low-temperature interaction with water prior to melting. Because we can also 'date' the zircon grains, we can place these conditions in a temporal context. What we find is that the oxygen isotope ratios (δ18O) in the oldest detrital igneous zircons record mantle-equilibrated values from 4400 to ~4325 Ma (e.g. ~5.3 to 5.4‰). From 4325 to ~4200 Ma the δ18O values of zircons are slightly elevated up to 6.3‰ (Note: the upper end of this range ~6.3‰ is higher than what is capable of being produced in a mantle melt, however the uncertainty in these analyses overlaps with the mantle). Just after 4200 Ma the story changes. Values of δ18O in the igneous zircons reach values as high as 7.3‰ with uncertainties that exclude a mantle source. We infer that to produce these 'high δ18O' zircons required that the igneous protolith of the zircon must have assimilated or re-melted crustal materials that were altered by low-temperature processes at or near Earth's surface. In other words, surface waters were present by at least 4200 Ma.

What does all this mean? There are now two hypotheses for when oceans originated on Earth.

Hypothesis 1: Oceans first formed at ca. 3800 Ma. The Isua BIF provides definitive 'ground truth' that surface water was indeed stable at 3800 Ma, however no 'boundary condition' can be defined by the Isua BIF. Simply put there is no way to determine if the Isua BIF was deposited in the first ocean on Earth. In that regard the Isua BIF is akin to a geologic 'snapshot'; we can't infer that water existed before the Isua BIF.

Hypothesis 2: Oceans formed much earlier by at least ca. 4200 Ma. The Jack Hills detrital zircons provide an actual timeline that records the magmatic oxygen isotope compositions of magmas on the young Earth. In this record, we can see a time before the influence of low-temperature weathering was recorded in magmas prior to ~4200 Ma and a definitive change in magmatic oxygen as recorded in elevated ?18O (zircon), after 4200 Ma. In this regard, the detrital zircons actually record a boundary condition that marks when surface weathering, and hence the presence of oceans, occurred.

How to link this topic to the classroom

References and other Resources

Cavosie A.J. Liu D. Wilde S.A. Weiblen P.W. Valley J.W. 2004 Internal structure and U-Th-Pb chemistry of Jack Hills detrital zircons: a mineral record of Early Archean to Mesoproterozoic 4348-1576 Ma magmatism. Precambrian Res. 135 251-279.

Cavosie A.J. Valley J.W. Wilde S.A. E.I.M.F 2005 Magmatic 94818O in 4400-3900 Ma detrital zircons: a record of the alteration and recycling of crust in the early Archean. Earth Planet. Sci. Lett. 235 663-681.

Nutman A.P. 2006 Antiquity of the oceans and continents. Elements 24 223-228.

Valley J.W. Lackey J.S. Cavosie A.J. Clechenko C.C. Spicuzza M.J. Basei M.A.S. Bindeman I.N. Ferreira V.P. Sial A.N. King E.M. Peck W.H. Sinha A.K. Wei C.S. 2005 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contrib. Mineral. Petrol. 150 561-580.

Valley J.W. 2006 Early Earth. Elements 24 201-204.

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