COMMENTARY Penciling in details of the Hadean Christopher H. House1 this new record of early carbon is exciting Department of Geosciences, Penn State Astrobiology Research Center, Pennsylvania State because it provides a new window into the University, University Park, PA 16802 seemingly accessible Hadean Earth. The Hadean graphite provides a new constraint on the carbon isotopic composi- Some truly remarkable graphite is described (3, 4). So far, these zircons and their inclusions tion of reduced carbon likely deposited in byBelletal.inPNAS(1).Graphiteis,of arecurrentlyouronlytangiblerecordofthe sediments at that time. In the upper left course, the same material as found in pencil first half a billion years of Earth history. portion of Fig. 1, an electron micrograph tips or in the anode of lithium ion batteries. Based in part on the extreme age of some of the Hadean zircon is shown along with Graphite is, however, also a very common of these zircon minerals, along with similarly the δ13Cvalueof−24±4‰, as measured material in Earth Science, and is often the old age dates for a Martian meteorite (5), the using an ion microprobe (1). In geochemis- form of carbon found in very old fossils that Planetary Science and Earth Science commu- try, δ13C values are reported relative to an have been subjected to substantial heat. The nities now appreciate that planets form and inorganic carbonate reference, meaning that graphite described in the Bell et al. article cool rather quickly (6). No longer is the this negative value is quite depleted in 13C is remarkable because it is exceptionally old, Hadean just a placeholder on our timelines relative to typical inorganic carbonate dating to the Hadean eon. Officially, the between the formation of the Earth and the rocks. In fact, the value is indistinguishable Hadean is defined as the time period from oldest known rocks. The Hadean Earth is from biological carbon fractionated by mi- the formation of the Earth until 4 billion y represented by tangible samples that appear crobial carbon fixation. During carbon fix- ago. Until recently, this has been a seemingly to have formed in a continental crust setting, ation, enzymes preferentially incorporate 12C, convenient definition, leaving it as the geo- likely above an active subduction zone (7). leaving inorganic carbon enriched in 13C. logical eon without a rock record on Earth. Specifically, Bell et al. (1) describe two sizable Thus, one of the most important geological Over the past quarter century, however, the graphite inclusions within a 4.1 billion-y-old records for understanding the long habitabil- discovery and exploration of detrital zircon zircon from the Jack Hills. This sample im- ity of the Earth is the record of the carbon minerals from the Jack Hills conglomerates plies that a Hadean chunk of organic material isotopic composition of organic material and of Western Australia (2) have provided a new was transported by geological processes (in- inorganic carbonates back through time window into this early time. Jack Hills zir- cluding subduction) to a granitic magma (8, 9). Broadly, this record convincingly cons crystallized in magma chambers at var- chamber, where it was incorporated into the shows the sustained impact of global marine ioustimesasfarbackas4.4billionyago crystalizing minerals. The sheer existence of carbon fixation on preserved sedimentary carbon going back at least 3.5 billion y. Throughout this long history, microbial life in the oceans would have had a δ13Cvalueof Graphite in Mars Highly about −25‰ (Fig. 1), quite distinct from in- 4.1 Ga Zircon Moon -20‰ 13 organic carbon with a δ C value of about -24±4‰ Reducing Highly -20‰ 0‰. This record of life has potentially been Bell et al. (1) Reducing pushed further back to ∼3.9 billion y ago Prebiotic with the discovery of 13C-depleted graphite Soup? in Greenland metasediments (10, 11). This new discovery of 13C-depleted graphite from 4.1 billion y ago potentially pushes biological Typical carbon into the previously uncharted Hadean. Diamonds The implication is that there was a substantial -5‰ Microbes amount of potentially biogenic carbon on the -25‰ Earth 4.1 billion y ago, 200 million y before the next known sedimentary carbon. Extend- Other Diamonds Moderately -25‰ ing the biological carbon cycle further back Reducing based on this graphite inclusion is simple, consistent, and in line with the principle of Fig. 1. Summary of the few carbon isotopic (δ13C) and redox constraints we have for the Earth’s earliest carbon uniformitarianism. Thus, this discovery sug- cycle. In the upper left is the newly reported carbon isotopic value for a graphite inclusion in a 4.1 Ga Hadean zircon gests life was flourishing in our oceans near (−24 ± 4‰). This remarkable graphite provides a new constraint on the global carbon cycle in the Hadean. Also the end of the Hadean, if not earlier. The shown are Precambrian diamonds (bimodal at −5‰ and −25‰) and Early Archean microbial biomass (scattered around −25%). The Hadean zircon hosting the graphite comes from a continental crust setting, shown as thick and dark brown. Typically, continental crust forms in magma chambers above a subducting slab of oceanic crust, shown Author contributions: C.H.H. wrote the paper. here in black. Finally, various measures of redox suggest that the Earth’s present upper mantle is mildly reducing and The author declares no conflict of interest. has been such for >3.8 billion y. (Inset) Similar constrains for magmatic carbon found in lunar rocks (around −20‰) and Martian meteorites (around −20‰). Both the Moon and Mars appear to have highly reducing mantles. (Inset See companion article on page 14518. image adapted from ref. 1.) 1Email: [email protected]. 14410–14411 | PNAS | November 24, 2015 | vol. 112 | no. 47 www.pnas.org/cgi/doi/10.1073/pnas.1519765112 Downloaded by guest on September 23, 2021 13 next clear step will be to test the continuity aspects of the Earth in its final stage of for- reducing and had significant C-depleted COMMENTARY of possible biogenic graphite preservation in mation. The simplest conclusion is that the carbon in its upper mantle. In other zircon. If this does indeed record ancient Earth’suppermantlewassignificantlymore words, the earliest Earth may have looked sedimentary carbon preserved despite a trip reduced in the beginning. Growing evidence like our present Moon. Certainly, during through a magma chamber, then such a re- indicates that magmatic carbon on Mars (17, the Hadean the Earth’s mantle changed to cord in zircon should be recoverable through 18) and the Moon (19) has a carbon isotopic become more oxidizing and to settle on an the subsequent 4 billion y of Earth history. upper mantle isotopic composition similar to This ancient carbon appears to have had a Bell et al. describe two today. In the beginning, however, a prebiotic wild ride, having been in a magma chamber, sizable graphite inclu- soup could have produced substantial sedi- but looking at carbon from extreme geo- sions within a 4.1 mentary carbon, and it is possible that the logical settings is required to understand the carbon was more 13C-depleted than we nor- ’ Earth s carbon cycle. This new isotope mea- billion-y-old zircon from mally consider. Bell et al. (1) cover their bases surement is one more constraint added to a the Jack Hills. by appropriately calling their graphite poten- substantial body of similar research on mate- tially biogenic. Is the graphite they found from rials extruded from deep within the Earth. composition of about −20‰.Thisissig- 13 a prebiotic soup? It is possible and would be For example, the new measurement is similar nificantly more C-depleted than that of equally exciting, but I do not think so given to some mantle diamonds. Fig. 1 includes Earth’s present upper mantle. At present, that the zircon’s crystallization age is over 400 a summary of several decades of carbon iso- the textbook answer is that the Earth and million y after the Earth’s formation. Four- topic measurements of diamonds extruded Mars started out with different carbon from the Earth’s mantle in violent explosive and that the Moon is just weird, poten- hundred million years is a long time. In terms eruptions. Diamonds show that the mantle is tially having been influenced by meteorite of geophysics, it is more time than plate tec- bimodal with respect to carbon isotopes. impacts. tonics took to both assemble and break-up Most diamonds have carbon isotope values A wild alternative suggestion is that the the supercontinent Pangea. However, the clustered at −5‰, but there is another less- bulk of Earth is actually more 13C-depleted Jack Hills zircons extend back to about abundant group of diamonds with carbon than we think, with an average carbon iso- 4.4 billion y ago, roughly 100 million y isotopic values spread around −25‰ (12, topic value around −20‰. For example, the aftertheMoonwasformed.Ifgraphite 13). Based, in part, on this evidence, the man- Earth’s core could contain significant C, continues to be found in more ancient zir- tle is believed to have an isotopic composition resulting in a fractionated mantle (20). In cons, distinguishing between a dense prebi- of −5‰, and the prevailing view is that the any case, I think it is worth considering otic soup and a productive carbon-fixing more 13C-depleted diamonds represent bio- that at its beginning, the Earth was highly biosphere could be a challenge. genic carbon that has been subducted into the mantle (14). In this interpretation, the 13 transition zone diamonds and the Hadean 1 Bell EA, Boehnke P, Harrison TM, Mao WL (2015) Potentially 10 Rosing MT (1999) C-Depleted carbon microparticles in biogenic carbon preserved in a 4.1 billion-year-old zircon.