Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago.

The history of the growth of continental crust is uncertain, and several different models that involve a gradual, decelerating, or stepwise process have been proposed1-4. Even more uncertain is the timing and the secular trend of the emergence of most landmasses above the sea (subaerial landmasses), with estimates ranging from about one billion to three billion years ago5-7. The area of emerged crust influences global climate feedbacks and the supply of nutrients to the oceans 8 , and therefore connects Earth's crustal evolution to surface environmental conditions9-11. Here we use the triple-oxygen-isotope composition of shales from all continents, spanning 3.7 billion years, to provide constraints on the emergence of continents over time. Our measurements show a stepwise total decrease of 0.08 per mille in the average triple-oxygen-isotope value of shales across the Archaean-Proterozoic boundary. We suggest that our data are best explained by a shift in the nature of water-rock interactions, from near-coastal in the Archaean era to predominantly continental in the Proterozoic, accompanied by a decrease in average surface temperatures. We propose that this shift may have coincided with the onset of a modern hydrological cycle owing to the rapid emergence of continental crust with near-modern average elevation and aerial extent roughly 2.5 billion years ago.

Affinities and architecture of Devonian trunks of Prototaxites loganii.

Devonian fossil logs of Prototaxites loganii have been considered kelp-like aquatic algae, rolled up carpets of liverworts, enormous saprophytic fungal fruiting bodies or giant lichens. Algae and rolled liverwort models cannot explain the proportions and branching described here of a complete fossil of Prototaxites loganii from the Middle Devonian (386 Ma) Bellvale Sandstone on Schunnemunk Mountain, eastern New York. The "Schunnemunk tree" was 8.83 m long and had six branches, each about 1 m long and 9 cm diam, on the upper 1.2 m of the main axis. The coalified outermost layer of the Schunnemunk trunk and branches have isotopic compositions (δ(13)CPDB) of -25.03 ± 0.13‰ and -26.17 ± 0.69‰, respectively. The outermost part of the trunk has poorly preserved invaginations above cortical nests of coccoid cells embraced by much-branched tubular cells. This histology is unlike algae, liverworts or vascular plants and most like lichen with coccoid chlorophyte phycobionts. Prototaxites has been placed within Basidiomycota but lacks clear dikaryan features. Prototaxites and its extinct order Nematophytales may belong within Mucoromycotina or Glomeromycota.

A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.

To understand better the link between atmospheric CO2 concentrations and climate over geological time, records of past CO2 are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO2 variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO2 concentrations, I reconstruct past atmospheric CO2 concentrations. For the past 300 Myr, only two intervals of low CO2 (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1-8 Myr) and early Permian (275-290 Myr) times. But for most of the Mesozoic era (65-250 Myr), CO2 levels were high (1,000-2,000 p.p.m.v.), with transient excursions to even higher CO2 (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO2 (refs 1, 2) and palaeotemperature records, but suggest that CO2 reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO2 in greenhouse climate warming over the past 300 Myr.

Paleoenvironmental reconstruction of middle Miocene paleosols bearing Kenyapithecus and Victoriapithecus, Nyakach Formation, southwestern Kenya.

Paleosols in the middle Miocene (15 Ma) Nyakach Formation at Kaimogool, near Sondu, southwestern Kenya have yielded specimens of the early cercopithecoid Victoriapithecus macinnesi and the early kenyapithecine Kenyapithecus africanus, and can be used as evidence for the environmental mosaic occupied by these primates. Five distinct types of paleosols (pedotypes) are recognized in the Nyakach Formation section at Kaimogool South. The most common paleosols are reddish brown, silty calcareous profiles with blocky structure and large root traces (Ratong pedotype) which are interpreted as soils of well-drained, dry bushland or thicket (nyika). Weakly developed paleosols associated with paleochannels (Dhero pedotype) represent wooded grassland early in the ecological succession from streamside flooding. One of these paleosols has yielded a fossil flora of grasses and small-leaved dicots like those of modern semi-arid wooded grassland. Crumb structured, calcareous paleosols with iron-manganese nodules (Yom pedotype) are interpreted to represent seasonally waterlogged, wooded grassland (dambo or vlei). Thick, red clayey, calcareous paleosols with blocky ped structure and large root traces (Tut pedotype) are interpreted as soils of well-drained dry woodland. Other blocky-structured, gray to brown calcareous paleosols with iron-manganese nodules (Chido pedotype) are interpreted as soils of seasonally waterlogged, riparian dry woodland. Fossil soils, plants and gastropods are evidence of an unusually dry (300-500 mm mean annual precipitation) habitat for apes, consisting of a vegetational mosaic dominated by dry woodland, bushland and thickets with few areas of seasonally waterlogged grassland. Fossils of V. macinnesi are rare from Nyakach, but were found in paleosols representative of bushland and thicket habitats (Ratong). Fossils of the ape K. africanus were found within paleosols indicative of dry woodland (Tut). Other paleosol types representative of seasonally dry dambo grassland (Yom), colonizing grassland (Dhero) or riparian woodland (Chido) are also represented, but have not yet produced primate fossils.

Permian-triassic life crisis on land.

Recent advances in radiometric dating and isotopic stratigraphy have resulted in a different placement of the Permian-Triassic boundary within the sedimentary sequence of the Sydney Basin of southeastern Australia. This boundary at 251 million years ago was a time of abrupt decline in both diversity and provincialism of floras in southeastern Australia and extinction of the Glossopteris flora. Early Triassic vegetation was low in diversity and dominated by lycopods and voltzialean conifers. The seed fern Dicroidium appeared in the wake of Permian-Triassic boundary floral reorganization, but floras dominated by Dicroidium did not attain Permian levels of diversity and provinciality until the Middle Triassic (244 million years ago).

Evidence from paleosols for the geological antiquity of rain forest.

Kaolinitic claystones in Paleozoic paleokarst underlying the Middle Pennsylvanian Fort Scott Limestone near Drake, Missouri, contain abundant fossil root traces. These include a surficial root mat as well as stout, woody, deeply penetrating root traces: a rooting pattern similar to that under rain forest. Also similar to soils of rain forest is the deeply weathered clay of the paleosol, in which minimal amounts of nutrient bases remain. Forest communities adapted to oligotrophic clayey substrates in humid climates existed at least 305 million years ago.

Reassessment of the paleoenvironment and preservation of hominid fossils from Hadar, Ethiopia.

Samples of paleosols from locality AL-333, known for numerous specimens of Australopithecus afarensis, were analyzed in order to reconstruct the original soils and environment of burial of the associated fossil hominids. The bones were found in swale-like features, within the calcareous and coarse-grained basal portion of a paleosol. This is more like an assemblage of bones buried during a single depositional episode, such as a flood, than an assemblage accumulated on a soil over a long period of time by carnivores or other means of death. What killed the hominids remains unclear, but considering the association of originally disarticulated bones of such hydraulically distinct types as phalanges and maxillae, it is very likely that they died and partially rotted at or very near this site. The paleosols at AL-333, here named the Fo and Go clay paleosols, have calcareous rhizoconcretions, granular surface horizons, prismatic peds, and shallow calcareous nodules and stringers like soils now supporting grassy woodland in semiarid regions. Although this group of hominids was buried in streamside gallery woodland, there is evidence from Laetoli, Tanzania, that A. afarensis ventured out into open wooded grassland as well. Evidence for this should be sought from other paleosols at Hadar.

Fossil soils and grasses of a middle miocene East african grassland.

Fossil soils and grasses from the well-known Miocene mammal locality of Fort Ternan, southwestern Kenya, are evidence of a mosaic of grassy woodland and wooded grassland some 14 million years ago. This most ancient wooded grassland yet known on the African continent supported more abundant and diverse antelopes than known earlier in Africa. Ape fossils at Fort Ternan, including Kenyapithecus wickeri, were associated with woodland parts of the vegetation mosaic revealed by paleosols. Grassland habitats were available in East Africa long before the evolutionary divergence of apes and humans some 5 to 10 million years ago.

Trace fossil evidence for late ordovician animals on land.

Fossil burrows within newly recognized buried soils in the Late Ordovician Juniata Formation, near Potters Mills in central Pennsylvania, represent the oldest reported nonmarine trace fossils. They are thought to have been an original part of the soil because their greater density toward the top of the paleosols corresponds with mineralogical, microstructural, and chemical changes attributed to ancient weathering and because about half the burrows are encrusted with nodular carbonate, interpreted as caliche. Associated fossil caliche, the size distribution of the burrows, and their W-shaped backfills are evidence that the burrows may have been excavated by bilaterally symmetrical organisms that grew in well-defined growth increments and were able to withstand desiccation. Among well-known soil organisms, millipedes are burrowing animals that satisfy these requirements, but have a fossil record not quite this old. This trace fossil evidence for animals on land, together with recent palynological evidence for land plants of a bryophytic grade of evolution during Late Ordovician time, are indications of terrestrial ecosystems of slightly greater antiquity and complexity than hitherto suspected.