Michael M. Tice

Oxygen and hydrogen isotope evidence for a temperate climate 3.42 billion years ago

Stable oxygen isotope ratios (δ18O) of Precambrian cherts have been used to establish much of our understanding of the early climate history of Earth and suggest that ocean temperatures during the Archaean era (∼3.5 billion years ago) were between 55 °C and 85 °C (ref. 2). But, because of uncertainty in the δ18O of the primitive ocean, there is considerable debate regarding this conclusion. Examination of modern and ancient cherts indicates that another approach, using a combined analysis of δ18O and hydrogen isotopes (δD) rather than δ18O alone, can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters. Here we show that δ18O and δD sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa are consistent with formation from waters at varied low temperatures. The most 18O-enriched Buck Reef Chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below ∼40 °C. Geochemical and sedimentary evidence suggests that the Buck Reef Chert was formed in shallow to deep marine conditions, so our results indicate that the Palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler (≤40 °C) than previously thought.

Continental arc–island arc fluctuations, growth of crustal carbonates, and long-term climate change

The Cretaceous to early Paleogene (ca. 140–50 Ma) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO_2. Hypotheses for the elevated CO_2 concentrations invoke an increase in volcanic CO_2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO_2 during this time interval. We show here that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. In particular, many of these continental arcs, unlike island arcs, intersected ancient continental platform carbonates stored on the continental upper plate. We show that the greater length of Cretaceous–Paleogene continental arcs, specifically carbonate-intersecting arcs, could have increased global production of CO_2 by at least 3.7–5.5 times that of the present day. This magmatically driven crustal decarbonation flux of CO_2 through continental arcs exceeds that delivered by Cretaceous oceanic crust production, and was sufficient to drive Cretaceous–Paleogene greenhouse conditions. Thus, carbonate-intersecting continental arc volcanoes likely played an important role in driving greenhouse conditions in the Cretaceous–Paleogene. If so, the waning of North American and Eurasian continental arcs in the Late Cretaceous to early Paleogene, followed by a fundamental shift in western Pacific subduction zones ca. 52 Ma to an island arc–dominated regime, would have been manifested as a decline in global volcanic CO_2 production, prompting a return to an icehouse baseline in the Neogene. Our analysis leads us to speculate that long-term (>50 m.y.) greenhouse-icehouse oscillations may be linked to fluctuations between continental- and island arc–dominated states. These tectonic fluctuations may result from large-scale changes in the nature of subduction zones, changes we speculate may be tied to the assembly and dispersal of continents. Specifically, dispersal of continents may drive the leading edge of continents to override subduction zones, resulting in continental arc volcanism, whereas assembly of continents or closing of large ocean basins may be manifested as large-scale slab rollback, resulting in the development of intraoceanic volcanic arcs. We suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.

DEEP-WATER INCISED VALLEY DEPOSITS AT THE EDIACARAN-CAMBRIAN BOUNDARY IN SOUTHERN NAMIBIA CONTAIN ABUNDANT TREPTICHNUS PEDUM

Abstract Valley-filling deposits of the Nama Group, southern Namibia, record two episodes of erosional downcutting and backfill, developed close together in time near the Ediacaran-Cambrian boundary. Geochronological constraints indicate that the older valley fill began 539.4 ± 1 Ma or later; the younger of these deposits contains unusually well-preserved populations of the basal Cambrian trace fossil Treptichnus pedum. Facies analysis shows that T. pedum is closely linked to a nearshore sandstone deposit, indicating a close environmental or taphonomic connection to very shallow, mud-draped sandy seafloor swept by tidal currents. Facies restriction may limit the biostratigraphic potential of T. pedum in Namibia and elsewhere, but it also illuminates functional and ecological interpretation. The T. pedum tracemaker was a motile bilaterian animal that lived below the sediment-water interface—propelling itself forward in upward-curving projections that breached the sediment surface. The T. pedum animal, therefore, lived infaunally, perhaps to avoid predation, surfacing regularly to feed and take in oxygen. Alternatively, the T. pedum animal may have been a deposit feeder that surfaced largely for purposes of gas exchange, an interpretation that has some support in the observed association of T. pedum with mud drapes. Treptichnus pedum provides our oldest record of animals that combined anatomical and behavioral complexity. Insights from comparative biology suggest that basal Cambrian T. pedum animals already possessed the anatomical, neurological, and genetic complexity needed to enable the body plan and behavioral diversification recorded by younger Cambrian fossils.

Palaeontology: Modern life in ancient mats

Microbial communities seem to have inhabited tidal sediments 2.9 billion years ago much as they do today — but what organisms were involved, and how they made their living, remain intriguing questions.