nai

Project Progress

Subproject 2, Neoproterozoic-Cambrian environmental change and evolution, has enjoyed the broadest participation of Harvard team members, and for good reason. The Proterozoic-Cambrian transition witnessed remarkable changes in tectonics, climate, atmospheric composition, and especially life. This is the interval during which animal life — and, hence, the prospect of intelligence, radiated on Earth. Harvard team researchers are studying the paleontology (Knoll, Grotzinger, Erwin), geochronology (Bowring, Grotzinger), tectonics (Hoffman, Bowring), and environmental changes (Hoffman, Schrag, Bowring, Grotzinger) of this interval, with an eye to constructing models of integrated change in the Earth system. Specific progress in Year 5 includes the following:

Using new carbon isotope data from carbonate-rich Neoproterozoic and Early Cambrian sections in Svalbard, Namibia and Morocco, Paul Hoffman’s group compiled the first high-resolution curve for Neoproterozoic-Cambrian seawater (Fig. 1). It highlights a series of high-amplitude biogeochemical anomalies that have no parallel in Phanerozoic (543 Ma to present) or earlier Proterozoic time. Two of these anomalies correspond to the Sturtian and Marinoan snowball Earth episodes, which were preceded uniquely by >100 myrs when fractional organic burial exceeded 0.4, or twice the modern value.

{{ 1 }}

Evidence for open water during Neoproterozoic glacial periods in different areas has been cited in opposition to the snowball Earth hypothesis. In Svalbard, Hoffman’s team documented an evaporitic carbonate lagoonal sequence within the Marinoan glacial sequence (Fig. 2). The stratigraphic and sedimentological contexts of this sequence suggest that it represents an oasis on snowball Earth. It formed when tropical sea-surface temperatures reached the melting point, causing shorefast ice to melt away, while the tropical ocean remained ice covered because of the inflow of thick sea ice (‘sea glaciers’) from higher latitudes. The glacial record of snowball Earth involves the different histories of three ice-mass types (Fig. 2): sea glaciers, shorefast sea ice (sikussak), and terrestrial ice domes.

{[ 2 }}

The existence of sand-wedge polygons formed in periglacial soils situated close to the paleo-equator during the Marinoan glaciation has long been cited as evidence for a high (>54 degrees) obliquity of the ecliptic during pre-Phanerozoic time, providing an alternative explanation to snowball Earth for low-latitude glaciation. A low-obliquity orbit does not produce sufficient seasonality at the equator to form 3-m deep cracks as observed. However, mechanical analysis reveals that such deep cracks can form by crack-propagation under diurnal forcing if soils are perpetually frozen (i.e., no summer active layer), as would be expected on a snowball Earth due to its high albedo. Thus, the observed sand wedges do not require high obliquity for their formation, which is not supported by any other data.

Hoffman’s group also finds low-temperature equilibrium fractionation of carbon and oxygen isotopes between coexisting dolomite and calcite in the Marinoan cap carbonate sequence in Namibia (Fig. 3 upper panel) and northwestern Canada. This suggests that dolomite formed in contact with seawater, unlike the normal Phanerozoic and Neoproterozoic ocean in which dolomite was kinetically inhibited. No similar fractionation is observed in other stratigraphic units (e.g. Fig. 3 lower panel). As sulfate ion is a known inhibitor of dolomite above 2 mM concentration (versus 28 mM in modern seawater), Hoffman proposes that low sulfate concentrations, evolved when the ocean was ice covered, led to sea-floor dolomite formation during the post-glacial transgression. Unlike normal marine carbonate phases, dolomite is stable and should be a more faithful carrier of isotopic information.

{[ 3 }}

Dan Schrag continued geochemical and geophysical modeling of the Neoproterozoic climate. The Snowball Earth hypothesis proposes that Neoproterozoic glacial deposits and associated “cap” carbonates represent a series of global glaciations followed by extreme greenhouse conditions. In the context of the hypothesis, a runaway ice-albedo feedback causes a global glaciation, with near-complete sea-ice cover, and a greatly reduced hydrologic cycle dominated by sublimation. Escape from this frozen state requires several to several 10’s of millions of years for carbon dioxide, released by magmatic outgassing, to build up in the ocean/atmosphere system, providing adequate radiative forcing to overcome the high planetary albedo. Meltback would be extremely rapid (i.e., hundreds of years), transforming the Earth from frozen to ultra-greenhouse conditions.

A research effort involving two members of our Astrobiology Group (Grotzinger, Bowring) has been able to provide important new constraints on biogeochemical events at the Precambrian-Cambrian boundary in Oman. Globally significant events of faunal turnover, tectonic reorganization, and biogeochemical change are now widely regarded to have closely coincided with the Precambrian-Cambrian boundary. Over the past decade, research on the boundary has provided constraints on the age and duration of these events, the possible phylogenetic relationships between faunal assemblages, and the magnitude and significance of contemporaneous shifts in the trace element and isotopic composition of the oceans. In turn, these results enable tighter focus on the links that existed between global biogeochemical events and episodes of faunal extinction and radiation. Biostratigraphic, carbon isotopic, and uranium-lead zircon geochronological data from Ara Group of Oman indicate an abrupt last appearance of Cloudina and Namacalathus coincident with a large-magnitude, but short-lived negative excursion in the carbon-isotopic composition of seawater that is globally coincident with the Precambrian-Cambrian boundary. U-Pb zircon age data from an intercalated ash bed directly constrain this negative excursion to be 542 Ma, consistent with previous constraints from Siberia and Namibia. The Oman age of 542 Ma constitutes the best independent evidence that this anomaly is global in extent. The stratigraphically highest fossiliferous sediments in Oman, which directly underlie the strata containing the negative excursion, contain an ash bed yielding an age of 543.2 ± 0.5 Ma. The absence of Cloudina and Namacalathus fossils in carbonate units above this negative excursion contrasts strongly with their great abundance in carbonate units below the negative excursion. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary. The most likely explanation — that Cloudina and Namacalathus disappeared as part of a global extinction — can account for the absence of those fossils in carbonates above the negative excursion in Oman. The simplest interpretation of the data presented here is that extinction occurred globally at the Precambrian-Cambrian boundary coincident with a global biogeochemical event marked by the negative isotope excursion. Given the extreme difficulty in preserving Ediacaran soft-bodied organisms as fossils, their stratigraphic distribution across the Precambrian-Cambrian boundary has provided ambiguous constraints on evolutionary models. In contrast, the disappearance of calcified Cloudina and Namacalathus above this critical level, despite constant depositional and taphonomic conditions, provides a more clearly resolved image of this global extinction.

A second research effort has resulted in the development of a digital model of a Proterozoic Thrombolite reef complex in Namibia. A terminal Proterozoic isolated carbonate platform resided downdip on a carbonate ramp of the Nama Group of southern Namibia. The overall geometric evolution of the studied platform from flat-topped to bucket with elevated margins is recorded in many Proterozoic and Phanerozoic isolated carbonate platforms with similar dimensions. The terminal Proterozoic, microbial-dominated, isolated carbonate platform of this study clearly illustrates that the answer to accommodation changes was already familiar among carbonate platforms before the dawn of metazoan-dominated platforms. The stratigraphic evolution of the platform was digitally reconstructed from an extensive dataset that was compiled by using digital surveying technologies. The platform comprises three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long-term accommodation increase. Aggradation and progradation during the first cycle resulted in a flat, uniform, sheet-like platform. The coarsening and shallowing-upward sequence representing the first cycle is dominated by columnar stromatolitic thrombolites and massive dolostones with interbedded mudstone-grainstone at the base of the sequence grading into cross-bedded dolostones. The second cycle features aggradation, formation of a distinct margin containing thrombolite mounds and domes, and the development of a bucket geometry. Columnar stromatolitic thrombolites dominate the platform interior. The final stage of platform development shows a deepening trend with initial aggradation and formation of well-bedded, thin deposits in the interior and mound development at the margins. While the interior drowned, the platform margin kept up with rising sea level, and a complex pinnacle reef formed containing fused and coalesced thrombolite mounds flanked by bioclastic grainstones (containing Cloudina and Namacalathus fossils) and collapse breccias. A set of isolated large thrombolite mounds flanked by shales indicates the final stage of the carbonate platform. During a progressive increase in accommodation, a flat-topped isolated carbonate platform becomes aerially less extensive by either backstepping or formation of smaller pinnacles or a combination of both.

In the past year, Sam Bowring’s group has worked on refining the history of glaciation in Newfoundland as well as in Namibia and Oman. At present we have evidence for glacial deposits that are >760 Ma, 711 Ma, 650-630 Ma, and 580 Ma. The out standing question is whether these are global or snowball glaciations or whether some are “normal” high-latitude glaciations. The other big question concerns synchronicity and duration. As reported last year and at the annual Astrobiology meeting in Tempe, the Gaskiers glacial deposit in Newfoundland has duration of less than 1 Ma, a duration at odds with models for snowball glaciation. While it could be dismissed as a non-snowball glaciation, it does have an associated cap carbonate and a low paleolatitude (30°). Another major effort has been to constrain the age and significance of the Cambrian/Precambrian boundary in Oman. Significantly, the boundary or extinction horizon corresponds to a sharp negative excursion in carbon isotopes that has duration of less than 1 Ma. It is quite possible that like other major extinctions in Earth history, post-extinction time was characterized by a period of rapid evolutionary change, which is often described as the Cambrian explosion. In complementary research, team member Knoll joined French colleagues in the radiometric analysis of Neoproterozoic phosphates from China. The rocks contain the oldest known fossil remains of animals; their recorded Pb-Pb age of 596+/-4 million years supports the hypothesis that, at 580 Ma, the brief Gaskiers glaication belongs to a third ice age that postdated the globally extensive Sturtian and Marinoan events.