F. A. Macdonald

Harvard University | Harvard · Department of Earth and Planetary Sciences

Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower...

The Ediacaran Shuram carbon isotope excursion (CIE) follows the regional Gaskiers glaciation and occurs before the appearance of macroscopic animal fossils. Previous interpretations for the Shuram CIE have proposed global perturbations to Earth's carbon cycle accompanied by significant climatic and environmental change. These studies assume that ca...

New analytical and field techniques, as well as increased international communication and collaboration, have resulted in significant new geological discoveries within the Appalachian-Caledonian-Variscan orogen. Cross-Atlantic correlations are more tightly constrained and the database that helps us understand the origins of Gondwanan terranes conti...

The rise of complex macroscopic life occurred during the Ediacaran Period, an interval that witnessed large-scale disturbances to biogeochemical systems. The current Ediacaran chronostratigraphic framework is of insufficient resolution to provide robust global correlation schemes or test hypotheses for the role of biogeochemical cycling in the evol...

The Humber Margin of Newfoundland preserves the most distal exposures of Proterozoic basement in northeastern Laurentia. Age uncertainty has permitted a range of hypotheses for its origin and links to subsequent tectonic events. One hypothesis has proposed large-scale orogen-parallel displacement between basement blocks in western Newfoundland. The...

The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16-18‰. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon i...

Neoproterozoic carbon isotope excursions are commonly attributed to changes in the global fraction of organic carbon burial associated with climate instability and/or oxygenation. Here we show that carbonate sediment deposited during the ca. 661-<651 Ma Cryogenian non-glacial interlude between the Sturtian and Marinoan glaciations exhibit lateral o...

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth’s climate1–3. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth’s surface environments4–12. Here we argue...

Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized sprea...

The Tonian supercontinent Rodinia is hypothesized to have included almost all Proterozoic continental blocks. Competing models variably place South China at the core or periphery of Rodinia or separated from it entirely. Tonian paleogeographic models also vary in whether they incorporate hypothesized large and rapid oscillatory true polar wander as...

Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group...

In the Eastern Cordillera of Peru, observations related to the accretion of the Arequipa Terrane to the Amazon craton are scarce and reactivation of sutures in a backarc basin may make interpretation more difficult. Cambro-Ordovician backarc successions located in proximity to a proposed suture in the Eastern Cordillera of southern Peru were invest...

One of the hypothesized effects of large igneous provinces (LIPs) is planetary cooling on million‐year timescales associated with enhanced silicate weathering of freshly emplaced basalt. This study combines reconstructions of the original surface extent and emplacement ages of LIPs, a paleogeographic model, and a parameterization of LIP erosion to...

Significance The Southeast Asian islands are a modern-day hotspot of CO 2 consumption via silicate weathering. Since ∼15 million years ago, these islands have been increasing in size at the same time that Earth’s climate has been cooling. Here, we test the hypothesis that this global cooling could have been driven by tectonic emergence of the South...

Significance A geochemical model shows that discrete volcanic perturbations, coupled to long-term changes in sulfur biogeochemistry result in apparent stepwise changes in the isotopic composition of marine sulfur, similar to the patterns seen in the marine barite record of the Cenozoic and Late Mesozoic. The perturbations required to reproduce this...

Carbon isotope values from shallow-marine carbonate rocks, including those from many dolomitized successions, are the primary lens through which we interpret the ancient carbon cycle. Carbon isotopes are typically regarded as being robust to alteration during dolomitization due to the high carbon content of the rock compared to the fluid. However,...

Death Valley (California, USA) hosts iconic Cryogenian snowball Earth deposits, but the lack of direct geochronological constraints has permitted a variety of correlations and age models. Here, we report two precise zircon U-Pb isotope dilution–thermal ionization mass spectrometry dates for the Kingston Peak Formation: a volcanic eruptive age of 70...

Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed ε o r g - c a r b ). Sedimentary deposits immediat...

The temperature and chemistry of early seawater have both been inferred from the isotopic composition of Precambrian chert (SiO2), a precipitated mineral formed on or within marine sediments. The δ18O of chert shows a robust quasi-linear increase through time - a signal that has been interpreted in a number of conflicting ways. For example, changin...

Significance Erosion below the Great Unconformity has been interpreted as a global phenomenon associated with Snowball Earth. Geological relationships and thermochronologic data provide evidence that the bulk of erosion below the Great Unconformity in Colorado occurred prior to Cryogenian glaciation. We suggest that there are multiple, regionally d...

The snowball Earth hypothesis predicts a strong hysteresis resulting in discrete multimillion-year glaciations followed by globally synchronous deglaciation. Here we present new U-Pb zircon and Re-Os sedimentary rock geochronology and Os isotope chemostratigraphy from post-Sturtian sequences in south China to test the synchroneity of deglaciation....

The oxygen isotope composition (δ ¹⁸ O) of marine sedimentary rocks has increased by 10 to 15 per mil since Archean time. Interpretation of this trend is hindered by the dual control of temperature and fluid δ ¹⁸ O on the rocks’ isotopic composition. A new δ ¹⁸ O record in marine iron oxides covering the past ~2000 million years shows a similar sec...

Paleogeographic models commonly assume that the supercontinent Rodinia was long-lived, with a static geometry involving Mesoproterozoic links that developed during assembly and persisted until Neoproterozoic rifting. However, Rodinian paleogeography and dynamics of continental separation around its centerpiece, Laurentia, remain poorly constrained....

We present a new sequence stratigraphic and facies architectural model for the ca. 900–850 Ma Hematite Creek and Katherine groups (lower and middle Mackenzie Mountains Supergroup, respectively) in the Wernecke Mountains of east-central Yukon, Canada. This ~3-km-thick internally conformable succession provides a virtually continuous tectono-sediment...

On multi-million-year timescales, Earth has experienced warm ice-free and cold glacial climates, but it is unknown if transitions between these background climate states were the result of changes in CO 2 sources or sinks. Low-latitude arc-continent collisions are hypothesized to drive cooling by uplifting and eroding mafic and ultramafic rocks in...

The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability with...

The North Slope subterrane of Arctic Alaska extends from the northeastern Brooks Range of Alaska into adjacent Yukon, Canada, and includes a pre-Mississippian deep- water sedimentary succession that has been historically correlated with units exposed in the Selwyn basin of northwestern Laurentia. Sedimentary provenance data, includ- ing Sm-Nd isoto...

Ediacaran Cloudina and Namacalathus are among the earliest shell-forming organisms. The debated carbonate phase of their skeletons, high-magnesium calcite or aragonite, has been linked to seawater chemistry and pCO2, yet independent constraints on the original mineralogy are lacking. We present a new method to distinguish primary skeletal mineralog...

Microfossil assemblages that include large acritarchs with complex processes, known as Doushantuo-Pertatataka-type acritarchs, are recovered from early Ediacaran successions globally. They are commonly found in shale and chert lithologies, but their diversity and palaeobiological significance is greatest when they are phosphatized. The best-known e...

Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to plac...

Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO 2 was 10 2 PAL (presen...

Phosphorites of the latest Ediacaran upper Khesen Formation in the Khuvsgul Group of northern Mongolia preserve a newly discovered, three-dimensionally phosphatized Doushan- tuo-type microfossil assemblage. Eight genera include the second occurrence of the putative multicellular fossil animal embryo Megasphaera outside South China, the Doushantuo-P...

Here we present a new U-Pb baddeleyite ID-TIMS age of 713.7 ± 0.9 Ma on the shoshonitic Tatonduk Suite near the Yukon-Alaska border in the Proterozoic Tatonduk inlier, which occur as mafic and intermediate dykes and a sequence of basaltic to andesitic volcanic flows immediately beneath iron formation-bearing Sturtian glacial deposits correlated wit...

Cryogenian strata of the Windermere Supergroup were deposited in tectonically active basins throughout the North American Cordillera from Alaska to Mexico. The Windermere Supergroup of Yukon, Canada, hosts key geochronological constraints on the start of the Cryogenian Period and the onset of the Sturtian Snowball Earth glaciation at ca. 717. Ma. A...

Archaeocyaths are an enigmatic group of calcifying sponges prevalent in early Cambrian (Terreneuvian to Series 2) successions around the world and preserved predominantly in reefal buildups, but also in adjacent reworked deposits. Here we report exceptionally preserved phosphatized archaeocyaths and small shelly fossils from phosphatized reef flank...

The Earth's climate cooled through the Ordovician Period leading up to the Hirnantian glaciation. Increased weatherability of silicate rocks associated with topography generated on the Appalachian margin during the Taconic orogeny has been proposed as a mechanism for Ordovician cooling. However, paleogeographic reconstructions typically place the A...

Ordovician strata of the Mohawk Valley and Taconic allochthon of New York and the Humber margin of Newfoundland record multiple magmatic and basin-forming episodes associated with the Taconic orogeny. Here we present new U-Pb zircon geochronology and whole rock geochemistry and neodymium isotopes from Early Paleozoic volcanic ashes and siliciclasti...

U-Pb dates on magmatic and detrital zircon from samples in the hinterland of the Taconic orogen place new constraints on the timing and plate tectonic geometry of terrane accretion and magmatic arc activity. The Moretown terrane, a Gondwanan-derived exotic block, extends from the Rowe Schist-Moretown Formation contact in the west to the Bronson Hil...

A global rise in oxygen levels has been proposed to coincide with the Ediacaran to Cambrian radiation of animals, yet the precise timing and nature of this change remains unresolved. One hypothesis is that the ocean/atmosphere system became temporarily well-oxygenated in the earliest Ediacaran, directly following the Marinoan Snowball Earth glaciat...

Cryogenian cap carbonates that overlie Sturtian glacial deposits were formed during a post-glacial transgression. Here, we describe microfossils from the Kakontwe Formation of Zambia and the Taishir Formation of Mongolia-both Cryogenian age, post-Sturtian cap carbonates-and investigate processes involved in their formation and preservation. We comp...

CAOB occupies a vast area that extends from the Urals to the Far East Asia and from the Siberian craton to the North China and Tarim cratons (Fig. 1, A). In order to better constrain Precambrian tectonic evolution of the CAOB, it is important to revisit Precambrian terranes of Mongolia as outlined in Badarch et al. [2002] that contain Archean to Pr...