Scott Angus MacLennan

Snowball Earth episodes, times when the planet was covered in ice, represent the most extreme climate events in Earth's history. Yet, the mechanisms that drive their initiation remain poorly constrained. Current climate models require a cool Earth to enter a Snowball state. However, existing geologic evidence suggests that Earth had a stable, warm, and ice-free climate prior to the Neoproterozoic Sturtian global glaciation (ca. 717 Ma). Here, we present eruption ages for three felsic volcanic units interbedded with glaciolacustrine sedimentary rocks from southwest Virginia, USA, that demonstrate that glacially influenced sedimentation occurred at tropical latitudes ca. 751 Ma. Our findings are the first geologic evidence of a cool climate teetering on the edge of global glaciation several million years prior to the Sturtian Snowball Earth.

The Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations—the Sturtian "Snowball Earth." Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution−thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0−2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ¹³C and ⁸⁷Sr/⁸⁶Sr data and U-Pb ID-TIMS ages from the Tambien Group are used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. Tambien Group δ¹³C data and U-Pb ID-TIMS ages reveal that a pre-Sturtian sharp negative δ¹³C excursion (referred to as the Islay anomaly in the literature) precedes the Sturtian glaciation by ∼18 m.y., is synchronous in at least two separate basins, and is followed by a prolonged interval of positive δ¹³C values. The composite Tonian ⁸⁷Sr/⁸⁶Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater ⁸⁷Sr/⁸⁶Sr rose ca. 880−770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, suggest that the ⁸⁷Sr/⁸⁶Sr increase was influenced by increased subaerial weathering of radiogenic lithologies as Rodinia rifted apart at low latitudes. The following ⁸⁷Sr/⁸⁶Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO₂ and contributing to the initiation of the Sturtian glaciation.

In order to understand the onset of Snowball Earth events, precise geochronology and chemostratigraphy are needed on complete sections leading into the glaciations. While deposits associated with the Neoproterozoic Sturtian glaciation have been found on nearly every continent, time-calibrated stratigraphic sections that record paleoenvironmental conditions leading into the glaciation are exceedingly rare. Instead, the transition to glaciation is normally expressed as erosive contacts with overlying diamictites, and the best existing geochronological constraints come from volcanic successions with little paleoenvironmental information. We report new stratigraphic and geochronological data from the upper Tambien Group in northern Ethiopia, which indicates that the glacigenic diamictite at the top of the succession is Sturtian in age. U-Pb zircon dates obtained from two tuffaceous siltstones that are 74 and 84 m below the diamictite are 719.68 ± 0.46 Ma and 719.68 ± 0.56 Ma (2σ), respectively. We also report a U-Pb date of 735.25 ± 0.25 Ma from a crystal-rich tuff located 2 m above the nadir of a high-amplitude, basin-wide, negative δ¹³C excursion previously correlated with the Islay anomaly. This age for the anomaly agrees with Re-Os age constraints from Laurentia, suggesting that the δ¹³C signal is globally synchronous and preceded the Sturtian glaciation by ~18 m.y. The interval between the Islay anomaly and Sturtian glaciation is recorded in the Tambien Group as an ~600 m succession of predominantly shallow-water carbonates and siliciclastics with δ¹³C values recording a prolonged period at +5‰, followed by an interval of lower, but still positive, values leading up to the glaciation. Our data are consistent with synchronous global onset of the Sturtian glaciation at ca. 717 Ma. Shallow-water carbonates in strata directly below the first diamictite suggest that glacial onset was rapid in terranes of the Arabian-Nubian Shield.

The Makhonjwa Mountains, traditionally referred to as the Barberton Greenstone Belt, retain an iconic Paleoarchean archive against which numerical models of early earth geodynamics can be tested. We present new geologic and structural maps, geochemical plots, geo- and thermo-chronology, and geophysical data from seven silicic, mafic to ultramafic complexes separated by major shear systems across the southern Makhonjwa Mountains. All reveal signs of modern oceanic back-arc crust and subduction-related processes. We compare the rates of processes determined from this data and balance these against plate tectonic and plume related models. Robust rates of both horizontal and vertical tectonic processes derived from the Makhonjwa Mountain complexes are similar, well within an order of magnitude, to those encountered across modern oceanic and orogenic terrains flanking Western Pacific-like subduction zones. We conclude that plate tectonics and linked plate-boundary processes were well established by 3.2–3.6 Ga. Our work provides new constraints for modellers with rates of a 'basket' of processes against which to test Paleoarchean geodynamic models over a time period close to the length of the Phanerozoic.

Sean and I started working together while we were both in graduate school, he at Lamont and myself at Princeton. We ended up dating a bunch of stuff together from around the world. This was a great step forward getting a big data set published from the White Mountains. The high precision TIMS geochronology was done at Princeton, while Sean raised money for a large amount of laser ablation zircon data. What we found was that initiation of magmatism in the White Mountains preceded the Central Atlantic Magmatic Province by about 10 million years and progressed for the next ca. 30 million years! The next step is to put this protracted magmatism into the context of plate motion models to see if there are any connections with Pangea breakup.

I am very happy to report that I am moving back to South Africa to start a faculty job at the University of the Witwatersrand. I am super excited to start new stratigraphy and tectonics projects in southern Africa working on Proterozoic and Archean rocks! Wits also has a lot of great new instrumentation getting installed soon: a Nu TIMS as well as a Nu ICP-MS with a collision cell!

My PI Mauricio Ibanez-Mejia got an offer to move to the University of Arizona last year, and after quite a few COVID and lab related delays I am now joining him and the rest of the group in Tucson! We just finished driving the lab equipment, rock samples and my furniture from upstate NY to Arizona.

When I got to the University of Rochester I got an office with another postdoc called Feng Cheng (now at University of Nevada, Reno). While we only saw each other for a couple months before COVID descended, we got on really well and Feng asked if I wanted to get involved in a summary paper on dust origin for the Chinese Loess plateau. That paper is out now and I found the experience with Feng super productive and useful!

This was a bit of a labor of love, but our paper documenting the first depositional age constraints on the Konnarock Formation from Virginia is out in Science Advances: read it here.

Working on this project was a real pleasure — a bunch of friends from Princeton went down to Virginia to drink beer, eat BBQ and collect rhyolites for high precision geochronology! This work was inspired by a blog post by Callan Bentley, who summarized that the age of the Konnarock glacial diamictites was essentially unconstrained. Arthur Merschat from the USGS was in the process of getting a new map together of the area and his intuition was that the rhyolites were part of the stratigraphy. After we got the dates and it was clear that the Konnarock diamictites did not correlate in time with either of the Neoproterozoic snowball Earths, the job was then to think what these rocks implied for pre-Sturtian climate! Nadir Jeevanjee and Gabe Vecchi at Princeton were very happy to discuss these data and I learned a bunch about modern climate such as polar amplification — the process where the poles heat up much faster than the tropics.

A bit of a life milestone — I finished my PhD! Now I'm off to the University of Rochester to start postdoctoral work with Mauricio Ibanez-Mejia and Francois Tissot (at Caltech). The end of the PhD process is tough...

Our research paper in Geology this month got a bit of coverage. Please take a look: "Ancient Earth froze over in a geologic instant" (Science).

The department went on a trip to Scotland! We saw some classic geology such as Hutton's unconformity, and Arthur's Seat.